Alice Delcour

Consequential environmental life cycle assessment of a farm-scale biogas plant.

Producing biogas via anaerobic digestion is a promising technology for meeting European and regional goals on energy production from renewable sources. It offers interesting opportunities for the agricultural sector, allowing waste and by-products to be converted into bioenergy and bio-based materials. A consequential life cycle assessment (cLCA) was conducted to examine the consequences of the installation of a farm-scale biogas plant, taking account of assumptions about processes displaced by biogas plant co-products (power, heat and digestate) and the uses of the biogas plant feedstock prior to plant installation. Inventory data were collected on an existing farm-scale biogas plant. The plant inputs are maize cultivated for energy, solid cattle manure and various by-products from surrounding agro-food industries. Based on hypotheses about displaced electricity production (oil or gas) and the initial uses of the plant feedstock (animal feed, compost or incineration), six scenarios were analyzed and compared. Digested feedstock previously used in animal feed was replaced with other feed ingredients in equivalent feed diets, designed to take account of various nutritional parameters for bovine feeding. The displaced production of mineral fertilizers and field emissions due to the use of digestate as organic fertilizer was balanced against the avoided use of manure and compost. For all of the envisaged scenarios, the installation of the biogas plant led to reduced impacts on water depletion and aquatic ecotoxicity (thanks mainly to the displaced mineral fertilizer production). However, with the additional animal feed ingredients required to replace digested feedstock in the bovine diets, extra agricultural land was needed in all scenarios. Field emissions from the digestate used as organic fertilizer also had a significant impact on acidification and eutrophication. The choice of displaced marginal technologies has a huge influence on the results, as have the assumptions about the previous uses of the biogas plant inputs. The main finding emerging from this study was that the biogas plant should not use feedstock that is intended for animal feed because their replacement in animal diets involves additional impacts mostly in terms of extra agricultural land. cLCA appears to be a useful instrument for giving decision-makers information on the consequences of introducing new multifunctional systems such as farm-scale biogas plants, provided that the study uses specific local data and identifies displaced reference systems on a case-by-case basis.

Sensitive parameters in local agricultural life cycle assessments: the illustrative case of cereal production in Wallonia, Belgium

PurposeLife cycle assessment (LCA) is a useful tool for investigating the environmental performance of agricultural products. For many crop-based products, the agricultural production step shows substantial impacts in LCA results. Using the illustrative case of cereal production in Wallonia, Belgium, the study uses sensitivity analyses to explore the parameters to be adjusted in priority when conducting a local LCA for crop production, taking into account uncertainties tied to input and output inventory data and impact characterization factors.MethodsThe analysis investigated local specificities in the production of eight cereal crops in Wallonia. Cropping systems were modeled according to accounting data from representative farms, including areas, yields, inputs, and machinery. Sensitivity and uncertainty analyses for input parameters and methodological choices were performed. The following parameters were evaluated: the influence of the functional unit (kg, ha, €), the link between yield and LCIA results, the relevance of using up-to-date data for mineral fertilizer production, the type of mineral fertilizer, the model for direct field emission calculation, and the allocation rules.Results and discussionWe observed that the use of several functional units embracing the multifunctional role of agriculture enables identifying crops with low impacts per kilogram, hectare, and euro at the same time. The cereal production steps with the greatest impact were mineral fertilizer production (up to 84% of the total impact, depending on crop and impact category) and emissions from the application of mineral (up to 65%) and organic (up to 94%) fertilizers. Sensitive parameters to be adjusted in priority were identified. Despite the high correlation between yield and results, simply extrapolating impacts on the basis of yield seemed mostly inappropriate. Using updated data for mineral fertilizer production reduced impacts by 6–160%. Shifting models for field emission calculation influenced acidification and eutrophication by 5–142%. Hypotheses on allocation factors affected results by 10–26%.Conclusions and recommendationsAn LCA for the production of major Walloon cereals was conducted taking into account local cropping practices and yields. The parameters relevant for conducting a regional LCA of crop production were identified and discussed in comparison with generic data and models. As LCA is an iterative process, further improvements would include the use of more specific models for direct field emission calculation. The results could serve as a reference for products using Walloon cereals and help enhance the quality of LCAs conducted for these products.