The chemistry behind why LNG is cleaner than LPG which is cleaner than coal.
Combustion happens when carbon meets oxygen to produce carbon dioxide, plus heat (energy). When hydrocarbons are involved, water is produced as well. It’s the same reaction that happens when you ingest carbohydrates (oh the familiar prefix and root word – yes, it’s exactly what its name says, carbon plus hydrogen) and fats, breathe in oxygen … and produce carbon dioxide (CO2).
Chemical composition of fats (check out all those carbon-hydrogen bonds!):
The equation for combustion looks something like this:
CH4 + 2O2 CO2 + 2H2O + heat
To understand why certain fuels are ‘cleaner’ than other fuels (ie, produce less CO2), we must understand the energy present in different types of bonds. For example, methane (CH4) has a carbon atom with 4 single bonds to hydrogen atoms.
Here’s a chart of the energy that’s locked in different types of bonds, courtesy of Univ of Massachusetts chemistry dept website:
In combustion, these bonds are broken and release their energy as heat.
So now that we have a chart of average bond enthalpies before us, let us figure out the energy involved in combustion of coal, methane (the primary gas of CNG/LNG), and propane (the primary gas of LPG).
Coal: C(s) + O2(g) CO2(g) ∆H (heat of this reaction) = -393.5 kJ/mol
Methane: CH4(g) + 2 O2(g) CO2(g) + 2H2O(l) ∆H = -890 kJ/mol
Propane: C3H8(g) + 5 O2(g) 3CO2(g) + 4H2O ∆H = –2220kJ/mol
Heat released per gram of fuel:
Coal: C(s) 34 kJ
Methane: CH4(g) 55.6 kJ
Propane: C3H8(g) 50.5 kJ
Coal produces approx 393.5 kJ of heat per mole of CO2 produced. Meanwhile, methane produces 890 kJ of heat per mole of CO2, more than twice that of coal. Propane produces slightly less than methane at 740 kJ per mole of CO2. Methane and propane deliver more energy for the CO2 released into the air than coal does.
As you can see, coal is less efficient than propane (LPG), which in turn is less efficient than methane (LNG). This is why natural gas is a cleaner fuel, and coal dirtier.