Coke oven flue gas denitrification technology - low nitrogen combustion technology
Nitrogen oxides, one of the harmful substances that pollute the atmosphere, have a serious impact on the environment and human health. They interact with hydrocarbons to form photochemical smog. According to the Coking Chemical Industry Standard GB (16171-2012), from October 1, 2012 to December 31, 2014, the existing coking enterprises implemented the atmosphere of nitrogen oxides of the chimney (NO2) 800mg / m3 Pollution discharge limits, the standard part of the enterprises to achieve compliance. As of January 1, 2015, the existing coking enterprises need to implement the emission limit of 500mg / m3. Only a few enterprises can meet the standard after the standards are severely restricted. However, for the standard "Special Air Pollution Emission Limit", the emission limit of nitrogen oxide of coke oven chimney should be set at 150mg / m3. If no measures are taken for existing and newly built coke oven, the flue gas will not meet the discharge standards.
Denitration technologies for industrial applications are mainly divided into: low nitrogen combustion technology, furnace injection denitration technology and flue gas denitration technology. Because furnace injection denitrification is only applicable to coal-fired or oil-fired boilers, it is not applicable to coke ovens. Therefore, only low nitrogen combustion technology and flue gas denitration technology are discussed.
1 low-nitrogen combustion technology Â
Low-NOx combustion technology uses various combustion methods in the furnace to control the formation of NOx in the combustion process, mainly including air staged combustion, re-combustion technology and low-NOx burner. The technology is mainly applied to coal-fired boilers and hot stoves.
Similar technologies that have been applied in coke ovens include staged combustion and exhaust gas recirculation, but can not be retrofitted to existing coke ovens. In order to control the generation of NOx, the formation of NOx can be reduced by controlling the combustion temperature and changing the fuel structure. Table 1 for different operating rates 7m coke oven turnaround time and straight temperature control standards.
Table 1JNX70-3 coke oven operating rate and turnaround time, straight temperature
As can be seen from Table 1, as the operating rate increases, the straight-line temperature increases. When the operating rate is 90%, the coke side straight furnace temperature has reached 1230 ℃. The normal continuous production rate of 110%, Jiao side straight temperature 1295 ℃. According to the formation mechanism of NO, when the combustion temperature exceeds 1227 ℃, the NO generation increases sharply and the reaction rate increases by 6 ~ 7 times for every 100 ℃ increase in temperature, indicating that the production of NO is much higher than the low operating rate When the amount of generation.
Taking into account the combustion characteristics of gas, blast furnace gas burning slow, long flame, large amount of exhaust gas. Coke oven gas burning fast, short flame, a small amount of exhaust gas. The use of pure blast furnace gas heating than the use of pure coke oven gas heating local temperature is less likely, the formation of thermal NOx less. At the same time, the generation of thermal NOx is also related to the residence time of the flue gas in the high temperature zone. The longer the residence time, the more NOx is generated. When the blast furnace gas is heated, the amount of gas and the amount of flue gas are larger than when the coke oven gas is heated, and the residence time in the combustion chamber is short, so the amount of NOx generated is small. Small and medium-sized coke oven furnace temperature than large coke oven low, by changing the fuel structure to reduce NOx generation.
2 flue gas denitration technology
Flue gas denitrification technology is the reduction or adsorption of tail NOx generated in the flue gas, thereby reducing NOx emissions. Flue gas denitration technology can be divided into two categories of dry denitration and wet denitrification.
2.1 dry denitration technology
There are engineering applications of dry denitration technology including selective catalytic denitration (SCR), selective non-catalytic denitrification (SNCR) and activated carbon method.
(1) SCR method. SCR method is the use of ammonia to reduce NOx to N2. Ammonia selectivity, only with the role of NOx, does not react with oxygen in the flue gas. The reduction reaction at a slow reaction temperature is very slow, in order to speed up the reaction rate can be added to the catalyst. According to the suitable flue gas temperature conditions of the catalyst, the SCR process is divided into high temperature (> 450 ℃), medium temperature (320 ~ 450 ℃) and low temperature (120 ~ 320 ℃). Currently more widely used in commercial temperature catalyst. The catalyst takes TiO2 as a carrier, and supports the main catalyst such as vanadium, tungsten and molybdenum or a catalytic promoter, and the denitration efficiency can reach more than 95%.
Taking into account the low temperature characteristics of coke oven flue gas, the SCR method used in two ways: First, the low temperature flue gas heating to meet the temperature conditions of the temperature SCR method to use the mature temperature catalyst denitrification, flue gas denitrification and then consider Waste heat recovery, it takes additional heat; the second is the use of low-temperature catalytic denitrification, but the commercial application of low-temperature catalyst is not much research is mainly concentrated in the metal oxide catalyst and carbon-based catalyst.
(2) SNCR method. SNCR method is the use of ammonia or urea as a reducing agent, the principle and SCR method the same, but the method does not use a catalyst. Since the optimal temperature for this reaction is 900 to 1000 ° C in the absence of a catalyst, a reducing agent must be added to the high temperature region of the flue gas. The method of reducing the use of large, at the same time must control the reaction temperature, and to ensure that the reducing agent has sufficient residence time. The denitrification efficiency of SNCR method is low, about 30% ~ 50%. It is mainly used in the hearth combustion of coal-fired boiler and is not suitable for the denitrification of coke oven flue gas.