CA1073211A

CA1073211A - Process for the preparation of synthesis gas

Info

Publication number: CA1073211A
Application number: CA235,741A
Authority: CA
Inventors: Godfried J. Van Den Berg; Pieter J. Halbmeyer
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1974-12-05
Filing date: 1975-09-16
Publication date: 1980-03-11
Also published as: DE2554419A1; PL100733B1; GB1528368A; NL189517B; ES443177A1; DD121797A5; AU8722775A; IN140976B; DE2554419C2; JPS6020436B2; IT1051040B; NL7415842A; FR2293483B1; BE835942A; CS199271B2; JPS5182301A; BR7508018A; NL189517C; FR2293483A1

Abstract

A B S T R A C T

Process for the preparation of synthesis gas by partial combustion of coal during which process tar is formed and separated from the gas. At least part of the tar is gasified by partial combustion, with formation of soot. The soot is separated from the gas and at least part of the soot is partially combusted together with the coal.

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Description

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The lnvention relates to a process for the preparation of synthesis gas by partial combustion of coal during which process tar is formed. This tar is separated from the resultant synthesis gas. At least part of the tar is gasified by partial combustion, with formation of soot. The soot is separated from the resultant gas. At least part of the soot is partially combusted together with the coal.
As feed for the process accorcling to the invention any solid carbonaceous material may be used. By solid carbonaceous material is meant any solid fuel which consists substantially of chemically bound or unbound carbon. In addition, this material may contain hydrogen, oxygen, sulphur, nitrogen, metals and/or ash. A starting material o~ this tyoe comprises, fcr example, lignite, bituminous coal, sub-bituminous coal, arthracite and coke.
Depending on the method followed to gasify the coal it is first rendered into small pieces or oowder form before being converted by partial combustion. The partial combuskion of the coal with formation of tar can be carried out in any desired manner, such as, for example, by means of the Lurgi process or the Synthane process. These processes have been described in "The Oil and Gas Journall' o~ 26th August, 1974, pp. 80 and 84. Preferably the Lurgi process is u~sed for the ~;; first stage of the process according to the invention.~ This process is extensivçly elucidated in "Ullmanns Enzyklopadie der technischen Chemie", Vol~. 10, pp. 418~ll24. During the partial combustion of the coal it is converted with oxygen, air or .: .
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oxygen enriched air into a gas mixture which mainly consists of hydrogen and carbon monoxide. In addition, larger or smaller amounts of nitrogen, C02, H20, CH4, H2S
and COS may occur in the gas mixture, depending on the nature of the coal, the composition of the oxidizirlg gas and the gasification process used. As oxidizing gas use is preferably made of oxygen since this produces a gas of high calorific value.
An amount of 0.2'0.4 Nm3 of oxygen is suitably used per kg of coal. As temperature moderator steam is prefer-ably introduced during the partial combustion of the coal.
An amount of 2-9 kg of steam per Nm3 of oxygen is suitably added to the reaction mixture.
After the coal has been gasified, pre~erably at a temperature in the range from 700 to 1100C, the resultant gas is withdrawn from the gasification reactor. This gas generally has a temperature in the range ~rom 370 to 600C
and in additlon to the desired H2,` C0 and CH4, it contains undesirable impurities such as tar, oil, naphtha~ phenols, water, ammonia, hydrogen sulphide, carbon dioxide and dust of coal and ash. In order to separate the impurities from the desired gas the crude gas is cooled to a temperature between 40 and 200C, preferably by indirect heat exchange ~ with water during which steam is generated. During this process the tar~ the oil, the naphtha~ the phenols and the water condense, while the coal and the ash are taken up in .

the condensing liquid. The condensed liquid is separated : :

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t from the remaining gas. Oil, naphtha, phenols and ash and coal containing tar are recovered from the separated liquid by means of fractional distillation. The gas may be purified in a known manner of hydrogen sulphide, carbon dioxide and any other impurities such as COS, CS2 and HCN.
At least part of the tar is gasified by partial combustion with formation of soot. For this partial com-bustion use may be made of oxygen or air, enriched or not - enriched with oxygen. The tar is preferably gasified with oxygen. This gasification may be carried out in any desired manner. A review of the known processes for the preparation of hydrogen and carbon monoxide containing gas by partial combustion of liquid fuels is given in 'IChemiker Zeitung" o~ 3rd March, 1972, pp. 123-134. The Shell Gasification Process is preferably used for the gasification --of the tar. This process is described in "Chemical Economy and EngineeFing Review", December 1973j Vol. 5~ No. 129 pp. 22-28.
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During the partia,l combustion of the tar steam is ~ preferably supplied as temperature moderator. Suitably 0.5-3 kg of steam~is~ added per kg of tar.
The bxygen or the oxygen-containlng gas is preferably preheated to a temperature between 200 and 500C before it is supplied to the reaction zone in which the partial com-`~25 ~ bustlon of the tar takes place. After~preheating,-the hot oxidizing gas is advantageously mixed with the tar and the gas~tar mixture is preferably atomized under high pressure -: ~ ;
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in a reactor. The excess pressure is advantageously 1-50 atm. higher than that in the reactor. This reactor suitably comprises an empty steel vessel, the interior of which is lined with heat-resistant material. A preferred reactor is described in the U.K. patent specifications 780,120; 832,385; 850,409 and 967,885.
The partial combustion of the tar is preferably carried out at a temperature in the range ~rom 1000 to 1500C, which temperature is the resu]t of the reaction between the tar and oxygen and optionally steam.
The pressure maintained in the reactor may vary within wide limits and is advantageously kept in the range ~rom 1 to 100 atm. abs. In order to convert into gas as much as possible o~ the tar introduced into the reactor, the tar droplets must be present in the reactor for a certain residence time. It has been found that a residence time between 1 and 15 seconds is sufficient to achieve this object.
After the tar has, been converted into gas~ this - ~ :
reaction product~ which consists substantially o~ H2, C0, -C02, N2 and H20, is discharged from the reactor.
This gas has a high temperature, generally a tempera-ture-above 1100C~, and contains;ash, soot and hydrogen sùlphide. Ruring the gasi~ication 0.05-0.20 kg o~ soot is ~ ;
generally formed per kg of tar. To permit the removal of - : ~
the impurities, the gas is ~irst cooled. This cooling is pre~erably effected by injection of water, steam and/or :

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part of the cooled and puri~ied product gas. According to another preferred embodiment of the process according to the invention, the cooling is entirely or partly carried out in a boiler in which steam is generated by means o~ the waste heat.
The gas is preferably cooled to a temperature in the range from 120 to 250C.
The solid matter content of the cooled gas is kept low by choosing the conditions in the reactor appropriately.
Nevertheless, a reduction in the solid matter content is desirable, for example if the gas has to be desulphurized, and also if in the gas carbon monoxide has to be converted with water into carbon dioxide and hydrogen. To this end the gas is preferably passed through a scrubber in which it is washed with a liquida preferably with water. An instal-lation of this type is described in the U.K. patent specification 826,209.
A washing treatment of this t~pe produces a gas which contains substantially no solid matter and which has a temperature between 20 and 80C. The dry gas freed of solid matter~has a composition which is usually between the following 1imits.

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% by volO

CH4 ~ 5 H2S 0.05 - 0~50 N2 ~ ~ 1 - 55 It is preferably further purified by removing H2S and posslbly at least part of the CO2. This purification may be carried out in a known manner.
H2S and CO2 are preferably~removed from the gas by , : means of the ADIP process or the SULFINOL process, which : ~ are described in the U.-K. patent specifications 1,444,963;
:15 ljl31,989; 965,358; 9579260 and 972,140.
: The dry gas~freed of solid matter and H2,S has a : composition between the ~ollowing~limits~
~ ~ :
TAB`~E II
- % by vo : ; ~ 15 - 50 CO :~ ~ ~ 15~ 30;

CH4 ~ ~ 5 H2S~ D.001 -~O.010 25~ N:2 ~ A ~ 55 :
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-As has been described above, -tar is formed as a by-product in the gasification of the coal. This tar generally contains 1-25% of solid matter, viz. ash and unconverted or partially converted coal particles.
Since a high solid-matter content impedes the subsequent gasification of the tar, at least part of the solid matter is preferably removed from at least part of the tar.
This may be effected in any suitable manner by allowing the solid matter to settle from one part of the tar into another part of the tar. Preferably, the solid matter is removed from 25-75% of the tar by allowing the solid matter to settle into 75-25% of the tar. Another advantageously used process for the removal of the solid matter from the tar comprises washing of the tar with hot water, preferably water at 125C.
The part of the tar from which the solid matter has been removed is particularly suitable for gasification by partial combustion. Conse~uently at least part of the said tar is preferably used for this purpose.
The part of the tar from which no solid matter has been removed or which as a result of the above settling process now contains even more solid matter than it originally con- --tained, is preferably recycled to the partial combustion of the coal with or without having been mixed with the coal intended as feed.
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According to a preferred embodlment of the process according to the invention,~25-75% of the tar is directly gasified and 75-25% of the tar is recycled to the pa~tial -:

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_ 9 _ combustion of the coal.
As has been mentioned above, soot is ~ormed during the gasification of tar by partial combustion. This soot is separated from the resultant synthesis gas and at least part of this soot is partially combusted together with the coal to obtain H2- and C0-containing gas. For the sepa-ration of the soot from the gas use is advantageously made of methods which have been described in the U.K. patent specifications 826,209 and 1,267,896.
If part of the tar formed during the partial com-bustion of the coal ls now also recycled to the partial combustion of the coal, it is preferred to admix at least part of the soot with at least part of the tar and the ~esultant tar/soot mixture is partially combusted together with the coal. To incorporate the soot it is advantageous to use that part of the tar which contains solid matter, namely ash and unconverted or at least partially converted coal. It is suitable to use 3-10 parts by weight of tar per part by weight of'soot. ~ ~
The soot may be directly incorporated lnto the tar or the soot may first be taken up in water and subsequently ~ in the tar. In the former case~the crude gas resulting ; from the partial combustion of part of the tar is contacted .
with another part of the tar? so that the soot is incorpo-~25 rated into this latter part of the tar. In this process the crude gas is preferably f1rst cooled to a temperature in the range from 50 to 200C, while the tar which is '. . ' :' . "

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intended for taking up the soot preferably has a temper-ature in the range from 50 to 200C. In the latter case soot-containing water which preferably has a soot content of 0.5-10% by weight and a temperature in the range from 100 to 150C is contacted with stirring with tar which preferably has a temperature in the range from 100 to 150C.
In the former case 0.001 m3 of tar is advantageously used per Nm3 of soot-containing gas.
In the latter case 1 m3 of soot-containing water is suitably contacted with 1 m3 of tar.
Starting from 1000 kg of coal lt is possible by means of the process of the invention and after drying and removal of H2S and CO2 to obtain 1500-3300 Nm3 of a synthesis gas having the following composition:
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% by vol.
H2 ~3 ~ 45 N ~ A ~ ~ 1 45 . -~ The invention will now be further elucidated with ~ ~
: . . . - :, , ~: reference to the~fo11Owine Example. ~ ~

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EXAMPLE
The starting material used was coal having the ~ollowing composition:
C 72.95 % by weight H 4.88 % " "
0 10.78 % " ~' S 1.36 % " "
N 1.56 % " "
ash 8.47 % " "
This coal contained 34.7% by weight Or volatile components and also 2% by weight of water. The coal was gasified by partial combustion together with a quantity o~
solid matter-containing tar from a subsequent stage of the - process. Per kg of water and ash-free coal o.o6 kg of tar was added and 0.356 kg of oxygen and 1.524 kg of steam were used. The oxygen supplied had a temperature of 250C.
The steam temperature was 263C.
The gasification was carried out at a temperature between 700 and 1100C and a pressure of 40 atm.abs.
After cooling of the resultant gas to 125C, tar was removed from the gas.
O.lO kg of tar was recovered per kg of coal.
The tar contained 15% by welght of solld matter.
After drying, the resultant gas had the following - , composition:

' : ' ;3~1 % by vol.

H2 29.3 CO ~5.5 C2 31.7 CH4 22.1 H2S 0.5 ' ' A quantity of 2.08 Nm3 of this gas wae formed per kg of water- and ash-free coal.
1~0 The solid matter was allowed to settle from 50% of the tar into the rest of the tar. The two portions of tar were subsequently separated. The pure tar was converted in a separate reactor by partial combustion with I.057 kg of oxy~en and 2.5 kg of steam per kg of tar into a soot-containing synthesis gas. The temperature was 1100C and ..
the pressure 40 atm.abs.

The crude synthesis gas was cooled to 180C by : . ~ . . . . : : .
indirect heat exchange with waterO The gas was subsequently~
washed with water as a result of whlch the soot was absorbed by the water. 0.15 kg of soot was ~ormed per kg of tar.
: ~ - - : : .
~ The dry synthesis gas which was ~reed o~ soot had the ~ ~ , . . . .
rollowing composition~
% by vol.

H2 ~8.0 CO ~ 25.2

2 ~ 25.6 1.0 :
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A quantity of 2.57 Nm3 of this gas was formed per kg of tar.
The part of the tar in which the solid matter from the other part had settled was intimately mixed by stirring with the soot-containing water at a temperature of 125C, the soot being incorporated into the tar. The soot- -containing tar and purified water were subse~uently sepa-rated from each other. The soot- and solid-matter-containing tar was recycled to the partial combustion of the coal.
The synthesis gas recovered directly from the coal was mixed with the synthesis gas recovered from the purified tar. Ultimately~ 2.228 Nm3 of dry synthesis gas was thus obtained per kg of coal, having the following :
composition:

% by vol.

C0 ~ - 16.1 C2 ~ ~ 31-3 ~20 ~ CH4 20.8 ~ - : - ; .

; N2 ~ A 0.9 ~ ~

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Claims

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

1. A process for the preparation of synthesis gas by partial combustion of coal during which process tar is formed and separated from the gas, characterized in that at least part of the tar is gasified by partial combustion, with formation of soot, the soot is separated from the gas and at least part of the soot is partially combusted together with the coal.

2. A process as claimed in claim 1, characterized in that the tar is gasified by means of oxygen.

3. A process as claimed in claim 1, characterized in that the tar is gasified in the presence of steam.

4. A process as claimed in claim 3, characterized in that 0.5-3 kg of steam is added per kg of tar during the gasification

5. A process as claimed in claim 1, 2 or 4, characterized in that the gasification of the tar is carried out at a tempera-ture in the range from 1000 to 1500°C and at a pressure in the range from 1 to 100 atm. abs.

6. A process as claimed in claim 1, characterized in that the tar contains 1-25% by weight of solid matter, and at least part of the solid matter is removed from at least part of the tar.

7. A process as claimed in claim 6, characterized in that the solid matter is removed from one part of the tar by allowing the solid matter to settle into another part of the tar.

8. A process as claimed in claim 7, characterized in that the solid matter is removed from 25-75% of the tar by allowing the solid matter to settle into 75-25% of the tar.

9. A process as claimed in claim 6, characterized in that the solid matter is removed from at least part of the tar by washing with hot water.

10. A process as claimed in claim 6, 7 or 8, characterized in that at least part of the tar from which the solid matter has been removed is gasified by partial combustion.

11. A process as claimed in claim 6, 7 or 8, characterized in that at least part of the solid-matter-containing tar is recycled to the partial combustion of the coal.

12. A process as claimed in claim 6, 7 or 8, characterized in that 25-75% of the tar is gasified by partial combustion and that 75-25% of the tar is recycled to the partial combustion of the coal.

13. A process as claimed in claim 1, characterized in that at least part of the soot is incorporated into at least part of the tar and that this mixture is partially combusted together with the coal.

14. A process as claimed in claim 13, characterized in that to incorporate the soot solid-matter-containing tar is used.

15. A process as claimed in claim 13 or 14, characterized in that 3-10 parts by weight of tar are used per part by weight of soot.