CFD Approach for Unburned Carbon Reduction in Pulverized Coal Boilers

Abstract

Low-NOx technologies are widely used in pulverized coal boilers, but they usually produce high levels of carbon in the fly ash. High levels of unburned carbon represent fuel loss, so the overall boiler efficiency is reduced. Additionally, the higher carbon content affects the suitability of fly ash for cement applications. The purpose of this paper is to provide a CFD approach for unburned carbon reduction by optimizing operating conditions. In this paper, three different boiler loads were simulated: 200 MW, 170 MW, and 140 MW. The air supply system was simulated previously for preparing as precise as possible boundary conditions. At last, the unburned carbon level of every burner was investigated, and the effects of residue residence time and the local fuel–air momentum ratio are discussed in detail. According to the predicted results, operating conditions and the residence time of the coal particles affects the unburned carbon level in fly ash. Operating conditions play a more significant role during the combustion process, while the residence time affects char burnout only when the burner’s location is low. Therefore, it is concluded that a cost-effective method could be developed for reducing the unburned carbon level in ash and correspondingly, the loss on ignition level. First, it is necessary to determine which burners are operating under poor conditions through CFD analysis. Then, the fuel air momentum ratios of those burners should be modified by changing the operating conditions, meanwhile increasing the residence time of coal particles to ensure complete combustion.