IMPROVING BIOGAS PRODUCTION BY ANAEROBIC DIGESTION OF DIFFERENT SUBSTRATES : Calculation of Potential Energy Outcomes

Sammanfattning: Global energy demand is rapidly increasing. In contrast, fossil fuel reserves are decreasing. Today, one of the major challenge is energy supply for the future. Furthermore, effects of global warming cannot be neglected anymore. Alternative energy sources such as biogas should be developed. The biomass has huge biogas potential. However, arable area in the world is limited. Therefore, substrate which will be used for biogas production should be chosen carefully. The objective of this study was to determine the biogas yields of different substrates. For this reason; red algae, green algae, mixture of brown and red algae, mixture of sugar beets and sugar beet leaves, mixture of straw and sugar beet leaves, mixture of maize and sugar beet leaves, straw, maize and ensiled ley were chosen to conduct a lab-based anaerobic digestion experiment. Biogas production and composition in mesophilic (37 OC) conditions during 25 days were measured and compared. The measurements were performed in a system consisting of 32, 1000 ml glass bottles with rubberstoppers. Potential energy production and energy requirements of each substrate were calculated. Methane yields ranged between 65.8 – 578.9 m3. t-1 VS (Volatile Solids). Whilst the highest methane yield was obtained from sugar beets, the lowest methane yields were obtained from the co-digestion of sugar beets and sugar beet leaves. The highest total energy potential for Sweden was obtained from ensiled ley and the lowest energy potential was obtained from maize. Sugar beet leaves were not good co-substrates, when they were digested with sugar beets, since they resulted in a decline in the methane yields. The highest total energy requirements for cropping and digestion were calculated for sugar beets. The lowest total energy requirements for cropping and digestion were calculated for ensiled ley. In the present study, digestion of sugar beets is suggested as good substrates for biogas production in Sweden, since it is more economical and helpful to solve the food&energy challenge. Although algae did not yield as much biogas as crops, they are interesting for biogas production since algae are considered a problem on the beaches and their high growth rates and abundance make them attractive for use in energy production. Due to lack of information, further studies are needed about economical aspects of algae for using in biogas plants.