两段式滴管内烟煤富氧空气分级燃烧NOx排放特性研究

Research on NOx emission characteristics of bituminous coal with oxygen enriched air staged combustion in two-stage dropper tube furnace

随着我国对大气污染物排放监管力度的日益严格,NOx控制技术已广泛应用于工业生产的各个领域。作为一种直接、简便的NOx排放控制技术,富氧空气燃烧技术已经出现在燃气锅炉和内燃发动机等行业,然而在燃煤锅炉行业中却鲜有应用。为了验证富氧空气燃烧技术在煤粉工业锅炉中的NOx减排效果,笔者以神府烟煤作为燃料,利用两段式滴管炉试验系统模拟煤粉在锅炉内燃烧的实际情况,采用热态试验方法,研究了烟煤富氧空气分级燃烧的NOx排放特性,并与单级供风、空气分级燃烧2种燃烧方式下的NOx排放情况进行对比。考察了主燃区温度、二次风配比(以主燃区过量氧气系数表示)、二次风氧浓度等关键因素对NOx排放的影响。结果表明:富氧空气分级燃烧的NOx排放显著低于单级供风燃烧,同时也低于空气分级燃烧的NOx排放。主燃区温度为1 300~1 500℃时,富氧空气分级燃烧的NOx排放减少比例比分级配风燃烧提高了6~12个百分点;富氧空气分级燃烧条件下,随主燃区温度升高,煤粉燃烧更加充分,燃料中N元素分解成NHi、HCN等大量中间产物,使主燃区气氛的还原性增强,被还原的NOx比例增加。因此,NOx排放降低且NOx排放减少比例呈现上升趋势;富氧空气分级燃烧的二次风配比对NOx排放具有显著影响,随着主燃区过量氧气系数的升高,NOx排放均呈现先降低后升高的趋势。因此存在最佳二次风配比,使NOx排放浓度最低。主燃区温度为1 300℃时,最佳主燃区过量氧气系数约为0.58;主燃区温度为1 500℃时,最佳主燃区过量氧气系数约为0.55;在主燃区过量空气系数给定的条件下,提高二次风氧浓度可以延长煤粉颗粒在主燃区的停留时间,并在煤粉颗粒表面形成局部富氧环境,促进煤粉充分燃烧,从而增强主燃区气氛的还原性,降低NOx的生成。因此,当二次风氧浓度为21%~31%时,NOx排放随二次风氧含量的升高而降低。随着二次风氧浓度的逐渐升高,NOx排放的降低趋势逐渐放缓。

With increasingly strict monitoring of air pollutant emissions in China,Nitrogen oxide control technology has been widely used in various fields of industrial production.As a direct and simple nitrogen oxide emission control technology,oxygen-enriched air combustion technology has been used in gas-fired boilers and internal combustion engines.However,it is rarely used in coal-fired boilers.In order to verify the NOx reduction effect of oxygen-enriched air combustion technology in pulverized coal industrial boilers,Shenfu bituminous coal was used as fuel,and the two-stage dropper tube furnace test system was used to simulate the actual situation of pulverized coal combustion in the boiler.The NOx emission characteristics of oxygen enriched air staged combustion of bituminous coal were studied by using the hot test method,and the NOx emission was compared with that of single-stage air supply and air staged combustion.The effect of key factors,such as temperature of main combustion zone,ratio of secondary air( expressed as excess oxygen coefficient in main combustion zone) and oxygen concentration of secondary air,on NOx emission were investigated. The results show that,compared with single stage air and air-staged combustion,the NOx removal effect of oxygen enriched air staged combustion is much more significant.When the main combustion zone temperature is in the range of 1 200-1 500 ℃,The NOx emission reduction ratio of oxygen-enriched air staged combustion is 6-12 percentage points higher than that of graded air combustion.Under the condition of oxygen enriched air staged combustion,and as the increase of the main combustion zone temperature,the coal combustion is more completed and the fuel nitrogen is decomposed into a large number of intermediate products such as NHi,HCN,etc,which makes the reducibility of the atmosphere in the main combustion zone enhanced and the proportion of NOx reduced increased.Therefore,NOx emission is reduced and the proportion of NOxemission reduction is on the rise;the ratio of secondary air in oxygen enriched air staged combustion has a significant impact on NOx emission.With the increase of excess oxygen coefficient in the main combustion zone,NOx emission is reduced first and then increased.Therefore,there is an optimal ratio of secondary air to minimize NOx emission concentration.When the temperature of the main combustion zone is 1 300 ℃,the optimum excess oxygen coefficient is about 0.58.When the temperature of the main combustion zone is 1 500 ℃,the optimum excess oxygen coefficient is about 0.55.At a certain excess air coefficient in the main combustion zone,increasing the concentration of secondary air oxygen can prolong the residence time of coal particles in the main combustion zone,and local oxygen-enriched environment is formed on the surface of the coal particles to promote the coal combustion,thereby enhancing the reducibility of the atmosphere in the main combustion zone and reducing the generation of NOx.Therefore,when the secondary air oxygen concentration is the range of 21%-31%,NOx emission decreases with the increase of the secondary air oxygen concentration. As the oxygen concentration of the secondary air gradually increases,the decreasing trend of NOx emission gradually slows down.