Computational fluid dynamics investigation on the effect of co-firing semi-coke and bituminous coal in a 300 MW tangentially fired boiler

In this paper, the effect of co-firing semi-coke in a 300 MW tangentially fired boiler was numerically investigated. The results indicate that the incomplete combustion heat loss and NO_x emission both increase with semi-coke co-fired ratio. Semi-coke may be injected into the furnace at a different height, which can lead to different thermal efficiency and NO_x emission. It is suggested that semi-coke should not be fed from the top or bottom layer burners, since this could give rise to high carbon content respectively in fly ash and bottom slag. In addition, injecting semi-coke from the top burners could significantly increase the NO_x emission. Under 1/2 co-firing ratio, the optimal fuel allocation is that feeding semi-coke from the B, D, and E layer burners. The growth in semi-coke particle size could increase the unburned carbon loss and NO_x emission. It is highly recommended to reduce the unburned carbon loss under semi-coke co-fired condition by increasing the stoichiometric ratio of primary air for semi-coke. As it is increased from 0.25 to 0.3, the combustion efficiency of the co-fired condition is 99.47%, the same as when only firing bituminous coal, and the NO_x emission is about 30% higher.