The effect of seasonal biomass availability and energy demand on the operation of an on-farm biomethane plant

Abstract

Globally, 10% of milk production occurs in countries whereby dairy livestock have pasture access for the majority of the year, leading to the seasonal collection and availability of dairy slurry. This can pose challenges if dairy slurry is used as a feedstock in on-farm biogas plants. On-farm biogas plants may also have limited gas grid access and have to facilitate seasonal gas demands of consumers in their locality. The effects of seasonal gas demand coupled with seasonal slurry availability have yet to be addressed in literature. This work aims to analyse the effects of such seasonal factors, and whether optimised digestate recirculation may alleviate some of these challenges. Countries which employ pasture-based dairy farming require such evidence to successfully develop on-farm biogas plants, reduce fugitive emissions resultant from current manure management practices, and furthermore, produce alternatives to fossil fuels. Ireland is used as a case study in this work and the effects of seasonal or constant slurry availability and seasonal or constant gas demand is investigated. The sustainability of such systems is also evaluated in terms of emissions reductions as per EU regulations. For a farm with slurry production from 100 dairy cows and 10 ha of grass dedicated for anaerobic digestion, the seasonal availability of slurry leads to a 21% reduction in total biomethane production; over 12 times the required digestate recirculation; smaller digester sizes; and increased variation in the organic loading rate throughout the year. Facilitating a seasonal gas demand leads to larger digester sizes due to periods of increased mass flow rates, and large peaks and troughs in biogas flow rates. Seasonal slurry availability increases greenhouse gas emissions by ca. 11 g of CO2 per megajoule of biomethane produced. Pasture-based AD systems may successfully manage the seasonal availability of slurry by optimising digestate recirculation to lower the solids content of the plant.