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Some Issues and Solutions in the Design of Coal Gasification Plants Author/Source: Zhao Lequn (Jiangxi Changyu Industrial Co., Ltd., Nanchang 330013, Jiangxi) 0 Introduction In recent years, due to the high market prices of anthracite coal, coal production enterprises and coal chemical companies have reorganized, resulting in more than 80% of such companies losing access to stable sources of high-quality anthracite coal as 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, 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 tons, while in poor companies it can be as high as over 2.0 tons. Some enterprises achieve annual economic benefits in the hundreds of millions thanks to briquette gasification, while others are dragged into trouble by 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 coal gasification plants. 1 The characteristics of gasifier-type gasification differ significantly from those of lump coal coking. The raw gas produced by coal gasification contains a high level of CO2, typically between 9% and 11%. To reduce the CO2 content to around 9%, it is necessary to increase the reducing layer in the gasification zone; therefore, the furnace chamber of the coal gasifier needs to be made taller ； Furthermore, briquettes have poor thermal stability, which requires lowering the furnace surface temperature as well as increasing the carbon layer thickness to raise the furnace volume. However, the higher the furnace height, the better it is not – exceeding the actual requirement can have negative effects. 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 coal gasification require a strong slag discharge capacity, while a very high slag breaking capacity is not necessary ； Since briquette ash has a high ash content and produces a large amount of slag, the slag particles are relatively soft and do not break easily; otherwise, an increase in fine slag powder would instead raise the resistance posed by the ash layer. Therefore, the design of the grates, slag breaking bars (plates), ash rakes, and ash discharge ports also needs to be modified accordingly compared to a lump coal furnace. The gasification intensity of briquetted coal is lower than that of lump coal, and its gasification temperature range is narrower. This characteristic requires larger air ducts, grates, and ventilation areas, as well as a more even distribution of air flow; otherwise, it can lead to localized overheating and incomplete combustion. The grates for lump coal are used for briquetted coal, and the distribution of air ducts is inappropriate; this only limits the intensity of gasification of the briquetted coal, leading to process instability. 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 dust composition in the fluidized bed gasification gas stream differs significantly from that in the fixed-bed gasification gas stream; therefore, there are differences in the material selection for designing dust collectors. The cyclone dust collector used in coal or coke gas furnaces can be equipped with a cast iron lining that offers good resistance to wear and erosion; when the airflow moves tangentially, the vortices and deceleration generated by the rough cast iron lining have little effect on the separation of large particles of dust. Briquette coal is different in this regard: its gas stream contains a high amount of dust, the dust particles are smaller than those in lump coal, and there is a high level of fine dust. If an iron cast lining is used, the inner surface of the cylinder becomes rough, which leads to the formation of gas vortices and a significant reduction in the tangential velocity of the gas flow; this is highly unfavorable for the settlement of dust. Therefore, for the cyclone dust collectors designed for coal gasification projects, their linings should preferably be made of materials with a smooth surface that do not easily form vortices and have low air resistance. Taking into account factors such as cost and service life, it is advisable to use a \"tortoiseshell 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, with its lower edge positioned slightly below that of the inlet, within 300 mm – this is determined by the characteristics of gasification in this type of system. 3 Waste heat boiler: In gasification projects of this type, waste heat boilers are chosen primarily to prevent dust from accumulating and to avoid blockages of the heat exchange elements, which could affect the efficiency of heat exchange. During design, ensure the gas conditions and flow rate (>3 m/s) inside the heat exchanger ； Heat exchange fins designed with longitudinal fins can achieve good anti-adhesion and anti-clogging effects. 4. Gas scrubber: The design of a gas scrubber requires not only effective dust removal but also prevention of dust from clogging the fillers in the scrubber, thereby avoiding an increase in resistance. Generally speaking, tower spraying can effectively prevent the increase in resistance caused by dust accumulation during coal gasification, but it tends to result in dust removal performance that does not reach an ideal level ； Packed towers offer good dust removal efficiency, but they tend to cause an increase in resistance. Based on production experience, the author believes that the scrubber towers in coal gasification systems should incorporate the advantages of the empty-tower spraying technology, such as nozzle design techniques, as well as the advantages of full gas-liquid contact in packed towers, in order to design a new type of scrubber tower – an anti-clogging, high-efficiency scrubber tower. Using this type of tower prevents an increase in resistance and a decline in washing efficiency, making it highly suitable for use in coal gasification projects. 5 Air blowers: For a long time, there has been a misconception that coal gasification is prone to being disrupted during the blowing process; therefore, it is not advisable to choose air blowers with high pressure and large flow rates. However, as demonstrated by recent practices in various companies, when using briquetted coal as a raw material for gas production, the air flow rate during the blowing stage is higher than that when using lump coal as the raw material. For example, the ammonia plant in Jiangxi uses coal briquettes as raw material; the primary air flow rate in its φ3.0m furnaces is around 35,000 m3/h, whereas that in furnaces using lump coal is only around 30,000 m3/h ； The gas generation furnace in the renovated Luoyang Nitrogen Fertilizer Plant uses coal balls as raw material; when the D800 air blower is in operation, the control valve is fully open, but this is not the case when using lump coal. Designing a new gas furnace using briquetted coal as raw material requires a higher volume of primary air flow compared to using lump coal. The main reason for this is that briquetted coal has good air permeability but low heat storage capacity; therefore, a process with a higher carbon layer thickness and greater air flow volume is necessary to meet the gasification requirements. Therefore, when designing and selecting air blowers, it is necessary to move away from the misconceptions associated with traditional briquette air selection blowers. Generally, for φ2.65m gasification furnaces, a blower set with an air pressure of 28 kPa, an air flow rate of 600 m3/min, and a motor power of 440 kW should be selected ； For gasification furnaces of φ3.0m to φ3.2 m type, a blower set with an air pressure of 28 kPa, an air flow rate of 800 m3/min, and a motor power of 630 kW should be selected ； For φ3.6 m gasification furnaces, a blower set with an air pressure of 28 kPa, an air flow rate of 1100 m3/min, and a motor power of 800 kW should be selected. The ideal air flow rates for the 3 types of gas stoves are respectively: 26,000 m3/h, 35,000 m3/h, and 45,000 m3/h; in other words, the gasification rate of these stoves is 4,700 m3/(h·m2). 6 Nitrogen-enriched air system: In the case of gasification using this method to produce semi-water gas, careful consideration should be given to the optimized design of the nitrogen-enriched air system during the top-blowing phase. Since the air volume required for blast air in coal gasification is large, it is difficult to supply it entirely with primary air; therefore, part of the air must be supplied by nitrogen-enriched air blown from above. To supply sufficient nitrogen-enriched air into the furnace, high-efficiency injectors are necessary. Generally, for φ2.65m gasifiers, the use of PS-Ⅱ type nitrogen injectors ensures an upper injection steam flow of 7 t/h and an air flow of 0–7000 m3/h ； For gasification furnaces of φ3.0 m to φ3.2m, the PS-Ⅱ type nitrogen injection unit is selected, which ensures an upper-blown steam flow of 9 t/h and an air flow of 0–10,000 m3/h ； For the φ3.6 m gasification furnace, the PS-Ⅲ type nitrogen injection nozzle is selected to ensure a top-blown steam flow of 11 t/h and an air flow of 0–12,000 m3/h. To ensure that the nitrogen content in the gas does not exceed the specified limits, companies with the necessary facilities can introduce oxygen-enriched air using nitrogen injection nozzles during the upward injection process, or employ such nozzles to achieve continuous oxygen enrichment of the gas. The latter method can be used to convert a fixed-bed batch gasification coal gasifier into an oxygen-enriched continuous gasification system. Sanying Chemical has already achieved continuous oxygen-enriched gasification of briquettes using this technology. 7 Hydraulic Control System and Valves 7.1 Basic Requirements The hydraulic control system of the coal gasification gas furnace is the same as that of other gas furnaces, and its main aspects are as follows. (1) The system oil pressure is low, with the main pipe oil pressure at 4.0 MPa. (2) The hydraulic-driven valve changes direction quickly. The direction change of all hydraulic control valves should be completed within 2.0 to 2.5 seconds. Among them: the φ2.65 m gas furnace completes the process in 2 seconds, while the φ3.0m–φ3.6m gas furnaces complete it in 2.5 seconds. (3) The oil pressure fluctuation is small, not exceeding ±0.2 MPa. (4) The temperature of the pressure oil in summer ≤ 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, shall be controlled by electro-hydraulic valves, with an oil passage diameter of ≥18 mm ； Other valves can be controlled by solenoid valves. (2) For DN250-DN700 automatically controlled valves, 80/40 cylinders should be selected for the hydraulic cylinders ； For the cylinders of DN700～DN1000 automatic control valves, 90/50～100/55 type cylinders should be selected. To avoid high oil pressure in the system, 63/45 cylinders cannot be used for all automatic control valves. (3) The accumulator of the system oil pressure main pipe should be selected to be appropriately large. (4) The heat exchange area of the oil cooler shall be ≥ 10 m2. (5) Oil pressure pump station configuration: 80 L/min pump station for φ2.65m furnace ； Furnaces with a diameter of φ3.0m to φ3.6m are equipped with a pump station of 140 L/min. 7.2 Installation of process valves: For furnaces with a diameter of φ2.65 m, 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 φ3.0m to φ3.2m, the large valves are generally of DN700 to DN900 size, while the valves for steam and nitrogen/air are of DN300 to DN400 size. For φ3.6 m furnaces, the large valves are generally of DN900–DN1000 size, while those for steam and nitrogen/air are of DN300–DN400 size. 8 Steam buffer tanks, φ2.65m; 1 set should consist of 3 to 4 boilers ； Furnaces with a diameter of φ3.0m to φ3.2m and φ3.6m should be grouped as 3 furnaces per group. The steam buffer tanks for each group of gas stoves should be installed as close as possible to the middle of that group of stoves, with a capacity of ≥20 m3. 9 Automatic coal feeder: The automatic coal feeder for gasifier-type coal furnaces differs significantly from that used in lump-coal furnaces, and these differences must be fully taken into account during design. 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 of type (2) is low; therefore, the hopper design needs to minimize drops and collisions, thereby reducing the rate of briquette fragmentation upon entry into the furnace ； (3) Briquettes are most sensitive to moisture absorption; it is necessary to keep them separated from the steam escaping from the furnace inside the automatic coal feeder ； (4) Briquettes tend to produce coal dust during transportation, and the coal dust generated has high viscosity; therefore, automatic briquette feeding machines must be equipped with anti-clogging fixed screens to reduce the amount of dust carried by the briquettes entering the furnace. 10 Processes and Piping Systems 10.1 Gas Production System For coal gasification, a relatively simple \"low-nitrogen process\" is generally preferred, that is, the \"four furnaces, one machine, one station, one boiler, and one tower\" process ； However, for furnaces with a diameter of 3.0m or more, the “three furnaces, one machine, one station, one pot, and one tower” process should be adopted ； For a φ3.6m furnace, the “three furnaces, one machine, one station, three boilers, and one tower” process should be adopted. Some people believe that the lower the resistance in a gas stove system during the gasification process, the better; therefore, they try to increase the diameter of the pipes in such systems. This is undoubtedly a misconception. The author believes that it is necessary to clarify the concepts first: the system pressure and system resistance of a gas stove are two different concepts; 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 exceeds this range, it not only results in wasted investment but is also detrimental to the process. To ensure safety in the production of gas furnaces, all oil pressure control valves that lead into the furnace should have high inlet and low outlet positions, in order to prevent process gases from forcing these valves (such as the steam main valve, primary air valve, nitrogen-enriched air main valve, etc.) to open in case of a sudden drop in oil pressure. For blowdown air recovery valves, upward gas valves, and downward gas valves, if they are installed with low inlet and high outlet positions, the hydraulic differential connection method is prohibited to prevent air leakage. For two-way valves that control high-temperature gases, such as blowout gas 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. 10.2 Steam System: In recent years, there has been a trend toward positioning the downward-flowing steam closer to the top of the gas furnace, which is undoubtedly an improvement. However, the closer to the top of the furnace, the more critical it is to address the issue of steam carrying water along with it as well as the uniform distribution of steam; otherwise, new problems will arise (such as damage to the refractory layer in the upper part of the furnace, leading to shutdowns for repairs), which will affect normal production. The steam system should be designed with one steam buffer tank for each group of gas burners; the steam generated internally is superheated before entering the steam buffer tank. Each set of pressure reducing valves can supply up to 2 groups of gas stoves; if there are more than 2 groups, an additional set of pressure reducing valves must be installed, and it should be connected to the middle part of the main steam pipe for the gas stoves. The pressure fluctuation in the steam system should not exceed ±0.001 MPa. Diameter of the low-pressure steam main for each group of boilers: For boilers with a diameter of φ2.65 m, the diameter should be ≥ DN500 ； For furnaces with a diameter of φ3.0m to φ3.2m, the pipe diameter should be ≥DN600 ； For a φ3.6 m furnace, the pipe diameter should be ≥DN800. The above measures are designed to ensure that the steam flow rate and pressure in the production of fixed-bed gasifiers meet the requirements of fixed-bed gasification. 10.3 Air System: In a production system with multiple furnaces, the connection between the air fans and the main air duct should not be made at one end; instead, it should be connected from the middle of the main air duct, in order 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 (also known as the safety relief valve) can be omitted. Main air duct: For φ2.65 m furnaces, it should be φ630 mm×8 mm pipes ； Furnaces with a diameter of φ3.0 m to φ3.2 m should use tubes of φ770mm×10mm ； The φ3.6m furnace should use φ920mm×10mm tubes. Central ash box of gas stove: For stoves with a diameter of φ2.65 m, it should be φ1000mm/1200mm; for stoves with a diameter of φ3.0m or φ3.2m, it should be φ1200 mm／1400mm or φ1200mm／1600mm ； The φ3.6m furnace should be φ1620mm/2000mm. Optimizing the air system pipes allows the gas stove to achieve the maximum air flow at the lowest wind speed when burning lump coal, ensuring stable air supply. The wind speed of the primary air in the furnace grating air chamber should be kept within 7 m/s. 10.4 Others (1) The critical parts of other system equipment are designed for a system major maintenance cycle of ≥24 months. (2) It is recommended to use 1Cr18Ni9Ti for the idler wheels and their shafts inside the gas holder, to ensure operation for over 30 months. (3) Electrostatic dust removal uses intermittent hot water flushing, focusing on flushing coal tar. (4) A gravity dust collector is installed in the lower gas duct of the gas stove, and the inner wall of the lower gas duct pipe is coated with an erosion-resistant material. (5) Sufficient spare rotary vane blowers should be provided to ensure timely cleaning and maintenance. 11 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 briquetted coal as a gasification feedstock will gradually optimize their systems to truly achieve economic benefits from briquetted coal gasification.