Combustion and life-cycle evaluation of torrefied wood for decentralized heat and power production

Torrefied wood pellets are produced from torrefied chips by thermo-chemical pre-treatment of biomass at 200-320°C in the absence of oxygen during about 15-30 minutes. Overall, the torrefaction process efficiency has been reported to be 90-95% % as compared to 84% for pelletisation. Torrefaction improves the biomass: 30% higher calorific value and 50% higher energy density resulting in much lower handling and transport costs. The fuel becomes hydrophobic making long term outdoor storage possible. The purpose of this project was to compare the combustion and emission characteristics of torrefied vs. normal wood pellets. With no modification to the feeding and the burner parameters, the ignition and combustion characteristics of torrefied pellets are found very similar to those of normal pellets. Particulate emissions per energy output were found very close and directly related to the ash content in the feedstock. Using the Taguchi approach, it was possible to establish a model of the boiler performance as a function of the input parameters. Further testing confirmed the validity of the model showing optimum performance with a defined value of primary and secondary air flow rates which minimized particulate emissions for both the normal pellets and the torrefied pellets.