Caustic oxidation as a pretreatment to high pressure bituminous coal gasification

Illinois No. 6 bituminous coal, which had been pretreated with potassium hydroxide, could not be efficiently gasified at the high pressure of 3.5 MPa in a fluidized-bed gasifier. The same feedstock performed well at a much lower operating pressure. However, at the high pressure the pyrolysed coal swelled significantly. The resulting low bed density produced a small carbon inventory in the gasifier. In addition, the coal char was very friable and large quantities of fine particulate material were entrained by the product gas. To prevent the swelling the coal was initially oxidized. A way was discovered to selectively convert the coal bitumen to humic acid, a substance that is rich in carboxylic acid groups. A direct correlation was found between the humic acid's carboxylate content and the swelling characteristics of the pyrolysed coal. The char bulk density was greatest when the humate carboxylic acid group content was maximized. It appears that when the coal was fed to the hot gasifier, these carboxylic acids decarboxylated and produced radical centres. The crosslinks that resulted from these radicals formed relatively stable carbon-carbon bridges, and this was sufficient to prevent the pyrolytic swelling.