The influence of high heating rate hydropyrolysis on the primary devolatilisation of coal
The hydropyrolysis of coal is considered to be an attractive future route for the conversion of coal to gaseous and liquid fuels and chemical feedstocks. This thesis comprises a study of the behaviour of coal under the conditions pertinent to those envisaged within a commercial hydropyrolysis process, that is, using pulverised coal, a short coal-reactor residence time, high hydrogen pressure and high heating rate. For this purpose, an electrically heated grid apparatus has been used. The experimental conditions have been weighted towards achieving a maximum loss of primary volatile products whilst minimising unwanted and poorly controlled secondary reactions such as char hydrogasification. In this way, it has been possible to compare the influence of a hydrogen atmosphere on primary devolatilisation for a range of coals. Overall, the total volatile release is enhanced by hydropyrolysis to a degree dependent upon the partial pressure of the hydrogen. The increased yield is due mainly to an increase of saturated hydrocarbons, although the production of some individual species such as carbon monoxide, carbon dioxide and unsaturated hydrocarbons are diminished with respect to helium pyrolysis. The magnitude of both total and individual product yields vary with coal type, as does the degree by which these yields are influenced by a hydrogen atmosphere. This influence is considered to depend specifically on the type and number of various reactive sites and functionalities within coal and also on their accessibility for hydrogen, which is in tum dependent upon the thermoplastic behaviour of the coal. A further factor governing both product yields and hydrogen influence is the coal petrographic composition. Inertinite macerals have been observed to be significantly more responsive than exinite or vitrinite to hydrogen, producing relatively high methane yields. Thus it has been shown that inertinite rich coals or enriched feedstocks may be suitable candidates for hydropyrolysis processing. The data obtained have also demonstrated how a number of coal properties obtained by standard analytical techniques may be used for coal selection and yield prediction. BS volatile matter, carbon content, vitrinite reflectance and both 0/C and H/C atomic ratios are the most useful of the properties investigated in this respect. This is considered to be an important step towards the foundation of a classification system for hydropyrolysis processing.