CPFD modeling of hydrodynamics, combustion and NOx emissions in an industrial CFB boiler

The ultra-low NOx emission requirement (50 mg/m3) brings great challenge to CFB boilers in China. To further tap the NOx abatement potential, full understanding the fundamentals behind CFB boilers is needed. To achieve this, a comprehensive CPFD model is established and verified; gas-solid flow, combustion, and NOx emission behavior in an industrial CFB boiler are elaborated; influences of primary air volume and coal particle size on furnace performance are evaluated. Simulation results indicate that there exists a typical core-annular flow structure in the boiler furnace. Furnace temperature is highest in the bottom dense-phase zone (about 950 °C) and decreases gradually along the furnace height. Oxygen-deficient combustion results in high CO concentration and strong reducing atmosphere in the lower furnace. NOx concentration gradually increases in the bottom furnace, reaches maximum at the elevation of secondary air inlet, and then decreases slightly in the upper furnace. Appropriate decreasing the primary air volume and coal particle size would increase the CO concentration and intensify the in-furnace reducing atmosphere, which favors for NOx reduction and low NOx emission from CFB boilers.