However, these possibilities have drawbacks. Electricity is an expensive and inefficient way of providing process heating. An additional renewable energy system sized to meet the entire process heat and electricity needs would be excessively expensive. Natural gas or LPG cannot be “re-credited” once they are consumed (not without complex upgrading of biogas for export to natural gas pipelines). Also, one of the objectives of the modelling is to find a way to reduce or eliminate the direct use of fossil fuels.
The preferred alternative for startup is to find a source of biomass to start the LY3039478 digester in advance of ethanol production. Options are likely to be available in a farm situation and the best choice will depend on what other activities are being carried out on or near the farm. For the purposes of the present modelling additional wheat straw will be used.
Allowing a four week period for the digester to come to the necessary productivity and stability to bring ethanol production on-line, means approximately 14.6 tonnes of wheat straw must be available for startup for case A (more than four weeks operating supply as the stillage is not available for digestion during this time). Averaging this figure over a 48 week period gives 300 kg per week or an additional 11%. An allowance is included for startup materials in the transport calculation as detailed below.