Oxidative synthesis of ammonia from briquetted coal

Comparison of advantages and disadvantages between intermittent gas generation and continuous oxygen-enriched oxidation (1) In the intermittent gas generation method, the temperature of the carbon layer varies greatly, and the gas flow direction changes periodically; therefore, high requirements are placed on the particle size of the fuel, its thermal stability, and its ash melting point. Continuous oxygen-enriched oxidation, due to constant bed temperature, medium flow direction, and flow rate, has lower requirements for fuel; it can handle small-grained fuels as well as briquettes with poor coal quality, and anthracite with low volatility and weak mechanical strength, whereas intermittent oxidation cannot. (2) In the batch method, in order to maintain the reaction temperature of the bed, air is blown in for combustion to raise the temperature; the exhaust gas carries away some of the heat, resulting in unnecessary loss of fuel. Moreover, the fluctuating temperature of the bed leads to low gasification efficiency. In continuous oxygen-enriched gasification, the heat generated by the oxidation reaction carried out in an oxygen-rich environment helps maintain thermal equilibrium in the gasification process; as a result, the bed temperature remains stable and heat loss is minimized, creating conditions for high gasification efficiency (the carbon conversion rate increases from 50%–60% to over 95% when coke is used). This leads to fuel savings, thereby significantly reducing the production costs and energy consumption associated with ammonia synthesis. (3) The intermittent gas generation process is carried out in a cyclic sequence of 6 steps, among which the air blowing stage is the phase for heating the material layer, with the blown air being vented. This phase accounts for 1/3 of the entire cycle time, which reduces the equipment utilization rate and lowers production capacity; it is generally only around 50% of that in continuous oxygen-enriched oxidation, resulting in high fuel consumption. (4) The intermittent process with a 6-step cycle involves complex programmable valves, controllers, and process pipelines; it features a high failure rate of equipment and valves, heavy maintenance workload, difficult operation, and difficulty in adjusting gas composition. Continuous oxygen-enriched oxidation simplifies the process flow, reduces the need for programmable valves, ensures stable and easy operation, minimizes maintenance requirements, and allows for easy regulation of gas composition. (5) During the blowing stage of the batch method, the combustion gases from the fuel are released into the atmosphere, causing 40% of the sulfides in the fuel, along with large amounts of CO2, some CO, and dust, to be emitted directly into the atmosphere, thereby causing severe pollution to the air. Continuous oxygen-enriched oxidation eliminates the blowing stage, with no emissions into the atmosphere, thereby preventing air pollution. (6) The staged operations of the batch method cause severe noise pollution in the operating environment due to the main air ducts, fans, programmable valves, and venting systems. Continuous oxygen-enriched oxidation is much quieter, creating a quieter environment for the gas production area. (7) The CO2 concentration in semi-water gas produced by continuous oxygen-enriched gasification is 6%–8% higher than that in batch gasification. In the non-ammonia synthesis process, since the (H2+CO)/N2 ratio is higher than the requirement for this ratio in pure ammonia production, a nitrogen compressor is needed in subsequent steps to supply additional nitrogen, and the load associated with carbon removal also increases ； In the process for diol alcohols, since the synthesis of methanol requires a large amount of H2+CO, no nitrogen supplementation is necessary at an appropriate alcohol-to-ammonia ratio. The impact on subsequent processes is that the load on the shift reaction decreases, while the load on the decarburization process remains unchanged; moreover, the total ammonia production per shift hour from the high-pressure compressor stays the same. Furthermore, the Ar content in semi-water gas produced by continuous oxygenation using a pressure swing adsorption oxygen enrichment unit is also slightly higher, which has a certain impact on subsequent processes; this issue is less prominent in plants equipped with hydrogen extraction units. Therefore, oxygen-enriched continuous gasification is particularly suitable for the process of producing lysoform; it meets the requirements of lysoform production for H2+CO while avoiding the adverse effects of high CO2 levels on subsequent processes. Based on the comparison of the above advantages and disadvantages, continuous oxygen-enriched oxidation is superior to the batch method in terms of technology, operation, maintenance, environmental protection, and other aspects.

I. Overview The oxygen-enriched continuous gasification technology for briquetted coal is a systematic engineering technology that integrates pulverized coal briquetting technology, fixed-bed gasifier technology, and air separation oxygen production technology. It utilizes coal powder shaping for gasification, enabling coal gasification enterprises to avoid the bottleneck of relying on high-quality lump coal as raw material and embark on a broader path that uses local coal powder as the gasification material, offering lower costs and abundant resources. At the same time, it also avoided taking the complicated path of adopting foreign powder coal gasification technology, which involves high investment, a long recovery period, and slow results. It has embarked on a path that requires less investment, yields quick results, and achieves good energy-saving and emission-reduction effects. China’s energy structure is quite unique: coal production accounts for 77% of the total energy production, crude oil production accounts for 12%, natural gas production accounts for 3%, hydropower production accounts for 7%, and the rest accounts for 1%. China’s national conditions dictate that coal must serve as the primary energy source for its economic development. At present, China’s nitrogen fertilizer manufacturers produce a total of 50 million tons of ammonia per year, ranking first in the world. The proportion of coal used as a raw material for producing synthetic ammonia exceeds 76%. Gasification is the main method for ammonia production in China. In the production of coal gasification plants, fixed-bed batch gasifiers account for over 70% of the total production capacity. It is evident from this that the fixed-bed batch gasifier holds an important position in China’s coal gasification industry and national economic development. In our country, there are approximately 500 enterprises that use fixed-bed batch gasification as their production process, with over 5,000 such fixed-bed batch gasifiers in use. Among them, there are about 110 furnaces with a diameter of 3.6m, 600 units in the Φ3.0m series, 3,500 units in the Φ2.6 series, and 300 other gas furnaces. The fixed-bed batch gasification process has high energy consumption and causes significant pollution, and is destined to fade from use. However, given our country’s national conditions, it is necessary to establish a connection and transition between the aforementioned outdated processing technologies and the advanced technologies available internationally; this is the historical mission of oxygen-enriched gasification of briquettes. The oxygen-enriched continuous gasification technology for briquettes is a technique that can be implemented by modifying existing equipment and upgrading technologies at many coal gasification plants. It is a combination of multiple mature technologies that have already been used in production, and it represents a risk-free technology. Following the \"Bali\" conference held in 2007, countries around the world made addressing greenhouse gas emissions a top priority in industrial development. Our country is at the forefront of development with a proactive attitude. China currently has nearly 5,000 atmospheric-pressure batch gasification furnaces used for gasification, which emit over 60 million tons of CO2 into the atmosphere each year; this represents a key issue in the country’s efforts to control greenhouse gas emissions. The oxygen-enriched continuous gasification technology for briquettes is precisely the key to addressing the emission of CO2 and other harmful gases from the 5,000 existing atmospheric-pressure intermittent gasification coal gasifiers in China. It is fully capable of achieving zero emissions as required by modern industrial development. In summary, the oxygen-enriched continuous gasification technology for briquettes is a comprehensive technology that suits China’s national conditions, aligns with the country’s industrial policies, and keeps pace with global trends; it is a technology with Chinese characteristics and strong viability. Oxygen-enriched continuous gasification (fixed-bed gasifier) in our country began in the late 1960s, evolving from fixed-bed batch gasifiers. Mainly used for UGI-type gas stoves. After more than forty years of development, there have been no significant technical advancements in the design of these furnaces; they still follow the basic design principles of UGI gas furnaces. The process flow has not changed much either. Even with the progress made in automation technology to date, the old manufacturing processes remain largely unchanged. Main reason: Historically, the production cost of oxygen-enriched gasification has been high, and it lacks competitiveness compared to other gasification technologies; as a result, it has not seen significant development. Its main disadvantages are: 1. The design of the device is unreasonable, resulting in high heat loss; the concentration of the useful gas components does not meet the required standards ； 2. Coal prices are low, while the cost of oxygen production is high; using oxygen at a higher price to reduce coal consumption is not cost-effective from a production perspective ； 3. High oxygen-enriched CO2 oxidation leads to increased costs for the C removal system. With the advancement of science and technology, a series of changes have emerged: 1. The technical complexity of these devices has increased significantly, including gas stoves, grills, automation levels, waste heat recovery technologies, automatic ash discharge systems, and pressure swing adsorption carbon removal technologies ； 2. The cost of oxygen production has dropped significantly, from 0.6 yuan/m3 to 0.3 yuan/m3 ； 3. Coal prices have quadrupled: the price of medium-sized coal has risen from 200 yuan per ton to 800 yuan per ton, while the price of pulverized coal has increased from 100 yuan per ton to 450 yuan per ton ； 4. Briquette manufacturing technology is developing at a rapid pace. New historical conditions and missions have determined the emergence and development of oxygen-enriched gasification technology for briquettes. II. Composition of the oxygen-enriched continuous gasification technology for briquettes 1. Briquette technology: Oxygen-enriched continuous gasification requires that briquettes possess the following properties: thermal stability ≥ 80%, heat strength ≥ 50 kg/cm2, fixed carbon content ≥ 60%, and good chemical reactivity. Currently, humic acid coal and coal with composite binders can both meet the above requirements. Furthermore, whether the briquettes are processed into balls or rods, continuous automatic control by an automatic coal feeder is achieved. 2. Fixed-bed batch gasification coal gas furnace technology: Oxygen-enriched continuous gasification is carried out using briquettes as raw material. The design of such gas furnaces needs to address the following issues: A. The height-to-diameter ratio issue – The CO2 concentration resulting from the oxygen-enriched gasification of briquettes is inevitably high; reducing this CO2 concentration can only be achieved by increasing the reduction layer within the gasification zone. However, the ash fusion point of briquetted coal is low, and the height design of jacketed boilers must take into account preventing scarring on the walls; therefore, gas furnaces need to address the above issues comprehensively. Through practical application, the problem of high aspect ratio design has been successfully resolved. B. Grate issue: The O2 concentration in the gasifying agent used for oxygen-enriched gasification of briquettes reaches around 58%. The reaction between O2 and high-temperature carbon is extremely violent. Using conventional circular air distribution with tower-shaped grills inevitably results in high temperatures in the center of the circle and lower temperatures at the edges; in particular, between the two ring zones, dead zones for the gasifying agent are formed, leading to weak carbon reaction and incomplete combustion ； The temperature in the center of the ring is high, exceeding the ash melting point, resulting in scarring and caking. In actual production, this phenomenon does occur. For example, in two enterprises with five furnaces for briquette oxygen-enriched gasification, once the above situation occurs, the only option is to reduce the production load to cope with it. For the grate used in the oxygen-enriched oxidation of briquettes, it is necessary to keep each air distribution unit as small as possible, minimize the gaps between these units, and ensure as even a distribution of air as possible; however, the ventilation area must be increased appropriately. At the same time, it must have strong slag-breaking capacity; otherwise, the slag that reaches a temperature above point T2 during oxygen-rich oxidation is molten slag with high hardness, resulting in greater slag-breaking strength. Requirements for the grate material: (1) It must be heat-resistant, as local temperature rises during oxygen-rich oxidation can easily exceed 600°C. (2) High material hardness and wear resistance. After melting and cooling, the slag becomes very hard, causing severe wear on the furnace grates; therefore, special materials are required for these grates. Currently, grates specifically designed for the oxygen-enriched continuous gasification of briquettes have been successfully put into operation. C. Ash discharge without stopping the operation and automatic continuous ash discharge: Oxygen-enriched gasification is a continuous process; it is not suitable to stop the operation for ash discharge, as each time the furnace is stopped, there is a need for time to adjust the gas-to-vapor ratio as well as the gasification layer. During these adjustments, imbalances can easily occur, leading to fluctuations in the process. Requirements for the non-stop ash discharge and automatic continuous ash discharge systems: First and foremost, safety is essential; it is necessary to completely prevent any contact between the gasifying agent and the ash in the ash hopper, with no leaks allowed. Secondly, it must be fast and flexible; the time required to complete the operation cannot be too long, with the process needing to be finished within 5 minutes. Automatic ash discharge requires continuity, balance, stability, and good adjustability. Finally, the device should have a long operating life, with continuous operation guaranteed for over a year. D. Automatic coal feeder: There is a significant difference between the automatic coal feeder used for the oxygen-enriched continuous gasification of briquetted coal and that used for the intermittent gasification of coal. In an intermittent gasification furnace, coal can be added to the furnace during the downward phase; even if there is a leak in the coal feeding mechanism, it will only be steam that escapes, so no accidents such as fires or explosions will occur. The automatic coal feeder for the oxygen-enriched continuous gasification furnace for briquettes must ensure that: (1) neither valve leaks air, otherwise the gas will come into direct contact with air, leading to combustion or explosion. (2) When briquettes come into contact with steam, their mechanical strength decreases, resulting in an increased fragmentation rate. (3) Oxygen-enriched oxidation requires a high temperature in the furnace, so certain components of the automatic coal feeding system must be able to withstand high temperatures. E. Device for free switching between continuous oxygen-enriched gasification and intermittent gasification. During continuous oxygen-enriched gasification, the oxygen-enriched air and vapor used as the gasifying agent mix together and enter the gasifier from its lower part; there, gasification occurs within a fixed carbon layer, and semi-water gas emerges from the upper part of the gasifier and enters devices such as cyclone dust collectors. Since the gasifying agent always flows from bottom to top, the temperature of the carbon layer in the furnace tends to rise, causing the temperature inside the furnace to increase gradually, often reaching 650°C–750°C. The high temperature in the upper part poses a significant risk, as it first leads to increased heat loss ； Second, it leads to an increase in the crushing rate of the raw materials, as well as an increase in the substances carried away by the gas ； Third, it reduces the service life of the equipment at high temperatures. To effectively address the aforementioned issues, the process design for oxygen-enriched continuous gasification can be switched freely between upward and downward gas production modes in order to reduce the high temperature in the furnace, after which oxygen-enriched continuous gasification can be resumed. This process not only allows for better utilization of heat, but also protects the equipment and maintains gas quality, thereby increasing the efficiency and range of available raw materials for gasification. Of course, this is on the condition that safety is ensured, as switching between oxygen-enriched air and gas poses a high risk of explosion. Therefore, the pipes, valves, hydraulic control systems, safety interlocks, and other components of the free switching device are subject to strict requirements, and it is essential that they function flawlessly. 3. Process flow: The process flow for oxygen-enriched gasification of briquettes differs from that of conventional oxygen-enriched gasification processes. Due to the poor thermal stability and heat strength of briquettes, if the temperature at the upper part of the furnace becomes too high during gasification, a powdery or mushy layer will form on the surface of the carbon layer inside the furnace, resulting in increased gasification resistance and a rapid deterioration of the process. To address the issue of excessive temperatures in the upper section caused by continuous upward gasification, a program for switching to intermittent gasification was specifically designed. That is, there is no intermittent gasification process with a blowing stage. Upward – purge – downward – upward; once the temperature at the upper part of the furnace drops, switch to oxygen-enriched continuous gasification. It can also be designed as an automated repetitive process: continuous vaporization upward – purging – downward movement – upward movement – purging – downward movement – continuous vaporization upward. Process pipes and process valves are installed in accordance with the process flow, and special attention must be paid to ensuring that there are no leaks from the process valves. The sealing surface should be protected from oxidation. The main gas components of the semi-water gas produced by the oxygen-enriched continuous gasification of briquetted coal are: CO2 at 14–16%, CO at 36–40%, H2 at 31–34%, O2 at 0.3%, CH4 at 1.5%, and N2 at 10–12%. After gas purification and decarburization, the percentage levels of these useful gas components exceed the requirements of the ammonia synthesis process, making them suitable for the production of ammonia and methanol. 4. Auxiliary equipment for the system: The auxiliary equipment for the oxygen-enriched continuous gasification of briquettes includes a cyclone dust collector and two-stage waste heat recovery boilers, namely a high-temperature stage and a low-temperature stage. Different heat exchange methods and areas are designed based on different locations. It can also be designed as one waste pot and two waste pots. Oxygen-enriched gases, steam, and CO2 require significant pressure differences in coal gasification; therefore, special feeding devices and mixers are necessary, otherwise it will lead to safety hazards and process deterioration. 5. CO2 gasification furnace: The raw material gas from the oxygen-enriched continuous gasification of briquettes contains a high level of CO2, typically ranging from 14 to 15%. After CO2 is removed in subsequent processes, it should be returned to the gas production unit for gasification, where it is converted into CO and some CO2 before being reintroduced into the system. This not only prevents CO2 release and thus avoids pollution of the atmospheric environment, but also increases the C conversion rate and reduces production costs. The CO2 gasification furnace is a specialized furnace, and the model and quantity of such furnaces to be installed are determined based on the scale of the system project. III. Air separation equipment: Generally speaking, the selection of air separation technology for a plant should be based on the actual production conditions of the enterprise, with either cryogenic air separation for oxygen production or pressure swing adsorption for oxygen production being chosen as appropriate. The production scale of air separation can be determined based on the scale of ammonia synthesis (methanol can be converted to ammonia). For each ton of synthetic ammonia, the required amount of pure O2 is between 5000 m3 and 550 m3 per TNH3; this figure is determined based on specific requirements. High-quality coal results in a higher production capacity and lower oxygen consumption ； Poor-quality coal is the opposite. Advantages of cryogenic air separation for oxygen production: higher purity of oxygen, numerous by-products such as liquid oxygen, liquid nitrogen, and liquid argon, with good economic value for these by-products. Compressors can use turbine technology instead of motors to achieve cost savings. Disadvantages: The manufacturing process is complex, requiring high levels of technical skill; it takes a long time to produce products after the system is started, about three hours. The speed under variable conditions is low; it takes 90 minutes. Advantages of pressure swing adsorption oxygen production: simple process flow, easy to operate, high production flexibility allowing operation at 40-110% of normal conditions, and fast response to changes in operating conditions, which can be achieved within 10 minutes. Low power consumption during operation. Disadvantage: Few by-products. Companies can choose air separation systems according to their own requirements. IV. Requirements for subsequent processes: For the oxygen-enriched continuous gasification technology of briquettes, pressure swing adsorption is the preferred method for carbon removal in the accompanying raw gas purification system. It can better adapt to feed gas with higher CO2 levels, resulting in lower C removal costs. V. Economic benefits of the oxygen-enriched continuous gasification technology for briquetted coal. There are currently about 20 fixed-bed gasifiers using oxygen-enriched continuous gasification in China, and this technology has not seen further development over the years. The main reasons for this are as follows: 1. The cost of producing O2 is high, exceeding 0.6 yuan per cubic meter. Before 1995, coal prices were low – briquetted coal cost only 300 yuan per ton, high-quality coke cost only 450 yuan per ton, and pulverized coal cost only 110 yuan per ton. As a result, companies preferred to use coal and coke, which are less costly, rather than O2, which is more expensive. 2. The oxygen-enriched gasification furnace relies on a fixed-bed batch gasifier; the equipment is outdated, having been in use for decades without any technical upgrades. To date, specialized grates for oxygen-enriched oxidizers have not been used. 3. The process flow is outdated; continuous upward gasification is not possible, which leads to excessively high temperatures in the upper section, significant heat losses, and poor economic efficiency. Today, significant changes have taken place in history: (1) the cost of producing O2 has dropped sharply, from 0.6 yuan/m3 to the current 0.25 yuan/m3. The prices of coal coke have increased several times: coke at 1,200 yuan per ton, lump coal at 800 yuan per ton, and pulverized coal at 450 yuan per ton. To produce one ton of synthetic ammonia using 550 m3 of O2, an additional cost of 165 yuan per TNH3 is incurred; however, it is possible to save 300 yuan in coal costs and 50 yuan in steam costs, resulting in a total cost savings of 350 yuan per TNH3. Therefore, 350 yuan – 165 yuan = 185 yuan. The cost per ton of synthetic ammonia has decreased by 185 yuan. (2) The oxygen-enriched oxidation process is more mature and flexible. It is possible to switch between upper and lower blowing gases, allowing for effective control of the temperature in the furnace. It makes it possible to apply the oxygen-enriched gasification process to low-quality coal. (3) The technical complexity of oxygen-enriched gas oxidation furnaces and their key components has increased significantly, thereby leading to substantial improvements in reliability and cost-effectiveness. (4) The development of O2 production technologies and equipment has enabled larger-scale air separation, resulting in a significant reduction in unit costs. (5) The development of CO removal technology enables the efficient removal of higher levels of CO2 from the feed gas, which can then be recovered and reused to achieve secondary gasification. (6) Oxygen-enriched gasification of briquetted coal has a high adaptability to different types of coal; almost all types of bituminous coal can be effectively utilized. Moreover, the residual carbon in the slag is very low. First, it can be “eaten up or burned completely”; second, the slag is popular as an industrial raw material in the market. For the above reasons, all the conditions necessary for the oxygen-enriched continuous gasification of briquettes are now in place, and this technology should see rapid development to bring benefits to society. Currently, several new construction projects are already using the oxygen-enriched gasification technology for briquettes, and this new technology will soon be widely adopted across the country.