Thermal events occurring during the combustion of biomass residue

The purpose of this study was to identify possible events taking place during thermal treatment of biomass residue (bio-sludge) in an oxidative environment. The bio-sludge sample was collected after biological treatment of de-inking waste generated by pulp and paper mills. Combustion tests were conducted in a high-temperature Cahn TG-171 thermogravimetric furnace (TGA) coupled with Mattson Galaxy 5020 Fourier transform infrared spectrometer (FTIR). A bio-sludge sample (1.1 g) was heated in the TGA furnace at a rate of 40°C min−1 until a maximum temperature of 1500°C was achieved. The sample was then “soaked” at this temperature for 10 min and subsequently cooled or quenched at a rate of 20 or 1800°C min−1, respectively. Temperature- and time-resolved profiles of sample weight loss were determined by the TGA while volatile release profiles were obtained from FTIR. Solid samples collected during the bio-sludge combustion process were analyzed with scanning electron microscopy, wavelength-dispersive and energy-dispersive X-ray spectrometry and electron probe microanalysis to determine morphology, particle size, composition and metals distribution in ash particles. The bio-sludge combustion process could be divided into four stages: (1) Initial Burning (T<600°C) dominated by moisture evaporation, fibers volatilization and pyrolysis. (2) Bio-sludge/ash Transition (600<T<1100°C) dominated by char oxidation, metals reduction and Al–Si spinel formation. (3) Solid particle Sintering (1100<T<1400°C) dominated by mullite formation, ash softening and metals segregation. (4) Ash Melting (T>1400°C). However, it is difficult to accurately establish a clear boundary between these stages because the regimes of volatiles release, char combustion and ash transformation are overlapping. Ash formed during quenching was a condensed and packed substance while during natural cooling it revealed dendritic character with needle-like features present on the particle surface. The former structure is better from the environmental point of view because it would not allow for leaching of toxic metals if ash was disposed of in a sanitary landfill.