Factors related to energy conservation and consumption reduction in the gas generation section

Thread Content

Factors Related to Energy Saving and Consumption Reduction in the Gas Production Section Author/Source: Wang Zhaohua (Gas Production Technology Department, Jinhao Foundry, Pizhou City) Date: 2009-2-27 China is a major coal-producing country with abundant coal resources in terms of quantity and variety. However, the reserves of high-quality coal are very limited and found only in a few areas; these reserves are far from sufficient to meet the needs of China’s rapid economic development. Furthermore, **from the perspective of safe production, some small coal mines that do not meet the required standards are shut down or converted; macro-control measures are also applied to the energy market. In particular, the approach to economic development in coal-producing areas is being changed and adjusted, with an emphasis on utilizing local resources locally. Many large-scale coal chemical enterprises and thermal power plants are being built or expanded, and the new coal chemical industry continues to develop. Therefore, the supply-demand imbalance and prices of raw coal are the main factors contributing to rising production costs in the chemical industry. For the survival and development of their businesses, many manufacturers have tried using inferior coal, briquetted coal, fine coal powder, and local coal by altering the raw material supply chain. Through these trials, they have gained a great deal of experience through exploration and research. However, some companies still hold misconceptions regarding how to innovate and optimize equipment and processing conditions in order to achieve energy savings and reduced consumption. I. In order to achieve energy savings and reduced consumption as well as lower production costs, we must first focus on scientific and advanced technologies and equipment – that is, what constitutes the most advanced technologies and equipment in the world today. As the gas generation section in coal chemical industry, the most ideal equipment and processes are water-coal slurry gasification, Ruhr pressure gasification, pulverized coal gasification, and oxygen-enriched gasification. These gasification technologies feature high carbon conversion rates, low heat loss, and good environmental performance. However, the investment required for these gasification facilities is quite substantial; ordinary companies cannot afford it without appropriate support and foreign investment, and they can only look on in envy. It becomes even more difficult if there are major external shocks. Since the end of last year, affected by the global financial crisis, two new space furnaces were installed in the country for the production of methanol. Due to the sharp drop in international oil prices, almost all companies involved in methanol production ceased operations, and as a result, these new space furnaces also had to stop working. This is because the subsequent processing steps of such furnaces cannot be integrated with the synthesis of NH3; only after modifications to reverse those processes can integration be achieved. But how much investment would be required for such modifications? What is the interest rate for a payment of 700 to 800 million per year? II. The atmospheric pressure fixed-bed batch gasification technology is a traditional coal gasification method that has been used in China since the 1930s. After more than 70 years of development and innovation, practice has shown that this gasification technique is mature, its process is feasible, it requires low investment and yields quick results, and it has a wide range of raw material compatibility. For a considerable period of time, fixed-bed batch gasification will remain the main approach for coal gasification in our country. Based on this gasification technology, we have been continuously researching, discussing, and exploring ways to achieve the most optimal results. Through modifications, innovations, optimizations, and improvements, we have reached ideal goals to varying degrees. III. Factors related to energy conservation and consumption reduction in the gas generation section 1. The hardware configuration of the system in the gas generation section. By hardware, we mean those essential components that are necessary, such as gas stoves and their associated stationary equipment; the operating equipment as well as the process pipelines must be properly arranged. ①How should the diameter of the gas stove be chosen? First, it should be determined based on the production capacity of the subsequent processing stage. For example, the air delivery volume of the compressor is related to the type of furnace used and the raw materials being vaporized; it is necessary to determine how many cubic meters of gas can be produced per furnace, and to ensure the proper operation of the entire plant by making reasonable arrangements. The second criterion is to choose based on the quality and properties of the raw materials: inferior coal should be used with a smaller furnace size, while high-quality raw materials warrant a larger furnace size. This helps to ensure stable operation, facilitates handling, and reduces consumption. ②For the outdoor equipment of gas stoves, combined waste heat recovery and superheating for multiple stoves should be adopted, and the process flow must be simplified, rational, and practical. Single furnaces, single kettles, and single towers are no longer suitable. ③The selection of fans should take into account the size of the furnace, the quality of the raw materials, the geographical location of the manufacturer, as well as the degree of temperature variation. Appropriate air volume and pressure need to be determined based on these factors. Often, due to differences in geographical location, large differences in day/night and seasonal temperatures, the air handling capacity of fans of the same model is around 5000 m3/h. ④The three gas generation systems must be configured in a balanced and rational manner. For the first gas system, the resistance should be appropriate; values that are too high or too low are not acceptable. In recent years, relevant publications have presented a variety of viewpoints. Some people say that the resistance should be high, as this promotes a good gasification reaction and results in better composition of the gas. It is recommended to apply pressure to the weights in the gas holder, which helps improve the air compression capacity of the Roots pump. Others suggest placing the resistance in the gas cleaning tower, as this facilitates safe production. Some people suggest that the resistance should be reduced, the water seal in the scrubber tower removed, a main gas valve installed, and the weights in the gas holder decreased. As a result, the gas production volume is low, CO2 levels are high, and there is plenty of residual ash. Quite a few people do not understand the reasons for this. The author believes that most of the system resistance should be attributed to the height of the carbon layer inside the furnace. Although increasing the carbon layer height affects the blowing efficiency, it is beneficial for improving the vapor decomposition rate, optimizing the composition of the gas produced, and increasing the gas output per furnace. When considering the level of resistance in such a system, it is necessary to distinguish between the advantages and disadvantages. A factory in Shaanxi installed control valves on the upstream and downstream pipelines in order to increase the gasification pressure inside the furnace; the purpose was likely to raise the pressure and facilitate the gasification reaction, but this approach did not work, so the valves were removed. This shows that adding or reducing resistance in the system is not arbitrary; it must be based on certain scientific principles. It is better to keep the resistance in the gas system between 500-600 mm of water column; if it is slightly higher or lower, there must be ways to address it. The second air system (including the blast air recovery system) requires an appropriate wind speed and flow rate; these parameters are determined based on factors such as the properties of the raw materials, the height of the carbon layer, and the resistance of the system. The presence or absence of an exhaust fan in the blast air recovery system affects the system’s resistance, so this aspect must be taken into careful consideration. Excessively high wind speeds and flow rates are not conducive to stable furnace operation; they cause the gasification layer to rise rapidly, result in poor heat retention, significant heat loss, an excessive amount of material being carried away, and prone to localized overturning. Generally, the air flow rate into the furnace should be 10–15 m/s. If this rate is too low, local heat cannot be removed, leading to scorching. The linear flow rate within the furnace should be less than 3 m/s. The wind volume needs to be adjusted according to the quality of the raw materials and the approximate range of gas production expected. In recent years, many new factories have been built, and air flow meters have been installed in their ventilation systems to facilitate digital measurement and management. However, due to the differences in the design of these flow meters, the actual amount of air that reaches the furnace is limited. A φ2650 gas furnace at a factory in Shanxi is equipped with a D-600-2650 fan; the raw material used is of medium to high quality. Based on an analysis of the furnace’s operating conditions and the percentage of air supplied, it is clear that there is insufficient airflow, a point also noted by the workshop managers, who plan to replace the fan with a larger model. After we went to the site for observation and analysis, we found that the air flow meter they were using was a conical flow meter. The diameter of the air inlet ducts was 600, while the actual internal diameter of the flow meter was less than φ500. Practical experience has shown that this has an impact on the amount of air entering the furnace; if such a flow meter is to be used, it is necessary first to consider whether its internal diameter is appropriate. For the third steam system, it is essential to ensure an adequate supply and stable pressure; the flow rate and pressure levels should be determined based on the factory’s raw material conditions, equipment status, and operating requirements. The capacity of the pressure relief valve, high and low pressure steam pipes, and buffer tank must all be properly configured. What size pressure relief valve should be selected to supply several boilers, thereby preventing competition among them for steam. The pressure relief valve should be as close as possible to the buffer tank, and the outlet of the buffer tank should be as close as possible to the low-pressure main pipe. The volume of the buffer tank should be around 8 m3 per furnace. The pressure after reduction should be directed into the buffer tank to prevent reaction delays. It is recommended not to install a flow meter on the steam entering the furnace, as the diameter structure of the flow meter varies and this affects the actual amount of steam that enters the furnace ； If installation is necessary, a type of \"Ainiuba\" flow meter is suitable, but its accuracy is not very high. This can also be resolved by extending the steam pipeline and increasing the diameter area of the flow meter. ⑤The DCS system must be configured in a proper and comprehensive manner, with its installation location chosen carefully. By utilizing the data provided by the instruments, we can gain a full understanding of the operating conditions of the gas furnace, which helps us improve our operational skills and achieve stable performance, high productivity, low consumption, and safety. For example, where should the temperature measurement point for the gray bin be installed? Some are installed in areas where the reaction is inaccurate, some in areas with reaction lag, and some in areas that are not resistant to wear; in any case, they must be installed in appropriate locations, otherwise they lose their meaning. Some factory furnaces are equipped with temperature measurement points inside their grates, but these points get damaged after just one or two months of operation. What is the reason for this? It’s not durable because the location and size where it is to be installed are incorrect. The data reflected in the DCS system serves as an eye and ear for operators; it helps us analyze and address difficulties in production, making it an essential part of the gas generation system. 2. Software configuration of the gas generation system: Software is something that is flexible. ①The quality of the raw materials used for gasification, as well as their input and output, often differ significantly from what theoretical calculations predict. Why is that? This is because the fixed carbon, ash melting point, ash content, and heat and cold strength of various raw materials vary. High-quality coal offers great operational flexibility, favorable conditions, stable furnace operation, and high gas production, resulting in lower consumption. Low-quality coal has low fixed carbon, a low ash melting point, high ash content, many other minerals, poor hot and cold strength, is difficult to handle, results in unstable furnace operation, large amounts of ash discharge, and high residual carbon in the ash; as a result, its consumption is relatively high. For example, when gasifying high-quality lump coal from Shanxi and lump coal from Ningxia, some of the gasification conditions are opposite to each other; if these two types of raw materials are gasified under the same conditions, unexpected results will occur. ②The technical competence of the operators in the gas production section plays a significant role; their skill level has a big impact on coal consumption. Under identical conditions, units with higher technical competence have lower consumption levels, with some achieving 1.05 T/TNH3, while others reach as high as around 2.0 T/TNH3. For example, while a gas stove was in use, the grates in its burner unit melted, eventually causing the entire bottom of the stove to be damaged. In some units, the gas stoves have developed scale that cannot be removed, eventually ending up as scale at the bottom of the stove. However, some manufacturers use steel rulers to measure in millimeters in order to control the amount of air and steam fed into the furnace; percentage adjustments are often made on a 1-second or 0.5-second basis, while temperatures are monitored within a range of ±5 degrees. This shows that the effects resulting from different levels of operational skill can be quite different. Some manufacturers in coal-producing areas don’t care about high consumption levels; after all, their coal price is low, and as long as the furnace operates steadily, gas production is sufficient. But in areas facing energy shortages, they regard coal as gold, demanding high gas production, low ash and residual carbon levels, minimal waste generated during the blowing process, strict control over the amount of coal fed into the furnace, as well as a system of substantial rewards and penalties. ③There is a certain relationship between the size of a gas stove and its fuel consumption. There are few manufacturers of φ3000mm series furnaces that have low energy consumption; there is a certain difference in consumption compared to smaller furnace models. Moreover, plants using medium-nitrogen processes possess strong technical capabilities – they have a large pool of skilled professionals, and their technical expertise is high both in theory and in practice. Then why has the issue of high energy consumption not been resolved? In recent years, there has been a shift from large furnaces to smaller ones. I have visited several ammonia plants, and their critical problems have not been properly resolved. The φ3000mm series of furnaces are developed by enlarging φ2745mm furnaces; however, these enlarged furnaces suffer from an unreasonable slag discharge system, an inappropriate height-to-diameter ratio, and unchanged traditional operating practices. So for the φ2000mm series of small furnaces, ranging from φ2260mm to φ2800mm, the consumption also varies; in other words, it is lower for the smaller ones and higher for the larger ones. Why? The diameter of the gas furnace increases gradually from φ2260mm to φ2800mm, while the installation at its bottom remains unchanged. The key issue is that the diameter of the ash tray does not change; as a result, the transition zone for ash and slag becomes smaller and smaller until it essentially disappears, which poses certain difficulties in operation. After the change in the raw material supply route, the use of large-sized furnaces to burn small-grained coal, coal balls and other materials led to problems such as furnace collapse, carbon slippage, uneven heating within the furnace, and high levels of residual carbon. As a result, various manufacturers carried out relevant modifications to their φ2600–φ2800 sized furnaces to suit the types of materials they used. To stabilize the furnace operation, minor modifications are first made to the lower part of the furnace, which is what is commonly referred to as the anti-flow system. Due to the expansion of the furnace chamber and the special nature of the raw materials used, it is impossible to operate the furnace properly without modifying the anti-flow system; it is also very difficult to stabilize the furnace’s operating conditions regardless of the process conditions employed. Anti-flow measures are necessary, but the specific way in which to make these changes should be determined after a comprehensive analysis of factors such as the quality of one’s raw materials, the type of furnace to be used, and one’s own skill level. Data show that the \"angle of repose\" of lump coal and coke, both in static and dynamic conditions, ranges between 27-45°C; guided by this principle, flexible measures can be taken to handle them properly. However, some organizations handle this in an inappropriate manner; they overdo it, failing to provide protection where it is needed and instead offering protection where it isn’t necessary. My view is that we should take proactive measures for prevention, rather than adopting a passive approach. However, other industry professionals hold different views; they say that the problem of the \"angle of repose\" of ash and slag can be resolved by increasing the diameter of the furnace bottom and the ash tray. In other words, if the value obtained by subtracting the diameter of the furnace chamber from the diameter of the ash tray and then dividing by 2 is 400 mm or more, then the flow behavior of the ash and slag will be properly managed. Some small and medium-sized enterprises are short of funds; they consider the cost of replacing the furnace bottom to be too high, so they have to find ways to prevent gas leakage in order to keep their gas furnaces operating properly, thus enabling them to produce normally with minimal investment. ④The raw materials used in gas stoves manufactured by various manufacturers across different regions of the country vary, so the operating methods and gasification conditions also need to change accordingly. The handling methods for high-quality coal and low-quality coal cannot be the same; since the particle size of the raw materials varies, the handling methods also differ. It is necessary to ensure that the particle size of the coal fed into the furnace is uniform, with minimal differences in size, otherwise the levels of ash, slag, and residual carbon will increase, leading to higher consumption ； Are there any differences in the gasification conditions and operating methods between dry coal pellets and wet coal pellets, as well as between high-quality coal pellets and low-quality ones, and between carbonized coal balls and sodium humate coal balls? Coal from Ningxia, Yunnan, Guizhou, and Sichuan each has its own regional characteristics; have the gasification process parameters been adjusted accordingly? Only by fully understanding the properties of various raw materials can one operate effectively and achieve desired results. ⑤To adapt to changes in raw material varieties and meet production demands, many scientists, engineers, and production managers have dedicated themselves to the research and improvement of gas furnace equipment, which has led to a significant enhancement in the overall performance of fixed-bed gas furnaces. Both the output per furnace and energy consumption have improved considerably. However, there are still those with different views on operating principles; some believe that intensive operation is better, while others think that operating the furnace at moderate load levels is optimal. For gas furnaces with a diameter of φ2800 as well, some are capable of producing over 70 tons of synthetic ammonia per furnace, while other manufacturers achieve around 50 tons of ammonia; apart from special factors, this difference is due to varying operational concepts. Factories that operate Taiping furnaces at light load levels, by running multiple furnaces and reducing the frequency of ash discharge in order to cut costs, follow a operational philosophy that is not suitable for the current requirements. The consumption within our own facility has decreased, but steam usage, electricity consumption, and machinery costs have increased; as a result, the total energy consumption remains high. This is a misconception that needs to be eliminated. ⑥Selection of the height-to-diameter ratio for the gas furnace and the water jacket: It is appropriate for the overall height-to-diameter ratio of the gas furnace to be 2.0–2.1:1, while the height-to-diameter ratio of the jacket should be 1:1. For the gasification of high-quality lump coal, the height-to-diameter ratio can be a bit smaller, but for the gasification of low-quality coal, this ratio must be higher. There is disagreement regarding the advantages of increasing the jacket height. From a theoretical perspective, it leads to greater heat loss; using white coal instead of steam is not cost-effective. However, it is beneficial for maintaining stable operation when gasifying low-quality feedstocks, enabling high-load operation, increasing production capacity, and reducing consumption. It is necessary to determine whether the benefits outweigh the drawbacks, but the height cannot be increased without limit – excessive increase is pointless and constitutes a waste. ⑦Selection of grates: Grates are the core component in gas stoves. Their excellent performance plays an important role in saving energy, reducing consumption, and maintaining stable operation, but they are not a panacea for all problems. An excellent-performing grate should meet the following conditions: first, the distribution of the gasifying agent must be even and reasonable ； Secondly, the ventilation area should be relatively larger, but it cannot be infinitely large ； Third, it has a strong slag discharge capacity and wide adaptability to raw material properties ； The ratio of the inner and outer air ducts should be appropriate, as well as the height of the upper and lower folds ； The amount of material blown out in the fifth stage should be low ； The material should be of high quality, resistant to heat and wear, and have a long service life. Currently, there are dozens of manufacturers of grates across the country, and any grate produced by them must meet the above requirements; otherwise, it cannot be considered a device with excellent performance. Currently, there are different opinions within the industry regarding the height of the grates to be used. Some people think that a low altitude is good. But what’s the advantage of being low? We believe that being too low is not a good thing; it is necessary to adapt to the current process conditions, and we hope to avoid going down the wrong path. In recent years, the approach of various factories to minor improvements and innovations has changed. Those working in the fields of technology and production have come to realize the advantages of increasing the height-to-diameter ratio, as this plays a significant role in burning lower-quality coal and briquettes, as well as in ensuring stable production and reducing consumption. Under such circumstances, using gas furnaces with elevated upper cylinders and jackets, along with grills of low height, would represent a step backward and a great waste. In the mid-to-late 1980s, furnaces with diameters of φ2260 and φ2400 were commonly used. These gas furnaces were only 4.07 meters in height overall, and the gas inlet was located on the side. Once the insulation material inside the upper cover and the height of the gas inlet were taken into account, the height of the furnace chamber became very low, resulting in a limited amount of coal that could be stored within it. The effective coal layer in the upper part of the jacket was approximately 0.7 meters thick. As the diameter of the furnace chamber increased and larger fans were used, the load on the gas furnaces rose, leading to higher temperatures; the surrounding areas would turn red, and slag accumulation on the walls often occurred. To address these issues, either more steam was used for downward blowing or the upward temperature was reduced. This, in turn, caused the downward temperature to rise. It was for this reason that some industry professionals at that time proposed that increasing the downward temperature was an effective way to boost the gas output per furnace. For this type of furnace during that period, special operating methods and equipment adapted to those process conditions were required, namely low-height grates. This equipment and operating method are still feasible for high-quality coal, but they are not suitable when switching to lower-quality coal. From the late 1980s to the early 1990s, most small ammonia fertilizer plants in the south used white clay coal balls and low-height grates; slag would often form in the upper part of these grates and not be able to flow down. To address this issue, several slag-scraping knives were installed in the upper section of the grates. Because the angles of each layer of the low-height grates are small, the shape of the grates becomes a parabola with a shallow curve, which makes slag removal difficult and leads to unstable furnace operation; this indicates that low-height grates are not suitable for burning low-quality coal or for operation under high load conditions. In recent years, due to significant changes in raw materials, gas has mostly been produced from low-quality coal. To accommodate the characteristics of gasification using such low-quality coal, new operational approaches have been adopted: increasing the height-to-diameter ratio of the gasifiers, raising the carbon layer thickness, increasing the amount of carbon stored within the furnace, and using equipment with excellent performance. All these measures are aimed at increasing the gas production per furnace while reducing consumption. As the effective carbon layer in the furnace increases, the height of the grate is gradually increased to meet the requirements of the process conditions. The grate height is increased accordingly under the new process conditions, which allows for an increase in the contact area between the grate surface and the raw materials, as well as an expansion of the gasification surface above the grate, thereby enhancing the gas generation rate. ⑧Selection and modification of coke feeders: The purpose of using coke feeders is to add material without shutting down the furnace, thereby increasing the gas production per furnace, as well as to reduce heat losses. However, it is also necessary to analyze the advantages and disadvantages of various coke adders. The advantage of the conical surface coal distribution and carbonization machine is stable carbon layers; the disadvantage is that there is a column of dark coal in the middle of the carbon layer, which affects the uniform distribution of the gasifying agent. The resistance in the middle is high, resulting in a sort of dead zone. Additionally, the natural accumulation angle of the carbon layer from the bottom of the distributor to the surrounding area of the furnace is too large. The larger the diameter of the gas furnace, the greater this angle becomes, which hinders the furnace’s ability to operate under heavy loads. For manufacturers that use this type of distributor, we can help you overcome the aforementioned drawbacks, increasing the gas generation rate per furnace by 5% to 10%. The original old-type spread-type charring machine also had unsatisfactory fabric handling, but it improved somewhat after modification. ⑨There are two types of equilibrium in the gasification process of gas stoves: one is the equilibrium between heat release and heat absorption, and the other is the equilibrium between material feeding and ash discharge. Controlling these two balances properly is an extremely complex issue, as there are many factors that affect them – both external and internal – and their elasticity is also high. After more than 50 years of hard work by several generations, small nitrogen fertilizers still have not reached an ideal state, and this requires our joint efforts to improve the situation. ⑩Regarding the level of workshop management, I have visited many manufacturers and found that their workshops have comprehensive rules and regulations, these are strictly enforced, and there are clear systems for rewards and punishments. The workshop supervisors wear suits and ties and work at their desks using computers to monitor production activities. However, there are also some manufacturers whose supervisors, driven by a simple passion for work and a spirit of leading by example, spend 80% of their time on the job site. The results in such cases are not satisfactory. As illustrated by these two approaches to management, both workshop leaders and employees need to be clear about their responsibilities and duties, so that there are guidelines to follow and rules to adhere to; only through unity between all levels can the work of the organization be done effectively. Each plant has its own characteristics, and each furnace presents its own conditions – this is an undeniable fact in gas production. Each person has their own method; trying to force everyone to use the same approach is the biggest mistake in gas production operations ； Three parts are the raw materials and seven parts are the processing techniques; three parts are traditional skills and seven parts are the operational procedures; three parts are the operations themselves and seven parts is management. This is experience gained through sweat and hard work. II. The relationship between the consumption in the gas production stage and subsequent stages: The consumption in gas production is generally calculated by working backwards from the final product, and such calculations can vary significantly. 1. The configuration of equipment in subsequent units, as well as the process flow and conditions, have an impact on consumption. For example, in a synthesis unit with a pressure of 31.4 MPa, 1 cubic meter of catalyst can produce around 30 tons of ammonia, while in a synthesis unit with a pressure of 25 MPa, 1 cubic meter of catalyst produces around 25 tons of ammonia. However, the total energy consumption differs; although the overall energy consumption is reduced, the energy required for gas production remains relatively high. 2. Severe leaks, spills, and other issues in the subsequent processing stages are also a reason for high consumption. There is new data showing that 3069 m3 of semi-water gas (theoretical value) is required to produce 1 ton of NH3. Some manufacturers consume 3500–3600 m3 of semi-water gas per ton of ammonia; for every additional 100 m3 of semi-water gas, an extra 30–50 kg of coal is needed. What does this figure mean? 3. Some manufacturers use a higher alcohol-to-ammonia ratio, which leads to increased consumption; the quality of the gas composition also affects consumption, and there are differences in the conversion methods used in various regions as well. In short, it is not an easy task for the gas production unit to achieve energy savings and reduced consumption; however, as long as the equipment, process flows, and operating conditions in this unit are managed properly, consumption can definitely be reduced. So it’s not the workshop that decides what raw materials each manufacturer uses; rather, it’s up to the higher-level management. The main task of the gas generation unit is to ensure that, regardless of the method used for gasification, as long as the process parameters are appropriate, the gas generator operates stably and there is an adequate supply of gas. This can be determined from the condition of the ash and slag resulting from gasification: the slag formation rate should be around 65%, with the carbon content in the ash and slag being less than 12%.