【摘要】 低氮燃烧改造是燃煤电厂降低氮氧化物排放最主要的策略之一。空气分级燃烧技术因其技术成熟、成本低廉等优势在燃用烟煤的锅炉中得到广泛应用。然而,随着煤/风比的进一步增加,NO_x降幅减小,未燃尽碳含量显著变大。与燃用烟煤的锅炉相比,燃用低挥发分煤种锅炉的低氮改造工作更加困难和复杂。四角切圆贫煤锅炉的三次风会影响风煤混合、燃烧气氛和温度,这些都会对煤粉燃烧过程和NO_x生成产生显著影响,若仅采用空气分级技术,并不能满足NO_x排放标准。因此,在低氮燃烧改造方案设计过程中,需寻求最佳的三次风布置方案以实现低氮高效燃烧。将一台300 MW四角切圆贫煤燃烧锅炉作为研究对象,采取CFD数值模拟方法,考察了三次风布置方式对锅炉燃烧特性的影响。结果表明:当三次风布置在燃烧区下部时,下层一次风和三次风中的煤粉迅速着火燃烧,温度攀升,火焰中心上移; NO_x还原区变长,此时炉膛出口NO_x浓度最低,为405 mg/Nm~3;三次风的下移导致炉膛主燃区中上部氧量较少,煤粉不充分燃烧,燃尽率降低。当三次风布置在主燃区中部时,由于三次风风温较低,导致炉膛燃烧温度下降,一定程度上抑制了热力型NO_x的生成,炉膛出口NO_x排放量减少;三次风的喷入增加了主燃区过量空气系数,有利于煤粉的充分燃烧,燃尽率提高。当三次风布置在主燃区上部时,随着三次风位置的升高,三次风煤粉整体燃烧燃尽区域上移,折焰角附近温度依次升高;三次风位置的上移增加了NO_x还原区的长度,三次风喷口位置越高,炉膛出口NO_x浓度越低;三次风上移导致三次风煤粉在炉膛的停留时间变短,造成燃烧不充分,飞灰含碳量增加,燃尽率降低。此外,对改造后飞灰及大渣含碳量,炉膛出口烟温和NO_x浓度等参数进行现场测量,NO_x排放浓度模拟值和测量值分别为445和448 mg/Nm~3,飞灰含碳量分别为1. 92%和1. 48%,数值模拟结果与现场测量结果吻合较好。更多还原

【Abstract】 Retrofitting of Low-NO_x combustion system in boilers is one of the main strategies for coal-fired units to reduce NO_x emissions.The air classification combustion technology has been widely used in bituminous coal-fired boilers because of its mature technology and low cost.However,with the further increase of coal/air ratio,the reduction of NO_x decreases and the content of unburned carbon increases significantly.Compared with bituminous coal fired boilers,there is a big difficulty for low volatile coal boilers to retrofit.The tertiary air of tangentially lean coal-fired boilers will affect the mixing of air and coal,combustion atmosphere and temperature,which will have a significant effect on the combustion process of pulverized coal and the formation of NO_x . If only air classification technology is used,it will not meet the emission standard of NO_x .Therefore,the modes of optimal tertiary air arrangement should be explored at retrofitting project design to get low NO_x emissions and high combustion efficiency.In this paper,based on a 300 MW tangentially lean coal-fired boiler,the influences of tertiary air arrangement on combustion characteristics of the boiler were studied by means of numerical simulation.The results show that when the tertiary air is injected at the bottom of the furnace,the pulverized coal in the primary and tertiary air of the lower part rapidly ignites and burns,the temperature rises,and the flame center moves upward.The NO_x reduction zone becomes longer,and the NO_x concentration at the furnace outlet is the lowest,at 405 mg/Nm~3.The downward movement of the tertiary air causes less oxygen in the upper and middle part of the main combustion zone of the furnace,resulting in a decrease in the burnout rate and insufficient of the pulverized coal and the air.When the tertiary air is injected in the middle of primary combustion zone,the combustion temperature of the furnace decreases due to the lower temperature of the tertiary air,which inhibits the generation of thermal NO_x to some extent,and reduces the emission of NO_x at the outlet of the furnace.The injection of tertiary air increases the excess air ratio in the main combustion zone,which is beneficial to the full combustion of the coal powder and the improvement of burnout rate. As the tertiary air is injected above primary combustion zone,with the increase of the position of the tertiary air,the overall combustion burnout area of the tertiary air pulverized coal moves up,and the temperature near the furnace arch increases in turn.The upward movement of the tertiary air increases the length of the reduction zone of NO_x ,and the higher the position of the tertiary air vent is,the lower the NO_x concentration at the furnace outlet is. The upward movement of the tertiary air causes the residence time of the third pulverized coal powder in the furnace to become shorter,resulting in insufficient combustion,and the carbon content of the fly ash is increased and the burn-out rate is reduced.In addition,the carbon content of fly ash and slag,flue gas temperature at furnace outlet and NO_x concentration were measured on site.The simulated and measured results of NO_x emission concentration were 445 and 448 mg/Nm~3,respectively.The numerical simulation results are in good agreement with the measured results.更多还原