What are the methods for desulfurization in gas?

This post was last edited by Shandong Huaxing on 2010-6-2 at 15:01. Take a look at the content below; I hope it will be helpful to you! ! Domestic flue gas desulfurization technology: China’s current economic and technical conditions do not permit the investment of large amounts of human and financial resources, as is the case in developed countries. The country started late in addressing the issue of sulfur dioxide emissions, and it is still in the exploratory stage. Most of the flue gas desulfurization systems used in domestic power plants are based on technologies imported from Europe, the United States, and Japan; they are either experimental in nature, or capable of handling only a small volume of flue gas, and thus not yet mature. However, due to the strict environmental regulations in recent years, desulfurization projects are a necessity for all newly built power plants. Therefore, our country has begun to gradually develop its own desulfurization technologies based on foreign technologies. The following are some of the more mature desulfurization technologies in use in China; due to limited information, only a few of them can be listed for readers’ reference. Limestone-gypsum flue gas desulfurization process: The limestone-gypsum desulfurization process is the most widely used desulfurization technology in the world; approximately 90% of the flue gas desulfurization systems used in thermal power plants in Japan, Germany, and the United States rely on this process. Its working principle is as follows: Limestone powder is mixed with water to form a slurry, which is then pumped into the absorption tower as an absorbent to come into full contact with the flue gas. Sulfur dioxide in the flue gas reacts with calcium carbonate in the slurry, as well as with the air blown in from the bottom of the tower, to produce calcium sulfate. Once calcium sulfate reaches a certain degree of saturation, it crystallizes to form gypsum dihydrate. The gypsum slurry discharged from the absorption tower is concentrated and dehydrated to reduce its water content to less than 10%, after which it is transported by a conveyor to a gypsum storage bin for accumulation. The flue gas after desulfurization has its mist particles removed using a demister, and then it is heated in a heat exchanger before being released into the atmosphere through a chimney. Since the absorbent slurry in the absorption tower is repeatedly circulated through a circulation pump to come into contact with flue gas, the utilization rate of the absorbent is very high, the calcium-sulfur ratio is low, and the desulfurization efficiency can exceed 95%. Rotary spray drying flue gas desulfurization process: The spray drying desulfurization process uses lime as the desulfurization absorbent. Lime is digested and mixed with water to form slaked lime slurry, which is then pumped into a atomization device located inside the absorption tower. There, the absorbent, in the form of fine liquid droplets, mixes with the flue gas; a chemical reaction takes place between this absorbent and the SO2 in the flue gas, resulting in the formation of CaSO3, thereby removing the SO2 from the flue gas. At the same time, the moisture brought in by the absorbent is rapidly evaporated, causing it to dry out, and as a result the temperature of the flue gas decreases. The desulfurization reaction products and the unused absorbent are carried out of the absorption tower with the flue gas in the form of dry particulates, and are collected in a dust collector. The flue gas after desulfurization is discharged after being cleaned by a dust collector. To improve the utilization rate of desulfurization absorbents, part of the dust collector collection is generally added to the pulp preparation system for recycling. This process offers two different atomization methods to choose from: one is rotary spray wheel atomization, and the other is gas-liquid two-phase flow. The spray drying desulfurization process features mature technology, a relatively simple process flow, and high system reliability, with a desulfurization rate of over 85%. This process has a certain degree of application in the United States and some countries in Western Europe (8%). Desulfurization ash can be used for brick manufacturing and road construction, but it is often discarded in ash dumps or used to fill old mine pits. Flue gas desulfurization process using ammonium phosphate fertilizer. The flue gas desulfurization technology based on ammonium phosphate fertilizer belongs to the recovery method, and is named after its by-product, ammonium phosphate. This process mainly consists of units such as adsorption (desulfurization using activated carbon to produce acid), extraction (decomposition of phosphate rock with dilute sulfuric acid to extract phosphoric acid), neutralization (preparation of phosphammonium neutralization solution), absorption (desulfurization of phosphammonium solution to produce fertilizer), oxidation (oxidation of sulfurous acid), and concentration/drying (production of solid fertilizer). It is divided into two systems: the flue gas desulfurization system – the flue gas passes through an efficient dust collector to reduce its dust content to less than 200 mg/Nm3; a fan is used to increase the pressure of the flue gas to 7000 Pa. The gas is then cooled and humidified using water sprayed from a venturi tube, before entering a set of four parallel activated carbon desulfurization towers (one of which is periodically switched for regeneration). This setup ensures that the primary desulfurization efficiency is 70% or higher, and sulfuric acid with a concentration of around 30% is produced. The flue gas after primary desulfurization enters the secondary desulfurization towers, where it is washed with phosphoric ammonium slurry to further remove sulfur compounds; the purified flue gas is then discharged after mist separation. Fertilizer preparation system — In a conventional single-tank multi-slurry extraction tank, dilute sulfuric acid produced in the first stage of desulfurization is used to decompose phosphate rock powder (with a P2O5 content of over 26%); after filtration, dilute phosphoric acid (with a concentration of over 10%) is obtained. By adding ammonia for neutralization, phosphammonium is produced, which serves as a desulfurizing agent in the second stage. The slurry resulting from the second stage of desulfurization is then concentrated and dried to produce phosphate-ammonium compound fertilizer. Flue gas desulfurization process with calcium injection in the furnace and flue gas humidification at the rear section. This process builds on the calcium injection desulfurization method used in the furnace by adding a humidification section at the back of the boiler, in order to improve the desulfurization efficiency. In this process, limestone powder is commonly used as the absorbent. It is injected into the furnace chamber at a temperature of 850–1150°C via pneumatic means; there, the limestone decomposes upon heating to form calcium oxide and carbon dioxide. Calcium oxide then reacts with sulfur dioxide in the flue gas to produce calcium sulfite. Since the reaction takes place between the gas and solid phases, it is affected by the mass transfer process, resulting in a slow reaction rate and low utilization efficiency of the absorbent. In the tail humidification activation reactor, humidified water is sprayed in mist form and comes into contact with unreacted calcium oxide to form calcium hydroxide, which then reacts with sulfur dioxide in the flue gas. When the calcium-sulfur ratio is maintained between 2.0 and 2.5, the desulfurization efficiency of the system can reach 65–80%. As the addition of humidifying water lowers the flue gas temperature, the outlet flue gas temperature is generally kept 10–15°C above the dew point temperature. The humidifying water evaporates rapidly due to the heating from the flue gas temperature, while the unreacted absorbent and reaction products remain in a dry state and are discharged with the flue gas, where they are collected by the dust collector. This desulfurization process is used in Finland, the United States, Canada, France, and other countries; the maximum capacity of a single unit employing this technology has reached 300,000 kilowatts. Flue gas circulating fluidized bed desulfurization process: The flue gas circulating fluidized bed desulfurization process consists of components such as absorbent preparation, absorption tower, desulfurization ash recycling, dust collector, and control system. This process generally uses dry slaked lime powder as the absorbent; other dry powders or slurries capable of absorbing sulfur dioxide can also be used as absorbents. The untreated flue gas discharged from the boiler enters at the bottom of the absorption tower (i.e., the fluidized bed). At the bottom of the absorption tower is a Venturi device; as the flue gas passes through this venturi tube, its velocity increases, and it mixes there with very fine absorbent powder. Intense friction occurs between the particles as well as between the gas and the particles, thereby creating a fluidized bed. With the addition of a uniform mist to lower the temperature of the flue gas, the absorbent reacts with sulfur dioxide in the flue gas to form CaSO3 and CaSO4. The flue gas, containing a large amount of solid particles after desulfurization, is discharged from the top of the absorption tower and enters the recirculation dust collector. The separated particles are returned to the absorption tower via an intermediate ash bin. Since these solid particles undergo repeated cycles of hundreds of times, the utilization efficiency of the absorbent is high. The by-products generated by this process are in dry powder form, and their chemical composition is similar to that of the desulfurization process using spray drying. They mainly consist of fly ash, CaSO3, CaSO4, and unreacted absorbent Ca(OH)2, making them suitable for use in backfilling abandoned mines and as road foundations. In a typical flue gas circulating fluidized bed desulfurization process, when the sulfur content in coal is around 2% and the calcium-sulfur ratio is no more than 1.3, the desulfurization efficiency can exceed 90%, with the flue gas temperature at approximately 70°C. This technology is currently used abroad in units with a capacity of 100,000 to 200,000 kilowatts. Due to its small footprint and lower investment requirements, it is particularly suitable for flue gas desulfurization of existing units. Seawater desulfurization process: The seawater desulfurization process is a desulfurization method that utilizes the alkalinity of seawater to remove sulfur dioxide from flue gas. Inside the desulfurization absorption tower, large amounts of seawater are sprayed to wash the coal-fired flue gas entering the tower; the sulfur dioxide in the flue gas is absorbed by the seawater and removed. The purified flue gas is then dehumidified using a demister and heated in a flue gas heat exchanger before being released. The seawater that has absorbed sulfur dioxide is mixed with large amounts of unsulfurized seawater, and then treated through aeration in an aeration tank; this process oxidizes the SO32- ions to stable SO42- ions. After the pH value and COD of the seawater are adjusted to meet the discharge standards, it is released back into the sea. The seawater desulfurization process is generally suitable for power plants located near the sea, with favorable dispersion conditions, which use seawater as cooling water and burn low-sulfur coal. The seawater desulfurization process is widely used in Norway for the desulfurization of flue gases from industrial furnaces such as aluminum smelters and oil refineries, with over 20 such desulfurization units in operation to date. In recent years, the application of seawater desulfurization technology in power plants has made rapid progress. The biggest problem with this process is the potential deposition of heavy metals after flue gas desulfurization, as well as its impact on the marine environment; it takes long-term observation to draw conclusions, so it should be considered carefully in areas where environmental quality is particularly sensitive and environmental protection standards are high. Electron beam desulfurization process: This process consists of steps such as pre-dust removal of flue gas, flue gas cooling, ammonia injection, electron beam irradiation, and capture of by-products. The flue gas emitted by the boiler passes through a coarse filtration stage in a dust remover before entering the cooling tower, where cooling water is sprayed to cool the flue gas to a temperature suitable for desulfurization and denitrification treatments (around 70°C). The dew point of the flue gas is typically around 50°C, and the cooling water sprayed in mist form evaporates completely within the cooling tower; as a result, no wastewater is generated. The flue gas exiting the cooling tower enters the reactor, where a certain amount of ammonia water, compressed air, and soft water are mixed and injected at the reactor inlet. The amount of ammonia added depends on the concentrations of SOx and NOx. After exposure to an electron beam, SOx and NOx generate intermediate compounds, sulfuric acid (H2SO4) and nitric acid (HNO3), under the action of free radicals. Then sulfuric acid and nitric acid react with the ammonia present to produce powdered particles (a mixed powder of ammonium sulfate (NH4)2SO4 and ammonium nitrate NH4NO3). Some of these powdered particles settle at the bottom of the reactor and are removed via a conveyor, while the rest are separated and captured by a by-product dust collector; after granulation, they are sent to a by-product warehouse for storage. The purified flue gas is discharged into the atmosphere through the chimney via a desulfurization fan. Ammonia wash desulfurization process: This desulfurization process uses ammonia as the absorbent, with sulfuric acid and nitrogen fertilizers as by-products. The flue gas emitted from the boiler is cooled to 90–100°C in a flue gas heat exchanger, and then enters a pre-washer where HCI and HF are removed through washing. The flue gas after washing passes through a droplet separator to have water droplets removed, before entering the main washer. In the pre-washer, ammonia water is sprayed from the top of the tower to wash the flue gas; the SO2 in the flue gas is washed away and removed. The washed flue gas is then discharged, and the water droplets carried in it are removed using a droplet separator before it enters the desulfurization washer. In this scrubber, the flue gas is further washed; the mist droplets are removed by the demister at the top of the scrubber tower, and the gas then enters the desulfurization scrubber. It is then heated by a flue gas heat exchanger before being discharged through the chimney. The sulfuric acid an solution with a concentration of about 30% produced in the washing process is discharged from the washing tower; it can be sent to a fertilizer factory for further processing or sold directly as liquid nitrogen fertilizer. Alternatively, this solution can be further concentrated, evaporated, and dried to produce granular, crystalline, or solid fertilizer for sale. Reference: http://www.cngspw.com/V30DataBan ... 2005M10D03H11m54s32

This post was last edited by Shandong Huaxing on 2010-6-2 at 15:02. Desulfurization methods can generally be divided into wet and dry types. The wet methods include: ammonia catalysis method, modified ADA method, tannin method, 888 method, sulfolane method, etc ； Dry methods include the iron hydroxide method, activated carbon method, zinc oxide method, molecular sieve method, cobalt-molybdenum hydrogenation method, etc.

This post was last edited by Shandong Huaxing on 2010-6-2 at 15:02. There are mainly two types: wet and dry. The original poster has covered everything very comprehensively; I’ll just share a few of my personal insights. 1. In my opinion, environmental considerations should be given top priority. 2. The scale of the enterprise should also be taken into account (in other words, financial resources). 3. For small and medium-sized enterprises, the limestone method and the ammonia method, which are more cost-effective, can be used as they require relatively less investment. I recommend the simple ammonia method, as it can replace absorption towers and also has strong absorption capabilities (the alkalinity can be increased slightly by adding a small amount of sodium hydroxide)

This post was last edited by Shandong Huaxing on 2010-7-18 at 22:50. The method used in coking plants is called HPF method, while it is commonly referred to as wet oxidation desulfurization in the case of small nitrogen fertilizer production. These two methods are essentially the same; it is recommended to use the wet oxidation desulfurization method. For catalysts, cobalt phthalocyanine types such as SDS, PDS, SY7, RTS, etc., are all good choices.

This post was last edited by Shandong Huaxing on 2010-7-18 at 22:51. There are also the NHD process and the low-temperature methanol washing process; the latter allows for simultaneous desulfurization and decarbonization. Currently, large-scale methanol plants, ammonia synthesis plants, coal-to-oil purification plants, coal-to-gas purification plants, and coal-to-olefins plants mostly use the low-temperature methanol washing process.