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The purpose and significance of gas furnace renovation: In recent years, due to the high prices of oil in the international market and the rapid development of domestic industry, the coal chemical industry in China has seen growth, which in turn has promoted 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 the gas furnaces so that the diameter is larger at the bottom and smaller at the top, while others make it larger at the top and smaller at the bottom... So, how should one choose a gas furnace? How should a fixed-bed gas furnace be modified correctly? It is indeed time for 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 reforms, when 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 be forced to use lower-quality raw coal; in such a situation, conventional fixed-bed gasifiers have no viable solution. Secondly, the objectives and directions of the technical transformation of gas stoves must be clearly defined. First, clarify the problems existing in the technology of existing gas stoves. Avoid blind technological upgrades, which result in the expenditure of human, material, and financial resources without yielding any results. Special care should be taken to avoid keeping one’s own \"damaged teeth\" while having the \"healthy teeth\" extracted. Secondly, it is necessary to objectively evaluate the better furnace types, avoiding the tendency to overlook all flaws just because of one advantage. By adopting a balanced approach, taking the best aspects and discarding 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. Different enterprises operate in various regional environments, have different production processes, and are in varying states of operational performance; as a result, the technical upgrades required for their gas furnaces also differ. 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 they are copied mechanically without considering practical considerations. 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 fertilizers to one based on using chemicals to produce fertilizers. The above-mentioned different situations require varying technical upgrades for gas stoves. Below, specific technical renovation methods for gas stoves are discussed in relation to several typical scenarios. 2 Ideas for technical upgrades of gas stoves 2.1 Main problems existing in various gas stoves The ф3.6m gas stove was introduced to China by the former Soviet Union in 1958. Since the original design used metallurgical coke as raw material, historically 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 coal pieces, and burning briquettes, especially coal sticks, is not suitable. The measure for the above situation is to modify the ash discharge system. The ash plow designed in the former Soviet Union is adjustable; the screw and the ash plow extend 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 lump coal; the \"angle of repose\" for these small coal lumps is only around 28°, whereas the discharge angle designed for the ф3.6m gas furnace is 38°. This angle prevents the ash residues from sliding away automatically, but it allows the small coal lumps behind the ash plow to slide easily, which in turn prevents the gas furnace from operating stably. 2.2 Control of the air velocity entering the furnace: The ф3.6m gas furnace was originally designed to use metallurgical coke, and the primary air velocity at the center tube during blowing was originally set at 10 m/s. 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 away. If the carbon layer is increased, it causes high blowing resistance, limiting the volume of primary air. Therefore, the central tube of the ф3.6m gas stove, as well as its grating gas chamber and ventilation ducts, need to be technically upgraded; otherwise, it cannot be used for burning low-quality coal or briquettes. Similarly, for gas stoves with a diameter of ф2.6m, ф3.0m, and ф3.2m, it is also necessary to control the air flow rate into the furnace when burning low-quality coal or briquetted coal. According to calculations, if the wind velocity reaches 5000 m3/(h·m2), the wind speed at the grate of a ф2.6m gas furnace will be 14.78 m/s, that of a ф3.0M gas furnace will be 21.6 m/s, and that of a ф3.2M gas furnace will be 24.69 m/s. When using low-quality coal or briquetted coal in a gas stove, it is necessary to have both sufficient blowing force 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. This thus imposes new requirements on the design of the gas stove’s air ducts, central ash box, grate gas chamber, and grate ventilation channels. This is also a key aspect of the technical upgrades we need to carry out on gas stoves. It should be noted that by resolving 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. 2.3 Flow velocity and direction of the blast air exiting the carbon layer While considering the flow velocity of the primary air through the charging grates, it is also necessary to address the flow 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 materials, 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 blown over. Gas production using low-quality coal and briquetted coal results in poor mechanical strength and thermal stability, which in turn leads to uneven pore structure 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 while controlling the blowing air velocity in the carbon output layer at ≤2.8 m/s; at the same time, minimizing the amount of material carried away by the blowing air is one of the key factors determining the performance of such furnaces. The gas flow rate mainly depends on the flow rate and the cross-sectional area of the flow path. In gas furnace production, it is not advisable to reduce the blowing air velocity by lowering the flow rate of primary air, as this reduces the production capacity of the gas furnace. We can only find a solution in terms of the cross-sectional area of flow. Make the gas burner in an inverted cone shape, that is, with a smaller diameter at the bottom and a larger diameter at the top, which can effectively reduce the flow rate of the blowing gas through the carbon layer. Taking a ф2.65m gas stove as an example, the explanation is as follows. If the blowing intensity reaches 5000 m3/h, the volumetric flow rate of the air in a single pass is 26600 m3/h. The airflow velocity at the coal outlet layer is 3.44 m/s. After the technical renovation of the gas furnace, the diameter at the lower part remains ф2.65m, it becomes ф2.8m in the middle section, and ф3.0m in the upper part. With the air volume remaining unchanged, the airflow velocity at the coal outlet layer drops 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. 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 the issue of the blowing air flow rate is resolved, 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 increases 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 in the blast air from being blown away, it is necessary to disrupt the flow of the blast air; by changing the direction of the airflow, the momentum of these materials is converted into potential energy, causing 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. At present, there are basically two types of roof designs for various furnace types in our country: one is a flat roof, and the other is a dome-shaped roof. Relatively speaking, a flat roof is more conducive to the settling of materials blown out by wind than a dome. The main reason is that the dome structure does not facilitate the formation of flow deflection. Therefore, there is excessive carryover, making it difficult to stabilize the blowing air. Upon investigation, in most enterprises across the country, the gasification efficiency of domed-structured gas stoves is generally lower than that of flat-roofed gas stoves. In recent years, some companies have installed the upper airway 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 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. 2.4 Slag discharge and removal issues in gas stoves Another important issue is slag discharge and removal. Low-quality coal has a high ash content and a low ash melting point, which objectively requires gasifiers to have a high capacity for slag breaking and removal; otherwise, high-load operation is not possible. Slag breaking and removal in gas stoves is a system-related issue that cannot be resolved solely by using grates. It also requires that other components designed to work in conjunction with the furnace grates be reasonably structured. Such as: ash discharge port, ash plow, ash tray, jacket, slag breaking plate, etc. In particular, the integrated design of the ash discharge ports and ash trays is often overlooked, or the original rational design is disrupted during the renovation of gas stoves, resulting in a new irrational design. The high point of the ash discharge port and the edge of ash discharge in the ash hopper form an angle, 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 carbon slippage in gas stoves and the 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 \"slag discharge angle\" being greater than the \"angle of repose\" of the ash and slag. 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 port inside 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 carbon slippage 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°; in some cases it is even above 70°. This is one of the main reasons for problems such as flow of coal inside the furnace, slippage of coal, and collapse of coal layers. 2.5 Operating method of the floating vaporization layer: The original ф2.6m gas furnace 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 “slag discharge angle” is already greater than 50°. To prevent carbon slippage, the slag-breaking strip is designed as a trapezoid that is wider at the bottom and narrower at the top; this not only enhances slag breaking in conjunction with the furnace grates but also effectively reduces the slag discharge angle. When the slag-breaking strip wears down to a certain extent, carbon flow 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 slag 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 from the gas furnace ； A high temperature in the gasification zone makes scarring more likely, leading to a deterioration in furnace conditions. 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 controlling the velocity of the primary air flowing into the furnace and the condition of the primary air flow 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 of the vaporization layer, which could otherwise lead to scarring and caking. Although this \"floating gasification layer\" approach 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 and 300°C, while when the \"floating gasification layer\" method is used, the grate temperature rises to between 200 and 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 beneath the furnace must be raised. Under these circumstances, new challenges have arisen regarding the technical modification of gas burners: components such as the burner 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.