How to prepare fire-proof and explosion-proof for coal powder in cement factoryDate: 2019-06-20 Views:
Cement industry is the main part of building materials industry, which is closely related to economic construction. Coal is one of the most commonly used fuels in cement production, especially since the worldwide oil crisis in 1973, the fuel composition of the international cement industry has undergone further changes, and many countries that used oil as the main fuel of the cement industry have also changed to raw coal as fuel. China's energy structure is characterized by more coal, poor oil and less gas, so the production of Portland cement clinker in China is all produced with coal as fuel.
When cement clinker is produced by precalciner kiln, 100 kg of standard coal is usually consumed for every ton of cement clinker produced. In order to speed up the chemical reaction rate of coal combustion process, meet the required combustion temperature, and form the high temperature state and environment required for raw meal calcination into clinker, raw coal must be ground into fine powder. Therefore, the fineness of pulverized coal is one of the controlling factors to ensure combustion speed and stable combustion.
In pulverized coal preparation system, grinding, storage, transportation and dust removal equipment are involved. Grinding equipment generally uses vertical grinding or air sweep tube grinding to grind pulverized coal.
In the design of pulverized coal preparation system in cement plant, although measures such as anti-combustion, explosion-proof, explosion release, explosion suppression, explosion arrest, static electricity and so on have been taken, advanced pressure, temperature, CO concentration detection and alarm devices have been set up, each key link of pulverized coal preparation system has been monitored and warned, and CO2 or N2 automatic fire extinguishing system has been set up at the same time, but because of the characteristics of easy combustion and explosion of pulverized coal, its preparation and transportation, Spontaneous combustion or explosion accidents are easy to occur in the process of storage and use, and the factors inducing accidents are caused by misoperation in the process of pulverized coal preparation, which makes the pulverized coal preparation system become a link with high frequency of safety production accidents in cement plants.
2. Safety measures for fire prevention and explosion protection
2.1 basic requirements
The main contents are as follows: (1) the safety management system of pulverized coal preparation system should be established in cement plant.
(2) the post personnel of pulverized coal preparation system must go through the training of post operation rules, hidden danger investigation, emergency rescue and accident emergency treatment before they can take up their posts after they have passed the examination.
(3) the pulverized coal preparation workshop shall set up a warning area to prohibit the storage of flammable and explosive articles, and the entry of unrelated personnel shall be prohibited without permission.
(4) the pulverized coal preparation workshop should be equipped with sufficient fire fighting equipment and facilities according to the requirements of GB 50016, the configuration of fire extinguishers should meet the requirements of GB50140, and the fire alarm devices should be installed in the field and control room. Fire fighting equipment and facilities should be inspected regularly and kept in good condition. Fire safety warning signs such as "no fireworks" should be set up in pulverized coal preparation workshop in accordance with the requirements of GB 13495.
(5) the cement plant should formulate a special emergency plan for fire prevention and explosion prevention of pulverized coal preparation system in accordance with the requirements of GB/T 29639, and organize the training and exercise of relevant personnel.
2.2 equipment and facilities requirements
(1)The design of pulverized coal preparation system shall meet the requirements of GB50016 and GB50116, GB50295, GB50577 and GB15577.
(2) there are not less than two hydrants on each floor of the coal mill workshop.
(3) the explosion relief valve should be set up in coal mill, powder separator, pulverized coal bin, coal mill dust collector and inlet dust collector process pipeline. The explosion relief valve should be opened and closed automatically, the installation position should be easy to repair, and the ejector should be avoided to the control room, electric oxygen equipment, cable bridge, staircase and main passage after explosion.
(4) the special bag dust collector for coal mill should be selected for dust removal equipment in pulverized coal preparation system, and there should be such facilities as fire protection, explosion protection, lightning protection, static electricity prevention and condensation prevention. The inlet and outlet of the dust collector should be equipped with a fast cut-off valve for automatic action under the condition of power outage, and a speed measuring device should be set up for the ash discharge rotary feeder.
(5) When the waste heat of the burning system is used as the drying heat source, the coal mill shall have a regulating valve and a cyclone (or settling chamber) before entering the coal mill. The waste heat of the waste gas of the kiln tail should be adopted for the coal grinding and drying heat source.
(6) The coal mill inlet and outlet air pipe, the pulverized coal bunker and the coal mill dust collector (inlet and outlet air pipe and ash bucket) shall be provided with the temperature monitoring device, the pulverized coal bunker and the air outlet pipe of the coal mill dust collector shall be provided with the CO monitoring device, and the temperature and the CO automatic alarm device shall be configured. During the operation and maintenance of the system, the monitoring and alarm devices shall be in the open state.
(7) dry powder fire extinguisher and gas automatic fire extinguishing system should be set up in coal mill, pulverized coal bunker and coal mill dust collector.
(8) the gas pipes leading to each protective area of the automatic gas fire extinguishing system shall be provided with control valves and the control valves shall be arranged centrally. Automatic control system should be equipped with automatic, manual mode switch and manual control valve; gas fire extinguishing system should be equipped with fire alarm device. Except for the maintenance operation of the associated equipment of the gas fire extinguishing system, the gas fire extinguishing system should always be in an automatic control state.
(9) the electric equipment in the closed factory building of pulverized coal preparation system should be explosion-proof.
(10) all equipment and pipes in pulverized coal preparation system should be reliably grounding.
(11) the cone slope of pulverized coal bunker should not be less than 70 °, and the pulverized coal exhaust device should be set up.
When the oxidation rate exceeds the heat dissipation rate, the heat accumulation of pulverized coal, such as some dead angles in dust collector and fine pulverized coal deposited in ash bucket, leads to spontaneous combustion and even explosion of pulverized coal. The spontaneous combustion temperature of pulverized coal is affected by the degree of oxidation of pulverized coal and the composition of surrounding media. Although it is lower than the normal ignition temperature of pulverized coal, it is not a fixed value. V.Fierro study shows that the critical temperature of spontaneous combustion of pulverized coal is in the range of 60 ℃ ~ 80 ℃. Therefore, preventing pulverized coal deposition in pulverized coal preparation system is an important measure to prevent spontaneous combustion.
When the pulverized coal is very fine, it is in direct contact with the air in the suspension state, and once it is ignited, the oxidation reaction will occur quickly and the explosion will occur. Generally speaking, the lower limit concentration of coal dust explosion is 30 × 50 g ·m ~ 3, and the upper limit concentration is 1000 × 2000 g ·m ~ 3. In the process of pulverized coal preparation, some volatile components of raw coal are precipitated due to the dual action of hot gas for drying and grinding, resulting in CO entering the air path of the system. When the basic conditions of combustion and explosion-oxygen, combustible matter (pulverized coal or CO), fire source (Mars or spark) exist at the same time, combustion or even explosion will occur. Therefore, pulverized coal preparation The operation of the system should pay special attention to the problems of fire protection and explosion protection.
When the concentration of oxygen is less than 14%, it is regarded as lazy gas, and the pulverized coal preparation system is relatively safe. Therefore, when the pulverized coal preparation system of cement plant is arranged at the end of the kiln and the flue gas of the kiln tail is used as the drying medium, the oxygen content in the flue gas is low and the explosion suppression effect is low. When the pulverized coal preparation system of the cement plant is arranged in the kiln head, using the hot air of the grate cooler or the hot air of the hot blast furnace as the drying medium, the oxygen content in the hot air will be as high as 21%. Have the condition of burning and blasting.
In general, the volatile matter of coal is not easy to explode when the volatile matter is less than 10%, and it is flammable when 25%-35%. It is very flammable when the volatile matter is above 35%. Therefore, when preparing anthracite pulverized coal, the hot air of kiln head grate cooler can be selected as drying heat source; when preparing bituminous coal pulverized coal, kiln tail exhaust gas should be selected as drying heat source; when preparing lignite pulverized coal, kiln tail flue gas should be selected as drying medium, and in order to reduce the explosion risk of pulverized coal preparation system, lignite should be avoided as far as possible.
In a word, the fire and explosion protection of pulverized coal preparation system in cement plant is a systematic project, which should include coal type selection, system design, operation, maintenance operation, safety protection, personnel management, angle treatment and so on.