Analysis of the safe operation of gas generators

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Since it was converted from CAJ software, some figures were not converted. It’s a pity! SHANXI SCIENCE AND TECHNOLOGY, Vol. 25, No. 3, 2010: Analysis of the Safe Operation of Gas Generators Zhang Jun (Taiyuan Gas Company, Shanxi Province, Taiyuan, Shanxi, 030024) Abstract: This article discusses the issues related to the safe operation of gas generators during the production process, analyzes the common accidents that occur while these generators are in use, and proposes corresponding preventive measures. It also explores the causes of gas explosions, explosion-proof measures, and gas leakage problems. Keywords: gas generator ； Accident ； Safe Operation. Library Classification Number: TQ54. Document Code: A. Article Number: 1004-6429(2010)03-0092-03. ● Applied Technology. Coal plays a dominant role in China’s energy structure; the sulfur oxides and hydrogen sulfides produced by its direct combustion severely pollute the atmosphere and are detrimental to environmental protection. Therefore, gas is widely used as a fuel in cities across China, which helps to reduce air pollution in these areas. Gas refers to an incomplete reaction between various types of coal (coke) and oxygen-containing gasifying agents (O2, H2O, CO2), which ultimately results in the formation of gas composed of CO, H2, CO2, CH4, N2, H2S, etc. Coal can be used as fuel gas for urban residents, and it can also be used to produce fertilizers, chemical products, and as a reducing agent in steelmaking. In the process of gas production, the products manufactured at gas stations are flammable, explosive, and toxic; any mistakes in operation and management can lead to severe consequences. The author has analyzed the issues related to the safe operation of gas generators and offered some personal insights. Common accidents in gas generators and preventive measures: Gas generators mainly include the Ruhr process furnace, Shell gasifier, End gasifier, two-stage gas generator, etc. A gas generator is a device that uses coal, along with air and steam as reactants, to produce gas. Although gas generator types vary, their basic structure consists of five parts: a furnace chamber, a grate, an air blower system, a coal feeding system, and a slag discharge system. Coal (coke) and the gasifying agent are introduced into the furnace through the coal feeding system and the furnace bottom respectively; they undergo redox reactions within the furnace to produce gas. The layer distribution within the furnace, from bottom to top, is as follows: ash layer (slag layer), oxidation layer (combustion layer), reduction layer, carbonization layer, drying layer, and empty space. For the normal gasification of coal in the furnace, a properly distributed and stable layer structure along with a high reaction temperature are necessary. Common accidents in gas generators include: burst explosion-proof membranes, gas leaks, combustion, explosions, etc. The corresponding preventive measures should include: (1) strengthening the regular inspection, testing, and replacement of safety accessories such as the explosion-proof membranes in gas generators, to ensure that such safety devices remain in good condition and are sensitive and reliable. (2) Regularly check the condition of the water seal to ensure it remains in an overflow state; also clean the dust accumulated in the seal groove periodically to maintain the appropriate height of the water seal. (3) Install gas detection and alarm devices and improve ventilation; personnel may enter only when the detection devices indicate no gas leaks and that the concentration is within safe limits. When entering areas with a risk of poisoning, it is necessary to wear gas protection equipment. (4) It is necessary to strictly enforce the approval and hierarchical management systems for hot work, and establish robust fire and explosion prevention measures. Before carrying out any welding work in the gas generator production area, it is necessary to monitor and analyze the mixture of gases present in that area; work can only proceed once the results are satisfactory. 3 Causes of gas explosions in gas generators and explosion prevention measures. Explosions are divided into physical explosions and chemical explosions. A physical explosion occurs when the pressure exerted on a pressurized device exceeds its mechanical strength limit, or due to excessive heating, corrosion, or reduced mechanical strength as a result of long-term use and lack of maintenance. A chemical explosion occurs when a substance undergoes a chemical change in an instant, generating large amounts of heat energy or gas, which causes its volume to expand rapidly and leads to a sudden increase in temperature and pressure, thereby producing a powerful explosive force. Gas is a multi-component mixture containing flammable and explosive substances such as CO, H2, CH4, and H2S. It is prone to chemical explosions; under certain conditions, such as in a closed system, when the flammable gases in the gas mix with air or oxygen and reach the explosive range, they will undergo an oxidation reaction rapidly upon exposure to the ignition temperature or a source of fire. This generates a large amount of heat, which in turn compresses and heats the adjacent flammable gases, causing the reaction rate to increase dramatically in an instant. More heat is released, which continues to heat and compress additional adjacent areas, resulting in a sharp rise in gas pressure within the closed system and thus a gas explosion. .3.1 Causes of producer gas explosions. The situations and causes of gas explosions generally include: explosions occurring due to an excessive oxygen content in the gas ； Gas was poured into the air duct and exploded ； The equipment exploded during maintenance ； Explosions caused by gas system leaks and explosions resulting from other circumstances. (1) Explosions of producer gas caused by excessive oxygen content in the gas: ① Explosions due to high oxygen content when starting up the furnace: When the producer gas furnace is started, the coal layer is thin and the furnace temperature is low, resulting in scorching inside the furnace ； Steam carrying water causes a sharp drop in furnace temperature ； Fumes entering the scrubber tower and cooling tower, as well as situations where the pipes are not fully purged after cleaning before gas is transferred, can all lead to explosions due to excessive oxygen levels. ②Explosion due to excessive oxygen levels during the operation of a producer: During production, issues such as an excessively low coal layer, scorching, high blast volume, or malfunctions of the automation systems can lead to explosions caused by excessive oxygen levels. ③Explosion during shutdown: Slow operation of the air supply cutoff mechanism or malfunction of the automatic system results in the failure to purge the gas production equipment (pipes) during shutdown. As the gas cools, negative pressure is created, which draws in air through any gaps in the equipment (pipes), leading to an excessive oxygen content and thus an explosion. ④Explosion during hot standby of the furnace: Gas outlet pipeline was not isolated and the gas was not vented ； Too much time passes, the furnace cools down, and air gets in ； The slag suddenly collapsed, creating a negative pressure in the furnace chamber; air was drawn in through the coal feeding bin and natural ventilation openings, leading to an explosion. (2) Gas being poured into the air pipeline results in an excessive oxygen content in the gas, leading to an explosion: ① Explosion in the air pipeline during startup: Before restarting the furnace after it has been shut down, the air pipeline is not purged or replaced; the gas that returns when the gas generator warms up or is shut down mixes with the air introduced during startup, reaching the explosive limit, and an explosion occurs upon contact with fire. ②Explosion during production: If the blower stops operating suddenly, the air pressure beneath the furnace ash tray drops immediately; gas then flows from the furnace chamber into the ash hopper, from there into the air intake duct, and finally out of the blower, resulting in an explosion. ③Explosion during shutdown: A water supply interruption due to reasons such as blocked water pipes caused the seal to lose its effectiveness; during shutdown, negative pressure in the furnace led to gas flowing back into the air duct fan, resulting in an explosion. (3) Explosion occurs during maintenance welding: The inert gas was not replaced thoroughly before welding, leaving dead corners in the equipment (pipes) where gas still remained ； Inaccurate sampling and testing analysis, or making decisions based on experience without ensuring that analyses show satisfactory results before proceeding with welding work, can also lead to explosions ； The gas supply connected to the production system has not been cut off ； If the interruption period between welding operations exceeds the specified time, or if analyses are conducted before resuming welding, this can lead to explosions. (4) Explosion due to a leak in the gas system: When there is a significant leak in a part of the gas system, and the area where the gas is leaking has poor ventilation, an explosion occurs upon contact with a fire source. (5) Explosion during furnace sealing: When sealing the furnace, the outlet pipeline is not blocked and the gas is not expelled; over time the furnace cools down, and air seeps in to form explosive substances, which explode when exposed to the flame. (6) Explosion upon power loss: When the power is cut off, the blower stops operating, and the air pressure in the furnace ash pit drops immediately. As a result, the gas pressure inside the furnace becomes higher than the air pressure in the ash pit, causing the gas to flow from the furnace into the ash pit, then into the air ducts and finally into the blower. There, it mixes with air to form an explosive mixture. When power is restored, the blower starts up again, forcing this explosive mixture back into the furnace where it is ignited by the flames. (7) Explosion due to water outage: A water outage mainly results in the gas scrubbing tank losing its water seal function. When the furnace is shut down, gas flows back into the main air duct and the blower, from where it is then sent into the furnace, leading to a gas explosion. (8) Explosion during maintenance: An explosion occurs due to the gas supply not being completely cut off and the equipment not being thoroughly cleaned, which leads to the initiation of a fire. .2 Measures for preventing gas explosions: Based on the reasons behind explosions caused by producer gas, the following measures can be taken to prevent such explosions: (1) Equipment and pipelines should be equipped with a comprehensive explosion prevention system consisting of control devices and monitoring instruments (see Figure 1). This includes instruments for monitoring air pressure, temperature, and flow rate at the inlet of the gas producer, as well as instruments for measuring gas pressure and humidity at the outlet, in addition to high and low pressure alarm devices for the gas ； Light signal, automatic coal feeding machine, ash discharge machine interconnection device, press and blower interlock device, as well as audio-visual alarm device ； Flammable and explosive gas detection and alarm device to prevent gas leaks and explosions ； The blast furnace air main, dust removal system, equipment after cooling and the semi-pure gas pipes, as well as the distribution main, should all be equipped with explosion-proof membranes. Check valves should be installed at both the front and rear of the gas system to prevent gas from flowing back in the main pipes when the furnace is shut down. Hazardous area (field) transmitter, safety barrier regulator, safety barrier actuator; non-hazardous area (control room). Power supply. Figure 1: Block diagram of the explosion-proof system. (2) During operation, the following steps must be followed: before starting up the furnace, it is necessary to adjust the settings of the alarm instruments and interlocks, set the pressure values for alarms and interlock actions, test individually whether each interlock valve operates correctly, and finally conduct several comprehensive overall interlock tests ； Oxygen content analysis should be conducted before connecting the gas to the grid, with the oxygen content to be less than 1% ； During the production process, special attention should be paid to changes in the temperature and pressure of gas and air; if any deviations from the specified process parameters are detected, they must be addressed promptly. (3) When performing maintenance work involving open flames, it is necessary to ensure that the gas supply to gas equipment and pipelines is reliably cut off, to purge and displace the gas inside such facilities, and to open access holes, inlet and outlet pipes, etc ； Air samples are taken for carbon monoxide level analysis to ensure that no explosive mixture is formed throughout the welding process. In the event of a fire, the gas supply must be shut off first; only after an analysis inside the furnace shows satisfactory results can welding resume. 4. Handling of gas leaks: In a gas generator, the pressure control parameters of the gas are important factors in the production process. If the gas pressure is controlled at too high a level, it will cause the gas to leak out of the furnace; on the other hand, if the pressure is controlled at too low a level, air will leak into the furnace from outside, thereby reducing the quality of the gas. 4.1 Detection of gas leaks: When carrying out maintenance work, it is necessary to have a sensitive pressure gauge, a calibrated gas detector, and leak detection fluid available. Under normal circumstances, leak detectors are very sensitive to leak detection fluids. During maintenance, a leak detector should be used first, followed by the leak detection fluid; all indicator lights must be turned off during testing. When entering the area where a leak may occur, it is necessary to inform the control center that one has arrived at that location. Before entering, prepare a gas detection device ； Be sure to stay in contact with the contact center to report emergencies. Maintain proper documentation; after any leak detection is completed, staff must thoroughly record the entire testing process. 4.2 Measures to prevent gas leakage: In the past, during gas production, the pressure of the gas in the coke ovens was kept at around 50–80 Pa as a result of basic operational practices, which led to significant gas leaks. It was also difficult to ensure that the oxygen content in the gas remained below 1%. The main reason for this was that the control systems were subject to too many interference factors on site, making it hard for them to achieve the goal of automatic control with a precision of 0±10 Pa. Since the 1990s, methods such as PLC control and distributed control have been widely used to regulate gas generators. In two-stage furnaces, a two-level regulation control system for generator gas pressure is employed (the system block diagram is shown in Figure 2). Once the system control parameters are set, high-performance automatic pressure control of 0±10 Pa can be achieved at the furnace outlet, effectively preventing gas leakage and ensuring that the oxygen content in the gas remains below 1%. M furnace gas tank, furnace gas tank, MPICR damper, PICR damper, PICR damper, cooler, blower, M1#2# electric butterfly valve to the chemical production section. Figure 2 shows the block diagram of the secondary pressure control system for generator gas. In Figure 2, PICR stands for pressure, display, control, and recording respectively. The damper does not require a power source or air supply during operation; it is installed in series between the pressure sampling point and the transmitter. The operating damping device can rapidly and smoothly suppress various external pulsating disturbances within 0.1 s, stabilizing the system’s pressure fluctuations. Changing the large reflux pressure tapping point to before the primary cooler helps to stabilize the pressure there. For example, when the pressure before the primary cooler is below the set value, the large reflux control valve quickly increases the amount of reflux flow, thereby bringing the pressure back to the set value; conversely, it reduces the reflux flow. Once the pressure before the primary cooler is stabilized, this creates favorable conditions for regulating the pressure of the gas inside the furnace. References: [1] Tang Guangjun, Zhang Feng. Selection of Coal Gasification Methods [J]. Journal of Jilin University, 2005(6): 335–337. [2] Wei Yushan. Gasification Principles of Gas Generators and Key Factors Affecting Gasification Efficiency [J]. Foreign Coking Chemistry, 2001(37): 1204–1206. [3] Luo Xiaoling, Xu Kunshan. Research on the Working Principle of Gas Generators [J]. Coal Engineering, 2009(8): 98–100. [4] Fei Donghui. Analysis of the Safe Use of Gas Generators [J]. Chinese High-Tech Enterprises, 2009(11): 40–41. Date of receipt: March 10, 2010. Author’s profile: Zhang Jun, male, born in 1969; graduated from Beijing Institute of Technology in 2005; engineer. Address: 030024, Taiyuan City, Shanxi Province. Safe Running of Gas Generators ZHANG Jun ABSTRACT: This paper explores the issues related to the safe operation of gas generators in industrial settings, analyzes various accidents that occur, proposes corresponding preventive measures, and discusses the causes of explosions, as well as explosion-proof measures and gas leakage issues. KEYWORDS: gas generator ； accident ； safe operation