Technical renovation of fixed-bed gas furnace

Technical Renovation of Fixed-Bed Gas Furnaces (II) Technical Renovation of Fixed-Bed Gas Furnaces (II) by Zhao Lequn In recent years, due to the high prices of oil on the international market and the rapid development of domestic industry, the coal chemical industry in China has seen growth, which in turn has accelerated the development of fixed-bed gas furnaces. For new construction projects, renovation and expansion projects, upgrading of existing systems, as well as major overhauls of individual furnaces in the coal chemical industry, it is essential to give priority to the technical upgrading of the existing fixed-bed gasifiers. In the renovation of gas furnaces, some fertilizer manufacturers are replacing their existing ф2.65M gas furnaces with ф2.8M ones, while some medium-nitrogen production companies are substituting their ф3.0M gas furnaces with ф2.65M ones. Some newly built coal chemical plants opt for ф3.6M gas furnaces, and some large-scale new projects choose ф2.65M gas furnaces. Some companies modify their gas furnaces so that the diameter is larger at the bottom and smaller at the top, while others do the opposite... So, how should one choose a gas furnace? How should a fixed-bed gas furnace be modified correctly? It is indeed time to conduct rational research and analysis. Firstly, in the face of increasingly fierce market competition, a shortage of high-quality coal in the coal market, and the inherent shortcomings of various furnace types such as ф3.6M, ф3.2M, ф3.0M, ф2.65M, and ф2.4M, it is necessary to carry out technical upgrades for fixed-layer gas furnaces. Without any changes, once international oil prices return to normal and foreign chemical products flood the domestic market, domestic coal chemical enterprises will face a shortage of high-quality coal and will be forced to use lower-quality raw coal. In this broader context, there is no way forward for the existing fixed-layer gas stoves without undergoing technical upgrades. Secondly, the objectives and directions of the technical renovation of gas stoves must be clear. First, clarify the problems existing in the technology of existing gas stoves. Avoid carrying out technical upgrades blindly, which wastes human, material, and financial resources without yielding any results. Special care should be taken to avoid keeping one’s own ‘diseased teeth’ while pulling out the ‘healthy teeth’. Secondly, it is necessary to objectively evaluate better furnace types, avoiding the tendency to overlook all flaws just because of one advantage; by adopting a balanced approach to take the best aspects and discard the worst, the goal of technical improvement can be achieved. Third, it is necessary to adapt measures to local conditions and to the specific circumstances of each factory, avoiding simple copying. Due to the different regional environments of various enterprises, as well as differences in their production processes and operational conditions, the technical upgrades required for gas furnaces also vary. For example: the Shanxi coal basin and the coal-deficient areas in the south of the Yangtze River ； The monolcohol process and the dialcohol process will definitely encounter problems if copied mechanically without considering practicality. Fourth, one must be forward-looking. It is necessary to analyze the medium- and long-term trends in the raw material market, as well as the development trends of the company’s product portfolio, so that the technical upgrades of gas stoves can meet the requirements of these broader trends. For example, some companies currently use coke and lump coal as raw materials, but in a few years they will have to change their raw material source. Or in a few years, the company’s product structure will shift from being dominated by chemical fertilizers to one based on using chemicals to produce fertilizers. These various situations and changes result in different requirements for the technical upgrading of gas stoves. Below, specific technical renovation methods for gas stoves are discussed in relation to several typical scenarios. The main problems with various gas stoves. The f3.6M gas stove was introduced to our country by the former Soviet Union in 1958. Since the original design used metallurgical coke as raw material, over time there was a shift from coke to coal, with high-quality lump coal from Shanxi being replaced by lower-quality local lump coal. During this process, although some modifications were made, problems still existed in production. For example, it is difficult to maintain stability when burning small pieces of coal; burning briquettes, especially coal sticks, is not suitable. Main reason: The ash discharge system needs to be modified. The ash plow designed in the former Soviet Union is adjustable; the screw and the ash plow are inserted obliquely from above the ash hopper into the ash tray, which inevitably creates a dead corner. This dead corner is the nemesis of burning small pieces of coal. Because after the ash plow discharges the ash residues, there are fewer residues behind it and more coal fragments; the \"angle of repose\" for these small coal fragments is only around 28°, while the angle designed for discharging residues in the ф3.6M gas furnace is 38°. This angle prevents the ash residues from sliding off automatically, but it allows the small coal fragments to flow away on their own, which in turn prevents the gas furnace from operating stably. The f3.6M gas stove was originally designed to burn metallurgical coke, with the primary air flow rate at the center tube reaching 10 m/s as per the original design. The primary air enters the grate chamber, first impacting the ventilation ducts above the grates; this is where the coal layer is thinnest, and in cases where low-quality coal or briquettes are used, it is easy for the coal layer to be blown over. If the carbon layer is increased, it results in high blowing resistance, which limits the volume of primary air flow. Therefore, the central tube of the ф3.6M gas stove, as well as its grate gas chamber and ventilation channels, require technical modifications; otherwise, they cannot handle low-quality coal or briquetted coal. Similarly, for gas stoves with ф2.6M, ф3.0M, and ф3.2M capacities, the air flow rate through the burner grates also needs to be controlled when burning low-quality coal or briquetted coal. According to calculations, if the wind speed reaches 5000 m3/hm2, the wind speed at the grate of the ф2.6M gas furnace will be 14.78 m/s, that of the ф3.0M gas furnace will be 21.6 m/s, and that of the ф3.2M gas furnace will be 24.69 m/s. When using low-quality burners or briquettes in a gas stove, it is necessary to have both sufficient blowing strength and an appropriate carbon layer thickness; neither can be lacking. Under these conditions, the most appropriate air velocity through the furnace grates should not exceed 7 m/s. Thus, new requirements have been imposed on the design of the gas stove’s air ducts, central ash box, grate gas chamber, and grate ventilation channels. This is also an important part of the technical upgrades we need to carry out on gas stoves. It should be noted that by addressing the aforementioned issues, adapting to the use of low-quality coal and briquetted coal, and simultaneously optimizing the process parameters for burning high-quality coal, this technical upgrade has improved the performance of fixed-bed gasifiers as a whole. While considering the primary air velocity through the furnace grates, it is also necessary to address the velocity and direction of the blast air exiting the carbon layer. In previous gas stove designs, since metallurgical coke was used as the raw material, there was no need to consider the airflow velocity in the carbon discharge layer. Especially during blowing, metallurgical coke has a very low dust content and excellent thermal stability; the flow rate of the blowing gas through the carbon layer is not important. However, when using low-quality coal or briquetted coal as raw material, it is necessary to consider the flow rate and direction of the blast air within the coal layer. Once the primary air enters the furnace, as the temperature of the airflow rises sharply, its volume expands exponentially, and the speed of the airflow inevitably increases. When the blowing air velocity reaches 3.5 m/s, it becomes a critical velocity for high-quality medium lump coal coke, at which point it is very easy to be overturned. Gas production using low-quality coal and briquetted coal presents difficulties due to their poor mechanical strength and thermal stability, as well as the uneven distribution of pores in the bed layer; as a result, it is easier for the bed layer to be overturned when air is blown through it. It is particularly important to control the flow rate of the blast air in the carbon output layer. It has been determined that the critical flow velocity of the blast gas exiting the carbonized layer when using low-quality coal as raw material is 2.8 m/s. When designing gas furnaces, it is crucial to ensure an adequate blowing intensity, control the airflow velocity in the carbon output layer at ≤2.8 m/s, and minimize the amount of material carried away by the blowing air – all of these factors are key determinants of the furnace’s performance. The gas flow rate depends primarily on the flow volume and the cross-sectional area through which the gas flows. In coal gas furnace production, it is not advisable to reduce the flow rate of the primary air in order to lower the gas flow velocity, as this will result in a reduction in the furnace’s production capacity. We can only find a solution in terms of the flow cross-sectional area. Making the gas stove in an inverted cone shape, that is, with a smaller diameter at the bottom and a larger diameter at the top, can effectively reduce the flow rate of the blowing gas through the carbon layer. Taking a ф2.65M gas stove as an example: if the blowing intensity reaches 5000 m3/h, then the volumetric flow rate of the primary air is 26600 m3/h. The air flow velocity in the coal outlet layer is 3.44 m/s. After the technical modification of the gas furnace, the diameter remains ф2.65M at the lower part, becomes ф2.8M in the middle section, and ф3.0M at the upper part ； With the air volume remaining unchanged, the airflow velocity at the coal outlet layer decreases from 3.44 m/s to 2.58 m/s. Obviously, with such a change, it is highly beneficial for burning low-quality coal or briquettes. At the same time, if high-quality coal is still used, the air volume can be increased further. When designing the ф3.6M gas stove, Soviet engineers followed this approach, making the bottom of the stove ф3.5M in diameter, the middle part ф3.6M, and the upper part ф3.67M. After resolving the issue of the blowing air flow rate, the direction of the blowing air flow also needs to be considered. Since the blowing gas contains ash, dust, particles, and even small pieces of raw material from the coal layer, an excessive amount of such entrained materials leads to increased coal consumption and simultaneously causes the coal layer to thin out rapidly in those areas, until it is overturned. Reducing the carryover in the blowing gas not only lowers consumption but also stabilizes the production process of gas furnaces. With a constant air volume, to prevent the materials carried by the blowing air from being taken away, it is necessary to disrupt the flow of the blowing air; by changing the direction of the airflow, the inertia of these materials causes their kinetic energy to be converted into potential energy, allowing them to settle on the surface of the carbon layer at the top of the furnace. This requires the furnace top and the exhaust outlet to form a deflection angle, with the exhaust outlet being positioned appropriately below the furnace top. Currently in China, there are basically two types of furnace roof designs for various furnace types: one is a flat roof, and the other is a domed roof. Relatively speaking, a flat roof is more conducive to the settlement of materials blown out by wind than a dome. The main reason is that the domed structure does not facilitate the formation of flow deflection. Therefore, there is excessive carryover, making it difficult to stabilize with air blowing. Upon investigation, in most enterprises across the country, gas stoves with a dome structure generally have a lower gasification efficiency than those with a flat roof structure. In recent years, some companies have installed the upper air duct at the top of the furnace. Experience has shown that such modifications are beneficial for increasing the furnace height, but they are detrimental to reducing the amount of waste material carried away. If, during modification, the upper airway is inserted into the furnace by 300–500 mm, the amount of material carried out will be significantly reduced, thereby stabilizing the blowing process. Another important issue is slag and broken rock removal. Low-quality coal has a high ash content and a low ash melting point, which objectively requires gasifiers to have high strength in breaking and discharging slag; otherwise, high-load production is not possible. The problem with debris in gas stoves relates to the system as a whole; it cannot be resolved merely by using grates. It also requires a rational design of other components that work in conjunction with the furnace grates. Such as: ash discharge ports, ash plows, ash trays, jackets, slag breaking plates, etc. In particular, the integrated design of the ash discharge ports and ash trays is often overlooked, or during the renovation of gas stoves, the original rational design is altered, resulting in a new irrational configuration. The high point of the ash discharge port and the edge of ash discharge in the ash hopper form an angle, commonly referred to as the \"ash discharge angle\". At present, the “slag discharge angle” of several important fixed-bed gasifiers in China is greater than the “angle of repose” of the ash and slag. The occurrence of carbon flow in gas stoves and an increased coking rate are mostly caused by human factors. For example, the diameter of the furnace chamber in the original UGI gas stove was ф2.74M; it was gradually increased to ф3.00M, ф3.20M, and ф3.30M. While the diameter increased, the diameter of the ash tray did not increase as well. Although some manufacturers installed ash baffles in the ash hopper, this still resulted in the \"ash discharge angle\" being greater than the \"angle of repose\" of the ash. It has been determined that the \"angle of repose\" of normal slag from gas stoves is 35° when in motion and 45° when at rest. As is well known, the ash at the ash discharge opening in the furnace is in motion during normal operation; therefore, the \"ash discharge angle\" should be equal to or less than the \"angle of repose\" to prevent uncontrolled ash discharge and subsequent flow of coals during production. Under normal circumstances, if the diameter of the furnace bottom and ash tray is not increased and the height of the ash discharge opening is ≥280 mm, the slag discharge angle for gas furnaces with diameters of ф3.0m, ф3.20m, and ф3.30m is above 50°, with the highest values even exceeding 70°. This is one of the main reasons for phenomena such as slag flow, carbon flow, and furnace collapse in gas furnaces. The original ф2.6M gas stove was developed from a ф2.2M model; it was first expanded to ф2.4M, and then further expanded to ф2.61M. The gray disk has not expanded, and the “scrap discharge angle” is already greater than 50°. To prevent slag flow, the slag breaking strips are designed as trapezoids that are wider at the bottom and narrower at the top; this not only enhances slag breaking in conjunction with the grate but also effectively reduces the slag discharge angle. When the slag strips wear down to a certain extent, carbon leakage occurs in the gas furnace. The fundamental solution to this problem is to provide the ф2.65M gas stove with a matching stove base and ash tray, so as to restore its normal ash discharge angle. In gas stove production, there is a significant difference between using low-quality coal and normal coal; low-quality coal has a higher ash content as well as a lower ash melting point. In the operation of gas stoves, special measures must be taken to achieve effective control. Due to the low melting point of low-quality coal ash, the temperature in the gasification zone has to be kept low, resulting in poor gasification efficiency and no gas production by the gasifier. If the temperature in the gasification zone is too high, scorching is likely to occur, leading to a deterioration in the operation of the furnace. In recent years, through repeated trials and experimentation, it has become possible to achieve high yields with low consumption even when using low-quality coal. In addition to properly controlling the primary air flow rate entering the furnace and the flow conditions of the primary air in the carbon output layer, it is also necessary to appropriately increase the temperature of the gasification layer and prevent caking. This causes the vaporization layer to move upstream when blown from above and downstream when blown from below, creating a larger moving range. This prevents localized overheating in the vaporization layer, which could otherwise cause scarring and caking. Although this \"floating gasification layer\" method solves the problem of producing gas from low-quality coal, it simultaneously leads to an increase in the temperature beneath the furnace. Under normal conditions, the grate temperature is between 200–300°C, while when the \"floating gasification layer\" method is used, the grate temperature rises to 200–380°C. Under these circumstances, the original gas stove cannot operate for extended periods of time. The main weakness is the reduced strength of the furnace skirt, also known as the \"ash bin\", which results in a shorter service life. Therefore, to burn low-quality coal properly, it is necessary to use the \"floating gasification zone\" method to increase the gasification intensity, and the temperature below the furnace must be raised. Under these circumstances, new challenges have arisen regarding the technical upgrading of gas stoves: components such as the stove skirt (ash bin), grates, and slides must be able to withstand high temperatures of 400°C, as well as operate over long periods of time. After repeated efforts, this issue has now been successfully resolved. The technically upgraded gas stove, when burning low-quality coal, maintains a temperature of no more than 350°C at the top of the stove, while the temperature at the bottom can reach 300–400°C, fully meeting the requirements of current developments. In summary, with the evolution of circumstances and advancements in science and technology, the technical upgrading of gas stoves has entered a new phase. Only by analyzing and studying problems from a systems theory perspective can we avoid taking detours in technological transformation.

Technical renovation of fixed-bed gas furnaces (III) At present, the main types of fixed-bed gas furnaces in China include those with diameters of ф3.6m, ф3.0m, ф3.2m, ф2.65m, ф2.61m, ф2.4m, etc. In actual production, there are certain differences among different furnace types in terms of productivity, gasification intensity, raw material consumption, gas composition, and other parameters. For several years, most people attributed this difference to the different models of gas stoves, particularly attributing it simply to the varying diameters of these stoves. Therefore, many companies adopt simple technical renovation measures by removing the existing gas stoves and rebuilding them with another model. Although certain results have been achieved, it has led to high actual costs and low returns. It is not the best choice for businesses. The best choice should be based on a rational analysis and study of the gas stove models and the processes used, from a systems theory perspective; this helps to identify the weaknesses of the existing models and the shortcomings of the current processes. Technical improvements can then be made on this basis, in order to achieve maximum benefits with minimal investment. It should be particularly noted that in the existing furnace types and production processes, even those with lower energy consumption and higher production intensity have their own shortcomings and deficiencies. Copying it verbatim will require additional technical modifications to the newly installed system, resulting in duplicate investment. Now, I can analyze UGI furnace types and process flows with high energy consumption. UGI gas burners refer to the ф2.74m, ф3.0m, ф3.2m, and ф3.3m series of gas burners; their processing technologies account for over 65% of those used in medium-sized fertilizer manufacturers in China. Their production capacity is generally low, with an average gasification intensity of less than 400 kg NH3/hm2, and the coal consumption per ton of NH3 produced is above 1400 kg/TNH3. There are two main reasons for the low production capacity and high consumption: First, the process flow is unreasonable. The process flow is as follows: Blowing air: fan → orifice plate flow meter → safety butterfly valve → primary air valve → gas furnace → combustion chamber → waste heat boiler → chimney for exhaust. Gas generation in the upward direction: steam main pipe → downward pipeline → gas furnace → combustion chamber → waste heat boiler → gas tee valve → gas cleaning box → steam main pipe. Gas generation in the downward direction: steam main pipe → combustion chamber → upward pipeline → gas furnace → downward pipeline → gas tee valve → gas cleaning box → steam main pipe. Analyzing the blowing air stage of the above process flow, the blowing intensity of UGI series gas furnaces is lower compared to other types of furnaces. First, in terms of fan configuration: the D700 fan ensures that the theoretical blowing strength for gas burners is ≤6000 m3/m2h, while for gas burners with a diameter of 2.4 meters, this value is 6637 m3/m2h. For gas burners with a diameter of 2.65 meters, it is 5650 m3/m2h in some cases, and 6780 m3/m2h in other cases. In actual production, due to the design of the blowing system in UGI gas furnaces, which includes orifice flow meters, safety butterfly valves, and numerous air duct elbows, the blowing resistance is relatively high; as a result, the actual blowing intensity is lower compared to other types of furnaces. In production using medium-lump Shanxi coal as raw material, the air volume for primary ventilation in gasifiers with a diameter of ф3.0m is generally within 30,000 m3/h, with an actual ventilation intensity of ≤4243 m3/m2h; whereas in gasifiers with a diameter of ф3.6m, this value is ≥4500 m3/m2h under the same conditions, and the same applies to gasifiers with a diameter of ф2.6m. In the UGI series of gas stoves, during the upward gas generation process, steam first fills the entire downward pipeline before entering the stove for vaporization. Compared with the ф2.6m gas stove, since the latter is equipped with a downward gas valve on its downward pipeline, the upward steam does not need to fill the entire downward pipeline; it only needs to fill the section of the pipeline between the downward gas valve and the gas stove in order for the gas to enter and be vaporized. Energy can be saved in terms of steam consumption, effective gas production time, and the amount of inert gas remaining in the container. The UGI series gas stoves suffer from particularly severe waste during the downward gas generation phase. It must first fill spaces such as the coal combustion chamber and the waste heat boiler, which are of larger volume, with steam before the gas can be introduced for gasification; these spaces require about 50 m3 more volume than those of a gas furnace with a diameter of ф2.6m. The steam in this area is needlessly forced into the scrubbing tank during the secondary blowing process; this not only results in waste but also raises the temperature of the water in the scrubbing tank’s circulation system, increasing the energy consumption of the cooling tower used for that water. In terms of the downward gas generation time, due to the large volume of the combustion chamber and waste heat boiler, there is a two-to-three-second gas filling time required for each cycle, during which no effective gasification takes place. This not only reduces the production efficiency of the gas furnace but also prevents the gasification layer from being properly concentrated, resulting in higher temperatures inside the furnace. Under the same conditions, the temperature on the furnace of ф3.0m series gas burners is generally 100°C–150°C higher than that of ф2.6m gas burners. It shows that UGI gas stoves have higher heat losses compared to gas stoves with a ф2.6m diameter, and their gasification layer is dispersed, resulting in lower gasification efficiency. For enterprises that produce water gas to manufacture methanol, TDI, etc., the UGI gas furnace process results in high levels of inert gases such as N2 and CO2 in the feed gas, which has a significant impact on subsequent processes and keeps the production costs of the products high. In summary, carrying out technical modifications to the process flow of UGI gas stoves has become an important factor affecting a company’s production capacity and costs, and it inevitably constitutes a significant decision for such companies. II. Unreasonable structure of the gas stove body: 1. The stove top is dome-shaped, or the air inlet is located on the top of the stove, which hinders the separation and settlement of dust particles carried in the blowing air; as a result, more debris is carried away by the blowing air, making it easy for the gas stove to tip over. 2. An imbalance in the height-to-diameter ratio is not conducive to intensified production. 3. The air inlet at the bottom of the furnace is too small and concentrated, causing significant localized impact on the carbon layer and resulting in uneven air distribution. The slag discharge port is too small, and at the same time the angle of repose of the slag is too large, causing sludge flow. 4. The transmission system is under excessive load, resulting in severe wear. For the specific details mentioned above, please refer to the article “Medium-Grade Nitrogen Fertilizer” and “Technical Renovation of Fixed-Layer Gas Furnaces”. There are three proposed options for the technical renovation of the UGI gas furnace process flow: A. The entire UGI gas furnace system is removed, and a new set of gas furnaces with a diameter of ф2.65m along with the corresponding process equipment and pipelines are installed. B. The UGI gas stove will not be used; other equipment and pipes will be removed, and new equipment and pipes will be installed according to the new process. C. Modify the UGI gas stove: upgrade the combustion chamber, convert the waste heat boiler into a combined waste heat boiler; remove the equipment and pipes associated with the combustion chamber, and install the newly modified combined waste heat boiler along with its pipes and valves according to the new process. Plan evaluation: Although Plan A underwent thorough modifications, it retained the shortcomings of the ф2.65m gas stove as well as those of the production process. Moreover, the investment is high, resulting in a decline in overall production capacity. Option B eliminates the two main units, namely the combustion chamber and the waste heat boiler, which results in higher investment costs; moreover, the shortcomings of the original gas furnace are not completely resolved, so the production performance does not reach an ideal level. Obviously, Option C involves modifying the main equipment of the original gas furnace while retaining it, resulting in the lowest investment required for technical upgrades. In terms of performance, Scheme C, when modifying gas stoves, incorporates the advantages of the ф2.65m gas stove, eliminates the disadvantages of the original UGI gas stove, and also avoids the shortcomings of the ф2.65m gas stove ； Regarding the modification of the UGI gas furnace itself, I have already explained it in \"Technical Modifications to Fixed-Layer Gas Furnaces (1) and (2)\\", so I will not repeat it here. The modification of the combustion chamber was carried out based on the existing equipment, **saving investment ； To modify the waste heat boiler, simply replacing the core is sufficient to achieve the effect of a combined waste heat boiler. New process flows are adopted for the installation of other pipeline valves. The aforementioned Option C can be combined with the exhaust gas recovery project, with two or three furnaces grouped together for retrofitting. Currently, some companies have carried out modifications according to Plan C, with very significant results – achieving maximum economic benefits with minimal investment. It should be noted that when Scheme C is implemented, it is advisable to set the upstream gas valve to open during gas production and close at the end of that process. Many companies do not set it such that when air is blown in downward to clean the system, the upward gas valve remains open. Although this poses no safety issues, it can easily lead to the accumulation of inert gases in the pipes downstream of the upward valve, affecting the quality of the gas. For the engineering design of blast gas recovery in UGI system gas furnaces, there are three types of blast gas intake locations: 1. Above the combustion chamber or dust collector. 2. At the front and upper air ducts of the dust collector. 3. After the waste heat boiler, before the chimney valve. Among the three gas extraction processes, the second one has more disadvantages than advantages and is not easy to adopt. Main problem: The purge air, which has not been dust-freeed, causes severe erosion of the valves. Especially at high temperatures above 300°C, the strength of the valve sealing surface decreases, and the purge gas carries particulate coal coke, which can cause the valve to leak in a very short time. The first gas extraction method is suitable for enterprises that produce water gas to use. Since the process requirements call for the use of a combined waste heat boiler, it is preferable that the section shared by the blowing gas and the water gas be as short as possible, in order to prevent residual inert gases in the blowing gas from entering the gas system. The disadvantage is that the existing safety procedures need to be modified significantly; otherwise, explosion accidents are likely to occur. More than 20 destructive explosion accidents have occurred in medium-sized nitrogen fertilizer companies after the implementation of air blowing recovery systems. Main reason: Leakage in the blower recirculation valve ； There is a leak in the upstream gas valve ； After the number of gas stoves in operation decreased, the load on the exhaust fans of the air recirculation system was not adjusted in a timely manner, resulting in excessive negative pressure above the gas stoves; air entered and mixed with the gas, leading to an explosion. One way to solve this problem is for enterprises that implement air blowing recovery to operate in accordance with the new process flow standards. Firstly, it is advisable to use variable-frequency speed control motors for the exhaust fans, linking the motor speed to the desired negative pressure level, so that the blowing air system remains under slight negative or positive pressure at all times to ensure safety. The third gas extraction method is suitable for the original UGI process flow. Its advantages lie in the fact that it not only protects the existing waste heat boiler but also enables the blowair recovery valve to be used safely over long periods of time. Moreover, no major changes need to be made to the existing safety procedures. The disadvantage is: the temperature of the blowing gas is low, requiring a special design for the blowing gas combustion furnace. The technical renovation of the UGI gas stove itself and its processing procedures is very challenging; modifying and formulating new safety operating procedures after implementing air blowback recovery is also complex, and correcting operational concepts and habits is a long-term task. This is a challenging task, as well as an effective technical improvement project. May peers in the Chinese nitrogen industry work together to meet this challenge.