Ramasubramania Iyer

Power station fly ash — a review of value-added utilization outside of the construction industry

The disposal of fly ash from coal-fired power stations causes significant economic and environmental problems. A relatively small percentage of the material finds application as an ingredient in cement and other construction products, but the vast majority of material generated each year is held in ash dams or similar dumps. This unproductive use of land and the associated long-term financial burden of maintenance has led to realization that alternative uses for fly ash as a value-added product beyond incorporation in construction materials are needed. Utilization of fly ash in such areas as novel materials, waste management, recovery of metals and agriculture is reviewed in this article with the aim of looking at new areas that will expand the positive reuse of fly ash, thereby helping to reduce the environmental and economic impacts of disposal.

The surface chemistry of leaching coal fly ash

The utilization of coal fly ash in the construction and non-construction areas has seen a rapid growth in the last decade. As production outweighs the utilization of fly ash, its disposal as a dilute or dense slurry is still practiced in coal fired power stations. In this review the surface chemistry of leaching coal fly ash is presented to highlight the role of mass transfer in providing resistance and consequently delayed leaching of elements, when fly ash is disposed or used for value addition.

The effect of water absorption and the role of fines on the yield stress of dense fly ash slurries

Fly ashes obtained from power stations burning Curragh and Tarong coal were aircyclosized into fractions of different particle sizes. The grading of fractions were done according to the percentage of fines (−10 μm at d32 [sauter diameter]). The fraction with maximum fines was mixed with the original fly ash samples in different proportions and water absorption determined. The yield stress of a slurry consisting of different proportions of fraction with maximum fines and the original fly ash was measured. In this work the effect of water absorption and the role of fines on yield stress is presented.

Solid-Liquid Mass Transfer During Leaching of Calcium from Dilute Slurries of Flyash

Flyashes obtained by burning Curragh and Tarong coal were air elutriated into seven size fractions. Samples were then leached with deionised water in shaken and stirred vessels. All size fractions were leached in an orbital shaker at 1:25 ash water ratio. The results suggest that ions from a surface shell and from the particle interior make distinct contributions to the conductivity of the leachate. The fine fractions ( d 32 ≈ 2 microns) were also leached in an unbaffled turbine mixer at a minimum speed, so that the particles remained in suspension. The leaching of calcium as calcium hydroxide into the diffuse double layer which forms around the negatively charged particles was examined. A mass transfer model was developed, based on rate expressions derived for the transport of calcium as calcium hydroxide from the surface of the particle into the diffuse double layer, from the diffuse double layer into the boundary layer and from the boundary layer into the bulk solution. Changes in the thickness of the diffuse double layer were taken into consideration. The model assumes rapid leaching of ions from the surface shell and a uniform calcium concentration across the diffuse double layer. From the experimentally measured concentration data for calcium ions in the diffuse double layer and in the bulk solution, transport values for the model were deduced. It was found that mass transfer from the surface of the flyash into the diffuse double layer is the rate controlling step.

Surface area of flyashes

The flow properties of cement or flyash slurries are partly determined by the surface area of the particles. This area is generally measured with the Blaine apparatus which relies on frictional drag of air flowing through a packed bed. A comparison is presented here between the areas of two flyashes, each measured by the Blaine method and also by calculation from mean particle diameter as determined by laser diffraction. There appear to be advantages in using the laser diffraction method.

The distortion of the diffuse double layer and its effect on flow properties of dense flyash slurries

Flyashes obtained from power stations burning pulverised Curragh and Tarong coals were air classified to obtain a fine fraction (- 10 mu m). When dense aqueous slurries of these fines were subjected to rheological measurement in a rotating viscometer, the two ashes exhibited different packing propensities. In interpreting the experimental flow data, a new volumetric packing factor Phi was defined to include the volume of the diffuse double layer. When the Krieger-Dougherty equation was fitted to the data, the maximum packing fraction (Phi(m)) redesignated as the quasi-maximum packing fraction was found to vary at low shear rates. It is proposed that the differences in rheological behaviour and variation in Phi(m) is due to the surface chemistry of fine particles. The two flyashes produce leachates of different ionic strength and hence generate different thicknesses of diffuse double layer. Shearing at close packing produces distortion in the diffuse double layer, especially in dilute leachates with thick diffuse double layers. A particle packing model based on the unit cell is proposed to quantify the distortion of the diffuse double layer, and to explain its influence on Phi(m) and flow behaviour

The issue of reducing or removing phospholipids from total lipids of a microalgae and an oleaginous fungus for preparing biodiesel

ABSTRACT This study is divided into three parts and examines recommendations made in the literature to remove or reduce phospholipids, a class of polar lipids, from microalgal and fungal lipids, so that transesterification of the neutral lipids with methanol improves the yield of fatty acid methyl esters (FAME) (biodiesel). Literature on the role of phospholipids lowering the yield of FAME during transesterification of vegetable oils is reviewed in the first part of this study. The second part of this study scrutinizes two published papers on the preparation and yield of FAME by transesterification of total lipids present in microalgae of Chlorella pyrenoidosa (CP) and oleaginous fungus Mucor circinelloides (MC) with or without biomass in different solvent systems. Mole balance between acyl groups present in total lipids of CP and MC with methanol in part 3 makes it possible to use the results reviewed in part 1 of reduction in the yield of FAME in vegetable oils due to phospholipids and excess methanol. In conclusion, it is recommended to separate neutral lipids from polar lipids/phospholipids present in the total lipids of microalgae and fungi by supercritical carbon dioxide to prepare FAME in improved yields and keep phosphorus content below the polluting limit in the biodiesel prepared.

A review on the role of allylic and bis allylic positions in biodiesel fuel stability from reported lipid sources

ABSTRACT Reported procedures of estimating allylic position equivalent (APE), bis allylic position equivalent (BAPE) and oxidizability is applied to reported work on autoxidation of methyl oleate with addition of methyl linoleate at 20 and 50  °C. Plot of reported peroxide value (PV) versus estimated oxidizability of autoxidation reactions indicates the effect of APE in rapidly increasing PV of neat methyl oleate at 50  °C. Plot of estimated oxidizablity versus estimated APE and BAPE of reported unsaturated fatty acid methyl esters (FAME) from four lipid sources results in novel insights of a linear relationship of oxidizability with BAPE and non-linear relationship with APE. Limitations of iodine value (IV) as an oxidation stability standard for biodiesel is highlighted by comparing with reported oxidation stability measurements of FAME from two lipid sources. Comments on thermal stability of biodiesel and antioxidants reveals challenges of preparing robust rugged antioxidants for oxidative as well as thermal stability.

Review of Surfactant Evaluation Methods and Perturbations of Components in Phases to Predict Onset of Emulsion Instability

Emulsions are a class of formulations used from paints to drug delivery in dilute to concentrated form, made by mixing two immiscible phases with an emulsifier. This presentation is divided into two parts. The first part is a review of methods adopted to evaluate surfactants/emulsifiers and the inadequacies existing to explain the role of impurities in flocculation, coalescence, and Ostwald ripening, leading to emulsion instability. In the second part, emulsion instability is predicted in a novel approach, wherein the purity of the components in oil and water phases, presented in two individual matrices, is perturbed. The addition of the perturbed matrices and the solution of its determinant is construed as the onset of emulsion instability.

The problem of purity in evaluating surfactant performance

Abstract Labeled surfactants have impurities in the form of un-reacted molecules or distributed products which is a constraint in evaluating its performance. This short review deals with the purity issue of surfactants and is divided into two parts. The first part deals with the methods of purification employed for obtaining a surface chemically pure surfactant for evaluation. In the second part the use of surface chemically impure surfactant in application areas of formulations is presented to highlight the impending impact on reproducibility of results.