Explosion of bag filter and improvement
1 The situation on the day before the accident
At 13:55 on September 14th, the fire was stopped and the kiln was stopped; the coal injection drying kiln was started at 17:30; the grate cooler was started at 17:45 to send the clinker. At 01:50 on September 14th, the HRM1700 vertical coal mill was turned on, and the pulverized coal silo was fully stopped at 11:20 on the same day. According to the operation record sheet and computer recorded data, the gas temperature at the inlet of the mill fluctuated between 360-380°C, and The recorded data before the grinding stop (11:00) are: the inlet temperature of the mill is 380°C, the inlet pressure is -330Pa, the outlet temperature of the mill is 77°C, the outlet pressure is -3310Pa, the pressure difference is 3010Pa, and the output per hour is 28t/h. From the temperature curve of the mill outlet recorded by the computer, the outlet air temperature was abnormal at 17:09; The outlet air temperature reached a maximum of 68.40 °C, and then at 17:54:50, the outlet air temperature gradually and steadily declined to the lowest (53.74 °C).
2 The situation at the time of the accident
17:30, coal injection kiln, 17:45 open the caster cooler to send clinker. At 17:55, the fan and valve of the coal dust collector are turned on, the valve opening is 50%, and the hot air valve from the grate cooler to the coal mill is turned on (100% opening). At 17:55:32, the coal grinding began. From the data curve recorded by the computer, starting from 17:58, the air temperature at the inlet of the bag filter rises from 47.81°C to 67.58°C (17:59:00); the temperature of the bag filter cone rises from 17:58:27 The 58.30°C of the temperature rises rapidly to 150°C at 17:58:57 (the maximum temperature in the computer is 150°C).
According to the analysis of data recorded in various places in the computer, it is believed that the explosion time of the TGM1500 bag dust collector was 17:58:50 on the 14th. After the bag filter exploded and caught fire, the equipment of the pulverized coal preparation system was shut down immediately. At 18:20, the fire and the kiln were stopped for maintenance because the pulverized coal weighing scale and the screw pump were blocked.
3 The situation after the explosion
After the accident, the trial production headquarters quickly organized personnel to put out the fire. After inspection, it was found that: 1536 filter bags were burned to varying degrees, 8 explosion-proof valves on the left and right sides were burst, 7 explosion-proof valves respectively, one of the explosion-proof valves on one side was completely exploded, and there were small coke lumps in the pulverized coal silo . Because the coke block blocked the rotor scale and the screw pump, all 28t of pulverized coal in the bin was discharged, and one thermal resistance was broken by the bottom of the dust collector.
4 Reason Analysis
After the accident, one view is that CO exploded, and another view is that it may be that Mars entered the bag dust collector and caused the explosion. No matter what kind of explosion, we must find evidence to prove it.
If it is a CO explosion, where does the CO come from? The hot air is the fresh air drawn from the caster cooler, so there should be no CO; even the CO generated by the incomplete oxidation of the pulverized coal in the bag filter will not cause an explosion. Because the exhaust fan is turned on at 17:55 and the valve opening is 50%, the inlet and outlet pressures of the bag filter are -2.81kPa and -3.75kPa respectively, and the inlet and outlet pressures at 17:58 are -3.17kPa and -3.98kPa. More than 3 minutes should have exhausted the CO in the dust collector. Therefore, the evidence of CO explosion is insufficient and cannot be established.
If the sparks caused an explosion, where did the sparks come from? The hot air was drawn from the upper part of the third and fourth chambers of the caster cooler. The kiln has been stopped for 4 hours, and there should be no high-temperature hot clinker powder. Judging from the previous experience of stopping the kiln for more than 3 hours, the temperature of the hot air drawn from the caster cooler to the HRM1700 did not exceed 60°C when the mill was first started, and it seems that there is no evidence of an explosion caused by a spark. On the morning of the next day (September 15th), the inspection of the HRM1700 hot air inlet found that a lot of coal crushed stone, some clinker powder and other residues had accumulated at the inlet. After observation, it was found that some coal crushed stones had already occurred Burning, but the coal on the grinding table has no burning traces, which means that the combustion of the coal grinding stone accumulated at the hot air inlet of the mill is not caused by the explosion, but the spontaneous combustion of the coal grinding stone here causes the exhaust fan to bring its sparks into it. into the baghouse and cause an explosion. From 01:50 to 11:20 on the 14th, the air temperature at the inlet of the mill was between 360 and 380 °C, and at 17:09 during the shutdown of the mill, the air temperature at the outlet of the mill began to change (increase), which indicates that in the At 17:00, the materials accumulated at the hot air inlet of the mill began to spontaneously ignite. Therefore, the view that the human Mars caused the bag dust collector to explode is established.
Then why can the coal grinding stone accumulate at the inlet, and then accumulate into the square and the expansion joint? The original HRM1700 scraper cannot scrape all the spit out into the slag outlet, and the coarse slag at the inlet ( There is no baffle block, which causes some coal grinding stones to enter the hot air duct of the mill, causing coal grinding stones and other residues to accumulate here.
After investigation and analysis of the cause of the accident, it is believed that the main reason is the unreasonable arrangement of the hot air inlet pipe of the mill and the location of the slag discharge port, which causes the accumulation of materials, and the secondary factor is that the inlet air temperature is too high (before the 14th, the inlet air temperature was all 350℃), the excessively high wind temperature induces the spontaneous combustion of the coal and dry rock.
5 Improvement measures
In view of the cause of the explosion and the potential hidden dangers, the following improvement measures have been implemented:
(l) Process management.
Strictly control the inlet air temperature below 350℃, and strictly prohibit over-temperature operation; in addition, it is stipulated that the hot air valve can only be opened after the coal mill runs for 15 minutes, and the inlet air temperature is strictly controlled not to exceed the limit.
(2) Improve the process piping layout.
Improvements to the angle of the hot air into the mill duct. Before the change, inspection found that there were a lot of crushed coal grinding stone and clinker dust at the gas inlet of the mill, at the place where the square becomes round, in the expansion joint and at the connecting elbow; Coal slammed stone and small coal particles are the main ones. There is a small amount of broken coal crushed stone and more clinker dust in the expansion joint, while there is more clinker dust in the elbow.
In order to prevent the crushed coal and crushed stone from falling into the hot air inlet of the mill and the expansion joint, and at the same time reduce the accumulation of clinker dust in the pipeline, the following measures were taken: First, a baffle (150mm high, and the length of the inlet) was installed at the inlet. The width is equal to 1960mm); the second is to add a cone slag discharge at the inlet, and the width is equal to the width of the air inlet; the third is to make the bottom of the square-to-round part and the bottom of the hot air inlet of the mill the same height or slightly higher; four is A dust collector is added before the hot air enters the mill. See Figure 1, Figure 2, and Figure 3 for the situation before and after the transformation.
Improve potential hidden dangers. The pulverized coal enters the bag filter together with the airflow. There is a flat section with a length of 500mm in front of the bag filter. In order to prevent the pulverized coal from accumulating here and causing spontaneous combustion, this flat section is removed, and the original angle is too flat ( 45°), changed to 30° after rectification, see Figure 4 before and after modification.
6 After the transformation
After the transformation, there is basically no accumulated material in the square and round of the inlet pipe and in the expansion joint, and it must be checked and cleaned after each shutdown; there is no coal dust accumulation in the inlet pipe of the bag filter. For 7 months, the system has been operating normally, indicating that the retrofit was a success.
7 Conclusion
The explosion accident of the pulverized coal bag dust collector caused a direct economic loss of more than 150,000 yuan. This accident also gave us inspiration: process design and layout are the keys to determining safe production; how important it is to strictly implement process control parameters, strengthen process monitoring and management.
