Desulfurization knowledge

1. What are the good methods for desulfurization


There are three major types of desulfurization processes: wet, semi dry, and dry.


The most common desulfurization methods are calcium based desulfurization and ammonia based desulfurization. The market for furnace spray calcium, plasma, seawater desulfurization, etc. is very small and only applicable to special situations. Wet desulfurization technology is relatively mature, efficient, and easy to operate.


The traditional limestone/lime gypsum flue gas desulfurization process uses calcium based desulfurizers to absorb sulfur dioxide and generate calcium sulfite and calcium sulfate. Due to their low solubility, scaling and blockage phenomena are easily formed in the desulfurization tower and pipeline. The dual alkali flue gas desulfurization technology was developed to overcome the disadvantage of easy scaling in the limestone lime method.


With the gradual implementation of the new environmental protection law, the requirements for desulfurization efficiency are also increasing. The only desulfurization methods that can meet desulfurization efficiency are calcium and ammonia methods. However, calcium desulfurization has problems such as complex processes, blockage, corrosion, and sulfur gypsum stacking, but it is still the current mainstream desulfurization method; The ammonia desulfurization process is relatively simple and does not generate any waste. The generated ammonium sulfate can be used as a composite fertilizer, but there are still problems with high investment and operating costs. Ammonia desulfurization is currently the least problematic desulfurization method and also the mainstream trend in the future. The new integrated technology of desulfurization and denitrification has gradually improved and can meet the ultra-low emission standards of the new environmental protection law.


2. Flue gas desulfurization method


The minimum amount of 0.27 yuan to open a library member, view the complete content>Original publisher: FX database Yan Zhongkai 1. Policy background of China's "12th Five Year Plan" flue gas desulfurization. Sulfur dioxide emission reduction is one of the most important tasks for reducing major pollutants in China's "12th Five Year Plan".


In March 2011, the State Council issued the "Twelfth Five Year Plan" outline, which regards sulfur dioxide as a binding indicator for controlling the total emission reduction of the main pollutant, with the goal of reducing it by 8%. In December 2011, the national "Twelfth Five Year Plan" for environmental protection was announced, and in order to achieve the goal of reducing emissions by 8%, sulfur dioxide emissions will be further reduced from 22.678 million tons in 2010 to 20.864 million tons by 2015.


At the same time, China's coal consumption is expected to increase from 3 billion tons in 2010 to around 3.8 billion tons in 2015. Therefore, the task of reducing sulfur dioxide emissions is extremely challenging.


In November 2011, the State Council issued the "Opinions of the State Council on Strengthening Key Environmental Protection Work" (Guo Fa [2011] No. 35), proposing to implement total control of sulfur dioxide emissions in the power industry, continue to strengthen desulfurization in coal-fired power plants, and build desulfurization and denitrification facilities simultaneously for new coal-fired units; Implement total control of sulfur dioxide emissions in the steel industry, and strengthen the governance of sulfur dioxide and nitrogen oxides in the cement, petrochemical, and coal chemical industries. Thermal power plants are the main source of sulfur dioxide emissions in China and the main battlefield for sulfur dioxide reduction.


The revised "Air Pollutant Emission Standard for Thermal Power Plants" (GB13223-2011) was issued in September 2011 and implemented from 2012. It is stipulated that the emission limit of sulfur dioxide for newly built coal-fired power plants is 100mg/m3 (200 mg/m3 in high sulfur coal areas); Implementation of 200mg/Nm3 for existing power plant renovation (400 for high sulfur coal areas); The coal fired power plants in key regions implement 50mg/Nm3 to control the sulfur content of coal. The 42 organic amine method developed by the Ministry of Environmental Protection of the People's Republic of China is a process for removing hydrogen sulfide in the chemical industry, which can also achieve.


3. Desulfurization methods


Flue gas desulfurization refers to the removal of sulfur oxides (SO2 and SO3) from flue gas or other industrial waste gases.


Catalog 1 Process Introduction 2 Basic Principles 3 Process Methods ? Method Introduction ? Dry desulfurization ? Spray desulfurization ? Coal ash desulfurization ? Wet desulfurization 4 process history 5 anti-corrosion protection 1 process introduction Edit flue gas desulfurization (FGD), [1] In FGD technology, according to the type of desulfurization agent, it can be divided into the following five methods: calcium method based on CaCO3 (limestone), magnesium method based on MgO, sodium method based on Na2SO3, ammonia method based on NH3, and organic alkali method based on organic alkali. [1] 2 Basic Principles Editing Chemical Principles: SO2 in flue gas is essentially acidic, and can be removed from the flue gas by reacting with appropriate alkaline substances.


The most commonly used alkaline substances for flue gas removal are limestone (calcium carbonate), quicklime (calcium oxide, Cao), and hydrated lime (calcium hydroxide). Limestone is abundant in production and therefore relatively cheap. Quicklime and hydrated lime are both produced by heating limestone.


Sometimes other alkaline substances such as sodium carbonate (soda ash), magnesium carbonate, and ammonia are also used. The alkaline substance used reacts with SO2 in the flue gas to produce a mixture of sulfites and sulfates (depending on the alkaline substance used, these salts may be calcium, sodium, magnesium, or ammonium salts).


The ratio between sulfites and sulfates depends on the process conditions, and in some processes, all sulfites are converted into sulfates. The reaction between SO2 and alkaline substances occurs either in alkaline solution (wet flue gas desulfurization technology) or on the wet surface of solid alkaline substances (dry or semi dry flue gas desulfurization technology).


In the wet flue gas desulfurization system, alkali substances (usually alkali solution, more often alkali slurry) and flue gas meet in the spray tower. SO2 in the flue gas dissolves in water, forming a dilute acid solution, and then undergoes a neutralization reaction with alkaline substances dissolved in water.


The sulfites and sulfates generated by the reaction precipitate from the aqueous solution, and the precipitation depends on the relative solubility of different salts present in the solution. For example, calcium sulfate has relatively poor solubility and is therefore prone to precipitation.


The solubility of sodium sulfate and ammonium sulfate is much better. In both dry and semi dry flue gas desulfurization systems, solid alkaline absorbents are used to inject flue gas into the flue gas flow through the alkaline absorbent bed, making it in contact with the flue gas.


In either case, SO2 directly reacts with solid alkaline substances to generate corresponding sulfites and sulfates. In order for this reaction to proceed, the solid alkaline substance must be very loose or quite finely broken.


In the semi dry flue gas desulfurization system, water is added to the flue gas to form a liquid film on the surface of alkaline particles, and SO2 dissolves into the liquid film, accelerating the reaction with solid alkaline substances. The commonly used commercial technology in the world is the calcium method, which accounts for over 90%.


Desulfurization technology can be further divided into wet method, dry method, and semi dry (semi wet) method based on the dry and wet state of the absorbent and desulfurization products during the desulfurization process. Wet FGD technology uses a solution or slurry containing an absorbent to desulfurize and treat desulfurization products in a wet state. This method has the advantages of fast desulfurization reaction speed, simple equipment, and high desulfurization efficiency, but it generally has problems such as severe corrosion, high operating and maintenance costs, and easy to cause secondary pollution.


The desulfurization absorption and product treatment of dry FGD technology are carried out in a dry state. This method has the advantages of no sewage and waste acid discharge, light equipment corrosion, no obvious cooling of the flue gas during the purification process, high flue gas temperature after purification, favorable for chimney exhaust diffusion, and less secondary pollution. However, it has problems such as low desulfurization efficiency, slow reaction speed, and large equipment. The semi dry FGD technology refers to the flue gas desulfurization technology in which the desulfurizer is desulfurized in the dry state, regenerated in the wet state (such as the water washing activated carbon regeneration process), or desulfurized in the wet state, and treated with desulfurized products in the dry state (such as the spray drying method).


Especially in the wet state, the semi dry method for desulfurization and the treatment of desulfurization products in the dry state has attracted widespread attention due to its advantages of fast wet desulfurization reaction speed and high desulfurization efficiency, as well as the advantages of dry method without sewage and waste acid discharge, and easy treatment of desulfurization products. According to the purpose of desulfurization products, they can be divided into two types: abandonment method and recovery method.


At present, the commonly used flue gas desulfurization methods at home and abroad can be roughly divided into three categories based on their processes: wet abandonment process, wet recovery process, and dry process. The application of frequency converters in equipment has made significant contributions to energy conservation.


[3] Dry desulfurization dry flue gas desulfurization process This process was first used for flue gas desulfurization in power plants in the early 1980s and has the following advantages compared to conventional wet scrubbing processes: low investment cost; The desulfurization product is in a dry state and mixed with fly ash; No need to install mist eliminators and reheaters; Equipment is less prone to corrosion, scaling, and blockage. Its disadvantage is that the utilization rate of absorbent is lower than that of wet flue gas desulfurization process; Poor economic performance when used for high sulfur coal; Mixing fly ash with desulfurization products may affect comprehensive utilization; High requirements for drying process control.


Spray desulfurization spray dry flue gas desulfurization process spray dry flue gas desulfurization (hereinafter referred to as dry FGD), a desulfurization process jointly developed by JOY Company in the United States and NiroAtomier Company in Denmark, was developed in the mid-1970s and rapidly promoted and applied in the power industry. The process uses atomized lime slurry to contact with flue gas in the spray drying tower. The lime slurry reacts with SO2 to generate a dry solid reactant, which is collected by the dust collector together with the fly ash.


China has conducted the pilot test of rotary spray dry flue gas desulfurization in Baima Power Plant in Sichuan Province, and obtained some experience, which provides a basis for the design of optimization parameters of rotary spray dry flue gas desulfurization on 200~300MW units. Coal ash desulfurization and fly ash dry flue gas desulfurization technology. Japan has been researching dry flue gas desulfurization technology using fly ash as a desulfurization agent since 1985. By the end of 1988, industrial practical tests were completed, and the first fly ash dry flue gas desulfurization equipment was put into operation in early 1991, with a flue gas treatment capacity of 644000Nm3/h.


Its characteristics include a desulfurization rate of over 60%, stable performance, and reaching the level of general wet desulfurization performance; Low cost of desulfurization agent; Low water consumption, no need for drainage treatment and smoke exhaust reheating, and the total equipment cost is 1/4 lower than that of wet desulfurization; Coal ash desulfurizer can be reused; No slurry, easy maintenance, and equipment.


4. What are the main categories of commonly used coal-fired flue gas desulfurization methods


The common desulfurization technology is flue gas desulfurization (FGD), which is an effective desulfurization method widely used in the industrial industry.


According to the form of sulfide absorbents and by-products, desulfurization technology can be divided into three types: dry method, semi dry method, and wet method. The dry desulfurization process mainly uses solid absorbents to remove SO2 from the flue gas. Generally, fine limestone powder is sprayed into the furnace to decompose into CaO under heat, absorb SO2 from the flue gas, and generate CaSO3. It is collected with fly ash in the dust collector or discharged through the chimney.


Wet flue gas desulfurization is a gas-liquid reaction using liquid absorbents under ionic conditions to remove SO2 from the flue gas. The equipment used in the system is simple, stable and reliable in operation, and the desulfurization efficiency is high. The biggest advantage of dry desulfurization is that there is no discharge of wastewater or waste acid during treatment, reducing secondary pollution; The disadvantage is low desulfurization efficiency and large equipment.


Wet desulfurization uses liquid absorbents to wash the flue gas to remove SO2. The equipment used is relatively simple, easy to operate, and the desulfurization efficiency is high; However, the flue gas temperature after desulfurization is relatively low, and the corrosion of the equipment is more severe than that of the dry method. [1] Limestone (lime) - gypsum wet flue gas desulfurization process Limestone (lime) wet flue gas desulfurization technology is widely used in the field of wet FGD due to its low cost and easy availability of absorbents.


The reaction mechanism using limestone as an absorbent is as follows: absorption: SO2 (g) → SO2 (L)+H2O → H++HSO3- → H++SO32- dissolution: CaCO3 (s)+H+→ Ca2++HCO3- neutralization: HCO3-+H+→ CO2 (g)+H2O oxidation: HSO3-+1/2O2 → SO32-+H+SO32-+1/2O2 → SO42- crystallization: Ca2++SO42-+1/2H2O → CaSO4 · 1/2H2O (s). The characteristics of this process are high desulfurization efficiency (>95%), high absorbent utilization rate (>90%), and adaptability. High concentration SO2 flue gas conditions Low calcium sulfur ratio (generally<1.05), desulfurization gypsum can be comprehensively utilized, etc. The disadvantages are high infrastructure investment costs, high water consumption, and corrosive desulfurization wastewater.


The seawater flue gas desulfurization process is a desulfurization method that utilizes the alkalinity of seawater to remove sulfur dioxide from flue gas. The desulfurization process does not require the addition of any chemical agents, nor does it generate solid waste. The desulfurization efficiency is>92%, and the operation and maintenance costs are relatively low.


After dust removal by the dust collector, the flue gas is sent to the gas-gas heat exchanger for cooling by a booster fan, and then sent to the absorption tower. In the desulfurization absorption tower, in contact with a large amount of seawater from the circulating cooling system, sulfur dioxide in the flue gas is removed by absorption reaction, and the seawater is discharged after oxidation.


The flue gas after removing sulfur dioxide is heated by a heat exchanger and discharged through the flue. The seawater flue gas desulfurization process is limited by the region and is only suitable for projects with rich seawater resources, especially for thermal power plants where seawater is used as circulating cooling water. However, it is necessary to properly solve the anti-corrosion problems in the absorption tower, the absorption tower drainage ditch and its rear flue, chimney, aeration tank and aeration device.


The process flow is shown in Figure 1. Spray drying process spray drying process (SDA) is a semi dry flue gas desulfurization technology, and its market share is second only to that of wet process.


This method is to spray the absorbent slurry Ca (OH) 2 in the reaction tower, and the droplets are evaporated by the hot flue gas while absorbing SO2 in the flue gas, generating solids and being captured by the dust collector. When the calcium sulfur ratio is 1.3~1.6, the desulfurization efficiency can reach 80%~90%.


The semi dry FGD technology combines the general characteristics of dry and wet methods. Its main drawback is that using hydrated lime milk as an absorbent can easily cause scaling and blockage in the system, and specialized equipment is required for the preparation of the absorbent, resulting in high investment costs; The desulfurization efficiency and absorbent utilization rate are not as high as the limestone/gypsum method.


Spray drying technology is widely used in small and medium-sized units burning low sulfur and medium sulfur coal. A medium-sized testing device was built in January 1990 at Baima Power Plant in China.


Later, many units also adopted this desulfurization process, and the technology has basically matured. The electron beam flue gas desulfurization process (EBA method) is a dry desulfurization technology that combines physical and chemical methods.


The process of this process consists of smoke exhaust pre dust removal, smoke cooling, ammonia injection, electron beam irradiation, and by-product capture processes. The flue gas discharged from the boiler enters the cooling tower after coarse filtration treatment by the dust collector. Cooling water is sprayed inside the cooling tower to cool the flue gas to a temperature suitable for desulfurization and denitrification treatment (about 70 ℃).


The dew point of flue gas is usually about 50 ℃. The flue gas after passing through the cooling tower flows into the reactor and is injected with a mixture of nearly stoichiometric ammonia, compressed air, and soft water. The amount of ammonia added depends on the concentration of SOx and NOx. After electron beam irradiation, SOx and NOx generate intermediate substances sulfuric acid and nitric acid under the action of free radicals.


Then sulfuric acid and nitric acid undergo a neutralization reaction with coexisting ammonia to form a mixture of powdered particles of ammonium sulfate and ammonium nitrate. The desulfurization rate can reach over 90%, and the denitrification rate can reach over 80%.


In addition, sodium based, magnesium based, and ammonia can also be used as absorbents. The mixed particles of ammonium sulfate and ammonium nitrate generated by the general reaction are separated and captured by the by-product dust collector, and the purified flue gas is pressurized before being discharged into the atmosphere.


5. What are the methods for flue gas desulfurization


The main technologies for industrialization include: ① wet lime/limestone gypsum method. This method uses lime or limestone slurry to absorb SO2 in flue gas, generate hemihydrate calcium sulfite, or oxidize it again to gypsum.


Its technical maturity is high, and its desulfurization efficiency is stable, reaching over 90%