Effects of blend ratio on thermal performance and NO emissions in biodiesel-coal co-combustion for coal-fired power units

Differences in the combustion characteristics of biodiesel and coal lead to combustion stability and NO emissions. This study employed computational fluid dynamics (CFD) to systematically investigate the influence of varying biodiesel blending ratios (0–25 %) on boiler combustion behavior and NO emissions. The results indicated that the temperature increased monotonically with higher biodiesel blending ratios. At a 25 % blending ratio, the average temperature reached 1543.21 K, representing a 69.42 K increase compared to pure coal combustion. This enhancement is primarily attributed to the high calorific value and elevated oxygen content of biodiesel. Notably, the NO emissions exhibited an inverse parabolic trend. The minimum NO emissions of 15.98 g/s occurred at a 20 % blending ratio, corresponding to only 34.34 % of the pure coal combustion. This trend can be explained by the initial facilitation of NO reduction to N2 and O2 in a high-temperature environment at lower blending ratios. However, when the blending ratio exceeded 20 %, the excessive generation of oxygen radicals intensified NO formation. Based on a comprehensive evaluation of combustion stability and emissions performance, the optimal biodiesel blending ratio was identified as 20 %. These findings provide theoretical and technical foundations for the large-scale application of biodiesel in power station boilers.