The numerous coal-fired grate furnaces in China, which were designed to partition primary air on the grate and arrange secondary air in the freeboard for air-staging combustion, suffered from poor burnout and high NO. However, secondary air was conventionally closed or set at marginal openings in real operations and no investigation was reported to explain why. Upon these off-design secondary-air circumstances, industrial-scale tests and modeling investigations were performed for a 75 t/h coal-fired reversal grate furnace at secondary-air ratios of 0 %, 3 %, 6 %, 10 %, and 16 %, respectively. The primary aims were to (i) uncover its combustion and NO formation characteristics with varying secondary air and (ii) confirm whether secondary air acted on a real air-staging function in reducing NO. Flow-field deflection and asymmetric combustion were found in the furnace, with the upward gas entirely deflecting towards the rear side. The front-half furnace was filled with a large but weak recirculation zone without effective combustion, acting on a huge dead zone to waste the furnace-space utilization. With increasing the secondary-air ratio, the upward gas deflection deteriorated continuously. Layer combustion worsened while suspension combustion first strengthened and then weakened. NO emissions displayed an ascent-to-descent trend. Carbon in slag raised while carbon in fly ash decreased. Among the five case setups, the 0 % setting (i.e., fully closing secondary air) exhibited the lowest NO emissions and burnout loss. Compared with the habitual 6 % setup, fully closing secondary air not only reduced NO emissions by 22 % but also improved a little burnout. These trends meant that opening secondary air only strengthened suspension combustion for burning well fly ash while raised both NO and total burnout loss. Secondary air thus failed to role as an air-staging function in reducing NO.

中文翻译：

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工业规模燃煤反转炉排炉悬浮燃烧二次风立式空气分级低NOx功能的判别


我国众多燃煤炉排炉，其设计是在炉排上隔开一次风，在干舷处布置二次风进行空气分级燃烧，但燃尽效果差，NO含量高。然而，在实际操作中，二次空气通常是关闭的或设置在边缘开口处，并且没有调查报告来解释原因。针对这些非设计二次风情况，对 75 t/h 燃煤反转炉排炉进行了工业规模试验和建模研究，二次风比例为 0%、3%、6%、10% 和分别为 16%。主要目的是 (i) 揭示其在不同的二次空气中的燃烧和 NO 形成特性，以及 (ii) 确认二次空气是否在减少 NO 方面发挥了真正的空气分级功能。炉内发现流场偏转和不对称燃烧，向上的气体完全向后侧偏转。前半炉膛内存在较大但较弱的再循环区，无法有效燃烧，作用于巨大的死区，浪费炉膛空间利用率。随着二次风比的增加，向上气体偏转不断恶化。层状燃烧恶化，悬浮燃烧先加强后减弱。 NO排放量呈现先上升后下降的趋势。炉渣中的碳增加，而飞灰中的碳减少。在五种情况设置中，0% 设置（即完全关闭二次空气）表现出最低的 NO 排放和燃尽损失。与惯用的6%设置相比，完全关闭二次风不仅减少了22%的NO排放，而且还改善了一点燃尽现象。 这些趋势意味着，打开二次风只会加强燃烧井飞灰的悬浮燃烧，同时增加 NO 和总燃尽损失。因此，二次空气未能发挥空气分级功能来减少NO。