Coal: an introduction

Coal is an organic sediment consisting of a complex mixture of substances. Visit http://www.uky.edu/KGS/coal/ and read about: What is coal? Coal forming conditions, Identification of coal components, and Classification and rank of coal

There are two broad categories of coal

• humic - More common and originates from peat deposits consisting mostly of organic debris deposited in situ (autochthonous).
• sapropelic - Derived from redeposited (allochthonous) resistant plant fragments such as spores or aquatic plants. The sapropelic coals can be further subdivided into:
  • cannel coal - Cannel coal is made up principally of uniformly sized plant fragments, such as spores.
  • boghead coal - Consists mainly of alginite (a coal maceral derived from algae).

Peat is formed from the deposition of organic material with a restricted supply of oxygen. Peat forming environments are known generally as mires.

Mires may be classified as:

• Ombrotrophic mires (those with surfaces elevated above the influence of groundwater, fed by rainwater; examples can be found in central American and southeast Asia)
• Rheotrophic mires (those fed by groundwater, such as the Okefenokee, or swamps where the dry season water table is always above the surface of the sediment).

The type of original plant input, the availability of nutrients, climatic conditions, the level of the water table, the pH and Eh conditions all help to determine  the type of peat that is formed. Every part of the ecosystem of the peatland or mire may be represented in the peat, including the large trees, herbaceous shrubs, grasses, aquatic plants and the micro-organisms that break down the organic material.

For a coal to be developed, the peat has to be buried and preserved. The process that converts peat to coal is called coalification. The degree of coalification which has taken place determines the rank of the coal.

Coal Ranking

The four recognized ranks of coal in the U.S. classification scheme are: anthracite, bituminous coal, sub-bituminous coal, and lignite. In the United States coal rank is classified according to its heating value, its fixed carbon and volatile matter content and, to some extent, its caking characteristics during combustion.

• Anthracite - Used primarily for residential and commercial space heating, anthracite is the highest rank of coal. Its hard, brittle, lustrous black texture contains a high percentage of fixed carbon and low percentage of volatile matter. It is often called "hard coal." Freshly-mined anthracite usually contains less than 15 percent moisture content. Its typical heat content is 15 million Btu per short ton or less.
• Bituminous - Dense coal of black or dark brown color and usually has well-defined bands of contrasting bright and dull material. It is used primarily to generate steam electric power. Substantial quantities are used for power and heat applications in manufacturing as well as to make coke. In active U.S. mining regions, bituminous coal is the most abundant coal. It has a a moisture content of less than 20 percent and a heat content range between 21 to 30 million Btu per ton in a moist, mineral-matter-free basis. Of the four ranks, bituminous coal accounts for over half (51 percent) of the demonstrated reserve base. Bituminous coal is concentrated primarily east of the Mississippi River, with the greatest amounts in Illinois, Kentucky, and West Virginia.
• Sub-bituminous - This coal can be dull, dark brown to black in color and soft and crumbly as one quality, to bright, jet black, hard and strong. Used primarily as fuel for steam-electric power generation, subbituminous coal has properties ranging from the properties of lignite or bituminous coal. The heat content of sub-bituminous coal consumed in the U.S. ranges from 17 to 18 million Btu per ton on a moist, mineral-matter-free basis. All sub-bituminous coal (38 percent of the demonstrated reserve base) is west of the Mississippi, with most of it in Montana and Wyoming.
• Lignite - This coal's brownish-black color has a high moisture content and is the lowest rank coal. It is often called "brown coal" and is used almost entirely as fuel for steam-electric power generation. It accounts for slightly less than 10 percent of the demonstrated reserve base and is found mostly in Montana, Texas, and North Dakota.

Coalification

The transformation of plant material into coal takes place in two stages, biochemical degradation and physico-chemical  degradation.

• Biochemical degradation involves chemical decomposition of botanical matter assisted by organisms.
  • In tropical environments, this process may be faster, since the warm moist conditions are ideal for the organisms that assist in this process such as bacteria and fungi. However plant growth is also more rapid and so the increased rate of decomposition may be balanced by plant growth. In tropical conditions high rates of evaporation need to be coupled with high precipitation to maintain plant growth and peat accumulation.
  • In cooler climates the growth rate of vegetation may be cyclical in nature and slower since the seasonal variation in conditions is greater. The conditions are less ideal for fungi and bacteria so the slower growth rate is matched by a slower rate of biochemical degradation.
  • Humification affects the soft contents of the plants cells before the cell walls, which consist of cellulose, hemicellulose and lignin which is the most resistant compound.
  • Humification begins with the oxidation of plant matter and attack by aerobic organisms such as fungi, insects and aerobic bacteria. Hydrocarbons are extracted from the tissue and the material left behind is relatively enriched in oxygen and carbon. Semifusinite, an inertinite maceral may be formed in this manner.
  • Various humic substances are formed at this time, these are acidic in nature. If this continues the plant material will be completely degraded into carbon dioxide and water. When the plant material or degraded plant material is buried below the ground water table aerobic organisms and oxidation can no longer attack the material. Anaerobic bacteria may still decompose the plant matter until it reaches a depth or conditions unsuitable for these organisms. Anaerobic bacteria utilise the oxygen in the plant matter, so all molecules may be attacked even the more resistant compounds. However the softer tissue may be more rapidly affected.
  • Biochemical coalification ends at the rank of sub-bituminous coal, when humic substances have polymerised.
• Physico-chemical degradation which follows is caused by conditions of burial.
  • The overburden which is deposited, the heat flows in the earth's crust and tectonic heat and pressure change the chemistry and structure of the altered organic material. The same conditions are applied to all the macerals.
  •  Water is squeezed out and pore size is reduced as pressure increases and oxygen and hydrogen are released during thermal cracking. Water and carbon dioxide are the first products released.
  • When rank reaches medium volatile bituminous coal demethanation begins.

Petrography is the microscopic study and description of coal and rocks. The petrography of coal is important since it affects the physical and chemical nature of the coal. Coal crushing, grinding, handling, washability, gasification, liquefaction combustion and carbonisation are affected by the petrography of the coal.

Coal 'type' refers to the petrographic constituents in coal.

Macerals

The smallest microscopically recognisable entities in coal are called macerals, they are analogous to minerals in rocks. However they differ since minerals have an homogeneous chemistry and an orderly internal structure, while coal macerals consist of a mixture of compounds.

The chemical and physical properties of macerals vary with coal rank. Coal macerals are distinguished by:

• their optical characteristics of colour
• relief of the polished surface
• morphology
• reflectance and fluorescence

Macerals differ because they represent different parts of the original plant material and micro-organisms that contributed to the peat. The mode of preservation, that is whether or not the organic fragments were oxidised before being preserved, is also considered in the classification of macerals.

There are three maceral groups vitrinite, inertinite and liptinite (exinite). These are all subdivided into maceral subgroups and macerals. The liptinite group is relatively enriched in hydrogen compared with the other two groups, inertinites have a greater carbon content and vitrinites have an intermediate chemistry. As rank increases the differences in the chemistry between the groups diminishes.

• The  vitrinite group generally represents the woody plant material (eg. stems , trunks, roots and branches). Liptinite includes the more resistant parts of plants like spores, cuticles and resins and inertinite is material that has been oxidised prior to coalification.
  • Vitrinite and inertinite are subdivided into maceral subgroups depending on size and the degree of gelification. Three prefixes are used to divide the macerals into maceral subgroups. Telo- and detro- differentiate between the size of the individual particles and gelo- means the material has been gelified.
  • Vitrinite in bituminous coal is dark to light grey in colour, depending on the rank. Telovitrinite represents intact fragments of plant matter, the original plant cell structure may be visible in this maceral subgroup. Detrovitrinite results from smaller fragments (must be >20æm in greatest dimension (AS3856-1986)) that often form a groundmass for other macerals. Gelified material produced before or during coalification becomes gelovitrinite which is relatively uncommon. The vitrinite maceral group is desirable technologically.
• The inertinite group originates from the same material as the vitrinite group however the oxidation endured before coalification has changed its optical properties and chemistry. It is much lighter coloured (varying from light grey, to white, to yellow) in comparison with vitrinite.
  • Cell structure is visible in telo-inertinites which are subdivided according to the degree of oxidation. These macerals may exhibit a high degree of relief. The oxidation may have occurred at any time before peat preservation. Forest fires which oxidise wood, can occur during peat accumulation, burnt leaves and wood (charcoal) result in the formation of the maceral fusinite.
  • Since it has a high carbon content to start with the composition of this maceral does not vary with rank Plant material may have already started to gelify and break down before oxidation, this oxidised gel can form the maceral macrinite.
• The liptinite group includes the parts of plants that because of their chemistry are more resistant to physical and chemical degradation. Quantitatively this maceral group is usually much less common, than the other two maceral groups.
  • Above approximately 1.25% mean random vitrinite reflectance liptinite is indistinguishable from vitrinite. Spores, cuticles (found on the surface of leaves and stems), waxes and resin are included in this group, as is alginite which is the remains of algae and rarer substances that may only be detected using fluorescence microscopy.
  • In general this group has a grey to brownish appearance with distinctive morphology and is highly fluorescent when irradiated with short wave (ultra violet or blue) light.

Using a microscope individual species of plants and micro-organisms that contributed to the peat deposit have been identified, by studying the type of spores and pollen, plant tissue or fungal spores. This can then be used to correlate some seams or seam splits, gain a rough estimate of the age of the deposit or provide some insight into the environment of deposition. Knowing the environment of deposition may be useful to predict changes in coal quality. For example anticipating high sulfur and nitrogen values and changes in ash content.

The quantity of each maceral group varies both between and within coal seams. Technologically vitrinite is usually the most desirable maceral group, since it contains more hydrogen and oxygen, but these elements decrease with increasing rank. The name inertinite is a misnomer as it is not all inert. The lower reflecting inertinite within a sample has been found to be reactive during carbonisation (Diessel & Wolff-Fischer , 1987).

Macerals differ in their specific gravity, this can be used to separate the coal from the mineral matter in crushed samples and also to separate out some inertinite (eg. for coal liquefaction). Liptinite macerals are the lightest group followed by vitrinite then inertinite. Fusinite the most carbon rich inertinite has a specific gravity >1.5.

The Hardgrove grindability of a sample is also a function of the petrographic composition, fusinite is the easiest to grind, vitrinite is the next softest while liptinite macerals are the hardest to grind because of their waxy nature (Tsai 1982).

Microlithotypes

Increasing in scale the macerals of coal form microlithotypes, these are bands >50æm wide. There are three main classes of microlithotypes those that contain one type of maceral (monomaceralic), two types of maceral both with a proportion >5% (bimaceralic) and >5% of all three maceral groups (trimaceralic). Microlithotype composition shown in Table 3 adapted from Stach et al. (1982).

The arrangement of microlithotypes is important technologically because of localised reactions during combustion, carbonisation, liquefaction and gasification. For example vitrinertite since it contains some inertinite is more likely to produce a stronger coke.

Lithotypes

In hand specimen coal is often banded, reflecting change in material and conditions in the mire. These bands are termed lithotypes and there are several systems of classification according to rank and preference.

Classification of brown coals may be based on colour, texture, desiccation pattern, strength and degree of gelification

Maceral Analysis

A maceral analysis is carried out on prepared polished grain mounts (or pellets), the coal is crushed and embedded in a mounting medium and the surface is polished for microscopy. The analysis usually involves counting a thousand points on a grain mount which is covered by evenly spaced traverses using a mechanical stage. Each time the centre of the image falls on a maceral, that maceral is entered into a point counter. The result is a volume percentage of each of the different macerals present in the sample.

Vitrinite Reflectance

For bituminous or black coals the most commonly used rank parameter is vitrinite reflectance, which is measured routinely. A measurement of the maturity of organic matter with respect to whether it has generated hydrocarbons or could be an effective source rock. The reflectivity of at least 30 individual grains of vitrinite from a rock sample is measured under a microscope. The measurement is given in units of reflectance, % Ro, with typical values ranging from 0% Ro to 3% Ro. The key attraction of vitrinite reflectance in this context is its sensitivity to temperature ranges that largely correspond to those of hydrocarbon generation (i.e. 60 to 120°C). 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 0.5-0.6% and the termination of oil generation with reflectance of 0.85-1.1%.

The quantity of the principal elements in coal; carbon, hydrogen, oxygen and sulfur, are determined. Usually the results of a proximate and ultimate analysis are enough to indicate how the coal may be utilised. Sulfur and nitrogen can be pollutants when the coal is combusted or carbonised. Alternatively some sulfur may be useful as a catalyst during coal liquefaction.

• Carbon and hydrogen are found in coal as hydrocarbons. The quantity of these is determined by heating an air dried sample in a stream of oxygen and collecting the CO2 and H2O produced.
• Nitrogen is determined using the Kjeldahl method. The nitrogen is converted to ammonium sulfate in sulfuric acid. Titration is then used to find the amount of ammonium sulfate.
• Four quantities are usually quoted for the sulfur content.
  • Total sulfur includes the three forms of sulfur measured.
  • Organic sulfur is that which is bound in organic molecules (hydrocarbons) and originates from the sulfur in the plant matter that contributed to the peat deposit.
  • Sulfide sulfur is that usually found in the form of pyrite and marcasite and is most easily removed by crushing, followed by washing or float sink methods.
  • Oxygen is usually calculated from the difference of the other elements from 100%

Mineral Matter

Mineral matter is different to the ash quantity of a proximate analysis. Mineral matter is the quantity of minerals observed in a grain mount and is usually included in a maceral analysis. Ash is a smaller quantity since it is found by heating the sample and the hydrous minerals are altered in the process.

Minerals may be washed into the mire during peat deposition, or may result from air falls due to volcanic activity. Other minerals precipitate out of ground water solution. Dissolved minerals may also occur in the surface and pore water of the sample. This type of mineral matter is referred to as 'adventitious'.

Plants themselves contain inorganic compounds and organo-metallic complexes that can be added to the peat. This type of mineral matter forms 'inherent ash'.

Mineral matter may occur in thin bands, fill cracks or fissures or be intimately associated with the coal matrix. Cell lumens often contain minerals or mineral matter may replace the cell structure eg. siderite and pyrite. It may also occur finely dispersed within the coal matrix. Discrete bands of mineral matter and infillings are more easily removed during coal washing.

Mineral matter in coal is incombustible and so is left as a residue from technological applications. Mineral matter affects the coal processing and handling. Hard minerals increase the wear and tear on equipment during handling and crushing. The quantity of ash and its composition is important to determine the method of its removal, either as a dry ash or a slag.

The composition of coal ash can affect the product formed. In the case of coking, the qualitity of the steel produced is affected by the elements in the ash (eg. phosphorous is undesirable). Other elements are pollutants eg. sulfur. Ten elements are routinely determined and are expressed as oxides, thes are SiO₂, Al₂O₃, CaO, Na₂O, K₂O, Fe₂O₃, TiO₂, MgO, P₂O₃ and SO₃.

The most abundant minerals in coal are clays, these are variable in their chemistry. Dominant minerals include kaolinite, illite, montmorillonite and illite-montmorillonite mixed layer clays. Bands of clay minerals are useful as marker beds to correlate seams across a coalfield. Clays which have swelling properties such as those in the montmorillonite group, expand in contact with water. This reduces strength and can be hazardous during mining. Clay minerals are converted to silica and alumina during ashing.

Carbonates are the next most common minerals. The main minerals are siderite, ankerite , calcite and dolomite. These decompose to give metal oxides and carbon dioxide.

Sulphides and oxides are commonly found in coal.