Combustion study of partially gasified willow and DDGS chars using TG analysis and COMSOL modeling

The combustion behavior of Dried Distiller’s grains with Solubles (DDGS) and willow chars, which were obtained from these fuels gasification on a 100 kWth steam-O2 blown circulating fluidized bed (CFB) gasifier, and pure charcoal was studied using thermogravimetric analysis (TGA) under both isothermal and non-isothermal conditions. The effects of combustion temperature (400–600, 750–900 °C), O2 concentration (7.5, 10, 15, 21vol. %), and heating rate (10, 30, 50 °C min−1) on the char-O2 reaction rate were studied. A 3D TG furnace model was built using COMSOL Multiphysics Software to understand better temperature and velocity profiles within the furnace. Char combustion rate increased with increasing O2 concentrations or temperatures. Within the temperature range of 750–900 °C, it was impossible to determine combustion kinetic parameters of DDGS and willow chars, but it was well possible for charcoal under 15 vol.% O2 (Ea ca.120 kJ mol−1 using the SCM). Within the temperature range of 450–600 °C, the Ea value obtained from DDGS char combustion was around 60 kJ mol−1 using the VRM, while it appeared to be impossible to determine properly combustion kinetic parameters of willow char probably due to its high reactivity. With increasing heating rate, the combustion of three chars occurred in a wider temperature range, and higher Ea values were obtained at lower conversion rates. Results predicted from the 3D TG furnace model agreed fairly well with the experimental ones. The velocity profile within the furnace was affected by the furnace temperature and vice versa, and they both were affected by the heat produced from char combustion as well.