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To improve the low-load stable combustion capacity of a low-grade coal-fired boiler, this study proposed to reform C-layer secondary air, which was then used to improve the ignition and combustion stability of coal/airflow of a 600 MWe Foster Wheeler down-fired boiler. Cold airflow experiments and industrial measurements were carried out to investigate the influence of the new C-layer secondary air on the aerodynamic and combustion characteristics at low load under different C-layer secondary air ratios and damper openings, respectively. The flow and mixing characteristics of the C-layer secondary air and coal/airflow, the flue gas temperature and NOx, O2, and CO concentration distributions in the furnace at low load were examined. With increase in the C-layer secondary air ratio, the flow range of coal/airflow gradually expanded. The mixing distance first increased and then decreased. At an air ratio of 25%, the mixing distance was the longest. Industrial measurements showed that compared with that at the C-layer secondary air damper opening of 0%, the coal/airflow ignited earlier at 40% (corresponding to the C-layer secondary air ratio of 23.73%), and the flue gas temperature in the furnace increased substantially. At C-layer secondary air damper opening of 0%, 20%, and 40%, carbon content in the fly ash was 4.62%, 5.32%, and 4.20%, respectively. The respective NOx emissions at the furnace exits were 450 mg/Nm3, 378 mg/Nm3, and 427 mg/Nm3, (O2 = 6%). In actual operation, 40% is recommended as the optimal C-layer secondary air damper opening. [ABSTRACT FROM AUTHOR]

A 46 MW pulverized coal industrial boiler with swirl burner had severe slagging and high NOx emission during actual operation. To solve these problems, a full-scale industrial test was carried out. The flue gas temperature and the composition of exhaust gas were obtained under different ratios of secondary and separated secondary air. The results shown that reducing the ratio of secondary and separated secondary air could delay the ignition of pulverized coal and weaken the flame deflection, which was conducive to slowing down the burner slagging problem. However, it will aggravate the erosion of high temperature flue gas on the back wall, and further aggravated the furnace slagging problem. Changing the air ratio couldn't substantially improve the problem of high exhaust temperature. Properly reducing the ratio of secondary and separated secondary air could effectively control NOx emission. When the ratio of secondary and separated secondary air was reduced from 73:27 to 52:48, the NOx emission was reduced by 44.8%. Reducing the ratio of secondary and separated secondary air leaded to the increase of carbon content in fly ash and slag, but the heat loss of exhaust gas was reduced and the thermal efficiency of boiler was increased. [ABSTRACT FROM AUTHOR]

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]

To effectively control the flow and combustion of pulverised coal in down-fired boilers and improve their thermal efficiency while ensuring low NOx emissions, on arch secondary air (OASA) combustion technology and a smoke-tracing analysis were proposed. The aerodynamic characteristics of a 300-MWe Foster Wheeler (FW) down-fired boiler with the OASA ratios (ROASA) of 0%, 10%, 20%, and 30%, the flow trajectories and mixing characteristics of OASA, and fuel-rich coal/air flow (FRA) and fuel-lean coal/air flow (FLA) were studied experimentally. For the ROASA of <10%, FRA and FLA flow to furnace centre soon after they are injected into the furnace. OASA and FLA may mix with FRA before ignition. For the ROASA of 20% or higher, FRA and FLA flow to the near-wall area soon after being injected into the furnace. FRA mixes with OASA after ignition. With the increase in ROASA, the mixing distance between OASA and FRA increases first and then decreases. The penetration depth of coal/air flow and furnace airflow fullness increases. For the ROASA of 30%, the dimensionless penetration depth of coal/air flow can exceed 0.7. Furnace dimensionless airflow fullness at all the dimensionless heights can exceed 0.6. In real operation, the recommended ROASA is between 20 and 30%. [ABSTRACT FROM AUTHOR]

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]

Number of down-fired boilers burning bituminous coal or anthracite blended with bituminous coal has exceeded 10% of the total down-fired boilers in China. However, lacking of the industrial-scale research on the performance of down-fired boiler burning bituminous coal or blended coal, which limits the actual operation adjustment. Industrial-scale experiments on a 300-MWe down-fired boiler fueled with bituminous coal and blended coal were performed, which boiler was originally designed to burn low-volatile coal. Without the over-fire air, air-staging combustion can still be realized when the down-fired boiler burning bituminous coal. The combustion performance in the down-fired boiler under different coal varieties was compared. Compared with the anthracite case, flue gas temperature is higher, coal flame fullness is greater, O2 consumption occurs faster and NOx concentration is lower in the primary combustion zone when burning bituminous coal. Strong reducing atmosphere is formed in the primary combustion zone. The NOx emissions are low at a level of 410 mg/m3 at 6% O2 at furnace exit and boiler thermal efficiency reaches 92.590% when burning bituminous coal. The NOx emissions cannot be further reduced by burning blended coal. In addition, the boiler thermal efficiency reduced by 1.108%. Without the over-fire air, compared with burning anthracite or lean coal, the NOx emissions and carbon in fly ash significant reduction by 70% and 90%, respectively, and the boiler thermal efficiency increased when burning bituminous coal. [ABSTRACT FROM AUTHOR]

• An understanding of the radial air staging method for PC utilization is improved and the key parameter (OFA distribution) is optimized. • Compared to the traditional axial air staging method, for the radial air staging method, the OFA distribution has a greater effect on the heat release from PC particles. • The increased R IO is not conducive to the rapid devolatilization from PC particles, resulting in poorer combustion stability of the boiler. • The increased R IO promotes the oxidation reaction of N released from PC particles, which is detrimental to boiler NO x emission reduction. • The advanced mixture of the OFA with the main airflow facilitates the burnout of the PC particles as the R IO increases. Radial air staging is an innovative method for reducing NO x emissions from coal-fired boilers compared to conventional axial air staging. However, an important issue in using this method is to set the over-fired air (OFA) distribution rationally to achieve good boiler performance. This paper aims to optimize the OFA distribution by evaluating important gas-particle (GP) parameters that influence the pulverized coal (PC) combustion performance. The GP flow experiment is conducted in a laboratory furnace model of a coal-fired boiler with different OFA distributions. The experimental results show that as R IO (Two-level OFA distribution ratio) changes from 0:1 to 1:0, the mixture of the OFA jet and airflow in the main combustion zone is advanced, which is conducive to PC burnout. Meanwhile, the size of the central recirculation zone decreases, resulting in poor PC ignition. It is worth noting that at R IO of 1:0, the central recirculation zone disappears at section y/d = 2.5, indicating that there is no high-temperature flue gas downstream to heat the fresh PC particles, which is extremely unfavorable for the stable combustion of the PC. In addition, at R IO of 0:1, compared to other cases, there are more particles in the central recirculation zone, which promotes the reduction reaction of N element released from PC particles, thus reducing NO x emission. According to the comprehensive evaluation, it is recommended that the R IO of the two-level OFA be set to 1:1 when using radial air staging in a coal-fired boiler. The results provide new insights into the application of the air staging method for efficient and clean PC utilization. [ABSTRACT FROM AUTHOR]

To achieve low-NO x emission in the coal-fired industrial boiler, the radial air staging method is proposed. The location of the over-fired air (OFA) is an essential parameter as it affects the combustion stability, NO x generation, burnout, and slagging trend. These are related to the gas-particle flow such as the recirculation region, gas mixture, and gas-particle mixture. Therefore, in this paper, taking a coal-fired industrial boiler with radial air staging method as an object, in a cold-modeling furnace, the gas-particle flow characteristics under various OFA locations (L D of 1.29, 1.43, 1.72, and 2.00) are studied. When OFA is radially arranged, the OFA jet diffuses towards the furnace center. As the L D increases, the size of the central recirculation region increases, thereby improving combustion stability. When the L D is 2.00, the mixing of the OFA jet and PCC jet obviously is delayed. Moreover, an increase in L D leads to more returned particles in the furnace center, promoting the reduction reaction of NO x. In addition, as the L D increases from 1.29 to 1.72, the gas-particle mixing uniformity is enhanced, which is beneficial to the uniform temperature distribution. Comprehensively, the L D of 1.43–1.72 is recommended when radial air staging is adopted in coal-fired industrial boilers. • The radial air staging method accelerates the mixing of OFA and the main airflow while reducing the mixing intensity. • With the increase of L D , the obstructing effect by radial OFA on the diffusion of the swirling main airflow is weakened. • The increased L D facilitates gas recirculation in the main combustion zone, which is conducive to stable combustion. • The mixing uniformity of GPF increases with L D in general, promoting the uniformity of temperature distribution. [ABSTRACT FROM AUTHOR]

Industrial pulverized coal boiler (IPCB) usually employ a pre-combustion chamber (PCC) to ensure stable combustion. However, there are potential slagging problems in the PCC, which threaten the safe operation of the boiler. Air-staged combustion technology is an effective method to reduce NO x emission, and different excess air coefficients affect slagging. CFD simulation is an important method for studying slagging in the furnace. In this paper, Using CFD simulation with slagging model embedded by user-defined function, the slagging trends in the PCC are studied under different excess air coefficients. The particles' collision rate (CR), deposition rate (DR), and sticking rate (SR) on the PCC walls are obtained by CFD simulation. As the excess air coefficient in the PCC (α PCC) increases from 0.3 to 1.1, the CR of particles on all walls of PCC (CR PCC) increases exponentially, indicating the abrasion degree on the PCC walls is growing rapidly. In addition, due to the sizeable annular recirculation zone when α PCC = 0.3, the DR of particles on all walls of PCC (DR PCC) and the SR of particles on all walls of PCC (SR PCC) are much larger than those under other conditions, the slagging trend is the worse. As the α PCC increases from 0.3 to 1.1, the slagging trends in the PCC first decrease and then increase. To improve the safety of boiler operation, it is recommended that the α PCC be adjusted between 0.5 and 0.9. • The effect of excess air coefficients (α PCC) on the slagging trends of PCC walls is studied by CFD simulation. • The relationship between the air-staged levels and CR PCC is established. • The entrainment effect of the secondary air on the particles is a crucial factor affecting slagging. • The annular recirculation zone distribution is the main reason for the maximum value of SR PCC at α PCC = 0.3. • The slagging trends first decrease and increase as the α PCC increases from 0.3 to 1.1. [ABSTRACT FROM AUTHOR]

To ensure good combustion stability, industrial pulverized coal-fired boilers normally employ a swirl burner with pre-combustion chamber (PCC). Owing to the high cross-sectional heat load of the PCC, there is potential for slagging, which can affect the safe and stable operation of the boiler, but can be avoided or reduced through the selection of suitable operating parameter values. Here, the effects on the slagging characteristics of PCC of varying the ratio of the inner to the outer secondary air mass flow rates in a swirl burner, R SA , from 0.00 to 0.83 were investigated using a computational fluid dynamics (CFD) simulation approach. The rates of collision, deposition, and sticking were analyzed. The results showed that as the R SA increases, the total collision rate of the pulverized coal particles in the PCC increases, and when R SA = 0.83, the total collision rate in the PCC is about 7 times of that when R SA = 0.00. Further, slagging tends to appear on the wall near the outlet of the PCC, and the degree of slagging in the PCC first decreases and then increases with increasing R SA. The optimum R SA range was found to be 0.22–0.38, which is conducive to the safe and stable operation of industrial pulverized coal-fired boilers. [Display omitted] • The slagging characteristics in the pre-combustion chamber were studied. • The rates of collision, deposition, and sticking were analyzed. • Slagging tends to appear near the exit area of the pre-combustion chamber. • The degree of slagging first decreases and then increases as R SA increases. [ABSTRACT FROM AUTHOR]

Radial air staging is a new air staging method for industrial pulverized coal boilers. In this paper, its impact on the gas-particle flow characteristics of industrial pulverized coal boilers was studied experimentally. The gas-particle flow experiments were carried out on a 1:6 scale boiler platform by a particle dynamics anemometer. The three-dimensional velocities of the gas-particle, average particle size and particle volume flux under different air stoichiometries in the furnace were measured. When the air stoichiometry was 0.2, the burner outlet could not form a stable reflux zone to establish a good combustion process. When the air stoichiometry is greater than 0.8, the maximum particle volume flow is also located near the water wall, increasing the slagging tendency on the wall. The appropriate air stoichiometry range for the new burner was 0.4–0.8. [Display omitted] • Radial air staging was a new air staging method. • Laboratory test and industrial application on the radial air staging were performed. • The appropriate range of air stoichiometry is recommended to be 0.4–0.8. • Anti-slagging and low NO x combustion are realized on industrial boilers. [ABSTRACT FROM AUTHOR]

In this study, a novel swirl burner for industrial pulverized coal boiler was studied by laboratory experiments and industrial application tests. First, a cold flow experiment was carried out on a 1:3 scale model experimental platform. The cold air velocities of burner outlet were measured in detail. Moreover, the influence of the mass flow ratio of the inner and outer secondary air on the shape of the central reflux zone and the reflux rate at the burner outlet was analyzed. It was found that a stable central reflux zone can be formed at the burner outlet. In addition, an industry application test was carried out in a 29 MW hot water boiler. The flue gas temperature at the burner outlet and the composition of exhaust gas were obtained under different operating parameters. The influence of inner and outer secondary air damper opening on burner performance (ignition performance, NO x emission characteristics, stable combustion performance, and anti-slagging performance) was analyzed. The influence of inner secondary air damper opening on burner performance was found to be greater than that of outer secondary air damper opening. The maximum concentration of NO x emission at furnace outlet was 364.9 mg/m3, and low-NO x combustion was achieved for industrial pulverized coal boilers. The minimum load without natural gas support was 3 MW. The load regulation rate was close to 10:1, and low load combustion stability was achieved. During three heating seasons, the normal operation of the boiler was not affected by slagging. • Laboratory experiments and industrial application tests are performed. • Influence of different inner and outer secondary air mass-flow ratio are investigated. • Low NO x combustion is achieved for industrial pulverized coal boilers. • Low load combustion stability is achieved for industrial pulverized coal boilers. [ABSTRACT FROM AUTHOR]

Through a gas-particle cold-modeling experiment, the particle dynamic analyzer (PDA) is used to investigate the effects of fuel-lean coal/airflow angle and ratio of oil secondary air to F-layer secondary air on the gas/particle flows characteristics in the furnace of a Foster Wheeler (FW) down-fired boiler with enhanced stable combustion technology. As the fuel-lean coal/airflow angle increases from 10° to 45°, the peak value of the vertical dimensionless velocity of the gas/particle flows gradually increases, and the peak position gradually moves to the near wall region. As the ratio of oil secondary air to F-layer secondary air (the secondary air ratio for short) increased, the peak value of the vertical dimensionless velocity of the gas/particle flows gradually increased and the peak value position of the dimensionless velocity gradually moved towards the near-wall region. In actual operation, for the perspective of performance and safety, the recommended fuel-lean coal/airflow angle and secondary air ratio is 30°and 1:2 respectively. The enhanced stable combustion technology had been applied to two 600 MW FW down-fired boilers, and these boilers can achieve stable operation when burning a wide range of coal quality under the load section from 200 MW to 600 MW. • A Foster Wheeler down-fired boiler retrofitted with novel low-load stable combustion technology is studied by PDA. • Effects of fuel-lean coal/airflow angle and secondary air ratio on the gas/particle flows characteristics were investigated. • The fuel-lean coal/airflow angle of 30° and the secondary air ratio of 1:2 is recommended. • This novel low-load stable down-fired boiler combustion technology is suitable for a wide range of coal quality combustion. [ABSTRACT FROM AUTHOR]

• A novel low-load stable combustion technology is applied on a 600MWe FW down-fired boiler. • 33% rated load stable combustion without oil support is achieved. • Full-scale industrial measurements are carried out at 200, 240 and 540MWe. • Combustion and NOx formation characteristics at high and low load are contrastively studied. The randomness and intermittence of solar and wind power generation require coal-fired thermal power boilers that have a low-load stable combustion capacity. This study proposed a novel low-load stable combustion technology in which the oil secondary air flow is arranged parallel to the fuel-rich coal/air flow and participates in pulverised coal combustion during boiler operation. The method was applied to a 600 MWe down-fired boiler manufactured with the technology of Foster Wheeler Corp. Full-scale industrial measurements of furnace flue gas temperature, fuel-rich coal/air flow ignition distance, and distribution characteristics of NOx, O 2 , and CO in the furnace at loads of 200, 240, and 540 MWe were performed. The results showed that the boiler can stably operate without oil support at 240 MWe (40% rated load) and 200 MWe (33% rated load) respectively burning a blended coal of anthracite and lean coal, and blending bituminous coal. At loads of 200, 240, and 540 MWe, the fuel-rich coal/air flow ignition distances were, respectively, approximately 1.4, 1.7, and 1.3 m. NOx emissions at the furnace exit were 406, 407, and 730 mg/Nm3 (O 2 = 6%), and the boiler thermal efficiencies were 92.42%, 92.45%, and 90.58%, respectively. This novel technology presents the characteristics of high efficiency and low NOx emission while realizing low-load stable combustion of down-fired boilers. [ABSTRACT FROM AUTHOR]

[Display omitted] • Laboratory experiments and industrial trials are conducted for a down-fired boiler. • Effects of air-staging degree (ASD) in furnace of the boiler at low load are studied. • On increasing ASD, the trend of particle diffusion to furnace center area increased. • Maximum value and deviation of water-wall temperature is large at low ASD settings. • Optimum coal consumption and NO x emission are as low as 337.36 g·(kW·h)−1 and 654 mg/m3. Low load operation of down-fired boilers has become the norm such that the comprehensive performances of such boilers at low loads need to be investigated to increase the energy conversion efficiency and reduce carbon and NO x emissions. In this work, cold-model air/particle flow experiments and industrial-scale trials were performed to study the effects of the air-staging degree (ASD) on the air/particle flow, combustion, water wall temperature, energy conversion and NO x emission characteristics of a 350 MW supercritical down-fired boiler at low loads. The experimental conditions are four different combinations of damper openings associated with the secondary-air, tertiary-air and OFA (i.e. 60%/40%/15%, 50%/50%/15%, 35%/70%/15% and 30%/55%/55%), and are referred to as ASD = I, II, III and IV, respectively. The results showed that, as the ASD was increased from I to IV, the injection effect of the secondary-air on the fuel-rich flow was weakened. In addition, the downward diffusion of the air/particle flow toward the furnace center increased, while the particle concentration near water wall decreased and the velocity decay was accelerated. Decreasing the ASD shortened the ignition distance of the fuel-rich flow, reduced the carbon in fly ash and slag and improved the boiler efficiency. At an ASD setting of I, a higher thermal load and increased temperature near the water wall were obtained in the lower furnace. These conditions gave the highest superheated and reheated steam temperatures of 568.1 and 570.7 °C, respectively. As the ASD was increased from II to III, the cooling effect of the tertiary-air reduced the maximum water wall temperature. The reduction, in turn, increased the regulating margin of superheating degree, which raised the steam temperature. Increasing the ASD from I to IV increased the specific coal consumption from 337.36 to 348.12 g·(kW·h)−1, indicating that the energy conversion efficiency of the unit was decreased. Additionally, the NO x emissions at the furnace exit were reduced from 770 to 528 mg/m3 at 6% O 2. On the basis of the present results, an ASD setting of II is recommended to ensure the safe and economical operation of such units while minimizing environmental impact. [ABSTRACT FROM AUTHOR]

To alleviate the over-temperature of water-wall and increase the energy conversion efficiency of supercritical down-fired boilers, cold-model air/particle flow experiments and in-situ trials were performed to investigate the effects of the secondary-air distribution wall-side deflection coefficient (w s) on air/particle flow and combustion characteristics. As w s increased from 0 to 0.33, the amount of air/particle flow diffused toward water-wall and the corresponding particle concentration both decreased, while the velocity decay was accelerated. At a w s of 0.16, due to prior ignition of fuel-rich flow and a longer flame stroke in lower furnace, the carbon in fly ash and slag were lowest. As w s increased, the thermal load was gradually focused at the furnace center, and the maximum value and deviation of water-wall temperature in lower furnace decreased. Consequently, the temperatures of superheated and reheated steam respectively increased from 542.7 °C to 543.9 °C–560.2 °C and 560.1 °C, respectively. Furthermore, increasing w s from 0 to 0.28 decreased the specific coal consumption from 346.29 to 341.09 g (kW h)−1, indicating the energy conversion efficiency was improved. However, adjusting w s had only a minimal effect on NO x emissions. A w s of 0.28 is recommended to optimize the water-wall temperature, pulverized-coal burnout, and economic performance. [Display omitted] • Laboratory and industrial experiments are combined performed for a down-fired boiler. • The wall-side deflection coefficient of secondary-air distribution (w s) is proposed. • On increasing w s , maximum value and deviation of water-wall temperature is lowered. • Specific coal consumption is reduced and energy conversion efficiency is improved. • The optimum carbon in fly ash and NO x emissions are as low as 4.92% and 654 mg/m3. [ABSTRACT FROM AUTHOR]