PURPOSE: To attain effective utilization of by-product gypsum and stable flow- down of intra-gasifier oven slag, by mixing by-product gypsum sludge with a high-temp. gas to elevate the temp. of the sludge, thereby realizing a high- temp. slurry form, and feeding the slurry to a high-temp. region at the lower portion of a gasifier oven so as to incorporate it into the slag within the oven.

CONSTITUTION: The following five steps are employed: (1) a step of oxidizing a coal-water slurry 1 in a desulfurization reactor 2, (2) a step of separating the product from the desulfurization reactor 2 in a thickener 5 into a thick desulfurized coal-water slurry 7 and a supernatant of dil. sulfuric acid, (3) a step of feeding the desulfurized coal-water slurry 7 mixed with a gasifying agent 8 into a gasifier oven 12, (4) a step of mixing the remaining dil. sulfuric acid with limestone 4 in a mixing tank 9 under agitation so as to effect neutralization and separating the deposit, by means of a gypsum thickener 10, into a thick gypsum sludge 11 and a supernatant, and (5) a step of mixing the gypsum sludge 11 with a high-temp. gas 20 (discharge gas from the desulfurization reactor 2) to elevate the temp. of the sludge and feeding the sludge to the high temp. region at the lower portion of the gasifier oven 12 so as to incorporate it into the slag within the oven 12.