Valves with Coal Water Slurry Gasification Device

Coal water slurry gasification technology is one of the most advanced entrained-flow coal gasification technologies in the world, which will provide key technologies for the development of coal liquefaction, integrated gasification combined cycle power generation (IGCC) and coal-based cogeneration technologies in China's coal chemical industry, greatly promote the development of China's coal chemical industry and promote the technological progress of related industries. In the process of production operation control, control valves play a very important role. According to the special conditions of gasification such as high temperature, high pressure, inflammable and explosive, violent reaction, slag washing, etc., suitable and high-quality oxygen, coal slurry and slag control valves are selected for the production operation and interlocking control process, so as to achieve safe and stable control of temperature, pressure and flow rate.
 
The combustion pressure in the gasifier is 4.0~6.3MPa and the temperature is 1,100 ~ 1,300℃. It is a high-temperature and high-pressure combustion tank and a highly dangerous device. Once an accident happens, the consequences are unimaginable, so its safety and reliability are the first priority.
 
Secondly, in order to maintain the normal combustion state, it is required that the flow rate of injected coal water slurry and the flow rate of mixed oxygen must match, otherwise, the pulverized coal may burn incompletely or produce useless gas, and the explosion may occur when the oxygen is excessive, so the automatic control system is required to have high control precision and good stability;
 
Thirdly, the gasifier is a continuous working device, which must have a long service life. Due to the above characteristics of the gasifier, a lot of technical requirements are put forward for its accessory equipment and related control instruments. The control valve directly controls the production medium and must have a special structure suitable for gasification. Several typical control valves are introduced as follows.
 
1. Oxygen cut-off valve and oxygen flow control valve
 
The gasifier burns at high pressure and high speed, and a large amount of oxygen enters. In order to ensure the normal combustion state and the safety of the gasifier, it is necessary to strictly regulate and control the flow rate of oxygen, and cut off the oxygen quickly and without leakage. Because of the high requirements for regulating and cutting off two functions, it is usually completed by three control valves, namely, oxygen flow control valve, the oxygen cut-off valve and the oxygen vent valve, which are collectively referred to as oxygen valves. Compared with general industrial process control valves, there are several special requirements for oxygen valves for gasifiers:
 
High-pressure difference: The closing pressure difference of the oxygen shut-off valve is the highest, reaching 5.0~7.0MPa. Under such a high pressure difference condition, the leakage level of oxygen shut-off valve is required to reach level VI at the same time. Generally, it is difficult to achieve it in the control valve room, and only O-shaped shut-off valve is used. Because of the high pressure difference, the lateral thrust borne by the ball greatly reduces the friction moment between the ball surface and the valve seat. Oxygen flow control valve mainly regulates the flow accurately, and it is required to have good flow characteristics. Secondly, it is the cut-off performance when it is closed, and the leakage level is VI, so the sleeve-type control valve is generally selected.
 
Material: High-pressure oxygen flows at high speed in the valve, which may cause friction with parts and generate heat, such as carbon steel. Nickel-based alloy (Inconel alloy) or ultra-low carbon austenitic stainless steel (such as 316L, 304L) shall be used, and MONEL material shall be used for the valve body, MONEL400/ALLY6 material shall be used for valve core and valve seat, in order to prevent the valve stem from scaling to death due to a constant opening for a long time, K500 material shall be used for valve stem.
 
Process requirements: Parts exposed to oxygen must be dehydrated and degreased to ensure cleanliness. Because the flow rate of high-pressure oxygen in the valve is 20~30m/s, if it encounters burrs, grooves, shoulders and other high-speed friction, the geometric shape of the inner wall of the flow passage and the valve internals should be smoothly transitioned and the surface is smooth. Therefore, in the design, processing, assembly, testing, transportation and other links, strict process regulations are formulated to ensure that the predetermined technical requirements are met. Therefore, the special requirements of oxygen valves lie in materials and processing.
 
Because of the explosive characteristics of high-pressure oxygen, the oxygen valve itself is an explosive dangerous device; To ensure safety, the nominal pressure of the oxygen valve should be increased by two pressure levels. If the pressure of the gasifier is 6.3MPa, then the nominal pressure of the valve should be PN160 (ANSI Class 900).
 
2. Slurry cut-off valve and slurry ash water control valve
 
It is a mixed medium of liquid phase and solid powder of coal water slurry, and the annular throttle formed between the valve core and the valve seat has a high flow rate. It faces three problems:
 
Coal slurry reaches a fairly high flow rate at the throttle of the valve, and its erosion destructive power is great because of its certain hardness. Coal slurry will also deposit and block the valve cavity, so it is required that the flow passage in the valve is unobstructed without dead angle; Coal slurry is very fine and can penetrate into the guide clearance of the valve core, which may cause the valve core to jam. Coal slurry has a certain adhesive force, which forms scale on the sealing surface of the valve core seat, which affects the leakage level when the valve is fully closed. Therefore, it is required that the control valve can automatically remove these scales.
 
Therefore, there are three special requirements for the grey water valve: erosion resistance, anti-blocking and self-cleaning.
 
Grey water is almost non-corrosive, so the valve body and valve cover can be made of carbon steel, the basic materials such as valve core and valve seat can be made of general austenitic stainless steel, and a layer of erosion-resistant cemented carbide can be hot-coated on its surface. The adhesion of the scale formed by ash water is less than that of the coking layer, so it is not necessary to use the actuator with large torque.
 
According to the above situation, the ash water control valve generally adopts the erosion-resistant eccentric rotary control valve. The valve body cavity of this valve is similar to a cylinder, which is unobstructed without dead angle, and coal ash can't accumulate in it. The valve core rotates, and in the gap between the guide sleeves on the two sides, the ash is difficult to penetrate into the interior. Even if the scale is formed, the valve core is easily crushed when rotating due to weak adhesion.
 
In addition, when the valve core is closed, when it touches the sealing surface of the valve seat, due to structural reasons, the valve core and the valve seat have a relative movement, which can grind off the scale on the sealing surface, so it has a self-cleaning function. Another important reason for choosing this control valve is its small size, light weight and convenient installation.
 
The quality of valves with the same caliber is only about one-third of that of angle control valves, and the valve body, bonnet and other parts can be made of carbon steel, so the price is much cheaper. Because the combustion rate of coal slurry in a gasifier is related to the reaction rate of other systems, oxygen supply, cooling, etc., in order to maintain a stable combustion state, it constantly adjusts and saves the flow rate of coal slurry, so it requires quite a high adjustment accuracy and usually adopts the equal percentage flow rate characteristic.
 
3. Slag lock valve
 
The process medium of slag valve locking is slag water, which contains 20% slag and is corrosive and volatile.
 
The flushing valve medium is grey water; The action frequency is switched 4 times per hour. The ball valve is selected, and the valve body is made of carbon steel runner surfacing Stellite alloy;
The valve stem shall adopt the structural design of the blowout preventer, made of 1Cr13 forging and plated with Ni and P.
 
The valve ball is made of fixed ball, and Satellite alloy is deposited on the sealing surface and flow passage, and the thickness of the Satellite alloy deposited on the lip of the valve ball is not less than 12mm. The whole ball is sprayed or deposited with cemented carbide.
 
The valve seat is made of 316 surfacing Stellite alloy, and the surfacing layer is not less than 1mm after processing. The material of the spring behind the valve seat is Inconel750, and there must be a sand control structure behind the valve seat to prevent the ash from entering the spring and causing the spring failure. The valve cavity has the function of automatic pressure relief. When the valve is closed, when the pressure in the valve cavity is higher than the closing pressure, it can automatically relieve the pressure to the side with low pressure. The filler is flexible graphite (with lubrication).
 
Hardness requirements are valve seat HRC52-54 and valve ball HRC58-62. The selection of the actuator has sufficient margin, and the number of switches is 1 million without faults; Double-cylinder double-acting electro-hydraulic actuator is adopted, which has high torque, high safety and torque protection function, and the valve stem will not be twisted or deformed. The slag lock valve adopts two-way hard seal ball valve, hardening treatment of valve ball and valve seat, three-stage fixed ball structure and floating valve seat, which ensures gasification slag discharge at regular time and safety interlocking operation.
 
4. Syngas butterfly valve
 
The flow rate of syngas at the outlet of gasifier is large, it moves quickly, and the leakage level is high. Therefore, butterfly valve is used to control the flow rate, and the actuator is electro-hydraulic actuator with fault position, which has large torque and small vibration, and the normal service life can reach 1 million times. Concentric structure design of valve body, valve plate and valve shaft, 100% bidirectional gas class VI seal; No bracket, compact structure, light weight and small volume. Pin-free connection is adopted, so the maintenance is simple and easy; Backrest-free design ensures that the torque will not be increased after the valve is installed. Butterfly valve is an imported high-performance spraying plate, which is more wear-resistant than ordinary plate, with longer service life and excellent flow characteristics.
 
Combined with experience, triple eccentric butterfly valve is selected. The structural characteristics of triple eccentric butterfly valve are that the conical axis of butterfly plate sealing surface is deviated from the cylindrical axis of the body while the shaft center of double eccentric valve stem is eccentric. After the third eccentricity, the sealing section of butterfly plate is no longer round, but oval, and its sealing surface shape is asymmetric because of this, with one side inclined to the center line of the body and the other side parallel to the center line of the body. The biggest feature of the third eccentricity is that it fundamentally changes the sealing structure and is no longer a position seal. Instead of relying on the elastic deformation of the valve seat, the sealing effect is achieved by using the contact surface pressure of the valve seat completely. Therefore, the problem of leakage of the metal valve seat is solved in one fell swoop, and the contact surface pressure is directly proportional to the medium pressure, and it is resistant to high pressure and high temperature.
 
5. Blackwater regulating valve
 
The raw gas and coal ash generated by combustion in the gasifier is discharged from the lower part respectively. After being cooled by spray water, most of the coal ash sinks into the lock hopper and is discharged. However, there is still a lot of coal ash in the cooling water, so the cooling water discharged from the gasifier is commonly called black water. After being regulated by the control valve, it enters the next process and is generally heated to about 250℃. At this time, many chemical components in the coal powder begin to decompose and chemically react with water to form a variety of highly corrosive substances. Therefore, the valve body and valve cover of the control valve and all parts in contact with the hot coal slurry must be made of anti-corrosion materials, especially the valve body and valve cover. Instead of ordinary austenitic stainless steel, ferrite-austenite duplex crystalline stainless steel is used. It not only has strong corrosion resistance, but also its mechanical strength is more than double that of austenitic stainless steel. According to the requirements of this technical characteristic, among many structural types, the angle control valve meets the requirements. The valve core of the angle control valve with a single-seat structure adopts the top guide structure, and there is a certain gap between the valve core handle and the guide sleeve. Although it is above the main stream, the black water will inevitably infiltrate into the guide gap, and once it stops, a coking film layer will be formed in the gap, which has strong adhesion. When restarting, the spool is often stuck. To prevent jamming, a scraping structure is designed in the guide section of the valve core, and the scraped coking powder can be discharged by itself through the escape groove, which means that the control valve also has self-cleaning ability. Because it takes a lot of thrust to eliminate coking, the electro-hydraulic actuator with large thrust is often used, and its thrust is far greater than that of ordinary pneumatic or electric actuators, and the thrust can be greater than 150kN.