Development of a simulation model for gasification of South African solid waste : waste tyres and agricultural residue

Abstract: Biomass is a potential fuel source that can contribute significantly to South Africa’s energy needs. Direct combustion is still the most used technology to utilize biomass high heating value. However this process suffers from significant inefficiencies and therefore creates the need for alternative processes to be analyzed. The present dissertation describes the study of fluidized gasification of biomass to produce syngas using ASPEN simulation software and the thermal decomposition of agricultural residues using thermogravimetric analysis (TGA). The TGA results confirmed that the thermal decomposition of two biomass types studied occurs in three stages. The first stage is the loss of moisture, occurring between 30oC and 120oC. The second stage is the devolution stage and constitutes the highest mass loss up to 65 wt%. This stage starts at 220oC for bagasse and 200oC for corncobs. The last stage is the thermal decomposition of lignin and the degration of heavier volatiles and the formation of char. This stage starts at 375oC for bagasse and 350oC for corncobs. The process decomposition of bagasse and corncobs shows good resemblance in all stages, therefore there are no kinetic limitations for co-processing of these materials in thermochemical processes such as gasification. These results were in agreement with those published in literature. The simulation software package Aspen plus was used to model the fluidized gasification of corncobs, bagasse and waste tyres. The gasification process was modelled by decomposing the dry feed of biomass into volatile components and ash followed by partial oxidation and gasification of the volatile components assuming the gasification reactions reached chemical equilibrium. The modelled data were compared to experimental results published in literature for processes where sugarcane bagasse and other biomass with comparable ultimate composition similar to bagasse were used as feedstock. The study focused on the effect of air (ER) and steam (SBR) on the composition of syngas and the gasification temperature. The results show that the optimum conditions for gasification of corncobs were ER of 0.34-0.36 and SBR of 0.8-1, these condition corresponded to a gasification temperature of 850-890oC. An ER of 0.33 to 0.38 and SBR of 1-2 were the optimum conditions to operate the gasification of waste tyres. The optimum conditions for bagasse gasification were found to be: ER 0.15-0.2 and the temperature of 800-900oC, SBR 0.9-1. However the simulation data under-predicted the CH4 formation and slightly over-predicted the H2 formation. However the overall results were in fair agreement with experimental data reported in literature.M.Tech. (Chemical Engineering