A comprehensive review on the applications of coal fly ash

TL;DR: In this paper, the authors present a review of coal fly ash at the global level, focusing on its current and potential applications, including use in the soil amelioration, construction industry, ceramic industry, catalysis, depth separation, zeolite synthesis, etc.

About: This article is published in Earth-Science Reviews.The article was published on 2015-02-01 and is currently open access. It has received 1167 citations till now. The article focuses on the topics: Fly ash & Coal combustion products.

Summary

1. Introduction

• Coal fly ash, a by-product of coal combustion in thermal power plants, is one of the most complex and abundant of anthropogenic materials.
• If not properly disposed of, it can cause water and soil pollution, disrupt ecological cycles and pose environmental hazards.
• Other uses include road base construction, soil amendment, zeolite synthesis, and use as a filler in polymers (Cho et al., 2005).
• These applications are not sufficient for the complete utilization of the fly ash generated.
• This paper will add to the further understanding of current utilizations of coal fly ash and identifying the promising applications.

2.1. Global generation

• Coal fly ash accounts for 5–20wt.% of feed coal and is typically found in the form of coarse bottom ash and fine fly ash, which represent 5–15 and 85–95 wt.% of the total ash generated, respectively.
• China's industrial growth depends on coal, which contributed to 68.49% of the total primary energy consumption in 2012 (see Fig. 2).
• Taking these data into account, a more up-to-date estimate of annual worldwide generation of fly ash is approximately 750 million tonnes (Blissett and Rowson, 2012; Izquierdo and Querol, 2012).
• The American Coal Ash Association (ACAA) surveys and published data on the generation and utilization of coal combustion products (CCPs) including fly ash, bottom ash, boiler slag, flue gas desulfurization (FGD) gypsum, FGD material wet/dry scrubbers, FGD other and fluidized bed combustion (FBC) ash.

2.2. Characterizations

• Understanding the physical, chemical and mineralogical properties of coal fly ash is important, as these properties influence its subsequent use and disposal.
• The specific properties depend on the type of coal used, the combustion conditions, and the collector setup, among other factors.
• The pH value of the ash–water system depends mainly on the Ca/S molar ratio in ash, although other minor alkalis or alkaline earth cations may also contribute to the balance (Ward et al., 2009; Izquierdoa and Querol, 2012).
• Micromorphology observation reveals that the fly ash particles are predominantly spherical in shape and consist of solid spheres, cenospheres, irregular-shaped debris and porous unburnt carbon (see Fig. 4).
• Commonly, elements such as Cr, Pb, Ni, Ba, Sr, V and Zn are present in significant quantities.

2.3. Hazards

• The environmental impact of coal fly ash has been fully recognized.
• Irregular accumulation and inappropriate disposal of fly ash will lead to its disposal over vast areas of land, with resultant degradation of the soil and danger to both human health and the environment.
• Neupane and Donahoe (2013) used batch and column leaching tests to evaluate the leachability of several elements (As, Co, Cr, Ni, Sb, Se, Ti, V, and Zn).
• Mathur et al. (2008) investigated the Rn exhalation rates in fly ash samples in India and observed that the activity concentration of radionuclideswas enhanced after coal combustion.
• From the various reports, it can be concluded that, drawing direct comparisons between results in various reports is challenging, because of the varying methodologies, sample sizes, material properties, and experimental goals.

3. Comprehensive applications

• Recycling coalfly ash can be a good alternative to disposal, and could achieve significant economic and environmental benefits as well.
• The global average utilization rate of fly ash is estimated to be nearly 25% (Bhattacharjee and Kandpal, 2002; Wang, 2008).
• Current utilization rates have been estimated at 50% for the US, over 90% for the EU and 60% for India.
• For China, this rate has been increasing annually but has remained around 67% in recent years.
• There is a contradiction in the reported utilization rate in China; Greenpeace reported that the practical utilization rate is only 30% (Greenpeace, 2010).

3.1. Soil amelioration

• They are not economic and environmentally friendly, and take a longer time to substantially improve the soil structure and physical properties.
• Coal fly ash, being mostly alkaline (depending on the coal source and operating condition of the plant), can be used for buffering the soil pH.
• Reductions in plant growth and yield were probably due to interactive effects of salinity and other elemental toxicity.
• Co-application of fly ash with these materials has more advantages: enhancing nutrient availability, decreasing bioavailability of toxic metals, buffering soil pH, enhancing soil organic matter content, indirectly stimulating microbial activity, overall improving soil health and increasing crop yield (see Fig. 6).
• Among the various amendments, the farm manure and sewage sludge are found to bemore effective.

3.2. Construction industry

• High calcium or Class C fly ash has considerable cementitious properties in addition to pozzolanic properties (ASTM, 2008), whereas Class F fly ash has mainly pozzolanic properties.
• Partial cement replacement by Class F fly ash reduces the heat of hydration and thus the risk of cracking in concrete in its early stage (Sarker and McKenzie, 2009).
• McCarthy and Dhir (2005) examined the use of high levels of low-lime fly ash as a cement component in concrete.
• It has been shown that fly-ash-based geopolymer concrete has similar strength and durability properties to those of traditional cement concrete.
• This study indicated that fly ash possessed low shrinkage and hence did not crack.

3.3. Ceramic industry

• Coal fly ash contains appreciable amounts of SiO2, Al2O3, CaO and Fe2O3, among other oxides.
• Due to their low or negative coefficient of thermal expansion (CTE), glass–ceramic materials based on Li2O–Al2O3–SiO2 ternary system have important industrial applications.
• He et al. (2005)preparedα-cordierite using coalfly ash, alumina and magnesium carbonate.
• In addition, Erol et al. (2008b) prepared glass, glass–ceramic and ceramic materials from fly ash with no additives.
• Jing et al. (2012) prepared ceramic granules using fly ash, clay and diatomite, and employed them in trickling filters.

3.4. Catalysis

• Metal andmetal oxides arewidely used as catalysts in various industrial applications.
• Fly ash mainly consists of various metal oxides with higher content of iron oxides and possesses higher thermal stability.
• Chakraborty et al. (2010) prepared a fly-ash-supported CaO catalyst for the transesterification of soybean oil.
• It could degrade phenol with the presence of oxone.
• Wang (2008) reported that fly ash can be used as an effective catalyst for reactions in gas-, liquid- and solidphases, such as gas phase oxidation of volatile organic compounds, aqueous phase oxidation of organics, solid plastic pyrolysis, and solvent free organic synthesis.

3.5. Environmental protection

• Most fly ash is alkaline, and its surface is negatively charged at high pH.
• As early as 1984, fly ash was considered as a potential adsorbent to remove Cu2+ from industrial wastewaters (Panday et al., 1985).
• Themaximumadsorption capacities of fly ash tometribuzin, metolachlor and atrazinewere found to be 0.56, 1.0 and 3.33 mg/g, respectively, by the Langmuir equation.
• Adsorption is considered to be one of the promising technologies for capturing CO2, SO2 and NOx from flue gases.
• A good removal capacity, which could be related to their textural properties, was shown by some samples.

3.6. Depth separation

• The recovery of cenospheres, unburnt carbon and magnetic spheres is one of the coal fly ash beneficiations, providing economic as well as environmental benefits.
• Air classification is one of the alternative techniques and a basic theoretical study comparing the efficiencies of dry separation with that of wet processes has been conducted (Hirajima et al., 2010).
• Recently, some researchers have studied the advantages of using cenospheres in composite materials by mixing it with metals and polymers to produce lightweight materials with higher strengths.
• Yang (2000) used froth flotation to separate the carbonaceous portion from low-carbon fly ash.
• The processing scheme was capable of producing magnetic material with an iron content of almost 75%.

3.7. Zeolite synthesis

• Synthesis of zeolites is gaining notice as one of the effective uses for coalfly ash, possibly due to the similar compositions of fly ash and some volcanic material, the precursor of natural zeolites.
• Zeolite synthesis is conventionally developed by hydrothermal crystallization under alkaline conditions (Querol et al., 2002).
• Murayama et al. (2002) conducted a series of hydrothermal experiments for fly ash in various alkali solutions (NaOH, KOH, Na2CO3, NaOH/KOH and Na2CO3/KOH) to clarify the mechanism of zeolite synthesis.
• When Na+ and K+ coexisted in the alkali solution, the crystallization rate decreased with an increase in K+ concentration.
• Li et al. (2014) prepared merlinoite and used it as a slow release Kfertilizer for plant growth.

3.8. Valuable metal recovery

• Besides certain heavy elements, coal fly ash also contains valuable metals, including germanium (Ge), gallium (Ga), vanadium (V), titanium (Ti) and aluminum (Al), which are extractable if an acceptable process can be developed.
• Fang and Gesser (1996) studied the Ga recovery from coal fly ash, which included acid leaching, impurity removal, foam extraction of Ga and further purification.
• Gawas extracted selectively from the base solution with LIX 54; the resulting stripped solution contained 83% of the Ga present in the leaching liquor.
• Since the 1980s, more researches have been conducted and new recovery technologies developed as well.
• It has been roughly calculated that 7– 10 tonnes of calcium silicates will be generated in recovering 1 tonne of alumina product during the lime sinter process.

4.1. China

• According to the annual report of China comprehensive resource utilization (2012) released by the National Development and Reform Commission (NDRC) of China, the coal fly ash generation and utilization were 540 and 367 million tonnes in 2011 respectively.
• Under a series of encouraged and preferential policies issued by the state and local government, the recovery of alumina is being aggressively undertaken in Inner Mongolia and Shanxi province, China. (2) The utilization of fly ash has been extended from conventional construction industry to depth separation and valuable metal recovery.
• (3) The total utilization has been increased although the utilization rate has remained 68% in recent years.
• (4) The fly ash recycling companies could offer synergetic advantages by producing more materials or having different types of businesses.

4.2. India

• Nearly 40% of the ash is still unused.
• Especially there is a wide scope for brick and title, roads and embankments, mine filling, and soil amendment as the present utilization in these sector are relatively little.

5. Conclusions

• The generation of coal fly ash is anticipated to increase for many more years, as a result of the world's increasing reliance on coal-fired power generation.
• The knowledge of the various ways to use fly ash, such as in soil amelioration, the construction industry, the ceramic industry and zeolite synthesis, is essential for better management of fly ash and the reduction of environmental pollution.
• Depth separation, including the recovery of cenospheres, unburnt carbon and magnetic spheres, can be beneficial and facilitate the utilization of the mineral content for cement production and zeolite synthesis.
• The treatment of residues should be also taken into account.
• What are the key barriers to adoption, and what changes should be done in strategy and action to overcome the identified barriers.

Figures

Fig. 6. The specific benefits of co-application fly ash with other amendments.

Fig. 9. Alumina recovery technologies from coal fly ash.

Fig. 1. Distribution of coal consumption worldwide in 2012.

Fig. 2. Energy consumption by source worldwide in 2012.

Table 1 The comparison of different applications of coal fly ash.

Fig. 3.The generation and utilization of coalfly ashworldwide (National Development and Reform Commission of the People's Republic of China, 2012).

Fig. 10.Modes of fly ash utilization in China, India, US and EU.

Fig. 8. SEM images of zeolites synthesized from coalfly ash (Belardi et al., 1998; Chareonpanich e Yao et al., 2013).

Fig. 4. SEM images of coal fly ash (Blissett and Rowson, 2011, 2012).

Frequently asked questions

Q1. What are the contributions in "A comprehensive review on the applications of coal fly ash" ?

In this paper, the authors investigated the fly ash generation in major countries ( China, India and US ), its physicochemical properties and hazards, and then focused on its current and potential applications, including use in the soil amelioration, construction industry, ceramic industry, zeolite synthesis, catalysis, depth separation, etc. 

Q2. What are the future works in "A comprehensive review on the applications of coal fly ash" ?

Further studies are needed to turn this research into commercial reality. The following recommendations can be made based on this research: ( 1 ) Investigate the distribution of coal rich in valuable elements and its flow. Economies of scale can be realized in depth separation and valuable metal recovery. The authors can use combinations of processes, such as predesilication of fly ash and a lime–soda sinter process.