Application of eccentric-swirl-secondary-air combustion technology for high-efficiency and low-NO_x performance on a large-scale down-fired boiler with swirl burners

A 300-MW_e anthracite and down-fired boiler equipped with swirl burners, which was retrofitted with the previously proposed deep-air-staging combustion technology to reduce particularly high NO_x emissions, still suffered from high carbon content in fly ash (despite this content being slightly lower than that before retrofit) on basis of significant NO_x reduction. To comprehensively produce a high-burnout and low-NO_x setting, a novel combustion technology was proposed in which relative to the axis of the primary air duct, the axes of the inner and outer secondary air ducts of the swirl burner shift away from the furnace center, and this technology is called an eccentric-swirl-secondary-air combustion technology. The coaxial symmetrical arrangements of primary and secondary air ducts for traditional swirl burners are broken. Full-scale industrial measurements under original, deep-air-staging and eccentric-swirl-secondary-air combustion technologies with respect to different loads (i.e., 180, 250, and 300 MW_e) were carried out to compare and analyze the validity of and improvement offered by the eccentric-swirl-secondary-air combustion technology. Under different boiler loads, especially at low and middle loads, compared with original and deep-air-staging combustion technologies, eccentric-swirl-secondary-air combustion technology markedly extends the penetration depth of coal/air flow and recirculation regions below arches. The thermal resistance between primary coal/air flow and high-temperature flue gas at the furnace center is reduced further, and the ignition of pulverized coal is timelier. The heating gradient in the burner outlet zone is maintained throughout, and the flame fullness in the primary combustion zone increases. The above three aspects contribute to pulverized coal burnout. Similar to deep-air-staging combustion technology, a low-oxygen and strong reducing atmosphere was formed in the primary combustion zone to reduce NO_x formation due to the introduction of overfire air. Compared with deep-air-staging combustion technology, for the eccentric-swirl-secondary-air combustion technology, the air staged combustion in the furnace is improved further, and pulverized coal combustion in the primary combustion zone is more well distributed, which is beneficial to further reducing NO_x emissions. The results of low-NO_x and high-efficiency performance show that compared with the original boiler, NO_x emissions and carbon content in fly ash for the boiler with the eccentric-swirl-secondary-air combustion technology are significantly reduced by above 42% and 32.5 to 20.7% at loads of 180, 250 and 300 MW_e, respectively.