BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an apparatus and a process for removing low-concentration, high-volume non- condensable gases produced in a kraft pulping process.

Background of the Invention

Pulp is a fibrous product derived from cellulosic fiber-containing materials used in the production of hardboard, fiberboard, paperboard, paper, and molded-pulp products. The objective of wood pulping is to separate the cellulose fibers one from another in a manner that preserves the inherent fiber strength while removing as much of the lignin, extractives, and hemicellulose materials as required by pulp end-use considerations. Wood is converted to pulp by a combination of mechanical and chemical steps which constitute the pulping process.

Pulping begins with receipt of the wood at the mill site. Pulp logs are conveyed to the debarking area, where they are cut to the proper length, if necessary, and sorted. Accepted logs are mechanically fed into a bark

remover. Removed bark is collected, shredded, and used as a fuel in steam boilers. The debarked wood is conveyed to a chipper for conversion into chips of the proper length for chemical treatment in a subsequent cooking operation. This cooking can be accomplished in either a batch digester or a continuous digester. In the digestion process, screened chips are conveyed from storage to a chip-supply bin associated with the digester. Chips feed by gravity from the bin to a chip meter, the speed of which determines chip and cooking liquor flow rates to the digester and pulp discharge rate. Metered chips drop to a low-pressure feeder valve, through which the chips are introduced into a steaming vessel, where the chips are preheated, air is expelled from the chip interior for impregnation, and chip moisture content leveled in preparation for impregnation with cooking liquor. Cooked chips are continuously being removed from the bottom of the digester and other chips pass downwards from above in the digester, replacing those discharged. As cooked chips reach the bottom zone of the digester, they are plowed to a central well in the bottom of the digester while being mixed with filtrate from the pulp washer for cooling. Mechanical forces exerted in the

transfer of the chips from the digester to the blow tank effect fiberization of the chips. This fibrous material collected in the blow tank is called pulp. The pulp (brown stock) discharged to the blow tank is in admixture with black liquor, a water solution of spent and residual cooking chemicals and dissolved wood materials . The fiber bundles left in the pulp after blowing must be fiberized, i.e. , separated into discrete fibers, and the black liquor removed in order for the pulp to be refined and formed into a fiber sheet on the linerboard machines. Pulp is diluted with filtrate from the pulp washer and fed to a fibrilizer which serves the purpose of metal trapping, fiber-bundle breaking, rough screening, and pumping. Removal of the black liquor from screened brownstock is usually accomplished on rotary drum vacuum filters, arranged for multistage countercurrent washing. At various points in this process, the woody material may be bleached by treatments with a variety of oxidizing agents.

During the pulping process, the reaction of the wood materials with various chemical components results in the production of numerous gaseous products. These gaseous products are released from a number of sources. For

example, the digester vents gases during heating. A further source is digester blow gases which are emitted when pressure is released upon completion of the digestion cycle. Further liberation of gases occurs during evaporation of the black liquor. Additionally, some gases are released during brownstock washing.

Because of the nature of the chemical agents that are commonly employed in the pulping process, these gaseous products often contain a variety of sulfur compounds, including various mercaptans. Some of these sulfur compounds are malodorous, while others are toxic. Environmental concerns prohibit the release of these gases to the atmosphere and require that the gaseous products be collected and processed. The gases generated during the pulping process may be classified into two categories: a high concentration, low volume stream (HCLV) and a low concentration, high volume (LCHV) stream. The first stream, having a high concentration of organic components and a small volume, resembles natural gas in that it can undergo self-sustaining combustion. Thus the sulfur compounds can be readily burnt off and this gaseous stream is easily disposed. The second

stream, having a low concentration of organic components and a high volume, has been more problematic. This gas is predominantly air admixed with a small amount of organic materials, including sulfur compounds. This gaseous stream will normally contain approximately 5 to 6% by volume of various mercaptans.

Conventionally, LCHV non-condensable gas streams had been vented to power boilers to be incinerated with the base fuel feeding the boilers. However, because the LCHV gaseous streams contained various sulfur oxides, which are extremely corrosive, this approach resulted in internal corrosion of the pipes in the gas supply system to the power boilers. This corrosion resulted in leakage from the supply system and hazardous release of these gases. This disposal method is not economical because r. method mandates that the power boiler rely upon natural gas to stabilize the flame and to provide a heat sink, ensuring stable combustion during the normally fluctuating LCHV flow. Moreover, in modern pulping systems the recovery boiler is self-sufficient with regard to steam generation. Thus, the output of a power boiler is no longer necessary. Thus, conventional processes of treating LCHV gases require the

operation and maintenance of an unnecessary steam generator and the expense of fuel employed to stabilize the flame in the furnace of the steam generator.

An alternate approach to handling the LCHV non- condensable gas stream was to vent the stream to a recovery boiler for incineration. It was believed that, because recovery boilers were designed to handle corrosive gases, this approach would successfully process the LCHV gε.ses . However, no attempt was made at engineering the system and the LCHV non-condensable gases were simply vented into the furnace. Accordingly, corrosion occurred downstream from the recovery boiler and the furnace was occasionally damaged upon explosive re-ignition of the components of the LCHV non-condensable gas stream. The venting cϊ the LCHV gas stream additionally adversely affected the combustion process in the furnace of the steam generator.

Various approaches to processing the LCHV stream have been exemplified in the prior art.

U.S. Patent No. 3,520,772 to Lindberg discloses a process for removing malodorous air and water pollutants produced in alkaline pulp cooking in which polluting gases are routed to a furnace via a single furnace feed without

passing through a condenser. In an optional embodiment, the gases pass through a superheater on the route to the furnace.

U.S. Patent No. 3,396,076 to Crosby et al . discloses a method of recovering chemical values from the alkaline effluent resulting from the bleaching stage of the kraft pulping process. In this process the relief gas from the digester, the blow tank, the evaporator, and the finisher are routed to the primary zone of the recovery furnace by means of a single supply port.

In Svensk Papperstidning arg. 69, Nr. 15, page 484-487 (August 15, 1966) , Lindberg describes procedures implemented at the Skoghall sulfate mill to eliminate malodorous air and water pollutants. In this process the non-condensable relief gases from the digester pass a steam injector and are introduced into the recovery furnace by means of a single supply port just above the tertiary air intake.

However, the conventional methods of the prior art did not correct the problems of corrosion, foul odors, high emissions, and explosive re-ignition associated with simple

venting of the relief gases from the pulping process to the furnace of a chemical recovery steam generator.

Thus, there remains a need for a well-engineered system for processing LCHV gases produced during the pulping process.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a novel apparatus for introducing a low concentration, high volume (LCHV) non-condensable gas into a chemical recovery steam generator furnace.

A further object of the present invention is to provide a process for treating a low concentration, high volume (LCHV) non-condensable gas generated during kraft pulping, which process substantially reduces the possibility of explosive re-ignition, corrosion, foul odors, and high emissions .

These objects and others have been obtained by an apparatus for introducing a low concentration, high volume non-condensable gas into a chemical recovery steam generator furnace comprising a plurality of gas inlet pipes, each gas inlet pipe having a source end and a nozzle end, in which

the source end of the pipe is connected to a source of said low concentration, high volume non-condensable gas and the nozzle end of the pipe, which includes a nozzle, vents to said chemical recovery steam generator furnace; a preheater positioned in close proximity to said pipe and upstream of said nozzle; a secondary air port with a source end and a furnace end, in which the source end of said port is connected to a source of such secondary air and the furnace end of said port vents to said chemical recovery steam generator furnace; wherein the area enclosed by said secondary air port is greater than the area of said nozzle and wherein said nozzle is positioned within the area enclosed by said secondary air port such that said pipe and said port are substantially co-axial with respect to one another such that the gas is aspirated into said furnace by said secondary air. Additionally, the above objects and others have been achieved with a process for treating a low concentration, high volume non-condensable gas produced during a pulping process, comprising the steps of preheating said low concentration, high volume non-condensable gas; conveying said preheated gas to a plurality of gas inlet pipes; each gas inlet pipe having a source end and a nozzle

end, in which the source end of the pipe is connected to a source of said low concentration, high volume non- condensable gas and the nozzle end of the pipe, which includes a nozzle, vents to said chemical recovery steam generator furnace; conveying said preheated gas through the nozzle to the furnace such that the gas is oxidized, wherein said nozzle is positioned relative to a secondary air port with a source end and a furnace end, in which the source end of said port is connected to a source of such secondary air and the furnace end of said port vents to said chemical recovery steam generator furnace; such that the area enclosed by said secondary air port is greater than the area of said nozzle and wherein said nozzle is positioned within the area enclosed by said secondary air port such that said p pe and said port are substantially co-axial with respect to one another.

Brief Description of the Drawings

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same become better understood by reference

to the following detailed descriptions when considered in connection with the accompanying drawings, in which:

Figure 1 depicts a plan view of a gas pipe for introducing a low concentration, high volume non-condensable gas into a chemical recovery steam generator furnace;

Figure 2 represents a section along line A-A in Figure 1; and

Figure 3 depicts a section along line B-B in Figure 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to Figure 1 thereof, numeral 1 represents the wall of the furnace of the chemical recovery steam generator, which in a preferred embodiment is constructed from tubes which can contain a mixture of steam and water. Numeral 2 represents the nozzle. Numeral 3 depicts the edge of the wind box. Numeral 4 represents a valve for closing off the gas inlet p pe from the nozzle, which valve permits cleaning of the nozzle. Numeral 5 depicts a flange, which can be removed to

allow entry to access clean-out pipe 6 during nozzle cleaning.

Proper incineration of LCHV non-condensable gases can occur, if a system is employed which avoids the pitfalls found in conventional incineration attempts that used the chemical recovery steam generator furnace as the "dump spot," the point to which the LCHV non-condensable gases were routed.

A major problem in the past was extensive corrosion of the LCHV non-condensable gas transport and delivery system. As the system corroded, highly malodorous components escaped from the system to the enclosed recovery work spaces. A second pitfall was a failure to distribute and disperse the gases as they were fed into the furnace; all were vented through a single opening on the furnace side wall . A third shortcoming was condensation and subsequent introduction of condensate and water into the recovery furnace. Entry of water into the furnace in this fashion can result in a catastrophic smelt water reaction. A fourth common pitfall was an absence of any interlocks on the delivery system, which interlocks could divert the LCHV non- condensable gas to an alternate destination such as a

holding tank, if the support load or the LCHV non- condensable gas temperature was too low. Additionally, conventional LCHV non-condensable gas incineration had made no provision to provide access to the LCHV non-condensable gas ports for inspection and cleaning of the LCHV non- condensable gas delivery system.

Other features of the invention will become apparent in the course of the following descriptions of exemplary embodiments which are given for illustration of the invention and are not intended to be limiting thereof.

In one embodiment of the present invention, the LCHV non-condensable gas is supplied to the furnace of the chemical recovery steam generator by means of a plurality of nozzles, which nozzles are arranged symmetrically around the furnace walls. In a preferred embodiment, the LCHV non- condensable gas nozzles are distributed symmetrically with the secondary air ports . In a more preferred embodiment there are 10 LCHV non-condensable gas nozzles arranged with 5 LCHV non-condensable gas nozzles on the front wall of the furnace and 5 LCHV non-condensable gas nozzles on the rear wall of the furnace. The symmetrical arrangement of the LCHV non-condensable gas nozzles ports permits an even

distribution of the gases around the furnace, reduces TRS spikes and aids in flame stabilization. In addition, the symmetrical distribution of nozzles minimizes the destabilizing effect these high volume gases previously had upon bed formation and furnace mixing.

In another embodiment of the present invention, a preheater is installed in the LCHV non-condensable gas supply line to the furnace upstream of the valve. This preheater comprises one or more methods of heating the fluid in the LCHV non-condensable gas supply line. Such methods include heating by means of electrical resistance, heating by means of a heated fluid surrounding the LCHV non- condensable gas supply line, and heating by means of a independent heat source such as a flame produced by burning a combustible material. Such methods are appropriately described in the conventional literature. In a more preferred embodiment, the preheater maintains the LCHV non- condensable gas at an exit temperature of 300° F.

In another embodiment of the present invention, the LCHV non-condensable gas supply to the furnace is provided with interlocks, which interlocks can either prevent the introduction of the LCHV non-condensable gas

into the chemical recovery steam generator or divert the LCHV non-condensable gas to an alternate destination such as holding tank, if the support load or the LCHV non- condensable gas temperature is too low. The interlock system comprises either means for switching on or off the

LCHV non-condensable gas supply to the one or more gas inlet pipes or means for switching the LCHV non-condensable gas supply line from supplying the gas to the one or more gas inlet pipes to supplying the gas to an alternate destination and back again. Such interlocks operate by monitoring either the support load or the temperature of the LCHV non- condensable gas. The interlock is installed in the LCHV non-condensable gas supply line upstream of the nozzle. Appropriate monitoring and switching systems are conventionally described in the literature.

The combination of preheater and interlock reduces the risk of condensation in the LCHV non-condensable gas supply and, thus, avoids the introduction of water into the furnace. In yet another embodiment of the present invention, the LCHV non-condensable gas is routed to the furnace of the chemical recovery steam generator only when

the generator is operating at a level of at least 60% of the maximum continuous power rating of the chemical recovery steam generator. At those periods when the power rating of the generator is below 60% of the maximum continuous power rating, the LCHV non-condensable gas is alternately routed to a holding tank. This procedure assures a stable heatsink for the gases and eliminates the risk of flame-out and explosive re-ignitions.

In addition, the LCHV non-condensable gas nozzles are arranged in the secondary zone of the furnace and each port is sized to fit within an area enclosed by a secondary air port, thus permitting secondary air to totally surround the LCHV non-condensable gas stream. This design provides aspiration of the LCHV non-condensable gas such that the combination of secondary air port and gas nozzle acts as an injector for the LCHV non-condensable gas into the furnace.

It should be noted that the chemical recovery stage is characterized by the large number of particulates generated by the combustion process, which particulates can plug the nozzles of the gas supply pipes. The present invention provides each gas supply line with a valve and a

clean-out, which allows each nozzle to be shut off individually and to be cleaned individually.

In another embodiment of the present invention, each gas inlet pipe is provided with a clean-out leg to permit cleaning and rodding. In a preferred embodiment of the invention, each gas inlet pipe is equipped with an shut- off valve to permit cleaning and rodding of the nozzle while the furnace is in operation.

In another preferred embodiment of the present invention, the system limits the input of the LCHV non- condensable gas to no more than 20% by volume of the total gas flow to the boiler. This is accomplished by means of a flowmeter regulating the volume of the LCHV non-condensable gas to 20% or less by volume relative to the volume of the total air flow of combustion air to the boiler. Such a flowmeter provides means for monitoring the total gas flow to the furnace and the LCHV non-condensable gas flow and means for regulating the LCHV non-condensable gas flow such that the LCHV non-condensable gas flow does not exceed 20% by volume of the total gas flow to the furnace. The flowmeter is installed in the LCHV non-condensable gas supply line upstream of the nozzle. Appropriate means for

monitoring and regulating gas flow are described in the conventional literature.

Because the LCHV non-condensable gas is corrosive and wet, it is preferred that the system supplying this gas stream to the chemical recovery steam generator is constructed from stainless steel piping.

In a particularly preferred embodiment, the LCHV non-condensable gas is introduced by means of a plurality of gas supply pipes symmetrically arranged around the periphery of the furnace and positioned in the hottest combustion zone, in contrast to conventional gas supply arrangements which routed the gas through a single opening.

In the present invention the area of the secondary air port is larger that the nozzle of the gas supply pipe and the nozzle introducing the LCHV non-condensable gas is positioned such that the secondary air stream issuing from the secondary air port surrounds and forms an annulus around the LCHV non-condensable gas stream. In a preferred embodiment, the center-line of the nozzle is arranged approximately with the center-line of the secondary air port . The result of this arrangement of the secondary air port and the nozzle is that the secondary air acts as an

aspiration jet, distributing and mixing the LCHV non- condensable gas .

Normally, the windbox rfhich supplies the air to the secondary air port is maintained at an air pressure of approximately 10 inches of water. On the other side of the secondary air port, the air pressure is approximately negative 1/2" of water. Thus, there is a differential head pressure between the windbox and the furnace .

Modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than is specifically described herein.