| China is a large coal producing and consuming country.The Chinese government has requested to further enhance the efficient and clean development of coal power,focusing on the proposal that coal-fired power plants should adopt more advanced combustion technologies.Down-fired combustion is one of the three boiler combustion modes in the coal-fired power plants,which is designed for burning anthracite,lean coal and other hard-to-burn coal.In recent years,with the change of coal market supply,the down-fired boiler began to burn blended bituminous coal or single bituminous coal.The existing swirl-flow down-fired boiler burned with blended bituminous coal has some problems,such as burner nozzle burnout,water-cooled wall slagging and denitrification inlet over temperature,which affects the safe and stable operation of the boiler.Therefore,in order to solve the above problems,it is necessary to develop a new combustion technology applicable to the blending of bituminous coal in the swirl-flow down-fired boiler.The causes of burner nozzle burnout,water-cooled wall slagging and denitrification inlet over temperature after burning blended bituminous coal in existing swirl-flow down-fired boiler were revealed through industrial experiments.Primary air biased combustion organization method was proposed for the swirl-flow down-fired boiler burning blended bituminous coal,and the primary air biased down-fired combustion technology（referred to as primary air biased technology）was developed.And through single-phase cold-modelling experiments,gas/particle two-phase cold-modelling experiments and numerical simulations,a comprehensive and systematic study of the flow,downwash and bituminous coal combustion characteristics in the swirl-flow down-fired boiler with primary air biased technology was conducted.Industrial experimental research was conducted on the temperature field and atmosphere field characteristics in the furnace of 300 MWe swirl-flow down-fired boiler with American B&W（Babcock&Wilcox）technology burning single bituminous coal and 600 MWe swirl-flow down-fired boiler with central air supplied technology of Beijing B&W company burning blended bituminous coal.It was found that after the swirl-flow down-fired boiler burning blended bituminous coal,the ignition distance of coal/air flow was generally less than 0.25 m,and it could achieve strong stable combustion,high efficiency and low-NOx combustion.However,due to the significant reduction in the ignition heat,the significant shortening of the ignition distance of the coal/air flow and upward shift of the flame center in the furnace,the boiler had experienced the burner nozzle burnout,the water-cooled wall slagging and denitrification inlet over temperature.To solve the above problems,the primary air biased combustion organization method was proposed for the swirl-flow down-fired boiler burning blended bituminous coal,and the primary air biased technology was developed.Single-phase cold-modelling experimental rig with a modelling ratio of 1:10 was established,and the single-phase flow characteristics in the swirl-flow down-fired boiler using the primary air biased technology and the central air supplied technology were carried out using an IFA 300 thermostatic hot-wire anemometer.And the advantages of the primary air biased technology in preventing slagging on the arch water-cooled wall and increasing the downwash of the coal/air flow were quantitatively analyzed.The dimensionless downwash depth of the main airflow was 0.430 and 0.525 under the central air supplied technology and primary air biased technology,respectively,and the downwash depth of the main airflow was greater under the primary air biased technology.By closing the vent air damper opening to enhance the primary air ratio of the burner,the dimensionless downwash depth at different primary air ratios were around 0.525.In the swirl-flow down-fired boiler with the primary air biased technology,reasonable cold flow field airflow folding upward position ensures that the coal particle cannot scour the hopper water-cooled wall even if the vent air damper opening is fully closed in actual combustion.Gas/particle two-phase cold-modelling rig with a modelling ratio of 1:20 was established,and the velocity field and particle field distribution in the furnace were measured using a 3D-phase Doppler Particle Anemometer（PDA）.Comparative study of the gas/particle phase flow characteristics in the swirl-flow down-fired boiler with the primary air biased technology and the central air supplied technology was carried out.The vent air nozzles location of the boiler with the primary air biased technology was optimized and the effect of the overfire air ratio on gas/particle two-phase flow characteristics was quantitatively analyzed.The results showed that compared with the central air supplied technology,the peak velocity of gas/particle two-phase in the vertical direction of the under-arch area was higher and decayed slower,and the diffusion range of the particle airflow in the horizontal direction was smaller and more concentrated in the down-fired boiler with primary air biased technology.The position of particle airflow folding upward was late,and the depth of downward impulse was large,and the volumetric flow rate decayed slow in the down-fired boiler with primary air biased technology.The primary air biased technology was beneficial for delaying the ignition of the coal/air flow in actual boiler,preventing slagging on the arch water-cooled wall,and reducing gas temperature of the denitrification inlet.The recommended better arrangement of the vent air nozzles for the primary air biased technology at a vertical distance of 145 mm from the lower edge of the arch（2.9 m in the actual boiler）.On the basis of the vent air nozzles arrangement at 145mm from the lower edge of the arch,with the increase of the overfire air ratio,the vertical velocity of the airflow and the particle volume flux decay become fast,the fold upward was advanced and the downward depth was reduced.Three-dimensional numerical calculation model was established,and the comparative study of the flow,bituminous coal combustion and NOx emission characteristics in the swirl-flow down-fired boiler with the primary air biased technology and the central air supplied technology was carried out.Quantitative analysis of the advantages of primary air offset technology in preventing burnout of the burner nozzle,preventing slagging on the arch water-cooled wall,and reducing gas temperature of the denitrification inlet.Optimized key operating parameters such as overfire air ratio and separate secondary air ratio under the down-fired boiler with the primary air biased technology.The results showed that,compared with the central air supplied technology,the swirl-flow down-fired boiler with the primary air biased technology had delayed ignition of the coal/air flow and gas temperature in the burner nozzle area was lower,which helped avoid burner nozzle burnout.The area near the water-cooled wall of the throat did not occur violent combustion of pulverized coal,the regional temperature was lower and the oxidizing atmosphere was stronger,which was conducive to preventing slagging of the water-cooled wall.The downward depth of the coal/air flow was obviously increased,and the flame center in the furnace was shifted downward,which was conducive to preventing slagging on the arch water-cooled wall and reducing gas temperature of the denitrification inlet under the primary air biased technology.Compared with the central air supplied technology,the NOx emissions,flue gas temperature and carbon in the fly at the furnace outlet were reduced by 37 mg/m3@6%O2,267 K and 1.31 percentage points respectively in the down-fired boiler with the primary air biased technology.Considering the flow field,temperature field and atmosphere field in the furnace,the better combination of operating parameters for the swirl-flow down-fired boiler with the primary air biased technology was 15%overfire air ratio,8%separate secondary air ratio and 16 m/s separate secondary air speed.The boiler with the primary air biased technology under better operating parameters can achieve the effect of large downwash depth of the coal/air flow,good effect of preventing burner nozzle burnout and water-cooled wall slagging,reasonable denitrification inlet gas temperature,low NOx emissions and high pulverized coal burnout ratio. |
