In-situ catalytic deNOx is a promising NOx control technology for circulating fluidized bed (CFB) boilers. In this application, matching the conditions between the catalyst and gaseous species is crucial. To understand this, a comprehensive computational particle fluid dynamics (CPFD) model was established; flow, combustion, and NOx emission characteristics in an industrial CFB boiler were elaborated; 20 catalysts with various sizes and densities were designed, and their degree of matching with the gaseous species was evaluated. The simulation results indicated that NOx was gradually produced at the bottom of the furnace and attained its maximum concentration at the elevation of secondary air; CO showed a high concentration in the bottom dense-phase zone; and the homogeneous NO-CO reaction is too weak to effectively reduce NOx. With catalyst application, the NO-CO reaction was evidently enhanced and the in-furnace NOx concentration decreased significantly. The 20 evaluated catalysts can be categorized as dipleg deposition, fluidization circulating, furnace suspension, and furnace deposition types. While the last three types of catalysts could match the spatial and temporal distribution of CO and NOx species well, the furnace suspension-type catalyst produced an optimal matching degree and maximum deNOx efficiency.

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.