BACKGROUND a. Field of the Invention. This invention relates to pollution control equipment, namely flue gas conditioning apparatuses used to increase the efficiency of electrostatic precipitators . b. Description of Related Art. Electric utilities, manufacturing plants and industrial facilities typically burn fossil fuels like coal and heavy oil to produce electric power and heat for process requirements. Coal and heavy oil combustion releases minerals into the air in the form of fine particulate matter, which ranges in size from tens of microns too less than one micron. The serious health consequences of inhaling particulate matter pollution are well known.

Minerals making up the bulk of particulate matter in fly ash produced by coal combustion are primarily oxides, such as silica (Si0 ₂ ) , alumina (A1 ₂ 0 ₃ ) , and iron oxide (Fe ₂ 0 ₃ ) . Collection of these materials is often accomplished through the use of electrostatic precipitators (ESP's) .

Particulate matter collection in an ESP takes place in several steps. First, particles are charged by the action of a corona discharge surrounding high voltage ESP wires. Second, the electric field established by the high voltage wires accelerates charged particles toward collecting electrodes, which are typically grounded metal plates. Third, particles adhere to the collecting electrodes forming a blanket, which gets thicker with time. Finally, they are removed from the electrodes by a mechanical shock, or

"rapping," which causes a collection of particles to fall into hoppers positioned below the plates.

Physical properties of particulate matter strongly affect the operation efficiency of an ESP, particularly size distribution of the particles. Large particles are captured more easily than smaller, sub-micron particles. However, small particles have been shown to contain a greater proportion of toxic heavy metals present in the ash, such as cadmium, selenium, arsenic, and nickel, presenting a greater risk to public health and the environment.

Electrical properties of particulate matter are crucial to proper operation of an ESP. It is well known that the electrical resistivity of the fly ash is of primary importance. Values on the order of 10 ¹⁰ ohm-cm are considered optimum for efficient collection. Resistivity values substantially below 10 ¹⁰ ohm-cm result in re- lntrainment of collected particles into the gas stream. It occurs because low resistivity particles easily give up their charge to the collecting electrode upon contact. Without charge on the particles, there are only weak surface tension forces to keep them on the collecting plate.

Electrical resistivity of the fly ash significantly above the optimum value of 10 ¹⁰ ohm-cm results in several problems. First, fly ash strongly adheres to the collecting plates, making it difficult to remove by rapping. Secondly, as the charged fly ash layer builds up on the plate the voltage increases across the ash layer causing a voltage drop across the gas space. A low voltage drop across the gas space reduces particle charging due to the weakened corona discharge that occurs.

High voltage across the ash layer also creates a strong electric field in the layer. An electrical discharge in the layer, known as back corona, can result, creating a hole in the layer and releasing charged species into the gas stream. These species (molecules and particles) are opposite in

charge to the normal airborne fly ash,- and can neutralize them upon contact, reducing collection efficiency. Resistivity is made up of two parts: (1) bulk resistivity, and (2) surface resistivity. Bulk resistivity is the resistance to flow of electrical current through the bulk of the material. It is strongly dependent on temperature, generally dropping several orders of magnitude as the temperature of the fly ash is increased from about 25 to above 230 degrees Celsius. At room temperature, resistivity values on the order of 10 ¹³ ohm-cm are typical of the mineral matter in fly ash.

Surface resistivity is resistance to the flow of current along the outer surface of the particulate matter. Characteristics of the surface, such as roundness and roughness, and material adsorbed to the surface of the fly ash are important factors in determining surface resistivity.

High temperature , or "hot side," precipitators operating at about 315°C collecting coal combustion-produced fly ash do not usually have problems with resistivity, since high temperature operation ensures low bulk resistivity. Low temperature, or "cold side," precipitators operating at about 150°C are much more susceptible to collection problems resulting from high resistivity. This problem has been exacerbated by increased use of low sulfur coal in power plants in order to reduce sulfur emissions and coal cost. Typically a small portion, approximately 1 - 2%, of sulfur burned during combustion is converted to sulfur trioxide (S0 ₃ ) instead of the more prevalent form, sulfur dioxide (S0 ₂ ) . Low sulfur coals yield commensurately lower S0 concentrations in the flue gas.

At 150°C, and in the presence of water vapor, S0 is converted to sulfuric acid (H ₂ S0 ₄ ) . Sulfur trioxide or sulfuric acid in low concentrations, about 10 pp ,

significantly increases the dew point of the flue gas stream, allowing for greater condensation of acid and water vapor on the surface of solid particles. A condensed water layer on the particles, containing ions created by dissociation of the strong acid and dissolved mineral salts, reduces surface resistivity of particulate matter.

Combustion of low sulfur coal results in low S0 ₃ concentrations in the flue gas, about 1 - 2 ppm, which is too low to greatly effect the dew point of the gas stream and resistivity of ash particles. Increasing S0 ₃ concentration of the flue gas is currently accomplished by one of two general methods. The most prevalent is combustion of elemental sulfur to S0 ₂ , followed by the catalytic oxidation of S0 ₂ to S0 ₃ . Catalytic conversion of S0 ₂ present in the flue gas is the second method. It requires that a portion of the flue gas be withdrawn front the gas stream, passed over a catalyst at high temperature (about 510 degrees Celsius), then re-injected into the bulk gas. There are many disadvantages in both of these methods. (1) Handling elemental sulfur is problematic, and the concept of increasing the sulfur content of the flue gas seems to reduce the environmental benefits of switching to low sulfur coal. (2) Sulfur is an expensive, consumable ingredient that must be purchased in addition to the fuel. Sulfur dust which escapes is a pollutant. When burned, sulfur creates S0 ₂ that causes acid rain.

(3) Catalysts are required in both systems, with their attendant plugging, deactivation, and disposal problems. Plugging, which can occur with elemental sulfur (as in the first method) or with fly ash (as in the second method, requires regeneration or replacement of the catalyst bed. If the catalyst bed becomes poisoned, the catalyst must be disposed of and replaced. Since they are typically composed of vanadium or other precious metals, replacement is

expensive. The catalysts are also hazardous wastes, which are expensive to properly discard. In addition, catalyst activity decreases over its lifetime. The result is that increasing amounts of sulfur must be added for the same conditioning effect, further defeating the advantages of using low sulfur coal.

(4) Injection of high temperature gases necessitated by operation of the catalyst bed increases fan power required for plant operation. This is a waste of power that could otherwise be sold or used in the plant.

For the foregoing reasons, there is a need for a method and apparatus for conditioning flue gas by the creation of S0 ₃ that does not use catalysts or require addition of elemental sulfur to the gas stream. A better process would be to use the S0 ₂ present in the flue gas and create sufficient quantities of S0 ₃ to optimize the fly ash resistivity for collection in an ESP.

SUMMARY The present invention is directed to a method and apparatus that satisfies these needs. The method and apparatus of the present invention converts a sufficient amount of S0 ₂ to S0 ₃ and H ₂ S0 ₄ by exposing a portion of the flue gas to a barrier discharge. The discharge is produced by applying high voltage, alternating current from a power source between electrodes separated by a gas space and a highly resistive, dielectric barrier. The barrier discharge, also known as a dielectric barrier or silent discharge, initiates a series of reactions, which lead to the production of S0 ₃ and H ₂ S0 ₄ . These and other features, aspects, and advantages of the present invention will become better understood with reference to the following drawings and description.

DRAWINGS

Fig. 1 is a perspective view of a section of barrier discharge apparatus, showing a cylindrical and planar electrode assembly.

Fig. 2 is a cut-away view of the cylindrical electrode assembly showing the conductive electrode inside a dielectric barrier.

Fig. 3 is a perspective view of a section of the barrier discharge apparatus, showing planar electrode assemblies . Fig. 4 is a cut-away view of the planar electrode assembly showing the conductive electrode surrounded by dielectric barrier material.

Fig. 5 is a cut-away view of the planar electrode assembly which can contain either a solid or w re mesh conductive electrode.

Fig. 6 is a cut-away view of the planar electrode assembly with spaced electrode conductors.

Fig. 7 is a perspective view of a complete barrier discharge apparatus positioned inside a flue gas duct. Fig. 8 is a side elevation of a version of the barrier discharge apparatus, with optional flow shield.

Fig. 9 is a cut-away view of a version with cylindrical high voltage and grounded electrodes.

DESCRIPTION

The present invention is a method and apparatus for converting S0 ₂ to S0 ₃ and H ₂ S0 using an electrical discharge and sulfur dioxide present in flue gas. The principle of operation is that a portion of the flue gas is exposed to a barrier discharge, which initiates a series of reactions leading to the production of S0 ₃ and H ₂ S0 ₄ . Ultraviolet light, ionizing radiation, pulsed electro-magnetic radiation, or microwave radiation may also be used instead of, or with, barrier discharge. A proposed reaction path is as follows:

( 1 ) H ₂ 0 - OH + H ⁺ + e ^■

( 2 ) OH + S0 ₂ → HOS0 ₂

( 3 ) HOS0 ₂ + 0 ₂ → H0 ₂ + S0 ₃ (4) HOS0 ₂ + OH→ H ₂ SO ₄ Additional reactions leading to the production of SO^ and H ₂ S0 ₄ are also possible, as is appreciated by those skilled in the art.

Reaction (1) is initiated by the action of the barrier discharge on water vapor present in flue gas. Subsequent reactions (2), (3) and (4) may take place within the discharge reactor, or after the gas exits the reactor. Reactions ^' (1) through (4) can be made to occur at low temperature (65 - 150 degrees Celsius), eliminating the need for flue gas heating and re-injection of the hot gases. Turning to Figs. 1 and 2, a barrier discharge is produced by the application of high- voltage, alternating current through the high voltage feed-throughs 12A and 12B to an electrode 16, which is adjacent to, or surrounded by, electrodes of substantially different voltage 14. Examples of electrodes 14 with substantially different voltage are those that are grounded or of opposite polarity to the high voltage electrode 16. In addition to the gas space between electrodes, a solid dielectric barrier 10 is required. The dielectric barrier 10 substantially surrounds the high voltage electrode 16, and should make a good electrical contact with it. Although the high voltage electrode-barrier assembly of Fig. 2 is shown as cylindrical, it can take on any number of configurations, several of which will be discussed. The dielectric barrier 10 alters the operation of the electrical discharge, producing higher energy electrons than possible in a corona discharge. High electron energies increase the production efficiency of the compounds initiating the conversion process, in this case, primarily

the production of hydroxyl radicals (OH) . Required properties for an effective barrier 10 are high electrical resistivity and high dielectric strength to prevent breakdown under the action of the discharge. The preferred embodiment uses fused silica tubing as such a barrier, although additional materials such as glass, alumina (A1 ₂ 0 ₃ ) , titania (Ti0 ₂ ) , other ceramics, or mica may be used.

As shown in Fig. 1, the configuration of a barrier discharge reactor allows for adequate spacing that eliminates the possibility of plugging, as can occur in catalytic reactors. Flue gas flows parallel to the plates 14 so there is minimum interruption to the flow in the gas stream and a maximum exposure to the barrier discharge.

In the preferred embodiment of the invention, as shown in Fig. 1, the planar electrodes 14 are plates, are approximately 30 cm square, and are held approximately 5 cm apart. The plates are electrically grounded, and are typically made of a conductive metal. Although more or fewer electrodes 16 may be used between each plate 14, the preferred embodiment uses a quantity of twenty. Fig. 1 shows a portion of only one cell. Also, the ground members 14 do not have to be flat. They may be concave shaped about each electrode or concave overall.

Parallel plate electrode configurations and assemblies are shown in Figures 3 through 6. A parallel plate electrode configuration is shown in Fig. 3 where both the high voltage 20 and grounded electrode assemblies 14 are in the form of flat plates. A planar high voltage electrode assembly 20 shown in Figs. 4 and 5 consists of a conductor 26 surrounded by dielectric barrier 20. The conductor 26 can take on the form of a solid metal plate or wire mesh screen. The conductor 26 may also be in the form of spaced strips as shown in the electrode assembly side view presented in Fig. 6.

As shown in Fig. 9, a coaxial cylinder electrode geometry may also be used where the high voltage electrode 16, surrounded by a dielectric barrier 10 is centered in a cylindrical grounded electrode 28. The inside surface of grounded electrode may also be covered with a dielectric insulating material 30 in the cylindrical or any other electrode configuration. The grounded electrode dielectric insulating material can be made from the same material as the high voltage dielectric barrier 10. In practice, a barrier discharge reactor for conversion of S0 ₂ to S0 ₃ and H ₂ S0 ₄ would consist of cells repeated across the length of a flue gas duct 18, as shown in Fig. 7. The cells 15 are arranged in modules 17 with several modules spanning the width of the flue gas duct 18. A section oi flue gas duct 18 suitable for placing a barrier discharge apparatus is placed after the burner but before the ESP in the flue gas stream. A typical cross sectional area is about 9 meters in width and about 3 meters in height. A stacked length of modules of the barrier discharge apparatus is preferably secured lengthwise across the duct 18.

Additional configurations of the barrier discharge reactor are also possible and will be appreciated by those having skill in the art. These configurations permit the apparatus to contact a sufficient proportion of the flue gas to enable it to produce the proper quantity of S0 for conditioning. In one embodiment, cells comprising sets of about thirty plates 14 and associated electrodes 16 and barriers 10 will be manufactured in 1.5 meter modules. These sections would be shipped individually and assembled end to end on site.

As shown in Fig. 8, one version of the present invention employs a flow shield 13. The flow shield 13 deflects particles in the flue gas that may damage the barrier 10.

The power supplied to the high voltage feed-throughs 12A and 12B of Fig. 1 is between about 15,000 and 50,000 volts RMS, and between about 50 Hz and 10 kHz. The preferred embodiment operates at line frequency (between about 50 or 60 Hz) eliminating the need for expensive frequency conversion. Power can be supplied in this embodiment with only a high voltage transformer and variable voltage power supply. The alternating current can be in the form of either a sine wave, square wave, or pulsed alternating current.

The dielectric barrier 10, if it has a cylindrical configuration, should have an inside diameter of between about 1 and 10 mm, and an outside diameter of between about 3 and 12 mm. The preferred embodiment has an inside diameter of about 4 mm, and outside diameter of about 6 mm.