By Jinder Jow
National Institute of Clean-and-Low-Carbon Energy (NICE),
China’s primary energy resources are fossil-based fuels: oil, natural gas, and coal, with coal being the least expensive. From a material aspect, coal has both organic and inorganic components, quite different from oil and natural gas which have only organic materials. Figure 1 depicts the process of a coal-fired power plant and its by-products—from coal mine to electricity or heat. The by-products are (1) NOx, sulfur oxides, Hg, particulate matter (PM), and CO2; (2) wastewater; and (3) fly ash, bottom ash, and flue-gas desulfurized gypsum when an external desulfurization process is used. The solid by-product with the largest volume is fly ash. The fly ash retains the inorganic components of coal after combustion.
Three different coal combustion processes are used to produce energy: pulverized coal (PC), circulating fluidized bed (CFB), and integrated gasification combined-cycle (IGCC). The first two are the most commonly used by coal-fired power plants. The PC process typically has a higher combustion temperature and efficiency than the CFB process, and produces less fly ash with better quality. Fly ash is mostly used in low-end applications, such as buildings and constructions, due to significant property variations that strongly depend on how each coal-fired power plant operates. This article describes the approach taken by the National Institute of Clean-and-Low-Carbon Energy (NICE), a subsidiary of Shenhua Group, to utilize and manage fly ash as a resource to increase its utilization volume and value. The same concept and approach can be applied to the utilization of fly ash or any other by-product related to coal-based energy. Coal-fired power can be cleaner if its by-products can be reduced or utilized.
FUNDAMENTAL PROPERTIES OF FLY ASH
Figure 2 depicts the three fundamental properties of fly ash: particle size distribution and morphology, chemical composition, and mineral composition. As noted, fly ash will differ in these properties, due to the operational differences of various coal-fired power plants. Factors that influence these properties include the coal type/source, pretreatment, combustion process, environmental control system, and the ash collection system.
Figure 3 shows how these three fundamental properties are linked with the operation of a coal-fired power plant. Several steps need to be taken to utilize fly ash as a resource. Identifying and understanding the properties of the fly ash is the first key step. Obtaining fly ash with consistent properties is the second step. Identifying suitable applications and development of specific products for different uses is the last step to maximize its properties and utilization value.
Particle size distribution of fly ash depends on the coal’s pretreatment, combustion process, and ash collection system. In general, fly ash has a particle size range of 0.1–600 µm. Fine coal particles produce finer fly ash. Higher combustion efficiency also tends to produce finer fly ash. Fly ash collected at the same plant using different electrostatic precipitators will have a different average particle size and distribution. Finer fly ash usually has a better utilization value due to its higher surface area and reactivity.1 The particle morphology depends on the combustion process. The PC process produces spherical particles due to natural cooling, whereas the CFB process creates irregularly shaped particles due to the fluidizing action.1 The images in Figure 4 show the differences in particle morphologies using a scanning electron microscope. A spherical shape has a better flow property but less aspect ratio effect than an irregular shape.
The chemical composition of fly ash depends on the coal type and the extent and temperature of combustion. The environmental control units used to remove NOx, sulfur, or Hg will also affect the composition. The major chemical compositions are dominated by SiO2 and Al2O3 as an aluminosilicate material followed by four secondary components, CaO, Fe3O4 or Fe2O3, SO3, and unburned carbon (loss on ignition, LOI). Combustion of lignite or subbituminous coal usually produces more fly ash, due to its high ash and CaO content, than does combustion of an anthracite or bituminous coal. The internal desulfurization process where lime is injected into the combustion process can also produce fly ash with high CaO content. Fly ash with more CaO tends to have higher cementitious reactivity. Typically, the PC process produces better fly ash quality with lower LOI, SO3, and CaO contents than the CFB process.
Mineral composition depends on the coal type, coal particle size, and boiler temperature. In general, higher boiler temperatures and smaller coal particles produce fly ash with higher glass content. Fly ash typically has a glass content range of 35–70%. Fly ash with more glass and smaller particle size has a higher pozzolanic reaction. Fly ash with high aluminum content tends to have lower glass content but higher mullite content in its crystalline phase.
FLY ASH UTILIZATION ISSUES
Two key issues for fly ash utilization are significant property variation and local supply-demand issues. In China fly ash is used in various applications, such as cement and concrete, walls and building materials, aggregates in road pavement, agricultural use, mine refilling, and mineral extraction.2 The building and construction sectors are major users of fly ash which can meet their low performance requirements. For example, China produced 540 million tons of fly ash in 2014 with a utilization rate of 70%, higher than the global average of 54%.3 The fly ash was used as follows: 60% for cement and concrete, 26% for bricks and walls, 5% for road pavement, 5% for agriculture and mine refilling, and 4% for mineral extraction and other applications. The utilizations are categorized into three types as shown in Figure 5: local massive utilization, high-value utilization, and local ecologic utilization.
Fly ash from coal-fired power plants located near metropolitan areas or large industrial complexes can be utilized to meet local demand in building and construction applications. However, these local applications are typically of low value (low price-to-performance) and only economic within a 100-km distance due to the transportation cost. Coal-fired power plants located in remote areas have limited options for fly ash utilization. Both high-value (high price-to-performance) and local ecologic utilizations become critical to increase its usage. Utilization and management of fly ash must be economically viable in remote locations or regions. In order to increase current utilization value and volume, efforts are underway to identify new applications for high-value and local utilizations. This requires an understanding of materials science and knowledge of possible applications.
APPROACHES TO ADDRESS FLY ASH UTILIZATION AND MANAGEMENT ISSUES
To address fly ash utilization and management issues, the first step is to characterize its fundamental properties from individual coal-fired power plants and re-characterize when the operational conditions change. The second step is to obtain fly ash with consistent property qualities through a cost-effective particle control system, particularly for particle size, LOI, and Fe3O4 content. The third step is to select suitable applications based on consistent fundamental properties of fly ash and to develop core technologies and products for full utilization of fly ash to achieve the maximum value. Figure 6 shows how these three steps address both property variation and supply-and-demand issues.
The fly ash R&D team at NICE has adapted this approach to characterize different types of fly ash and establish a cost-effective particle control system. This particle control system has obtained at least three grades of fly ash with consistent particle size distribution used to develop four products: hydraulic fracturing proppants, fillers, highly active supplemental cementitious (HASC) products, and river sand (RS) products (see Table 1). All products are based on at least one of these three fly ash grades, which are produced from the PC process. The processes of making fly ash-based products do not generate any by-products and consume less energy than the existing products to be replaced.1
HASC products can replace up to 50% cementitious materials including cement used in concrete. Concretes using HASC products have higher compressive strengths, including three-day compressive strength which is one of the most important properties of concrete.3 The RS product fully replaces ultrafine sand used in mortar. Fillers can fully replace CaCO3 and other inorganic fillers (2500 mesh or above) used in polymers with better flow property. When the polymers are molten and pushed to flow, spherically shaped fillers help the molten polymer flow better than do irregularly shaped fillers. Fly ash-based proppant properties are either equivalent to or better than three commercially available bauxite-based proppants, identified as SG overseas, YT China, and CQ China, as shown in Table 2.4
The three cases described below demonstrate how these products increase the utilization rate and value in local massive and high-value utilizations. The fly ash reference case was obtained from a pulverized coal-fired power plant. The fly ash is rated as Class II according to Chinese National Standard GB1596-2005 for concrete and mortar uses. For the particle size requirements, the GB 1596-2005 standard specifies fly ash with particle size greater than 45 µm and no higher than 25% by weight as Class II fly ash, while ASTM C618 specifies no higher than 34% by weight as Class F. The fly ash cost reference is assumed to be RMB50/ton from a coal-fired power plant and sold to a concrete producer at RMB150/ton, resulting in a gross margin of RMB100/ton.
Case I demonstrates two fly ash-based products used for concrete and mortar as an example of local massive utilization. Case II shows the viability of fillers for high-value utilization along with two products used for concrete and mortar for local massive utilization as a mixed example. Case III maximizes the utilization value and rate by making fillers and proppants using fly ash with an Al2O3 content of at least 35% as the example of high-value utilization only.
Case I: The reference fly ash is classified and converted into a highly active supplemental cementitious (HASC) product to replace 50% cement in concrete, Class II fly ash as an existing product, and a river sand (RS) product to fully replace ultrafine river sand used in mortar at a price of RMB350/ton, RMB150/ton, and RMB50/ton, respectively, under a product split ratio of 20%, 75%, and 5%. The average cost of conversion is RMB80/ton. The calculated gross margin is RMB105/ton. The market size of Class II fly ash used in concrete is assumed to be 71 million tons. The expected fly ash volume processed is 84 million tons to achieve a total extra gross margin of RMB420 million in China. The extra fly ash volume is 21 million tons used for HASC and RS products.
Case II: The same fly ash is classified and converted into fillers, Class II fly ash, and an RS product priced at RMB1000/ton, RMB150/ton, and RMB50/ton, respectively, under a product split ratio of 30%, 60%, and 10%. The average cost of conversion is still RMB80/ton. The calculated gross margin is RMB265/ton. The market size of fillers is assumed to be 1.8 million tons. The total fly ash volume processed is 6 million tons to achieve a total extra gross margin of RMB990 million. The total extra fly ash volume is 2.4 million tons used for filler and RS products.
Case III: Fly ash with high Al2O3 content is classified and converted into fillers and proppants priced at RMB1000/ton and RMB2500/ton under a product split ratio of 30% and 70%, respectively. The average cost of conversion rises to RMB800/ton. The calculated gross margin is RMB1250/ton. The market size of proppants is assumed to be 1.4 million tons in China. The extra fly ash volume is 2 million tons used for both proppants and fillers to achieve a total extra gross margin of RMB2300 million.
All prices stated are a reference for economic comparison and not necessarily the actual prices. Table 3 summarizes the extra fly ash volume and margin created by these three cases. As expected, high-value utilization creates more value and consumes less fly ash volume, while local massive utilization consumes more fly ash volume but creates less value.
The development of options to make coal-fired power cleaner by reducing or utilizing more waste by-products is critical to maintain long-term sustainability. Coal has the organic component used to generate heat or electricity while its inorganic component is converted into fly ash through the combustion process. This article discusses options to increase the utilization of fly ash from coal-fired power generation. The fundamental properties of fly ash are particle size distribution and morphology, chemical and mineral composition, and significant variability depending on the operational conditions of individual power plants.
This article demonstrates how to increase fly ash utilization volume and value based on understanding the fundamental properties of fly ash and their property-driven applications for high-value and local building materials uses. Local ecologic utilizations are other options to increase volume and add value to fly ash, including mine refilling, agricultural use, land reclamation, and road construction. These usages are of extremely low value but useful in achieving full utilization, particularly in remote regions. How to achieve positive economic benefits for any ecologic utilization is another important and challenging goal. Resource utilization and management of fly ash requires collaborative efforts among local coal-fired power plants, governments, R&D teams, and enterprises to achieve a full utilization with an overall positive economic benefit in each region.
- A. High-value utilization includes fillers, flame retardants, low-density foam for fire protection, thermal insulation, and industrial ceramics. Local massive utilization includes building materials for cement, mortar, and concrete, pre-cast, wall materials, and high-density foam. Local ecologic utilization includes mine refilling, aggregates for road pavement, land reclamation, and agricultural use.
- Dong, Y., Jow, J., Su, J., & Lai, S. (2013). Fly ash separation technology and its potential applications. Paper presented at the 2013 World of Coal Ash Conference, 22–25 April, Lexington, Kentucky.
- National Development and Reform Commission. (2013, 18 February). Fly ash comprehensive utilization management regulation (translated from Chinese).
- Jow, J., Dong, Y., Zhao, Y., Ding, S., Li, Q., Wang, X., & Lai, S. (2015). Fly ash-based technologies and value-added products based on materials science. Paper presented at 2015 World of Coal Ash Conference, 5–7 May, Nashville, Tennessee.
- Ding, S., Gao, G., & Jow, J. (2016). Resource utilization of high-alumina fly ash: High performance proppant application and development. Paper to be presented at 2016 Asia Coal Ash Conference. Shuozhou, China.
The content in Cornerstone does not necessarily reflect the views of the World Coal Association or its members.
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