COAL PROPERTY:

Physical Property:

Coal is a brownish black sedimentary, organic rock formed from accumulation of plant remains modified by chemical, biological and physical processes during and after burial.  Coal analysis techniques are specific analytical methods designed to measure the particular physical and chemical properties of coals. These methods are used primarily to determine the suitability of coal for coking, power generation or for iron ore smelting in the manufacture of steel. The property of coal is broadly classified as:

1. Physical Property
2. Chemical Property

 

1.   Physical Property of Coal:

 

Physical property of coal includes moisture, fixed carbon, volatile matter, ash, and heating value. All these parameters are calculated with the help of proximate analysis of coal.

Proximate analysis of coal is a method of determining the approximate amounts of different components in coal, such as moisture, ash, volatile matter, and fixed carbon. It is used for various purposes, such as coal exploration, mining, and preparation. It can also help estimate the heating value, combustion behaviour, and environmental impact of coal.

Proximate analysis of coal is performed using an automated instrument called a proximate analyzer. 

a)  Moisture:

Moisture occurs in a range of forms in 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 analysed quantitatively. Moisture may occur in four possible forms within coal:

·         Surface moisture(also termed Adventitious Moisture): This moisture occurs on the surface of coal and in open pores and macerals. Free Moisture which is the moisture yield determined at approximately 40°C.

·         Hygroscopic moisture: This moisture is held by capillary action within small pores and microfractures within the coal. The stronger intermolecular force produces a lower vapor pressure and therefore higher temperatures are required to evaporate this moisture. Obviously, there is a gradational boundary between Surface Moisture and Hygroscopic Moisture. Moisture approximates Residual Moisture which is the moisture yield determined at approximately 105°C. At this temperature there may also be some Decomposition Moisture and some Water of Crystallization in the evaporated moisture.

·         Decomposition moisture: As coal is heated to higher temperatures, water is formed from the decomposition of the organic part of the coal.

·         Mineral moisture: water which comprises part of the crystal structure of hydrous silicates such as clays

 

 

Total moisture is analysed by loss of mass between an untreated sample and the sample once analysed. This is achieved by any of the following methods;

1.    Heating the coal with toluene

2.    Drying in a minimum free-space oven at 150 °C (302 °F) within a nitrogen atmosphere

3.    Drying in air at 100 to 105 °C (212 to 221 °F) and relative loss of mass determined

Methods 1 and 2 are suitable with low-rank coals, but method 3 is only suitable for high-rank coals as free air drying low-rank coals may promote oxidation. Inherent moisture is analysed similarly, though it may be done in a vacuum.

The water content in coal, which decreases its heat value and increases its weight. Reduced moisture content of coal increases the efficiency of power plant, decreases transportation costs, decreases ash disposal requirements and decreases power plant emissions.

Typical range of moisture is 0.5 to 10 %

b)  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. Volatile matter also evaluates the adsorption application of an activated carbon. 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 min. Also as the rank of coal increases the volatile matter decreases. It also content small amount of incombustible gases like carbon dioxide and nitrogen.

The higher the volatile matter content, the lower the minimum ignition temperature of the coal dust, the easier it is burn and faster spreading of the explosion flame.

Typical range of volatile matter is 20 to 35 %

c)  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.

d)  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. After the coal thoroughly burnt and the ash material expressed as a percentage of the original weight. It can also give an indication about the quality of coal. Ash content may be determined as air dried basis and on oven dried basis. The main difference between the two is that the latter is determined after expelling the moisture content in the sample of coal.

 

The inorganic material in coal that does not burn, which increases the cost of transportation and disposal and affects the efficiency and emissions of coal combustion. Ash represents parts of the original mineral matter in coal that does not combust.

As the ash content increases, the lower burning rate of fuel results in char particle leaving the grate without being fully burned, causing a loss of combustible material and therefore reducing the combustion efficiency.

Typical range of ash is 5 to 40%.