R Nagarajan

The present work investigates utilization of ultrasound in reagent-based coal de-ashing and de-sulfurization. The coal under study was received from Girald mine, Rajasthan, India. Three different ultrasonic frequencies (25kHz, Dual (58/192kHz) and 430kHz) and three reagents (HCl, HNO3 and H2O2) were used. The study employed a Taguchi fractional-factorial L27 DOE. Experimental data were used to derive an empirical model for the prediction of total sulfur removal. The model incorporates cavitational intensity, reagent concentration, sonication time, coal particle size and coal concentration as key parameters. Effects of above factors on reagent-based ultrasonic coal-desulfurization are presented here. An optimum set of process parameters are identified and validated. Larger-scale trial with high-ash and high-sulfur coals is strongly recommended. © 2011 Elsevier B.V.

Deposition of fly ash particles onto heat-transfer surfaces is often one of the reasons for unscheduled shut-downs of coal-fired boilers. Fouling deposits encountered in convective sections of a boiler are characterized by arrival of ash particles in solidified (solid) state. Fouling is most frequently caused by condensation and chemical reaction of alkali vapors with the deposited ash particles creating a wet surface conducive to collect impacting ash particles. Hence, the amount of alkali elements present in coals, which, in turn, is available in the flue gas as condensable vapors, determines the formation and growth of fouling deposits. In this context, removal of alkali elements becomes vital when inferior coals having high-ash content are utilized for power generation. With the concept of reducing alkali elements present in a coal entering the combustor, whereby the fouling deposits can either be minimized or be weakened due to absence of alkali gluing effect, the ultrasonic leaching of alkali elements from coals is investigated in this study. Ultrasonic water-washing and chemical-washing, in comparison with agitation, are studied in order to estimate the intensification of the alkali removal process by sonication.

Enhancement of heat transfer from a heat source to a flowing fluid within a tube is a challenging problem with many practical applications. In this paper, experimental investigation of a low-frequency (20-33. kHz), high-intensity (500-1000. W) ultrasonic field as a potential heat-transfer " process intensifier" is undertaken. Heat-transfer enhancement data collected in a miniaturized furnace tube over a range of flow conditions and ultrasonic process parameters indicate that sonication provides significant augmentation only under near-static (e.g., stagnant) and low-Reynolds number flow conditions. With increasing flow velocity, cavitational and acoustic-streaming fields associated with ultrasound are rapidly diminished in importance, hence playing no role in bulk fluid heat transfer (unless input power levels or frequencies are suitably increased). However, the relevance to some locations, such as those under porous deposits in water-wall tubes of boilers near the goose-neck portion, can spur further study to exploit the impact of ultrasonic heat-transfer enhancement. The critical parameter that determines the efficacy of ultrasonic enhancement of heat transfer appears to be the ratio of the characteristic ultrasonic field velocity (sum of cavitational and acoustic streaming velocities) to the prevailing flow velocity. © 2012 Elsevier B.V.

Stratification is one of the main causes for vaporization of cryogens and increase of tank pressure during cryogenic storage. This leads subsequent problems such as cavitation in cryo-pumps, reduced length of storage time. Hence, it is vital to prevent stratification to improve the cost efficiency of storage systems. If stratified layers exist inside the tank, they have to be removed by suitable methods without venting the vapor. Sonication is one such method capable of keeping fluid layers mixed. In the present work, a mechanistic model for ultrasonic destratification is proposed and validated with destratification experiments done in water. Then, the same model is used to predict the destratification characteristics of cryogenic liquids such as liquid nitrogen (LN2), liquid hydrogen (LH2) and liquid ammonia (LNH3). The destratification parameters are analysed for different frequencies of ultrasound and storage pressures by considering continuous and pulsed modes of ultrasonic operation. From the results, it is determined that use of high frequency ultrasound (low-power/continuous; high-power/pulsing) or low frequency ultrasound (continuous operation with moderate power) can both be effective in removing stratification. © 2013 Elsevier B.V. All rights reserved.