PRODUCTION OF HIGH QUALITY LIQUID FUELS FROM COAL BY MILD PYROLYSIS OF COAL-LIME MIXTURES

Abstract

The most recent studies of coal devolatilization processes have emphasized maximisation of liquids yield by use of extreme process conditions such as high heating rate. The liquids produced by such processes are often unsuitable for utilization without extensive upgrading. The aim of the study reported here was to optimize product quality by use of relatively mild conditions. Low-temperature devolatilization (slow heating to 500° to 650°C in a batch reactor) of coal in the presence of CaO (10 to 20 weight percent) can produce a relatively high-quality liquid fuel (containing low sulfur, low oxygen, low viscosity, low Conradson carbon residue, and low pour point). Compared to that occurring in rapid heat-up processes, the residence time of both the liquid and gaseous products is significantly greater in a batch or fixed-bed slow-heating system. It is proposed that this increased residence time facilitates greater secondary reactions of the primary products in the coal bed with the added CaO. The H/C (atomic) ratio of the liquids generated in such a relatively simple devolatilization system can vary between 1.3 and 1.5 for high-volatile coals such as Illinois No. 6 or Pittsburgh No. 8. The presence of CaO in the fuel bed markedly reduces H₂S in the gas phase and decreases sulfur and oxygen heteroatoms in the liquids.