Effects of Impregnation of Coal with Potassium and Sodium Salts

摘要

Much interest has developed over the addition of potassium salts to potentially caking coals prior to their gasification. It is found that such addition markedly reduces the caking tendency of the coals, enabling them to be more easily handled in reactors. An extra bonus is found in that potassium is a good catalyst for both gasification of carbon in the presence of steam and the production of hydrogen via the water-gas shift reaction. In this report, mechanisms of how potassium salts may affect the fluidity of coal upon heating are considered. But first, what is known about the phenomenon of coking in the absence of potassium is reviewed at some length. The addition of potassium salts to caking coals is thought to reduce their fluidity primarily by increasing the number of cross-links which normally exist between the aromatic and hydroaromatic building blocks in the coal. Such an increase results, in turn, in an increase in molecular weight of the coal, decrease in its fluidity upon heat treatment, and the consequent decrease in mobility of planar regimes preventing their alignment to form anisotropic coke substance. It is suggested that the presence of potassium results in a higher oxygen content being present in the coal upon heating, either by reducing the rate of oxygen evolution from the coal as CO, CO sub 2 , and/or water or by acting as an intermediate to extract additional oxygen from the steam added as a reactant to the system (that is, steam gasification). Thus, an increased oxygen content results in more crosslinking in the structure probably via ether linkages between aromatic and/or hydroaromatic regimes. (ERA citation 03:049360)