Rhode Island’s Smart Choice

Compressed Natural Gas

A recent article in the Providence Journal caught my attention: Another fuel to power your car arrives in R.I.

Some excerpts from the article:

May 24–WARWICK — Hate the gas-guzzling SUV? Worried about greenhouse effects and smog? Fearful that we’ll someday run out of oil? Rhode Island’s eco-conscious, your day has come.

Environmentalists have long offered the benefits of compressed natural gas vehicles as a solution to all of these problems. The engines burn immaculately clean. Vehicles powered by CNG produce only 10 percent of the carbon monoxide and particle discharge of gasoline-powered engines, and half the nitrogen oxides. Carbon dioxide discharge is reduced by 30 to 40 percent.

The fuel, which is primarily methane, is cheaper than gasoline — at T.F. Green, the natural gas will retail for $2.69 for the equivalent of one gallon — and natural gas-powered cars get better mileage.

According to the Natural Gas Vehicle Coalition, there were 130,000 natural gas vehicles operating in the United States and 5 million worldwide as of last year.

The states have been the leaders in buying them: In 2005, Governor Carcieri signed an order mandating that 75 percent of all new state vehicles purchased use some sort of alternative fuel — CNG, hydrogen, ethanol, biodiesel, or others. Rhode Island chose to use CNG.

“Initially, Rhode Island invested very heavily into natural gas,” said David R. Sheldon, principal engineer with the Department of Administration’s Environmental Compliance Unit.

While the gasoline-powered Civic GX gets an average highway mileage in the high 20s, the CNG version will average in the mid 30s, officials said.

I have long maintained that converting vehicles to natural gas (NG) makes more sense than converting that NG to diesel via the gas to liquids process (GTL) or ethanol via the corn to liquids process. Note that Brazil, which we immediately associate with ethanol, has over 8 times the CNG fleet of the U.S., despite having a population 100 million less than the U.S.

Consider the options for NG. If we convert it to diesel, we are going to consume about 40% of our initial BTUs in the conversion process, as shown here. But, since the diesel engine is around 35% more efficient than the combustion engine, we have an approximate wash. We will get around the same ultimate fuel efficiency from directly burning the natural gas as we will from processing it into diesel. (But, of course the advantage of GTL is the ability to develop stranded gas reserves).

If we convert it to ethanol, the BTUs in and out are close to a wash. According to the 2002 USDA study Estimating the Net Energy Balance of Corn Ethanol, it takes 77,228 BTUs of fossil fuel inputs – primarily natural gas – to make 83,961 BTUs of ethanol (and a BTU co-product credit of 14,372 BTUs). This is a gain of 8% for fossil fuels in and ethanol out, or a gain of 27% if we include the co-products. But to earn that modest energy gain we mine the topsoil, apply herbicides and pesticides – some of which end up polluting waterways, and we have to build an ethanol refinery.

Also note that the Union of Concerned Scientists report that CNG vehicles “achieve green house gas emission reductions in the range of 5 to 25 percent compared to conventional passenger gasoline vehicles.” The above article from the Providence Journal reported a 30-40% reduction in carbon dioxide emissions. Corn ethanol is reported by Daniel Kammen (the Berkeley ethanol advocate interviewed on the recent 60 Minutes piece on ethanol) to achieve a reduction of 10 or 15 percent over gasoline in terms of greenhouse gas production. So, natural gas is at least as good as ethanol with respect to greenhouse gas emissions, but probably better as a whole for the environment when you consider the environmental aspects of corn farming.

Natural Gas Sources

As I indicated in my essay on XTL, the estimated 3,000 trillion cubic feet of stranded natural gas is enough to produce 300 billion barrels of fuel. However, if we were to burn this gas directly in CNG vehicles, instead of converting it into fuel via the GTL process, we could expect that stranded gas to provide the energy equivalent of over 500 billion barrels of fuel. At current world energy usage rates of 84 million barrels/day of oil, that is enough BTUs to supply us for over 16 years. Consider that if ¼ of the world’s BTUs came from stranded natural gas, the stranded natural gas could contribute to the world energy portfolio for over 60 years at today’s consumption rate. Also remember, we are only talking about stranded reserves. There are another 3,200 trillion barrels that are not considered to be stranded.

However, if you are concerned about global warming, as I am, converting all of that natural gas into carbon dioxide may not be too appealing. Fortunately, natural gas can also be renewable if we make if from biomass. Natural gas is created by the anaerobic decomposition of biomass. It can be made from sewage sludge, municipal solid waste, or biomass crops grown specifically for that purpose.

According to the company Gas Separation Technology, between 450 and 650 billion cubic feet per year of methane leeches out of landfills into the atmosphere. This is the energy equivalent of around 75-100 million barrels of oil, floating up into the atmosphere each year. While this is only equivalent to about 1% of U.S. oil demand, according to the EPA methane is 21 times more potent as a greenhouse gas than carbon dioxide. By capturing some of this landfill gas, we can make a small contribution toward our energy requirements while trading methane emissions for carbon dioxide emissions. That’s a good potential source of natural gas, and 395 landfill energy projects are already in place, demonstrating the feasibility of the technology.

Of course we are going to need more methane than we can get from landfills. As I mentioned above, we can generate methane from sewage sludge, any waste source of biomass, or from crops grown specifically for methane generation. It is unlikely that we will be able to generate enough methane to maintain our current levels of consumption, but with a major conservation push, methane can be a nice fraction of the energy pie.

EROI

Ah, but what about the EROI? That’s an obvious question, and I don’t have a good answer. I simply haven’t found a good energy analysis. But due to the fact that biogas consists mostly of methane and carbon dioxide, and carbon dioxide has a much higher solubility in water than methane, the separation should not have a high energy requirement. If anyone knows of any literature on this, I would be interested in reading it.

Footnotes

This essay was also posted at The Oil Drum. As always, peer review is appreciated. If you spot an error, I will correct it.

Note that I have used methane and natural gas interchangeably. Natural gas consists primarily of methane (~92-97%), but natural gas from gas wells can contain nitrogen, carbon dioxide, hydrogen sulfide, ethane, propane, and even trace longer chain hydrocarbons.

Finally, I want to note that the article above says that the price of NG right now is $2.69 for a gasoline gallon equivalent (GGE). To put that in perspective, a gallon of ethanol on the CBT today closed at $3.41, and is around $3.70 in California on the spot market. On a GGE basis, this would be $5.00-$5.50 a gallon – twice the price of the NG.

9 thoughts on “Rhode Island’s Smart Choice”

  1. What I think is interesting about the possibilities of a transportation infrustructure based more and more on CNG is that it may have some “fuel cell flexibility.” That is, direct methane/methanol fuel cells already exist today, and I suspect (but I’m no engineer) that the efficiency of a methane powered fuel cell driving an electric motor is going to be better than methane going into an ICE. As more CNG vehicles hit the road, and more CNG refilling stations arrive, it seems to me that it would be easy to get a fuel cell infrastructure off of that quite easily. Correct me if I am wrong here, but can’t methane be stored efficiently at lower pressures than hydrogen? The only downside of methane against hydrogen is that it can’t be generated on the spot from electricity and water. That and the fact that most of today’s methane wouldn’t be produced in a C02 neutral fashion, I guess.

  2. Regarding biogas upgrading, you can use this source but it’s in Swedish:

    Persson, M., 2003. Utvärdering av uppgraderingstekniker för biogas. Rapport SGC 142, Swedish Gas Centre, Malmö, Sweden.

    🙂

    More seriously, try this reference:

    S.S. Kapdi, V.K. Vijay, S.K. Rajesh and R. Prasad, Biogas scrubbing, compression and storage: perspective and prospectus in Indian context, Renew Energy 30 (2005), pp. 1195–1202.


    http://dx.doi.org/10.1016/j.renene.2004.09.012

    BTW, I disagree that natural gas is cheaper than gasoline. On a per unit energy basis, it wasn’t last winter. North America is probably going to run out of significant volumes of natural gas soon. Canada is predicted to consume its existing conventional reserves by 2012. If Russian gas goes to Europe, and Iranian gas to China, that leaves us with Quatar?

  3. On the minus side, CNG has a couple of inherent problems when compared with XTL: lack of infrastructure, limited range, slower refueling, and more expensive retrofitting.

    Home refueling can mitigate this a bit, but you run into many of the same limitations that battery-electric vehicles face, i.e. they’re only good for about 90% of the public.

  4. BTW, I disagree that natural gas is cheaper than gasoline. On a per unit energy basis, it wasn’t last winter. North America is probably going to run out of significant volumes of natural gas soon.

    NG prices did spike up very high in the fall last year, but for the most part have been cheaper per BTU than gasoline. There is still a lot of stranded gas out there, and it will be imported to the U.S. via NGL technology. I would much rather see us burn it directly as opposed to inefficiently turning it into ethanol.

    RR

  5. RR said, “I have long maintained that converting vehicles to natural gas (NG) makes more sense than converting that NG to diesel via the gas to liquids process (GTL) or ethanol via the corn to liquids process.”

    Me too Robert.

    If a bunch of smart people started with a blank piece of paper to design a process to use natural gas for a transportation fuel, they would never say, “Hey, I’ve got an idea, let’s use that natural gas to make fertilizer. Then we can grow corn with the fertilizer and then use more natural gas to turn that corn into ethanol.”

    The only reason we do it that way now is because of decades of entrenched government subsidies for agribusiness and Corn Belt politics. We pay those factory farms to grow so much corn, we have to use it for something.

    Unfortunately, politics always trumps science and almost always trumps good reason.

    By the way: I saw Alan Greenspan testify before the Senate Foreign Relations Commitee on oil and energy economics today. He was very cool on corn ethanol, and very upbeat about making liquid fuels directly from natural gas.
    He was also upbeat about the potential of celluosic ethanol, while emphasizing that is still an unproven technology.

    Best,

    Gary Dikkers

  6. “If we convert it to diesel, we are going to consume about 40% of our initial BTUs in the conversion process, as shown here. But, since the diesel engine is around 35% more efficient than the combustion engine, we have an approximate wash.”

    Yes, but it has much lower volatility, and therefor is less polluting and less likely to result in an explosion.

    “The estimated 3,000 trillion cubic feet of stranded natural gas is enough to produce 300 billion barrels of fuel. However, if we were to burn this gas directly in CNG vehicles, instead of converting it into fuel via the GTL process, we could expect that stranded gas to provide the energy equivalent of over 500 billion barrels of fuel.”

    Yes, but liquifying and transporting the gas is expensive. Further there are safety concerns about LNG terminals, that are much less important about diesel.

  7. Yes, but liquifying and transporting the gas is expensive. Further there are safety concerns about LNG terminals, that are much less important about diesel.

    Well, it isn’t going to be either/or. Both GTL and LNG are scaling up right now. But I think the economics for LNG are a bit more favorable under most circumstances, so I expect to see more growth there.

    RR

  8. Regarding liquified natural gas, remember that some of the energy used to liquify it can be regained at the terminal.

    You can run a heat engine using the LNG as the cold sink and the ocean as the hot sink. The Carnot efficiency for a heat engine running between 275K and 100K is 68 %.

  9. But between now and whenever (at least past 2010, if not much further out) a globalized gas infrastructure is put in place (which is also of questionable merit since it will have much the same issues as our oil problem does now–ie unfriendly regimes, long supply lines) what are we going to be burning? I guess we can get over the hump and into the LNG era, but it seems like a tremendous investment for something that will again end up being quite transitory (though it certainly will last longer than oil, see TOD on North Field/South Pars for concern about gas volumes). Again, CONSERVATION, CONTRACTION, real renewables (so not ethanol… but that is truly beating a dead horse in these circles).
    -David H.

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