Yeah, it's a huge difference, and all you need is high school chemistry: - gas is CH4 (four hydrogens for every carbon). Oil is approximately CH2 (in reality,

Yeah, it's a huge difference, and all you need is high school chemistry: - gas is CH4 (four hydrogens for every carbon). Oil is approximately CH2 (in reality, it's slightly higher than two). Coal is approximately CH. - a CH bond has about 410 kJ / mol; a CC bond has about 350 kJ / mol (http://en.wikipedia.org/wiki/Bond-dissociation_energy http://en.wikipedia.org/wiki/Bond-dissociation_energy) - thus, you can calculate approximately how much energy is stored in each of the above fossils per mole of carbon dioxide: - methane = 4CH = 1640 kJ / mol CO2 - petrol = 2CH + CC = 1170 kJ / mol CO2 - coal = CH + CC = 760 kJ / mol CO2 This is obviously a gross simplification, but hopefully you'll see the outline of why gas is much cleaner than coal. Methane, since it's a gas, also burns much cleaner than liquids or solids since you can better mix the fuel with oxygen, so methane tends to produce much less particulate matter, soot, and other products of incomplete combustion. EDIT: spenrose below has a great point about uncombusted methane in the atmosphere. Methane is a NASTY greenhouse gas, so it's absolutely worth taking into account how natural gas production affects methane levels in the atmosphere. Namely, there was a worrying metastudy a few months ago about how natural gas production is quite leaky, which has nontrivial greenhouse gas considerations: http://news.stanford.edu/news/2014/february/methane-leaky-gas-021314.html http://news.stanford.edu/news/2014/february/methane-leaky-ga...