Biogas from agricultural waste : Turning unavailable residues into accessible resources
Sammanfattning: Moving from a fossil dependent to a fossil free economy requires increased energy production from renewable resources. This thesis discusses the utilization of agricultural waste streams, such as straw and manure, for biogas production. The first part of the research presented focuses on pretreatment of straw with the aim to reduce the handling issues concerning straw and improve the degradation of the material during anaerobic digestion. The second part concerns process design of agro-based biogas production plants with the aim to find process configurations and feedstocks that lead to high bioenergy yields. The effect of co-digesting a manure-rich stream with a carbohydrate-rich stream and the role of the degree of carbohydrate accessibility on the methane production is also discussed.Wheat straw is a problematic material to digest due to its high porosity which causes it to float and makes it hard to pump/feed. Mechanical pretreatments that applied higher shearing to the straw, such as pelletization and extrusion, led to reduced floating layers. Particle size reduction of wheat straw impacted the methane production rate below 3 mm but did not have an impact on the methane yield. The particle size was, however, not the only factor affecting the methane production rate. Hammer milled straw and extruded straw had a similar particle size but the degradation of extruded straw was faster. To increase the methane production rate, a shearing effect of the pretreatment may be more important. To solve only the handling issues of the straw, it may not be worth the high energy demand of those pretreatments.Wheat straw cannot be digested without the addition of nutrients. Co-digestion with manure or animal bedding is thereby a promising solution. By washing the animal bedding, it was possible to separate out the fibers and subject them to pretreatment with similar yields as pretreatment of wheat straw. Such a process design opened up for parallel production of biogas from the manure-fraction and fiber hydrolysate, and bioethanol from the steam pretreated fibers. Co-digestion of manure and readily available hydrolysate led to an increased initial lag phase and additional studies presented in this thesis showed that a too high degree of carbohydrate accessibility will increase the risk of process instability due to volatile fatty acids accumulation. Further, because part of the carbohydrates was diverted for yeast fermentation, the C/N ratio in the anaerobic digestion step became low. To solve this, cow manure and additional wheat straw were added to the production process. Energy balances and estimated energy demands over the process, in comparisons with other designs, showed that biofuel production was more energy efficient without co-production of ethanol. However, because of the recovery of lignin, there is a great potential of covering most of the energy demand by on-site steam production. Like so, the energy efficiency would much improve.
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