Overview of Flue Gas Desulfurization Technologies

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To address the deteriorating atmospheric environment, it is imperative to control SO2 emissions, and China has carried out research and application of various desulfurization technologies. Coal desulfurization can be divided into three main categories depending on the specific circumstances: pre-combustion desulfurization, in-combustion desulfurization, and post-combustion desulfurization. Desulfurization before combustion: Methods of desulfurization before combustion include mechanical desulfurization, chemical desulfurization, electromagnetic desulfurization, bacterial desulfurization, ultrasonic desulfurization, etc. Mechanical desulfurization methods have been applied in practice, such as jig desulfurization, flotation desulfurization, shaker table desulfurization, cyclone desulfurization, and spiral concentrator desulfurization. Mechanical desulfurization involves removing sulfur-containing compounds such as iron sulfide sulfur (FeS2) from coal based on the difference in density between these compounds and coal. The efficiency of this method in removing sulfur depends on the particle size of substances like FeS2 as well as the amount of inorganic sulfur present in the coal. The washing method cannot remove organic sulfur and iron sulfide sulfur that is finely dispersed in coal. Chemical methods, such as coal pyrolysis and hydrothermal pyrolysis for desulfurization, rely on the fact that the chemical bonds responsible for sulfur presence in raw coal—Fe-S and C-S—are less stable compared to C-C bonds; these bonds can easily break down under pyrolytic conditions, resulting in the formation of gaseous sulfides such as H2S or CaS. Coal pyrolysis and hydrothermal pyrolysis take advantage of this property to remove sulfur from coal. The electrochemical method utilizes the electrochemical oxidation and reduction reactions of coal in an electrolytic cell to oxidize pyrite and organic sulfur in coal into soluble sulfides, or to reduce and hydrogenate the coal, thereby achieving desulfurization. Biological desulfurization technology makes use of microorganisms to participate in the oxidation of iron and sulfur compounds, thereby degrading pyrite. Bacteria oxidize Fe2+ to Fe3+, and elemental sulfur is oxidized to sulfuric acid as a result of bacterial action. This process has not yet been adopted on a large scale, but with the breakthrough developments in biotechnology, it holds good prospects for future development. The microwave method desulfurizes coal by using microwave energy to stimulate the reaction of sulfides in the coal with the leaching agent. Desulfurization during combustion, as well as desulfurization after combustion – that is, flue gas desulfurization – is generally achieved by adding desulfurizing agents in the combustion chamber and in the exhaust flues. Flame desulfurization occurs in the high-temperature atmosphere generated by combustion, where a desulfurizing agent reacts chemically with SO2 gas molecules; therefore, flame desulfurization takes place simultaneously with the combustion of the fuel. In flue gas desulfurization, limestone (whose main component is CaCO3) is generally used as the desulfurizing agent. After being crushed to an appropriate particle size, it is injected into the boiler. At high temperatures, CaCO3 decomposes into CaO and CO2; the SO2 in the flue gas reacts with CaO, thereby completing the absorption of SO2 within the boiler. In a reducing atmosphere, lime or limestone reacts with H2S produced by coal combustion to form CaS, which is then oxidized to CaSO4 when exposed to oxygen. Limestone generally achieves the highest desulfurization efficiency at 800–850°C; when the furnace temperature exceeds 1200°C, the already formed CaSO4 decomposes to release SO2, resulting in suboptimal desulfurization effects. The best combustion method for adding limestone to the furnace for desulfurization is Fluidized Bed Combustion (CFBC) technology. Limestone only needs to be injected directly during its combustion; this process involves low investment and operating costs, and a sulfur fixation rate of over 70% can be achieved when the Ca/S ratio is 2. Post-combustion desulfurization: Flue Gas Desulfurization (FGD) is a widely used method for controlling SO2 emissions around the world today. Flue gas desulfurization refers to the removal of SO2 pollutants from the flue gas emitted by boilers; this desulfurization method does not affect combustion and heat exchange within the furnace. Based on the physical state of the reaction products (liquid or solid), flue gas desulfurization can be divided into three types: wet, semi-dry, and dry FGD. The diagram shows the possible approaches for flue gas desulfurization when desulfurizing agents are added in different temperature zones of the furnace and flue ducts of coal-fired boilers. Wet flue gas desulfurization technologies account for about 85%, of which the limestone-gypsum method makes up approximately 36.7%, other wet desulfurization technologies account for about 48.3%, and the spray drying desulfurization technology accounts for about 8.4% ； The absorbent regeneration desulfurization method accounts for about 3.41% ； In-furnace injection of absorbents and tail wetting-activated desulfurization technology account for about 1.9% ； Other technologies include circulating fluidized bed flue gas desulfurization, electron beam desulfurization, activated carbon adsorption desulfurization, seawater desulfurization, copper oxide method, and catalytic oxidation and reduction desulfurization. Wet flue gas desulfurization involves using a slurry to wash the flue gas at the end of the flue; both the desulfurizing agent and the desulfurization products are in a wet state, and the reaction takes place in a solution. This method offers high calcium utilization rates, with a desulfurization efficiency of over 90%. It is currently the preferred desulfurization technique for large-scale boilers both domestically and internationally. However, it requires significant investment, high operating costs, and presents challenges in treating wastewater; therefore, mist eliminators or specialized reheating devices must be installed. The main wet flue gas desulfurization processes include the limestone/lime-gypsum (discarded) method, simple wet method, double-alkali method, seawater desulfurization, magnesium oxide method, wet ammonia method, lime-magnesium method, and basic aluminum sulfate method. The desulfurizers used include calcium-based, magnesium-based, amino-based, sodium-based desulfurizers, etc. In the dry flue gas desulfurization process, both desulfurization absorption and product treatment take place in a dry state. It offers advantages such as reduced equipment corrosion, a higher temperature of the flue gas after purification, which facilitates its dispersion through the chimney. However, it has a low desulfurization efficiency, a slow reaction rate, and large-sized equipment. Most dry desulfurization technologies can also remove NOX contained in flue gas, enabling integrated desulfurization and denitrification, and thus hold broad application prospects. Processes of this type that are currently in use or under development include in-furnace or flue gas injection desulfurization, the copper oxide method, electron beam irradiation, activated carbon adsorption, and catalytic oxidation and reduction methods. The semi-dry flue gas desulfurization technology utilizes the principle of spray drying; after the absorbent slurry is injected into the absorption tower, the absorbent reacts chemically with SO2 in the flue gas to produce solid ash ； On the other hand, the flue gas transfers heat to the absorbent, keeping it continuously dry. The waste residue formed after the desulfurization reaction inside the tower is in the form of solid dust; part of it is separated within the tower, while the other part enters the electrostatic precipitator along with the flue gas after desulfurization. This process includes important steps such as the atomization of the slurry (water), the drying of the slurry droplets, the reaction between SO2 and the desulfurization agent (including gas-liquid reactions and gas-solid reactions), and the separation and recycling of the desulfurization ash. Spraying powdered desulfurization agent and humidifying water separately into the desulfurization reaction tower, or first preparing a slurry from the desulfurization agent and water before spraying it into the tower, both fall under the category of semi-dry flue gas desulfurization; the former method is known as powder spraying with humidification, while the latter is known as slurry spraying. Foreign researchers have been studying semi-dry flue gas desulfurization technology for over a decade, developing various semi-dry desulfurization processes that serve as a useful complement to the limestone-gypsum wet desulfurization method. These processes have been put into commercial use, allowing for the accumulation of extensive operational experience. Half-dry flue gas desulfurization technologies include the Lime Injection in Furnace with Afterburner Wetting and Activation technology (LIFAC), developed jointly by the Finnish company Tampella Boiler Manufacturing and IVO Power; the Limestone Injection Multi-stage Combustion technology (LIMB), developed by American companies Babcock Wilcox (B&W), Ohio Edison, and CONSOL.Inc; the Circulating Fluidized Bed Flue Gas Desulfurization technology (CFB-FGD) from the German company Lurgi; the Recirculating Fluidized Bed Flue Gas Desulfurization process (RCFB) from the German company Wulff; the Gas Suspension Absorption method for flue gas desulfurization (GSA) from the Danish company FLS; the Rotating Spray Drying flue gas desulfurization technology (SDA) from the Danish company NIRO; the new integrated NID flue gas desulfurization unit from the Swedish company ABB; the CZD-FGD technology for localized flue gas desulfurization developed by Bechtel; the Powder-Particle Spouted Bed flue gas desulfurization process; as well as the “Cool Side” process developed by Consol, the “HALT” process developed by D.B., and the “Dry Injection” process developed by EPRI. Many domestic institutions have successively developed semi-dry flue gas desulfurization technologies suitable for China’s national conditions. Based on laboratory research, these technologies have been applied in a number of demonstration projects for desulfurizing small and medium-sized boilers. Examples include Tsinghua University’s medium-temperature dry flue gas desulfurization and steam activation technology, Southeast University’s CFB-FGD technology, and the technique of injecting calcium inside the furnace followed by water injection for humidification and activation developed by the Harbin Power Plant Equipment Design Institute. There is also Shandong University’s self-developed dual-circulation fluidized bed flue gas desulfurization technology, as well as the composite spouted fluidization flue gas desulfurization technology developed by Harbin Institute of Technology. Zibo Ruimai Industry & Trade Co., Ltd. specializes in the production of desulfurization agents. www.zbruimai.com