| The air pollution in our country is typical pollution of coal-burning. When anthracite and lean coals are taken as the fuel of the boiler with swirl burners, it is found that NOx emissions remain at a high level. By technology of OFA system, NOx emissions can be decreased to meet requirements of environment protection, but at this time, it is hard to form the recirculation zone, so the ignition and combustion stabilization of low-volatile coals become weakened, leading to a series of problems including water-cooled wall slagging, high temperature corrosion and boiler efficiency depression. Aiming at the above problems, an IFA300 constant temperature hot-film anemometer system and a temperature tracing device were used to do the bench-scale cold modeling tests. And meanwhile, numerical simulations were performed to study the characteristics of combustion and NOx emissions in furnace. According to the studies, the CFR swirl burner which uses fuel of low-volatile coals under the deep air-staged condition was developed. And then industrial cold state aerodynamic field tests and hot state tests were performed after the improved burners were applied on five 300 MW utility boilers which burns low-volatile coals.The results of cold modeling test for single-phase airflow indicated that, a steady heart-shaped central recirculation zone, substantially symmetric with respect to the burner axis, can always be formed closely next to the outlet of burners under different operating parameters and configuration parameters, and that there is always a certain distance between each beginning point and the outlet of OSA. In addition, under different OFA ratio, different ratio of inner and outer SA and different vane angle of ISA and OSA, the maximum length of the recirculation zone is between1.7D1 and 2.1D1, the maximum diameter is between 0.8D1 and 1.0D1, and the corresponding spread angle of jet between 64° and 93°. The results of in situ cold state aerodynamic field tests on 300 MW boiler units indicated that, under different SA ratio and vane angle of OSA, the maximum length of the recirculation zone is between 1.4D and 1.9D, the maximum diameter is between 0.8D and 1.4D,and the corresponding spread angle of jet is between 67° and 96°. Accordingly, the problem is overcome that recirculation zone in boilers with swirl burners is difficult to form under deep air-staged condition.A modeling test for single-phase airflow shows that: in cases that secondary air flow decreases, ratio of inner and outer secondary air increases, ISA vane angles decrease and OSA vane angles increase, the corresponding swirl numbers all reduce,the size of recirculation zones all decrease, the peaks of axial, radial and tangential velocity and relative recirculating rates all tend to decrease, and the maximum value of relative recirculating rates is between 1.31-3.58. The mixing degree of primary and secondary air weakens, and the concentration of primary air shows an increasing trend, while the maximum mixing rate shows an decreasing trend. And the maximum axial mixing rate is between 0.65-1.35 on the measuring cross section along the direction of the jet.The influence of boiler load, rate of OFA and the oxygen content at the outlet of the furnace to combustion and NOx formation characteristics in furnace is given, by means of full furnace thermal state simulation for a 300 MW boiler which burns mixing coals of anthracite and lean coal. Results of simulation and that of industrial experiment are in great agreement. After using CFR swirl burners suitable for burning low volatile coal in the condition of deep air staging, the combustion in furnace is stable and the emission of nitrogen oxide decreases under situation that OFA rate and oxygen in the furnace outlet are both reasonable. When the OFA rate is17.74% and 27.50% under 300 MW load, the temperature field near the nozzle of the burner is relatively symmetrical. When the OFA rate is 0, 7.74% and 37.74%, the temperature field appears slightly skewed, and the secondary air rate deviates from the normal operating range of the burner. In the later stage of coal combustion, with the OFA rate decreasing and the oxygen near the furnace outlet increasing, oxygen supply in the main combustion region is more adequate and the concentration of it decreases slowly; the concentration of carbon monoxide decreases, and the concentration of nitrogen oxides increases; the temperature in upper part of the main combustion region and lower part of the burnout region shows an increasing trend,and the range of high temperature flue gas expand. With the height of furnace increasing, the difference of the oxygen concentrations between different OFA rates decreases. Under load of 300 MW, with the OFA rate increasing from 0% to 27.5%,the content of combustible matter in fly ash increases from 3.87% to 4.57%, the growth rate of which is only 0.7%, while nitrogen oxide emission decreases from1008.8mg/m3(O2=6.0%) to 845.1mg/m3(O2=6.0%), with a high drop rate of 19.4%.When burning mixing coals of anthracite and lean coal, nitrogen oxide emission of the 300 MWe boiler which adopts the swirl combustion technology from Babcock& Wilcox Co is over 1250mg/m3(O2=6.0%). When using mixing coals of lean coal and bituminous coal, its nitrogen oxide emission is up to 950mg/m3(O2=6.0%) or more with high temperature corrosion and slagging happened. Applying the CFR swirl burner which uses fuel of low-volatile coals under the deep air-staged condition to four 300 MW boilers which burns mixing coals of anthracite and lean coal and a 300 MW boiler which burns mixing coals of lean coal and bituminous,main steam pressure and temperature both reached the designed value, and problemsof sharp drop in nitrogen oxides emission, combustible matter in fly ash being essentially the same and high temperature corrosion and slagging are all eased. The hot-state experiments showed the influences of the OFA rate, vane angles of OSA,boiler’s load on the combustion and nitrogen oxides formation in the furnace. When the fuel was made up with anthracite and lean coal at the ratio of 7:3, the following results is concluded:(1) As the OFA rate increase or vane angles of OSA(in the range of 25°-40°) decrease, rising speed of gas temperature is as high as approximately 1650°C/m, corresponding ignition point located between0.2≤X≤0.4m(X is the distance to the primary air outlet surface along the axial direction of the burner), and oxygen consumption rates were accelerated,concentration gradient changed faster, carbon monoxide concentration increased,and the nitrogen oxide concentration reduced at X≥0.8m region and flattened. With the increase of the OFA rate, temperature reduced in the main combustion zone while it grew in burnout zone; combustible matter in fly ash increased from 5.18%to 6.84%, NOx emissions reduced from 833.4mg/m3(O2=6.0%) to 769.9mg/m3(O2=6.0%), and boiler efficiency was dropped down slightly from 91.53% to90.99%.(2) At load of 150 MW, rising speed of gas temperature is as high as approximately 1004°C/m and relevant ignition point locates near X=0.532 m.Combustible matter in fly ash is 3.91% and NOx emitted is 743.7mg/m3(O2=6.0%)with boiler efficiency being 91.29%. The boiler can operate normally without injecting oil.While mixing coals of lean coal and bituminous coal by ratio 1:1 are used, the following results are concluded: At load of 300 MW, rising speed of gas temperature is as high as approximately 1511°C/m and relevant ignition point locates near X=0.331 m. Combustible matter in fly ash is 4.78%. NOx emitted is 593.4mg/m3(O2=6.0%) with boiler efficiency being 91.81%. The boiler can operate normally without injecting oil at 150 MW load. |
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