Publication History: This article is based on "Crain's Data Acquisition" by E. R. (Ross) Crain, P.Eng., first published in 2010. This webpage version is the copyrighted intellectual property of the author.

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PROXIMATE ANALYSIS / COAL ASSAY  BASICS
Coal is a term used to describe a wide range of organic compounds composed of macerals (as opposed to minerals). A maceral is a component of coal or oil shale. The term 'maceral' in reference to coal is analogous to the use of the term 'mineral' in reference to igneous or metamorphic rocks. Examples of macerals are inertinite, vitrinite and liptinite. Macerals are forms of kerogen with varying carbon, oxygen, and hydrogen content. The kerogen in coals are mostly type 3 and 4 kerpgen.

 

 Coal macerals are Type 3 and Type 4 Kerogens. Coals are often described by their common names instead of maceral type.

 


 

 

Bituminous coal is an organic sedimentary rock formed by diagenetic and submetamorphic compression of peat bog material. Il has been compressed and heated so that its primary constituents are macerals. The carbon content of bituminous coal is around 60 to 80%; the rest is composed of water, air, hydrogen, and sulfur, which have not been driven off from the macerals. Bituminous coal or black coal is relatively soft, containing a tarlike substance called bitumen. It is of higher quality than lignite coal but of poorer quality than anthracite coal.

Lignite, often referred to as brown coal, is a soft brown fuel with characteristics that put it somewhere between coal and peat. It is considered the lowest rank of coal, used almost exclusively as a fuel for steam-electric power generation. Lignite has a carbon content of around 25 to 35%, a high inherent moisture content sometimes as high as 66%, and an ash content ranging from 6% to 19% compared with 6% to 12% for bituminous coal.

Anthracite is a hard, compact variety of mineral coal that has a high luster. It has the highest carbon content, between 92% and 98%, and contains the fewest impurities of all coals, despite its lower calorific content. Anthracite is the most metamorphosed type of coal. The term is applied to coals which do not give off tarry or other hydrocarbon vapours when heated below their point of ignition.


Coal rank depends on thermal maturity


Coal rank is determined by the BTU heating content or by the fixed carbon content, on a dry, ash-free basis.


Proximate analysis
Proximate analysis of coal is a simple laboratory method for determining the components of coal, obtained when the coal sample is heated (pyrolysis) under specified conditions. The coal sample is extracted from a core and placed quickly in a canister to preserve as much gas as possible.

As defined by ASTM D 121, proximate analysis separates the coal into four groups:
    1. moisture,
    2. volatile matter, consisting of gases and vapors driven off during pyrolysis,
    3. fixed carbon, the nonvolatile fraction of coal
    4. ash, the inorganic residue remaining after combustion.

Fixed carbon is also called carbon, dry coal, pure coal, or dry ash-free coal. The latter term is the most descriptive - dry ash-free is often abbreviated as "daf" or "DAF".

Moisture is an important property of coal, as all coals are mined wet. Groundwater and other extraneous moisture is known as adventitious moisture and is readily evaporated. Moisture held within the coal itself is known as inherent moisture and is analyzed quantitatively. Adventitious moisture is removed in the lab by evaporation in air.

Moisture may occur in four possible forms within coal:
  1. surface moisture: water held on the surface of coal particles or macerals
  2. hydroscopic moisture: water held by capillary action within the micro-fractures of the coal
  3. decomposition moisture: water held within the coal's decomposed organic compounds
  4. mineral moisture: water which comprises part of the crystal structure of hydrous silicates such as clays

Total moisture is analyzed by loss of mass between an air-dried sample and the sample after driving off the inherent moisture with heat. This is achieved by any of the following methods;
  1. heating the coal with toluene
  2. drying in a minimum free-space oven at 150 C (302 F) within a nitrogen atmosphere
  3. drying in air at 100 to 105 C (212 to 221 F)

Methods 1 and 2 are suitable with low-rank coals but method 3 is only suitable for high-rank coals as free air drying low-rank coals may promote oxidation. 

Volatile matter in coal refers to the components of coal, except for moisture, which are liberated at high temperature in the absence of air. This is usually a mixture of short and long chain hydrocarbons, aromatic hydrocarbons, and some sulfur. In Australian and British laboratories, this involves heating the coal sample to 900 5 C (1650 10 F) for 7 minutes in a cylindrical silica crucible in a muffle furnace. American procedures involve heating to 950 25 C (1740 45 F) in a vertical platinum crucible. These two methods give different results and thus the method used must be stated.

Fixed carbon content of the coal is the carbon found in the material which is left after volatile materials are driven off. This differs from the ultimate carbon content of the coal because some carbon is lost in hydrocarbons with the volatiles. Fixed carbon is used as an estimate of the coke yield from a sample of coal. Fixed carbon is determined by subtracting the mass of volatiles, determined above, from the original mass of the coal sample.

Ash content of coal is the non-combustible residue left after coal is burnt. It represents the bulk mineral matter after carbon, oxygen, sulfur and water (including from clays) has been driven off during combustion. Analysis is fairly straightforward, with the coal thoroughly burnt and the ash material expressed as a percentage of the original weight.


Example of Proximate Analysis of several coal seams - data is in Weight %


Well log showing location of coal layers analyzed by proximate analysis. Log curves are GR, CAL, PE, neutron, density, density correction.

Float Sink Analysis is used to separate non-coal cavings from cuttings samples. The crushed material is placed in a liquid with a density of 1.75 g/cc. The coal fraction is floated off and the non-coal sinks and is removed. Some mineral (ash) in the coal may sink, reducing the apparent ash content. By comparing the ash analysis to the float sink analysis with that from core analysis, the gas contents can be normalised to reflect true ash contents of the coal cuttings.
 

Vitrinite is the most common component of coal. It is also abundant in kerogen, derived from the same biogenic precursors as coals, namely land plants and humic peats. Vitrinite forms diagenetically by the thermal alteration of lignin and cellulose in plant cell walls. It is therefore common in sedimentary rocks that are rich in organic matter, such as shales and marls with a terrigenous origin. Conversely, carbonates, evaporites, and well-sorted sandstones have very low vitrinite content. Vitrinite is absent in pre-Silurian rocks because land plants had not yet evolved.


Vitrinite reflectance was first studied by coal geologists attempting to determine the thermal maturity, or rank, of coal beds. More recently, it is used to study sedimentary organic matter from kerogen. It is sensitive to temperature ranges that correspond to hydrocarbon generation (60 to 120C). This means that, with a suitable calibration, vitrinite reflectance can be used as an indicator of maturity in hydrocarbon source rocks. Generally, the onset of oil generation is correlated with a reflectance of 0.5 to 0.6% and the termination of oil generation with reflectance of 0.85 to 1.1%

 

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