The combustion characteristics of high-heating-rate chars from untreated and torrefied biomass fuels
Torrefaction of biomass is of great interest at the present time, because of its potential to upgrade biomass into a fuel with improved properties. This study considers the fundamentals of combustion of two biomass woods: short rotation willow coppice and eucalyptus and their torrefied counterparts. Chars were prepared from the untreated and torrefied woods in a drop tube furnace at 1100 °C. Fuels and chars were characterised for proximate, ultimate and surface areas. Thermogravimetric analysis was used to derive pyrolysis and char combustion kinetics for the untreated and treated fuels and their chars. It was found that the untreated fuels devolatilise faster than their torrefied counterparts. Similarly, the chars from the untreated biomass were also found to be more reactive than chars from torrefied fuels, when comparing reaction rates. However, the activation energy value (Ea) for combustion of the untreated eucalyptus char was higher than that for the torrefied eucalyptus chars. Moreover, the eucalyptus chars were more reactive than the willow char analogues, although they had seen a lower extent of burn off, which is also a parameter indicative of reactivity. Similar trends in were also observed from their intrinsic reactivities; i.e. chars from the untreated fuel were more reactive than chars from the torrefied fuel and eucalyptus chars were more reactive than willow chars. Chars were also studied using scanning electron microscopy with energy-dispersive X-ray analysis. This latter method enabled a semi-quantitative analysis of char potassium contents, which led to an estimation of potassium partitioning during char formation and burnout. Results show a good correlation between potassium release and percent burnout. With respect to the effect of torrefaction on fuel-N, findings suggest that torrefaction would be beneficial for pf combustion in terms of nitrogen emissions, as it resulted in lower fuel-N contents and ∼72–92% of the fuel-nitrogen was released with the volatile fraction upon devolatilisation at 1100 °C.