A Novel Approach Towards Waste Treatment in FBC

Waste combustion has the potential to play an important role in the energy production despite its contribution to heavy metals emissions. A new multi-zone temperature combustion technique, known as a Low-High-Low (LHL) temperature method, was developed to reduce pollutant emissions, particularly heavy metals, from FBCs. This paper focuses on the environmental impacts of biowaste combustion at different FBC conditions with emphasis on gas and solid emissions. The biowaste (de-inking sludge) studied contained 15% moisture, 27% carbon, 18% oxygen, and 35% ash. Ash elemental analysis shows a dominance of SiO2, Al2O3 and CaO (38%, 28% and 19%, respectively) with selected alkalis Na2O and K2O (0.3% and 0.2%, respectively). The used biowaste material had a heating value of 10,000 kJ/kg, which indicates that its combustion may be used to treat a portion of the total solid waste produced, while generating energy. The paper reports the following results of LHL vs. Classical FBC: (1) average axial profiles of gas concentrations (NO, NOx, and CO2) as well as their final averages at the exhaust, (2) final heavy metals leachability from generated fly ash. During the multi-temperature combustion experiments (LHL), the final average gas measurements for NO, NOx, and CO2 were 91 ppm, 175 ppm, and 6.1%, respectively. As for the classical FBC experiments, the final average gas measurements were similar (94 ppm, 141 ppm and 5.9% for NO, NOx and CO2, respectively). The final fly ash sample had leachability rates of 0.14 ppm and 0.061 ppm for Cd and Cr, respectively. Such low leachability rates are due to the LHL’s ability to form dense and compact final fly ash structures. On the contrary, 30.7 ppm and 14.3 ppm of Cd and Cr leached out of the porous no-LHL final fly ash structures, respectively. These results confirm that the LHL combustion could be considered as an effective waste-to-energy approach.