Article pubs.acs.org/EF
Mineralogy of Furnace Deposits Produced by South African Coals during Pulverized-Fuel Combustion Tests Ratale H. Matjie,*,† Zhongsheng Li,‡,⊥ Colin R. Ward,‡ Johan Kosasi,§ John R. Bunt,† and Christien A. Strydom∥ †
Energy Systems, School of Chemical and Minerals Engineering, North-West University, Potchefstroom 2520, Republic of South Africa ‡ School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney NSW 2052, Australia § ALS-ACIRL Pty. Ltd., P. O. Box 242, Booval Qld 4304 Australia ∥ School of Physical and Chemical Sciences, North-West University, Potchefstroom 2520, Republic of South Africa ABSTRACT: Pilot-scale combustion tests were carried out on coals with low (13.5%) and high (39.6%) ash percentages from the Highveld Coalfield of South Africa, using the ALS-ACIRL 100−200 kW Combustion Test Facility. The mineral matter in the feed coals and also the chemistry and mineralogy of the ash deposits from different parts of the test furnace were analyzed using quantitative X-ray diffraction and X-ray fluorescence techniques, and the results were used to develop an understanding of the mineralogical processes taking place in the different parts of the pulverized-fuel combustion system. Low-temperature oxygenplasma ashing and quantitative X-ray diffractometry showed that the mineral matter of both coals contained abundant kaolinite, lesser but still very significant proportions of quartz and carbonates (ankerite and calcite), and minor proportions of pyrite and other minerals. A small but significant proportion of nonmineral Ca also occurs in the organic matter, especially of the low-ash coal. The chemical composition of the combustion products taken from the different sampling points in the test facility (slagging panels, furnace ash, and fouling probes, etc.) was overall very similar for each coal sample. However, the percentages of the different crystalline minerals in the combustion products showed a wide range of variation at the different sampling points. Anorthite, derived from interaction between Ca and the aluminosilicate components, was formed from both coals at temperatures above 1300 °C, mostly as part of sintered aggregates that built up and became detached from the slagging panel in the highest temperature part of the combustion system. Anhydrite was formed by interaction of Ca with SO2; lime and periclase were also formed from the low-ash coal, where insufficient sulfur was available for complete sulfate development. Low-viscosity ash may adhere to the fire-side surfaces of the boiler tubes and form slag deposits.5,9 Slagging usually refers to the formation of molten deposits in the areas directly exposed to the flame radiation (furnace walls and widely spaced pendant superheaters).2,3,5,9,14,15 Fouling, on the other hand, is the accumulation of ash deposits in the downstream (cooler) convective heat exchange region of the boiler. Ash deposit formation may cause damage to the tubes, block hoppers, and build up layers on the wall that are difficult to clear by sootblowing.16,17 The role of fluxing elements (e.g., Ca, Mg, and Fe) in the formation of aluminosilicates and glasses in coal ash has been extensively studied.2,9,18,19 However, little such research has been conducted on Highveld and other South African coals, especially products of different quality (ash yield) derived from the same source, to evaluate the nature of the mineral matter in those coals and the reactions occurring in that mineral matter during pf combustion processes. Although significant work has been done at a generic level on the mineralogy of fly ash,20,21 only limited research has been carried out into the mineralogy of the slagging, fouling, and other deposits that may form in the
1. INTRODUCTION When coal is combusted in pulverized-fuel (pf) boilers to produce electricity, mineral matter in the coal is transformed at elevated temperatures to produce ash and related materials (e.g., slagging and fouling deposits) of varying chemical composition, mineralogy, and physical characteristics (size, shape, viscosity, and density).1−9 While some minerals, such as quartz, may be essentially nonreactive at the temperatures and exposure times associated with combustion,9 the clay minerals (e.g., kaolinite, illite, and montmorillonite) are reactive phases and usually start to lose water of hydration at temperatures below 500 °C.10 Carbonates such as calcite, dolomite, and siderite are also reactive, decomposing at temperatures typically between 400 and 1000 °C to liberate CO2 and produce metal oxide residues (e.g., CaO, MgO, and FeO).11 Depending on the temperature, and also on opportunities for contact, residues derived from those carbonates may interact with metakaolin, derived from thermal decomposition of kaolinite, to form lowviscosity silicates (glass) and/or crystalline aluminosilicate phases such as anorthite.9,12,13 Pyrite decomposes at elevated temperatures to form pyrrhotite, which is subsequently oxidized to iron oxide and sulfur oxide.10 The iron oxide may remain as a solid phase (e.g., magnetite or hematite) and be otherwise nonreactive,9,13 but may also react with other residues to form glassy aluminosilicate materials. © 2015 American Chemical Society
Received: April 30, 2015 Revised: November 6, 2015 Published: November 24, 2015 8226
DOI: 10.1021/acs.energyfuels.5b00972 Energy Fuels 2015, 29, 8226−8238
Article
Energy & Fuels
Figure 1. Schematic of pilot-scale boiler simulation furnace (BSF). Image reprinted with permission from ACIRL Pty. Ltd.
Figure 2. Detailed view of the ash sampling locations and temperature profiles observed in the combustion tests. Only the upstream bank of fouling probes (right-hand side of tunnel in the diagram) was used in the experiments.
different parts of a combustion system and on the linkages between feed coal mineral matter and the mineralogy of the associated furnace deposits.9 The first objective of this study was to evaluate the mineral matter of two different feed coals and the mineralogy of the ash deposits formed from each coal in different parts of the same test furnace that had been exposed to similar temperatures and dynamic conditions. Although both coals were derived from the same mine source, one had an ash yield similar to those of coals
exported from South Africa for power generation and one had an ash yield similar to those of coals used in South African domestic power plants. The second objective of the study was to establish the links between feed coal and ash/deposit mineralogy in the different parts of a pf furnace, based on quantitative X-ray powder diffraction (XRD) analysis integrated with chemical studies. This was intended to investigate the mineralogical reactions that take place during pulverized coal combustion at a more 8227
DOI: 10.1021/acs.energyfuels.5b00972 Energy Fuels 2015, 29, 8226−8238
Article
Energy & Fuels
poorly crystalline silica phase in the Siroquant database to represent the amorphous component.20
generic level, based on a comparison of the ashes and other deposits derived from two coals with different quality characteristics burned under essentially the same combustion conditions.
3. FEED COAL PROPERTIES Proximate analysis (Table 1) indicates that the low-ash coal has a 13.5% ash yield, while the high-ash coal has almost 40% ash
2. MATERIALS AND METHODS A run-of-mine sample of South African Highveld coal, crushed to
