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In-depth analysis of the conceptual differences and formation mechanisms of \"re-coking\" and \"re-carbonization\" in gas generation ash residues Tian Shouguo, National Gas Generation Technology Advisory Department Keywords: How to distinguish between re-coking rate, re-carbonization amount, as well as the distinctly different concepts of \"scars\" and \"ash\"； Abstract: The difference between back-focusing and back-carbonization lies in ； The temperature to which the lump coal is heated in the furnace, and the duration for which it remains under high temperatures. The condition of the ash and slag is the ultimate result of the gasification process; by examining this result, it is possible to analyze the gasification conditions within the furnace and determine the reasons for its formation ； Large temperature differences across the plane within the gasification zone, as well as uneven combustion and ash distribution, are persistent problems that affect normal production when the furnace size is increased. “\"Technological progress is the fundamental way to save energy and reduce consumption.\" The level of success a company achieves in saving energy and reducing consumption largely reflects the state of technological progress in that company. In the gas production process, the quality of the ash and slag from the gas generator is the ultimate indicator of whether the gasification conditions inside the furnace are favorable or not. With the high price of raw coal today, the quality of the slag produced by a manufacturing plant determines its economic profitability. These days, almost every company includes an assessment of the \"coke regeneration rate from gas generation ash\" as part of its internal production evaluations. However, in some places there are still some vague understandings regarding this aspect. For example, some manufacturers have always used the \"ash and slag carbonization rate\" to evaluate the quality of gas generation ash and slag. This term is not entirely incorrect, as gas generators do indeed emit \"char,\" but this only occurs under conditions of extreme abnormality within the furnace, and it is something that should be avoided at all costs. To manage gas production effectively and conduct proper evaluations, it is necessary to first understand the differences between \"coke return\" and \"carbon return,\" so as to analyze problems more thoroughly and accurately. 1. The fundamental difference between “back-firing” and “back-carbonization”: Back-firing is an inevitable outcome of the normal operation of a gas generator. Why is that? In UGI fixed-bed batch gas generators that use solid slag removal, the overall design parameters determine that a certain amount of unfully gasified material will inevitably be present in the slag. The coked particles that result from this are referred to as \"regenerated coke,\" and it is necessary to minimize the amount of such regenerated coke in the ash and slag. “\"Back coking\" occurs in gas generators through two pathways: one is the coke residue left after bituminous coal undergoes carbonization during the dry distillation process in the gas generator, followed by high-temperature gasification in the oxidation layer. The second major aspect is also a problem that cannot be fully resolved under the existing configuration ； It is due to the installation of a water-tank type jacketed boiler in the lower half of the gas generator’s body. Due to the wall cooling effect of the jacketed boiler, the gasification temperature in the area of the gas generator that is close to the edges of the jacketed boiler is much lower than that in the outer ring zone, inner ring zone, and central zone (Note: the furnace is divided into four ring zones) ； (Central zone, inner ring zone, outer ring zone, peripheral zone); in some carbon blocks, heat exchange occurs on the inner wall of the jacket, preventing the formation of high-temperature conditions, which results in a slow gasification rate and incomplete gasification, thus leading to the formation of some already coked material that re-cokes. Of course, the jacket boiler as a heat transfer mechanism is essential for this type of furnace. “\"Back carbonization\" refers to the discharge or outflow of unvaporized lump coal from the slag discharge port; it is a result of extremely abnormal gasification conditions within the gas generator. It is caused by factors such as scarring inside the furnace, localized scarring, excessive temperature differences across the furnace interior, uneven distribution of heat and ash, localized collapse of the carbon layer, the formation of air pockets in certain areas, and dead zones where no combustion takes place. Such phenomena should be prevented as much as possible. The formation of back carbon indicates that the gasification conditions inside the furnace have deteriorated, and the order of the various layers has become disrupted. In fact, “returning to carbon” refers to the situation where coal lumps introduced into the furnace are ejected from it again; they have not undergone a complete carbonization process and thus remain uncoked, with their surface still retaining its original luster and smoothness. Sometimes, even if the surface color of the carbon blocks changes slightly to a whitish-brown, they should still be classified as “return carbon” since they have not yet undergone coking. 2. The fundamental difference between “scar” and “residue”: The characters “scar” and “residue” represent two entirely different vaporization conditions, in terms of their physical shape ； The scabs in the ash have a lava-like surface and cross-section; their structure is dense and pore-free, with a specific gravity more than twice that of the ash fragments. Scars form when, due to factors such as excessive blowing efficiency and insufficient steam usage, the furnace loses its thermal balance; this causes the temperature in the gasification zone to exceed the ash melting point T3 of the coal, resulting in the coal melting and solidifying into hard scar masses. Second is the scarring that often occurs after the diameter expansion renovation of modern gas stoves; both these \"scars\" and \"carbon regression\" are products of extremely abnormal operating conditions in the stove. Therefore, it is highly undesirable in normal production. “\"Slag\" is a by-product of the normal operation of a gas stove, and it is desirable for the slag formation rate to reach its maximum possible level. Slag is formed during the normal gasification process, as the carbon molecules on the surface of multi-particle coal or lump coal are transferred to the gas phase as gasification proceeds. The remaining ash retains a certain degree of viscosity and plasticity under high temperatures; as a result, the ash keeps peeling off while also sticking together, thus forming slag masses. The amount of slag produced is what is referred to as the \"slag formation rate\". The slag formation rate is adjustable; it depends on the temperature of the gasification layer. A higher temperature in the environment where ash forms slag leads to a higher slag formation rate. However, when the temperature reaches the material’s T3 temperature, the material melts and this deteriorates the conditions inside the furnace. The optimal temperature for the gasification zone is between the material’s T2 and T3 temperatures; but since current temperature detection technologies are not advanced enough, it is difficult to determine this exact temperature value. As a result, the temperature of the gasification zone is generally set at the material’s T2 temperature. And if the temperature of the slagging environment for the ash content is low ； As a result, the slag formation rate is low, the amount of fine ash is high, and the coking return rate is high. Of course, the level of slag formation is also influenced by the chemical reactivity of the different raw materials and the elemental composition of their ash. 3. Analysis of the condition states with a high rate of coking return: As mentioned earlier, it is normal for a certain amount of coking return to be present in the ash discharged from gas generators. Why do I say that? The reason is: first, the gasification unit is equipped with a jacketed boiler that creates a wall cooling effect. Secondly, the particle size of the coal fed into the furnace is not uniform, resulting in inconsistent gasification rates for different coal lumps. Third, the coal fed into the furnace is a mixture from multiple mining sites, resulting in mixed characteristics; the ash fusion points and chemical activities of the coal vary, leading to inconsistent gasification rates. Fourth is the issue of uniform distribution of the gasifying agent determined by the furnace grates. Fifth is the issue of process conditions and operational control conditions. The presence of these factors determines that it is not possible to reduce the combustible components in the ash to levels as low as those in the slag from new gasification methods. In other words, the coking rate due to slag discharge from the gas generator is merely a matter of more or less. The degree of slag formation in gas stove ash, as well as the rate of coking regression, are related not only to the chemical reactivity of the coal and the composition of the ash contained in it, but also to the gasification temperature inside the furnace and the duration for which the coal is exposed to high-temperature gasification. Therefore, high load and high gasification layer temperature are the primary conditions for increasing the slag formation rate and reducing the coking return rate. Furthermore, the three height-to-diameter ratios of the gas furnace (the gas furnace itself, the water jacket, and the carbon layer) are optimal, which increases the effective gasification space. The raw coal has sufficient time within the furnace for drying, dry distillation, and gasification, and it also enjoys high-temperature gasification conditions. Under such conditions, the slag formation rate inevitably increases while the coking back rate decreases. Currently, there are many terms used to describe the condition of ash and slag. For example: back-focusing rate, carbon return amount, residual carbon amount, combustible content in ash and slag, etc. Of the terms mentioned above, only two are exact: one is the \"backfocusing rate\" in physical analysis methods, and the other is the percentage of carbon content in the ash residue in chemical analysis methods. And “combustible ash” generally refers to the carbon content in the slag discharged from steam boilers. By observing and analyzing the changes in the morphology of the slag, and by studying the reaction mechanism behind coking, it is possible to determine the extent of the influence of the wall cooling effect ； When the process conditions, raw material conditions, and operational controls are all at optimal levels, and the furnace operation is stable. Under conditions of a high slag formation rate, very little fine ash, and a large amount of fine slag, there is still a small quantity of coke particles with relatively large sizes, ranging from 15 to 30 mm in diameter, accounting for less than 15% of the total; extremely few tiny coke particles are present. In such conditions, very little coke returns are generated in the gasification zone, and over 95% of the coke returns in the ash and slag originate from the edge zone, which is mainly a result of the wall cooling effect. This slagging condition also indicates that the operating conditions inside the gas generator are excellent, and the load has reached its limit. ? If the amount of fine-grained coking residue in the ash is high, it indicates certain issues with control, management, and oversight. The fine coke particles carried away in the ash significantly affect the conversion efficiency of the raw materials, and these particles are difficult to recover and reuse for gasification in the furnace. Moreover, the amount of this combustible material is sometimes greater than the amount of coke that can be recovered, resulting in serious waste of raw materials for many manufacturers. Even if the steam boiler is used again, although it replaces some of the coal as fuel, it still results in some of the raw coal being used in a lower quality form as fuel, which has a significant impact on the overall costs. Using coal in this reduced quality manner does not conform to the principles of a circular economy. And this problem can also be reduced through human control. More precisely, fine cinders can cause them to appear in small amounts or not at all. A large amount of backfiring at small particle sizes is a problem present in many manufacturers, and this phenomenon is one of the main reasons for the low conversion efficiency of the raw coal. Smokeless lump coal undergoes a qualitative change after passing through the carbonization zone, transforming from \"coal\" to \"coke\". The resulting coke is formed when the volatile components are removed from the lump coal due to heating in the carbonization zone; subsequent surface oxidation and exothermic reactions, along with high-temperature calcination, leave behind the residual coke mass. After being converted from lump coal into coke, the vitrine layer with its high surface smoothness is lost due to carbonization; the coke changes from oil-black to gray-brown, and its surface becomes rough. Its shape also loses its sharp edges, becoming round, oval, or pillow-shaped. Due to the loss of volatile substances, the density decreases, and the pores formed by surface vaporization become clearly visible. Due to its rough surface, backfiring results in high flow resistance; it has poor sliding capacity, and the angle of repose at its edges in a naturally piled state is greater than that of lump coal. Under normal operating conditions of the gas stove, the increased roughness on the surface of the lump coal after dry distillation and coking reduces the fluidity of the materials within the gasification layer, which helps to stabilize the position of this layer and lowers the likelihood of ash accumulation and carbon breakdown. (Generally, the fluidity of materials inside the furnace can be classified into the following four levels: lump coal > coked material > fine ash slag > slag lumps and scar lumps). 4. Analysis of the condition states with high carbon return rate: As mentioned earlier, the level of coke return rate is influenced not only by the characteristics of the gasification equipment and the physical and chemical properties of the feedstock, but also by the operating conditions. However, in comparison, regardless of the level of backfocusing, it at least indicates that the furnace conditions are basically normal, that they can be adjusted, and that the situation can be reversed. A high “return carbon” content is a sign or result of extremely abnormal operating conditions in the furnace, critical deterioration, or already existing deterioration. “\"Back-to-carbon\" refers to those mixed in the ash and slag that have not been fully carbonized or not carbonized at all, have not undergone complete coking, and still appear black on the surface. Coal blocks whose surface gel layer has not lost its luster completely, with no changes in their appearance such as edges and corners. The main causes of carbon return ； First, under high-intensity gasification conditions, improper control of the fire layer position leads to its dispersion and the formation of local scabs; as a result, the distribution of the fire layer within the furnace is uneven. The coal gas generator operates in an abnormal manner, with inconsistent rates of bed layer descent. Scabbed blocks are formed on one side of the two ash bins, while carbon flows out on the other side due to the complete removal of the ash layer. The second is scorching inside the furnace ； A wind tunnel is formed in the gap between the scar marks, allowing the coal pieces to leak onto the ash tray and be discharged. Third, it is caused by equipment defects ； The unreasonable design and configuration of the slag discharge and ash removal mechanisms result in uneven heating and ash distribution inside the furnace, leading to carbon reformation. Fourth, the grate has poor slag-breaking capacity; the size of the discharged slag particles is too large, and when the slag blocks are removed, the loose carbon in the low-temperature area at their back collapses as well. Fifth, the gas generator is in a low-load gasification mode ； At low temperatures, the slag formation rate is low, the coking return rate is high, the stability of the ash and slag layer is poor, and its fluidity is high; even minor deficiencies in anti-flow measures can easily lead to carbon flow collapse. Sixth, the level of operational control is low; improper operation leads to the complete removal of the ash layer and the fuel layer reaching the bottom. Furthermore, at low temperatures, the metabolic rate inside the furnace is too slow, which makes certain areas prone to overheating and the formation of large slag masses. Areas with such slag masses descend more slowly, while areas without slag masses descend more rapidly due to the loose ash and slag at low temperatures, resulting in uneven distribution of ash and heat. In summary, high carbon return is caused by unreasonable process conditions, excessive accumulation of ash and slag which leads to a shift in the position of the fire layer, resulting in uneven heat distribution, local scorching, disordered layer structure, uneven distribution of fire and ash, or equipment defects. Sometimes, due to untimely handling or improper methods for addressing certain issues during the operation process, these problems worsen, resulting in the formation of voids in the lower parts of the bed layer and ultimately leading to collapse. Under abnormal operating conditions, the entry of coals of different particle sizes at the bottom of the bed reduces the mobility of the ash layer. The presence of returned coal has a significant impact on the stability of the ash layer, increasing its fluidity and greatly raising the likelihood of coal collapse; the risk of failure is particularly high. (In abnormal conditions, the fluidity of the materials inside the furnace is classified into six levels: lump coal > returned carbon > fine carbon > fine ash > slag lumps > scarred lumps.) 5. Utilization of returned coke: Under normal conditions, during the gasification process, gasification starts from the surface of the coal lumps; as gasification proceeds, the ash is stripped off, and under high temperatures slag is formed. The size of the coal lumps gradually decreases until complete gasification occurs and slag is produced. However, due to the cooling effect of the jacket boiler walls, as well as the presence of some lump coal with excessive particle size in the area where gasification occurs normally, and influenced by factors such as the gasification rate and reaction rate, some coke lumps are inevitably discharged from the furnace before they have been completely gasified. However, the carbon content of the residual coke lumps did not decrease, and after being exposed to high temperatures, the volatiles, tar, sulfur, and other hydrocarbon substances contained in the lump coal were essentially gone; as a result, the quality of the recycled coke is higher than that of the lump coal fed into the furnace. From this perspective, when a small amount of coked material is blended into the combustion process, there is no need to change the existing operating procedures or process conditions. However, when a large amount of coked material is used for gasification, since there is no carbonization process involved with such materials, the upward movement of the flame layer accelerates. The gasification characteristics of carbonaceous feedstocks are evident, and the process conditions should be adjusted accordingly. Under normal circumstances, the chemical analysis results of returned coke are as follows (Note: the calorific value is given in the unit of KJ/kg). Analysis parameters include moisture content, carbon content, ash content, volatile matter content, and calorific value. Analysis results: 1.05%, 76.10%, 21.51%, 2.18%, 24539 KJ/kg. Comprehensive analysis: The above analysis outlines the fundamental differences between \"returned coke\" and \"returned carbon\", as well as the conditions under which each occurs, and their respective impacts on the operating conditions within gas furnaces. It can be seen that the properties and formation conditions of back-focusing and back-carbonization are entirely different; the former occurs when the furnace operating conditions are basically normal according to the process requirements and operational standards, but not optimized. In the latter case, the operating conditions inside the furnace are chaotic, with uneven fuel distribution and ash distribution; it may even be on the verge of further deterioration or already be in a state of deterioration. These two issues are primary problems that are directly related, to varying degrees, to the conversion and utilization efficiency of raw materials; yet to resolve them, entirely different corresponding measures must be taken. Solve the problem of high back-focusing rate ； First, it is necessary to ensure the reasonable determination of the process parameters for gas generators. Second, it is necessary to determine operation methods that are suitable for this system and that are scientifically sound and reasonable given the internal and external objective conditions. Third, it is necessary to ensure proper control of process conditions and stable operating conditions. Fourth, ensure proper control and screening of the particle size of the coal used as feedstock in the furnace, and combine it reasonably based on its properties. Fifthly, it is necessary to establish scientific, reasonable, as well as strict and detailed rules and regulations to standardize operations, thereby optimizing process conditions, stabilizing furnace performance, and reducing the rate of slag return and coking in gas generators as well as overall consumption. To address the issues of uneven combustion, excessive ash formation, and high carbon reformation, it is first necessary to conduct a realistic and in-depth analysis of the actual internal and external problems existing in this system, such as its technical equipment, process conditions, and operating methods. Further categorize to identify the key points, focus on the critical issues and take appropriate actions; meanwhile, technical upgrades in both software and hardware aspects must be carried out simultaneously. For the grates used in gas generators, it is necessary for enterprises under any internal or external conditions to select them specifically based on their existing conditions and to require specialized design. Due to the influence of external conditions and the characteristics of the bed distribution in gas generators, the resistance in different zones within the gas generator bed varies. Additionally, issues such as the composition of the raw materials, the method of feeding coal, the performance of fans, and system resistance contribute to these differences; as a result, different manufacturers have their own unique features, and even gas generators produced by the same manufacturer can differ from each other. Therefore, there has long been the saying that \"each factory has its own conditions, and each furnace has its own situation.\" Therefore, it is necessary to select grates suitable for the actual conditions of the plant in order to accommodate different resistance characteristics. Due to different coal feeding methods, various coal qualities, distinct physical and chemical properties, as well as different fan performances, the resistance characteristics within the bed vary from case to case; therefore, a grate with fixed performance parameters cannot suit all situations. It is essential to do a good job in slag breaking and flow prevention in gas generators; first, attention must be paid to controlling the particle size of the slag, and second, measures must be taken to prevent, block, and delay flow. “Flow prevention measures such as internal barriers, downward resistance, and extension all have their advantages and disadvantages; it is necessary to select the methods and measures suitable for the specific conditions of this system from among various available options. During the operation of a gas generator, it should be avoided to operate it for extended periods under conditions of high airflow and overload, or for extended periods under conditions of low airflow and light load. Regarding the maintenance of the ash discharge mechanism, it is necessary to properly match the furnace grates, and at the same time ensure that the ash plow and the slag-breaking bars distributed around it are in good condition, so as to maintain uniformity in slag breaking and ash discharge around the perimeter. To control the ash discharge rate, it is essential to prevent excessive fluctuations in the rotation speed of the grate machine; therefore, it is necessary to follow and actively apply the \"new ‘Four Stability Operation Method’\" proposed by the author. The slag particle size is adjustable, yet it has a significant impact on the process conditions. The particle size of the slag at 200–300 mm has a vastly different impact on the amount of carbon returned, compared to the two slag conditions where the particle size is 150 mm or less. When lumps and scars form, air cannot pass through beneath them; as a result, a large amount of air can only flow along the edges of these scars. This causes the heat release at the edges of the scars to be greater than in other areas, leading to higher temperatures there. The slag formed in these areas tends to remain in a molten state and sticks to the existing scars, causing them to grow larger over time. When large-sized scars leave the gasification zone, they are supported on their backside by a region of lower temperature. The coal particles in this low-temperature region have lower temperatures due to their location, which results in smooth surfaces and high fluidity. Sometimes the position of the slag masses also moves in the direction of rotation of the ash duct; this is mainly because large slag masses slide above or between the highest layer of the grate and the slag-breaking ribs, making it difficult for them to break apart. As a result, these slag masses are pushed around along the ash duct, and there is always a low-temperature area on the back side of these slag masses. Due to the slow movement of the scar mass, the rotation of the gray disk removes the ash beneath it, resulting in the formation of voids under the scar mass and localized suspension of material. The scarred masses fall into the lower, suspended ash bin, causing varying degrees of collapse within the bed layer; the burned or raw coals then fall into the ash bin as well. The area where the collapse occurs moves together with the scarred masses toward the ash discharge outlet. Large scarred masses can lead to extensive collapses, and it becomes difficult to deal with flowing burned coals or even raw coals. The immediate collapse at the slag outlet results in the outflow of burned carbon (also known as “red carbon”) ； In the gray bin area, bed collapse occurs; when the collapse surface reaches the slag discharge outlet, what flows out is extinguished green coke. Therefore, if the scar mass is not broken up and removed as soon as possible, it will grow larger. To remove the scabs as quickly as possible and to ensure that the size of the slag particles remains uniform and not too large, careful consideration and serious attention must be given to the design of the furnace grates as well as to the accompanying technologies for slag removal, with a focus on achieving a reasonable combination of these elements. If the slag-breaking conditions of the design meet the requirements, the slag particle size is adjusted appropriately. In the slag breaking zone, large slag pieces are crushed to around 100 mm, and the gasification effect changes as a result. After the slag lumps break apart, there is a layer of ash and slag in the lower ash channel, so they do not fall directly onto the ash tray. Once the slag lumps crack, ventilation and steam flow can occur between the cracks. Therefore, after the scar masses cracked, the raw coal on the back side was rapidly gasified, forming a new layer of ash and slag. With this process, fluctuations in furnace conditions are greatly controlled; the results are, of course, completely different from those under conditions of excessively large slag granules. In the production of gas generators, when there are no slag lumps inside the furnace, the air distribution is controlled by the characteristics of air distribution through the furnace grates, resulting in a uniform distribution of air volume and a balanced flow rate. And when there are scar masses, the flow rate of the vaporizing agent within the gaps of those scars is greater than in the normal areas. 7. Conclusion: The high rates of \"coking back\" and \"carbonization back\" in the ash and slag of gas generators are two major problems that severely affect the efficiency of raw material conversion; they also represent significant obstacles to reducing coal consumption and maintaining stable furnace operation. Significant efforts must be made to address these issues, eliminate equipment defects, and improve process standards in order to achieve stable operation of the gas generator. Only with a solid foundation, and by optimizing processes and operations on that basis, can further reduction in consumption be considered. Determining and controlling the process conditions and operating conditions of gas generators is an extremely complex task. First, it is necessary to understand the principles; having a clear grasp of the distinct concepts of \"backfocusing\" and \"backcarbonization\" will make it easier to identify the situation, analyze the causes, and apply the right solutions.