**Mineralogy and Geochemistry of Sub-Bituminous Coal and Its Combustion Products from Mpumalanga Province, South Africa**

S. A. Akinyemi, W. M. Gitari, A. Akinlua and L. F. Petrik

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/50692

#### **1. Introduction**

46 Analytical Chemistry

729.

iv, 124 p.

[93] Andersen, C. M. l.; Mortensen, G., Fluorescence Spectroscopy: A Rapid Tool for Analyzing Dairy Products. *Journal of Agricultural and Food Chemistry* 2008, 56, (3), 720-

[94] Bro, R.; Kiers, H. A. L., A new efficient method for determining the number of components in PARAFAC models. *Journal of Chemometrics* 2003, 17, (5), 274-286. [95] White, J. W., *Composition of American honeys*. U. S. Govt. Print. Off.: Washington,, 1962; p

> Coal forms from the accumulation and physical and chemical alteration of plants remains that settle in swampy areas and form peat, which thickens until heat and pressure transform it into the coal we use. The coal we use is combustible sedimentary rock composed of carbon, hydrogen, oxygen, nitrogen, sulphur, and various trace elements (it has a carbonaceous content of more than 50 % by weight and more than 70 % by volume). As much as 70 % of the South African estimated coal reserve is located in the Waterberg, Witbank, and Highveld coalfields, as well as lesser amounts in the Ermelo, Free State and Springbok Flats coalfields. However, the Witbank and Highveld coalfields are approaching exhaustion (estimated 9 billion tons of recoverable coal remaining in each), while the coal quality or mining conditions in the Waterberg, Free State and Springbok Flats coalfields are significant barriers to immediate, conventional exploitation [1]. South Africa is the third largest coal producer in the world, and coal accounts for 64 % of South Africa's primary energy supply [2]. Electricity generation accounts for 61 % of the total coal consumption in South Africa and more than 90 % of the country's electricity requirements are provided for by coal-fired power plants [2]. South African coals are generally low in sulphur, nitrogen and phosphorus, and in the case of the first two the contents are dependent on maceral composition and rank [3, 4].

> The inorganic elements in coal can have profound environmental, economic, technological and human health impacts [5, 6]. Consequently, knowledge of their concentration is necessary when evaluating coals for combustion and conversion and also to evaluate potential negative environmental and health impacts resulting from coal use. Trace elements

© 2012 Akinyemi et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Akinyemi et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

in coal are emitted into flue gas, fly ash or bottom ash of combustion plants. In a flue gas stream, trace elements are fixed in ash particles and in by-products such as gypsum and sludge if wet flue gas desulphurization unit is equipped.

Mineralogy and Geochemistry of Sub-Bituminous Coal and Its Combustion Products from Mpumalanga Province, South Africa 49

, CO2, F-

, Cl-

, S; in parts also K+ and

*2.1.2. Pressed pellet method for Trace element analysis* 

Mix thoroughly with 3 drops of Mowiol wax binder

of contributions from the volatile compounds H2O+, OH-

theta scale at intervals of 0.02 and counted for 0.5 sec per step.

Dry in oven at 100 ºC for half an hour before analysing.

Loss on Ignition (LOI) is a test used in XRF major element analysis which consists of strongly heating a sample of the material at a specified temperature, allowing volatile substances to escape or oxygen is added, until its mass ceases to change. The L.O.I. is made

Na+ (if heated for too long); or alternatively added compounds O2 (oxidation, e.g. FeO to Fe2O3), later CO2 (CaO to CaCO3). In pyro-processing and the mineral industries such as lime, calcined bauxite, refractories or cement manufacture, the loss on ignition of the raw material is roughly equivalent to the loss in mass that it will undergo in a kiln, furnace or

Coal samples and its combustion products were analysed for mineralogical composition by X-ray diffraction (XRD). A Philips PANalytical instrument with a pw 3830 X-ray generator operated at 40 kV and 25 mA was used. The pulverised samples were ovendried at 100 °C for 12 h to remove the adsorbed water. The samples were pressed into rectangular aluminium sample holders using an alcohol wiped spatula and then clipped into the instrument sample holder. The samples were step-scanned from 5 to 85 degrees 2

**2.4. Scanning Electron Microscopy and Electron Dispersive X-ray Spectroscopy** 

Microstructural and chemical composition investigations of coal and coal ash were carried out by scanning electron microscopy/electron dispersive x-ray spectroscopy (SEM/EDS). For SEM/EDS aluminium stubs were coated with carbon glue; when the glue was dry, but still sticky; a small amount of powder residue samples was sprinkled onto the stub. The excess residue sample powder was tapped off and the glue allowed complete drying. The residue samples were then coated with carbon in an evaporation coater and were ready for analysis with the SEM. The SEM is an FEI Nova NanoSEM (Model: Nova NanoSEM 230); The EDS analyses were determined at 20 Kv and 5 mm working distance. The EDS detector is an Oxford X-Max (large area silicon drift detector) using the software program INCA-(INCAmicaF+ electronics and INCA Feature particle

Press pellet with pill press to 15 ton pressure

Weigh 8 g ± 0.05 g of milled powder

**2.2. Loss on ignition determination** 

smelter.

**2.3. XRD analysis** 

**(SEM/EDS)** 

analysis software).

Coal fly ash is a solid residue from the combustion processes of pulverised coal for the production of electrical power in power generating stations, especially when low-grade coal is burnt to generate electricity [7, 8, 9]. The coal burning power stations in the Mpumalanga Province, South Africa generates over 36.7 Mt of fly ash annually in which only 5 % is currently utilized, the rest being disposed of in the ash dams, landfills, or ponds [9, 10]. During combustion, mineral impurities in the coal, such as clay, feldspars, and quartz, are fused in suspension and float out of the combustion chamber with exhaust gases. As the fused material rises, it cools and solidifies into spherical glassy particles called fly ash [11]. The properties of the coal fly ash depend on the physical and chemical properties of the parent coal, coal particle size, the burning process and the type of ash collector.

This article presents results obtained from mineralogical and chemical characterization of coal and its combustion products from a coal burning power station in the Mpumalanga Province, South Africa. The main objective of the study is to understand the role of combustion process, chemical interaction of fly ash with ingressed CO2 and percolating rain water on the mineralogy and chemical compositions of fly ash.
