Abstract
The production of biogas from six agricultural crops was analysed regarding energy efficiency and greenhouse gas (GHG) performance for vehicle fuel from a field-to-tank perspective, with focus on critical parameters and on calculation methods. The energy efficiency varied from 35% to 44%, expressed as primary energy input per energy unit vehicle gas produced. The GHG reduction varied from 70% to 120%, compared with fossil liquid fuels, when the GHG credit of the digestate produced was included through system expansion according to the calculation methodology in the ISO 14044 standard of life cycle assessment. Ley crop-based biogas systems led to the highest GHG reduction, due to the significant soil carbon accumulation, followed by maize, wheat, hemp, triticale and sugar beet. Critical parameters are biogenic nitrous oxide emissions from crop cultivation, for which specific emission factors for digestate are missing today, and methane leakage from biogas production. The GHG benefits were reduced and the interrelation between the crops changed, when the GHG calculations were instead based on the methodology stated in the EU Renewable Energy Directive, where crop contribution to soil carbon accumulation is disregarded. All systems could still reach a 60% GHG reduction, due to the improved agricultural management when digestate replaces mineral fertilisers.
Highlights
Methane of fossil origin is increasingly used as vehicle fuel worldwide, a development driven by environmental benefits, such as reduced local air pollution and greenhouse gas (GHG) emissions, compared with petrol and diesel
The present study presents results from the interdisciplinary research collaboration “Crops 4 Biogas”, where the overall aim was to establish and compare land use, energy, GHG and economic efficiency for biogas as vehicle fuel derived from six different crops representing food, feed and energy crops
The energy efficiency performance of the crop-based biogas systems included in this assessment varied from 35% up to 44%, expressed as primary energy input per energy unit of vehicle gas produced
Summary
Methane of fossil origin is increasingly used as vehicle fuel worldwide, a development driven by environmental benefits, such as reduced local air pollution and greenhouse gas (GHG) emissions, compared with petrol and diesel. Using biomass-based methane instead of natural gas as vehicle fuel to replace petrol and diesel will lead to even greater. The increased demand for biogas in the transport sectors has resulted in a need of developing new biogas systems based on other feedstock, such as biogas crops, since the amount of wastes and by-products available as biogas feedstock is limited. Large-scale commercial biogas plants producing vehicle fuels from biogas crops are being built in Sweden. There is a need to increase knowledge about the GHG and energy performance of biogas vehicle fuels based on various agricultural crops. The purpose of this study is to fill this knowledge gap
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