Definition and physicochemical properties of water-coal slurry

Water-coal slurry is a low-pollution, high-efficiency, pipeline-transmissible coal-based fluid fuel that is produced through physical processing from approximately 65% coal, 34% water, and 1% additives. I. Overview of Coal Water Slurry Coal water slurry (abbreviated as CWM) is obtained by grinding coal into powder of a certain fineness, mixing it with water, and adding additives. In actual production, operations generally meet the design requirements, but long-term operational experience has shown that there are still areas that need improvement in the storage and transportation process. II. Overview of the Composition of the Water-Coal Slurry Storage and Transportation System The water-coal slurry storage and transportation system consists of two parts: the storage of water-coal slurry and its transportation. Transportation is further divided into railway transportation systems, road transportation systems, and pipeline slurry transport systems: 1. The railway slurry unloading system transports the water-coal slurry produced in the slurry plant by rail to railway stations; by taking advantage of the height difference in the railway track, the water-coal slurry is transferred from the train tanks into storage tanks. It is then pressurized using slurry pumps and sent through pipelines to the storage tanks for later use ； 2. The vehicle slurry unloading system utilizes the height difference between the slurry unloading platform and the slurry tank to allow the water-coal slurry to flow automatically from the vehicle unloading platform into the slurry tank, and then it is pressurized by a slurry pumping unit and sent directly into the slurry storage tank in front of the furnace. 3. The pipeline transportation system makes use of the fluid properties of water-coal slurry, relying on the mechanical thrust of screw pumps to transport it. Generally, we use water-coal slurry from Datong, Shanxi, and the transportation process is different, which leads to different problems. III. Challenges in the storage and transportation of water-coal slurry: Since its commissioning, the water-coal slurry storage and transportation system has been operating normally, capable of supplying fuel to the boilers on a continuous basis and meeting the design requirements to a large extent. However, in the application of water-coal slurry, since it is a new technology, people’s knowledge and understanding of it are still in the exploratory stage. The biggest concern when storing water-coal slurry is sedimentation. Through operation, we found that there are some problems and defects in the water-coal slurry storage and transportation system. The main issue is the occurrence of \"hard sedimentation\" of the water-coal slurry in the storage tanks, slurry transfer pumps, and pipelines, which leads to blockages at the outlet of the storage tanks, the inlet of the slurry transfer pumps, and within the pipelines, thereby causing system failures on multiple occasions. Therefore, the water-coal slurry storage and transportation system needs further improvement and refinement. l) Sedimentation of water-coal slurry in the vehicle-based slurry discharge tank: After operating for a certain period of time, and due to the open nature of the discharge process which exposes it to air, water-coal slurry tends to settle within this tank. As a result, the bottom of the tank rises over time, which affects the slurry discharge process. 2) Blockage of the pipelines in the train slurry unloading system. After operating for a certain period of time, it was found that due to the low slope of the slurry unloading pipelines, the flow rate of the slurry during unloading was slow; as a result, a large amount of solid deposits of water-coal slurry accumulated inside the pipelines over time. 3) Blockage in the slurry delivery and supply pipelines. The biggest problem in pipeline transportation is sedimentation when the water-coal slurry stops flowing or operates at low flow rates. From storage to the furnace, and all the way until combustion, water-coal slurry relies on pipeline transportation. Prolonged intermittent operation results in a decreasing flow area of the pipelines, eventually leading to insufficient flow capacity; in such cases, the system has to be shut down in order to clear the pipelines. 4) Solid precipitation in the slurry storage tank: The tank is equipped with heating and insulation pipelines, and it also contains a water-coal slurry mixer as well as a slurry discharge pump. The slurry enters the tank directly from the top via the upper slurry collection pipeline. After the trial operation, it was found that the biggest problem was the sedimentation of the water-coal slurry in the slurry storage tank. The main reason is: A) The slurry mixer has almost no effect ； B) Mixed storage of various water-coal slurries ； C) The rinse water enters the slurry tank ； The quality of water coal slurry itself is unstable. After the stored water-coal slurry settles, it blocks the outlet of the slurry tank, preventing the slurry pump from pumping out the water-coal slurry and severely affecting the normal operation of the boiler. Water-coal slurry storage tanks need to be designed and manufactured specifically; it is advisable to install a high-power vertical mixer at the center of the tank, as well as test mixers at the inlet and outlet points for the slurry flow. Water-coal slurry can easily settle and cause blockages in pipes and storage tanks during operation. From the perspective of water-coal slurry applications, it is not desirable for high-concentration water-coal slurries to settle during storage; therefore, they must maintain a certain level of stability. During pipeline transportation, it is desired that the water-coal slurry have good fluidity, that is, a low viscosity. Due to the thixotropic properties of water-coal slurry, it must be stirred during storage, both to prevent sedimentation and to ensure that the pipelines can draw up the slurry properly. Blockages in the pipes can cause serious disruptions to production: Even mild blockages, such as the deposition of water-coal slurry around the pipes, reduce the flow cross-sectional area of these pipes, thereby decreasing the flow rate of the water-coal slurry and affecting the system’s capacity to handle loads ； Severe blockages, namely a complete blockage of a partial or the entire pipeline, will result in zero flow within the entire pipeline, **undermining the economic efficiency of system operation and threatening its stable functioning. Cleaning blocked pipes is not only laborious and time-consuming but can also cause serious environmental pollution. Therefore, sufficient attention must be paid to various aspects such as design, installation, and operation. IV. Improvements Implemented In response to the actual problems identified during operation, the following improvements were made to the system and processes: ① The slurry transfer system is equipped with a comprehensive water flushing system and an air purge system. Due to the inevitability of slurry sedimentation and the unpredictability of pipeline blockages, water flushing systems and air purging systems for pipelines are essential. While introducing a water flushing system and an air purging system into the water-coal slurry pipeline, the outlet valves were arranged appropriately; flexible connections and inspection holes were installed, and discharge valves were added to remove the flushing water and compressed air that enter the pipeline, thereby preventing the water and air from mixing with the water-coal slurry. Reduce the stability of water-coal slurry. ②Operate scientifically. Since coal water slurry pipelines are prone to clogging, it is not allowed for them to remain unused for long periods of time. Even during shutdown for maintenance, the storage tanks and pipelines must be operated in a recirculation mode, or the pipelines must be thoroughly cleaned before they can be shut down. At the same time, strengthen winter and routine management. ③ Install viewing ports and inspection/cleaning sections in the middle of the pipelines. Furthermore, train transportation should use dedicated water-coal slurry tank cars equipped with complete heating systems. ④Ensure the recirculation pipeline is unobstructed. The scouring capacity generated by the flow of water-coal slurry is utilized to enhance the self-cleaning of the pipelines, and pipeline recirculation helps to avoid the existence of local dead zones, preventing blockages in the pipelines due to a lack of flow over extended periods. ⑤Install a cleaning system; after the slurry has been discharged, rinse it promptly to keep the pipelines clean and prevent sedimentation. Additionally, raise the height of the pipeline appropriately to ensure that the water-coal slurry inside flows completely through, thereby preventing sedimentation. ⑥Valve modification ； Special slide valves are used in place of regular valves; moreover, since the slurry deposits first on rough surfaces and irregular areas, the use of unnecessary valves is reduced, thereby minimizing the chances of slurry deposition. ⑦Storing coal water slurry mixtures with different properties can lead to \"soft precipitation\" of the coal water slurry; it is advisable to store different types of coal water slurry separately. If there is mixed slurry in the slurry storage tank, try to use it up as quickly as possible. ⑧The water-coal slurry in the storage tank is stirred regularly, but air inevitably gets mixed into it; these gases are broken up by stirring, resulting in many bubbles that are evenly distributed throughout the water-coal slurry. Due to the high concentration and high viscosity of the coal slurry, it is difficult for these microbubbles to escape, resulting in a coal slurry with three phases—gas, liquid, and solid—that has virtually no fluidity. Therefore, during the mixing of water-coal slurry, efforts should be made to prevent air from entering, or an appropriate amount of defoamer should be added. When storing water-coal slurry under normal conditions, it should also be stored in a sealed manner as much as possible to prevent the slurry from coming into contact with air. ⑨When flushing the coal slurry pipeline, the flushing water should be prevented from entering the storage tank ； Water destroys the stability of the coal slurry and accelerates its sedimentation. ⑩The pipeline should have a certain slope, and bends should be used as little as possible; where they are necessary, bends with a large radius of curvature should be chosen to reduce pipeline resistance. ⑾In the design and use of slurry pipelines, attention should be paid to the circulation pipes. In this way, even when the boiler is not using slurry, normal flow of the slurry within the pipelines can be ensured, thereby reducing sedimentation inside the pipes. In summary, in the design of the storage and transportation of water-coal slurry, efforts should be made to minimize the use of elbows and valves; specialized butterfly valves should be chosen for these purposes, and they should be installed on vertical pipes. The installation location of the valves should take into account minimizing the length of dead zones in the pipes ； The pipes after use should be rinsed thoroughly with water or compressed air. During operation, the load should be maintained at over 60% as much as possible. At the same time, the quality of pipeline layout, the choice of pipeline routing, the design of valves and their installation locations, all have an impact on slurry transportation through the pipelines ； V. Conclusion. Water-coal slurry is a solid-liquid mixture that does not maintain homogeneity easily, and solid-liquid separation tends to occur frequently. It is generally required that no \"hard precipitates\" form during storage and transportation. The so-called “hard precipitate” refers to a precipitate that cannot be restored to its original state by stirring the water-coal slurry. The ability of water-coal slurry to remain free from the formation of hard precipitates is known as the “stability” of the water-coal slurry. The required storage period depends on the user's requirements, usually being three months. Water-coal slurry with poor stability will severely impact production if sedimentation occurs during storage and transportation. During the storage and transportation of water-coal slurry, the biggest problem is \"hard sedimentation\". To prevent and address \"sedimentation,\" the main measure during storage is to stir the water-coal slurry. During transportation, the key approach is to keep the pipes clean in order to avoid \"hard sedimentation.\" Special attention should be paid to the choice of pipe cleaning methods, the type of pipe connections, as well as providing access points for clearing blockages in the pipes. At the same time, scientific water-coal slurry storage, transportation, and operation technologies should be adopted. Since people tend to consider slurry storage and transportation to be a generic technology and thus pay little attention to it, although this aspect accounts for a small portion of the overall technology related to slurry boiler operation, the slurry storage and transportation system still belongs to the fuel system. If there are failures in this system, it will directly affect the proper supply of fuel to the boiler, thereby impacting its stable operation. Therefore, it is essential to pay close attention to it.