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Advanced Coal Science

Coal – How It Is Formed

Coal is made from Peat that has been compressed and then subjected to high temperature ( + 100° Celcius) for a few million years to drive off moisture, methane and carbon dioxide bearing molecules.  This process causes the peat to progress through brown coal, lignite, sub-bituminous coal and then to bituminous coal (the category most of Australia’s coal currently mined belongs to) and finally anthracite.  Through this process, the carbon content of the coal increases as does the energy content of the coal.  Commensurate with this process is a reduction in the moisture content and oxygen content of the coal.

The process by which peat accumulates is complex but in general it:
• requires a source of vegetal material (plant material in the form of trees, branches, leaves, spores and resins),
• requires a preserving medium (normally water) where microbial activity is restricted.
• requires a sinking floor or rising water table for the duration of the peat formation
• requires a long period of time where quiescent condition predominate

Where Does Coal Form?

Coal, as with most organic sediments, tends to accumulate in structures known as basins.  These structures are normally controlled by a series of faults that provide the constant sinking of the floor of peat swamps.  These basins may be several thousand kilometres long and several hundred kilometres wide and are normally associated with “Plate Tectonics” at the time of formation.  These basins are often associated with the accumulation of many thousands of metres of sediment and are found behind zones of subduction where ocean floor sinks beneath crustal rocks depiceted below.


Schematic Diagram of Coal Basin Setting

This type of peat-forming environment is responsible for the Sydney-Bowen Basin that hosts the majority of Australia’s bituminous coal.  Most of the major coalfields around the world have been developed as a result of similar types of tectonic environment.

 

Coal Types

Although all coal can be described by the basic constituents of moisture, volatile matter, fixed carbon, ash and sulphur, a microscopic evaluation of coal reveals the plant type and depositional environment that the coal was formed from.  The components in coal that can be recognised by microscope are described very simply by coal specialists as macerals and are divided into 4 groups shown below.

 

Coal Maceral Groups

Maceral Type Description Additional Information
Vitrinite The vitreous part of coal that is normally represented as bands. This material is formed from woody material This maceral group has the lowest ash content and is responsible for the coking properties for coals that meet the coking coal criteria.
Liptinite Derived from resin bodies, waxy cuticles and spores from plants. Produce large quantities of volatile matter.
Inertinite As the name suggests this group is inert (to the coking process) and does not soften or swell on heating This maceral group comprises charcoal and products of fungal breakdown of woody tissue. It contains low volatile matter and is responsible for the sooty effect of coal.
Mineral Matter Comprises the non-organic component of coal such as clay, quartz, and minerals introduced later. This material makes up most of the ash of a coal, where mineral matter comprises mostly clay the ash is very refractory, when is comprises mostly carbonate minerals the ash melts readily

 

Examples of the various macerals groups are shown below where banding of coal from the Gunnedah area is clearly visible with dull inertinite at the top, a mixture of vitrinite and liptinite in the middle and pure vitrinite at the bottom of the picture.

 

Gunnedah Coal Showing Three Maceral groups

Source Organic Petrology G Taylor, M Teichmuller, A Davis & C Diessel et al. 1998

Coal Rank Progression

Coal is a sedimentary deposit (laid down in water) that comprises preserved plant material from ages past.  Over time a great thickness of plant material accumulates that is broken down into peat.  This represents a storehouse of organic matter produced via photosynthesis by plants using the sun’s energy.  In this form even the peat is a source of preserved solar energy.
Peat contains a lot of water and subsequent burial of the peat results in the water being squeezed out.  This process normally extinguishes bacterial activity and as temperatures rise with increasing depth of burial, the coalification processes begin to transform the peat to brown coal, then lignite, sub-bituminous coal, bituminous coal and finally to anthracite.
This coalification process is accompanied by a decrease in moisture, a decrease in volatile matter and increase in carbon and an increase in calorific value and a commensurate decrease in oxygen until the end of the bituminous rank shown in the table below.

Changes in Coal Properties as Rank Increases

  Moisture Volatile Matter Carbon Content Calorific Value Oxygen Content
Coalification Stage As Recieved Dry Ash Free  Dry Ash Free Asrecieved Dry Ash Free
Peat ~75% 69 - 63% <60% 3,500 kcal/kg >23%
Lignite 35 - 55% 63 - 53% 65 - 70% 4,000 - 4,200 kcal/kg 23%
Sub-bituminous C 30 - 38% 53 - 50% 70 - 72%  4,200 - 4,600 kcal/kg 20%
Sub-bituminous B 25 - 30% 50 - 46% 72 - 74% 4,600 - 5,000 kcal/kg 18%
Sub-bituminous A 18 - 25% 46 - 42% 74 - 76% 5,000 - 5,500 kcal/kg 16%
High Volatile Bituminous C 12 - 18% 46 - 42% 76 - 78% 5,500 5,900 kcal/kg 12%
High Volatile Bituminous B 10 -12% 42 - 38% 78 - 80% 5,900 - 6,300 kcal/kg 10%
High Volatile Bituminous A 8 - 10% 38 - 31% 80 - 82% 6,300 - 7,000 kcal/kg

8%

Medium Volatile Bituminous 8 - 10% 31 - 22% 82 - 86% 7,000 - 8,000 kcal/kg 4%
Low Volatile 8 - 10% 22 - 14% 86 - 90% 8,000 - 8,600 kcal/kg 3%
Semi-Anthracite 8 - 10% 14 - 8% 90% 7,800 8,000 kcal/kg 3.5%
Anthracite 7 - 9% 8 -3% 92% 7,600 - 7,800 kcal/kg 4.5%
Meta-Anthracite 7 - 9% 8 - 3% >92% 7,600 kcal/kg 5%

 

 

Coal Formation

Plant material normally accumulates in peat swamps that are low lying areas shielded from the incursion of sediment bearing streams by vegetative barriers.  A peat swamp normally accumulates 7 – 20 metres of peat to produce 1 metre of bituminous coal and the rate of peat accumulation is extremely slow, measured in mm per year.  The peat comprises fallen trees, leaves, branches, spores and pollen that can accumulate at rates varying from 1 mm/year in reed swamps to 4 – 5 mm year in tropical forest swamps.  The formation of a coal seam therefore requires very stable conditions for long periods of time – about 2,500 - 6,000 years per metre of coal.


To prevent the peat swamp drying out it is necessary for either the water table to rise at the same rate as peat accumulates or for the stratum that the peat swamp rests upon to subside below the water table at the rate of peat accumulation.
The peat continues to accumulate until it is either drowned by a rising water table or it is covered by sediments from fluvial channels. Drowning normally results in the peat being covered by mud where as an approaching stream channel will normally be associated with sandstones and other coarse sediments deposited on the top of the peat as a result of flooding activity.


While accumulating in the peat swamp, the plant material is broken down to peat by bacterial, actinomycal and fungal activity while the peat has access to the air and anaerobic bacteria once the peat swamp has been buried. This process is controlled by temperature and the degree of acidity.  Highly acid peat swamps do not promote bacterial activity while more alkaline peat swamps promote bacterial activity.  This process is called peatification.

Effects of Climate

Peat accumulation requires that the rate of inflow of plant material into the swamp is greater than the rate of decay of the plant material.  Cellulose decomposing bacteria are the most active at temperatures in the range of 35 – 40 °C.  Peat accumulated under such conditions is normally degraded and partially homogenised forming coal that is devoid of the characteristic banding found in cool climate coals such as those formed in Gondwana Land during the Permian Period.


Permian coal seams from Australia, South Africa, Madagascar, India and Antarctica were all deposited during cold conditions.  As a consequence peat accumulated normally at a faster rate than subsidence and the peat swamps were exposed to the atmosphere for long periods of time. As a consequence fungal activity and fires converted much of the woody material near the top of the seam to either charcoal or material from which the soft cell lumens had been consumed by fungi leaving behind the lignin rich precursors of inertinite.


Worldwide the major coalfields reflect a diversity of climatic environments under which they formed and as time has progressed they also represent a greater diversity of plant life. (Table 13).

 

Coal Forming Periods

Age Coal Basins Climate / Dominant Plant Types
Carboniferous (360 – 290 my BP) Kuzbass (Russia) Donets (Ukraine), Kazakhstan coalfields, Saar-Lorraine coalfields, UK/French coalfields, Appalachain coalfields, Cape Bretton/Newfoundland coalfields Warm climate, moist,tropical/sub-tropical. Coal made from Lycopods (Lepidodendron and sigolaria), Gymnosperms (Cordaites) and Cycadophytes.
 
Permian (290 – 251 MY BP) East Coast Australia, South Africa, India, Madagascar, South America, Antarctica. Zimbabwe, China Climate considered to be cold with warm wet summers and freezing winters. Main plants Gymnosperms (Glossopteris and Gangangopteris)
Triassic (251 – 205 MY BP)  Callide and Tarong Australia Cool climate warmer than Permian with similar plants
Jurassic (205 -  141 My BP) Gunnedah, Walloon, Milmerran basins Australia, Yakutia and Pechora Basins Russia Appearance of flowering plants such as Angiosperms, but gyymnosperms and cycads remain the major peat forming plant
Cretaceous (141 – 65 MY BP) Canadian, Wyoming, Colorado, Spitzbergen, New Zealand, Venezuela Cool Climate to warm, Angiosperms predominate
Tertiary
(65 – 1.78 MY BP)
Indonesia (Eocene & Miocene), New Zealand (Paleocene, Eocene, Miocene, Oligicene), Australia (Eocene, Oligocene Miocene), China, Germany, Japan, USA, Canada Warm – Angiosperms predominate.

 

USAGE OF COAL

As discussed previously the volatile matter and fixed carbon of the coal contain energy and this can be harnessed for the purposes of raising steam for electricity generation and general boiler use or for the calcining of limestone in the manufacturing of cement.  The sale of coal for thermal purposes constitutes the major use of coal, but there are other uses for coal that are broadly termed metallurgical where the coal is used in some way to reduce metallic oxides to metals.

  • Metallurgical coals


Metallurgical coals generally fall into two categories:
Coking coals and non-coking coals.  The coking coals are used to make coke, a hard porous substance that comprises about 90% carbon with the balance being ash (non-combustible material), volatile matter and other impurities such as sulphur and phosphorus.  The non coking coals are those coals used in other processes that are used in the reduction of metallic oxides to metals.


Coking coals are those coals that soften, swell and then solidify as they are heated through the temperature range 350°C to 550°C.  By definition these coals all have a low ash content (1 – 10%), have low permeability as determined by inherent moisture, moderate vitrinite content (to provide volatile matter) and volatile matter in the range 18 – 45%


The reflectance of the maceral vitrinite is also used as a measure of a coal’s suitability for coking.  Reflectance measures the amount of light that is reflected from a polished piece of vitrinite and for coking coals it is in the range 0.6% through 1.8% (range of bituminous coals).  The coals with the lowest reflectance have the lowest rank and highest volatile matter.  Reflectance measures the degree of graphite crystallite order within the coal.  These are a precursor to the degree of graphitisation that is achieved when the coal is coked.


Low rank coals have few aromatic ring structures and none of these are aligned.  As the rank of the coal increases the number and alignment of these ring (graphitic structures) increases.  These increase through the condensation of alpiphatic compounds and the loss of methane and carboxyl groups until graphite has been produced.  Aliphatic compounds are long chain hydrocarbons that break and condense as rank increases releasing methane and soluble metallic carboxyl groups.


Lower rank coking coals have a lot of oxygen containing groups that link the aromatic ring structures and on rapid heating (as in a coke oven) these oxygen linkages cleave only at elevated temperatures where maximum volatile matter release occurs and consequently the plastic properties are short lived.  As the rank increases the oxygen content of the vitrinite component of the coal decreases and the temperatures required to create a plastic mass are reduced, as a consequence the plastic range increases.  Once coals reach the top end (low volatile matter) of the bituminous rank, the coals have very low oxygen content and also low volatile matter content, and consequently the plastic range begins to decrease again until no plasticity occurs at about 16% volatile matter.


The type of carbon produced in the resultant coke is a function of the rank of the precursor coal. Coals with more ordered graphitic structures produce cokes with highly ordered and unreactive carbon (mosaic).  Mosaic is the term used to define the graphitic structure observed in cokes.  The larger the domain of each of the mosaic units the higher the rank (lower oxygen content) of the precursor vitrinite.  Crude oil has a low oxygen content and the coke produced after cracking out all the liquid and gaseous products produces a very unreactive form of carbon.

 

Increase and Alignment of Graphitic Ring Structures as Rank Increases


Mosaics from three coal seams located in the northern Bowen Basin are shown below with rank increasing from Q Seam (Upper Goonyella equivalent) 0.8% Rv max through E Seam (Middle Goonyella equivalent) to 1.3% Rv max in the Blake seam at the base of the Collinsville Coal Measures.  The size of the mosaic domains increases with increasing rank.

 

Figure 6: Q Seam - Moranbah Coal Measures Rv Max 0.8%, note fine "Equant"Mosaic fining toward inertinite.  Field of view 66 microns


The mosaic shown in Figure 6 is typical of a soft coking coal and contrasts with the mosaic from a seam with 1.0% reflectance (Figure 7).

 

Figure 7: Large Flame Mosaic from the E Seam (Moranbah Coal Measures) Rv max 1.0%, Field of View 165 microns – Note large gas vesicles

Figure 7 shows the effects of higher rank and lower oxygen content on the morphology of the coke mosaic – note the presence of large gas vesicles.  This mosaic is more ordered and thus less susceptible to attack by carbon dioxide in a blast furnace.

 

Figure 8:Ropey Mosaic Derived from Blake seam Coal with 1.3% Rv max - note small gas vesicles- Field of view 165 microns

 

The structure and size of the graphitic crystallites is not the only factor impacting on the reactivity of the coke, the composition of the ash of the coal that made the coke is also of prime importance to the manufacturers of blast furnace coke.  Alkali and alkali Earth metals are known to catalyse the reactions in the blast furnace that lead to premature oxidation of the coke.

Blast furnace operators prefer coke to remain inert until it reaches the base of the blast furnace where temperatures are high enough to melt iron (1,538°C).  Prior to this temperature any coke that reduces carbon dioxide to carbon monoxide is effectively lost to the system.  Two factors influence the propensity of a coke to react with carbon dioxide before 1,538°C, the size (and thus reactivity) of the mosaic and the presence of catalysts that promote the carbon solution reaction at lower temperatures.  The two important reactions in a blast furnace are simply expressed below:


Equation 1: Carbon Solution Reaction

  • C (coke) + CO2 (gas)   →    2 CO

Since carbon monoxide is the reductant for Iron ore, Equation 1 must occur if the ore is to be reduced.  The reduction reaction is simply represented in Equation 2.

 

Equation 2: Reduction of Iron Ore

  • FeO + CO  →   Fe (liquid) + CO2 (gas)

 

The main function of coke in a blast furnace is to provide an open structure to allow the reacted gases to move up the blast furnace. When the coke reacts prematurely with the carbon dioxide in the blast furnace atmosphere, the coke becomes weakened and finally crushes reducing the permeability and thus productivity of the blast furnace.

The secondary functions of coke are to provide both heat and carbon dioxide to drive the reduction reactions that produce the iron at the bottom of the furnace. Since coke is a very expensive commodity (up to US$400/tonne), blast furnace operators prefer to use it for creating permeability in the blast furnace and they use pulverised coal injection (PCI) at the base of the blast furnace to provide both heat and a source of carbon dioxide.  Enhancement of the carbon dioxide generation is attained by use of low volatile matter coals for PCI which coincidentally have very high specific energy values and are able to provide conditions hot enough to allow the unreactive coke to react with CO2 at the base of the blast furnace.

Manufacturers of blast furnace coke use a combination of coals to make up the coke oven blend and in addition to this they also use various techniques to maximise the coke strength and reduce reactivity of the coke.  Coking coals within the rank range 1.15 – 1.4% reflectance are considered prime coking coals if the ash content is < 9.5% and sulphur approximates 0.6%.  Coal such as this is the most expensive and coke manufacturers try and reduce costs by using coals classed as soft coking (make a weaker coke) and semi-soft coking (are weakly coking) coals.  Soft coking coals by definition tend to have a high volatile matter content and produce a weak and frothy coke.  To overcome these short comings coke manufacturers add inert material in the form of coke breeze or petroleum coke to reduce the overall volatile matter of the blend. The coke oven blend is stamp charged to remove air between coal particles to allow for better bonding.  Finally the coal charged is dried to ensure faster heating rates (that promote plasticity in the coal mass) and to prevent dissolution of the coke through the water shift reaction.

Where coke manufacture is undertaken as part of an integrated steel mill, the production of coke oven gas for use in the rolling mills and for generation of electricity becomes an important consideration when choosing coals to use in a coke oven blend.  Steel mills that have smaller blast furnaces are able to use coals with a lower coke strength after reactivity (CSR) and as a consequence have a wider range of cheaper coals to choose from.

 

Non-Blast Furnace Route for Metals Production

 

Iron

In India, South Africa, Korea, China and New Zealand, hot iron metal is produced from routes other than the blast furnace route through the employment of direct reduction rotary kilns. This system uses similar chemical reactions but does not require the coal to be converted to coke.  Low ash coal that is reactive is charred in a multi hearth kiln to remove volatile matter before it is mixed with iron ore (normally magnetite) and allowed to react in a counter-current kiln where carbon from the char reacts with carbon dioxide to form the reducing gas carbon monoxide which reduces the iron ore fines.  This process does not require the melting of the iron, but produces a sponge iron instead. The sponge iron is melted in an electric furnace where it is converted to steel.  Typically the process requires highly reactive (lower rank) coals that have low ash, low sulphur and low phosphorus contents.

Other metallic oxides such as lead are reduced using foundry type cokes (large lumps of coke) as the reductant in a process similar to a blast furnace.  The requirements of the coke are less stringent and operating temperatures lower than iron blast furnaces.

Silicon

Silicon metal is a major component in electronics (photovoltaics), as the basis for the production of silicones and as a hardening agent for aluminium. It is produced by removing the oxygen content from quartz (SiO2) and is a very endothermic process requiring a large amount of energy.  Very pure coal is added to sized pure quartz and large amount of electricity produced from a carbon anode are used to produce a submerged arc that provides heat at 2,000 °C. The coal is required as a source of reactive carbon to remove oxygen from the quartz in Equation 3:

 

Equation 3: Quartz to Silicon Metal Reaction

  • SiO2 + C (from coal)   2,000°C →  Si (metal) + CO2

The requirements for the coal are very stringent as it is the primary source of any contaminants.  Typically chemical grade silicon must have the following minimum qualities:

Silicon  98.5%
Iron    0.5%
Calcium   0.07%
Aluminium   0.2%

Aluminium Grade Silicon must have the following

Silicon  98.5%
Iron    0.35%
Calcium   0.07%

As a consequence the coal used must have very low quantities of iron, calcium and aluminium which are normally achieved through use of coal with <1% ash. To achieve the requisite reactivity the coal normally has 28 – 40% volatile matter.

Other uses for coal

Activated carbon

Carbon has a positive charge and when coal has been charred and then steam activated it develops a very large surface area that can be used to attract negatively charged anions such as chlorine to purify water or act as a substrate onto which gaseous or liquid hydrocarbons can be attached such as dry-cleaning vapours and petrol vapours that can be collected in activated carbon filters to prevent them entering the atmosphere.  Activated carbon is also used to take molasses out of sugar syrup to produce white sugar.

 

Anthracite Uses

Anthracite is very high rank coal in which there is virtually no volatile matter and consequently has a high percentage of carbon.  Very high rank anthracites (meta-anthracite) which have volatile matter of <3% are used extensively in the reduction of silica to silicon metal and in the production of ferro-silicon.  Anthracites are also used extensively for water treatment both as a filtration medium and as a scavenger for unwanted chemicals in the water.

Meta-anthracites are also used in the manufacture of carbon anodes and cathodes as it has good electrical conductivity and good abrasion resistance.
Anthracites are also used extensively as a smokeless fuel.

 

Thermal coals

All metallurgical coals can be used as thermal coals, but competing pressures from other consumers of more specialised coals results in only the cheapest coals being used for thermal purposes.  The greatest use of thermal coal is in the generation of electricity via the pulverised fuel method.
Although this market sector is the repository of most of the coal mined in New South Wales and Indonesia the requirements on a power-station by power-station basis are often quite exacting and governed by the equipment at the power station.  Factors that affect coal fired power stations are:

 

  • Free moisture in the coal – this affects handling of the coal
  • Total moisture of the coal – this impacts on the drying capacity of the grinding circuit
  • Hardness of the coal – this affects the mill and power requirements to reduce the coal to 70% passing 75 microns
  • Ash fusion characteristics – normally the higher the temperature at which the ash fuses (melts) the better as ash deposits that form on the steam tubes act as insulators and reduce the efficiency of the transfer of heat energy from the burning of coal into steam energy
  • Volatile Matter – this is the most readily combustible part of the coal and high volatile matter coals readily evolve large amounts of gas and tar leaving behind particles of char that are oxidised.  Low volatile matter coals require special furnaces to achieve good char burnout
  • Sulphur content of the coal is converted on combustion to sulphur dioxide which is a pollutant gas.  Some dust removal systems work best with some sulphur in the coal.
  • Ash contents should generally be lower than 30% and this level is dependent on the economics of transport the coal from the mine to the power plant.  Ash is a dilutent and requires transportation while contributing nothing to the process of generating electricity.  Following combustion, ash collected from baghouses or electrostatic precipitators must be disposed of. Coal ash with high silica and alumina content produces fly ash that can be used as an additive to concrete.  Fly ash addition can increase the density of concrete, reduce shrinkage and reduce heat during the setting process. The use of fly ash reduces the quantity of cement required to attain a certain concrete strength
  • Specific energy of coal – the specific energy of a coal is that amount of energy contained in the organic matter less the dilution caused by mineral matter (ash) and moisture.  Clearly the higher the energy content of the coal the better it is from the economics of running a power station, less coal is required to be transported to the power station site, less coal requires grinding and as a consequence there is less ash to be disposed of. Energy content of the coal is measured in 4 different ways:
    • Dry basis (db)– an American method that uses dry basis energy and treats the effect of moisture separately
    • Air-dried basis (adb) – a British based system that uses the energy of a coal that has been air-dried and so retains the inherent moisture of the coal.
    • As received basis (ar) – The energy contained in coal as it is delivered to a power station, this includes all the moisture and ash.
    • Net as-received basis (nar) – This basis determines the amount of usable energy a coal contains and is the as-received energy less the amount of energy required to vaporise the moisture in the coal and moisture formed from the combustion of the hydrogen in the coal.