Co-combustion characteristics and kinetics of sludge/coal blends in 21%–50% oxygen-enriched O2/CO2 atmospheres

This research investigated the thermogravimetric analysis (TGA) and combustion kinetics of sewage sludge and Australian sub-bituminous coal and their blends with different mixing ratios (sludge/coal = 100/0, 80/20, 50/50, 20/80, 0/100 by weight) under air atmosphere (O2/N2 = 21/79 by volume), oxy-fuel combustion (O2/CO2 = 21/79, 25/75, 35/65, and 50/50 by volume), and pyrolysis (pure N2 atmospheres). The gaseous products that evolved from the TGA experiments were also analyzed using a Fourier transform infrared (FTIR) spectrometer under pyrolysis conditions. Results showed that the mixed fuel with higher coal blending ratio had higher ignition temperature, lower burnout temperature and larger combustion characteristic index. Furthermore, TGA–FTIR experiments of the 20 % sludge + 80 % coal blended fuel under nitrogen atmospheres showed more gasification components in pure coal than in sludge. The combustion kinetics analytical results via the Flynn–Wall–Ozawa method indicated that the larger the conversion degree, the smaller the obtained activation energy value. Under the oxy-fuel combustion conditions, when the oxygen concentration increased from 21 % to 50 % for the 20 % sludge + 80 % coal blended fuel, the ignition and burnout temperatures decreased, and the combustion characteristic index increased nearly three times. Oxygen enrichment can clearly improve combustion characteristics and flammability.