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Four key issues and countermeasures for stable operation under high-load conditions file:///E:/DOCUME~1/zzjjaa/LOCALS~1/Temp/ksohtml/wps_clip_image1.wmf Today, the technical level and degree of automation of fixed-bed batch gasifiers are far superior to those in the past. The level of manufacturing technology and operation control techniques have also developed in parallel. It has also become the foundation and guarantee for the effective use of new types of equipment and automatic control technologies. The enlargement and improved performance of gas stoves and their supporting facilities have also created the conditions for generating gas at high temperatures and under high loads using these stoves. The high-load gas production method has played a significant role in helping many ammonia synthesis plants run the furnaces less frequently while operating them more often, thereby improving the efficiency of heat conversion and reducing production costs. The emergence and application of each new method represent a process of replacing the old with the new. In recent years, there has been rapid development and innovation in gas generation theory, as well as advancements in process technologies. A considerable portion of the theoretical foundations and operational methods that were widely used from the 1980s to the early 1990s has been replaced by new processes and methods arising from entirely new concepts. For example, many industry professionals have put forward opposing views on hot-air gas generation technology and superheated steam gas generation technology. Gas generation technology has moved beyond the stage of exploring ways to increase the hydrogen content in semi-water gas; it has now advanced to the point where it is possible to deliberately control the methane content in semi-water gas. The operation of gasifiers has evolved toward stable operation with high gasification efficiency over extended periods of time. However, since gas stoves operating under high load conditions present greater challenges in terms of management and operational control, a set of appropriate operating methods is necessary to ensure their stable and optimal performance. Recently, it has been observed that some manufacturers experience significant fluctuations in furnace conditions when using high-load gas production methods, making it difficult to maintain stable production. After investigating the situations of several such manufacturers, it was found that in some cases, the instability is due to the characteristics of the coal used, which prevent relative stability. Another common issue is that, in terms of management and operation, the most critical problems under high-load conditions have not been identified or addressed. I often end up focusing on the wrong things; in short, I fail to identify and prioritize what’s important. We present the “four key elements” for maintaining stable furnace conditions under high-load conditions, in the hope that our peers will find them useful, and that we can work together to explore and exchange ideas in more in-depth and broader areas, striving for common progress. Stable carbon layer: On the basis of a reasonable determination of the carbon layer height, and during normal operation once all process parameters have been set, it is essential that the carbon layer be kept stable. This is a key requirement for maintaining stable conditions inside the furnace and optimizing the process parameters. Large fluctuations in the carbon layer caused by improper operational control are a major drawback in gas generation processes. During normal operation of the gas stove, if the carbon layer control falls below the specified range, the temperature rise on the upper part of the furnace will be indicated by the furnace temperature gauge; the coal addition cycle shortens, the temperature at the lower part of the furnace begins to drop, and a difference in temperature occurs between the upper and lower parts of the furnace. The volume of gas released will also gradually decrease. The principle behind this phenomenon is as follows: when the carbon layer thickness decreases, the resistance in the bed layer falls, its heat storage capacity declines; an increase in wind speed causes the flame layer to rise and thin out, resulting in greater heat loss. It reduces the amount of gas emitted. If operational errors are not corrected in a timely manner, the result will be increased coal consumption and reduced production capacity. Conversely, if the control of the carbon layer exceeds the process requirements, the established blowing rate is disrupted; the blowing resistance increases, the furnace temperature gradually drops, the slag formation rate of the ash decreases, more fine ash accumulates within the ash layer, the blowing resistance rises further, the production load declines gradually, and heat balance is lost. The quality of the gas produced also decreases as a result. It can be seen that fluctuations in the carbon layer caused by improper operation have a rather severe impact on the gasification conditions; therefore, in terms of operation and management, stabilizing the carbon layer height should be regarded as a key process parameter. However, when the height of the carbon layer is properly determined and the operational control is stable, changes in the particle size of the coal fed into the furnace can still cause variations in the bed resistance and the heat storage conditions; as a result, inadequate processing of the raw materials will complicate the gas production process further. Maintaining stability in the characteristics and particle size of the coal fed into the furnace will greatly assist in stabilizing the operating conditions within the furnace and optimizing the process parameters. Therefore, classifying the raw coal entering the plant and using it in accordance with the national standards for coal used in gas production during the processing stage is a measure that helps to stabilize furnace operation and reduce coal consumption. This increases the workload associated with processing management and raises processing costs slightly, but the benefits achieved in gas production far outweigh those increased costs. In recent years, there have been differing opinions within the industry regarding the choice of carbon layer height; supporters of high, medium, and low carbon layer heights can be found. The author believes that this important process parameter cannot be applied in a rigid, absolute manner, and that each manufacturer should determine it appropriately based on the specific characteristics of its equipment (taking into account factors such as fan capacity, raw material properties, process characteristics, pipeline resistance, and furnace design). The principle for correctly determining the height of the coal layer is to maximize the capacity of the fan; it is not acceptable for an overly high coal layer height to affect the blowing efficiency, but at the same time, a too-low chosen coal layer height can cause the coal layer to topple over, disrupting operations. To handle these two points appropriately, one should set the level high when it’s appropriate to do so, and low when that’s appropriate; there should be no dogmatism. In short, a stable and reasonable coal seam thickness is a prerequisite for stabilizing various processes. The stable ash layer: the ash layer is an inactive, unreactive and ineffective layer zone. Its thickness and the stability of its control both have a significant impact on the gasification conditions. Even after the carbon layer height becomes stable, it is not enough for all layers within the bed to remain in their optimal positions; reasonable process adjustments and scientific, effective operational controls are still required. The main zone within the bed is the gasification zone (commonly known as the fire zone), and the proper selection of the location of this fire zone as well as its stability depend on the appropriate selection of the ash layer thickness and its stable control; these two issues are highly interrelated. Adjusting the position of the fire layer is primarily achieved by controlling the upper and lower blowing times as well as the amounts of steam used for blowing, but what helps to stabilize the position of the fire layer is also the stability of the ash layer thickness. In other words, the condition that has the greatest impact on the position of the fire layer during normal operation is the change in the ash layer. Many manufacturers neglect the proper selection of the ash layer thickness and overlook the importance of changes in ash layer thickness on the gasification conditions. While constantly striving for favorable gasification conditions with moderate temperatures at both the top and bottom of the furnace, it is essential to scientifically determine the appropriate thickness of the ash layer when establishing the gas production capacity. If these process conditions are not selected properly, it will be impossible to increase the gas production load and optimize the gasification conditions. For example, under high load conditions, if a thick ash layer is mistakenly chosen or the temperature at the bottom of the furnace is low, then even if the height of the carbon layer is set appropriately, the excessive thickness of the ash layer will reduce the available space for the burning zone inside the furnace. As a result, the burning layer moves upward, increasing the blowing resistance and reducing the efficiency of air supply. These issues limit the full utilization of the production capacity. To reasonably determine the thickness of the ash layer, it is also necessary to establish the process parameters for the temperature below the furnace. This important process parameter should have its fluctuation range minimized as much as possible, given the allowable external conditions and operational skills, in order to reduce fluctuations in the ash layer. The goal of improving gas generation technology requires, first and foremost, more stable control parameters with a reduced range of fluctuations, so as to gradually achieve constant values for all process parameters. The method to control the ash layer is to carefully regulate the ash discharge rate in order to achieve a balance between the formation and removal of the ash layer. In addition to regular ash sampling, the main criteria for operating the furnace are the changes in the temperature at the bottom of the furnace and in the ash layer. Under normal operation, when the thickness of the ash layer remains constant, the gasification rate may increase suddenly due to sudden changes in the properties of the feed material, resulting in a rapid decrease in the coal layer thickness. In such cases, it is not advisable to try to increase the coal layer thickness by reducing the ash discharge rate; instead, it is necessary to maintain a stable ash layer thickness. It is important to determine the impact that changes in coal quality will have on furnace operation, and to address the issue by shortening the coal feeding interval or adjusting the amount of steam supplied to the furnace. This phenomenon usually occurs because the replacement feed material has higher reactivity and a lower melting point. Under these conditions, localized or widespread melting has begun to occur within the oxide layer. If no reasonable measures are taken in a timely manner to reduce the speed of the rod-reducing machine, the ash layer will thicken, the flame zone will become more concentrated, and the temperature of the gasification zone will rise as well, further exacerbating the formation of molten material. Therefore, the control of the gray layer should remain unchanged in the face of changing circumstances, unless it is time to make comprehensive adjustments to various processes and reset the parameters accordingly. In terms of operations, specific circumstances must be taken into account and tailored measures implemented to avoid disrupting the overall plan. As long as the ash layer remains unchanged, the position of the fire layer will stay stable, and the reduction layer, carbonization layer, and drying layer will also remain stable. It also creates favorable conditions for stabilizing the entire operating condition; therefore, the key to stabilizing each layer zone lies in having a stable ash layer first. Stabilizing agents: The stabilization of the carbon layer and the ash layer lays a solid foundation for maintaining the position of each layer zone and ensuring their uniform distribution across the same cross-section. However, to achieve stability in the gasification temperature as well as optimization and stability of the composition of semi-water gas, it is also necessary to strive for stability in the air volume fed into the furnace, as well as in the steam pressure and flow rate fed into the furnace. To ensure stability in the gasification temperature inside the furnace, it is first necessary to maintain a stable blowing rate in each cycle. Given a fixed blowing time, operational errors can have a significant impact on the amount of air supplied to the furnace. In addition to the effects of the changes in carbon layer thickness and coal particle size mentioned earlier on the blast rate, the method of nitrogen addition also plays an important role in influencing the blast rate. Some plants can only adjust the hydrogen-to-nitrogen ratio by adjusting the recovery time, and they make large adjustments to the recovery process, adding nitrogen in bulk. This results in large fluctuations in furnace temperature. The amount of air fed into the furnace during the recovery phase is only 30% of that in the blowing phase; therefore, this nitrogen addition method is not conducive to increasing the gas generation rate or stabilizing the gas composition. It is even less conducive to improving the quality of semi-water gas. There are two ways to solve this problem. One is to change the method of nitrogen addition, making full use of the effects of top and bottom nitrogen injection; by focusing on these methods for nitrogen supplementation, only a small amount is left to adjust and balance the hydrogen-to-nitrogen ratio through recovery. And efforts should be made to maintain stability, achieving fine-tuned recovery. Secondly, the optimization function of microcomputers is utilized to automatically compensate for the increased amount of air fed into the furnace as a result of reduced recycling. As proven through our factory’s applications, the functional design of Yifeng brand microcomputers is scientific and effective. As the recovery time increases, the blowing time also increases accordingly, to maintain a constant air volume entering the furnace throughout the entire cycle. At the same time, the upper and lower blowing times are also adjusted appropriately to ensure stable furnace temperature and prevent changes in the position of the flame layer as a result. It is very beneficial for improving the quality and yield of semi-water gas. The method of manual intermittent coal feeding has a certain impact on the amount of primary air supplied to the furnace. To maintain the production load, the blowing time must be increased by 1–2 seconds compared to gas stoves with automatic coal feeding. In contrast, gas stoves with automatic coal feeding create four favorable conditions for stabilizing the furnace operation and improving gas production efficiency. First, it eliminates the furnace temperature fluctuations caused by manual shutdowns for coal addition and the interruption of primary air flow ； Secondly, it eliminated the impact of a large amount of cold material being added on the furnace temperature ； Third, it eliminated the impact of large-scale material feeding on carbon layer fluctuations on blowing efficiency ； Fourth, it effectively improved the automation level of gas stoves, significantly increasing their continuous operation time. The gas generation rate per furnace increases by over 10%. The composition of semi-water gas has also seen significant improvement and stabilization. However, to date, gas stoves equipped with coal feeders used in the small nitrogen fertilizer industry account for only about 30% of the total, which is due to both a lack of awareness and financial constraints. A major drawback of intermittent gas stoves is their short operating time. For example: stopping the furnace to add coal, stopping it to remove ash, stopping it for temperature checks, and stopping it to observe the color of the flames – these factors have a significant impact on the production capacity of gas furnaces. The focus of current improvements in gas generation technology should be on increasing the degree of automation of these furnaces and extending their continuous operating time. At present, technologies such as new type ash boxes that allow automatic ash discharge when the furnace is shut down, and spherical valves installed on upward-flow dust collectors to enable ash discharge during operation, are already well-developed. There is a solid foundation in terms of the processes involved and how to operate them; if these three technologies are used together in gas furnaces, those furnaces can operate continuously for extended periods of time. There is no longer any need for shutting down the furnace for cleaning tasks, which will surely increase the gas production per furnace by over 20%; achieving this goal is just within reach. However, there has yet to be a manufacturer that applies all of these mature technologies simultaneously. Some manufacturers don’t use any of them at all; this is still a matter of awareness and mindset. In the process of promoting new technologies, it is often outdated concepts that hinder technological progress. Stabilizing the amount of steam entering the furnace in each cycle is another key factor in maintaining thermal balance. To determine the load of a gas furnace, the method of first determining the air volume and then the steam volume is generally used. That is, based on the amount of gas required for production – following the basic principle that 0.9~1.0 m3 of air is needed to produce 1 m3 of semi-water gas – the air volume required per furnace is determined first; thereafter, the pressure and flow rate of steam supplied to the furnace are determined to maintain thermal balance. It is important to maintain stable steam pressure and ensure that the same amount of steam enters the furnace in each cycle. Only in this way can heat be balanced, allowing the operating conditions to remain stable. The method of manually adjusting the steam pressure entering the furnace has long been abandoned. Instead, a steam control valve that exerts direct force downstream of the valve is used, and this control method is relatively reliable. After the promotion, significant progress was made in stabilizing the steam pressure entering the furnace and maintaining a stable furnace temperature. It played a significant role in the advancement of gas generation technology. With the advancement of electronic control technology in our country, steam automatic regulation technology controlled by microcomputers has also been gradually adopted and widely used. As the gas generation process progresses and the furnace temperature drops, the amount of steam fed into the furnace is gradually reduced; this helps to minimize fluctuations in furnace temperature and improve the steam decomposition rate. However, not many manufacturers employ this technology, and it is rarely used in the small-scale nitrogen fertilizer industry. However, as this technology continues to improve, its enhancement and gradual adoption will replace other adjustment methods. While control measures are important for stabilizing the steam pressure entering the furnace, the design and configuration of the low-pressure steam system for gas production are also crucial. Whether it is reasonable or not has a significant impact on maintaining the steam pressure. Many manufacturers have carried out expansion modifications based on the Φ2260 type gas furnace, but a common problem is that the low-pressure steam system has not been modified accordingly; as a result, steam pressure builds up and is released during blowing, while insufficient compensation occurs during gas production. Thus, it has a certain impact on the gasification conditions and gas production efficiency. To address this issue, our company adopted a solution in 2000 by connecting the steam buffer tanks of multiple boilers in series, changing from a single-boiler, single-steam system to a shared common system for multiple boilers. The characteristic of alternating blowing with multiple furnaces is utilized to achieve complementarity between production and consumption. It has played a significant role in stabilizing the steam pressure entering the furnaces. Recently, it has been observed that some small nitrogen fertilizer manufacturers have also carried out modifications to combine and use the low-pressure steam systems of their various furnaces; however, some of these factories have still not achieved satisfactory results. After investigating several manufacturers on-site, it was found that the problem lay in the fact that they had installed only one steam make-up station for their steam systems that accommodated four or more boilers. Since the steam generated by each boiler alone was not sufficient to meet its own demand, and given the varying efficiencies of waste heat recovery in different factories, some manufacturers needed a large amount of external steam supply. With multiple boilers requiring make-up steam, having only one make-up point that served both to reduce steam pressure and to stabilize it led to insufficient steam supply due to the large volume of make-up steam and the long length of the pipelines. The principle is that during the normal operation of each furnace, if one furnace stops to discharge ash or load coal, the steam supply to that furnace is suspended. As a result, the pressure of the remaining steam increases, and this rise in pressure is transmitted to the control mechanism. Just as the external steam supply is reduced, all the furnaces start generating steam at the same time, leading to high demand for steam. Due to the limited number of steam supply points and the long steam pipelines, it becomes impossible to compensate for the loss of steam, causing the pressure of the steam supplied to the furnaces to drop too low. This cycle repeats, making it difficult to maintain stable steam pressure. Therefore, when modifying the steam system, it is necessary to install more steam supply points. At least one unit per 2 furnaces is required; in any case, ensuring a stable supply of both air and steam as gasifying agents to the furnaces is crucial for maintaining a stable gasification temperature as well as for improving the yield and quality of semi-water gas. The return on investment in hardware facilities in this area is quick, and the benefits are quite substantial. Stable operation: The improvement and stabilization of the effective components in semi-water coal during normal production is a direct reflection of the stability of the gasification conditions in the gas furnace and the optimization of various processes. To achieve this goal, in addition to scientifically and reasonably determining various process parameters, it is also necessary for operators to improve their technical skills. Only through reasonable, timely, and proactive regulatory control to maintain stability in various process parameters can the gasification conditions be kept stable and gradually optimized. To achieve this goal, in addition to enhancing the professional skills training of employees, it is also necessary to establish scientific and thorough management systems and enforce them strictly in order to improve technical and production standards. The detection methods for intermittent gas stoves are not yet perfect; the values obtained from measuring the gas temperature at various detection points, as well as the temperature of the ash discharged, can only serve as a basis for assessing changes in operating conditions and for guiding operations. It cannot reflect the changes in the temperature and thickness of the vaporization layer, and the temperatures at various points cannot be used as an absolute basis during operation. It is only after conducting a thorough quantitative analysis that takes into account changes in gas production and composition, as well as changes in raw materials and the shape and quality of ash residues, along with empirical data, that an approach to operation can be determined. The operation and management of gas furnaces are far more difficult than any process in the ammonia synthesis system. In fact, the process adjustment, operational control, and management of gas stoves constitute a long and complex systematic project. The processes and operating methods for producing gas have never remained fixed for an extended period; they are constantly under process of exploration, improvement, optimization, and enhancement. In practical operation, a stable carbon layer, ash layer, furnace temperature, gasifying agent, etc., can all be achieved only under reasonable control and steady operation by humans. Therefore, stable operation is very important. If the operational skills do not meet the requirements for stable operation, adjustments often lag behind, and mistakes occur frequently; as a result, any good process conditions will be disrupted, and even the best operating methods will become ineffective due to instability. This undermines the principle of seeking optimization through stability in the operation and management of gas furnaces. The key to stable operation lies in requiring operators to be able, through analysis and judgment of changes in various operating conditions, to make appropriate adjustments in advance to prevent potential problems before they occur. Ensure stable gasification conditions. Therefore, the level of operational skill reflects the level of ability to anticipate problems. Therefore, operators need to improve their ability to summarize experience, compare it with reality to identify problems, and analyze those problems. It is necessary to develop the ability to anticipate problems and eliminate them in advance. To keep things at the level where workers only learn *how to handle certain types of problems*. In conclusion, the above analysis shows that the conditions of the gas generation process are highly interrelated, with many factors influencing one another. This requires us to clarify our thinking in terms of management and make scientific use of these interactions. With the characteristic of both restricting and promoting each other, by focusing on key issues and leveraging them to influence other aspects, the four elements proposed in this article represent the most crucial aspects of gas production. By mastering and firmly holding onto these four elements, gas production can be put on a path of healthy development.