Prize-winning Quiz on Coal Chemical Engineering Knowledge 3

1. Ash fusion temperature is an important indicator for coal used in power generation and gasification. Coal ash is a mixture of various minerals; it does not have a fixed melting point, but rather a range of melting temperatures. Coal ash fusibility is also known as ash melting point. Due to the different mineral components of coal, its ash fusion point is lower than that of any single component within it. The common method for determining the ash fusion point is the cone method, as specified in GB219-74. Coal ash is mixed with dextrin to form pyramidal shapes, which are then heated in a high-temperature furnace; the DT (deformation temperature), ST (softening temperature), and FT (melting temperature) are determined based on the changes in the shape of these ash pyramids. ST is generally used to evaluate the fusibility of coal ash. Visco-temperature properties of ash: Viscosity is a key parameter for measuring the flow properties of fluids. In order to enable the ash to be discharged smoothly from the gasification furnace in a liquid state at the gasification temperature, the viscosity must be within an appropriate range – it is necessary to ensure the formation of an effective protective layer of ash on the surface of the refractory bricks, while also maintaining a certain level of fluidity. According to domestic and international research on liquid slag discharge boilers, the viscosity of the ash sludge should be between 25 and 40 Pa•s to ensure smooth slag discharge; for water-coal slurry gasifiers, it is appropriate to keep the viscosity of the ash sludge at 25–30 Pa•s at operating temperatures. The main factor affecting the viscosity of ash slurry is the composition of coal ash, that is, the ash components. The main mineral components of coal ash are Al2O3, SiO2, MgO, etc. Studies have shown that Al2O3 is the primary component responsible for increasing the melting point of the ash and reducing its viscosity. The higher the Al2O3 content, the higher the flow temperature of the coal ash ； When the Al2O3 content is higher than 40%, the flow temperature of the coal ash is greater than 1500°C. The MgO content is generally low, and MgO forms low-melting-point silicates with SiO2, which helps to reduce the melting temperature of the ash. SiO2 is the most abundant component in coal ash, which deteriorates the ash fusion properties of coal and increases its viscosity. However, it can form low-melting substances with other components (such as CaO); therefore, by controlling its content, CaO can be added within a certain range to mitigate the effect on ash viscosity. CaO is a component that lowers the ash fusion point, as it forms low-melting-point silicates with SiO2; therefore, CaO is the most commonly used fluxing agent. However, if its content is too high, CaO monomers will precipitate, which in turn raises the ash fusion point and increases viscosity. Its addition amount should be controlled at around 20% of the ash content. Fe2O3 is also a component that reduces the ash melting point and the viscosity of the ash, as it is reduced to FeO by H2 or CO in a reducing atmosphere; FeO then forms low-melting eutectics with SiO2 and Al2O3 present in the ash. For coal ash with an Fe2O3 content of less than 20%, for every 1% increase in Fe2O3 content, the softening temperature of the coal ash decreases by an average of 18°C. An increase in the contents of K2O and Na2O leads to a significant decrease in the melting temperature of coal ash; for every 1% increase, the flow temperature of the coal ash decreases by an average of 16°C. 2. Reactivity of coal: The gasification process of coal is very complex; in order to design gasification reactors or to evaluate existing gasification units, it is necessary to know the gasification reactivity of various feedstocks. Reactivity is an important indicator of the reaction rate. Knowing the reaction rate allows one to determine the amount of coal that can react per unit time and volume under certain conditions. A high reactivity leads to a faster combustion reaction, resulting in a lower ignition point. Therefore, the ignition point can also be used to relatively indicate the reactivity of a fuel. 3. Carbon conversion rate: The percentage of carbon elements in the products relative to the total amount of carbon elements in the reactants. 4. C ； 5、A

1. Ash fusion point: The fusibility of coal ash is conventionally measured by four temperatures, namely the initial transformation temperature of the ash (IT or T1), the softening temperature (ST or T2), the hemispherical temperature (HT or T3), and the flow temperature (FT or T4). The ash fusion point of coal generally refers to the flow temperature, and its value is closely related to the chemical composition of the ash. As can be seen from conventional coal ash analysis and Table 1, the components SiO2, Al2O3, CaO, and Fe2O3 account for approximately 90% to 95% of the ash composition. Relative changes in their contents have a significant impact on the ash fusion point; therefore, many scholars use the tetrad system SiO2-Al2O3-CaO-Fe2O3 to study the viscosity-temperature properties of ashes. Table 1 Typical composition of ash in % (mass fraction) Component SiO2 Al2O3 TiO2 Fe2O3 CaO MgO K2O Na2O P2O3 SO3 Composition 37~60 16~33 0.9~1.9 4~25 3~15 1.2~2.9 0.3~3.6 0.2~1.9 0.1~2.4 Generally, the higher the contents of iron oxide, calcium oxide, and magnesium oxide in the ash, the lower its melting point ； The higher the contents of silica and alumina, the higher the ash melting point. However, ash does not exist in the form of a simple physical mixture; rather, it crystallizes into mixtures with different structures. The melting points of these ashes vary significantly depending on their crystal structure (see Table 2), so this cannot be used as the sole criterion for identification. Formula 1 is commonly used to roughly determine the ease with which the ash of different coal types melts: Acid-base ratio = (SiO2 + Al2O3) / (Fe2O3 + CaO + MgO). A ratio between 1 and 5 indicates easy melting, while a ratio greater than 5 indicates difficult melting. Table 2 Melting points of various mixtures in ash Component Melting point/°C Component Melting point/°C Component Melting point/°C SiO2 crystals 1723 3Al2O3 2SiO2 1850 CaO + Al2O3 + 2SiO2 1553 Al2O3 2020 2FeO + SiO2 1065 2CaO + Al2O3 + SiO2 1590 CaO 2570 CaO + SiO2 1544 2CaO + FeO + 2SiO2 1203 MgO 1380 CaO + Al2O3 1605 CaO + FeO + SiO2 1208 Some experts use the ratios SiO/Al2O3 and SiO2/(SiO2+Fe2O3+CaO+MgO) to study the relationship between ash composition and ash melting points, suggesting that the former ratio should not be less than 1.6 and the latter should not be greater than 0.9; otherwise, it is necessary to add Fe2O3 or CaO, or to mix in other types of coal in order to adjust the ash composition and facilitate melting and slag removal. The viscosity-temperature properties of ash refer to the relationship between the viscosity of molten ash and temperature. The viscosity of molten slag is a physical property of the slag; once the ash composition of the coal is determined, it depends only on the temperature under which the operation takes place. 2. Reactivity of coal: The gasification process of coal is very complex; in order to design gasification reactors or to evaluate existing gasification units, it is necessary to know the gasification reactivity of various feedstocks. Reactivity is an important indicator of the reaction rate. Knowing the reaction rate allows one to determine the amount of coal that can react per unit time and volume under certain conditions. A high reactivity leads to a faster combustion reaction, resulting in a lower ignition point. Therefore, the ignition point can also be used to relatively indicate the reactivity of a fuel. 3. Carbon conversion rate: The percentage of carbon elements in the products relative to the total amount of carbon elements in the reactants. 4. C ； 5. B. This post was last edited by Dinosaurs on 2009-4-4 11:22]

1. Ash fusion temperature is an important indicator for coal used in power generation and gasification. Coal ash is a mixture of various minerals; it does not have a fixed melting point, but rather a range of melting temperatures. Coal ash fusibility is also known as ash melting point. Due to the different mineral components of coal, its ash fusion point is lower than that of any single component within it. The common method for determining the ash fusion point is the cone method, as specified in GB219-74. Coal ash is mixed with dextrin to form pyramidal shapes, which are then heated in a high-temperature furnace; the DT (deformation temperature), ST (softening temperature), and FT (melting temperature) are determined based on the changes in the shape of these ash pyramids. ST is generally used to evaluate the fusibility of coal ash. Visco-temperature properties of ash: Viscosity is a key parameter for measuring the flow properties of fluids. In order to enable the ash to be discharged smoothly from the gasification furnace in a liquid state at the gasification temperature, the viscosity must be within an appropriate range – it is necessary to ensure the formation of an effective protective layer of ash on the surface of the refractory bricks, while also maintaining a certain level of fluidity. According to domestic and international research on liquid slag discharge boilers, the viscosity of the ash sludge should be between 25 and 40 Pa•s to ensure smooth slag discharge; for water-coal slurry gasifiers, it is appropriate to keep the viscosity of the ash sludge at 25–30 Pa•s at operating temperatures. 2. Reactivity of coal: Reactivity is an important indicator of the reaction rate. Knowing the reaction rate allows one to determine the amount of coal that can react per unit time and volume under certain conditions. A high reactivity leads to a faster combustion reaction, resulting in a lower ignition point. Therefore, the ignition point can also be used to relatively indicate the reactivity of a fuel. 3. Carbon conversion rate: The percentage of carbon elements in the products relative to the total amount of carbon elements in the reactants. 4. C ； 5、C

Ash fusion temperature is an important indicator for coal used in power generation and gasification. Coal ash is a mixture of various minerals; it does not have a fixed melting point, but rather a range of melting temperatures. Coal ash fusibility is also known as ash melting point. Due to the different mineral components of coal, its ash fusion point is lower than that of any single component within it. The common method for determining the ash fusion point is the cone method, as specified in GB219-74. Coal ash is mixed with dextrin to form pyramidal shapes, which are then heated in a high-temperature furnace; the DT (deformation temperature), ST (softening temperature), and FT (melting temperature) are determined based on the changes in the shape of these ash pyramids. ST is generally used to evaluate the fusibility of coal ash. Visco-temperature properties of ash: Viscosity is a key parameter for measuring the flow properties of fluids. In order to enable the ash to be discharged smoothly from the gasification furnace in a liquid state at the gasification temperature, the viscosity must be within an appropriate range – it is necessary to ensure the formation of an effective protective layer of ash on the surface of the refractory bricks, while also maintaining a certain level of fluidity. According to domestic and international research on liquid slag discharge boilers, the viscosity of the ash sludge should be between 25 and 40 Pa•s to ensure smooth slag discharge; for water-coal slurry gasifiers, it is appropriate to keep the viscosity of the ash sludge at 25–30 Pa•s at operating temperatures. Reactivity of coal: The gasification process of coal is highly complex; in order to design gasification reactors or to optimize existing gasification units, it is necessary to know the gasification reactivity of various feedstocks. Reactivity is an important indicator of the reaction rate. Knowing the reaction rate allows one to determine the amount of coal that can react per unit time and volume under certain conditions. A high reactivity leads to a faster combustion reaction, resulting in a lower ignition point. Therefore, the ignition point can also be used to relatively indicate the reactivity of a fuel. Carbon conversion rate: The percentage of carbon elements in the products relative to the total amount of carbon elements in the reactants. CA. Since the materials used for thermocouples are generally expensive (especially when precious metals are used), and the distance from the temperature sensing point to the instrument is quite large, in order to save on thermocouple material and reduce costs, compensation wires are typically used to extend the cold end (the free end) of the thermocouple into a control room where the temperature is more stable, thereby connecting it to the instrument terminals. It must be noted that the function of a thermocouple compensation wire is merely to extend the thermal electrode and move the cold end of the thermocouple to the instrument terminals in the control room; it cannot eliminate the impact of changes in the cold end temperature on temperature measurement, and thus it does not serve a compensating role. Therefore, other correction methods must also be employed to compensate for the effect of the cold-end temperature t0≠0℃ on temperature measurement. This post was last edited by Sun Yongkang on 2009-4-4 11:55.]