Combustion properties of torrefied willow compared with bituminous coals

Thermal pre-treatment, or torrefaction, is a process that can improve the handling and grinding properties of biomass for combustion and co-firing. This paper compares the combustion properties of raw and torrefied Short Rotation Coppice Willow (SRC), with those of typical bituminous power station coals. The fuels were analysed using a number of standard fuel characterisation tests. Willow SRC was torrefied at 290 °C for two reaction times (10 min (short) and 60 min (long)). During torrefaction, longer reaction time promotes loss of nitrogen from the solid product. Chars were produced from the fuels under high heating rates up to 1000 °C and the char yields and nitrogen partitioning were determined. Results show that the nitrogen partitioning for both raw and torrefied biomass favours release of nitrogen into the volatiles during rapid pyrolysis. In contrast, rapid pyrolysis of the coals favours nitrogen retention in the char. The chars were subjected to thermogravimetric analysis-mass spectrometry (TGA-MS) combustion tests, which were used to calculate the char reactivities and fate of char nitrogen. The char reactivities follow the order coal char < torrefied willow char < raw willow char. The release of volatile-nitrogen species during char combustion occurs at higher temperatures for the torrefied willow chars compared to the raw willow chars due to their relatively lower reactivities. Like coal char, nitrogen is concentrated in the char (both raw and torrefied willow chars) as combustion proceeds and NO and N2 species are released towards the end of char burnout.