Bureau Veritas has years of experience in providing petrographic analysis to the coal industry. Our highly qualified and experienced team of technical staff and operators are accredited by the international committee of coal and organic petrology. Bureau Veritas is dedicated to delivering a service of high standard and add value to your business with responsive turn around time. Our Petrography Specialist Mr Willem Swanepoel wrote this article to introduce this valuable service to the industry and explain the relevance of coal petrography.
What is petrography?
Petrography is a microscopic investigation to determine a coal’s degree of coalification or maturity (rank), and type (classification of organic and inorganic constituents in a coal). A Petrography block is polished to a surface suitable for microscopic analyses under oil immersion, at a magnification of X50.
To determine a coals rank or degree of coalification from lignite/brown coal to bituminous coal and anthracite, measurements are made on Vitrinite particles to determine the percentage reflectance, expressed as Random reflectance of Vitrinte (RoV random). This reflectance percentage is used to classify a coal as low, medium or high rank. As reflectance increases, volatile decreases and carbon content increases, this is indicative of a more mature coal.
To determine a coals type, different macerals are assigned to a certain group according to their reflectance, morphology, colour, texture and structure.
Macerals belong to the organic group; they derived from plant and vegetation tissues such as roots, stems, barks, leaves, waxes, spores and other organisms such as fungi and algae.
Different macerals have different properties, which cause them to behave differently when exposed to high temperatures in boilers or ovens. Some macerals have the ability to swell and become plastic during carbonization, these are assigned to the reactive group of macerals, whereas other does not change much and is assigned to the inert group. These different properties determine if a maceral will be assigned to the reactive or inert group. The total reactives are obtained by the sum of Vitrinite, Liptiniet and Reactive semifusinite.
The inorganic group consists of minerals, which include the following: Clay, Iron disulphides, Carbonates, Salts, Silicates, Oxides, Hydroxides and Sulphides. All minerals are assigned to the inert group.
What are the benefits of Petrography to the industry?
Petrography is useful to detect coal contamination. This this is done by measuring vitrinite particles of higher or lower rank not inherent in the coal sample. These reflectance outliers as seen below are the result of contamination.
Coal Blend Analyses
The same technique is used to determine the number of coals in a blend sample. Reflectance of vitrinite particles of the same origin will be grouped together. Not only is the number of coals identified but the percentage of each different coal in the blend is quantified. Below is an example of a blend containing coals of 3 different origins.
Abnormal Condition Analyses
Petrography can also be used to determine abnormalities in coal such as primary or secondary weathering in the form of oxidation and discoloration and heat affected material. Oxidation has a negative impact on the reactive macerals, this influences the plasticity and swelling ability.
What is Coke Petrography?
Coke petrography is a carbon form analyses which consist of binder and filler phases. These are classified according to texture, structure, isotropic and anisotropic properties.
Binder phase carbons derived mainly from parent coal reactive macerals such as Vitrinite, Liptinite and reactive semifusinite. The reactive macerals soften and become plastic during carbonization and produce binder phase carbon forms which bind around inert material. These are classified into the following categories, isotropic, incipient and circular, lenticular and ribbon anisotropic domains.
Filler phase derived mainly from parent coal inert materials. This phase includes those inertinites and minerals that didn’t soften appreciably during carbonization. This phase is classified into the following categories, organic and inorganic inerts, additive carbons, depositional carbons and other miscellaneous inerts and materials such as reactives that are non-coking due to weathering and oxidation.
Coal through a microscope is not just a black rock; each coal particle has its own unique characteristics!
For any coal petrography enquiries, please contact
Mr Willem Swanepoel,
ICCP/SCAP-183/AB, Single Coal Accreditation Programme
ICCP/DOMVR-183, Dispersed Organic Matter Vitrinite Reflectance Accreditation Programme
ICCP/CBAP-183, Coal Blend Accreditation Programme
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