The presented discussion paper contributes to an assessment of the option "bio-methane for injection into the natural gas grid" by taking a close look to the greenhouse gas (GHG) balance of the process chain as well as at the environmental aspects of cultivation biogas substrates. Through the ongoing research of the authors the results will embedded into be broader context of further assessment of the possible role of bio-methane in a sustainable energy system. The knowledge of the climate and environment impacts of bio-methane is an important element for an integrated assessment. Biogas is produced through fermentation of various wet biomass. After upgrading to the quality of natural gas (mostly by desulphurisation, drying and CO2-sequestration) the subsequent so-called bio-methane can be used as a perfect substitute for natural gas.
In the presented paper, five different locations throughout Germany were selected to analyse regionally adjusted crop rotation systems and whether it was possible to cultivate biogas substrates in addition to conventional crops. The substrates evaluated in this paper - aside from maize - are rye, sorghum, whole-crop-silage from triticale and barley, and the innovative options of agricultural grass (Landsberger Gemenge),as well as a combination of maize and sunflower. At the five locations, it was looked at the effects on the balance of humus, the coverage of soil (protection against erosion) and biodiversity. The paper arrives at the conclusion that, while applying good agricultural praxis, no negative effects to the environment are to be anticipated by the cultivation of biogas substrates.
For the calculation of GHG emissions, two different types of biogas plants have been in the focus of the study: (1) the current state-of-the-technology as an industrialised, but average efficient biogas plant in the year 2008 and (2) a new, large-scale plant with optimised technology, representing already the next generation of biogas plants by widely exploiting the optimisation potential of the near future. Compared to the former plant, GHG emissions can be decreased by about 30%, from 97 g CO2eq/kWh to 67 g CO2eq/kWh, if optimised technology is applied. Small-scale agricultural and older biogas plants have not been in the focus of the presented work; their emission factor can by all means significantly vary from the here given results.
For the medium term up to 2030, taken into consideration the already achieved progress changing from the current state-of-the-technology to an optimised technology, it can be estimated that the GHG balance of a large-scale industrialised plant amounts to about 53 g CO2eq/kWh. The optimisation potential originates mostly from an integrated, comprehensive process management.
Armin Vetter, Karin Arnold:
Klima- und Umwelteffekte von Biomethan
Anlagentechnik und Substratauswahl
Wuppertal Paper no. 182 (February 2010)