Oxygen-enriched gas production in intermittent gas furnaces
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Could someone talk about the changes in the manufacturing process, as well as the gas production from the gas furnaces and the changes in the equipment?
In my opinion, the use of oxygen-enriched vaporization or pure oxygen vaporization, along with adiabatic jackets, represents the future direction for the development of fixed-bed vaporizers. Fixed-bed vaporizers are constrained by the quality of the feedstock and the amount of gas produced, which results in high production costs; under the current market conditions, many enterprises are unable to survive. The cost of installing coal slurry or pulverized coal gasification units is beyond the reach of many small and medium-sized enterprises. From what I’ve read, the benefits of oxygen-enriched gasification are quite significant. I hope those who are involved in oxygen-enriched gasification will share their experiences and insights on its practical application, so that we can gain a better understanding of this technology and accelerate the process of upgrading gas production facilities.
Prospects for Oxygen-Enriched Continuous Gasification of Coal in a Fixed-Layer System at Atmospheric Pressure Sun Zhengtai, Zhou Mengren (China Tianchen Chemical Engineering Company, Tianjin 300000) Introduction According to statistics and plans from relevant authorities, the production capacity for nitrogen fertilizers in 1995 was 21 million tons (in terms of pure N; the same hereafter), with actual production amounting to 17.68 million tons. By the year 2000, the production capacity was 25.77 million tons, while the actual production was 21 million tons. There are nearly a thousand ammonia synthesis plants that use coal coke as raw material, including almost forty medium-sized nitrogen fertilizer plants, distributed across various regions of the country. Ammonia production from small and medium-sized nitrogen fertilizer plants across the country accounts for about 2/3 of the total ammonia production. These plants consume more than 40 million tons of coal per year, making them the second-largest energy consumers after the power generation industry. These small and medium-sized fertilizer plants that are still in operation and use coal coke as raw material generally have outdated production technologies; their products require high amounts of energy, resulting in high production costs. What’s more serious is the high volume of waste emissions, which cause severe environmental pollution. Most of these plants use U.G.I. and Д-type gas generators from the United States and the former Soviet Union from the 1940s and 1950s, which are atmospheric-pressure fixed-bed reactors. The gas production process is mostly based on intermittent gas generation techniques, which involve high raw material consumption, low gas output from the equipment, complicated operations, and severe environmental pollution due to the direct release of purge gas into the atmosphere. Direct cooling is commonly used for the cooling and dust removal of gas, which results in cyanide (CN-) in the gas being released into the atmosphere along with the cooling water cycle, while certain harmful substances such as coal dust and tar are discharged along with the wastewater. These factories mostly use only simple graywater sedimentation treatment for gas generation wastewater treatment; a few have installed biological treatment systems, but due to the large volume of water, treatment proves difficult and such systems end up being ineffective. Therefore, it can be said that the gas generation systems of the nearly thousand existing fertilizer plants that use coal coke as raw material are not only major consumers of raw materials and energy but also significant sources of pollution to the atmosphere and water bodies. Amid the rapid development of technology today, as our country is set to join the WTO, the fertilizer industry faces a highly competitive market and an extremely stringent environmental protection environment. The old intermittent gas generation process severely restricts the production and development of the aforementioned small and medium-sized nitrogen fertilizer plants; it is therefore imperative to modernize these old plants by adopting new process technologies and raw material routes. The following discusses, from economic and environmental perspectives, the methods for upgrading batch gas generation using oxygen-enriched continuous gasification, as well as the prospects for this approach. 1 Application of atmospheric-pressure fixed-bed oxygen-enriched continuous gasification process technology. The world’s earliest gas generators appeared around the 16th century, mainly used in the metallurgy and glass industries. Initially, charcoal and firewood were used as fuel, and later coal and coke were adopted. Entering the 20th century, with the development of synthetic ammonia and the chemical industry, large-scale gas generators saw rapid advancements in technology and equipment. The first gas generators used in China’s ammonia synthesis industry were the U.G.I reactors manufactured in the United States, introduced by Nanhua Company in the 1930s, with a diameter of φ2745; as well as the Д-type reactors manufactured in the Soviet Union, introduced by Taiyuan Fertilizer Plant in the 1950s and 1960s, with a diameter of φ3600. After being modified through digestion and absorption, all of these devices were manufactured in China and became the standard equipment for the provincial fertilizer plant of that time (Zhongniu Plant). 1.1 Advantages of the atmospheric pressure fixed-bed oxygen-enriched continuous gasification process technology: At present, most medium-nitrogen fertilizer plants use coke or Shanxi anthracite (white coal) as raw materials, and employ batch process technologies to produce semi-water gas for the synthesis of ammonia. At the end of the 1960s, Jilin Fertilizer Factory began to utilize the excess O2 produced by its air separation unit to carry out continuous oxygen-enriched air–steam gasification on its old batch gas generators in order to produce raw material gas for synthetic ammonia production; this approach was successful, doubling the production capacity of each generator with very significant results. Subsequently, Huainan Fertilizer Plant adopted the same approach, using its excess O2 to conduct experiments on continuous oxygen-enriched gasification of coke and white coal in the old system, and all such experiments were successful. Both theoretically and in practice, it has been found that oxygen-enriched continuous gasification offers the following advantages over batch gas production: (1) In the batch method, the temperature of the carbon layer fluctuates greatly, and the direction of gas flow changes periodically; as a result, high requirements are placed on the particle size of the fuel, its thermal stability, and its ash melting point. In contrast, with oxygen-enriched continuous gasification, since the layer temperature, the direction of the medium flow, and the flow rate remain constant, lower requirements are imposed on the fuel, allowing the use of smaller fuel particles, whereas the batch method does not permit this. (2) In the batch method, in order to maintain the reaction temperature of the material layer, air is blown in for combustion to raise the temperature; however, the exhaust gas carried away some of the heat, resulting in unnecessary loss of fuel. Moreover, the alternating temperature of the material layer results in low gasification efficiency. In continuous gasification, the oxidation reaction carried out in an oxygen-rich environment generates heat, which helps maintain thermal balance in the gasification process. As a result, the bed temperature remains stable and heat loss is minimal, creating conditions for high gasification efficiency (the efficiency increases from 50-60% to 80-84% 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 six 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. As a result, equipment utilization is reduced and production capacity declines, usually to only about 50% of that in continuous gasification, with high fuel consumption. (4) The intermittent six-step cycle programmable valve, control system, and process pipelines are complex; the failure rate of equipment and valves is high, maintenance work is extensive, operation is difficult, and it is hard to adjust the gas composition. In contrast, continuous gasification simplifies the process flow, reduces the need for programmable valves, ensures stable and easy operation, reduces maintenance workload, and allows for easy adjustment of the 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 gasification eliminates the blowing stage and does not release any emissions into the atmosphere, thereby preventing air pollution. (6) The staged operations of the batch method result in severe noise pollution in the operating environment due to the main air ducts, fans, programmable valves, and venting systems. Continuous gas production is much quieter, creating a quiet environment at the gas generation station. Based on the comparison of the above advantages and disadvantages, oxygen-enriched continuous gasification is superior to the batch method in terms of technology, economics, operation, maintenance, environmental protection, and other aspects. It is evident that the intermittent method is an outdated and obsolete technology; it is inevitable that it will be phased out and replaced in today’s era of rapid technological advancement. However, there are still dozens of large nitrogen plants in the country, and hundreds of small nitrogen fertilizer plants continue to use the batch process for production, as continuous gasification requires oxygen generation equipment to produce oxygen-enriched or pure oxygen (O2>50%). Oxygen production equipment requires additional investment and higher electricity consumption (1 Nm3 of pure oxygen consumes 0.4–0.5 kWh of electricity). This is the only reason for intermittent gas production, and this issue will gradually be resolved as technology advances. In 1998, we built a synthetic ammonia plant with an annual production capacity of 180,000 tons for a chemical factory in Heilongjiang. It uses coke as raw material, featuring seven φ3m oxygen-enriched continuous gasification furnaces along with an oxygen production system capable of producing 12,000 Nm3/h of oxygen; this results in low coke consumption and reduced total investment. After being put into use in 1999, this device performed very well, completely transforming traditional intermittent gas production operations and management, and achieving initial success. 1.2 Oxygen-enriched continuous gasification process: After being crushed and screened, the raw coke is transported by belt conveyors to the coke bin at the top of the gas generator, where it is fed into the fixed-bed gas generator in a regular and continuous manner through a feeding hopper and an automatic coke feeder. Steam and oxygen-enriched air are continuously fed into the furnace, where coke and the oxygen-enriched air undergo incomplete combustion, generating a large amount of heat. As the temperature rises, the steam decomposes within the hot coke to produce semi-water gas. The air from the air blower enters the mixing tank simultaneously with oxygen at over 99% purity from the air separation system, where they are mixed to form oxygen-enriched air with a concentration of around 50–55%. This oxygen-enriched air is then mixed again with steam coming from the steam superheater before entering the bottom of the gas generator. The temperature of this mixture is between 110–140°C. The oxygen-enriched air and steam react with the carbon present in the furnace to produce semi-water gas, which exits through the top of the furnace and enters the waste heat boiler. The semi-water gas, with a temperature of around 700°C, has a volume of 471.56 Nm3; per furnace, the consumption is 1964.83 Nm3/h. This process generates medium-pressure steam with a pressure of 2.5 MPa, as well as 1.1 tons and 27.5 tons of by-products, which are used for urea production. The amount of semi-water gas produced is 3241.76 Nm3 per unit, and 81044.14 Nm3 in total, which is used for ammonia and methanol synthesis. The gas production capacity per furnace (with a diameter of 3 meters), with 6 units in operation and 1 in standby, is 13507.36 Nm3 per hour, resulting in 4.167 tons of NH3 per hour per unit. 2. Future prospects for fixed-bed oxygen-enhanced continuous gasification technology: Oxygen-enhanced continuous gasification has a history of over a hundred years, spanning from theoretical research to industrial application. However, in China, hundreds of small and medium-sized fertilizer plants operate thousands of fixed-bed gas generators, most of which use intermittent gasification processes. To enable these companies to upgrade their technologies and equipment, thereby reducing raw material and energy consumption, increasing gas production, and minimizing environmental pollution. Here, the economic efficiency of continuous gasification, automatic furnace condition monitoring, and zero pollution emissions are highlighted as future development directions. 2.1 Oxygen-enriched continuous gasification replaces air-based intermittent gasification; through a technical and economic comparison of the two gasification methods, oxygen-enriched continuous gasification proves to be a highly promising option. (1) Comparison of technical parameters for the two gasification methods Table 2-1-1 Ratios of technical parameters for the two gasification methods Item Oxygen-enriched continuous gasification Air intermittent gasification Remarks Carbon conversion rate Over 90% –60% Coke consumption 1.314 t/tNH3 1.50 t/tNH (coke fed into the furnace); ratio 1:1.14 Gasification efficiency 80% 75% Steam decomposition rate –55.0% –45.0% Gas production capacity per furnace (for comparison) 2–2.5 1 (baseline); varies depending on the raw material As can be seen from Table 2-1-1, the technical parameters for oxygen-enriched continuous gasification are all superior to those of air intermittent gasification. Oxygen-enriched continuous gasification features a high carbon utilization rate and no blowing phase, which results in about 14% less coke per ton of ammonia produced, thereby **reducing energy consumption. Although the CO2 content in the semi-water gas produced by oxygen-enriched continuous gasification is about 5–6% higher than that in gasification using the batch method, which increases the load on the decarburization equipment and raises the energy consumption associated with unnecessary work done by the compressors, the overall energy consumption remains lower than that of air-based batch gasification, as can be seen in the relevant technical documents. Continuous gasification is superior to batch gasification in terms of adjusting the H2/N2 ratio, gas composition, operational management, and avoiding environmental pollution. (2) Comparison of energy consumption between the two gasification methods: Details are provided in Table 2-1-2, which shows the energy consumption for producing ammonia using these two methods (in terms of tons of ammonia).
| Parameter | Unit | Energy value (KJ×106) | Energy value (kcal×106) |
|-----------|------|-----------------------|--------------------------|
| Coke (bituminous coal) fed into the furnace | t | 27.5 | 1.314 |
| | | 36.13 | 8.63 |
| | | 1.5 | 41.25 |
| | | 9.85 |
| Circulating water | m3 | 0.0025 | 60.4 |
| | | 0.151 | 0.036 |
| | | 68.42 | 0.17 |
| | | 0.041 |
| Oxygen | m3 | 0.0095 | 453.37 |
| | | 4.30 | 1.03 |
| | | – | – |
| Electricity | lWh | 0.01184 | 10.6 |
| | | 0.13 | 0.03 |
| | | 40.6 | 0.48 |
| | | 0.11 |
| Steam | t | 2.738 | 1.23 |
| | | 3.37 | 0.80 |
| | | 1.77 | 4.85 |
| | | 1.16 |
| Deoxygenated water | t | 0.0285 | 1.68 |
| | | 0.045 | 0.011 |
| | | 1.85 | 0.066 |
| | | 0.016 |
| By-product steam | t | 2.802 | 1.60 |
| | | –4.48 | –1.07 |
| | | 1.8 | –5.04 |
| | | –1.20 |
| Total | | 39.65 | 9.47 |
| | | 41.78 | 9.98 |
(3) Comparison of investment costs between the two gasification methods: The raw materials are bituminous coal or coke, with an annual production capacity of 180,000 tons of synthetic ammonia. For the air-based intermittent gasification method, 9 gasifiers with a diameter of 3600 mm are required, with a production capacity of 3.50 tons of ammonia per gasifier per hour ； The oxygen-enriched continuous gasification furnace requires 7 sets of gasification furnaces with a diameter of φ3000 mm, with a production capacity of 4.2 tons of ammonia per set per hour. It is equipped with 1 set of air separation unit capable of producing 12,000 cubic meters of air per hour, to supply the oxygen needed for creating an oxygen concentration of 50%. Investment details are shown in Table 2-1-3 below. Table 2-1-3: Comparison of Investments for Two Gasification Methods. Item: Oxygen-enriched gasification; Batch gasification. Remarks: 1. Production capacity: tons of ammonia per day – 600, 600. 2. Investment in gas generation equipment (in ten thousand yuan): 7 furnaces and related facilities; 9 furnaces and related facilities. Related equipment, electricity, instrumentation, water, etc. φ3m furnace; φ3.6m furnace. Full set of facilities (6 in operation, 1 on standby); (8 in operation, 1 on standby or 7 in operation, 2 on standby). Equipment cost: 1782.66, 2666.2. Installation cost: 1169.16, 1856.4. Construction cost: 395.66, 873.6. Total: 3347.48, 5396.2. 3. Investment in air separation equipment (in ten thousand yuan): Air compressors and separation units – no such unit required. (1) Air compressors: Equipment cost: 2206.79; Installation cost: 235.32; Construction cost: 147.49. Subtotal: 2589.60. (2) Air separation unit: Equipment cost: 2096.05; Installation cost: 216.33; Construction cost: 125.64. Subtotal: 2438.02. (3) Total for air separation equipment: 5027.62. 4. Total investment in the gas generation system (in ten thousand yuan): 8375.1, 5396.2. Difference: 2978.9. (4) Cost comparison of the two gasification methods (per ton of ammonia) is shown in Table 2-1-4. Table 2-1-4 Cost Comparison of Two Gasification Methods (per ton of ammonia) | Item | Unit Price (yuan) | Oxygen-enriched gasification | Batch gasification | Remarks | Unit consumption cost (yuan) | Unit consumption cost (yuan) | Coke or brown coal – small particles: 300; 1.314; 394.2. Mixed use of small particles and large/medium-sized particles: 380; 1.5; 570. Oxygen – Nm3: 0.2; 453.37; 90.67. – – The air compressor is driven by a back-pressure steam turbine, with the back-pressure steam being used for gas production. Circulating water, t: 0.1, 60.4, 6.04, 68.42, 6.84; Deoxygenated water, t: 2.00, 1.68, 3.36, 1.85, 3.7; Electricity, kWh: 0.25, 16.6, 2.05, 40.6, 10.15; Steam at 0.3 MPa, t: 35, 0.73, 25.55 – used internally; wages: 1.28, 1.49; Wages for operators; Steam supplied externally at 2.5 MPa, t: 50, 1.1, –55.0; Selling price of externally supplied steam; Total cost per ton: NH3: 443.20, 592.18; Difference: 148.98 per ton of ammonia; The investment made in the air separation unit can be recovered after 1.12 years of operation. 2681.56 ten thousand per year (5) Comparison of environmental pollution caused by the emissions of “three wastes” from the two processes is shown in Table 2-1-5 below. The calculations are based on an annual ammonia production of 180,000 tons (600 t NH3 per day). Table 2-1-5: Item, Unit – Oxygen-enriched gasification, Batch gasification; Remarks: Hourly amount, Annual amount; Hourly amount, Annual amount. 1. Raw material consumption (white coal), tons: 32.85, 23.66×10^4; 37.5, 27×10^4. 2. Ash and slag emissions, with residual carbon ≤15%, tons: 6.58, 4.74×10^4; 7.95, 5.72×10^4. 3. Exhaust gas volume, Nm3: None, None; 82,750, 59,580 million m3. After passing through the combustion chamber: CO at 0.4%, Nm3 – – 331, 2.38×10^6; CO2 at 19%, Nm3 – – 15,722.5, 113.2×10^6; SO2 at 1.72 g/Nm3, kg – – 143.15, 1,030.7 tons; Dust at 7.5 mg/Nm3, kg – – 2.06, 14.9 tons. 4. Cooling water: A zero-emission process is used; zero emission. The recycled water is treated in biochemical membrane cooling towers, but the treatment effect in factories in the north is generally poor, rendering such systems ineffective. CN: 5–10 mg/l kg – – 10–20; 72–144 tons. S: 5–20 mg/l kg – – 10–40; 72–288 tons. NH3-N: 5–250 mg/l kg – – 10–500; 72–3600 tons. As can be seen from the table above, the purge gas generated during batch gasification is released directly into the atmosphere, with levels of CO and SO2 exceeding the environmental standards, thus having a very negative impact on the atmospheric environment. CO and SO2 are toxic to humans, and acid rain caused by them poses a serious threat to life on Earth. Currently, the cooling water in atmospheric pressure gasification processes is mostly cooled by direct cooling. Toxic substances such as CN-, S2-, and NH3-N dissolved in the water are treated in biological towers, but the effectiveness of this treatment is poor. In the northern regions, where winter temperatures are low, the activity of the biological membranes is reduced, rendering the treatment systems ineffective; as a result, some of the wastewater from the circulating water system contaminates river waters. Given the pollution caused by the intermittent gasification process to the atmosphere and water bodies, environmental regulations demand that it be modified and treated. Adopting the oxygen-enriched, atmospheric-pressure continuous gasification process to transform the batch method is a way that makes full use of existing process equipment, requires low investment, yields quick results, helps revive existing fertilizer enterprises, and is a transformation approach suitable for China’s national conditions. 2.2 Use of an automatic furnace monitoring system: In recent years, our research department has developed various methods for automatically measuring the height of the carbon layer inside the furnace, the shape of the material surface, as well as the temperature of each layer. Through mathematical models and computer processing, it is possible to automatically control the amount of material fed in, the amount of ash discharged, the speed of the grate, the steam-to-gas ratio, and the furnace temperature. This replaces the traditional method of using probe rods to manually check the layer conditions, as well as the need to stop the furnace for feeding material or removing ash, thereby reviving the old technologies used in fixed-bed atmospheric pressure gasification furnaces. Leverage its advantages of low cost and rapid results, and overcome its shortcomings of low automation levels and outdated techniques relying on manual control. Currently, many small chemical and fertilizer factories in China are using the gas generation furnace condition monitoring system developed by Wuhan Radar Institute, with excellent results. However, most of these factories rely only on layer thickness measurement and do not use microcomputer systems for the automatic optimization control of various parameters; manual adjustment is still employed, and this represents the direction in which manufacturers and research and design departments should work together in the future. 2.3 Temperature measurement in gas generation furnaces Over the years, many companies have carried out extensive work on measuring the temperature in gas generation furnaces; these methods include direct measurement, indirect measurement by sampling the gas flow, and calculating the furnace temperature based on component analysis. The Wusong Gas Plant in Shanghai and the Shanghai Institute of Industrial Automation Instruments have done significant research on developing devices for measuring the temperature in the gasification zone. The Lurgi gasification technology introduced in our country operates at a pressure of 2.73 MPa. The gasifier furnace is equipped with three layers of thermocouples, totaling 10 in number: 4 at the bottom (in the ash layer) at temperatures of 280–320°C, 4 in the middle (above the fire zone) at temperatures of 760–820°C, and 2 at the top at temperatures of 200–210°C. The temperature of the ash at the bottom is -280°C. The gas is led out after passing through the waste boiler; the steam generated by this boiler has a pressure of 0.73 MPa. The thermocouple sleeves in this furnace are made of Cr25N20 heat-resistant stainless steel, and each thermocouple lasts for 6–8 months before needing replacement. Regular replacement of these sleeves ensures stable operation, which is beneficial for production management. The oxygen-enriched continuous gasification furnace is equipped with three layers of thermocouple temperature measurement points, based on Lurgi furnace technology; its service life is estimated at 6 months. It is believed that with proper temperature monitoring, the need for manual shutdowns to check for flames can be eliminated, thereby increasing the furnace’s gasification capacity and reducing the complexity associated with such shutdown procedures. Thanks to the temperature indication, accidents caused by burning out the grate are avoided, thereby enabling fully automatic optimized control of feeding, load, temperature, and gas composition. 2.4 Gas generation waste heat boiler: The fire-tube type waste heat boiler used in U.G.I furnaces has a service life of around one year due to erosion by airflow and ash, corrosion caused by gas condensation, as well as stress corrosion resulting from changes in pressure and temperature during intermittent gasification; this significantly impacts production. In recent years, some factories have adopted heat pipe technology, which increases the steam pressure generated as a by-product and extends the equipment’s service life. However, heat pipe technology is relatively complex; it has a poor ability to withstand sudden temperature rises and over-temperature conditions, and further improvements are needed to enhance its adaptability. By drawing on foreign technologies to improve the structure and materials of waste boilers, it is possible to generate medium-pressure steam at pressures above 2.5 MPa, thereby increasing the value of thermal energy for use in urea stripping units; this helps factories **improve the economic efficiency of thermal energy utilization. 2.5 Eliminating toxic pollution in the atmosphere and wastewater, and achieving zero-cyanide emission technology: In recent years, in line with global trends, restrictions on harmful substances such as CO, H2S, and CN- have become increasingly strict; as a result, clean technologies that prevent the release of toxic substances into the environment have emerged. In recent years, China’s Dongfang Fertilizer Company has developed technologies for high-temperature dust removal from gases, indirect cooling, and converting two types of cooling water (turbid water and clean water) into one type of cooling water (only clean water for indirect cooling). The CN- ion (cyanide) in gas is removed during the desulfurization process, and sodium thiocyanate is recovered through regeneration and sold as a useful chemical product, thus truly turning waste into treasure. In recent years, we have also proposed a method for the effective biochemical treatment of harmful substances such as cyanide in concentrated scrubbing water; it is a more reliable, low-risk, and less complex approach from a technical standpoint, making it an excellent option for future adoption and expansion. 2.6 Developing various oxygen production technologies to support oxygen-enriched gasification: As we all know, oxygen-enriched continuous gasification is effective, but it is limited by the equipment available for oxygen production. The currently common cryogenic air separation method for oxygen production requires high investment and results in high oxygen costs. The advent of pressure swing adsorption oxygen production technology has reduced the cost of oxygen generation equipment, lowering the costs of pure oxygen and enriched oxygen, **which in turn has promoted the development of continuous enriched oxygen oxidation technology. This should serve as an important direction for the technological upgrading and transformation of small and medium-sized nitrogen fertilizer plants in order to expand their production capacity. 2.7 For the improvement and renovation of existing gas generation systems, measures such as addressing leaks in various oil-pressure and air-pressure gas generation valves to extend their service life and reliability, improving the diameter and material of the pipes in the gas generation furnace, enhancing the air pressure provided by air blowers, replacing the packing in the gas cleaning tower with sieve trays, using new high-efficiency and low-resistance cyclone separators for dust removal, and making more scientific improvements to the grates, furnace chamber, and refractory linings – all these are key areas for further development of fixed-bed gasification furnaces in the future. Through everyone’s efforts, this outdated and backward technology is expected to turn into a new process technology with high technical standards and good economic benefits. Conclusion: At present, thousands of atmospheric-pressure fixed-bed gas generation furnaces are in use at hundreds of small and medium-sized fertilizer plants in the country. It is impossible to completely eliminate them; the only viable approach is to continuously upgrade them using new technologies and materials, turning them into innovative technologies that remain effective over time. Let’s work together to achieve this goal. It is hoped that the leaders, technical personnel in the fertilizer industry, and all industry practitioners will take immediate action to upgrade the existing batch gasification units, so as to give new life to older enterprises. This post was last edited by Dopoda on 2009-3-31 11:33]
| Parameter | Unit | Energy value (KJ×106) | Energy value (kcal×106) |
|-----------|------|-----------------------|--------------------------|
| Coke (bituminous coal) fed into the furnace | t | 27.5 | 1.314 |
| | | 36.13 | 8.63 |
| | | 1.5 | 41.25 |
| | | 9.85 |
| Circulating water | m3 | 0.0025 | 60.4 |
| | | 0.151 | 0.036 |
| | | 68.42 | 0.17 |
| | | 0.041 |
| Oxygen | m3 | 0.0095 | 453.37 |
| | | 4.30 | 1.03 |
| | | – | – |
| Electricity | lWh | 0.01184 | 10.6 |
| | | 0.13 | 0.03 |
| | | 40.6 | 0.48 |
| | | 0.11 |
| Steam | t | 2.738 | 1.23 |
| | | 3.37 | 0.80 |
| | | 1.77 | 4.85 |
| | | 1.16 |
| Deoxygenated water | t | 0.0285 | 1.68 |
| | | 0.045 | 0.011 |
| | | 1.85 | 0.066 |
| | | 0.016 |
| By-product steam | t | 2.802 | 1.60 |
| | | –4.48 | –1.07 |
| | | 1.8 | –5.04 |
| | | –1.20 |
| Total | | 39.65 | 9.47 |
| | | 41.78 | 9.98 |
(3) Comparison of investment costs between the two gasification methods: The raw materials are bituminous coal or coke, with an annual production capacity of 180,000 tons of synthetic ammonia. For the air-based intermittent gasification method, 9 gasifiers with a diameter of 3600 mm are required, with a production capacity of 3.50 tons of ammonia per gasifier per hour ； The oxygen-enriched continuous gasification furnace requires 7 sets of gasification furnaces with a diameter of φ3000 mm, with a production capacity of 4.2 tons of ammonia per set per hour. It is equipped with 1 set of air separation unit capable of producing 12,000 cubic meters of air per hour, to supply the oxygen needed for creating an oxygen concentration of 50%. Investment details are shown in Table 2-1-3 below. Table 2-1-3: Comparison of Investments for Two Gasification Methods. Item: Oxygen-enriched gasification; Batch gasification. Remarks: 1. Production capacity: tons of ammonia per day – 600, 600. 2. Investment in gas generation equipment (in ten thousand yuan): 7 furnaces and related facilities; 9 furnaces and related facilities. Related equipment, electricity, instrumentation, water, etc. φ3m furnace; φ3.6m furnace. Full set of facilities (6 in operation, 1 on standby); (8 in operation, 1 on standby or 7 in operation, 2 on standby). Equipment cost: 1782.66, 2666.2. Installation cost: 1169.16, 1856.4. Construction cost: 395.66, 873.6. Total: 3347.48, 5396.2. 3. Investment in air separation equipment (in ten thousand yuan): Air compressors and separation units – no such unit required. (1) Air compressors: Equipment cost: 2206.79; Installation cost: 235.32; Construction cost: 147.49. Subtotal: 2589.60. (2) Air separation unit: Equipment cost: 2096.05; Installation cost: 216.33; Construction cost: 125.64. Subtotal: 2438.02. (3) Total for air separation equipment: 5027.62. 4. Total investment in the gas generation system (in ten thousand yuan): 8375.1, 5396.2. Difference: 2978.9. (4) Cost comparison of the two gasification methods (per ton of ammonia) is shown in Table 2-1-4. Table 2-1-4 Cost Comparison of Two Gasification Methods (per ton of ammonia) | Item | Unit Price (yuan) | Oxygen-enriched gasification | Batch gasification | Remarks | Unit consumption cost (yuan) | Unit consumption cost (yuan) | Coke or brown coal – small particles: 300; 1.314; 394.2. Mixed use of small particles and large/medium-sized particles: 380; 1.5; 570. Oxygen – Nm3: 0.2; 453.37; 90.67. – – The air compressor is driven by a back-pressure steam turbine, with the back-pressure steam being used for gas production. Circulating water, t: 0.1, 60.4, 6.04, 68.42, 6.84; Deoxygenated water, t: 2.00, 1.68, 3.36, 1.85, 3.7; Electricity, kWh: 0.25, 16.6, 2.05, 40.6, 10.15; Steam at 0.3 MPa, t: 35, 0.73, 25.55 – used internally; wages: 1.28, 1.49; Wages for operators; Steam supplied externally at 2.5 MPa, t: 50, 1.1, –55.0; Selling price of externally supplied steam; Total cost per ton: NH3: 443.20, 592.18; Difference: 148.98 per ton of ammonia; The investment made in the air separation unit can be recovered after 1.12 years of operation. 2681.56 ten thousand per year (5) Comparison of environmental pollution caused by the emissions of “three wastes” from the two processes is shown in Table 2-1-5 below. The calculations are based on an annual ammonia production of 180,000 tons (600 t NH3 per day). Table 2-1-5: Item, Unit – Oxygen-enriched gasification, Batch gasification; Remarks: Hourly amount, Annual amount; Hourly amount, Annual amount. 1. Raw material consumption (white coal), tons: 32.85, 23.66×10^4; 37.5, 27×10^4. 2. Ash and slag emissions, with residual carbon ≤15%, tons: 6.58, 4.74×10^4; 7.95, 5.72×10^4. 3. Exhaust gas volume, Nm3: None, None; 82,750, 59,580 million m3. After passing through the combustion chamber: CO at 0.4%, Nm3 – – 331, 2.38×10^6; CO2 at 19%, Nm3 – – 15,722.5, 113.2×10^6; SO2 at 1.72 g/Nm3, kg – – 143.15, 1,030.7 tons; Dust at 7.5 mg/Nm3, kg – – 2.06, 14.9 tons. 4. Cooling water: A zero-emission process is used; zero emission. The recycled water is treated in biochemical membrane cooling towers, but the treatment effect in factories in the north is generally poor, rendering such systems ineffective. CN: 5–10 mg/l kg – – 10–20; 72–144 tons. S: 5–20 mg/l kg – – 10–40; 72–288 tons. NH3-N: 5–250 mg/l kg – – 10–500; 72–3600 tons. As can be seen from the table above, the purge gas generated during batch gasification is released directly into the atmosphere, with levels of CO and SO2 exceeding the environmental standards, thus having a very negative impact on the atmospheric environment. CO and SO2 are toxic to humans, and acid rain caused by them poses a serious threat to life on Earth. Currently, the cooling water in atmospheric pressure gasification processes is mostly cooled by direct cooling. Toxic substances such as CN-, S2-, and NH3-N dissolved in the water are treated in biological towers, but the effectiveness of this treatment is poor. In the northern regions, where winter temperatures are low, the activity of the biological membranes is reduced, rendering the treatment systems ineffective; as a result, some of the wastewater from the circulating water system contaminates river waters. Given the pollution caused by the intermittent gasification process to the atmosphere and water bodies, environmental regulations demand that it be modified and treated. Adopting the oxygen-enriched, atmospheric-pressure continuous gasification process to transform the batch method is a way that makes full use of existing process equipment, requires low investment, yields quick results, helps revive existing fertilizer enterprises, and is a transformation approach suitable for China’s national conditions. 2.2 Use of an automatic furnace monitoring system: In recent years, our research department has developed various methods for automatically measuring the height of the carbon layer inside the furnace, the shape of the material surface, as well as the temperature of each layer. Through mathematical models and computer processing, it is possible to automatically control the amount of material fed in, the amount of ash discharged, the speed of the grate, the steam-to-gas ratio, and the furnace temperature. This replaces the traditional method of using probe rods to manually check the layer conditions, as well as the need to stop the furnace for feeding material or removing ash, thereby reviving the old technologies used in fixed-bed atmospheric pressure gasification furnaces. Leverage its advantages of low cost and rapid results, and overcome its shortcomings of low automation levels and outdated techniques relying on manual control. Currently, many small chemical and fertilizer factories in China are using the gas generation furnace condition monitoring system developed by Wuhan Radar Institute, with excellent results. However, most of these factories rely only on layer thickness measurement and do not use microcomputer systems for the automatic optimization control of various parameters; manual adjustment is still employed, and this represents the direction in which manufacturers and research and design departments should work together in the future. 2.3 Temperature measurement in gas generation furnaces Over the years, many companies have carried out extensive work on measuring the temperature in gas generation furnaces; these methods include direct measurement, indirect measurement by sampling the gas flow, and calculating the furnace temperature based on component analysis. The Wusong Gas Plant in Shanghai and the Shanghai Institute of Industrial Automation Instruments have done significant research on developing devices for measuring the temperature in the gasification zone. The Lurgi gasification technology introduced in our country operates at a pressure of 2.73 MPa. The gasifier furnace is equipped with three layers of thermocouples, totaling 10 in number: 4 at the bottom (in the ash layer) at temperatures of 280–320°C, 4 in the middle (above the fire zone) at temperatures of 760–820°C, and 2 at the top at temperatures of 200–210°C. The temperature of the ash at the bottom is -280°C. The gas is led out after passing through the waste boiler; the steam generated by this boiler has a pressure of 0.73 MPa. The thermocouple sleeves in this furnace are made of Cr25N20 heat-resistant stainless steel, and each thermocouple lasts for 6–8 months before needing replacement. Regular replacement of these sleeves ensures stable operation, which is beneficial for production management. The oxygen-enriched continuous gasification furnace is equipped with three layers of thermocouple temperature measurement points, based on Lurgi furnace technology; its service life is estimated at 6 months. It is believed that with proper temperature monitoring, the need for manual shutdowns to check for flames can be eliminated, thereby increasing the furnace’s gasification capacity and reducing the complexity associated with such shutdown procedures. Thanks to the temperature indication, accidents caused by burning out the grate are avoided, thereby enabling fully automatic optimized control of feeding, load, temperature, and gas composition. 2.4 Gas generation waste heat boiler: The fire-tube type waste heat boiler used in U.G.I furnaces has a service life of around one year due to erosion by airflow and ash, corrosion caused by gas condensation, as well as stress corrosion resulting from changes in pressure and temperature during intermittent gasification; this significantly impacts production. In recent years, some factories have adopted heat pipe technology, which increases the steam pressure generated as a by-product and extends the equipment’s service life. However, heat pipe technology is relatively complex; it has a poor ability to withstand sudden temperature rises and over-temperature conditions, and further improvements are needed to enhance its adaptability. By drawing on foreign technologies to improve the structure and materials of waste boilers, it is possible to generate medium-pressure steam at pressures above 2.5 MPa, thereby increasing the value of thermal energy for use in urea stripping units; this helps factories **improve the economic efficiency of thermal energy utilization. 2.5 Eliminating toxic pollution in the atmosphere and wastewater, and achieving zero-cyanide emission technology: In recent years, in line with global trends, restrictions on harmful substances such as CO, H2S, and CN- have become increasingly strict; as a result, clean technologies that prevent the release of toxic substances into the environment have emerged. In recent years, China’s Dongfang Fertilizer Company has developed technologies for high-temperature dust removal from gases, indirect cooling, and converting two types of cooling water (turbid water and clean water) into one type of cooling water (only clean water for indirect cooling). The CN- ion (cyanide) in gas is removed during the desulfurization process, and sodium thiocyanate is recovered through regeneration and sold as a useful chemical product, thus truly turning waste into treasure. In recent years, we have also proposed a method for the effective biochemical treatment of harmful substances such as cyanide in concentrated scrubbing water; it is a more reliable, low-risk, and less complex approach from a technical standpoint, making it an excellent option for future adoption and expansion. 2.6 Developing various oxygen production technologies to support oxygen-enriched gasification: As we all know, oxygen-enriched continuous gasification is effective, but it is limited by the equipment available for oxygen production. The currently common cryogenic air separation method for oxygen production requires high investment and results in high oxygen costs. The advent of pressure swing adsorption oxygen production technology has reduced the cost of oxygen generation equipment, lowering the costs of pure oxygen and enriched oxygen, **which in turn has promoted the development of continuous enriched oxygen oxidation technology. This should serve as an important direction for the technological upgrading and transformation of small and medium-sized nitrogen fertilizer plants in order to expand their production capacity. 2.7 For the improvement and renovation of existing gas generation systems, measures such as addressing leaks in various oil-pressure and air-pressure gas generation valves to extend their service life and reliability, improving the diameter and material of the pipes in the gas generation furnace, enhancing the air pressure provided by air blowers, replacing the packing in the gas cleaning tower with sieve trays, using new high-efficiency and low-resistance cyclone separators for dust removal, and making more scientific improvements to the grates, furnace chamber, and refractory linings – all these are key areas for further development of fixed-bed gasification furnaces in the future. Through everyone’s efforts, this outdated and backward technology is expected to turn into a new process technology with high technical standards and good economic benefits. Conclusion: At present, thousands of atmospheric-pressure fixed-bed gas generation furnaces are in use at hundreds of small and medium-sized fertilizer plants in the country. It is impossible to completely eliminate them; the only viable approach is to continuously upgrade them using new technologies and materials, turning them into innovative technologies that remain effective over time. Let’s work together to achieve this goal. It is hoped that the leaders, technical personnel in the fertilizer industry, and all industry practitioners will take immediate action to upgrade the existing batch gasification units, so as to give new life to older enterprises. This post was last edited by Dopoda on 2009-3-31 11:33]