Study of chlorine behavior in a simulated fluidized bed combustion system

Fluidized bed combustion techniques have been widely used throughout the world in an effort to reduce sulfur oxide emissions, especially from burning high-sulfur coal. However, in the utilization of FBC systems for co-firing high chlorine coals with municipal solid waste (MSW) there are some concerns about the possible emission of polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). PCDD/Fs may be produced from the reaction of volatile organic compounds (VOCs) and molecular chlorine under relatively low combustion temperature conditions. In oxygen-rich conditions during combustion molecular chlorine can be formed through the Deacon Reaction when the temperature is around 600 °C. It is also likely that chloride might affect the detailed chemistry of desulfurization in FBC processes. In order to better understand the behavior of chlorine in an FBC system during combustion processes, a comprehensive study was carried out in a simulated FBC system with an on-line feeder at Western Kentucky University. Conditions used simulated the flue gas and operating conditions of an FBC system. Optimum operating conditions to suppress HCl, Cl₂ and SO_x emissions from FBC systems were determined. A better understanding of the chlorine behavior during combustion will help in controlling possible PCDD and PCDF formation and reducing corrosion in FBC systems. The test results indicated that the formation of molecular chlorine is favored at temperatures above 600 °C, in oxygen-rich atmospheres, and in relatively high HCl concentrations. The reaction temperature plays a key role in the capture of HCl. The optimum combustion conditions for controlling PCDD/Fs formation in FBC systems is to maintain combustion temperature around 850 °C in the bed area, 600 °C in the freeboard area, low oxygen concentrations in the flue gas and enough residence time for fine particles in the freeboard.