Analysis and control measures of combustion and explosion in vertical coal mill system

Date: 2019-07-04 Views:

I. basic conditions for combustion and explosion of pulverized coal

1.1 ignition characteristics of gases

Pulverized coal is a kind of inflammable and explosive material. Its ignition characteristics and explosibility are related to its fineness, volatile mass fraction, coal concentration in gas and CO volume fraction, etc. Many of these factors are determined by production conditions. For example, coal fineness is determined according to the combustion requirements of rotary kiln.Studies show that when the concentration of pulverized coal is less than 100g/m ^ 3 or more than 1,500 g/m ^ 3, no combustion or explosion will occur.The fluctuation range of this limit range is large, and it changes with the variation of volatilization, ash and dispersion of pulverized coal.

1.2 the volume fraction of oxygen in the gas

The higher the volume fraction of oxygen in the gas, the faster the oxidation and heating rate of pulverized coal will be, and the higher the probability of combustion and explosion of the system will be, especially under the condition of rich oxygen.

1.3 gas temperature (or ignition source)

The faster the oxidation rate of pulverized coal, the greater the flammability.In particular, it is more dangerous to take gas from the kiln head.In a mixture of pulverized coal and air, if a source of fire ignites in one place, it will spread to other places and set the whole mixture on fire.

Generally, the source of fire is caused by the following aspects: the hard friction of the metal structure in the mill produces sparks; the metal striking the coal bunker produces sparks;Open flame generated during welding;Due to the strong friction between the moving gas and the components, the static electricity will accumulate to a certain extent, and the spontaneous combustion of pulverized coal will occur.

II.Control measures to prevent the combustion and explosion of the system

It can be known from the above that the formation of combustion and explosion conditions can be prevented and restrained by taking corresponding measures to eliminate or control the combustion and explosion conditions in each link of the system.

2.1 operational control of temperature

The fuel and its mixture of gases will ignite without external action when the ignition temperature is exceeded and oxygen is enriched.The higher the volatile content of the fuel, the lower the ignition temperature.The ignition temperature of bituminous coal is 350 ~ 500℃.In actual production, if the mill inlet temperature exceeds 300℃.Mars may be visible in the scraper chamber.This is caused by the ignition of some coal particles under the action of high-temperature exhaust gas, which may cause the combustion and explosion of coal particles, such as the dust collector with bag or the grinding chamber (only the coal particles falling into the scraping chamber are generally not easy to be taken into the grinding chamber).Therefore, it is necessary to reduce the gas temperature to prevent the combustion of pulverized coal.

(1) control the temperature of grinding gas.Try to control the temperature of hot air of mill population < 220-260 ℃, generally not more than 300℃.In specific production, the temperature of human grinding gas can be controlled by adjusting the opening of hot air valve and cold air valve according to the change of water content of raw coal, so as to meet the requirements of drying and avoid excessive temperature of grinding gas, so as to ensure safety.

(2) control the gas temperature out of the mill.The outlet temperature of the mill should be controlled at 65 ~ 75℃, not exceeding 85℃, otherwise the mill and exhaust fan will automatically stop.When the feeding quantity is too large or too small, more strict control of gas temperature, to prevent the system spontaneous combustion explosion.

(3) control the temperature of the bag dust collector.The temperature of the bag filter should be controlled above the dew point temperature.Under normal circumstances, the exit temperature of the bag dust collector is slightly lower than the inlet temperature, we should pay close attention to the change of the dust bucket cone temperature of the bag dust collector, it should be controlled at less than 65℃ (over 65℃ will alarm, over 80℃ will stop).During shutdown, the system should stop and close the valve, spray inert gas, exhaust pulverized coal, and check the filter bag.

(4) control the temperature of pulverized coal bin.Under normal conditions, the temperature of the cone part of coal powder warehouse is controlled at about 50℃, and the high material level (85%) is maintained for production.If the temperature of the bin cone continues to rise, the bin level should be reduced;If the temperature exceeds 85℃ and the temperature rises, it indicates that coal powder has spontaneously ignited, so it should be sprayed with inert gas immediately to prevent combustion and take disposal measures such as warehouse release.

(5) control the outlet temperature of the main exhaust fan of the system.In the specific operation, according to the change of the water content of the raw coal entering the mill, and under the condition of ensuring the production and quality, the wind should be used reasonably, especially to ensure the temperature of the grinding gas is less than 75℃.In the investigation and analysis of the combustion and explosion accident of the bag filter of various manufacturers, it is found that some coal mill operators, in the process of kiln operation, blindly emphasize the requirements of the drying effect of coal, adjust the temperature of the grinding gas to more than 100℃, under the cooperation of other conditions, the combustion and explosion accident occurred, resulting in losses.

2.2 settlement and adhesion of pulverized coal in the control system to prevent spontaneous combustion

The coal powder concentration of the vertical mill system is about 650g/m cubed, generally no combustion and explosion will occur. However, when the coal powder settles, sticks and oxidize in some parts, it will spontaneously ignite, causing the system combustion and explosion accident.The actual production of bag dust combustion is mostly caused by this reason.Pulverized coal has a large specific surface area and a strong oxidation heat generation capacity.The rate of oxidation heating increases with the increase of coal powder content and fuel concentration (especially volatile matter) in the waste gas and the increase of coal powder storage, dispersion and specific surface area in the system.According to laboratory tests, the reaction rate decreases with temperature rise when the temperature is below 80℃, while the activity increases with temperature when the temperature is above 80℃.

In addition, the amount and change of water content in coal is also a major factor affecting the spontaneous heat of coal. Theoretically, a 1% increase in water content will increase the coal temperature by 17℃.At the same time, the high speed of the exhaust gas carries coal powder will also take away a lot of heat.At low speeds, the opposite is true, and does not carry away the heat produced by spontaneous combustion.Specific control can be carried out from the following aspects:

(1) strengthen the sealing of the system to prevent fire and explosion accidents caused by air leakage and condensation and rainwater infiltration.Due to wear, foundation settlement and other reasons, resulting in lax sealing and air leakage, system temperature decreases, and then condensation;In rainy days, rain water enters the system with air leakage, which reduces the temperature of the system and makes coal powder adhere to the wall of the system. Once the conditions are appropriate, it will cause local oxidation and spontaneous combustion and burn down the filter bag of the dust collector.The three explosions at two factories involved in the investigation occurred after a rainy day.

(2) regularly inspect the ash discharge valve to prevent the coal accumulation and oxidation spontaneous combustion accidents caused by the dust bucket due to wear or stuck.

(3) strengthen system heat preservation, prevent condensation caused by system temperature reduction, and make pulverized coal bond.

(4) during the trial production, all dead corners of the system should be completely filled with uncombustible substances (such as limestone powder), to prevent the accumulation of pulverized coal during production, causing spontaneous combustion.

In addition, the discharging device under the ash bucket should adopt the continuous discharging and conveying device, and the capacity should be greater than the capacity of ash bucket to prevent the accumulation of pulverized coal in the ash bucket from oxidation and spontaneous combustion.

2.3 prevent the ignition source from being generated in the system or brought into the operation of naked flame.But must pay attention to the system seal, prevents the air leakage phenomenon.

(1) prevent red hot clinker particles from being brought into the mill by hot wind.If the hot air needed for the preparation of pulverized coal is extracted from the cooling machine, it should be noted that when the cooling machine is not working normally, some small red hot clinker particles are pumped into the coal grinding system and cause open flame combustion, especially when the grinding is stopped and the pulverized coal and combustible gas in the system are not emptied, it is easy to cause open flame combustion when the grinding and ventilation are resumed.

(2) prevent the generation of electrostatic sparks.Connect the filter bag to the pipe and the wall to ground together to prevent ignition and explosion caused by electrostatic spark.

(3) prevent sparks from hard friction.Strengthen patrol inspection, prevent metal operation according to operating rules, strengthen the connection between the site and the central control operator of the kiln to cause sparks caused by hard friction between the grinding disc and roller, and report any abnormal problems in time.

(4) prevent open fire during equipment maintenance.The overhaul of the coal mill system should be carried out in strict accordance with the operating procedures. Welding is not allowed when the safe and stable operation of the vertical coal mill system is related to the production condition of the whole plant.

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