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1. Traditional integrated flue gas desulfurization and denitrification technology: The integrated desulfurization and denitrification technology widely used at home and abroad today is mainly the wet-FGD + SCR/SNCR combination technique, which combines wet flue gas desulfurization with selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR) technologies for denitrification. The calcium-based method using lime or limestone is commonly employed in wet flue gas desulfurization, with a desulfurization efficiency of over 90%. Its disadvantages include large-scale infrastructure requirements, high initial investment and operating costs, as well as the potential for secondary pollution. When the temperature for selective catalytic reduction denitration is between 250 and 450°C, the denitration rate can reach 70% to 90%. This technology is mature and reliable, and it is 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 denitration technology: The integrated dry flue gas desulfurization and denitration technology comprises four aspects: solid-phase absorption/regeneration method, simultaneous gas/solid catalytic desulfurization and denitration technology, absorbent injection method, and high-energy electron-activated oxidation method. (1) Solid adsorption/regeneration method: Carbon-based material adsorption method. Depending on the type of adsorbent, it can be further divided into activated carbon adsorption and activated coke adsorption methods, both of which operate on similar principles for desulfurization and denitration. 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 It achieves a high desulfurization rate (98%) and a high denitrification rate (80%) at low temperatures (100–200°C); ② The flue gas does not need to be heated before being released; ③ No water is used, so there is no secondary pollution; ④ The adsorbent is readily available, and there is no risk of poisoning – only the consumed portion needs to be replaced; ⑤ It can remove SO2, which is difficult to remove using wet methods; ⑥ It can eliminate pollutants such as HF, HCl, arsenic, and mercury from exhaust gases, making it an effective advanced treatment technique; ⑦ It has dust removal capabilities, with the dust concentration at the outlet being less than 10 mg/m3; ⑧ By-products can be recovered, such as high-purity sulfur, concentrated sulfuric acid, liquid SO2, and chemical fertilizers; ⑨ The construction cost is low, operating costs are economical, and it 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 around 20 μm, thereby greatly increasing the adsorption 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 denitrification. This technology uses activated carbon as a carrier for nitrogen oxides, and by utilizing microwave energy, it can achieve a desulfurization and denitrification rate of over 90%. No×So method: The American company No×So began researching desulfurization and denitration technologies using activated alumina adsorption in 1982. The adsorbent of this method uses r-alumina as a carrier; the carrier is sprayed with a solution of alkali or salts containing alkaline components, and the resulting adsorbent is then heated and dried to remove any residual moisture. Once the adsorbent becomes saturated with adsorbed substances, it can be regenerated. The regeneration process involves feeding the saturated adsorbent 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 steady state, and a chemical equilibrium is established. In this way, nox will no longer be produced and only N2 will remain, 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 involves using CuO/Al2O3 or CuO/SiO2 as adsorbents (with a CuO content of typically 4%-6%) for desulfurization and denitrification. The entire reaction takes place 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; since both CuO and the resulting CuSO4 exhibit high catalytic activity in the reduction of nitrogen oxides by NH3, this method is combined with the SCR process for denitrification. 2) In the regenerator: The CuSO4 formed after the adsorbent becomes saturated is sent to the regenerator for regeneration. This regeneration process typically involves using H2 or CH4 to reduce CuSO4, and the resulting sulfur dioxide can be recovered to produce acid through a Claus process. The metallic copper or Cu2S obtained as a result of reduction is oxidized to CuO in an adsorbent treatment unit using flue gas or air, and the resulting CuO is then reused in the absorption and 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 advances in research, methods have emerged that combine activated coke/carbon (ac) with CuO. By combining the two, a catalytic absorbent with an appropriate active temperature can be produced, overcoming the drawback of AC having a low operating temperature and CuO/Al2O3 having a high active temperature. Liu Shoujun and others studied the use of CuO/AC for the low-temperature removal of SO2 and NOx from flue gas. The new CuO/AC catalyst exhibited high desulfurization and denitrification activities at flue gas temperatures of 120–250°C, which were significantly higher than those of AC and CuO/Al2O3 at the same temperatures. Pahlman process: The American company EnviroScrub Technologies has developed a new process called the Pahlman process, which uses a one-step dry scrubbing method to remove over 99% of sulfur oxides from flue gases. It is also capable of removing 99% of nitrogen oxides either selectively or simultaneously, ensuring that the exhaust gases meet all environmental standards. Since it uses inorganic compounds as absorbents instead of ammonia in traditional processes, its by-products are recyclable nitrates and sulfates, rather than gypsum by-products that pollute the environment and need to be landfilled. This process is suitable for power plants fueled by natural gas or coal; it is still in the experimental stage and has not been put into industrial use. (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 SNox process is a combined desulfurization and denitrification technology developed by the Danish company Haldor Topsoe. It involves oxidizing SO2 to SO3, which is then processed into sulfuric acid for recovery; meanwhile, NOx is removed through 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 has high desulfurization and denitrification efficiency, causes no secondary pollution, is simple in technology, and has low investment and operating costs, making it suitable for the renovation of existing plants. The SNRB process is a new type of high-temperature flue gas purification process developed by B&W Company. 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 purification process has reached the pilot scale stage, with a removal efficiency of over 99% for SO2 and NOx in flue gas from coal-fired boilers. This process catalytically reduces SO2 to H2S, NOx to N2, and the remaining oxygen to water in a separate reduction step; H2S is recovered from the exhaust gas of the hydrogenation reactor; and elemental sulfur is produced from the H2S-enriched gas. Flue gas circulating fluidized bed (CFB) combined desulfurization and denitrification 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 denitration technology was developed by Lurgi GmbH. In this method, slaked lime is used as an absorbent for desulfurization to remove sulfur dioxide, with the main products being CaSO4 and 10% CaSO3. For denitration, ammonia is used as a reducing agent in a selective catalytic reduction reaction; the catalyst is an active fine-powder compound of 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) Desulfurization and denitrification technology by absorber injection: Dry powders such as alkalis or urea are injected into the furnace, flue gas ducts, or spray dry scrubbers, and under certain conditions, sulfur dioxide and nitrogen oxides can be removed 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. Furnace lime (stone)/urea injection process: The simultaneous desulfurization and denitrification process using furnace lime (stone)/urea injection was developed jointly by institutions such as the Mendeleev Institute of Chemical Technology in Russia. This process combines in-furnace calcium injection and selective non-catalytic reduction (SNCR) to simultaneously remove 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 extensive experimental studies using a 14.7 kW natural gas combustion apparatus. Due to the extremely low volume of flue gas it can handle, this process fails to meet the requirements for industrial use, and therefore requires further improvement. The overall dry 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: Electron beam irradiation. A high-energy electron beam is generated by cathode emission and acceleration by an electric field; when these electron beams irradiate the flue gas, free radicals are produced, which then react with SOx and NOx to form sulfuric acid and nitric acid. 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, Ehara Corporation in Japan has gradually progressed from laboratory-scale trials to industrialization. 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 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. The combination of pulsed corona with traditional liquid-phase (calcium hydroxide or ammonium bicarbonate) absorption technologies improves the efficiency of removing sulfur dioxide and nitrogen oxides from flue gases, enabling integrated desulfurization and denitration. Pulse corona discharge for desulfurization and denitrification has prominent advantages; it holds great potential in energy conservation and has no adverse effects on the safe operation of power plant boilers. 3. Integrated wet flue gas desulfurization and denitration technology: The wet 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. Wet simultaneous desulfurization and denitration methods are currently mostly in the research stage, including oxidation methods and wet complexation methods. Oxidation method: The 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 piece of equipment. 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 into nitric acid 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 nitrates. This approach achieves a removal efficiency of over 95% for sulfur dioxide and nitrogen oxides. However, yellow phosphorus is flammable, unstable, and somewhat toxic; pre-treatment methods are required to address these issues. Wet complex absorption process: The wet complex absorption process generally uses iron or cobalt as a catalyst. After adding a chelating agent that can complex with NO to an aqueous solution, it binds to form a complex. 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 the regeneration of the absorbent by removing hyposulfite, sulfate, and N-S compounds from it, as well as reducing the ferric chelate to a ferrous chelate. 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 focus of research in various countries aimed at controlling flue gas pollution. At present, most such integrated processes are still in the research phase; although a few demonstration projects have been implemented, high operating costs hinder their widespread adoption. Developing an integrated desulfurization and denitration technology that suits China’s national conditions, features low investment, low operating costs, high efficiency, and the utilization of by-products as resources, has become a key focus for future development.