Analysis of the causes of hydrogen fire in electrolyzers
Thread Content
This post was last edited by sunjl1981 on 2013-1-6 at 20:04. I. Brief account of the incident: At 20:05 on November 1, 2006, during the startup of Cell No. 4 in a chlor-alkali plant, the current level rose to 6 KA. The electrolysis operator noticed a slight leak in the cell while adjusting the pressure difference for Cell No. 4; he immediately notified the production supervisor via walkie-talkie, went back to the control room to get a flashlight, and reported the situation to the team leader. When the staff returning to the electrolyzer found that the leakage was already severe and the cathode gas side of the electrolyzer had turned red, they grabbed a fire extinguisher. By the time they brought it, flames were already emerging from the electrolyzer, so they promptly put out the fire. At this point, flames also appeared at the gas phase end on the north side of the cell, so fire fighting was carried out at the northern end of the cell as well. The production scheduler summoned personnel from other sections to the scene, and after about 15 minutes of efforts to extinguish the fire, it was put out. The company’s equipment was insured, so no significant direct losses were incurred. This indirectly caused the shutdown of Cell No. 4 for 34 hours, resulting in a reduction in production of 44 tons of caustic soda (on a percentage basis) and 40 tons of liquid chlorine. Upon inspection of the accident site, an explosion occurred inside cell No. 47 of electrolyzer No. 4, causing the entire cell to rise by 10 mm. The electrode grids collapsed to varying degrees, and there were 3 cracks on the ion membrane, each approximately 2 cm long. . Note ← ) # ← , .
I. Analysis of the accident cause: Firstly, a leakage in the cell gasket led to a short circuit, which in turn caused a fire. In ion-exchange membrane caustic soda electrolyzers, the cathode chamber contains NaOH solution and hydrogen, while the anode chamber contains NaCl solution and chlorine. If the gaskets are not installed properly and come into contact with each other, the displacement of these gaskets can result in the stretching and tearing of the ion-exchange membrane. If a leak occurs during operation, it may lead to \"dry burning\" inside the electrolyzer; the ion membrane will be damaged, the electrode plates will melt, and in extreme cases, combustion and explosion may occur. Two days before the fire incident in the electrolyzer, gasket leakage occurred during two operations (on October 30th, at 22:05, the current rose to 9.6 KA; an emergency shutdown was triggered due to gasket leakage at 22:30) ； At 22:13 on October 31, the current just rose to 9.6 KA, resulting in an emergency shutdown due to gasket leakage). Apart from improper pasting, failures of the extruder or incorrect operation can also cause leaks in the gaskets, as well as excessive pressing force that leads to damage to the rubber gaskets and membranes.
Secondly, the ion membrane is torn or punctured, causing hydrogen and chlorine to mix and resulting in a flash explosion. The ion membrane gets torn or punctured due to the displacement of gaskets or when the gaskets leak and catch fire, leading to deformation of the tank frame. Under positive pressure, hydrogen diffused rapidly toward the anode; when the mixture of Cl2 and H2 reached the explosive limit of 5%-87.5%, a flash explosion occurred. The shock wave resulting from this flash explosion caused the gaskets in the northern area to leak and catch fire, leading to severe deformation of the electrode grids in electrolysis cell No. 47.
Furthermore, there is a possibility of leakage in the pole plate or the ion membrane of cell #47. Cell #47 is a newly repaired electrolyzer that was installed on October 30, and has been in operation for about 9 hours so far. It is also highly likely that the cell will explode due to leaks in the anode and cathode plates or in the ion membrane. However, no obvious signs of such a leak were observed when the fire broke out at the scene
This is because the main signs of leakage in the anode and cathode plates of the electrolyzer as well as the ion membrane are: (1) the color of the anode solution outlet hose changes to milky white, pink, or purple. (2) The hydrogen content in chlorine increases to over 0.3%. (3) Abnormal single-slot voltage. (4) Abnormal flow in the corresponding outlet hose. (5) The voltage difference reading fluctuates significantly. Upon checking the data from the control system, it was found that only the potential difference increased from 0 mv to 0.6 mv within those 30 seconds; the operating current, the flow rates of the anode and cathode fluids, the pressure difference, the gas phase pressure, and the voltage of tank #4 were all within normal ranges. Afterward, the purity of the original chlorine was found to be 98%, with 0.13% hydrogen contained in it; the purity of the residual chlorine was 53% and 78% respectively. There was also no evidence of discoloration in the outlet hose for the anode solution. However, it cannot be ruled out that such leaks may occur over a short period of time, with little noticeable sign, and that timely handling results in no significant impact on the system.
III. Thoroughly learn from the lessons of this accident and carry out the following tasks: 1. Provide employees with training on the safety and operation aspects related to electrolytic cells, especially regarding membrane replacement and gasket application. During the installation of electrolytic cells, the sealing gaskets of these cells are the key components for ensuring sealing; therefore, applying these gaskets properly is a task that requires strict attention during the assembly process. Before applying the adhesive gaskets, first distinguish between those for the cathode side and those for the anode side, and then differentiate between the membrane side and the tank frame side. For the anode gaskets, it is important to identify the upper and lower sides; the upper part of the anode gasket is narrower, while the other three sides have equal width. When applying the gasket, the recommended ratio of diluent is approximately 1.0:1.5. After applying it evenly, let it air-dry for 5–10 minutes; it can be glued on only when it no longer feels sticky to the touch. If bonding is attempted too soon before it has dried, the adhesive will act like a lubricant; not only will the bond be weak, but it may also cause the gasket to slip and shift during the operation of the electrolyzer. Therefore, after pasting, check that the corners of the gasket have not shifted due to contraction; if their position is incorrect, remove it and paste it again.
1. Inspect the #4 hydraulic system and train employees on the correct operation of the extruder; improper operation of the extruder often leads to potential hazards such as damage or misalignment of electrolytic cells, gaskets, ion exchange membranes, etc. Before the electrolyzer is powered on, there is no gas pressure inside the cell; the compressive force of the electrolyzer acts entirely on the rubber gaskets of the cell frame, thereby providing a sealing effect. The oil pressure required at this time is 7 MPa. When the electrolytic reaction begins after power is supplied, the gas pressure inside the cell rises in accordance with the system’s control pressure, which offsets part of the compressive force and reduces the pressure exerted on the rubber gaskets of the chamber frame. Therefore, the compressive force at this point should be increased to 12.5 MPa (9 MPa for natural circulation; the same applies hereafter). During stable operation, the electrolyzer can be locked using a pressing force of 12.5 MPa from the tightening screw installed on the extruder, after which the oil pressure can be reduced to 7 MPa. One pump from the oil pressure supply always remains in operation, serving as a backup mechanism. When parking, the electrolytic reaction stops and the pressure inside the tank drops. At this point, it is necessary to promptly release the locking of the screw using a compression force of 12.5 MPa, and then reduce the compression force to 7 MPa, in order to avoid damage to the rubber gaskets and membranes caused by excessive compression force.
When using the chain with the tank, it is necessary to reduce the sealing compression force of the electrolyzer to 1 MPa before tensioning the 4 chains. If the pressure is not reduced, the chain will be subjected to tens of tons of pulling force when using a hydraulic device to open the unit slot. First, a link in the chain may break; secondly, the four stress points on the chain will cause severe deformation and structural damage to the separated cell grooves, leading to leaks, and the corresponding ion membrane will also be damaged by the bulging electrode mesh. Chlor-alkali manufacturers have reported that the continuous damage to the ion exchange membranes installed in the same location after replacement is caused by the aforementioned improper operations.
Additionally, when tightening the chains, the two chains on the movable side bracket of the unit slot should be tightened slightly more than the chains on the other side. This way, when the unit slot is pulled apart, the positioning steps of the fixed side bracket and the side rod guide will help to guide the unit slot as it slides, preventing it from shifting laterally toward the fixed side bracket during pulling. This prevents potential problems such as misalignment of the seals between adjacent unit slots due to further compression.
1. Improve the ability to prevent and handle emergency incidents. When hydrogen leaks, there is a high risk of fire, and the flames are not easy to detect; therefore, personnel working with electrolyzers must be thorough and careful during their inspections. In the event of a fire, notify the dispatch center while promptly extinguishing the fire. Upon receiving the notification, the control system promptly reduces the current while ensuring a positive pressure of hydrogen; when a slight positive pressure is present, it quickly fills the system with nitrogen and initiates an emergency shutdown. In particularly severe cases, use nitrogen-based emergency shutdown.
1. Improve the ability to prevent and handle emergency incidents. When hydrogen leaks, there is a high risk of fire, and the flames are not easy to detect; therefore, personnel working with electrolyzers must be thorough and careful during their inspections. In the event of a fire, notify the dispatch center while promptly extinguishing the fire. Upon receiving the notification, the control system promptly reduces the current while ensuring a positive pressure of hydrogen; when a slight positive pressure is present, it quickly fills the system with nitrogen and initiates an emergency shutdown. In particularly severe cases, use nitrogen-based emergency shutdown.
During parking for maintenance, the electrolyzer has a certain electromotive force due to the \"battery effect\". Even after the anode solution was completely drained, it continued to flow out intermittently. The catholyte has not yet been completely drained; a small amount of it remains at the anode gaskets. If there are pinholes in certain membrane gaskets or if liquid is present only in localized areas, the resistance to current flow in those spots is extremely low. Under the influence of the electromotive force generated by the \"battery effect\", the current density becomes very high. If the rectifying DC switch is not turned off to break the current circuit, arcing will occur at those points, resulting in electric sparks, which still pose a risk of combustion and explosion (such as the flash explosion incident at Kaifeng Dongda in 2003). Therefore, during parking for maintenance, it is necessary not only to fill with nitrogen in a timely manner but also to disconnect the rectifier DC switch promptly.