Analysis of Coal
There are two methods
1. Proximate analysis and
2. Ultimate analysis.
The objective of proximate analysis indicates the percentage by weight of the Fixed Carbon, Volatiles, Ash, and Moisture Content in coal. The amounts of fixed carbon and volatile combustible matter directly contribute to the heating value of coal. Fixed carbon acts as a main heat generator during burning. High volatile matter content indicates easy ignition of fuel. The ash content is important in the design of the furnace grate, combustion volume, pollution control equipment and ash handling systems of a furnace.
The definition, importance and measure of coal parameters are explained as follows
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.
Typical range of Moisture content is 0.5 to 10%.
Moisture may occur in four forms within coal:
Water held on the surface of coal particles or minerals.
Water held by capillary action within the micro fractures of the coal
Water held within the coal’s decomposed organic compounds
Water which comprises part of the crystal structure of hydrous silicates such as clays.
Measurement: Determination of moisture is carried out by placing a sample of powdered raw coal of size 200-micron size in an uncovered crucible and it is placed in the oven kept at 108+2 C along with the lid. Then the sample is cooled to room temperature and weighed again. The loss in weight represents moisture.
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.
Typical range of volatile matter is 20 to 35%.
Measurement: Fresh sample of crushed coal is weighed, placed in a covered crucible, and heated in a furnace at 900 + 15ºC. For the methodologies including that for carbon and ash, refer to IS 1350 part I:1984, part III, IV. The sample is cooled and weighed. Loss of weight represents moisture and volatile matter. The remainder is coke (fixed carbon and ash).
Ash and Fixed Carbon:
The 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. Typical range Ash content is 5 to 40%.
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. It gives a rough estimate of heating value of coal.
Measurement: The cover from the crucible used in the last test is removed and the crucible is heated over the Bunsen burner until all the carbon is burned. The residue is weighed, which is the incombustible ash. The difference in weight from the previous weighing is the fixed carbon. (In actual practice Fixed Carbon or FC derived by subtracting from 100 the value of moisture, volatile matter and ash).
The objective of ultimate analysis is to determine the amount of carbon (C), hydrogen (H), oxygen (O), sulfur (S), and other elements within the coal sample. The determination of the carbon and hydrogen in the material, as found in the gaseous products of its complete combustion, the determination of sulfur, nitrogen, and ash in the material as a whole, and the estimation of oxygen by difference. The carbon determination includes that present in the organic coal substance and any originally present as mineral carbonate. The hydrogen determination includes that in the organic materials in coal and in all water associated with the coal. All nitrogen determined is assumed to be part of the organic materials in coal.
For practical reasons, sulfur is assumed to occur in three forms in coal: as organic sulfur compounds, as inorganic sulfides, which are mostly the iron sulfides pyrite and marcasite, and as inorganic sulfates. The total sulfur value is used for ultimate analysis.