Steam gasification kinetics of biochar at elevated pressures

Abstract

Coal remains a valuable resource in the generation of energy and transportation fuels, however the world's coal resources are depleting and its impact on the environment and human health raises concern. The investigation into alternative resources, such as biomass, have become more relevant, and one of the short term promising processes for biomass utilization is co-gasification with coal. In this study the steam gasification kinetics of a locally available biomass derived char at elevated pressures is investigated to further understand the possible application of biomass as gasification resource in the industry. A waste product biomass (eucalyptus nitens) sample was received and charred to produce the biochar sample. A comparison between the raw biomass and biochar was completed with the use of several characterisation techniques. A mass loss of 83.4% was obtained during char preparation, while an O/C ratio of 0.66 and 0.03 and H/C ratio of 1.42 and 0.03 was obtained for biomass and biochar respectively. For the gasification experiments, the biochar sample was prepared to a size of -150+75 µm. The char reactivity experiments were carried out in a laboratory-scale high pressure fixed bed reactor in the temperature range of 700 to 730˚C to ensure experiments are completed in the chemical reaction controlled regime. The steam partial pressure was varied from 1 to 10 bar steam, and the CO and CO2 products were measured and used to determine the specific reaction rate of biochar. From the obtained results it was found that the specific reaction rate increased with an increase in conversion, temperature and steam partial pressure. The increase in steam partial pressure had a significant effect on the reaction rate up to 10 bar steam, where it was observed that the formation of CO2 contributed more to the specific reaction rate than that of CO, and that the selectivity of CO2 increased over the entire steam partial pressure range. The effect of temperature on product selectivity was found to be less significant than that of steam partial pressure. From the comparison between biochar and coal char it was observed that the biochar had a higher reactivity than that of a typical Highveld derived coal char. Both the LH and PL models were able to accurately predict the specific reaction rate at 20% conversion, with the average error estimated at 4.1% and 3.9% respectively. The activation energy was also determined by the use of these kinetic models and the results were found to be consistent with literature. In general, progress has been made in the study of biochar as a viable option for a co-source in steam gasification, however more research is required on the processing of this alternative source in existing commercial gasifiers.