In-depth analysis of the conceptual distinction and formation mechanism of "return to coke" and "return to carbon" in gas generation ash

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In-depth analysis of the conceptual distinction and formation mechanism of "return to coke" and "return to carbon" in gas generation ash and residue Tian Shouguo National Gas Generation Technology Consulting Department Keywords: How to distinguish the completely different concepts of refocus rate, carbon return amount, "scar" and "slag" ； summary: The difference between returning coke and returning carbon is that ； The temperature at which lump coal is heated in the furnace and the length of time it lasts under high temperature conditions. The condition of ash and slag is the final result of the gasification process. From the gasification results, the gasification conditions in the furnace can be analyzed and the cause of the formation can be analyzed. ； The large plane temperature difference in the gasification layer and the partial fire and gray are the "stubborn problems" that affect normal production after the furnace type is enlarged. “"Technological progress is the fundamental way to save energy and reduce consumption." The status of an enterprise's energy saving and consumption reduction work also reflects the status of technological progress to a large extent. In the gas production process, the quality of the ash and slag of the gas generator is the ultimate reflection of the quality of the gasification conditions in the furnace. Today, when raw coal prices are high, the quality of slag produced by a manufacturer also determines the manufacturer's economic benefits. Nowadays, almost every enterprise has an assessment of "ash and slag re-coking rate" in the internal production evaluation of gas production. However, there are still some vague understandings in this regard in some places. For example, some manufacturers have been using the "ash slag return rate" to evaluate the quality of gas-making ash. This term is not completely wrong, because the gas generator does discharge "charcoal" from time to time, and this can only occur under extremely abnormal working conditions in the furnace, which is very undesirable. To manage and properly assess gas production, we must first understand the difference between "returning to coke" and "returning to charcoal" so that we can analyze the problem more carefully and accurately. 1. The fundamental difference between "returning to coke" and "returning to charcoal": Return of coke is an inevitable product of the normal operation of the gas generator. Why do you say this? Because the UGI fixed-bed intermittent gas generator with solid slag discharge, the overall supporting design conditions determine that there must be a certain amount of incomplete gasification in the slag discharge, but the coked coke is called "return coke", and the amount of recoke in the ash and slag is required to be reduced as much as possible. “"Return" is produced by two ways in the gas generator: One is the coke that remains after a piece of coal has been coked in the gas generator through the process of carbonization and gasified at high temperature by the oxide layer. Second, the main aspect is also a problem that cannot be completely solved under the existing configuration conditions. ； This is because the lower half of the gas generator body is equipped with a water tank jacketed boiler. Due to the wall cooling effect of the jacketed boiler, the gasification temperature in the edge area of ​​the gas generator close to the jacketed boiler is much lower than that of the outer ring area, inner ring area, and central area (note: Division of four rings in the furnace ； Center area, inner ring area, outer ring area, edge area), part of the carbon block cannot form high temperature conditions due to heat exchange on the inner wall of the jacket, and the gasification speed is slow and the gasification is not complete, thus producing a part of the coked back coke. Of course, the heat conduction mechanism of the jacket boiler is indispensable for this furnace type. “"Charcoal return" refers to ungasified coal lumps being discharged or flowing out from the slag outlet. It is the product of extremely abnormal gasification conditions in the gas generator. It is caused by factors such as scarring in the furnace, local scarring, excessive temperature difference in the plane of the furnace, partial fire and gray, coal collapse in local areas, wind tunnels in local areas, dead fires and other factors. It should be avoided as much as possible. The production of carbon return indicates that the gasification conditions in the furnace have deteriorated and the layers in each layer are disordered. In fact, "char return" refers to the fact that although the coal entering the furnace is added to the furnace and discharged, it has not undergone a complete carbonization process and has not been coked at all. Its surface still maintains its original gloss and smoothness. Sometimes even if the surface color of the carbon block changes slightly to white-brown, it should be classified as "charcoal return" because it has not yet been coked. 2. The fundamental difference between “scar” and “scum”: “The two words "scar" and "slag" represent two completely different gasification conditions. From the physical shape ； The surface and cross-section of the scars in the ash are lava-like, with a dense and non-porous structure, and the specific gravity is more than twice that of the slag. The first cause of scarring is that the furnace temperature loses thermal balance due to excessive blowing efficiency, low steam consumption, etc., causing the temperature in the gasification layer to exceed the ash melting point T3 of coal, causing the coal to melt and condense to form hard scars. The second is the scars caused by local scarring that often occurs after the expansion and transformation of today's gas furnaces. "Scars" and "charcoal return" are both products of extremely abnormal furnace conditions. Therefore, it is highly undesirable in normal production. “"Slag" is a product of the normal operation of the gas furnace, and it is best to hope that the slag formation rate reaches the highest limit. Slag is the continuous gasification of multi-granular coal or lump coal during the normal gasification process. The carbon molecules on its surface are transferred to the gas phase. The remaining ash still has a certain degree of viscosity and plasticity under the action of high temperature. Therefore, the ash is continuously peeled off and bonded to form slag blocks. The amount of slag formed is the "slag formation rate". The slagging rate is adjustable and depends on the temperature of the gasification layer. The higher the temperature of the ash slagging environment, the higher the slagging rate. However, if the temperature reaches the T3 temperature of the raw material, the raw material will melt and the furnace condition will deteriorate. The optimal temperature is to control the gasification layer temperature between T2 and T3 of the raw material. However, because the current temperature detection technology has not reached the corresponding level, the temperature value is not easy to determine accurately. Therefore, the current gasification layer temperature is generally at the T2 temperature of the raw material. And if the ash slagging environment temperature is low ； The slag formation rate is low, the amount of fine ash is large, and the refocusing rate is high. Of course, the slag formation rate is also related to the different chemical activities of the raw materials and the elemental composition of the ash. 3. Conditional state analysis of high refocus rate: As mentioned earlier, it is normal for a certain amount of back coke to be included in the ash discharged from the gas generator. Why do you say that? The reason: First, the gasification device is equipped with a jacketed boiler to produce wall cooling effect. Second, the particle size of coal entering the furnace cannot be uniform, and the gasification speed of lump coal is inconsistent. Third, the coal entering the furnace is mixed from multiple mineral points and has mixed properties. The coal has different ash melting points and chemical activities, and the gasification speed is inconsistent. The fourth is the issue of uniformity of gasification agent distribution determined by the grate. Fifth, there are issues with process conditions and operating control conditions. The existence of these factors determines that the combustible components in the ash cannot be reduced to as low as the carbon content of the slag produced by the new gasification method. In other words, the slag recoking rate of the gas generator is just a matter of more or less quantity. The slag formation rate and coking rate of gas furnace ash are not only related to the chemical activity of the coal and the ash content contained in the coal, but also related to the gasification temperature in the furnace and the length of time it continues to withstand high-temperature gasification. Therefore, high load and high gasification layer temperature are the primary conditions for increasing the slagging rate and reducing the refocus rate. In addition, the three height-to-diameter ratios of the gas furnace (gas furnace, water jacket, and carbon layer) are reasonable, the effective gasification space is increased, the raw coal has sufficient drying, carbonization, and gasification time in the furnace, and there are high-temperature gasification conditions. Under these conditions, the slag formation rate will inevitably increase and the coking rate will decrease. At present, there are many terms used to describe the problem of ash and slag conditions. For example: Return coke rate, returned carbon amount, residual carbon amount, ash combustible content, etc. Only two of the above titles are accurate, one is the "refocus rate" based on the physical analysis method, and the other is the percentage of "carbon content in the ash" based on the chemical analysis method. The "ash and slag combustibles" generally refers to the carbon content of steam boiler slag. By observing and analyzing the morphological changes of ash and slag and analyzing the reaction mechanism of back-coke formation, we can basically determine the extent of the wall cooling effect. ； When the process conditions, raw material conditions and operation control are all in ideal conditions, and the furnace conditions are in a stable state. Under the conditions of high slag formation rate, little fine ash, and large amount of fine slag, there is still a small amount of back coke with relatively large particle size, with a particle size of 15 to 30 mm, and a quantity of less than 15%. There are particularly few fine coke particles. Under such conditions, very little back coke is generated in the gasification zone. More than 95% of the back coke in the ash and slag is generated in the edge zone, which is mainly a reflection of the wall cooling effect. This slagging state also shows that the working conditions in the gas generator are excellent and the load has entered the limit state. ? However, if the amount of fine-grained refocusing in the ash is large, there are certain problems in control, control, and management. The fine coke particles brought out from the ash and slag greatly affect the conversion and utilization rate of raw materials. It is difficult to recover these coke particles and reuse them in the furnace for gasification. Moreover, the amount of this part of the combustible material is sometimes greater than the amount of coke that can be recycled, which causes serious waste of raw materials for many manufacturers. Even if it is reused in a steam boiler, although a part of the fuel coal is replaced, part of the raw coal is devalued into fuel coal, which has a great impact on the overall cost. The devalued use of energy is not in line with the concept of circular economy. This problem can also be reduced through human control. To be more precise, the fine coke particles can make it appear in small amounts or not at all. Small particle size and large amount of returned coke exist in many manufacturers. This phenomenon is one of the main reasons for the low conversion and utilization rate of raw coal. The anthracite lump coal undergoes a qualitative change after passing through the carbonization layer, and is transformed from "coal" into "coke". Return coke is the coke that remains after the lump coal loses volatile matter after being heated through the carbonization layer, undergoes surface oxidation and exothermic reaction in the gasification layer, and is calcined at high temperature. After the lump of coal is converted into returned coke, the smooth colloid layer on its surface is lost due to retorting, and the coke block also changes from oily black to grayish brown, and its surface becomes rough. The shape also loses its edges and corners, becoming round, oval or pillow-shaped. Due to the loss of volatile matter and the smaller specific gravity of water, the pores produced by surface vaporization are clearly identifiable. Due to its rough surface, the back-coke has greater flow astringency and poor sliding ability. The angle of repose at the edge of the coal in the natural accumulation state is larger than that of lump coal. Under normal operating conditions of the gas furnace, the surface astringency of the lump coal increases after retort coking, which weakens the fluidity of the materials in the gasification layer, which is beneficial to the stability of the gasification layer and reduces the probability of graying and coal collapse. (Generally, the strong and weak fluidity of materials in the furnace can be divided into the following four levels, namely: lump coal>return coke>fine ash>slag block, scar block). 4. Analysis of conditions with high carbon return amount: As mentioned in the previous article, the level of refocusing rate is not only affected by the characteristics of the gasification equipment and the physical and chemical characteristics of the raw materials, but also related to the operating control conditions. However, in comparison, no matter how high or low the refocus rate is, it at least shows that the furnace condition is in a basically normal state, is adjustable, and can be reversed. The high content of "returning carbon" is a sign or result of extremely abnormal working conditions in the furnace, critical deterioration, or deterioration. “"Return to carbon" refers to the carbon that is mixed in the ash residue and has not been completely retorted or not retorted at all, and is not completely coked, and the surface is still black. The surface gelatin layer has not completely lost its gloss, and the appearance of edges and corners has not changed. The main reason for carbon return ； First, under high-intensity gasification conditions, due to improper control of the position of the fire layer, the fire layer is scattered and partially scarred. The fire layer in the furnace is unevenly distributed. The gas generator is in a partial operation state, and the bed layer descends at different speeds. One side of the two ash bins discharges scar blocks, and the other side discharges charcoal due to the ash layer being drained. The second is scarring in the furnace ； A wind tunnel is formed in the gap between the scars, and the coal leaks to the ash pan and is discharged. Third, equipment defects caused ； The design and configuration of the slag-breaking and ash-discharging mechanisms are unreasonable, resulting in partial fire and partial ash in the furnace, resulting in carbon return. Fourth, the grate has poor slag-breaking ability, and the slag particle size is too large. When the slag block is discharged, the loose combustion charcoal in the low-temperature area on its back collapses. Fifth, the gas generator is in low load gasification condition ； Under low temperature conditions, the slag formation rate is low, the coking rate is high, the stability of the ash and slag layer is poor, and the fluidity is strong. If the flow prevention measures are slightly lacking, carbon collapse will easily occur. Sixth, the level of operation control is low, the operation is improper, the ash layer is drained out, and the fire layer falls to the bottom. In addition, under low temperature conditions, the metabolism rate in the furnace is too slow, and local areas are prone to overfire, forming large low-temperature slag blocks. Areas with slag blocks decline more slowly, while areas without slag blocks form gray and partial fires due to low-temperature ash loosening and rapid decline. In short, the high amount of carbon return originates from unreasonable process conditions, excessive accumulation of ash and slag, which causes the position of the fire layer to drift, resulting in uneven heat distribution, local scarring, disordered layer areas, partial fire and partial gray, or equipment defects. Sometimes during the operation, some problems are not handled in time or handled improperly, which aggravates the problem, causing holes to form in the middle and lower parts of the bed, causing collapse. Under abnormal working conditions, due to the different particle sizes of coal entering the bottom of the bed, the movement of the ash layer is reduced. The presence of returned carbon has a great impact on the stability of the ash layer, which enhances the fluidity of the ash layer, greatly increases the probability of coal collapse, and the probability of collapse is particularly high. (The fluidity of materials in the furnace is divided into six levels when it is abnormally strong or weak.: Lump coal > Returned char > Fine char > Fine ash > Cinder block > Scar block) 5. Utilization of returned coke: Under normal conditions, the gasification process of coal lumps starts from the surface of the coal lumps. While gasification, the ash is peeled off and formed into slag under the action of high temperature. The particle size of the coal lumps gradually decreases until the gasification is complete and slag is formed. However, due to the cooling effect of the jacketed boiler wall and the presence of some oversized coal lumps in the normal gasification area, coupled with the influence of factors such as gasification speed and metabolism speed, there will always be some coke that is discharged out of the furnace before gasification is complete. However, the carbon content of the residual coke has not been reduced, and the volatile matter, tar, sulfur and other hydrocarbons contained in the coal after high temperature have basically disappeared. Therefore, the grade of the returned coke is higher than that of the coal entering the furnace. From this point of view, when a small amount of coke is mixed with burnt coke, there is no need to change the original operating rules and process conditions. If a large amount of coke raw material is mixed with gasification, the fire layer position will move upward faster because the coke raw material does not have a carbonization process. The gasification characteristics of coke raw materials are reflected, and the process conditions should be changed accordingly. Generally, chemical analysis results of refocusing (note: Calorific value is the application base unit: KJ/kg) Analysis items Moisture content Carbon content Ash content Volatile calorific value analysis results 1.05% 76.10% 21.51% 2.18% 24539 6. Comprehensive analysis: The above analysis has analyzed the fundamental difference between "returning coke" and "returning carbon", as well as the differences in the conditions for their respective production, as well as the different impacts on the working conditions in the gas furnace. It can be seen that the properties and formation conditions of return coke and return char are completely different. The former is that the working conditions in the furnace are in a state where the process conditions and operating conditions are basically normal but not optimized. The latter means that the working conditions in the furnace are in disordered layer areas, with partial fire and partial gray, and have even begun to develop from the critical line of deterioration or have already deteriorated. These two problems are the primary issues directly related to the conversion and utilization of raw materials to varying degrees, but to solve these two problems, completely different corresponding measures must be taken. Solve the problem of high refocus rate ； First, we must do a good job in reasonably determining the process indicators of gas generators. The second is to determine a scientific and reasonable operating method that is suitable for this system, internal and external objective conditions. The third is to ensure reasonable control of process conditions and stable operating conditions. The fourth is to do a good job in particle size control and screening of raw coal entering the furnace, and to rationally mix it according to its characteristics. The fifth is to establish scientific and reasonable, strict and detailed rules and regulations to standardize operations, thereby optimizing process conditions, stabilizing furnace conditions, reducing the slag and coking rate of gas generators, and reducing consumption. To solve the problems of partial fire, partial ash, and excessive carbon return, we must first conduct a realistic and in-depth analysis of the actual internal and external problems that exist objectively such as the technical equipment, process conditions, and operating methods of the system. Then identify the key points by category, seize the key issues and prescribe the right medicine, and also adopt technical transformation in both software and hardware at the same time. Regardless of the internal and external conditions of the enterprise, it is necessary to specially select and require special design based on the existing conditions for the grate used in the gas generator. Due to the influence of external conditions and the distribution characteristics of the gas generating furnace bed layer, the resistance of each ring zone in the gas generating furnace bed layer is inconsistent. In addition, there are problems in the raw material structure, coal addition method, fan performance, system resistance, etc. Different manufacturers have different characteristics, and even the gas generating furnaces of one manufacturer have different characteristics. Therefore, there has long been a saying that "one factory has one emotion, one furnace has one situation". Therefore, it is necessary to choose a grate suitable for the actual situation of the factory to suit different resistance characteristics. Because of different coal adding methods, different coal qualities, different physical and chemical properties, and different fan performances, the resistance characteristics in the bed have different characteristics. A grate with fixed performance cannot cover everything. The slag breaking and flow prevention work of the gas generator must be done well. First, we must pay attention to controlling the particle size of the slag. Second, we must do a good job in preventing, blocking, and extending the flow. “Flow prevention measures such as "internal barrier", "lower barrier" and "external extension" have their own advantages and disadvantages. It is necessary to choose the methods and measures suitable for the supporting conditions of this system among many measures. During the operation of the gas generator, it should be avoided to operate under the condition of large air volume and overload for a long period of time, or to operate under the condition of low air volume and low load for a long period of time. In terms of maintenance of the ash discharge mechanism, while reasonably matching the grate, it is also necessary to ensure that the ash plow and the slag breaking strips distributed around it are neat and intact to ensure the uniformity of slag breaking and ash discharge around the circumference. The control of ash discharge speed must absolutely prevent the speed adjustment range of the grate machine from being too large, and the "new 'four stable operation methods'" proposed by the author should be promoted and actively applied. The slag particle size is adjustable, but the size of the slag particle size has a great impact on the process conditions. The impact of the two slag discharging states on the amount of carbon returned is very different between the two slag discharging states where the block diameter is 200-300mm and the block diameter is within 150mm. When agglomeration and scarring occur, air cannot pass under the scar, so a large amount of air can only pass through the edge of the scar, causing the exothermic reaction at the edge of the scar to be stronger than in other areas, so the temperature in this area is higher than that in other areas, and the slag generated in this area can easily become It is in a molten state and adheres to the original scar block, so the scar block becomes larger and larger. When the large-grained scar block leaves the gasification layer, its back is always holding a low-temperature mass. The lump coal in the low-temperature mass has a smooth surface and strong fluidity due to the low temperature within the location range. Sometimes the position of the scar block will move in the direction of the rotation of the ash pan. This is mainly because the large slag block is not easily broken when it slides above or between the largest layer of the grate and the slag breaking ribs. Therefore, the slag block is pushed to rotate around the ash channel, and there is always a low temperature zone on the back of the slag block. Because the scar block moves slowly, the rotation of the ash disk removes the ash residue under the scar block, thus forming a cavity in the lower part of the scar block and partially forming suspended material. The scar blocks fall into the suspended ash bin at the bottom, causing varying degrees of collapse in the bed. The burning or raw charcoal then falls into the ash bin, and the collapse surface moves to the ash discharge port along with the scar blocks. The discharge of large scar blocks can easily cause large-scale collapse, and red or even raw charcoal will be difficult to clean up. The immediate collapse that occurs at the slag outlet is burning charcoal (also called "red charcoal"). ； When the bed collapses within the ash silo, the burnt charcoal flows out when the collapse surface turns to the slag outlet. Therefore, if the scar is not broken and eliminated as soon as possible, it will become larger and larger. In order to remove the scars as soon as possible and control the particle size of the slag discharge to not be too large and uniform, the design of the grate and the matching technology of the slag breaking ribs must be carefully considered and taken seriously, and the matching should be reasonable. If the designed slag breaking conditions meet the requirements, the slag particle size adjustment is reasonable. The large slag blocks are broken to about 100mm in the slag breaking area, and the gasification effect is different. After the slag block breaks, there is a ash and slag pad in the lower ash channel, so it will not fall onto the ash pan immediately. After the slag block cracks, there will be ventilation and steam ventilation conditions in the gaps between the slag blocks. Therefore, after the scar block cracks, the raw coal at the back can be quickly gasified to form a new ash layer. With this process, the fluctuations in furnace conditions are controlled to a large extent, and the results are of course completely different from those under conditions where the slag particles are too large. In the production of gas generators, when there are no slag blocks in the furnace, the air distribution is controlled by the characteristics of the grate air distribution. The air volume is evenly distributed and the flow rate is balanced. When there is a scar, the gasification agent flow rate in the scar gap is greater than in the normal area. 7.Conclusion: The high rates of "returning coke" and "returning carbon" in the ash and slag of gas generators are two serious problems that seriously affect the conversion and utilization rate of raw materials, and are important obstacles to reducing coal consumption and stabilizing furnace conditions. It is necessary to make great efforts to rectify, eliminate equipment defects and improve the level of technology, so that the operation of the gas generator can be stabilized. Only by laying a solid foundation and optimizing processes and operations can we further reduce consumption. The determination and control of the process conditions and operating conditions of the gasifier is an extremely complex task. First of all, we need to understand the principle. We should clearly understand the different concepts of "return to coke" and "return to charcoal", which will make it easier to identify the situation, analyze the cause, and prescribe the right medicine.