Chemical Composition of Coal:

Each type of coal has a certain set of physical parameters which are mostly controlled by moisture content, volatile matter content and carbon content.

 

 

Moisture: Moisture 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 analysed quantitatively. Moisture may occur in four possible forms within coal:

 

i)     Surface moisture: water held on the surface of coal particles or macerals.

ii)    Hygroscopic (Inherent) moisture: water held by capillary action within the

microfractures of the coal.

iii)   Decomposition moisture: water held within the coal’s decomposed organic

compounds

iv)   Mineral moisture: Water which comprises part of the crystal structure of hydrous silicates such as clays, quartz.

There is a correlation between the moisture content and the rank or maturity of coals. Generally, the moisture content of low rank coals (lignite) is greater than 20% where as in the highest rank coal anthracite moisture content is generally 1-2%. Strongly coking coal also contain moisture less than 2%.

 

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.

 

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.

Volatile matter (VM) is the percentage loss in mass, adjusted for moisture, when coal is heated out of contact with air under standard conditions. In this test, 1 g of sample is placed into a lidded crucible (to prevent ingress of air), which is placed in a furnace at 900 °C for 7 min or 950 °C if using the ASTM method

Volatile matter is one of the factors governing the ignition and flame stability, the reactivity and burnout of chars, and the amount of unburned carbon in the fly ash.

Coal with higher volatile matter can be beneficial for ignition and combustion processes because they are easier to ignite, generally have better flame stability and improved carbon burnout, and lower NOx emissions have been observed with coals with higher volatile matter.

While using a coal with lower volatile matter than the design value, the required coal fineness needs to be increased (i.e., higher power consumption, more mill wear) to maintain a low carbon loss and improve combustion efficiency

  

Table 1.1: Rank of Coal and VM content: (dry ash free basis)

 

Rank stages	% VM (daf)
Wood	65
Peat	60
Brown coal	52
Sub-bituminous coal	40
High volatile bituminous coal	31
Medium volatile bituminous coal	22
Low volatile bituminous coal	14
Semi-anthracite	8
Anthracite	2

 

VM supports ignition of the coal, therefore it is favourable for coal burning. When VM decreases and the fuel ratio (fuel ratio = fixed carbon/VM) increases, the coal becomes harder to ignite and burns more slowly. VM content is often used as an indication of the ease of combustion of a coal. The fuel ratio is often calculated to assist in boiler design.

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. It gives an indication about the quality of coal.

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 a measure of the amount of non-volatile carbon remaining in a coal sample. It is a calculated value determined from other parameters measured in a proximate analysis, rather than through direct measurement (ASTM method D3172-07a; American Society for Testing and Materials, 2013, p. 492-493). Fixed carbon is the calculated percentage of material that was lost during the testing for moisture, volatile matter, and ash:

Weight % fixed carbon = 100 – weight % moisture + weight % volatile matter + weight % ash

 

Fixed carbon from proximate analysis is a different value than total carbon from ultimate analysis. Total carbon includes some organic carbon that escapes as volatile matter emissions during combustion. Fixed-carbon content increases with rank, and is used to define ranks above medium-volatile bituminous coal. Fixed carbon has the opposite trend of volatile matter with increasing rank because increases in the amount of volatile matter driven off of coal increase the relative amount of carbon (Stach and others, 1982). Fixed-carbon content is also an important criteria for estimating the amount of coke that can be distilled from a coal (Zimmerman, 1979). Coke is a high-carbon product used in steel production.

 

  

 

 

 

Reference:

 

1. K. Miller, in The Coal Handbook: Towards Cleaner Production: Coal Production, 2013.