Technical and environmental assessment of energy and material production from rural and urban residual biomass

This thesis covers studies carried out to investigate the technical and environmentalaspects of utilising residual biomasses. The utilisation pathways include energy generation and activated carbon production. The role played by integrated generation of solid fuel andbiogas from biomass (IFBB) in influencing the technical and environmental aspects of utilising residual biomasses was the focus of the respective studies. Residual biomasses from less intensively managed grassland sites and green spaces are harvested for nature conservation and landscape management. These biomasses are generated across Europe and the utilisation pathways are limited to composting or disposal in most cases. Utilising residual biomass to fulfil energy and material demand would not only provide an effective utilisation pathway, but also substitute fossil-based resources presently used to fulfil the demand. Therefore, utilising residual biomasses could be crucial in achieving targets set in the EU renewable energy directive and Bioeconomy strategy. IFBB converts biomass into a useful solid fuel and an energy-rich press fluid through mashing and mechanical separation. After mechanical separation, a solid fuel that has a lower concentration of harmful elements and a liquid fuel that can be co-digested in biogas plants are obtained. The effectiveness of IFBB has been proven on a laboratory and prototype scale, thus the first study in this thesis aimed to investigate the properties of the fuel obtained using a commercial scale IFBB process. The reduction of N, S, Cl, K and ash was higher using commercial scale IFBB, thereby resulting in a better solid fuel compared to prototype scale. As a result of the reduced concentration of the harmful elements, emissions of NOx and SOx were well below the German threshold limits. Extensive grasslands affected by species invasion are also a potential residual biomass that can be found in mountainous areas across Europe. These need to be harvested for maintaining biodiversity and ecosystem services, and the residual biomass thus obtained can be used for energy generation. IFBB and anaerobic digestion (AD) are two potential techniques to convert the residual biomass into fuel. Therefore, the environmental and primary energy balances of using these techniques were investigated and compared in the second study. Though greenhouse gas (GHG) emissions and primary energy usage was higher for IFBB, the better energy conversion efficiency for IFBB resulted in higher GHG and primary energy savings compared to AD. The savings for GHG and primary energy could be generated from replacing grid based sources of heat and electricity sources using the respective fuels obtained from IFBB and AD. Additionally, presscake drying was found to be the most GHG and primary energy intensive process in the IFBB system. However, the GHG emissions associated with presscake could be lowered using alternative energy sources. Activated carbon (AC) treatment is used in wastewater treatment plants for removal of organic micropollutants (OMPs) from wastewater. But AC used presently is mostly derived from fossil-based sources and needs to be transported across long distances. Residual biomasses that were delivered to the wastewater treatment plants were used as a raw material to produce AC. The third study investigated and compared the environmental and cumulative energy demand balances involved in the usage and production of AC from residual biomass-based and fossil-based sources for micropollutant removal at the Baden-Baden WWTP. GHG and non-renewable cumulative energy demand balances achieved using residual biomass AC were lower compared to the fossil AC. Utilisation of waste heat from pyrolysis and activation process of the biomass as well as the electricity produced from the CHP using press fluid methane played a key role in in achieving the lower balances. A lower social risk was an additional advantage of using residual biomass AC produced at the Baden-Baden WWTP. Hence, producing AC from residual biomass using IFBB and a state-of-the-art pyrolysis and activation unit combined with energy recovery would aid WWTPs to become self-sufficient in terms of raw materials and provide a sustainable alternative for micropollutant removal at WWTPs.