Energy conversion of biogas from livestock manure to electricity energy using a Stirling engine

Abstract

• Stirling engine performance prediction is evaluated, and specific biogas consumption, biomethane production and number of animals required for a continuous operation are calculated. • When biogas is composed of 60% of methane and engine is heated to 1000 K, 88 pigs or 26 dairy cows are needed to produce 2.1 kW of shaft power or 50 kWh per day. • When biogas is composed of 60% of methane and engine is heated to 1000K, 710 and 1090 liters of daily water feeding rate and 190 and 340 kg of daily biomass feeding rate are required in a biodigester with a capacity of 26 and 40 m 3 when operating with pig and cow waste, respectively. • When the engine hot chamber is at 1000 K the output power is 2089 W. In this condition, the required mass flow is 20.0 m 3 /h for biogas with 80% of CH4 and 47.0 m 3 /h, in case of having low quality biogas (e.g. 40% concentration). • The mathematical modelling proposed in this work allows the extrapolation for other Stirling engines and other operating conditions. The use of biogas can present an interesting alternative for energy production, especially in regions far from conventional centers of power production and distribution, such as estates with agricultural and livestock activity, in addition to representing an environmentally adequate manner of dealing with its bio-waste. In order to prove this potential, the present work evaluates, through simplified mathematical models, a decentralized biogas production system from the livestock tailings biodigestion and its use for power generation using a Stirling engine. For this, the Stirling engine, the burner combustion and the bio-digestion, which work in a dependent way, were modeled. Thus, the biogas consumption, the cattle or swine population size necessary to supply the biomass demand, the amount of water needed in the process and the biodigester volume were estimated. The influence of the methane concentration and air humidity in combustion are evaluated, as well as the convective coefficient of heat transfer between the combustion gases and the engine during energy generation. Through this analysis, it is concluded that when biogas is composed of 60% of methane and engine is heated to 1000 K, 88 pigs or 26 dairy cows are needed to produce 2.089 kW of shaft power or 50 kWh per day, with 535 and 325 liters of daily water feeding rate and 180 and 325 kg of daily biomass feeding rate in a biodigeste with a capacity of 18.7 and 11.3 m 3 when operating with pig and cow waste, respectively.