Zhongsheng Li

Quantitative evaluation of minerals in fly ashes of biomass, coal and biomass-coal mixture derived from circulating fluidised bed combustion technology

The chemical and mineralogical composition of fly ash samples collected from laboratory scale circulating fluidised bed (CFB) combustion facility have been investigated. Three fly ashes were collected from the second cyclone in a 50 kW laboratory scale boiler, after the combustion of different solid fuels. Characterisation of the fly ash samples was conducted by means of X-ray fluorescence (XRF), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Quantitative analysis of the crystalline (mineral) and amorphous phases in each ash sample was carried out using the Rietveld-based Siroquant system, with an added spike of ZnO to evaluate the amorphous content. SiO(2) is the dominant oxide in the fly ashes, with CaO, Al(2)O(3) and Fe(2)O(3) also present in significant proportions. XRD results show that all three fly ashes contain quartz, anhydrite, hematite, illite and amorphous phases. The minerals calcite, feldspar, lime and periclase are present in ashes derived from Polish coal and/or woodchips. Ash from FBC combustion of a Greek lignite contains abundant illite, whereas illite is present only in minor proportions in the other ash samples.

Mineralogy and Leaching Characteristics of Coal Ash from a Major Brazilian Power Plant

The feed coals, fly ashes and bottom ashes collected from seven different units in a major Brazilian PF power plant have been subjected to comprehensive mineralogical, geochemical, and petrographic studies, to investigate the links between feed coal and ash characteristics. Ashes from two of the units were collected while the coal was being co-fired with oil as part of the boiler start-up procedure, allowing the impact of oil co-firing on ash characteristics also to be evaluated. High proportions of unburnt carbon and high proportions of retained sulphur were found in the fly ashes produced during oil co-firing, probably reflecting less efficient combustion and associated lower combustion temperatures. Higher concentrations of a number of relatively volatile trace elements were also noted in these fly ashes, compared to the fly ashes collected from units under normal operating conditions. The fly ashes produced during oil co-firing gave rise to acid pH conditions in water-based leaching tests, in contrast to the alkaline pH associated with fly ashes produced during normal operations. This probably reflects higher SO3 contents relative to total CaO + MgO for the co-fired ash samples. Many trace elements that are typically mobilised as cations were also more abundant in leachates from the co-fired fly ashes. This is due, most likely, to the more acid pH conditions involved. Despite similar or even higher total concentrations, however, elements that are typically released from coal ash as oxy-anions were less mobile from the co-fired fly ashes than from the normally-fired fly ash materials. f 2010 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association All rights reserved. A R T I C L E I N F O Article history: Received 28 July 2010; Received in revised form 17 August 2010; Accepted 23 August 2010

Crocoite: an unusual mode of occurrence for lead in coal

Abstract What is believed to be a very unusual mode of occurrence for lead in coal has been identified as crocoite (PbCrO 4 ). As part of a larger study on trace elements and mineralogy in the Cretaceous Main Seam in New Zealand, crocoite was found in raw coal samples within the lower part of the coal seam. X-ray diffraction (XRD) and bulk chemical data from a SEM equipped with an energy dispersive X-ray analyser (EDXA) have confirmed the identity of this mineral. This is apparently the first time that crocoite has been reported in coal. Crocoite usually occurs only in the oxidised zone of lead mineral deposits. The occurrence of this mineral in the Main Seam coal implies that the deposit was exposed to an oxidising environment at some stage, most likely after coalification.

Leaching of inorganics in the Cretaceous Greymouth coal beds, South Island, New Zealand

Abstract Leaching processes are believed to be responsible for the unusually low-ash content (sometimes less than 1%) of the thick (up to 35 m) Cretaceous coals located in the Greymouth coalfield, South Island, New Zealand. Although leaching of inorganics in peat is a generally accepted process, little is known about leaching after burial. The “Main” and “E” seams in the Greymouth coalfield show good correlation between low ash and bed thickness. The ash content, however, is often less than 1%, which is lower than most known modern analogues (i.e. peat). There are several lines of evidence that suggest that mineral matter may have been removed from the coal not only in the peat stage but also after burial. For example, etching features found in quartz grains and clay aggregates indicate that some leaching processes have taken place. In addition, liptinitic material (e.g., bitumen) in the cleat networks supports the conclusion that there has been some movement of solutions through the coal after burial. These solutions may have helped to remove some of the inorganics originally within the Greymouth coals.

Mineralogical Transformations in Coal Feedstocks during Carbon Conversion Based on Packed-Bed Combustor Tests: Part 1. Bulk Coal and Ash Studies

Mineralogical and inorganic geochemical studies have been carried out on composite samples of six different coarse-crushed (,120 mm) feed coals tested in a pilot-scale packed-bed combustor designed to simulate conversion and some fixed-bed combustion processes, and also on composite samples of the ashes produced. The mineral matter of the coals consisted mainly of quartz, kaolinite, and Ca-bearing carbonate minerals (calcite, dolomite, aragonite), with minor proportions of illite, pyrite, and several other phases, along with some Ca in an organically associated form. The base to acid ratio of the coal ashes ranged from 0.1 to 0.3, due mainly to variations in relative abundance of Ca-bearing carbonate minerals. The minerals in the feed coals reacted at temperatures of up to 1250uC in the packed-bed combustor to form cristobalite, mullite, anorthite, and other Ca-silicates, as well as anhydrite, iron oxides, and amorphous material. The proportions of the different crystalline phases in the ashes produced from each of the coals in the packed-bed combustor are related to the proportions of relevant minerals in the respective feed-coal materials. Comparisons of feed coal and packed-bed combustor ash mineralogy suggest that the cristobalite in the packed-bed combustor ashes mainly represents a high-temperature product derived from quartz in the feed coals. Mullite in the combustor ash is formed by high-temperature reactions from the mainly kaolinitic clay component. The proportions of anorthite and other Ca-silicates in the combustor ashes have been related to the proportions of Ca-bearing carbonate minerals and organically associated Ca in the feed coals. The proportion of iron-oxide minerals is broadly related to the proportion of pyrite in the respective coal samples. These observations provide a basis for assessing the mineralogical processes that take place during conversion of different Highveld coals, and possibly for predicting ash properties based on coal mineral matter characteristics. f 2012 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association All rights reserved. A R T I C L E I N F O Article history: Received 1 February 2012; Received in revised form 5 June 2012; Accepted 11 June 2012

Mineralogical Transformations in Coal Feedstocks during Carbon Conversion, Based on Packed-Bed Combustor Tests: Part 2. Behavior of Individual Particles

Handpicked carbon-rich particles (coal or char), mineral-rich particles (heated stones), and bonded stone aggregates (clinkers) from the upper and lower parts of a pilot-scale packed-bed combustor bed, designed to simulate conversion or combustion processes, were subjected to mineralogical and inorganic geochemical studies, in order to evaluate the reactions that may occur during carbon conversion at a particle-by-particle scale. The mineral matter of the carbon-rich particles was found to have higher proportions of carbonate minerals and kaolinite than the mineral-rich (stone) particles in the coal feedstocks used in the study, and to form phases such as anorthite, gehlenite, pyrrhotite, troilite, and vaterite, as well as mullite and cristobalite, in the high-temperature parts of the combustor column. The clinkers and the heated stones from the hightemperature parts of the bed both contain similar minerals (quartz, mullite, cristobalite, K-feldspar, and hematite), but the clinkers have higher proportions of anorthite and amorphous material. From the study, it is suggested that melting of Ca-rich ash remaining after destruction of the organic matter in the carbonrich particles takes place at high temperature (up to 1250uC) in the combustor bed. Melting of Ca-rich ash from the carbon-rich particles, with a high base to acid ratio, represents a key process in clinker formation. Anorthite crystallizes from this melt as it cools. Before cooling, the melt may envelop still-intact heated stone and, in some cases, char particles, allowing the resulting anorthite-bearing glass to bond those particles together and form clinkers within the combustor ash. The mineralogical changes observed in the individual particles in this study complement those identified from analysis of composite feed coal and ash samples and, thus, provide a key to better understanding ash formation and clinker formation processes associated with the inherently heterogeneous, coarse coal feedstocks used in some conversion or combustion systems. f 2012 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association All rights reserved. A R T I C L E I N F O Article history: Received 1 February 2012; Received in revised form 5 June 2012; Accepted 13 June 2012