Characterization and computer simulation of corn stover / coal blends for co-gasification in a downdraft gasifier

Abstract

The need for sustainable alternative energy technology is becoming more urgent as the demand for clean energy environment increases. For centuries, electricity in South Africa has been derived mostly from coal with results growing in multifold annually due to concerns about the impact of fossil fuel utilization related to emission of greenhouse gasses. It is practically impossible at the moment to replace coal with biomass resources because of the low energy value of biomass. However, the conversion of coal has experienced some challenges especially during its gasification which includes, but are not limited to a high reaction temperature exceeding 900°C which most gasifiers cannot achieve, and if achieved in most cases, combustion of the resulting syngas usually occur, leading to low conversion efficiency and the risk of reaching extremely high temperatures that may result in pressure build up and explosion may also occur. Therefore, this study sought to investigate the possibility of co-gasifying corn stover with coal with the ultimate aim establishing the best mixing ratio that would result in optimum co-gasification efficiency after computer simulation. Proximate and ultimate analysis, including energy values of corn stover and coal as well as their blends were undertaken and results showed significant differences between the two feedstocks and narrow range composition betwee their blends in terms of properties and energy value. Corn stover showed a higher fraction of volatile matter and lower ash content than coal, whereas those of their blends vary considerably in terms of physical properties. Differences in chemical composition also showed higher fraction of hydrogen and oxygen, and less carbon than coal while those of their blends vary according to the ratio of corn stover to coal and vice versa in the blends. The thermal stability of corn stover and coal as well as their blends were also established and the maximum temperature reached for thermal degradation of their blends was 900°C as depicted by TGA analysis. The SEM results revealed no changes in morphology of the pure samples of corn stover and coal which was due to the fact that a pre-treatment of the samples were not undertaken, whereas the blends showed significant changes in morphology as a result of blending. However, luminous and non-luminous features were noticed in both SEM images of the blends with the 10% coal/90% corn stover blend having higher percentages of luminosity as a result of higher quantities of coal in the blend. The energy density of the samples were also measured and found to be 16.1 MJ/kg and 22.8 MJ/kg for corn stover and coal respectively. Those of their blends varied from 16.9 to approximately 23.5 MJ/kg. These results were used to conduct computer simulation of the co-gasification process in order to establish the best blend that would result in maximum co-gasification efficiency. The blend 90% corn stover/10% coal was found to be the most suitable blend for co-gasification resulting in an efficiency of approximately 58% because its conversion was efficiently achieved at a temperature that is intermediate to that of coal and biomass independently. The simulation results were, however, compared with experimental data found in the literature and results showed only slight variation between them.