Behaviour of calcium as a steam gasification catalyst

Abstract

The catalytic coal gasification process for synthetic natural gas, which is being developed by the Exxon Research and Engineering Company, uses a potassium salt to catalyse the steam gasification of coal while it simultaneously catalyses methanation reactions in the same vessel. The catalyst is recovered from the gasifier residue and recycled, but some is lost. These relatively small, but inevitable, losses from the recovery process provide an incentive to find a lower-cost catalyst. Exploratory catalyst screening studies have shown that calcium compounds provide good catalytic activity under certain conditions. Essentially, the calcium must be atomically dispersed throughout the char to obtain good activity. Potassium compounds tend to be mobile and distribute themselves during gasification even when applied to the coal in a dry, mechanical mixture. Calcium, in contrast, is poorly active unless it is very well dispersed by chemical reaction with the organic matter. For example, it is an active catalyst when it is transferred to acid sites in coal by ion exchange. Sufficient sites occur naturally in lower-rank coals, and some of these coals have undergone ion-exchange with calcium naturally. Such coals show high reactivities during gasification for this reason. Higher-rank coals have fewer ion-exchangeable acid sites, but these can be created by a mild air-oxidation. This paper describes the experimental techniques used, to develop low-cost, calcium catalysts for the steam gasification of coal and presents the results obtained in this work.