DME poses a challenge to CNG

If there’s an Achilles’ heel to the efforts being made to introduce compressed natural gas (CNG) into the country’s vehicles, it is that somebody is going to come along with a liquid fuel that works much better.

CNG has many things going for it. Natural gas is now abundant and promises to stay that way for a long time. That puts the price around $2 a gallon, which is a big savings when gas costs $3.50 and diesel costs $3.70 per gallon. Trucks — mid-sized delivery trucks and big 18-wheelers — are the target market. Delivery vans usually operate out of fleet centers where a central compressor can be installed to service many vehicles. Meanwhile, pioneering companies such as Clean Energy Fuels are busy building an infrastructure at truck stops along the Interstate Highway System to service long-hauling tractor-trailers on their cross-country routes.

But there is a weakness. As a gas, CNG requires a whole new infrastructure. Compression tanks must be built at gas stations, much stronger than ordinary gas tanks and tightly machined, so gas does not escape. Even under compression, CNG has a much lower energy density than gasoline. This requires special $6,000 tanks that must still take up more space. In passenger vehicles they will devour almost all the trunk space, which is why vendors are concentrating on long-distance tractor-trailers.

As a result, there always seems the chance that some liquid derivative of methane is going to come along and push CNG off the market. Methanol has been a prime candidate since it is already manufactured in commercial quantities for industrial purposes. M85, a mixture of 85 percent methanol and 15 percent gasoline, is legal in the United States, but has not been widely adopted.

Now a new candidate has emerged in the long-distance truck competition — dimethyl ether or “DME.” Two methane ions joined by a single oxygen molecule, DME is manufactured from natural gas and has many of the same properties as methanol. It is still a gas at room temperature but can be stored as a liquid at four atmospheres or -11o F. It can also be dissolved as a gasoline or propane additive at a 30-70 percent ratio. In 2009 a team of university students from Denmark won the Shell Eco Marathon with a vehicle running on 100 percent DME.

So is it practical? Well, we’ll soon find out. Volvo has just announced it will release a version of its D13 truck in 2014 that runs on DME. At the same time, Volvo pushed back the launch of its natural gas version of the same line, meaning it may be changing its mind about which way the technology is going to go. In case you haven’t been keeping abreast, Volvo is now the largest manufacturer of heavy trucks in the world, having acquired Mack, America’s oldest truck company, in 2000.

So does that mean that CNG may turn out to be a dead end and Clean Energy Fuels is going to get stuck with a lot of unused compressor pumps? Well, hold on a minute. Technology does not stand still.

Last week at the Alternative Clean Transportation Expo in Long Beach, Calif., Ford and BASF unveiled a new device for the Ford F-450 CNG fuel tank. It’s called a Metal Organic Framework (MOF), a complex of clustered metal ions built on a backbone of] rigid organic molecules that form one-, two-, or three-dimensional structures. Lots of surface area is created, making MOFs porous enough to hold large amounts of gaseous material such as methane.

MOFs create the possibility that on-board CNG tanks will not have to operate under extremely high pressure or extremely low temperatures. Like a metallic sponge the high-surface material soaks gas right up, where it can be easily dislodged as well. According to BASF and Ford, the same amount of natural gas that requires 3,600 pounds per square inch (PSI) can be stored in an MOF tank at close to 1,000 PSI. That makes a big difference when it comes to designing an automobile.

So does that mean natural gas is going to be able to hold its own against DME and other liquid competitors? Well, wait a minute, there’s still more. Not only is MOF technology good at storing methane, it also works with hydrogen! That means the hydrogen-fuel cell — still the favorite among Japanese manufacturers — may be able to work its way back in the game as well.

In fact, Ford isn’t playing any favorites. Equipped with its new MOF tanks, the F-450 will offer drivers a choice of seven — that’s right, seven — different fuel options using the same internal combustion engine. “Ford has no idea which of these fuels will make the most sense,” Ford’s Jon Coleman told Jason Hall of Motley Fool. “So we need to build vehicles that have the broadest capability and the broadest fuel types so our customers can choose for themselves.”

That’s the name of the game. It’s called Fuel Freedom.

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