WO2000065149A1

WO2000065149A1 - Sulfur recovery from spent liquor gasification process

Info

Publication number: WO2000065149A1
Application number: PCT/US2000/010777
Authority: WO
Inventors: Jerry D. Blue; William Downs; Timothy A. Fuller; Christopher L. Verrill
Original assignee: Mcdermott Technology, Inc.; The Babcock & Wilcox Company
Priority date: 1999-04-23
Filing date: 2000-04-21
Publication date: 2000-11-02
Also published as: CA2370964A1; AU4653600A

Abstract

A method and apparatus for processing a waste stream from digestion of lignocellulosic material to form useful products. The waste stream (1) can comprise black liquor, red liquor, alkaline, acidic or neutral sulfite spent liquor, or polysulfide spent liquor. Partial oxidation (100) is performed on the waste stream to form hot gases and molten salts. These are cooled using a quench liquor (102) to form quenched gas and carbonate liquor from which particles are removed to form a raw fuel gas. H2S is removed from the raw fuel gas using an H2S removal process (105) which is more selective for H2S than it is for CO2, following the particle removing step (104), thereby forming usable fuel gas (5) as one useful product, and acid gases (6). Further processing is performed on the acid gases to produce additional useful products.

Description

SULFUR RECOVERY FROM SPENT LIQUOR GASIFICATION PROCESS

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to the corresponding U.S. patent application of William Downs, titled ULTRA-HIGH PARTICULATE COLLECTION OF SUB-MICRON AEROSOLS, U.S. Serial No. 0?laJYHZ filed April 23, 1999 and the U.S. patent application of Jerry D. Blue, William Downs, Timothy A. Fuller, Christopher L. Verrill, Paul S. Weitzel, and Phung H. M. Chan, titled GASMC AΗON PROCESS FOR SPENT UQUOR AT HIGH TEMPERATURE AND HIGH PRESSURE, U.S. Serial Ho. øfl 77,531 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 those applications are valid for this disclosure also.

FIELD AND BACKGROUND OF THE INVENΩON

The present invention relates in general to sulfur recovery and in particular to a new and useful method and apparatus for recovering sulfur and other useful products from spent liquor gasification systems. There is a large body of prior art relating to the removal and/or recovery of H₂S from petroleum and natural gas processes and from pulp and paper spent liquor chemical recovery processes. The motivation for removing H₂S from petroleum and natural gas processes is singularly to improve the quality of the product. Usually, these processes convert the H₂S to solid elemental sulfur because it facilitates storage and transportation. Most sulfur thus produced is ultimately converted to sulfuric acid at the point of use. In a few instances, H₂S is converted to sulfuric acid directly. Prior art for H₂S recovery in the pulp and paper industry varies according to the specifics of the process. U.S. Patent No. 3,323,858 deals with the absorption of H₂S with carbonate liquor. The carbonate liquor is then causticized to caustic liquor. U.S. Patent No. 4,297,330 uses hot potassium carbonate to produce an acid gas stream containing H₂S, CO₂ and H₂O. The selectivity of that process for H₂S recovery over CO₂ recovery is only about 12 to 1. By comparison, as set forth in the DESCRIPTION OF THE PREFERRED EMBODIMENTS of the present invention, the selectivity of H₂S recovery over CO₂ recovery according to the present invention must be typically better than 100 to 1. The process described in U.S Patent No. 4,297,330 is not capable of achieving that degree of selectivity. U.S. Patent No. 4,609,388 describes a process that separates all of the components of a fuel gas into separate pure component streams. This process requires the complete dehydration of the fuel gas. This fact alone makes this process inappropriate for a spent liquor gasification process. U.S. Patent No. 5,205,908 deals directly with the issue of absorbing H₂S from a fuel gas generated by gasification of spent liquor. It is quite specific in stating that the absorption of H₂S is done with an alkaline wash solution that is not green liquor and has a composition where the mole ratio OH7HS^' is greater than 8. This patent does not deal at all with the issue of the co-absorption of CO₂ and therefore is missing primary elements for its practical application. U.S. Patent No. 5,556,605 uses carbonate hquor to absorb both H₂S and CO₂ followed by steam stripping out the H₂S and using it outside the kraft pulping process in a process such as the Neutral Sulfite Semi-Chemical (NSSC) pulping process. Finally, U.S. Patent No. 5,660,685 deals with spent liquor gasification in such a way that H₂S is removed from the fuel gas and then returned to the gasifier so that the carbonate liquor produced by dissolving the molten salts from the gasifier has a very high sulf-dity, and little carbonate. In the extreme, this approach has the possible advantage of eliminating the causticizing step. Although this idea has certain appeal, it has some significantly difficult steps; e.g., a Claus Reactor, H₂S compression and re-injection, and would be very difficult to implement. SUMMARY OF THE INVENTION

The approach of the present invention to H₂S recovery and reuse differs significantly from the prior art.

An object of the present invention is to provide a method and apparatus for processing a waste stream from digestion of lignocellulosic material to form useful products, 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; removing H₂S from the raw fuel gas using an H₂S removal process which is more selective for H₂S than it is for CO₂, the removing step forming usable fuel gas as one useful product, and acid gases; and further processing the acid gases to form additional useful products.

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 DRAWINGS

In the drawings: Fig. 1 is a flow chart showing the apparatus and method of the present invention;

Fig. 2 is a graph plotting carbonate content against the hydrogen sulfide-carbon dioxide ratio; Fig. 3 is a flow chart showing a typical proprietary SELEXOL process used in accordance with the present invention; Fig. 4 is a flow chart similar to Fig. -1, but showing a conventional process; and

Fig. 5 is a flow chart showing an alternative embodiment employing a plurality of absorption-stripping units connected in series. _

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. 1 in particular, 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 (Stream 1) resulting from the digestion of wood or other lignocellulosic materials. An oxidant (Stream 2) 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 hquor 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 hquor, 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 "hgnocellulosic" 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* 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. This process takes place in suspension in a gasifier vessel 100 that is operated at above- atmospheric pressure, typically up to 800 psia, preferably between 300 and 600 psia. The hot fuel gases produced proceed to a quench zone 102 where a spray conφrising process water and condensate (Stream 12), preferably a sulfide-lean quench liquor, rapidly cools the fuel gases. These quenched, sour, dirty fuel gases (Stream 3) will have sufficient heating value for use in a gas turbine, schematically indicated at 110. However, they will also contain alkali fume, carbonaceous aerosols, and reduced sulfur compounds that must be removed before the fuel gas can proceed to the gas turbine.

The particulate in the fuel gases will be predominantly sub-micron aerosol. The fuel gas first proceeds to a particulate removal stage 104 where up to 99.9999% (six nines control) of the alkali fume and carbonaceous aerosol are removed. Although this level of particulate removal is extreme, it is necessary to meet the very tight specification for alkali contamination of fuel gases entering the gas turbine 110. This particulate cleanup stage 104 will comprise a combination of one or more inertial-type dust collectors and may include an electrostatic dust collector/agglomerator to meet the most severe particulate requirements. For details of one such type of particulate removal equipment, reference is made to the aforementioned U.S. Patent application of William Downs, titled ULTRA-HIGH PARTICULATE COLLECTION OF SUB-MICRON AEROSOLS. Upon exiting from the particulate removal stage 104, the fuel gases (Stream 4) will then proceed to a system generally designated 105 for removal of H₂S from the fuel gas and which is designed for high selectivity of H₂S over CO₂. System 105 includes a process unit 106 designed to remove H₂S from the fuel gas, and preferably comprises an absorption step or H₂S scrubber and one or more stripping steps at 108. The fuel gases, after passing through the H₂S absorption step (Stream 5), will proceed into the gas turbine 110 or other suitable power generation equipment such as a steam generator. In the power generation equipment, any residual H₂S in the fuel gas will be oxidized to SO₂. SO₂ emissions resulting from the power generation step will be held below environmental emission limits by controlling the efficiency of the upstream H₂S removal system 105. In the gasifier 100, where the organic portion of the waste was gasified by partial combustion and the water shift reaction, the inorganic alkali portion of this stream 112 will be liberated as a stream of molten salts. In the context of a kraft recovery process, the molten salt stream at 112 is referred to as smelt. This stream 112 consists principally of sodium carbonate and sodium sulfide. Much of the molten salts will impinge on the walls of the gasifier 100 and flow by gravity towards the quench zone 102. Some relatively coarse droplets of molten salts will remain suspended in the fuel gas, but both of these streams will be effectively captured in the quench zone 102. The fume and carbonaceous aerosol will not be efficiently captured in the quench zone 102 but will instead proceed along with the fuel gas and be collected by the particulate removal stage 104 described above. The molten salts produced by this high temperature, high-pressure gasification process will be lean in sodium sulfide when compared with those produced in a conventional Tomlinson boiler. The aqueous fluid stream 12 used for quenching the fuel gas will consist of condensate containing dissolved fume (Stream 11) and a weak alkaline process water stream commonly referred to in the industry as weak wash (Stream 10). This stream 10, in turn, comes from the washing with fresh water at 116 of the calcium carbonate precipitate (a.k.a. lime mud) that is created from a causticizing operation 118 to be described. The fluid used in the quencher 102 is thus a sulfide-lean quench liquor.

The sulfide-lean quench liquor 12, when combined in the quencher 102 with the molten salts at 112 from the gasifier 100, will form a solution of principally sodium carbonate, sodium disulfide and either sodium bicarbonate or sodium hydroxide. This solution is known in the kraft pulp and paper industry as green hquor or, more broadly, as carbonate liquor. Since the molten salts from which the carbonate liquor is formed are lean in sodium sulfide, so is the carbonate liquor (Stream 8), especially when compared to the carbonate liquor formed in the conventional kraft recovery process. This sulfide-lean carbonate hquor (Stream 8) is next taken to the causticizing plant 118 where the carbonate liquor first contacts powdered lime (Stream 15) in a conventional slaker-causticizer. The purpose of the slaker-causticizer 118 is to react slaked lime (calcium hydroxide) with aqueous sodium carbonate to form solid calcium carbonate and aqueous sodium hydroxide. A competing and undesirable reaction is between solid calcium hydroxide and aqueous sodium sulfide to form solid calcium sulfide and aqueous sodium hydroxide. Since the carbonate liquor (Stream 8) of the invention is lean in sodium sulfide, the causticizing is therefore more efficient when compared to a conventional kraft recovery process. Therefore, the amount of undesirable carbonate that stays with the caustic liquor (a.k.a. white liquor) (Stream 9) following the causticizer 118 will be less here than in a conventional process.

The caustic liquor (Stream 9) produced in this causticizer 118 is deficient in sulfide (i.e., sulfide-lean) when compared to conventional kraft recovery processes. For some pulping processes this would be a desirable trait. However, for the conventional kraft recovery processes, high sulfidity caustic hquor is preferred. Sulfidity is an industrial term, and is commonly defined as the molar ratio of HS^' to (HS^" + OH^"). To recover this sulfur value to the caustic liquor, it will be necessary to contact a portion of this caustic hquor (Stream 9) with the acid gases from the H₂S stripper 108 (Stream 6). In order to do this without overly carbonating the caustic liquor (Stream 9), it is necessary that the molar ratio of H₂S to CO₂ in Stream 6 coming from the H₂S stripper 108 be greater than about 2. The influence of the H₂S over CO₂ ratio on the caustic liquor (Stream 9) composition can best be illustrated with an example. If a tray type absorption column is used to scrub the H₂S and if the selectivity of H₂S over CO₂ is say 10, then an absorption column that is designed to remove 99% of the H₂S will remove approximately 37% of the CO₂ in that gas. In this example, it is assumed that the sulfide-lean caustic liquor (Stream 9) has a sulfidity of 12.3% and a carbonation extent of 13.7%. If that caustic liquor in Stream 17 contacts an acid gas (Stream 6) containing an H₂S to CO₂ ratio of 2.0 in an H₂S caustic liquor scrubber 114, then the sulfide-rich caustic liquor (Stream 19) leaving the H₂S contactor or scrubber 114 in this example would have a sulfidity of about 32.5 % and a carbonation level of about 17.3%. The influence of the H₂S to CO₂ ratio entering the caustic hquor scrubber 114 on the caustic liquor composition leaving the scrubber is illustrated in Fig. 2. The amount of carbonation of the caustic liquor (Stream 19) will depend therefore on the ratio of H₂S to CO₂ in the acid gas (Stream 6) entering the caustic liquor scrubber 114. It also depends on the selectivity of that scrubber 114 to absorb H₂S in preference to CO₂. Any number of commercially available absorption columns can be used for the selective absorption of H₂S over CO₂. The caustic liquor, (sulfide-rich white liquor - Stream 19) upon leaving the scrubber 114, is suitable for use as pulping liquor without any further treatment.

A typical fuel gas (Stream 4) composition entering the H₂S removal system 105 is depicted in Table 1.

Table 1

The ratio of H₂S to CO₂ in this example is 0.0866. Recall from above that the H₂S to CO₂ ratio needs to be about 2.0 or higher before contacting the caustic liquor (Stream 17). The absorption-stripping operation therefore has two distinct -functions. The first is to reduce the H₂S concentration of the fuel gas 110 sufficiently so that when combusted in the gas turbine 110 the SO₂ concentrations in the turbine exhaust will be environmentally acceptable. The second function is to produce an acid gas stream with an H₂S to CO₂ ratio of at least 2.0. This means that the H₂S selectivity over CO₂ must be very high. Selectivity in this context is defined as the ratio of mass transfer coefficients, e.g. K_ga_HjS I K_ga_COι . Using σ to represent this selectivity, it can be shown that the selectivity can be expressed in terms of transfer units, NTU where NTU can be approximated by:

NTU„ _s = -\n(\ - ε)_Hv Then,

NTU σ = H,S

NTV_COι

If in this example the H₂S concentration leaving the scrubber 106 must be lowered from 2.32% to 100 ppm, that will require a removal efficiency of ε = 1-.0001/.0232 = .9957 or 99.57%. Conversely, the CO₂ removal efficiency must be exceedingly low. If the acid gas is to have a ratio of H₂S to CO₂ of 2, then no more than 2.32 x .9957/2 moles of CO₂ can be absorbed (about 1.16 moles CO^. Therefore, the CO₂ removal efficiency must not exceed 1.16/26.79, or 4.33 % . Then the required selectivity must be:

-ln(l-.9957) σ = = 123.1

-ln(l-.0433)

This selectivity of 123.1 is beyond the capability of conventional absorption-stripping processes known to the inventors. A conventional absorption-stripping process or system is meant to imply a single absorption tower coupled with a single stripper tower. Even sterically hindered tertiary amines are capable of H₂S to CO₂ selectivities of no better than about 30.

A system that is capable of achieving adequate selectivity is the SELEXOL process. SELEXOL is a trademark of UOP Canada Inc., Toronto, Canada, for its process of scrubbing H₂S. This commercially available process incorporates the use of a physical solvent and therefore absorbs various acid gas compounds in proportion to their partial pressure. The SELEXOL solvent itself is proprietary. Solvent regeneration is by pressure letdown of rich solvent. The solvent can be regenerated without heat. However, to reduce treated gas contaminants to low concentration, the solvent can be regenerated by a stripping medium such as an inert gas, or regeneration can be enhanced by the application of heat. Additional information concerning the publicly available SELEXOL process can also be found in HYDROGEN PROCESSING, April 1998, page 123.

A generalized SELEXOL process flow diagram is depicted in Fig. 3. Feed gas enters an absorber 201 where contaminants are absorbed by the SELEXOL solvent. Rich solvent from the bottom then flows to a recycle flash drum 202 to separate and compress 203 any co- absorbed product gas back to the absorber. Further pressure reduction on the drum 204 releases off gases. In some applications, the solvent is regenerated in a stripper column 205. The regenerated solvent is then pumped through a cooler 206 and recycled back to the absorber 201.

The gases leaving the H₂S removal process such as the SELEXOL process are passed to a tower (114 in Fig. 1) where they are contacted with a portion of the sulfide lean caustic liquor (Stream 17) in Fig. 1. By proper design of this caustic liquor absorption tower 114, a selectivity of H₂S over CO₂ of about 10 to 15 is achievable. By designing this absorption tower to remove 99⁺% of the H₂S, the tail gases leaving this tower (Stream 7) can be taken directly to the lime kiln 119 for incineration or they can be delivered to the pulp mill's non- condensible odor control system.

The SELEXOL solvent and process can be obtained from UOP Canada Inc. of Toronto Canada. There are SELEXOL processes which are available and which can be tailored to specific applications to enhance process performance. In the particular case of spent liquor gasification, the feed gas typically has a H₂S/CO₂ ratio which is less than 1:20. The requirement is for selective H₂S removal to less than 100 ppmv in the product gas while minimising CO₂ co-absorption, such that the resulting acid gas to sulfur recovery has a H₂S/CO₂ ratio of at least about 1:1. See Fig. 2. In order to accomplish these goals effectively, the basic SELEXOL process is modified to a more specialized process illustrated in Fig. 3 that involves both selective absorption and selective desorption/regeneration.

The person having ordinary skill in this art can therefore practice the SELEXOL process, H₂S removal aspects of the present invention based on pubhcly available information.

An alternate to the SELEXOL process which removes more H₂S than CO₂ is to subject the gases to a plurahty of absorption-stripping units connected in series. For example, suppose that a conventional absorption-stripping system based on methyldiethanolamine (MDEA) were designed to contact the H₂S bearing fuel gas to achieve the desired level of H₂S control. If the fuel gas contained 1 part H₂S per 23 parts CO₂, the acid gas evolved from the stripper portion of the absorption-stripping unit could achieve a H₂S to CO₂ ratio of about 1 part H₂S to 1.8 parts CO₂. If this acid gas is now taken as the feed gas to a second absoφtion-stripping set, then the H₂S to CO₂ ratio achievable could be about 1.9:1. If a still higher ratio of H₂S to CO₂ is desired before contacting the acid gas with caustic liquor, as in tower 114 of Fig. 1, then a third absoφtion-stripping unit could be used. Fig. 5 illustrates one form of such a plurality of absoφtion-stripping units connected in series. As shown, stream 22 is the acid gas product from the first absorber-stripper set. Stream 22 becomes the feed to the second absorber- stripper set, whose output is the acid-gas stream 6 provided to the H₂S caustic hquor scrubber 114.

The principal improvement of the process of the invention is the ability of this gasification system to recover the H₂S from the fuel gas generated by gasifying spent liquor without increasing the burden on the causticizing system. The principal advantage has to do with savings in energy, i.e. fuel oil, that is required to calcine calcium carbonate that is produced in the causticizer. This advantage is derived by adding an intermediate step in the H₂S recovery system, i.e. the SELEXOL process or equivalent, that first creates an acid gas stream with a high H₂S to CO₂ concentration before contacting the acid gas with caustic hquor. A more conventional approach to H₂S recovery is depicted in Fig. 4. Here the fuel gas stream 400 is contacted directly with a mixture of weak wash 402 and caustic liquor 406 in a multistage tower 404. Because the CO₂ concentration is so much higher than the H₂S concentration, most of the caustic that was produced in the causticizer 118 is consumed by the absoφtion of CO₂. Therefore, that extra CO₂ must be recycled to the causticizer through the quencher via streams 412 and 414 and that CO₂ is therefore discharged through the lime kiln stack. Calcining of calcium carbonate is a highly energy intensive process and therefore creates a significant burden on the energy efficiency of this process. Moreover, many kraft pulp and paper mills have limited lime processing capacity in their rotary kilns 410. The additional amount of calcium carbonate that must be handled may require additional capital investment.

A second advantage of the present invention concerns the ability to produce both high sulfidity and low sulfidity caustic liquor. The portion of sulfide-lean caustic liquor used to recover sulfur from the acid gas becomes saturated with HS^" ion. This sulfide rich caustic liquor can be used advantageously to improve pulp properties by application to wood early in the kraft digestion process. It can alternately be blended with lean caustic liquor (Stream 18 in Fig. 1) to produce a conventional caustic liquor (Stream 20) of typical sulfidity and carbonate content.

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 processing a waste stream from digestion of lignocellulosic material to form useful products, 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; removing H₂S from the raw fuel gas using an H₂S removal process which is more selective for H₂S than it is for CO₂, the removing step forming usable fuel gas as one useful product, and acid gases; and further processing the acid gases to form additional useful products.

2. 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.

3. The method according to claim 2, 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.

4. The method according to claim 3, comprising supplying the weak wash as part of the quenching liquor for cooling of the hot gases.

5. The method according to claim 4, comprising forming a condensate with dissolved fumes while removing particles from the quenched gas, and combining the condensate with dissolved fumes and the weak wash to form the quench liquor.

6. The method according to claim 2, comprising calcining the lime mud in a kiln to produce calcium oxide.

7. The method according to claim 6, comprising recycling the calcium oxide from the kiln to the causticizer.

8. The method according to claim 1, comprising the step of recovering sulfur from the raw fuel gas as the H₂S is removed from the raw fuel gas.

9. The method according to claim 1, comprising processing a waste stream of black hquor.

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

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

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

13. An apparatus for processing a waste stream from digestion of lignocellulosic material to form useful products, 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; particle removing means for removing particles from the quenched gas to form a raw fuel gas;

H₂S scrubbing means for removing H₂S from the raw fuel gas using an H₂S removal process which is more selective for H₂S than it is for CO₂, the removing step forming usable fuel gas as one useful product, and acid gases; and means for further processing the acid gases to form additional useful products.

14. The apparatus according to claim 13, 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.

15. The apparatus according to claim 14, 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.

16. The apparatus according to claim 15, comprising means for supplying the weak wash as part of the quenching liquor for cooling of the hot gases.

17. The apparatus according to claim 16, comprising means for foπning a condensate with dissolved fumes 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.

18. The apparatus according to claim 14, comprising means for calcining the lime mud in a kiln to produce calcium oxide.

19. The apparatus according to claim 18, comprising means for recycling the calcium oxide from the kiln to the causticizer means.

20. The apparams according to claim 13, comprising means for recovering sulfur from the raw fuel gas as the H₂S is removed from the raw fuel gas.

21. The apparatus according to claim 13, wherein the waste stream comprises black liquor.

22. The apparatus according to claim 13, wherein the waste stream comprises red liquor.

23. The apparatus according to claim 13, wherein the waste stream comprises one of alkaline, acidic, and neutral sulfite spent liquor.

24. The apparatus according to claim 13, wherein the waste stream comprises polysulfide spent liquor.