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In-depth analysis of three new issues in the development of gas generation technology in coal chemical industry. Author/Source: Tian Shouguo (Fertilizer Factory, Yiyuan County, Shandong Province, 256100). Date: 2009-2-2 -------------------------------------------------------------------------------- 1. Two problems related to the height-to-diameter ratio of gas generators. 1.1 The height-to-diameter ratio of gas generators. Regarding this issue, many experienced professionals in the industry have expressed their opinions; they share the same views but propose different theoretical values. This move has played a significant role in prompting industry professionals to pay attention to the height-to-diameter ratio issue and to take action to address it. The author has also participated in discussions on this issue, believing that a reasonable value is needed for the height-to-diameter ratio, but this value will not be an absolute one. A reasonable value for the height-to-diameter ratio is defined as one that is suitable for the material properties of most enterprises, the capacity of the fans, the load requirements for individual furnaces, the capabilities of the thermal network, and that can adapt to the level of operation. This value is a combination of empirical and theoretical figures, and it is not based on mere speculation. At present, for most enterprises under the existing conditions, the reasonable height-to-diameter ratio for gas stoves is 1:2. The starting and ending points of the calculation are from the bottom seal of the water jacket to the top cover of the furnace; thus, the value obtained by multiplying the inner diameter of the water jacket by 2 represents the appropriate value. However, this value is not absolutely suitable for all enterprises; it is not appropriate for those with special raw material properties and limited system support capabilities. Due to the special nature of the raw materials, it is not feasible to control the high-carbon layer; therefore, it is unnecessary for the furnace height to be too great, as this would have negative effects. In recent years, only after it became apparent that the height of gas stoves was insufficient due to certain expansions carried out on them, was the term height-to-diameter ratio applied to gas stoves. An unreasonable height-to-diameter ratio restricts the thickening of the carbon layer, resulting in a bed structure with a wide radial extent but insufficient height. Such conditions are unfavorable for the distribution of the gasifying agent; it is extremely difficult to optimize the air distribution design of the grate, making it hard to increase the gas production capacity. High-efficiency fans cannot operate at full capacity, which in effect limits the production capacity of coal gas furnaces. Therefore, the issue of height-to-diameter ratio has received considerable attention. In recent years, most companies have carried out height-increasing renovations on the gas burners installed at the time of plant construction; however, due to limitations imposed by the factory buildings or insufficient renovation efforts, the heights of these gas burners vary. For both the expansion of old systems and new chemical engineering projects, the height-to-diameter ratio of the gas furnaces used can reach reasonable values, and the benefits resulting therefrom are evident. Over the past few years, in the process of upgrading old gas furnaces (referring to those of low height that were built at the time of construction or put into use in previous years), many companies have changed the location of the upward outlet of the gas furnace from the side to the top. This creates the conditions for increasing the height of the carbon layer. Similarly, the carbon layer height can also be increased to a value close to that of a gas furnace with a reasonable height-to-diameter ratio. For example, in a gas stove whose furnace body is raised by 300 mm, the total height H of the furnace body is 4,325 mm. By modifying the blocking of the rear outlet for ejection, the carbon layer can be raised significantly. Increasing the height of the gas furnace by 300–500 mm allows the effective carbon layer to be increased to 2,300–2,400 mm. The gas stove of the old system can achieve even greater adjustability by fully utilizing the potential of the space above the furnace, changing the location of the upward outlet, and increasing the height of the carbon layer – an adjustability that is even greater than that achieved by raising the height of the furnace itself. For gas furnaces with a reasonable height-to-diameter ratio, the effective carbon layer height should generally be maintained between 2,600 and 2,700 mm. Solving the problem of high height-to-diameter ratio is aimed at creating conditions to increase the height of the carbon layer, as well as the volume and heat capacity of the contents within the furnace, thereby providing a foundation for further enhancing the gasification intensity and the thermal conversion rate. However, considering the actual application conditions of some enterprises at present, there are still vague understandings. For example, a certain plant has two gas generation systems, one new and one old. The new system features a reasonable diameter ratio, while the length of the feeding cylinder in the homemade simple coke feeder is 2,400 mm, which increases the height of the furnace and thus expands the space above it. This phenomenon exists to varying degrees in some enterprises; in some companies, the capacity of the fans limits the thickness of the carbon layer, while in other companies the airflow is reduced in order to thin out the carbon layer. Of course, the operating concept of the latter is incorrect, while the former fails to fully utilize the equipment’s potential due to inadequate supporting facilities; efficient fans that suit the conditions should be installed or used instead. The design of the gas generation system should be based on the principle that the performance of all auxiliary equipment must meet the requirements of the gasification conditions for gas furnaces. 1.2 The height-to-diameter ratio issue of the jacketed boiler: Due to the low gasification intensity of the batch coal gasifier used in the initial design, the height of the jacketed boiler (hereinafter referred to as the jacketed boiler) is relatively low (it accounts for only 45% of the total height of the furnace). In today’s environment, where the production load per furnace is high and the gasification intensity is high, the design principle for the height of the cladding should be that the greater the gasification intensity and the worse the raw material conditions, the higher the cladding height must be. Moreover, as the overall height of modern gas stoves is increasing, the height of the pot holder should also increase proportionally; otherwise, the height of the pot holder will limit production capacity. Some companies today continue to use low-quality coal over the long term, and attempt to increase the boiler drum height to address the issue of slag accumulation on the walls. Currently, the clamping height of some enterprises has been increased to 2,645, 2,745, and 3,000 mm respectively, and those that took the bold step to do so have reaped benefits from these improvements. The original design height of the kettle could only meet a gasification intensity of ≤1,000 m3/(m2·h) in the 1970s and 1980s. At present, the gasification rate of coal furnaces ranges from 1300 to 1500 m3/(m2·h). At present, for the gas furnaces in the Ø2,000 mm series in the small nitrogen fertilizer industry, over 90% of them still have a pot clamp height of 2,134–2,345 mm. Under heavy load, even slight changes in operating conditions or raw materials can lead to scaling. Once sticking to the wall occurs, gas deviation, a decrease in gas generation rate, and an increase in coal consumption are inevitable consequences. Enterprises with poor coal quality resort to reducing air supply and lowering the coal layer thickness to maintain production, while those with heavy gas supply demands must shut down the furnaces twice a month to remove deposits from the walls. For most enterprises, with the improvement in gas generation levels, it is easier to maintain heat balance, while the problem of fouling on the walls remains difficult to resolve. Today, automatic coal feeders are widely used, which increases the difficulty of dealing with coal sticking to the walls. To ensure safety, it is necessary to keep the load at a lower level and increase the steam consumption; a conservative operating mode is adopted, and traces of underutilizing the furnace can be seen to some extent. Ultimately, it is actually the excessively low height of the burner that limits the full potential of the gas stove. In particular, gas furnaces with a reasonable height-to-diameter ratio have greater potential to be limited by the pot clamping height. From the perspective of industry demands, companies with high-quality coal need to increase the height of their boilers in order to take full advantage of their advantages and enhance gas production, while those using lower-quality coal must overcome the practical operational challenges they face in order to boost their production capacity. For most enterprises, increasing the thickness of the cladding is a necessary measure to cope with future raw material diversification and declining coal quality. It is also a measure to reduce operational complexity, increase gasification intensity, and ensure the furnace can operate under high loads for extended periods of time. There is also the issue of determining a reasonable value for increasing the height of the clamping pot; this was determined by analyzing the conditions of those enterprises that have already carried out such modifications, as well as by considering the raw material conditions of most enterprises and their future trends, while taking into account the overall system requirements of these enterprises. It is considered that the reasonable height-to-diameter ratio for the kettle is 1:1, that is, the inner diameter of the kettle to its reasonable height. 2 Issues related to the thermal conversion efficiency of gas furnaces (1) It must be acknowledged that measures such as the expansion of the gas furnace’s diameter, modifications to the height-to-diameter ratio, and proper control of temperatures and loads during operation are all aimed at effectively improving the thermal efficiency of gas furnaces. The technical renovation of gas stoves starts with enlarging the diameter of the stove. The fundamental purpose of this modification was to increase the gasification cross-sectional area, thereby increasing the volume of material within the furnace and thus expanding the amount of material involved in the gasification and related reactions, with the aim of increasing the gas production per furnace. At that time, there was no consideration given to ways of reducing the temperature of the gases exiting the furnace, nor to using a higher carbon layer to retain more heat within the bed; as a result, the greater the diameter, the lower the height of the carbon layer became. In fact, the initial expansion modification was merely aimed at concentrating and increasing the amount of raw material used for gasification, thereby raising the gas production per furnace and enabling fewer but more frequent operations of the furnaces. However, using high-capacity equipment to gasify the raw materials collectively is itself a measure to improve thermal efficiency. (2) The modification of the height-to-diameter ratio of the gas furnace was undertaken after a period of operation with a low carbon layer and a large cross-sectional area, during which it was found that the results were not satisfactory. It is an effective measure taken to further improve the gasification intensity and thermal efficiency, while also enhancing the gasification intensity. In production practice, if the height-to-diameter ratio is used appropriately, its advantages can be fully utilized, with the upper and lower temperatures being significantly lower than those of untreated gas furnaces. This is the manifestation of the improved thermal efficiency. However, traditional views once undermined some people’s confidence in modifying the height-to-diameter ratio of gasifiers, as it was believed that increasing the CO content in the blast air for the carbon layer would lead to higher levels. It is puzzling that, for many years, efforts have been made to reduce carbon emissions and maintain wind speeds in order to lower the CO content in the blast air, yet the coal consumption per ton of ammonia produced has not decreased ； Conversely, production capacity increases and coal consumption per ton of ammonia decreases. From the perspective of the working principle of gas stoves, the wind speed is determined by the capacity of the fan; the concepts of reducing carbon deposits and maintaining a constant wind speed are incorrect guiding principles. Today’s efficient fans have solved this problem, yet some people still hold old-fashioned ideas. Even after the carbon layer is increased, the high wind speed achieved when the low-carbon layer was under control can still be maintained and even exceeded. This is the fundamental difference between solving the problem by leveraging the capabilities of supporting equipment and by sacrificing the gasification conditions. By taking advantage of the high height-to-diameter ratio, increasing the thickness of the carbon layer is equivalent to expanding the gasification reaction area in a vertical direction, which is akin to creating conditions for enhancing gasification efficiency by changing the operating methods – an effect equivalent to that of increasing the furnace diameter through additional investment. Of course, applying this method to large-scale furnaces can also maximize the return on investment for the renovation. (3) Regarding the issue of increasing heat loss by raising the pot holder height. One view is that any increase in the heat exchange area leads to increased heat loss. Another view is that while increasing the pot height does indeed lead to a certain amount of heat loss, it creates conditions for improving the gasification intensity, resulting in higher gasification intensity, optimized gas composition, and lower residual carbon content. Moreover, the elevated part of the clamping pot is located in the dry distillation zone; under normal conditions, the heat conduction capacity in this area at 500°C is much lower than that in the oxidation zone around 1,300°C. Previously, experts have conducted quantitative analyses of the amount of increase or decrease, proving that it is small. Every technical improvement related to gas production comes with its own advantages and disadvantages. If the height of the furnace chamber is not increased according to the requirements of the vaporization conditions, the load per furnace is limited. If the conditions for large-capacity, high-load gasification cannot be met, the primary limitation is the inability to increase the coal layer height and production load. It can be said qualitatively that increasing the pot holder height indirectly improves the thermal efficiency of the gas stove, rather than merely increasing heat loss. (4) Concept regarding the selection of upstream and downstream gas temperatures. The ideal condition is that both the upper and lower gas temperatures of the gas stove are not high, and this has been the goal pursued for many years. Methods to reduce the upper and lower temperatures: ① Reduce the load by using a lower furnace temperature and a thinner fire layer ； ②Create conditions for heat storage, storing more heat within the bed under high loads. The former has been proven to be unsuitable through production practice, actually because the current production load per furnace does not permit it. In the mid-1990s, a new operating method broke away from the long-held convention that the rising temperature had to be higher than the falling temperature. It also overcame the standard parameters that were in use at most companies at the time – namely, a rising temperature of 350°C (with about 40% of companies actually using temperatures above 350°C) and a falling temperature of 200–250°C – and set the new parameters at a rising temperature of ≤300°C and a falling temperature of around 350°C. This operating method was suitable for the actual conditions at that time, where small nitrogen fertilizer plants were gradually expanding their production capacity, the production load per furnace was increasing, and the poor quality of coal made it urgent to increase the gas generation rate. After some enterprises adopted it, the gasification conditions improved significantly, and the gasification intensity increased markedly. It was soon adopted by most companies in the small nitrogen fertilizer industry. This operating method addresses the root causes of poor coal quality that leads to easy pulverization, as well as the problems that arise when the load increases, such as overheating, ash sticking to the walls, ash being blown away, and difficulty in increasing the load; it focuses on solving the issue of the fire layer being positioned too high. With the increase in downstream temperature, higher requirements are placed on the heat resistance, crack resistance, and wear resistance of the grates and furnace bottom equipment, which has led to an overall improvement in their design quality, manufacturing standards, and material selection. However, after more than 10 years of development, given the current equipment conditions, this operating method should be re-evaluated. Today, many gas stoves have a reasonable height-to-diameter ratio, resulting in increased mass and heat capacity. Technologically and mechanically, the conditions required to reduce the heat carried away by the gas, which have been sought after for many years, are now available; therefore, a scientific approach is needed when using these gas furnaces to improve the downward temperature control method. From the perspective of energy savings in gas stoves, it is necessary to create the conditions necessary, and once they are available, to minimize and reduce the sensible heat carried away by the gas, thereby improving thermal efficiency. Meanwhile, some companies continue to use the same operating method even when conditions permit, still requiring a cooling temperature of 350°C. If the upstream temperature is reduced to below the temperature of the steam entering the furnace, the downstream temperature still fails to reach 350°C; in such cases, it is considered that the modification was not successful. In fact, the resistance of the ash layer is not proportional to its thickness; it also depends on the slag formation rate, as there is a significant difference in air permeability between slag and ash. When the gas furnace with enhanced carbon layers reaches the required load, the lower and upper temperatures being lower than before the modification is a sign that its advantages are being fully utilized; this is evidence of the improved heat efficiency resulting from the enhanced heat storage capacity, and it also creates conditions for further increasing the load. For gas furnaces with a reasonable height-to-diameter ratio, determining the downward temperature should be adjusted to meet the requirement of controlling the position of the combustion zone; the method of drastically reducing the ash layer to maintain an upward temperature can no longer be used. Since the upward temperature has decreased due to the increased bed thickness and improved heat storage capacity, the operation method with a high downward temperature remains useful for gas furnaces whose height-to-diameter ratio has not been changed. 3 System resistance issue: \"Reducing system resistance and increasing gas generation volume\" is the goal pursued by enterprises. In the annual major maintenance plans or technological upgrade plans of the vast majority of enterprises, there are plans to reduce gas generation equipment, simplify processes, and thicken process pipelines. This work should be analyzed, summarized, and studied in detail. At present, there is no widespread consensus on this issue. First, it is necessary to analyze the overall situation: the technological upgrades carried out by different enterprises are not synchronized, which results in significant differences in the level of technical equipment among these enterprises. Some enterprises indeed still face problems of excessive systemic resistance that restricts their production capacity, while those that have advanced quickly in reforms have gone too far in this regard; this is also an objective reality. A few years ago, the author proposed that in system upgrades, the blower system and the gas system should be considered separately. Reducing resistance in the blower system helps to maximize the performance of the fans, providing a certain degree of compensation for companies that have fans with limited capacity. The main goal is to create conditions for high air volume and short blowing times. At present, after some enterprises have carried out extensive reforms to reduce resistance in their gas systems and brought them into operation, they have found that it is extremely difficult to control the furnace conditions. Local scarring, caking, and uneven ash distribution occur frequently; regardless of the load level, it is hard to maintain stability in the furnace operations, resulting in a decrease in gas production and an increase in coal consumption. Such results left these companies puzzled, as to explain the increase in gas output, investments were made to thicken the gas pipelines to Ø800 mm, and corresponding resistance-reduction modifications were carried out on the main gas pipes and the ballast of the gas holders. This gasification method relies on constant pressure; even slight changes in the pressure and pressure difference within the furnace have a significant impact on the gasification conditions. This is the issue of determining the optimal balance point for gas generation resistance. Excessive resistance leads to low gas generation efficiency, which is not suitable for today’s demands of intensified production; on the other hand, too low resistance introduces new problems. Changes in system resistance can be detected, firstly, through the pressure gauge of the single furnace system, and secondly, via the pressure difference at the top and bottom of the furnace. System resistance increases, the pressure difference decreases, and the pressure rises ； The resistance decreases, the pressure difference increases and the pressure drops, especially the pressure in the back-gas direction is significantly low. Of course, this pressure difference is also affected by the furnace conditions, the height of the carbon layer, and the particle size of the raw materials; careful attention must be paid when observing in order to find representative and accurate values. An excessively low resistance has a significant impact on the distribution of steam into the furnace, which is different from the blowing stage. Since 1 m2 of air is required to produce 1 m2 of semi-water gas, the volume of air passing through the carbon layer during the 20%–30% of the cycle time must be equal to the total flow rate of the medium during the remaining time periods. The flow velocity and volume of the air entering the furnace are more than 4 times those of the steam; therefore, once the air enters the carbon layer, it has a strong diffusivity and is capable of penetrating areas with high resistance. The gas production stage is quite different; in this stage, the steam flow is minimal and the flow rate is slow. Under conditions of low system resistance, there is a large pressure difference at both ends of the gas furnace, and the resistance within the bed layer is low, resulting in a fast steam flow rate. As a consequence, the steam stays in the carbon layer for a short time, and a low decomposition rate is inevitable. What’s worse, the steam flow rate is low; as a result, steam diffusion and penetration are poor, and in areas with high resistance, less steam enters, preventing the endothermic reactions on the bed surface from proceeding in equilibrium. This results in an uneven vapor distribution across the bed cross-section, sharply increasing the likelihood of localized overheating and sintering, especially when the particle size of the raw material is inconsistent. Uneven steam distribution leads to an uneven heat distribution within the bed layer, and the re-injection of air continues to enter the high-temperature areas due to its strong diffusivity and penetration ability, where it undergoes reaction and releases heat; as a result, localized sintering is inevitable. After the resistance to gas generation was reduced, there were also changes in the steam entering the furnace. In one company, after the resistance in its gas system was lowered, the opening degree of the steam valve handle returned to its level before the modification. The pressure of the external high-pressure steam remains unchanged, at 0.1 MPa as per the design specifications. During operation, it was observed that the flow rate of the external steam increased significantly; the furnace temperature was difficult to raise, the carbon content in the low-temperature slag was high, and the volume and quality of the gas were far inferior to those before the modification. Local scorching occurred, and the operational flexibility was extremely low. This is a manifestation of the effect of excessively low gas generation resistance. By reducing the steam pressure entering the furnace, the steam velocity and flow rate decrease, as does the diffusion force; in order to achieve equilibrium in terms of heat, the steam pressure is adjusted to 0.06 MPa, after which the furnace temperature rises to the desired level. 4 Conclusion: Those in the gas production industry should reflect on the past and approach the development of gas production technologies with a scientific, pragmatic, innovative, proactive, and responsible attitude, focusing on innovation and striving for technological progress. Lacking innovative thinking means merely repeating the past, which hinders technological progress and the development of industries. Through the hard efforts of several generations in China, intermittent gasification furnaces have moved beyond the technical limitations of their initial development stage; however, their current state does not mean they are fully perfected, and it is up to our generation to complete this task.