Sep
9
Reality Check – Where to Put All That Methane?
September 9, 2008 | 12 Comments
With natural gas production growing, landfills getting “drilled” for methane and the sure to come list of new research on making methane from organics the question coming up for fuel cells, motorcycles, cars and just huge storage and transport for methane is going to come rushing at us and soon.
Today methane is simply compressed or compressed and chilled to a liquid state. The compression is usually to 3000 psi on up to 4000 psi. To get to the liquid state the compression is light, about 3.6 psi and cooled to –260 degrees F or -163 degrees C. Compression can get better than 1/100th the volume. Liquefied methane gets to 1/600th the volume of free gas. Those ratios of volume are important.
Tanks are enclosed volumes. The 20 gallon gasoline tank isn’t especially large and holding a liquid it can be molded to fit cavities so making good use of an available volume. But when you get pressured up the tanks need cylindrical shapes with spherical ends to keep the materials used minimized so controlling the cost. That’s when the volume within a vehicle becomes an issue. They must be integrated early in design to avoid foregoing some other features.
The U.S Department of Energy has a storage goal stated as 180 v/v or standard temperature and pressure equivalent volume / volume of the absorbing material. Roughly speaking the USDOE number is 1/180 or so. With that in mind we can make some sense of the year’s research so far in new technologies for storage.
The next issue to address is the cost of tanks. The 20-gallon gas tank would only cost a few dollars. As progress is made on Compressed Natural Gas tanks the cost will come down, but for today we’ll look at cost issues and volume effectiveness. The last aspect in the cost issue will be the difference between CNG, gasoline and diesel at fill up. With prices wavering about with such volatility and the vast differences from location to location for natural gas prices, an old number of natural gas being about 2/3rds or so the cost of gasoline or diesel can be used. So, a more expensive tank can be expected, and to some as yet unknown extent, justified.
Route from Corncob to Stored Methane
Intuitively, the low cost leader might be the University of Missouri-Columbia and Midwest Research Institute in Kansas City created carbon briquettes with complex nanopores capable of storing natural gas at an unprecedented density of 180 times their own volume and at one-seventh (about 500 psi) the pressure of conventional natural gas tanks. 500 psi could be low enough to make more adaptive shapes. The raw material is corncobs, the center cylinder of the corn plant that holds the kernels as they grow. Currently they are usually just pitched out the back of the farmer’s combine as the kernels are harvested. This has to be a cheap resource that has collection and transport costs before the processing into the briquettes.
Peter Pfeifer of MU says “We are very excited about this breakthrough because it may lead to a flat and compact tank that would fit under the floor of a passenger car, similar to current gasoline tanks. Such a technology would make natural gas a widely attractive alternative fuel for everyone.” Pfeifer believes the absence of such a flatbed tank has been the principal reason why natural gas, which costs significantly less than gasoline and diesel and burns cleaner, is not yet widely used as a fuel for vehicles.
“Our project is the first time a carbon storage material has been made from corncobs, an abundantly available waste product in the Midwest,” said Pfeifer. “The carbon briquettes are made from the cobs that remain after the kernels have been harvested. The state of Missouri alone could supply the raw material for more than 10 million cars per year. It would be a unique opportunity to bring corn to the market for alternative fuels–corn kernels for ethanol production, and corncob for natural gas tanks.”
A test pickup truck, part of a fleet of more than 200 natural gas vehicles operated by Kansas City, has been in use since mid-October. The researchers are monitoring the technology’s performance, from mileage data to measurements of the stability of the briquettes and have started work on the next generation of briquettes to store more at lower briquette production costs.
A Methane Cage of Nano-sized Crystals
Hong-Cai Zhou and colleagues at Miami University published a report describing development of a new type of MOF (metal organic frameworks), called PCN-14, (porous coordination network) that has a high surface area of over 2000 m2/g. Laboratory studies show that the compound, composed of clusters of nano-sized cages, has a methane storage capacity 28 percent higher than the DOE target (230.4 v/v), a record high for methane-storage materials. Published in the Jan. 23 issue of ACS’ Journal of the American Chemical Society the costs are not explored, although the temperature and pressure are at standard. Note that this solution would be 38% of liquid without pressure or chilling. The costs are well worth exploring in this idea too.
Dry Water - Composition Graphic - Time to Capacity
In a report in Nature News, Andrew Cooper and his colleagues at the University of Liverpool, UK, have found that they can trap methane in a bizarre material dubbed ”dry water”, a mixture of silica and water that looks and acts like a fine white powder. The methane reacts with the water to produce a crystalline material called methane gas hydrate, in which individual methane molecules sit inside ice-like cages of water molecules. A liter of methane gas can be stored in about 6 grams of the material that they say, is very close to the target set by the US Department of Energy. This material is sourced from silica, the base material of sand helping to make this method economical relative to other, more exotic potential methane-capture materials.
This looks great, but the reality check is the density of the fuels. Natural gas would I suspect, needs cleaned to be stored by these tanking systems all the way to near pure methane, a not particularly difficult or costly enterprise but one that yields about 87% of the original natural gas. The other products like ethane and propane are valuable hydrocarbon products as well. Nor is there discussion about the price to get methane in and out of these solutions.
Methane is only 1,000 BTUs per standard cubic foot, so needing 125 cubic feet to equal the BTUs of one gallon of gasoline. But the volume of a gallon tank to the volume of a cubic foot is 1/7.48 – a 7.5 to 1 volume increase to maintain the total energy on board. So a 180 v/v storage solution (about a 50% increase in methane to volume) would be about a 1 of 5 reduction in range for the same tank volume. Or the 20-gallon gas tank replaced by methane at 180 v/v would be equal to 4 gallons of gasoline. Even at the liquid state, methane isn’t equal to gasoline. We’re going to have to get used to stopping to fill more often.
This will work. In the U.S. natural gas has a historic price advantage to gasoline. Methane is abundant, can be found, made, scavenged, and converted from oil. Even more important over time is that methane makes a fine fuel cell fuel. Swap out that internal combustion engine for a fuel cell and there might be a 3, 4 or better multiple of efficiency. If methane can be marketed at a 1/3 lower cost than gasoline, and be used four times as efficiently, the cost per mile compared to gasoline will be very low indeed.
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