On the quality of ash in fixed-bed gasifiers and the conversion efficiency of feed coal

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On the Quality of Ash and Slag in Fixed-Bed Gasifiers and the Conversion Efficiency of Raw Coal / Author/Source: Tian Shouguo. The author published an article titled \"On the Quality of Ash and Slag and Operating Conditions in the Furnace\" in 2000; in this article, a systematic approach to determining operating conditions in the furnace based on the quality of the ash discharged was discussed, and the gasification conditions corresponding to different ash and slag conditions were analyzed in detail. The states of the discharged ash were classified into thirteen types. After its publication, it attracted widespread attention in the fertilizer industry; to this day, colleagues continue to send letters and make calls to share their thoughts and opinions. They also express the hope for more in-depth analysis regarding the quality of ash residues and the utilization of raw materials fed into furnaces. This is indeed an issue that receives widespread attention under the current circumstances. In the ammonia synthesis system, the conversion efficiency of the feed materials fed into the furnace determines the level of production costs; rising coal prices increase the proportion of costs associated with coal consumption. In actual operation, the degree of conversion and utilization of the feedstock fed into the furnace can be analyzed indirectly, not only based on the heat loss indicated by the temperatures at both ends of the gas furnace but also according to the composition of semi-water gas. However, the most indicative indicator is the analysis of the ash output quality, which reflects the results at the final stage of the gasification process. The amount of ash discharged has now attracted the attention of most manufacturers, but as for how to address this from a management perspective, the approaches adopted in the industry vary greatly, with different results. Currently, some manufacturers have changed the methods used to evaluate gas production, shifting from simply assessing the coal consumption of each team to evaluating the furnace temperature curve, the temperatures at various control points, and the rate of ash discharge and coke return. This is linked directly to the employees’ salaries, which represents an improvement in gas production management; however, there are still aspects that need further refinement and improvement. The following provides a systematic analysis of how to accurately determine the carbon content in ash and slag, as well as how to reduce this carbon content. 1 Scientific analysis helps to determine the quality of ash and slag, providing an accurate basis for operation and management. Analyzing and evaluating the coking rate of ash and slag from gasifiers is a evaluation method that has been used since the early stages of development in the fertilizer industry; a standard of coking rate below 20% has been adopted, and this has long served as a basis for analyzing the gasification conditions within the furnace as well as the conversion efficiency of the coal fed into it. The so-called backfiring rate mentioned earlier refers to the proportion of the amount of backfiring manually separated from the ash to the total amount of ash. Manual sorting has requirements regarding particle size; generally, only coking coals with a particle size of 2 cm or more are sorted. Under complex gasification conditions, the condition of ash deposition varies greatly; sometimes, fine coke particles make up a larger proportion than the coke pieces that can be recovered. Therefore, relying solely on the rate of coke recovery cannot accurately reflect the coal utilization efficiency. During the operation of a gas stove, the losses caused by the ash discharge process are as follows in five aspects. The first is the sensible heat loss carried away by the ash. The higher the gasification intensity, the higher the slag discharge temperature becomes, resulting in greater heat loss. However, this can be controlled as close to the minimum level as possible through equipment modifications and adjustments to the process and operations. The most fundamental solution is to change the height-to-diameter ratio of the gas furnace. The second is coke of a size that can be recycled; since the UGI gasification method cannot completely digest the feed material in one go, it is necessary to minimize the amount of coke that is carried away, and this can be achieved through adjustments to the process and operations. Third are the small coked particles carried away in the ash. This portion of coke particles is difficult to recover and cannot be reused in the furnace. Moreover, under certain conditions, the amount of combustible material in this portion is often greater than the amount of coke pieces that can be recovered (many manufacturers suffer from severe waste of raw materials as a result), and this issue can also be reduced through manual control. More precisely, it can be made to not appear. Fourth is the black carbon powder carried away in the ash. This phenomenon occurs under poor gasification conditions, and it is also quite common. This kind of ash, regardless of how it is reused, already represents significant waste, and such practices should be strictly prevented. Fifth are the coke lumps enclosed within the slag masses. This is also a hidden channel for loss of benefits, and it is difficult to reuse this portion of combustible materials. Some plants discard large slag lumps due to the difficulty in crushing them; instead, they grind the smaller-sized ash and slag before feeding it back into fluidized bed boilers for reuse. In fact, the phenomenon of coke pieces being enclosed within slag masses is not inevitable; although differences in the particle size and chemical properties of the raw materials can have an impact, the underlying cause lies largely in the gasification conditions and operating conditions. Tests have shown that in severe cases, the carbon content in these slag masses is around 20%, and sometimes even higher. The analysis results shown in Table 1 reflect the ash condition in a certain plant, which is characterized by low gasification intensity and an improper ratio of top to bottom blowing. As can be seen from Table 1, the main characteristics of the slag condition in this plant are a low slag formation rate and a high coking return rate; the fine slag contains a large amount of coal powder, and it is also black-brown in color. The conversion efficiency of the coal fed into the furnace is low, and overall analysis indicates that the carbon content in the slag is approximately 40%. file:///E:/DOCUME~1/zzjjaa/LOCALS~1/Temp/ksohtml/wps_clip_image1.jpg In actual production, the most efficient way to utilize the coal fed into the furnace is to determine the process parameters and implement appropriate operational controls in order to create favorable gasification conditions, thereby enabling the maximum possible release of the heat contained in the coal – in other words, striving to increase the conversion rate of the reaction in its first stage. No matter what method is used for the secondary utilization of ash, it cannot make up for the losses caused by the low conversion rate. As can be seen from the above analysis, the carbon loss in the ash does not occur solely in the form of returned coke; manual sorting cannot recover all the carbon carried away by the ash. Calculations show that, under normal conditions, only 55% to 60% of the carbon carried away by the ash is recovered as returned coke. It can be seen that using the refocusing rate for a qualitative analysis of coal utilization is insufficiently comprehensive. The analysis of slag quality should be changed to measuring the carbon content in the ash and slag; a carbon content of ≤20% is considered the normal range, ≤18% is an excellent value, and ≤15% represents the target to be achieved. 2 Analysis of the reasons for high carbon content in slag 2.1 Low furnace temperature fails to provide the conditions for high-temperature conversion; slag formation at low temperatures results in a large amount of carbon reversion, and the carbon content in the slag at these low temperatures is also high, resulting in a lower efficiency of coal conversion compared to high-temperature gasification conditions. Reasons for the low temperature: First, limitations in the equipment’s supporting capabilities ； Secondly, due to differing opinions, there is an intentional reduction in load in order to maintain a stable furnace condition, resulting in the creation of a \"stable furnace\"” ； Third, one fan supplies air to four gas stoves; under the \"one belt for four\" configuration, a 120-second short cycle is used, which limits the blowing time to less than 25%. Additionally, the competition between the processes of cleaning and nitrogen injection for air supply restricts an increase in load. 2.2 Unstable furnace conditions result in large and frequent changes in gasification conditions. The main reasons for unstable furnace conditions lie in the gas production operations and management, as well as the impact of changes in raw material properties on the gasification conditions. The author has previously stated: “Managing raw materials well is the first step to ensuring efficient gas production.” The processing, screening, and classified utilization of raw materials are indeed fundamental tasks for improving gas production levels; every enterprise should attach great importance to them and ensure they are properly carried out. Furthermore, deficiencies in process conditions can be appropriately compensated for through proper adjustment and control ； A low level of operational control prevents the equipment and processes from achieving their intended performance, let alone unlocking potential capabilities and benefits. Changes in operating conditions are mainly caused by control lag and errors. In this regard, new breakthroughs can only be achieved through skill training, technological innovation, and improved management. 2.3 The rationality of the process conditions is insufficient; the appropriate level of determination of the upper and lower blowing ratios plays an important role in the efficiency of coal conversion. Regardless of the load level, whether these ratios are set appropriately or not has a direct impact on the quality of the ash produced. During the operation of a gas stove, if the amount of gas flowing through the bed layer from bottom to top in each stage – secondary upward blowing, blowing, cleaning, upward blowing, and recovery – becomes unbalanced with the amount of gas flowing downward, it will directly cause the position of the flame zone to change. Currently, it has been found that many manufacturers experience a high incidence of excessive upward gas flow, which is also related to the effects of changes in raw material quality. The high proportion of upward gas flow directly leads to changes in operating conditions, disrupting the metabolic rate of the material layer inside the furnace. The raw materials fed into the furnace do not have enough time to undergo gasification reactions, resulting in a reduced amount of material transitioning to the gas phase and an increased amount of carbon being discharged as a solid phase. An excessive upward gas flow can sometimes also be caused by overblowing; there is an emphasis on increasing the blowing time to raise the furnace temperature, without paying attention to the effective establishment of heat storage conditions. As a result, the gas generation rate still does not increase. The only solution is to blow more air, which leads to a thinner fire layer. Although the peak temperature of the oxidation layer is high, the heat storage capacity of the bed is low, resulting in large fluctuations in furnace temperature and significant heat losses, and thus the gasification intensity still cannot be improved. These gasification conditions are particularly suitable for the formation of the \"slag-coated coke\" phenomenon. During the middle and later stages of the blowing process, the temperature of the oxide layer reaches T2, but the blowing continues. Small-sized lumps, due to their larger reaction surface area, experience a rapid rise in temperature; when the temperature reaches T3, they stick together with the coke lumps, encasing some of the larger-sized coke lumps within them. Under these conditions, the material layer in the furnace undergoes rapid metabolism, and as the flame layer rises quickly, the high-temperature masses that have stuck together are swiftly drawn into the ash layer. Although they are at a high temperature, they do not undergo sufficient gasification; instead, they are cooled by steam and air to below their active temperature before being discharged. Under suitable gasification conditions, although it is not possible to achieve complete uniformity in the particle size of the feed materials entering the furnace, or when the coal types are varied and the activation temperature and melting point of the coal vary as well, the issue of slag encapsulating the coke can be eliminated if the bed thickness and the ratio of air supply from above and below are appropriate – resulting in a thicker high-temperature zone with higher temperatures and more heat, thereby providing sufficient time for the gasification reaction of the feed materials in that zone. Due to the influence of the particle size and chemical properties of the feedstock, inconsistencies in the gasification rate are inevitable. However, under favorable gasification conditions, the carbon molecules in those parts of the feedstock with smaller particle sizes and higher activity convert to the gas phase first, while the ash forms porous, high-temperature slag that sticks together to form flakes or lumps. Although it also contains some unfully vaporized carbon pieces, this type of slag mass is formed by the sticking together of several slag particles; it is loose and porous, highly brittle, and the gasifying agent can still pass through it. Even the thicker parts will break under the pushing force of the furnace grates. As the carbon molecules in the carbon chunks within the slag mass gradually move into the gas phase, the portion of ash that has been stripped off remains attached to the hot slag chunks, or it is carried away by the gasifying agent to form small slag particles. Once the combustible material in the carbon blocks has been vaporized, the separated ash condenses into slag, reducing in volume; the slag mass develops several irregular holes. Under favorable gasification conditions, the slag mass is light and porous, with a large amount of small slag particles – this is the principle behind the formation of high-quality ash and slag. Conversely, when the gasification conditions are poor, the slag lumps formed are large, have a high density, and are dark in color; these lumps contain unvaporized carbon pieces, which is due to the poor gasification conditions preventing steam from undergoing sufficient gasification and endothermic reactions with the hot slag mass. Not only is the carbon in the slag not completely vaporized, but the carbon chunks within the slag mass are also hindered from coming into contact with the gasifying agent due to the high density of the slag. The enclosed carbon chunks undergo coking under oxygen-deficient conditions, and are then discharged after being cooled by steam and air cooling. In the operation and management of gas stoves, the goal of optimizing the process and operating conditions should be to increase the slag formation rate of the ash; the level of this slag formation rate reflects the degree to which the raw materials are utilized effectively. By adjusting the gasification conditions, the ash is deeply concentrated and melted into slag, while carbon molecules are transferred to the gas phase to the greatest extent possible; this allows the feed materials to be completely processed in a single pass, resulting in the lowest possible residual carbon content. This represents the most economical operating condition for the UGI gasifier. Data show that for every 1% increase in the amount of ash and char, coal consumption per ton of ammonia increases by 5%; this is due to both the carbon loss during gas production and the low efficiency of the system. 3 Measures to address high carbon content in ash 3.1 Factors related to the equipment: One is that the height-to-diameter ratio of the gas furnace is too small. Judging from the current development of gas generation fans, it can be said that they meet the requirements for technological upgrades in the gas generation process. One of the main factors limiting the increase in the gasification intensity of coal gas furnaces is the type of furnace that has been modified from φ2260 mm coal gas furnaces while retaining the original design height of the furnace body. Due to its insufficient total height, this limits the possibility of increasing the height of the carbon layer, resulting in an inadequate load-bearing capacity of the bed layer. When attempting to increase the load, issues such as large fluctuations in furnace conditions, bed inversion, poor slag quality, significant changes in furnace operation, and increased temperatures at both ends occur. High heat loss significantly affects the establishment of high-temperature gasification conditions, limiting the increase in the production capacity of a single furnace. A too-thin bed results in poor heat retention, causing the flame layer to rise rapidly; this easily leads to imbalances in the metabolic processes, as well as rapid ash discharge that affects the quality of the slag produced. Secondly, the air distribution across the grate is unreasonable; the air flow in a certain ring area is too concentrated, causing the fire layer in that area to rise. This results in an increased loss of heat, leading to a rapid rise in the temperature of the furnace. In some cases, this can even cause the fire to stick to the furnace walls or cause it to topple over. Third, the design of the slag discharge port is unreasonable: the slag discharge angle is too large, and the flow distance is inappropriate, which causes the ash and slag at the discharge port to descend rapidly, resulting in uneven heating of the slag, as well as a higher risk of carbon collapse. 3.2 Maximize furnace temperature through process control. Under high-temperature conditions, the reactivity of coal increases, and the gasification rate accelerates. The ash that peels off from the surface of the coal particles melts into slag under these high temperatures, which further reduces the resistance posed by the ash layer and improves the air permeability of the bed layer. This facilitates an increase in furnace temperature and thus enhances the gasification efficiency. Therefore, increasing the furnace temperature not only raises the gas generation volume and the composition of the useful gases, but also helps to improve the coal conversion efficiency and reduce the amount of residue. 3.3 It is necessary to understand the relationship between the blowing load, the ratio of upper to lower blowing, and the total amount of steam supplied to the furnace. Even when the load is sufficient, it is important to control the position of the flame layer and to allocate the steam used for upper and lower blowing in a scientific manner; there is a very delicate relationship involved here. That is, with a constant total amount of steam supplied to the furnace, if the proportion of steam injected from above is set too high, it results in an increased amount of steam used, a lower slag formation rate, and a higher CO2 content in the semi-water gas. When the proportion of downward blowing is increased, the amount of steam used becomes lower; some of the ash deposited develops hard lumps, and the slag formation rate increases. This shows that excessive upward blowing increases heat loss, deteriorates the heat storage conditions, and causes the temperature of the gasification layer to drop. Increasing the downward airflow ratio to a moderate level allows the blowing air and the upward-moving gas stream to absorb heat from the upper part of the bed layer and push it back to the middle and lower sections of the bed layer, thereby enabling the vaporization layer to accumulate more heat and raise its temperature. After the upper and lower blowing rates are adjusted appropriately, the temperature of the gasification layer gradually rises during the blowing process until it reaches saturation. As a result, heat spreads to both the upper and lower ends of the gasification layer, leading to an increase in its thickness and heat storage capacity. This creates conditions that facilitate faster and more complete conversion of the raw materials, thereby improving the quality of the slag produced. This is the positive impact of maintaining an optimal ratio between upward and downward blowing on achieving a high conversion rate of raw materials. 3.4 Breakdown of stable operating conditions. Sudden changes in the pressure of the steam pipeline network or unreliable steam control methods can also cause fluctuations in furnace temperature, thereby disrupting the normal conditions for gasification. Therefore, to stabilize the furnace conditions, it is necessary to start with stabilizing the operating conditions and the flow rate of the gasifying agent. 3.5 Scientific and strict management: By using institutional constraints to stabilize and optimize operating conditions, the gasification conditions can also be stabilized and optimized. At the same time, urge relevant departments to provide favorable conditions for gas production operations. The conditions related to gas generation are complex and variable, making it easy for them to have an impact on and impose constraints on operations. Therefore, the system must be scientific and meticulous, and its implementation must be solid and effective, so as to ensure an orderly and stable operation of gas production. 4 Ideal slag formation condition (1) We strive to increase the slag formation rate, but it is unrealistic to expect all of it to form into uniform slag masses. Due to the complex gasification conditions in the furnace, the particle size of the slag inevitably varies between large, medium, and small. The maximum particle size of the slag lumps is determined by the width of the annular gap between the uppermost layer of the grate and the slag-breaking strip at the lower part of the jacket, as well as the height of the slag discharge port. Slag lumps with a particle size of φ100–150 mm can be considered medium-sized, but such large particle sizes are not formed uniformly during the gasification process. Slag formation within the gasification zone is essentially in patches; under normal conditions, these irregularly shaped slag pieces are porous and somewhat brittle. The fluctuating forces generated by the rotation of the grate as it pushes the ash cause these pieces to break, and they then roll and grind into spherical shapes as they descend. For small slag pieces smaller than φ50 mm, under favorable gasification conditions, it can be observed that they form without sticking to other slag pieces. It forms in the lower part of the gasification zone. Carbon particles that leak into the gaps between the slag particles experience a rapid increase in temperature due to the fast air flow, high volume flow, and high oxygen content in those gaps. In a state of softening and deformation, carbon molecules quickly transition into the gas phase, while the carbon itself is rapidly melted and condensed into slag particles that contain almost no combustible materials ； Secondly, under the action of grates with good ash pushing capacity, some coal lumps form small slag after complete gasification; as these lumps move downward, the ash that has separated from other coal lumps adheres to them to form small slag masses. The higher the proportion of such small slag masses, the better the gasification conditions. It can be observed that, under conditions of aged grates and poor gasification conditions, the amount of fine slag is very small. The characteristics of the slag output are extreme: either there are many large-sized slag lumps, or there is a lot of fine ash, with a significant amount of fine coal dust mixed within this fine ash. File:///E:/DOCUME~1/zzjjaa/LOCALS~1/Temp/ksohtml/wps_clip_image2.jpg The data shown in Table 2 are the results of sampling and analysis conducted when a small nitrogen fertilizer plant was operating well; they reflect the condition of the slag after the plant had undergone comprehensive adjustments and optimizations to its process parameters, followed by 3 days of stable operation. It can be seen that the fine ash contains a relatively high carbon content. Under conditions of a high slag formation rate, the carbon content in the fine ash cannot be reduced. This is related to the deterioration of the grate’s ability to push and spread ash due to its aging, or to the fact that the grate itself is not designed with sufficient such capabilities; it is also associated with the grate being positioned slightly too high. The overall carbon content of the 3 samples in Table 2 is 22.9%, which is close to the normal value; with appropriate adjustments to the control of the furnace grates and the fire layer, it can reach the normal range and is expected to attain an even better value. (2) Among various slag discharge forms, “scar” and “slag” are two entirely distinct concepts. Scars form when the local material reaches a temperature of T3 without undergoing sufficient vaporization and reaction with steam. After being removed from the vaporization layer, it is cooled by the vaporizing agent to below its active temperature; it becomes hard, has a high density, a surface that is pore-free or has few pores, and a high structural density. The formation of such scarred masses is a sign of poor gasification conditions inside the furnace, and it occurs mainly under conditions of excessive air supply or insufficient steam usage. But sometimes it can also occur when raw materials with a low ash fusion point are suddenly replaced without timely adjustment of the process conditions. Under conditions of inadequate processing and operating parameters, this phenomenon occurs regardless of the type of raw material used; it simply happens less frequently when the coal quality is good. The lumped scar areas result in a more uneven distribution of the gasifying agent. Due to the obstruction caused by these scars, the air flow around the perimeter of these scar areas becomes concentrated, with flow rates that exceed those in other areas with normal air permeability; as a result, the heat generated in these areas is greater than in the other areas. Thus, the temperature at the bottom of the scar mass as well as its perimeter is also higher than in other areas; this continues to cause melting and adhesion, leading to an increase in the size of the scar mass. Eventually, a high-temperature mass is formed, and the heat it contains cannot be absorbed or dissipated by the normal steam supplied. Therefore, it is necessary to increase the steam consumption, lower its temperature, and accelerate ash discharge to facilitate its reduction. That is why the lumps formed under such superheat conditions are called scar lumps. As for the slag mass, under normal gasification conditions, due to the certain viscosity and plasticity of the ash at temperature T3, the ash peels away from the coal mass while forming slag, thereby maintaining favorable gasification conditions. If the gasification temperature is low, the ash does not reach the temperature at which viscosity and plasticity can be exhibited; a large amount of fine ash hinders full contact and adsorption between the gasifying agent and the carbonized material. Oxygen cannot participate fully in the reactions within the oxidation layer. Once it reaches the reduction layer where there is less ash, extensive oxidation reactions occur, releasing heat that is then transmitted upward through the bed. This is the mechanism behind the high amount of fine ash in the slag, significant coking, and large temperature differences between the upper and lower layers under conditions of low gasification temperature. (3) Striving for a high slag formation rate means aiming for favorable gasification conditions. Based on an analysis of the current technical equipment and raw material conditions in the fertilizer industry, a slag formation rate of around 70% is considered an ideal value, while 75% represents the optimal level. Pursuing an even higher slag formation rate will affect the amount of gas produced per furnace and prevent the furnace from operating stably. By using high-quality raw materials and ensuring a reasonable height-to-diameter ratio for the gas furnace, the slag formation rate can reach the optimal value of 75%, while the carbon content in the ash can attain the ideal level of ≤15%. “Although “scars” and “residues” represent two different gasification conditions, in practical production it is necessary to achieve an optimal integration of the two in order to bring the gasification conditions to their best state. When the slag mass is light, porous, and brittle, its slag formation rate is not high. Although it shows that the gasification conditions inside the furnace are stable with no risk of deterioration, it also exhibits signs of a \"peaceful furnace\". In an era that advocates for boosting production and increasing the output per furnace, we cannot be satisfied with this situation. The optimal slag formation rate and an ideal low ash content along with low carbon content are achieved under conditions of high furnace temperature and high load; to attain a high slag formation rate, it is also necessary to maintain a certain level of \"scar formation\" within the furnace. Among the discharged slag pieces, 15% to 20% of them should show signs of being molten. This indicates that the gasification conditions operate at the limit temperature; it can be said that only under temperature conditions where there is slight local scarring can significant slag formation occur. Under such gasification conditions, careful attention must be paid to monitoring the furnace operation; since the furnace operates under extreme conditions, process control becomes more difficult, requiring predictive adjustments. Although the operational difficulty has increased, the furnace operation has entered the optimal economic operating load range, which represents the best conditions for achieving efficiency. In summary, the ideal slag formation condition is an slag formation rate of 70%–75%, wherein slag lumps with traces of melting account for 15%–20%, large and medium-sized slag lumps account for 40%–45%, small slag lumps and particles account for about 30%, and the rest consists of fine ash and a small amount of recycled coke. Under these conditions, there are no fine coking particles; only about 10% of the coke pieces have a size of around 20 mm, which is a result of the cooling effect of the jacket wall. Fine ash should be gray in color; colors that are too white or too dark are abnormal. The fine ash should contain small particles showing signs of scarring, grinding, abrasion, and peeling, rather than being entirely in a fine, powdery form. The proportion of scar masses with melting traces can only be 15% to 20%; a higher proportion indicates an excessively intense operating condition in the furnace, and the stability of furnace operation cannot be guaranteed. 5 Reuse of Recycled Coke 5.1 Quality Requirements for Recycled Coke Strict quality requirements must also be established for the selection of recycled coke: firstly, it is necessary to minimize the content of slag in this recycled coke, as the reintroduction of slag into the furnace can have a significant impact on the gasification conditions ； The second is to minimize the amount of ash-containing residue resulting from defocusing ； Thirdly, it is necessary to prevent the recycling of gangue; the contents of slag, fines, and gangue must all be strictly kept below 6%. If the quality of the recycled coke cannot be guaranteed, the presence of large amounts of impurities in the furnace will disrupt the gasification conditions and lead to losses; in comparison, it is better to abandon such recycled coke if its quality cannot be ensured. Therefore, it is necessary to carry out this work meticulously and thoroughly to ensure quality; only then can the value of re-focusing, sorting, and reusing be realized. 5.2 Combusting returned coke does not require changing operating conditions. For many years, gas production operators at most manufacturers have followed an operating method passed down orally by several generations of masters: “When burning returned coke, it is necessary to increase feed rates and extraction rates significantly.” However, back-focusing is not a low-quality material; then why implement methods to accelerate the rate of bed layer metabolism? The root cause lies in the lack of understanding of the back-firing phenomenon; there is concern that it might affect the furnace operation, and as a result, excessive use of back-firing material leads to a decline in the quality of the slag. Backfiring refers to the coke chunks that remain after the lump coal added is coked, as the gasification does not occur completely; they are called coke because they have been exposed to high temperatures. During the gasification process, lump coal begins to gasify from its surface, with the ash being stripped away; the size of the coal lump gradually decreases until complete gasification occurs. In contrast, for coal with a larger particle size, due to factors such as gasification rate and metabolic rate, some of it is discharged from the furnace before it has been fully gasified; as a result, the carbon content of the remaining coke blocks does not decrease. Moreover, after being exposed to high temperatures, the volatile substances, tar, sulfur, and other hydrocarbon compounds contained in the coal are essentially eliminated. From this perspective, its quality is higher than that of the raw coal fed into the furnace; therefore, there is no need to change the existing operating procedures and process conditions when blending it with returned coke. 6　Personalized development of operating methods and conceptual differences: Some manufacturers, under their own objective conditions and through long-term practical experience, gradually develop a set of dry-processing methods that even they themselves cannot fully explain. Yet it considers this to be the best way of operating and adheres to it consistently – this is personalization. This is also why the operational concepts in the fertilizer industry, particularly in the production of ammonia from nitrogen sources, vary so greatly. Some of these views are difficult to communicate, while others indeed contain unique and insightful elements. Through years of communicating with numerous manufacturers, the author has come to realize that this field truly is a platform where a wide variety of ideas flourish, and many issues can inspire reflection and deep consideration. However, once personalized, it becomes deeply rooted and difficult to change. By adhering excessively to individualism, refusing to adopt the best from others and rejecting new ideas, one will forever remain trapped in misconceptions. For many years, there has seemed to be an insurmountable gap between the φ3000 mm series gas furnaces and the φ2000 mm series gas furnaces in terms of operation and technology. In fact, the base model for the φ3000 mm series of gas furnaces is the UGI φ2745 mm gas furnace. Moreover, the previous φ2000 mm series of gas furnaces has been expanded to φ2800 mm, exceeding the cross-sectional area of the φ2745 mm furnace and approaching that of the φ3000 mm furnace; their operating characteristics are similar, with the only difference lying in the approach to operation. Currently, the medium-nitrogen industry is also trying to adopt high-load gas production methods for small nitrogen plants, but due to various constraints, it is difficult to increase the load. On the other hand, some smaller nitrogen production plants, after expanding their scale and setting up multiple furnaces, adopted the traditional medium-nitrogen production methods, operating at low loads to maintain stable furnace conditions; the gasification intensity was kept within the range of 800–900 m3/(m²·h), with careful control to ensure that both the temperature at the top and bottom of the furnaces remained low. Some have restored the process conditions of the 1970s, with a temperature of 350°C above the furnace and 200°C below it; the quality of the slag produced under such conditions is predictable. 6.1 Personalization is reflected first in the differences in the selection of temperatures at both ends of the gas furnace (it should be noted that it was through trial and error that a lower upper furnace temperature was chosen, which in turn contributed to an overall improvement in the gas production capabilities of the small nitrogen fertilizer industry); the choice of temperatures at the upper and lower parts of the furnace reflects the distribution ratio of upward and downward gas flow. The second difference lies in the methods of nitrogen addition; top and bottom blowing for nitrogen addition have a solid theoretical and practical foundation, and there is no doubt regarding this ； Yet now about 50% of the factories do not use it. Thirdly, some plants employ intermittent operation of the grate machine; there are also gas furnaces equipped with ash removal systems that allow the furnace to remain in operation continuously, except during shutdowns for maintenance. This represents the optimal way of using such furnaces. Never stop the strand machine from operating in order to increase the thickness of the ash layer – this is a practice that should definitely no longer be used. Fourth is the selection of the bed thickness; adjusting the height-to-diameter ratio to a reasonable level is essential for further improving the gasification intensity and the efficiency of raw material conversion. Yet, currently, there is an increasing trend of using shut-off valves to reduce the carbon layer. Fifth is the difference in the choice of cycle time; regardless of objective conditions, it is often believed that a shorter cycle time can solve all problems, with cycle times being reduced to extreme levels of 90–100 seconds. Sixth, the positions of process valves are moved back and forth; efforts are made to shift these valves in order to produce more gas and save steam. Too many ineffective actions are taken, without any consideration for process technology, operational techniques, safe production, or the service life of the valves. Seventh, there is a lack of emphasis on carbon layer stability; manufacturers that add coal manually still face the problem of excessive fluctuations in the carbon layer. Even manufacturers that use automatic coal feeding encounter the problem of large fluctuations in the coal layer height for various reasons. The effect of variations in the particle size of the raw materials fed into the furnace alone on the bed resistance makes it difficult to manage operations effectively; if the carbon layer is not properly controlled as well, it becomes even harder to stabilize the conditions inside the furnace, let alone optimize them. These problems arise due to insufficient attention and inadequate operational management. 6.2 New Concepts: Personalized operations often focus on only one aspect, failing to take everything into account and achieve a balanced approach; as a result, it is difficult to achieve optimal operational conditions. When adjusting gas production, it is important to \"focus on the key aspects, emphasize the main elements, and take all factors into consideration.\" Personalization tends to be superficial on key issues, yet it involves \"extremely in-depth research\" on minor details. In some aspects, it can be said to have a certain advantage, but the final production results are not satisfactory. Most manufacturers in the fertilizer industry have gone through a challenging journey of over 40 years, and several generations of gas production experts have never ceased to explore and refine the technologies and operational methods related to gas production. As mentioned earlier, each factory has its own capabilities and strengths; there is no factory that is completely useless. However, since the fertilizer industry has not provided any standardized vocational skills training for several years, the technical materials available to various factories are only at the level suitable for junior workers. There is no in-depth research on gas generation process technologies and control technologies, which has led to a situation where various approaches are pursued independently. The current situation is that some factories possess good manufacturing processes and operational methods, but they fail to recognize their advantages; instead, they give them up and try to adopt what seem to be the latest trends in operations. As a result, things get worse and worse, to the point where it becomes very difficult to return to the previous level of performance. To further improve and develop gas generation technology, it is necessary to draw on the strengths of various approaches, and integrate scientific principles related to gas furnace operation and gasification processes to foster innovation. Guided by these innovative concepts, the best elements can be selected to create a sophisticated set of new principles that have been refined through practical application. These principles can then be used to standardize the gas generation processes and operations in fertilizer manufacturers, reducing or eliminating individual differences, thereby narrowing the gap in performance between different manufacturers. To achieve this goal, it is of great significance in improving the efficiency of energy conversion as well as enhancing the economic and social benefits of the industry. 7. Conclusion In this paper, from the perspective of improving the quality of slag produced by gas furnaces, the author outlines how to increase the conversion efficiency of the raw materials fed into the furnace, reduce the carbon content in the slag, and eliminate the resulting waste of raw materials as well as the negative impact on economic efficiency. In a market scenario characterized by high coal prices and tight supply, the fertilizer industry must rely on existing fixed-bed gasification technology and conduct in-depth research on energy-saving technological innovations. The top priority in terms of technical innovation currently lies in finding ways to reduce coal consumption and improve the conversion efficiency of the raw materials fed into the furnace. Achieving new breakthroughs in this area is an important step toward further enhancing the utility of batch gas furnaces. At present, there are over 9,000 intermittent gasifiers in operation across the country’s coal chemical industry, with an average carbon content in the ash at around 23%. If efforts are made to reduce this figure by another 10%, it will yield significant economic and social benefits.