Triple Pundit: AskPablo: Clean Natural Gas?

One 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.
» Pablo Paster
Comments
A comparison between CNG and modern low-emission diesel-powered vehicles would be interesting. Most of the active promotion of CNG these days is for heavy-duty fleet vehicles - particularly public transit buses - because the cost of CNG fueling stations makes CNG impractical for vehicles that aren't kept at a central yard.
» Michael at November 6, 2006 08:20 AM
Michael, that is an excellent topic. I will put that on my list and will start doing some research. Thanks for reading AskPablo!
» Pablo at November 6, 2006 09:26 AM
Thanks for looking into Natural Gas in such detail; bang-up analysis Pablo. Sounds like it's cleaner for the ride but particularly harmful when processed and transported.
In a future column, it would be interesting to learn more about the emissions of processing and transporting Natural Gas as well as the environmental impacts of extraction.
I'd like to know the real skinny on the Life Cycle of Natural Gas beyond the tailpipe impacts. Natural Gas has many uses of which vehicle fuel is one. Too much for one column, I know. But I'd love a follow up post.
Many thanks,Jason
» Jason Smith at November 6, 2006 10:32 AM
My understanding is that Liquified Natural Gas also poses a fairly large safety risk, with LNG tankers and offloading facilities being especially juicy terrorist targets and prone to accidents... thoughts?
» Nick Aster at November 6, 2006 03:11 PM
A number of relatively recently completed life cycle analyses for various alternative vehicle fuels all show the same ranking: CNG-fueled vehicles have the third lowest (total) greenhouse gas emissions. First is hybrid gasoline followed by diesel. CNG also compares well when so-called criteria pollutant (NOx, PM, NMVOC and CO) emissions (also total) are considered. CNG ranks first for NOx and PM, and places second for NMVOC and CO with diesel taking the top spot. These studies come from academia, government and industry groups in US, Europe and elsewhere over the period 2000 to 2004.
However, that's not the whole story. CNG usage is extremely small. In the US, CNG-fueled vehicles represent about 0.05% of the total (243 million registered vehicles). In California, the corresponding figure is 0.1%, about 25,000 CNG-fueled vehicles out of a total (registered) populaion of 23.5 milllion.
The story for LNG is more complex, because its composition varies more and standards are just being developed. However, early LCA studies suggest, CNG ranks higher.