The combustion of torrefied biomass in commercial-scale domestic boilers

Abstract

The use of solid renewable fuels in domestic-scale boilers, DSB is especially common in places where the distribution of other renewable energy sources is strongly limited (mountain and foothill regions). High moisture content and limited ventilation have a destructive impact on wood pellets' stability. Torrefaction of biomass is recognized as a possible way to enhance the wood pellets’ hardness under wet conditions. It was found that the maximal water uptake for torrefied biomass after its water immersion was only 18.7 wt%. In this work, the combustion performance and emission of main pollutants during the combustion of thermally–treated biomass are analyzed using two types of boilers: coal-fired 25 kWth (retort burner) and biomass-fired 10 kWth (pellets burner). The best results were obtained during the combustion of torrefied biomass in a 10 kWth DSB, i.e. emission of CO 218 g/GJ, SO2 2.5 g/GJ, NOx 111 g/GJ, and particulates 10.5 g/GJ, boiler efficiency of 90.23%. The combustion of torrefied biomass met the requirement of a 5th-class boiler according to the PN-EN 303-5 standard. Thermally–treated biomass can be used as a substitute fuel in DSB. Nevertheless, the boiler construction can be improved to obtain lower emissions of CO.