International biogas applications
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Full Text E-thesis
Date
2018
Authors
Chan Gutierrez, Enrique
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Publisher
University College Cork
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Abstract
AD plants for the production of biogas have grown steadily in recent years. Animal slurry and sewage sludge have been traditionally used as feedstock for AD, however, substrates such as the organic fraction of municipal solid waste (OFMSW) and energy crops are preferred due to their higher methane yields. Concerns about the sustainability of the use energy crops for biogas production have reduced the use of these substrates worldwide, which has encouraged the research on algae feedstock for AD. In the first part of this thesis, the biogas and economic potential from wastes and energy crops in a Mexican context are analysed. In Mexico AD from food waste has the potential of producing 42 PJ and to reduce 17.9 MtCO2e on a yearly basis. An urban centralised plant for the co-digestion of food waste and sewage sludge for the production of biomethane as a transport fuel can be economically feasible. The co-digestion of food waste and pig slurry appear less profitable due to higher operational costs and reduced gate fees. The implementation of economic instruments can help increasing the economic attractiveness of these plants. In the case of rural digesters, on-site co-digestion of pig slurry and grass can be economically feasible if the biogas produced is used to generate electricity. Centralised biogas plants appear less profitable, requiring higher tariffs to break even. The production of biomethane as a transport fuel is not economically viable in rural plants. In the second part of the thesis the biogas potential of algae in an Irish context is analised through laboratory research. A special reactor was design and build for the co-digestion of Ulva lactuca, dairy slurry and grass silage. Continuous co-digestion of 25% U.lactuca, 5% dairy slurry and 70% grass silage based on volatile solids (VS) yielded 288 LCH4/kgVS, at an organic loading rate (OLR) of 2 kgVS/m3 /d equating to 89% of the biomethane potential test value. When digested at an OLR of 3 kgVS/m3 /d, methane yields decreased due to an accumulation of volatile fatty acids. A biomethane plant in the coastal town of Timoleague, Cork co-digesting beach cast U.lactuca, dairy slurry and grass silage would have a methane production of approximately 1.19 Mm3 /year (42 TJ/year). A two-stage continuous fermentative hydrogen and methane co-production using macro-algae (Laminaria digitata) and micro-algae (Arthrospira platensis) was tested in laboratory trials. The hydraulic retention time (HRT) of the H2 reactor was set at 4 days. The highest hydrogen yield was obtained at an OLR of 6 kgVS/m3 /d. In the second-stage Methane reactor a HRT of 12 days was used, reaching a specific methane yield of 245 L/kgVS at an OLR of 2kgVS/m3 /d. The energy yield was calculated at 9.4 kJ/gVS. Nevertheless, when the OLR was increased a reduction in H2 and methane was observed. A one-stage anaerobic reactor was run as a comparison to the two-stage system at an HRT of 16 days. At an OLR of 1 a methane yield of 204 L/kgVS was achieved. When the OLR was increased to 2 kgVS/m3 /d, the yields decreased due to volatile fatty acid accumulation. The two-stage system offered better performances in both energy return and process stability. The energy potential of this algal mixture may reach 213 GJ/ha/yr.
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Keywords
Biomethane , Biogas , Economic analysis , Waste , Algae
Citation
Chan Gutierrez, E. A. 2018. International biogas applications. PhD Thesis, University College Cork.