For this article, I only accounted for the greenhouse gases released by the two fossil fuels, so that’s why I didn’t include mercury. I should have included methane released from coal, thanks for pointing that out. I ran some quick numbers to see how that might look:
Coal: 25g CO2 released per MJ of energy
Natural Gas: 15g CO2 released per MJ of energy
Coal releases roughly 1.91g of methane per kg of surface-mined coal and 4.23g per kg of underground-mined coal.
Roughly 40% is surface-mined and 60% underground, so we can say the “average” methane released per kg of coal mined is 3.30g.
Coal produces roughly 2.4 kWh of energy per kg, or 8.64 MJ. This means coal releases 0.38g of methane per MJ of energy. Multiplied by 86, this gives us 32.68g of CO2 equivalent.
So coal is now at 57.68g CO2 equivalent per MJ of energy (25+32.68)
Now for natural gas…a cubic meter produces about 6.6 kWh of electricity or 23.76 MJ. The density is about 0.8 kg per cubic meter, so we’ve got to divide 23.76 by 0.8 and we get 29.7 MJ of energy per kg of natural gas.
The real leakage rate of natural gas is 2.3%. That means for every kg of natural gas, 20g of methane is released (1000g x 0.023 x 0.87 [natural gas is 87% methane]). 20/29.7= 0.67g of methane released per MJ of energy produced. Multiply by 86, this gives us 57.62g of CO2 equivalent.
So natural gas is now at 72.62g CO2 equivalent per MJ of energy produced, compared to 57.68g CO2 equivalent for coal.
That’s using a 20 year time period. If we want to see what it would be over 100 years we need to use the 34x multiplier.
Coal: (0.38g x 34) + 25 = 37.92g CO2 equivalent released per MJ of energy
Natural Gas: (0.67g x 34) + 15 = 37.78g CO2 equivalent released per MJ
So it appears that natural gas is significantly worse than coal over a 20-year period and about equal over a 100 year period, which means natural gas is a little worse than portrayed in my article (I wrote that coal was cleaner over a 100-year period). I should have included that in the original article, thanks for pointing it out.
The takeaway from the article should not be that coal is an acceptable fuel source, but that both coal and natural gas are very dirty and unacceptable fuel sources.
