Discussion on the Application of Water-Coal Slurry

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Discussion on the Application of Coal Water Mixture Abstract: Coal water mixture possesses good fluidity and stability; it can be stored and transported like oil, and can also be burned or gasified directly. Water-coal slurry also features high concentration, fine particle size, high burnout rate, high combustion efficiency, and low pollutant emissions. It focuses on explaining the characteristics, current technical status, production process, and application status of water-coal slurry as an efficient and clean coal-based fuel alternative to oil. It also discusses the factors restricting its development as well as its future prospects. Keywords: water coal slurry ； Boiler ； Combustion: Our country is rich in coal but short of oil; coal reserves account for over 90% of the total energy reserves, and coal makes up as much as 75% of the energy consumption structure. The rapid economic development in our country has led to an increasing demand for energy, particularly for liquid fuels. At present, China’s dependence on oil imports exceeds 40%, and the energy issue has become an important strategic concern. Currently, international oil prices remain high, and the unstable international situation in oil-rich regions means that prices are not likely to drop significantly in the near future. As people’s living standards improve, their expectations regarding the environment also increase. However, the direct combustion of coal releases large amounts of acidic gases such as SO2 and NOX, causing severe damage to the environment. Therefore, it is imperative to promote efficient coal combustion technologies and carry out projects to replace oil with coal and convert coal into oil. The “coal-to-oil” technology is not yet mature in China; it is still in the demonstration phase, and it requires substantial investment. To prevent domestic companies from making blind investments and to avoid investment risks, the National Development and Reform Commission has issued a regulation (Document No. 1350) to restrict its development. This brings new hope for the promotion and application of coal-water slurry technology, which requires low investment and features mature technological processes. 1 Current Status of Water-Coal Slurry Technology: Water-coal slurry is a new coal utilization technology developed as a substitute for oil during the global oil crisis of the 1970s. It is created by grinding and refining coal, water, and small amounts of additives, and mixing them thoroughly with water; under the action of chemical additives, it becomes a flowable coal-based liquid fuel similar to oil. Water-coal slurry possesses good fluidity and stability; it can be stored and transported just like oil, and it offers advantages such as safety and low environmental pollution. It is currently the most suitable clean coal fuel to replace oil for China’s national conditions. It is currently used in power station boilers, industrial boilers, and industrial furnaces as a substitute for oil, gas, and coal in combustion; it can also be used as a feedstock for gasification to produce synthetic ammonia, synthetic methanol, synthetic oils, etc. 2 Properties and Parameter Indicators of Water-Coal Slurry 2.1 Concentration of Water-Coal Slurry The concentration of water-coal slurry refers to the mass concentration of solid coal; it directly affects the ignition properties and calorific value of the slurry. The higher the concentration, the easier it is to ignite, and the higher the heat output. However, an increased concentration affects the fluidity of the water-coal slurry; typically, the concentration is kept between 60% and 75%, depending on actual requirements and the characteristics of the coal. 2.2 Particle size of coal in water-coal slurry The particle size of coal in water-coal slurry has a significant impact on the rheology, stability, and combustion properties of the slurry. Coal particles of different sizes can fill each other, reducing the gaps between them and achieving a better packing efficiency. It features high packing efficiency and few voids, which reduces water consumption and facilitates the preparation of coal water slurry at higher concentrations. Under normal circumstances, the maximum particle size of coal does not exceed 300 μm, and the proportion of particles smaller than 74 μm is not less than 75%. 2.3 Rheological properties of water-coal slurry The rheological properties are used to describe the flow characteristics of heterogeneous fluids; they are important factors that affect the storage stability of water-coal slurry, its fluidity during transportation, as well as its atomization and combustion performance. These properties are generally expressed through the shear stress-shear rate relationship, with viscosity being the commonly used parameter. To enable water-coal slurry to achieve high concentration, high stability, and good rheology, certain chemical agents—dispersants and stabilizers—must be added during its preparation. Dispersants are certain surfactants that can significantly reduce the surface tension of a solution, improve the wettability of coal particles, lower the viscosity of the solution, and enhance the fluidity of the liquid. Stabilizers are used to improve the stability of water-coal slurry, so that it maintains a uniform composition during storage and transportation. 2.4 Stability of water-coal slurry As a solid-liquid mixture, water-coal slurry is prone to solid-liquid separation and the formation of precipitates. The stability of water-coal slurry refers to its ability to avoid the formation of hard precipitates; hard precipitates are those that cannot be restored to a homogeneous state by stirring, whereas those that can be restored in this way are referred to as soft precipitates. General industry requires a stabilization period of three months. Furthermore, the choice of raw coal has a significant impact on the properties of water-coal slurry. The slurry-forming properties of different coal types vary greatly. Generally, the lower the coal rank, the less internal moisture it contains; the ratio of O to C in the coal is smaller, there are fewer hydrophilic functional groups, the porosity is less developed, the grindability index is higher, and there are fewer soluble heavy metal ions in the coal, making pulp production easier. Before pulping, the type of coal must be selected as required. Factors such as its ash content, sulfur content, calorific value, volatile matter, and ash fusion point have a significant impact on whether water-coal slurry can burn stably, on the combustion efficiency, and on pollutant emissions. Since volatile matter affects whether water-coal slurry can ignite and burn stably in the furnace, it is generally required that the volatile matter content of the coal used for slurry preparation be greater than 25% for use in boiler combustion, and greater than 15% for use in furnace combustion. If impurities such as ash and sulfur are present in excessive amounts, they must be removed through washing and sorting during the preparation process. The main parameter values of water-coal slurry are shown in Table 1. Table 1: Main parameter values of water-coal slurry. Parameters include concentration, viscosity, ash content, average particle size, calorific value, and stability. Units: %, mPa·s, %μm, MJ/kg. Values: 60–70, 800–1200, <10, <50, 17–19.5; Stability period: >3 months. 3. Process flow for preparing water-coal slurry: The processes used to prepare water-coal slurry can be divided into three categories: dry method, dry-wet method, and wet method. Due to the high energy consumption of the dry and dry-wet methods, as well as their inferior performance compared to the wet method, these approaches are rarely used in industry these days. Figure 1 is a flowchart of the wet pulping process. Coal → Crushing → Ball milling or rod milling → Water → Slurry filtration → Mixing tank → Stabilizer → Storage tank → User → Dispersant. Figure 1 shows the process flow diagram for wet slurry preparation. After the raw coal is washed to remove impurities such as ash and sulfur, it is crushed into coal particles with a size of less than 6 mm. These particles are then fed into ball mills (or rod mills), where water and dispersant are added to grind them into a slurry. The slurry is pumped to a slurry filter to remove any unground coarse particles and impurities, after which it is sent to a mixing tank. Stabilizers are added there, and the viscosity of the coal-water slurry is adjusted before it is stored in a storage tank for later use. 4 Advantages of water-coal slurry combustion technology 4.1 Good combustion performance: The viscosity of water-coal slurry is lower than that of heavy oil, making it easy to adjust; the minimum load can be reduced to 40%. It can replace heavy oil in boilers for combustion, with a combustion efficiency of 96%-99%. The thermal efficiency of the boiler exceeds 86%, reaching a level comparable to that of fuel boilers; it features easy combustion control and stable, reliable operation. 4.2 Significant environmental benefits: Since the combustion temperature of water-coal slurry is between 1200–1300°C, which is 100–150°C lower than that of fuel oil and pulverized coal, and since refined coal used in water-coal slurry has low sulfur and ash content, the emission concentrations of SO2 and NOX resulting from its combustion are reduced. Environmental dust and noise levels are low. It has good slag discharge efficiency; the ash remaining after combustion can be used to produce building materials such as cement and bricks, with no secondary pollution. 4.3 The cost of equipment modification is low. By switching from fuel boilers to low-water coal slurry, existing equipment can be fully utilized, the production process is simplified, and investment costs are reduced. The cost of switching to coal water slurry is only 1/3 to 1/2 of that of switching to pulverized coal. 4.4 Low operating costs for the equipment: 2 tons of water-coal slurry can replace 1 ton of fuel oil, saving approximately half of the fuel costs. Table 2 shows a comparison of the operating costs for several types of fuel-fired boilers, using a 4T/h boiler as an example (saturation steam pressure of 1.25 MPa, steam temperature of 194°C, 2717 MJ of heat required per ton of steam, feedwater temperature of 20°C). Table 2 Comparison of Operating Costs for Boilers Using Various Fuels
Comparison Item | Unit | Water-coal slurry | Coal | Heavy oil | Light oil | City gas | Natural gas |
Unit price (yuan/ton) | 600 | 450 | 2000 | 3500 | 1.6 yuan/m³ | 2.5 yuan/m³ |
Boiler efficiency (%) | >86 | <80 | >90 | >90 | >90 | >90 |
Fuel combustion rate (%) | >98 | <80 | >99 | >99 | >99 | >99 |
Calorific value (MJ/kg) | 20.9 | 19.6 | 37.6 | 41.8 | 16.7 MJ/m³ | 35.1 MJ/m³ |
Fuel consumption per ton of steam (kg) | 154 | 216 | 827 | 318 | 0 m³ | 86 m³ |
Fuel cost per ton of steam (yuan) | 927 | 916 | 425 | 528 | 821 | 55 |
Applications of water-coal slurry: The applications of water-coal slurry can be roughly divided into three categories: direct combustion, gasification, and pipeline transportation. Direct combustion is widely used in power plant boilers, industrial boilers, and industrial furnaces both domestically and internationally. Slurry coal gasification has also been in practical use for many years in equipment such as the Texaco gasifier and the Yankuang multi-nozzle opposed gasifier. Pipeline transportation has successful commercial applications in the United States, the former Soviet Union, and other countries. Compared to other modes of transport, it offers significant advantages in terms of continuity, reliability, and security; it causes little environmental impact, produces no noise pollution, and results in no transportation losses. It also facilitates mechanization and automation, and its transportation costs are lower than those of railways, roads, and waterways. 6 Constraints on the Development of Water-Coal Slurry Research on water-coal slurry began in the 1970s, and the technologies for its production and use have become quite mature. The main reason why it has not been put into widespread industrial use is that the world economy was growing relatively slowly at that time, resulting in lower demand for liquid fuels; oil supplies were abundant, international oil prices remained low, and fuel was therefore relatively inexpensive. Secondly, countries have **lower environmental protection requirements, and the direct combustion of pulverized coal (pulverized coal boilers) is dominant. Thirdly, the preparation and transportation of water-coal slurry require significant amounts of electricity and water resources; 1 ton of water-coal slurry consumes 40–60 kWh of electricity. At the same time, the preparation of water-coal slurry requires high standards for coal quality; low-ash and low-sulfur bituminous coal is needed, and using water-coal slurry as a substitute for pulverized coal has no economic value. Driven by economic considerations, companies are reluctant to switch to coal-water slurry boilers. Fourthly, the main equipment used in the preparation and application of water-coal slurry, namely coal grinders and combustion nozzles, are prone to failure, which also hinders the widespread use and development of water-coal slurry. 7 Prospects for the Development of Water-Coal Slurry: The shortage of petroleum resources and high oil prices have led many fuel boilers to be replaced by water-coal slurry boilers. **Environmental regulations are becoming increasingly strict; some cities in China have restricted or even banned the use of coal-fired boilers, forcing existing coal-fired boilers and industrial furnaces to switch to coal water slurry as a fuel source. The rise of large-scale coal chemical industries such as coal-based methanol, synthetic ammonia, and synthetic oils, along with the widespread development of water-coal slurry gasifiers, has promoted the use of water-coal slurry. Key technological breakthroughs in the manufacturing of ball mills, rod mills, and coal-water slurry combustion nozzles have further promoted the industrial development of coal-water slurry technology. Industrial wastewater such as mine water and papermaking wastewater can be used to produce water-coal slurry, which not only saves on wastewater treatment costs but also conserves water resources. Slurry production plants should be located at coal mine entrances and in areas where slurry users are concentrated, in order to make comprehensive use of mine water and industrial wastewater. Pipeline transportation should be used as much as possible to protect the environment and reduce transportation costs. Water-coal slurry plants should develop in a direction toward scale and larger size in order to reduce manufacturing and usage costs. The rapid development of China’s economy, the call to build an \"environmentally friendly and resource-efficient society,\" as well as the introduction of **\"energy conservation and emission reduction\" measures, will undoubtedly spur the advancement of the industry related to water-coal slurry as a high-efficiency and clean alternative fuel. References: Yao Qiang, et al. Clean Coal Technology. Beijing: Chemical Industry Press, 2005. He Yongde, et al. Handbook of Modern Coal Chemical Technology. Beijing: Chemical Industry Press, 2004.