Coal; also called the “black gold” has earned its name from its indispensability in nearly every industry either directly or indirectly. One of the biggest sources of power across the globe and an excellent reducing agent, coal is found in many forms:
It is an important property of coal, as all coals are mined wet. Groundwater and other extraneous moisture are known as adventitious moisture and are readily evaporated. Moisture held within the coal itself is known as inherent moisture and is analyzed quantitatively. Moisture may occur in four possible forms within coal:
- Surface moisture: water held on the surface of coal particles or macerals
- Hydroscopic moisture: water held by capillary action within the microfractures 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
Proximate Analysis enables us to understand the percentage by weight of Fixed Carbon, Volatile Matter, Ash and Inherent Moisture content of a sample of coal.
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. Any coal which has high volatile matter content:
- Can ignite easily.
- Burns with long smoky yellow flame.
- Has low calorific value.
- Needs large furnace volume for combustion.
- High tendency of catching fire.
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. Identification of Ash content is necessary because it directly affects the combustion and boiler efficiency.
Coal containing high ash:
- Is Harder and Stronger.
- Has Low calorific value.
- Produces slag.
- Causes clinker formation.
Mineral Matter: There are two types: (1) Inherent (2) Extraneous - Inherent comes from inorganic constituent of plant materials, but the amount is less.
- Extraneous comes from the amount that gets associated with substances during its conversion process.
These are associated with internal structures and amount is high. It is difficult to remove them.
It is the main heat generator during the process of burning. 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.
Fuel ratio is the ratio of FC to VM i.e. FC:VM. According to their fuel ratios, coals have been classified as anthracite, at least 10; semianthracite, 6 to 10; semibituminous, 3 to 6; and bituminous, 3 or less.
Gross Calorific Value
GCV is determined to measures the heat value that coal can generate. The GCV of a coal sample is ascertained by LECO AC 500.
Ash Fusion Test
This is determined to understand the coals suitability to the boiler and maintain the efficiency of the boiler. At MSK we use the best in the technology which is the LECO AFT Machine to check the same. At high temperatures, the heat from burning coal is often sufficient to melt its own ash which, When cooled solidifies into clinker. In industrial coal burning application, clinkers can be extremely large and can cause both disposal problem and operation downtime. Coal Ash Fusion testing is therefore required to determine the ash fusion ability characteristics of the fuel.
Ultimate Analysis Test is done to determine the element composition of coal which includes carbon, hydrogen, nitrogen, sulphur and oxygen. We at MSK determine the each element through chemical analysis and also with LECO CHNS Analyser.
With the help of Ultimate analysis and GCV we can determine the NCV or Net Calorific Value of Coal.
Hardgrove Grindability Index
This was developed as an empirical test to indicate how difficult it would be to grind a specific coal to the particle size necessary for effective combustion in a pulverized coal fired boiler or blast furnace.
In case of coking coal we analyze the sample of Crucible Swelling Number (CSN)
PLASTIC PROPERTIES When coal is heated, it passes through a transient stage which is called as plastic state (caking). If a particular coal does not pass through a plastic state, it is called sintered mass (non-coking). Plastic properties of coal are determined by caking index test, free swelling test, GKLT, Plastometer etc.
Caking Index: It is the measure of binding or agglutinating property of coking coal. Coal with ash higher than 17% must be washed before testing.
Crucible Swelling Number
One of the most common, simple caking tests Finely crushed coal is heated rapidly in a crucible and the coke button obtained is compared with a series of standard profiles to give a number, which is the crucible swelling number (CSN). Values range from 0 (no caking characteristics at all) to 9 then 9+ (superior coking properties). Coal having free swelling index between 4-5 are taken for coke making.