System studies of biogas production : comparisons and performance

Abstract: Biogas has the potential to be part of the transition towards a more sustainable energy system. Biogas is a renewable energy source and can play an important role in modern waste management systems. Biogas production can also help recirculate nutrients back to farmland. Besides all this, biogas is a locally produced energy source with the potential to increase global resource efficiency, since it can lead to more value and less waste, as well as decreased negative environmental effects. However, biogas production systems are complex, including different substrates, different applications for biogas and digestate, and different technology solutions for digestion, pre-treatment and for upgrading the raw gas. To increase the development of biogas production systems, knowledge sharing is a key factor. To increase this knowledge sharing, comprehensible analysis and comparisons of biogas production systems are necessary. Thus, studies are needed to verify the resource efficiency of biogas production systems from different perspectives.The aim of this thesis is to perform a systems analysis of biogas production systems and to explore how to analyse and compare biogas production systems. An additional aim is to study biogas production systems from a systems perspective, with a focus on environment, energy and economy. Studying biogas production systems from different system levels, as well as from different approaches, is beneficial because it results in deeper knowledge of biogas systems and greater opportunities to identify synergies.Systems studies of biogas are important, since biogas systems are often complex and integrated with other systems. In this thesis, biogas systems analyses are performed at different levels. In the widest system study, classifications of different biogas plants are analysed and classifications in different European countries are compared, with the prospect of paving the way for a new common classification for biogas plants in Europe. Today, classifications vary between countries, and hence comparisons of plants in different countries are difficult. In the narrowest system study, a new methodology for analysing energy demand at different biogas production plants has been developed. The aim was to develop a methodology that is applicable for all kinds of biogas plants with energy inputs. The methodology describes the process of analysing energy demand and allocating energy to sub-processes and unit processes.Further, an approach for assessing the resource efficiency of different treatment options for organic waste was designed. The approach includes environmental, economic and energy perspectives, and was applied to five different regions with several food manufacturing companies. A study of treatment options for organic waste from a single food company was also conducted. The results showed that biogas production is a resource-efficient way to treat waste from the food industry. The approach enables a wider analysis of biogas systems, and the results from the applications show the complexity of assessing resource efficiency. It is also shown that it is important to understand that the resource efficiency of a system is always in relation to the substituted system.In this thesis, three different approaches to analysing biogas production systems are presented: categorization, resource efficiency analysis and energy demand analysis. These approaches all contribute to the understanding of biogas systems and can help, in different ways, to increase knowledge about biogas systems in the world. If knowledge about different biogas systems can be easily disseminated, more of the unused potential of biogas production may be realized, and hence more fossil fuels can be replaced within the energy system.