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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]