Greenhouse gas and energyassessment of the biogas from co-digestion injected into the natural gas grid: A Swedish case-study including effects on soil properties

In this study, a large, farm-based, co-digestion plant in southern Sweden, using manure and various food industry wastes is investigated concerning its use of energy and its emissions of greenhouse gases from a life cycle perspective based on measured, site-specific data. The biogas is upgraded and utilized as a vehicle fuel, distributed via the natural gas grid. The case-study also includes a novel approach in which potential changes in soil compaction and soil carbon levels are assessed, based on farm-specific conditions, when digestate replaces mineral fertilizer. An additional objective is to identify potential technical improvements leading to further GHG reductions, and the cost of such measures. According to this case-study, biogas produced from food industry waste and manure in a modern co-digestion plant could reduce GHG emissions by approximately 90% compared to conventional fossil fuels. The corresponding energy input:output ratio is calculated to be about 25%, where the use of electricity in the biogas process, upgrading and pressurisation is the dominating energy input. Finally, several possible technical improvements to further reduce GHG emissions were identified. The economic prerequisites of the specific improvements varied, from profitable from a business perspective to unprofitable from a socio-economic point-of-view.