Numerical study on combustion characteristics and NOx emissions in a 600 MWe boiler under half load: a study of single and blended coal types

Blended coal combustion represents a promising strategy for achieving global carbon neutrality. Significant differences are observed in the combustion characteristics among various coal types. Lean coal exhibites poor combustion performance, characterized by delayed ignition and high NO x emissions. In contrast, bituminous coal demonstrates favorable combustion behavior but is prone to localized high temperatures and severe slagging/fouling issues. Co-firing these coals is shown to compensate for their respective limitations and achieve synergistic emission reduction effects. This study employed three-dimensional numerical simulations to systematically investigate the combustion characteristics of different coal types and their blending schemes in a 600 MWe wall-fired opposed boiler. The numerical model incorporated the Realizable k-ε turbulence model and the P1 radiation model to accurately capture the rotational flow field and radiative heat transfer within the furnace. A total of 13 operating conditions were simulated, including four types of coal under single-fuel combustion (low-volatile bituminous coal (BLV), lean coal A (LA), lean coal B (LB), and high-volatile bituminous coal (BHV)) and nine co-firing conditions. The results indicated that the single-fuel combustion of BLV yielded the lowest NOx concentration at the furnace outlet (195.18 mg/m 3 ). Among the blends of LB and BLV, the 50% LB + 50% BLV mixture showed the best NO x reduction performance, with an emission level of 300.03 mg/m 3 , representing a 7.34% reduction compared to pure LB combustion. This improvement was attributed to the strong CO-rich reducing atmosphere formed under this condition, which significantly promoted the rate of homogeneous reduction reactions. For the blends of BHV and BLV, the 50% BHV + 50% BLV blend achieved the most notable results, reducing NO x emissions to 295.29 mg/m 3 —an 11.2% decrease compared to pure BHV combustion, demonstrating a significant synergistic emission reduction effect. The findings of this study provide important theoretical support and practical guidance for optimizing coal blending strategies and achieving stable, clean combustion in wall-fired opposed boilers.