Analysis on the safe use of gas generator

Analysis on the safe use of gas generator

1. Development Overview
As one of the most important energy sources in the world, coal has been widely used in industrial production. The technology of coal gasification into coal gas has been used for more than 100 years. With the deepening of research and the development of science and technology, the technology of coal gasification has made great progress. The gas generator has been developed to be smaller, simpler, and lower in production, which greatly reduces energy consumption, production costs, and pollution emissions. The improved gas generator is widely used in various industries, so it is very important to improve the safety of the gas generator.

2. Working principle
The gas generating furnace is mainly composed of a mechanical feeding system, a gas generating system, a steam generating system, a slag discharge system, etc. The core of which is the gas generating system. The gasification of coal occurs in the gas generation system. It is a chemical process that gasifies solid coal into combustible gas under the condition of high temperature and with the help of the chemical action of the gasification agent. According to the coal gasification process, the coal in the furnace can be divided into an ash layer, an oxidation layer, a reduction layer, a retorting layer, and a drying layer from bottom to top. As shown in Figure 1:

The physical and chemical reactions that occur in each coal seam are different, and they also have different effects on the entire gasification process.
1. Ash layer. The gasification agent composed of air and water vapor is preheated in the ash layer and enters the oxidation layer uniformly through the ash layer. At the same time, the ash layer also plays a role in protecting the ash tray, keeping the temperature within a certain range during its operation. Excess ash is discharged from the gas generator through the ash tray.
2. Oxide layer. The oxide layer is the key part to produce gas and heat, and its height is generally about 150mm. The carbon in the coal in the oxide layer is oxidized by the oxygen in the gasification agent to produce CO2 and a small amount of CO, and at the same time release a lot of heat. The highest temperature in the oxide layer, generally up to 1100 ℃ ~ 1200 ℃. Steel plates in contact with the oxide layer are most susceptible to corrosion.
C + O2 → CO2 + CO + heat 3． Reduction layer. The reducing layer is above the oxide layer and is the main part of producing gas. The reducing layer is heated by the oxide layer, and the temperature of the reducing layer reaches above 1000 ° C. The carbon in the coal reacts with CO2 and water vapor to produce CO and H2 +, while absorbing a lot of heat.
Heat + C + CO2 → CO
Heat + C + H2 + O → CO + H2 +
4. Dry distillation layer. A small amount of coal gas can also be produced in the pyrolysis layer. After heating the coal in the pyrolysis layer to 700 ° C, the coal begins to crack and disintegrate, and at the same time methane, CO, hydrogen, tar and other gases are carbonized.
5. Dry layer. The drying layer is actually the place where coal is dried and preheated. After the coal is added from the top of the gas generator, it is quickly heated to about 500 ° C. The moisture on the surface of the coal quickly evaporates into water vapor and is discharged outside the furnace together with the gas.

3. Inspection case
In the first half of 2007, we carried out the first comprehensive inspection of a 3-year-old gas generator in a Ningbo foundry. During the macro inspection, we found that the bottom of the cylinder bulged slightly. After asking the equipment administrator, we learned that After the gas generator was put into use, there was a jacket water shortage accident. In the case of jacket water shortage and steel plate overheating, the operator did not take measures such as emergency shutdown of coal slag and other measures, but went to the gas generator. Misoperations such as emergency watering of the bag cause the internal pressure of the jacket to increase suddenly, causing deformation of the bottom of the jacket. After understanding the situation, we asked the user unit to open the manhole and remove the internal coal slag, and found that the bottom of the inner tank was not deformed, but the corrosion was serious. After grinding and measuring the thickness, the steel plate in the oxide layer was found to be the most corroded, 14mm steel plate It has been corroded by about 7mm, and the corrosion is the most serious at 200mm below the manhole. The wall thickness is only 7.1mm, and the steel plate above the reduced layer hardly corrodes. After magnetic particle inspection of the weld and penetration inspection of the corrosion area, no defects such as cracks were found.
According to GB150-1998 "Steel Pressure Vessel", the strength verification of the gas generator is taken, the calculated length of the inner tank is L = 2350mm, the outer diameter of the inner tank is DO = 1628mm, C = (14-7.1) /3=2.3 mm / y, δe = 7.1-2.3 = 4.8mm,
L / DO = 2350/1628 = 1.44, DO / δe = 1628 / 4.8 = 339.2
From GB150-1998 "Steel Pressure Vessel", B = 22, so [P] = B / (DO / δe) = 22 / 339.2 = 0.06MPa.
According to the strength verification results and the deformation of the jacket, and in accordance with the “Regulations for Periodic Inspection of Pressure Vessels”, the safety level of this gas generator is set at level 5, and the equipment shall be rejected.

4. Reason analysis
1. The temperature of the tank in the gas generator is extremely high, and the internal reaction is complicated. During daily production, the oxide layer releases a lot of heat, so that the temperature of the inner tank of the gas generator can reach more than 1200 ℃, and the temperature of the center of the inner tank can even reach about 1400 ℃. At this time, the cooling water in the jacket plays a vital role in the protection of the inner tank steel plate. In the absence or absence of water in the jacket, the steel plate of the furnace body will be overheated or even deformed. If the operation is wrong at this time, emergency water replenishment will cause the jacket pressure to rise suddenly, causing the jacket to deform and even the furnace The body exploded, causing casualties and economic losses.
2. In daily production, the inner tank steel plate is in a high temperature and high humidity environment for a long time, the mechanical properties and corrosion resistance of the steel plate are greatly reduced, and the humid water environment makes the steel plate prone to electrochemical corrosion, and a large number of anions (such as Cl-) After being adsorbed on the metal surface, it quickly destroys the passivation film on the surface of the steel plate. After the passivation film is damaged and the steel plate lacks self-passivation ability, the steel plate surface corrodes. The passive film is still in a passive state, which forms an active-passive corrosion battery. Since the anode area is much smaller than the cathode area, the anode current density is very large, and corrosion continues to develop deeper. It will be corroded into small holes soon, forming pitting corrosion.
3. The sulfur content of coal in China is generally high. Under high temperature and humidity conditions, a series of redox reactions occur between sulfur and the oxygen in the gasification agent and the carbon in the coal in the oxidation layer and the reduction layer, generating SO2 and H2 + S, in the water environment, sulfuric acid, sulfurous acid and hydrogen sulfuric acid are generated, and the steel plate is quickly corroded by the acid solution. At the same time, due to the decrease of PH value and the increase of temperature, this increases the possibility of pitting corrosion, and the Fe3 + generated after corrosion of the steel plate can promote the occurrence of pitting corrosion, so the steel plate in the oxide layer will continue to pit corrosion.

4. At the same time when pitting corrosion occurs, sulfur and H2 + S and other sulfides in coal will undergo high-temperature sulfidation corrosion with the steel plate at the bottom of the jacket under high temperature conditions, H2 + S + Fe → FeS + H2 +, S + Fe → FeS . Under the combined action of pitting corrosion and sulfidation corrosion, the steel plate at the bottom of the jacket is rapidly thinned.

V. Preventive measures
According to the working principle of the gas generator and the main cause of corrosion, we can improve the safety and corrosion resistance of the gas generator from the following aspects:
1. An automatic water inlet device is installed in the steam drum above the gas generator. In the case of low water, the water inlet valve automatically opens to ensure that the jacket can work normally. This not only improves the safety of the gas generator, but also reduces The workload of the operator.
2. Strengthen the safety training and education of operators, improve the safety awareness of relevant personnel, formulate emergency plans for accidents, and the operator can take correct operations in case of overheating of the jacket water shortage furnace body, instead of adding water urgently, thus preventing accidents .
3. Large-diameter rupture discs are installed on the steam drum above the gas generator and the top of the jacket. When the jacket pressure suddenly increases, the rupture disc ruptures in time, thereby controlling the jacket pressure within the normal range, not because of the pressure Excessive height can cause deformation or rupture of the cylinder and cause a safety accident.
4. In the daily production, add alkaline water (such as lime water) to the ash tray to neutralize the acid liquid, so as to achieve the purpose of protecting the steel plate and reducing the corrosion rate.
5. Due to the inevitable corrosion, the steel plate between the oxide layer and the dry distillation layer at the bottom of the inner tank can be lined with a layer of steel plate of about 4mm to cut off the contact between the acid liquid and the steel plate of the inner tank body, thereby protecting the steel plate of the inner tank body the goal of.
6. Strengthen the inspection and shorten the overall inspection cycle. When it is found that the remaining amount of the lining board after corrosion cannot meet the next comprehensive inspection cycle, replace the lining board in time to avoid the corrosion of the jacket steel plate. This can increase the life of the gas generator and reduce the production cost of the enterprise.

6. Conclusion
The gas generator is widely used in casting, glass, chemical industry, metallurgy and other industries. After adopting the above protective measures, the safety and service life of the gas generator can be effectively improved. After being promoted and used in a foundry, it has been well received. Effect.

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