Aiming at Foster Wheeler (FW) technology down-fired boiler with horizontal F layer secondary air, in which the strong horizontal momentum of the secondary air seriously pounds the down flowing coal gas flame, making the problems of short trip of coal flame and low flame fullness of the furnace, and this leads to higher carbon content of fly ash and so on. A designed adjusting device making the declivitous angle of secondary air is erected in the F layer wind box of a FW down-fired furnace, so the declivitous angle of secondary air can be regulated freely. By this means, the declivitous angle of the F layer secondary air is able to be refined depending on the real combustion situation to achieve the best configuration of the momentum radio by downward momentum from the arch and horizontal momentum from the front/rear wall. As a result, it is able to adapt the change of the furnace combustion situation, extending the flame travel, and improving the combustion process and efficiency. This method was applied to a 300MW grade FW down-fired boiler, and the cold flow field results by fireworks showed the down flow depth of the primary air increasing with the declivitous angle. The best flow field was achieved as the declivitous angle was set to 45°. Based on the cold flow field experiments, combustion experiment was accomplished to compare with the coal combustion performance before the declivitous refinement of the F layer secondary air. It was shown that the carbon content of the fly ash was greatly decreased after the refinement, and the combustion efficiency was increased by 3.3%.

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