Anaerobic Digester Calculator
The anaerobic digestion calculator has been designed to carry out full energy and financial balances for a proposed plant. It is primarily designed to analyse single and two stage mesophilic plants, but could also analyse a thermophilic plant with some minor work. The inputs to the calculator include the demographics of the area surrounding the plant, parameters defining the plant design, and economic considerations. The calculator sizes the waste streams appropriately for the local populations, while taking into account their geographic positioning for transport calculations. The plant is sized appropriate to the waste stream magnitudes; the plant size affecting the digester sizes (and hence heat losses), capital cost, biogas engine size, and also the electrical process loads due to mixing and pumping. The input mixed waste stream is analysed for dry solids content, and also for C:N (carbon-nitrogen) ratio. The biogas yields are calculated simultaneously with the plant thermal operation characteristics. This is a complex iterative task as the digester temperatures influence the biogas yield, the biogas yield influences the biogas engine power available, and this in turn influences the plant thermal characteristics.
The outputs from the analysis include:-
- plant sizing including digester sizes and biogas engine size
- biogas yield output
- electrical surplus available for export
- CHP heat surplus available for export
- process electrical loads
- process heat loads
- transport energy and costs
- overall energy balance
- greenhouse gas contribution (relative to conventional procedures)
- overall financial viability calculation, via net present value (NPV) and profitability index methods.
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