Biogas production from a systems analytical perspective

Anaerobic digestion and the production of biogas can provide an efficient means of meeting several objectives concerning energy, environmental and waste management policy. Interest in biogas is increasing, and new facilities are being built. There is a wide range of potential raw material, and both the biogas and digestates produced can be used in many different applications. The variation in raw materials and digestion processes contributes to the flexibility of biogas production systems, but at the same time makes their analysis and comparison more complicated. In this thesis, the energy performance in the life cycle of biogas production is assessed, as well as the environmental impact of introducing biogas systems to replace various fuels and existing strategies for the handling of various raw materials. The energy performance and environmental impact vary greatly between the biogas systems studied depending on the raw material digested and the reference system replaced. The results are largely dependent on the methodological assumptions made, for example, concerning focus, system boundaries, and how the energy required in joint operations is allocated to the raw materials digested. Many of the environmental implications depend on how changes resulting from non-energy-related aspects of the implementation of biogas production can be taken into account. For example, changes in emission of methane and ammonia from the handling of the raw material or changes in nitrogen leaching from arable land. There are several potential barriers to the successful implementation of biogas production. The aspect to which most attention was devoted here was the prospect of using digestate from large-scale biogas plants as a fertilizer in agriculture. Reliable and generally accepted disposal of the comparatively large amounts of digestate produced is necessary if biogas production is to be implemented. Agriculture is currently the most common, and sometimes the only suitable means of disposal of the digestate. Serious resistance to or problems associated with this use could therefore jeopardise the development of biogas systems. (Less)