Influence of process conditions on the pyrolysis of Pakistani oil shales

Abstract

Oil shale samples of two geological ages, Infracambrian and Eocene from two regions of Pakistan were pyrolysed in a thermogravimetric analyser (TGA) and a fixed bed reactor to determine the influence of temperature and heating rate on the thermal degradation of the samples. The heating rates investigated in the TGA were, 5°C–40°C min −1 to a final temperature of 950°C. It was found that for the oil shale samples analysed in the TGA, increasing the heating rate shifted the reaction to higher temperatures. The main region of weight loss corresponding to hydrocarbon oil and gas release was between 200°C and 620°C, and at higher temperatures, significant weight loss was attributed to carbonate decomposition. Two of the oil shale samples were also investigated in a fixed bed reactor to determine the influence of heating rate and temperature of pyrolysis on the yield of products and composition of the gases evolved. The pyrolysis reactor was 200 cm 3 volume, constructed of stainless steel and externally electrically heated. The samples were heated to 520°C at heating rates similar to those of the TGA; in addition, the influence of pyrolysis temperature between 400°C and 650°C was also investigated. The maximum oil yield was optimised in terms of heating rate at 20°C min −1 and in terms of temperature at 500°C. Maximum oil yields were between 12.0 and 16.5 wt% for the two oil shales used representing a considerable potential source of liquid hydrocarbons for Pakistan. The main gases produced were H 2, CO, CO 2 and CH 4 and also minor concentrations of alkene and alkane gases. Gaseous yield increased linearly with both increasing heating rate and increased pyrolysis temperature. There was a corresponding decrease in spent shale. Alkene/alkane gas ratios were determined and were linked to secondary reactions, which were discussed in relation to the formation of oil.