## 3 Results
### GHG Emission Modeling  
The environmental impact of biogas production is often described by the amount of GHG emissions produced, with methane leakage during AD and storage being the major emission source. The LCA according to DIN EN ISO 14040 [60] and 14044 [61] considers the complete product chain, from raw material to waste removal [62]. This includes, for example, N2O emissions from the agricultural production of energy crops. Emissions of different GHGs are commonly summarized as CO2 equivalent or global warming potential. Calculating the impact of energy production and its resulting impact on the environment, LCA based on biogas energy systems as research topics were deeply discussed in several studies including different aspects, and were reviewed by several authors, for example [22; 23; 24; 25; 26; 27]. Using the LCA database GaBi [63], Reference [15] evaluated a LCA for maize to generate electrical energy considering its regional impact, such as soil, weather and crop management conditions, for the districts of Hildesheim, Gottingen and Celle, Germany. Biomass in this study was calculated according to the regional biomass yield. The electrical and thermal energy production was calculated considering constant efficiencies based on dry and organic matter and methane yield. Net GHG emissions varied from 0.05 kg CO2 eq/MJel in Celle to 0.016 kg CO2 eq/MJel in Gottingen due to different maize cultivation systems and heat utilization from CHPs. The results of acidification and eutrophication show similar results for these regions. The LCA software GaBi was also used in [64] to investigate the influences of liquid manure and energy crops (maize silage and grass silage) as feedstock in four different types of biogas plants. The methane yield was calculated for different mixing ratios of input materials. The energy conversion included different sizes (50, 150, 500 and 2000 kWel) and efficiencies (electrical: 36, 38, 40 and 41%, thermal: 46, 46, 43 and 44%) of CHPs. Liquid manure showed a reduction of GHG emissions for small biogas plants (0.2 kg CO2 eq/MJel) but a low effect for bigger plants (0.08 kg CO2 eq/MJel) when comparing a co-digestion of 35% of manure and 65% energy crops with a mono-digestion of energy crops. An LCA for a demand-driven biogas system using maize, grass, rye silage, sunflowers, biogenic wastes and manure as feedstock was performed in [65]. Two superior scenarios were compared: A flexible demand-oriented and a traditional baseload operation. A substitution of maize with sunflowers, manure and biogenic wastes was obtained in the flexible feedstock operation. The methane yield of the feedstock input was calculated by using dry and organic content ratios with its mass-specific methane yield. The LCA data of the tractor, CHP unit, energy, fuel and fertilizer were taken from the Ecoinvent [66] database and SimaPro [67] as the LCA software environment. Calculating the fermenter volume with an HRT taken from literature led to a mass flow of feedstock into the digester. A flexible feedstock input led to a 16% higher energy consumption of plant operation compared to the baseload energy production. A higher energy consumption of feedstock transport and storage caused this additional demand. However, a global warming potential reduction of up to 45% was achieved with a flexible feedstock load against baseload production due to a substitution of maize with biogenic wastes. Based on an overall mass-specific energy content, Reference [68] used different types of pretreated grass as feedstock and their energy potential to model a process chain of biogas production, including cultivation, harvesting, storage of feedstock, digestion and digestate use. Three different case studies were performed: fresh grass, dried grass (bay) with storage, and ensilaged grass with storage. The studies were done without a specific software solution to obtain LCA results and included all process parameters, such as methane yield, gas leakage and CO2 emissions.