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Author/Source: Xu Hongdong, Men Changgui (Northwest Research Institute of Chemical Engineering, Xi’an 710054). Fluidized-bed pressurized gasification technology is one of the best options for the large-scale and efficient utilization of coal. One of the keys to pressurized gasification technology is solving the technique for feeding pulverized coal into the furnace under pressure. The industrially applied fluidized bed coal pressurized feeding technology is mainly divided into dry pressurized feeding technology and wet pressurized feeding technology. Dry pressurized feeding generally employs a hopper system to achieve intermittent pressurization of coal powder and its continuous feeding into the furnace ； Wet pressurized feeding relies on a high-pressure slurry pump to pressurize the coal slurry and feed it continuously into the furnace, while both feeding methods have their own advantages and disadvantages. The author will introduce the characteristics of the two feeding methods, as well as the new technologies and processes developed based on them. 1 Dry powder pressurized feeding technology 1.1 Dry powder pressurized feeding technology The basic principle of dry coal powder pressurized feeding technology is intermittent pressure increase followed by continuous pressurized feeding; typical examples of this technology are the Shell process and the GSP process (the process flow is shown in Figure 1). After the raw material enters the storage hopper, valve 1 between the storage hopper and the locking hopper opens, while valve 2 between the locking hopper and the bottom feed hopper closes. When a certain amount of raw material has been added to the lock hopper, valve 1 closes, raising the pressure in the coal lock from atmospheric pressure to the pressure in the feed hopper. Then valve 2 opens, allowing the solid material to fall into the feed hopper, and the raw material is sent into the gasifier by the blowing gas. Once the level of material in the lockhopper drops to a certain value, valve 2 is closed to relieve pressure in the lockhopper, after which the next cycle begins. The difference between the Shell and GSP feeding processes is that the Shell gasifier uses a bottom-discharge feed tank feeder combined with a pipeline dense-phase conveying system for coal supply ； GSP uses a feed tank with an upper discharge port for feeding, as well as a pipeline dense-phase conveying system for coal supply; there are fluidization plates and agitators at the bottom of the feed tank, while the coal conveyance pipes extend from the top of the tank to the nozzles at the top of the gasifier. Shell’s dry coal powder pressurized gasification technology is well-developed; it has been successfully applied in the feeding process of gasifiers used in IGCC power generation units, and such systems operate smoothly with a low failure rate. 1.2 Factors restricting the expansion of the application scope of dry feeding systems. Gasification processes that use dry feeding systems as the conveying method offer numerous advantages such as high efficiency in cold gas utilization, as well as low consumption of oxygen, coal, and water. However, due to factors such as pressure and the properties of the conveying gas, this feeding method cannot be used in all types of fluidized-bed gasification processes. 1.2.1 Pressure factors The selection of the pressure for the gasification process depends primarily on the demand for syngas from downstream processes. For high-temperature pure oxygen fluidized bed gasification units, a syngas pressure of 10 MPa is more than sufficient to meet the pressure requirements of the vast majority of downstream processes. The experimental setups for wet-pressure fluidized-bed gasification and fixed-bed pressure gasification have completed tests at a gasification pressure of 10 MPa. The situation of the dry coal powder pneumatic conveying system differs from the previous two; due to issues related to the air flow conveying system and the performance of the conveying equipment, the maximum test pressure for this system is approximately 5 MPa. For processes requiring a syngas pressure of over 5 MPa, dry powder pressurized feeding fails to leverage the advantages of reducing reactor size, lowering costs, and saving compression energy at specific production capacities. Furthermore, increasing the pressure will inevitably place stricter requirements on conveying machinery, valves, instruments, etc. 1.2.2 Gas transport factors: The nitrogen obtained from the air separation unit is often used as a carrier gas in lock-hopper systems and for dense-phase transport to nozzles downstream. The product gas is utilized in IGCC power generation or for the production of synthetic ammonia; however, when it is used to produce other chemical products such as hydrogen and methanol, it has a significant negative impact on the quality of the syngas, especially at high pressures. For example, when the operating pressure is increased from 3 MPa to 7 MPa at the same temperature, the amount of nitrogen required to transport each kg of coal will rise from approximately 0.09 kg to 0.21 kg; accordingly, the proportion of nitrogen in the produced gas will increase from 2.7% to 5.1%, **increasing the amount of inert gas that needs to be removed in subsequent synthesis systems. Using syngas to replace nitrogen for the dense-phase transport of pulverized coal into the furnace can **reduce the problem of nitrogen contaminating the syngas** ； However, using syngas as the carrier gas for transporting coal is also not an ideal choice, as it contains a large amount of flammable and explosive components. The main advantage of using CO2 as the carrier gas over nitrogen is that it does not dilute the syngas with an inert gas. Its advantage over the syngas itself is that it is non-toxic and can reduce the demand for process steam to a certain extent; moreover, under identical temperature and pressure conditions, CO2 has no significant difference in terms of its impact on process efficiency and the load on the syngas cooler compared to nitrogen ； However, the CO2 suitable for process requirements must be produced by adding additional equipment. The liquefaction of CO2 under high pressure, as well as the formation of dry ice under variable pressure conditions, are also issues that prevent the widespread use of CO2 as a transport gas. Furthermore, when CO2 is used as the carrier gas, the H2/CO and H2S/CO2 ratios in the syngas decrease. When the feedstock contains iron and nickel under high pressure conditions, this facilitates the formation of carbonyl groups and formic acid, thereby having some adverse effects on downstream conversion processes, acid gas removal processes, and material selection. 1.2.3 Other factors: The sealing technology for rotating equipment under high pressure, as well as the measurement and control technology for coal powder flow, are relatively complex ； The structure and material requirements for various valves in the sealed hopper system are stringent. 2 Wet pressurized feeding technology: This is a wet feeding method in which water (or oil) serves as the continuous phase and is pressurized by pumps; both in terms of principle and practice, it has greater advantages over hopper pressurized coal transportation (dry feeding). Compared with dry powder pressurized feeding, the wet feeding method achieves a higher pressure, does not require an inert carrier gas, simplifies operation and metering control, and ensures continuous feeding. The downside is that only a portion of the water in the slurry is required for gasification, while the rest acts as a burden on the gasification process. Since the excess water must be evaporated and heated to its vaporization temperature, oxygen consumption is higher than that of dry coal powder, resulting in lower efficiency of cold gas. Furthermore, characteristics such as the concentration, viscosity, and particle size distribution of water-coal slurry vary significantly, posing instability issues for the pressurized transportation of such slurry. 2.1 Influence of the transport medium The rheological properties of water-coal slurry are the most critical factors affecting its transportation. Many researchers have studied the rheological properties of water-coal slurry, representing the experimental data either through curves showing changes in shear stress or changes in some kind of \"representative viscosity\" relative to the shear rate, or through \"rheological models\" that use empirical equations for curve fitting. These large amounts of data provide a basis for a comprehensive understanding of the rheological properties of water-coal slurry, but it is difficult to delve deeper into them, which affects the analysis of how these rheological properties influence transportation conditions ； Due to the high viscosity of high-concentration water-coal slurry, pipeline transportation must be carried out at low speeds in a laminar flow state to minimize pressure losses; a flow rate of 0.8 m/s is generally appropriate. As a result, the diameter of the transport pipes is large, and thicker pipe walls are required to withstand the erosion caused by the slurry ； Furthermore, the stability of water-coal slurry can affect the uniform distribution of coal particles in water during storage, transportation, and conveyance; the better the stability, the more favorable it is for the long-term storage and long-distance transport of the slurry. 2.2 Impact of the equipment: In the wet pressurized feeding method, the slurry is pressurized by a pump and then injected into the gasification furnace through nozzles; therefore, the performance of the high-pressure slurry pump plays a crucial role in the stable operation of the feeding system. The vast majority of devices use plunger diaphragm pumps as tools for pressurizing and transporting slurries. The vulnerable components of a conventional diaphragm high-pressure pump are the valve and the diaphragm, with an average service life of approximately 6,000 hours to 8,500 hours. The high-pressure pumping action of modern advanced hose-diaphragm piston pumps is achieved through changes in the internal volume capacity resulting from the contraction of the hose. Unlike mechanically driven peristaltic hose pumps, this mechanism allows them to have a longer service life than traditional diaphragm pumps. The wet pressurized feeding system is equipped with shut-off valves on the slurry feed line to the furnace and the slurry return line. The medium being transported has characteristics such as high viscosity, high concentration, strong abrasiveness, and a tendency to cause blockages. Therefore, the opening area of the valve must be equal to the inner diameter of the pipe. Sealing cannot rely on the contact between mechanically processed metal surfaces, nor should there be any dead corners where solid particles can accumulate or areas where the cross-section narrows. In most cases, fittings for flushing and draining should be installed so that solid particles on the valve body can be washed away after the valve is operated. 3 Other feeding methods: Due to the shortcomings of both conventional dry coal powder pressurized feeding and slurry pumping pressurized feeding processes, research institutions around the world have in recent years embarked on the development and application of new pressurized feeding processes and technologies. Methods under investigation include wet hopper feeding, high-position hopper pressurization, hot gas-assisted slurry feeding, and high-pressure powder pumping. The two most important of these methods are described below. 3.1 Wet lock-hopper feeding method: To overcome the high energy consumption associated with using water as a carrier for coal powder during coal slurry pressurization, some research institutions are exploring a process in which the coal slurry is preheated before being fed into the gasifier. Preheating the coal slurry has the following advantages: (1) The water is preheated to about 300°C, and the latent heat of vaporization decreases at high temperatures ； (2) Hot water containing coal particles yields good atomization effects, enhancing the reactivity of the coal and increasing the gas production rate ； (3) For gasification, the reaction volume increases relatively, resulting in an increased carbon conversion rate ； (4) Reduced oxygen and coal consumption, increased cold gas efficiency ； (5) An increase in water temperature will allow good pasting properties to be maintained with less water ； (6) As the temperature increases, with the shear rate remaining constant, both the shear stress and the viscosity of the slurry decrease significantly, thereby reducing the resistance during transportation and the wear on the pipelines. The schematic diagram of the process principle for the wet lock-hopper feeding method is shown in Figure 2. Open the top valve 1, close the bottom valve 2, and the slurry is added to the lockhopper ； Close valve 1 at the top; the lockhopper is pressurized by high-pressure gas from the source via open valve 3. Once the pressure stabilizes, valve 2 at the bottom is opened, allowing the slurry to flow out of the lockhopper and into the high-pressure system. When the material level in the lockhopper drops to a certain level, valve 2 is closed and valve 4 at the bottom is opened, allowing high-pressure water to enter the lockhopper and push the gas inside back into the high-pressure gas space. Close all valves; open the top vent valve 5 to relieve pressure in the lockhopper. Water is discharged from the lockhopper through valve 6 at its bottom, after which one cycle is completed and the next cycle begins. This process has extensive experience in both hopper systems and plunger pump systems. Furthermore, the viscous coal slurry before reaching the nozzle becomes less viscous due to the thermal expansion of water, which further enhances the reliability and stability of pumping. With this feeding process, it is theoretically possible to pressurize the coal slurry to 20 MPa. At this pressure, water exists in a liquid state and can be heated to near its critical temperature of 350°C; the coal can also be heated to above 300°C. At such high temperatures, the coal particles become viscous and more turbulent in a fluidized state. This process is currently under trial, and the issue that needs to be addressed is that if the temperature of the water-coal slurry exceeds 140°C, the properties of conventional water-coal slurry additives change, causing the slurry to become gel-like; as a result, it loses its original fluidity. Therefore, research on additives suitable for high temperatures is crucial for determining the feasibility of this process ； The suitability of the materials used for equipment such as pipelines, valves, pumps, and nozzles for transporting slurry at 300°C also requires further testing and verification. 3.2 High-position hopper pressurization method: The high-position hopper pressurization method makes use of a so-called power hopper system to allow pulverized coal to enter from the top of the high-position hopper and fall along it into the pressurized container (see Figure 3). The pressure inside the container should be slightly less than or equal to the weight of the coal column in the hopper. Furthermore, the downward velocity of the pulverized coal should be higher than or equal to the velocity at which the gas in the pressurized container moves toward the areas with lower pressure between the coal particles at the top of the coal column; if this velocity is too low, the gas may expand, and the coal column could be blown out of the hopper. The relevant formulas are as follows: △P＜ρh (1) Vcoal≥V＝△Pε2／（5hηS2） (2) Here, △P is the pressure difference between the two, ρ is the bulk density of the coal, h is the height of the hopper, V is the average velocity of the gas in the voids, η is the dynamic viscosity of the gas, S is the surface area of the coal particles, and ε is the porosity between the coal and the gas. The density of coal is 1000 kg/m3 ； The minimum height required for a hopper to overcome a pressure difference of 1 atmosphere is 10.33 m; thus, take h = 10.33 m ； The η value is 1.5×10－6 kg/s·m ； Let S be 40,000 m2/m3 ； By taking ε as 0.4, it can be derived that Vcoal = 0.13 m/s. This means that for a plant with a coal feeding rate of 2000 t/d, the amount of coal flowing into the gasifier is 2000000/[(24×3600)×1000] = 0.023 m3/s. Therefore, the coal hopper needs to have an internal cross-sectional area of 0.023/0.13 = 0.18 m2, which corresponds to an inner diameter of around 500 mm. The calculation results show that such a hopper is very compact, allowing it to be easily installed near the device or integrated with tall structures. Coal can be conveyed to the top of the hopper either by pneumatic or mechanical means. Since the hopper operates near the initial fluidization point, it is not easy for the material inside the hopper to adhere to the walls and become suspended. Additionally, an annular design of the inside of the hopper can be used to slow down rotation in its central portion, thereby preventing bridging or clogging. One disadvantage of using a high hopper is that the maximum pressure achievable in a single-stage operation is 2 MPa, and a hopper with a height of nearly 200 m is required. In light of this, multipole devices capable of achieving higher pressures have been proposed; a two-stage device with a total height of 200 m can achieve a pressure of 4 MPa, which is sufficient for most gasification devices. The high-position hopper pressurization method is simple in process, but it requires a relatively long hopper. 4 Conclusion The dry coal powder pressurized feeding technology and the wet slurry pump pressurized feeding technology are widely used in current fluidized bed gasification technologies. However, in practical application, the dry coal powder pressurized feeding technology faces issues such as low conveying pressure, the carrier gas used for transportation reducing the effective components of the raw gas and increasing the amount of gas released from the system, as well as complex requirements regarding the sealing and metering of the transportation system ； Pressurized feeding using wet coal slurry pumps results in higher coal and oxygen consumption due to the high water content of the medium being transported, leading to relatively low effective gas content and cold gas efficiency. In response to the problems and shortcomings identified in the practice of older processes, new processes such as slurry preheating feeding and feeding without inert carrier gas are gradually being developed and refined. While addressing the issues posed by older processes, these new processes still require further optimization and improvement. For wet feed, future research and development efforts should focus on addressing issues such as additives for high-temperature coal slurry and the optimal selection of pipes for high-temperature coal slurry, in order to improve the properties of the material fed into the furnace and enhance gasification efficiency. For dry feeding, more research should be conducted on the sealing of the conveying systems and the material metering devices, as well as on reducing the amount of inert carrier gas used or replacing it. Additionally, research should be carried out on the appropriate applications for the pressurization method using high-level hoppers, as well as on the processes and equipment involved in pressurized conveying using dry coal powder pumps.