CA1077271A - Coal gasification - Google Patents

Abstract

ABSTRACT
Coal gasification process in which coal is pyrolyzed at a temperature below 700°C to form aqueous vapors and a char, the char is reacted with steam in a gasi-fication zone to produce carbon monoxide and hydrogen, and the vapors are condensed to produce an oil and waste water containing ammonia and phenolic compounds.
This waste water is vaporized and substantially all the resulting steam containing ammonia and phenolic compounds is fed to the gasification zone.

Description

1 O ~ ~ 1 FMC 5544 This invention relates to a process in which coal is subjected to low temperature carbonization and the resulting char is then reacted with steam to produce a combustible gas, mainly carbon monoxide and hydrogen.
In the low temperature carbonization of the coal, pyrolysis reactions release tars, oils, tar acids and bases, water, hydrogen sulfide, organic sulfides, amnlonia and organic nitrogen compounds.
Durir.g the gasification step the main gasifica~
tion reaction is one between carbon and steam.
C + H20 ~ CO + H2 This reaction i8 endothermic. Heat can be supplied indirectly by a heat transfer medium or directly by the addition of oxygen to the gasifier, or by gasi-fying at high pressures where the exothermic reaction between carbon and hydrogen is thermodynamically favored.
C + 2H~- ~CH4 In the gasification zone other reactions occur. Thus oxygen, nitrogen and sulfur compounds in the char can r~act to form water, ammonia and hydrogen sulfide.
The gaseous streams taken from the gasification zone and from the preliminary pyrolysis zone or zones I contain water, including unreacted steam from the gasi-fication zone. Condensation of this water, in the course of purifying the gaseous streams, results in the forma~
~; tion of highly contaminated waste water containing particulate matter, dissolved carbon dioxide, hydrogen sulfide and ammonia, and, depending on the particular process, dissolved organics (tar acids and bases) and : , ~

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traces of oil. Disposal of this waste water through ordinary channel~ can create serious environmental problems.
In accordance with the process of this invention, contaminated waste water containing ammonia and phenolic ;; compounds (produced by the carbonization step) is vaporized, and substantially all the resulting steam containing said ammonia and phenolic compounds is fed to the gasification zone. In the gasification ~one the ammonia is to a large extent decomposed to form hydrogen and nitrogen and the phenolic compounds are also decom-posed. The proportion of ammonia in the vaporized waste water and the proportion of waste water are ~, generally such that the added nitrogen contributed by , ,,~ .
~'1 the decompo~ition of said ammonia is well below 1%
(preferably less than 1/2~) of the final product gas j stream.
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, In one procedure for treating the waste water in accordance with this invention the waste water is treated to remove H2S and a substantial proportion of its ammonia, while leaving sufficient ammonia in the water containing -~l the phenolic impurities to.maintain a pH of at least :' 8 and up to about 10.5, preferably about 8.5 to 9.0, and the ammoniacal water is then flashed into a stream ;l of superheated steam being fed to the gasification zone.
In another procedure for treating the waste water in accordance with this invention the contaminated ~- waste water is fed to a pebble heater which is supplied with preheated pebbles. The waste water is distributed onto these pebbles, as by spraying, and as a result it : .

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vaporize~ and is superheated.
The gasification reaction generally is effected at ; a temperature of about 750 to llOO~C preferably about 850 to 1000C at pressures from atmospheric up to about 1000 pounds per square inch gauge, psig, (70.31 kilograms per square centimeter gauge, kg/cm2g) or more.
In one preferred process the pressure is about 50 to 100 psig (3.52 to 7.03 kg/cm2g). In other known gasi-fication processes considerably higher pressures are used, e.g. about 300 to 400 psig (21.09 to 28.124 kg/cm2g) (such as about 350 psig (24.61 kg/cm2g)) in one case and about 1000 psig (70.31 kg/cm2g) in another case.
The heat for the endothermic gasification reaction may be supplied by a heat carrier material, such as solid heat-transfer particles which are heated by the combus-tion of uel in a combustion zone 11 (Figure 1). The resulting hot particles are circulated to the gasifi-cation zone 12 and then back to the combustion zone 11, the zones 11 and 12 being maintained at about the same pressure.
For the heat transfer medium circulated in the com-bustion zone and gasification zone one may employ, for instance, materials known in the art such as inert refractory pebbles, e.g. of alumina or mullite, agglomerated ash from the burning of coal, calcined dolomite which undergoes an exothermic reaction with CO2 in the gasifi-cation zone, char, coke, etc. The gasification zone preferably comprises a fluidized bed of the char or other carbonaceous material into which the steam is fed. The arrangement of gasification and combustion '' zones is, as previously indicated, preferably such that these zones are maintained at substantiall~ the same pressure, so that little if any of the gas made in the gasifier tends to flow into the combustor and vice versa. For instance the pressure differences between these zones may be about 5 psi (0.352 kg/cm2) or less.
Part of the heat for the endothermic gasification reac-tion can be supplied by the superheat of the steam fed thereto.
In one embodiment of the invention the flue gas from the combustion zone 11 is fed to a pebble heater, indicated generally as 13. The hot flue gas passes through a first pebble zone 14 where it serves to pre-heat the pebbles, and then leaves the first pebble zone.
The resulting preheated pebbles pass to a second pebble zone 16 to which the contaminated waste water is fed.
The resulting steam is supplied to the gasification zone 12 and the pebbles are returned to the first pebble zone 14, all the zones 11, 12, 14, 16 being maintained at a pressure which is approximately the same throughout said zones. The pebbles are returned by means of a conveyor 17 (which may be a mechanical or pneumatic lift which is insulated so as to conserve the heat in the pebbles).
Pebble heaters and the pebbles used therein are well known in the art. See for instance Findley and Goins, Advances in Petroleum Chemistry and Refining 2, (published by Interscience, 1959) chapter 3, p. 127-206 entitled "Pebble Heaters". See also the articles by C. L. Norton Jr. in Chemical & Metallurgical Engineering, July 1946, p. 116-119 and M. O. Kilpatrick et al in Petroleum Refiner April 1954, p. 171-174. In the prac-tice of this invention the zone in which the pebbles are heated (e.g. zone 14 of Fig . 1) and the one in which the waste water is vaporized (e.g. zone 16) are at substantially the same pressure so that little if any of the heating gas mixes with the steam and vice versa.
For instance, the pressure differences between these zones may be about 2 to 5 psi (0.141 to 0.352 kg/cm2).
In one embodiment the bottom of zone 14 is at a pressure of about 54 psig (3.797 kg/cm2g) and the top of zone 16 is at a pressure of about 52 psig (3.656 kg/cm2g), the two zones being connected by a pebble-filled tube 17 into which is injected a small stream 18 of a seal gas (preferably steam) at a higher pressure than that in the bottom of zone 14 to prevent, or diminish, the transfer of flue gas from zone 14 into the steam generated in zone 16. The hot flue gas fed from the combustion zone to the pebble heater is usually at a temperature in the range of about 900 to 1100C and the difference between that gas temperature and the tempera-ture to which the steam is heated in the pebble heater is about 100 to 200nC. The pebbles are preferably spherical balls of heat-resistant inert material such as alumina, mullite or stainless steel, having a diameter of about 1 cm.
It is also within the broader scope of the inven-tion to employ the pebble heater for treating the waste water to produce steam for the gasifier in processes in which the heat for gasification is supplied autothermally ~ , 107727~ , within the gasification reactor. In that case, instead of feeding flue gas, made in the combustion zone, to the pebble heater one feeds the synthesis gas (i.e. the gas produced in the reactor). Typically the temperature of this gas is about 850 to 1000C and the difference between that gas tempera~ure and the temperature to which the steam is heated in the pebble heater is about 100 to 200C.
In a less desirable embodiment of the invention there is a separate heat supply for the pebble heater which produces steam from the waste water. That is, fuel and oxygen (usually air) (instead of flue ~as or s~nthesis gas) are supplied to the first pebble zone 14.
In this case the pebbles can be heated there to a higher temperature, such as about 1000 to 1650C preferably about 1000 to 1400C, to give steam in the second pebble zone 16 at a temperature at about 800 to 1200C, the pres-sure in the first pebble zone being about the same as that in the second pebble zone which is in turn at a slightly higher pressure than that in the gasification zone (e.g. the pressure differential is sufficient to cause the steam to flow to the gasification zone through the unobstructed pipes leading thereto from the second pebble zone).
Instead of using a pebble heater, in which the pebbles are maintained in contact with each other in the pebble-heating and steam-generating vessels, one may employ, less desirably, a fluidized bed of heat-transfer particles (pebbles) of fluidizable size, as in the embodiment shown in Fig. 3. Thus the pebble-heating :.

107~271 zone 21 is supplied with hot gas (e.g. flue gas or ma~e-gas, as previously discussed) which passes upwardly and serves to fluidize the pebble~, the heated pebbles from zone 21 are transferred to ~one 22 where they are fluidized by a gas which is compatible with the gasifi-cation reaction such as steam, carbon monoxide, hydrogen or carbon dioxide or a mixture of two or more of these gases which are sub~tantially free of inert gas such as nitrogen. Waste water is injected into zone 22 and steam generated therefrom (mixed with said fluidizing gas, when the latter is not steam) is taken from the upper part of zone 22 and fed to the gasification zone.
The cooler pebbles from zone 22 are transferred back to zone 21 for reheating. Slnce the amount of waste water is usually insufficient to generate all the steam needed in the gasification reaction the re~uired fresh steam may be conveniently employed as the fluidizing medium.
Preferably the fluidizing gas is substantially free of nitrogen which (passing to the gasification zone) would undesirably dilute the make-gas.
In another embodiment, illustrated in Fig. 4, the pebbles are not circulated but are present as essentially stationery beds 31, 32 in a multi-zone recuperative stove. The hot gas is fed to one zone to heat the pebbles therein while waste water is fed to the second zone, which contains previously heated pebbles. Then, the three-way valves 33, 34, 35, 36 are reset so that the hot gas flows to the now-cooler second zone and the waste water is injected into the now-hotter first zone, this alternation being repeated continually.

10772'71 In the pebble heaters, hydrogen sulfide dissolved in the waste water is vaporized, passes to the gasi-fication zone and appears in the synthesis gas together with additional hydrogen sulfide formed in the gasifi-cation reaction. Carbon dioxide in the vaporized waste water reacts with char in the gasification zone to form CO. The acid gas constituents are then removed in the gas purification section. The ammonia dissolved in the waste water is also vaporized in the pebble heater (or decomposes therein if the temperature is above about 550C) and decompose to form hydrogen and nitrogen in the gasification zone; the hydrogen enriches the resulting gas while the nitrogen acts as a diluent.
Other constituents in the waste water, such as trace tar and dissolved or dispersed organics, are decomposed, at least in part, during vaporization in the pebble heater, decomposition being completed in the gasification zone.
Inorganic non-volatile constituents such as particu-; late matter or salts in the waste water may form liquid or solid deposits on the pebbles, e.g. a molten saltphase on the surfaces of the pebbles leaving the steam-producing zone. To keep these deposits from building up a slip stream of the pebbles (e.g. a minor proportion, such as 5% of the main pebble stream) may be withdrawn from the main pebble stream, either continuously or intermittently, and treated to remove the deposits, as by passing through a cooler abrading zone (e.g. zone 19 in Fig. 1) such as a rotating drum in which th~ pebbles rub against each other at a temperature at which the deposit is solid, e.g. about 300C.

~077271 In the embodiment in which the waste water is pre-treated to remove H2S before feeding the water to the gasification zone, it is preferred to remove both H2S
and part of the ammonia by stripping with steam. Strip-ping processes of this type are known in the art (see, for instance the article by R. J. Klett in Hydrocarbon Processing Oct. 1972 p. 97-99); in such processes the water is fed to a distillation column having a reboiler and operated so that the impurity (H2S or NH3 or both) 0 i5 taken overhead while water of lowered impurity content is taken from the bottom of the column. In a preferred procedure two stripping columns are employed, the first one, A, being operated at a pressure (e.g. about 75 to 125 psig (5.273 to 8.789 kg/cm2g), such as about 100 psig ~7.031 kg/cm2g)) such that the overhead is substantially anhydrous hydrogen sulfide gas having a relatively low NH3 content ~such as less than about 100 parts per million (ppm), e.g. less than 50 ppm) and a low water content (such as less than 2%, preferably less than 1%, e.g.
below about 1/2%) and the second column, B, generally being operated at a higher pressure (e.g. about 175 to 230 psig (12.304 to 16.171 ky/cm2g), such as about 200 psig (14.0~2 kg/cm2g)) such that the overhead is sub-stantially anhydrous ammonia containing less than about 1% water (e.g. about 0.2 to 0.5% water) while the stream from the bottom of the column is the hot water under pressure and having an H2S content of less than about 50 ppm, e.g~ about 0 to 20 ppm and a pH of at least 8 which is to be flashed into the stream of super-~; 30 heated steam. The temperature of the stream of hot water .~ , , , , :.

1(~77'~71 will be dependent on the pressure employed in theammonia stripping column; when that is operated at 200 psig (14~062 kg/cm2g) (measured at the top of the column, as is conventional) the stream of water from the bottom is at a temperature of about 200 to 21~C.
Externally produced team i5 preferably fed to each stripper column A and B as indicat~d on the drawing.
It will be understood that the use of two stripper column~, one for H2S and the other for NH3, enables one to recover two relatively pure useful products and is therefore desirable. It is, however, also within the broader scope of this invention to use a single column taking off both H2S and NHg overhead, e.g. at a pressure of about 200 psig ~14.062 kg/cm2g).
The stripped ammoniacal water will preferably con-tain less than about 6% NH3, such as about 2 to 5% or less. This ammonia as present in association with the phenolic compounds in the water.
The stream of superheated steam into which the , 20 str~pped ammoniacal water tcontaining organic impurities) is fed may be generated in an ordinary steam boiler (as , by heat-transfer from furnace-heated solid metal tubes).
Its temperature may be within the range of about 300 to 600C, preferably about 350 to 450C and its pressure preferably may be within the range of about 150 to 550 psig (10.547 to 38.671 kg/cm2g), more preferably about 200 to 300 psig (14.062 to 21.093 kg/cm2g). The propor-tions and temperatures are such that the resulting mixture (after the flaQhing of the ammoniacal water) is super-heated steam having a temperature preferably within the .i . . .

range of about 200 to ~50C and more preferably about 250 to 350C and a preqsure within the range of about lO0 to 450 psig (7.031 to 31.640 kg/cm2g) and, preferably about 150 to 250 psig ~10.647 to 17.578 kg/cm2g) con-taining an amount of ammonia preferably in the range of about 0.1 to l.0 weight percent (wt. %) more preferably below 0,3 wt. ~; this mixture is fed directly to the gasifier.
The hot ammoniacal water may be flashed into the superheated steam in any suitable manner, as by spraying it or otherwise feedlng it (as through a suitable pressure reducing valve when the pressure of the steam stream is below that of the water stream and the temperature of the water stream is above its boiling temperature at the pressure of the steam stream).
As previouqly mentioned the organic impurlties in the waste water are decomposed in the gasifier. Ammonia may be decomposed there to form nitrogen and hydrogen, but the amount of ammonia in the feed to the gasifier is not so large that a significant undesirable dilution of the resulting product gas by nitrogen occurs, the ~2 adds to the fuel value.
Because of the retention of some ammonia in the stripped waste water the latter will not be unduly corrosive to steel processing equipment (such as pipes and valves) with which it comes into contact, both in its liquid and vaporized state (e.g. in the lines to the gasification zone).
In one preferred process, shown in Figure 2, vapors resulting from low temperature pyrolysis at 39 may be .~ , .

7 7Z~71 led to a separation zone 41 in which they are cooled to condense oily liquids and water and the aqueous phase is separated from the oily phase. This aqueous phase i5 typically about 4 to 12~ by weight of the coal fed to the pyrolysis zone and contains fairly high concen-trations of water-miscible organic compounds (such as phenol, cresols, xylenols, resorcinol, methyl dihydroxy-benzene), hydrogen sulfide (e.g. in amounts in the range of about 0.1 to 1%, such as about 0.3 to 0.5%) and ammonia (e.g. in amounts in the range of about 0.1 to 0.5%, such as about 0.2 to 0.4%), together with water-dispersed higher alkylated phenols, such as a broad spectrum of mixed phenols of the type having two or more carbons in one or more substituents (which substituents :
may be cyclic) and/or three or more methyl substituents, the individual components of this mixture being pre~ent in:such small proportion as to be dispersed or dissolved ; in the water. Thus, such compounds as ethyl phenol, propyl phenol, hydroxyindane, dihydroxy ethyl methyl indene, dihydroxyl naphthalene, trimethyl phenol tetra-methyl phenol and dimethyl ethyl phenol may be present, among others.
In the process illustrated in Fig~ 2, the oily liquids are then purified at 42 to remove heteroatoms and reduce their viscosity. One method for doing this involves hydrogenation which converts combined nitrogen, oxygen and sulfur to ammonia, water and hydrogen sulfide, respectively, and yields a two phase mixture comprising an aqueous phase and an organic phase, the latter being a combustible light hydrocarbon oil, which may be further .
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refin~d or treated to produce typical petroleum products such as gasoline, etc. Processes of this type are known in the art, as in "hydro-cracking" (such as described in The Oil and Gas Journal April 25, 1966 pages 146-167).
The separated aqueous liquor contains water~miscible or water-dispersed organic compounds, such as phenols (e.g.
in amount up to about 3%), organic bases (e.g. in amount up to about 1%), hydrogen sulfide (e.g. in amount in the range of about 0.1 to 1%, such as about 0.3 to 0.5%) and ammonia (e.g. in amount in the range of about 0.1 to 0.5~, such as about 0.2 to 0.4%).
The pyrolysis of the coal by low temperature carboni-zation, e.g. at a final char temperature up to about 700C, is described in ahapter 10 ~by Wilson and Clendenin entitled "Low-Temperature Carbonization") of Chemistry of Coal Utilization by H. Y. Lowry, Supplementary Volume pu~lished 1963 by Wiley, New York, U.S.A.
In one preferred process for carrying out the pyrolysis at 39, the coal is passed through a series of fluidized beds (not shown) at progressively higher temperatures to devolatilize the coal. The process involves partial oxidation of the material only in the ; very last stages of the process, after about all the conden~able volatiles have been removed. Examples of such processes are found in Eddinger, Jones and Seglin U. S. patent 3,375,175 of March 26, 1968.
The synthesis gas stream produced by the gasifica-tion (at 43) of the char contains not only carbon monoxide and hydrogen but unreacted steam, particulate material ,, . ' , 1~77Z71 (~uch as char fines), CO2, a little ammonia, hydrogen sulfide (e.g. up to about 1% depending on the sulfur content of the coal) and traces of phenolic materials.
In the embodiment illustrated in Fig. 2 this gas stream is subjected at 44 to a purification step after it has been mixed with uncondensed material (gas) from the separation step applied to the volatilized products of the pyrolysis; the latter gas (from the separation step) may contain Cl-C4 hydrocarbons, CO, H2, CO2, H2S, NH3, COS.
' 10 The purification at 44 may be effected, for instance, ' by scrubbing and cooling the gas with plain water (e.g.
to reduce the gas temperature to a temperature of about 25 to 200C, preferably about 40C, at a pressure of about 25 to 150 psig (1.758 to 10.547 kg/cm2g), prefer-ably about 50 psig (3.516 kg/cm2g)); this yields an aqueous waste stream containing dissolved hydrogen i sulfide, carbon dioxide, ammonia and particulates and, often, water-soluble or water-dispersed organic com-pounds (such as phenolic compounds) and traces of water-insoluble oily material. After scrubbing and cooling, ; the gas still contains such impurities as H2S and it is preferably given a further treatment, e.g. a solvent extraction (using such solvents as potassium carbonate '~ solution or alkanolamine solutions; see the processes described for instance in the'series of articles entitled "~ease-Gas Sweetening" which appeared in The Oil and Gas Journal in 1967, August 14, 21 and October 9 and in 1968 January 8, June 3 and 17). The gas may then be subjected to a shift reaction, desirably after re-ducing the sulfur content of the gas to a very low level ' .

10'77Z 71 as by contact with a suitable material such as ~inc oxide.
~ s illustrated, in one preferred embodiment a portion 46 of the ~rude synthesis gas stream (e.g. about 15 to 30%, such as about 25%, thereof) from the gasifi-cation zone is fed to one or more of the pyrolysis zones 39 to serve as a fluidizing medium therein and its con-stituents will thus be incorporated with the pyrolysis products.
Also, instead of adding the relatively impure pyrolysis gas from separation zone 41 to the synthesis gas, the pyrolysis gas may be separately treated for removal of H2S (and C2), e.g. by solvent extraction as described above, and then washed, as with a liquid hydrocarbon, to remove C2-C4 hydrocarbons. The resulting purified pyrolysis gas may then be mixed with the purified synthesis gas and the resulting gas mixture may then be subjected to a shift reaction, desirably after reducing the sulfur content to a very low level as by contacting the gases, individually or in admixture, with a suitable material such as zinc oxide.
The shift reaction is carried out at a temperature of about 250 to 550C desirably at a relatively high pressure, such as 500 psig (35.155 kg/cm2g), in the presence of added steam to convert some of the carbon monoxide in the gas to carbon dioxide and hydrogen~ e.g.
to give a 1:3 CO:H2 mol ratio. The gas may then be cooled to condense out some of the water content to adjust the water content prior to methanation.
The gas may then be subjected to methanation in ~77Z71 which the carbon monoxide and hydrogen react in the presence of a suitable catalyst (such as the known nickel catalyst) to form methane and water. The gas is then cooled to condense out the water. Owing to ~he purity of the feed gas at this stage, the condensed water is relatively pure and suitable for use in a conventional steam boiler to make steam for the process.
In addition the methanation reaction is very exothermic and may be used as a source of heat (by conventional heat-exchange) to produce steam for the process.
The methanation reaction is preferably carried out in stages, as is known in the art. Thus the feed gas stream may be divided into several smaller substreams.
One substream is diluted with a stream of recycled methane and fed to a first methanation reactor. The hot gaseous product at a temperature of about 500C
is then cooled, ~y heat-exchange, to a temperature of about 300~, and the second substream of feed gas is mixed therewith and fed to a second methanation reactor, and so forth.
For each 100 parts by weight of water fed to the gasifier, the amount of the waste water which is most highly contaminated with organic compounds, i.e. the aqueous pyrolysis liquor generated from the pyrolysis of the coal, generally is in the range of about 8 to 15 parts (e.g. about ll parts). When a water-containing gas is used for fluidization in the pyrolysis step, ~such as the gas stream 46 from the gasification zone) the amount of waste water from the pyrolysis step, e.g. the aqueous phase from separation zone 41, may be 10'77'~7~

about doubled, e.g. it now amounts to about 15 to 20 parts per 100 parts of water fed to the gasifier. The amount of waste water from the pu~ification of the crude gas (e.g. from purification 44) may be in the range of about 20 to 30 parts; the total amount of waste water from these three steps (pyrolysis, hydrotreating, crude gas purification) is generally below 50 parts such as in the range of about 35 to 45 parts (again per 100 parts of water fed to the gasifier) and the amount of relatively pure water from the methanation step may be relatively large such as about 25 to 35 parts.
The pyrolysis liquor from low temperature carboniza-tion contains a signiflcant proportion of pheno:Lic com-pounds which have a higher content of alkyl substituents and are much less biodegradable than the phenolic mixture ~containing xylenols and cresols) produced by higher temperature carbonization.
In spraying the waste water onto the pebbles it is preferable to have the spray noæzle situated so as to Z0 inject the water into the lower portion of the mass of pebbles, e.g. at the bottom of the second pebble zone 16.
In Fig. 4 the multi-zone recuperative stove may contain brick or ceramic checkerwork instead of pebbles.
Like the pebbles, this checkerwork is a solid heat-transfer material which is substantially inert in the process and which is preheated, before contact with the waste water, to a temperature of above 500C preferably above 600C such as about 800C or higher, e.g. 1000C, or more, the solid heat-transfer material being repeatedly 1077'~71 reheated after contact with the waste water and repeatedly recycled (after reheating) into contact with : additional quantities of the waste water.
In this application all proportions are by weight unless otherwise indicated.

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Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for production of combustible gas in which coal is pyrolyzed at a temperature below 700°C to form aqueous vapors and a char, the char is reacted with steam in a gasification zone, the gasification reaction being effected at a temperature of about 750 to 1100°C and at a pressure from atmospheric up to about 1000 pounds per square inch gauge to produce carbon monoxide and hydrogen, said vapors are condensed to produce an oil and waste water containing ammonia and phenolic compounds, wherein the improvement comprises vaporizing said waste water and feeding substantially all the resulting steam containing ammonia and phenolic compounds to said gasi-fication zone.

2. Process as in claim 1 in which said gasification zone is at a temperature of 850 to 1000°C.

3. Process as in claim 1 in which said gasification zone is at a temperature of 850 to 1000°C, the gas from said gasi-fication zone is cooled to form a stream of impure water, and said impure water stream is blended with said waste water prior to the vaporizing thereof.