洁净煤技术

2019, (05) 73-79

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420 t/h四角切圆锅炉低氮燃烧改造数值模拟研究
Numerical simulation of low NOx burner retrofit of a 420 t/h tangentially lean coal-fired boiler

陈涛;于洁;赵鹏;孙路石;
CHEN Tao,YU Jie,ZHAO Peng,SUN Lushi

摘要(Abstract):

为达到严格的超低排放标准,目前国内绝大部分电站锅炉均实施了NOx排放控制技术改造。针对一台燃用烟煤的420 t/h四角切圆煤粉锅炉,将原双通道燃烧器改造为水平浓淡燃烧器并加装3层燃尽风(SOFA),从而达到低氮燃烧的效果。应用数值模拟方法进行方案论证,研究了一次风浓淡比、SOFA风率和SOFA风射流角度等参数对锅炉燃烧状况及NOx排放规律的影响,并提出最佳改造方案。随着浓淡比的增加,炉膛出口温度逐渐增加,而NOx含量逐渐降低。浓淡比为4∶1时,飞灰含碳量最低。随着浓淡比增大,CO浓度升高,增强了主燃区域的还原性,抑制挥发分含氮中间产物氧化成NO;另一方面,浓淡比增大使浓煤粉气流挥发分析出速率加快,强化挥发分含氮中间产物HCN和NH3将已生成的NO还原为N2;同时,淡侧气流煤粉浓度低,含氮基团析出量变小,与氧反应生成NO的量减少。随着SOFA风率的增加,炉膛出口烟温、飞灰含碳量增加,20%SOFA风率时,NOx浓度较高,SOFA风率由30%增加到40%时,NOx浓度基本保持不变。随着SOFA风率的增加,主燃区形成的低O2高CO浓度的强还原性气氛抑制了HCN及NH3被氧化成NO,反而促进了其与已生成的NO发生反应生成N2。此外,高SOFA风风率下,主燃区高温区缩小,生成的热力型NOx也相应减少。随着SOFA风射流角度上扬,还原区加长,有利于降低NOx浓度,但燃尽区的火焰中心会上升,煤粉燃尽时间变短,炉膛出口温度和飞灰含碳量上升。随射流角度增加,O2浓度降低而CO浓度升高,这是由于射流角度增大延迟了煤粉燃尽过程,增加了化学不完全燃烧损失;这种低氧高CO的强还原性气氛大大抑制了NOx生成。根据数值模拟结果,确定试验锅炉的低氮燃烧改造方案为:选择浓淡比为4∶1的水平浓淡燃烧器作为改造燃烧器,SOFA风率定为30%,SOFA射流角度上扬15°。改造后锅炉燃烧稳定,NOx排放显著降低,为220 mg/Nm3左右(降幅达65%~70%),而飞灰含碳量保持在3%~4%,表明改造方案可达到良好的低氮燃烧效果。
In order to meet the strict emission standards,most of the power station boilers have implemented NOx emission control technology. A 420 t/h tangential coal fired boiler of utilizing bituminous coal was retrofitted by replacing original burners with horizontal bias combustion(HBC)ones as well as introducing separated over-fire air(SOFA)to achieve the effect of low nitrogen combustion. The numerical simulation method was used to demonstrate the scheme. The effects of the primary air concentration ratio,SOFA air rate and SOFA air jet angle on the combustion status and NOx emission of the boiler were studied,and the optimum modification scheme was put forward. As the rich-lean ratios increasing,the furnace outlet temperature gradually increases,while the NOx content gradually decreases. When the rich-lean ratios is 4∶1,the carbon content in fly ash is the lowest. As rich-lean ratios increasing,the concentration of CO increases,which enhances the reduction of the main combustion zone and inhibits the oxidation of volatile nitrogen-containing intermediates into NO. Moreover,the increase of rich-lean ratios makes the rate of volatilization of concentrated coal powder increase. The volatile nitrogen-containing intermediates HCN and NH3 are used to reduce the produced NO to N2. At the same time,the lean side gas pulverized coal concentration is low,the amount of the nitrogen-containing group precipitation is small,and the amount of NO formed by reaction with oxygen is reduced. With the increase of SOFA ratios,the flue gas temperature of the furnace exit and the carbon content of the fly ash increase,and the NOx concentration is higher at the 20% SOFA ratios,and the NOx concentration remains basically unchanged when the SOFA wind rate increases from 30% to 40%. With the increase of SOFA ratios,the strong reducing atmosphere with low O2 and high CO concentration formed in the main combustion zone inhibits the oxidation of HCN and NH3 to NO,which in turn promotes the reaction with the generated NO to form N2. In addition,under the high SOFA ratios,the high temperature zone of the main combustion zone is reduced,and the thermal NOx is also reduced accordingly. As the SOFA jet angle increases,the reduction zone lengthens,which is conducive to reducing the NOx concentration,but the flame center in the burnout zone rises,and the pulverized coal burnout time become shorter,and the furnace exit temperature and fly ash carbon content increase. As the jet angle increases,the O2 concentration decreases and the CO concentration increases. Because the increase of jet angle delays the pulverized coal burnout process and increases the chemical incomplete combustion loss. The strong reducing atmosphere with low O2 and high CO concentration greatly suppresses NOx generation. According to the numerical simulation results,the low-nitrogen combustion reforming scheme of the boiler is determined as:a horizontal rich-lean burner with a ratio of 4∶1 is selected as the transformation burner,the SOFA ratios is set at 30%,and the SOFA jet angle is raised by 15°. The on-site measurement after retrofitting showed that combustion characteristic of the furnace was stable. The NOx emissions decreased significantly to 220 mg/Nm3(The decline is 65%-70%). The unburned carbon content in fly ash was as low as 3%-4% at the same time. It shows that the retrofitting scheme can achieve good low nitrogen combustion effect.

关键词(KeyWords): 四角切圆锅炉;低氮燃烧;水平浓淡燃烧器;SOFA;热态试验
tangential coal fired boiler;low NOx combustion;horizontal bias combustion;separated over-fire air;in-situ measurement

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作者(Author): 陈涛;于洁;赵鹏;孙路石;
CHEN Tao,YU Jie,ZHAO Peng,SUN Lushi

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