Discussion on Issues Related to the Burners in Water-Coal Slurry Gasification Furnace Processes

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Due to its high efficiency and cleanliness, the pressurized gasification technology for water-coal slurry has received significant attention and widespread adoption in China. However, the continuous service life of the water-coal slurry gasification burner is relatively short, which limits the long-term operation of the plant and affects economic efficiency. To this end, domestic research institutions and users of burners have carried out extensive work to make improvements in various aspects such as structure, design, and materials, in an effort to extend their service life. The author of this article has been engaged in the development of burners for water-coal slurry gasification furnaces for many years. As the project leader, they took charge of the project titled \"Special Contract for Major Technical Equipment Development Project (Science and Technology Research Program): Development of Burners for Water-Coal Slurry Gasification Furnaces\", which was successfully applied in the domestic pressure gasification facility for water-coal slurry at Shandong Hualu Hengsheng Chemical Co., Ltd. We also supply the burners for the water-coal slurry staged gasification technology (also known as the “non-slag–slag” coal gasification technology), which was jointly developed by Beijing Dalike Technology Co., Ltd., Tsinghua University, and Shanxi Fengxi Fertilizer Group. This technology passed the expert evaluation organized by the Sinopec Association on December 6, 2007. The author of this article was involved in the development of this project throughout its entire course. They provided suggestions regarding the selection of solutions, patent applications, the determination of the structure of the gasification furnace, the design and configuration of process burners, as well as the design and configuration of secondary oxygen supply burners. The specialized process burners and secondary oxygen supply burners developed as part of this work made significant contributions to the industrial implementation of this technological process. The author of this article provides a brief overview of some of the work and considerations carried out in the development of burners for water-coal slurry gasification furnaces, and also offers some personal views on the improvement of such burners, for the reference of colleagues. 1 Design of the process burner The head structure of the burner commonly used in current gasification furnace processes is shown in Figure 1. The factors to consider in the design of a burner include the following. Figure 1 Typical structure of the burner head in the water-coal slurry gasification furnace process. (1) The structural format is a concentric triple-tube design. The outlet of the central oxygen tube in the burner is designed to be constricted, with the aim of accelerating the central oxygen. Meanwhile, its end face is set at a certain distance inward relative to the reference plane of the burner’s cross-section, thereby creating a pre-mixing chamber for the water-coal slurry and the central oxygen. The outlet pipeline for the water-coal slurry is also designed to be constricted, so that the water-coal slurry entering the pre-mixing chamber has a certain velocity. In the premixing chamber, central oxygen is used to dilute and initially accelerate the water-coal slurry, thereby improving its rheological properties; this is done to ensure good atomization of the water-coal slurry after it leaves the burner. The retraction amount of the external oxygen inlet is greater, with the aim of providing a higher flow rate of oxygen to ensure good atomization of the water-coal slurry mixture passing through the premixing chamber, thereby achieving optimal gasification effects in the gasifier. (2) Flow channels: The flow area for both central oxygen, coal water slurry, and external oxygen must meet the flow requirements of their respective media. Where supply pressure permits, strive to achieve good mixing and atomization effects. However, the proportion of central oxygen is subject to certain limits, generally ranging from 5% to 25% of the total oxygen amount, with the remainder being peripheral oxygen. The oxygen content at the center cannot be too low, otherwise it will not be possible to achieve dilution and acceleration of the coal slurry. The oxygen concentration at the center should also not be too high. On one hand, a high level will cause the flow velocity of the mixture in the premixing zone to increase significantly, leading to increased wear at the outlet of the central tube and reducing the burner’s overall service life ； On the other hand, an increase in the oxygen content at the center inevitably leads to an increase in the axial velocity component of the material at the burner outlet and a decrease in the radial velocity component. As a result, the flame at the burner outlet becomes more slender and is unable to fit with the internal geometry of the gasification furnace; this causes coal powder particles with a larger diameter to stay in the furnace for a shorter time, increases the carbon content in the slag, and reduces the gasification efficiency. Moreover, it causes the flame to shoot straight toward the bottom of the furnace, affecting the operation of the cooling ring at the furnace bottom. (3) Flow rates: The outlet flow rate of oxygen at the center is generally 150–180 m/s, the outlet flow rate of coal slurry is generally 2–4 m/s, the average outlet flow rate in the premixing chamber is generally 12–20 m/s, and the outlet flow rate of external oxygen is generally 160–200 m/s. 2 Discussion on the main factors affecting the service life of burners. Based on years of experience in the development of burners for water-coal slurry gasification furnaces, as well as practical insights from industrial operations, there are mainly several ways in which these burners can be damaged: namely, damage to the cooling water coils ； Physical abrasion of the middle nozzle ； Heat, chemicals, and stress damage the external nozzle. The following is an analysis one by one. 2.1 Damage to the cooling water coil: The function of the cooling water coil is to protect the burner (the part that is exposed to high-temperature process gases), and the environment surrounding the cooling water coil is harsh. The common ways of damage are as follows. (1) Thermal stress damage at the weld between the cold zone water coil and the external nozzles. The reason is that the connection between the two components is achieved through fillet welding; there is a significant difference in wall thickness, and the materials used are different as well. Additionally, since they are located at the end of the burner, cracks (mainly due to thermal stress) are likely to occur during use, leading to damage. The improvement we implemented was to use the same material at the fillet welds, which yielded some results. As shown in Figure 2. Figure 2: Improved cooling water coil (2). If the temperature of the cooling water inside the coil is not properly controlled, it can lead to low-temperature corrosion on the surface of the coil. It is generally appropriate to keep the cooling water temperature above 170 °C; however, in actual process design, the temperature of the cooling water is usually below 50 °C, which results in low-temperature corrosion on the coil surface. Additionally, the coil should be made of a material with stable high-temperature performance; currently, Inconel 600 is the best choice for such materials. (3) During the bending process of the cooling water coil, it is necessary to control the heating temperature and bending speed, as well as the degree of deformation and thinning of the pipe material, in order to ensure the overall strength and stiffness of the coil after it is formed. (4) During normal operation, due to strong gas backflow at the end face of the process burner, slag accumulation often occurs in the gap between the process burner and the inner wall of the gasifier. This slag can also cause damage to the coiled tubes when the burner is removed; increasing the wall thickness of these coiled tubes can help reduce such damage. 2.2 Physical abrasion of the mid-nozzle Physical wear is a critical weakness of the burners in water-coal slurry gasification furnaces, and it is also one of the main factors affecting the continuous operation of these furnaces as well as the entire process. Under normal circumstances, the burners in water-coal slurry gasification furnaces need to be shut down for maintenance and replacement after operating continuously for 30 to 60 days. Therefore, a backup furnace is necessary for the water-coal slurry gasification process, and this is one of the main reasons for increased investment and operational costs. As mentioned earlier, in order to ensure thorough gasification of the coal powder in the water-coal slurry within the gasifier, a certain amount of oxygen must be used to atomize the water-coal slurry. Figure 3 shows the relationship between the size of the atomized droplets in the water-coal slurry and the flow rate of the atomizing gas (i.e., the gas velocity). To achieve good atomization, the gas flow rate must reach a certain value. In other words, the flow rate of the mixture (water-coal slurry, oxygen) within the premixing chamber must reach a certain value. Due to the large amount of coal powder solid particles in the mixture, wear occurs in the inner cavity of the middle nozzle. To enhance the wear resistance of the inner cavity of the middle nozzle, using materials with good wear resistance is currently the only feasible approach; of course, oxidation resistance also needs to be taken into account. The commonly used materials are GH188 and UMCo-50, but their continuous operation time is only 30 to 60 days. There are many factors that affect the operation cycle, mainly including coal type and production load. It is reported that the relevant organizations have successfully developed nozzles made of ceramic materials with a longer lifespan, but there have been no public reports on this yet. We sprayed a physically abrasion-resistant cemented carbide on the inner surface of the middle nozzle, which helps to increase the continuous operating time of the burner (see Figure 5b); relevant testing is still in progress. The flow rate of the mixture in the premixing chamber cannot be too low, as a too low flow rate will result in poor atomization and affect the overall carbon conversion rate. The flow rate should not be too high either; if it is too high, the axial velocity component of the mixture increases, causing the flame to become elongated or even extend straight down to the bottom of the furnace. This affects the residence time of some of the materials as well as the carbon conversion rate. Moreover, as can be seen from Figure 3, once the mixture flow rate reaches a certain value, its impact on the size of the atomized droplets becomes very small. Figure 3 Relationship between the atomization performance of water-coal slurry and the atomization oxygen flow rate. 2.3 Effects of heat, chemistry, and stress on the outer nozzle: Another form of damage to the burners in water-coal slurry gasification plants is the appearance of radial cracks and irregular fissures on the end face of the outer nozzle. After the burner has been operating normally for a period of time, dense radial cracks and irregular fissures appear along the edges of the outer nozzle orifices, as shown in Figure 4. 2.3.1 Causes of crack formation There is currently no authoritative conclusion regarding the causes of crack formation; the author believes that the main influencing factors are as follows. Figure 4 Typical damage patterns of external nozzles (1) Effect of thermal shock. Since the end face of the external nozzle faces the high-temperature process gas inside the furnace, which is typically between 1200 and 1500 °C, metal materials operating under such high temperatures for extended periods, along with being subjected to the impact of high-speed coal slurry, oxygen, and recirculating process gas, will gradually reveal all defects present during the smelting and forging processes; this leads to the formation of irregular cracks. (2) Chemical effects. Due to the high oxygen concentration at the outer nozzle, the metal material undergoes oxidation reactions in a high-temperature oxidizing environment. Under high temperatures, metal materials can also undergo a certain degree of carburization, which alters their composition and properties. Furthermore, sulfur in coal can also cause high-temperature sulfidation corrosion on metal surfaces. (3) Stress effect. The source of radial radiating cracks is the orifice of the external nozzle, which is a stress concentration area resulting from the mechanical processing of the component. Under high-temperature conditions, stress release is the cause of the formation of these radiating cracks, and it is also the main factor leading to damage of the external nozzle. A high jet velocity exists at the outlet of the process burner; this is also the area where the backflow speed of the high-temperature process gas is highest. The end face of the external nozzle is subjected to the impact of high-temperature process gas containing solid coal powder particles, which is another factor contributing to damage. This is also evidenced by the fact that the refractory bricks in the gasifier arch are prone to damage. 3 Exploration of ways to improve the lifespan of process burners As mentioned earlier, the factors that affect the service life of process burners in water-coal slurry gasification furnaces are mainly two: the physical wear of the middle nozzle, and the damage to the outer nozzle caused by heat, chemicals, and stress. Below, the author presents some views and ideas for reference by fellow experts and technical personnel. 3.1 Physical wear of mid-mounted nozzles: We can take action in the following areas to improve their service life. (1) Under the premise of meeting the flow and atomization requirements, minimize the exit flow velocity of the premixing chamber. For general metal materials, when subjected to the scouring by a fluid containing solid particles, the relationship between the wear rate and the fluid flow velocity is shown in Figure 5. It can be seen that when the fluid flow velocity reaches a certain value (which can be referred to as the critical velocity), the wear rate increases significantly. By keeping the flow velocity at the outlet of the premixing chamber below the critical velocity for material wear, the service life of the intermediate nozzle can be extended. (2) It can also be seen from Figure 5 that different materials exhibit certain differences in wear resistance. Although Material D has poorer wear resistance at low speeds compared to Material C, it possesses a higher critical speed; its wear resistance at high speeds is significantly better than that of Material C. Therefore, once the minimum flow velocity at the exit of the premixing chamber is determined, it is highly practical to find a material with a high critical wear speed (of course, for water-coal slurry) to use in manufacturing the mid-nozzle, so that its critical wear speed is higher than the flow velocity at the exit of the premixing chamber. Figure 5 Relationship between material wear rate and particle flow velocity (3) Optimizing the structural dimensions can also improve the wear resistance of the middle nozzle. Figure 6 shows the relationship between the impact angle of solid particles and the wear rate. It can be seen that the impact angle of the solid particles has a significant effect on the wear rate of the material surface, with the wear rate being highest at a certain angle (which can be referred to as the critical angle). When designing the inner surface of the nozzle, trying to keep the impact angle away from the critical angle can also reduce material wear. Figure 6 Relationship between material wear rate and particle impact angle (4) Improving surface structural properties. First, it is necessary to ensure the smoothness of the material surface ； Secondly, it is necessary to ensure uniformity in the internal quality of the material ； Another important point is to spray an anti-wear material (such as cemented carbide) on the surface of the inner hole of the middle nozzle in order to improve its anti-wear properties. The medium-nozzle shown in Figure 7b is used for spraying superalloys, and it can be seen that it yields quite noticeable results. It should be noted that there is a certain limit to the thickness of the coating; if it is too thick, it may peel off. Therefore, this improvement also has its limitations. 3.2 Measures to extend the lifespan of external nozzles The most direct way to address the problems associated with external nozzles is to improve the material’s resistance to high-temperature corrosion and thermal shock, while also carrying out stress-relief treatment after the nozzles have been manufactured. The currently recognized best materials are GH188 and UMCo-50, but even nozzles made from these two materials have an operational life that is far from meeting the requirements of production. To address this issue, researchers have considered and attempted approaches in the following areas. (1) Replace the material of the external nozzle with porous ceramic or porous metal, and direct some of the oxygen through the porous medium into the vaporization furnace; this ensures effective cooling of the end surface and prevents all problems caused by high temperatures. If porous ceramics can be used, the problems of chemical corrosion and physical wear can also potentially be solved simultaneously. The principle of implementation is proposed in the U.S. patent (patent number CN1056916C), but how porous materials (especially ceramics) can ensure sealing and pressure resistance will be a new challenge. To date, no products in actual operation have been seen. (2) A heat protection plate is installed on the end face of the external nozzle; the material selection and connection of this protection plate are the main issues that need to be addressed. The U.S. patent (patent number CN1110358C) proposes using a special material to adapt a heat protection plate according to the shape of the end face of the external nozzle, thereby bypassing the limitations imposed by sealing conditions; however, there are no reports of its actual use. (3) By using thermal spraying technology to apply special heat-resistant and corrosion-resistant alloy powders to the end face of the outer nozzle, good heat and corrosion protection can be achieved, **extending the service life of the outer nozzle. By selecting special alloy powders and improving the thermal spraying process, it is believed that this approach can achieve satisfactory results. At present, we have already begun to implement this method, and the results of its industrial-scale operation are satisfactory. As shown in Figures 7a and 7b, the performance of the external nozzle improved significantly after thermal spraying. Figure 7a: The burner after one cycle of normal use. http://www.nmtech.com.cn/JISHUWANG/ADMIN/Southidceditor/uploadfile/20081219150832142.jpg Figure 7b: The burner after one cycle of use following thermal spraying. 3.3 Other factors Through the above analysis, it can be concluded that the external reasons for the short lifespan of the burners in water-coal slurry gasification furnaces are, firstly, the physical wear on the central nozzle caused by the water-coal slurry containing solid particles ； Second is the thermal, chemical, and stress damage to the external nozzles caused by high-temperature process gases. The physical wear of the middle nozzle has been discussed in considerable detail earlier. Although the mechanism of chemical corrosion of the outer nozzle in high-temperature gas environments has not yet been determined, reducing the gas temperature and oxygen concentration at the tip of the outer nozzle, as well as the backflow velocity of the process gas, will certainly reduce the rate and severity of thermal, chemical, and stress-induced damage to the outer nozzle. While ensuring the overall performance of the gasifier, if it is possible to meet the requirements regarding the atomization degree of the water-coal slurry imposed by the process burner through other means, the flow rate of the mixture at the outlet of the premixing chamber can be reduced. This will inevitably lower the physical wear rate of the middle nozzle. If the temperature at the tip of the burner’s outer nozzle can be reduced through other means, it is also possible to slow down the rate of thermal, chemical, and stress-induced damage to the outer nozzle. 3.4 Examples of burner improvements The water-coal slurry staged gasification technology (also known as the “non-slag–slag” coal gasification technology; the feeding method of the gasifier is shown in Figure 8), developed jointly by Beijing Dalike Technology Co., Ltd., Tsinghua University, and Shanxi Fengxi Fertilizer Group, represents improvements made in these two areas. Figure 8 Feeding method of the gasifier in the slurry coal gasification technology by staged gasification: (1) Reduce the oxygen flow rate of the process burner to about 80% of that in a conventional gasification process (also known as primary gasification). After the oxygen is reallocated between outer oxygen and central oxygen, the flow velocity in the premixing chamber decreases significantly, by about 20%; as a result, the physical wear rate of the middle nozzle also decreases. The amount of decrease can be roughly estimated using the following impulse formula (ignoring frictional losses and mixing losses). G0V0 = GsVs + GyVy, where G0, Gs, and Gy are the mass flow rates of the mixture, water-coal slurry, and oxygen at the center, respectively ； V0, Vs, Vy are the flow velocities of the mixture, coal water slurry, and central oxygen, respectively. (2) With the water-coal slurry flow remaining constant, since the oxygen flow rate through the process burner decreases by about 20%, the backflow rate of the high-temperature process gas at the burner outlet also decreases, and the amount of this decrease is calculated as follows. Ge/G0+1=0.32(ρe/ρ0)X/d0, where Ge and G0 represent the flow rates of the high-temperature process gas used for recirculation and the total flow rate of the process burner, respectively. The overall result is that, on the one hand, the decrease in the total oxygen amount supplied by the process burner leads to a reduction in the oxygen concentration at the outlet nozzle surface, thereby weakening the local oxidation exothermic reactions ； On the other hand, as the total flow rate of the process burner decreases, the amount of high-temperature process gas flowing back is reduced, resulting in a decrease in the gas temperature at the outlet tip of the burner. The temperature drop measured by Tsinghua University in the laboratory was approximately 200 °C (see Figure 9), which is very important for improving the working conditions of the nozzles outside the process burners. The actual drop in production may differ from the experimental values, but the effects of heat, chemicals, and stress on the end surface of the nozzles outside the burners will be significantly reduced. Figure 9 compares the temperature fields of staged gasification (two-stage gasification) and single-stage gasification (continuous gasification). The remaining 20% of oxygen, which should otherwise enter the gasifier through the process burners, is supplied to the gasifier via the two secondary oxygen injection burners shown in Figure 8. This not only meets the total oxygen requirement of the gasifier but also utilizes the high-speed injected oxygen to further atomize the unatomized water-coal slurry emitted by the process burners. In addition to resolving the issue of the service life of the process burner, the temperature field inside the gasifier has also been improved. Due to the secondary addition of oxygen, the progression of the oxidation reaction is relatively slowed down; therefore, it is necessary to increase the axial dimension of the furnace to compensate for this. Typically, the axial dimension of the furnace needs to be extended by about 0.5 m. Commissioned by the above three parties and in accordance with the requirements of the aforementioned improvement plans, we have developed a slurry coal firing nozzle as well as an oxygen firing nozzle for secondary oxygen supply. On December 6, 2007, this project passed the expert evaluation organized by the China Petroleum and Chemical Industry Association, and the two types of burners we developed received high praise. Field tests conducted by Shanxi Fengxi Fertilizer Group have shown that the middle nozzle of the gasification furnace burner can operate for over 100 days, the outer nozzle can operate for over 200 days, and the oxygen burner is expected to last for more than 5 years. After the spraying tests of the wear-resistant material for the middle nozzle and the chemical-resistant material for the outer nozzle are completed, a process burner with staged gasification will be fabricated, which will have an even longer continuous service life. The water-coal slurry four-counterflame burner technology developed by East China University of Science and Technology suffers from rapid wear of the refractory bricks at the top of the gasifier and overheating at the furnace roof [3]. Since the material flowing out of the burner undergoes secondary counterflow inside the furnace, it is possible to reduce the exit flow velocity of each burner. This not only helps to reduce wear on the middle nozzles but also, by lowering the exit flow velocity of each burner, the counterflow velocity is reduced as well, which in turn reduces the erosion of the bricks at the top of the gasification furnace and extends their service life. Additionally, by changing four counterflow burners to two, the counterflow effect can still be achieved, while investment can be saved and the number of control points reduced. 4 Conclusion Relatively speaking, the water-coal slurry pressurized gasification technology has advantages such as high gasification pressure, a simple gasifier structure, and low investment costs. One of its disadvantages is the short service life of the burner, which affects the continuous operation of the gasification furnace. This article presents a series of personal opinions and suggestions for extending the service life of burners; some of these may be immature, and many studies and tests are still required to implement them, particularly regarding the use of new wear-resistant materials for the middle nozzle, the optimization of the channel geometry, the spraying of a wear-resistant layer on the middle nozzle, and the spraying of high-temperature corrosion-resistant materials on the outer nozzle. It is believed that through the joint efforts of numerous scientists, engineers, and technical professionals, the pressurized gasification technology using water-coal slurry will play an even greater role in China’s coal chemical industry. Acknowledgments This paper was completed with the generous support of Dr. Zhang Jiansheng from the Department of Thermal Energy Engineering at Tsinghua University, as well as the assistance of Engineer Zhang Fan from my institution; we express our sincere gratitude to them. References: [1] He Yongde. Modern Coal Chemical Technology Manual [M]. Chemical Industry Press, 1st edition, March 2004. [2] Tang Hongqing. Illegal content should be removed [M]. Editorial Department of Nitrogen Fertilizers and Methanol, 2006. [3] Zhang Ronglin. How to Select Gasification Process Technologies [J]. Nitrogen Fertilizers and Methanol, 2007, 2(2): 1–7. [4] Liu Xiaodi. Burners for Water-Coal Slurry Gasifiers [C] // Proceedings of the 25th National Conference on Technical Reform Experience in Nitrogen Fertilizer Plants, November 2007. [5] Wei Hua Science and Technology [J]. Supplementary issue of 2000. [6] Valve Technology Manual [M]. Machinery Industry Press, 1991. [7] Li Bingke et al. Summary of Expansion and Renovation Projects for Gasifier Burners [C] // Compilation of Papers from the National Conference on Nitrogen Fertilizers and Methanol Technology Exchange and Business Negotiations, 2008, 9: 55–58.