The heating value or calorific value of a substance, usually a fuel or food, is the
amount of heat released during the combustion of a specified amount of it. The
calorific value is a characteristic for each substance. It is measured in units
of energy per unit of the substance, usually mass, such as: kcal/kg, kJ/kg, J/mol,
Btu/m³. Heating value is commonly determined by use of a bomb calorimeter.
Proximate Analysis
A typical proximate analysis includes the moisture, ash, volatile matter, Ash and fixed
carbon content.
1. Moisture
Moisture is an important property of coal, as all coals are mined wet.
Groundwater and other extraneous moisture is known as adventitious moisture and
is readily evaporated. Moisture held within the coal itself is known as inherent
moisture and is analyzed. Moisture may occur in four possible forms within coal:
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Surface moisture: water held on the surface of coal particles or macerals. |
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Hydroscopic moisture: water held by capillary action within the microfractures of the coal |
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Decomposition moisture: water held within the coal’s decomposed organic compounds |
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Mineral moisture: water which comprises part of the crystal structure of hydrous silicates such as clays. |
2. Volatile matter
Volatile matter in coal refers to the components of coal, except for moisture, which are liberated at high temperature in the absence of air. This is usually a mixture of short and long chain hydrocarbons, aromatic hydrocarbons and some sulfur. The volatile matter of coal is determined under rigidly controlled standards. 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.
3. Ash
Ash content of coal is the non-combustible residue left after coal is burnt. It represents the bulk mineral matter after carbon, oxygen, sulfur and water (including from clays) has been driven off during combustion. Analysis is fairly straightforward, with the coal thoroughly burnt and the ash material expressed as a percentage of the original weight.
4. Fixed carbon
The fixed carbon content of the coal is the carbon found in the material which is left after volatile materials are driven off. This differs from the ultimate carbon content of the coal because some carbon is lost in hydrocarbons with the volatiles. Fixed carbon is used as an estimate of the amount of coke that will be yielded from a sample of coal. Fixed carbon is determined by removing the mass of volatiles determined by the volatility test, above, from the original mass of the coal sample.
Coal Ultimate Analysis
Similar to coal proximate analysis, the objective of coal ultimate analysis is
to determine the constituent of coal, but rather in a form of its basic chemical
elements. The ultimate analysis determines the amount of carbon (C), hydrogen (H),
oxygen (O), sulfur (S), and other elements within the coal sample. These variables
are also measured in weight percent (wt. %).
Ash Analysis
| Oxides |
wt.% of ash(Calculated) (Desirable Limits) |
Elements | wt.% of ash(Measured) |
| Na2O | 0.35 | Na | 0.26 |
| MgO | 0.48 | Mg | 0.29 |
| Al2O3 | 20.0 | Al | 10.6 |
| SiO2 | 74.1 | Si | 34.6 |
| P2O5 | 0.05 | P | 0.05 |
| K2O | 1.1 | K | 0.92 |
| CaO | 0.68 | Ca | 0.49 |
| TiO2 | 0.80 | Ti | 0.48 |
| Mn3O4 | 0.06 | Mn | 0.05 |
| Fe2O3 | 3.25 | Fe | 2.28 |
Ash Fusion Temperature
The behaviour of the coal's ash residue at high temperature is a critical factor in selecting coals for steam power generation. Ash fusion temperatures are determined by viewing a moulded specimen of the coal ash through an observation window in a high-temperature furnace. The ash, in the form of a cone, pyramid or cube, is heated steadily past 1000 °C to as high a temperature as possible, preferably 1600 °C (2900 °F).
