The gasifier is the main equipment of the water-coal slurry pressurized gasification device, and the quality of the refractory bricks in the gasifier is the main factor affecting the operation cycle of the gasifier. The operation of the gasifier requires high temperature and high pressure. Oxygen and coal slurry are sprayed into the gasifier through the process burner. The impact of the spray forms 6 flow zones with different characteristics, which intensifies the scouring of the refractory firebricks and causes a sharp change in furnace temperature during start-up and shutdown. Therefore, the lining is required to have high resistance to slag erosion and permeability, high hot strength, and good high-temperature volume stability. The gasifier furnace is divided into three parts, the upper part is the vault part, the middle part is the cylinder part, and the lower part is the cone bottom and slag mouth part. The three parts are independent of each other, which is conducive to the removal or replacement of each part. The corrosion rate of refractory bricks in different parts is not consistent. According to operating experience, it is found that the refractories bricks in the vault part have a faster ablation rate.
Through the study of the flow field distribution in the gasifier and the structure of refractory fire bricks, combined with the working conditions in the furnace, the causes of refractory brick wear were analyzed in many aspects and corresponding measures were taken.
01 Structural design reasons
1. The thickness of the fire-facing bricks in the vault is insufficient. The thickness of the fire-facing bricks is 200mm. When the thickness of the fire-facing bricks is reduced to 1/3 of the original thickness, the bricks reach their service life and cannot be used. According to the actual thinning speed of the vault bricks on site, it can be seen that the thin thickness of the refractory fire bricks and the fast thinning speed are the key reasons for the short life of the overall refractories bricks. After the improvement, the refractory bricks in the vault part of the new gasifier were changed from the original three layers to two layers, the inner layer is the fire-facing bricks, the outermost layer is the heavy castable, and the backing bricks in the middle layer were cancelled. After the transformation, the fire-facing bricks replaced the original backing bricks, thereby increasing the thickness of the fire-facing bricks, extending the ablation rate, and thus extending the service life of the refractory fire bricks in the vault part.
2. The structure of the plugging brick is unreasonable. The plugging brick is designed as a cylinder. Its main sealing surface is the side of the plugging brick and the B brick is a gap seal. The design gap is 2mm. In fact, there are certain errors in the manufacture and masonry of refractories bricks. Especially after the gasifier is used, the secondary installation of the plugging brick cannot completely clean the molten slag on the sealing surface of the refractory brick. The plugging brick is a casting, and the manufacturing size error is about 2mm. According to the above situations, the actual reserved gap of the plugging brick is greater than 4mm, otherwise it cannot be installed smoothly. Due to the large gap, the sealing effect is poor, and the vault neck is repeatedly overheated. The service life of the gasifier vault prefabricated parts is short. The structural form of the gasifier top sealing brick is modified: 1) The gasifier top prefabricated parts are changed from the original cylindrical boss type to the conical boss type. 2) The B brick is thickened, the size of the preheating port is reduced, and the preheating port is changed from a cylindrical hole to a conical hole. The design of the A brick close to the B brick is changed to an A1 brick in order to protect the B brick. 3) Through repeated inspections and summaries of the gasifier's fire-facing bricks, it was found that the vault bricks B to K were corroded too quickly, which was the weak point of the gasifier. We redesigned and improved the vault refractory bricks, changing the original vault firebricks from one mother-and-child groove to two, and adding a line of defense against brick seam erosion. Through the above-mentioned transformation, the phenomenon of gas blowby and overheating at the vault neck was effectively improved, extending the service life of the gasifier vault prefabricated parts.
02 Raw material reasons
1. The influence of coal ash melting point Simply put, the ash melting point is the temperature at which the ash melts. The silicon, aluminum, iron, magnesium, potassium, calcium, sulfur, phosphorus and other elements contained in coal and carbonates, silicates, sulfates and sulfides constitute the ash content of coal. The ash melting point of coal determines the operating temperature of the gasifier. If the ash melting point is low, the operating temperature is relatively low, which is conducive to the protection of firebrick; if the ash melting point is high, the operating temperature must be relatively high, and the heat radiation in the furnace is large, which accelerates the thermal erosion of refractory bricks. The size of the ash melting point is related to the composition of the ash. The greater the proportion of SiO2 and Al2O3 in the ash, the higher its melting temperature; and the higher the proportion of alkaline components such as Fe2O3 and MgO, the lower the melting temperature. It can be adjusted by adding flux. Most coal ash slag is acidic slag, and flux is often adjusted by alkaline CaO or CaCO3 produced by pyrolysis. Coal blending technology can also be used to control the melting point of coal ash entering the furnace. The ash melting point of gasification coal is generally controlled below 1300℃.
2. Influence of ash viscosity The multi-nozzle opposed new gasifier adopts liquid slag discharge. The operating temperature increases and the ash viscosity decreases, which is conducive to the flow of ash. However, if the ash viscosity is too low, the refractories bricks will directly contact with high-temperature gas, and erosion and peeling will be aggravated; if the operating temperature is low, the ash viscosity increases, which is not conducive to the flow of ash, and it is easy to accumulate slag and block the slag mouth. Only when operating within the optimal viscosity range can a certain thickness of ash protective layer be formed on the surface of the fire bricks, which prolongs the service life of the fire bricks without blocking the slag mouth. Therefore, in order to prevent the erosion of the refractory bricks by high-temperature gas, it is necessary to maintain a layer of ash film on the surface of the fire bricks. Therefore, the optimal operating temperature of the multi-nozzle opposed new gasifier is determined according to the viscosity-temperature characteristics of the ash, and the general viscosity is below 250P.
Process operation reasons
1. The oxygen flow rate out of the burner is unreasonable. The unreasonable oxygen flow rate will not only affect the atomization effect, but also accelerate the erosion of the fiire bricks near the burner. Control the load and pressure of the gasifier without changing the overall structure of the gasifier. According to the results and calculations of the hot mold experiment of East China University of Science and Technology, the operating load corresponding to the process burners of different assembly sizes under different operating pressures is formulated. Make the oxygen flow rate ≤140m/s.
2. Frequent start and stop of the gasifier will cause a sharp change in furnace temperature, which will cause a sharp change in the thermal stress of the refractory firebricks, resulting in cracks in the furnace lining, aggravating the erosion rate of the refractory firebricks and reducing the service life of the fire bricks. The operating conditions should be kept stable to avoid fluctuations and minimize the number of start and stop times of the gasifier.
3. Operating temperature The operating temperature of the gasifier is generally controlled at 50-100°C above the ash melting point to ensure that the coal is fully gasified and the slag can be discharged smoothly. If the temperature is too low, the ash and slag cannot be discharged smoothly, causing the slag mouth to be blocked; if the temperature is too high, the ash and slag will increase the erosion and penetration of the refractory bricks. For every 100°C increase in operating temperature, the erosion rate of the refractories bricks will increase by 3-4 times. Excessive temperature will reduce the Cr2O3 in the refractory firebricks, resulting in structural damage. Therefore, the operating temperature should be strictly controlled. The lower limit of the temperature should be higher than the temperature corresponding to the slag viscosity of 250P; the upper limit of the temperature should be the temperature corresponding to the slag viscosity of 30-50P, and large temperature fluctuations should be avoided.
4. Operating pressure Operating pressure fluctuations will affect the joints of refractory fire bricks, causing gasification of refractory fire bricks and reducing the service life of refractory fire bricks. Therefore, when the system is started and stopped, it should be operated according to the pressure increase and decrease curve to avoid too fast pressure increase and decrease; in normal operation, the pressure should be kept stable to avoid pressure fluctuations.
