"COAL" is the general term for the rocks formed from the fossilled remains of fresh water plants in dense swamps or logs. When the plants died, they fell into the water where they were soon engulfed in the black, oxygen-deficient mud. Here they were protected from further decay & did not rot away. The mud itself was largely made of bacterially decomposed vegetable matter subsequent burial beneath sand or mud compressed or consolidated the planty layer, driving out water & other volatile substances. (The vegetable matter underwent a series of changes as this proceeded). The most of World's coal beds were deposited millions of years age in warm, humid regions. They have since been compressed by the weight of overburden or by earth movements to less than 1/20th of the thickness of the original deposit.
Types of Coal
From the view of value as fuel, the coals are classified according to the degree of change that they have undergone. With greater compression the vegetable material is reduced in volume becomes blacker, harder, more brittle and the individual plant fragments become more difficult to distinguish. Such changes are said to indicate and increase in rank. In other terms, rank refers to the MOISTURE, VOLATILE MATTER and FIXED CARBON in the coal and it increases with the proportions of FIXED CARBON present.
Normally, the coal is of two types, Coking and Non-Coking (Anthracite, Bituminous, Sub-bituminous and Lignite).
Calculation to Different Basis
M = moisture%
A = ash%
MM = mineral matter%
ar = as received basis
ad = air dried basis
d= dry basis
For conversion to and from d.a.f. and d.m.m.f. bases, corrections need to be made to some analyses prior to conversion to the required basis. For wxample, both carbon & volatile matter need correction if significant amounts of carbon dioxide are present. These corrections are in most cases not significant.
Basis of Analysis
The Proximate Analysis of any coal i.e. the % content of Moisture, Ash (A),
Volatile Matter (VM), Fixed Carbon (FC) – also Sulphur (S) and Calorific
Value (CV) – can be expressed on any of the above bases.
Metric ton (t) = tonne = 1000 kilograms (= 2204.6 lb)
Imperial or long ton (lt) = 1016.05 kilograms (= 2240 lb)
Short (US) ton (st) = 907.19 kilograms (= 2000 lb)
From long ton to metric ton multiply by 1.016
From short ton to metric ton multiply by 0.9072
Mt million tonnes
Mtce million tonnes of coal equivalent (= 0.697 Mtoe)
Mtoe million tonnes of oil equivalent
kcal/kg – kilocalories per kilogram
MJ/kg* – Megajoules per kilogram
Btu/lb – British thermal units per pound
* 1 MJ/kg = 1 Gigajoule/tonne (GJ/t)
Gross & Net Calorific Values
Gross CV or ‘higher heating value’ (HHV) is the CV under laboratory conditions.
Net CV or ‘lower heating value’ (LHV) is the useful calorific value in boiler plant.
The difference is essentially the latent heat of the water vapour produced.
Conversions – Units
From kcal/kg to MJ/kg multiply kcal/kg by 0.004187
From kcal/kg to Btu/lb multiply kcal/kg by 1.8
From MJ/kg to kcal/kg multiply MJ/kg by 238.8
From MJ/kg to Btu/lb multiply MJ/kg by 429.9
From Btu/lb to kcal/kg multiply Btu/lb by 0.5556
From Btu/lb to MJ/kg multiply Btu/lb by 0.002326
Conversions – Gross/Net (per ISO, for As Received figures)
kcal/kg: Net CV = Gross CV - 50.6H - 5.85M - 0.191O
MJ/kg: Net CV = Gross CV - 0.212H - 0.0245M - 0.0008O
Btu/lb: Net CV = Gross CV - 91.2H - 10.5M - 0.34O
– where M is % Moisture, H is % Hydrogen, O is % Oxygen (from ultimate analysis*, also As Received).
*Ultimate analysis determines the amount of carbon, hydrogen, oxygen, nitrogen & sulphur.
For typical bituminous coal with 10% M and 25% Volatile Matter, the differences between gross and net calorific values are approximately as follows:
1 MWh = 3600 MJ
1 MW = 1 MJ/s
1 MW (thermal power) [MWth] = approx 1000 kg steam/hour
1 MW (electrical power) [MWe] = approx MW (thermal power)/3
A 600 MWe coal-fired power station operating at 38% efficiency and 75%
overall availability will consume approximately:
– Bituminous coal (CV 6000 kcal/kg NAR*): 1.5 Mt/year
– Brown coal (CV 2250 kcal/kg NAR*): 4.0 Mt/year *Net As Received
Quick Review on Coal
Anthracite is the highest rank coal and is characterized buy low volatile matter (always less than 10%) and high carbon content. It has a semi- metallic luster and is capable of burning without smoke. Semi- anthracite is coal midway between low volatile bituminous coal and anthracite.
Bituminous coal is that coal which in rank is between sub-bituminous coal and semi- anthracite. Volatile matter on dry ash free basis range from between 10% and 14% to 40% and over. Usually divided into three sub groups- low volatile medium volatile and high volatile.
SUB- BITUMINOUS COAL
Sub- bituminous coal is the next highest coal in rank after lignite. Typical bed moisture levels are 10-20%, and calorific value also enters in to the classification scheme.
Lignite is a low rank coal containing high moisture. Generally a coal is considered to be a lignite if it contains greater than 20% bed moisture (classification schemes for lignite are generally based on calorific value). Other characteristics of lignite are low reflectance, high volatile matter as high oxygen and low carbon and low carbon level and often the presence of some woody structure. In general the term is synonymous with brown coal.
Peat is the first stage in the conversion of vegetable matter to coal. Bed moistures are high, often greater than 75% and plant remains are clearly visible.
The inorganic residue after the incineration of coal to constant weight under standard conditions. Is less than the mineral matter because of the chemical changes occurring during incineration, with most important differences being loss of water of hydration, loss of carbon dioxide, and loss of sulphurous gases from sulphides.
A component of the proximate analysis calculated by differenece, i.e., 100% less the sum of moisture, ash and volatile matter, intended to give a indication of yield. Not to be confused with carbon in the ultimate analysis.
A term primarily used only Japan and equal to fixed carbon divided by volatile matter.
GROSS CALORIFIC VALUE
The amount of heat liberated during laboratory testing when coal is combusted under standardized conditions. With the temperature of starting materials and products being approximately 25 C. During actual combustion in boilers the gross value is never achieved because some of the products most importantly water, are lost in the gaseous state with their associated heat of vaporization.
The maximum achievable calorific value under these conditions is the net calorific value. Calorific values is also know as Specific Energy and the gross and net CV are known as the higher and lower heating value in some countries.
HARDGROVE GRINDABILITY INDEX
Indicates the relative grind ability or ease of pulverization of a coal in comparisons to coal in comparison to coals chosen as standards. High values indicate a coal easy to pulverize and low values indicate a coal hard to pulverize. Hard grove grindabilty index is rank dependent and increases as does rank, although anthracites have low hard grove grindability indices.
Often used to indicate air dried moisture, particularly in Australia. In ASTM equivalent to bed moisture or equilibrium moisture.
The analysis of coal or coke in term of moisture, ash, volatile matter and (by difference) fixed carbon.
RUN-OF-MINE RESERVE (ROM)
The quantity of coal potentially available to be delivered to a coal preparation facility or stockpile after mining. This is sometimes used synonymously with mine able reserves but will normally show an increase caused by extra unwanted stone inevitable taken in mining and moisture.
An open cast mine where the overburden is removed (stripped) off from the coal using heavy equipments which expose the coal for loading operations.
The moisture in the coal as sampled and removable under standard conditions.
The analysis of coal expressed in term of carbon, hydrogen, nitrogen, sulphur and oxygen, the analysis refers to the carbonaceous material only and hence is often expressed on a dry ash free basis or dry mineral matter free basis. Oxygen is generally estimated by difference although there are methods for its determination.
The loss is mass, less that due to moisture, when coal is heated under standard conditions and out of contact with air. This test is very empirical and results are very sensitive to operating conditions. Results obtained from different standard methods will not necessarily give the same result. The ASTM method generally gives a higher result than other methods.