CA1274355A - Gasification of black liquor - Google Patents

Abstract

Abstract of the Disclosure A concentrated aqueous black liquor containing carbonaceous material and alkali metal sulfur compounds is treated in a gasifier vessel containing a relatively shallow molten salt pool at its bottom to form a combustible gas and a sulfide-rich melt. The gasifier vessel, which is preferably pressurized, has a black liquor drying zone at its upper part, a black liquor solids gasification zone located below the drying zone, and a molten salt sulfur reduction zone which comprises the molten salt pool.
A first portion of an oxygen-containing gas is introduced into the gas space in the gasification zone immediately above the molten salt pool.
The remainder of the oxygen-containing gas is introduced into the molten salt pool in an amount sufficient to cause gasification of carbonaceous material entering the pool from the gasification zone but not sufficient to create oxidizing conditions in the pool. The total amount of the oxygen-containing gas introduced both above the pool and into the pool constitutes between 25 and 55% of the amount required for complete combustion of the black liquor feed. A combustible gas is withdrawn from an upper portion of the drying zone, and a melt in which the sulfur content is predominantly in the form of alkali metal sulfide is withdrawn from the molten salt sulfur reduction zone.

Description

35~i GASIFICA~IQN OF BLACK LIQUOR
Arthur L. Kohl The United States Governme.nt has rights in this inv~ntion pursuant to subcontract STR/DOE 12 of Contract.DE-AC05-80C~40341 awarded by the U. S. Department of En~rgy.

Background of the Invention l. Field of the Invention This invent;on relates to the gasific~tion of black liquor. More part~cularly, th~s invention relates ~o an apparatus and process for 10 controllably gas~y~ng an aqueous black liquor by means o~ a molten salt to produce a combust~ble gas. -

2. Prior Ar~
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.;In the production of pulp and paper using the ~od~um-based sulf~te and sulfite processes, digestion of wood with aqueous alkaline solutions results ~n ~he production of a byproduct which is known as spent or black -l~quor, hereinafter referred to as black liquor. In order ~o realize -economies in the overall pulp~ng process, this byproduct may not be d~sposed of as a waste material but ~nstead mus~ be converted into useful products. In par~icular, it is des~red to regenerake sod~um sulfide3 which can be used to reconstitute ~ct~ve solutions for the pulp d~gestion step of the process. In addit~on, it is desirable ~o u~ ze the black liquor as an energy source.

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The most widely practiced method of process~ng black 11quor makes use sf the Tomlinson recovery furnace (also re~erred to as the Tomlinson recovery boiler). In this system concentrated black liquor is burned in the furnace of a specia11y des~gned boiler to produce steam; a mo1ten salt product generally referred to as "s~elt" sr "melt", which contains sodium carbonate and sodium sulfide; and non-combustlble flue gas which, after suitable cleaning, is vented to the atmosphere. The process using the - Tomlinson boiler has served the pulp and paper industry for about fifty years, yet it has serious deficiencies. The large volume of flue 3as is difficult to clean and can constitute an environmental problem; all recovered energy is ln the form of steam which has limited utility;
explosions can occur if the boiler tubes leak and cause water to contact the smelt; and the reduction of sulfur compounds to sulfide is incomplete.
Yarious processes involving alternatives or improvements to the Tomlinson furnace have been used or proposed for converting black liquor to useful products.
U. S. Patent No. 1,808,773 discloses a process whirh util k es a black liquor recovery furnace having two zones sf combus~ion. In the first high temperature combustion zone, black liquor sprayed tnto the furnace is dehydra~ed and substantially comple~ely burned. In the second zone, located be~ween the first zone and ~he bottom of the furnace, an additional quantlty of black l~quor is sprayed into the furnace along wi~h sodium sul~ate. In this second zone, water is removed from the black liquor by evaporation. Partial combustion of ~he black iiquor results in the formation in the bottnm of the furnace of a sol~d smelting bed of spongy carbon, mixed with alkali residues from black liquor and added sodium sulfate. Reducing conditions maintained in the bottom of the furnace result in the reduction of sulfate to sulfide. The molten salts trickle downward through the bed of spongy carbon and leave the furnace v~a a bottom drain. Although this process provides an alternative to use of the Tomlinson recovery boiler~ the necessity for two discrete combustion zones requires a cumbersome apparatus. Also, the absence of . . : . .
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79!;~55 ~ny provision for heat recovery results in the loss of the heating value of the black liquor. Further, while there is conversion of sodium sulfate ~o sodium sulfide and the combustion of black l~quor, the percentage of unconverted sulfate is relatively h~gh, ranging from 8 to 12%.
U. S. Pa~ent No. 1,931,536 describes a process for ~ontrolling the zone of combustion of both sprayed black liquor and black liquor powder in a smelting furnace. An inert gas is introduced into the smelting furnace at or near the point of entrance of the sprayed black liquor or dried black liquor powder. This inert gas retards the combustion of the volatile constituents of the black liquor and perm~ts the s~rayed concentrated liquor or the dried black liquor powder to be projected into the smelting furnace for some distance before combust~on of the organic and carbonaceous content of the black liquor occurs in a relatively deep bed in the smel~ing furnace. This process represents an fmprovement over the conventional Tomlinson reco~ery boiler but has the same basic limitations; the black liquor undergoes complete combustion to produce a large volume of impure flue gas, and only steam is produced.
U. S. Patent No. 2,056,266 describes the use of a combined smelter and boiler, much like the Tomlinson boiler, for recoYering alkall metal values from black liquor and utilizing the heat conten~ thereof. Dried black liquor solids are fed ~o a solid fuel bed zone where they are burned in a reducing atmosphere wi~h the result ~hat partially burned ~ases rise from the fuel bed. These partially burned gases then are completely combusted by introducing a stre~m of air into a combustion zone above the -- 25 bed. The combustion zone contains boiler tubes for the production of steam. Flue gases produced in the combustion zone are allowed to rise, - and an inert gas is blown down on the fuel bed to prevent entra;nment of solids in the gases rising from the fuel bed and to create a distinct line of separation between ~ones. F~sed alkaline values are drained fro~ the bottom Df the bed. This process requires conversion of black liquor to - black liquor solids prior to Introduction into the fue1 bed zone. In addition, the apparatus necessary for carrying out ~he process is co~plex and requires a separate means of drying black liquor.

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4 85R045 U. S. Patent No. 2,182,428 discloses a process for drying waste llquors by spraying the liquor to b~ evaporated upon ~he surface of a heat transfer medium such as oil, tar, pitch, asphalt or wax. S~nce the heat transfer medium is inert and no combustion or reduction reactions occur, the waste liquors are merely evaporated wi~hout recovering any useful product from the evaporate~ liquors.
U. S. Patent Nos. 3,639,111 and 3,718,446 disclose a process for producing a clean-burning fuel by the high temperature distillation and pyrolysis of an organic material such as kraft black liquor. In order to achieve the requlred cracking temperatures in the pyrolysis zone, a controlled amount of an oxygen-contain~ng gas (up to about 15% o~ that required for complete combustion) fs introduced during the cracking operation. Because the oxYgen-contalning gas, pyrolyz~ng black liquor and product gases flow concurrently through the system and the product gas leaves at the full reaction ~emperature without giving up heat to ~ncoming materi~l, the process is thermally inefficient. Further, the requirements for both indirect heat exchange and direct combustion result in ~he need for relatively large complex equipment.
U. S. Patent No. 3,916,617 describes the use of a ~olten salt to produce a low Btu gas from the gasification and partial oxidation of a carbonaceous material. Carbonaceous material is maintained in the molten salt zone in order to provide the desired reducing atmosphere when air is passed into this molten salt zone for partial combustion of the carbonaceous material. When air and black liquor are ~ntroduced into a molten salt reaction zone, the heat required to evaporate water in the black liquor must be supplied by combustion reactions. This results in the requirement for a high air/black liquor ratio and the production of low quali~y gas ~typically less than 70 BtuJscf). As a result the process of this patent is primarily useful for gasifica~ion of coal and other relatively dry carbonaceous materi~ls.
U. S. Patent No. 4,441,959 discloses a process for recovering heat and chemical values frcm spent pulping liquors which utilizes a fluidized bed reaction chamber. A concentra~ed spent pulping liquor is combusted with air in a fluidized bed comprislng a plurality o~ inert olid .

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-5- 85R045 particulate materials, at le~st one of wh~ch ~s a f~ner partlcle s ke than another. Follow;ng combust~on, the part~culate materi~ls of finer particle sk e are tre~ted ~n ~n cxternal fluid ked bed heat exchanger to recover heat and to separate the Piner particles from gaseous and solid products produced in the combustion. The sol~d products are thereafter sub~ected to treatment in a molten salt reducer, which results in the product~on of a smelt conta~ning sodium sulfide and other salts. The gaseous products essentially comprise a noncombust;ble flue gas, the heat content of which is used to produce steam. The result;ng cooled flue gas, following su;table purification, can be released to the atmosphere.
Although this process recovers some of the heat and ehemical values from spent pulping l~quors, the sol;d combustion products are not reduced In the fluid k ed beds. Therefore a separate molten salt reducer ~s requlred, adding to the complexity of the process.
15None of the processes previously avallable are therefore seen as being capable of conveniently and efficlently recovering substantially the entire energy and chemical content of black liquor as high value products.
While not considered part of the pr~or art, the present inventor and - his associates have previously proposed other processes for the gasification of black liquor.
Thus it has been suggested that dried black liquor so~ids be gasi~ied ~n a molten salt pool. In such a process, ~ combust~ble offgas ;s produced and a high level of reduction of the sulfur content of the black liquor solids to sulfide ~s realized. However, it is flrst necessary to dry the bldck liquor to form the black liquor solids requ~red as feed to the molten salt pool. Th;s ~ncreases the complex;ty and cost of the process.
In Can.Pat.appln. S.N~ 489,41S, filed August 26, 1985, the present inventor has proposed a process for recovering the energy and chemical content Q~ an aqueous black liquor by utilizing a reactor , ~

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-6- 85R045 contalnlng a drylng 20ne locsted ~bove a 9aslflc~t1On zone. ~he re~ctor conta~ns ~ be~ of porous sol~d c~rbon~ceous materlal 1n the gas1f1ca~10n ~one. An oxygen-contalning gas ls lntroduced 1nto the 9~s1flc~tion ~one 1n a substo khlometr1c amount to produce partlal combustlon ~nd g~slflcation reace~ons sufflclent to ma1rt~1n the temperature ~t an upper - surface of the bed of sol1d c~rbon~ceous materl~l ~n ~he gas1f~catlon zone In the range of from ~bout 870' to 1200-C and to form a hot combust~ble gas whlch rlses from the gas~flcatlon 20ne. A concentrated black l~quor cont~ning ~lkall metal oxysulfur compounds ts Introduced ~nto the dry~ng : 10 zone, and the water contalned thereln ls e~aporated by contact ~th the hot gases rlslng from the gaslficat~on 20ne, In the drylng 20ne there ~s produced ~ reduced-temperature product gas and dry black l~quor sollds -whlch fall ont~ ehe surface of the bed ~n the gasif~catlon .zone. The dr~ed black llquor sollds are conYerted lnto the hot combustfble gas, hhlch r~ses from the gas~f~catlon ~one~ and alkall Rletal sal~s, ~h~ch n~elt ~nd permeate t'nrough the bed. The product 9~ ses ~re w~ thdrawn from an upper portion of the dry~ng zone. A :nelt ln wh~ch the sulfur content ~s at least about ~OS ~n the forn~ of alkall met~l sulflde ls w~thdrawn from a lower port~on of the gaslfk~tion zone. Desp~te the ~dvantageous features 20 of th1s process 1n promot~ng gas~f~c~t~on ~nd sulfur reduction reactlons, the react10ns that ocsur are lnefflc~el~t because o~ the relat~vely poor çontact between the a1r ~nd sol~d car~on. Also, oper~t~ng charac~erl t~es are uncerta~n ~n th~t the bed of solld c~rbonaceous m~ter1al c~n change he~gh~ w~th m~nor fluctuatlons 1n operat1ng con~lt10ns.
The present inventor, in Canadian Letters Patent No.1,222,604, has described the gasification of aqueous black liquor using a molten salt pool. An oxygen-containing gas is introducea beneath the surface of the molten salt pool, which comprises an alkali metal carbonate and an alkali metal sulfiae contained within an 30 enclosed gasifier vessel, at a rate sufficient to produce a high degree of turbulence in the molten salt pool. Black liquor in the form of a coarse spray is introduced into the rising hot gases above the pool. Thereby, - ~ -- , ' . ' .

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-7- 85R045 ~ater 1s evapor~te~ from the 2queous bl~ck l~quor into the hot g~ses ~o produce ~ reduced-temperature product gas 3nd dr~ed blaek llquor sol1ds, ~h1ch fdll onto the surface of the pool and ~re dlspersed thereln. 7he dr1ed black llquor sollds are converted In the pool tnto a hot combustlble S gas, whlch rlses out of the pool~ ~nd tnto ~lkal~ metal salt5, ~h1ch merge ~th the ex~stlng salts ~n the pool. A stream of product gas w~h a ~ry bas~s h~gher heating value IHHY) of ~t least about 90 8tuJscf 1s wlthdrawn from the gas~fler vessel together wtth a mo1ten s~lt ~roduct ln ~hlch the sulfur content ts at least about 90~ ln the form ~f alkall metal sulf~de.
10 Al~hough the process of this lnvent~on ls of utlllty ln productn~ the desired results of prov~ding a combust~ble gas and ~ mo1ten salt product In wh~ch alkall metal sulf~de predom~nates, the process 1s su~ect to cèrtaln problems, Corroslon and destructlon of ~onta~nment ~aterlals ~e generally ~nherent ln the use of turbulent pools of molten salts. A15G~
entratnment of molten salts may occur tn the gases rlstng out of the pool. ~hls may requ~re ltmltlng ~he gas veloctty through th~ pool. It has further been found that some of the car~ondceous matter ~n the btack llquor Is gas~f~ed before the part~cles reach the pool. As a result, only a port~on of ~he carbonaceous matter enters ~he pool. If all of ~he ~Ir requ~red for gasificat~on of the bl~ck llquor ls fed t~ the pool benea~h Its surface, cond~tlons ~lth1n the pool ~ay be too strongly ox1d~1ng ~or effect1ve reductlon of sulfur compounds to occur.

Summary of the Invent~on The present invention constitutes an improvement over the 25 inventions disclosed in Can.Pat.appln. S.N. 489,416 and in Canadian Letters Patent No.1,222,604. It retains the advant-ageous features of the basic black liquor gasification processes, while alleviating the above-mentioned problems. It provides the further significant advantage of permitting operation at a very 30 high gasification rate for a given gasifier size.

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-8- 85R045 In its broadest aspects, the invention comprises an ~mproved apparatus and process for the gdsification of black l~quor ~n whieh a combustible gas is produced and the sulfur content of the black liquor ~s substantially completely converted to sulfide. An enclosed vertically elongated gasif~er vessel has provis~on therein for maintaining in a sump at its bottom a relatively shallow pool of ~olten salt comprising an alkali metal carbonate and an alkali metal sulfide. Concentrated aqueous black liquor is sprayed into the vessel near its top. Means are provided for controllably feeding an oxygen-containing gas, preferably air9 ~nto the gasifier vessel a~ two distinct loca~ions -- one beneath ~he surface of the molten salt pool and the other above the ~elt surface into the lower portion of the gas space.
The distribution of the air ~eed into the gasifier is controlled so as to uniquely produce three zones in the gasifier vessel: ~l) a black liquor drying zone located in the upper part of the vessel; (2~ a black liquor solids gasification zone located below the drying zone, and (3) a molten salt sulfur reduction zone comprising the molten salt pool.
It is a key feature of this invention that the oxygen-containing gas, preferably air, is controllably fed both directly into the body of the molten salt pool (the sulfur reduct~on zone) and also above the surface of ~he pool into the lower portion of the gas space (the gasification zone).
The distribution of the air feed is controlled so that only that amount of air is fed into the sulfur reduction zone in the molten salt pool which is required to assure complete destruction of the carbonaceous material which actually enters the melt pool. Typically, this represents about 30-70~ of the total amount of air fed in~o the gasifier ~essel. The balance of the air is fed into the gasification ~one immediately above the pool. Thus, of the total amount of air fed to the gasifier vessel, about 30-70g of the total w~ll be fed to the gas~ficat~on zone. Since the purpose of the present process is to gasify the black liquor to produce a combustible gas, ~s well as to recover sulfur values, the total amount of a~r fed to the gasifier vessel will typically be about 25-55% of the amount required .
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9 85R045 for complete combustion of the black liquor feed. Typically, the total air feed to the gasifier will be preheated to a temperature in the range of about 120-450C (250-840~F), preferably 150-400C (300-750F).
The combustible gas, whose combustible components are principally hydrogen and carbon monoxide, produced in the gasffication zone can, a~er suitable cleanup, be used in a gas turbine in order to utilize the energy values of the black liquor feed to the maximum extent. The alkali metal sulfide produced in the molten salt pool can be recovered as an aqueous solution and recirculated to the papermaking process as green liquor.

Brief Description of the Drawin~
FIG. 1 is a schematic flow d~agram illustrating a preferred e~bodiment of the process of the present invention.
FIG. 2 is an elevational view partly ~n cross section of the gasifier vessel and assoclated quench tank of the present ~nvention. These are utilized in carrying out the process of the present invention.

Description of the Preferred Embodiments Black liquor, typically obtained from a wood-pulping opera~ion as part of a paper~aking process, contains combustible organic material~
alkali metal sulfide and alkali metal hydroxide, as well as various alkall 20 metal oxysulfur compounds. Typically, these oxysulfur compounds will be the sulfate, thlosulfate, and sulfite of sodium. The economjcs o~ the papermaking process require that substantially all of the combustible material be removed~ the alkali metal and sulfur values be recovered frum the black llquor9 and the oxysul~ur compounds be cnnverted to alkali metal 25 sulfide for return to the process without ox~dation of the alkali metal sul~ide init~ally present.
In prac~icing ~he present invention, a concentrated aqueous black l~quor containing at least about 45~ by weight solids and hav~ng a higher heating value (HHY) of at least about 3200 Btu/lb. ~s introduced into the 30 drying zone, typically as a coarse spray. The drying zone provides . , ...... . : ~ "
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-10- 85R0~5 direct-contact heat exchanye beSween the falling drops of black liquor and the gas stream rising from the gasificatlon æone. Wa~er is evaporated from the concen~rated black liquor due to the absorption of hea~ by ccnvection from the risin3 gas stream. Heat is also supplied to the drops by radiation from high temperature components. Gas temperatures in this drying zone range from about 500C to about 800C near the top of the zone and from about 800C to about 1000C near the bottom.
Drled black liquor solids are formed in the drying zone as a result of the aqueous black liquor being contacted in the drying zone by the 5ases rising from the gasification zone. In the ~lack liquor solids gasification zone, a signif~cant portion of the carbonaceous material in the dried particles falling from the dry~ng zone ~s converted to gas. The gas-producing reactions include pyrolysis with the release of volatile hydrocarbons, gasification of carbon wlth carbon dioxide and s~eam to produce carbon monoxide and hydrogen, and combustion of both gas and solid-phase components by reaction with the oxygen present in the oxY9en-containin9 gas injected into this zone. As a resul~ of the partial combustion reactions, hea~ is released to produce elevated tempera~ures in this zone ~n the range of 9~0-1200C. Since the solid particles are subjected ~o reaction only for the rela~ively short time they are falling through the gasification zone, they are only par~ia71y gasiPied.
Typically, 25-75~ of the organic carbon present in the black liquor entering the gasifier vessel is gasified in the 3asification zone. A
similar or higher percentage of the organic hydrogen is also conver~ed to gaseous components in this zone. The unreacted carbonaceous material 9 together with most of the inorganic material from the original black liquor ~eed, falls out of this zone into the sulfur reduction zone. The sulfur reduction zone comprises a pool of molten alkali metal salts contained in a shallow sump at the bottom of ~he gasifier vessel.
Essentially, all of the sulfur compounds present ~n the falling particles are reduced to sulfide in the molten salt sulfur reduction zone~
or maintained as sulfides ~f they are already present in this form. Air ~;
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~ 85R045 - 1s fed 1nto thls ~one tn an ~mount sufficlent to provlde the oXygen requ1red to gasI~y all of the enterlng carbonaceous ~atter but slgn~flcantty less than the amount requlred for eomplete combustlon, ~yplcally, the ~mount of oxygen requ~red 1s 1n the r3nge of 25-558 o~ that requ1red for complete combust~on of carbon to c~rbon d~ox~de, hydrogen to water, ~nd sulfur compounds to the sulf~te form. By providlng only thls amount of oXygen, conditlons ~n the melt pool ~re hlghly reduc~ng, ~nd at least about 90~ of the sulfur compounds In the product melt wlll be ln ~he form of sul~ide.
The presen~ ~nventlon provldes seYeral stgn~flcant advantages. Slnce only ~ poriIon of the carbonaceous mater~ n the black llquor feed enters the molten salt pool and only ~ portlon of the a~r ls fed ~nto the pool to react w~th lt, the pool c~n be relatively shallow--typlcally 1 to 4 feet ~n depth. The use of a shallow pool reduces the ~mount of expens~ve melt-reslstant refractory whlch must be used to l~ne the gas1f~er. ~nother advantage of thls lnventlon ls an Increase ~n gaslf~er capaclty. In a pool-type gaslfler, the gas rate 1s normally l~m~ted by the problems of excesslve entralnment and turbulence when the superf~clal veloc~ty of gas leavlng the pool exceeds 2-~ feet per second. Thls, ~n 20 turn, llm~ts the capac~ty of a glven diallleter ~aslf~er. In the present ~nvent~on only a portlon of the gas passes through lthe pool. ~hus9 for example, 1f one-h31 f of the gas passes through the pool at ~ superfk131 veloclty at the surface of 2.5 feet per second and the other h~ s formed ~n the gasff~ca~on zone, the total gas ~fll enter the drying 20ne 25 at a superfic~al velo~lty of about 5 fee~ per second, wh~ch ~s ~u~e acceptable for gas/sol~d or g2s/l~qu~d reactors 1n ~hlch gas ls the continuous phdse, As a result, the capac~ty of ~ g~ven d~ameter gasif~er - ls greatly 1ncreased by ~c~rpor~ting th~s 1mprovement ~n the invention coYered by Canadian Letters Patent No.1,222,604.
As described in Canadian Letters Patent No.1,222,604 and in application No. 489,416, heat losses from the reaction zones ~ust be minimized. Thus operation at elevated pressure is preferred since this reduces the size of the gasifier vessels required thereby reducing heat loss as well as cost.
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A typical system ut~lizing the molten salt gasifler vessel and the process of the present inventlon will now be described by reference to the drawing.
Referring to FIG. 1 of the drawing, there is shown a molten salt gaslfier vessel used with a gas turbine combined cycle system representing a preferred e~bodiment of the present invention. Wood waste from any suitable source is introduced via a conduit 2 and a valve 4 into a lock hopper 6. From there the wood waste passes via a valYe 8 into a second lock hopper 10. The lock hoppers are operated with a pressurizing gas in the conventional manner used to feed solids lnto a pressurized receiver.
From lock hopper 10 the wood waste passes via a conduit 12 and a feeder valve 14 to a valved conduit 16 through which compressed air is flowing.
The wood waste is conveyed by the compressed alr and iniected together with it below the surface of a molten salt pool 18 contained in a sump in the bottom of a mol~en salt gasifier vessel 20. Air is also injected above the surface of the melt ~hrough a valved conduit 21. The amount of air fed into the melt via conduit 16 is typically 30-70X of the ~otal air fed to the gasifier vessel. The amount of air fed to above the melt sur~ace via conduit 21 represents the balance of 30-70~ of ~he total.
Molten salt gasifier vessel 20; wh;ch uniquely enables practice of the process of the present invention, will be described fn greater detail in connection with FIG. 2.
Referring again to FIG. 1, black l~quor from a paper making process is sprayed into vessel 20 near the top of the vessel via a conduit 22 and a nozzle 240 The sprayed aqueous black liquor typically has a concentration of about 45-75X solids. The gaseous product from vessel 20 exits via a conduit 26 to a heat removal system 28, which may include a steam generator~ and thereafter through a conduit 30 to an absorber 32.
Absorbent is introduced into absorber 32 vla a conduit 34. The absorbent ` 30 may be an alkaline paper mill liquor or a conventional absorbent such as ethanolamine solution. This absorbent is used to remove H2S and other undesirable components from the ~as. The absorber system used is : .
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preferably des~gned for select~ve absorption of H~S in the presence of C2 slnce the produc~ gas con~ains sign~icant quantities (10-15%) of C02. Spent absorbent ex~ts absorber 32 via a conduit 36. Partially purified gases from absorber 32 are conducted via a conduit 38 to a fume scrubber 40 for further purif kation. Water ~s introduced into fume scrubber 40 thrsugh a conduit 42 and exits via a conduit 44. Scrubbed gases exi~ via a conduit 46 to a gas turbine combustor 48. The fume scrubber 4Q is shown as a typ~oal device for removing very flne particles of soluble salts. Other deYices such as fabric filters may also be used for removlng the fine particles. Air is supplied to combustor 48 via a conduit 50, a compressor 52 and a conduit 53. A~r from compressor 52 is also fed via a conduit 54 to a booster compressor 56 and then to a compressed a1r line 58. This ~eeds conduits 16 and 21, whioh introduce air into and above the mol~en salt pool, respectlvely. Hot, clean combustion gases exlt combustor 48 via a conduit 59 and are fed to a gas turbine 60, which powers a generator 62 and compressor 52. Expanding gases from gas turb~ne 60 are conducted via a conduit 64 to a waste heat boiler 66 into which water is introduced via a conduit 68 for conversion to steam. The stea~ produced in waste heat boiler 66 exits via a conduit 70 to a steam ~urbine 72, which powers a generator 74. Process steam is furnished from steam turbine 72 v~a a conduit 75. Exhaust gases from waste heat boiler 66 exit via a conduit 76 to a stack 77 for release to the atmosphere.
- Overflow melt from vessel 20 flows via a condui~ 7~ into a quench ~ank 80. Water ~s introduced into quench tank 8D via a conduit 82. The aqueous solution resulting from quenchin~ the melt is removed from quench tank 80 via a conduit 84, a pump 86 and a condu~t 88. Part of the removed aqueous solution is recycled to quench tank 80 Yia a conduit 90 and serves ~o break up the falling stream of melt as it exits conduit 78. Ano~her part vf the solution is fed ~rom conduit 88 via a conduit 92 to green liquor storage tank 94. A conduit 96 conducts the g~en liquor ~rom storage tank 94 to an appropriate point in the papermaking process, for example, the caust~cizing stage of a sulfate ~kraft) process plant.

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FIG. 2 describes the molten salt gasifier vessel of the present invention and the associated quench tank and their operation in greater detail. A gasifier vessel 100 contains a black liquor drying zone 102 located in the upper portion of the vessel, a black liquor solids gasification zone 104 located below the drying zone, and a sulfur reduction zone 106 comprising a molten salt pool 108 located in a ~ump at ~he bottom of the vessel. The vessel 100 is shown consis~ng of an outer wall metal containment shell 110 which is lined w~th an insulating refractory 112 capable of withstanding the temperatures and env~ronment within the vessel 100. Insulating refractory material 112 is provided in sufficient thickness to minimize1 to the extent practical, heat losses from within vessel 100. The molten salt pool 10~ is further contained within a smelt-reslstant refractory liner 113 which extends upward at least part way through the gas~fication zone 104.
A black liquor 114 to be treated is introduced (from a source not shown) through a conduit 116 to a pump 118. From pump 118 the black liquor is introduced into vessel 100 v~a a spray system 120 which injects the concentrated aqueous black liquor as a coarse spray via a plurality of spray nozzles 122 into an upper portion of drying zone 102.
A gas supply system for vessel 100 ~s provided which includes an inlet conduit 124 for an oxygen-containing gas ttypically air) which 7eads intG a compressor 126 driven by a motor 128. Advantageously, by . compressing the oxygen-containing gas, the temperature of the gas is :: increased thereby~ Alternatively, a gas heater may be used to raise the - 25 temperature of the oxygen-containing gas to the desired level. Thepressurized oxygen-containing gas leaves compressor 126 ~ia a conduit 130 to an air flow distribution control system 132 containing one or more prop~rtioning valves. These serve to selectively control the flow of compressed air Yia a conduit 134 directly into reduction zone 106 (molten salt pool 108) and via a conduit 136 into the yasification zone above the molten salt pool. Conduits 134 and 136 generally constitute a circumferential array of qas injection ports for the respective feeding of the compressed air ~o the reduction ~one 106 and gasifica~ion zone 104.

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-15- 85R04~

In accord~nce with a key feature of this invention, the d;str~bution of the air feed is so controlled that only ~hat amount cf air required to assure destruction of the carbonaceous material actually entering the melt pool ~s fed into the molten salt reduction zone 106. This is typically 30-70X of the total amount of air fed to vessel 100. The balance of the air fed to the vessel ~s fed ~nto the gasification zone 104 immediately above the pool. Since a combus~ible gas ~s being produced, the total air -- fed to the vessel is typically 25-55~ o~ the amount requlred for complete combustion of the total feed materials. As noted in FIG. 1, other material, such as wood waste, may be fed by means of compressed a~r d;rectly into the molten salt pool.
An overflow melt outlet 1~8 leads into an enclosed quench tank 140.
Dur;ng normal operation, molten salt product melt 141 is d;scharged from pool 108 through melt outlet 138 into quench tank 140. Water or a }5 suitable salt solution such as recycled green liquor is introduced into the quench tank 140 v;a a conduit 142. The water serves to shatter and quench the melt entering the quench tank to form a pool of green liquor 144 containing reduced chemical salts from the black liquor. The green liquor is withdrawn via a conduit 146, typically for return to a pulping process. A portion of the green liquor product may be recycled to conduit 142 for aid ~n breaking up mel~ 141. During the quenching of melt 141, there is produced a hot gas principally comprising water vapor which ls withdrawn from quench tank 140 via a conduit 148. A minor portion of the gas produced in gasifier vessel 100 may be allowed to flow through melt outlet 138 together with the discharged melt, and this gas is also wlthdrawn from quench tank 140 Yia conduit 148. The total gas stream withdrawn from quench tank 140 through conduit 148 is preferably ~dded to the ma;n product gas stream by directing the gas in condu~t 148 to an appropriate point in the product gas cooling and clesning system, such as into conduit 30 of FIG. 1 or into the gas stream before its f~nal cooling stage.

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Referrlng back to vessel 100, the hot product gas ~s removed from the vessel v~a a gas outlet conduit 150 located ln the upper end of the vessel above the drying zone. As noted ~n FIG. 1, the product gas ex~t~ng vessel 100 may be passed through a heat recovery system and thereafter through an absorber to remove H2S and other undes~rable components from the gas.
The heat remo~al system 28 in FIG. 1 may include a stea~ generator, feed water heater, or other heat-exchange means. Typically, the final stage of heat removal is accomplished by heat exchange with cooling water and results in the condensation of water vapor to fonm liqu~d water. Th~s condensed water is preferrably returned tD quench tank 140 via conduit 142.
As noted in copending Can.Pat.appln. S.N. 48~,4167 it may be advantageous to provide the gasifier vessel 100 w~th a burner assembly for providing a stream of hot gas into vessel lQ0 to preheat it prior to starting operation and optionally for prov~ding an additional source of heat dur~ng opera~on. Also, as shown ~n FIG. 1, an acid gas-absorbing device 32 may be provided for providing contact between an absorbent and the product gas to remove noxious acid gases such as H2S and the like from the produc~ gas so that ~t may be rendered su~table ~or use as a fuel for a gas turb~ne or other purposes.
Inasmuch as the operation of heaters~ steam generators, condensers and absorbers are state-of-the-art, these assoc~ated components which are utilized with the black liquor gaslfication system need not be discussed ln any detail.
It is desirable during operation of the process that a relatively constant ~emperature be maintained in gasification 70ne 104, for example 1000C in that part o~ the zone adjacent to the upper surface of mol~en salt pool 108. This can be accomplished by adjusting the air/black liquor ratio up or down to ralse or lower the temperature as requ~red to maintain the desired value. If other parameters such as black l~quor compos~ion, air preheat, and heat losses are not varied, this mode of operation ~ill result in the product~on of a product gas of relatlvely constant composition and heating value. The product gas heating value can be - ~ . . . .

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' ~743S~

~ncreased, if desired, by introducing a high heatin~ value fuel such as oil or petroleum coke into the gasification zone; ~ncreas~ng the temperature of the air feed; or reducing heat 7Osses, by adding insulation, for example. A gaseous fuel such as natural gas or volatlle hydrocarbons can, of course, be added directly to the product gas to raise it5 heating value.
The molten salt produst melt 141 which flows ou~ of vessel 100 to - quench tank 140 is d~ssolved in water to forn green liquor. It is advan~ageous to operate ~he quench tank at the same pressure as the gasifier to avoid the requirement for a pressure control valve operating on molten salt. The green liquor, which contains dissolved sodium sulfide, may be recycled to the pulping process or used for other purposes.
The gas rising from gasification zone 104 contains C0, H2, ~i20~
C02, CH4 and, if air is used, N2 plus various trace componen~s and 15 impurities and is at a temperature in the range of about 870 to 1200C
(1600 to 2200F), Two impurities of special interest are H2S, derived from sulfur in the black liquor feed, and fine particles of sodium salts, such as sodium carbonate and sodium sulfide, produced by vaporization and reaction phenomenaO As the gas then passes through drying zone 102, ~t is 20 cooled to a temperature in the range of about 350 tn 8S0C depending upon its temperature entering the drying zone, the water content of the black liquor and related factors. Preferably the gas is cooled to a temperature - at which the particles of sodium salts are solid, which is below about 790C for typical salt compositionsO
As pointed out above, an oxygen-containing gas is controllably in~roduced into gasification zone 104 and reduction zone 106 of vessel 100 in order to cause partial oxidatlon of the carbonaceous material in the black liquor, generate the required high temperature, and produce the desired products. The oxygen-containing gas i~ suitably ~nd preferably air; if desired, oxygen-enriched air or pure oxygen can be used. Although pure oxygen may be utilized in the process of this invention, ~t ~s less desirable than air or oxygen-enriched a~r because of the higher cost of - - . .' - . ' . ~ .
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oxygen and the requirement for locating an oXygen plant near the black l~quor gasif~cation system. In ~eneral, the upward veloci~y of the gas leaving the gas~fication zone should not exceed about 20 ft/sec and preferably should be in the range of 2 to 15 ft/sec.
The pressure wlthin gasificatlon vessel 1~0 should be within the range of about 1 to 50 atmospheres, with superatmospheric pressure particularly desired. Preferably a pressure of about 3 to 30 abmospheres should be used. The use of superatmospher~c pressure is desirable for a number of reasons. Safety of the process ~s enhanced by the use of superatmospheric pressure because explosions which may occur when mixing melt and water in the ~rocess of quench~ng the melt are ~nh~bited by increased pressure. The product gas volume and consequently the size of the equipment necessary for conducting the process is reduoed by a factor of as much as about 20:1 when superatmospheric pressures are used. Thls reduces both cost and heat losses. In add~tion, salt vaporizat;on is reduced, elimlnating the necessity for extensive cleanup of the gas produced in the process. The removal of vapor-phase impurities such as hydrogen sulfide from the product gas by use of absorption or adsorption processes is facilitated by increased pressure. Another advantage of operating the process under pressure is increased thermal efficiency of the process due to partial recovery of ~elt thermal energy which is made possible by the increase in bofling po~nt of the quench tank solution as ~he pressure is increased. Another advantage 1s that the product gas is available at the pressure required for use in subsequent operations, such as at ~he inlet to a gas turbineO
Temperatures in the gasification zone 104 adjacent the upper surface of ~he molten salt pool 108 are maintained in the range of about 870-1200C ~160~-2200F) and preferably in the range of about 900-1070C
- (1650-1950F). It should be noted that the gasification zone does not operate at a completely uniform temperature. The highest temperature in ~h~s zone is normally near ~he surface of the molten salt pool where ; injected oXygen reacts w~th carbonaceous material. Te~peratures near the top of the gasificat~on zone decrease as the gas approaches the drying zone.
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The high-temperature gases rising from the gasif~cat~on 2cne are cooled to a temperature of about 350 to 8500 during passage through the drying zone. This cooling effect represents an additional benefit of this ~nvention in that lt causes droplets of molten salt which might be entrained in the rising gas to be solidified before leaving the reac~or.
The resulting sol;d particles do not adhere to or corrode heat transf2r surfaces and other equipment ~n the product gas processing system.
Tempera~ures ~n the molten sal~ pool reduction zone may be somewhat lower than those in the gasification zone due to the endothermic sulfur reduction reactions occurring in ~he reduction zone. However, temperatures in ~he reduct~on zone must be maintained at a sufficiently high level to assure that solidification of the sal~s does not occur and the reduction reactions can proceed at a hlgh rate. A range of about 860-1100C (15~0-2000F) is useful, and the preferred range is about 870-1050C (1600-1920F~ for the molten salt pool reduction zone.
It is very important that heat be retained within the gasification and reduction zones. Otherwise heat losses will require a higher air-to-black l~quor feed ratio to malntain temperature. As ~he air-to-black liquor ratio is increased, more complete combustion uccursi particularly the highly exothermic reactions to C02 and H20 from CO
- and H2. This compensates for heat losses but reduces the heating value of the product gas. It is somewhat less important that heat losses be minimized from the dry~ng zone because heat losses frnm this zone act primarily to reduce the temperature but not the heating value of ~he product gas. Heat losses from all three zones are reduced by the use of ~nsulating material 112. Any convenient insulation can be used for this purpose. For example, insulating blankets, castable refractory, fire hrick, fiberglass and tile are suitable. Materials which are in contact with h~gh temperature molten salt and salt vapors must be resistant to attack by these agents. High purity fusion cast alumina blocks for example have been found to be quite effective for use as smelt resistant refractory liner 1l3.

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The control of heat losses is an important feature of the present invention and is in sharp contrast ~o the pract~ces util k~ng the - Tomlinson boiler or an equivalent thereof in which the heat produced in the combustion of black liquor fs used to convert water to steam in bo~ler 5 tubes present in the reactor. Rather than remoYing heat in this manner, in order to produce a combust1ble g3S product having the desired higher heating value, it has been found essential to prevent the heat from being lost. In particular, where it is desired to have a higher hea~inq value (HHV) for ~he product gas of at least about 90 Btu/scf, it is necessary to lO design the system so that the total heat loss from the gasiffcation and reduction zones is less than about 600 Btu per pound of black liquor feed ~ and preferably less than 500 Btu/lb.
e In order to limit heat loss from these zones by radiation upward into the cooler drying zone, it is desirable that the cross sect~onal area of : 15 the vessel at the top of the gasification zone be limited. For ex~mple, a cross sectional area less than about 0.009 ft /lb/hr of black liquor feed will limit radiation losses to less th3n about 500 B~u/lb of black liquor for typical operating conditions. Since some heat losses by conduction through the walls and floor of the vessel can also be expected, a cross sectional area less than about 0.008 ft2/lb/hr of black liquor feed is ordinarily required. Thus a commercia~ unit ~o handle lO0 tcns/day of black liquor feed (8333 lb/hr) ~ould require a cross sectional area at the top of the gasification zone less than 66.7 ft2, or an inside diameter less than about 9 ft for a circular cross-sec~ion. Even smaller cross sectional areas are preferred (e.g., less than about 0.006 ft /lb/hr) and can conveniently be attained with acceptable gas velocit~es by operating at elevated pressures. Reducing ~he cross sect~onal area necessarily results in an increase ln gas velocity in the gasifier ~f other conditions are not changed. Thus in order ~o avoid excessive velocities wh~le operating with a cross sectional area ln the preferred range it is desirable to operate the gasifier a~ an eleva~ed pressure.

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The heat ~oss or heat removal referred to in the above discussion refers only to heat which leaves the ~asjfication and reduction zones by rad~a~ion upward or conduction ~nto or through the walls and wh~ch is ~herefore controllable by proper system design. In addition, i~ ~s ~mportant that the black liquor be almost completely dried before it enters the gasification zone so that heat will not be consumed evaporating water, and that the air feed to both of the lower zones be preheated to minimize the heat required to raise ~ts tempera~ure. Certain heat losses are unavoidable, however, and set an upper limit of about 75~ on the heat~ng value of the black liquor that can be converted to product gas heating value. The unavoidable heat losses include sensible heat in the product gas and product melt and the heating value of sulfide in the melt.
In order to achieve the desired gasification of aqueous ~lack liquor in the process of the present invention, aqueous black liquor is introduced into drying zone 102 of vessel 100 in a manner that provides an adequate area of black liquor surface in direct contact w~th the rising stream of hot gas and an adequate contact time. The black liquor may be sprayed into the vessel t3 form falling drops which are dr~ed by the gases rising from the gasification zone~ with the wa~er being vaporized from ~he black liquor before the black liquor leaves the drying zone. Spray drops - may 81 so strike the inner walls of the vessel ~n the drying zone where they adhere and are dried to form deposits o~ carbonaceous materia1 and salts which subsequently ~all from the walls into the gasification and reduction zones. However, ît is not desirable to in~roduce the black liquor in so fine a spray that the spray droplets or the resultant dried, finely divided ~lack liquor sollds are entrained in the hot gases rising through the gasifier vessel. The coarseness of ~he spray is adjus~ed so that adequate drying with minimum entrainment occurs.
The gas produced as a result of ~he gasificat10n of the black liquor solids has a dry basis higher heating value of at least about 90 Bku/scf primarily due to the presence of C0, H2 and CH4. As the product gas rises through the black liquor drying zone, Its wa~er vapor content - , . . : .. . - . .
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3~

increases and its temperature decreases as a result of evaporation of water from the black liquor. In additiQn, the increase in water vapor causes the water gas sh~ft reaction to occur as ~ollows:

CO + H20 = ~2 + H2-This results in a change in gas composition so ~hat the gas leaving the top of ~he drying zone contains less CO and more H2 than that leaving the gasification zone. However, the higher heating value is not materially chansed by the reaction.
Gas leaving the drying zone may be processed in a number of ways.
Preferably, its sensible heat is utilized for the produc~on of steam in a steam generator or other heat~ng serv~ce. For most applicat~ons, it is desirable to remove water vapor~ fine salt particles~ and H2S from the gas before it is used. These steps may be accomplished in conventional equlpment such as a condenser to remove water vapor, absorption contactors employing alkaline solutlons to absorb H2S~ and fume scrubbers or fabric filters to remove particulate matter. The water9 salt, and sulfur recovered in such steps can be recycled to ~he pulp mill or gasification process. In some cases it may be desirable to purify ~he produc~ gas as it leaves the yasifier without further cooling so that the sens~ble heat and compression energy in the gas and in the wa~er vapor may be utilized - in a gas turbine or other energy conversion system.
As pointed out, the discharged melt 141 flows from vessel 100 via conduit 138 ~nto quench tank 140 where it is dissolved in water at gasifier pressure. The melt will solidify and block the flow path if it ~s permitted to cool below about 760C tl400F) while ~n contact wi~h the discharge noz71e. It is therefore desirable to allow a portlon of the h~gh temperature gas from the gasification zone to flow through the melt discharge line to help maintain a high temperature tn th~s ~ine. This gas will flow into quench tank 140 from which it can be vented to the product gas system at a point downstream of the gaslfier. Other means may be used to maintain a clear pa~h for melt flow including auxiliary burners and mechanical breaker systems.
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~7~35~

The ~ollowings examp1e are illustrative nf ~hls invention bu~ are no~
intended ~o restr~t the scope thereof.

EXAMPLES

The basic process chemistry involved in the molten salt gasification of concentrated aqueous black liquor was previously demonstrated by the present inventor in a series of bench-scale testsO These were conducted in a six-inch ID bench-scale gasifier installed with an electric ~urnace that could be operated to minimize heat losses through the wall~. Product gas higher hea~ing values (~HV, dry basis) ranqed from abou~ 120 ~o 140 Btu/scf depending on the black liquor composition and other variables.
Sulfur recovered from the melt was generally over 90% in the ~orm of sodium sulf~de. The effects of pressure on the basic chemistry were also previously demonstrated by test programs.
To further demonstrate the commercial potential of the molten sa1t black liquor gas~fication process> a mult~purpose molten salt test facility tMSTF) was modified to provide a black liquor gaslfier vessel capable of demonstrating the present process at a pilot plant level. The modification provided a three-zone gasif~cation vessel consisting of an aqueous b1ack liquor drying zone, a black liquor solids gasification zone, and a molten salt sulfur reduction zone. The MSTF used oonsists of a vessel of about 33 inch ID by about 167 inch inside he~ght. The lower 96-inch section is lined wi~h fused cast alumina bricks about 6 inches thick, which are backed by about a half ~nch of high alumina cas~able refractory. These ~aterials are highly resistant to attack by the high temperature molten sal~, but are not effective as thermal insula~lon. To reduoe heat losses from the gas~fk a~ion and reduction zones, a 1/8 inoh thick layer of mineral fiber insul~tlng paper was ~nstalled on the outside of the metal vessel; however9 a more effective thic~er layer could not be ~sed without causing the allowable temperature of the metal vessel to be exceeded.

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~2~ 3~

In prinr testing and analytical s~udies of black liquor gasification~
the present ~nventor had demonstrated that a key requirement for producing a combustible gas hav~ng an HHY greater than lO0 Btu/scf and melt reduction greater ~han 90~ reduction ~o sulfide required that heat lost from the combined gaslfication and reduction zones should preferably be less than about 500 Btu/lb feed for a typical black llquor composition.
Since the original purpose oP the MSTF vessel was to test chemical waste disposal by co~plete combustion, ~n order to maximize throughput the uni~
was designed to permit a very high rate of heat loss through the walls (about S00,000 - 800,000 Btu/h). Acsordingly, because of the original high heat loss design of the MSTF vessel, the key objectives of the black liquor gasification program at the pilot plant level was limited to demonstratlng ~he operability of relatively large-scale equipment and establishing the predictability of performance based on bench scale tests and analy~ical studles.
Two key structural modifications were made in the MSTF vessel in accordance with the present invention. The melt removal port located 76 inches above the floor of the vessel was plugyed with a ceramic insert and covered wi~h a blanking flange. A new melt overflow spout was designed and fabrica~ed ~or the test operation and installed 14 inches aboYe the vessel floor. By lowerlng the melt removal port, melt inventory was reduced and a relatively shallow pool was provided.
In addition to the four existing nozzles used for air injection into the molten sal~ pool, six new nozzles were provided at an elevation of 20 inches above ~he vessel floor so as to permit a portion of the ~njec~ed air to be injected above the melt pool. These newly provided nozzles were evenly spaced around the circumference of the vessel and po~nted down and inward at a 45 angle so that the air was directed toward the surface of the molten salt pool. Balancing orifices were used at each nozzle to prov~de an even air distribution to ~ndividual air ports. Changes were also made to the black liquor ~njection system aimed at increasing and maintaining the black liquor flow.

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~h~
-25- 8~045 The total run time conslsted of about 46 hours of operation from the initial black l~quor feeding to system shut-down and ~ncluded 14 tests.
About 19,000 lbs of black l;quor were gasified; however, bl~ck liquor flow ~as not cont~nuous during ~he entire run.
5The gasifier was started up by first setting the air flows to the nominal ~alues for full load conditions, e.g., for a nominal superficlal gas velocity of five fps at 980C (1800F). The total air distribution to the gasif;er vessel was init;ally set to provide about 40% of the air to - the top six nozzles (above the melt) and 60X of the a;r to the bottom four nozzles (~nto the melt). However~ this ratio was reversed for ~ests 10 through 14. The upper six no~zles received preheated air; the bottom four nozzles rece;ved ambient temperature air. A temporary natural gas burner was ~nstalled on the vessel head for preheating the unit. The gasifier was preheated to 930-980C (1700-1800F) prior to the run. Table 1 shows an analysis of the black liquor used in the tests.
Analysis of the test results showed that the product gas had a maximum HHV, dry, of 52.3 Btu/scf during steady-state operation and a maximum reduction of sulfur in the melt of 67.4g. As noted, because of the design of the MSTF vessel, it was not feasible to increase these values significantly during the run by changes which would permit operation at a lower air/fuel ratio such as by providing additional insulation to the vessel or by increasing the black l~quor feed rate.
Tests 10 and 13 (see Table 2) are typical examples of the performance of the MSTF in the conf~guration of this invention. For comparison, the results of a previous test, desiqnated as test A, with a different configuration are included in the table. During this previous tes~1 all of the air was fed beneath the surface of a deep molten salt pool.
A comparison of tests 13 and A, which operated at approximately the same air/black liquor ratio, ;ndicates that reducing ~he melt pool depth from 76 to 14 inches had no adverse effec~ on the product gas heating value. The sulfur reduction efficlency is seen to be significantly higher in both tests 10 and 13 than ;n test A. This is attributed to the divided : . , -.
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7~ 5 ANALYSIS OF BLACK LIQUOR USED ~N MSTF RUN
_ ._ _ ~et Basis Dry Basis _ _ Solids concentration, ~t. S 66.47 100.0 pH 12.8 Density g/cm @ 25C 1.41 Heat of Combustion, Btu/lb 4311 6485 Element analysis, wt ~
Carbon~ 24.~0 37.31 - 10 ~Iydrogena 2.27 3.41 Organic carbon 25.46 38.30 Sodium 13.90 ~0.91 Potassium 1.24 1.87 : Calcium 0.02 0.03 Magnesium 0.01 0.02 Iron 0.01 . 0.01 Aluminum ~c o.Ol ~ 0.01 Total sulfur 2.71 4.07 Elemental sulfur 0.0& 0.12 Polysulfide sulfur 0.05 0.07 Co~pounds, wt. X
NaOH 0.37 0.55 ~a2S 4.11 6.18 . Na2~03 4.63 6.97 ~la25D4 2.68 4.03 Na SO 0.01 0.01 Na2s o . 1.38 2.07 : ~ Na~212 3 0.09 0.14 ~a2C204 0.93 1.40 ~Iethoxyl (O-CH3) 3.16 4.76 Tall oil 0.56 O.RS
Yolatile Doids 7.06 10.62 . , ~ _ _ .
aSample dried before ~nalysisi may have lost vol~tile organics.
34 bDoes not include hydro~en in water, ' ~
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MSTF TEST RESULTS

Run Nos. 10 13 A

Melt Pool Depth, ~nches 14 14 76 Air Distribution, ~
into melt pool 37 37 100 above pool sur~ace 63 63 0 Black Liquor Feed, lb/h 844 674 530 A~r Feed, lb/h 1469 1426 1180 Air/Black Liquor wt ratio 1.74 2.12 2.23 Temperatures, C (F) melt pool 886 (1627) 964 (1767)993 ~1820) feed air 230 ( 44S) 231 ( 448)~62 ( 864) black liquor 102 ( 216) 94 ( 201)77 ( 170) Product Gas Analyses9 vol ~ dry H2 7.7 5,0 4,8 C2 17.3 1~.3 16.~
Ar 0.8 0O9 0.9 N2 67.2 73,3 74.3 CH4 0.6 0,3 0.5 C0 6.4 4,3 3.2 ~ ~ Product Gas HHV, Btu/scf 52.3 33,0 31.5 :` Melt Composition, wt ~
Na2C03 .68.4 75.3 74.0 :: 25 Na2S 16.6 8.9 0.2 Na2S3 0-7 1 1 0.1 Na2S04 14.3 28 1 25.7 : Reduction Efficiency, X 67.4 38.6 1.4 .
: -: :-: 4055P

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air feed arrangement for tests 10 and 13 whereby only 37~ of the air passed through the melt pool, wi~h the balance injected ~nto the gasification zone. This arrangement also permitted more total a~r ~and therefore more black liquor) to be fed into the gasifier during gests 10 and 13 w~thout excessive entrainment of melt droplets. As a result the unit could be operated at a lower air/black liquor ratio during test 1 to produce a higher he~ting value gas than possible with the configuration used for test A.
Test 10 represents the maximum steady~state operating capabi7ity of the MSTF in the final configuration with regard to throughput, product ~as heating value, and sulfur reduct~on. The throughput is limited by the allowable gas Yeloc~ty and could be increased by increasing the operating pressure or, to a lesser extent, by operating with a lower air/black liquor feed ratio. The product ~as hea~ing value and sulfur reduction efficiency could also be increased by operat~ng with a lower air/black - liquor ratio; however thi~ mode of operation would cause the system temperature to drop unless the heat loss per pound of feed is reduced.
This can be accomplished by either reducing the total heat loss (e.g., by the use of additional insulation) or by increasing the allowable feed rate (e.g. by increasing the pressure).
At conditions obtainable in the MSTF vessel~ the results indicate that operation with a significant portion (30-70g) of the air injected above the pool of molten salt results ~n more efficient sulfur reduction than operation with 100~ of the air 1njected beneath the pool surface and also permits operation at a higher gas production rate. The results al50 show that a very shallow pool of melt (nominal depth about 14 inches) is as effective for black liquor gasification as a deep pool t76 inches).
The present tests, compared with previous tests at both the bench scale and pilot scale level, demonstrate that decreasing the air to b1ack l~quor ratio results ~n an increase in both the product gas ~HY and the melt sulfur reduction efficiency. The data indicate that a sulfur-reduction efficiency of over 9Q~ will be obtained when the . .

.. ..
.: . ,' a~r/black liquor ratio is decreased to the point where the gas HHV exceeds about 60 Btu/scf. This enables projectlon of the pilot-scale resul~s ~o ~nd~cate that commercial plants will operate to produce gas w~th an HHV
over 100 Btu/scf and melt in which the sul~ur content is over 90X in the form of sulfide.
It will be recognized that the kraft pulp product~on process is about lO0 years old. Because the chemicals used ~n the cooking liquor compositlon for the treatment of the cellulosic raw material are too expensive to discard, from the ~nception of the kraft process many attempts have been made to recover these cooking ma~er~als, wi~h incidental recovery of heat through burning l~quor organic matter dissolved from wood. The Tomlinson boi1er was introduced about 50 years ago to accomplish the desired recovery Because of the previously ment~oned disadvantages of the Tomlinson bo~ler~ many ~odifications and replacements for it have been proposed. The present process avoids the disadvantages of other proposed processes in that It US2S the ~dentical concentrated black liquor feedstock without the requirement to predry~
oxidize, hydrolyze or otherwise prepare the feedstock. Also~ the present process produces a smelt which is essentially identical to that produced by the Tomlinson bo~ler. Because of the above advantageous features~ as well as Its use of a single component vessel, the present process can be ~ readily integrated into existing pump mill systems to replace or supplement Tomlinson boilers.
- It will be realized that various modifications utilking the 2~ long-standing teachings in the black liquor rerovery field can be made to -~ the design of the vessel and the operation of the process of this - invent10n without departing from the sp~rit thereof. For example, the vessel may be designed with a smaller diameter in the drying zone than ~n the gasification and reduction zones in order to reduce thermal radiation from these latter zones. Also, o~her gas~fier vessel shapes may be used lnstead of the constant diameter Yertically elongated walls shown.
Further, the black liquor feed may be broken up by a ~pinning disk - ~ : - . .. . :.
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7~35.~

atomizer, steam atomizer, or flow distrfbution system ~nstead of spray noz~les as illustrated. Thus9 ~hile the princ~ple, preferred design and mode of operatfon of the Invention have been explained and what is now oonsidered to represent its best embodiment has been illustrated and described, it should be understood that, within the scope of the appended claims, the invent~on can be practiced otherwise than as speciffcally illustrated and describedO

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Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for treating a concentrated aqueous black liquor carbonaceous material and alkali metal sulfur compounds to form a combustible gas and a sulfide-rich melt comprising:

(a) providing a gasifier vessel maintained at a pressure of from about 1 to 50 atmospheres and containing a relatively shallow molten salt pool at its bottom within a sump equipped with an overflow discharge, said vessel having (i) a black liquor drying zone at its upper parts, (ii) a black liquor solids gasification zone located below the drying zone, and (iii) a molten salt sulfur reduction zone comprising said molten salt pool;

(b) introducing into the top of said drying zone the concentrated aqueous black liquor containing carbonaceous material and alkali metal sulfur compounds;

(c) evaporating water from said concentrated aqueous black liquor in said drying zone by direct contact of said aqueous black liquor with the hot gas rising from the gasification zone to produce dried black liquor solids, which fall into said gasification zone, and a cooled combustible gas containing water vapour said combustible gas being at a temperature below the melting point of entrained droplets of the molten salt causing solidification of said droplets;

-31a- 85R045 (d) introducing a first portion of an oxygen-containing gas into the gas space in the gasification zone located below the drying zone immediately above the molten salt pool to partially oxidize and gasify a fraction of the carbonaceous material in said dried black liquor solids falling through said zone to form a hot combustible gas.

(e) introducing a second portion of said oxygen-containing gas beneath the surface of said molten salt pool in an amount sufficient to cause gasification of essentially all carbonaceous material entering the pool from the gasification zone but not sufficient to create oxidizing conditions in the pool, the formed gas rising from said pool, the total amount of said first and second portion of oxygen-containing gas constituting between 25 to 55% of the amount of oxygen-containing gas required for complete combustion of the black liquor feed and representing the total amount of oxygen-containing gas fed to said gasifier vessel, (f) withdrawing said cooled combustible gas having a higher heating value of at least about 90 Btu/scf (dry basis) from an upper portion of said drying zone; and (g) withdrawing from said overflow discharge in the molten salt reduction zone a melt in which the sulfur content is predominantly in the form of alkali metal sulfide.

2. The process of Claim 1 wherein each of said first and second portions of the oxygen-containing gas constitutes from 30 to 70% of the total amount of oxygen-containing gas fed to the vessel.

3. The process of Claim 2 wherein said oxygen-containing gas comprises air.

4. The process of Claim 1 wherein said gasifier vessel is maintained at a pressure in the range of from about 3 to 30 atmospheres and wherein the concentrated aqueous black liquor fed to the vessel comprises at least 45 wt% solids and has a higher healing value of at least about 3200 Btu/ lb .

S. The process of Claim 1 wherein the hot gas leaving the gasification zone is at a temperature of 870-1200°C and the cooled combustible gas leaving the drying zone is at a temperature of 350-850°C.

6, The process of Claim 1 wherein the temperature in the reduction zone is 860-1100°C.

7. The process of Claim 1 wherein the total heat loss from said gasification and reduction zones is less than about 600 Btu per pound of black liquor fed to the gasifier vessel.

8. A device for treating a concentrated aqueous black liquor to form a combustible gas and a sulfide-rich melt comprising:
an enclosed vertically elongated gasifier vessel having (i) a black liquor drying zone at the upper part of the vessel, (ii) a black liquor solids gasification zone below said drying zone, and (iii) molten salt sulfur reduction zone below said gasification zone;

a sump region in the bottom of said vessel adapted for containing a pool of an alkali metal molten salt, said molten salt pool comprising said molten salt sulfur reduction zone;
inlet means disposed in the upper part of said vessel for feeding said black liquor to said drying zone;
outlet means disposed in the lower part of the vessel above said sump region serving to define the height of the molten salt pool and to remove overflow of molten salt from the sump region;
a first set of gas inlet means located in the gasification zone for feeding an oxygen-containing gas into the gas space of the gasification zone immediately above the molten salt pool;
a second set of gas inlet means disposed in the sump region of the vessel for feeding an oxygen-containing gas directly into the molten salt pool; and outlet means disposed in the top part of the vessel and communicating with the upper part of the drying zone for removing a combustible gas from the vessel.

9. The device of Claim 8 further including means for controllably proportioning the feed of the oxygen-containing gas to the first and second sets of said oxygen-containing gas inlet means.

10. A device according to Claim 9 wherein at least one air compressor provides a pressurized oxygen-containing gas for feed to the first and second sets of gas inlet means.

11. The device of Claim 8 wherein the outlet means from the sump region is connected at its other end to a quench tank for receiving the melt overflow from the sump region of the vessel.

12. A device according to Claim 11 wherein the quench tank contains an inlet conduit for feeding a source of water to the quench tank and further contains gas outlet means for withdrawing gases from the quench tank.

13. The device of Claim 8 wherein the black liquor inlet means includes a spray system for injecting the concentrated aqueous black liquor as a coarse spray into the upper part of the drying zone.

14. The device of Claim 8 wherein the vessel has an outer wall metal containment shell lined with an insulating refractory material for withstanding the temperatures and environment within the vessel and for minimizing heat losses therefrom.