Bioenergy potential of millet chaff via thermogravimetric analysis and combustion process simulation using Aspen Plus

Abstract

Millet chaff constitutes one of the most abundant agro-residues in the sub-Saharan Africa and its utilisation as a feedstock in developing sustainable bioenergy solutions is very sketchy. This study presents the first comprehensive physicochemical and combustion characteristics of millet chaff via thermogravimetric analysis and process simulation using Aspen Plus. The millet chaff sample was collected and assessed as received for proximate and ultimate analyses. The results showed the biomass has 71.25 wt%, 15.35 wt%, 13.40 wt% and 13.15 MJ/kg for volatile matter, fixed-carbon, ash content and higher heating value respectively. The material consists of low nitrogen and sulphur content with potassium, aluminium, magnesium, calcium, iron and sodium as the inorganic components. Kinetic study using distributed activation energy model (DAEM) revealed an average frequency factor and activation energy of 1.41 × 1018(s−1) and 149.39 kJ/mol. Ignition and burnout temperature in the range of 232-244°C and 430-489°C were recorded. The average combustion thermodynamic parameters; ΔH, ΔG and ΔS were found to be 144.75 kJ/mol, 167.12 kJ/mol and -40.08 J/mol. The combustion process analysis coupled with steam turbine cycle via process simulation revealed an excellent combustion efficiency at air-fuel ratio of 5.14. (stoichiometric air). The power generation and electric efficiency of 0.7kWh/kg and 21.07% respectively were recorded at 24% excess air with minimal environmental impacts. This suggests that millet chaff is a good biomass feedstock suitable for clean bioenergy production.

Keywords

• Millet chaff
• Characterisation
• Proxanal
• Ultanal
• Combustion
• Thermogravimetric