Flue gas desulfurization(FGD) refers to a boiler flue gas purification technology to remove nitrogen oxides, sulfur oxides (SO2 and SO3) from flue gas or other industrial waste gas. Nitrogen oxides and sulfur oxides are one of the main sources of air pollution. Therefore, the application of this technology has a lot of benefits for environmental air purification.
At present, there are dozens of types of flue gas desulfurization technology. According to whether water is added to the desulfurization process and the dry and wet form of the desulfurization product, flue gas desulfurization is divided into three categories: wet flue gas desulfurization, dry flue gas desulfurization and semi dry fgd.
Wet desulfurization technology is mature, high efficiency and simple operation.
Advantages: wet flue gas desulfurization technology for gas-liquid reaction, reaction speed, high desulfurization efficiency, generally higher than 90%, mature technology, wide application. Wet desulfurization technology is relatively mature, safe and reliable production and operation, and has always occupied a dominant position in many desulfurization technologies, accounting for more than 80% of the total installed capacity of desulfurization.
Disadvantages: the product is liquid or sludge, difficult to deal with, the equipment is seriously corrosive, the flue gas needs to be reheated after washing, high energy consumption, large footprint, high investment and operating costs. The system is complex, the equipment is huge, the water consumption is large, the one-time investment is high, and it is generally suitable for large power plants.
Classification: The commonly used wet flue gas desulfurization technology include limestone &#; gypsum method, indirect limestone &#; gypsum method, lemon absorption method and so on.
1. Limestone -gypsum wet flue gas desulfurization process
Limestone gypsum desulfurization system includes flue gas heat exchange system, absorption tower desulfurization system, desulfurization slurry preparation system, calcium sulfite oxidation system, gypsum dehydration system, etc. This process is the most mature and widely used technology in the world.
The desulfurization process is as follows: the flue gas enters the desulfurization tower through the dust collector and the heat exchange system, contacts with the lime emulsion in the absorption tower, and the slurry absorbs the SO2 in the flue gas to generate CaSO3, and then is oxidized into CaSO4, that is, gypsum, by the CaSO3 oxidation system. The desulphurization efficiency of this process can reach more than 95%, which has wide application range, mature process and stable operation, and is one of the preferred methods for desulphurization of large and medium-sized coal power plants. The process flow is shown in the figure below:
Limestone -gypsum wet flue gas desulfurization process2. Magnesium oxide desulfurization process
The basic principle of magnesium oxide desulfurization is similar to that of limestone (lime) method, that is, the SO2 in the flue gas is absorbed by magnesium oxide slurry, which mainly generates trihydrate and polyhydrate magnesium sulfite, and then generates stable and dissolved magnesium sulfate through oxidation, and then concentrates and crystallizes magnesium sulfate, and finally produces MgSO4·7H2O finished products. Its brief process flow is shown below.
3. Dual alkali scrubbing FGD process
The dual alkali scrubbing FGD process uses soluble alkaline clear liquid as absorbent to absorb SO2 in the absorption tower, and then drains most of the absorption liquid out of the absorption tower and uses lime milk to regenerate the absorption liquid.
Because two different types of bases are used in the absorption and absorption solution treatment, it is called the double alkali method. The double alkali process includes a variety of different double alkali processes such as sodium calcium, magnesium calcium and calcium calcium. The sodium-calcium double alkali method is one of the more commonly used desulfurization methods, which has been successfully applied to power stations and industrial boilers.
dual alkali scrubbing FGD process4. Wet ammonia flue gas desulfurization process
Ammonia desulphurization is a process that uses ammonia as absorbent to remove SO2 from flue gas. The process is generally divided into three steps: sulfur absorption, intermediate product treatment, by-product manufacturing; According to the different processes and by-products, it can be divided into ammonia-ammonium sulfide fertilizer method, ammonia-ammonium phosphate fertilizer method, ammonia-acid method, ammonia-ammonium sulfite method, etc.
The process is mainly composed of desulfurization washing system, concentration system, flue gas system, ammonia storage system, ammonium sulfate production system (if not ammonia-ammonium sulfide rule is the by-product manufacturing system corresponding to its process), electrical automatic control system.
Wet ammonia flue gas desulfurization process5. Calcium carbide slag &#; gypsum flue gas desulfurization technology
Calcium carbide is an important raw material in the organic synthesis industry, mainly used for the production of acetylene, further production of polyvinyl chloride (PVC), vinyl acetate (VAc), neoprene rubber (CR) and other chemical products and metal processing (cutting and welding, etc.). Calcium carbide slag is the waste slag produced when calcium carbide produces acetylene, the main component in addition to Ca(OH)2, but also contains Fe2O3, SiO2, Al2O3 and other metal oxides, hydroxide and a small amount of organic matter.
A large amount of Ca(OH)2 contained in calcium carbide slag is strongly alkaline and is a good sulfur dioxide absorber. The results show that the desulphurization capacity of calcium carbide slag is 20% higher than that of commodity Ca(OH)2, and the product cost is only one-third of that of commodity Ca(OH)2.
The process flow is basically the same as the limestone-gypsum method, including flue gas system, desulfurizer preparation system, absorption and circulation system, by-product treatment and electrical automatic control system.
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6. Paper white mud &#; gypsum flue gas desulfurization process
The main components of white mud for paper making include CaCO3, MgO, SiO2, etc. CaCO3 and MgO are soluble in water and alkaline after being dissolved in water. They are the main components used for desulfurization.
The process flow is basically the same as the limestone-gypsum method, including flue gas system, desulfurizer preparation system, absorption and circulation system, by-product treatment and electrical automatic control system.
Advantages: dry flue gas desulfurization technology for gas reaction, compared with the wet desulphurization system, simple equipment, small footprint, low investment and operating costs, easy operation, low energy consumption, easy disposal of products, no sewage treatment system.
Disadvantages: But the reaction speed is slow, the desulfurization rate is low, and the advanced can reach 60-80%. However, at present, the desulfurization efficiency of this method is low, the utilization rate of absorbent is low, the wear and scale phenomenon is serious, the maintenance of equipment is difficult, the stability and reliability of equipment operation is not high, and the life is short, which limits the application of this method. Classification: Commonly used dry flue gas desulfurization technology is activated carbon adsorption method, electron beam radiation method, charged dry absorber injection method, metal oxide desulfurization method.
A typical dry desulfurization system is to inject a desulfurizer (such as limestone, dolomite, or slathered lime) directly into the furnace. Taking limestone as an example, when calcined at high temperature, the desulfurizer forms porous calcium oxide particles after calcination, which reacts with SO2 in the flue gas to form calcium sulfate to achieve the purpose of desulfurization.
Dry FGD technology has been applied to large converter and blast furnace in the steel industry, but it is not suitable for small and medium blast furnace. The advantages of dry desulfurization technology are simple process, no sewage, dirty acid treatment problems, low energy consumption, especially the high temperature of the purified flue gas, which is conducive to the diffusion of chimney exhaust, will not produce &#;white smoke&#; phenomenon, the purified flue gas does not need secondary heating, and the corrosion is small; Its disadvantages are low desulfurization efficiency, huge equipment, large investment, large footprint, and high technical requirements for operation.
Common dry desulphurization technologies including:
Semi-dry desulphurization includes spray drying desulphurization, semi-dry and semi-wet desulphurization, powder and particle spouted bed desulphurization, flue jet desulphurization and so on.
1. Spray drying desulphurization process
Spray drying desulfurization method is the use of mechanical or air force to disperse the absorber into very small mist droplets, mist droplets and flue gas to form a relatively large contact surface area, in the gas-liquid phase between the heat exchange, mass transfer and chemical reaction desulfurization method. The general absorbent is lye, lime milk, limestone slurry, etc. At present, most of the devices use lime milk as an absorber. Under normal circumstances, the desulfurization rate of this method is 65% ~ 85%. Its advantages: desulfurization is carried out in the three-phase state of gas, liquid and solid, the process equipment is simple, the product is dry CaSO, CaSO, easy to handle, no serious equipment corrosion and blockage, and less water consumption. Disadvantages: automation requirements are relatively high, the amount of absorbent is difficult to control, and the absorption efficiency is not very high. Therefore, selecting and developing reasonable absorbent is a new problem to solve this method.
2. Semi-dry and semi-wet desulphurization process:
Semi-dry semi-wet method is a kind of desulfurization method between wet method and dry method, and its desulfurization efficiency and desulfurizer utilization rate are also between the two. This method is mainly suitable for flue gas treatment of small and medium-sized boilers. The characteristics of this technology are: less investment, low operating costs, although the desulfurization rate is lower than the wet desulfurization technology, it can still reach 70%tn, and the corrosion is small, the footprint is small, and the process is reliable. Compared with the wet desulphurization system, the semi-dry and semi-wet desulphurization system commonly used in industry saves the pulping system, and the wet desulphurization system is sprayed into Ca(OH) : aqueous solution instead of CaO or Ca(OH) : powder and water mist. Compared with the dry desulphurization system, it overcomes the disadvantages of low reaction efficiency and long reaction time of SO2 and CaO, improves the utilization rate of desulphurization agent, and has a good development prospect because of the simple process.
3. powder-particle spouted bed half-thousand flue gas desulfurization process:
Principle: The flue gas containing SO2 enters the powder particle spouted bed through the preheater, and the desulfurizer is made into powder form and mixed with water in advance. It is continuously sprayed into the bed from the top of the spouted bed in the form of slurry, and is fully mixed with the spouted particles. By means of contact with the hot flue gas, desulfurization and drying are carried out at the same time. The product of the desulfurization reaction is blown out of the separator in the form of dry powder. This technology uses limestone or slaked lime as a desulfurizer. It has a high desulfurization rate and desulfurizer utilization rate, and has little impact on the environment. However, there are strict requirements between the intake air temperature, the relative humidity in the bed and the reaction temperature. When the moisture content of the slurry and the reaction temperature are not controlled, there will be a phenomenon of desulfurizer sticking to the wall.
4. flue gas injection semi-dry desulfurization process
This method uses the flue between the boiler and the dust collector as the reactor for desulfurization, and does not need to add an additional absorption vessel, so that the process investment is greatly reduced, the operation is simple, the need for small space, and is suitable for development and application in China. Semi-dry flue gas desulphurization means that the absorbent slurry is sprayed into the flue, the slurry evaporates while reacting, and the reaction product comes out of the flue as dry powder.
The desulfurization and denitrification technology of boiler enterprises, the existing boiler manufacturers mostly use coal or gas as the combustion medium, for coal-fired boilers, the most mature process is FGD method (using absorbent or adsorbent to remove sulfur dioxide in flue gas) desulfurization technology; The selective catalytic reduction SCR technology is the main method for denitration.
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