The amount of carbon produced by the US has dropped slightly in recent years because we are burning more natural gas (methane) and less coal than we used to. I am interested to see just how much of a difference this makes.
Wikipedia lists the heat of combustion of various fuels, which is an approximation to the amount of energy that can be extracted from those fuels.
|fuel||HHV MJ/kg||% carbon|
|methane||55.5||12/16 = 75%|
|gasoline||47.3||~85%||gasoline is a mixture of varying
components and amounts so
this percentage is approximate
|32.5||~95%||coal usually contains impurities,
so it isn’t quite 100% carbon
The interesting question is: For a given amount of energy output, how much carbon will be put in the atmosphere? The actual amounts are immaterial to me, I’m interested in the relative amounts, so I arbitrarily am using 1 Mega-Joule as a reference.
(Most sources quote the amount of carbon dioxide produced rather than the amount of carbon, the conversion is fairly simple, there are 12g of carbon in 44g of carbon dioxide.)
|fuel||g carbon/MJ||g CO2/MJ|
|methane||.75/55.5 = 13.5g||49g|
|gasoline||.85/47.3 = 17.9g||65g|
|anthracite coal||.95/32.5 = 29.2g||107g|
So switching from coal to methane while producing the same amount of power would reduce carbon output to 13.5/29.2 = 46%. If we fueled our cars with methane (somehow, rather than gasoline) that would reduce carbon output to 13.5/17.9 = 75%.
Now the IPCC said back in 2007 that if we wanted to keep warming less than 2°C we would have to reduce our annual carbon production to ~15% of then current production by 2050. A reduction to 46% is nowhere near enough.
Doubtless many have argued that 46% is better than nothing, but the problem is that once you build a power plant it tends to continue working for decades, thus many plants built now will probably still be working in 2050. The decisions we make now will keep us from doing what needs to be done by then. If we were actively pursuing solar, wind, geothermal on a large scale then building the occasional natural gas fired power plant would not be a problem. But currently natural gas seems to be looked on as the main solution — and it isn’t.
After writing this I learned of another problem with natural gas: Leaks. You don’t have to worry about coal leaking into the atmosphere, but any badly sealed pipe anywhere in the production chain will leak methane into the atmosphere. Unfortunately methane is a considerably more potent greenhouse gas than CO2. According to Wikipedia it’s direct effect is 72 times more potent than CO2 and there are indirect effects that are harder to quantify. Basically if even 1% of natural gas leaks on its way to being burnt then you are no better off than burning coal. A recent study in Science concludes that after taking leaks into consideration natural gas is worse than diesel but better than coal. (If you don’t have access to Science here’s a summary article in the New York Times.)
Of course some of these leaks can probably be fixed, and various states are attempting to require this.