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About
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- Coal is simply old plants buried inside earth for millions of years to become a rock (sedimentary). The elemental composition of the plants, which includes carbon, hydrogen, and oxygen, transforms into coal over time. This transformation occurs in the absence of oxygen.
- With time, plant loses its oxygen and water content. The longer the coal is underground, the more concentrated the hydrocarbons become, resulting in a higher quality fuel.
- Accordingly, coal is classified peat, lignite, bituminous, and anthracite, with anthracite being the highest quality and most valuable type of coal. See table below for details.
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Composition
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- Coal or any hydrocarbon is primarily made of carbon, hydrogen oxygen. In addition, since they are just old plant matter, they should contain some nitrogen, sulphur and phosphorous. (remember all life is made of CNOS majorly)
- In addition, there is some moisture, heavy elements as they are buried deep.
- Carbon, hydrogen and oxygen are all combustible. Meaning when they mix with oxygen, they release some heat. This is the heat we are all interested in to do work in the powerplant.
- Note that even oxygen is combustible, but the proportion is so low we don’t bother.
- Note its carbon and hydrogen that decides the amount of heat we can generate. If you look at chemical composition, it is C137H97O9NS for bituminous coal and C240H90O4NS for anthracite. (You don’t need to remember the numbers its only for you to know why anthracite is better)
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Burning and Combustion
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- What do mean by burning and combustion?
- Burning of any fuel simply means you are adding heat to break the bonds between carbon and hydrogen.
- Once you do that you add oxygen(air) in order for carbon and hydrogen to combine with. This gives oxides of carbon and hydrogen and in the process give out heat. (you always add heat to break the bonds and you always get heat when bonds are made)
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Burning of Coal: Steps
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- Coal when burnt, 1st thing that comes out is water. As dug from the ground coal has some amount of moisture.
- At slightly above 1000C (boiling point of water) causes water to evaporate leading to loss of weight.
- The left-over dry coal is further heated but now in the absence of air (we don’t want burning). The matter that comes out is called volatile matter.
- Note that volatile matter is also hydrocarbon (aromatic rings) that evaporates when heated in the absence of oxygen.
- The left-over coal is called fixed carbon which is what we “burn” in coal plants.
- The carbon content now ranges from 50% to about 95%.
- Now we burn this fixed carbon coal in presence of oxygen to derive useful heat.
- After all carbon and hydrogen is ‘burnt’, the left-over non-combustible solid matter is called ash.
- Ash is simply left-over incombustible solid made of inorganic contents. This is the main useless and hence undesirable thing in solid fuels in general. (more on this later)
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Process Of Burning
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- In coal-fired plant fuel is coal and source of oxygen is air
- Complete combustion (allow all coal to burn in air)
- Coal + Oxygen —–> Carbon dioxide + other gases (mixture is called flue gas)
- 1 gram of Carbon —–> 3.6 gram of CO2 and 9 gm of N2
- Incomplete combustion (don’t allow all coal to burn by restricting the supply of air)
- Carbon + Oxygen —–> Carbon Monoxide (if you add more air it can ‘combust’)
- 1 gram of Carbon —-> 4.5 g of CO and 5.5 g of N2
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