Air and Air-Steam Gasification of Coconut Shell in a Fluidized Bed

Abstract

Coconut shells’ abundance in tropical countries, along with its high volatile combustible matter (83.51%) and energy content (18.68 MJ kg-1) make it a good biomass resource and a promising feedstock for gasification. In gasification, different mediums such as air, steam, oxygen, or their combinations can be used to react with the solid carbon and heavy hydrocarbons of biomass. Hence, the effects of using an air-steam mixture as a gasifying agent for the bench-scale fluidized bed gasification of coconut shells were studied. The steam-to-carbon ratio (SCR) was varied to evaluate its effect on the resulting syngas quality and gasification performance, while the equivalence ratio was maintained at 0.25 and the bed temperature was kept at 700°C. Results revealed that the optimum SCR is 0.6, where peak values of hydrogen (H2) and methane (CH4) content, hydrogen-to-carbon monoxide ratio (H2/CO), higher heating value (HHV), cold gas efficiency (CGE), and carbon conversion efficiency (CCE) were observed. Furthermore, the use of the air-steam mixture (SCR 0.6) as a gasifying agent was compared with the conventional air gasification. The study also showed a significant increase in syngas quality in terms of H2 content (4.70 - 5.82%), H2/CO (0.30 - 0.39), and CH4 content (3.53 - 4.38%). The syngas heating value and gasification performance revealed a statistically similar improvement: HHV from 4.95 to 5.41 MJ Nm-3, CCE from 77.64 to 81.75%, and CGE from 47.99 to 51.01%. Moreover, air-steam gasification produced less CO2 (13.42 CO2mol kgbiomass-1) and had higher energy recovery (49.67%).