锅炉低氮燃烧技术研究进展

Research progress of low-NO;combustion technology for boilers

随着国家对氮氧化物(NO_(x))排放要求越来越严格,提高NO_(x)治理水平现已成为环保领域的研究重点。概述了锅炉NO_(x)生成机理,对低氮燃烧技术特点进行分析,重点讨论了富氧燃烧技术的机理、特点和应用。NO_(x)的生成机理复杂,与燃料特性、氧气含量、氮气含量、温度及炉膛结构等因素密切相关,锅炉燃烧过程产生的NO_(x)根据其生成机理不同可分为燃料型、热力型、快速型、NNH型及N;O型5种。不同种类的锅炉可应用不同低氮燃烧技术或其结合来实现NO_(x)减排。低氮燃烧技术具有种类多样化、适应范围广、成本较低等优点,是实现NO_(x)减排的重要技术之一,目前应用较为广泛的低氮燃烧技术主要包括空气分级燃烧技术、燃料分级燃烧技术与烟气再循环技术。其中空气分级燃烧技术运行经验多,适合锅炉改造项目,但其减排效果有限且存在燃烧不充分及腐蚀问题;燃料分级燃烧技术中以天然气为代表的气体再燃燃料降低NO_(x)效果较好,但更适用于新建锅炉;烟气再循环技术对于现有锅炉改造较容易,但单独使用时NO_(x)减排效果有限且对锅炉燃烧稳定性及燃烧效率有不利影响,宜与其他技术配合使用。MILD(Moderate and Intense Low Oxygen Dilution)燃烧技术、多孔介质燃烧技术、富氧燃烧技术及MILD富氧燃烧技术作为新兴的低氮燃烧技术,可实现NO_(x)超低排放,应用前景十分广阔。MILD燃烧技术的燃烧稳定性好、燃烧效率高且NO_(x)排放低,在不预热空气或全预混条件下均可实现气体、液体及粉状固体燃料的MILD燃烧,对于低热值燃料燃烧也同样具有显著优势。多孔介质燃烧技术燃烧效率高、NO_(x)排放低且贫燃极限宽,可实现高炉煤气、生物质气及挥发性有机物(VOCs)等低热值气体燃料的燃烧。富氧燃烧技术是一种可同时实现超低污染物排放与碳捕集的清洁燃烧技术,具有十分可观的经济效益与社会效益。MILD富氧燃烧技术兼具MILD燃烧技术与富氧燃烧技术2者的优点,并可进一步降低NO_(x)排放量,是一种新型、高效的燃烧技术,可实现煤或天然气等化石燃料的“近零排放”。

With the increasingly strict national requirements for nitrogen oxide(NO_(x)) emissions, improving the level of NO_(x)control has become a research focus in environmental protection field.The NO_(x)generation mechanism of boilers was summarized, the characteristics of low-NO_(x)combustion technologies was analyzed, mechanism characteristics and appliation of oxy-fuel combusiton was discussed. The results show that NO_(x)formation mechanism is quite complex, which is closely related to fuel characteristics, oxygen content, nitrogen content, temperature, boiler structure and so on. Based on the different formation mechanism, the NO_(x)produced in boilers can be divided into five types: fuel-NO_(x),thermal-NO_(x),prompt-NO_(x),NNH route and N;O-intermediate route. Different low-NO_(x)combustion technologies or their combination can be applied to reduce NO_(x)emission of different boilers. Low-NO_(x)combustion technology has the advantages of diversified types, wide adaptability and low cost, which is one of the important technologies to realize NO_(x)emission reduction. At present, the widely used low-NO_(x)combustion technologies include air staged combustion technology, fuel staged combustion technology and flue gas recirculation technology. A wealth of operation experience has been accumulated for air staged combustion technology, which is suitable for boiler renovation projects, but the NO_(x)emission reduction effect is limited and the problems of inadequate combustion and corrosion cannot be ignored.For fuel staged combustion technology, the NO_(x)emission reduction effect is significant when gas fuels represented by natural gas are used as reburning fuels, and it is more suitable for new boilers. It is easy to retrofit the existing boilers for flue gas recirculation technology, but the combustion stability and efficiency is not very good. The NO_(x)emission reduction effect of flue gas recirculation technology is so limited when used alone that it should be combined with other technologies simultaneously. Nevertheless, the ultra low NO_(x)emission can be realized by emerging technologies such as MILD combustion, porous medium combustion, oxy-fuel combustion and MILD oxy-fuel combustion, thus having a very wide application prospect. MILD combustion technology has the advantages of good combustion stability, high combustion efficiency and low NO_(x)emission. MILD combustion can be achieved for gas, liquid and powder solid fuels in both fully premixed and air unpreheating conditions. It also has significant advantages for the combustion of low calorific value fuels. Porous media combustion technology has the characteristics of high combustion efficiency, low NO_(x)emission and wide lean combustion limit are, which can realize combustion of low calorific value gas fuels such as blast furnace gas, biomass gas and VOCs. As a clean combustion technology, oxyfuel combustion technology can achieve ultralow pollutant emissions and carbon capture at the same time, of which the economic and social benefits are of great worth. Combining the advantages of MILD combustion technology and oxy-fuel combustion technology, MILD oxy-fuel combustion is a new and efficient technology that can achieve near zero pollutant emissions for fossil fuels such as coal or natural gas.