Nitrogen oxide emission control technologies for coal-fired boilers 【Daily Question 20090209】

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Nitrogen oxide emission control technologies for coal-fired boilers. Nitrogen oxides (NOX) are a major type of air pollutant emitted by coal-fired boilers; they are the primary cause of photochemical smog and pose serious hazards to human health. Global anthropogenic NOX emissions are on the rise year by year. Many **are considering implementing strict legislation to limit their emissions. The United States has the largest installed capacity of thermal power plants in the world (according to recent United Nations statistics, the installed capacity of thermal power plants in the U.S. is 566 GW, while the total capacity of all power plants there is 770 GW). The basic heat energy in the United States is primarily supplied by coal-fired boiler power generation units. Therefore, the NOX emissions from coal-fired thermal power plants in the United States are the highest in the world, with coal-fired plants accounting for about 90% of total NOX emissions into the atmosphere. The total installed power generation capacity of China’s power system currently stands at 339 million kW, ranking second in the world after the United States. Going forward, more than 10 million kW of new generating capacity will be added each year, and since 1996, China has maintained its position as the world’s second-largest country in terms of power generation capacity. The emission of harmful gases from coal-fired power plants is causing increasingly severe environmental pollution; therefore, controlling the harmful gases emitted by the boilers in such plants is a long-term, challenging, and urgent task aimed at protecting the ecosystem and improving the environment. The utilization of clean coal has been designated as a strategic approach for the coordinated development of energy and the environment in China. GB13223-1996 sets an upper limit of 650 mg/m3 for NOX emissions from coal-fired power plants; power generation boilers with a capacity of over 1000 t/h, built after 1997, are required to comply with these regulations, and such boilers should adopt advanced NOX emission control technologies. The United States defines the \"first\" denitration method as the use of \"low NOX\" (LNB) burners and \"two-stage\" torch combustion (OFA) in coal-fired boilers. Both utilize staged combustion of the torch to suppress NOx. Most power plants use this method to meet the NOx emission standards (requirement of Phase I). It is said that with this technology applied to 177 boilers, NOX emissions are 33%–48% lower than those in 1990. With LNBs, it is 40% lower than in 1990. Improvements to the “first” denitration method—LNB, multi-layer installation of tertiary air at different heights along the furnace, and combined use with the ROFA system. When the previous technique was applied in a furnace burning semi-bituminous coal, the NOX concentration was reduced to 0.0645 mg/m3; the subsequent technique achieved a 50% reduction. “The second “denitration method” refers to the use of “three-stage” torch combustion, selective non-catalytic reduction SNCR, and selective catalytic reduction SCR technologies in coal-fired boilers. The “three-stage” combustion technology has been applied to 14 boilers, reducing NOX concentrations by 39%–67% (several more are under installation). SNCR technology is applied to 22 boilers. NOx concentrations decreased by 22% to 62%. In the past 3 years, SCR has almost become the preferred choice. Improvements have been made to the “second” denitration method, and it is currently in the testing phase. The AGR and AEFLGR methods represent a combined application of the \"three-stage\" flare combustion technique and the SNCR method, with the potential to achieve significant denitration. For example, by applying the AGR method in a 105MW furnace, the NOX concentration is reduced by 68% to 76%. (Ammonia leakage > 10ppm) ； By applying the FLGR method on 5 boilers, the NOx concentration was reduced by 27% to 40% ； The AEFLGR method was applied to 4 boilers, reducing NOX by 50%–70%. “The HSR method catalyzed by \"mixing\" is SNCR+SCR; its denitration performance can fully reach the level achieved by the SCR method (when ammonia leakage is <5 ppm), resulting in significant cost savings with 45% less reagent required. The “In-duct SCR” method, which places the catalyst in a spacious part of the flue, is suitable for use in situations where space is limited on site. When this method is combined with the SNCR method, the NOx levels at the inlet of the air preheater can be reduced by 85%–90% when ammonia leakage is <10 ppm. Using the SCR method to produce ammonia through urea hydrolysis can reduce safety risks associated with transportation, storage, and ammonia overloading; this method has been validated using a 565MW load-variable fuel furnace. Therefore, it is extremely urgent to understand the domestic treatment processes, equipment, as well as investment and operating costs related to nitrogen oxides. Everyone is welcome to share their views, especially regarding the selection of processes and the technical and economic evaluation of nitrogen oxide emission control technologies for small and medium-sized boilers.