Note: Descriptions are shown in the official language in which they were submitted.
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.
i!
, 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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~077Z7~
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
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1077Z71
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,
1077Z71
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
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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
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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.
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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.
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~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
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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
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1077Z71
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
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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
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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
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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
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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
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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
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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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:~ 30
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