Article pubs.acs.org/EF
Generation of Ultra-Clean Coal from Victorian Brown Coal: Effect of Hydrothermal Treatment and Particle Size on Coal Demineralization and the Extraction Kinetic of Individual Metals Niken Wijaya, Teck Kwang Choo, and Lian Zhang* Department of Chemical Engineering, Monash University, GPO Box 36, Clayton, Victoria 3800, Australia S Supporting Information *
ABSTRACT: This paper addressed the influences of hydrothermal treatment and particle size on the demineralization extent of Victorian brown coal for the generation of ultra-clean coal to burn directly in a gas turbine combined cycle. Extraction kinetics of individual metals have been investigated. The results for four different sizes of two Victorian brown coal samples showed that, for brown coal that is rich in the aluminum-/silicon-bearing mineral grains, its demineralization extent was dependent on coal particle size, showing the best result for the coal size of 150−300 μm. In contrast, the ash removal efficiency of brown coal rich in organically bound metals remained unaffected by particle size, substantiating a uniform distribution of ash-forming metals on the coal surface as a weak association with the oxygen-containing functional groups. The elution of most metals followed a pseudosecond-order with a confidence interval ≥90%. The activation energy and pre-exponential factor varied significantly with element type and coal sample. Irrespective of coal sample, the extraction of sodium (Na) was achieved instantaneously upon acid attack, relative to iron (Fe) demonstrating an intraparticle diffusion controlling extraction with an activation energy less than 20 kJ/mol. The coal sample rich in mineral grains exhibited an elution behavior limited by both surface reaction and intraparticle diffusion control with respect to a variety of metals. The mineral-grain-rich coal was further treated with pyroligneous acid, citric acid, and Na-EDTA followed by pyroligneous acid at 200 °C to maximize its demineralization extent. As has been confirmed through the use of these three reagents, the overall ash content in coal has been reduced to ∼1.59, 0.95, and 1.17 dry basis (db)-wt %, respectively, as opposed to 2.49 db-wt % in the corresponding raw coal. This study has demonstrated the potential use of waste pyroligneous acid and citric acid for brown coal leaching to generate ultra-clean coal.
1. INTRODUCTION Ultra-clean coal (UCC) is a coal-derived solid fuel with an overall ash content on the order of 0.1 wt %, which has potential to directly burn in gas turbine combined cycle (GTCC) or direct carbon fuel cell (DCFC) for a net power generation efficiency greater than 48%. In comparison to raw coal combustion, the use of UCC in GTCC has been proposed to mitigate CO2 emissions by 25−35%.1−5 This is crucial for sustainable utilization of coal as a low-emission energy source in the carbon-constrained future economy. Chemical leaching is a well established method to reduce the inorganic impurities in coal.6,7 As a major feedstock of power generation worldwide, high-rank coal such as bituminous coal has been tested extensively in the literature. However, little attention has been paid to low-rank coals such as brown coal (i.e., lignite). The authors are conducting a systematic investigation on Victorian brown coal demineralization via chemical leaching. The motivation is tied to the fact that Victorian brown coal is the single largest energy source in Victoria, Australia, contributing to >90% of the local electricity consumption.8 Increasing the net power generation efficiency is a key concern, particularly with regard to the growing concern over carbon dioxide emissions and climate change. The abundant moisture in brown coal has been causing a notoriously large greenhouse gas emission rate from combustion. Additionally, Victorian brown coal is rich in water-soluble species and species organically associated with oxygen-containing functional groups, relative to bituminous © 2012 American Chemical Society
coal which shows prevalence of discrete aluminum- and siliconbearing minerals. The organically bound species have proven to be easily removed by weak acids at room temperature.9 Continuing from previous work on speciation of the modes of occurrence of individual metals and room temperature leaching,9 the present paper aims to introduce the up-to-date efforts conducted to clarify the leachability of individual inorganic elements in two typical brown coals. Use of a variety of reagents at elevated temperatures, their variation with coal particle size as well as the extraction kinetics of individual metals were investigated. At elevated temperatures, coal particles are expected to soften due to thermal relaxation of its cross-linked structure, which in turn allows the leachant to penetrate and attack the mineral grains embedded within a coal matrix.3,9,10 Moreover, a hydrothermal treatment in hot water has proven effective in upgrading a low-rank coal to increase its heating value.11 In this study, leaching temperatures up to 200 °C have been tested. Coal particle size is another influential factor affecting the degree of coal demineralization, and it is expected to have the opposite effect from temperature. A small particle size favors the coal demineralization process particularly for the mobilization of trace elements, as they are surfaceenriched and are prone to be concentrated in small coal particles.12 However, very finely ground coal particles may lead Received: March 30, 2012 Revised: June 4, 2012 Published: June 5, 2012 5028
dx.doi.org/10.1021/ef3005459 | Energy Fuels 2012, 26, 5028−5035
Energy & Fuels
Article
Table 1. Properties of Victorian Brown Coal Samples Studied coal A
a
particle size (μm) ash, db-wt %
