Search

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]

A new overfire air (OFA) technology was proposed by the authors for a Foster Wheeler down-fired boiler, and a small-scale cold experimental system for a 660-MWe unit was established. The velocity field distribution was measured in the furnace to study the characteristics of single-phase flow in the furnace at different OFA nozzle angles. The furnace temperature and distributions of the O2 concentration and NO concentration at different OFA nozzle angles were simulated using Fluent 6.3.26, and the average temperature of the furnace outlet, the O2 and NO x concentrations and carbon content in the fly ash were calculated. As the OFA nozzle angle increased, the vertical penetration depth of OFA increased and the horizontal penetration depth of OFA gradually decreased. The carbon content in the fly ash and NO x concentration initially rose and then fell at the furnace outlet, and they were lowest when the OFA angle was set at 20°. Thus, according to the findings of this article, an optimized OFA angle of 20° was chosen. [ABSTRACT FROM AUTHOR]

Abstract: A new over-fire air (OFA) program, which called arch-supplied OFA, is proposed for Foster Wheeler (FW) type down-fired boiler in this paper. The OFA nozzles are setting on arches near the furnace center and emitting OFA flow into the furnace at an inclined angle. Cold airflow experiments were conducted to determine the effect of the OFA ratio on the aerodynamic field in a small-scale furnace modeled for a 660-MWe down-fired boiler. As the OFA ratio increases, the penetration depth of the OFA flow increases in the level direction and the turning point from downward to upward of the OFA flow moves to the furnace center. At the same time, the velocity of the F-tier airflow decreases and its ability to bring the arch-injected airflow to deeper positions weakens and the turning point from downward to upward of the F-tier airflow moves to the wall. When the OFA ratio is 20%, the OFA flow could penetrate to the furnace center in the level direction and mix with the upflowing gas completely, while the arch-injected airflow have sufficient travel distance and enough residence time in the lower furnace. Thus, considering the penetration depth of arch-injected flow in the vertical direction and the OFA flow in the level direction, an OFA ratio of 20% is recommended for the new OFA arrangement according to the present work. [Copyright &y& Elsevier]

Abstract: Using an IFA300 constant temperature anemometer system, cold air experiments on a scale-model of a down-fired pulverized-coal 300MWe utility boiler with swirl burners were performed to investigate the influence of the staged-air flow ratio on flow characteristics in the furnace. This ratio was increased in incremental steps over the range of 7%–20%. The flame reach of the downward airflow was decreased with the ratio increased. A shallower reach is accompanied by a weaker mixing of vent and staged-air. At lower ratios, mixing is good and thus favors coal combustion but weak mixing at intermediate ratios produces a delay in the mixing point. This is compensated by an increasing recirculation zone under the arch that benefits ignition and the appearance of an upward airflow below the mixing point due to an increasing influence of vent air. At higher ratios still, this upward flowing disturbance is centered higher up in the furnace generated primarily by mixing with staged-air as vent-air mixing weakens. [Copyright &y& Elsevier]