A computational fluid dynamics (CFD) model of a 200 MW multifuel tangentially fired boiler has been developed using Fluent 6.3.26, which is able to model the three-fuel combustion system of coal, blast furnace gas (BFG), and coke oven gas (COG) with an eddy-dissipation model for simulating the gas-phase combustion. A level of confidence in the current CFD model has been established by carrying out a mesh independence test and validation against the experimental data obtained from the boiler for case study. The validated CFD model is then applied to investigate the effects of different BFG and COG flow rates on the boiler performance. It is found that increasing the BFG flow rate brings negative effects on the ignition of primary air and pulverized-coal mixture, pulverized-coal burnout, and heat transfer in the furnace and, consequently, decreases the thermal efficiency. However, increasing the COG flow rate can increase the thermal efficiency via improving the pulverized-coal burnout and heat transfer. Increasing both the BFG and COG flow rates is favorable for reducing NO emissions. The results also indicate that co-firing pulverized coal with BFG of about 20% heat input and COG of about 10% heat input is an optimal operating condition for improving the boiler performance at 180 MW load. The present study provides helpful information for understanding and optimizing the multifuel combustion of the boiler.

The optimum temperature within the reagent injection zone is between 900 and 1150°C for the NOX reduction by SNCR (selective non-catalytic reduction) in coal-fired utility boiler furnaces. As the load and the fuel property changes, the temperature within the reagent injection zone will bias from the optimum range, which will reduces significantly the de-NOX efficiency, and consequently the applicability of SNCR technology. An idea to improve the NOX reduction efficiency of SNCR by regulating the 3-D temperature field in a furnace is proposed in this paper. In order to study the new method, Computational fluid dynamics (CFD) model of a 200 MW multi-fuel tangentially fired boiler have been developed using Fluent 6.3.26 to investigate the three-fuel combustion system of coal, blast furnace gas (BFG), and coke oven gas (COG) with an eddy-dissipation model for simulating the gas-phase combustion, and to examine the NOX reduction by SNCR using urea-water solution. The current CFD models have been validated by the experimental data obtained from the boiler for case study. The results show that, with the improved coal and air feed method, average residence time of coal particles increases 0.3s, burnout degree of pulverized coal increases 2%, the average temperature at the furnace nose decreases 61K from 1496K to 1435K, the NO emission at the exit (without SNCR) decreases 58 ppm from 528 to 470 ppm, the SNCR NO removal efficiency increases 10% from 36.1 to 46.1%. The numerical simulation results show that this combustion adjustment method based on 3-D temperature field reconstruction measuring system in a 200 MW multi-fuel tangentially fired utility boiler co-firing pulverized coal with BFG and COG is timely and effective to maintain the temperature of reagent injection zone at optimum temperature range and high NOX removal efficiency of SNCR.