Can biomass be satisfactorily gasified under pressure using an aqueous slurry feed? Examination by simulation

A major hurdle for the gasification of biomass under pressure is the need to introduce a fibrous biomass feed material such as straw, switchgrass or miscanthus into a pressure vessel. One proposed solution is to prepare a dense aqueous slurry from the biomass and then use a conventional high pressure pump. The production of syngas from wheat straw and subsequent power generation is examined theoretically in an Australian context. A slurry of concentration of 50% biomass by volume, which is regarded as the maximum pumpable value, is dried with superheated steam and the biomass gasified at 2 MPa with steam only in cyclones. It was found that the thermal deficiency of introducing excessive liquid water is considerable, rendering the process unsustainable. The problem can be overcome by employing a subsidiary fuel such as natural gas, but even with the minimal amount of water, the required energy input is equivalent to that of the straw. The net electrical efficiency of the process based on both fuels is 33.5%, so that the approach would be contemplated only if the over-riding consideration was the use of the straw for energy generation. The zero net present value cost of power production is 125 $ MWh-1 for 90 Gg of straw consumption per annum.