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• 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]

• Industrial experiments of down-fired boiler with bituminous coal under various loads are taken. • The ignition heat of coal/air flow under different coal varieties and loads is given. • Burning bituminous coal has a large furnace flame fullness under the low-load. • Strong combustion stability is determined by the low ignition heat and high furnace temperature under the low-load. • The combustion performances of bituminous coal and anthracite under the low-load are analyzed and compared. The new energy generating capacity is increasing year by year in the electric industry structure, but it is greatly influenced by the natural conditions. Coal-fired power generation unit boilers connected to the grid have the ability of flexible peak-load regulation and long-term low-load operation is required. However, the combustion stability is weak under the low-load when the coal-fired boiler burning anthracite. Down-fired boiler fueled with bituminous coal gradually attract the attention. Ignition and combustion stability, burnout, and NO x emissions are experimentally studied under the various loads on a 300 MW down-fired boiler fueled with bituminous coal, and the results compared with anthracite under the low-load. The ignition distance of the coal/air flow burning bituminous coal is gradually advanced with the reduction of load. And under the corresponding load, the ignition heat of the coal/air flow when burning bituminous coal is more than 37% lower than that burning anthracite. With the reduction of load, the decreasing rate of the overall furnace temperature and the cross-sectional heat load in the primary combustion zone decreases, and the furnace temperature in the primary combustion zone is high under the low-load when bituminous coal is burned. Short ignition distance, low ignition heat and high furnace temperature ensures the strong combustion stability of burning bituminous coal under the low-load. Moreover, under three various loads with burning bituminous coal, the carbon in the fly ash are all less than 1.1%, and the NO x emissions at the furnace exit are 410, 397 and 288 mg/m3 @ 6% O 2 , respectively. [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]

[Display omitted] • Effect of fuel-lean flow damper opening (FLD) in a down-fired boiler is investigated. • Relation of water-wall and steam temperature in a supercritical boiler is discussed. • On decreasing the FLD, heating of the fuel-rich coal/air flows gradually accelerates. • Specific coal consumption is lowered and energy conversion efficiency is improved. • The optimum NO x emissions and carbon in fly ash are as low as 672 mg/m3 and 4.51%. In-situ trials were carried out to investigate the effects of the fuel-lean coal/air flow damper opening (FLD) on the pulverized-coal combustion and energy conversion efficiency in a 350-MW supercritical down-fired boiler. This boiler was operated with inferior coal, having a high ash content and low calorific value. The influence of combustion in furnace on steam temperature and coal consumption of a supercritical down-fired boiler were discussed innovatively. The experimental results indicate that, as the FLD was decreased from 100% to 25%, heating of the fuel-rich coal/air flow accelerated such that the overall furnace temperature increased and the thermal load gradually focused at the furnace center area. As a result, the maximum temperature of the water-wall in the lower furnace was reduced and the superheating degree rose significantly. Thus the superheated and reheated steam temperatures increased from 550.4 and 545.8 °C to 558.2 and 555.0 °C. Reducing the FLD also lowered the carbon in the fly ash and slag from 8.17% and 7.70% to 4.51% and 3.43%, respectively. Furthermore, the increase of boiler efficiency and the decrease of specific heat consumption both contributed to the reduction of the specific coal consumption, from 354.45 to 340.43 g·(kW·h)−1, thus the energy conversion efficiency of the unit was improved. However, adjustment of the FLD had only a minimal effect on NO x emissions, which remained in the range of 654–672 mg/m3 at 6% O 2. Based on optimizing coal combustion characteristic and economic performance of the unit, an optimal FLD of 25% was recommended. [ABSTRACT FROM AUTHOR]