Converting to CNG?

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ttugrad

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Re: Converting to CNG?

I did do a search and some items came up about cars and trucks, but nothing specifically about boats.

Care to share the link?
 

QC

Supreme Mariner
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Re: Converting to CNG?

There are some misconceptions in this thread, but there is also some pretty good info. Some key points to clarify:

As others have mentioned, methane (primary constituent of natural gas) is lighter than air at ambient temperature and pressure. For vehicle storage there are two methods, compressed (CNG) and liquefied (LNG). CNG is typically stored at either 3000 or 3600 PSI, and this is the critical safety issue. LNG is methane that is super cooled to -259 F and stored in cryogenic vessels (dewars). Both are used extensively on-highway, and yes . . . drum roll . . . some in Marine.

The most popular use in Marine is probably with mega LNG tankers. They use the boil off gas (another long discussion regarding LNG) to supplement their large diesel engines in a combustion process called dual-fuel. This combustion process, used for on-highway trucks, is what I do for a living. Another long discussion that has been covered here I think in Dockside and Non-Boating Technical. The LNG tankers (ships) use it because it is kind of free. The fuel boils off during transport anyway, so they inject it into the engine and light it off with diesel or preferably bunker fuel (big ships):

4_stroke_dual_fuel_diesel_gas_cycle.gif


There are also LNG ferries operating in Norway where this is becoming fairly popular. There is a CNG Ferry in British Columbia, but I am not sure it is still in operation.

So somehow, some way, there has been marine certification for both although these were compression ignition (diesel) engines in all cases (no spark plugs).

I also know of some LPG boats in the UK. I've actually seen them for rent and assume there are more applications there, but I have actually seen these. So again, somebody insured, and hence "approved" them. But I have never seen CNG in a pleasure boat and there are quite a few reasons why. Let's assume you could get an engine "kit" for a 5.7 (you can), and a bigger assumption is that you could make it legal. Here's why it might not be that great:

1) Fuel density

The gallon equivalency info in post #8 is a little confusing. Here for reference is the questionable piece:

Fuel Type BTU Per US Gal/Equiv.

Regular Unleaded Gasoline
114,100
. . .

Compressed Natural Gas (CNG) 87,600

The problem is they are showing a "gallon" of CNG and they've got it wrong. Since CNG is a compressed gas, how can they define a "gallon"? The other fuels shown are liquid and can be compared by that volume unit, but can't really do that with CNG. LNG yes, at -259 F, LPG yes at -44 F which are their respective boiling points, and how they might be able to be counted as a "gallon". If you look at the comparison of those fuels though there is a very useful piece of info, BTUs per gallon. And, for simplicity, CNG is usually sold in Gasoline Gallon Equivalents (GGE). So you have to figure out how to store (and buy) 114,100 BTUs of natural gas to get the same energy as a gallon of gasoline. If you look at your gas bill, you probably pay in Standard Cubic Feet (SCF). An SCF is around 1000 BTUs, so you need 114 SCF to get that GGE. That's basically a 5 ft cube at ambient pressure. Not really practical for anything. But when squeezed to 3600 PSI we can get a GGE into the same space as around 4 gallons of gasoline. In other words you need 4 times the space for the same amount of fuel. This is why you usually only see CNG on light duty vehicles.

2) Weight

Since it's lighter than air, the more you store, the lighter your boat, right? Well, of course not. When you "weigh" the CNG in a GGE it is actually about the same as a gallon of Gasoline. What, say 7 pounds? That's not the issue. Four times the tankage, and those tanks must be able to safely handle 3600 psi and they're suddenly VERY heavy. Say 500 lbs for 25 gallons . . . :eek:

3) Cost

Carbon Fiber tanks are expensive and you have to buy the fuel system and you have it installed if you could. Let's use $5000, but it would probably be twice that.

4) Payback . . . (the killer of almost all pleasurecraft fuel saving ideas)

Let's say we save 1.00 on a GGE, could be as good as $2.00, but use $1.00 and if you want to cut the payback period in half, that's OK, but pretty aggressive. So let's say you use 25 gallons a weekend, and you boat 20 weekends a year = 500 gallons. This is a lot, but it's a simple example. If we save that buck, it would take us 10 years to get even, and that does not take into account the time value of money. If you use less fuel per year, then payback period is longer. Baaaaaad investment, put the $5K in the bank and buy gasoline. Done.

P.S. We can talk LNG if you want, but basically you need your own station and that would require a use of around 10,000 LNG gallons per week to be very practical. This is why I chase trucks. A fleet of 20 can do that in some cases.
 
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Ned L

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Re: Converting to CNG?

Thanks for correcting me on natural gas being lighter than air. I should have looked it up before commenting. That does open it up to being an interesting concept.
 

awsadler

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Jun 28, 2011
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Re: Converting to CNG?

Using this chart, CNG has roughly 77% of the energy per gallon equiv. of the E10 gas that is used now, for my purposes I'll call it 2/3 the energy. If CNG was used and took up the same space as gasoline, and I am not sure if it does, you would need to have a 1/3 larger "tank" to get the same distance as a tank of gasoline.

I get what you are saying about refueling, but you could carry smaller tanks of CNG to and from your boat as you would gas cans, hook them up in-line and be on your way.

This would actually require a tank 50% larger.
 

smokeonthewater

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Re: Converting to CNG?

welcome to iboats and kudos for searching but this is an old thread and nobody is asking anymore
 
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