Improving the carbon efficiency of two-stage DME synthesis based on wheat-straw gasification

Abstract

For a future fossil-free energy system, solutions for hard-to-abate sectors like maritime transport need to be found. Dimethyl ether (DME) produced from sustainable biomass, like wheat straw, is a promising fuel for substituting fossil Diesel fuels in marine engines. In this work, biorefineries based on wheat straw gasification were designed and analysed. The biorefineries produced DME via two-stage synthesis, where syngas was first synthesized to methanol, before methanol was dehydrated to DME in a second step. In addition, bio-ash was produced, which acts as a soil amendment and carbon sink, when returned to the fields. The biorefineries included alkaline electrolysers for producing hydrogen to increase the carbon and energy efficiency of the plants. In this work, the addition of hydrogen at different process locations was investigated: 1) at high temperatures, enabling the conversion of CO2 to CO via the reverse water gas shift reaction or 2) at low temperatures. In addition, the amount of hydrogen added through the electrolysers and the amount of gas (containing mainly H2, CO and CO2) purged after the methanol reactor were varied. By increasing the amount of hydrogen in the system, carbon efficiencies >98 % were achieved, independent of the location of the hydrogen addition and purge ratios used. Reducing the purge ratio enabled to produce more DME at the same amount of hydrogen added, leading to higher energy efficiencies. Hydrogen addition at low temperature achieved higher energy efficiencies compared to hydrogen addition at high temperatures, because it enabled the operation of the electrolysers at high pressures of 30 bar and reducing the electricity consumption in the compressors. In general, any measures of increasing the energy and carbon efficiency led to an increase in the size of the methanol reactor, leading to a trade-off between investment and operational costs.