Technical and environmental feasibility of co-firing torrefied biomass in a coal-fired power plant

Abstract

Biomass torrefaction is a thermochemical pretreatment process performed at a temperature range of 200oC- 300oC in an inert environment. Moisture and part of the hemicellulose are released during the process resulting an energy densified solid fuel. In this study, rubberwood chips were torrefied at a temperature of 300 oC for 60 minutes. The results show that the lower heating value of biomass increase from 14.58 MJ/kg to 16.38 MJ/kg after torrefaction. Further, a 300MWe coal-fired power plant was simulated using Aspen Plus software and co-firing possibilities of both raw and torrefied rubber wood were evaluated. The results reveal that up to 10% of thermal input can be substituted by raw rubberwood whereas torrefied rubberwood has the potential to substitute around 20% of thermal input without compromising the electrical efficiency. The total input mass flow rate is a crucial factor when co-firing is to be implemented in an existing plant even though it gives the expected energy output with a further increase of torrefied biomass share. With increasing co-firing ratio, it is observed that the total NOx and SOx emissions are reduced due to low N and S content of biomass. Further, combustion efficiency is increasing with increasing co-firing ratio of torrefied biomass.