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To further reduce the NOx emissions of the down‐fired boiler while improving its low‐load stable combustion capacity, a new low NOx combustion organization mode that utilizes the high velocity parallel oil‐secondary air to improve the pulverized coal combustion process was proposed based on a novel stable combustion technology. We carried out cold airflow experiments under different oil‐secondary air and F‐layer secondary air distributions (mass flow rate ratios of 1:9, 1:3, 2:3) based on a 300 MWe Foster Wheeler down‐fired boiler to study its influence on the flow and mixing characteristics of the coal/airflow. Results show that the mixing between the oil‐secondary air and the fuel‐rich coal/airflow is gradually delayed with the increase of the secondary air distribution. When the secondary air distribution reaches more than 2:3, the oil‐secondary air starts to become the dominant airflow that influences the whole flow field in the furnace. Based on the result of the cold airflow experiment, we carried out industrial measurements under different parallel oil‐secondary air and F‐layer secondary air distributions (4:6, 5:5, 6:4) on a 600 MWe FW down‐fired boiler under the condition of using the high velocity oil‐secondary air. The ignition distance of the fuel‐rich coal/airflow, the flue gas temperature, the O2, CO, and NOx concentrations in the furnace were measured. At the secondary air distribution of 4:6, the ignition distance of the fuel‐rich coal/airflow was about 1.31 m. When the secondary air distribution was increased to 5:5 and 6:4, the ignition was put off, but the pulverized coal/airflow could still catch on fire in time. With the increase of the parallel oil‐secondary air proportion, the flame center in the furnace did not move downward substantially. In three cases, the lowest O2 concentrations in the inspection port area under the burner were all below 1% (peak CO concentration values were all above 28,000 ppm). The pulverized coal all burned in a strong reducing atmosphere. At secondary air distributions of 4:6, 5:5, and 6:4, NOx emissions of the boiler were, respectively, 825.83, 819.48, and 853.86 mg/Nm3 (O2 = 6%). The thermal efficiency was, respectively, 90.09%, 89.16%, and 90.98%. In actual operation, the recommended secondary air distribution is 6:4. [ABSTRACT FROM AUTHOR]

Effects of adjusting the inner-secondary-air damper opening among 100%, 70%, and 50% on the flow, combustion, and NO x emissions of a 600-MWe down-fired boiler incorporating multiple-injection and multiple-staging technology were investigated. Both 1 : 20-scale aerodynamic tests and industrial experiments were conducted. The small-scale tests showed that symmetric flow patterns formed in all cases, and the velocities of boundary and outer secondary air decayed more quickly than the velocity of inner secondary air. As the inner-secondary-air damper opening decreased from 100% to 50%, the average dimensionless depth of downward airflow reduced from 0.830 to 0.788 while the horizontal velocity in the recirculation zone increased. Industrial experiments demonstrated that, as the inner-secondary-air damper opening decreased from 100% to 50%, the heating rate of fuel-rich coal/air flow increased from 572 to 687°C/m, the corresponding ignition distance shortened from 1.36 to 1.11 m, and the hopper near-wall temperature decreased by approximately 50°C. Carbon in fly ash decreased from 7.31% to 6.93%, and NO x emissions rose from 343 to 371 ppm at 6% O2. The boiler efficiency improved from 89.78% to 89.92%. An inner-secondary-air damper opening of 70% is more reasonable than 50% and 100% considering the ignition distance, carbon in fly ash, and NO x emissions. Copyright © 2017 Curtin University of Technology and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Numerical simulations are carried out to investigate the influence of burner nozzle–organization–mode (N–O–M) on flow field distribution and combustion characteristics of a 300‐MWe subcritical down‐fired boiler. Three typical N–O–Ms respectively designed with Mitsui Babcock Energy Limited (MBEL), fuel‐lean coal/air flow down‐setting (FD), and multi‐injection multistaging combustion (MIMSC) technology are studied, plus industrial‐size measurements on the original MBEL boiler. Vertical velocity attenuation index (η) and maximum dimensionless penetrating depth (γ) are introduced to estimate the transfer effect and penetrating capacity of pulverized coal/air flow. Results uncover that under the N–O–M based on MBEL technology, flow field and temperature field are deflective. η is 9.257. Under N–O–Ms based on FD and MIMSC technology, flow field and temperature field present obvious symmetry. η respectively are 4.365 and 2.921. γ are correspondingly about 0.8 and 1.36. Carbon content in fly ash and NOx emissions at furnace outlet are lowest (5.77% and 735.21 mg/m3 (O2 = 6%)) under the N–O–M with MIMSC technology. The bigger the η is, the worse penetrating characteristics of coal/air flow is, resulting in poor burnout of pulverized coal. Therefore, MIMSC technology is recommended for the boiler improvement, and η should be taken into account in the design of new down‐fired boiler N–O–Ms. [ABSTRACT FROM AUTHOR]