Seeking advice on flue gas denitration technology for coal-fired boilers

This post was last edited by ming*ang77777 on 2017-2-26 08:05. As two major technologies for flue gas denitration, SNCR and SCR require that an appropriate denitration technique be selected at the initial stage of building a flue gas denitration system. SNCR is a denitration technology that uses ammonia or urea as a reducing agent to react with nitrogen oxides in flue gas, without the use of a catalyst. Compared with the SCR selective catalytic reduction denitration technology, its features are as follows: 1. SNCR does not require the use of a catalyst ； 2. The reducing agent involved in the reaction with SNCR technology can be ammonia or urea. Since the temperature of the flue gas in an SCR system is low, urea must be converted into ammonia before it can be injected into the flue gas ； 3. The reaction temperature for SNCR is relatively high; when ammonia is used as a reducing agent, the reaction temperature range is 850°C to 1100°C. When the flue gas temperature exceeds 1050°C, ammonia begins to be oxidized into nitrogen oxides; at 1100°C, the oxidation rate increases significantly, on one hand. It reduces the denitration efficiency; on the other hand, it increases the amount of reagent used as well as its cost ； And when the flue gas temperature is below 870°C, the denitrification reaction rate decreases significantly ； 4. Due to the reaction temperature range, the reaction time, as well as the setting and adjustment of the ammonia injection point, are constrained by the layout of the heating surfaces in the boiler furnace ； 5. The ammonia escape rate for SNCR technology is generally between 5 and 10 ppm, while it is kept between 2 and 5 ppm in SCR systems ； 6. Due to limitations related to the temperature range and ammonia escape, the SNCR denitration efficiency is generally between 30% and 50%; for large boilers, this efficiency is usually below 40% ； The denitration efficiency of SCR has virtually no technical limitations, with a typical guaranteed performance level abroad being 90% ； 7. With the catalyst used in SCR, some of the SO2 is converted into SO3; SNCR does not require concern regarding this issue ； 8. Compared to SNCR, the domestic SCR denitration technology is not yet fully developed. SNCR technology is a mature denitration technique. Compared with SCR denitration technology, it features lower investment costs, reduced operating expenses, and shorter implementation periods. When combined with low-nitrogen combustion technology, it not only ensures high denitration efficiency but also helps to cut operating costs. Denitration technologies: Low-nitrogen combustion, SCRSNCR, Low-nitrogen combustion + SNCR. Denitration efficiency: %50, 80–95; 30–50, 60–75. Investment cost: yuan/Kw – 15, 25, 050, 65. Operating cost: fen/Kwh – 0, 20, 0.3, 0.3. It can be seen that for some small and medium-sized boilers, choosing the denitration technology that combines low-nitrogen combustion with SNCR not only ensures high denitration efficiency but also **reduces denitration costs.

SCR is used in coal-fired boilers, while SNCR is used in circulating fluidized bed boilers

But it seems that SCR is the trend now