Proximate analysis is the most commonly used chemical analysis conducted
on coals, and is also the simplest to perform. A typical proximate analysis
includes determination of moisture, volatile matter, ash, and fixed carbon
content after the coal sample has been ground to pass 0.212 mm sieve. This
test is used to ascertain the ‘Rank’ of coals, and also to establish the ratio
of combustible to incombustible constituents. These data give substantial
indication about the heating/fuel value of coal, which is a very important
indicator in the process of selling and buying of coals.
The importance of determining ‘Moisture’ in coal arises from the fact that
all coals are mined in very wet conditions. This moisture is water held
within the coal itself, and is also known as ‘inherent moisture’.
‘Adventitious moisture’ refers to Groundwater and other extraneous moisture,
which gets readily evaporated. Moisture may occur in four major forms within
- Surface moisture: water held on the surface of coal particles or macerals.
- Hydroscopic moisture: water held by capillary action within the micro-fractures of the coal.
- Decomposition moisture: water held within the coal’s decomposed organic compounds.
- Mineral moisture: water which comprises part of the crystal structure of hydrous silicates
such as clays.
Moisture decreases the heating value of coal, as it decreases the combustible matter of coal.
2. Volatile matter
Volatile matter in coal serves as an index of the inherent gaseous
fuels present. Basically, it refers to the components of coal,
excluding moisture, which are liberated at high temperatures in the
absence of air. This is usually a mixture of aliphatic or aromatic
hydrocarbons, like methane, benzene, xylene, and compounds
containing sulfur and oxygen. The determination of volatile matter
of coal is conducted under rigidly controlled experimental
conditions, as mentioned in various national and international
standards. For example, 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 Standard procedures involve heating to 950 ± 25 °C (1740 ± 45 °F) in a vertical platinum crucible. Volatile matter leads to proportionate increase in flame length, and helps in easier ignition of coal.
Ash content of Coal is the non-combustible powdery residue left
after burning of coal. It represents the total inorganic/mineral
matter present in coal, after expulsion of all organic compounds,
i.e. compounds containing carbon, oxygen, sulfur etc. The analysis
involves the complete combustion of coal at a particular
temperature, and the ash material is expressed as a percentage
of the original mass of the coal sample. The test results are
generally expressed in terms of metallic oxides, like Na2O, K2O, SiO2, TiO2 etc.
Below is given an example of Coal analysis, where along-with the composition of the various metallic oxides the composition of the corresponding trace metals are also given. The analysis of the coal ash can be done with the use of instruments like ICP-OES and AAS. Data from such analysis can be used for environmental impact modeling.