Prospects for Flue Gas Desulfurization and Denitration Technologies in Industrial Furnaces — An Introduction to the One-Step Desulfurization and Denitration Technology Using Urea Aqueous Solution

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This post was last edited by newsdom on 2019-3-10 09:47. Building on traditional and mature ammonia-based desulfurization methods, new wet denitration techniques using urea aqueous solutions, as well as other related unit technologies, in-depth research and analysis were conducted. By taking into account the reaction mechanisms of ammonia-based desulfurization and urea aqueous solution denitration, the process flow was scientifically optimized, and the layout of the components inside the tower was repeatedly refined. This approach enabled high desulfurization efficiency (not less than 99%) while significantly improving denitration efficiency (not less than 90%), thereby maximizing the performance of the multi-functional integrated tower. It ensured that the functions of washing for denitration, absorption for desulfurization, evaporation and concentration, and oxidation conversion were all fully utilized, thus minimizing the investment and operating costs of the facility. The wet denitration technology using urea aqueous solution is based on the fact that, when the flue gas temperature is below 200°C, the oxidation rate of NO increases exponentially, resulting in the formation of HNO2, which is soluble in water. Urea aqueous solution can react rapidly with HNO2 produced from the dissolution of NOx in water, thereby directly generating N2 and H2O; in some cases, *ao acid an may also be formed. By integrating the ammonia-based desulfurization and urea-based denitrification unit technologies, the waste liquid generated from urea-based denitrification can be used as a supplementary raw material for ammonia-based desulfurization, thus making full use of it. Compared with traditional SNCR high-temperature denitration, SCR medium-temperature denitration, and SCR low-temperature denitration, it offers significant advantages in terms of water requirements, the recovery rate of waste heat from flue gas after the furnace, and the consumption of water, electricity, and steam in the system. Since it is a natural oxidation process that requires no external oxidants, its operating costs are significantly lower compared to forced oxidation denitration technologies that are already in industrial use, such as those based on ozone or NaClO/NaClO2. It does not cause a sharp increase in the Cl- concentration in the desulfurization and denitration circulation fluid, as is the case with the NaClO/NaClO2 forced oxidation method; it also does not accelerate material corrosion caused by high Cl- concentrations in the system, nor does it increase the amount of wastewater generated by the system. 　 For the control of SO2, NOx, and dust in furnace exhaust gases, traditional methods typically address these issues separately. In terms of desulfurization techniques, there is the limestone-gypsum method used in large coal-fired power plants, as well as the calcium carbide slag-gypsum method for smaller plants; there is also the double-alkali method that does not require forced oxidation, the magnesium oxide method, the ammonia method which produces sulfuric acid, an agricultural fertilizer, along with other desulfurization methods ； 　　Mature traditional or established denitration methods include 1) the ammonia injection non-catalytic selective reduction method (SNCR) at the rear of the furnace, which is suitable for flue gas denitration in circulating fluidized bed boilers. For coal-fired boilers of other structural types, the denitration efficiency is low; it usually does not exceed 40% in large units, and it is even lower than 20% in small boilers. The denitration efficiency of chain boilers is even worse. It is reported that although this method requires low investment and has low operating costs (no need to purchase or replace catalysts), it reduces the thermal efficiency of the boiler as well as the safe and stable operation of the equipment in related areas; currently, it can only be considered as an option for partially retired units with a limited remaining operational life ； 2) The catalytic selective nitrogen removal technology (SCR), which has been widely used in recent years and operates at moderate temperatures (with a maximum temperature of not less than 280°C, usually around 350°C), suffers from a significant reduction in its efficiency in some units. This is because, due to the need to reduce the load of these units, the temperature near the denitration reactor often drops below 320°C or even 280°C, preventing the achievement of the required emission standards ； 3) In recent years, thanks to the relentless efforts of various research institutions, SCR denitration technology that utilizes low-temperature catalysis over a wide temperature range of 200–470°C (with some cases allowing temperatures as low as 130°C, depending on the composition of the flue gas) has been adopted and applied in China. The denitration efficiency can even reach over 90% in the initial stages of use in certain applications. For some projects, such as the sintering gas from steel plants, the flue gas temperature is too low; therefore, a flue gas heating exchanger must be added before the catalytic denitration reactor, which is considered to increase production costs. Since 2016, the **Ministry of Environmental Protection and the environmental protection authorities in various provinces have successively introduced target requirements for ultra-low emissions of flue gases. Units with a capacity of over 300MW are equipped with tube-type high-efficiency mist and dust removal integrated units at the top of the desulfurization tower, which impose strict requirements on the dust content in the flue gas entering the tower (below 30–10 mg/Nm3) ； Small and medium-sized furnaces can also meet the requirements for ultra-low emissions by installing wet electrostatic dust removal units at the top of the desulfurization tower; however, the investment and operational maintenance costs are relatively high.