Overview of integrated flue gas desulfurization and denitration technologies

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This paper introduces the research progress of integrated flue gas desulfurization and denitrification technologies at home and abroad, analyzes the basic principles of various processes as well as the problems existing in their application, and provides guidance for the practical use of integrated desulfurization and denitrification. 1. Traditional integrated flue gas desulfurization and denitrification technologies 2. Dry-type integrated flue gas desulfurization and denitrification technologies: (1) Solid adsorption/regeneration method; (2) Simultaneous desulfurization and denitrification via gas/solid catalysis; (3) Simultaneous desulfurization and denitrification through absorbent injection; (4) High-energy electron-activated oxidation method 3. Wet-type integrated flue gas desulfurization and denitrification technologies 4. Conclusions and recommendations 1. Traditional integrated flue gas desulfurization and denitrification technologies: The integrated desulfurization and denitrification technologies widely used today, both domestically and internationally, are mainly the wet-FGD + SCR/SNCR combination technology, which involves the combination of wet flue gas desulfurization with selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR) technologies for denitrification. The calcium method using lime or limestone is commonly employed in wet flue gas desulfurization; it achieves a desulfurization efficiency of over 90%. Its drawbacks include large-scale engineering requirements, high initial investment and operating costs, as well as a tendency to cause secondary pollution. When the temperature for selective catalytic reduction denitration is between 250 and 450°C, the denitration efficiency can reach 70% to 90%. This technology is mature and reliable, and it is currently widely used around the world, especially in developed countries. However, it requires significant investment in equipment, involves preheating of flue gas, uses expensive catalysts that have a short lifespan, and is subject to issues such as ammonia leakage and equipment corrosion. The temperature range for selective non-catalytic reduction is 870–1200°C, with a denitration rate of less than 50%. The disadvantages include high investment in processing equipment, the need for preheating of flue gas, and susceptibility of the equipment to corrosion. II. Integrated dry flue gas desulfurization and denitrification technology: The integrated dry flue gas desulfurization and denitrification technology comprises four aspects: solid-phase absorption/regeneration method, gas/solid catalytic simultaneous desulfurization and denitrification technology, absorbent injection method, and high-energy electron-activated oxidation method. (1) Solid adsorption/regeneration method: The carbonaceous material adsorption method can be further divided into two types—activated carbon adsorption and activated coke adsorption—depending on the adsorbent used. Their principles of desulfurization and denitrification are essentially the same. The entire desulfurization and denitration process using activated carbon adsorption is divided into two parts: the adsorption tower and the regeneration tower. The activated carbon adsorption method uses only one adsorption tower, which is divided into two layers: the upper layer is used for denitration and the lower layer for desulfurization. The activated carbon moves up and down within the tower, while the flue gas flows horizontally through it. The main advantages of this method are: ① a very high desulfurization rate (98%) and a high denitrification rate (80%) at low temperatures (100–200°C) ； ②The treated flue gas does not need to be heated before emission ； ③No water is used, resulting in no secondary pollution ； ④Adsorbents are available from a wide range of sources, pose no risk of poisoning, and only need to be replenished when they are used up ； ⑤It can remove SO2 that is difficult to remove by wet methods ； ⑥It can remove pollutants such as HF, HCl, arsenic, mercury, etc. from exhaust gases; it is a advanced treatment technology ； ⑦It has dust removal functionality, with the dust emission concentration at the outlet being less than 10mg/m3 ； ⑧By-products can be recovered, such as high-purity sulfur, concentrated sulfuric acid, liquid SO2, chemical fertilizers, etc ； ⑨It has low construction costs, economical operating expenses, and requires little space. I. Mochida from Japan proposed a new activated carbon fiber technology for desulfurization and denitration. This technology transforms activated carbon into fiber-like structures with a diameter of about 20 μm, thereby greatly increasing the adsorption surface area and enhancing the adsorption and catalytic capabilities. Through development, the desulfurization and denitration efficiency of this technology can now reach 90%. In recent years, some researchers have combined activated carbon adsorption with microwave technology to propose a microwave-induced catalytic reduction technique for desulfurization and denitration. This technology uses activated carbon as a carrier for nitrogen oxides, and by utilizing microwave energy, it can achieve a desulfurization and denitration rate of over 90%. The no×so method: The American company no×so began researching desulfurization and denitrification technologies using activated alumina adsorption in 1982. The adsorbent of this method uses γ-alumina as a carrier; the carrier is sprayed with a solution of alkali or alkali-containing salts, and then the soaked adsorbent is heated and dried to remove any residual moisture. Once the adsorbent becomes saturated with adsorbed substances, it can be regenerated. In this regeneration process, the saturated adsorbent is fed into a heater, where it is heated to around 600°C to release the NOx, which is then recycled back into the burner of the boiler. In the burner, the concentration of NOx reaches a stable state, and a chemical equilibrium is established. In this way, NOx will no longer be generated; only N2 can be produced, thereby suppressing the formation of NOx. By introducing a reducing gas into the regenerator, a mixture of high-concentration SO2 and H2S gases is generated, and sulfur can be recovered using the Claus process. The CuO adsorption method for desulfurization and denitrification uses CuO/Al2O3 or CuO/SiO2 as adsorbents (with a CuO content of typically 4%-6%) to carry out these processes. The entire reaction occurs in two steps: 1) In the adsorber, at temperatures ranging from 300°C to 450°C, the adsorbent reacts with sulfur dioxide to produce CuSO4 ； Due to the high catalytic activity of CuO and the resulting CuSO4 in the reduction of nitrogen oxides by NH3, it is combined with the SCR method for denitration. 2) In the regenerator: The CuSO4 produced after the adsorbent becomes saturated is sent to the regenerator for regeneration. The regeneration process typically involves reducing CuSO4 using H2 or CH4. The regenerated sulfur dioxide can then be recovered and used to produce acid via a Claus unit ； The reduced metallic copper or Cu2S is oxidized to CuO in an adsorbent processor using flue gas or air, and the resulting CuO is then reused in the reduction process. This process can achieve a sulfur dioxide removal rate of over 90%, as well as a nitrogen oxide removal rate of 75% to 80%. The cuo adsorption method requires a high reaction temperature, necessitates a heating device, and the production cost of the adsorbent is high. In recent years, with the progress of research, methods of combining activated coke/carbon (ac) with CuO have emerged. The combination of the two can produce catalytic absorbents with an appropriate activation temperature, thereby overcoming the drawbacks of AC having a relatively low operating temperature and CuO/Al2O3 having a relatively high activation temperature. Liu Shoujun et al. studied the low-temperature removal of SO2 and NOx from flue gas using CuO/AC. The novel CuO/AC catalyst exhibits high desulfurization and denitrification activities at flue gas temperatures ranging from 120 to 250°C; these activities are significantly higher than those of AC and CuO/Al2O3 under the same temperature conditions. The Pahlman process: Enviroscrub Technologies, a U.S.-based company, has developed a new process—the Pahlman process. This one-step dry scrubbing method can remove over 99% of sulfur oxides from flue gas. It can also selectively or simultaneously remove 99% of nitrogen oxides. The emitted exhaust gas fully complies with environmental standards. Since it uses inorganic compounds as absorbents instead of ammonia in traditional processes, its by-products are recyclable oxalates and sulfates, rather than gypsum by-products that pollute the environment and need to be landfilled. The process is applicable to power plants fueled by natural gas or coal; however, it is still in the experimental stage and has not been used industrially. (II) Gas/solid catalytic simultaneous desulfurization and denitrification technology: This type of process uses catalysts to reduce the reaction activation energy, thereby facilitating the removal of sulfur dioxide and nitrogen oxides; it offers a higher efficiency in removing nitrogen oxides compared to traditional SCR processes. The SNX process is a combined desulfurization and denitrification technology developed by the Danish company Haldor Topsoe; it involves oxidizing SO2 to SO3 to produce sulfuric acid for recovery, while NOx is removed using selective catalytic reduction (SCR). This process can remove 95% of SO2, 90% of NOx, and almost all particulate matter. The Desonox process was jointly developed by Degussa, Lentjes, and Lurgi. In addition to converting SO2 in flue gas into SO3 to produce sulfuric acid, as well as using SCR to remove NOx, this process can also oxidize CO and unburned hydrocarbons into CO2 and water. This process features high desulfurization and denitrification efficiency, no secondary pollution, simple technology, and low investment and operating costs; it is suitable for the retrofitting of existing plants. The SNRB process is a new type of high-temperature flue gas purification process developed by B&W. This process can remove sulfur dioxide, nitrogen oxides, and dust simultaneously, all in a high-temperature dust collection chamber. The SNRB process reduces costs and minimizes floor space by concentrating the removal of three types of pollutants in a single unit. Its drawback is that, as the required flue gas temperature ranges from 300°C to 500°C, special filter bags made of high-temperature resistant ceramic fibers are necessary, which increases the cost. The Parsons flue gas cleaning process has reached the pilot scale stage, with a removal efficiency of over 99% for SO2 and NOx in flue gases from coal-fired boilers. This process catalytically reduces SO2 to H2S, NOx to N2, and the remaining oxygen to water, all in a single reduction step ； Recovering H2S from the exhaust gas of the hydrogenation reactor ； Production of elemental sulfur from H2S-enriched gas. Flue gas circulating fluidized bed (CFB) combined desulfurization and denitration process: Circulating fluidized bed technology was originally a semi-dry desulfurization technique developed by the German company LLB (Lurgi Lentjes Bischoff). This technology has developed rapidly in recent years; it is not only mature and reliable but also features significantly reduced investment and operating costs. To develop more economical, efficient, and reliable methods for simultaneous desulfurization and denitrification of flue gas, circulating fluidized bed technology has been incorporated into such systems. The flue gas circulating fluidized bed (CFB) combined desulfurization and denitrification technology was developed by Lurgi GmbH. This method uses slaked lime as a sorbent for desulfurization to remove sulfur dioxide; the main products are CaSO4 and 10% CaSO3 ； The denitration reaction employs ammonia as a reducing agent in a selective catalytic reduction process; the catalyst is the active fine-powder compound FeSO4·7H2O, and no support carrier is required, with an operating temperature of 385°C. Results from the operation of this system in Germany show that at a Ca/S ratio of 1.2–1.5 and an NH3/NOx ratio of 0.7–1.03, the desulfurization efficiency is 97% and the denitrification efficiency is 88%. (III) Simultaneous desulfurization and denitrification technology via absorbent injection involves spraying dry powders such as alkali or urea into the furnace, flue duct, or spray-dry scrubber; under certain conditions, this can remove both sulfur dioxide and nitrogen oxides simultaneously. The denitration rate mainly depends on the ratio of sulfur dioxide to nitrogen oxides in the flue gas, reaction temperature, particle size of the absorbent, and residence time, among other factors. However, when the sulfur dioxide concentration in the system is low, the removal efficiency of nitrogen oxides is also low. Therefore, this process is suitable for the treatment of flue gas from high-sulfur coal. The furnace lime (stone)/urea injection process, or the simultaneous desulfurization and denitrification process using furnace lime (stone)/urea injection, was jointly developed by the Mendeleev Institute of Chemical Technology in Russia and other organizations. This process combines in-furnace calcium injection and selective non-catalytic reduction (SNCR) to achieve the simultaneous removal of sulfur dioxide and nitrogen oxides from flue gas. The sprayed slurry is composed of a urea solution and various calcium-based absorbents, with a total solid content of 30% and a pH value of 5–9. Compared to the method of spraying dry Ca(OH)2 absorbents, slurry spraying enhances SO2 removal, likely due to the absorbents being ground finer and thus more active. Gullett et al. conducted numerous experimental studies using a 14.7 kW natural gas combustion device. Due to the extremely low volume of flue gas it can handle, this process fails to meet the requirements for industrial use, and therefore needs further improvement. The overall dry-type SO2/NOx emission control system utilizes Babcock & Wilcox’s low-NOx DRB-XCL bottom-mounted burners, which suppress the formation of nitrogen oxides by injecting a portion of coal and air into an oxygen-deficient environment. The introduction of excess air is intended to complete the combustion process and further remove nitrogen oxides. Low-nitrogen oxide burners are expected to reduce nitrogen oxide emissions by 50%, and with the introduction of excess air, they can cut NOx emissions by over 70%. Whether it is an integrated dry SO2/NOx emission control system or individual technologies, they can be applied to power plants or industrial boilers, and are mainly suitable for older small and medium-sized units. (IV) High-energy electron activation oxidation method: The electron beam method utilizes electrons emitted from the cathode and accelerated by an electric field to form high-energy electron beams. When these electron beams irradiate the flue gas, free radicals are generated, which then react with SOx and NOx to produce sulfuric acid and *AO acids. When ammonia (NH3) is introduced, by-products such as ammonium salts like (NH4)2SO4 and NH4NO3 are formed. After more than 20 years of research and development, the Japanese company Ebara has progressed from pilot-scale production to industrial-scale manufacturing. The desulfurization rate is over 90%, and the denitrification rate is over 80%. However, it consumes a large amount of electricity (about 2% of the plant’s total power consumption), resulting in high operating costs. The pulsed corona plasma method (PPCP): Masuda and others discovered in 1986 that corona discharge can simultaneously remove sulfur dioxide and nitrogen oxides. Due to its advantages such as simple equipment, easy operation, significant effects in sulfur and nitrogen removal as well as dust removal, and the possibility of recycling by-products as fertilizer, this method has become a forefront in international research on sulfur and nitrogen removal. Both pulsed corona plasma technology and electron beam methods belong to plasma-based techniques. By combining pulsed corona with traditional liquid-phase absorption technologies (such as calcium hydroxide or ammonium bicarbonate), the efficiency of removing sulfur dioxide and nitrogen oxides from flue gases is improved, enabling integrated desulfurization and denitration. Pulse corona discharge for desulfurization and denitrification has notable advantages; it holds great potential for energy conservation and has no adverse effects on the safe operation of power plant boilers. 3. Integrated wet-process flue gas desulfurization and denitration technology: The wet-process flue gas desulfurization and denitration process typically oxidizes NO to NO2 in the gas/liquid phase, or increases the solubility of NO by adding additives. Currently, most methods for wet simultaneous desulfurization and denitrification are still in the research phase, including the oxidation method and the wet complexation method. The oxidative chloric acid oxidation process (also known as the Tri-Nox-NoxSorb process) utilizes a wet scrubbing system to simultaneously remove sulfur dioxide and nitrogen oxides from flue gas in a single unit. The Tri-Nox-Noxsorb process utilizes a two-stage process involving an oxidation absorber and an alkaline absorber; it removes toxic trace metal elements such as As, Be, Cd, Cr, Pb, Hg, and Se while also removing sulfur dioxide and nitrogen oxides. Isabelle et al. studied the process of oxidizing NOx and SO2 to *AO acids and sulfuric acid using hydrogen peroxide under acidic conditions. The yellow phosphorus oxidation method involves oxidizing NO to NO2, which then reacts with a liquid alkaline absorption slurry to produce sulfates and *AO acids; this approach achieves a removal efficiency of over 95% for sulfur dioxide and nitrogen oxides. However, yellow phosphorus is flammable, unstable, and somewhat toxic, so pretreatment methods are required to address these issues. The wet complexation absorption process generally uses iron or cobalt as a catalyst. After adding a complexing agent capable of complexing NO to an aqueous solution, it forms a complex with NO. NO combined with the chelating agent can react with SO32-/HSO3- in the solution to form a series of N-S compounds, thereby regenerating the chelating agent. This process requires regenerating the absorption solution by removing dithionates, sulfates, and N-S compounds from it, as well as reducing trivalent iron chelates to ferrous chelates. The wet complexation absorption process can simultaneously remove sulfur and nitrogen oxides, but it is still in the experimental stage at present. The main obstacles to its industrial application are the loss of chelates during the reaction process, as well as the difficulty in regenerating metal chelates and their low utilization efficiency, which result in high operating costs. IV. Conclusions and Recommendations
The integrated desulfurization and denitrification process has become a research hotspot in various countries for controlling flue gas pollution. Currently, most integrated desulfurization and denitrification processes remain at the research stage. Although there are a few demonstration projects in operation, their large-scale application is hindered by relatively high operating costs. Developing an integrated desulfurization and denitrification technology that suits China’s national conditions—featuring low investment, low operating costs, high efficiency, and the resource utilization of by-products—has become a key focus for future development.