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Down-fired boilers used to burn low-volatile coals have high NO(x) emissions. To find a way of solving this problem, an overfire air (OFA) system was introduced on a 300 MW(e) down-fired boiler. Full-scale experiments were performed on this retrofitted boiler to explore the influence of the OFA ratio (the mass flux ratio of OFA to the total combustion air) on the combustion and NO(x) emission characteristics in the furnace. Measurements were taken of gas temperature distributions along the primary air and coal mixture flows, average gas temperatures along the furnace height, concentrations of gases such as O(2), CO, and NO(x) in the near-wall region and carbon content in the fly ash. Data were compared for five different OFA ratios. The results show that as the OFA ratio increases from 12% to 35%, the NO(x) emission decreases from 1308 to 966 mg/Nm(3) (at 6% O(2) dry) and the carbon content in the fly ash increases from 6.53% to 15.86%. Considering both the environmental and economic effect, 25% was chosen as the optimized OFA ratio.

Abstract: An OFA (over-fire air) program and matched measures supporting efficient-combustion are proposed for the FW (Foster Wheeler)-type down-fired boiler, where the OFA is supplied from the arches of the boiler. A gas/solid cold test was conducted at a scale of 1:15, using a two-dimensional laser phase Doppler analyzer to measure parameters in the model furnace, such as the two-phase gas/solid flow velocity, fluctuation velocity, particle size, particle volume flux and particle number concentration. The effects of the OFA ratio on the gas/solid flow characteristics in the furnace were investigated. With an increase in the OFA ratio, the jet depth of the fuel-rich flow decreased. For an OFA ratio of 25%, the jet depth of the fuel-rich flow was much less than that for other OFA ratios, and the residence time of the gas/solid flow in the lower furnace was shorter. An increase in the OFA ratio can increase the penetration depth and allow fuller mixing with the upward airflow, while an OFA ratio that is too great will hinder the upward flow back to the burner nozzle region. An OFA ratio of 20% is considered appropriate for the proposed OFA technical scheme. [Copyright &y& Elsevier]

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]