Further Investigation into the Formation Mechanism of Ash Cenospheres from an Australian Coal-Fired Power Station

Abstract

This study continues the investigation into the ash cenosphere formation mechanism using a series of narrow size-fractioned ash cenosphere samples separated from the fly ash of an Australian coal-fired power station. The gas products locked inside various ash cenosphere size fractions are dominantly CO2 and some N2. With increasing ash cenosphere particle size from 63–75 μm to 150–250 μm, the average gas pressure decreases from 0.227 atm (at normal temperature and pressure, NTP) to 0.172 atm while the amount of CO2 and N2 locked in ash cenospheres increases significantly. The SiO2/Al2O3 ratio decreases with increasing ash cenosphere size, accompanied with an increase in the sum of TiO2 and Fe2O3 contents. Thermomechanical analysis further shows that ash cenospheres of different size fractions cannot achieve full melting at 1600 °C, suggesting that the formation of these ash cenospheres requires higher temperatures. Further analysis based on ash chemistry of individual cenospheres suggests that the optimum particle temperature for cenosphere formation is ∼1640–1850 °C. The growth of cenosphere precursors is governed by a wide range of viscosity of molten cenosphere precursors together with the force of surface tension, which is inversely proportional to the viscosity of molten droplets, producing ash cenospheres with various wall thicknesses. The data suggest that, apart from Fe2O3, TiO2 may play an important role in the formation of ash cenospheres.

Dissolution mechanism of fly ash to quantify the reactive aluminosilicates in geopolymerisation

“…Figure 5a shows the as-received particles of FA. It can be seen that the particles are round since the most stable shape (spherical) is obtained as result of equilibrium between surface tension forces on molten droplets and the intrinsic pressure of the internal hot gas during coal combustion (Li et al, 2013;Ranjbar and Kuenzel, 2017a). Figure 5b shows the undissolved residue of FA particles which were exposed to 8 M NaOH for 2 h at 25 °C.…”

Separation Process and Microstructure-Chemical Composition Relationship of Cenospheres from Lignite Fly Ash Produced from Coal-Fired Power Plant in Thailand

“…) is probably due to a phenomenon observed in many studies; the SiO 2 and Al 2 O 3 contents reduced with increasing the particle size, while the extent of Al 2 O 3 content reduction was measured in a very low amount with respect to the SiO 2 content [13,34,50] were approximately in the close range (2.20-2.23) when using the fly ash/medium ratios from 1:2.5 to 1:10 [28]. This clearly confirms that cenospheres originated from the same pulverized coal combustion and the combustion process condition have the same major chemical composition.…”

“…is probably due to a phenomenon observed in many studies; the SiO2 and Al2O3 contents reduced with increasing the particle size, while the extent of Al2O3 content reduction was measured in a very low amount with respect to the SiO2 content [13,34,50]. The small particles (<45-250 µm, , , ) having lower SiO2/Al2O3 ratio (~2.1-2.2) have a single-ring shell structure, whereas the larger cenospheres particles (>250 µm, ) having higher SiO2/Al2O3 ratio (~2.3) have a porous wall shell structure, more likely in a network shell formed structure.…”

Study on the Compressive Strength and Water Absorption Characteristics of Mortar Blocks with Cenosphere as Partial Replacement for Cement

“…The chemical properties of cenosphere help in the pozzolanic reaction and this binder property increases with the decrease in particle size and high temperature [4,5]. Many research works have been done using cenosphere as partial substitute for cement, sand, filler material and concluded that cenosphere as filler material can be used in lightweight cement composites application and also have good thermal and acoustical properties [6]. Baduge et al [7] used three types of cenosphere and reported that cenosphere is a sustainable alternative for lime binder and cement binder.…”