WO2000065150A1

WO2000065150A1 - Gasification process for spent liquor at high temperature and high pressure

Info

Publication number: WO2000065150A1
Application number: PCT/US2000/010995
Authority: WO
Inventors: Jerry D. Blue; William Downs; Timothy A. Fuller; Christopher L. Verrill; Paul S. Weitzel; Phung H. M. Chan
Original assignee: McDermott Engineers and Constructors Canada Ltd; Babcock and Wilcox Co; McDermott Technology Inc
Current assignee: Jacobs Canada Inc; Babcock and Wilcox Co; McDermott Technology Inc
Priority date: 1999-04-23
Filing date: 2000-04-21
Publication date: 2000-11-02
Anticipated expiration: 2001-10-23
Also published as: WO2000065150A9; AU4659600A; CA2370968A1; CA2370968C

Abstract

A method and apparatus for producing clean, sweet, fuel gas for use in a combustion process by processing a waste stream from digestion of lignocellulosic material partially oxidizes the waste stream to form hot gases and molten salts and cools the hot gases and molten salts using a quench liquor to form quenched gas (76) and carbonate liquor (30). Particles are removed from the quenched gas to form a raw fuel gas by subjecting the quenched gas to a multi-step fume reduction process (80) which includes heat extraction (91) from the quenched gas to reduce particulate load and water content of the quenched gas to form a low fume fuel gas. H2S is removed from the low fume fuel gas using an H2S removal process (105) which is more selective for H2S than it is for CO2, the removing step forming clean, sweet, fuel gas and acid gases.

Description

GASIFICATION PROCESS FOR SPENT LIQUOR AT HIGH TEMPERATURE AND HIGH PRESSURE

CROSS-REFERENCE TO RELA TED APPLICA TIONS

Reference is made to the U.S. patent application of William Downs, titled ULTRA-HIGH PARTICULATE COLLECTION OF SUB-MICRON AEROSOLS, U.S. patent application Serial No. 09/298,778, filed April 23, 1999 and the U.S. patent application of Jerry D. Blue, William Downs, Timothy A. Fuller, and Christopher L. Verrill, titled SULFUR RECONERY FROM SPENT LIQUOR GASIFICATION PROCESS, U.S. patent application Serial No. 09/298,974, filed April 23, 1999, the text of which are hereby incorporated by reference as though fully set forth herein. Unless otherwise stated, definitions of terms in these applications are valid for this disclosure also.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates in general to pulp and paper spent chemical recovery processes, and in particular to a new and useful spent liquor gasification process, such as a black liquor gasification process, which provides a more efficient utilization of the spent chemical's fuel value in the production of electric power.

A schematic representation of a conventional kraft recovery process is depicted in Fig. 1. This process consists of several unit operations. The key ones are the combustion of black liquor in a process recovery boiler 10, the conversion of green liquor to white liquor in a slaker/causticizer 12, the production of steam 14 for both process use and electric power production, and the calcination of calcium carbonate to lime (calcium oxide) in a lime kiln 16. Gases from the recovery boiler 10 are passed to a dust collection apparatus

18, such as an electrostatic precipitator (ESP), and yield flue gas at 20 which can be released to a stack (not shown). Salt cake is returned at 22 to the inlet of recovery boiler 10 where it is mixed with incoming black liquor and combusted with air. Smelt at 24 from boiler 10 is passed to a dissolving tank 26 where it is contacted with weak wash 28. Dissolving tank 26 discharges green liquor 30 for input into the slaker/causticizer 12. White liquor 32 from the slaker/causticizer 12 is supplied to a digester (not shown) while reburned lime at 34 from kiln 16 is provided into the slaker/causticizer 12 where it reacts with sodium carbonate to produce the white liquor and solid calcium carbonate 36. The white liquor and solid calcium carbonate 36 are separated on a filter, 38, into a filter cake called lime mud 40 and the filtrate, i.e. the white liquor. The lime mud is washed with water, and the filtrate from the washing step is called weak wash 28. The lime mud 40 is provided to the kiln 16 which discharges flue gas 42 to a kiln stack (not shown).

This is a very mature process with few improvements over the past 40 years. Although no full scale, high pressure, high temperature, oxygen blown black liquor gasification (BLG) processes have been built to date, process concepts have been published. A generic process is depicted in Fig. 2. Fig. 2 illustrates one of the principal advantages of black liquor gasification over the conventional kraft recovery process, i.e., a substantial improvement in cycle efficiency through the use of combined cycle technology in the form of a gas turbine/steam turbine couple 50. In Fig. 2 and the remaining figures, the same reference numerals will be used to designate the same, or functionally similar parts.

One of the consequences of BLG is that a substantial portion of the sulfur in the black liquor is partitioned to the gas phase when compared to the conventional kraft process. In an H₂S scrubber 58 of the system depicted in Fig. 2, sulfur in the form of H₂S is reabsorbed into a mixture 52 of weak wash 28 and white liquor 56.

CO₂ will also absorb into this solution. In fact, most of the free hydroxide in the mixture 52 of weak wash 28 and white liquor 56 will be consumed by the CO₂. It is therefore necessary to recycle an effluent 54 from the H₂S scrubber 58 back through the causticizing plant 12 (via the quencher 26) as shown in Fig. 2. This has the undesirable effect of increasing the burden on the causticizing plant 12 and lime kiln 16 by as much as 50% and results in a very large thermal penalty to the black liquor gasification process.

The black liquor gasification process can also generate a significant quantity of sub-micron sized alkali fume and soot. Alkali fume is extremely damaging to gas turbine blades. For this reason, it is vitally important that alkali fume and soot be controlled to a significant extent, perhaps to a removal efficiency as high as 99.9997% (five nines control). The conventional BLG process purports to do adequate control of fine particulate in a single venruri scrubber 60. However, the energy required to achieve adequate fume control in the single venturi scrubber 60 again places a significant energy penalty on the BLG process.

SUMMARY OF THE INVENTION

The system utilized in the improved BLG process of the present invention described below, solves the aforementioned problems in an energy efficient manner.

An object of the present invention is to provide a method and apparatus for producing clean, sweet, fuel gas for use in a combustion process by processing a waste stream from digestion of lignocellulosic material. The invention includes partially oxidizing the waste stream to form hot gases and molten salts, and cooling the hot gases and molten salts using a quench liquor to form quenched gas and carbonate liquor. Particles are removed from the quenched gas to form a raw fuel gas by subjecting the quenched gas to a multi-step fume reduction process which includes heat extraction from the quenched gas to reduce particulate load and water content of the quenched gas to form a low fume fuel gas. H₂S is removed from the low fume fuel gas using an H₂S removal process which is more selective for H₂S than it is for CO₂, the removing step forming clean, sweet, fuel gas and acid gases. Finally, the clean, sweet, fuel gas is conveyed to a combustion process. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRA WINGS In the drawings:

Fig. 1 is a flow chart showing a prior art kraft recovery apparatus and method; Fig. 2 is a flow chart showing a prior art black liquor gasification apparatus and method; Fig. 3 is a flow chart showing the apparatus and method of the present invention; and Fig. 4 is a schematic diagram of a fume and soot collection system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings generally, wherein like reference numerals designate the same or functionally similar elements throughout the several drawings, and to Fig. 3 in particular, there is shown the process and apparatus of the present invention. The apparatus includes several elements that are common to the generic BLG process depicted in Figure 2. In its broadest form the process of the present invention begins with the atomization, partial combustion and gasification of a mixed organic/inorganic waste stream resulting from the digestion of wood or other lignocellulosic materials. An oxidant such as air or oxygen is used for the partial combustion. Of course, while the method and apparatus of the present invention will likely find first commercial application to the processing of black liquor produced in the well known kraft pulping and recovery process, the present invention is not limited to that particular type of pulping process. For example, the present invention can also be applied to process alkaline, acidic, or neutral sulfite spent liquors, as well as polysulfide spent liquors. As is known to those skilled in the art, the terms "black liquor" or "smelt" are commonly used in connection with the kraft pulping process, while sulfite spent liquors are commonly called "red" liquors and not "black", and polysulfite pulping liquor is commonly called " orange" liquor and not "white" liquor. Accordingly, it will be understood that while the terms black liquor, smelt, green liquor, white liquor, lime mud, and weak wash have been employed in the Figures and in the following description of the preferred embodiment of the invention, persons skilled in the art will appreciate that the invention is not limited merely to the kraft pulping process. Corresponding broader terminology such as spent liquor, molten salts, carbonate liquor, caustic liquor, and calcium carbonate solids may be substituted, respectively, for those terms as applicable, together with the same term weak wash depending upon the particular type of pulping process that is involved. Such broader terminology has been employed in the claims appended to and forming a part of this specification.

Similarly, the present invention employs the term "lignocellulosic" to encompass all of the various types of feed stocks which one might want to employ in a pulping process, to broadly include woody and non- woody plants, whether or not the kraft type pulping process or other types of pulping processes are employed. For further details of the various aspects of pulping processes used in the paper industry, the reader is referred to STEAM Its Generation and Use. 40^th Ed., Stultz and Kitto, Eds., © 1992 The Babcock & Wilcox Company, particularly to Chapter 26 - Chemical and Heat Recovery in the Paper Industry, the text of which is hereby incorporated by reference as though fully set forth herein. A gasifier 70 of the invention is required, and preferably comprises a water jacketed, refractory lined vessel where a waste stream produced by the digestion of lignocellulosic materials, such as black liquor 72, typically containing less than 40% water is atomized and partially combusted with an oxidant 74, such as air or preferably oxygen or mixtures thereof. The gasifier 70 operates in a pressure range from atmospheric up to 800 psia, and preferably 300 to 600 psia. A stream 78 leaving the gasifier 70 will be in the temperature range of 1600 to 2200 F, typically about 1800 F and comprises fuel gas 76 constituents, molten salts referred to in the pulp and paper industry as "smelt" (in a kraft recovery process), and a sub-micron sized mixture of alkali fume and soot. The molten smelt comprises relatively large molten droplets and or streams which flow down the walls of the gasifier 70 towards a quencher 26. In the quencher 26, a relatively coarse spray comprising a mixture 29 of weak wash 28 (a slightly alkaline solution) and condensate with dissolved fume 100, is used to cool the fuel gas 76 to its adiabatic saturation temperature, which is typically in the range of about 300 to 400 F. While the evaporative cooling takes place in the quencher 26, the smelt constituents are collected quantitatively by the quench spray and dissolved therein. The resulting mixture 30 is sulfide lean and is known as green liquor. This stream 30 of green liquor will be subsequently causticized to white liquor in the slaker/causticizer 12. That portion of the process will be described in more detail later.

The raw fuel gas 76, upon leaving the gasifier quencher 26, will typically contain about 3500 ppm of H₂S, about 8% CO₂, a water content of over 1.75 lbs H₂O/lb dry gas, and a fume concentration as high as 800 grains per ft³ of fuel gas 76.

To be an acceptable fuel gas for a gas turbine, the fume concentration in the fuel gas

76 must be reduced to less than about 0.0024 grains/ft³. To accomplish that feat, a three-step process 80 (shown in greater detail in Fig. 4) is employed according to the present invention. Referring now to Fig. 4, in the three-step fume reduction process and apparatus 80, a first venturi scrubber 82 first contacts the fuel gas 76. The venturi scrubber 82 depicted in Fig. 4 will typically operate at a pressure drop of about 2 to 5 psi. Scrubber 82 uses a circulation of water 88 from a circulation pump 90, for example, through venturi 86 where the aqueous spray contacts the fuel gas 76 in the throat of the venturi 86. The water 88 may be taken from the quencher stream 100. The relatively high gas velocity (> 200 ft/sec) and low liquid flow rate (<1 lb liquid/lb gas) promotes atomization of the liquid in the venturi throat and subsequent inertial impaction between the liquid droplets and the fume particles in venturi scrubber enclosure 84. The purpose of this first venturi scrubber 82 is to reduce the dust loading of the fuel gas 76 to less than about 8 grains/ft³. Following this venturi scrubber 82 the fuel gas 76 is cooled (schematically indicated at 91 in Fig. 4) and the water content is reduced to less than about 0.35 lb H₂O/lb dry gas. Three separate approaches are available for this task. First, the fuel gas 76 can be cooled in a heat exchanger 91 , advantageously a condensing heat exchanger, using cold, high-pressure process water from the mill or another source such as a cooling tower. After exiting the heat exchanger 91 , the cooling water will be heated sufficiently for use in resaturating the fuel gas 76 after the H₂S removal operation (at 106) as described below. Thus, energy removed from the fuel gas 76 during the particulate control operations can be efficiently returned to the fuel gas later in the process. Alternatively, the fuel gas 76 can be cooled in the first venturi scrubber 82 by using cooled water from 89 as the aqueous spray 88. The heat absorbed by this water in the venturi scrubber 82 can be rejected by a liquid-to-liquid heat exchanger. The water condensed from the fuel gas 76 in this manner leaves the venturi 86 as blowdown 101. In a further alternative, the fuel gas 76 can be passed through a boiler 91 to generate steam for use within the paper making process. The reduction in water content of the fuel gas 76 from 1.75 to 0.35 lb H₂O/lb dry gas represents a greater than 50% reduction in total volumetric flow rate of fuel gas 76. Thus, the fuel gas treatment equipment to follow is greatly reduced in size.

The fuel gas 76 with the dust loading reduced to less than 8 grains/ft³ enters an electrostatic agglomerator-venturi scrubber arrangement depicted in Fig. 4. The electrostatic agglomerator (ESA) 92 is sized to collect at least 99.9% of the fume and soot not captured by the first venturi scrubber 82. The ESA 92 is designed to temporarily retain the collected material on the walls of its collection tubes 94 whereby it is re-entrained by the suitable use of rappers (not shown). A second venturi scrubber 96 subsequently captures the agglomerated solids. A source of water from quench stream 100, and a blowdown stream 103 may be employed as described above in connection with the first venturi scrubber 82. For details of one such type of particulate removal equipment 80, reference is made to the aforementioned U.S. Patent application of William Downs, titled ULTRA-HIGH PARTICULATE COLLECTION OF SUB-MICRON AEROSOLS.

Upon leaving the particulate control section 80, sour fuel gas 98 will contain about 7700 ppm H₂S and about 17% CO₂. The temperature will be about 325 F. This fuel gas 98 temperature is too hot to be effectively treated by conventional absorption-stripping processes for H₂S removal.

To accomplish adequate H₂S removal performance requires that the fuel gas 98 be cooled further. This cooling preferably takes place in another heat exchanger 99 again preferably a condensing heat exchanger, to cool the fuel gas stream 98. Leaving the heat exchanger 99, the fuel gas 98 water content has been lowered to about 0.01 lb H₂O/ lb dry fuel gas. The H₂S concentration is raised to 10,900 ppm and the CO₂ concentration is 24.4%. The water condensed from the fuel gas stream in the two heat exchangers 91, 99 (or alternate devices as described) and blowdown streams 101, 103 from the two venturi scrubbers 82, 96 are collected and sent to the quencher stream 100. The purpose of this aqueous stream is to control the concentration of salts in the green liquor 30. The pulp and paper mill requires that the green liquor salt concentration be within a specified range; typically 120-130 g/L as Na₂O. The water collected from the two venturi scrubbers and the heat exchangers is also contaminated with low levels of salts from the alkaline fume carryover. By taking this water back to the quencher 26, all alkali compounds are returned to the process. The sour fuel gas 98 next enters an absorption column 102 (part of a sulfur removal system generally designated 105) where H₂S is preferentially absorbed from the fuel gas 98. The column 102 is designed to remove over 99% of the H₂S in the fuel gas. Some CO₂ is also removed from the fuel gas 98 in this absorption column.

Chemical solvents such as methyldiethanolamine, MDEA, or a physical solvent such as the SELEXOL solvent can be used here.

The fuel gas 98 leaving the absorption column at 104 is next contacted with a stream of hot water and/or steam at 106. The hot water can come from the condensing heat exchanger 91 after the first venturi scrubber 82 as described earlier.

The purpose is to produce reheated and humidified fuel gas 109 which is provided into a gas turbine 110. The fuel gas 109 is heated to a suitable temperature for the selected gas turbine. For example, for a General Electric Model 6FA gas turbine the fuel gas 109 is reheated to about 385 F, a water content of about 0.5 lb H₂O/lb dry fuel gas, and a pressure of about 300 psia. At the gas inlet of the turbine combustor 112 these gases are throttled before mixing with the combustion air in such a manner that the combustor 112 operates at about 50 psi below the fuel gas 109 inlet pressure.

The gas turbine 110 is coupled to both an electric generator 117 and to a compressor

118 which supplies combustion air 116 to the combustor 112 at the operating pressure of the combustor 112. The system is flexible and can easily accommodate other gas turbine requirements. The hot exhaust gases 120 leave the gas turbine 110 at a temperature above about 1000 F. These gases 120 contain in excess of 10% oxygen. Since these gases 120 are well above the auto-ignition temperature of natural gas, additional heat can be added to the turbine exhaust gases 120 by contacting these gases with natural gas in a duct burner 122. In this manner, the gases can be heated to above 1500 F before entry into a waste heat boiler 124. This permits the generation of high pressure steam at, for example, 1250 psi and 925 F, but again other steam cycles can be used. This steam is suitable for use in a back pressure steam turbine 126 to produce electric power via electric generator 127, and also process steam.

The recovery and reuse of sulfur that began with the H₂S absoφtion column

102 described above is more particularly accomplished in the manner described here.

The solvent used in the absoφtion step is transferred to a stripping column 107 where the pressure is reduced preferably to less than 30 psia. The solvent is then regenerated by heating. The combination of low pressure and high temperature causes the absorbed H₂S and CO₂ to evolve from the solvent into the gas phase. This unit operation is known as solvent regeneration. The gas 134 that evolves from the stripping column 107 is referred to as acid gas since it contains mostly H₂S and CO₂.

The lean solvent leaving the stripping column is cooled and pumped to the pressure of the absoφtion column 102 for reuse. The cooling is accomplished by contacting the lean solvent with the rich solvent in a liquid-liquid heat exchanger or an air cooled heat exchanger.

The H₂S selectivity of the absoφtion-stripping process is critical. The ratio of H₂S to CO₂ in the raw, sour fuel gas 98 entering the absoφtion column will be on the order of 1 part H₂S to 20 parts CO₂ on a molar basis. The H₂S to CO₂ ratio in the acid gas leaving the steam stripper must be at least 1 part H₂S to 1 part CO₂. A ratio of 4 parts H₂S to 1 part CO₂ or higher is preferred. The SELEXOL process, available from UOP Canada Inc., Toronto, Canada, is one of the processes capable of this degree of H₂S selectivity. SELEXOL is a trademark of UOP Canada Inc. for its process and a solvent used in the process. The acid gases 134 are next contacted with sulfide lean white liquor 140 in an absoφtion column 136 designed to have a high selectivity for H₂S absoφtion over CO₂. In this manner, the sulfide can be returned to the pulping liquor stream as white liquor 138 without significant carbonation thereof. The white liquor 138 can in this manner be used directly in the pulping process without further chemical processing.

As described above, the production of pulping liquor for the digestion of wood chips begins (in the context of a black liquor gasification process) with the production of green liquor 30 in the quencher 26. Recall from above that the green liquor 30 was produced when the smelt constituents dissolved into the mixture 29 of weak wash 28 and condensate with dissolved fume 100 and recycle water during the quenching operation. The green liquor consists of a mixture of sodium carbonate, sodium hydroxide, and sodium sulfide. The weak wash 28 is produced when fresh water is used to wash white liquor from the filter cake of calcium carbonate that is produced in the causticizing operations at 12. This weak wash is shown as stream 28 in Fig. 3. From a chemical composition standpoint, the weak wash 28 can be thought of as dilute white liquor. The recycled water contains low levels of alkali compounds from the fume carryover. The recycled water is shown as stream 100 in Fig. 3. The calcium carbonate is produced as a suspended solid when green liquor contacts an aqueous suspension of calcium hydroxide in the causticizing plant 12. The solid calcium hydroxide reacts with the dissolved sodium carbonate to produce dissolved sodium hydroxide and solid calcium carbonate.

Soot (a sub-micron sized carbonaceous aerosol) that is caught by the venturi scrubbers 82 and 96 is removed from the aqueous phase at the mud filter 38 leaves the process by combustion in the lime kiln 16.

The green liquor stream 30 leaving the quencher 26 differs substantially from that of a conventional kraft recovery process as illustrated in Fig. 1. Green liquor stream 30 in Fig. 3 will contain as little as 50% of the sulfide of a conventional green liquor. That situation results from the much greater partition of sulfur as H₂S in the gas phase of the gasification process. This is one of the principal differences between black liquor gasification (BLG) and conventional kraft recovery. This difference provides the opportunity for several process improvements over conventional kraft recovery. First, the BLG scheme offers a causticizing improvement. One of the means by which sulfur is lost from the conventional kraft recovery process is through the precipitation of calcium sulfide (CaS). This precipitated CaS is separated along with the calcium carbonate (lime mud 40) on the vacuum filter 38 from the white liquor. This lime mud is then separated and taken to the rotary lime kiln 16 where any calcium sulfide co-mingled with the calcium carbonate is decomposed to CaO and SO₂. In the case of the present invention, since the green liquor is lean of sulfide, there is a proportionate reduction of calcium sulfide mixed with the lime mud sent to the kiln 16. Another advantage of the low sulfidity green liquor in the BLG process is the fact that the white liquor produced from this green liquor will have a proportionately lower sulfidity. If all of this sulfide lean white liquor in stream 140 of Fig. 3 is used to contact the acid gas in stream 134, then by material balance constraints, the sulfidity of the white liquor in stream 138 would be equivalent to that of the conventional kraft recovery cycle. But, if only a portion of stream 140 contacts stream 134, then a white liquor stream with a higher sulfidity can be generated along with a stream of lower sulfidity white liquor. This offers the plant operator certain pulping options that are not available with the conventional kraft recovery process. There are various advantages of the present invention. The Tomlinson recovery boiler is the current standard method and apparatus for chemical recovery in the kraft pulping process. Recovery boilers are costly, prone to corrosion and catastrophic smelt-water explosions and are limited to relatively modest steam temperatures and pressures. These limits constrain the ability of this standard technology to effect improvements in electric power production. Black liquor gasification is widely viewed as the technology most likely to replace the recovery boiler.

If pressurized and coupled with gas turbines, BLG systems can provide more efficient utilization of black liquor fuel value and produce more electrical power relative to steam. This is an attractive feature for future mills where higher electrical usage will be required to operate mechanical pulping and pollution control equipment. Smelt-water explosions are a serious risk associated with recover)' boilers and most BLG concepts eliminate the possibility of these catastrophic events. Unlike recovery boilers, BLG systems recover sodium and sulfur as separate streams which can be blended to produce a wide range of pulping liquor composition. This increased process flexibility of BLG may be a significant asset in future kraft pulping operations.

In addition to the general advantages of black liquor gasification, the specific embodiment of the present invention offers advantages that constitute improvements over other BLG systems. Other BLG processes can cause a significant burden on the causticizing plant, because of co-absoφtion of CO₂. This process avoids that problem by including the absoφtion-stripping process that greatly increases the ratio of H₂S to CO₂ in the gas that contacts the white liquor. This process provides means to eliminate alkali fume problems that could be a problem in other BLG processes.

The inventors believe that the fume generation and control shown in other BLG process patents significantly understate the severity of this problem. The process and apparatus described in the present invention can realistically reduce fume and aerosol emissions below the gas turbine allowable limits.

The rapid quench of fuel gas from 1800 F to about 400 F by adiabatic humidification represents a significant portion of the chemical energy in the black liquor, or other type of waste stream. The means used to reclaim that energy into useful form is a challenge for any BLG process. The use of a heat exchanger or boiler to raise process steam between the first venturi 82 and the ESA 92 is an efficient way to recover nearly all of that waste heat as low-pressure (nominally about 80 psig) steam. This steam can be used as process steam throughout the pulp mill.

Within the framework of the apparatus and process described by Fig. 3, several alternatives are possible. All involve suspension gasification followed by a rapid quench to saturation with water. How the heat is recovered from the process is one area where several options exist. A condensing heat exchanger could be used to raise the temperature of high-pressure water from about 130 F to about 340 F. This hot, high-pressure water can then be contacted with the fuel gas in a saturator as at 106. The excess water that is not evaporated into the fuel gas is cooled in a liquid-to-liquid heat exchanger, increased in pressure and sent back to the condensing heat exchanger thus forming a closed-loop system. In this embodiment, the heat that is not returned to the fuel gas in the form of water vapor is simply rejected from the system. In an alternate embodiment an economizer section from a boiler could be used in place of the condensing heat exchanger. Here, the fuel gas would generate low-pressure steam that could be used elsewhere in the paper plant.

The manner in which the fume and soot are collected in the process described in Fig. 4 is designed for the most severe range of particulate emissions likely to be encountered in black liquor gasification. If the dust loading from a particular gasifier were about two orders of magnitude below the estimate used for the design of this gasifier 70, then the first venturi scrubber 82 could be eliminated.

The process by which the H₂S is scrubbed from the fuel gas and delivered to the white liquor is subject to various possibilities. Although the SELEXOL process is the preferred means, other absoφtion-stripping processes are suitable for this gasification process. Sterically hindered tertiary amines such as methyldiethanolamine are one such compound that can be used in a conventional absoφtion-stripping process.

While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

Claims

CLAIMSWe claim:

1. A method for producing clean, sweet, fuel gas for use in a combustion process by processing a waste stream from digestion of lignocellulosic material, comprising: partially oxidizing the waste stream to form hot gases and molten salts; cooling the hot gases and molten salts using a quench liquor to form quenched gas and carbonate liquor; removing particles from the quenched gas to form a raw fuel gas by subjecting the quenched gas to a multi-step fume reduction process which includes heat extraction from the quenched gas to reduce particulate load and water content of the quenched gas to form a low fume fuel gas; removing H₂S from the low fume fuel gas using an H₂S removal process which is more selective for H₂S than it is for CO₂, the removing step forming clean, sweet, fuel gas and acid gases; and conveying the clean, sweet, fuel gas to a combustion process.

2. The method according to claim 1, wherein the multi-step fume reduction process comprises passing the quenched fuel gas through a first venturi scrubber, an electrostastic agglomerator, and a second venturi scrubber in series.

3. The method according to claim 1 , comprising subjecting the carbonate liquor to a causticizer to form a caustic liquor and lime mud, the lime mud comprising a suspension of calcium carbonate, and further processing the acid gases by combining the caustic liquor with the acid gases in a caustic liquor scrubber to form a tail gas and a sulfur-rich caustic liquor.

4. The method according to claim 3, comprising filtering the lime mud to separate the caustic liquor from the lime mud and washing the lime mud with water to produce weak wash.

5. The method according to claim 4, comprising supplying the weak wash as part of the quench liquor for cooling of the hot gases.

6. The method according claim 5, comprising forming a condensate with dissolved fumes in the multi-step fume reduction process while removing particles from the quenched gas, and combining the condensate with dissolved fumes and the weak wash to form the quench liquor.

7. The method according to claim 3, comprising calcining the lime mud in a kiln to produce calcium oxide.

8. The method according to claim 7, comprising recycling the calcium oxide from the kiln to the causticizer.

9. The method according to claim 1, comprising the step of recovering sulfur from the low fume fuel gas.

10. The method according to claim 1, comprising processing a waste stream of black liquor.

1 1. The method according to claim 1 , comprising processing a waste stream of red liquor.

12. The method according to claim 1, comprising processing a waste stream of one of alkaline, acidic, and neutral sulfite spent liquor.

13. The method according to claim 1, comprising processing a waste stream of polysulfide spent liquor.

14. The method according to claim 1 , comprising conveying the clean, sweet, fuel gas to a combustor of a gas turbine coupled to an electric generator.

15. The method according to claim 14, comprising producing hot exhaust gases in the gas turbine and conveying the hot exhaust gases to a waste heat boiler and producing steam in the waste heat boiler.

16. The method according to claim 15, comprising conveying steam from the waste heat boiler to a steam turbine coupled to an electric generator.

17. The method according to claim 1, comprising contacting one of hot water and steam obtained from the quenched gas as it is processed in the multi-step fume reduction process with the clean, sweet, fuel gas to increase the heat and water content of the clean, sweet, fuel gas.

18. A apparatus for producing clean, sweet, fuel gas for use in a combustion process by processing a waste stream from digestion of lignocellulosic material, comprising: gasifier means for partially oxidizing the waste stream to form hot gases and molten salts; quenching means for cooling the hot gases and molten salts using a quench liquor to form quenched gas and carbonate liquor; multi-step fume reduction process means for removing particles from the quenched gas to form a raw fuel gas which includes means for extracting heat and water from the quenched gas to reduce particulate load and water content of the quenched gas to form a low fume fuel gas;

H₂S scrubbing means for removing H,S from the low fume fuel gas using an H₂S removal process which is more selective for H₂S than it is for CO₂, the removing step forming clean, sweet, fuel gas and acid gases; and means for conveying the clean, sweet, fuel gas to a combustion process.

19. The apparatus according to claim 18, wherein the multi-step fume reduction process means comprises a first venturi scrubber, an electrostastic agglomerator, and a second venturi scrubber in series.

20. The apparatus according to claim 18, comprising means for providing the carbonate liquor to causticizer means to form a caustic liquor and lime mud, the lime mud comprising a suspension of calcium carbonate, said processing means including means for combining the caustic liquor with the acid gases in a caustic liquor scrubber to form a tail gas and a sulfur-rich caustic liquor.

21. The apparatus according to claim 20, comprising means for filtering the lime mud to separate the caustic liquor from the lime mud and means for washing the lime mud with water to produce weak wash.

22. The apparatus according to claim 21 , comprising means for supplying the weak wash as part of the quench liquor for cooling of the hot gases.

23. The apparatus according to claim 22, comprising means for forming a condensate with dissolved fumes in the muti-step fume reduction process while removing particles from the quenched gas, and means for combining the condensate with dissolved fumes and the weak wash together to form the quench liquor.

24. The apparatus according to claim 20, comprising means for calcining the lime mud in a kiln to produce calcium oxide.

25. The apparatus according to claim 24, comprising means for recycling the calcium oxide from the kiln to the causticizer means.

26. The apparatus according to claim 18, comprising means for recovering sulfur from the low fume fuel gas.

27. The apparatus according to claim 18, wherein the waste stream comprises black liquor.

28. The apparatus according to claim 18, wherein the waste stream comprises red liquor.

29. The apparatus according to claim 18, wherein the waste stream comprises one of alkaline, acidic, and neutral sulfite spent liquor.

30. The apparatus according to claim 18, wherein the waste stream comprises polysulfide spent liquor.

31. The apparatus according to claim 18, comprising means for conveying the clean, sweet, fuel gas to a combustor of a gas turbine coupled to an electric generator.

32. The apparatus according to claim 31 , wherein the gas turbine produces hot exhaust gases and comprising means for conveying the hot exhaust gases to a waste heat boiler to produce steam in the waste heat boiler.

33. The apparatus according to claim 32, comprising means for conveying steam from the waste heat boiler to a steam turbine coupled to an electric generator.

34. The apparatus according to claim 18, comprising means for contacting one of hot water and steam obtained from the quenched gas as it is processed in the multi-step fume reduction process with the clean, sweet, fuel gas to increase the heat and water content of the clean, sweet, fuel gas.