Process for the gasification of coal

Continuous process for the gasification of particulate coal with steam wherein SO.sub.2 is present, preferably through introduction into the steam, thereby enabling the steam to react with the coal at considerably lower than conventional temperatures such as temperatures ranging upwards of 1200.degree. F, preferably 1400.degree. F.

BACKGROUND OF THE INVENTION 
Heretofore, coal has been gasified with stream to make methane, or fuel for 
power plants, and, in general, the uses of coal gas have paralleled those 
of natural gas. However, all previous conventional methods have usually 
generated coal gas by contacting coal with steam or hydrogen at high 
temperatures and pressures in accordance with the following equations: 
EQU C(amorph.) + H.sub.2 O(g) CO + H.sub.2 + some CH.sub.4 
thereafter, CO is reacted with H.sub.2 as follows: 
EQU CO + 3H.sub.2 CH.sub.4 + H.sub.2 O 
it has not been possible heretofore to achieve satisfactory rates of 
reaction when reacting coal with steam at relatively low temperatures, 
such that coal gasification would be practical at such low temperatures. A 
significant advantage, however, afforded by the present invention resides 
in its ability to carry out this reaction at much lower temperatures than 
had been previously thought possible. Thus, in accordance with the present 
invention, sulfur dioxide, which is normally considered a pollutant, can 
be used to promote reaction of steam and coal at significantly lower 
temperatures. 
In the past, gasification of coal has been carried out by the reaction of 
steam on incandescent coke or coal at temperatures around 1000.degree. C. 
and higher where the reaction rate and equilibrium have been favorable. By 
using a higher molar volume ratio of steam relative to coal of 2:1, it has 
been possible to conduct this reaction at temperatures several hundred 
degrees lower. 
Since the reaction of coal with steam is an endothermic one, which tends to 
cool off the coal or coke fed into the gasifier, it has been known to use 
calcium carbonate as a source of CO.sub.2 in the gasification process 
since such use accomplishes the following results: (1) it restores heat to 
the gasifier in view of the fact that the reaction of coal with CO.sub.2 
is exothermic; and (2) it generates CO.sub.2 which is able to react with 
the coal to provide carbon monoxide, itself an intermediate in the 
preparation of methane. 
However, it has not been previously possible, under conditions that would 
justify commercial operation, to conduct coal gasification at temperatures 
as low as those just upwards of 1200.degree. F., preferably temperatures 
approximating 1400.degree. F., at atmospheric pressure. By means of the 
present invention, however, low temperature gasification of coal with 
steam is now made possible. 
SUMMARY OF THE INVENTION 
In accordance with illustrative embodiments demonstrating features and 
advantages of the present invention, there is provided a process for the 
gasification of coal which comprises contacting a particulate form of coal 
with steam in the presence of sulfur dioxide. In this manner, reaction of 
the coal and steam can be effected at much lower temperatures than 
heretofore had been thought possible, temperatures just upwards of 
1200.degree. F., between 1300.degree. F. and 1450.degree. F., and 
preferably a temperature of about 1400.degree. F. The present invention 
thereby enables the conversion of coal to a combustible gas, preferably 
with high B.T.U. content, which can be used as a fuel for power plants, 
pipeline gas, etc. This process also provides the operator with the option 
of either conducting the gasification of coal at low temperatures, or of 
increasing its output capacity or downgrading the size and capacity of his 
equipment at the same throughput levels.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now specifically to the drawing, there is schematically shown a 
process for the gasification of coal. The coal is introduced into a coal 
gasifier and the source of coal is a conventional one and can include any 
of the usual types of carbonaceous materials subjected to gasification 
such as lignite, sub-bituminous coal, bituminous coal, super-bituminous 
coal or coke. 
Preferably, the coal is in particulate form in a pulverized or crushed 
state. However, it should be noted that particles that are too fine are 
not preferred since expensive grinding or milling equipment is required to 
produce them, and simple crushed bituminous coal is preferable. The 
particulate or granular coal feed which is used, is introduced 
continuously into the gasifier, and a suitable source of superheated 
steam, such as that generated from a boiler or reboiler, is fed therein. 
Preferably prior to introduction of the steam into the coal gasifier, an 
effective amount of sulfur dioxide, is added to the steam, preferably 
between 0.1 and 20% vol/vol [SO.sub.2 /H.sub.2 O(g)], and the steam-sulfur 
dioxide admixture is fed into the gasifier. 
The reaction is effected at temperatures upwards of 1200.degree. F. such as 
between 1300.degree. F. and 1450.degree. F., preferably at a temperature 
approximating 1400.degree. F. Temperatures of upwards of 1200.degree. F. 
are necessary in order to enable the gasification process to proceed at a 
satisfactory rate. In general, while theoretically there is no upper limit 
as to the temperature for gasification, short of pyrolysis, the higher the 
temperature, the less economical coal gasification processes become. 
Accordingly, it has been found suitable to operate at temperatures between 
1300.degree. and 1450.degree. F. for bituminous coal, preferably at a 
temperature approximating 1400.degree. F. Suitable pressures approximate 
atmospheric pressure and the process can also operate at higher pressures, 
such as to 1000 psi. Suitable amounts of sulfur dioxide include, as 
indicated, 0.1-20% vol/vol (SO.sub.2 /steam), preferably 8-10%. 
The gas residence time approximates 6 seconds for the following conditions: 
1200.degree. F, atmospheric pressure, and 8% SO.sub.2. However, at a 
temperature of 1400.degree. F., 1 atm., and 8% SO.sub.2, the gas residence 
time is 5 seconds. 
The SO.sub.2 necessary for gasification process is produced in the 
regeneration portion of the H.sub.2 S removal system. 
Upon completion of the gasification, the solid residues, i.e., coal ash and 
non-volatile materials such as tar are removed from the coal gasifier and 
the effluent gases comprising carbon dioxide, hydrogen, water vapor, 
carbon dioxide, methane, and hydrogen sulfide are subjected to 
conventional means for removal of hydrogen sulfide, such as wet or dry 
scrubbing. 
The process stream recycled from the hydrogen sulfide removal step contains 
sulfur. This sulfur containing effluent, in accordance with the present 
invention, is converted to sulfur dioxide. Thus, the effluent of the 
hydrogen sulfide removal step is passed into a sulfur dioxide generator 
and the sulfur in such effluent is thereby converted to sulfur dioxide. 
All the other materials that were present in the effluent gases except for 
hydrogen sulfide are taken off in a product stream, which can be used as 
fuel for power plants or processed into pipeline gases. In accordance with 
the foregoing, a quantity of sulfur dioxide, equivalent to the sulfur 
content of the coal initially fed into the coal gasifier, is removed from 
the system and the remaining SO.sub.2 in the generator is then recycled 
back into the coal gasifier via introduction into the steam input line 
thereto. 
A latitude of modification, change and substitution is intended in the 
foregoing disclosure, and in some instances, some features of the 
invention will be employed without corresponding use of other features. 
Accordingly, it is appropriate that the appended claims be construed 
broadly and in a manner consistent with the spirit and scope of the 
invention herein.