No abstract

In dedicated wood pellet combustion and cofiring with coal in large pulverized fuel furnaces, poor grindability and low bulk density of biomass are important issues for lowering the unburned carbon in ash and achieving high cofiring ratios with coal for pulverized fuel combustion furnaces. In this study, the torrefaction of wood pellets was investigated for improvement of energy density and grindability. The torrefaction tests were performed using a fixed bed reactor for a temperature range of 210–310 °C and holding time of 15–60 min. The mass yield varied from 86.18 to 39.46% accompanied by an increase in the carbon content and heating value. The properties of torrefied wood pellets (TWP) were correlated with the mass yield for use with different time–temperature histories. The bulk density decreased by the mass yield rose to a power of 0.538. The energy density of TWP was higher in the initial torrefaction stage with a peak of 10.41 GJ/m3 but was below that for the original pellets when the mass yield was approximately ≤60%. The grindability of TWP increased almost linearly with the degree of torrefaction, and the mass yield of 80% attained the lower range of the grindability of coal.

No abstract

No abstract

For renewable electricity production, biomass can fully displace coal in an existing power plant with some equipment modifications. Recently, a 125 MWe power plant burning mainly anthracite in Korea was retrofitted for dedicated wood pellet combustion with a change of boiler configuration from arch firing to wall firing. However, this boiler suffers from operational problems caused by high unburned carbon (UBC) contents in the bottom ash. This study comprises an investigation of some methods to reduce the UBC release while achieving lower NOx emissions. The computational fluid dynamics approach was established and validated for typical operating data. Subsequently, it was applied to elucidate the particle combustion and flow characteristics leading to the high UBC content and to evaluate the operating variables for improving the boiler performance. It was found that the high UBC content in the bottom ash was a combined effect of the poor fuel grindability and low gas velocity in the wide burner zone originating from the arch-firing boiler. This prevented the operation with deeper air staging for lower NOx emissions. Reducing the particle size to <1.5 mm by modifying mills or pretreating the fuel using torrefaction was the only effective way of lowering the UBC and NOx emissions with deeper air staging while increasing the boiler efficiency.