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Need for coal conversion
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- Coal is basically a complex molecule that is made of long chains of hydrocarbons arranged in the form of rings. (see figure).
- As a result, it takes a lot of heat to burn coal and in-turn lot of heat is released when carbon in coal oxidizes with air.
- As a result, the temperature in a coal power plant is extremely high which leads to two problems a) thermal pollution and b) oxides of nitrogen are formed. (remember otherwise stable nitrogen, oxidizes at very high temperatures)
- In addition, burning of coal as in case of any solid fuel leaves solid residue like ash.
- In order to tackle all these issues coal conversion is resorted to.
- Coal conversion simply means converting combustible solids (coal) to combustible liquids or combustible gases.
- Accordingly, coal conversions include liquefaction and gasification.
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Gasification Of Coal
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About
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- While the goal of combustion is to produce the maximum amount of heat possible by oxidizing all the combustible material, the goal of gasification is to convert most of the combustible solids into combustible gases such as carbon monoxide, hydrogen, and methane.
- See how this works.
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Coal gas/Producer gas
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- Burn coal with air: complete combustion —-> CO2
- Burn coal with controlled oxygen: incomplete combustion —-> [CO + N2]. This is called coal gas or producer gas.
Advantage
- Fly Ash is absent.
- Less NOx formation due to low temperature of incomplete burning
- Carbon sequestration is easy (easy to remove nitrogen from flue gas as compared to oxides of nitrogen)
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Syn Gas
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- Alternately burn Coal with Steam —–> CO + H2 (Synthesis Gas/Syngas)
- Aptly this is called steam reforming.
- You can make variety of things using Syngas like Methanol, Hydrogen, Methane (methanation).
- Syngas can be used to make ammonia-based fertilizer.
- N2(g) + 3 H2(g) —–> 2 NH3(g)
Advantage
- Reduced CO2 emissions.
- Syngas is combustible.
- NOx is not formed.
- Fly Ash is absent.
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Syngas can be used to make methane in a process called Methanation/ Hydrogasification
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Advantage
- CH4 is combustible.
- CO2 is not produced.
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Liquefaction Of Coal
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About
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- In liquefaction the trick is to break the long chains of hydrocarbon rings and add hydrogen at high pressure.
- Depending on the type of coal used we get various coal liquids like gas oil, gasoline, kerosene.
- Note that the main difference between coal and petroleum products is the size of hydrocarbon molecule. (coal is long chain with 140-250 carbon atoms, petrol has 4-10, diesel has 8-14)
- The main advantage is low-quality coal is easy to liquify.
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Advantage
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- Heating value is doubled (because C-H ratio is increased)
- CO2 emissions are reduced.
- Fly Ash is absent.
- Since Indian coal is low in sulphur, methanol when used in vehicles can reduce SOX and NOX emissions and hence low PM pollution,
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Methanol
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- Another approach to make liquid fuel is compress the syn gas obtained from gasification process. The mixture of CO and H2 is adjusted in its pressure and temperature to form methanol. (CH3OH)
- CO(g) + 2 H2(g) —-> CH3OH(l)
- Methanol is also called wood alcohol. (More on methanol in Gas-to-liquid alternative under transportation fuels)
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