| The structure of Chinese energy is currently in a period which movies to renewable source from fossil fuel.The primary aim of energy transformation is to develop renewable energy rapidly.The establishment of the clean,low-carbon,safe,efficient,and modern energy system depends on the consumption of renewable energy.It is also an important interim of the process to the construction of ecological civilization and a beautiful China.Aiming to improve the consumption percentatge of renewable energy in China,it is necessary to improve the adjustment capability of power system and excavate the peak load regulation ability of existing system.For thermal power units,one of the most important points is to improve the minimum load combustion stability of coal-fired boilers.As the core equipment of pulverized coal combustion system,burner plays a key role in the stable combustion characteristics of boilers.In view of the limitation of peak load regulation ability of existing conventional thermal power units,this paper takes a new type of low NO_x swirl burner as the research object,using field test,cold-state test,numerical simulation and other research methods to investigate the effects of the parameters such as the stabilizer shape,the concentrator position,the ratio of air speed(air volume)and swirling intensity on its characteristics about stable combustion.Through field test of peak load regulation for a lignite boiler equipped with a new type of low NO_x swirl burner,the temperature field of burner nozzle flame was measured on-line by a two-color spectral radiation temperature measurement system,to investigate the influence of operation parameters on flame temperature.The results show that,the average temperature of burner nozzle increases 96°C with the opening of inner secondary air volume increases from 40%to 100%,reducing the fineness of pulverized coal and increasing the swirling intensity could increase the flame temperature in varying degrees.Through the fine operation adjustment,the on-line measurement and accurate feedback of two-color spectral radiation temperature measurement system,the low load peak regulation ability of the unit itself can be excavated.The following problems occurred during the test,some burners have low intensity of fire detection signal,poor combustion state and risk of extinguishing fire.while 30%rated load test period of unit.In order to further improve the combustion stability of boiler,a cold test bench should be built for the burner to investigate the influence of its key parameters on the nozzle flow field.A cold-state burner test bench was built to conduct a cold-state trial for the 4:1 reduction model of the prototype burner.The field characteristics of the burner nozzle were analyzed in detail by using particle image velocimetry(PIV)and fireworks tracer method.The results show that the toothed flame stabilizer and the cross bluff body flame stabilizer expand the reflux area to 0.08 m~2 and 0.02 m~2 compared with the prototype burner,the position of concentrator moves forward from?=0.19 to?=0.14,no obvious reflux zone is formed.The high temperature flue gas return rate can be improved by increasing the ratio of inner secondary air to primary air and increasing the swirling intensity,the inner secondary air speed increases from 18.07 m/s to 21.60 m/s,the reflux area enlarges by 0.04 m~2,the blade opening increases from 30%to 60%,the swirling intensity of outer secondary air increases from increases from 0.69 to 1.94,and the reflux area enlarges by 0.14 m~2.The increase of swirling intensity will also increase the airflow expansion angle.A three-dimensional model of the new type low NO_x swirl burner was established.and the numerical simulation method was used to calculate the flow field,temperature field and CO concentration field of the burner nozzle,the results were compared with the test results.The numerical calculation has the same trend as the burner nozzle velocity distribution in the cold test,and is close to the average temperature range of the nozzle measured in the field test.It shows that the numerical calculation can obtain reliable calculation results through the selection of suitable mathematical physical model.The toothed flame stabilizer and the cross bluff body flame stabilizer can move forward the position of reflux zone,pyrolysis of pulverized coal and ignition position compared with the prototype burner,the ignition position moved forward about 0.24 m.The ratio of inner secondary air to primary air has a significant influence on the reflux zone,the reflux velocity and the combustion chamber temperature,the primary air speed decreases from 19.60 m/s to 12.10 m/s,the maximum reflux velocity is increased by 1.5 m/s,and the maximum temperature in the combustion chamber is increased from 1420 K to 1557 K.The swirling intensity mainly affects the flame spreading angle and the reflux velocity,the blade opening increases from 30%to 60%,the swirling intensity of outer secondary air increases from 0.69 to 1.94,the flame spreading angle is increased by 70°,the maximum reflux velocity is increased by 2.7 m/s.Comparing the CO concentration field of the prototype burner and the toothed flame stabilizer,for the prototype burner,the area where CO concentration increased significantly(mole fraction of CO>0.01)is near the outer secondary air nozzle while the toothed flame stabilizer is located 0.04 m after the stabilizer,which is about 0.2 m ahead of the prototype burner.The combustion stabilizer advances the pyrolysis position of the pulverized coal,and reduce the CO concentration in the combustion chamber,which is beneficial to alleviating the problem of high reducing atmosphere on both sides of the furnace and high temperature corrosion of the water wall.Aiming at improving the combustion stability of the new low NO_x swirl burner,firstly,the regulation of thermal power generation units operation should be more specific to explore the potential of thermal power generation units for peak adjustment under low load condition.The main principles of regulation consist of decreasing the fineness of pulverized coal,decreasing the primary air speed suitably,increasing the secondary air speed,and increasing the swirling intensity suitabily.To futher improve the deep peak adjustment capability of generation units,the sturecture of the burner could be re-designed.A toothed or bluff body stabilizer is installed at the primary air nozzle,and the pulverized coal concentrator can be arranged far away from the primary air nozzle while a primary air guide ring is installed to improve the reflux of high-temperature flue gas and ensure the pulverized coal separation of high concentration and low concentration. |
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