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Abstract A new air-staged and low-NO x emission combustion technology has been applied to a 300 MW e anthracite- and down-fired boiler with swirl burners. To achieve the optimum air distribution in the primary combustion zone and further reduce NO x emissions, full-scale industrial experiments varying the ratios between burner secondary air ratio (R bsa) and staged air ratio (R sa) (i.e., R bsa / R sa ratios of 42.9/17.8, 47.1/13.5 and 51.5/9.3), on the premise that the overfire air (OFA) ratio was approximately 20%, were performed to evaluate the overall performance of retrofitted boiler. The improved ignition of coal/air flow was at a distance of 0.8–1.4 m to the burner outlet with increased R bsa / R sa ratio. Compared with the boiler before retrofit, under R bsa / R sa ratios of 47.1/13.5 and 51.5/9.3, the flue gas temperature in the primary combustion zone of the retrofitted boiler increased slightly. Oxygen and carbon monoxide concentrations in the near-sidewall region revealed the furnace flame fullness varied under different R bsa / R sa ratios. With increased the air staged level in the lower furnace, NO x emissions decreased continually and the carbon content in fly ash did not necessarily increased. Under the optimum R bsa / R sa ratio of 47.1/13.5, the NO x reduction efficiency was approximately 46% with increasing boiler thermal efficiency slightly. Highlights • Low-NO x combustion technology was applied to a down-fired boiler with swirl burners. • As NO x emissions increased, carbon content in fly ash did not necessarily decreased. • NO x reduction of 46% was achieved without negative effects at the optimal condition. • Coal ignition and flue gas temperature in the primary combustion zone were improved. • Furnace flame fullness under different operation conditions was revealed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

To burn anthracite and reduce NO x emissions, a new combustion system was applied to a 300 MW e Babcock & Wilcox (B&W) down-fired boiler that included overfire air (OFA) and a decreased flow area of the inner and outer secondary air ducts of the installed swirl burners. Industrial-scale measurements (adjusting OFA ratios from 15.4% to 22.6%) were conducted to evaluate the overall performance of the retrofitted boiler. The experimental results demonstrated that the new combustion system promoted coal/air flow ignition and combustion stability. When the OFA ratio was less than 19.7%, the influence of the burner secondary air ratio on coal/air flow ignition was greater than that of the recirculation region below the arches. The overall combustion level in the primary combustion zone depended on the oxygen content and the recirculation region in the lower furnace instead of the early or late ignition of the coal/air flow. Oxygen and carbon monoxide concentration measurements in the near-sidewall region revealed that the fullness degree of the coal flame varied under different OFA ratios. Considering both environmental and economic effects, 19.7% was chosen as the optimal OFA ratio, thereby achieving a significant NO x reduction of 47% without increasing carbon content in the fly ash. [ABSTRACT FROM AUTHOR]

A new combustion system has been applied to a 300 MW e down-fired boiler with swirl burners to reduce NO x emissions. The unit provided the introduction of overfire air (OFA) and a decrease in the flow area of the inner and outer secondary air ducts of the swirl burners. Industrial experiments on the retrofitted boiler were performed at different loads. Full-scale measurements of the flue gas temperature distribution in the burner outlet region, the furnace temperature distributions measured by a pyrometer and the local mean gas species concentrations in the region near the sidewall were made at loads of 180, 250, and 300 MW e . The results show that the ignition distance increased with decreasing load, especially as the load decreased from 250 MW e to 180 MW e . At three different loads, the retrofitted coal/air flow could be all ignited in time at a distance in the range of 0.6–1.4 m from the burner outlet. Compared with the original combustion system, the ignition distance of the coal/air flow was significantly reduced at a load of 300 MW e . In addition, at a load of 300 MW e , the temperature of the boiler hopper was much higher than that at loads of 180 MW e and 250 MW e . Compared with the original combustion system, the upper furnace temperature decreased slowly with increasing measurement height at a load of 300 MW e after the retrofit. Measurements of the O 2 and CO concentrations in the region near the sidewall indicate that the fullness degree of the coal flame in the furnace at different loads was different. After the retrofit, the reheat steam temperatures reached the design temperature of 541 °C at loads of 180 MW e and 250 MW e , and the average reheat steam temperatures increased by approximately 13 °C. Compared with the original combustion system, a significant NO x reduction (more than 40%) at different loads was achieved without increasing the levels of unburnt carbon in the fly ash. [ABSTRACT FROM AUTHOR]

To achieve significant reductions in NO x emissions without increasing the levels of unburnt carbon in the fly ash, a new combustion system was applied to a 300-MW e Babcock & Wilcox (B&W) down-fired boiler installed with swirl burners. The unit featured introduced overfire air (OFA) and decreased outlet area of the inner and outer secondary-air ducts of the swirl burners. Full-scale measurements (adjusting the inner secondary-air vane angle to 35°, 45°, and 55°) revealed that the influence of the high-temperature recirculating region under the arch upon the combustion and NO x emission characteristics of the boiler is greater than that of the high-temperature flue gas entrained by the swirl burner itself. The ignition distance of the coal/air flow is reduced by at least 1.8 m compared with that of the original combustion system. For the inner secondary-air vane angle of 35°, the coal/air flow ignites earlier than for the vane angles of 45° and 55°. The measurements of the gas species concentrations in the zone near the sidewall indicates that at inspection port 1, the coal flame of the burners does not spread across the entire furnace cross-section for all three vane angles; however, for the vane angle of 35° the flame is spread across the entire furnace cross-section at inspection port 2. For this optimal (35°) inner secondary-air vane angle, the NO x emissions and carbon content in the fly ash reached levels of 674 mg/m 3 (6% O 2 ), and 11.4%, respectively, achieving a significant NO x reduction of 51.9% without increasing the levels of unburnt carbon in the fly ash. [ABSTRACT FROM AUTHOR]

Deep peak shaving requires extremely high requirements for low-load combustion stability of boilers. In this study, a novel swirl burner (NSB) with an eccentric secondary air arrangement was proposed, and the validity and the progressiveness of the NSB in achieving ultra-low-load combustion stability for down-fired boilers (DFBs) were confirmed from laboratory experiments to industrial applications. Firstly, the cold-modelling experiments of gas/particle (GP) two-phase flow characteristics involving two combustion systems (i.e., the DFB with traditional swirl burners (TSBs) and the DFB with NSBs) were performed at an ultra-low load of 90 MW e. Compared with the original boiler with TSBs, the maximum horizontal recirculation velocity and the area of recirculation zone below arches significantly increase for the improved DFB with NSBs. The particle number concentration near furnace center for the improved DFB with NSBs is much higher than that for the original DFB with TSBs. The above two aspects will effectively guarantee timely ignition of anthracite. The downward depth of GP flows and the space utilization ratio of the lower furnace increase, which will be beneficial to promoting volume heat load and heat flux density in primary combustion zone. In addition, full-scale industrial-sized measurements aiming at a 300-MW e DFB improved by NSBs were carried out at ultra-low loads of 100 and 90 MW e. For the original DFB after improved by NSBs, the minimum load for stable combustion without oil support is reduced from 150 to 90 MW e , and the ultra-low-load combustion stability is achieved. For the improved DFB with NSBs, pulverized coal ignition distances for operating burners are about 2 and 2.6 m, respectively, and the signal strength of flame detectors for all operating burners is above 95% at 100 and 90 MW e. The flame fullness and combustion stability are good at the initial combustion stage. At ultra-low loads, furnace negative pressure, superheat steam pressure and oxygen concentration at furnace outlet fluctuate slightly. The temperature at air preheater inlet meets the needs of normal operation of denitrification system, and there is no problem of low temperature corrosion on the surface at flue gas side of air preheater. The unburned carbon in fly ash is about 4%. The maximum concentrations of NO x emission at furnace outlet are 714 and 687 mg/m3 (O 2 = 6%) at 100 and 90 MW e , respectively, and ultra-low emission of NO x after denitrification system is achieved. • A novel swirl burner with an eccentric secondary air arrangement was proposed. • Laboratory and industrial experiments were performed. • Performances of flow, mixture, ignition and combustion are all improved. • Ultra-low-load combustion stability is achieved for the improved down-fired boiler. [ABSTRACT FROM AUTHOR]

Abstract: Using an IFA300 constant temperature anemometer system, cold air experiments on a scale-model of a down-fired pulverized-coal 300MWe utility boiler with swirl burners were performed to investigate the influence of the staged-air flow ratio on flow characteristics in the furnace. This ratio was increased in incremental steps over the range of 7%–20%. The flame reach of the downward airflow was decreased with the ratio increased. A shallower reach is accompanied by a weaker mixing of vent and staged-air. At lower ratios, mixing is good and thus favors coal combustion but weak mixing at intermediate ratios produces a delay in the mixing point. This is compensated by an increasing recirculation zone under the arch that benefits ignition and the appearance of an upward airflow below the mixing point due to an increasing influence of vent air. At higher ratios still, this upward flowing disturbance is centered higher up in the furnace generated primarily by mixing with staged-air as vent-air mixing weakens. [Copyright &y& Elsevier]

Abstract: Industrial experiments were performed on a down-fired pulverized-coal 300MWe utility boiler with swirl burners. Gas temperature, concentrations of gas components (O2, CO, CO2 and NO x ) in the burning region and carbon content in the fly ash were measured with outer secondary-air vane angles of 25°, 32.5° and 50°. Results indicate that with increasing vane angle, NO x emission and boiler efficiency decrease. Overall evaluation boiler efficiency and NO x emission, the vane angle of 32.5° is optimum. Using an IFA300 constant-temperature anemometer system, cold air experiments on a quarter-scaled burner model were also carried out to investigate the influence of various outer secondary-air vane angles on the flow characteristics in the burner nozzle region. No central recirculation zone appeared for vane angles of 25° and 32.5°. Most of the pulverized-coal was ignited in the external recirculation zone. For vane angles of 45° and 55°, a central recirculation zone could be observed, and air flow rigidity and axial velocities decreased rapidly. [Copyright &y& Elsevier]