Exergy-based comparison of three wheat straw based biorefineries

For a rapid transition to a fossil-free society, the production of transport fuels from sustainable biomass, like wheat straw will be crucial. The so-called Polygas system enables the efficient conversion of wheat straw to bio-ash, bio-oil and a tar-free gas, which can be used for electricity production or synthetic fuel production. It is based on the low-temperature circulating fluidized bed (LT-CFB) gasifier combined with a catalytic reactor.
Three different plants based on the Polygas system were investigated using wheat straw as feedstock. Their main products were electricity, synthetic natural gas (SNG) and dimethyl ether (DME), respectively. Additionally, each plant produced bio-oil, process heat and district heating. Electrolytic hydrogen from alkaline electrolysis was used in the SNG– and DME production plant for boosting the fuel yield. The systems were simulated using the software Aspen Plus and were compared using exergy analysis.
The results showed that the SNG production plant reached the highest exergy efficiency (64 %), followed by the DME production plant (52 %) and the electricity production plant (44 %). For the SNG production plant, the electrolyser was the component with highest exergy destruction. This could be improved by replacing the alkaline with solid oxide electrolysers. In the DME production plant, the combustion of off-gas from the DME synthesis in a gas engine lead to a high exergy destruction. This should be avoided by optimizing the plant and reducing the amount of hydrocarbons in the off-gas. For the electricity production plant, no simple improvement measure was identified.