Life cycle assessment of the supply and use of bioenergy: impact of regional factors on biogas production

Purpose

This article evaluates the parameters that influence the results of a life cycle assessment (LCA) of biogas production from maize and the conversion of biogas into electricity. The environmental impacts of biogas vary according to regional farming procedures and, therefore, the soil, climate conditions, crop yield, and cultivation management. This study focuses on these regional parameters and the existing infrastructure, including the number of installed biogas plants and their share of used heat.

Materials and methods

To assess the regional impact, the LCAs of maize cultivation, on the one hand, and the production and use of biogas, on the other, were performed for three different areas. These areas were the administrative districts of Celle, Hildesheim, and Goettingen; all located in the south of Lower Saxony, Germany. The areas differed in geographic location conditions, crop yield, and the number of installed biogas plants. The necessary data for modeling the cultivation of maize were derived from the specific regional and local parameters of each area. The most important parameters were the soil characteristics and the climate conditions for cultivating maize. The share of used heat from combined heat and power unit (CHP) was another relevant factor for biogas production and use.

Results

Our results demonstrate significant differences among the investigated areas. The smallest environmental impact of all the considered categories occurs in Goettingen and the largest in Celle. The net greenhouse gas emissions vary from 0.179?kg CO₂ eq./kWh_el in Celle to 0.058?kg CO₂ eq./kWh_el in Goettingen. This result is due to the maize cultivation system and the different credits for using heat from the CHP. Variances in energy crop cultivation result from different nitrogen and irrigation demands. In addition, despite higher applications of nitrogen fertilizer and irrigation, the maize yield is lower in Celle. The impact category of total fossil energy shows similar results to that of the greenhouse gas (GHG) emissions. The results range from ?0.274 to 0.175 kWh/kWh_el. The results of acidification and eutrophication vary from 1.62 in Goettingen to 1.94?g SO₂ eq./kWh_el in Celle and respectively 0.330 to 0.397?g PO ₄ ^3? eq./kWh_el. These differences are primarily caused by maize cultivation, especially irrigation.

Conclusions and perspectives

Cultivating maize and using waste heat from the CHP were identified as the most influential parameters for the GHG emissions and total fossil energy demand. Regarding acidification and eutrophication, the most relevant factors are the application of digester output and the emissions from the CHP. Our results show the need to consider regional parameters in the LCA of bioenergies, particularly biogas production and use, especially if the LCA studies are used for generalized evaluations such as statements on the climate protection potential of biogas.