Background: There are large amounts of manure and crop residues that could be used for for the generation of renewable energy. Anaerobic digestion [AD] converts the easily degraded part of these materials into bio-methane and other gases. Bio-methane can be used for the generation of heat and electricity. Only combinations of manure slurries and crop residues are economic substrates for AD. Crop residues are ligno-cellulosic materials that need to be macerated for efficient conversion into biogas.
Methods: A literature study was made.
Results: Methane yields for different straw lengths and energy requirements for maceration were compiled. The need for sufficient bioavailable micronutrients was established and methane yields for combinations of manure slurries and straw are given. Harvesting methods for crop residues are discussed and a model is developed for the effective organic matter in the effluent of the AD plant.
Conclusion: AD of straw and manures is with the present subsidies economic only where AD is done in cooperatives with the members delivering straw and manure and taking back the effluent.
Large scale of AD of manures and crop residues requires reductions in the costs of harvesting transport and storage of crop residues and a solution to the phosphate surplus in the regions with a large concentration of animals.