Safety analysis of the oxygen content in gas in electric tar collectors

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Safety analysis of the oxygen content in gas in electric tar capturers Chen Rongqing (North China Design and Research Institute for Municipal Engineering, Tianjin 300074). A portion of the tar produced during gas production exists in the gas as extremely small droplets, with particle sizes ranging from 1 to 7 μm. The tar mist in the gas is washed away during subsequent gas purification processes, entering the solution or adhering to pipes and equipment, thereby causing contamination of the solution, a decline in product quality, and blockages in the equipment and pipes. 1 Safety operation requirements for electrostatic tar catchers. Devices used to capture tar mist in gas include organic tar catchers and electrostatic tar catchers; in China, electrostatic tar catchers are primarily used for this purpose. Based on the structure of the precipitation electrode, electric tar catchers can be classified into types such as tubular, honeycomb, concentric circular, and plate types. Electrostatic tar collectors utilize high-voltage electrostatic fields to create positive and negative poles; as the gas passes through these collectors, the tar mist present in the gas is captured due to the effect of the high-voltage electric field. Since gas is flammable and explosive, it is necessary to ensure the safe operation of the electrostatic tar catcher. Furthermore, a corona exists between the electrodes of the electrostatic precipitator, which may lead to spark discharge. If oxygen is mixed in with the gas, an explosion will occur when the mixture ratio of gas to oxygen reaches the explosive limit. 2 Control of oxygen content in gas The main sources of oxygen in gas include air that enters as a result of leaks in equipment and pipelines during the production process ； Second, there is an excess of gasifying agent or a short circuit in the gasification process ； Third, during the gas production process, a certain amount of air enters the gas. To ensure that the air mixed with gas does not reach the explosive limit, the oxygen content in the gas must be controlled. The \"Code for Design of Town Gas\" (GB 50028-2006) stipulates that an alarm signal should be issued by the electrostatic tar catcher when the volume percentage of oxygen in the dry distillation gas exceeds 1%. When the volume percentage of oxygen reaches 2%, measures should be in place to immediately cut off the power supply. This requirement is also specified in the \"Safety Regulations for Gas in Industrial Enterprises\" (GB 6222-2005). All these regulations are based on the criterion that the volume percentage of oxygen in gas must not exceed 1%. However, this threshold is relatively conservative, making the operations in actual production processes quite difficult. 3 Relationship between oxygen content in gas and explosion limit. The explosion limits of different gases vary; the explosion limits of various synthetic gases are shown in Table 1. Table 1 Explosive limits of various types of artificial gas (vol%): Gas type, Explosive limit of gas in air, Explosive limit of air in gas, Upper limit/Lower limit/Upper limit/Lower limit/Upper limit/Lower limit for oxygen content in gas. Coke oven gas: 35.8/4.5/64.2/95.5/13.5/20.1; Upward furnace gas: 40.9/4.9/59.1/95.1/12.4/20.0; Generator gas: 67.5/21.5/32.5/78.5/6.8/16.5; Water gas: 70.4/6.2/29.6/93.8/6.2/19.7; Oil-based gas: 42.9/4.7/57.1/95.3/12.0/20.0. As can be seen from Table 1, for coke oven gas, oil-based gas, and upward furnace gas, an explosive mixture is formed when the volume percentage of oxygen in the gas reaches 12%–13.5% (which corresponds to a volume percentage of air in the gas of around 60%). Under normal production conditions, the amount of air in the gas cannot reach such a high level; therefore, the control limit of less than 1% volume percentage oxygen in the gas can be appropriately relaxed. For producer gas and water gas, the explosive limit is reached when the volume percentage of air in the gas reaches around 30% (i.e., when the volume percentage of oxygen in the gas exceeds 6%). Taking water gas, which has the widest explosive limit range, as an example, if the volume percentage of oxygen in the gas is kept at ≤3%, this corresponds to a volume percentage of air in the gas of ≤14.3%. In this case, there is still a considerable distance from its upper explosive limit (a volume percentage of air of 29.6%), meaning there is ample margin for safety. Therefore, from the perspective of the explosion limit, it should be safe to control the volume percentage of oxygen in gas at ≤3%. 4 Recommendations: In actual production, it is difficult to keep the volume percentage of oxygen in the gas below 1%. Many companies use a control system that shuts off the power supply when the oxygen volume percentage reaches 1%, which often leads to power outages and shutdowns, thereby affecting the normal operation of subsequent processes. With the advancement of processing technologies, equipment, and control systems, as well as the improvement in the skills of operators, a considerable number of enterprises are able to keep the volume percentage of oxygen in the gas at ≤1%. For example, several gas plants and coking plants in Shanghai can maintain the volume percentage of oxygen in the gas coming from electric tar collectors at ≤1%. However, most related enterprises in the country report that it is difficult to keep the volume percentage of oxygen in the gas from electric tar collectors at ≤ 1%, with most enterprises achieving levels between 2% and 4%. Through years of actual operation at home and abroad, no explosions in electric tar collectors have occurred due to excessively high oxygen levels in the gas. From theoretical analysis and years of production experience of enterprises at home and abroad, it is feasible to control the volume percentage of oxygen in the gas from electric tar catchers at ≤3%. To meet the requirements for safe operation, it is recommended to trigger an automatic alarm when the volume percentage of oxygen in the gas is ≥2%, and to cut off the power supply when this percentage reaches 3%. For low-calorific-value gas used in carbon monoxide conversion, an automatic alarm should be triggered when the volume percentage of oxygen exceeds 0.5%, and the volume percentage of oxygen in the gas should be controlled to be ≤1%. This is due to the requirements for the oxygen content in gas when using nickel-based catalysts.