Economic and Environmental Analysis of Small-Scale Anaerobic Digestion Plants on Irish Dairy Farms

Abstract

The European Union’s (EU) climate and energy package requires all EU countries to reduce their greenhouse gas (GHG) emissions by 20% by 2020. Based on current trends, Ireland is on track to miss this target with a projected reduction of only 5% to 6%. The agriculture sector has consistently been the single largest contributor to Irish GHG emissions, representing 33% of all emissions in 2017. Small-scale anaerobic digestion (SSAD) holds promise as an attractive technology for the treatment of livestock manure and the organic fraction of municipal wastes, especially in low population communities or standalone waste treatment facilities. This study assesses the viability of SSAD in Ireland, by modelling the technical, economic, and environmental considerations of operating such plants on commercial Irish dairy farms. The study examines the integration of SSAD on dairy farms with various herd sizes ranging from 50 to 250 dairy cows, with co-digestion afforded by grass grown on available land. Results demonstrate feedstock quantities available on-farm to be sufficient to meet the farm’s energy needs with surplus energy exported, representing between 73% and 79% of the total energy generated. All scenarios investigated demonstrate a net CO2 reduction ranging between 2059–173,237 kg CO2-eq. yr−1. The study found SSAD systems to be profitable within the plant’s lifespan on farms with dairy herds sizes of >100 cows (with payback periods of 8–13 years). The simulated introduction of capital subvention grants similar to other EU countries was seen to significantly lower the plant payback periods. The insights generated from this study show SSAD to be an economically sustainable method for the mitigation of GHG emissions in the Irish agriculture sector.

The Role of Anaerobic Digestion in Reducing Dairy Farm Greenhouse Gas Emissions

“…To determine the biogas production potential, a literature review will be conducted to draw together the published knowledge on the topic and predict the yield based on the VS content of the farm slurry and silage. Once complete, the factors will be loaded into the model that assumes the gas produced is used in a CHP engine, for which a typical thermal efficiency of 55% and electrical efficiency of 30% can be expected [32]. Heat consumption of 32 kWh/tonne and electricity demand of 5.4 kWh/tonne [18] are used to calculate the energy requirement of the AD systems.…”

“…Improving the efficiency of CH 4 usage could also bring advantages. As the CHP operates at a combined efficiency of 85% [32] to produce power and heat beyond the farm needs, diverting a percentage of CH 4 for use in tractors, road-going vehicles and other machines to tackle the 40,000 L of diesel consumed annually on the farm and potentially avoid the 110 t CO 2 impact (at 2.76 kg of CO 2 eq per litre [43]) would bring immediate benefit. However, whilst methane-powered tractors are nearing commercialisation, there is an obvious need for a small-scale CH 4 purification plant to make a farm-based system achievable.…”

Optimization of biogas production through anaerobic digestion of municipal solid waste: a case study in the capital area of Reykjavik, Iceland

“…In terms of total kilograms of CO 2 eq per kilogram of feedstock, the single‐stage model values are in the range 0.080 to 0.111; the two‐stage AD model approach scenarios are in the range 0.00034–0.049 kg CO 2 eq kg −1 waste. These results are better than those for dairy cow AD plants that are in the range 0.67 to 2.5 kg CO 2 eq kg −1 waste 50 …”

Development of the Technical Structure of the “Cow Energy” Concept

“…However, it was not possible to cover the entire range of possible technical systems. For the production of solar energy and wind energy [24] as well as biomass energy [25], there are also case studies on dairy cows. These are with predefined framework conditions and are not transferable to the general public.…”