FLY Ash Cenosphere Waste Formation in Coal Fired Power Plants and Its Applicationas A Structural Material- A Review

FLY Ash Cenosphere Waste Formation in Coal Fired Power Plants and Its Applicationas A Structural Material- A Review

Joseph. K. V, Finjin Francis, Joyson Chacko, P.Das And G. Hebbar Dept of Mechanical Engineering, Christ University, Kombalgadu, Bangalore

ABSTRACT

Ash cenosphereis part of fly ash exhibiting spherical shape. It isproduced frompulverized coal combustionin coal-fired power stations. The term cenosphere was originated from Greek kenos (hollow) and sphaira (sphere), firstly used by Sinnatts research group, to describe hollow char particles formed during coal pyrolysis. Ash cenospheres refer to ash particles that are hollow, of a low bulk density (lowerthan that of water),andis typically collected from the surface of ash lagoons.India produces a considerable amount of fly ash from the coal-fired power generation industry and is one of the countries with the lowest ash utilization.Only 10% of fly ash produced is used, mainly in the building and construction industry.The large quantities of fly ash impose significant pressure on environment and waste management. The impact ,however, could be minimized by effective ash utilization. A range of potential industry applications of cenosphere is reviewed in this article

(key words- cenosphere ,coal, pulverized, construction, pyrolysis )

1 Introduction

Ash cenospheres are hollow lightweight micro-spheres(see Fig1(a)) as part of fly ash produced during pulverized coal combustion for power generation.36. They have superior properties including a light weight, good packing factor, enhanced insulation, improved flow characteristics, reduced shrinkage, less water absorption, excellent mechanical strength, chemical inertness, good thermal resistance, and good electrical properties.Ash cenosphere utilization offer significant manufacturing advantages, product improvement, and cost reduction in a wide range of industrial processes. Ash cenospheres can be used to produce various lightweight construction products, including lightweight cements, 7 aggregates in lightweight concrete, 810 and porous glass crystalline molded blocks for the removal and solidification of liquid and other hazardous wastes.11They have also found extensive applications in manufacturing lightweight composites, including polyurethane composites,12 polyester composites,13 functionally gradient materials,14 poly(vinyl chloride) composites, nylon composites,17 syntactic polymer foam,1821 metal-matrix composites,2224 glass fiber/epoxy resin composites,25 and conducting polymers.2628.

The properties and performance of the lightweight products utilizing ash cenospheres depend largely on ash cenosphere characteristics, particularly ash cenosphere physical structure such as diameter, particle size distribution, wall thickness, and shape.8,1319,21,29 For example, when used as fillers, ash ceno-spheres of thicker walls are favored to increase the mechanical strength of the finished products.8,21 On the other hand, the shape of ash cenospheres may also be an important factor16 In comparison to other shapes, spherical ashcenospheres have the lowest surface area to volume ratio and, thus, require lessbinder, and water to wet the surface. Besides, spherical particles offer the advantages of reduced shrinkage as well as improved workability and ease of use.19,29 Apart from these properties, the particle size distribution of ash cenospheres affects the mechanical properties of the manufactured products.13,14,18 Therefore, a good understanding of ash cenosphere characteristics and its formation mechanisms is essential for development of effective strategies to enhance and optimize ash cenosphere production in power plants, without affecting the main business of electricity generation. In practice, floaters, that is, ash particles that have a bulk density less than that of water and therefore float on the top of ash ponds, are collected as ash cenospheres.3,34,35 In other words, regardless of whether the particles are of spherical shapes or nonspherical, of a network structure, inorganic particles which have apparent density less than that of water are all considered to be ash cenosphere products. Therefore, it becomes essential to carry out a systematic study on the structure of the collected ash cenospheres as the quality of the products utilizing these collected ash cenospheres are largely determined by the structure of these ash cenospheres

Fig1(a)- Spherical particle shape formation in fly ash cenospheres

Fig 1(b) – Xray Diffraction analysis of Fly ash cenospheres

1. Sample Collections and Preparations of Fly ash samples

   Ash cenosphere samples were collected from five power stations, In practical applications ash cenospheres are harvested from the surface of ash lagoons and have a density less than that of water. Therefore, usually , a sinkfloat method using water as the medium is employed to separate ash cenospheres from the bulk fly ash samples.The float samples are then heated to 830°C and held at that temperature for1hr inafurnace, with air continuously flowing through the furnace to remove unburnt carbon. The yield of the float samples after density separation and heat treatment is taken as the ash cenosphere yield. Further sample preparation may also be carried out to sieve ash cenospheres from different liquid mediaµm flotation and subsequently with different size fractions.(2545µm, 4563µm, 6375µm,7590µm, 90 106µm, 106125µm, 125150µm, 150250µm, and>250µm) for subsequent characterization

   1. Sample Characterization

2.1.1 Properties of Ash Cenospheres

The propertiesof ash cenospheresare generallycharacterized into four aspects:physical(andmorphological)structure, mineralogy, chemistry, and thermal properties.

1. Physical(and Morphological) Properties.

   While the real density ofash materials in ash cenospheres is> 2.0 g cm -3 , the apparent density of ash cenosphere is generally in the range of 0.4 to 0.8 g cm -3 and the bulk density varies from

   0.2 to 0.5 g cm -3(much lower than that of water).13, 17, 20, 21, 26, 30, 40-48 Therefore, ash

   cenospheres can be easily separated and collected from fly ash in ash lagoons as ash cenospheres have low apparent density and float on the lagoons' surface.20, 24, 25 Ash cenospheres may have particle sizes ranging from 5 to 500 µm , typically 20 200 µm 13, 17, 19-22,

   25, 30, 42, 43, 45, 46, 48-53 The particle surface isgenerally smooth, with uneven texture or having

   blisters. The shell thickness of ash cenospheres can vary from 1 to 20 µm . The apparent ratio of wall thickness to particle diameter is 3 16%,( calculated based on the estimation of bulk ash cenosphere samples).20, 21, 30, 45, 48, 51, 53, 54 However, the direct measurement of true diameter and shell thickness of ash cenosphereson a particle-by-particle basis via a novel techniquedeveloped recently show that such a ratiois limited between an upper bound of

   ~10.5% and a lower bound of ~2.5%,irrespective of ash cenosphere size.19

   The common perception is that ash cenospheres are generally hollow with thin shells so that the cross-section of an ash cenosphere has a ring structure.19, 20, 30, 43, 44, 48, 54 However, the recent study 19 has shown that the ash cenospheres separated from fly ash via density separation using water as a media consist of two different types of structures. One is spherical and of a single-ring structure(see Figure 2-1a)and the other is irregular and of a network structure(see Figure 2-1b)

2. Mineralogy.

   The mineralogy of ash cenosperes variesfrom sample to sample. It is reported that mullite and/or quartz are the main crystalline phases identifiable in the shells of ash cenospheres, based onXRD analysis.17, 41, 44, 46, 50, 55(See Fig-1(b))The minor or accessory mineral phases in the shells of ash cenospheres are plagioclase, K-feldspar,dolomite and some gypsum and Fe

   oxides.22 However, ash cenospheres from the combustion of solid fuels are generally amorphous aluminosilicate glass phases (70-90 wt%).

3. Chemistry.

   The chemical compositionsof ash cenospheres also vary fromsample to sample, depending on the power station where the sample was collected. Table 1 lists the chemical compositions of bulk ash cenospheres and fly ash reported in the literature.

   Table 1. Chemical Composition of Fly Ash Samples from Different Power Stations ( bulk fly ash (wt %))

   Chemical composition	A1	A2	A3	A4
   SiO2	54.00	65.10	68.60	62.60
   Fe2O3	10.50	2.00	1.30	0.54
   Al2O3	28.30	27.30	25.50	34.90
   TiO2	1.70	1.20	1.10	1.90
   P2O5	1.10	0.14	0.10	0.04
   CaO	1.70	0.37	0.22	0.03
   MgO	1.20	0.35	0.25	0.11
   Na2O	0.28	0.36	0.37	0.05
   K2O	0.73	3.20	2.50	0.19
   V2O5	0.03	0.03	0.04	0.03
   BaO	0.35	0.06	0.04	0.03
   SrO	0.26	0.04	0.03	0.03

   Table 2. Chemical Compositions of Ash Cenospheres from Different Power Stations cenospheres (wt %)

   CHEMICAL COMPOSITION	A1	A2	A3	A4	A5	A6
   SiO2	51.60	57.90	58.10	67.40	62.40	60.60
   Fe2O3	5.10	1.20	0.73	0.94	0.49	0.50
   Al2O3	36.20	34.80	36.70	26.20	34.60	36.80
   TiO2	1.50	1.10	0.85	1.08	1.31	1,43
   P2O5	0.84	0.10	0.04	0.01	0.03	0.04
   CaO	0.88	0.29	0.19	0.03	0.04	0.04

   MgO	0.87	0.88	0.22	0.01	0.02	0.04
   Na2O	0.23	0.31	0.23	1.30	0.30	0.01
   K2O	1.70	3.50	2.70	0.03	0.35	0.35
   SO3	<0.1	<0.1	<0.1	BDL	BDL	BDL
   BaO	0.25	0.23	–	–	–	–
   SrO	0.13	0.20	0.20	ND	ND	N.D

   The chemistry of ash cenospheres varies from power plant to power plant. Such variations in the chemistry of ash cenospheres are expected, depending on mineralogy of ash-forming species in the parent coals and combustion conditions.Secondly,Si and Al oxides are dominant (> 80%by weight) in the chemistry of ash cenospheres,with SiO2 /Al2O3 ratio varying from 1.3 to 2.5.Such chemistry is in accordance with the mineralogyof ash cenospheres which contain commonly aluminosilicate glass phases plus mullite and quartz as the main crystalline phases

   Lastly,there hasalso been debate on whether iron species are essential to ash cenosphere formation. Raask 20 suggested that a minimum ~5% ferric oxide (Fe2O3 ) is essential to ash cenosphere formation, as it provides the required source of oxygen for gas generationinside the molten silicate particlesfor expansion. This conclusion appears to be suggested by the majority of the published data in the literature. However, there were several studies 19,51 , in which it is clearlyshown that high yield of ash cenosphereswere produced when iron oxide wasvery low (<0.5%). It is therefore clear that during pulverized fuel combustion, there are different mechanisms responsible for ash cenosphere formation. Iron may be one of the important factors for enablinggas generation within the molten precursor droplets. Other possible sources of gas generation including decomposition of carbonates, dehydration and dehydroxylation of clay minerals, evaporation of pore water and fusion of silicates should be consideredas suggested in the literature. 20, 21, 25, 56

4. Thermal Properties.

Various methods were used to investigate the thermal behaviour of ash cenospheres from solid fuelcombustion, such as TGA/DTA analysis,22, 50 DTA/DTG analysis 58 and a Leitz heating microscope.20De crepitation of ash cenospheres was observed totakeplace at 230 330 °C, due to the release of water dissolved in the silicate glass 20, 22, 50 and/or some gases escaped from the hollow ash cenospheres,22 as suspected. Therefore, ash cenospheres should be finely ground for thermal analysis to avoid the samples loss due to the decrepitation.Additionally, an exothermic effect may also take place at 260 600 °Cand cause weight loss, 22, 58 due to the oxidation of residual unburnt carbon materials in the ash particles.This fact also suggests that ash cenospheres harvested from ash lagoons are necessary to be heat-treated under oxidizing atmosphereto remove possible unburntcarbonmaterials for further analyses.19, 52Thermal

analysis on several ash cenospheresamples by Raask 20 also indicated that the sintering of ash cenospheres commenced at 1200 °C a temperature higher than the sintering temperature of dense fly ash. For other ash cenosphere samples, Vassilev and co-authors 22 found that the sintering and softening started at1010 1330 °C while complete fusion was reached at 1315 1450 °C.

1. Key Factors Influencing the Formation of Ash Cenospheres

   The formation of ash cenospheres during pulverized coal combustion can be governed by a number of important factors, assummarized below.

   Coal Properties. These include three aspects: particle size, properties of coal organic matter and properties of coal inorganic matter. Coal particle size was reported to be an important factor in the formation 20, 22 and size 14, 37 of ash cenosphere. According to Raask,20 mineral particles smallerthan 10 µm are incapable of expanding to stable cenospheres. The speculated reason is that carbonaceous matter is completely separated from ash particles due to extensivemilling, or as result of excessively vigorous gas evolution and expansion inside small ash droplets, resulting intheir bursting to form dense particles The properties of coal organic matter are also believed to be critical in ash cenospheres formation because they have very different organic properties, resulting in substantial difference in their devolatilization behavior. This includes the swelling characteristics of coal particles,thus influencing the combustion condition hence ash cenospheres formation.The properties of mineral matter in coal, including the content and mineralogy of mineral matter play important roles in the formation of ash cenospheres. Ghosal 79 showed thatashcenosphere yield appears to be positively correlated with the total ash content of coal. In addition, it was also indicated by the studies that the mineral type present in coal, such as clay mineral, mica, zeolite, feldspar and anauxite, will favor the cenosphere formation.17, 22, 25, 55, 79, 83, 85, 86 Furthermore, Kruger and Toit 17 claimed that alkalis in the mineral matter can act as precursors for the generation of cenospheres at the prevailing combustion

   Characteristics of Ash Droplets -.Generally speaking, these include threefactors, i.e.carbon in ash droplets, iron content of ashdroplets , viscosity and surface tension of ash droplets.Carbon in Ash Droplets was considered to be one of the favorable conditions for cenosphere formation by Raask 20, 21as observed in Leitz heating microscope, it was found that a minimum of 1 % by weight of carbon should be incorporated or attached to fused silicate particles as one of the

   conditions for cenosphere formation along with Iron Contentin Ash Droplets. The gaseslocked inside the ash cenospheres werefound to beCO2 and N2with traces of O2and CO Based on ash cenosphere chemistry and the possible formation routes, Raask ruled out the possibility of dissociation of carbonates and combustion of carbonaceous matter being the origin of CO 2. Via experiments on gas evolution by heating variousminerals in the Leitz heating microscope, Raask

   20, 21originally suggested that the presence of iron can catalyze the reactions to produce CO2 and CO inside the fused ash particles. Rassks estimation based on his experiments results suggested that generally, a minimal ~5% wt of iron oxide is required for the formation of substantial quantities of ash cenospheres. More than 7 % iron oxide is required for the formation of a large amount of ash cenospheres from bituminous coal combustion, while for some sub-bituminous coal, 3-4% iron oxide is sufficient.While the requirement of high iron oxide contents for the formation of substantial ash cenospheres is generally supported by the

   published experimental data, there have been various exceptions. For example,Ngu and co- authors 19reported the high ash cenosphere yield from an Australian power station which burns coals with negligible iron contents in ash (~0.6 wt % on average). Else, some studies 18, 51 alsosuggested that high iron contents do not always lead to high ash cenospheres yield. As shown in Morrisonssurvey,51 there were even cases that have Fe2O3 contentsup to 25% but do

   not produce a largequantity of floaters. Similar phenomenon was also reported by Lauf 18 when

   the contentof Fe 2O3 in ash is >20%.

   Viscosity and Surface Tension of Ash Droplets- Viscosity and surface tension of molten ash droplets control the expansion of the precursor ash droplet and determine the survival of ash cenospheres.14, 17, 18, 20-22, 33, 54, 79The change rate of a hollow sphere radius is governed byviscous relaxation. Viscosity and surface tensionof the precursor ash droplets determine the final size of ash cenospheres. In order to sustainthe high intrinsic pressure of expanding gas bubble within the precursor ash droplet, the precursor droplets should have relatively high viscosity. The cenosphere precursor will collapse if the forceas results of surface tension is too low to

   maintain the equilibrium pressure between the internal and external gas. This was supported by the work of Lauf 18 who investigated ashcenospheres produced by melts with relatively high viscosities from ash samples ofseven different locations. Calculations alsosuggest that only those molten droplets that aresufficientlyviscous can survive long enough to be resolidified into ash cenospheres. Unfortunately, there are few published data in terms of the value of viscosity and surface tension of molten precursors responsible for ash cenosphere formation.

   Combustion Conditions -.These mainly include threekey parameters, i.e. furnace temperature, residence time and cooling rate. Furnace Temperatureisconsidered to be a critical factor in ash cenosphere formation during pulverised-coal combustion. 14,17, 19, 22, 37, 38, 87 Itaffects not only the rate of gas released from the ash particle, but also the formation of molten ash droplets(hence theviscosity and surface tensionof these ash droplets). There isstill debate on the effect of furnace temperature. For example, it was reported that the yield of ash cenospheres increased with combustion temperature and reached the highest at 1600 °C.87 However, Vassilev and co-authors 22 suggested that the suitable furnace temperature for ash cenosphere formation is 1315 1500 °C.Considering the viscosity and surface tension of fused

   silicate glass, as well as the rate of gas production, Raask 20 estimatedthatthe optimal temperature for ash cenosphere formation is 1400 °C.

   Residence Time.A proper residence time is required for the formation of ash cenospheres with different sizes, governed by the rate of gas evolution and viscous relaxation of the molten ash droplets. Sufficiently long residence time of ash particles in the boiler during combustion is required to enable the molten ash droplets formation and trapping an appropriate amount of expanding gas to form ash cenospheres. However, it should not be excessiveas further expansion of gas inside the meltswould lead to burstingbefore solidifying of the precursors to form ash cenospheres.

   Cooling Rate- During cooling stage, ash precursor droplets of cenospheres solidify and ash cenospheres survive eventually. Uponcooling, the molten ash droplets will contractdue to a reduction in gas pressure within the cenospheric particle. The viscosity and surface tension will change upon cooling. Therefore, the net balance between the internal pressure and the strength of the shell of molten droplets may lead to the breakage of the particles hence reduce ash enosphere formation.88 Rapid cooling of molten ash droplets is critical for ash cenospheres formation.22, 51 However, cooling should not be too rapid. It is logic to reason that dueto the thin wall thickness of cenosphere precursor, the high internal pressure will cause the rupture of the cenospheres.Another opinion based on calculation suggested that there is no significant contraction of cenospheres uponrapid cooling to about 1000 °C in boiler flue gas.20 After this rapid cooling in high-temperature flue gas, the pressure inside the frozen cenospheres will reduce steeply, causing the possible breakage of ash cenospheres, although there is no experimental evidence to support so

2. Separation of Cenospheres from Fly Ash and the Typical Yields

The most common method for separating ash cenospheres from fly ashisthe sink-float method that uses water as the medium for density separation because ash cenosphere have bulk density lowerthan that of water (1 g cm -3 ).18-20, 22, 24, 48, 54, 55, 57, 76-78 Brieflythe method involves dispersing the dry fly ash samples collected from the power plants in water (distilled water and/or tap water) at room temperature. After continuous stirring and settling for 24 hours, the float ash samples above water arecollected viadecantation followed by filtration. The float samples are then heated to 800-900 °C in a furnace with air continuously flowing through for 2hours in order to remove unburnt carbon. The yields of ash cenospheres using thi separation procedure vary strongly with thesources of fly ash, up to 35.6 % wt, with the most common values are 0.1 -5.0 % wt. 11, 19-22, 25, 26, 48, 49, 53, 57, 63, 77,78 It should be noted that, apart from using water as medium, other solvent liquids were also used for separating ash cenospheres from ash,such as ethanol and hexane,55, 71, 72 acetone containing 1% triethanol amine lauryl sulphate 46 and n- heptane.52.The other method is centrifugal separation that can be used to determine

the amount of ash cenospheres in fly ash. Various liquid medium may be considered, including water,50 distilled water and lithium metatungstate solutions of density 1.5 and 2.0 g cm-3 , and liquid of density 2.2 g cm -3 prepared by mixing carbon tetrachloride, dibromomethane and di- iodomethane in appropriate proportions.79, 80 However, it isobvious that the ash cenospheres separated with the last two liquids have density greater than that of water.Lastly, a combination of hydro-and aerodynamic separation can also beemployed to isolate the concentrates of magnetic microspheres and cenospheres.13.

3.0 Advantages and Applicationsof Ash Cenospheres

Ash cenospheres have unique properties such as being spherical, lightweight, heat resistant, enhanced insulation, good thermal stabilityandchemical inertnessetc.These materials have a wide varietyof applicationson the area of industry, agriculture,recreation and others . Ash cenospheres can be used in manufacturing value-added products such as lightweight materials

,Lightweight Construction Products. Lightweight construction products have many benefits including low density, excellent mechanical strength and reasonable cost. Ash cenospheres can be used as fine aggregate in the manufacture of lightweight concrete and as additive for producing lightweight cements.

Cenospheres can be used as aggregate to produce lightweight concrete,resulting in significantly-improved thermal conductivity of the lightweight concreteand significantly decreased weight of products. It was also suggested that ash cenosphere additives can improve the crystallization of hydration products. Theacoustic behavior of the product also does not deteriorate compared with the lightweight concrete manufactured with other aggregate.As additives of lightweight cement, ash cenospherealso leads to the changes in the structure of lightweight cement and raises hardening temperatureHowever, the addition of ash cenosphere does lead to some strength loss.Fortunately, such strength loss canbe recovered by improving the interfacial strength between the cenospheres and the cement.It should be noted that special consideration needs to be given to the excessive moisture uptake/loss by cenospheres (hence the water uptake/loss in cenosphere-concrete products) when ash cenospheres are used to replace fine sand to be the aggregates in concrete. The work of Barbare et al showed

that the equilibrium moisture content of cenospheres is 18 times higher than that of sand

indicating substantial water should be added prior to curing the concerete Ash Cenosphere Formation during Solid Fuels Combustion 18 times higher than that of sand, suggesting that substantially more water should be added prior to curing the concrete.

Composites.

Polymer Composites. As results of unique properties such as spherical shape,controlled particle size, porous structure, low density, good insulationand low cost,ash cenospheres are extensively introduced as lightweight fillers into various polymer composites.

For example, ash cenosphere particles coated with Cu could be utilised for manufacturing conducting polymers for EMI shielding applications.59 While the use of cenospheres in polypropylene and polyethylene is limited due to high-shear conditions in extrusion and injection moulding, new manufacturing techniques may be employed to address this aspect by controlling cenosphere particle size.24 Lightweight, high strength polyester materials canalso be produced by the incorporation of a controlled size, surface modified cenospheres into a polyester matrix.27 Comparison of the mechanical properties of both epoxy resin and high density polyethylene shows the similarity with the two fillers.17 It is known that significant advantage could be achieved by filling ashcenospheres into the high density polyethylene since the wider variety of coloursand more importantly, less pigment required.

Syntactic Foams.

Syntactic foams are made by mixing hollow cenosphere particlesin a matrix material, achieving various advantages including excellent buoyancy, low moisture absorption, high compressive strength, high energyabsorption during deformation, and high damage tolerance.51, 61-64 Such materialsare extensively applied in automotive industries,66 in marine and aerospace industries for core material in sandwich structured composites,65 and as underwater buoyant structures which can withstand high hydrostatic pressures of deep ocean. 63 It was found that the density and mechanical properties of syntactic foam can be affected by ash cenosphere sizes and specimen aspect ratio. 61 An increase in compressive strength and modulus can be achieved as the internal radius of cenospheres decreases.

Ceramic Foams.

Ceramic foam has advantages including lightweight, high compressive strengthand good resistance towards gas erosion. It can be easily shaped,making this material desirable for producing high performance products, includingAsh Cenosphere Formation during Solid Fuels Combustion 17 sensors, heat dissipaters, thermal insulators etc. 67Russak and co-authors 52demonstrated that a ceramic closed cell foam insulation material made from ash cenospheres could meet all the requirements for developing a fully reusable ceramicheat shield of space shuttle to protect the vehicle prime structure during the reentry maneuver.

Polyvinyl Chloride (PVC) and Ethylene Vinyl Acetate (EVA) Foams.

PVC foam is an insulation material with various characteristics of light, fire-proof, water-proof, heat-resistant, sound-insulating, non-flammable and non-toxic.48EVA foam has unique properties of lightweight, elasticity and insulation and is mainly used to make sports shoe bottoms. For manufacturingPVC foams, CaCO3 and talc are the two major constituents. For manufacturingEVA foams, a rather high proportion of CaCO3is added as the filler. Considering the vast economic benefit,ash cenospheres can beadded to replace the talc present in 5% of the mixture to make PVC foams. For EVA foams, ash cenospheres can be used to completely replace CaCO3 present in the mixture. It was found that such replacement resulted in excellent

strength and insulation properties compared to the original products48

Functionally Gradient Materials.

Functionally gradient materials arecomposites made of varied material compositions to optimize the performance so as to achieve specific applications. These materials are extensively used in applicationssuch as internal combustion engines, insulating and thermal coatings for turbine blades, military armor and other engineering applications 69 Parameswaran and Shukla68 developed a technique to prepare a model functionally gradient materialusing polyester resin and ash cenospheres. The results showed that increasing cenosphere volume fractioncan reduce the density of the functionally gradient material by 20%.

Metal-matrix Composites.

Ash cenospheres have desired characteristics to be used as fillers or reinforcement in aluminium, lead, magnesium and several other nonferrous metal matrices, e.g. spherical shape, low density, especially the high melting point 30, 31, 70 It was found that the aluminum composites with 50-60 vol.% of ash cenosphere exhibit sound microstructure except near the regions of contacting particles.70

Other Applications. Ash cenospheres can also find applications in making catalytic supports, absorbents and catalysts,22, 40, 71, 72as results of their spherical shape,chemical inertness, and thermal stability. For example, catalysts were developed based on ash cenospheres for deep methane oxidation.

Ash cenospheres are also used to manufacture special ceramics such as fire-, heat-, and sound- insulating materials, taking advantage of their heat-resistant, thermally-stable,and sound- insulation features.22Ash cenospheres can also be incorporated into mortars as binders to produce ceramicproducts such as man-made marbleand insulation/refractory ceramics.24

Another application of ash cenospheres is for manufacturinganti-corrosive paint andwear resistant coating due to their excellent mechanical properties and chemical inertness.73, 74 Ash cenosphereisalso agood option for producing fire retardant and insulation paintsas these hollow particles are non flammable and high temperature resistant.

The gas trapped within enclosed ash cenospheres becomesan excellent insulating medium, which makes cenospheres having excellent heat insulation properties.Therefore, cenospheres are ideal materials for heat-insulating refractory products.17, 24,43

Test has shown that the mixture of ash cenospheres and clays can be used to produce various refractories for metallurgical industry.

Pearlite microspheres with low density were found as the sensitizer of emulsion explosives after investigations of the physical-chemical and explosive properties of emulsion compositions contained pearlite microspheres 75

Apart from the various industrial applications, ash cenospheres can also be used to manufacture materials used in recreation and home. For example, ash cenospheres could be used for the manufacture of sportsequipments such as kayaks, surfing boards and other floaters due to their low density and excellent buoyancy properties. 46 Interestingly, ash cenospheres combined with other floating materials are helpful togrow a special instant turf.51 This type of turf isthinner and therefore lighter than natural turf, and has an intact root system. Such instant turf also has a long duration of keeping moist and thus saveswater and costs.

4. Conclusions

7.2.1 Possible Formation Mechanisms of Ash Cenospheres from a Coal-fired Power Station

Thermomechanical analysis shows that the fusion characteristics of ash cenospheres are dependent on the particle size of ash cenospheres. However, the various size-fractioned ash cenospheres do not melt at 1600 °C, suggesting that these ash cenospheres from coal-fired power station are impossible to be formed at temperatures <1600 °C. The chemistry of ash cenospheres of various size fractions is dominantly SiO2 Al2O3, TiO2 and Fe2O3 , which comprise

>99.3 wt% of the total ash chemical compositions. SiO2/Al2O3ratio decreases with increasing ash cenosphere size, accompanied with an increase in the sum of TiO2and Fe2O3contents (i.e.TiO2+Fe2O3) The gas products locked inside ash cenospheres of all size fractions aredominantly CO2and some N2, with trace amount of CO. The average gas pressure inside the individual ash cenosphere particles decreases from 0.227atm to 0.172atm (NTP) as particle size increases from 63-75 µm to 150-250µm The characteristics of molten ash cenosphere precursors derived from the ash chemistry of individual cenospheres indicate that the optimum particle temperature for cenosphere formation is from ~ 1640 to 1800 °C. Under these

conditions, molten ash droplets can be formed and trap a certain amount of gas generated within the ash droplets

Ash cenosphere yield and chemistry vary across the power stations. Ash partitioning occurred in the process of ash cenosphere formation during combustion. Iron is not necessarily an essential requirement for ash cenospheres formation. This study has also developed an ash cenosphere characterization technique which is capable of measuring true ash cenosphere diameter and wall thickness on a particle-to-particle basis. It has been demonstrated that the ash cenospheres can be made use of in various areas of application like composites ,foams functionally graded materials, and metal matrix composites.

ACKNOWLEDGEMENT

The authors wish to thank the management of Christ University for allowing publication of this article.

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