New Pathways in Clean Combustion of Biomass and Coal via Partial Gasification

Abstract

This chapter addresses studies conducted on a new approach to clean combustion via gasification process progressing on the earlier work on packed bed reverse downdraft (REDS) combustion. The additional element is the development of continuous combustion device. The studies are aimed at the use of prepared (in terms of size and dryness) biomass in a broad range of densities (100 to 1000 kg/m$$^3$$ ) in a newly conceived scalable combustion scheme. The range of power levels includes domestic demands ($$\sim $$ 1 kg/h), semi-industrial needs (3 to 20 kg/h), and larger industrial requirements (50 kg/h and more). System can deliver hot gases at a flame temperature from 1150 to 1200$$^\circ $$ C. In domestic stoves, CO emissions are within the permissible limits (CO:CO$$:2$$ ratio of 0.006 ± 0.002) and PM2.5 emissions showed incremental steady values of a maximum of 30$$\upmu $$ g/m$$^3$$. An important aspect addressed here concerns the mode of assessment of efficiency and emissions from these stoves. It is suggested that recent expectations of domestic stove emissions need revision in favor of known concepts from other combustion devices. The second part is concerned with the use of coal of permitted ash content (of 21%, but up to 34%) sized to 2–8 mm for thermal applications and clean cold combustible gas applications. Studies on the flame propagation behavior in packed beds in REDS with air show rates about half of that with biomass. With air–steam mixtures, carbon conversion beyond 99% and avoidance of ash fusion are achieved. Operation of the bed with heated coal ($$\sim $$ 120$$^\circ $$ C) and air up to 160$$^\circ $$ C are considered beneficial to reduce the flaming time and char conversion times. The fixed bed studies provide inputs for evolution to mildly fluidizing strategy for complete conversion of coal without ash fusion.