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Analysis of the structure and technology of several fixed-bed gasifiers Analysis of the structure and technology of several fixed-bed gasifiers At present, there is great activity in the technological upgrading of fixed-bed batch gasification gasifiers. Successful technological upgrades are intertwined with those that yield no results or even have the opposite effect, creating a complex and confusing situation. How can one use a discerning eye to see through this chaotic array of technological upgrades, helping businesses avoid detours and achieve the best results in the shortest possible time? We must conduct a comprehensive comparative analysis of several gas stoves in terms of their basic structure and manufacturing processes. There are many types of fixed-bed batch gasification coal gasifiers in our country. φ1.98m, φ2.24m, φ2.4m, φ2.61m, φ2.65m, φ2.8m, φ2.74m, φ3.0m, φ3.2m, φ3.3m, φ3.6m, φ3.8m, and so on; however, there are basically only three types of furnaces: φ1.98m, φ2.745m, and φ3.6m. All other furnace types are derived from these three basic types. I. Process operation from the structural relationship between furnace diameter and ash pan diameter. The diameter of the ash tray for the φ1.98m gas stove is 2820 mm; the difference between this value and the relevant parameter is: (2820 – 1980) ÷ 2 = 420 mm. For the φ2.74m gas stove, the diameter of the ash tray is 3360 mm, and the difference is: (3360 – 2740) ÷ 2 = 310 mm. In the case of the φ3.6m gas stove, the diameter of the ash tray is 4440 mm, with a difference of: (4440 – 3600) ÷ 2 = 420 mm. Why is the diameter of the ash tray for these gas stoves 400 mm larger than one side of the furnace chamber? (The ash tray of the φ2.74mm gas stove is designed as a concave groove to prevent carbon flow); this is because the materials used, the operational principles, and the amount of ash generated during gasification are roughly the same for several types of gas stoves, and a ash discharge opening sized at 300mm is sufficient for production purposes. In accordance with the requirement of a \"stacking angle of repose\" of 35° for the material ash, the ash tray should be about 400 mm wider than one side of the furnace chamber. It is clear that the original designs of these several basic furnace types were scientific and rigorous. However, over time, several basic furnace types underwent technical modifications in production practice. The furnace chamber of the φ1.98m gas stove was enlarged to φ2.24m, φ2.4m, φ2.61m, 2.65m, until it reached φ2.8m. Because during the diameter-expansion renovation, the ash tray and furnace bottom were hardly enlarged. To prevent the occurrence of flowing or collapsing coal, corresponding technical improvements such as \"trapezoidal breakage strips,\" \"flow prevention plates,\" and \"fake ash trays\" have been developed. In gasifiers derived from the φ1.98m gasifier, as the furnace chamber was expanded time and again, the ash tray continued to use a diameter of φ2820mm, which disrupted the \"angle of repose for ash accumulation.\" Therefore, in order to prevent phenomena such as \"flowing coals\" and \"collapse of coals\" during operation, the process parameters were gradually adjusted to increase the temperature at the bottom of the furnace, thereby raising the viscosity of the ash and the coal layer below, so that the materials could stick together and support one another, minimizing their fluidity as much as possible. This gives rise to the operating characteristic of small gas stoves: high temperature at the bottom of the stove and low temperature at the top. The advantage of this operation is a high ash formation rate, a low coking return rate, and a high gasification efficiency in coal gasifiers. This is also the main reason why the \"small nitrogen\" furnace has a high gasification efficiency and low consumption over the years. The negative effects of this operating process cause the grates and the bottom drive mechanisms to operate at high temperatures, resulting in a short overhaul cycle, generally within one year. Reasons for the short operating cycle of the Φ2.6M series gas stoves and measures to address the issue. 1. Reason: ① Design issue. a. For the gas furnace with a diameter of Φ1980 mm, the bottom of the furnace has a diameter of 3240 mm, while the ash tray has a diameter of 2820 mm; this design is continued even in gas furnaces with a diameter of Φ2650 mm. The load on the furnace interior has increased from 12 T to 25 T, meaning the load has more than doubled. As a result, the transmission system at the bottom of the furnace is overburdened and suffers severe wear. A major repair must be carried out within one year. b. The rolling guide ring design does not allow for proper positioning, and the sealing is inadequate. Oil is applied only once; no further oiling is done thereafter. The positioning system also requires lubrication, but there are no measures in place for this, which prevents long-term operation. c. The furnace rod machine has a defective design, which easily leads to water leakage in the main shaft, severely affecting the operational cycle. d. The design of the furnace bottom center positioning system is unsound, resulting in easy displacement and severe wear. The positioning system cannot be separated from the guide ring. e. A continuous oil injection lubrication system at the furnace bottom was not designed. ② Equipment manufacturing issues: the material of key components does not meet the design specifications. The guide ring should be made of 55° cast steel, and the steel balls should be made of bearing steel; however, the manufacturers actually use ball-milled cast iron and ordinary cast steel. The large gear should be made of 45° cast steel; in practice, it is made of ordinary ZG30 cast steel, as many manufacturers lack the equipment for annealing, and high-grade cast steel cannot be processed. The pinion should be made of forged steel, but cast steel is actually used. The worm gear should be made of 9-4 bronze; in reality, it is made of ball-milled cast iron. The furnace grates, ash plows, and ash trays should be made of heat-resistant cast steel, ZG30Cr7Si2; in practice, they are made of cast iron. The combined effect of the above issues results in a short operating cycle for the Φ2.6M series gas stoves. 2. Methods for solving problems: The new Φ2.6M series gas stoves retain and build upon the advantages of the original Φ2.6M series gas stoves, while eliminating their existing shortcomings. Examine the design of the Φ2.6M gas furnace from a systems engineering perspective. To maximize the process advantages of the new Φ2.6M gas furnace while achieving optimal cost-performance in terms of equipment. That is, to maximize satisfaction of the requirements for long-term operation of the equipment, without resulting in excessive performance in certain areas. It is advisable to ensure a major maintenance cycle of 24 months. This new Φ2.6M series of gas stoves eliminates the problems of \"carbon collapse\" and \"stove failure\" that plagued older models; they have a much greater ability to handle low-quality coal, resulting in an increase in output of over 10%. The maintenance cycle has been extended from one year to more than two years. The φ2.74m gas stove was also modified with an expanded furnace chamber. However, its approach is completely different from the diameter-enlargement renovation of the φ1.98m gas stove. The diameter expansion of the φ2.74m gas burner also disrupted the \"ash accumulation angle of repose,\" leading to \"flowing coals\" and \"coal collapse.\" To solve this problem, the methods of reducing the height of the ash discharge port and adding a \"fake ash tray\" are employed. As a result, the phenomena of \"flowing carbon\" and \"crumbling carbon\" were prevented, but the high-intensity production of gas stoves was sacrificed. At medium to low load levels of production, a low level of balance was achieved. The ash discharge height of the gas furnaces with diameters of φ3.0m, φ3.2m, and φ3.3m at the Zhongniu Plant is designed to be between 240mm and 280mm, which is 20–60mm less than the original design value of 300mm for the gas furnaces with a diameter of φ2.74m. The width of the ash discharge opening is only 760 mm; the ash plow occupies one-third of this space, leaving an effective width that is half that of a gas furnace with a diameter of φ2.6 m. To accommodate the aforementioned technical upgrades, the process operation had to adopt the method of operating the open Tai Ping furnace. The primary air volume is low; the blowing intensity is around 3200 m3/m2. The ash particles are relatively fine, with fewer particles of about φ200mm in size, and the rate of coke return is high. Both are diameter-expansion renovations, but the methods used to address the issues differ, resulting in completely opposite outcomes. II. Process operation based on the height-to-diameter ratio of the gas furnace: The height of a φ2650mm gas furnace is generally between 5600mm and 6000mm, resulting in a height-to-diameter ratio of 2.0–2.2 : 1. The height of a φ3000mm gas furnace body is generally 5445mm, with a height-to-diameter ratio of 1.8:1. The height of the furnace body for a φ3600mm gas furnace is generally 6225mm, with a height-to-diameter ratio of 1.73:1. Due to differences in the height-to-diameter ratio, there are variations in the blowers selected for gas stoves. For the φ2.65m series gas stoves, the D600 type fan is selected, with an air supply rate of 6780 m3/m2h ； For the φ3.0m series gas stoves, the D700 type fan is selected, with an air supply rate of 6000 m3/m2h ； For the φ3.6m series gas stoves, the D1100 type fan is selected, with an air supply rate of 6600 m3/m2h ； It is obvious that the air supply intensity of the φ2.65m series gas stoves is higher than that of the other two stove types, and their blower head pressure is also higher than that of the other stove types. In actual production, due to differences in the aforementioned conditions, there are significant variations in the blowing intensity of gas furnaces. The blowing strength of φ2.65 series gas burners is generally around 4520 m3/m2, while that of φ3.0m series gas burners is around 3800 m3/m2/h. The blowing strength of φ3.6 m gas burners is approximately 4300 m3/m2/h. Blowing air is the driving force and foundation for vaporization; obviously, in the φ3.0m series of gas stoves, since their furnace structure still relies on that of the φ2.74m gas stoves, the blowing air forms a constricted stream at the upper part of the furnace, resulting in a lower intensity of the blowing air. In the φ3.6m gas stove, part of the furnace structure is of a “dome” type; as a result, the blast air flow does not receive adequate buffering in the upper part of the furnace, and the substances carried out by this airflow fail to settle properly. Moreover, the intensity of the blast air is significantly lower compared to other types of stoves. In recent years, some people have mistakenly believed that burning low-quality raw materials or briquettes can easily cause fans that are not suitable for high wind pressure and large flow rates to fail. As is well known, low-quality raw materials, as well as briquettes, are characterized by a low calorific value and a low ash fusion point. Under such circumstances, to achieve high-volume production, it is necessary to improve the gasification efficiency. Therefore, it is necessary to increase the effective carbon layer and boost the heat storage capacity of the carbon layer, which requires an increase in the pressure and flow rate of the fan. At the same time, the height-to-diameter ratio of the gas stove should be 2:1 or more. Especially in the case of burning briquetted coal, since briquetted coal is flammable, has a high porosity, and good ventilation, it is more suitable for operations with high carbon layers and large air volumes. The new type of gas stove that burns briquettes, with a higher primary air flow rate compared to those that burn lump coal, is a clear example of this. 3. Gas furnace jacketed boilers and process analysis: The main function of the jacketed boiler in a gas furnace is to prevent the materials in the high-temperature gasification zone within the carbon layer from adhering to the walls. However, in recent years, some technical upgrades to gas stoves seem to have forgotten the original purpose of the jacket, treating it as a steam boiler whose main function is to produce large amounts of steam. If the height of the jacketed boiler is increased unrealistically to an infinite extent, even to the point of being a \"fully jacketed\" boiler. The production of gas stoves is a process of energy conversion. The law of conservation of energy is a fundamental scientific law. The fundamental goal is to use the limited energy to maximize the gasification reaction. Excessively large jacketed boilers, or even \"fully jacketed\" boilers, inevitably use a portion of the heat to produce steam rather than gas. Especially during the downward blowing phase, steam fills the upper space of the furnace; it would be necessary to increase the steam temperature in this area in order to improve the gasification efficiency. However, due to the excessively high height of the jacketed boiler, the steam temperature not only failed to increase but actually decreased. (Because the jacket temperature is lower than the steam temperature entering the furnace). This will inevitably lead to a decrease in the gasification rate during the downward blowing phase, affecting both the yield and quality of the gas. IV. Analysis of the structure and process of the cone furnace. In recent years, a \"cone furnace\" has appeared for φ2.65m gas stoves. This is undoubtedly the product of bold innovation. Practice has shown that under certain conditions, the \"cone furnace\" increases the gasification area of the original furnace type, resulting in an increase in gas production. The basic structure of the cone furnace is such that the upper part of the furnace remains unchanged; instead, the lower part of the jacketed boiler is expanded taking advantage of the opportunity for a major overhaul of the furnace, with the upper part remaining intact and connected to the original upper furnace body. The enterprise carries out renovations without altering the structure of the plant, increasing the gasification area of the gas furnace and thereby boosting production. Clearly, this technology is feasible for upgrading the original φ2.65m gas furnace. The negative impacts of this technical modification are as follows: 1. The upper structure of the gas stove remains unchanged; the stove is larger at the bottom and smaller at the top. During the blowing phase, the airflow inevitably has to accelerate inside the furnace chamber. For any raw material, there is objectively an issue of a \"limit wind speed\" at the surface of the carbon layer; if this limit wind speed is exceeded, the gas furnace will be overturned. Therefore, the \"cone furnace\" is larger at the bottom and smaller at the top, which is not conducive to the intensified production of low-quality raw materials. 2. The lower part of the cone furnace has been expanded to φ2800mm, while the ash tray is φ2820mm; this inevitably poses the risk of \"flowing coals\" and \"coal collapse\" during operation, and any process fluctuations can make it difficult to maintain control. Due to the aforementioned negative effects, it is not advisable to use a \"cone furnace\" for new gas stoves. There are no restrictions on the structure of new gas stove factories; attempting to create \"cone-shaped stoves\" would instead waste their unique advantages and resources, focusing on the wrong things. V. Structural analysis of the “double-volleyball rolling slide”. Changing the drive mechanism under the gas stove from \"sliding\" to \"rolling\" represents a significant technical advancement. The purpose of the sliding track is to minimize frictional resistance, reduce operating load, and extend the service life of the furnace bottom ash tray. “When the \"double-volleyball rolling slide\" was first introduced, it attracted the attention of many companies. After two years of operational experience, the following problems have been identified: 1. The \"frictional resistance\" of the double-row volleyball slide is greater than that of the single-row volleyball slide. It goes against the design of the sliding track. 2. The wear on the double-volleyball slides is not only significant but also increasing. Due to the rigidity of the sliding tracks, double-row volleyballs still experience the same forces as single-row volleyballs; since the diameter of double-row volleyballs is smaller, the force exerted on each ball is greater, which in turn increases wear and tear. 3. The positioning grooves for double volleyball systems can only be designed in a horizontal direction; however, the forces acting on them are a combination of gravity acting vertically downward and horizontal thrusts, which result in forces directed diagonally downward. Therefore, the design of these positioning grooves should be in line with the direction of this combined force, rather than being horizontal. It is difficult to locate in the horizontal direction. 4. Due to their already wide design, double-volleyball slides do not allow for the creation of two sealing grooves; only one sealing groove is possible, which makes it easier for dust to get in compared to single-ball slides that have two sealing grooves. Therefore, the once-famous \"double-volleyball rolling slide\" failed to stand up to practical testing and ultimately proved to be a short-lived phenomenon. In summary, the technical renovation of gas stoves should be analyzed using scientific principles, rather than following trends blindly. One should understand not only how all technologies work, but also why they work that way. Only in this way can one grasp the essence of the technical renovation of gas stoves. Understanding the essence, advantages, and disadvantages of various gas furnaces, and designing gas furnaces that suit the specific conditions of a company based on its actual situation, is the current trend in the technological upgrading of gas furnaces. Some companies have recognized this and opted for newly designed gas burners with diameters of φ2.8m, φ3.0m, φ3.2m, and φ3.3m, achieving success in doing so. The benefits are particularly evident when burning low-quality coal and briquetted coal. Practice has shown that the basic structure of a gas stove is what matters truly, while its diameter is merely a superficial aspect. By mastering the essential aspects, one can make the best use of any gas stove, whether it is large or small.