Renovation of Low Nitrogen Burners in Coal Fired Boilers
At present, coal is still the main resource composition in China, and production and daily life heavily rely on traditional coal resources, causing significant pollution to the atmospheric environment. Coal series is mainly composed of coal, and during combustion, pollutants such as sulfides and nitrogen oxides are produced. In order to solve the contradiction between energy supply and environmental protection, reduce the cost of denitrification transformation and operation, the transformation of low nitrogen burners is imperative.1、 Introduction to Low NOx Combustion TechnologyReducing NOx emissions by changing combustion conditions is a low NOx combustion technology. Low nitrogen combustion technology is currently the simplest and most economical method widely used to reduce nitrogen oxide emissions. At present, low NOx combustion technologies mainly include the following:1. Low excess air coefficientLow excess air coefficient is a convenient method for optimizing combustion and reducing NOx production. The reduction of nitrogen content in flue gas can inhibit the generation of nitrogen oxides. Without changing the structure of the combustion device, NOx emissions were reduced and the economic efficiency of the device operation was improved. For coal-fired boilers, their limitations are mainly due to the low air excess coefficient, which can cause bonding slag and corrosion on the heating surface, temperature changes, and an increase in combustible fly ash, leading to a decrease in economic efficiency. Therefore, power plant boilers cannot be significantly adjusted in actual operation.2. Air staged combustionThe basic principle of gas staged combustion is to complete the fuel combustion process in stages. Firstly, reduce the amount of air supplied by the main combustion engine to the furnace to 70-75% of the total combustion air (equivalent to 80% of the theoretical air volume), so that the fuel is first burned under low oxygen and rich fuel combustion conditions. This method compensates for the simple disadvantage of low excess air combustion. The smaller the excess air coefficient in the first stage combustion zone, the better the effect of suppressing NOx generation. However, the more combustion products there are, the lower the combustion efficiency, and the greater the possibility of slagging and corrosion. Therefore, in order to ensure NOx emissions and the economy and reliability of boiler combustion, it is necessary to organize the air staged combustion process reasonably.3. Fuel classificationDuring the combustion process, nitrogen oxides and hydrocarbon roots, as well as incomplete combustion products CO, H2, C, and CnHm, undergo a NO reduction reaction. Using this principle, 80-85% of the fuel is transported to the primary combustion zone α> Burning at 1 hour produces carbon dioxide. The fuel sent into the primary combustion zone is called primary fuel, while the remaining 15-20% of the fuel is sent into the secondary combustion zone at the upper part of the main burner α< Under the condition of 1, a strong reducing atmosphere is formed, causing the NOx generated in the first stage combustion zone to be reduced to nitrogen molecules in the second stage combustion zone. The second stage combustion zone, also known as the reburning zone, and the fuel sent into the second stage combustion zone is also called the secondary fuel, or reburning fuel. The reburning zone not only reduces the generated NOx, but also suppresses the production of new NOx, further reducing the emission concentration of NOx.4. Flue gas recirculationCurrently, the flue gas recirculation method is also widely used for low NOx combustion technology. It directly sends a portion of low-temperature flue gas into the furnace, or mixes it with primary or secondary air and sends it to the furnace. This not only reduces the combustion temperature, but also lowers the oxygen concentration, thereby reducing the NOx emission concentration.5. Low NOx burnerThe burner is one of the key equipment in the boiler combustion system. During the combustion process of a boiler, the performance of the burner directly affects the reliability and economy of the boiler. According to the mechanism of NOx generation, most NOx is generated during the ignition stage of coal powder. Therefore, by designing a special burner structure and adjusting the air coal ratio of the burner, the concentration of oxygen and temperature in the ignition zone can be reduced, thereby achieving the goal of suppressing NOx generation.6. Low NOx combustion systemCombining low NOx burners with low NOx combustion (air classification, fuel classification, and flue gas recirculation) to form an ultra-low NOx combustion system, in order to better reduce NOx emissions and carbon content in fly ash.2、 Issues to be noted during renovation:(1) In terms of fire safety: Gas boiler rooms belong to Class A factories and have a fire resistance rating of no less than Level 2. Fire safety is much higher than that of coal-fired boiler buildings. Therefore, when transforming coal-fired boilers into gas boilers, the boiler buildings should be renovated according to regulations, and attention should be paid to the selection of explosion-proof and fire-resistant grades.(2) In terms of automatic control, special attention should be paid to the automatic interlock protection of boiler water level, as well as the automatic shutdown protection of gas boilers, and the automatic interlock protection of steam pressure, gas pressure, wind pressure, etc. In terms of automatic control degree, gas boilers are much higher than coal-fired boilers.(3) Two combustible gas alarm devices should be installed in the boiler room and pressure regulating room, and the alarm system should be interlocked with the emergency shut-off valve and forced exhaust facilities.