Summary

Interest is growing in extending fluidized bed combustion (FBC) to fuels that are difficult to handle and those that present difficulties because their combustion is associated with particularly challenging air pollution problems. Such fuels include biomass (such as straw), plastic wastes, black liquors and heavy liquid fuels. As all of these have very high volatiles contents, they tend to be treated as easy to burn, and instead solid fuels and char combustion have received more attention in the literature. Nonetheless, understanding the gas-phase chemistry of such fuels is helpful in optimizing their combustion. This paper presents a study of natural gas combustion in a fluidized bed, as a model system for investigating the gas-phase reactions involving C/H/N/O chemistry taking place in the absence of char. The experimental work was conducted using a pilot-scale mini circulating FBC (CFBC) unit of 0.1 m diameter and 5 m height. Combustion characteristics and emissions were investigated by varying the operating conditions and in particular the combustion temperature, fluidizing velocity and bed material. The results fit with the general current consensus that FBC chemistry is associated with super-equilibrium free radical processes, similar to high-temperature flame systems. A CFBC model has been developed based on the general kinetic model and a NO/N2O formation model. It uses the semi-theoretical approach with some measured parameters as inputs and appears to be capable of providing a reasonable description of the nitrogen chemistry and the concentration profiles of NH3, HCN, NO, and N2O for the case of burning natural gas.