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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]

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]

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]

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]