Fly ash characterization by SEM–EDS Barbara G. Kutchko * , Ann G. Kim 1 US Department of Energy, National Energy Technology Laboratory, 626 Cochrans Mill Road, P.O. Box 10940, Pittsburgh, PA 15236, United States Received 27 October 2005; received in revised form 2 May 2006; accepted 13 May 2006 Available online 15 June 2006 Abstract A study conducted at the National Energy Technology Laboratory characterized twelve Class F fly ash samples from nine PC power plants in PA, WV, MD and TN, using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Both the surface and internal structure of fly ash particles were analyzed. All of the fly ash samples were comprised mainly of amorphous alumino-silicate spheres and a smaller amount of iron-rich spheres. The majority of the iron-rich spheres had two components: iron oxide and amorphous alumino-silicate. Both materials were apparent on the particle surface, and cross-sections clearly showed that the iron oxide and alumino- silicate were mixed throughout the fly ash particles. Calcium, the fourth most abundant element in the fly ash, was associated with oxy- gen, sulfur or phosphorous, not with silicon or aluminum. The calcium-rich material was distinct in both elemental composition and texture from the amorphous alumino-silicate spheres. The elemental concentrations, as determined by EDS, were consistent with ICP-OES and X-ray diffraction data. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: SEM–EDS; Coal by-products; Fly ash 1. Introduction Coal is composed of combustible organic matter with a variable amount of inorganic mineral matter. Of the one billion metric tons of coal mined annually in the United States, approximately 90% is burned to generate electricity [1]. The coal combustion process produces solid coal utili- zation by-products (CUBs) from the non-combustible por- tion of the coal. Over 100 million tons of CUB (fly ash, bottom ash, slag, and flue gas desulfurization products) are generated annually [2]. The physical and chemical char- acteristics of CUB are controlled by the coal, the boiler and its operating conditions, and post-combustion parameters [3–6]. During combustion, minerals in coal become fluid at high temperature and are then cooled [7]. In a pulverized coal (PC) fired boiler, the furnace operating temperatures are typically in excess of 1400 °C(2500 °F). At these tem- peratures, the mineral matter within the coal may oxidize, decompose, fuse, disintegrate or agglomerate. Rapid cool- ing in the post-combustion zone results in the formation of spherical, amorphous (non-crystalline) particles (Fig. 1). Expansion of trapped volatile matter can cause the particle to expand to form a hollow cenosphere. Minerals with high melting points may remain relatively unchanged. The heat- ing and cooling have a significant effect on the composition and morphology of each particle. The purpose of this investigation was to use scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) to characterize the morphology and determine the chemical phases of a random group of fly ash samples from a large number of widely dispersed sources. 2. Materials and methods The PC fly ash samples were analyzed at the National Energy Technology Laboratory (NETL) using the Personal 0016-2361/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2006.05.016 * Corresponding author. Tel.: +1 412 386 5149; fax: +1 412 386 4579. E-mail addresses: barbara.kutchko@netl.doe.gov (B.G. Kutchko), ann.kim@netl.doe.gov (A.G. Kim). 1 Tel.: +1 412 386 6724; fax: +1 412 386 4579. www.fuelfirst.com Fuel 85 (2006) 2537–2544