Discussion on the selection of equipment such as fluidized bed, fixed bed, and fluidized-bed reactors for coal-to-methanol synthesis processes

2 Technical Analysis of the Gasification Process 2.1 Technical Introduction First, in fixed-bed gasification, the raw material is bituminous coal. This technology utilizes air, steam, and oxidizers to convert coal into gases composed of carbon monoxide, methane, and hydrogen, which are then used as fuel to release heat. In the fixed-bed gasification process, the gasification pressure is required to reach 4 MPa  ； The operating temperature is between 850 and 950 °C ； The particle size of the raw material ranges from 4 to 28 mm ； The oxygen consumption is 280~290 km3  ； The typical production capacity per furnace is 600 t. Secondly, in a fluidized bed, a gasifying agent is injected into the gasifier, after which small particles of coal are burned to keep the fuel in a boiling state, thereby enabling gradual gasification reactions. The gases generated as well as the coal ash are removed from beneath the furnace; as a result, the coal gas produced contains virtually no tar-like components. The gasification pressure required for this process is 0.098 MPa  ； The operating temperature is 900 ℃  ； The particle size of the raw material is 1~9 mm ； Oxygen consumption is 302–318 km3 ； The productivity per furnace ranges from 350 to 450 tons. Finally, there is the fluidized bed system, in which coal powder is carried into the gasification furnace using a gasifying agent; the high temperature inside causes the coal powder to burn instantly, releasing thermal energy to produce syngas. Because the temperature of the gasifier itself is extremely high, the reaction time of the coal powder is very short; the gasification reaction usually ends in about 3 seconds. Moreover, the fluidized bed imposes no strict restrictions on the type of coal added; chemical reactions can occur with most types of coal. Assuming the particle size is relatively large, a relatively longer gasification time is required. However, if a smaller particle size is chosen, the reaction time can be significantly shortened. In practical production, small-particle raw materials are generally chosen, as this not only saves time costs but also ensures a high conversion rate of the raw materials. The gasification pressure standard for this process is 3.6~6.3 MPa  ； The operating temperature is between 1,450 and 1,550 °C ； The particle size of the raw material is less than 0.19 mm ； The oxygen consumption is 312~318 lm3  ； The production capacity per furnace is 2,000 t. 2.2 Comparison of advantages and disadvantages: Among the three coal gasification technologies, the one with relatively high equipment costs is the fluidized bed; the cheapest is the fixed bed. In terms of raw material conversion efficiency, the order from highest to lowest is fluidized bed, fixed bed, and fluid bed. Among them, although the cost of a gas-flow bed is relatively high, this technology enables enhanced treatment of the raw materials by raising the operating temperature, ensuring a conversion rate of over 99%; it can thus compensate for the shortcomings of a fluidized bed. Apart from considerations regarding construction costs, each of the three has its own advantages and disadvantages. Fixed-bed reactors are relatively suitable for small-scale gasification plants, as they require less energy and have lower initial investment costs. However, its drawbacks are also quite apparent: the methanol synthesis rate is limited, it has a significant impact on environmental conditions, and the composition of the output gas is complex. Fluidized bed reactors are suitable for small and medium-sized gasification plants. This is because it has low requirements for raw materials and also offers a high yield. However, the drawback is that the gasification operation temperature is not high, which makes it difficult to achieve a high conversion rate. Gas flow beds are more suitable for large-scale coal gasification plants, as they can utilize high-temperature conditions to process the feedstock, thereby significantly increasing the conversion rate.

3 Selection of Equipment for Coal-to-Methanol Synthesis Technology: Based on the key production stages in coal-to-methanol production and the more advanced technical methods available in current coal gasification processes, this section examines the equipment selection for each of the gasification and synthesis steps. 3.1 Gasification stage Taking the entrained flow reactor as an example, the available process equipment includes: (1) Dry powder entrained flow reactor. This is a relatively common gasification method in recent years. The existing dry powder fluidized bed equipment has slightly different technical parameters.  ①The raw material is crushed into particles of 0.09 mm size; after ensuring it is dry, it is placed in the coal bin, and inert gas is used to transfer the coal powder into the gasification furnace. By maintaining a high temperature of 1,400 ℃, the gas inside the furnace reacts with coal powder; thereafter, the resulting gas is cooled to 300 ℃. After impurities in the gas are removed using a dust collector, it is sent to the gasification furnace. ②Coal powder with a particle size of less than 0.2 mm is directly fed into the gasification furnace, where the pressure inside the furnace is maintained at 2.6–4 MPa. Finally, coal powder is gasified through quenching, thereby producing methanol. ③It is produced through a cold-shock type thermal reflux process; its basic principle is similar to that of the first type of fluidized bed. The main difference between the two lies in the fact that for this process, the particle size of the raw materials required is 0.075 mm, the pressure inside the furnace must be between 3.6 and 4.2 MPa, and the temperature should be maintained at 1,400 ℃. Ultimately, methanol is obtained through cooling treatment. Such gasification technology equipment enables effective control of energy consumption and helps to reduce the environmental impact of production processes. (2) Water-coal slurry fluidized bed. Currently, there are mainly two types of large-scale water-coal slurry gasification units, both of which rely on a quenching stage. ①Water-coal slurry is added to the gasifier along with oxygen, and gasification is achieved through quenching to produce methanol. Through years of optimization, it is now being used by several countries, and my country is one of them. Under normal conditions, the particle size of coal powder used in this process equipment should not exceed 0.2 mm, while the operating pressure inside the gasifier ranges from 2.7 to 3.9 MPa; the media and slag are extracted from within the gasifier. It is worth noting in particular that the gas temperature at the outlet of the gasifier should be maintained around 215 ℃. ② The IGTI gasifier also uses water-coal slurry. The overall operation process can be divided into two stages: heating, during which the thermal energy is used to gasify the coal slurry; and then rapidly cooling the gas at a temperature of 1,000 ℃ in order to produce methanol through water separation.

3.2 Synthesis stage: First, the tubular synthesis tower. This type of synthesis equipment is capable of optimizing heat exchange; clean water is injected into the heat transfer tubes, and in cases where there is excess energy, it can be quickly transferred to these tubes to produce medium-pressure steam, with a pressure of around 2.5–4 MPa. At the same time, hot water is added to the inner tube, which helps to increase the heat transfer coefficient and control the range of heat transfer. Under such an operating structure, the synthesis tower can make effective use of energy and avoid additional consumption, making it a type of synthesis tower equipment favored by some factories. Secondly, the cold-tube synthesis tower. The operating principle of this equipment differs slightly from that of other synthesis towers; it utilizes regulated cold air to facilitate methanol synthesis, thereby improving the energy conversion rate. The internal components of the equipment are essentially equivalent to an additional heat exchange device incorporated into the contact module. Generally speaking, the type of cooling tubes installed in such synthesis towers also varies, namely counter-current, co-current, and U-shaped configurations. The first type of cold tube removes heat directly, and there is a restraining effect between this and heat release. This structure also has shortcomings; if the temperature difference is too large, it cannot be used in the production of methanol. Therefore, relevant manufacturers usually choose the other two types of cold pipes, so as to prevent a range of problems caused by large temperature differences. Furthermore, this type of reaction tower operates at an isothermal condition; the reaction process takes place near the optimal temperature range. The ammonia concentration at the outlet is relatively high, resulting in stable operation with minimal variations. However, the drawbacks are also quite apparent: the operating performance of the cold tubes can interfere with coal reduction, preventing the full manifestation of the raw material’s activity. Moreover, both coal loading/unloading and the installation of internal components present certain operational difficulties. Third, multi-bed internal heat exchange type. As the methods for methanol synthesis continue to be improved, the ammonia synthesis towers, which had previously performed well, gradually become inadequate for modern production requirements. Through rational modifications, this type of synthesis tower was developed; it uses equipment with internal heat exchange mechanisms, and catalysts can be added, allowing the reagents to be used for a longer period of time. The heat exchange devices among them are located at the central axis of the synthesis unit, specifically installed in each separate catalytic bed. The heat exchange equipment used in such synthesis towers includes both tube arrays and coiled tubes, and these synthesis towers offer numerous advantages: their internal structure is extremely simple, which reduces equipment costs; at the same time, they also exhibit excellent performance in terms of production conversion rates. However, its drawbacks cannot be ignored either; the reactants must be processed before medium-pressure steam can be generated. Fourth, fixed tube sheet shell and tube. Unlike the tubular and multi-bed internal heat exchange types of synthesis towers discussed earlier, in the production using this type of synthesis tower, the catalyst added reacts rapidly with high-temperature water to produce medium-pressure steam; ultimately, through counterflow heat exchange, methanol synthesis is achieved, which helps to optimize the energy conversion efficiency. However, such synthesis tower equipment has an extremely complex internal structure due to the constraints imposed by the length and diameter of the pipes; if only one such synthesis tower is used, the required production level cannot be achieved. Assuming that it is required to produce 2,000 tons of methanol each time, several tubular synthesis reactors must be installed. Only after they are connected in parallel can the equipment withstand the corresponding production pressure. These reactors are typically made of stainless steel; given the special properties of this material, their manufacturing costs are relatively high. As the demand for methanol production continues to rise, the number of tubular reactors connected in parallel also needs to increase accordingly. Consequently, this type of reactor is currently the most costly among similar types of equipment. Fifth, cold-shock type. This type of synthesis tower was the earliest to be developed; when cold air is introduced into it, it carries away the heat generated by the reactions taking place inside. Several adiabatic sections are arranged on the reaction bed in the tower; between them, low-temperature feed gas must be added. As a result, the overall temperature distribution of the reaction gas is suboptimal. Under conditions where the required methanol output remains constant, this leads to a relatively higher consumption of catalyst. The structure of such reaction tower units is relatively simple, equipped only with a mixed distribution module for coolers and heaters. Depending on the actual amount of catalyst added, the synthesis tower can be used a varying number of times, and it is also convenient to regulate the cold gas flow. On a plane within the adiabatic section, the temperature conditions are fairly similar, so there is no need to give too much consideration to airflow resistance. However, this type of synthesis tower also has significant drawbacks: the cooling gas serves only to lower the temperature and is not involved in the catalyst reaction; moreover, the temperature of the exhaust gas is lower compared to that of the aforementioned type of synthesis towers, resulting in low heat recovery efficiency. Each instance of cooling leads to a decrease in the ammonia content. These days, due to the poor efficiency of cold-shock synthesis towers in producing methanol and their low production capacity, they have been completely phased out from the market. By analyzing existing synthesis tower equipment, it can be seen that cold-tube and cold-jet types have limited production capacities and are not capable of meeting the demand for methanol products in the current market; as a result, almost no factories choose these two types anymore. As for the other three types of devices, they all perform relatively well in terms of production volume, and are considered to be popular categories. In comparison, in terms of methanol production alone, equipment of the fixed-tube-sheet shell-and-tube type performs best; however, in practice, parallel units must be added continuously based on production requirements, which increases the total investment amount. Although the other two types of synthesis towers have lower production capacities, they are cheaper to construct, and their output is generally sufficient to meet the needs of modern methanol production; therefore, they are more suitable for most factories to choose.

4 Issues and Countermeasures in the Selection of Equipment for Coal-to-Methanol Synthesis Technology 4.1 Selection Issues Due to problems with the selection of production equipment, the technical processes cannot fully realize their potential. Therefore, to improve the efficiency of methanol synthesis, the most fundamental approach is to ensure that the right equipment is selected, so as to increase the conversion rate of raw materials. Today, coal-to-methanol technology requires further improvement in management and research; the practical selection process often involves blindness, resulting in outcomes that fail to meet the acceptance standards set by relevant authorities and thus cannot be put into actual production use. Furthermore, some selection projects failed to take into account the market competitiveness of the equipment units. Therefore, proper attention must be paid to the selection of equipment for coal-to-methanol production, in order to ensure the efficiency of the plant and prevent any impact on the quality of methanol production resulting from poor selection decisions. 4.2 Response Measures Methanol products play a crucial role in many modern industries, and various derivatives also hold great potential for further enhancing the utility of methanol. This helps to continuously raise the awareness of relevant industry sectors and practitioners regarding coal-based methanol, thereby enabling a reasonable increase in funding for equipment selection. In addition, policy guidance should also be given attention to. At all levels, **can use macro-policy tools to guide the industry. Following a standardized construction approach, it ensures the operational efficiency of production facilities and guides relevant factories in selecting the most suitable equipment. From the perspective of enterprises and factories, it is necessary to update their concepts regarding equipment manufacturing, build a skilled workforce in mechanical equipment, and actively collaborate with institutions such as universities and research institutes. 5 Conclusion In the production of methanol from coal, the selection of gasification equipment and synthesis towers are key preliminary design considerations; appropriate equipment not only helps to increase output but also enables control of energy consumption during the production process. In summary, the fluidized bed is a relatively suitable gasification device at present. When selecting a synthesis tower, apart from the cold-tube type and cold-jet type, the other three types require companies to conduct thorough consideration based on their own production needs.

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