Biomass-based negative emission technology options with combined heat and power generation

Biomass-based combined heat and power (CHP) generation with different carbon capture approaches is investigated in this study. Only direct carbon dioxide (CO2) emissions are considered. The selected processes are (i) a circulating fluidized bed boiler for wood chips connected to an extraction/condensation steam cycle CHP plant without carbon capture; (ii) plant (i), but with post-combustion CO2 capture; (iii) chemical looping combustion (CLC) of solid biomass connected to the steam cycle CHP plant; (iv) rotary kiln slow pyrolysis of biomass for biochar soil storage and direct combustion of volatiles supplying the steam cycle CHP plant with the CO2 from volatiles combustion escaping to the atmosphere; (v) case (iv) with additional post-combustion CO2 capture; and (vi) case (iv) with CLC of volatiles. Reasonable assumptions based on literature data are taken for the performance effects of the CO2 capture systems and the six process options are compared. CO2 compression to pipeline pressure is considered. The results show that both bioenergy with carbon capture and storage (BECCS) and biochar qualify as negative emission technologies (NETs) and that there is an energy-based performance advantage of BECCS over biochar because of the unreleased fuel energy in the biochar case. Additional aspects of biomass fuels (ash content and ash melting behavior) and sustainable soil management (nutrient cycles) for biomass production should be quantitatively considered in more detailed future assessments, as there may be certain biomass fuels, and environmental and economic settings where biochar application to soils is indicated rather than the full conversion of the biomass to energy and CO2.