| China is one of the few countries in the world with rich reserves of lean coal and anthracite which are hard to burn coal,and more than 40% of the power generation in China comes from firing hard-to-burn coal.Down-fired boiler,as an important boiler type for firing this kind of coal,has received great development in China.The down-fired multi-injection multi-stage combustion technology proposed by Harbin Institute of Technology has shown many advantages in firing hard-to-burn coal,including timely coal ignition,symmetric and stable combustion,slight slagging,and the NOx emission levels are significantly lower than similar technologies.While there are still rooms for improvement on its actual application to the 600 MW supercritical down-fired boiler,that is,the water wall in the lower furnace was vulnerable to overheating,the carbon in fly ash and NOx emissions still had a large room for reduction?the optimal NOx emissions and carbon in fly ash were 878 mg/m3?O2=6.0%?and 9.81%,respectively?.Supported by the National Natural Science Foundation of China,this paper aims to conduct a comprehensive study on the above three aspects.First,in the case of a 600 MW supercritical down-fired boiler with 12 burner groups?each group has a centralized pattern of two single burners?uniformly arranged on arches,the full-scale cold air trajectory tracer tests indicated that,increasing the tertiary-air damper opening from 55% to 100% influenced less on the airflow downward depth,the downward airflows all had large penetration depths.On increasing the tertiary-air damper opening from 55% to 100% in the industrial thermal experiments,the measurement results indicated the fuel-rich flow containing around 55% of the total fed coal ignited at a distance of around 1.4 – 2.3 m,the fuel-lean flow containing around 45% of the total fed coal did not ignite within a distance of 4.4 m,the highest temperature only reached 771 °C.A large share of unburnt coal fell into the hopper zone and ignited,the hopper near-wall temperatures were as high as 1200 °C.Carbon in fly ash was in the range of 7.02% – 9.53%,the corresponding NOx emissions were in the range of 801 – 1023 mg/m3?O2=6.0%?.Therefore,it is necessary to further optimize the airflow downward depth,enhance the coal ignition,and deepen the combustion of fuel rich/lean staging and air staging.The numerical simulation and industrial experiment were used to study the stoichiometric ratio of fuel-rich flow.Increasing the stoichiometric ratio of fuel-rich flow from 0.46 to 0.55 facilitated the coal burnout and decreased the downward flame depth effectively,while the NOx emissions kept a high level of 860 – 1045 mg/m3?O2=6.0%?.The experimental scheme of burner arrangement and parameters under 24 burners uniformly arranged on arches was proposed.The 1:20 scale single-phase cold tests were conducted to obtain the in-furnace aerodynamic characteristics.Considering of forming the symmetrical flow pattern,moderate airflow downward depth and the air flux into the hopper zone,the optimal parameters were obtained: the distance between adjacent burners was 1687 mm,the velocities of fuel-rich and fuel-lean flows were 13.11 and 17.49 m/s,the mass ratio of pulverized-coal in fuel-rich flow to that in fuel-lean flow?MRL?was 9:1,the secondary air velocity was 37 m/s,the secondary air ratio was 39%,and the tertiary air ratio was 22.74%.Under the combination of above optimal parameters,the flow field presented good symmetry.Compared with the results under the 12 burner groups uniformly arranged on arches,the front-arch airflow downward depth and air flux into the hopper zone reduced by 6.3% and 14.9%,respetively.On this basis,reducing the inner-secondary-air ratio from 15.16% to 9.41%,the results indicated that the velocities of boundary and outer secondary air decayed more quickly than the velocity of inner secondary air,the average dimensionless depth of downward airflow reduced from 0.830 to 0.788 while the horizontal velocity in the recirculation zone increased.A 1:40-scaled two-phase cold system and the Phase Doppler Anemometer?PDA?were used to investigate the gas/solid flow characteristics under 24 burners uniformly arranged on arches.Upon increasing the MRL from 6:4 to 9:1,the maximum particle volume flux under the fuel-rich flow nozzle significantly raised,the velocity decay of the downward gas/solid flow was accelerated.The particle volume flux in the hopper zone gradually reduced while the location of the maximum particle volume flux gradually moved away from the hopper declined wall.On reducing the inner-secondary-air ratio from 19.66% to 7.66%,the downward entrainment effect of the inner secondary air on the fuel-rich flow gradually weakened,the fuel-rich flow turned to the furnace center ahead.Reducing the inner-secondary-air ratio delayed the dilution of inner secondary air on the fuel-rich flow and simultaneously slowed down the decay of particle volume flux in the near-burner region,which was conducive to the ignition of fuel-rich flow in the actual boiler.Numerical simulations were conducted to predict the combustion and NOx formation characteristics in the furnace under 24 burners uniformly arranged on arches.Upon increasing the MRL from 6:4 to 9:1,the ignition distance of fuel-rich flow shortened from 1.2 to 0.7 m.The flame center in the lower furnace moved upwards,the area of high-temperature zone in the furnace hopper decreased and the high-temperature appearance position gradually moved away from the hopper wall.The carbon in fly ash and NOx emissions respectively reduced from 5.87% to 5.52% and from 778 to 662 mg/m3?O2=6%?.With the inner-secondary-air ratio decreasing from 19.66% to 7.66%,the ignition distance of fuel-rich flow shortened from 1.0 to 0.8 m.The airflow downward depth reduced,and the hopper near-wall temperatures maintained in the range of 600 – 800 °C.Reducing the inner-secondary-air ratio enhanced the coal burnout,while was adverse to inhibit the NOx production,the NOx emissions increased from 662 to 726 mg/m3?O2=6.0%?.The industrial experiments were conducted under the 24 burners uniformly arranged on arches.The results indicated that,compared with the results under 12 burner groups uniformly arranged on arches,the tendency of downward airflow flowing to the furnace center enhanced,the airflow downward depth significantly reduced,and the phenomenon of downward airflow washing the hopper declined wall disappeared;On increasing the overfire air damper opening from 20% to 70%,the fuel-rich flow containing around 90% of the total fed coal ignited at a distance of 0.66 – 1.05 m,the fuel-lean flow containing around 10% of the total fed coal still had no ignition within a distance of 4.4 m.The hopper near-wall temperatures were in the range of 700 – 800 °C,which were around 400 °C lower than the temperatures under 12 burner groups uniformly arranged on arches,the previous water-wall overheating phenomenon in the lower furnace disappeared.The NOx emissions decreased from 702 to 575 mg/m3?O2=6.0%?,and the carbon in fly ash increased from 5.65% to 6.4%;On increasing the tertiary-air damper opening from 40% to 70%,the ignition distance of fuel-rich flow shortened from 1.25 to 0.87 m.The hopper near-wall temperature gradually reduced,with a reduction of around 50 °C at the same measuring position.The corresponding NOx emissions increased from 593 to 641 mg/m3?O2=6.0%?,and carbon in fly ash decreased from 6.91% to 6.15%;Upon reducing the inner-secondary-air damper opening from 100% to 50%,the ignition distance of fuel-rich flow shortened from 1.36 to 1.11 m,the downward flame depth gradually reduced.Carbon in fly ash decreased from 7.31% to 6.93%,and the NOx emissions increased from 703 to 706 mg/m3?O2=6.0%?.Taking all this into account,the optimal secondary-air,tertiary-air,overfire-air and inner-secondary-air damper openings of 95%,70%,55% and 100% were recommended,respectively.Under the combination of these optimal damper openings,the NOx emissions and carbon in fly ash attained levels of 589 mg/m3?O2=6.0%?and 6.18%,respectively.Compared with the results under 12 burner groups uniformly arranged on arches,achieving a significant reduction in NOx emissions and carbon in fly ash of about 33% and 37%,respectively. |
PDF Full Text Request