A Review of the Current Research Status on Circulating Fluidized Bed Ash Residues
Abstract
Against the backdrop of global climate change and China's "Dual Carbon" goals, the efficient utilization of industrial solid wastes like coal ash from power plants has become a key issue for green transformation and sustainable development. Circulating Fluidized Bed (CFB) combustion technology is widely used in China due to its advantages, including broad fuel adaptability, high combustion efficiency, and low pollution emissions. However, with the proliferation of CFB technology, the treatment and resource utilization of its by-products—CFB ash and slag—have gradually become a research focus. This paper systematically reviews the composition and characteristics of CFB ash and slag, focusing on their expansion properties and loose structure. It summarizes the current status of low-carbon applications and high-value utilization research in fields such as cement-based materials, geopolymers, road base materials, aerogel preparation, and zeolite preparation. Studies show that CFB ash and slag have significant advantages in enhancing the mechanical properties, optimizing microstructure, and improving the durability of cement-based materials. Despite challenges like expansion issues caused by high sulfur and calcium content, the potential of CFB ash and slag as geopolymers and high-value functional materials is gradually being unlocked through physical or chemical modification technologies, significantly enhancing their application value. In the future, researchers should intensify efforts in developing modification technologies and exploring resource utilization pathways to promote the application of CFB ash and slag in a low-carbon circular economy, providing a theoretical basis and technical support for the sustainable use of industrial solid waste.
1 Composition and Characteristics of CFB Ash and Slag
CFB ash and slag mainly include two types: Circulating Fluidized Bed Fly Ash (CFBFA) and Circulating Fluidized Bed Bottom Ash (CFB-BA). CFBFA is fine particulate matter discharged with flue gas during combustion and captured by dust removal devices, while CFB-BA is larger unburned material discharged directly from the bottom of the furnace .
The main chemical components of CFBFA include SiO₂, Al₂O₃, Fe₂O₃, CaO, and small amounts of MgO, Na₂O, and other trace elements. Due to the use of lime as a desulfurizer in CFB boilers, CFBFA contains higher levels of CaO and SO₃. Its primary mineral compositions are anhydrite (CaSO₄), calcite (CaCO₃), free lime (f-CaO), and quartz (SiO₂). Unlike ordinary fly ash, it does not contain a mullite phase. This difference is attributed to the lower combustion temperature (850–900°C) of CFB boilers, which prevents the high-temperature phase transition of clay minerals to form mullite. CFBFA has smaller particle size, larger specific surface area, and irregular particle morphology with a loose and porous surface structure. These physical characteristics affect its rheological properties in cement, such as increasing the contact area between particles and enhancing the system's reaction rate. The active silicon-aluminum components in CFBFA enable it to participate in the hydration reaction of cement, forming C-S-H and C-A-S-H gels. The hydration of f-CaO generates Ca(OH)₂, causing volume expansion, while calcium sulfate reacts with calcium aluminate to form ettringite (AFt), contributing to early strength .
CFB-BA has a wide particle size distribution, with relatively coarse particles that are rough, irregular, loose, and porous, correlating with its high water absorption. Due to this microstructure, the standard consistency water demand of CFB-BA is typically high, about twice that of ordinary fly ash. CFB-BA exhibits self-hardening properties, forming a stable state with certain hardness, and shows strong expansibility during hydration, generating large volumes of AFt and Ca(OH)₂. It contains active Al₂O₃, f-CaO, and CaSO₄, giving it certain cementitious activity. Under appropriate conditions, it can participate in the hydration reaction of cement. However, due to its high calcium and sulfur characteristics, CFB-BA is difficult to use directly as a cement raw material and requires modification or mixing with other materials to enhance its utilization value .
Compared to fly ash from pulverized coal boilers (PCCFA), CFB ash and slag have significant differences: CFB desulfurization ash consists of non-molten particles without shape, with coarse ash quantity greater than fine ash quantity, higher calcium content, and relatively large loss on ignition. PCCFA fly ash is spherical, containing porous carbon particles and glassy bodies, with fine ash quantity greater than coarse ash quantity, lower calcium content, and relatively small loss on ignition. The high sulfur and high calcium characteristics of CFB ash are due to the addition of large amounts of limestone or dolomite as desulfurizers during coal combustion in CFB technology, which introduces CaO. This CaO combines with SO₃ in the coal to form gypsum, achieving sulfur fixation. The loss on ignition is large because the lower furnace temperature (850–900°C) results in more unburned carbon .
2 Low-Carbon Utilization of CFB Ash and Slag in Building Materials
2.1 Supplementary Cementitious Materials
CFB ash and slag, as supplementary cementitious materials, can partially replace cement or slag, reducing cement usage, lowering production costs, and decreasing CO₂ emissions. Research shows that adding CFB ash and slag to high-volume slag cement mortar can effectively activate slag hydration, improve initial strength, refine pore structure, reduce critical pore size and total porosity, enhance compactness, and lower water permeability. For instance, mixed cement samples with 10% CFB ash and slag and 20% slag showed compressive strength increases of 7.3% and 5.0% at 7 and 28 days, respectively, compared to pure cement paste. Additionally, CFB ash and slag help improve the dry shrinkage resistance of cement-based materials, inhibit crack formation, and enhance durability, including freeze-thaw resistance, sulfate erosion resistance, and carbonation resistance. Hydration products like AFt and C-S-H gel fill pores, reduce porosity, optimize microstructure, and improve overall performance .
Physical activation, such as fine grinding, can disrupt the core-shell structure of f-CaO and type II CaSO₄ in CFB ash and slag, increase specific surface area, and significantly enhance pozzolanic activity. This reduces the heat release in the initial hydration stage of cement, promotes early AFt formation, improves microstructure, and allows cement with 50% ultra-fine CFB ash and slag to achieve later strength comparable to pure cement. Proper grinding can improve the application value of CFB ash and slag as cement admixtures, achieving effective resource utilization. It also prolongs the induction period of hydration in cement-CFB ash composite materials, reduces cumulative heat release, and increases the amount of hydration products like C-S-H gel and AFt .
2.2 Concrete
Given their unique physical and chemical properties, CFB ash and slag show great potential as concrete admixtures. Studies using low-sulfur-calcium and high-sulfur-calcium CFBFA as raw materials found that low-sulfur-calcium CFBFA can improve the compressive strength of foam concrete, while high-sulfur-calcium CFBFA shows an initial increase followed by a decrease. AFt and C-S-H gel in CFBFA fill pores, increase the proportion of small pores, reduce porosity, enhance compressive strength, and decrease water absorption. When CFBFA replaces PCCFA in autoclaved aerated concrete (AAC), the delayed formation characteristics of AFt are observed. Although the hydration products of AAC with PCCFA and CFBFA are similar, differences in composition lead to distinct performance; for example, CFBFA's higher standard consistency water demand and carbon content make AAC prone to water saturation, reducing frost resistance. However, with high CFBFA content (50%), AAC can still exhibit good mechanical and durability properties, meeting standard requirements and passing toxicity leaching tests .
CFB ash and slag concrete, prepared using crushed stone as coarse aggregate, CFB-BA as fine aggregate, and CFBFA as a mineral admixture, shows better chloride ion penetration resistance compared to machine-made sand concrete. The high activity of CFB ash and slag results in a denser cementitious material system with lower porosity, reducing chloride ion diffusion channels. Additionally, CFB ash and slag exhibit strong adsorption characteristics, adsorbing Cl⁻, lowering internal Cl⁻ concentration, reducing electric flux, and improving chloride ion penetration resistance. Thus, CFB ash and slag provide a new pathway for efficient solid waste utilization and can enhance various concrete properties .
2.3 Geopolymers
The micro-expansion characteristics of CFB ash and slag can cause expansion in cement-based materials, potentially leading to structural damage if excessive. To mitigate this, chemical activation is used to disrupt the dissolution balance of II-CaSO₄ and f-CaO in CFB ash and slag, accelerating hydration and rapidly releasing harmful expansion. Geopolymers, composed of aluminosilicate minerals, form three-dimensional network structures under the action of alkaline solutions (e.g., NaOH, KOH) and alkaline activators like sodium silicate. Compared to ordinary Portland cement, geopolymers exhibit higher mechanical strength, excellent chemical durability, inherent fire and heat resistance, low thermal conductivity, and low shrinkage .
Studies on geopolymers prepared from CFBFA with different calcium contents show that precisely controlling Na/Al and Si/Al ratios can effectively improve performance. Appropriate calcium content aids the formation of C-A-S-H gel, but excessive calcium may adversely affect later strength due to AFt decomposition. Using sodium silicate and NaOH as activators, factors like water-binder ratio, alkali activator content, sodium silicate modulus, and curing system affect the strength of CFB ash and slag geopolymer mortar. After alkali activation, the quartz crystal structure is destroyed, lime and anhydrite participate in reactions, forming large amounts of gel-like substances and needle-bar-shaped C-A-S-H crystals. Besides chemical activation, physical activation like fine grinding can enhance the activity of CFB ash and slag, improving the mechanical and durability properties of resulting geopolymers. However, excessive grinding may lead to overly small particles causing electrostatic agglomeration, reducing specific surface area. Composite modification with physical grinding and chemical activation can significantly improve the fluidity and mechanical properties of CFB ash and slag-based materials, reducing expansion rates .
It is noted that low-calcium and low-sulfur CFB ash are typically used to prepare geopolymers, or other materials are added to dilute the total calcium in the system, usually keeping the system CaO below 3.5%. This is because as the calcium content increases, geopolymerization reactions decrease, and hydration reactions increase .
2.4 Road Base Materials
CFB ash and slag demonstrate potential in road base materials due to their physical and chemical properties, soil improvement capabilities, micro-expansion characteristics, and as substitutes for traditional materials, offering environmental and economic benefits. Research indicates that CFB ash and slag, used as stabilizers, can effectively treat local sandy soil and pavement base materials. Their high calcium and sulfur content and unique micro-expansion properties provide additional value to pavement structures, such as enhancing soil bearing capacity, reducing crack formation, and improving overall performance .
3 Low-Carbon Utilization of CFB Ash and Slag in Other New Materials
3.1 High-Value Applications
CFB ash and slag show potential for high-value utilization in areas such as waste gas absorbents, aerogels, and zeolites. An improved CFBFA recycling method involves mixing CFBFA with water or aqueous solutions containing additives to form slurry, which is reinjected into the CFB combustion chamber. The high temperature causes flash hydration, dehydration, and agglomeration into larger particles. These agglomerates have longer residence times, and the hydrated free lime becomes more active, improving calcium utilization, reducing carbon content in CFBFA, and lowering the calcium-sulfur ratio and operating costs. When using limestone and water as hydrating agents, the formed CFBFA agglomerates exhibit high SO₂ absorption efficiency .
Using CFBFA and natural soda ore as raw materials, combined with ambient pressure drying technology, SiO₂ aerogels with good thermal stability, high hydrophobicity, and high specific surface area can be prepared. The main component of CFBFA is amorphous metakaolin with higher dissolution activity, significantly reducing energy consumption compared to traditional fly ash. By mixing CFBFA with sodium carbonate, sintering at 750°C, and then using sulfuric acid leaching to obtain silicon-aluminum sol, silicon-aluminum composite aerogels can be prepared via sol-gel method combined with trimethylchlorosilane modification and ambient pressure drying. Simultaneously, the acid leaching residue dried under specific conditions can yield hemihydrate gypsum meeting building gypsum requirements .
CFBFA, rich in silicon and aluminum, is an ideal raw material for synthesizing zeolites. It can be converted into high-value zeolite materials under different conditions via hydrothermal synthesis, alkali fusion activation, etc. Without high-temperature roasting pretreatment, CFBFA can be directly mixed with alkali solution to synthesize P-type or X-type zeolite molecular sieves under specific conditions. CFBFA rich in silicon tends to form P-type sieves, while that rich in aluminum favors X-type sieves. High-purity Na-X and Na-P type zeolite molecular sieves can be synthesized directly from acid leaching residues. Na-X zeolite requires longer crystallization time and relatively lower temperatures, while Na-P zeolite can fully crystallize in shorter time at higher temperatures. Zeolite-like materials synthesized by low-temperature synthesis have better performance, larger specific surface area, and higher thermal stability than those synthesized by fusion .
4 Comprehensive Comparison of CFB Ash and Slag in Different Application Scenarios
4.1 Energy Consumption
In different applications, the use of CFB ash and slag impacts energy consumption differently. For example, in cement-based materials, replacing part of cement reduces thermal energy consumption and CO₂ emissions during cement production. In contrast, preparing geopolymers requires additional alkaline activators, potentially increasing initial energy for chemical reactions. However, in the long term, due to excellent durability and low maintenance needs, the overall energy consumption of geopolymers made from CFB ash and slag might be lower .
4.2 Economic Costs
Economic cost is a crucial factor determining the application potential of CFB ash and slag. Costs from raw material procurement to processing and final product formation vary significantly across stages, affecting market competitiveness. Research shows that incorporating an appropriate amount of CFB ash and slag in concrete and road base materials saves raw material costs and improves product quality. Although the synthesis of high-value products like aerogels or zeolites requires substantial upfront investment, the high added value brought by the unique properties of CFB ash and slag offers considerable economic benefits .
4.3 Commercial Potential
Considering support from environmental policies and growing societal demand for green building materials, CFB ash and slag as new building materials show broad commercial development prospects. Especially under the current "Dual Carbon" goals, developing low-carbon, efficient solid waste resource utilization technologies has become an industry trend, providing vast space for diversified applications of CFB ash and slag. In the short term, applying CFB ash and slag to ordinary building materials like cement-based materials and road bases is more practical and economically feasible. Transforming them into high-value functional materials like geopolymers, aerogels, or zeolites is expected to achieve higher market returns and technological breakthroughs in the future .
5 Conclusion and Outlook
This review summarizes the compositional properties, physicochemical characteristics, and application research progress of CFB ash and slag in building materials, aerogels, zeolites, and other fields. The following main conclusions are drawn:
Composition and Characteristics: Due to unique physicochemical properties (e.g., high CaO, high SO₃, micro-expansion), CFB ash and slag show great potential as supplementary cementitious materials. Their application in cement-based materials can significantly improve early strength and durability.
Applications in Building Materials: CFB ash and slag can partially replace cement or aggregate, reducing CO₂ emissions from cement production and offering significant low-carbon benefits. They also notably improve concrete's resistance to chloride ion penetration and freeze-thaw cycles.
Potential for High-Value Utilization: The richness in silicon and aluminum makes CFB ash and slag promising for preparing high-value materials like aerogels and zeolites. These applications facilitate effective resource reuse, reduce environmental impact, and promote green sustainable development.
Challenges in Resource Utilization: Current applications still face technical bottlenecks, including expansion issues from high sulfur/calcium and cost optimization in processing.
Future research should focus on:
Development of Multifunctional Materials: Applications could extend beyond traditional building materials to include high-performance adsorbents for water/air treatment and self-healing concrete using their micro-expansion properties.
Role in Low-Carbon Circular Economy: Developing efficient, low-cost modification technologies can enhance the application value of CFB ash and slag, enabling effective reuse and reducing environmental burden.
Incentive Mechanisms: Governments should introduce policies (subsidies, tax incentives, green credit) to encourage enterprises and promote technological innovation.
Standardization: Establishing sound quality standards and technical specifications for different application scenarios is necessary to ensure safe and reliable use.
Life Cycle Management: Implementing life cycle assessment methodologies from raw material acquisition to final disposal aims for optimal resource allocation.
Information Platform: Building a dedicated information service platform for data collection (production, composition, applications) can support decision-making and serve as a supply-demand bridge.
Through deepened research and promoted industrial application, the resource utilization of CFB ash and slag is expected to provide crucial support for achieving zero solid waste discharge and developing a low-carbon circular economy.