Investigation of bioenergy potential of cassava and yam crops wastes in Benin

This thesis aimed to investigate the energy potential of cassava and yam crop wastes produced in Benin. In this context, firstly, the thermal behavior of cassava, yam, coal, and their blends was analyzed by using thermogravimetric analysis (TGA) in a nitrogen gas environment at a heating rate (β) of 10, 20, and 40 °C min-1 and atmospheric pressure. Then, the TGA data was used for kinetic and thermodynamic analysis using FWO and KAS models. Secondly, fast pyrolysis process parameters such as temperature (T) and carrier gas flow rate for cassava, yam, and their blend were optimized to maximize bio-oil yield using Response Surface Methodology with Central Composite Design. The TG and DTG curves showed first mass loss of feedstocks between 200 and 250°C, which indicates moisture content. The second mass loss was between 430 and 530°C, accounting for about 50-60%. Based on TG curves, degradation for cassava, yam, and their blends was approximately between 400-700ºC. Whereas coal stayed stable until ~ 600°C, with gradual mass loss from 600 to 700°C. Cassava exhibited relatively low Ea of 124.28 and 121.20 kJ mol-1 for FWO and KAS models respectively. For the FWO model, the mean Ea of yam was slightly higher at 129.15 kJ mol-1 and the KAS model had a mean Ea of 126.53 kJ mol-1. For coal, both models showed a higher Ea, particularly at high α (0.9), where Ea reached 384.34 kJ mol-1 for FWO and 387.26 kJ mol-1 for KAS, signifying more energy-demanding pyrolysis at later stages. The optimum conditions for fast pyrolysis of biomass were determined as 400oC, with 200 mL min-1 for cassava and 500 mL min-1 for yam. The highest bio-oil yield of 47.83% was obtained in the fast pyrolysis of biomass blend at 460°C and 300 mL min-1. When the compounds of bio-oil samples obtained at carrier gas flow rate of 200, 500, and 300 mL min-1 were analyzed, it was determined that the bio-oil had more balanced hydrocarbon and acid content as a result of the application of fast pyrolysis at 300 mL min-1. In general, using optimized parameter values for cassava and yam appears to be suitable for producing biofuels or valuable chemicals.