Gasification question bank sharing

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I. Essay Questions 1. Explain the concept of the grindability index (Hartig grindability index)? What is the relationship between the grindability index, that is, the Haas index, and coal crushing and grinding? Does a higher Haas index mean lower power consumption? Answer: The grindability index is a measure of the wear resistance of raw coal during the grinding process. When determining the grindability index of coal in the laboratory (i.e., the Hardgrove index), 1 gram of bituminous coal sample and 5 grams of anthracite sample are thoroughly mixed, then carbonized under strictly specified conditions. The resulting coke is subjected to grinding tests in a specific drum to measure its wear resistance, which is expressed as an index. The higher the index, the easier it is to grind the coal; in other words, the lower the hardness of the coal, the easier it is to grind it into a suitable coal slurry. It can be inferred from this that the higher the Ha’s index, the greater the mill output, i.e., the production volume. In terms of output per unit time, it also consumes less electricity. 2. What is the recommended solid content percentage in the coal slurry by Texaco? What are the effects of being too high or too low on the transportation, storage, and gasification of coal slurry? Answer: The recommended solid content range for the coal slurry by Texaco is 60.5–63%. An excessively high solid content in the coal slurry—that is, a higher coal slurry concentration—is beneficial for improving gasification efficiency. However, under such conditions, the slurry exhibits high viscosity and poor fluidity, making it difficult to pump; it also tends to settle and stratify during storage. When the slurry concentration is too low, it facilitates the transportation by the slurry pump and the storage of the slurry; however, the effective component that enters the gasifier (i.e., the dry coal) decreases, the calorific value drops, and the evaporation of water absorbs a large amount of heat, thereby reducing the gasification efficiency. 3. What is the function of additives? Describe their physical properties? Answer: The function of additives is to improve the fluidity and stability of water-coal slurry. As surfactants, they have three functions: a) to wet the coal particles, thereby facilitating the combination of water and coal ； b) Disperse larger gelatinous coal particles ； c) Provide cations as the equilibrium ions of the hydration film to serve an electrolytic function. Its physical properties are as follows: the additive has a dispersing effect; it can adjust the hydrophilicity and charge level of the coal particle surface, reduce the forces between the hydration film on the coal particle surface and ions, and cause the water trapped on the coal particle surface to overflow. 4. What are the units of viscosity? How do they make the conversions? Answer: The unit of viscosity is derived from Newton’s law; the physical unit is poise, denoted by the symbol P. Newton’s law of viscosity states that the shear stress between fluid layers is equal to the product of the fluid’s viscosity and the velocity gradient. The viscosity unit poise, derived from this law, is relatively large; the viscosity values of substances expressed in terms of it are very small. (For example, at 20°C, the viscosity of water is 0.0102 poise.), Therefore, it is usually expressed as 1/100 of a poise; centipoise (CP) is used as the unit of viscosity. Other major units of viscosity include the international unit Pa•S, the engineering unit kgf•s/m2, and the unit cm2/s. The conversion relationships are as follows: 1 Pa•S = 10 P = 103 CP; 1 kgf•s/m2 = 98.1 P = 9810 CP. What is viscosity? What is the viscosity of water at 20°C? The physical quantity that indicates the degree of viscosity in a fluid is called viscosity. It describes a physical property of fluids, and its definition can also be derived from Newton’s law of viscosity. The physical meaning of viscosity is the shear stress that facilitates fluid flow resulting from a unit velocity gradient; it only becomes apparent when the fluid is in motion. When analyzing the behavior of stationary fluids, this factor of viscosity need not be considered. Viscosity is one of the physical properties of fluids, and its value is determined experimentally. At 20°C, the viscosity of water (in physical units) is 1.02 CP. 6. Why should no iron pieces, wooden blocks, pipes, wires, etc. get into the mill inlet? Answer: The mill crushes materials through the friction between the grinding particles, as well as those particles that are thrown up by the cylinder. If iron parts or wires get mixed in, they not only cannot be ground into dust, but they also accelerate the wear of the material being ground, as well as damaging the lining plates. Flexible materials such as wood blocks have good flexibility; impacts and friction cannot grind them down. Stones are hard materials and difficult to grind; *tubes, on the other hand, are easier to handle. When mixed in, they rub against and impact the steel rods, which can easily lead to explosions. Additionally, when the aforementioned hard-to-grind materials enter the drum screen of the mill, they can clog the screen mesh; therefore, it is necessary to prevent such materials from getting into the mill. 7. What is the maximum coal slurry viscosity recommended by Texaco? Answer: The maximum value for the viscosity of coal slurry recommended by Texaco is 1400 cP. 8. What is the recommended slurry flow rate for the coal slurry pipeline proposed by Texaco? Why? Answer: Texaco recommends a slurry flow rate of 0.6 to 0.9 meters per second for the slurry pipeline. This is because when the slurry flow rate is below 0.6 meters per second, coal particles tend to precipitate, while at higher flow rates, the wear on the slurry pipes becomes severe. 9. Why must operators strictly control the concentration of the coal slurry and prevent large solid particles from entering the system? Answer: The Texaco gasification process requires a coal slurry concentration of 63%, which is one of the stringent process parameters for gasification. It affects factors such as the quality of gasification, efficiency, operating conditions, and the transportation of the coal slurry; any fluctuations in this concentration will inevitably lead to instability in production, so it is essential to keep this parameter within the specified range. If large solid particles get into the system, they can easily clog the inlets and outlets of slurry pumps, valves, slurry pipelines, and the burner of the gasifier. They also accelerate wear and tear on the slurry system, leading to incomplete combustion and low gasification efficiency. 10. What is the O2 pipeline flow rate recommended by Texaco? Answer: Texaco requires that O2 flow at low speeds; when the O2 pressure exceeds 40 kg/cm2, its maximum flow rate must not exceed 12 m/s. To prevent damage to the O2 check valve disc due to impact and to avoid uneven oxygen supply to the gasification furnace, the minimum flow rate should also be greater than or equal to the minimum flow rate recommended by the valve manufacturer. 11. What issues should be considered during the commissioning, start-up, and shutdown of the O2 system? Answer: The assembled oxygen delivery pipeline must be purged with dry nitrogen and subjected to a leak test and pressure test. During testing, apply grease-free soapy water to all fittings and connection points; no bubbles should appear within 1 minute. When testing or starting up the oxygen delivery system, open the valves gradually to adjust the flow rate slowly. It is strictly prohibited to open and close the valves abruptly to avoid adiabatic compression or excessively high instantaneous flow rates. For pipes that are in long-term use, it is necessary to remove fittings such as valves, elbows, and tees before starting the machine, and check for any signs of wear, thinning, or accumulation of iron particles; if such issues are found, they should be replaced and cleaned promptly. 12. Why does a burner need a cooling water coil, and what is its function? Answer: Since the burner extends into the furnace, where the temperature is around 1450°C, without any other cooling method the burner would be quickly burned through; therefore, soft water is used as the cooling fluid. The function of the cooling water coil is to protect the burner head from being damaged by high-temperature heat radiation. 13. Describe the working principle of the startup extractor and the separation tank? Answer: (1) The starting extractor consists of a steam nozzle, a reducing tube, a mixing chamber, and an expanding tube. The working steam is ejected at high pressure through a steam nozzle at a speed of 1000–1400 m/s; its static pressure energy is converted into kinetic energy, thereby creating a negative pressure in the expansion chamber that draws in the fuel gas. The incoming gas mixes with the steam in the mixing chamber before entering the expansion chamber, where its speed gradually decreases and the pressure increases. Once again, kinetic energy is converted into static pressure energy, and the mixture is then discharged from the outlet. (1) The gas resulting from the mixing of steam and combustion gases (flue gas) in the separation tank enters the tank along the tangent direction within it. After entering, the airflow swirls downward along the inner wall inside the container, forming a helical flow pattern. Due to the effect of centrifugal force, when the gas strikes the wall of the container, some of the steam is cooled into water (as a result of wall cooling), and this liquid flows along the wall toward the bottom of the container. Meanwhile, the fuel gas is separated and discharged through the central tube of the container, thereby achieving gas-liquid separation. 14. After successful ignition of the burner, how should the oxygen and slurry flow rates into the furnace be adjusted? Answer: From the reaction of partial oxidation of carbon to produce CO (2C + O2 = 2CO + Q), the theoretical amount of oxygen required should be such that the oxygen-to-carbon atom ratio is 1:1. Therefore, when the oxygen-coal ratio is too high, part of the carbon burns completely to produce CO2, raising the temperature and resulting in an increased CO2 content in the gas, which in turn reduces the efficiency of the cold gas. If it is too low, the reaction is insufficient, which also leads to a reduced carbon conversion rate. However, in actual reactions, there are interactions and constraints from other reactions, as well as heat loss and other factors; as a result, the oxygen-coal ratio is generally slightly higher than the theoretical value. According to the above theory, when increasing the gasification load, it is necessary to keep the oxygen-coal ratio as close as possible to the theoretical value in order to maintain a relatively stable operating temperature. When an increase is required, first increase the coal slurry flow rate, and then increase the oxygen flow rate (the opposite is true when reducing the load); this helps to avoid overheating of the gasifier caused by an excessively high oxygen-to-coal ratio. 15. Why are surface thermocouples installed in the vaporizer? How many points were measured in total? What does a local increase in temperature indicate? Answer: The purpose of installing surface thermocouples on the gasifier is to monitor the surface temperature of the gasifier shell. If the surface temperature is too high, it can cause changes in the internal structure of the material constituting the gasification furnace, leading to a reduction in stress and strength; in severe cases, this may result in the explosion of the gasification furnace. The surface thermocouple for a single gasifier is 1,200 feet long. A Φ3mm metal wire has about 30 temperature measurement zones. The local temperature rise is caused by the detachment of refractory bricks and improper installation. When refractory bricks detach, the total thermal resistance at that location decreases, resulting in an increase in heat transfer and thus a rise in temperature. If installed improperly, air leakage to the furnace wall can also cause a temperature rise. 16. What is the size of the slag outlet in the gasification furnace? What factors cause blockage at the slag outlet of the gasifier? By what phenomenon can it be determined? Once it’s found, how to deal with it? Answer: The size of the slag outlet of the gasifier is 625mm ； The reason for the poor flow at the slag outlet is that the operating temperature of the gasifier is too low, resulting in increased viscosity of the slag (the viscosity of ash is inversely proportional to temperature). On the one hand, the synthetic gas carrying slag impacts against the area surrounding the slag outlet as it passes through it, resulting in accumulation there. On the other hand, the slag that impacts against the refractory bricks on the fire-facing side also flows downward and accumulates at the slag outlet, until it causes the outlet to become blocked. This phenomenon can be detected using the differential pressure gauge PDI203 (or PDI253). When blockage occurs, the pressure difference increases. In such cases, the following measures can be taken: (1) Increase the oxygen-coal ratio to raise the furnace temperature and reduce the viscosity of the ash ； (2) Appropriately reduce the operating pressure of the gasifier to lengthen the flame, thereby raising the temperature at the slag tap and causing accumulated slag to melt and flow away (this method is generally not used). 17. What are the requirements for verticality, levelness, concentricity, etc., when laying bricks? Answer: To ensure the quality of assembly of the refractory materials in the gasification furnace, it is required that each layer of bricks be at the same level during construction and perpendicular to the center of the gasification furnace. The roundness of the inner surface of each layer of bricks is also checked, ensuring that its center point coincides with the center line of the gasification furnace. The specific requirements are: verticality of ±5mm/m, levelness of ±4mm/m, and maximum concentricity of 5mm/m. 18. What is the function of the quench chamber water bath? Answer: The water bath in the quench chamber serves two purposes: (1) When the high-temperature syngas enters the quench chamber, it comes into direct contact with the quenching water, causing the slag carried in the gas stream to solidify and settle at the bottom of the quench chamber; simultaneously, the high-temperature syngas is also cooled. (2) It supplies saturated steam (steam generated when the quench water is heated) to the CO conversion system, saving energy; this is also a major advantage of the quenching process. I. Fill in the blanks: 1. The five process parameters that are closely related to the operation of the lockhopper valve in a lockhopper logic system are (lockhopper pressure ≤ 0.28 MPa), (lockhopper pressure difference ≤ 0.18 MPa), (low level in the flushing water tank), (high level in the flushing water tank), and (full level in the lockhopper). 2. The lockhopper safety valve will only close automatically in an emergency condition, namely when the liquid level in the quench chamber is very low. 3. When inspecting the burner, special attention should be paid to measurements such as the wear condition of the nozzle’s wear-resistant components and the condition of the cooling water coils. The normal service life of the burner is (3 months). 4. The purpose of adding water to the vacuum pump’s separation tank is to compensate for water loss and to lower the temperature of the liquid used as water for the vacuum pump. 5. The functions of the flare water seal tank are: (water sealing), (gas-liquid separation), and (pressure buffering). 6. Factors affecting the viscosity of coal slurry: (amount of additive), (particle size distribution of coal slurry), (concentration of coal slurry). 7. The operating temperature of the gasifier is determined by (the temperature corresponding to the optimal slag viscosity of the gasifier slag). 8. The operating pressure of the final stage of flashing is determined by the pressure corresponding to the temperature of the sedimentation tank. 9. The purpose of the bypass switch in the safety system settings is (to repair faulty detection elements without affecting the gasification operation). 10. Requirements for the pre-crushed particle size of raw coal: ≤10 mm; requirements for the pre-crushed particle size of limestone: ≤30 mm. 13. The function of the flare sealer: (preventing air from entering the flare) (separating the condensate contained in the exhaust gases). 14. Factors that may cause the high-pressure coal slurry pump to shut down during normal operation include: (shut-down trigger signal), (manual shutdown), (gasifier shutdown), (low instrument air pressure), (power loss), (low lubricating oil pressure), (driving fluid failure), (high pump outlet pressure). 15. Principles for temperature control during the preheating of the gasification furnace: (Raising temperature: First increase the exhaust volume, then increase the gas supply volume) ； Cooling down: First reduce the amount of gas, then decrease the extraction volume. 16. Principles for increasing and decreasing the load in the gasification furnace: (Increasing load: First add coal slurry, then increase the oxygen supply) ； Load reduction: First reduce the oxygen supply, then reduce the coal slurry. 17. The main factors affecting slurry concentration: (amount of additive used), (inherent moisture content in coal), (particle size distribution). II. Essay questions
1. What are the control indicators for the surface moisture content of coal entering the mill? What is the impact of excessive surface moisture on the grinding process? Answer: Control index for surface moisture of coal entering the coal mill