AskPablo: Clean Natural Gas?

CNG.jpgOne of my readers has asked “are Natural Gas cars really cleaner?” Well, what do you think? Many people believe that CNG stands for “Clean Natural Gas” but it actually stands for “Compressed Natural Gas”, no doubt a result of clever marketing. CNG is compressed methane (CH4) gas, which is extracted from the ground, often along with oil. CNG is not to be confused with LNG, which stands for “Liquid Natural Gas.” The main difference is that LNG is liquid wheras CNG is gaseous. Natural Gas is usually transported as LNG because it is less voluminous, but it is also more costly to cool it enough to make it liquid.


Anyway, is it actually cleaner than gasoline? If you didn’t get a chance to read my “AskPablo: The Tailpipe Mystery” you might want to go back and read it first.
OK, did you do it? Well, here we go… Methane, CH4, has molar mass of 16g/mol. When CH4 is combined with O2 and combusted it makes H2O and CO2. CO2 has a molar mass of 44g/mol. One kg of CH4 contains 62.5 mol (1000g / 16g/mol) and results in 2750g of CO2 (62.5 mol x 44g/mol), or 2.75kg of CO2 per kg of CH4. If you recall the result of “AskPablo: The Tailpipe Mystery” 1kg of gasoline creates 3.087kg of CO2. So, CNG creates less CO2, per kg of fuel. End of story? No, not quite.
Since CNG and gasoline don’t have the same energy density we can’t make such a simple comparison. Gasoline has an energy density of 44MJ/kg, while CNG has an energy density of 55MJ/kg. So, while CNG creates only 89% as much CO2 as gasoline (2.750/3.087), when compared by energy density CNG creates only about 71.5% as much CO2.
In “AskPablo: The Tailpipe Mystery” we assumed that my car averages 30 miles/gallon and I drive 18,000 miles per year. We calculated that I would need to buy 1,654 kg of fuel (30,000 km / 18.14 km/kg). Based on the factor that I derived this amounts to carbon dioxide emissions of 5,105kg, or roughly 5.1 metric tons. Now, if my car were powered by CNG my CO2 emissions would be 3,646kg, or 3.6 metric tons (the results from the DriveNeutral calculator are again quite close to this result). So, yes, the CNG fueled car is cleaner in terms of CO2 emissions. But there must be more to it…
Why yes there is… In addition to producing less CO2, CNG also creates less particulate matter and other types of emissions, such as NOx. Quantifying these other gaseous emissions of both CNG and gasoline will have to be left for a future column.
This discussion would not be complete if I didn’t mention a recent article in the newspaper of the Association of German Engineers (VDI). While the article states that CNG fueled cars emit 25% less CO2 than gasoline fueled vehicles ( 28.5% by my calculations) and up to 99% less particulate emissions, Prof. Hans Lenz maintains that CNG is not better. The reason is that there are significant emissions created in processing and transporting the fuel. Much of this comes from the process of liquefying the gas for transport in LNG tankers. So, in light of this information I will conclude that CNG-fueled cars emit less locally-harmful emissions (including locally emitted CO2) but the net CO2 emissions results are inconclusive.