Proximate Analysis
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.
1. Moisture The important 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 coal.
- 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.
- Minerals moisture: water which comprises part of the crystal structure of hydrous silicates such as clays.
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, xylem, 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 0C (1650 ± 10 0F) for 7 minutes in a cylindrical silica crucible in a muffle furnace. American Standard procedures involve heating to 950 ± 25 0C (1740 ± 45 0F) in a vertical platinum crucible. Volatile matter leads to proportionate increase in flame length, and helps in easier ignition of coal.
3. ASH – 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, SiO2, TiO2 etc.
