Abstract
Hydrothermal carbonisation (HTC) has been identified as a potential route for digestate enhancement producing a solid hydrochar and a process water rich in organic carbon. This study compares the treatment of four dissimilar digestates from anaerobic digestion (AD) of agricultural residue (AGR); sewage sludge (SS); residual municipal solid waste (MSW), and vegetable, garden, and fruit waste (VGF). HTC experiments were performed at 150, 200 and 250 °C for 1 h using 10%, 20%, and 30% solid loadings of a fixed water mass. The effect of temperature and solid loading to the properties of biocoal and biochemical methane potential (BMP) of process waters are investigated. Results show that the behaviour of digestate during HTC is feedstock dependent and the hydrochar produced is a poor-quality solid fuel. The AGR digestate produced the greatest higher heating value (HHV) of 24 MJ/kg, however its biocoal properties are poor due to slagging and fouling propensities. The SS digestate process water produced the highest amount of biogas at 200 °C and 30% solid loading. This study concludes that solely treating digestate via HTC enhances biogas production and that hydrochar be investigated for its use as a soil amender.
Highlights
The key to realising the potential of biomass resource lies in the integration of processes, and maximising outputs to create the sustainable production of bioenergy and bio-products
The increase in plants is largely due to financial incentives that were available for anaerobic digestion (AD) operators to encourage renewable electricity, heat, and transport fuel generation through the feed-in tariff (FIT), renewable heat incentive (RHI), and renewables transport fuel obligation (RTFO)
The results show that the effect of solid loading on solubilisation is feedstock dependent
Summary
The key to realising the potential of biomass resource lies in the integration of processes, and maximising outputs to create the sustainable production of bioenergy and bio-products. The biorefinery concept integrates two or more biomass conversion processes to provide increased system efficiency, greater yields of products, and multiple energy vectors. Integration of biological and thermochemical biomass conversion processes (i.e., a hybrid energy system), can provide improved material management, the production of different bioenergy vectors, and value-added products. The increase in plants is largely due to financial incentives that were available for AD operators to encourage renewable electricity, heat, and transport fuel generation through the feed-in tariff (FIT), renewable heat incentive (RHI), and renewables transport fuel obligation (RTFO). A number of barriers are emerging with the expansion of this technology. These include operational and policy facets such as shrinking gate fees for raw materials, increased costs for digestate recycling, uncertainty about the long-term sustainability of recycling digestate to land and availability of financial incentives
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