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Some Issues in the Design of Gas Production Projects Using Coal Gasification Some Issues in the Design of Gas Production Projects Using Coal Gasification Abstract: This article identifies the problems that arise in the design of gas production projects using coal gasification, proposes design solutions, and provides measures to help address these issues for those involved in such project design as well as related chemical industry companies. Keywords: briquette, gasification, gas stove, engineering design, height-to-diameter ratio, gas scrubber. Author profile: Zhao Lequn, male, with a university education, born in November 1949. Currently the Chief Engineer at Jiangxi Changyu Industrial Co., Ltd. In recent years, due to the high market prices of anthracite, coal production companies and coal chemical enterprises have restructured, resulting in over 80% of such enterprises losing stable sources of high-quality anthracite raw material. The dual challenges led coal chemical enterprises using lump coal coke as raw material to choose the path of gasification. However, in actual production and operation, the performance of different types of enterprise gasification varies greatly. In good companies, the ammonia consumption per ton of briquetted coal used in the furnace is ≤1.6 T, while in poor companies it exceeds 2.0 T. Some enterprises achieve annual economic benefits in the hundreds of millions of yuan through coal briquette gasification, while others are dragged into trouble by coal briquettes, incurring costs and losses without seeing any returns. By summarizing the successful experiences and failed lessons of dozens of coal chemical enterprises that use briquetted coal as raw material, we find that the design of briquetted coal gasification projects is the key factor determining the efficiency of production and operation, and it represents an important issue affecting the survival and development of such enterprises. This paper analyzes the issues in the design of gasification gas production plants using coal. 1.1 The characteristics of gasifier-type gasification differ significantly from those of lump coal coking. The raw gas produced by coal gasification contains a relatively high level of CO2, typically around 9% to 11%. To reduce the CO2 level to about 9%, it is necessary to increase the reducing layer in the gasification zone; therefore, it is required to raise the height of the furnace chamber. Briquettes have poor thermal stability, which requires lowering the furnace surface temperature as well as increasing the carbon layer thickness to raise the furnace height. However, the higher the furnace height, the better it is not. If the height exceeds what is actually necessary, it will have negative effects instead. At the same time, different models of gas stoves also have varying actual design \"height-to-diameter ratios\". “The \"height-to-diameter ratio\" should be designed in accordance with the requirements of systems engineering, rather than being uniform for all types of furnaces and all enterprises’ gas furnaces. The design of the slag discharge system for briquette gas stoves differs significantly from that of lump coal stoves. Gasifiers for briquette gas require a high slag discharge capacity; however, strong slag crushing is not necessary, as briquettes have a high ash content and produce large amounts of slag. Yet this slag is relatively soft and does not easily break apart, resulting in more fine slag particles that increase the resistance posed by the ash layer. Thus, corresponding design is required for the grates, slag strips (plates), ash plows, and ash discharge ports. The gasification strength of briquetted coal is lower than that of lump coal, and the gasification temperature range is narrower. This requirement necessitates large furnace air ducts, grates, and ventilation areas, as well as a more even distribution of airflow within the ducts. Otherwise, it will cause local overheating and partial failure to burn through. The grate for lump coal is used for briquettes, and the distribution of air ducts is inappropriate. It can only limit the intensity of gasification, resulting in process instability. 1.2 Dust collectors: During gasification, there is a significant amount of dust, and the tar content in the gas stream is high; as a result, the requirements for the blowing air used in gasifiers and for dust removal from the gas are much higher than those for lump coal. Cyclone dust collectors are generally used in gasification of fixed-bed gasifiers. The design of dust collectors also has its particularities. The dust composition in the batch gasification gas stream differs significantly from that in the continuous gasification gas stream. A cyclone dust collector for burning coal briquettes can use an iron cast lining, as wear and erosion resistance are very important. When the airflow moves tangentially, the vortices and deceleration caused by the rough cast iron lining have little impact on the separation of large particles in coarse dust, and thus do not draw much attention. Briquette coal is different; it has a high dust content, and the quality of that dust is lower than that of lump coal, with a higher level of fine dust. If a cast iron lining is used, the rough inner surface of the cylinder leads to gas vortices and a significant reduction in tangential velocity, which is highly detrimental to the settling of dust. Therefore, for the cyclone dust collector designed in coal gasification plant projects, the lining should be made of a material with as smooth a surface as possible, one that does not easily form vortices and has low air resistance. At the same time, taking factors such as cost and service life into account, it is advisable to design a \"tortoise-shell mesh corundum lining\". To ensure the dust removal efficiency of the cyclone dust collector, the insertion depth of the outlet pipe must be strictly controlled, so that the lower edge of the outlet pipe is slightly below that of the inlet, within 300 mm. This is determined by the characteristics of coal gasification. 1.3 Waste Heat Boilers: In gasification projects of this type, waste heat boilers are selected primarily to prevent dust from adhering to and clogging the heat exchange elements, which could affect the efficiency of heat exchange. During design, it is necessary to ensure the gas conditions and flow velocity inside the heat exchanger shell; the flow velocity should be above 3 m/s. The heat exchange fins should be designed as longitudinal fins in order to achieve an effective prevention of fouling and blockage. 1.4 Scrubber Towers: In gasification projects of this type, scrubber towers are designed to achieve effective dust removal, while also preventing dust from causing blockages in the packing inside the towers and thereby increasing resistance. Generally speaking, tower spraying can effectively prevent dust accumulation during coal gasification, which would otherwise lead to increased resistance, but it can make it difficult to achieve an ideal dust removal effect. Packed towers offer good dust removal efficiency, but they tend to cause an increase in resistance. Based on production experience, the scrubber towers in coal gasification projects should adopt the advantages of empty-tower spraying technology, such as nozzle design techniques. At the same time, by taking advantage of the thorough gas-liquid contact in packed towers, a new type of gas scrubber has been developed, namely the anti-clogging high-efficiency gas scrubber. It neither causes an increase in resistance nor a decline in cleaning efficiency, making it highly suitable for use in coal gasification projects. 1.5 Air blowers: For a long time, there has been a misconception that coal gasification is prone to being disrupted during the blowing process; as a result, it is not advisable to choose air blowers with high pressure and large flow rates. As verified by recent practices in various enterprises, when using briquetted coal as raw material, the air flow rate during the blowing stage is higher than that when using lump coal as raw material. For example, the ammonia plant in Jiangxi uses coal briquettes as raw material; the air flow rate for furnaces with a diameter of φ3.0m is around 35,000 m3/h, while that for furnaces using lump coal is only about 30,000 m3/h. The furnace at the Luoyang Nitrogen Fertilizer Plant, after being modified, uses coal balls as raw material, and the control valve of the D800 air blower is kept fully open during air supply. And burning a lump of coal doesn’t achieve full operation. A new gas furnace designed using briquetted coal as raw material requires a higher volume of primary air flow compared to coal in lump form. The main reason for this is that briquetted coal has good air permeability but low heat storage capacity; therefore, a process with a high carbon layer thickness and high air flow rate is necessary to meet the requirements for gasification. Under such circumstances, it is necessary to move away from the misconceptions surrounding the air selection fans used for briquettes. Generally, a φ2.6m gasifier requires a blower with an air pressure of 28 Kpa, a flow rate of 600 Nm3/min, and a motor power of 440 KW ； For coal gas furnaces of the φ3.0m~φ3.2m type, a blower with an air pressure of 28 Kpa, an air flow rate of 800 Nm3/min, and a motor power of 630 KW is required ； A φ3.6m gas furnace requires a blower with an air pressure of 28 Kpa, an air flow rate of 1100 Nm3/min, and a motor power of 800 KW. Using briquettes as raw material, the ideal air flow rates for the three types of furnaces should be 26,000 m3/h, 35,000 m3/h, and 45,000 m3/h respectively; in other words, it is preferable for the gas furnace to have an air flow rate of 4,700 m3/hm2. 1.6 Design of nitrogen-enriched air: In coal gasification projects where the feed gas is semi-water gas, careful consideration should be given to the optimized design of the nitrogen-enriched air system during the top-blowing stage. Because coal gasification requires a large amount of air for blowing, it is difficult to rely solely on the flow rate of primary air. Therefore, some of the air needs to be transferred to the top blowing with N2. To supply sufficient N2 air to the furnace, high-efficiency injectors are necessary. Generally, for φ2.65m gasification furnaces, a PS-I type unit combined with an N2 injector should be selected to ensure that a steam supply of 7 T/h and an air supply of 0–7000 m3/h can be achieved ； For φ3.0~3.2m gasification furnaces, a PS-II type unit with an N2 injector should be selected to ensure that 9 T/h of steam and 0~10000 m3/h of air can be supplied ； For the φ3.6m gasification furnace, a PS-III type unit with an N2 injector should be selected to ensure that a steam supply of 11 T/h and an air flow rate of 0~12,000 m3/h can be achieved. To ensure that the N2 content in the gas does not exceed the specified limits, companies with the necessary facilities can introduce oxygen-enriched air by using N2 injectors, or employ such injectors to achieve continuous oxygenation of the gas. This method enables the transformation of fixed-bed batch gasification gasifiers into oxygen-enriched continuous gasifiers. For example, Sanming Chemical has already achieved continuous oxygen-enriched gasification of briquettes using this technology. 1.7 Hydraulic control system and valves 1.7.1 The hydraulic control system of the coal gasification gas furnace is the same as that of other gas furnaces. Basic requirements for the hydraulic pressure control system: ① The system’s hydraulic pressure is low; the pressure in the main pipeline is 4.0 Mpa ； ②The hydraulic-driven valve changes direction quickly. The direction change of all hydraulic control valves should occur within 2.0~2.5 seconds. Among them: gas burners with a diameter of φ2.65m should complete the process within 2 seconds, while those with a diameter of φ3.0~φ3.6m should do so within 2.5 seconds ； ③The oil pressure fluctuation should be small, at ±0.2 Mpa ； ④The temperature of the pressure oil in summer shall be ≤60°C. To meet the above requirements, the hydraulic system must be configured as follows: ① Valves with a diameter of DN500 or larger, including DN500 itself, must be controlled by electro-hydraulic valves, and the oil passage diameter must be ≥18 mm. Other valves can be controlled by solenoid valves ； ②For valves with a diameter of DN250~DN700, cylinders of the 80/40 type should be used, while for valves with a diameter of DN700~DN1000, cylinders of the 90/50~100/55 type should be employed. To avoid high oil pressure in the system, all automatic control valves should not use cylinders with a ratio of 63/45 ； ③The accumulator of the system oil pressure main pipe should be sized appropriately. It should generally be ≥100L per furnace. ④The heat exchange area of the oil cooler is ≥10 m2. ⑤The oil pressure pump stations are as follows: for furnaces with a diameter of φ2.65m, the flow rate is 80 L/min; for furnaces with a diameter of φ3.0~3.6m, the flow rate is 140 L/min. 1.7.2 Selection of process valves: For furnaces with a diameter of φ2.65m, the large valves are generally of DN600~DN800 size, while those for steam and nitrogen/air are of DN250~DN300 size. For furnaces with a diameter of 3.0m to 3.2m, the large valves are generally DN700 to DN900, while those for steam and nitrogen/air are DN300 to DN400. For φ3.6m furnaces, the large valves are generally DN900~DN1000, while those for steam and nitrogen/air are DN300~DN400. 1.8 For steam buffer tanks with a diameter of φ2.65m, 3 to 4 boilers should be grouped together; for boilers with a diameter of φ3.0–φ3.2m or φ3.6m, 3 boilers should form one group. The steam buffer tank for each group of gas boilers should be installed as close as possible in the middle of that group, with a capacity of ≥20m3. 1.9 Automatic coal feeder: The automatic coal feeder for gasifiers of this type differs significantly from those used in lump-coal furnaces. The main difference is that briquette coal has a large volume, requiring the coal storage tank of the automatic coal feeder to be ≥1.2 m3. The mechanical strength of briquettes is low; therefore, the hopper design needs to minimize drops and collisions in order to reduce the rate of briquette fragmentation upon entry into the furnace. Briquettes are most sensitive to absorbing water; therefore, in the production of automatic briquette feeding machines, it is necessary to separate the briquettes from the steam leaking into the furnace. Briquettes tend to produce dust during transportation, and the dust generated has high viscosity. Therefore, the automatic briquette feeding machine must be equipped with a clog-preventing fixed screen to reduce the dust content in the briquettes fed into the furnace. 2. Design of the process flow 2.1 For coal gasification, a relatively simple “low-nitrogen process” is generally adopted, namely a process consisting of four furnaces, one machine, one station, one boiler, and one tower. However, for furnaces with a diameter of 3.0m or more, one machine, one station, one pot, and one tower should be provided per three furnaces. For a φ3.6m furnace, it should be equipped with one machine, one station, three boilers, and one tower per three furnaces. In recent years, the location where the downward steam enters the furnace has shown a trend of moving closer to the top of the gas furnace, which is undoubtedly an improvement. However, the closer to the top of the furnace, the more important it is to address the issue of steam carrying water, as well as the uniform distribution of steam. Otherwise, new problems will arise that affect normal production. For example: it damages the refractory layer at the upper part of the furnace, forcing a shutdown for maintenance. 2.2 Design of the steam system: The steam system should be equipped with one steam buffer tank for each group of gas burners, with pressure reducers capable of serving a maximum of two groups of gas burners per unit. The steam produced is superheated before entering the steam buffer tank. For more than two groups of gas stoves, an additional pressure reducing valve must be installed, which should be connected to the middle part of the main steam pipe of the gas stoves. The pressure fluctuation in the steam system should be within ±0.001 Mpa. For φ2.65m furnaces, the low-pressure steam main per furnace set should be ≥DN500mm. For furnaces with a diameter of φ3.0~3.2m, the low-pressure steam main for each furnace set should be ≥DN600mm. For φ3.6m furnaces, the main low-pressure steam pipe per furnace set should be ≥DN800mm. The above measures ensure that the steam flow and pressure in the production of modular gasifiers meet the requirements of modular gasification. 2.3 Installation direction of process valves: To ensure safety in the operation of gas furnaces, all process oil pressure control valves that lead into the furnace should have the inlet at a higher level than the outlet, in order to prevent the process gases from opening the valves in case of a sudden drop in oil pressure. Such as the main steam valve, primary air valve, main N2 air valve, etc. Blower recirculation valve, upward gas valve, downward gas valve; if the installation position is low inlet and high outlet, the hydraulic differential connection method must not be used to prevent air leakage. For two-way valves that control high-temperature gases, such as blowback recovery valves, upper and lower gas valves, and main gas valves, it is advisable to arrange the sealing surfaces of these valves to face away from the direction of the flowing gas, in order to prevent direct erosion of the sealing surfaces and thus reduce the service life of the valves. 2.3 Design of the air system: In a production system with multiple furnaces, the air fans and the main air ducts should not be connected at one end; instead, they should be connected in the middle of the main air duct to prevent the occurrence of weak gas furnaces. When a two-way seat valve is used for the primary air valve, the safety baffle valve (or safety relief valve) can be eliminated. For the main air duct, it should be φ630×8mm for furnaces with a diameter of φ2.65m, φ770×10mm for furnaces with a diameter of φ3.0~φ3.2m, and φ920×10mm for furnaces with a diameter of φ3.6m. For gas stoves, the diameter of the central ash box should be φ1000/1200 mm for stoves with a diameter of φ2.65 m; it should be 1200/1400/1600 mm for stoves with a diameter of φ3.0(2) m; and φ1620/2000 mm for stoves with a diameter of φ3.6 m. Optimizing the air system pipes allows the gas stove to achieve the maximum air flow at the lowest wind speed when burning briquettes, ensuring stable airflow. The wind speed of the primary air in the furnace grate air chamber should be kept within 7 m/s. 2.4 Design of system gas pipelines Recently, some people have believed that the lower the resistance in a gas stove system during the gasification process, the better; therefore, when designing the pipes for such systems, efforts are made to increase their diameter as much as possible. This is undoubtedly a misconception. First, it is necessary to clarify the concepts: the system pressure and system resistance of a gas stove are two different things; the pressure difference between them represents resistance, not pressure itself. In the gasification process, increasing the pressure in a gas stove is beneficial for gasification; it is not increasing resistance that helps with gasification. At the same time, a certain level of resistance is also necessary for gasification, as any equipment or pipes will result in some degree of resistance. The design principle for gas system pipelines is to ensure that the gas flow rate remains within an appropriate range. If the diameter of the main gas pipeline is designed beyond this range, it not only results in wasted investment but is also detrimental to the process. 3. The critical parts of other system equipment are designed for a system major maintenance cycle of ≥24 months. 3.1 The material of the idler wheels and idler wheel shafts inside the gas holder is 1Cr18Ni9Ti, to ensure operation for over 30 months. 3.2 The electrostatic precipitator is cleaned using intermittent hot water flushing, with a focus on cleaning coal tar. 3.3 A gravity dust collector is installed in the lower gas duct of the gas stove, and an anti-erosion material is sprayed on the inner wall of the lower gas duct pipe. 3.4 Sufficient spare rotary vacuum pumps should be provided to ensure timely cleaning and maintenance. 4. Conclusion: The design of gas generation through coal gasification is a systematic project, in which each component must be adapted to and support the entire project. It is precisely through these incremental optimizations that the optimization of the entire project is achieved. It is hoped that enterprises using briquettes as gasification feedstock will gradually optimize their systems to truly achieve economic benefits from briquette gasification.