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To address the problem of late ignition and poor burnout of Foster-Wheeler down-fired boilers, this study proposed a new combustion technology, which combines adding on arch secondary air and alternately arranging the fuel-lean coal/airflow (FLCA) and fuel-rich coal/airflow (FRCA). The coal particle trajectories of FLCA and FRCA and overall aerodynamic characteristics in the furnace were studied at FRCA and FLCA ratio (RRL) of 5:5, 7:3, and 9:1. The results showed that even at a low FRCA ratio, the new combustion technology could still effectively ensure the penetration depth of pulverised coal/airflow. At different RRL values, the main airflow from the arch appeared in the near-wall area and could penetrate downwards into the cold ash hopper area. With an increase in the value of RRL, the deflection degree of FLCA towards the furnace wall increased, while that of FRCA decreased. At RRL of 7:3, both the penetration depth and deflection degree of FRCA and FLCA were at a high level, which was conductive to promoting the burnout and recommended in practical applications. [ABSTRACT FROM AUTHOR]

• A novel low-NO x technology is proposed for a down-fired boiler with swirl burners. • Gas/particle flow characteristics are studied under two combustion technologies. • Downward depth of gas/particle flows improves for the improved boiler. • Primary air is effectively ejected by burner secondary air for the novel technology. • Turbulent intensity in the recirculation region increases for the improved boiler. To confirm the validity and progressiveness of the eccentric-swirl-secondary-air combustion technology (ESSACT) in improving the economic and environmental performance of the down-fired boiler (DFB) with swirl burners, the gas/particle (GP) cold-modelling experiments are conducted by using a particle dynamic analyzer (PDA) measurement system in a 1:10 scale model of 300-MW e DFB applying original Babcock & Wilcox combustion technology (BWCT) and novel ESSACT, respectively. The distributions of average velocity, fluctuation velocity and particle size are compared and analyzed under the two combustion technologies in this study, in addition to the decay of particle volume flux and the jet trajectory of downward GP flows. In comparison with the BWCT, the rotation intensity of the burner secondary air is greatly weakened, and the vertical velocities of downward GP flows decay more slowly under the ESSACT. Although the recirculation regions are both formed below the arches under the two combustion technologies, the horizontal velocities and the horizontal fluctuation velocities of GP flows in the recirculation regions are both larger for the ESSACT, which benefits pulverized coal ignition. The particle size distributions under the two combustion technologies reveal that the mixing between the primary air and burner secondary air is obviously delayed and the burner secondary air effectively ejecting the primary air downward is realized by the ESSACT, which is advantageous to improve pulverized coal burnout and inhibit NO x formation. In addition, the maximum particle volume flux in the lower part of the lower furnace under the ESSACT is obviously higher than that under the BWCT, and the downward depth of GP flows and the space utilization ratio in the lower furnace increase for the original BW DFB retrofitted by the ESSACT. [ABSTRACT FROM AUTHOR]

The eccentric-swirl-secondary-air combustion technology (ESSACT) is an innovative technology to comprehensively solve high NO x emission and unburned carbon content in fly ash of the down-fired boiler with swirl burners. In this study, a breakthrough idea (i.e. decreasing burner injection angle (θ b)) is proposed to further greatly improve coal burnout of the improved boiler. Using a laser particle dynamic analyzer, the distributions of average and fluctuation velocities of gas/solid (GS) flows, distribution and decay of particle volume flux and jet trajectory of burner primary air in a 1:10 scale model of full-scale furnace at different θ b of 0°, 8°, 15° and 25° are acquired. The effects of θ b on GS flow characteristics (i.e. downward depth of GS flows, recirculation zones below arches and air flow fullness in the lower furnace) in the furnace are intensively studied. As the θ b decreases, the maximum vertical velocity of GS flows increases in the whole lower furnace, and the vertical fluctuation velocity of GS flows and the turbulent intensity and diffusion also increase. With decreasing θ b , maximum recirculation velocity and dimension of recirculation zone below arches both increase continuously, which is conducive to improving ignition and burnout of pulverized coal. Especially, the maximum recirculation velocity with θ b decreasing from 25° to 15° and the dimension of recirculation zone below arches with θ b decreasing from 15° to 8° increase more obviously. With decreasing θ b , the ejection effects of vent air and staged air on GS flows from arches are more effective. In the region below staged air, the maximum particle volume flux for the θ b of 0° and 8° is more than twice that for the θ b of 15° and 25°. As a result, the downward depth of GS flows and air flow fullness in the lower furnace both increase with decreasing θ b. To improve coal burnout of the down-fired boiler applying ESSACT, the θ b is recommended to be set below 8°. Image 1 • A novel low-NO x technology is proposed for a down-fired boiler with swirl burners. • Gas/particle flow characteristics are studied at various burner injection angles. • Recirculation zone and downward depth of flows both increase with decreasing angle. • Particles are more effectively ejected by vent and staged air with decreasing angle. • The burner injection angle is recommended to be set below 8° for the boiler. [ABSTRACT FROM AUTHOR]

Deep peak shaving requires extremely high requirements for low-load combustion stability of boilers. In this study, a novel swirl burner (NSB) with an eccentric secondary air arrangement was proposed, and the validity and the progressiveness of the NSB in achieving ultra-low-load combustion stability for down-fired boilers (DFBs) were confirmed from laboratory experiments to industrial applications. Firstly, the cold-modelling experiments of gas/particle (GP) two-phase flow characteristics involving two combustion systems (i.e., the DFB with traditional swirl burners (TSBs) and the DFB with NSBs) were performed at an ultra-low load of 90 MW e. Compared with the original boiler with TSBs, the maximum horizontal recirculation velocity and the area of recirculation zone below arches significantly increase for the improved DFB with NSBs. The particle number concentration near furnace center for the improved DFB with NSBs is much higher than that for the original DFB with TSBs. The above two aspects will effectively guarantee timely ignition of anthracite. The downward depth of GP flows and the space utilization ratio of the lower furnace increase, which will be beneficial to promoting volume heat load and heat flux density in primary combustion zone. In addition, full-scale industrial-sized measurements aiming at a 300-MW e DFB improved by NSBs were carried out at ultra-low loads of 100 and 90 MW e. For the original DFB after improved by NSBs, the minimum load for stable combustion without oil support is reduced from 150 to 90 MW e , and the ultra-low-load combustion stability is achieved. For the improved DFB with NSBs, pulverized coal ignition distances for operating burners are about 2 and 2.6 m, respectively, and the signal strength of flame detectors for all operating burners is above 95% at 100 and 90 MW e. The flame fullness and combustion stability are good at the initial combustion stage. At ultra-low loads, furnace negative pressure, superheat steam pressure and oxygen concentration at furnace outlet fluctuate slightly. The temperature at air preheater inlet meets the needs of normal operation of denitrification system, and there is no problem of low temperature corrosion on the surface at flue gas side of air preheater. The unburned carbon in fly ash is about 4%. The maximum concentrations of NO x emission at furnace outlet are 714 and 687 mg/m3 (O 2 = 6%) at 100 and 90 MW e , respectively, and ultra-low emission of NO x after denitrification system is achieved. • A novel swirl burner with an eccentric secondary air arrangement was proposed. • Laboratory and industrial experiments were performed. • Performances of flow, mixture, ignition and combustion are all improved. • Ultra-low-load combustion stability is achieved for the improved down-fired boiler. [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]

In this paper, based on the MIMSC (multi-injection and multi-stage combustion) technology, new burner arrangement and air distribution parameter settings were proposed for a 300-MW subcritical down-fired boiler originally using MBEL (Mitsui Babcock Energy Limited) combustion technology. Numerical simulations were conducted to study the influences of the secondary-tertiary air proportion on the airflow mixing effect in the furnace, the ignition and pulverized coal combustion characteristics. The airflow mixing effect in the furnace is characterized by the size of the dimensionless vertical velocity decay area (V da) and the fluctuation of the maximum vertical velocity decay curve (V dc). During the research, the sum of the secondary and tertiary air rate remained constant, and the secondary air rate was set to 30.54%, 33.54%, 36.54%, 39.54%, and 42.54%. It was found that under the condition of using new type burner, with the secondary air rate increased from 30.54% to 42.54%, the ignition distance of pulverized coal decreased from 1.30m to 0.84m, the dimensionless penetration depth decreased from 1.46 to 1.27, and the NO x emission and carbon in the fly ash decreased first and then increased. The variation of V dc showed the following regulation. At the secondary air rate of 30.54% to 33.54%, increasing the secondary air rate decreased the fluctuation amplitude. While at the secondary air rate of 39.54% to 42.54%, increasing the secondary air rate increased the fluctuation amplitude. Only at the secondary air rate of 36.54%, V dc in the tertiary air mixing area was smooth. When V dc was smooth, V da was small at 0.043, the carbon in fly ash at the furnace outlet was the lowest at 4.19%, and the NO x emission was low at 675.9 mg/m3 at 6% O 2. An optimal secondary air rate of 36.54% is recommended. For the subsequent design of burner structure and parameters, it is suggested that V dc in the tertiary air mixing area should be smooth. [ABSTRACT FROM AUTHOR]

Fuel rich/lean combustion is an important means of reducing NO x in coal-fired boilers. The mass ratio of pulverized-coal in fuel-rich flow to that in fuel-lean flow (MRL) is a key parameter in fuel rich/lean combustion that affects the gas/particle flow characteristics and in turn the coal ignition and NO x formation. In this work, using a 1:40 scale model furnace and a particle dynamics anemometer measurement system, the effects of various MRLs (6:4, 7:3, 8:2 and 9:1) on the gas/particle flow characteristics in a 600 MWe down-fired boiler incorporating multiple-injection and multiple-staging were studied. The results demonstrate that airflows injected from the front-arch burners flow downwards along the front wall and then turn upward in the furnace center zone, forming a U-shaped flow pattern. Increasing the MRL significantly raises the maximum particle volume flux under the fuel-rich flow nozzle, which favors advanced pulverized-coal ignition and improves the fuel rich/lean combustion in the boiler. The velocity decay of the downward gas/particle flow is also accelerated. The particle volume flux in the hopper zone is gradually reduced while the location of the maximum particle volume flux gradually moves away from the hopper declined wall. This scenario avoids the downward flame flushing the furnace hopper and prevents both hopper wall overheating and tube bursts in the actual down-fired boiler. Based on these results, an MRL of 9:1 is recommended as optimal. [ABSTRACT FROM AUTHOR]