Key points for producing inert gas using a fixed-bed gasifier

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Summary of Key Points for Fixed-Bed Gas Production to Produce Inert Gases by Qin Jianding (Hebi Bama Fertilizer Technology Co., Ltd., Hebi 458008, Henan) 0. Introduction: In coal chemical plants, system safety purging is necessary during shutdowns for maintenance and startup procedures. Currently, there are basically four methods of displacement: the first is to use steam for displacement ； Although this method is convenient, it has certain limitations; it can only be used for replacing systems prior to the converter, and the cost is high. The second method involves using wood to produce inert gases; it yields better results in terms of performance, but it is not practical. The main issue is that little amount of inert gas can be produced from wood, and it takes a long time to do so. Especially these days, automatic coal feeding is used, making it difficult to use wood for this purpose. The third method involves using a nitrogen generation system to produce nitrogen for replacement, but it has more limitations and requires large-scale nitrogen generation and air separation equipment. None of these three can be used as common methods for producing inert gases. The fourth type is the commonly used fixed-bed gas generator, which uses coal as raw material to produce qualified inert gases to meet the replacement needs of the entire plant. Although most plants use fixed-bed gas generators to produce inert gases from coal as raw material, they each employ different methods, and the time required to produce qualified inert gases varies; in some cases, it takes more than 6 hours to obtain such gases, resulting in unnecessary increases in consumption and a waste of maintenance time and progress. This article provides an overview of our factory’s experience in producing inert gases over the past twenty years, for reference by peers in the industry. 1. Inerting process 1.1 Process selection: Coal is used to produce inert gas, with two methods available – a three-step method and a five-step method. The three-step method is an old processing technique; the temperature at the upper part is high, which poses a significant risk to the equipment (the temperature there can reach over 600°C, and the entire upper pipeline turns red). Moreover, it is difficult to control the composition of the product, so this technique has been phased out. Before 1997, our factory used a three-step inerting process, but thereafter it has used a five-step inerting process. It has advantages such as short duration, quick results, and no damage to equipment. Modern computers come equipped with a five-step inerting procedure, allowing for direct switching. What’s important is that the control of process parameters is closely related to the operating condition of the equipment. Truly qualified inert gas should have CO+H2 levels of ≤5% and O2 levels of ≤0.5%, in order to ensure safety during maintenance, replacement tasks, and welding operations. 1.2 Process principle: The fixed-bed reactor uses coal as the raw material and air as the gasification agent to produce high-quality inert gases. Main reactions: 2C + O2 = 2CO ①; C + O2 = CO2 ②; CO2 + C = 2CO ③. In the blowing stage mentioned above, reactions ① and ② are the main reactions in the oxidation zone, while reaction ③ is the main reaction in the reduction zone. According to the reaction principles, to obtain more CO2 and reduce the formation of CO, it is necessary to operate at low temperatures, with a low carbon layer, and over short cycles. Low temperatures can increase CO2 levels and reduce the reduction reaction of CO2. However, the temperature should not be too low, as excessively low temperatures can lead to incomplete combustion of the O2 in the air, resulting in an increased oxygen content in the inert gases. Temperature control: Generally, the rising temperature should be between 190–220°C, while the falling temperature can be between 250–280°C. The low-carbon layer can increase the air flow rate, thereby reducing the contact time between the gas and carbon. However, if the carbon layer is too low, it is prone to cause furnace overturning and incomplete combustion with the O2 in the air; it is advisable to keep the effective height of the carbon layer between 1800 and 2200 mm (it is possible to set it 200 mm lower than usual). If it falls below 1600 mm, the inert gas becomes unstable, resulting in inconsistent performance. From a demand perspective, it is better to have as few products as possible for reactions ① and ③. The longer the blowing time, the higher the temperature inside the furnace; as a result, the CO2 content in the blowing gas gradually decreases while the amount of CO produced through reduction increases. Therefore, the cycle time should be kept between 120 and 150 seconds – it should not be too long. During the purging phase, the time should be set to 5–10 seconds. Since the purging process during inerting belongs to the venting stage, it is used to discharge the gases with a low furnace temperature and high O2 content at the beginning, and it also serves to completely displace CO+H2 from the secondary blowing phase. As air continues to flow in, the furnace temperature gradually rises; the purging time is adjusted based on the current furnace temperature and by analyzing the O2 content in the inert gas. 2. Process Operations 2.1 Shutdown and Inerting: Before shutting down, select 2–3 normal furnaces from among all the furnaces to be maintained; (if an analysis of the purge gas is conducted on these furnaces prior to shutdown, it will be easier to make the necessary adjustments.) ) When reducing the number of machines and the volume of production in the subsequent section, do not shut down the furnace; instead, switch to the inert gas generation mode and release all the gas. Analyze the data and make adjustments in advance to buy time for preparation. Those with experience can also determine the amount of steam used and the furnace temperature by observing the gases being released. During the inerting process, steam is blown from above and below to stabilize the vaporization layer and reduce the furnace temperature; therefore, the steam pressure entering the furnace should be higher than that under normal operating conditions, and it should not be lower than 0.1 Mpa. No additional adjustment is required for the valves used for blowing steam from above and below, and it is advisable to keep the temperature at the upper part 20°C lower than that during normal gas production operations. Sampling and analysis should be carried out continuously without interruption, in order to stabilize the process conditions as quickly as possible, make timely adjustments, and minimize the production time. Since the temperature of the inert gas generation layer during parking is normal, it is easy to produce qualified inert gas, and compliance is often achieved already with the first analysis. 2.2 Driving-in inerting: If the shutdown period exceeds 15 days, driving-in inerting must be carried out about 6 hours in advance to prepare the furnace; for shorter shutdown periods, the preparation time can be reduced accordingly. If the gas generator is to have its slag refilled and ignited, even more preparation is needed in advance. Only when the effective carbon layer reaches a certain temperature can qualified inert gases be produced. The most important factor is the temperature at the lower level, which must remain within the specified normal range; it is preferable if it is 20°C higher than that during shutdown idling, and the wind speed should not be too high. The air volume at the fan outlet can be controlled based on the O2 content in the inert gas; the required air volume is approximately 50–65% of that during normal gas production. The blowing time can be appropriately extended for temperature control, with a cycle time of 150 seconds being ideal. Generally speaking, in inert gas generated by shutdown, the CO level tends to exceed the permissible limits, while in inert gas generated by startup, the O2 level tends to exceed those limits; this is mainly due to the temperature inside the furnace. 2.3 Reasons for gas non-conformity: 2.3.1 Main reasons for high CO levels in inert gases: ① Excessively high furnace temperature ； ②、Excessive blowing time ； ③The total steam pressure is too low ； ④、the carbon layer is too thick ； ⑤The medication has expired. 2.3.2 Reasons for high O2 content in inert gases: ① The downward valve does not close properly, allowing air to take a shortcut ； ②The furnace temperature is too low, resulting in incomplete combustion of the O2 in the air ； ③The carbon layer is too low, or the furnace conditions are poor, resulting in furnace overturning ； ④Air got in during sampling ； ⑤The drugs are substandard. If the analysis of the O2 content in the inert gas continues to show unsatisfactory results, it must be a problem with the valve, and action should be taken immediately. 2.4 Precautions: ① The furnace operation should be adjusted as needed; measures such as reducing air supply or lowering load can be taken in advance if necessary ； ②Given the accuracy and timeliness of the analysis, two sets of analytical instruments and four sets of sampling tools should be prepared, and the personnel responsible for sampling must be skilled workers ； ③Ensure that the moisture and ash content of the coal fed into the furnace are within the specified limits ； ④Before starting the inerting process, it is necessary to ensure the sealing performance of the valve as well as its reliability in reaching the correct position. ⑤During the inerting process, only the person in charge of supervision is responsible for adjusting the parameters; the others may only give suggestions and are not allowed to make arbitrary changes ； ⑥During the production process, detailed records should be kept of parameters such as temperature, steam consumption, fan airflow control, and air pressure throughout the adjustment process; these records serve as a basis for gaining experience, enabling faster and better performance in future instances.