Causes and Analysis of Start-up and Shutdown Accidents in Gasifiers

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Causes and Analysis of Start-up and Shutdown Accidents in Gasifiers: Causes and analysis of start-up and shutdown accidents in gasifiers from 2007 to the end of 2008: 1. Problems with coal slurry – Substandard coal slurry led to poor performance of the large coal slurry pump. One or more cylinders of the slurry pump fail frequently. This caused the triplex selection of slurry flow rate, leading to the shutdown of the gasifier. 2. Problems with the addition of mill balls: The poor quality of these mill balls, along with improper addition methods, result in the fragmentation of the balls; such fragments, together with the resulting water-coal slurry, clog the check valves of the coal slurry pump once they enter the large tank. It causes the gasification furnace to shut down. 3. Drum level: During the initial startup, the interlock was not disabled due to the high drum level. The high-temperature gas generated after feeding, after exchanging heat with the water wall, causes the drum liquid level to rise suddenly. Chain gasifier trip. Before subsequent feedings, the high drum level interlock has been disabled. This issue has been resolved. 4. Gasifier pressure: During normal operation of the gasifier, perform maintenance and welding work on the frame. The pressure gauge of the gasifier fluctuates due to electromagnetic interference. This causes the gasifier pressure to trigger a tri/2 high-high trip. After the accident, the instruments conducted multiple shielding tests on the gasification instruments. It has been resolved. 5. Tripping due to extremely low level of waste boiler liquid. This is because the lock-hopper system has always used manual slag discharge. Plus, the lockout valve switch isn’t working properly. During slag discharge, the lockhopper pressure relief valve was not closed properly. The lockout isolation valve opened, causing a sudden drop in the water bath level of the gasification furnace, which led to an emergency shutdown due to the abnormally low water bath level. After the accident occurred. The workshop and the instrumentation team conducted tests on xx-xxx. The interlock value for the water bath level in the gasification furnace is 30; therefore, YV-214 automatically shuts down when the water bath level falls below 40. This issue has now been resolved. 6. Dead zone issue: If the dead zone temperature is too high, one of the two vaporization furnaces will shut down. Due to coking issues. The slag from the gasifier flows downward in a molten state at the throat. The dead zone is blocked, resulting in a very high temperature in the dead zone and an automatic shutdown. There is a purging pipeline leading to the dead zone. Due to the blockage, it is not possible to purge with a higher dead zone temperature. The high dead zone temperature caused the gasification furnace to trip in a cascade manner. 7. Oxygen flow rate: When one cylinder of the large slurry pump fails, the furnace temperature rises rapidly. The operator carried out oxygen reduction. Because the furnace temperature rises very high. When oxygen is reduced too quickly, the low oxygen flow rate causes the gasification furnace to shut down. This accident was a low-low trip caused by the operator’s negligence regarding the oxygen flow rate. Oxygen was removed too quickly during the process. 8. Oxygen cut-off valve failure. After feeding the gasifier, a fault signal from the oxygen cut-off valve caused the gasifier to shut down. After this incident, the instrumentation staff carried out repairs, and the issue has now been resolved. 9. Burner cooling water: The cause of the accident was that the liquid level in the burner cooling water tank was set to manual control. The operator failed to detect the drop in the tank level in time, resulting in delayed water replenishment and a tripping of the system based on the three-to-two criterion. We need to pay more attention to the burner cooling water system in the future. In particular, the crew members keep a close watch on the footage. Address issues promptly once they are identified. 10. Shutdown of the coking system in the gasification furnace – Process characteristics of coking. The initial process parameters associated with coking include a sudden increase in the level and temperature of the water bath, as well as a gradual rise in the turning point temperature; solid slag was detected on site ； The intermediate process parameters for coking are as follows: the inflection point is close to the outlet temperature, the dead zone temperature rises, and it was confirmed on site that the amount of slag has decreased ； The process parameters in the later stage of coking are characterized by a sharp rise in dead zone temperature and a sudden drop in inflection point temperature, with no slag discharge confirmed on site. Due to coal quality, coking occurs in the gasifier because of the excessively high ash melting point of the coal. The adjustment of the furnace temperature does not match the melting point of the coal ash actually used; the gap between the nozzles affects the atomization effect, and the oxygen supply at the center also influences the atomization effect. 1. References regarding the ash content and melting point of coal: According to the plant’s regulations, raw coal with an ash content of less than 12% is used for gasification, while raw coal with an ash content higher than 12% is used for boilers. The ash fusion point has been changed to refer to the ash fusion point of the coal used in the gasification coal feeding belt. The ash fusion point can only serve as a reference; it cannot be used alone to control the temperature of the gasification furnace. 2. Stability of pressure and temperature: The stabilization of pressure can currently only be achieved through the operations in the synthesis workshop; contacting that workshop is necessary to maintain stable system pressure. As for the stabilization of furnace temperature, it relies entirely on the operators in my gasification workshop. 3. The thermocouples are in good condition and fairly accurate. Thermocouple-related issues represent a challenge in measuring the temperature of our plant’s gasification furnace; after the furnace has been operating for some time, the thermocouples can no longer display the true temperature, so other parameters must be used to regulate the furnace, which makes those other parameters quite important. At the moment when material is first fed into the gasifier, the thermocouples provide fairly accurate readings; therefore, we need to adjust the oxygen-to-coal ratio based on the coal slurry concentration and furnace temperature at that time. Once this ratio is determined, the furnace temperature can be controlled in subsequent operations only by relying on that oxygen-to-coal ratio and the coal slurry concentration. Here, it is required that each shift adjust the slurry concentration in the large coal slurry tank as a reference for adjusting the oxygen-to-coal ratio. As for the high methane value of raw gas, it is not possible to determine the furnace temperature; since a high methanation value, along with changes in the slurry concentration, can both affect the methane value, it is not feasible to adjust the furnace temperature based solely on a high methane value. 4. The importance of throat temperature and inflection point temperature: In Vaporizer A, all the thermocouples located at the throat area fail to provide readings; therefore, it is necessary to rely on the inflection point temperature and the exit temperature to determine the actual temperature inside the furnace. An increase in these temperatures indicates that the furnace temperature is rising, and prompt control measures must be taken. 5. Slag discharge volume and slag morphology: The central oxygen valve has been changed from xx to xx. The above are the main problems encountered in our plant’s gasification operations. Problems with the air separation system led to low oxygen pressure; however, thanks to the prompt action of the operators, no shutdown was required. We are now considering whether it would be advisable to add alarms for low oxygen pressure in the gasification section and to establish corresponding operating procedures. Since the gasification furnace already has a interlock mechanism for low oxygen flow, we ask everyone to share any gasification-related accidents that have occurred in their respective units so that we can exchange information together!