Comparison between Shell gasification and Texaco gasification processes

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Comparison between Shell Gasification and Texaco Gasification Processes Introduction Both the Shell pressurized gasification method and the Texaco slurry pressurized gasification method are among the more advanced gasification technologies in the world today, and both belong to the category of fluidized bed pressurized gasification methods. Their common feature is that the process has a wide range of compatibility with different types of coal; pulverized coal can be used, each gasification furnace has a high production capacity, the gasification operates at high temperatures, and liquid slag is produced. It features a high carbon conversion rate and good quality of gas, with a low methane content; it does not produce pollutants such as tar, naphthalene, or phenols. The furnace ash can be used as raw material for cement and construction materials. The treatment of waste gases, wastewater, and solid waste is simple, making it easy to meet environmental protection requirements. The level of production control is high, facilitating process automation and computer-based control. However, the two gasification technologies have distinct characteristics in many aspects. This article compares the gasification technologies of SHELL and TEXACO from the following aspects. I. Comparison of SHELL and TEXACO processes: The basic approach to coal gasification at Shell is as follows: (1) entrained flow; (2) pure oxygen as the gasifying agent; (3) jacketed water-cooled walls; (4) liquid slag discharge; (5) symmetric tangential feed nozzles; (6) dry powder feeding. Process overview: The raw coal is delivered to the coal grinder, which crushes it to an appropriate size for gasification (particles with a 90% mass fraction being smaller than 100 µm). While the coal is being crushed, it is dried using an inert gas, which carries away the evaporated water vapor. After separation in an internal separator, the suitable coal powder is collected in a sedimentation tank. The oxygen required for gasification is supplied by an air handling unit, while the nitrogen from this same unit, after compression, provides low-pressure and high-pressure nitrogen for the coal transportation system. The dried and qualified coal powder is transported by nitrogen to the coal pressurization and feeding system. The pressurized coal powder, oxygen, and steam enter the gasification furnace through paired nozzles. The operating pressure in the gasification furnace ranges from 3.0 M Pa to 4.0 M Pa, while the reaction temperature reaches up to 1,400 °C to 1,700 °C. The slag flows out from the lower part of the gasification furnace, comes into contact with water, forms solid particles, and is discharged through the ash lock. The exhaust gas at 1,500 ℃ is mixed with cooled gas, resulting in a temperature of 900 ℃ before it enters the waste heat boiler, where heat is recovered. As a result, the temperature of the syngas drops to 2,500 ℃. A ceramic filter is then used to reduce the dust content in the syngas to between 3 mg/m3 and 5 mg/m3. The syngas subsequently enters a water washing tower, which further reduces the dust content to 1 mg/m3 before it is sent to subsequent processing steps. Process features: It offers great operational flexibility, wide compatibility with various raw materials, and excellent environmental performance. It can handle a wide range of coal types, with a carbon conversion rate of > 99.5%; the volume fraction of (CO + H2) in the gas output is > 90%. The efficiency of producing cold gas is high, ranging from 80 to 83% ； Oxygen consumption is low; compared to Texco water-coal slurry gasification, it is at least 15% lower. The TEXACO water-coal slurry gasification process was developed by the American company TEXACO in the 1970s. The basic principles of this process are as follows: (1) feeding of water-coal slurry; (2) pressurized gasification; (3) use of pure oxygen as a gasifying agent; (4) molten slag discharge. A brief description of the process: The properly prepared water-coal slurry is pressurized by a pump and then, together with oxygen, is injected into the gasifier through special nozzles. There, the water-coal slurry is finely atomized and its moisture is removed; subsequently, it undergoes complex oxidation-reduction reactions with oxygen to produce water gas and slag. Both the water gas and the slag proceed to the quenching chamber located below the gasifier. Once the slag has cooled and solidified, it is collected and removed via a slag hopper, while the water gas, after being quenched, is sent to a washing and dust removal system ； Process features: wide range of suitable raw materials, carbon conversion rate of 94% to 98%, effective gas composition of 84%, low waste emission, and high production capacity. Disadvantages: High oxygen consumption, high technical costs, and large investment requirements. II. Comparison of the preparation, transportation, and suitability of raw coal: In Shell and Texaco gasification processes, the pressurized transportation of dry coal powder and coal water slurry are prerequisites for the stable operation of the gasifier; therefore, stability is one of the important characteristics of the coal supply system. In the coal gasification process, the raw coal is pre-crushed before entering the coal drying system, where it is dried using high-temperature inert gas to reduce its moisture content to less than 2%. After that, it goes into a coal grinder to be converted into coal powder, and once in powder form, it is sent to the coal powder silo using N2 gas. It then enters a 2-stage pressurized lockhopper system, where high-pressure N2 gas is used to deliver the coal powder to the 4 gasification furnace nozzles at a high solid-to-gas ratio. Due to SHELL’s high requirements for the uniformity of the coal powder particle size distribution (90% ≤ 90 mesh) and the fact that high-pressure nitrogen serves as the continuous phase, there is a significant degree of compression between the gas and solid phases. As a result, when there are leaks or fluctuations in certain parts of the conveying system, the impact on the entire system is minimal (i.e., it has good operational flexibility), without affecting the stability of the feed to the burner. The Texaco coal gasification process uses a wet feeding method: the raw coal is crushed and mixed with water to form a water-coal slurry with a concentration of typically 60%–68%. This slurry, along with oxygen, is injected from the top of the furnace where it undergoes gasification reactions. Due to the different structural properties of water and pulverized coal, various coal types exhibit different levels of hydrophilicity, as well as optimal particle size distributions and concentrations. A lower concentration of coal slurry will result in an increase in the amount of moisture that enters the gasifier; to maintain the furnace temperature, more oxygen must be used. On the other hand, an excessively high concentration of coal slurry increases its apparent viscosity, reducing its fluidity and affecting both its pumping capacity and atomization efficiency. Therefore, in order to obtain a coal slurry with the desired concentration and good fluidity, it is necessary to add a certain amount of dispersants and surfactants. The following shows a comparison of the pressure drop fluctuations in the two conveying systems, as illustrated in Figure 1: Shell’s dry coal powder conveying system and Texaco’s slurry coal conveying system. Figure 1 It can be concluded from Figure 1 that the pressure difference curve for Shell’s conveying system has a very steep slope, indicating good start-up performance of the coal supply system, which reaches the rated conveying flow rate within a few seconds. Once this rated value is attained, the pressure difference curve becomes horizontal with a relatively stable average value; this reflects, on one hand, that the stability of coal supply in this system is superior to that of Texaco’s slurry coal conveying system. The differences in structure and performance between Shell and Texaco gasifiers result in different requirements regarding the type and quality of raw coal, which plays an important role in determining the gasification process chosen by ammonia producers. The adaptability of coal to Texaco and Shell gasification processes is shown in Table 1. Table 1 Comparison of the adaptability of the two processes to raw coal and carbon conversion rates

| Parameter | Shell Texaco Process |
|-----------|---------------------|
| Coal types | Lignite, subbituminous coal, bituminous coal, anthracite, petroleum coke |
| Ash content | 8%–20% | ≤15% |
| Particle size | 90% of particles ≤90 mesh |
| Concentration of water-coal slurry | 60%–68% | ≤200 mesh |
| Ash fusion point | ≤1500°C | ≤1350°C |
| Moisture content | 35% | ≤2% |
| Carbon conversion rate | 99.5% | 98% |

As can be seen from the table, due to the high gasification temperature, the Shell powder gasification process is capable of gasifying coals with high ash fusion points; therefore, it has a wider range of adaptability to different coal types. Coals such as lignite, subbituminous coal, bituminous coal, anthracite, and petroleum coke can all be used, and even coals with high moisture content, high ash content, high sulfur content, and high ash fusion points can generally be gasified. This facilitates manufacturers in selecting coal locally, thereby reducing production costs ； II. Comparison of Gasifier Performance and Structure The gasifier is the core component of the coal gasification process, and its characteristics influence the choice of the entire gasification process. At present, the various gasifiers under development are to a large extent constrained by physical, chemical, mechanical and other factors, such as the method of feeding pulverized coal into the gasifier, the way in which pulverized coal comes into contact with the gasifying agent, the method of slag removal, and the methods used to prevent corrosion of the inner wall of the furnace. The Shell and Texaco coal gasification processes have distinct features in terms of gasifier structure, as shown in Table 2 below. Table 2 Comparison of Two Types of Gasifiers: Shell Gasifier vs. Texaco Gasifier
Feeding Method: Pulverized coal transported under high-pressure N2; Water-coal slurry transported via pump
Gasifier Type: Gas-solid fluidized bed/cold-wall type; Gas-liquid fluidized bed/hot-wall type
Ash Discharge Method: Liquid slag discharge from the bottom
Pressure for Liquid Slag Discharge (MPa): 2.0–4.0; 4.0–6.5
Reaction Temperature (°C): 1400–1700; 1300–1500
Location of Gas Outlet: Top; Bottom
Maximum Production Capacity per Day (t/d): 1900–2000; 900–1500

The SHELL gasifier is a vertical cylindrical gasifier, with a membrane-type water cooling wall made up of boiling water-cooling tubes surrounding its furnace chamber. During gasification, the molten slag forms a liquid film on the inner surface coating of this water cooling wall, and then flows down along the wall for separation. An anti-corrosion method that uses slag to protect against further corrosion is employed, which effectively solves the problems of severe damage to high-temperature refractory materials and frequent maintenance needs. The circular channels that make up the heat exchange surface of the membrane-type water wall are the internal components. The inner surface is protected by a heat-resistant lining of 14–20 mm thickness. During the first start-up of the gasifier, measures are taken to protect the furnace wall from slag accumulation, thereby preventing the outer cylinder of the gasifier from overheating, recovering some heat, and protecting it against gas erosion – this is a notable feature of Shell’s gasification process. An annular space is provided between the water wall and the cylindrical shell, which facilitates the placement of the inlet water collection pipes and outlet steam collection pipes, as well as simplifies the inspection and maintenance of the water wall ； The furnace is equipped with four symmetric tangential coal powder feeding nozzles, which helps to increase the degree of reaction of the reactants inside the reactor while reducing backmixing of the products. The Texaco gasifier uses coal slurry as the feed material, operates at high pressures, and employs a liquid slag discharge method. However, since the refractory lining of a hot-wall furnace must not only withstand high temperatures but also resist the erosion caused by coal slag (containing SiO2, Al2O3, MgO, etc.), it is expensive and has a short service life ； Furthermore, because the feed contains a large amount of water, the oxygen consumption is high; compared to SHELL powder gasification, the oxygen consumption is at least 15% higher. III. Comparison of the structure and operating positions of coal burners 1. Working principle of Shell coal burner The simple structure of the Shell coal burner is shown in Figure 2, while the design parameters are listed in Table 3. The basic working principle is as follows: The pressurized pulverized coal is ejected at high speed from the pulverized coal burner, where it mixes with oxygen vapor to form a fine mist; this mixture then burns at high temperatures inside the furnace to produce process gas. Table 3 Design process parameters for Shell coal burners – Material name: Overall burner; Oxygen/steam; Pulverized coal/N2; Circulating cooling water. Design pressure (MPa): 5.2, 6.1, 5.8, 6.9. Design temperature (°C): 350, 450, 130, 260. Design flow rate (m3/h): (1) The gasifier is equipped with a membrane-type water wall, enabling it to withstand high gasification temperatures. There are fewer restrictions on the ash fusion point of the raw coal. 　　(2) High carbon conversion rate, greater than 99%. 　　(3) Since dry powder is fed, the volume fraction of the useful gases (CO+H2) in the crude syngas is higher than 90%, with a low CO2 content. 　　(4) High gasification efficiency and high raw material utilization rate. 　　(5) Large single-furnace capacity, long operating cycle, and no need for a backup furnace. 　　(6) With a multi-group counterarranged burner configuration, the output volume of syngas can be adjusted by shutting down one or more groups of burners, providing great operational flexibility. 　　(7) Syngas is relatively clean; the gas produced by high-temperature gasification does not contain harmful substances such as tar and phenols, which facilitates its purification. 　　(8) During normal production, almost no waste gas is emitted ； Since the vast majority of process water can be reused, less wastewater is discharged. The slag produced at high temperatures has stable properties, causes little environmental impact, and can also be used as a building material. This process is environmentally friendly.