Lng/cng

[rocketmanpete]

Really I do know what the difference between the L and C. But what is the application difference and how is it dispensed. Which is a better system? I would think the liquid is better,am I wrong?

[John Mitton]

Re: Lng/cng

Pete,

CNG = Compressed Natural Gas. A gaseous fuel requiring large high-pressure storage containers. The most common natural gas transportation fuel as it can be placed on many types of vehicles.

http://en.wikipedia.org/wiki/Compressed_natural_gas

LNG = Liquefied Natural Gas. A liquid fuel stored at extremely low temperatures, requiring smaller storage containers than CNG relative to the energy contained. Found on some transit buses and long-haul trucks. It is a use-it-or-lose-it fuel as it will evaporate over a relatively short time.

http://en.wikipedia.org/wiki/Lng

[rocketmanpete]

Re: Lng/cng

Thanks John, what dose not make sense is why dose it evaporate if it is a sealed tank.

[larrycng]

Re: Lng/cng

LNG is considered a cryogenic liquid at -260 deg f. As the container be warm the liquid changes state to gas. The pressure of the gas increases with temperatue. If the Ng from LNG were confined in the tank it would reach about 4,000 psi. The Lng storage tank is an overside thermos bottle. Eventually as with any thermos bottle whatever is inside will come to ambient temperature. LNG "Dewars" are designed to have a relief valve setting of about 235 psi to protect the storage container.

Under normal vehicle operating conditions (while driving) the pressure will stay around 125 to 150 psi (I remember correctly). Under ambient temperatures of around 70 deg F it will usually take 48 hours or so for the pressure to blow the relief - and this is normal system operation.

Some companies use cng from lng to fuel vehicles (LCNG). They just run it through a heat exchanger and you have 4,000 psi cng for fueling you cng vehicles

If you familiar with propane, you know that as you can keep pressure on it, the boiling point goes up and it will stay mostly liquid. You would need an awful heavy and thick tank to do the same with LNG

I'll try an attach a picture of an LNG tank I cut-a-way.

Larrycng

[rocketmanpete]

Re: Lng/cng

Thanks Larry,the reason I brought up the question is because when I was young and lived in N.Y. back in the 70's with the gas crunch,they were also talking about NG being in short supply. There solution was to build a ship port in Jersey and bring in tankers loaded with LNG. Guess that didn't work out. Thanks!

[rtry9a]

Re: Lng/cng

I wonder if it would be possible to add a small lng tank to an existing cng system- either as a mileage extending add-on or as another alternate fuel source. LNG has to be more much compact per btu than cng is.

As the lng warms and builds up pressure, it could fill the normal cng tank up to the cng tank blow-off valve pressure point, possibly venting extra gas (pressure).

The trick would be to know how much room to leave in the cng tank for the lng-sourced gas and/or to size the lng tank (or limit LNG fill amounts) appropriately to fill the existing cng tank to full pressure.

[Curtis]

Re: Lng/cng

Just for hypothetical giggles, lets look at this more closely.

Help me out here. The LNG wouldn't create enough pressure on it's own to be able to apply much pressure to a CNG tank, right. Maybe enough to run the car as it gasifies, maybe a few hundred lbs pressure supplied to the regulator.
Would that even be enough pressure to keep the car running at freeway speeds?

You'd need an onboard compressor to fill the high pressure CNG tank. It would also need a one way valve system because an LNG tank couldn't withstand the cng tank pressure coming back into it.

Perhaps one of our experts could lay out the logistics of this hypothetical mixed temp/pressure system. I'm sure that if it was easy, they'd already be out there.