Numerical investigation of oxygen-enriched combustion for 600 MW opposed firing boiler under ultra-low load

Poor combustion stability is generally encountered during low-load boiler operation under deep peak regulation. Oxygen enrichment of the combustion air is regarded as an effective approach to enhancing combustion and improving combustion stability at low loads. A 600 MW opposed firing boiler is investigated using computational fluid dynamics (CFD). After the numerical model is thoroughly validated, the effects of the O₂ volume fractions in the primary stream, inner secondary stream, outer secondary stream, and central stream, as well as the ratio of overfire stream to the total secondary stream flow rate, are systematically evaluated at 20% load. The results demonstrate that primary stream oxygen enrichment provides the most pronounced improvement in combustion stability, with both the peak temperature and the temperature in the main combustion zone being significantly increased; however, the O₂ volume fraction in the primary stream should not exceed 25%. Ignition near the burner root is enhanced by increasing the O₂ volume fraction in the inner secondary stream, although the improvement in combustion stability is less pronounced than that achieved by primary stream oxygen enrichment. Only a limited influence on the peak temperature is observed with outer secondary stream oxygen enrichment, whereas the overall furnace temperature is increased and burnout is promoted. The smallest contribution to combustion stability is obtained by central stream oxygen enrichment. In addition, the NO mass concentration at the furnace outlet is significantly reduced by increasing the overfire stream ratio. Specifically, when the primary stream O₂ volume fraction is increased from 21% to 25%, the peak temperature is increased from 1742 to 1812 K; however, the outlet NO mass concentration increases at the O₂ volume fraction of 25%. When the inner secondary stream O₂ volume fraction is increased from 21% to 25%, the peak temperature reaches 1780 K, while the NO mass concentration first decreases and then increases, with a minimum value of 276 mg/Nm³ at an O₂ volume fraction of 22%. When the outer secondary stream O₂ volume fraction is increased from 21% to 25%, the NO mass concentration also first decreases and then increases, with a minimum value of 317 mg/Nm³ at an O₂ volume fraction of 22%. When the overfire stream ratio is increased from 20% to 36%, NO emissions are reduced by 34.9%. Overall, O₂ volume fractions of 24% in the primary stream and 22% in both the inner and outer secondary stream, together with an overfire stream ratio of 36%, are recommended to simultaneously ensure stable oxygen-enriched combustion and effective NO control under ultra-low-load conditions. The results provide theoretical support for by-product oxygen utilization from hydrogen production using renewable energy and for the application of oxygen-enriched combustion technology to deep peak regulation of boilers.