Analysis of the causes of high combustible content in Texaco coal gasification slag and countermeasures

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Zhang Jizhen1, Wang Yankun2 (1. Yankuang Cathay Chemical Co., Ltd., Tengzhou 277527； 2. Yankuang Guohong Chemical Co., Ltd., Zoucheng 273512) China's first Texaco coal-water slurry pressurized gasification demonstration unit was completed and put into operation at the Lunan Fertilizer Plant. The original design capacity was 80kt/a ammonia synthesis gas production. With the change of gasification coal types and the improvement of operation level, the production capacity of a single furnace has now reached 145% of the original design capacity. However, the combustible content in the slag increased from 24% (average value) in the initial stage to about 55% (average value). The original designed combustible content in the slag was 13.5%. In more than 10 years of operation, the combustible content in the slag rarely reaches the design value. Why is the combustible content in the slag so high as the coal type changes and the production load increases? To this end, the author analyzed the reasons for the high combustible content in the slag, proposed methods and measures to reduce the combustible content in the slag and improve the gasification efficiency, and analyzed the operating economy under high load and low furnace temperature operations. 1 Texaco coal gasification process of Lunan Fertilizer Plant Coal, water and additives are added to the mill at the same time and ground into coal slurry for pumping, with the mass fraction controlled at around 65%. Coal slurry pressurized to 5.0MPa is mixed with oxygen through the outlet of the process burner, and after entering the gasifier for limited atomization, a partial oxidation-reduction reaction occurs under the heat radiation of the furnace wall to produce coal gas (synthetic gas). After the coal gas is quenched by water between the downcomer and riser of the gasifier, water gas is generated and exits the gasifier. The water gas is humidified and dusted through venturis and scrubbing towers, and then sent to the purification process for transformation. After transformation, desulfurization, decarbonization and purification, CH3OH is synthesized, and NH3 is synthesized after mixing with N2. The ash is collected by the lock bucket and discharged from the system regularly. The black water recovers heat through three-stage flash evaporation. After settling, the clean water is returned to the system for use. The black water is discharged into the slag tank. After settling and collecting the slag, the overflow water goes to the sewage treatment system to process and recover the sewage as pulping process water to achieve clean production. The operating pressure of the gasifier is 2.9MPa, and the operating temperature is about 1300°C. 2 Analysis of the reasons for the increase in combustibles in gasifier slag discharge The main reasons for the increase in combustibles in slag are:: The gasification load increases and exceeds the design capacity ； Gasifier operating temperature decreases ； Coal slurry particle size distribution is unreasonable ； Changes in coal quality and equipment and operation reasons. 2.1 Increase in production load. The original coal input amount for a single furnace was 348t/d, and now the coal input amount for a single furnace is 456t/d, an increase of 35%. ； The original design of a single furnace for gas-to-synthetic ammonia production was 240t/d, and the current single-tobacco furnace for gas-to-synthetic ammonia production is 330 t/d, an increase of 37.5%. Due to the expansion and transformation of methanol production (double furnaces are turned on, and the other gasifier is used for the production of 130kt/a methanol system), the coal input per furnace is further increased. As the amount of coal input for gasification increases, the average residence time of the reactants coal and water in the furnace shortens, from about 8 seconds in the original design to about 6 seconds now. Under the same reaction pressure and temperature conditions, the reaction time shortens, the conversion rate of reactants decreases, and the combustibles in the slag increase. 2.2 Unreasonable slurry particle size distribution. The original designed coal slurry particle size is less than 325 and should be greater than 55%. The actual size is about 40%. ； Less than 200 mesh should be greater than 70%, now it is about 55%. As the production load increases, the mill load increases and the coal slurry particle size increases. At the same pressure, temperature and residence time, the reactant particle size increases, which reduces the gas-solid phase contact area, reduces the reaction rate, decreases the reactant conversion rate, and increases the combustibles in the slag. However, due to the large kinetic energy of large particle coal after being ejected from the nozzle, it runs faster and its residence time in the furnace is shortened. It leaves the reaction zone after the reaction is completed and is discharged with the slag, which also leads to an increase in combustibles in the slag. 2.3 Changes in coal quality and coal type The reaction time of coal particles in the gasifier is 6s to 8s. Young bituminous coal with high reactivity is selected as the raw material, and the combustible content in the slag is reduced. When gasifying coal that has been stored for a long time, the reaction activity of the stored coal is reduced due to long-term weathering, and the combustible content in the slag increases. ； In addition, due to the popularity of coal in recent years, there are many newly developed raw coal types, the coal quality is unstable, and the operation and adjustment process parameters cannot keep up, which is also the reason for the increase in combustible content in the slag. From gasified raw coal to gasified refined coal, the ash content drops from 16% to 25% to 9% to 15%, and the combustibles in the slag are relatively increased. In addition, components such as Na and K in the ash play a role in accelerating the gasification reaction rate. 2.4 Decreasing the operating temperature of the gasifier The operating temperature of the gasifier changes with the change of the ash melting temperature of the coal. In order to ensure that the inner wall of the gasifier has an appropriately thick slag layer (3mm~5mm), while ensuring the fluidity of the slag and smooth slag discharge, the operating temperature of the gasifier is generally required to be about 50°C higher than the FT temperature. In order to extend the service life of the refractory bricks and the operating cycle of the gasification device, the Lunan Fertilizer Plant selected a clean coal blend with low ash melting temperature as raw material. As the FT temperature of the clean coal used decreased, the operating temperature of the gasifier gradually decreased (from the originally designed 1400°C to the current 1300°C), and the gasification reaction rate decreased. Under the same residence time and pressure, the conversion rate of reactants decreased and the combustibles in the slag increased. 2.5 Fluctuation of ash melting temperature. Multiple coal types are mixed and burned, and the coal blending is uneven, causing large fluctuations in ash melting temperature. Since there is no online ash meltability temperature analysis instrument for coal slurry, manual analysis is relatively lagging behind. The ash meltability temperature analysis results are only fed back to guide production. In practice, the operating furnace temperature and the ash meltability temperature changes of coal slurry are generally analyzed and judged by the pressure difference at the slag port and the CH4 and CO2 content in the gas. Human judgment errors lead to the operating furnace temperature being lower than the optimal temperature for matching the ash meltability of the coal slurry, causing an increase in combustibles in the slag. When production is unstable, the combustible content in the slag can reach as high as 68%. 2.6 Unbalanced oxygen-to-coal ratio Due to the limited oxygen production capacity of the air separation unit, it cannot meet the increase in gasification production load. The large amount of gasified coal slurry is invested, resulting in an oxygen-to-coal ratio that is too small (<0.98). The normal situation is 490m3/m3 coal slurry, and the actual operation is 475.6m3/m3 coal slurry. The relative lack of oxygen causes incomplete reaction after the coal slurry is put in, a large excess of coal slurry, and an increase in ash combustibles. In addition, in order to be safe and avoid oxidation of the purification shift catalyst, O2 is not allowed to exist in Texaco gas. Therefore, the amount of coal slurry is required to be relatively excessive, and all O2 reacts to avoid explosion due to overoxygen in the scrubber, or burning of the purification shift catalyst due to oxidation. In this way, the gasified slag must contain a certain amount of combustibles, and the design value is 13.5%. However, when the oxygen-to-coal ratio is excessively imbalanced, the combustible content is much greater than the design value. 2.7 Changes in operating pressure Changes in pressure, especially during startup and shutdown and when the production load is unstable, pressure fluctuations or depressurized operation can cause an increase in the amount of residual carbon in the slag and an increase in combustibles. 2.8 Changes in the atomization angle of the burner and matching issues with the geometry of the gasifier. Current domestic burners can generally run for 90 days. In the later stages of the use of the burner, the coal slurry channel is abraded by the high-speed coal slurry jet, the flow area increases, and the cross-section changes from an annular shape to an irregular shape, causing the coal slurry to eject a biased flow, the coal slurry is unevenly distributed, and the flow rate decreases, which reduces the mixing effect of gasifying agent oxygen and pulverized coal. The atomization effect becomes worse, and the gas-solid phase reaction conversion rate decreases, resulting in an increase in combustibles in the slag. As the service life of the burner continues to extend, the ash combustibles further increase. Sometimes due to the error in the fastening force during the assembly of the burner and the influence of thermal stress after operation, the three-channel burner quickly becomes out of center (the cross-section ring of the burner is not concentric), causing the coal slurry to flow out of the burner. The atomization effect is extremely poor, causing the combustibles in the slag to rise, and the torch to deviate to the side of the furnace. The erosion and abrasion of the refractory bricks in the deviated part are accelerated, and in severe cases, local overtemperature of the furnace wall is caused. 2.9 Short circuit effect Texaco gasifier is a single-nozzle gasifier, with the nozzle at the top center and the slag port at the bottom center. In the flow field area in the center of the gasifier, the coal particles are short-circuited, and the gasification reaction time is extremely short (3s to 4s). They are discharged with the slag before conversion, resulting in an increase in the amount of residual carbon in the slag and an increase in combustibles. In addition, the atomization effect of the designed burner, the concentration of coal slurry, the degree of wear of the refractory bricks of the gasifier, the size of the slag mouth, etc. will affect the gasification reaction rate to varying degrees, that is, the combustibles in the slag. When the discharged slag is in the shape of fine, black slag and the coarse slag is in the shape of glass balls, the operating conditions are better, the production is stable, and the combustibles in the slag are low. When the coarse slag component is plastic slag in the plastic zone, it is difficult to control and it is easy to block the down pipe. The temperature has to be raised to form small slag, but a large amount of glass filament ash will be formed, making the production unable to operate stably. 3. Measures to reduce the combustibles in the slag. In order to simply reduce the combustibles in the slag and improve the gasification efficiency, the following approaches can be taken in view of the factors causing the high level of combustibles in the slag. 3.1 Increase the operating temperature of the gasifier. In April 2000, the Lunan Fertilizer Factory conducted an industrial online test to increase the operating temperature of the furnace. The purpose is to examine the relationship between the changes of combustibles in the slag with the furnace temperature, and to evaluate the specific oxygen consumption and specific coal consumption under different combustibles content in the slag. At that time, the Texaco gasification system had been operating for more than 20 days, various indicators had been optimized, and the test results should be representative. 3.1.1 Test procedures 3.1.1.1 Starting from the operating condition (CO + H2 = 83.85%), gradually reduce the speed of the coal slurry pump and increase the O/C ratio so that the volume fraction of (CO + H2) in the syngas is between 82% and 83%. ； 3.1.1.2 In the second stage, the speed of the coal slurry pump is reduced so that the volume fraction of (CO + H2) in the syngas is between 81% and 82%. ； 3.1.1.3 In the third stage, the speed of the coal slurry pump is reduced to make the volume fraction of (CO + H2) in the syngas between 80% and 81%. ； 3.1.1.4 In the fourth stage, the speed of the coal slurry pump is reduced to make the volume fraction of (CO + H2) in the syngas between 79% and 80%. ； 3.1.1.5 Analyze the combustibles in the slag. If the mass fraction drops to 20%, the test will end and the original working conditions will be restored. The test selected the content of (CO + H2) to calibrate the furnace temperature. 3.1.2 Test process 3.1.2.1 Phase 1 April 5, 2000 8: 00, start to raise the furnace temperature, gradually reduce the coal slurry pump speed, slowly increase the O/C ratio, adjust the gasifier control parameters within the index range, to 10: 00 is basically adjusted. The target value is that the volume fraction of (CO + H2) is 82% to 83%, and stable operation is maintained for 24 hours. Since high-temperature thermocouples generally fail after one week of operation, the exact temperature increase range cannot be determined, and changes in reaction furnace temperature can only be indirectly determined by changes in gas composition. 3.1.2.2 Phase 2 April 6, 2000 8: Start raising the furnace temperature from 00 to 10: 00 is basically adjusted, the target value is (CO + H2) volume fraction of 81% ~ 82%, but on the day of 15: 00 jumps and stops the test. Experimental steps ③~⑤ are to further verify the rules of temperature raising operation and operational economic issues. However, since the direct economic loss of a trip is 600,000 yuan, the experiments of steps ③~⑤ were not carried out after the gasifier tripped. The conclusion drawn from the experimental data of steps 1 and 2, combined with the experimental data of the small thermal model, should be credible. 3.1.3 Comparison of test data (see Table 1) http://pub2.hi2000.com/upload1/0712271533101056.jpg3.1.4 Test Conclusion Although this test was terminated after only two stages, the data has shown trends and can illustrate the problem. The following conclusions are drawn. 3.1.4.1 In this test, under the condition that the oxygen flow rate, coal slurry concentration, and ammonia production amount are basically unchanged, when the furnace temperature is increased, the specific oxygen consumption increases, the specific coal consumption decreases, and the ash and slag combustibles decrease. The more oxygen consumption per 1000m3 (CO+H2) is 2.3m3 and 7.5m3 respectively, and the less coal consumption is 11.3kg and 24.3kg respectively. It can be seen that appropriately raising the furnace temperature can improve the utilization rate of coal and reduce production costs. However, factors such as the increase in specific oxygen consumption and the life of the refractory bricks must be considered, so it should not be raised too high. There is an optimal operating temperature. 3.1.4.2 In order to ensure that the thickness of the slag layer on the wall of the furnace is 3mm to 5mm, the FT temperature of the coal must be increased. The result is that the combustibles in the slag are reduced, and at the same time, the oxygen consumption and coal consumption are increased, the effective gas components are reduced, the output is reduced, the service life of the refractory bricks is shortened, and the reliability and stability of the operation are reduced. Therefore, it is necessary to choose an optimal operating temperature. From many years of exploration, the optimal gasification operating temperature is to increase the FT temperature by 30°C to 50°C. 3.1.4.3 The operation of the Texaco coal-water slurry pressurized gasifier should be operated at the optimal temperature to maximize benefits, and the highest carbon conversion rate cannot be pursued one-sidedly. 3.2 Adjust the production load to match the design. The original designed gas production volume of a single furnace was 37509m3/h. The current gas production volume of a single furnace is about 50000m3/h. The volume and operating pressure of the gasifier remain unchanged, and without considering other factors, the average residence time of the reactants is relatively shortened to about 5 seconds, and the conversion rate is reduced. If you want to improve the carbon conversion rate, you can reduce the amount of coal input and reduce the production load (but it is uneconomical from all aspects). After the third set of flash evaporation system is completed, the simultaneous operation mode of three gasifiers can reduce the load of the gasifier to the production load matching the original design to meet the high-load production of the methanol and synthetic ammonia dual systems. By then, the combustible content in the slag can be reduced to about 20%. 3.3 Improving the quality of coal slurry 3.3.1 Selection of coal for pulping For the Texaco coal-water slurry gasifier that uses liquid slagging, the coal requirements for pulping: (1) The ash melting temperature FT of coal is between 1100°C and 1300°C. Too high or too low is not conducive to gasification. The optimal operating temperature determined by coal quality analysis is within the allowable range of the Texaco gasification device. (2) The higher the fixed carbon (FC) content of coal, the better, and the calorific value Qnet ≥ 25.121 MJ/kg to ensure the thermal balance of the gasifier, and the higher the better. (3) Ash content Aad≤20%, the lower the better. (4) In the FT state, the viscosity of the liquid slag is maintained at 15 Pa·s ~ 25 Pa·s to ensure normal liquid slag discharge, and the liquid ash is less corrosive to the refractory lining. (5) The greater the grindability index, the easier it is for coal to be ground, which can increase the output of the coal mill and reduce the power consumption rate. It is best to control the grindability index to ≥56%. (6) The lower the internal water content, the better (preferably ≤8%). The slurry concentration of coal decreases as the internal moisture content increases. Coal with low internal moisture is easy to prepare high-concentration coal-water slurry. As the concentration of coal-water slurry increases, the effective gas components in the coal gas increase, the gasification efficiency increases, and the oxygen consumption decreases. (7) Coal has good reactivity and high activity. According to the reactivity analysis at 1250°C, the CO2 reduction rate is ≥98%. (8) The content of harmful elements in coal is low to reduce the load on the purification system and avoid catalyst poisoning. 3.3.2 Control the quality of coal imported into the mill. Control the quality of raw coal feeding to prevent large pieces of coal, gangue and other impurities from entering the mill. Strengthen inspections of pulping stations to prevent blockage of the mill feed pipe. When flushing the feed pipe, use the correct method and appropriately reduce the amount of water added. When purchasing raw coal, control the ash content. When the ash content in coal is high, the trommel screen will be seriously scaled and the screen holes will be blocked. When the coal slurry concentration is high, the trommel screen will overflow more, resulting in a waste of raw materials, especially in rod mills. When air-separated coal is used, the ash content in the coal is relatively high, and the slurry concentration of the rod mill drops from over 63% to 57% in less than 10 days on average. Therefore, the ash content of the raw coal should be strictly controlled, water-washed coal should be used as much as possible, and air-separated coal with higher ash content should be used less. Control the ash content of clean coal to ≤9%. When purchasing raw coal, control the moisture content in the coal. When the moisture content in coal is high, the water-to-coal ratio entering the mill is difficult to adjust, and the coal slurry concentration fluctuates greatly. Coal types with high internal water content are more difficult to produce high-concentration coal-water slurry. When coal is wet, it is easy to adhere to the feed pipe of the mill, causing the feed pipe to become clogged. Every time the feed pipe is blocked, it takes 20 to 30 minutes to clear it and flush it with plenty of water. When the coal is relatively wet, in order to prevent the feed pipe from being blocked, the operator must flush the coal adhering to the pipe wall with water every 30 minutes or so, which will undoubtedly cause fluctuations in the concentration of the coal slurry. Therefore, the external water content of coal must be controlled within the specified indicators, and the moisture content Mad ≤ 9%. Monitor the operation of the electromagnetic iron suction device in the feeding system, and clean the ferromagnetic material on the electromagnet in a timely manner once every shift. 3.3.3 Uniform feeding and crushing load Strictly control the amount of coal feeding and appropriately extend the coal feeding time. Due to the small design load of the crusher, the normal feeding load is not allowed to exceed 60t/h, and the average feeding time per operating shift is 7 hours (before taking measures, feeding per shift was about 5 hours, and the feeding load was high up to 120 t/h). In this way, after appropriately extending the coal feeding time, the load can be evenly crushed and the particle size of the coal imported into the mill can be reduced. Strictly control the inlet coal ≤10mm. More crushing and less grinding is an effective measure to improve pulping quality and reduce energy consumption. 3.3.4 Control the quality of additives and optimize the dosage of additives. Fluctuations in additive quality will affect changes in viscosity. When the quality of additives is poor, the viscosity of the coal slurry will be larger and the stability will be poor. The coal slurry will accumulate on the trommel screen and cause serious overflow of the trommel screen. The amount of overflow will increase, which will affect the increase in coal slurry concentration and cause the coal slurry to undergo hard precipitation of water on the vibrating screen. Therefore, the quality of additives should be controlled well. Appropriate additives can achieve the effect of thickening and reducing viscosity. Due to the additives imported into the company, the content of the active ingredient lignin often appears to be low (the mass fraction of lignin is required to be ≥8%), but about 1/4 of the time it fails, which has a serious impact on the stability of the coal slurry. At present, there is only one supplier and does not have the conditions for selection. The development of new additives with high quality and low price and its customers is an urgent problem that needs to be solved. Strictly control the dosage of additives. Under normal circumstances, the total dosage of the two mills does not exceed 2.5m3/h. If the dosage of additives needs to be increased due to increased load or changes in coal quality, the approval and approval of the technician in charge should be reported first. Based on the analysis and evaluation of the coal slurry, a reasonable dosage should be issued. Experiments have found that there is an optimal value for the amount of additives added. If not enough or too much is added, the rheological properties of coal-water slurry will deteriorate. This optimal value should be determined based on analysis and evaluation of coal slurry and combined with empirical judgment. 3.3.5 Reasonable and timely adjustment of the water-to-coal ratio. According to the coal moisture content, coal feeder operation and coal feeding amount, reasonable and timely adjustment of the water-to-coal ratio, increase the stability and concentration of the coal slurry, and control the overflow of the vibrating screen and trommel screen are very important for controlling the quality of the coal slurry. 3.3.6 Optimize the gradation of rods. Appropriate particle size distribution can increase the concentration of coal slurry. Due to the unreasonable gradation of steel rods in the rod mill, the particle size distribution of the coal slurry is unreasonable, the stability is poor, and the sieve holes of the drum screen are easily blocked, which limits the increase in coal slurry concentration. Therefore, adjusting the steel rod ratio of the rod mill based on the rod matching test is a realistic solution to improve the particle size distribution of the coal slurry. For one-stage grinding pulping, according to the grindability index of the coal, adjust the grinding body ratio, select the appropriate particle size distribution, and control the particle size of the pulverized coal according to the reactivity of the coal. On July 15, 2003, based on the rod matching test, the ratio of rod mill rods was adjusted. In order to make use of old rods and gradation needs, φ60mm rods that were not included in the original design were added, with a total installation capacity of 48.25t. The assembly is as follows: φ75mm:φ65mm:φ60mm:φ50mm is 11.88:8.15:13.61:14.61. When changing the rod, I found that the φ50mm rod broke more seriously, especially when it was ground to about φ40mm. For this reason, it is recommended to steel the surface of the rod. In terms of operation, adding steel rods on time to ensure the normal number and gradation of steel rods in the mill are necessary measures to ensure the stability of the coal slurry particle size and concentration. In addition, the idling time of the mill must not exceed 30 minutes. However, in actual operation, especially when the moisture content of the raw coal exceeds the standard and the discharge port is blocked, the idling time often exceeds 1 hour, which is another reason for broken rods. 3.3.7 Optimizing ball gradation Based on the ball matching test, the following ball gradation was determined: φ100mm:φ80mm:φ60mm:φ40mm:φ20mm is 4.125:3.795:18.425:16.555:12.1. In daily operation, steel balls are added in time according to the summarized steel ball wear, and the ball gradation is adjusted to ensure the normal number and specification of steel balls in the mill. When the coal quality changes, the ball gradation is adjusted in real time according to the particle size distribution. If necessary, a ball distribution test is conducted to classify and re-proportion the steel balls. 3.3.8 The coal crushing load moves forward due to the increase in production load, resulting in an increase in the load on the coal loading system. When the single gasification furnace was originally designed to operate, the coal loading and crushing load was less than 20t/h. Now that the double furnace is operating, the load has increased to more than 60t/h. The capacity of the original designed crusher is far from meeting the needs of coal crushing, which makes the coal particle size at the entrance of the mill larger. 10% to 30% of the coal particles are between 20mm and 30mm, and particles above 40mm account for about 5%, resulting in an increase in the crushing load of the mill. Therefore, an additional stage of pre-crushing and screening is added to move the crushing load forward. In the coal loading system, connecting one crusher in series can save investment and floor space, and can ensure that the particle size at the entrance of the mill is <10mm, and the expected coal particles at the entrance of the mill are <8mm. 3.3.9 Cleaning of the trommel screen When cleaning the trommel screen, the mill needs to be stopped, which will cause the loss of coal slurry when flushing and startup are unqualified, causing waste of coal slurry and fluctuations in coal slurry quality. Therefore, cleaning should be based on the coal slurry concentration trend in the mill discharge trough. The mass fraction of coal slurry in the discharging trough of the rod mill drops to about 61%, and the screen is cleaned when the slurry overflows seriously. ； When the mass fraction of coal slurry in the ball mill drops to about 63.5% and the slurry overflows seriously, the mill should be stopped for cleaning. During routine inspection of the mill, the sieve should be thoroughly cleaned to maintain the stability of the coal slurry concentration. 3.3.10 Solve the problem of the blocking of the discharge port of the mill. The gasification coal of Yankuang Cathay Chemical Co., Ltd. is washed clean coal with high moisture content. The received base moisture is often above 10%. If it rains and the relative air humidity is high, the feeding belt system Above, the water in the coal is non-volatile but absorbs moisture, and the moisture content is higher. Blockage often occurs at the discharge port of the mill. After the blockage, water flushing is used to clear it, resulting in an imbalance in the water-coal ratio of the mill, a decrease in coal slurry concentration, and large fluctuations, which affects system production. It is recommended to modify the inlet discharge pipe of the mill, thickening it from the original design of DN300mm to DN400mm, and lining it with a 10mm thick wear-resistant polymer material skateboard (this polymer material skateboard has been used in the gasification raw material coal bunker and discharge pipe of Yankuang Cathay Chemical Co., Ltd., with very good results) to reduce the frictional resistance and magnetic force adsorbing fine coal particles during the falling movement of coal. Adjust the slope of the feed pipe of the mill to a suitable position, move the water distribution pipe up to the upper part of the feed pipe, and play the role of hydraulic coal flushing. Install a 0.5MPa or 2.5MPa N2 pneumatic coal pulling device (or air cannon) at the feed port. Determine a reasonable inclination angle, form a layer of air film in the feed pipe and have the ability of pneumatic transportation. Use pneumatic transportation to ensure that the feed port is evenly fed without clogging. At the same time, N2 plays a drying role on the surface of wet coal particles. It can be turned on continuously when the coal is wet, but not turned on when the coal is dry. If it is switched to N2 or air pneumatic dredging and transportation, it can avoid the imbalance of the water-coal ratio caused by water flushing and blockage, as well as the mismatch of coal addition and water distribution, avoid fluctuations in coal slurry concentration, ensure the quality of coal slurry, maintain high concentration and appropriate viscosity of coal slurry, and keep coal slurry stable. However, the safety of using N2 must be considered. It is better to use compressed dry air. 3.3.11 Start the new vibrating screen as soon as possible. Since the pulping load has increased to 3 times the original value, but the vibrating screen has not been expanded, the thickness of the slurry layer on the vibrating screen has increased and the residence time has been extended, which provides conditions for water to escape from the coal slurry. The particle size distribution and viscosity of the first stage of the pulping process determine the poor stability of the coal slurry, causing the coal slurry to form water-separating hard precipitates on the vibrating screen, reducing the concentration of the coal slurry, and increasing the amount of slurry overflowing at the overflow port of large particles in the vibrating screen, which is flushed into the sedimentation tank, causing coal slurry loss. In 2003, a new domestic vibrating screen was installed. After a period of trial operation, it was found that although there were some defects, it could maintain operation. At present, it has not been opened due to the lack of a spring component. In addition, changing the vibrating screen on the high-level coal slurry tank to a new drum screen can reduce the hard precipitation of coal slurry and the power consumption of pulping, reduce the amount of slurry overflow, and improve the concentration and stability of coal slurry. The new drum screen designed to replace the vibrating screen has been very successfully used in the coal slurry preparation section of the new four-nozzle gasifier of Yankuang Cathay Chemical Co., Ltd., and its performance indicators and operation cycle are better than the imported vibrating screen. 3.3.12 Reduce the load on the pulping system. The vertical mill jointly developed by the University of Science and Technology Beijing and Yankuang Cathay Chemical Co., Ltd. has been installed for one year. However, it has not been successfully tested due to reasons such as low motor load and stringing of shafts. After the vertical mill is successfully tested, the coal input amount can reach 12t/h ~ 15t/h, which reduces the load on the existing mill, makes the particle size distribution of the slurry produced by the mill more reasonable, and the stability and rheological properties of the coal slurry are better. 3.3.13 Extend the operation cycle of the trommel screen. After cleaning the trommel screen of the rod mill, the slurry overflowed seriously after 7 days of operation. The reason was that the screen holes were blocked. Adding a φ40mm rod to the trommel screen to vibrate and stir the coal slurry in the trommel screen can extend the operation cycle of the trommel screen to 1 month. The effect is obvious. It is recommended to add a φ30mm rod to the trommel screen of the ball mill to avoid slurry overflow and extend the service life of the trommel screen. Since there are almost no large particles in the rod mill, the risk of adding rods is less ； There are many large particles in the ball mill, so adding rods is risky. This requires that when adding rods to the drum screen of the ball mill, the vibrating screen at the entrance of the large coal slurry tank must be kept open and intact to prevent large particles from entering the coal slurry tank and then entering the high and low pressure coal slurry pumps, damaging the diaphragm of the pump, blocking the outlet of the pump, and negatively affecting the stable operation of the coal slurry pump. Another negative impact of adding rods is that the particle size distribution of the coal slurry changes slightly, the distribution of large particles increases slightly, and the overflow of the vibrating screen on the high-level coal slurry tank increases. However, it can obviously increase the concentration of the coal slurry and extend the operation cycle of the mill drum screen, and the operational benefits of improving the production system are considerable. 3.3.14 Establish a dynamic reward and punishment system for coal slurry quality. Determine scientific evaluation standards for coal slurry quality, establish a dynamic reward and punishment system for coal slurry quality, and formulate strict coal slurry quality management and assessment regulations.: The mass fraction of coal slurry in the large coal slurry tank is >63.5%, the mass fraction of coal slurry in the discharge chute of ball mill is >64.5%, the mass fraction of coal slurry in the discharge chute of rod mill is >63%, the viscosity of coal slurry is 0.9MPa·s～1.4MPa·s, and other indicators are qualified. The monthly average coal slurry mass fraction is ≥63.5%. 3.3.15 To prevent fluctuations in coal slurry quality during shift handover, strictly enforce the handover system for pulping positions, stabilize the concentration of coal slurry during shift handover, and formulate assessment measures for coal slurry fluctuations. 3.3.16 Replacement of crusher blow bar The coal crusher used by Yankuang Cathay Chemical Co., Ltd. is a blow bar type crusher. After the front side is worn to a certain extent, the blow bar can be replaced and used on the reverse side. After the back side is worn to a certain extent, a new blow bar can be replaced. 3.3.17 The gangue and other impurities can be removed by manually sorting gangue, wood blocks, gun wires and other impurities to reduce the gangue content and impurities in the coal, control the quality of coal in the coal yard and feeding system, and avoid affecting the grinding efficiency of the mill. 3.3.18 To solve the problem of analysis lag, use an online coal slurry density automatic analyzer to timely and accurately measure the coal slurry concentration and change trend curve, which can be used to quantitatively guide coal slurry preparation. The operator adjusts the water-to-coal ratio according to the trend, controls the slurrying process in a timely manner, and improves and maintains the stability of the coal slurry concentration. 3.4 Optimize the burner atomization effect and its matching with the furnace type. According to the coal slurry quality, promptly adjust the ratio of the burner center oxygen amount and the annulus oxygen amount, optimize the operation, achieve a good atomization effect of the burner, increase the average residence time of coal in the gasifier and uniform mixing with the gasification agent, and improve the carbon conversion rate. 3.5 Reduce the slag opening and appropriately extend the reaction time of coal particles. Use the optimal slag opening size, appropriately increase the residence time of coal in the gasifier, reduce the proportion of coal with too short residence time, and improve the carbon conversion rate. The slag opening of the gasification furnace of Lunan Fertilizer Plant has been reduced from the original φ800mm to φ625mm, and the effect is obvious. In addition, adjusting the furnace temperature can also fine-tune the size of the slag opening. When the furnace temperature is increased, the slag opening becomes larger. When the furnace temperature is lowered, the slag opening becomes smaller. 4. Operational economic analysis under high load and low furnace temperature operation. As the ash melting temperature of the coal type used decreases, the operating temperature of the gasifier decreases relatively. Due to the reduction of ash content in clean coal, oxygen consumption decreases and gasification load increases. Correspondingly, NH3 and CH3OH production increases, power consumption and coal consumption decrease, the effective components of gas increase, the service life of refractory bricks is extended, the safety, stability and reliability of operation are improved, and production costs are reduced. Fixed cost allocations are reduced. 4.1 Increased output and reduced costs Due to the increase in gasification load, the gas generation volume increased, and the ammonia output of a single furnace increased by 37.5%. From the analysis of the complete cost composition of synthetic ammonia, gasification maintained a production load of 10 t NH3/h in 1999, and the operating cost per ton of ammonia (raw material consumption, power and electricity, and deducting by-products) was 597 yuan, and the complete cost after adding fixed costs (direct labor, manufacturing costs, management costs and financial cost sharing) was 1,536 yuan. Compared with the current gasification production load of 13.75tNH3/h, the complete cost after fixed cost allocation is 1,207 yuan/tNH3, and the ammonia cost is reduced by 329 yuan (1999 price). Therefore, increasing output can significantly reduce the cost of producing ammonia per ton. 4.2 Reduced oxygen consumption and coal consumption. Due to the gasification of clean coal, the ash content is reduced to 9%. The FT temperature of Beisu and Luoling clean coal is reduced by 100°C to 150°C. The operating temperature of the gasifier is correspondingly reduced by 100°C to 150°C. The oxygen and coal combustion reaction is converted into CO2 and the heat released to maintain the blast furnace temperature is reduced, that is, the energy consumption is reduced, so the oxygen consumption and coal consumption are reduced. This is reflected in the reduction of power consumption by 317kW·h/tNH3 (88.76 yuan/tNH3) and the reduction of coal consumption by 140kg coal/tNH3. 4.3 Increase in effective gas components As the operating furnace temperature decreases by 100°C to 150°C, the raw gas CO+H2 (dry gas) increases by 5% and CO2 decreases by 5%. Considering the carbon balance, the effective gas component converted from carbon increases by about 5%. 4.4 Extending the service life of refractory bricks According to the usage characteristics of refractory bricks, the corrosion rate of refractory bricks is reduced by 1 times for every 44°C decrease in operating temperature. Considering the influence of other factors, judging from the current situation, the service life of the refractory bricks has been increased from the original 4000h to the current 10600h, and the annual refractory brick service life has been extended from half a year to one year, saving 2 million yuan in refractory brick costs every year. The cost per ton of ammonia is reduced by 20 yuan. It also extends the operating cycle of the gasifier, reduces the number of shutdowns and increases the operating rate. 4.5 Improved operation reliability Due to the reduction of operating furnace temperature, the service life of refractory bricks, burners, quench rings, and downcomers is extended, the accident rate is reduced, the operation reliability is improved, and the cycle is extended. The operation cycle of a single furnace is 100d 16h, reaching the world's advanced level. The operation cycle of the blackwater flash evaporation system has been extended from 3 months to 7 months, and the maximum operation cycle has been cleaned once every 9 months, which has improved the operation quality. 4.6 The increase in combustibles in the slag has no significant impact on the carbon conversion rate. Currently, the ash content of coal is 7%, the combustibles in the slag are 55%, and the carbon conversion rate is 91%. In contrast, although the increase in combustibles in the slag is greater than in the original design, the carbon conversion rate decreases slightly, and the Texaco process is acceptable. This kind of slag can be mixed into the fuel coal of the circulating fluidized bed boiler for comprehensive utilization of the slag, and the operation economy is reasonable. 5 Conclusion The problem of high combustibles in the slag must be analyzed in terms of comprehensive benefits, seeking the best coal type, improving the quality of the coal slurry, operating the gasifier at the best operating temperature, optimizing the burner design and matching the geometric size of the furnace body slag port, and achieving a reasonable gasification flow field distribution to reduce the combustibles in the slag, improve the carbon conversion rate and effective gas composition. Based on comprehensive considerations such as gasification load, power consumption, coal consumption, service life of refractory bricks, system operation reliability, operation cycle length and other factors, the process control conditions for optimal operation are determined to ensure the normal operation of the Texaco gasifier and the optimization of the gasification process to achieve the purpose of economic operation and maximization of benefits. This post was last edited by lcs000212 on 2008-1-18 16:43 ]