Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to gasifying fossil fuels
and more particularly to gasifying such fuels in the presence
of a catalyst.
In the early sixties of this century, the energy
market was characterized by large amounts of inexpensive
crude oils, but the recent energy crisis has again direc-ted
attention to coal. The statistics regarding world energy
reserves on fossil fuels prove that of the three kinds of
fossil fuels, namely, coal, petroleum and natural gas, the
coal resources are 5 to 10 times higher than those of petro-
leum and natural gas. For various economic systems t the
availability of corresponding individual types of energy is
also of importance. While large petroleum and natural gas
aep~sits exist only in relatively few regions of the world,
coal deposits can be found in almost every country. The di~-
ferences in reserves of fossil fuels make it clear that with
an increasing shortage of petroleum and natural gas, greater
reliance will be made on coal. Methods of liquefying or gas-
ifying coal in order to convert one primary source of energy
into a secondary source of another type will thus aid in re-
lieving the demand for petroleum and natural gas.
The recent, steadily increasing prices of liquid
and gaseous fuels have given a new incentive ~o the develop-
ment of methods for gasifying and liquefying coal. The con-
version of coal into a gas rich in methane is especially
interesting.
Methane as a substitute for natural gas can be pre-
pared from coa]L by one of the following two reactions:
(1) The first method involves three steps: ;
(a) comp]Lete gasification of coal to CO + H2;
. .
;. '-' . '' . :,-, ~ . . : ' '
(b) adjustment of the CO/H2 ratio of 1:3 necessary
for methanization by means of CO-conversion; and
(c) catalytic methani~a-tion.
The gross reactions upon which the individual steps are
based are shown below together with their reaction
en-thalpies:
2 C + 2 H2O ~2CO + 2H2 + 56.6 kcal
CO -~ H20 ~H2 + C2 ~ 10.1 kcal 2
CO + 3H2 ~CH4 + H20 - ~9.2 kcal 3
2 C + 2H20 ~CH4 + CO2 - 2.7 kcal 4
(II) The second method consists of the following steps:
(a) complete gasification of coal or coke to CO + H2;
(b) conv~rsion of CO to H2; and
(c) hydrogenation of fresh coal to CH4 with the aid of
the hydrogen generated.
The gross reactions on which the individual steps are
based are shown below together with their reaction
enthalpies:
C + H20 ~ CO + H2 + 28.3 kcal 5
CO + H20 -~ H2 + C2 ~ 10.1 kcal 6
C + 2H2 ~ CH4 - 20.9 kcal 7
- - :
2C + 2H2O CH4 + CO2 - 2.7 kcal 8
Both methods convert coal into methane as the final
product. Therefore, the sum of reaction enthalpies is equal.
In Method II, the heat of formation of methane by hydrogen
generated at relatively high temperatures during the course
of the gasification is used much more advantageously for the - -
total process than the higher value of Method I which is ~
.
, c 2 -
4~
liberated at relatively lower temperatures during the
methanization step after gasification. Therefore, the
effectiveness of Method II is better in practice. Moreover,
Method II requires only half as much energy from outside
sources as Method I. Considering both methods to be equally
efficiant, this means less exchange area for heat transfer
and, in autothermic processes, Less oxygen consumption,
respectively. It is thus apparent that for the preparation
of synthetic gas, one should produce as much methane as
1~ possible by hydrogenation rather than by synthesis from
CO ~ 3H2.
By conducting the reaction in a suitable manner,
gasification under pressure in a counter current of coal and
g sifying agent or stepwise gasification, the thermodynamic-
ally advantageous formation of methane by hydrogenation may
be utilized in practice.
The main process steps for the conversion of coal
into a gas rich in methane are pretreatment of the coal, gas-~
ification, conversion, methanization and hydrogenation.
The coal is pretreated by separating it from pieces
of rocks and granulating it in a manner suitable or gas-
ification. An important factor is the coking capacity of the
coal which often can be influenced only with difficulty or
to an insufficient degree. Granulation and coking capacity
of the available coal determine whether gasification can be
carried out in a solid bed or in suspension.
Counter-current gasification in a solid bed pro-
vides better heat utilization and more favorable kinetic
conditions. Gasification of suspended coal dust particles in
a fluidized bed working with parallel streams requires more
heat and leads tô less favorable kinetic conditions because
:~V'~'4~ ~
of the decreasing concentration of carbon and gasifying agent.
In the most important known methods such as the
Lurgi pressure gasification, the Hygas process of the Insti-
tute of Gas Technology, the Koppers-Totzek process, etc.,
gasification is carried out at pressures of about 30 bar and
temperatures of 800C. to 1100C.or more. The Lurgi process
is an economically approved method. The Hygas process which
is still under test promises to lower the costs of gas pro-
du~tion. The Koppers-Totzek process and the Hygas process
work with steam gasification in a fluidized bed while accord-
ing to the Lurgi process coal is gasified in a solid bed by
countercurrent.
Mixtures of oxygen and steam are employed for gas-
ification. Lean gases may also be prepared with mixtures of
steam and air. ~y burning part of the coal, the added oxygen
provides the reaction temperature and heat required for the
endothermic reaction between coal and steam. It is also
possi~le to supply the necessary reaction heat from other
sources, nuclear heat, for example.
In the Lurgi process, coal is gasified in a descend-
ing bed by an ascending stream of oxygen and steam. Ashes are
removed by a rotating grid at the bottom of the gasifying
zone. The crude gas leaving the head of the gasifier is washed
with oil to remove tar, heavy oil and entrained solids.
The gas composition is corrected by modifying the ratio of 2
to steam. After CO2 and H2S have been separated, the gas is
methanized to produce a substitute for natural gas.
The Hygas process gasifies coal in two fluidized
beds connected in series. Coke residues from the coal gas-
ification are used to provide the hydrogen needed for further
gasification steps. The crude gas from the reactor is quenche~
to remove oil whereupon it is freed of acidic gases. Then CO
is partially converted and methanized after separation from
the CO2 formed. Because of the increased pressure of 75 bar
in the reactor and temperatures of 650C and 950C to 1000C,
respectively, hydrogenation of the coal plays a noticeable
part in this reaction.
According to the Koppers-Totzek process, coal dust
and oxygen are gasified with the addition of steam in a "reac-
tion flame" in front ofa nozzle. The oxygen serves as a
carrier gas for the coal dust. This process is applicable
to all kinds of coal, irrespective of their coking capacity.
Gasification is carried out at normal pressure. The reaction
flame has a very high temperature so that the C4 content of
the crude gas remains under 0.1% C~4. The entire methane
concentration required if the gas is to be used as a substitute
for natural gas must be formed by methanization.
The crude gases contain varying amounts of carbon
monoxide, hydrogen, methane, carbon dioxide, and unreacted
steam, depending on the gasifying agent, high concentrations
of nitrogen and the process selected. A major part of the !.
sulphur contained in the fuel charge can be found as hydrogen
sulphide and, to a lesser extent, as carbon oxide sulphide
in the final gas and must be removed by washing.
Carbon dioxide is washed out together with the gas-
eous sulphur compounds.
In general, the following reactions are important
for gasification:
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o ~ o
+ ~ ~ ~
O U N O N
U ~ ~ ~ :~
OP~ ~ O ~ ~ ~ O
~:
h O Q~ ~ ~1 ot` ~
,a ,, ~, :r: ~1 ~1 o ~oo ~ n ~D ~J
O
t~ S1) ~ 1~ 1 ~ ~ ~ I CO o
d 1;1 ~) K cn N ~r ~1 ~
I + I I + i I
:
E~ ~ 3 ~i
~ ~ rl rl O ~1 "
O ~ ~ ~ ~1
r~ (d a~
h
S S ~ S S ,~ ~ ~ S
,:
E~ P~ C m ~
.. ..:.. . .
H OC)
C) ~
h ~ h
~ ~ s ~
E~ a~ s ~ ~ h S
:~ ~ O O ~ ~ :
C ~ ~ S O '
O X ~ ~ ~0 0~X.U
r~ Xrl E~
o ~1~1 ~ h ~~D h
s
S
O O O ~ ~ O O ~
h h h la O h ~ O ::
O X ~~:: X
3 ~ u~
:,
O ~ ':
m ~ m ~ o
m u
+ o + + +
+ ~, O
~ ~ 8 o 8 ~ u m~ m~
o o ~ ~ u . ..,, v o
o ....
o ~~ o ~
o ~ ~ m ~ ~ 5
o ~ m ~ ~ m
o ~ } o ~ ~
+ + + ~a + +
o ~ o ~ .........
U U:r U C~ V ~
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The above reaction enthalpies refer to coke carbon
and differ by about 3Kcal/mol from the standard enthalpy of
graphite.
According to Table I J the main reaction of gas-
ification, namely the reaction C ~ H2O -~ CO ~ H~, is strony-
ly endothermic and proceeds at a practical rate only at temp-
eratures above 750C to 800C. The heat required for the
reaction may be supplied in the following ways:
a) Burning part of the solid fuel by means of
oxygen or air (autothermic gasification);
b) Heating the reactor from the outside by heat
transfer through the reactor walls or by means
of special heating elements; and
c) Circulating solid, liquid or gaseous heat
carriers.
The above possibilities have all been tested and
tried in practice. Autothermic gasification with o~ygen and
steam is widely employed. A method of supplying high temp-
erature nuclear heat by means of outer heating elements is
still in the experimental stage. Special attention has also
been given to increasing the throughput rate. While the known
Lurgi pressure gasifier works under pressures of 30 bar and
at temperatures of 700C to 1000C, new efforts in the United
States aim at carrying out the gasification under the pres-
sures re~uired for feeding the gas into a long distance pipe
line, i.e. under 70 bar or moreipressure.
Gasification under higher steam pressures does, how-
ever, generally result in lower reaction rates. ~or this rea-
~; son, the use of higher steam pressures was not previously con-
sidered to be useful.
_ 7 -
.
The present invention provides novel methods where-
in selected compounds are added -to the steam which dissolve in
the steam and catalyze the reaction between coal and steam so
that at elevated steam pressures reaction rates are obtainable
which are even higher than the known rates at low pressures
and the same temperature. Preferably, the amount of catalyst
in th~ steam corresponds to or approaches the saturation point.
The catalysts employed in accordance with the pre-
sent invention are alkali salts which are easily soluble in
high pressure steam. The most preferred are alkali and alka-
line earth metal hydroxides, borates, (including tetraborates),
carbonates and bicarbonates. The reaction can also be accel-
erated with alkali halides, especially chlorides. Carbonates
and hydroxides of potassium are most preferred since they com-
bine good results with favorable economics. For instance, 0.007
grams of potassium hydroxide per 1000 grams of water will dis-
solve in the steam at 825C. and 150 bar. Although the amount
of salt dissolved in the steam under these circumstances is
very small, the throughput is increased by the factor 4 com-
pared to gasification under a steam pressure of 30 bar. Theincrease in reaction rate achieved by adding the catalysts in
accordance with the present invention corresponds to that which
can be obtained by raising the temperature by about 150C to
200C. The alkali compounds, mainly silicates, which are pre-
.. ~ ,.. . .
sent in the coal ash rather inhibit the xeaction.
The foregoing provisions and advantages of the in-
vention may be ac~ieved by recourse to a method of gasifying
fossil fuels by contacting the fuels with steam at an elevated
temperature and pressure in the presence of catalytic amounts
of at least one of, ammonium, alkali and alkaline earth salts
; and hydroxides which are soluble in the steam.
~? 8 -
~7~ 7
Surprisingly, it has been found that the method of
the present invention leads to a change in the composition of
the crude gas since more hydrogen and carbon dioxide and less
carbon monoxide than usual are formed during the reaction. The
carbon monoxide content of the crude gas drops to less than
2 vol. %. Thus, the c~ude gas is especially suited for pre-
paring synthetic gas in accordance with the thermodynamically
more advantageous Method II. Apart from this fact, the method
of the present invention permitsp without loss in gasification
capacity, the use of the high gasification pressures desired
for further utilization of the finished product. Furthermore,
the heat balance of the gasifier is improved since Reaction 7
is preferred over Reaction 3.
According to the present invention, in a gasifier
working ~ith a counter current of coal and gasifying agent,
high steam pressures will prevail in the lower part where the
gasifying agent is introduced and the catalyst will be dissol-
ved in the gaseous phase. As the reaction progresses the as-
cending gas streamwill contain less steam, thereby increasing
the concentration of the salts in the gas stream. Ultimately,
the entrainedcatalysts precipitate and deposit in very finely
divided form on the coal. The coal carries the catalyst back
into zones of high partial steam pressures where the catalyst
again dissolves. The method of the present invention,thus,
results in circulation of the catalyst. The only catalyst
which needs to be replaced is that lost through adsorption on
ash or by entrainment in the form of dust in the crude gas
stream. The loss in catalyst through adsorption on or reaction
with ash can be minimized by adding lime to the coal.
The finely divided catalyst precipitated onto the
F~ 9-
7 4 r~
fresh coal is also effective in catalyzing the reaction be-
tween coal and steam.
The following non-limiting examples are given by
way of illustration only. Unless stated o~herwise, all per-
centages given are by volume.
EXAMPLE 1
Steam saturated with potassium hydroxide is reacted
with ground lignite briquettes in an autoclave under a pressure
of 600 bar and a temperature of 660C. After remov~l of the
steam, 4 liters NTP of a gas per literofIignite charge per
minute are obtained, the gas containing 23% methane, 0.7%
ethane, 34% hydrogen and 42% carbon dioxide. Minor amounts
of higher hydrocarbons are also formed. By way of comparison,
gasification at the same temperature and under a pressùre of
70 bar results, per minute and per liter of lignite charge,
in 2.5 liters NTP of a gas containing 46% of hydrogen, 6% of
carbon monoxide, 11% of methane and 37% of CO2.
EXAMPLE 2
Steam saturated with potassium hydroxide (about
0.6 grams/100 grams of H2O) is reacted with ground lignite
in an autoclave under a pressure of 400 bar at a temperature
of 630C. Per liter of lignite charge and per minute, 2.5 lit-
ers ~TP of a gas are obtained which, after removal of the steam,
contain 20% methane, 3~% hydrogen, 41% carbon dio~ide and
1% ethane. Minor amounts of higher hydrocarbons are also
formed. By way of comparison, gasification at the same temp~
erature and under a pressure of 70 bar leads to 2 liters NTP
of a gas per minute and per liter of lignite charge, the gas
.' ' ,.'
..
- 1 0 - ' ~ '
~)743 ~'~
containing 4% methane, 61% hydrogen, 34% carbon dioxide and
1% carbon monoxide.
EXAMPLE 3
Steam saturated with sodium hydroxide is reacted
with ground pit coal coke in an autoclave under a pressure of
300 bar and a temperature of 750"C. Per liter coke charge and
per minute, 1.9 liters MTP of a gas are obtained which, after
condensation of the steam, contain 1% methane, 65% hydrogen
and 33~ carbon dioxide. Minor amounts of higher hydrocarbons
are also formed. By way of comparison, gasification without
sodium hydroxide at the same temperature and under a pressure
o~ 70 bar results, per minute and per liter o~ coke charge,
in 0.4 liters NTP of a gas containing 3% methane, 59~ hydrogen,
11~ carbon monoxide and 27% carbon dioxide.
EXAMPLE 4
Steam saturated with potassium hydroxide is reacted
with ground pit coal coke in an autoclave under a pressure of
300 bar and a temperature of 770C. Per minute and per liter
coke charge, 6.1 liters NTP of a gas are obtained which, after
condensation of the steam, contain 5~ methane, 60% hydrogen
and 35~ carbon dioxide. Minor amounts of higher hydrocarbons
are also formed. By way of comparison, gasification without
potassium hydroxide at the same temperature and under a pres- -
sure of 300 bar results in 0.5 liters NTP of a gas per minute
and per liter coke charge. The gas contains 4% methane, 58%
hydrogen, 26% carbon dioxide and 12% carbon monoxide.
~ _ 11-
EXAMPLE S
Steam saturated with potassium hydroxide is reacted
with ground pit coal coke in an autoclave under a pressure o~
70 bar and a temperature of 750C. Per liter coke charge and
per minute, 1.6 liters NTP of a gas are obtained which, after
condensation of the steam, contain 1% methane, ~4% hydrogen,
2% carbon monoxide and 32% carbon dioxide. Minor amounts of
higher hydrocarbons are also formed. By way of comparison,
gasification without potassium hydroxide at the same temper-
ature and under a pressure of 70 bar results in 0.4 liters NTPof a gas per minute and per liter coke charge, the gas con-
taining 3% methane, 59% hydrogen, 11% carbon monoxide and 27%
carbon dioxide.
- EXAMP~E 6
Steam saturated with potassium hydroxide is reacted
with ground pit coal coke in an autoclave under a pressure of
150 bar and a temperature of 8~5C. Per minute and per liter
of coke charge, 5.1 liters NTP of a gas are obtained which,
after condensation of the steam, contain 1.5% methane, 64%
hydrogen, 2% carbon monoxide and 32.5% carbon dioxide. Minor
amounts of higher hydrocarbons are also formed. By way of
comparison, gasification without potassium hydroxide at the
same temperature and a pressure of 150 bar results in 1.2
liters NTP of a gas per minute and per liter coke charge, the
gas containing 2% of methane, 61% hydrogen, 11% carbon mon-
oxide and 26% carbon dioxide.
EXAMPLE 7
. . .
Steam saturated with potassium hydroxide is reacted
with ground pit coal coke in an autoclave under a pressure of
300 bar and at a temperature of 825~. Per liter coke charge
and per minute, 10.3 liters NTP of a gas are obtained which,
- - 12 -
.
~)7~
after condensation of the steam, contain 1.8% methane, 64%
hydrogen, 1.55~ carbon monoxide and 32.7% carbon dioxide.
Minor amounts of higher hydrocarbons are also formed. By way
of comparison, gasification without potassium hydroxide at the
same temperature and a pressure of 300 bar results in 1.6
liters NTP of a gas per minute and per liter coke charge, the
gas containing 2.5% methane, 62% hydrogen, 6~ carbon mono-
xide and 29.5% carbon dioxide.
EXAMPLE 8
An ~queous ammonia solution containing 11% by weight
of N~3 is reacted with ground lignite in an autoclave under a
pressure of 600 bar and a temperature of 620~C. Per liter
lignite charge and per minute, 0.85 liters NTP of a gas are
obtained which contain 38% hydrogen, 42% carbon dioxide, 14~
methane and 6% ethane. Minor amounts of higher hydrocarbons
are also formed. By way of comparison, gasification without
ammonia at the same temperature and pressure results in 0.5
liters NTP of a gas per minute and per liter lignite charge,
20 the gas containing 32% hydrogen, 43% carbon dioxide and 25
methane.
EXAMPLE 9
Steam saturated with potassium hydroxide is reacted
with ground pit coal coke in an autoclav,e under a pressure of
300 bar and a tlemperature of 1050C. Per liter coke charge
and per minute, 61 liters NTP of a gas are obtained/ the gas
containing after condensation of the steam 1.8% me~hane, 62%
hydrogen, 4% carbon monoxide and 32% carbon dioxide. By way
30 of comparison, repeating the process a~ the same temperature
.''~' ' .
.
. . .-, ., . .- . : . . ,
~L~7~
and under the same pressure but without potassium hydroxide
results, per minute and per liter of ground coke charge,
in 19 liters NTP of a gas containing 3~ methane, 60~ hydrogen,
4% carbon monoxide and 33% carbon dioxide.
EXAMPLE 10
Steam saturated with lithium hydroxide is reacted
with ground pit coal coke in an autoclave under a pressure of
3~0 bar and at a temperature of 825C. Per liter coke charge
and per minute, 2.1 liters NTP of a gas are obtained which
contain 2~ methane, 63% hydrogen, 31% carbon dioxide, and 4~
carbon monoxide aftex removal of the steam. If the process is
repeated at the same temperature and under the same pressure
but without lithium hydroxide, 1.6 liters NTP of a gas are
obtained per minute and per liter of ground coke charge, the
gas containing 2.5~ methane, 62~ hydrogen, 6% carbon monoxide
and 29.5% carbon dioxide.
.
EXAM2LE 11
Steam saturated with calcium hydroxide is reacted
with ground lignite coke in an autoclave under a pressure of
140 bar and a temperature of 750~C. Per liter coke charge and
per minute, 22 liters NTP of a gas are obtained which contain
42~ of hydrogent 15~ methane, 3% ethane, 34% carbon dioxide
and 6% carbon monoxide after removal of the steam. If the
process is repeated without calcium hydroxide at the same
temperature and under the same pressure, 15 liters NTP of a
gas are obtained per minute and per liter of ground lignite
coke, the gas containing 45~ hydrogenl 13% methane, 2% ethane,
35~ carbon dioxide and 5% carbon monoxide.
"~ '
~ . . ,
~ 14 -
.
1~'74~
EXAMPLE 12
Steam saturated with potassium hydroxide is reacted
with ground plt coal coke in an autoclave under a pressure of
70 bar and a temperature of 890C. Per liter of coke charge
and per minute, 21 liters NTP of a gas are obtained which
contain 2% methane, 64% hydrogen, 3% carbon monoxide and 31%
carbon dioxide after removal of the steam. If the process is
repeated at the same temperature and under the same pressure
but without potassium hydroxide, 6.3 liters NTP o~ a gas are
obtained per minute and per liter of ground pit coal coke, the
gas containing 3~ methane, 61% hydrogen, 6~ carbon monoxide and
30% carbon dioxide.
EXAMæLE 13
Steam saturated with potassium hydroxide is reacted
with ground pit coal coke in an autoclave under a pressure of
150 bar and a temperature of 1200C. Per liter of coke charge
and per minute, 110 liters NI~P of a gas are obtained which con-
tain 2~ methane, 61% hydrogen, 5% car~on monoxide and 32%
carbon dioxide after removal of the steam. If the process is
repeated at the same temperature and under the same pressure
but without potassium hydroxide, 90 liters NTP of a gas are
obtained per minute and per liter of ground pit coal coke, the
gas containing 3% methane, Ç0% hydrogen, 8% car~on monoxide and
29% carbon dioxide.
EX~MPLE 14
Steam saturated with potassium hydroxide is reacted
with ground lignite coke which has been admixed with 1% by
weight of ground limestone in an autoclave at a pressure of
~(~7~
140 bar and at a temperature of 750C. Per liter of coke
charge and per minute, 29 liters NTP of a gas are obtained
which contain 15% methane, 42% hydrogen, 3~% carbon dioxide,
3% ethane and 6% carbon monoxide. If the process is repeated
under the same conditions but without adding ground limestone,
25.5 liters of a gas are obtained per minute and per liter of
ground lignite coke, the ~as containing 11% methane, 45%
hydrogen, 6% carbon monoxide, 0.5% ethane and 38% carbon
dioxide.
EXAMPLE 15
Steam saturated with rubidium carbonate is reacted
with ground pit coal coke in an autoclave under a pressure of
300 bar and at a temperature of 755C. Per liter of coke
charge and per minute, 1.3 liters NTP of a gas are obtained
which contain 1.5% methane, 64% hydrogen, 1.5% carbon monoxide
and 33~ carbon dioxide. If the process is repeated at the
same temperature and under the same pressure but without the
presence of rubidium carbonate, 0.5 liters NTP of a gas are
obtained per minute and per liter of ground pit coal coke, the
gas containing ~% methane, 60% hydrogen, 3~ carbon monoxide
and 33% carbon dioxide.
EXAMPLE 16
Steam saturated with potassium borate is reacted
with ground pit coal coke in an autoclave under a pressure of
300 bar and a tlemperature of 750~C. Per minute and per liter
coke charge, 2.2 liters NTP of a gas are obtained which, after
condensation of the steam, contain 5% methane, 60~ hydrogen
and 35~ carbon dioxide. Minor amounts o~ higher hydrocarbons
- 16 -
.. ~ :. ..
,
. . ~-- . . ..
~L0~7~
are also formed. By way of comparison, gasification without
potassium borate at the same temperature and under a pressure
of 300 bar results in 0.4 liters NTP of a gas per minute and
per liter coke charge which contain 4% methane, 58% hydrogen,
12~ carbon monoxide and 26~ carbon dioxide.
