Ultrasonic coal washing to leach alkali elements from coals

Abstract

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.

A Review of the Application of Ultrasound in Bioleaching and Insights from Sonication in (Bio)Chemical Processes

“…The effect of ultrasound is attributed mainly to cavitation, microstreaming and instabilities produced due to the vibrating motion of particles. Similarly, the improved extraction of alkali metal and decreased fouling of boilers have been observed on ultrasonic washing of coal with water or a chemical reagent [51]. During cavitation, each collapsing bubble can be considered a micro reactor in which temperatures of several thousand degrees and pressures higher than one thousand atmospheres are created instantaneously [18].…”

Separative Refining of Coals through Solvolytic Extraction under Milder Conditions: A Review

“…The effect of both microwave and ultrasonic irradiation on an Indian coking coal has been studied at the bath scale level by Chandaliya et al where these pretreatments followed by extraction in NMP containing a small amount of EDA or MEA led to almost 3–4% and 10–15% increase in the extraction yield, respectively. Ultrasonic treatment has also been used to improve the water washing and chemical washing of coals that have resulted in reduced amounts of the alkali metals that result in fouling of the boiler parts used for power generation …”

The standardization and application of an external (in air) particle induced gamma emission (PIGE) method for the rapid and non-destructive quantification of light elements at major to trace concentrations in coal, bottom ash and coke samples

“…The leaching of alkali metals via ultrasonic coal washing becomes important for inferior-quality coal with a high ash content. 30 Elements like Al, Si, and F, when present in excess, can have adverse effects on the environment and human health, causing soil and water contamination and posing an inhalation hazard to exposed workers. A high concentration of Al in fly ash can lead to acid rain.…”