Swelling and plastic properties of coal devolatilized at elevated pressures of H 2 and He : Influence of potassium and silicon additives

Abstract

The influences of SiO 2 and potassium additives on the swelling and plastic properties of a low volatile bituminous coal have been characterized at elevated pressures of H 2 and He using a high-pressure microdilatometer. The results suggest that non-porous SiO 2 serves as a ‘diluent’ as it has only a minor influence on the nature of the thermoplastic properties of coal. In marked contrast, K 2CO 3 or KOH significantly reduced the swelling of coal at low pressures (< 1.0 MPa). The effectiveness of K 2CO 3 and KOH as de-caking additives increased with the increase in loading of the additives. In addition, the effectiveness of potassium additives depended on the type of anions present. While K 2CO 3 or KOH served as strong decaking additives, KCl showed little effect. At elevated pressures of H 2 or He (>2.0 MPa), the swelling behaviour of K 2CO 3 or KOH loaded coal was reduced only slightly compared with the behaviour without additives. The influence of potassium additives is a function of coal particle size, heating rate and mode of addition (dry-mixed or solution impregnation). The presence of K 2CO 3 or KOH resulted in increased coke yield (i.e., reduced weight loss of coal during pyrolysis). This increase in coke yield was accompanied by a slight reduction in total light gases monitored (primarily C 2C 4). It is suggested that char-forming crosslinking reactions that can be catalysed by K 2CO 3/KOH facilitate increased thermosetting solid yield. At elevated pressures of H 2, the maximum swelling parameter in the presence of these potassium compounds was further increased compared with that noted in He. This behaviour is explained by suggesting that a hydrogen atmosphere reduces the extent of cross-linking reactions.