Effects of Thermal or Steam Pretreatment on the Hydrogenolysis Reactivities of Coals

Abstract

It has been widely reported that the hydrothermal pretreatment of coal promotes the subsequent conversion of coal. In this study, the effects of dry thermal (DT) or steam (ST) pretreatment of Adaro subbituminous and Huaipei bituminous coals on the chemical changes of oxygen containing functional groups in coals were discussed with the yields of the subsequent hydrogenolysis reactions of coals. As discussed below, it was considered that the effects of pretreatments varied with the treated temperature and the coal rank. During DT pretreatments of Adaro coal below 150°C and Huaipei coal below 200°C, the cleavage of hydrogen bonds did not affect the subsequent hydrogenolysis reactions. In DT pretreatments of Huaipei coal at 235°C and Adaro coal at 200°C and 250°C, the crosslinking formations among the acidic groups (carboxyl and phenolic hydroxyl groups) in coals developed, followed by the suppression of hydrogenolysis reactions of coals. The contents of the acidic groups in lower rank Adaro coal were higher than those of higher rank Huaipei coal. It was indicated that the crosslinking formations of Huaipei coal occurred at around 235°C, while those of Adaro coal started at the lower temperature, around 200°C. However, the crosslinking formations occurred less during ST pretreatments of coals below 250°C because the hydrogen bonds were freshly formed between the acidic groups and water molecules. Therefore, there was no change between the yields of hydrogenolysis reactions of ST pretreated coals and those of the respective raw coals. Accordingly, for both coals, the hydrolyses of ester and ether crosslinks in coal structures also occurred in ST pretreatment at 300°C, which resulted upgrading on the subsequent hydrogenolysis reactions. However, the crosslinking formation occurred in ST pretreatments of both coals at 350°C, followed by the suppression of the upgrading on the hydrogenolysis reactions of the pretreated coals.