| In recent years,the smog weather has occurred frequently in China.Nitrogen oxides(NO_x)are one of the main precursors of smog formation.Chinese government has issued the most stringent emission standard for coal-fired power plants.The ultra-low emissions of coal-fired power plants need to be achieved by 2020,in which NO_x emission concentration is not higher than 50mg/m3(O2=6%).The reserve of anthracite is abundant and globally distributed in China.The down-fired boiler has an irreplaceable position for burning anthracite because of its own structural characteristics,which prolongs the residence time of pulverized coal particles in the furnace,so the down-fired boiler is widely applied in power plants.As one of the two types of down-fired boilers,the down-fired boiler with swirl burners manufactured with Babcock & Wilcox(B&W)company in the United States has a large market share.However,NO_x emission and carbon content in fly ash are extremely high,and ultra-low-load combustion stability cannot be realized for this boiler.It is necessary to study gas/particle flows,combustion,and NO x formation and control of the down-fired boiler.A new combustion technology for the anthracite-and down-fired boiler needs to be further developed,and achieves high-efficiency and low-NO_x combustion and ultra-low-load combustion stability.To realize low-NO_x emissions of a down-fired boiler with swirl burners,a deep-air-staged combustion technology was proposed and applied to a 300-MW B&W down-fired boiler.By industrial experiments,the effects of different overfire air ratios and secondary and staged air distributions on pulverized-coal ignition,burnout and NO_x emission characteristics of the down-fired boiler with deep-air-staged combustion technology were investigated.The key parameters affecting carbon content in fly ash and NO_x emissions were revealed.The optimal operation parameter settings were also determined.The overfire air ratio is 19.7%,and the ratio between secondary and staged air amount is 47.1%:13.5%.To reduce carbon content in fly ash,an eccentric-swirl-secondary-air combustion technology(ESSACT)was further developed based on above-mentioned deep-air-staged combustion technology.The comprehensive and systematic research was carried out through gas/particle two-phase experiments,industrial experiments and numerical simulations.By establishing a single-phase cold-modeling experimental system and using a constant-temperature hot wire anemometer,the axial and tangential velocity distributions of air flows at the outlet of the eccentric-secondary-air swirl burner were measured.At the same time,and the air flow characteristics in the down-fired boiler with respect to B&W combustion technology and ESSACT were measured and compared.It was determined that the ESSACT can promote pulverized coal burnout and reduce NO_x formation.Finally,the velocity decay of air flows from the arches,the symmetry of flow field in the furnace and the downward depth of air flows at front wall and rear wall sides at different primary air ratio were studied.When the primary air ratios are 17.09% and 19.08%,the flow field in the furnace is deflective.When the primary air ratio increases from 21.11% to 24.75%,a symmetrical flow field in the furnace is formed.With the primary air ratio increasing from 17.09% to 21.11%,the dimensionless downward depth of air flows at front wall side increases linearly from 0.364 to 0.521.The gas/particle two-phase cold-modeling experimental systems were set up for a down-fired boiler with respect to the eccentric-secondary-air swirl burners and B&W swirl burners,respectively.Through the particle dynamic analyzer(PDA),the gas/particle flow characteristics under the two combustion technologies at a full load and an ultra-low load(30% rated load)were comprehensively compared and analyzed,respectively.The effectiveness and advancement of the ESSACT in promoting pulverized coal burnout,reducing NO_x formation,and improving low-load combustion stability were determined.At the same time,the effects of vent-air inclination angle,burner injection angle and overfire air ratio on the gas/particle flow characteristics of the down-fired boiler with ESSACT were studied.With increasing vent-air inclination angle and decreasing burner injection angle or overfire air ratio,the recirculation zone and recirculation velocity below the arches increase continuously.When the vent-air inclination angle increases from 28° to 50°,the particle volume flux below vent air nozzles increases continuously,and the downward ejection ability of vent air to the air flows from the arches increases.With decreasing burner injection angle,the maximum vertical velocities of gas/particle flows in the lower furnace increase continuously.In the region below the staged air,maximum particle volume flux with burner injection angles of 0° and 8°is more than two times of that with burner injection angles of 15° and 25°.It is recommended that the vent-air inclination angle and the burner injection angle be set at 40° to 50° and below 8°,respectively.By comprehensive industrial experiments(including flue gas temperature in the burner outlet region,flue gas temperature distribution in the furnace,flue gas species concentrations in the primary combustion zone,and NO_x and CO emission concentrations at the furnace outlet,carbon content in fly ash and the exhaust gas temperature),anthracite combustion and NO_x emission characteristics under B&W technology,deep-air-staged technology and ESSACT at different loads are analyzed in detail firstly.At the same time,by increasing the output of the primary air fan and decreasing the vent air opening,the primary air velocity of the burner was greatly increased to further reduce carbon content in fly ash.When the primary air ratio is 18.47%,flue gas temperature in the furnace hopper is seriously asymmetric.When the primary air ratio increases from 22.96% to 23.85%,carbon content in fly ash decreases continuously,and NO_x emission concentration at the furnace outlet increases significantly.With decreasing vent air damper opening from 100% to 40% and increasing primary air velocity of the burner from 23.9 to 32.6 m/s,the boiler thermal efficiency increases from 91.0% to 92.3%,and NO_x emissions at furnace outlet slightly increase.It is recommended that,the primary air ratio is 22.96% and the vent air damper opening be set at 40%.Compared with B&W technology,carbon content in fly ash is reduced by four percentage points and NO_x emission concentration is reduced by 43.3% for ESSACT.The industrial experiments also verified that ultra-low-load combustion stability was successfully realized by the ESSACT.By numerical simulations,the effects of different primary and vent air amount distributions and overfire air ratios on flow,anthracite combustion and NO x emission characteristics were studied.With increasing the ratio between primary and vent air amount increasing from 4:6 to 6:4 and primary air velocity from 20.48 to 30.72 m/s,carbon content in fly ash decreases from 8.97% to 5.35%,and NO x emissions at furnace outlet increase slightly from 672 to 729 mg/m3(O2=6%).When the overfire air ratio increases from 20% to 25%,carbon content in fly ash increases obviously.When the overfire air ratio increases from 15% to 20%,NO x emission concentration decreases obviously.It is recommended that the ratio between primary and vent air amount and the overfire air ratio be set at 6:4 and 20%,respectively. |
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