FLUE GAS CONDITIONING SYSTEM

Technical Field

This invention relates to systems for treating boiler flue gas to improve the removal of particulate matter contained therein by electrostatic means and, more particularly, relates to a flue gas conditioning agent production systems for converting sulfur dioxide in the flue gas into sulfur trioxide to provide conditioning agent to the flue gas particulate matter prior to its passage through an electrostatic precipitator.

Background Art

The increasing demand for electrical power has forced electrical utilities to burn increasing quantities of fossil fuels such as coal; however, electric utilities also face increasing environmental standards imposed upon their operation. In order to reduce air pollutants, electrical utilities have turned to burning low-sulfur coals to fire their boiler furnaces to generate the steam needed for generating electrical power. In addition, electrical utilities generally use a flue gas treatment system to remove the majority of the particulate matter in the gas effluent. A flue gas treatment system generally comprises an electrostatic means, such as an electrostatic precipitator, and a conditioning agent source for introducing a conditioning agent into the flue gas stream to enhance the efficiency of the precipitator in removing particulate matter.

The efficiency of an electrostatic precipitator in removing particulate matter from the boiler flue

gas is partially dependent upon the electrical resistivity of the entrained particulate matter in the boiler flue gas. The entrained particulate matter expelled from a boiler fired with low-sulfur coal, i.e., coal having less than 1 percent sulfur, has been found to have a resistivity of approximately 10 ¹³ ohms/cm. It has been determined that the most efficient removal of particulate matter by electrostatic precipitation occurs when the particulate matter resistivity is approximately 10 ⁸ ohms/cm. Therefore, to obtain more effective use of an electrostatic precipitator, the resistivity of the entrained particulate matter from low-sulfur content coals must be reduced. Electrical utilities have long used conditioning agents introduced into the flue gas flow upstream of the electrostatic precipitator to reduce the resistivity of the entrained particles. Various chemicals, such as water and anhydrous ammonia, sulfuric acid, sulfur trioxide and phosphoric acid and various ammonia-bearing solutions have been used as conditioning agents.

A variety of flue gas conditioning systems have been disclosed to automatically add conditioning agent, most generally S0 ₃ , into a flue gas stream to condition entrained particulate matter. Such prior systems include those which control the quantity of conditioning agent injected into the flow of flue gas by using the quantity of coal being burned or electrical power generated (U.S. Patent Nos. 3,689,213; 3,772,178; and 3,993,429) or by using parameters of the exhaust system, such as precipitation power, exhaust gas flow rate, or opacity (U.S. Patent Nos. 4,770,674; 4,779,207; 5,032,154). Such prior systems also include those referred to as "slipstream" systems in which a portion of the flue

gas is removed from the flow of flue gas, passed across a catalytic converter where S0 ₂ contained in the flue gas portion is converted to S0 ₃ and returned into the flow of the gas (U.S. Patent Nos. 3,581,463; 5,011,516). In addition, multi boiler flue gas conditioning systems have been disclosed, such as that proposed in U.S. Patent No. 4,333,746.

A controller commercially available from Castlet (Electronic Engineers) Ltd. , of Lincoln, England, can control an electrostatic precipitator by detecting the presence of deleterious back ionization and intermittently applying voltage to the charging electrodes of the precipitator to minimize the back ionization phenomenon. The Castlet controller detects back ionization by interrupting the applied charging voltage at its peak value and comparing, after a preset time, the actual charging electrode voltage with a programmed charging electrode voltage to identify excess charging electrode decay rate, which is indicative of back ionization. The Castlet controller uses the difference in the actual and the programmed charging electrode voltage to determine a rate of application of voltage to the charging electrodes in an effort to optimize precipitator operation in the presence of back ionization. Other conditioning systems are shown, for example, in U.S. Patent Nos. 3,686,825; 3,893,828; 4,042,348; 4,284,417; 4,466,815; 4,533,364; and 4,624,685.

Disclosure of Invention

The present invention includes systems, methods and apparatus for conditioning flue gas with sulfur trioxide for removal of entrained particles with an electrostatic precipitator.

Some systems of the invention feature a self- contained sulfur burner control loop that operates to maintain a constant desired temperature supply of sulfur dioxide in air by varying the rate of supply of sulfur fuel to the sulfur burner and the absence of a device in the sulfur flow path to measure or monitor sulfur flow rate. In such systems of the invention, a conditioning agent demand signal operates either a means to supply the sulfur burner with a flow of air, or an air heater in the air flow path upstream of the sulfur burner, or both, to provide conditions that would lower or raise the temperature of the sulfur burner output absent the self-contained sulfur burner control loop, and as a result of the operation of the self-contained sulfur burner control loop, such systems provide a desired flow of sulfur dioxide and air at a constant desired temperature to a catalytic converter for conversion to sulfur trioxide in response to a conditioning agent demand signal. Other systems of the invention feature a control apparatus for generating a conditioning agent demand signal in response to a sensed discharge rate of a charged electrode of the electrostatic precipator. Further systems of the invention feature conversion means for generating conditioning agent movable between an operative position where flue gas flows through the conversion means for generation of conditioning agent and an inoperative position where the glue gas flows through the conversion means. Still further systems of the invention feature a single integrated means for

generating flows of S0 ₂ and air and a plurality of controllable remotely located catalytic conversion means to convert the S0 ₂ to S0 ₃ conditioning agent adjacent to each of a plurality of flue gas ducts to condition the flue gas from a plurality of boilers. Of course, some systems of the invention feature a combination or combinations of such features and those further described below.

Systems of the invention can thus preferably comprise first means for providing a flow of sulfur dioxide in air at a desired temperature to a catalytic converter, which includes at least a sulfur burner, means for providing said sulfur burner with a flow of air, means for providing said sulfur burner with a flow of sulfur, temperature sensor means for sensing the temperature of said flow of sulfur dioxide in air, and a control for said means for providing said sulfur burner with a flow of sulfur or said means for providing said sulfur burner with a flow of air, or both, that responds to said temperature sensor means to control the flow of sulfur, or the flow of air, or both, and maintain the flow of sulfur dioxide in air at the desired temperature for conversion to a flow of sulfur trioxide conditioning agent. In such systems of the invention, a sulfur trioxide demand signal means can provide a signal corresponding to a demand for sulfur trioxide to a system controller, and the system controller can be connected with the means for providing the sulfur burner with a flow of air and programmed to operate the means for providing the sulfur burner with a flow of air to vary the air flow into the sulfur burner in response to the sulfur trioxide demand signal to meet the sulfur trioxide demand.

Systems of the invention can further comprise means for heating the flow of air to the sulfur burner, means for bypassing a portion of the air flow around the sulfur burner, means for heating the portion of the air flow bypassing the sulfur burner and means for directing heated bypass air into the flow of sulfur trioxide conditioning agent and/or directing the heated bypass air through the catalytic converter. In such further systems of the invention, the sulfur trioxide demand signal can be used by the system controller to vary the heat added to the flow of air to the sulfur burner and the heat added to the bypass air, either alone or in combination with variation of the air flow through the sulfur burner. In such further systems, an air heater in the flow path to the sulfur burner may be operated only to preheat the sulfur burner, and an air heater in the flow path of the bypass air may be operated only to preheat the catalytic converter and to keep the flow of sulfur trioxide and air from the catalytic converter to the injection site above the condensation temperature.

Systems of the invention can also preferably comprise means for controlling the generation of conditioning agent by a sulfur trioxide demand signal generated from a rate of discharge of a charged electrode of the electrostatic precipitator. It has also been discovered that the rate of electrode discharge, such as the voltage decay of a charged precipitator electrode, can be used to indicate when conditioning agent will be needed and to provide an effective flow of conditioning agent to improve removal of entrained particulate matter from a boiler flue gas, and to preclude a wasteful flow of conditioning agent when it is not needed. In such

preferred systems of the invention signal means, connected with the electrostatic precipitator, sense a voltage discharge rate of an electrostatic precipitator charging electrode and generate a conditioning agent demand signal for use in producing a flow of conditioning agent. In one embodiment of the invention, the signal means for generating a conditioning agent demand signal can include a voltage sensing circuit connected with the charging electrode to provide a voltage decay rate signal, and can further provide rate-of-change circuit means to generate the conditioning agent demand signal. The signal means can also generate a conditioning agent demand signal by comparing a charging electrode decay rate signal with a pre-programmed decay rate, for example, by comparing after a programmed time, a voltage proportional to the voltage of a charging electrode with a programmed voltage.

Systems of the invention can also comprise apparatus and method to generate within the main flue gas duct for the boiler flue gas and from the flowing flue gas an S0 ₃ conditioning agent to enhance the electrostatic removal of particulate matter entrained in the flue gas. In such systems of the invention, a movable catalytic converter is operatively associated with the main flue gas duct and is moved between an inoperative position where flue gas does not pass through the catalytic converter and an operative position where a substantial portion of the flue gas passes through the catalytic converter for the generation of S0 ₃ conditioning agent. In preferred such system, a catalytic converter is moved between an inoperative position outside of the flue gas ductwork and an operative position within the flue gas ductwork. Furthermore, in such preferred system, the

catalytic converter may be enclosed in a housing adjacent the flue gas ductwork in the inoperative position, and the housing may be isolated from the flue gas ductwork and opened to permit maintenance and regeneration of the catalyst.

The invention also provides S0 ₂ /S0 ₃ converters particularly effective for use in such in-duct flue gas conditioning systems of the invention. Such S0 ₂ /S0 ₃ converters provide a plurality of open, generally parallel paths for flue gas that are formed by a S0 ₂ /S0 ₃ catalyst effective over a wide temperature range for the conversion of preferably low percentage of the S0 ₂ in the flue gas stream to S0 ₃ conditioning agent. Systems of the invention can also provide sulfur trioxide conditioning agent to multiple flue gas streams of multiple boilers from a single sulfur dioxide source (hereafter referred frequently as a "cross-tie system") . The cross-tie systems of this invention can overcome disadvantages of prior multi- boiler flue gas conditioning systems. In preferred cross-tie systems of the invention, there is no loss of control flexibility and each separate boiler flue gas stream is provided with a variable individual conditioning agent flow which can satisfy the desirable operating conditions of the electrostatic precipitator for the flue gas from that individual boiler. With the cross-tie system of this invention, the flue gas of any number of boilers can be conditioned using either S0 ₂ or elemental sulfur as feedstock sulfur source. A significant advantage of such systems is its versatility, which is based on using or generating gaseous S0 ₂ and delivering gaseous S0 ₂ to a plurality of catalytic converters, with each catalytic converter being located adjacent the site of

injection of sulfur trioxide into the flue gas conduit of a different boiler. At each sulfur trioxide injection site, the S0 ₂ may be mixed with heated air, and sulfur trioxide (S0 ₃ ) may be generated at each of the separate catalytic converters to suit the individual needs of the flue gas streams from the individual boilers connected with the system. In such systems of the invention, a sulfur furnace can supply S0 ₂ for conversion to S0 ₃ for a multiplicity of boilers and the difficult transmission of S0 ₃ can be avoided. Hot S0 ₂ generated at the sulfur furnace can be delivered long distances without risk of pipe corrosion.

Such preferred systems of the invention for conditioning a plurality of flue gas streams from a plurality of boilers can comprise means for providing a single flowing mixture of sulfur dioxide gas and air, means for dividing the single flowing mixture of sulfur dioxide gas and air into a plurality of flows of sulfur dioxide gas and air in a plurality of distribution conduits, with each of the distributing conduits extending from the dividing means to adjacent one of the plurality of flue gas conduits and an adjacent one of a plurality of catalytic converters for converting sulfur dioxide and air into sulfur trioxide conditioning agent. Each one of the plurality of catalytic converters is connected with a different one of the distributing conduits at its input end and with means to inject sulfur trioxide into one of the plurality of flue gas streams from one of the plurality of boilers at its output end.

Such systems can further comprise means for providing a single flow of air, means for dividing the flow of air into a plurality of flows of air in a plurality of air distributing conduits; each of the

air distributing conduits extending from said air dividing means to adjacent one of the plurality of flue gas conduits and an adjacent one of a plurality of temperature controlled air heaters, with each one of said plurality of air heaters having its input connected with one of the air distributing conduits and its output connected with the input of one of the catalytic converters. A plurality of flow control valves, one for each of the sulfur dioxide distributing conduits and air distributing conduits, can be operated to control the generation of sulfur trioxide and for each one of the flue gas streams by the plurality of control outputs from one or more controllers. Each of the plurality of control outputs is connected to one of said flow control valves and is generated by a controller, which is adapted for determining the need for conditioning agent for the flue gas flowing in the flue gas conduit with which it is connected. In preferred such systems, one or more of the plurality of catalytic converters can provide two- stage conversion of the sulfur dioxide and air flowing through the converter. Such two-stage catalytic converters can comprise two individual catalytic converter assemblies interconnected with a cooling conduit and installed adjacent a flue gas conduit to which it is connected.

The foregoing brief general disclosure of major features of the invention cannot include, disclose or describe all the inventive methods and apparatus and combinations of inventive features of the invention. Various embodiments of systems incorporating many preferred inventive features of the invention are illustrated on the drawings and disclosed in more detail below, including descriptions of their

inventive methods of operation and inventive combinations of apparatus. While not included in this section of the description for purposes of brevity, it should be understood that method and apparatus shown in the drawings and disclosed below in more detail are part of the invention.

Brief Description of Drawings

Fig. 1 is a diagrammatic representation of a flue gas conditioning system of the present invention.

Fig. 2 is another diagrammatic representation of a conditioning agent production system of the present invention.

Fig. 3 shows a simplified schematic representation of the electrical characteristics of particle charging in an electrostatic precipitator to help explain the invention.

Fig. 4 shows a general diagrammatic representation of a flue gas treatment system of the invention embodying an alternative charging electrode decay rate sensing means.

Fig. 5 is a block diagram of a device for generating a signal indicative of the resistivity of the particulate matter of the system. Fig. 6 is a diagrammatic representation of a preferred condition agent production system of the present invention.

Fig. 7 shows a more detailed diagrammatic representation of a flue gas treatment system incorporating the present invention.

Fig. 8 shows a diagrammatic representation of a flue gas treatment system with an alternative embodiment of the present invention.

Fig. 9 is a general schematic view of a flue gas conditioning system provided by this invention.

Fig. 10 is a partially exploded perspective view of an in-duct flue gas conditioning system provided by this invention shown in working arrangement adjacent a vertical flue gas duct. Fig. 11 is a perspective view of the in-duct flue gas conditioning system of Fig. 10 showing catalyst beds disposed in operative positions within the flue gas duct.

Fig. 12 is a perspective view of the in-duct flue gas conditioning system of Fig. 10 showing a single catalyst bed disposed in an inoperative position outside the flue gas duct.

Fig. 13 is an enlarged partial cross-sectional view showing a catalyst bed of this invention as depicted in Fig. 12 in an inoperative position outside of the main flue gas duct.

Fig. 14 is a view of the system of Fig. 12, from the left of Fig. 13, showing a damper system and access means provided by this invention. Fig. 15 is a further enlarged isolated cross section of Fig. 13 showing a catalyst bed and means provided by this invention for driving the catalyst bed between operative and inoperative positions.

Fig. 16 is a partial cross-sectional end view of the roller assembly provided by a preferred embodiment of this invention taken from line 16-16 of Fig. 15.

Fig. 17 is a perspective view of a further embodiment of an in-duct flue gas conditioning system provided by this invention shown in a working arrangement adjacent a vertical flue gas duct.

Fig. 18 is an enlarged isolated cross-sectional view showing one means by which the catalyst bed of the system of Fig. 17 can be pivotably moved between a first operative position within the gas flow and a second inoperative position outside of the gas flow.

Fig. 19 is a perspective view of a still further embodiment of the in-duct flue gas conditioning system of Fig. 17 shown in a working arrangement adjacent a horizontal flue gas duct. Fig. 20 is a partial perspective view of the induct flue gas conditioning system of Fig. 10 using the S0 ₂ /S0 ₃ converters of this invention.

Fig. 21 is a perspective view of an S0 ₂ /S0 ₃ converter of this invention. Fig. 22 is a partial perspective view of a portion of the S0 ₂ /S0 ₃ converter of Fig. 21.

Figs. 23 and 24 are partial plan views of the S0 ₂ /S0 ₃ converter with flue gas flow paths of different cross-sections. Fig. 25 is a diagrammatic drawing of another system of this invention.

Fig. 26 is a diagrammatic drawing of still another system of the invention.

Fig. 27 is a cross-sectional view through the center of a catalytic converter adapted to supply a single injection site in system of the invention.

Fig. 28 is a drawing illustrating an example of an installation of a catalytic converter at an injection site for a flue gas conduit.

Best Models) for Carrying Out the Invention

Fig. 1 shows system 10 of the invention which features a self-contained sulfur burner control loop, encompassed within the dashed line box 43, to maintain a constant temperature supply of sulfur dioxide in air to catalytic converter 44.

System 10 includes a conditioning agent production system 12 coupled to conditioning agent injectors 16 which are positioned within duct 18. The conditioning agent production system 12 is operated by

a conditioning agent system controller 39. A sulfur source 41 provides a supply of liquified sulfur to sulfur pump 40 which in turn delivers a flow of sulfur from sulfur source 41 to a sulfur burner 42. Catalytic converter 44 converts the products of combustion of sulfur from sulfur burner 42 into sulfur trioxide. The sulfur trioxide from catalytic converter 44 is directed to conditioning agent injectors 16 for introducing sulfur trioxide into boiler flue gas stream 14 for conditioning its entrained particulate matter in preparation for removal by electrostatic precipitator 20.

The self-contained sulfur burner control loop of Fig. 1 includes temperature sensor means 56 connected with a control 45 for the sulfur pump 40. Temperature sensor means 56 measures the temperature of the sulfur dioxide and air leaving the sulfur burner and provides an output signal if the sulfur dioxide-air mixture departs from a selected desired temperature set point. Control 45 operates the sulfur pump 40 in response to the signal from temperature sensor means 56 to increase the flow of sulfur to the sulfur pump if the temperature of the sulfur dioxide- air mixture leaving the sulfur pump is less than the desired temperature and to decrease the flow of sulfur to the sulfur pump if the temperature of the sulfur dioxide-air mixture leaving the sulfur burner is higher than the desired temperature. Thus, in systems of the invention, the sulfur burner is operated in a simple manner, like many furnaces, to increase or decrease the fuel flow (i.e., sulfur flow) to maintain a desired temperature at its output, or downstream of its output, for example, at the catalytic converter 44. Unlike prior art systems, the sulfur burner pump 40 in systems of the invention is not operated

directly in response to a conditioning agent demand signal, and no means is necessary to sense sulfur flow rate. The elimination of sulfur flow sensing means eliminates a significant source of maintenance and unreliability in the gas conditioning system.

Where the heat losses experienced by the sulfur dioxide-air mixture in travelling between the sulfur burner output and the catalytic converter input are predictable, the temperature sensor means 56 is preferably located at the output of the sulfur burner to avoid a conduit installation that may be necessary when the catalytic converter 44 is installed, remotely from the sulfur burner 42. Where the catalytic converter 44 is remote from the sulfur burner 42 and catalytic converter temperature cannot be reliably predicted, temperature sensor means 56 can be located at and operatively associated with the catalytic converter (see, for example, 56A of Fig. 6) . In either such arrangement, the self-contained sulfur burner loop can automatically operate to maintain the operating temperature of the catalytic converter 44 within a temperature range for efficient conversion of sulfur dioxide to sulfur trioxide.

In the invention, the sulfur pump of the self- contained sulfur burner control loop can also be operated to provide conversion of sulfur to sulfur dioxide in varying amounts to satisfy the sulfur trioxide demands of the system as a result actions of system controller 39 in response to a conditioning agent demand signal 24A by producing input conditions to sulfur burner 42 that would reduce or increase the temperature of its sulfur dioxide-air output in the absence of the self-contained sulfur burner control loop 43. For example, the conditioning agent production means 12 of Fig. 1 includes a means 46 for

providing the sulfur burner 42 with a flow of air and a means 48 for heating the flow of air to the sulfur burner 42. In the simplest method of operation of the system of Fig. 1, this air flow heating means 48 is operated only to bring the sulfur burner 48 up to sulfur burning temperature and is inoperative after sulfur flow is commenced to sulfur burner 42, and controller 39 operates the means 46 for providing the sulfur burner 42 with a flow of air to increase air flow through the sulfur burner in response to a conditioning agent demand signal indicating an increased demand for conditioning agent and to decrease the air flow through the sulfur burner 42 in response to a conditioning agent demand signal indicating a reduced demand for conditioning agent. In the system of the invention, the self-contained sulfur burner control loop 43 will increase the flow of sulfur to the sulfur burner as air flow through the sulfur burner increases and decrease the flow of sulfur to the sulfur burner as air flow through the sulfur burner decreases to maintain the desired output temperature of its sulfur dioxide-air output. Another desirable feature of the system of the invention is that its operation tends to automatically maintain a constant sulfur dioxide-air concentration for delivery to the catalytic converter and permit effective and efficient operation of catalytic converter 44 through a supply of sulfur dioxide in air at a substantially constant concentration and desired temperature. Means 46 for providing the sulfur burner with a flow of air can be a variable speed blower, a constant flow blower with either an input or an output damper, or any other such means for providing varying flow rates of air.

The turndown ratio of such systems can be extended by operating air heating means 48 at low sulfur flow rates. In addition, in such systems where the air heating means 48 continues to operate, controller 39 can vary the operation of air heating means 48, reducing the heat introduced into the air flow into the sulfur burner 42 if the conditioning agent demand signal indicates an increased demand for conditioning agent and increasing the heat introduced into the air flow into the sulfur burner 42 in the event of a decreased demand for conditioning agent. In addition, both air flow means 46 and air heating means 48 (which combine to provide means for providing the sulfur burner with a flow of heated air 49) can be operated by controller 39 in response to a conditioning agent demand signal. As explained above, in response to such changing inputs to the sulfur burner, the self-contained sulfur burner control loop will vary the sulfur flow into the sulfur burner 42 to maintain the desired temperature of its sulfur dioxide-air output and thereby meet the varying demands for conditioning agent flow.

Fig. 1 has been simplified to clarify the description of the invention. Commercial system will include additional components apparent to those skilled in the art.

Conditioning agent controller 39 is preferably an industrial quality programmable controller capable of storing, loading, and executing program instructions for retrieving input information and controlling output devices. Controller 39 receives at least one load demand signal 24A related to a demand for sulfur trioxide. Pre-programmed instructions stored in controller 39 perform the desired operations on the demand signal input to formulate a desired output for

operation of air flow means 46 or air heating means 48, or both. Controller 39 then outputs a signal to either air flow means 46 or air heating means 48, or both, thereby indirectly controlling the sulfur dioxide output of sulfur burner 42. Conditioning agent controller 39 thus responds to conditioning agent demand signals to satisfy a demand for sulfur trioxide and the self-contained sulfur burner control loop 43 maintains automatically the operating temperature of catalytic converter 44 within acceptable limits.

Sulfur source 41 can be either an insulated, steam-heated, steel container or a concrete-lined storage pit placed largely underground and is preferably adapted to contain liquified sulfur. The tank or concrete pit can contain a heater or heat exchanger in intimate contact with the sulfur to liquify the sulfur and to keep the liquified sulfur at the preferred temperature for minimum viscosity and pumping. The heat exchanger with sulfur source 41 may be any heat exchanger suitable for this purpose and may be provided with any source of heat, such as steam, electric, or the output of a suitable oil or gas burner. Sulfur pump 40 for delivering sulfur from the source 41 to sulfur burner 42 is preferably a positive-displacement pump, such as gear pump or vane pump driven by a variable speed electric motor to deliver a flow of liquified sulfur at a controllable rate. In preferable systems, the positive displacement pump may be immersed within the liquified sulfur to simplify the installation, improve operating characteristics and eliminate pump seal problems. Preferably, sulfur pump 40 will supply a flow of sulfur at rates of, for example, one to five pounds

per minute (0.45-2.26 kg/min.) and at a pressure of 60 to 100 pounds per square inch absolute (4218-7030 gm/cm 2) . Since li.qui.fi.ed sulfur is easy to pump, e.g., having a viscosity on the order of water and being non-abrasive, it will be apparent to those skilled in the art that a number of commercially available positive displacement pumps may be used.

Sulfur burner 42 and catalytic converter 44 are the type known to those skilled in the art. The sulfur burner can be the type frequently referred to as a "checker work" or a "cascade burner" or can be an atomizer type. Such sulfur burners are operable preferably in the range of 750° F to 850° F (400° C to 454° C) to oxidize the liquified sulfur into sulfur dioxide through combustion.

Catalytic converter 44 is a structure and converter well known in the art which is capable of catalytically converting sulfur dioxide to sulfur trioxide through the action of a vanadium pentoxide or other catalyst. Converter 44 contains such catalyst generally applied to the surface of ceramic materials; and as sulfur dioxide passes through the catalytic converter, it is exposed to the catalyst and is converted into sulfur trioxide. It is well known in the art that the catalytic converters preferably operate at a temperature range from about 750° F to about 1075° F (399° C to 579 °C) , and preferably at about 850° F (454° C) . It is also well known in the art that below temperatures of about 750° F (399°C) and above temperatures of about 1100° F (593°C) such catalytic converters are not efficient in converting sulfur dioxide into sulfur trioxide.

As shown in Fig. 1, such systems also include a forced air supply 46 for providing a flow of air to sulfur burner 42 and catalytic converter 44. The

forced air supply 46 may be a commercial air blower known to those skilled in the art. The size of the blower and its electric motor drive will depend on the capacity of conditioning agent production system 12. The air flow from blower 46 is directed through heater 48 and then into sulfur burner 42 for pre-heating the sulfur burner to the combustion temperature of sulfur. Once sulfur flow to sulfur burner 42 begins and combustion occurs, burner start-up heater 48 may be de-energized thereby saving energy as its operation may not be needed in systems of this invention.

Fig. 2 is a diagrammatic representation of a system of the invention with a preferred system and method of generating a conditioning agent demand signal 24A. The system 10 includes an electrostatic precipitator system 11 and a conditioning agent production system 12. Electrostatic precipitator system 11 produces an electric field between charging electrode 30 and collection electrode 32 for electrostatically charging entrained particulate matter passing therebetween. Conditioning agent production system 12 introduces a flow of conditioning agent, preferably sulfur trioxide, into a flow of particulate laden boiler flue gas (indicated generally by arrow 14) through injectors 16 positioned within a duct or conduit 18 upstream of an electrostatic precipitator 20 to reduce the resistivity of the particulate matter, thereby aiding the removal of particulate matter from the boiler flue gas prior to its expulsion to the atmosphere from a stack 22.

Signals relating to a demand for sulfur trioxide are coupled to conditioning agent system controller 39. Conditioning agent system controller 39 controls the conditioning agent production system 12 to provide a

flow of conditioning agent so as to maintain an acceptable particulate matter output from stack 22.

Electrostatic precipitator system 11 includes an electrostatic precipitator controller 24 which supplies an AC output to a high voltage transformer 26. The AC output of high voltage transformer 26 is then converted to DC by rectifier 28. An output of rectifier 28 supplies DC high voltage and current to charging electrode 30. Collection electrode 32 is grounded. The DC voltage and current supplied to charging electrode 30 creates a high voltage electric field between charging electrode 30 and collection electrode 32 which causes corona and an electric discharge current to flow therebetween. The corona discharge current charges the particulate matter of the flue gas passing through the electric field, and the charged particulate matter is then attracted to collection electrode 32.

Fig. 3 depicts a simplified series circuit representation of the electrical relationship between charging electrode 30 and collection electrode 32. Rl represents the resistance of an air gap 36 between charging electrode 30 and an outer surface of collected particulate matter 34 collected on collection electrode 32. R2 represents the resistance of particulate matter 34 accumulated on collection electrode 32. As shown, the total charging electrode voltage (VT) applied to charging electrode 30 is equal to the sum of the effective voltage (Vair) across air gap 36 and the voltage drop (Vpar.) across collected particulate matter 34.

When the resistivity of the collected particulate matter 34 is high, collected particulate matter 34 begins to act like an insulator. As the resistivity of the particulate matter 34 increases, a surface

charge develops and the voltage dropped across collected particulate matter 34 increases, thus reducing the effective voltage across the air gap (Vair), i.e., between charging electrode 30 and the outer surface of collected particulate matter 34. A high effective air gap voltage is required to maintain an effective particle changing current, which will drop significantly if the air gap voltage becomes too low. As the surface charge and voltage across the collected particulate matter 34 increase, the dielectric strength of the layer of collected highly resistive particulate matter is exceeded and back ionization occurs. Back ionization substantially reduces the charging and collection of particulate matter and generally indicates the need for more conditioning agent in the flue gas stream to reduce the resistivity of entrained particulate matter.

As indicated above, a prior art Castlet controller detects back ionization by interrupting the AC input to the transformer 26/rectifier 28 set (shown in Fig. 2) at the peak AC (which corresponds to the peak DC voltage on charging electrode 30) and by comparing actual charging electrode voltage decay with a programmed charging electrode voltage decay. The Castlet controller executes pre-programmed instructions to determine the existence of an excessive voltage decay rate of an electrostatic precipitator charging electrode by interrupting the applied charging electrode voltage at its peak, monitoring the decay of the charging electrode voltage and comparing the charging electrode voltage with a programmed voltage after a pre-programmed time. The Castlet controller uses the results of the comparison to determine a frequency of voltage application at which the charging electrodes are recharged. A high

number of charging voltage applications per second (high frequency) is intended to produce a relatively high effective voltage between the precipitator charging and collection electrodes (i.e., across the air gap) and corresponds to a lack of back ionization. For example, if the charging application is continuous, i.e., a 60 Hz AC application in a half wave rectifier system or a 120 Hz AC application in a full wave rectifier system, the effective voltage is at a maximum and discharge rate of charging electrodes is low. A low number of charging voltage applications per second (low frequency such as 6 voltage applications per second or 6Hz) is intended to operate at a lower average voltage to avoid back ionization. Therefore, the frequency of the charging voltage application is indicative of the discharge rate of charging electrodes.

The frequency of voltage applications of a Castlet controller can be used to indicate the resistivity of the flue gas particulate matter collected by the electrostatic precipitator and the need for conditioning agent. In the invention, a method and apparatus are provided to monitor the frequency of application of voltage of a Castlet controller and/or provide a "decay rate signal", preferably in the form of a standard 4-20 milli-ampere or digital control signal, which can be interpreted to indicate the need, or lack of need, for conditioning agent and the flow rate of conditioning agent needed to provide a particulate resistivity for effective particle charging and removal, for precluding back ionization and for maximizing air gap collection voltage.

In a system of the invention shown in Fig. 2, the discharge rate of a charging electrode 30, can be

determined by a controller 24, such as a Castlet controller provided with a signal means 137, and the controller and signal means can provide a signal 24A, which will be related to the resistivity of collected particulate matter 34. As explained above, such a signal indicative of the voltage discharge of a precipitator charging electrode can be indicative of the resistivity of the collected particles conditioned and/or unconditioned. A controller 24 adapted to provide an output signal 24A related to discharge rate of charging electrode 30 can provide a signal indicative of the variable that conditioning systems seek to control, the resistivity of the particulate matter. Thus, in the invention, an electrostatic precipitator system 11 including such a signal means 137 can provide a signal 24A based upon the indirect determination of the resistivity of the particulate matter in the flue gas treatment system which can be used to produce a conditioning agent demand signal 24A.

Either controller 24 or controller 39 can be adapted to compare a discharge rate signal with pre¬ programmed levels indicating the need for and the lack of need for conditioning agent, and adapted between these pre-programmed levels to calculate or to look up, in a programmed look-up table, the operating conditions for air flow means 46 or air heater means 48, or both, that are needed under the indicated conditions of the flue gas treatment system to satisfy a conditioning agent demand.

An alternate means for determining the decay rate of charging electrode 30 is also shown in Fig. 4 and is depicted by dashed lines. Controller 24, like the Castlet controller, is programmed to interrupt the voltage applied to the charged electrodes of the

electrostatic precipitator at the peak applied voltage level. A first terminal 38a and a second terminal 38b of a signal means 38, such as a voltage divider, are coupled between an input of changing electrode 30 and ground, respectively. A third terminal 38c of signal means 38 is coupled to a first means 137. The voltage signal from signal means 38 indicates the voltage of charging electrode 30 as a function of time.

Controller 24 is thus adapted with first means 137 for generating a signal 24A indicative of the resistivity of the particulate matter in the system. Fig. 5 shows one embodiment of such a means 137. As shown in Fig. 4, the signal 38C from signal means 38 can be directed to an input of a gating circuit 137a. The gating input to gating circuit 137a can be generated by a gate pulse generating circuit connected with the output of controller 24. The gating circuit input is applied as the voltage that is applied to the charged electrodes of the electrostatic precipitator is interrupted, and the gating circuit input is applied to the gating circuit for a predetermined measurement period, or until voltage is again applied to the electrostatic precipitator electrodes. During the gating period, with no voltage applied to the charging electrodes, the signal from signal means 38C is passed by the gating circuit to a voltage rate of change circuit 137b (such as a differentiator circuit or a circuit which compares the voltage signal with a programmed voltage after a programmed time) . The voltage rate of change circuit 137b provides a voltage rate of change output (i.e., electrode voltage discharge rate) to an output signal circuit 137c. Output signal circuit 137c may use the output from the rate of change circuit 137b and provide either an analog or a digital output signal that may be

converted to a conditioning agent demand signal by either the controller 24 or system controller means 39. In preferred systems of the invention, however, a signal 24A indicative of the resistivity of the particulate matter and the need for conditioning agent is provided to the system controller 39, which determines the operation of sulfur trioxide production means 12.

Other conditioning agent demand signals such as signals indicating the quantity of coal burned, the opacity of the flue gas effluent and/or the precipitator power can also be supplied to system controller means 39 if advisable.

Fig. 6 is a diagrammatic drawing of another system of the invention including components to provide a bypass air flow path for a portion of the air flow from air flow means 46 around the air heater 48 and sulfur burner 42, and both through and around the catalytic converter 44. The control of the portions of the air flow from air flow means 46 that are directed through the sulfur burner 42 and through the bypass air flow path is effected by operation of valve means 60 and 62 by the system controller 39. The control of the air portion in the bypass air flow path to direct it either through the catalytic converter 44 or around the catalytic converter or both, is effected by operation of valve means 52 and 54 by the system controller 39.

The system of Fig. 6, like the system illustrated in Fig. 1 includes a self-contained sulfur burner control loop (within dashed box 43) which operates to maintain a flow of sulfur dioxide in air at a substantially constant concentration and at the desired temperature for effective conversion of the sulfur dioxide to sulfur trioxide. In the system of

Fig. 5, system controller 39 receives a conditioning agent demand signal 24A from a means 137 for determining the discharge rate of the charged electrodes 30 of electrostatic precipitator 20 in the manner described above, and controller 39 determines, from the conditioning agent demand signal 24A, operating conditions for valve means 60 and/or air heater 48 to provide input conditions to sulfur burner 42 that result in a sulfur flow into the sulfur burner 42 and a production therein of sulfur dioxide that provides a satisfactory flow of sulfur trioxide in response to the demand for conditioning agent.

In the system of Fig. 6, blower 46 also supplies a flow of air, controlled by valve 62 and controller 39 through converter air heater 50 to pre-heat catalytic converter 44 to a preferred temperature range of 750° to 1000° for conversion of sulfur dioxide into sulfur trioxide. Once the catalytic converter 44 reaches its operating temperature and sulfur flow is initiated to sulfur burner 42, valve 52 is closed and valve 54 is opened thereby diverting the air flowing through converter air heater 50 around catalytic converter 44 to be mixed with the sulfur trioxide-air mixture leaving catalytic converter 44 to maintain a relatively constant rate of gas flow through the injectors 16 that direct sulfur trioxide into flue gas stream 14.

In a preferred method of this invention, air is supplied through the heater 48 from blower 46 to pre- heat the sulfur burner 42 to the combustion temperature of sulfur and there is no bypass air flowing. System controller 39 then applies power to the sulfur pump 40 and initiates a flow of liquid sulfur through the sulfur burner 42 and at the same time de-energizes the heater 48. The heat generated

from the sulfur combustion continues to ignite the flow of sulfur as it arrives in sulfur burner 42. Air flow through sulfur burner 42 is adjusted in response to the sulfur trioxide demand by operating the means for providing a flow of air to the sulfur burner (for example, valve means 60 in Fig. 6) with controller 39, and sulfur flow adjusts through the action of the self-contained sulfur burner control loop 43 and satisfies the demand from sulfur trioxide. As indicated above, air flow into the sulfur burner 42 is related to sulfur trioxide demand.

In another method of the invention, all of a constant air flow enters the heater 48 and there is no bypass air flowing. Temperature at the outlet of the heater 48 is controlled by the sulfur trioxide demand. Temperature at the output of heater 48 is inversely related to demand. That is, at maximum sulfur trioxide demand, the temperature setpoint at the outlet of heater 48 is minimum. At minimum demand sulfur trioxide, the temperature setpoint of heater 48 is maximum. Temperature into the converter 44 is controlled by the adjustment of the sulfur flow rate by the self-contained sulfur burner control loop 43 to maintain a constant temperature at the output of the sulfur burner and the input to the converter 44. In such embodiments of the invention, system controller 39 controls the rate of flow of sulfur trioxide from signal means 137 generating a sulfur trioxide demand signal 24A, preferably from a signal relating to the voltage decay rate of an electrostatic precipitator charging electrode 30, as illustrated and described above. Controller 39 is provided with sulfur trioxide demand 2 A and may be provided with other signals such as the rate of coal consumption,

the flue gas opacity, and precipitator power consumption.

In another method of operation of the system of Fig. 6, the bypass air flow through catalytic converter heater 50 is not totally diverted around the catalytic converter 44 by valves 52 and 54, but a portion of the heated air flow from catalytic converter heater 50 is directed through the catalytic converter 44, and the catalytic converter heater 44 is operated by controller 39 over a control connection (shown as dashed line 50A in Fig. 6) to maintain the catalytic converter operating temperature within a desirable range. In such operations, the system of Fig. 6 is provided with the temperature sensor 56A at the inlet of catalytic converter 44. In this method, the temperature setpoint at the discharge of the bypass heater 50 is varied inversely with sulfur trioxide demand. At maximum S0 ₃ demand, the temperature will be at the minimum setting and at minimum S0 ₃ demand, the temperature setpoint will be at the maximum setting. Where heated bypass air from heater 50 is introduced ahead of the converter 44, it will be introduced ahead of the converter inlet temperature sensor 56A. As before, control of the converter inlet temperature occurs by adjustment of the sulfur flow rate, which in the system of the invention, also meets the demand for conditioning agent.

In still another method of the invention, the temperatures at the outlets of the sulfur burner and bypass heaters, 48 and 50 respectively, and air flow through the sulfur furnace and in the bypass path are controlled by the conditioning demand signal from controller 59. Minimum flow limits are placed on air flows, and the sulfur burner and bypass burner output

temperatures are adjusted to compensate. In the system, sulfur flow varies to maintain a substantially constant converter temperature.

In each of the methods of invention, there is no need to measure sulfur flow. Measuring devices for sulfur flow are expensive and in many cases require frequent maintenance. Proper sulfur flow can be determined by the stable control of the converter temperature. Failure of sulfur flow will be quickly detected. A low temperature at the converter will show flow failure. Prior art systems control sulfur burner discharge and converter inlet temperatures by adjusting heat input ahead of the sulfur burner. In such systems, these temperatures will be at setpoint regardless of sulfur flow. Hence pump failure will not be detected until discovered by operator monitoring or by improper precipitator operation caused by loss of conditioning.

In one embodiment of Fig. 6, sulfur burner 42 and catalytic converter 44 are located remote in relation to one another to allow for their convenient placement. For example, sulfur burner 42 may be located at ground level near sulfur source 41 and sulfur pump 40. Catalytic converter 44 can be conveniently mounted in an elevated location adjacent to flue duct 18. In such an arrangement, sulfur burner 42 and catalytic converter 44 are coupled by a thermally insulated delivery conduit 59, and the sulfur dioxide-air mixture produced in sulfur burner 42 is transferred via insulated delivery conduit 59 to catalytic converter 44 for conversion into sulfur trioxide. In such embodiments, a trim signal which relates to the temperature of catalytic converter 44 can be calculated based upon the temperature of the sulfur dioxide-air mixture exiting sulfur burner 42.

A temperature sensor 56 mounted near an output of sulfur burner 42 supplies a temperature signal related to the temperature of sulfur burner 42. The temperature of the sulfur dioxide-air mixture exiting sulfur burner 42 will be higher than the temperature of the sulfur dioxide-air mixture entering catalytic converter 44 because of thermal losses in conduit 59. However, the temperature of catalytic converter 44 can be determined by system controller 39 because the thermal losses in conduit 59 can be calculated.

System controller 39 may calculate with acceptable accuracy the catalytic converter temperature and provide a trim signal based upon the output temperature and flow rate of sulfur burner 42, values related to, for example, the thermal dissipation of delivery conduit 59 and the applicable ambient temperature and pre-programmed algorithms for thermal losses from conduit 59. System controller 39 can use the calculated trim value in conjunction with one or more sulfur trioxide demand signals to adjust the input condition to the sulfur burner 42 and maintain the operating temperature of catalytic converter 44 within a desirable operating temperature range.

Thus, as indicated above, in the invention, a conditioning agent system comprises means 40, such as a pump providing liquid sulfur to a sulfur burner 42 to maintain a substantially constant temperature at the output of the sulfur burner 42 and/or at the input of a catalytic converter 44. In the invention, the air flow and/or heat input to the sulfur burner 42 is varied in response to the demand for sulfur trioxide conditioning agent, and as the sulfur pump adjusts sulfur flow rate to maintain the constant desired temperature, the sulfur burner 42 provides a varied flow of sulfur dioxide, and thus a varied flow of

sulfur trioxide into the boiler flue gas in response to demand where it conditions entrained particles for removal by electrostatic precipitator 20.

In one method of implementing this invention by the system of Fig. 6, air supplied from blower 46 is supplied through the sulfur burner start-up heater 48 to pre-heat the sulfur burner 42 to the combustion temperature of sulfur. Air is also supplied through catalytic converter air heater 50 to pre-heat the catalytic converter to a temperature range of between 750° F and 1000° F. System controller 39 coupled to sulfur pump 40 initiates a flow of liquid sulfur through the sulfur burner 42 in proportion to one or more sulfur trioxide demand signals, at which time the sulfur burner start-up heater 48 is de-energized. The heat generated from the sulfur combustion continues to ignite the flow of sulfur as it arrives in sulfur burner 42. Once the flow of liquified sulfur has begun, the air flow from blower 46 through catalytic converter air heater 50 is diverted around the catalytic converter, and mixed with the sulfur trioxide at the output of catalytic converter 44. The air flow is then adjusted through sulfur burner 42 in proportion to the sulfur trioxide demand by operating valves 60 and 62 with controller 39 to vary the air flow through the sulfur burner 42 while maintaining a relatively constant flow of S0 ₃ and air through the injectors 16. The sulfur flow is adjusted by the self-contained sulfur burner control loop 43 to maintain a satisfactory operating temperature in catalytic converter 44 and meet the sulfur trioxide demand.

In such a method for implementing this invention, a discharge rate of a charging electrode 30 of electrostatic precipitator system 11 can be determined

by signal means 38 as shown in Fig. 4 and described above, and a sulfur trioxide demand signal is generated by signal means 137 which relates the charging electrode discharge rate to a demand for sulfur trioxide. The demand signal is used by system controller 39 to adjust the flow of sulfur trioxide in the system.

In one variation of a method of operation of the system of Fig. 6, heated air from catalytic converter heater 50 is not entirely diverted around the catalytic converter 44 and the temperature at the discharge of catalytic converter heater 50 is reduced inversely with sulfur trioxide demand when the demand for sulfur trioxide is high. This method offers energy savings by reducing the heat to converter air heater 50 at times of high sulfur trioxide demand.

Another variation of an operating method of the system of Fig. 6 includes varying the temperature setpoint of the sulfur pump control 45 with controller 39 as the demand for conditioning agent varies to provide in improved control of the catalytic converter operating temperature. Where the conditioning agent demand signal is used by controller 39 to vary the operation of heater 48 and the temperature of the air flow into sulfur burner 42, variation of sulfur flow rate to maintain the sulfur burner output temperature or catalytic converter input temperature substantially constant will vary the concentration of sulfur dioxide in air and may require adjustment of the desired temperature output of the sulfur burner for more effective conversion of sulfur dioxide to sulfur trioxide. For example, as the sulfur trioxide demand increases, sulfur pump 42, in response to a decreased input temperature, increases the flow of liquified sulfur to sulfur burner 42, thus "richening" the

air/sulfur mixture burned, and the concentration of sulfur dioxide air mixture leaving the sulfur burner and entering the catalytic converter 44, where the conversion of the increased concentration of sulfur dioxide may elevate the catalytic converter 44 beyond its desirable range as a result of the exothermic conversion reaction.

Still another variation of the operating methods of the system of Fig. 6 inpludes varying the temperatures at the outlet of sulfur burner start-up heater 48 and catalytic converter air heater 50 and varying the air flow through both the sulfur burner 42 and converter air heater 50 based upon the demand for sulfur trioxide. Systems of the invention, including such methods and apparatus are capable of an effective supply of sulfur trioxide for conditioning flue gas prior to its passage through an electrostatic precipitator. Sulfur flow need not be measured and proper flow can be determined from a stable control of the catalytic converter temperature. Therefore, expensive measuring devices for determining sulfur flow which often require frequent maintenance are thereby eliminated. Furthermore, failure of sulfur flow can be detected quickly since a low temperature at the catalytic converter will indicate sulfur flow failure. Therefore, the invention provides an improved and simplified apparatus and method for effectively supplying sulfur trioxide for conditioning boiler flue gas or demand while maintaining the operating temperature of the catalytic converter within acceptable limits.

Fig. 7 shows a simplified representation of a conditioning agent system 212 operated by a conditioning agent control means 239 connected with a

signal means 237, like that described above with respect to Figs. 2-5, for monitoring a rate of discharge of a charged electrode 230 of the electrostatic precipitator 220 and providing therefrom a signal 224A which relates to the decay rate of charging electrode 230 and which indicates a demand for sulfur trioxide. Signal means 237 can include a Castlet controller 224 which has been supplemented to generate signal 224A. Conditioning agent control means 239 uses signal 224A to control the flow of conditioning agent from system 212 as described above. As shown in Fig. 7, a conditioning agent system 212 comprises means 240, such as a pump, to provide liquid sulfur to a sulfur burner 242. Such a system converts the combustion products of sulfur burner 242, mostly S02, into a sulfur trioxide conditioning agent in a catalytic converter 244. The S0 ₃ conditioning agent is directed to injectors 216 which introduces the conditioning agent into the flow 214 of boiler flue gas where it conditions entrained particles for removal by electrostatic precipitator 220. Of course, in such systems other means for producing a flow of conditioning agent may be used, such as those using sources of liquid or gaseous S0 ₂ or other sulfur bearing substances, or sources of non-sulfurous conditioning agents.

In such systems as that shown in Fig. 7, the signal 224A is coupled to input 239A of conditioning agent system control means 239 of conditioning system 212. Conditioning agent control means 239 uses the signal 224A to generate a conditioning agent demand signal to operate the sulfur pump 240. Where conditioning agent is needed for effective removal of entrained particles from the flue gas, sulfur pump 240 supplies a flow of liquified sulfur to sulfur burner

242 where the sulfur is burned in air supplied by a blower 243 to form a quantity of sulfur dioxide in air which passes through catalytic converter 244 thereby producing sulfur trioxide. As described above, control means 239 and signal means 237 can coact to operate sulfur pump 240 only when a rate of discharge of voltage of the charged electrode 230 indicates conditioning agent is needed (generally by the presence of back corona) or can adjust the flow of sulfur in variable steps to provide a variable rate of conditioning agent flow to improve the electrostatic removal of entrained particles by the precipitator 220.

In another system, as shown in Fig. 8, conditioning agent system control means 239 can determine a conditioning agent demand signal from a sensing means 238, such as a voltage divider, connected with the charging electrode 230. Information on the rate of discharge of the voltage of charging electrode 230 can be determined by conditioning agent control means 239, which then produces a conditioning agent demand signal 212C related to a demand for conditioning agent and sends the signal from the control means 239 to input 212A of conditioning agent production system 212. In still another alternative embodiment, a discharge rate signal 248C for controller 239 can be generated by an electrical resistor 248 connecting a collector electrode 232 of the electrostatic precipitator to ground. Control means 239 is preferably a programmable digital control device with an A-D converter to convert the discharge rate information from sensing means 238 into digital form for use with a programmed algorithm to generate a conditioning agent demand signal for improved operation of the

precipitator. The conditioning agent demand signal 212C is shown coupled to conditioning agent system input 212A. Conditioning system 212 uses the demand signal for the generation and adjustment of the flow of conditioning agent produced thereby and introduced into flue gas stream 214 through injectors 216 to reduce the resistivity of the entrained particulate matter in the flue gas.

The invention thus provides an improved method for supplying a flow of conditioning agent for removal of entrained particles with an electrostatic precipitator. In the method of the invention, a rate of discharge of a charging electrode of an electrostatic precipitator, preferably a voltage discharge rate, is determined, and a signal is generated from the discharge rate of said charged electrode for providing and controlling a flow of conditioning agent in response to the signal to maintain removal of entrained particles within acceptable limits. In the method, the rate of discharge of said charged electrode can be determined by interrupting power to said charged electrode and sensing the rate of voltage decay of said charged electrode. The rate of discharge of the voltage of the charged electrode can be determined by determining the voltage of the charged electrode at a preset time after interruption of power to the charged electrode and comparing the determined voltage with a pre¬ selected voltage. In the method power to the charging electrode can be interrupted after variable periods, which are determined from the sensed rate of voltage decay of said charged electrode, and the discharge rate signal can be generated from the frequency of the variable periods of power interruption. In another variation of the method, the conditioning agent demand

signal can be determined by differentiation of the rate of discharge of the voltage of the charging electrode by the use of differentiating circuits, as well known in the electronic art. It is well known in the art that the resistivity of particulate matter can be reduced by introducing a conditioning agent into the flue gas stream. The flow rate of conditioning agent produced has been frequently selected by the quantity of coal burned, flue gas opacity, etc. By using information on the discharge rate of the charged electrode of an electrostatic precipitator in deriving a conditioning agent demand signal, the present invention provides a signal for use by a conditioning system which is almost dependent upon the resistivity of the particulate matter being conditioned, which is the variable that flue gas conditioning systems seek to control.

In addition, the use of the invention permits a determination of satisfactory resistivity condition of particulate matter collected within the electrostatic precipitator which can permit the conditioning agent producing means to be shut down for periods of several hours, thus providing substantial savings in operating costs, which are not available with existing conditioning agent control systems.

Fig. 9 shows a flue gas conditioning system 310 providing means and method for converting sulfur dioxide (S0 ₂ ) contained in the particulate-laden boiler flue gas, indicated generally by arrow 312, to sulfur trioxide (S0 ₃ ) within a duct or conduit 313 upstream of an electrostatic means 314 to improve the removal of the particulate matter from the boiler flue gas 312 prior to its expulsion to the atmosphere from a stack 315. Precipitator 314 can include a charged

electrode 314a and a collector electrode 314b. Conversion of the S0 ₂ in the boiler flue gas to S0 ₃ can be effected by an assembly 320 arranged adjacent duct 313 upstream of the precipitator 314. Assembly 320 provides an inoperable position when conditioning agent is not generated and an operable position for converting a small portion of the S0 ₂ contained in the flue gas 312 to S0 ₃ , which serves to condition the flue gas 312 prior to electrostatic precipitator 314. Such a system is preferably designed to provide conditioning agent sufficient to condition the particulate matter of a boiler flue gas ranging from 3 g/m ³ stp to about 10 g/m ³ stp, and to provide a conditioning agent concentration of generally less than 40 ppm, preferably 20 to 30 ppm. As set forth below, assembly 320 is controlled by a controller 316 to provide conditioning agent for the removal of particulate matter from the stack effluent when needed. Referring now to Figs. 9-16, and particularly to

Figs. 10-12, assembly 320 can include catalytic conversion means 322, represented for simplicity in Fig. 9 by bi-directional arrow 322, movable between an operative position within the duct 313 and an inoperative position outside of duct 313 by any convenient operating means, such as an electric or hydraulic motor. When the catalytic conversion means 322 is located in the operative position within the duct 313, it positions an S0 ₂ -S0 ₃ catalyst so that a substantial portion of the flue gas 312 passes through the catalyst conversion means 322 and so a portion of the S0 ₂ contained in the flue gas 312 can be converted to S0 ₃ . When the catalytic conversion means 322 is located in the inoperative position outside of the duct 313, the flue gas 312 cannot pass through the

catalytic conversion and, consequently, no conditioning agent is generated.

As shown in Figs. 10-12, systems of the invention can comprise a pair of assemblies 320 and catalytic conversion means 322 located on opposing sides of the flue gas duct 313. While a separate assembly 320 is shown disposed on each opposing side of duct 313 in Figs. 10-12, each assembly 320 and its elements are substantially identical, and for simplicity, reference will be made to only one of the assemblies. The discussion should be understood, however, to apply to each of the opposing assemblies 320.

Conversion means 322 can comprise S0 ₂ -S0 ₃ catalytic material 324 carried and supported within a carriage means or frame 326 that is movable by any motor means, which may include, as further set forth below, hydraulic or electric motors (not shown) and a rack-and-gear assembly 330, as shown in Figs. 13 and 15. Each assembly 320 may further comprise, as shown in Figs. 10-14, an insulated steel housing 323 built onto the existing flue gas duct 313, generally at a right angle to the path of gas flow 312, for housing the conversion means 322 while in the inoperative position. For the use of currently available S0 ₂ -S0 ₃ catalytic materials housing 323 is preferably installed on the flue gas duct where flue ^' gas temperature is generally in the range of about 750° F. to about 1075° F. (399° C. to 579° C.) and preferably approximately 850° F. (454° C). Access to the interior of duct 313 from housing

323 is provided through opening 325, shown best in Fig. 10.

As shown in Figs. 13-15, each assembly 320 can further comprise a plurality of rollers 334 upon which the carriage means 326 is supported and provide a

steel track means 336, which spans the flue gas duct 313, upon which the carriage means 326 travels back and forth (Figs. 13 and 15). As shown in Figs. 10-13, housing 323 preferably extends outwardly from the main flue gas duct 313 and the track 336 extends into the housing 323 to allow the catalytic carriage frame 326 to travel and be positioned within the housing 323 when travelling from the operative position within the flue duct 313 to the inoperative position outside the flue gas duct 313.

Each assembly 320 can further comprise a guillotine damper 340 (Figs. 13 and 14) to close off opening 325 in the flue gas duct 313 to isolate the housing 323 and carriage frame 326 from the duct 313 for maintenance or replacement of catalyst material or for routine inspections, all while the system 310 is in full operation. If desired, soot blowers can be installed within the housing 323 to clean the carriage frame 326 periodically or if the operating pressure drop across the catalyst material 324 increases. As shown in Figs. 13 and 14, guillotine damper 340 is positioned adjacent housing 323 and is driven by drive means 342, which may be provided by an electric motor, which, in turn, reversibly drives shaft 343 carrying pulley wheels 344 that are connected to damper 340 by cables 346. Damper drive 342 reversibly drives shaft 343 to raise and lower damper 340 as determined by controller 316. Motor 342 can be supported on a shelf-and-bracket structure 342a secured to the outside wall of main duct 313, or may be secured by any other conventional means for securing motor 342 in a fixed position to move damper 340 in a vertical path to close and open opening 325 as needed.

As shown in Figs. 10-14, the outward side of housing 323 is provided with an opening 350, which is

formed with a perimeter flange 352, to allow access into the interior of housing 323 and specifically to the carriage frame 326 for inspection or maintenance or replacement of catalyst material 324. During such times, guillotine damper 340 will be moved into position to close off opening 325 into the main duct 313. Fig. 14 is a view of the assembly 320 showing the housing 323 extending outwardly from the main duct 313 toward the reader with the access door 354 removed for clarity. Access door 354 may be detachably secured to perimeter flange 352 by a variety of conventional means, such as by threaded fasteners in threaded bore 352a.

As noted above, the apparatus of this invention can comprise a first housing 323 adjacent to and opening into one side of the main flue gas duct 313 and a second housing 323 disposed adjacent to and opening into the main flue gas duct 313 on the side opposite the first housing, with track 336 spanning across the main flue gas duct 313 and extending into each of the housings. A first movable carriage frame 326 is adapted to travel upon track means 336 and be positioned within a first housing 323 on one side of the duct 313 when travelling from a first operative position within the main flue gas duct 313 to an inoperative position outside of the main flue gas duct 313. Likewise, a second movable carriage frame 326 is adapted to travel upon track 336 and be positioned within the second housing 323 on the other side of duct 313 when travelling from a first operative position within the main flue gas duct 313 to an inoperative position outside of the main flue gas duct 313. In their respective operative positions within the duct 313, first and second carriage frames 326 are located adjacent one another and span substantially

across the entire interior of the duct 313 (as shown in Fig. 11) so that substantially all of the flue gas flow 312 passes through the catalytic material 324 carried by the carriage frames 326. As shown in Figs. 13, 15 and 16, one preferred operating means for moving the carriage frames 326 between their operative and inoperative positions comprises motor driven rack-and-pinion assemblies 330 including a drive gear 331, preferably a 4 inch spur gear, and a rack 333, preferably a No. 5 pitch rack. Gear 331 is preferably driven by a conventional electric motor (not shown) controlled by controller 316 to move in reverse directions, as indicated by reference arrow "a", so that a plurality of teeth 331a provided about the perimeter of gear 331 engage a flat rack 333 provided with a plurality of corresponding teeth 333a. Rack 333 is securely affixed to carriage frame 326 so that as gear 331 rotates in engagement with rack 333, carriage frame 326 is moved back and forth along track means 336 on rollers 334 which are securely affixed to track means 336 so that the carriage frame 326 and rack 333 are movably supported thereon. In one exemplary assembly 320, rollers 334 are preferably forged steel wheels having dimensions of four inches in diameter and one and one-half inches in thickness. Track means 336 can be supported within housing 323 upon spanner plates constructed of 2" x 1-V flat stock. Distance Dl of carriage frame 326 is preferably seven feet, six inches, which substantially corresponds to the interior distance D2 (Fig. 13) from the inside wall of main duct 313 to the center line 313a of duct 313, where the interior cross-distance of flue duct 313 is approximately 15 feet. In this assembly, the distance D3 from the inward surface of

perimeter flange 352 and the outside wall of duct 313 is preferably eight feet.

Fig. 16 shows an enlarged partial cross-sectional end view of the rack-and-pinion assembly 330 and track means 336 and rollers 334 provided by this invention to move catalytic carriage frame 326 between its operative and inoperative positions. Such structure more particularly includes a pair of angle brackets 350 secured to the spanner plates 323a. A channel guide 352 is affixed to the underside of catalyst frame 326 adjacent its edge. As shown in Fig. 16, channel guide 352 extends partially beyond the edge of catalyst frame 326 to provide additional support for rack 333. Angle bracket support 354 provides additional support for rack 333.

Track means 336 is formed by angle supports 350 which extend into the housings 323 and span across the ^" main flue gas duct 313. As shown in Figs. 15 and 16, the plurality of rollers 334 are rotatably carried between the angle supports 350. Each roller 334 rotates about an axle 356 which is welded to angle supports 350 at reference points 358. Channel guide 352 extends fully along the length of the underside of carriage frame 326 and is guided by and rolled over rollers 334 as shown in Figs. 15 and 16. Alignment rods 360 are provided in the upper corners of channel guide 352 to provide better alignment and smoother movement of the catalyst frame 326 across the plurality of roller wheels 334. An arrangement like that shown in Fig. 16 is provided at the opposite edge of carriage frame 326, without however, the structure to support rack 333. If desired, however, a rack-and- pinion assembly can be provided on each side of carriage frame 326 to drive the frame back and forth. Also, if desired, a similar roller assembly can be

provided along the central portion of the catalyst frame 326 to provide additional support if needed.

A further embodiment of this invention is shown in Fig. 17. As shown in Figs. 17 and 18, a movable carriage means 426 of a catalytic conversion means 322 of an assembly 320 may be pivotally supported and disposed in a housing 423 generally parallel to the flue gas duct 313 while in the inoperative position. Carriage means 426 can be moved pivotally, indicated by reference arrow "b", between an inoperative position shown in solid lines in Fig. 18 and an operative position 426a substantially normal to the flue gas flow 312 within the main flue gas duct 313, as shown in phantom lines in Fig. 18. As shown in Fig. 17, such a system can be installed on both sides of duct 313. The embodiment of Figs. 17 and 18 includes a motor-driven curved rack-and-pinion assembly 430 comprising toothed gears 431 affixed to gear rod 432 and curved rack members 433 which engage and are driven by gears 431. Assembly 430 may include transverse support members 436 to ensure the rigidity of the assembly. An idler arm 440 is affixed in position within annex housing 423 to ride on the inward side of assembly 430 to keep the assembly 430 moving in a fixed curvilinear linear path. Gear rod 432 and gears 431 can be driven by operating means such as a conventional electric motor (not shown) . Gears 431 engage the rack members 433 of assembly 430, which is connected to catalyst frame 426 at one or more brackets 428 at its end opposite pivotal end 427. In use, when actuated by control means 316, gears 431 are driven in a counter-clockwise direction, as shown in Figs. 17 and 18, which drives assembly 430 and catalyst frame 426 in a clockwise direction, as shown in Fig. 18, to its operative position 426a,

shown in phantom lines in Fig. 18, generally normal to the gas flow 312 within duct 313. In the operative position 426a, a substantial portion of the gas flow 312 passes through the catalytic material 424 carried within frame 426.

Upon being directed by control means 316 to move catalyst frame 426 to its inoperative position, the electric motor (not shown) drives gears 431 and gear rod 432 in a clockwise direction, as observed in Figs. 17 and 18, which drives assembly 430 and catalyst frame 426 along a counter-clockwise path until catalyst frame 426 is in an inoperative position as shown in solid lines in Fig. 18. If desired, catalyst frame 426 may be provided with a roller or wheel means 442 provided at its end opposite pivotal end 427 to be received in a track 424 provided in the floor of housing 423.

The embodiment of Figs. 17 and 18 can further include a damper 340 to close off opening 425 in the flue gas duct 313 to isolate housing 423 and catalyst frame 426 in its inoperative position from the interior of duct 313. As with the previously described embodiment, damper 340 may be driven by an electric motor 342, supported upon bracket support shelf 342a, that drives rod 343 carrying pulley wheels 344 which, in turn, reel in or let out cable 346 to move damper 340 in a vertical path to open or close opening 425. An access panel 454 can be provided in a side wall of annex housing 423 to provide access to catalyst frame 426 for inspection or maintenance or replacement of the catalyst material.

Fig. 19 shows the embodiment of this invention shown in Figs. 17 and 18 having a curved rack-and- pinion assembly oriented in a working arrangement adjacent a horizontal main flue gas duct 313. The

embodiment of Fig. 19 is substantially identical to the embodiment of Figs. 17 and 18 except for a different orientation of damper drive motor 342. Motor 342 is preferably supported on a base 342a secured to duct 313 and drives a drive shaft 342b having a gear 342c at its end which engages corresponding gear 343a disposed normally thereto at the end of rod 343. Rod 343 carries a spur gear 344a and guide roller 344b which move damper 340 through rack 346 and guide channel 347. The end of rod 343 rotates freely within a yoke bracket 348. In all other aspects, the embodiment of Fig. 19 is identical to the embodiment of Figs. 17 and 18.

In some installations, pivoting catalyst conversion means, such as those shown in Figs. 17-19, may be pivotally moved within the flue gas duct and driven from outside of the flue gas duct between an inoperative position parallel to and adjacent the inside walls of the duct where the catalyst is out of the flow of flue gas and an operative position extending across the duct where the catalyst is in the flow of flue gas. In such installations, removable panels may be installed in the main duct to provide access to the catalyst and catalyst conversion means. In such installations, the rack members 433 that drive the catalyst carriage 426 may extend through sealed openings in the duct.

Conversion means 322 may be activated to move between the operative and inoperative positions upon receiving a signal from a remote control means 316

(Fig. 9) based upon the need for conditioning of the flue gas 312. When a need for conditioning of the flue gas is determined by control means 316, it directs a signal to actuating means 321 of assembly 320 to move conversion means 322 to the operative

position within the duct 313 so that a substantial portion of the flue gas flow 312 passes through the catalytic material 324 carried by frame 326. Conversely, when control means 316 determines that conditioning of the flue gas 312 is not needed, it directs a signal to actuating means 321 of assembly 320 to move conversion means 322 to the inoperative position where flue gas does not pass through the catalyst. Operation of the catalyst conversion means 322 between the operative and inoperative positions may be preferably effected by a conditioning agent demand control apparatus like that disclosed above and shown, for example, in Figs. 2-5, 7 and 8. The combination of such control apparatus with the in-duct flue gas conditioning systems in this invention permits a determination of a satisfactory resistivity condition of the particulate matter collected within the electrostatic precipitator 314, which can permit the catalytic conversion means 322, 326, 426 to remain in its inoperative position for periods of several hours. In such systems, the control means 316 for assembly 320 can be connected with an electrostatic precipitator control adapted to sense a discharge rate of a charged electrode 314a as disclosed, for example, in Figs. 2-8.

Other control means can be used, however, for operating assembly 320. For example, as shown in Fig. 9, the control means 316 can monitor the operation of the electrostatic precipitator 314 and the opacity of the flue gas emitted from the stack 315. To this end, control means 316 can be coupled to the electrostatic precipitator control 317 and determine the power used by the electrostatic precipitator in removing particulate matter from the

flue gas. Control means 316 can also be coupled to an opacity meter 315a that provides output signals proportional to the opacity of the effluent from stack 315. Control means 316 may thus determine changes in the opacity of the effluent from stack 315 that accompany such transient conditions as boiler upsets due to the incomplete combustion of coal or carbon carryover from coal combustion, ash-handling problems, precipitator rapping, boiler soot blowing and other such transient changes in operation which cannot be corrected by S0 ₃ -conditioning methods and provide a conditioning agent demand signal for operation of assembly 320 as disclosed, for example, in U.S. Patent No. 5,032,154. A controller suitable to serve as control means

316 is Allen-Bradley's T30 Plant Floor Terminal Programmable Controller, or Bristol-Babcock's Network 3000 Compatible Intelligent Controller, but other equivalent programmable controllers can also be used. Indeed, control means 316 can be provided by a programmable microprocessor and random access memory. The preferred catalyst material 324 suitable for use with this invention comprises a honeycomb ceramic sold by Applied Ceramics of Atlanta, Georgia 30359 under their trademark Versagrid which has been coated with vanadium pentoxide. Such catalyst material works effectively in particulate-laden flue gas and operates at high gas velocity with low pressure drop. Its wide operating temperature range allows the catalyst material 324 to be located in variable positions within the duct 313 between the economizer outlet and the airheater inlet. Depending on the amount of conditioning agent needed, several batteries of conversion assemblies 320 can be installed in the duct 313 a certain distance apart, or the depth of the

catalyst can be varied. Catalyst may also be available from Monsanto Enviro-Chem, St. Louis, Missouri.

The catalyst material 324, which can be changed (more or less catalyst) and cleaned all with the system 310 on line, is preferably carried by carriage means 326 in replaceable perforated plate containers carried by carriage means 326, which are constructed of steel suitable for high-temperature operation. Catalyst frame 326 can be constructed of stainless expanded steel in sizes of approximately ten feet (10') in width, and seven feet six inches (7'6") in depth and one foot (1') tall. As shown in Figs. 10- 12, frame 326 can be constructed to carry therein three rows of bed baskets of catalyst material 324 with three baskets in each row. After the catalyst material is loaded into frame 326, its top is fastened to secure the baskets within the frame 326.

The invention also provides an S0 ₂ /S0 ₃ converter that is particularly effective for use in in-duct flue gas conditioning systems. Such an S0 ₂ /S0 ₃ converter provides a plurality of open generally parallel paths for flue gas that are formed by an S0 ₂ /S0 ₃ catalyst effective over a wide temperature range for the conversion of preferably low percentages of the S0 ₂ in the flue gas stream to S0 ₃ conditioning agent.

The "open" generally parallel catalyst-formed flow paths of the S0 ₂ /S0 ₃ converters are particularly effective in the in-duct flue gas conditioning systems of the invention. The plurality of open and generally parallel flow paths provide, in aggregate, an open, or unobstructed flue gas path across the cross-section of the flue gas conduit and generally perpendicular to the direction of flow of the flue gas, and having an area in excess of about 67% of the total cross-

sectional area across the flue gas flow path. The aggregate open area of the plurality of the gas flow paths preferably lies in a range of about 70% to greater than about 80% of the total cross-sectional area of the flue gas flow path. Each of the unobstructed, open and generally parallel flow paths have preferably an area that lies in the range of about 0.01 square inches (0.06cm ² ) to about 1.00 square inch (6.45cm ² ) . Such unobstructed, open and generally parallel flow paths impose little or no pressure drop on the flue gas stream as it passes through the S0 ₂ /S0 ₃ converter, yet expose a sufficient amount of the flue gas to the catalyst for the effective in-duct conversion of a significant percentage of S0 ₂ to S0 ₃ . Fig. 20 shows an in-duct catalytic conversion means such as the in-duct catalytic conversion means of one of the in-duct flue gas conditioning systems of Figs. 9-19. As an example, Fig. 20 shows an in-duct catalytic conversion means 322 for the flue gas conditioning system of Figs. 9-14 described above, but with one or more S0 ₂ /S0 ₃ converters 500 of the invention in use as the catalytic material 324.

A preferred S0 ₂ /SO ₃ converter 500 is shown in greater detail in Figs. 21-24. The preferred S0 ₂ /S0 ₃ converter of Figs. 20-23 may be formed by a foraminous ceramic substrate 501 which forms the plurality of open and generally parallel flow paths 502 for the flue gas. The ceramic from which the substrate is formed may be any ceramic material which provides the structural integrity and temperature resistance for durability in an application where the temperature cycles from ambient temperatures to temperatures on the order of 1000° F. or greater, such as Cordierite or Mullite or any of a number of other alumina and zirconia materials. It is desirable that the surfaces

forming the open and generally parallel flue gas flow paths be somewhat porous.

The open and generally parallel flow paths can have any of a variety of cross-sectional shapes such as polygonal, including triangular, rectangular or square, hexagonal and circular. Such ceramic substrates for the preferred S0 ₂ /S0 ₃ converters are available from Applied Ceramics, Inc. of Atlanta, Georgia 30359 under their trade name Versagrid. The catalyst of the S0 ₂ /S0 ₃ converters 500 can be a composite material, such as the CS210 catalyst available from Monsanto Enviro-Tech Corp. of St. Louis, Missouri, and equivalents which will bond to the surfaces of the foraminous ceramic substrate 501 and provide conversion of the S0 ₂ in the flue gas flowing through the S0 ₂ /S0 ₃ conversion device to S0 ₃ in low percentages, such as from about 2% to in excess of about 10% and preferably at such low percentages as from about 3% to about 5%. Such low percentage direct conversion of the S0 ₂ of the flue gas flowing in the flue gas conduit leading from the boiler provides a desirable S0 ₃ concentration for conditioning the flue gas for subsequent electrostatic removal. The catalyst will form a thin coating on the surfaces of the ceramic substrate 501. Because the catalyst coating is thin, it is not shown in Figs. 21-24, but it is to be understood that the internal surfaces forming the passageways 502 are coated with such catalyst. Fig. 21 shows an example of one of a plurality of

S0 ₂ /S0 ₃ converters 500 that can be placed in the catalytic conversion means 322. Such an S0 ₂ /S0 ₃ converter 500 of Fig. 21 can, for example, have a width of 9 inches (22.9cm) , a length of 12 inches (30.5cm) and a thickness of 3 inches (7.6cm) and the

open and generally parallel flow paths 502 formed thereby can have a square cross-section with sides 501a of 3/4 inch (1.9cm). As noted above, the cross- section of the open and generally parallel flow paths 502 can have other shapes and cross-sectional areas than that shown and described in Fig. 21, as shown in Figs. 23 and 24, for example, 3/8 inch by 3/8 inch, and each So ₂ /S0 ₃ converter can be made in any other convenient shape, for example, circular, and with any larger or smaller dimensions.

As shown in Fig. 20, a plurality of such S0 ₂ /S0 ₃ converters can be easily carried by the catalytic conversion means 322 between its operative and inoperative positions. As indicated above, the catalytic conversion means 322 can be moved to an operative position inside the flue gas conduit 313 (Fig. 11) and position the plurality of S0 ₂ /S0 ₃ converters 500 across the flow of flue gas in conduit 313 with their open and generally parallel flue gas flow paths 502 generally parallel to the flow of flue gas in conduit 313, thus imposing little pressure drop on the flue gas as it flows through the catalytic converter means 322 but converting a small but effective percentage (e.g., about 3% to about 5%) of the S0 ₂ within conduit 313 into S0 ₃ flue gas conditioning agent that is mixed with and conditions the particulate matter being carried by the flue gas for electrostatic removal. Such S0 ₂ /S0 ₃ converters work effectively in particulate-laden flue gas and operate at high gas velocity with low pressure drop. The catalyst applied to the foraminous ceramic substrate 501 can be composed of vanadium pentoxide and potassium and cesium-based material to provide controlled conversion of S0 ₂ to S0 ₃ over a wide operating temperature range that allows the S0 ₂ /S0 ₃

converters 500 to be located in variable positions within the duct 313 between the economizer outlet and the airheater inlet. In addition, the S0 ₂ /S0 ₃ converters 500 are easily removed from the catalytic conversion means 322, when in the inoperative position, for replacement and/or reconditioning.

Figs. 25 and 26 are diagrammatic drawings to illustrate cross-tie systems of the invention for conditioning a plurality of flue gas streams from a plurality of boilers or generating units including a sulfur source (e.g., 620) for providing a single flowing mixture of sulfur dioxide and air, one or more flow dividers (e.g., 617) for dividing the single flowing mixture of sulfur dioxide and air into a plurality of flows of sulfur dioxide and air in a plurality of distributing conduits 615, 615a, and a plurality of catalytic converters 632, 632' connected ^* with the distributing conduits at their input ends and with injectors 636, 636' to inject sulfur trioxide into the plurality of flue gas conduits 637, 637' for a plurality of boilers or generating units.

As shown in Figs. 25 and 26, such systems can include a blower 612 for providing a single flow of air, one or more air dividers 619 for dividing the flow of air into a plurality of flows of air in a plurality of air distributing conduits 613a, and a plurality of air heaters 634 having their inputs connected with the air distributing conduits 613a and their outputs connected with the inputs 635 of the catalytic converters. As indicated in Figs. 25 and

26, each one of the plurality of catalytic converters 632 and an associated one of the plurality of air heaters 634 can form a sulfur dioxide conversion assembly 630 which is located adjacent one of the conduits 637 of generating units 1-3, and the

distributing conduits for sulfur dioxide and for air 615a extend to the sulfur dioxide conversion assemblies 630 near each generating unit conduit 637. In addition, the flow of sulfur dioxide and air to each of the catalytic converters 632 is controlled by a controllable flow control valve 652 connected in each distributing conduit 633 for sulfur dioxide. Each of the controllable flow control valves 652 is operated by a controller 622 that is connected with sensors installed for each generating unit (not shown) and its precipitator and/or other such sensors that are used to determine the need for conditioning agent, as well known in the art.

In each of the systems of Figs. 25 and 26, the sulfur trioxide is mixed with the boiler flue gas of each unit 1-3 and its entrained particulate matter to condition its particulate matter for removal by an electrostatic precipitator before the flue gas is emitted into atmosphere from the stack. As indicated above. Figs. 25 and 26 illustrate further embodiments provided by this invention for providing flue gas conditioning agent for multiple boilers or generator units from a single S0 ₂ generating source. As shown in Fig. 25, such a system 600 can include one S0 ₂ generator skid 610, a separate S0 ₂ /S0 ₃ converter assembly 630 for each of the generator units 1, 2 and 3, a feedstock storage means 640 connected to the S0 ₂ generator skid 610, and a plurality of control valves 650, 652 and mass flow meters 658. The S0 ₂ generator skid 610 can include a process air blower 612 that is equipped with variable speed drive, and a furnace blower 614 equipped with a controllable automatic valve 616, an air heater 618, which may be either electrical or gas-fired, for the

furnace air from blower 614, a sulfur furnace 620, and a controller 622.

The process air blower 612 is equipped with variable speed drive and supplies air to the individual S0 ₂ /S0 ₃ converter stations 630. The size of the process air blower 612 is selected to provide about fifty percent (50%) of the total air volume needed to generate S0 ₃ conditioning agent for all of the generator units 1-3, and the variable speed drive of blower 612 allows air volume to be varied to meet the variable operation needed to satisfy the flue gas condition requirements of the boiler or generator units in service, and to maintain high S0 ₂ /S0 ₃ conversion efficiency as the S0 ₃ demand varies. The furnace blower 614 is connected to a controllable automatic valve 616 for controlling the air flow to sulfur furnace, and is controlled to deliver that amount of air to the sulfur furnace 620 for sulfur combustion and an S0 ₂ /air mixture with an approximately ten percent (10%) maximum S0 ₂ concentration. The output of furnace blower 614 can be varied to deliver the desired S0 ₂ concentration and to minimize the energy consumed by the system 600. The outputs of process air blower 612 and furnace blower 614 are combined at the converter inlet 635, and the process air blower 612 cooperates with furnace blower 614 to deliver an S0 ₃ air mixture to the probes 636 at a concentration of approximately two to six percent. The furnace air heater 618 raises the air provided by the furnace blower 614 to a temperature of approximately 700° F. to initiate sulfur combustion within the sulfur furnace 620 as sulfur spontaneously ignites at this temperature range. When a sufficient amount of sulfur is burned to maintain a desirable

furnace temperature, control 622 deactivates the furnace heater 618. At high S0 ₂ concentrations, the furnace heater 618 may be deactivated as the sulfur combustion process itself supplies all the heat needed for the process to be self-sustaining.

Sulfur furnace 620 may be a spray burner type of multiple ball-type furnace. The specific design of sulfur furnace 620 is dependent upon the required S0 ₃ output of the system. The sulfur furnace is preferably sized to produce the maximum rate of S0 ₂ needed to provide the maximum expected consumption of S0 ₃ conditioning agent for all of the generator units tied in with the cross-tie system. For example, if each of the three generator units 1-3 may each require 20 ppm S0 ₃ , the sulfur furnace 620 will be designed to produce sufficient S0 ₂ for conversion to supply S0 ₃ with a concentration of 60 ppm, plus an operating margin. The sulfur furnace 620 preferably operates to produce an output of S0 ₂ in air at about ten percent maximum concentration.

Control means 622 can include the total system controls, MCC, local power supplies, and the like. A fully automated control for all of the components making up the cross-tie system, plus full control of the S0 ₂ generating skid 610, can be made available at the skid 620. Alternatively, controls for the components and systems can be provided remote from the S0 ₂ generating skid 610.

Each S0 ₂ /S0 ₃ converter assembly 630 can include an electric or gas-fired heater 634 and one or more

S0 ₂ /S0 ₃ catalytic converters 632. The output of each S0 ₂ /S0 ₃ converter assembly is connected with injection probes 636, which accommodate the size of the flue gas conduit 637 of each boiler or generator unit. The S0 ₂ /S0 ₃ converter 632 can have an S0 ₂ /S0 ₃ conversion

efficiency rate of 94 to 98 percent, and high efficiency converters with a conversion rate efficiencies greater than 98 percent can be used, with two converter beds with inter-bed cooling. As shown in Fig. 25, sulfur source 640 is connected to S0 ₂ generator skid 610 and supplies liquified elemental sulfur for combustion directly to furnace 620. As noted above, however, the sulfur source feedstock suitable with this invention may be provided by liquid or gaseous S0 ₂ . The storage and handling of the feedstock is effected by well-known methods as described above.

Control valves 650 provided in the process air distribution conduits 613a from the process air blower 612 control the amount of process air entering the individual S0 ₂ /S0 ₃ converter assemblies 630. Control valves 652 provided in the S0 ₂ distributing conduits 615a from the sulfur furnace 620 control the amount of S0 ₂ delivered to the individual S0 ₂ /S0 ₃ converter assemblies 630. Mass flow meters 658 can be provided in the S0 ₂ distributing conduits 615a to measure the flow rate of S0 ₂ delivered to each S0 ₂ /S0 ₃ converter assembly 630.

The converter heaters 634 can heat the air provided by the process air blower 612 via distribution conduits 613a to a temperature of about 800° F. This process air passes through the converter 632 and the probes 636 and heats the components of the converter station 630 to a temperature suitable for S0 ₂ /S0 ₃ conversion. Converter 634 is preferably located close to the converter inlet 635 to conserve energy and reduces the warm-up time. The S0 ₂ /S0 ₃ converters assemblies 630 are preferably located immediately adjacent the probes 636.

The cross-tie systems of Figs. 25 and 26 employ two process supply conduits for each boiler or generator unit in the system. First, one conduit 613 delivers air from the process air blower 612 to one or more air dividers 617, which divide the flow of process air into a plurality of flows in a plurality of air distributing conduits 613a which extend from the divider to each S0 ₂ /S0 ₃ converter assembly 630 adjacent the flue gas conduits 637 for each unit. The conduits for process air 613 and 613a can be provided by uninsulated carbon steel piping.

Second, hot S0 ₂ /air from the sulfur furnace 620 is delivered through a single conduit 615 to another divider 619, which divides the flow of S0 ₂ /air mixture into a plurality of flows of S0 ₂ in a plurality of . distributing conduits 615a, which extend from divider 619 to each S0 ₂ /S0 ₃ converter assembly 630 adjacent the flue gas conduits 637. S0 ₂ supply conduits 615 and 615a and divider 619 are preferably constructed of insulated stainless steel. Dividers 617 and 619 are preferably located on skid 610.

Initially, each converter assembly 630 receives the same volume of process air from the blower 612. Subsequently, the volume of process air delivered to each converter station 630 may be individually controlled by control valves 650. The furnace blower 614 is a constant volume blower with an automatic control valve 616 controlled to generate desirable S0 ₂ /air concentrations for conversion to S0 ₃ . The flow rate of S0 ₂ supplied to the individual S0 ₂ /S0 ₃ converter assemblies 630 is individually controlled by control valves 652 and is metered by mass flow meters 658.

The delivery of sulfur from feedstock source 640 to the sulfur furnace 620 is accomplished via delivery line 642 and may be controlled by control valve 644.

Sulfur source 640 includes a controllable pump 641 to provide a continuous flow of sulfur into delivery line 642. In the event sulfur is not needed at sulfur furnace 620, control valve 644 may be closed and the sulfur is recirculated via line 646 back into the feedstock storage 640.

The cross-tie conditioning system of this invention described above has four operating modes: Warm-up; Ready; Process; and Off. In the initial "Warm-Up" mode, the system is prepared from a cold start for the production of S0 ₂ , with a condition at "Ready" where the operating temperatures are sufficient for sulfur burning but where no S0 ₂ is being produced. In the "Process" mode, S0 ₂ is being produced from a combustion of sulfur in the sulfur furnace 620. If no conditioning agent is required, the sulfur furnace 620 may go into the Ready mode where the temperatures are sufficient for sulfur combustion, but where no S0 ₂ is being produced, as noted above. In the "Off" mode, the supply of sulfur from feedstock source 640 is discontinued, the furnace blower heater 618 and the furnace blower 616 are shut down.

Process air to the S0 ₂ /S0 ₃ converter assemblies 630 is also controlled in the four operating modes of the system. In the Warm-up to Ready mode, where the system is activated from a cold start, process air is heated and delivered to the inputs of the catalytic converters 632, and the converters 632 and injection probes 636 are brought to temperatures suitable to S0 ₂ /S0 ₃ conversion and injection without condensation S0 ₃ gas in the catalytic converter 632 and associated S0 ₃ transport conduits and injection probes 636. In the process mode, the S0 ₂ /S0 ₃ conversion assemblies 630 are converting S0 ₂ to S0 ₃ and delivering the S0 ₃ through the injection probes 636 in response to the need for

conditioning agent in each flue gas conduit 637 for each boiler generator unit of the system.

In the Purge mode, the flow of S0 ₂ from boiler furnace 620 is discontinued, but the process air blower 612 and air heaters 634 remain activated to deliver heated air through the S0 ₂ /S0 ₃ converter assemblies 630 and injection probes 636 to thereby remove S0 ₃ from each converter 632 and its associated conduits and probes. An acceptable "purge" may be accomplished in 60 to 90 minutes.

After the Purge cycle has been completed, the process air loop is shut down and the system is in the "Off" mode.

Both the cross-tie sulfur furnace operating loop and the process air operating loop can be provided with emergency stop (E-Stop) switches on the control panel for emergencies. A plant controller 622, which may be located at the skid 610 or remote station, or automatically control all of the operations of the cross-tie conditioning system 600. The sulfur furnace 620 supplies S0 ₂ for all of the operating boiler or generator units 1-3, but the system is operated to achieve individually controlled S0 ₃ production for the operating needs of each boiler or generator unit. To activate system 600 and to begin the "Warm-up" mode, both blowers 612 and 614 are activated at the S0 ₂ generator skid 610. During the Warm-up mode, the process air blower 612 delivers equal amounts of process air to each of the three converter stations 630. The flow of the process air is controlled by control valves 650 provided in each of the air distributing conduits 613a. If desirable, an automated valve (not shown) may also be provided at the process air blower 612 to control its flow. The conversion assembly air heaters 634 are also

activated, and the process air temperatures at the outlets of converters 632 is continuously increased until they reach an acceptable operating temperature of approximately 800° F. and until the inlet temperatures at the injection probes 636 exceed 600° F.

At this point, the furnace blower 614 is activated and begins providing a flow of air, controlled by automatic control valve 616, to sulfur furnace 620 for sulfur combustion. The furnace air heater 618 is also activated and brings the furnace air and furnace temperature to an acceptable operating temperature of approximately 700- 800° F.

When all set point temperature alarms are clear, i.e.. when all of the temperatures are at operating levels, the system is then ready to burn sulfur and provide S0 ₂ to the S0 ₂ /S0 ₃ converter assemblies 630. The system is then in the "Ready" mode.

The system 600 is now ready to begin the "Process" mode upon command. Upon being activated, the sulfur pump 641 pumps sulfur from feedstock storage 640 to sulfur furnace 620 through delivery line 642 and valve 644 to be burned at a rate to accommodate the total demand for conditioning agent by each precipitator (not shown in Fig. 25) associated with each flue gas conduit 637. Load demand is measured by summing individual unit load signals generated, for example, by conventional 4-10 ma. control signals derived from steam flow, boiler load, electric power demand and the like from each boiler or generator unit in the system. The need for conditioning agent for the flue gas of each of the boilers or generator units in the system may be determined by monitoring one or a combination of the following: fly ash resistivity; precipitator power

consumption; concentration of S0 ₂ in the flue gas emitted into the atmosphere; and/or opacity of the stack effluent.

The control means 622 of the conditioning system considers the sulfur as a fuel so that the sulfur combustion at sulfur furnace 620 can be controlled by two established fuel-control principles. In this case, the two "fuels" may be elemental sulfur (which generate 4,000 BTUs/poun ) , and the furnace heating energy added to the sulfur furnace 620 by air heater 618, for example, by electrical energy, fuel oil, or gas. Thus, as the conditioning agent demand for the connected units 1-3 increases, more sulfur is delivered to and burned by sulfur furnace 620 and less added energy is required to maintain furnace operation at an acceptable operating level. Where conditioning agent demand and the associated burning of sulfur can maintain a satisfactory operating temperature of the sulfur furnace 620, the control means 622 deactivates the furnace heater 618 as no additional heat is required. When conditioning agent demand decreases, the furnace air heater 618 is energized by control means 622 to maintain the temperature of the sulfur furnace 620 at an acceptable operating level. Such a method of two fuel combustion control is simple and economic, and does not require accurate measurement of elemental sulfur consumption at the sulfur furnace 620.

The S0 ₂ generated by the sulfur furnace 620 flows via distribution conduits 615, 615a to each S0 ₂ /S0 ₃ converter assembly 630. Each of the plurality of flows of S0 ₂ through each of the plurality of distribution conduits 615a is controlled by a control valve 652 to provide the conditioning agent needed for effective operation of the electrostatic precipitator

of each boiler or generating unit. The flows of S0 ₂ and heated process air are controlled so they may be combined to provide a correct mixture of S0 ₂ and air at the correct temperature at each converter inlet 635 of each S0 ₂ /S0 ₃ converter assembly 630 for effective S0 ₂ /S0 ₃ conversion.

Operation of each of the systems to supply conditioning agent to each of the flue gas conduits 637 is preferably effected by controller 622 and one or more of conditioning agent demand control apparatus like that disclosed, for example, in Figs. 1-8 and the accompanying text. The combination of such conditioning agent demand control apparatus in this invention permits a determination of a satisfactory resistivity condition of the particulate matter collected within each electrostatic precipitator associated with each flue gas conduit 637, and may permit closing of one or more control valves 652 for periods of several hours. In such systems, the control means 622 for the system can be connected with the electrostatic precipitator control of each boiler or generator unit in the system, which can be adapted to sense a discharge rate of a charged precipitator electrode as disclosed above. Other control means can be used, however, for the system. For example, for each connected boiler or generator unit the control means 622 can monitor the operation of its electrostatic precipitator and the opacity of the flue gas emitted from its stack. To this end, control means 622 can be coupled to each electrostatic precipitator control in the system and determine the power used by each electrostatic precipitator in removing particulate matter from the flue gas. Control means 622 can also be coupled to each opacity meter in the system and determine the

opacity of the effluent from each stack. Control means 622 may thus determine changes in the opacity of the effluent from each stack that accompany such transient conditions as boiler upsets due to the incomplete combustion of coal or carbon carryover from coal combustion, ash-handling problems, precipitator rapping, boiler soot blowing and other such transient changes in operation which cannot be corrected by S0 ₃ - conditioning methods and provide a conditioning agent demand signal for operation of assembly 630 as disclosed, for example, in U.S. Patent No. 5,032,154. In operation of the system, the S0 ₂ flow to each S0 ₂ /S0 ₃ converter assembly may be determined by its associated mass flow jneter 658, each of which may be connected to controller 622.

A controller suitable for inclusion in control means 622 is Allen-Bradley's T30 Plant Floor Terminal Programmable Controller, or Bristol-Babcock's Network 6000 Compatible Intelligent Controller, but other equivalent programmable controllers can also be used as set forth above. Indeed, control means 622 can be provided by a programmable microprocessor and random access memory.

The operation of each S0 ₂ /S0 ₃ converter assembly 630 can be effected as follows. As noted, S0 ₂ flow can be measured by mass flow meters 658 and controlled by control valves 652 in response to unit load and trim signals. Mass flow meter 658 can measure the S0 ₂ flow rate and direct this information to control means 622, which then compares the measured S0 ₂ flow rate with the temperature required at the converter inlet 635 by using look-up tables stored in its memory. Depending on the S0 ₂ concentration and the flow rate of the process air, the control means 622 operates the converter heater 634 to adjust the air temperature

entering the converter 632 to effect efficient S0 ₂ /S0 ₃ conversion. As more S0 ₂ is directed into the S0 ₂ /S0 ₃ converter assembly 630, the process air control valve 650 will be operated to provide a correct S0 ₂ /air mixture for conversion, and the output of converter heater 634 can be adjusted to improve S0 ₂ /S0 ₃ conversion. If desirable, the volume of process air blower 612 can also be reduced while maintaining preferred S0 ₂ concentrations at the converter inlets 635 to reduce energy consumption by the system 600. As described above, each converter station 630 works independently of the others in effecting S0 ₂ /S0 ₃ conversion for proper conditioning of the flue gas from the power plant unit to which it is adjoined. In the event S0 ₃ is no longer required for conditioning a boiler or generating unit because of unit load and/or favorable precipitator operating conditions, the S0 ₂ control valve 652 closes, thereby discontinuing the generation of S0 ₃ . At this point, the process air control valve 650 opens fully and,the system 600 is in the Purge mode where hot air flows through the converter assembly 630 and connected injectors 636 and the station remains in a Ready state with set point temperatures suiting S0 ₃ production. In this "Ready" mode, S0 ₃ can be made at once upon command. Because one or more of the converter stations is no longer generating S0 ₃ , the control means 622 reduces the total demand for S0 ₂ to match the new lower S0 ₂ requirement, and less sulfur is therefore admitted into and combusted in the sulfur furnace 620. When any one of the boilers or generating units is shut down and its supply system is in the "Purge" cycle, hot process air from process air blower 612 is directed through its S0 ₂ /S0 ₃ converter assembly 630 at temperatures of approximately 800° F. and above.

During this cycle, its converter heater 634 is activated to maintain the elevated temperature of the process air as it flows through the converter assembly 630 and connected injection probes. When the purge cycle ends, the converter heater 634 is deactivated, the process air control valve 650 is closed and a vent valve located at the converter inlet 635 (not shown) is opened, all by control means 622. At this point, the individual S0 ₃ converter assembly 630 is out of service and control means 622 consequently reduces the variable speed drive output of the process air blower 612 to allow for the reduced demand for process air. The output of the sulfur furnace 620 is thus continually varied to generate a sufficient amount of S0 ₂ to suit the demands of one or a combination of all of the power-generating units in the cross-tie system of this invention.

Fig. 26 shows a further embodiment of a cross-tie flue gas conditioning system provided by this invention. While Fig. 26 shows three power-generating units, units 1-3 as an example, any number of power- generating units can be tied in with and conditioned by the cross-tie conditioning system of this invention just like the system of Fig. 25. In contrast to the cross-tie embodiment shown in Fig. 25, the cross-tie system 660 shown in Fig. 26 uses S0 ₂ as a source of sulfur, compared with system 600 of Fig. 25 which uses elemental sulfur as the feedstock and a sulfur furnace to convert it to S0 ₂ . The cross-tie system 660 using S0 ₂ is of similar design to sulfur-burning cross-tie system 600 of Fig. 25 except that S0 ₂ from a source of liquid S0 ₂ is piped as a gas directly to each S0 ₂ control valves 652. The S0 ₂ control valves 652 meter S0 ₂ into the converter station 630'. System 660 further includes an equipment skid 662, which can

carry a variable speed drive process air blower (not shown) , as described above in relation to process air blower 612 in Fig. 25. This process air blower directs process air through air distribution conduits 613' and 613a', which are preferably constructed of uninsulated carbon steel. The S0 ₂ is delivered to the S0 ₂ /S0 ₃ converter stations 630' through distributing conduits 615' and 615a', which are preferably constructed of insulated stainless steel piping. As before, each of the S0 ₂ control valves 652 is independently controlled by a control means as described above to provide the desired S0 ₃ conditioning agent for each of the power-generating units 1-3. The operating mode of system 660 is similar to that of the sulfur-burning system 600, described in relation to Fig. 25 and a detailed discussion will not be presented. In the system 660 of Fig. 26, the S0 ₂ feedstock is preferably stored in liquid state in a remote storage facility (not shown) and can be vaporized at the storage facility or at the skid 662. Each of the S0 ₂ /S0 ₃ converter assemblies 630' can include a plurality of S0 ₃ injection probes 636' and, in this embodiment, a separate S0 ₂ /S0 ₃ converter 632 can be provided for each of the plurality of injection probes 636'. If desired, however, a single converter 632 may be provided to supply conditioning to all of the injection probes 636' situated in the flue gas conduits of power-generating units 1-3.

The cross-tie conditioning system of this invention shown and described in relation to Figs. 25 and 26 have the same flexibility and control as individual S0 ₃ generating plants, and their designs are simple, flexible and economical.

The invention thus provides an improved apparatus for conditioning boiler flue gas with sulfur trioxide

for removal of entrained particles with an electrostatic precipitator. One such improved apparatus includes an integrated assembly adapted for providing a flow of air and sulfur dioxide at a temperature in excess of the condensation temperature of sulfurous acid. The integrated assembly includes first means for providing a flow of sulfur dioxide, second means for providing a flow of heated air, third means for dividing the flows of sulfur dioxide and air into a plurality of flows of sulfur dioxide and air for conversion to sulfur trioxide and injection into the boiler flue gas at a plurality of injection sites upstream of the electrostatic precipitator, and fourth means for supporting and carrying said first, second and third means as an integrated assembly. In a preferred integrated assembly of such apparatus, the first means comprises a sulfur burner having a sulfur input and air input and a sulfur dioxide output, and an insulated conduit interconnecting the sulfur burner with a sulfur dioxide flow divider, an air blower and air flow divider connected with the air blower and having a plurality of first outputs.

In such apparatus, one or more sulfur dioxide conversion assemblies, such as the two-stage catalytic converters 700 shown in Fig. 27, may be located remotely from the integrated assembly adjacent the injection sites and away from work areas. In the improved apparatus, a plurality of such compact sulfur dioxide conversion assemblies can be adapted for support and location remote from the integrated assembly adjacent a plurality of flue gas conduits at injection sites for sulfur trioxide upstream of their electrostatic precipitators. An example of such installation is shown in Fig. 28. Each such sulfur dioxide converter (as shown, for example, in Fig. 27)

is adapted for connection with one of the plurality of flows of sulfur dioxide and air. The heaters and catalytic converters have a physical size and a heating and conversion capacity permitting their close location to one of the injection sites for sulfur trioxide.

Fig. 27 shows an example of a two stage S0 ₂ conversion assembly 700 that may be used in the invention. As shown in Fig. 27, the catalytic converter includes an inlet 701 which may be connected with a flow of sulfur dioxide and air from a distributing conduit 615a as described above. The second stage S0 ₂ conversion assembly 700 comprises a first stage catalytic converter 702 and a second stage catalytic converter interconnected with a cooling conduit 704. Each catalytic converter 702, 703'can comprise an identical canister 705 constructed of a stainless steel, which may have a diameter of six to eight inches and a length of 18-24 inches. Each catalytic converter 702, 703 contains a catalyst bed 706, 707 of vanadium pentoxide which has been applied to ceramic substrates, as well known in the art. The catalyst is held in the catalytic converter by foraminous spacers 710 and 711, as shown in Fig. 27. An output 708 extends from the second stage canister 703 and is adapted to be connected to one or more injection probes. The catalytic converter shown in Fig. 27 may, of course, be made larger or smaller canisters 705 and catalyst beds and with different size canisters 705 for each stage to accommodate its application. As shown in Fig. 27, two such catalytic converter canisters 705 may be interconnected with an intervening cooling conduit 704 to provide two-stage conversion of S0 ₂ to S0 ₃ . In the two-stage conversion apparatus of Fig. 27, the intervening cooling loop can

be designed to provide, corresponding to the temperature rise in the first stage canister 702 due to the exothermic catalytic conversion S0 ₂ to S0 ₃ , an interstage temperature reduction for example 160- 200° F. In addition a temperature sensor 709 may be placed in the interstage conduit 704 and connected with the controller 622 for control of the interstage temperature. The interstage cooling may be provided with a single conduit loop or may require a small heat exchanger in the interstage connectors.

Fig. 28 shows an exemplary installation of a small remote catalytic converter 632, 632' adjacent a flue gas conduit 637, 637'. Of course, because of the flexibility resulting from the small and convenient catalytic converter sizes used in the invention, the method of installation is not limited to that shown in Fig. 28.

Systems of the invention, including the new control methods and apparatus, are uncomplicated, are capable of an effective supply of sulfur trioxide for conditioning boiler flue gas prior to its passage through an electrostatic precipitator, and are controllable to maintain minimal opacity of the flue gasses that pass into atmosphere from the boiler stack. The invention provides a non-complex, direct system for providing minimal opacity of stack effluents and minimal pollution from boiler flue gas particulate matter in installation with a plurality of boilers and power-generating units. For example, the process air lines 613, 613a,

613' and 613a' can be made of uninsulated carbon steel to reduce construction costs. The system 600 can utilize the heat generated by sulfur combustion for much of the heat necessary to generate S0 ₂ at satisfactory temperatures, thereby reducing the need

for activating furnace heater 618 and the overall energy consumed by system 600. The S0 ₂ -generating skid 610, shown in Fig. 25, and the equipment skid 662, shown in Fig. 26, can be located virtually anywhere in relation to the boilers or power-generating units, thereby providing ease and versatility in the installation of the cross-tie system 600. Further, in system 600, the converter heaters 634 can be arranged near the catalytic converters 632 in assemblies to conserve energy and reduce warm-up times. Similarly, the S0 ₂ /S0 ₃ converter assemblies 630 and 630' can be arranged near the S0 ₃ injection probes for each unit to minimize the distance and reduce the energy necessary to operate the system at suitable operating temperatures, and each individual S0 ₂ /S0 ₃ converter assembly can include one or more S0 ₂ /S0 ₃ converters in a simple assembly providing efficient two-stage conversion at S0 ₂ to S0 ₃ , other such important features and advantage are apparent from the drawing and foregoing description of the invention.

While several systems and preferred embodiments of the invention have been described and illustrated above, it will be clear to those skilled in the art that the various methods, apparatus and features of the invention included in the described and illustrated systems and preferred embodiments can be used, individually, and can be combined, in other systems and embodiments of the invention without departing from the scope of the following claims.