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.

20 responses

  1. 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.

  2. 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,

  3. 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.

  4. One more out of left field. Natural gas in pipelines contains significant levels of mercury. When the gas leaves a major trunk line for domestic distribution lines (to homes) the mercury, if present above a certain level, is scrubbed out. However, where the natural gas is headed for a commercial user like a power plant it is NOT scrubbed out unless by the customer. Because transportation uses are commercial we should assume that CNG as presently distributed is a source of mercury emissions. (NOTE: gas fired electricity generation plants do indeed have mercury emission limits in their permits. Check one if you think I am making this up!) Very few people know about this by the way because the gas distrubution companies are not excited about consumers fretting over indoor emissions, etc.

  5. John, I can’t speak for all sources of vehicle CNG but a close friend of mine fills up at a Pacific Gas and Electric (PG&E) lot. The CNG he gets there is presumably of the residential type. But even so there is still a residual amount of mercury in the fuel. Your gas company may warn you of this in their fine print and that is why you shouldn’t cook your food directly over a natural gas flame (sorry, s’mores are for camping…) and one of the reasons you shouldn’t head your house with the stove or oven (aside from the whole carbon monoxide issue). Have you seen any numbers on the mercury content of gasoline? I would assume that, since CNG and gasoline ultimately come out of the same hole in the ground, they probably both contain comparable levels of mercury. I could be wrong on this. What do you think?

  6. One factor that makes CNG engines possibly better than your numbers show is that they run at a higher compression ratio. A higher compression ration enables an engine to get more energy out of the fuel it burns.
    My Civic GX (I am Pablo’s mystery friend) has an engine that runs at a 12.5:1 compression ratio. Most car engines are around 8:1 or 9:1.

  7. I think that you need to make a more even analysis when comparing compressed natural gas and gasoline. Even though much is mined for in this county you must realize that most shipped natural gas would be wasted in the production of oil. In a process called flaring. Methane is a greenhouse gas and in naturally occurring. Before flaring it was released directly into the atmosphere. Therefore the collection of the from other industries is considered carbon neutral and green house effect negative. Remember even ethanol and of course oil also has incredible transportation and production costs in energy. So to compare a negative statement about CNG transport you need to include what you are comparing it to. Also methane can be collected from other sources as well such as farm waste, landfills and sewage facilities which actually prevents greenhouse gas potency. Methane is considered 20 times more potent of a green house gas than CO2.
    Methane is considered an easy and source for processing hydrogen when the government is looking at coal and nuclear.
    Remember a natural gas vehicle can also be made into a hybrid vehicle.
    I do not know what kind of vehicles you are comparing but as far the California Air Resource Board and most car comparison data available compares compact CNG verses gasoline powered vehicles rates the CNG at about 30% the emissions of the gasoline powered vehicle.
    And if you ever lived by an oil refinery you would know that producing that gasoline endangers all life in the surrounding area. CNG is compressed not does not need processing.

  8. As a director of a nuetral foundation that reviews various clean vehicle technologies for academia, industry, government and policymaking groups, I find your chat discussion interesting…. some of it is accurate and well informed, some of the submissions are off the mark entirely… wish there was a way to do a peer review by scientists/analysts of everything you post. Case in point: Most CNG comes from pipeline gas, which IS processed, cleaned etc of its minor amount of polutant constituents before it gets to your home, business or – in the case of this discussion – to your local CNG station compressor – so the mercury discussion is a red herring. As for transport, less than 20% of the NG used for transportation is in the form of LNG, which is transported by truck from 4 locations to about 75 west coast and Texas accounts (i.e. there are the effects of the transport truck’s pollution – albeit nothing like the effects of the large impact of thousands of gasoline and diesel tankers that deliver fuel from the tank farm to the local gas station or fuel depot). As for the references to diesel VOCs, etc – you reference academic and industry reports from 2000-2004. The only one that I know of that came to the errant conclusion about diesel being cleaner than NG was a preliminary draft done by CARB on two sets of buses…the prelim report was widely criticized by CARB’s own review board for apples to oranges comparisons of old outdated CNG bus technology without catalysts taken off LA streets versus finely-tuned and tweaked beta test diesel units using ULSD fuel that wasn’t even available on the market at the time. In addition, the report came to conclusions about VOCs that were later thrown out because the error factor on the testing equipment used was more than twice the differential seen between the two sets of vehicles. The result? The CARB prelim report was withdrawn, further test were done and a new report was published that received good reviews from the nuetral peer review committees that evaluated it…yet International Truck and the Diesel Technology Forum folks continue to this day to cite the prelim (withdrawn) report and folks continue to propagate this misinformation (deliberately and/or inadvertenatly) on web sites and chat rooms
    As to the reference to nomenclature that CNG gets confused for Clean Natural Gas (instead of compressed natural gas) is “no doubt a result of clever marketing..” – your conjecture shows bias… the term CNG was adopted by “geeky” gas company engineers working on their own vehicles in the mid-1970s just to prove that they could get their vehicles to run on NG and to see what the pereformance and emissions issues would be — CNG was not marketed to outside fleets until nearly 5 years later, and then, only to a very limited number of customers and local government accounts who liked the fact that it was cheaper and domestic….you give the ytilities far too much credit for being “clever marketers” – the engineers called it what it was – compressed naturla gas.
    Thanks for your discussion group – I’ll keep an eye on it and participate occasionally as time permits.
    Reply to the Q about total resource efficientcy of natural gas “from well to burner tip”: There are numerous peer-reviewed published studies available via Internet… about 7% of the inherent Btu value of natural gas is used extracting it from the ground, processing/cleaning it, transporting it across the interstate pipeline system and delivering it to your meter….from there, multiply the efficient of the appliance. By comparison, electricity (from the cleanest power plants – i.e. NG turbines) is about 40% – most of the loss is at the plant itself with all that wasted thermal energy going up the stack, then you lose quite a bit over the power line distribtion system, especially during summer daytime hours when we need the most power.

  9. I was wondering if anyone has factored in losses of NG to the environment from leakage? If 1% of all NG produced is lost to the atmosphere (i have no idea if this is a realistic proportion), What would the effect be in terms of equivalent CO2 emmisions?

  10. Leakage of NG is called “fugitive emissions” and is factored in to the emissions inventory of major natural gas companies as well as national GHG inventories. The GHG potential of NG, which is methane, is 23x that of CO2. So, just take the assumed amount of leakage (in metric tons) and multiply by 23 to get CO2e.

  11. My understanding is only new cars with the natural gas burning system factory installed are practical.And it may not be practical for the semi long distance trucks. We have over 5,000 CNG vehicles in Utah and a number of stations. And it is about 80 cents a gallon. it is much cleaner than gasoline.. After all we use it in the kitchen. And there is plenty of it to allow a few years to get solar, wind, sea algae oil, etc practical. And it is domestic.

  12. Is CNG really cleaner???? It is my understanding that methane is a material byproduct of CNG. Methane is also ~20x (there seem to be variations on this multiple but its still significant) more potent a greenhouse gas than CO2. The permafrost in Siberia, Canada, etc. contains billions of tons of methane which is already being released due to higher temperatures.
    Last point: there were multiple studies conducted on the impact of reduction of particulate pollution on global tempratures … (most recently, to my knowledge, after 9/11/01 triggered by the halting of air traffic over the US .. Google “Global Dimming”). The basic premise was that particulate pollution acts as a sunshade and helps cool the Earth offsetting the impact of greenhouse gasses. The studies concluded that if we were to reduce particulate pollution w/o reducing greenhouse gasses in appropriate proportions, we would see an acceleration of global warming.
    So does CNG really reduce CO2 and other gas emissions enough to more than offset the production of methane (adjusting for the above multiple)? Also does it reduce overall “adjusted” emissions vs. using gasoline/diesel enough to where switching over would give us an economic and environmental benefit, all things considered??? And then there is the offsetting impact of particulates which I don’t have the acumen to quantify….

  13. People haven’t responded to Vik’s comment above, and it exactly encompasses my concerns about CNG. Particulate pollution has already masked the severity of global warming through global dimming. Are any methane emissions accounted in these calculations that profess CNG to be the cleanest option?

  14. Is CO2 really dirty?  – or is that just the latest fashionable hobgoblin. All plants would die without this colourless, odourless trace gas, and so would all animal life, including us.

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