Wobbe Index Determination of Cattle Dung Biogas

Abstract

This paper is aimed at determining the Wobbe Index (W I) of cattle dung biogas which is the amount of energy delivered to a burner through an injector or orifice especially during combustion. The biogas was produced by anaerobic digestion of cattle dung in a batch digester within two weeks. Pressure and temperature of the digester were monitored at 12 noon daily. The analysis of the combustible biogas using thermal conductivity detector (TCD) revealed CH₄ at 0.562, CO₂ 0.395, H₂S 0.018, CO 0.019 and 0.005 moles for NH₃ by volume. Its calorific value (C V) using gas calorimeter Junkers was 19.5 MJ/m³.  The cumulative specific volume of the biogas rose from zero through day 1 to 0.622732 m³/kg on the sixth day before recording daily marginal increments with the rest of the days of incubation. The relative density of the biogas also had a sharp rise on the second day at 5.039208 before dropping to 2.51126 and then gradually decreased daily to the end of digestion. The Wobbe Index equally rose from zero on the first day through to 12.08434 MJ/m³, 14.07783 MJ/m³ and 15.87665 MJ/m³ on days 2, 3, 4 and 5 respectively after which it had logarithmic increments. The volatile solids destroyed also recorded logarithmic rise while the bacterial growth rate followed the zigzag pattern as those of internal energy and enthalpy. It is therefore concluded that parameters like the cumulative specific volume and Wobbe Index increased throughout the incubation period while those that are directly temperature dependent varied with the temperature of the digester.

Keywords: Wobbe Index, calorifc value, cattle dung, biogas, batch digester, specific volume, relative density, internal energy, enthalpy