An Agricultural Biogas Plant as a Thermodynamic System: A Study of Efficiency in the Transformation from Primary to Secondary Energy

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Abstract
Using a wide range of organic substrates in the methane fermentation process enables efficient biogas production. Nonetheless, in many cases, the efficiency of electricity generation in biogas plant cogeneration systems is much lower than expected, close to the calorific value of the applied feedstock. This paper analyses the energy conversion efficiency in a 1 MWel agricultural biogas plant fed with corn silage or vegetable waste and pig slurry as a feedstock dilution agent, depending on the season and availability. Biomass conversion studies were carried out for 12 months, during which substrate samples were taken once a month. The total primary energy in the substrates was estimated in laboratory conditions by measuring the released heat (17,760 MWh·year−1), and, in the case of pig slurry, biochemical methane potential (BMP, (201.88 ± 3.21 m3·Mg VS−1). Further, the substrates were analysed in terms of their chemical composition, from protein, sugar and fat content to mineral matter determination, among other things. The results obtained during the study were averaged. Based on such things as the volume of the biogas, the amount of chemical (secondary) energy contained in methane as a product of biomass conversion (10,633 MWh·year−1) was calculated. Considering the results obtained from the analyses, as well as the calculated values of the relevant parameters, the biomass conversion efficiency was determined as the ratio of the chemical energy in methane to the (primary) energy in the substrates, which was 59.87%, as well as the electricity production efficiency, as the ratio of the electricity produced (4913 MWh·year−1) to the primary energy, with a 35% cogeneration system efficiency. The full energy conversion efficiency, related to electricity production, reached a low value of 27.66%. This article provides an insightful, unique analysis of energy conversion in an active biogas plant as an open thermodynamic system.

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