Trends in predicting and controlling ash vaporization in coal-fired utility boilers

The past decade has seen a dramatic increase in the use of computational fluid dynamics (CFD) in the solution of problems related to the design and operation of pulverized coal-fired utility boilers. These tools have been increasingly used to simulate the performance of utility boilers, primarily for NOx control and associated problems of unburned carbon in fly ash and in water-wall corrosion. These models are extended to calculate the emissions of submicron particles by the vaporization and condensation of ash constituents. A single particle model for the vaporization of minerals in coal was first calibrated with results for vaporization of 14 coals in a laboratory reactor. The calibrated model was then applied in the simulation of the ash vaporization in a 500-MW opposed-wall fired boiler with 12 burners on each of the front and rear walls, and for cases before and after retrofitting the boiler with burner technology to reduce NOx emissions. The simulations showed that the ash vaporization occurred primarily during short intervals along particle trajectories when the particle temperatures increased above 1800 K. The ash vaporization decreased slightly on retrofitting the boiler, and the contributions by different burners to the total amount vaporized varied widely, particularly after the low-NOx retrofit.