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Influence of over Fired Air on Gas-Solid Flows in a W-Shaped Flame Boiler

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Tutor: ZhaoGuangBo
School: Harbin Institute of Technology
Course: Thermal Power Engineering
Keywords: Over fired air(OFA),penetrating depth,injection angle,jet structure,direct flow,
CLC: TK229
Type: Master's thesis
Year:  2008
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Abstract:
W-shaped flame boiler, which adopts long-flame, air zoning and high concentration factor of pulverized coal, is the preferred type for anthracite coal combustion. Over fired air (OFA) technology is an effective and extensive way to reduce NOx emission. Considering the large amount of NOx coming from W-shaped flame boiler, it is necessary to study the effect of OFA on the NOx reduction in the boiler furnaces.In order to study the effect of OFA on the gas-solid flow field characteristic in the lower boiler furnaces and the effect of different injection angle and different structure of nozzle of OFA on the over fired field flow characteristic of the upper chamber of furnace, the gas-solid two phase testing system is designed according to similarity-simulation principle, and the gas-particle two phase flow field is measured by Phase Doppler Anemometry system.By using OFA, there’s no essential change in the flow field of the arch chamber of furnace and the first air can form w-shape no matter the secondary air flow down or not. With OFA construction being installed, the reducing of the first air will diminish in the vertical direction, and the deep of the first air will increase, the center of W-shape flame will get down and pulverized coal will stay longer in the chamber of furnace. All of these will help the complete combustion of pulverized coal.When the proportion is 30% and the injection angle of OFA is 0°, 15°, 30°and 45°, the horizontal jet route can arrive at the center of the furnace, which will improve the mixing properties of OFA jet with flue gas in different section and depth. When the injection angle of OFA is bigger, the position of the mixed point of OFA jet with flue gas will be lower. This will increase the time of the combustion of pulverized coal without oxygen. If the injection angle is too big, the vertical penetrating depth won’t change a lot, but the speed of the horizontal direction will reduce. As a result, it can’t reach the center of the furnace. So the injection angle of OFA is 30°.The experimental study unveils the direct central flow and exterior swirling structure of OFA can help realize the fullness of radial direction, so OFA can reach every field between the two adjoining OFA jet. While the complete direct central flow structure will result in the poor radial direction. So OFA can’t reach every field between the two adjoining OFA jet. Therefore the direct central flow and exterior swirling structure is better.
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