A chemistry lesson.
A colleague recently asked at work about which engines can run CNG, LNG, and LPG. This got me thinking, though I report on these fuels, I’m not yet clear which engines exactly run them.
Well the short answer to that is that natural gas vehicles take CNG and LNG, except LNG is heated to return to a gas before being combusted like CNG. LPG is simply propane, like the stuff you fire up your grill with, and is increasingly being used as “autogas” for specially-designed vehicles – not available at your local Ford showroom, but can be ordered. I’ll like to do a post later on on how exactly these different engines function, but for now I think it’ll be helpful for readers to do a review of these three fuels and the chemistry behind them (luckily I have a chemistry background).
CNG – compressed natural gas (mostly methane)
LNG – liquefied natural gas (mostly methane)
LPG – liquefied petroleum gas (propane or butane as well as mixture of other gases)
Fossil fuels, from natural gas to diesel, are hydrocarbons. This means they are composed of carbon atoms bonded to hydrogen atoms. Carbon atoms can be bonded to other carbon atoms as well. The number of carbon atoms bonded together affects the physical properties of the resulting hydrocarbon. The more carbons, the heavier the hydrocarbon.
Hydrocarbons can exist as gas, liquid, or solid depending on how many carbons they have – gas has the fewest, solids the most. However, as the ideal gas law demonstrates, if you mess with pressure and temperature you can force hydrocarbons into different states, i.e. gas can turn into liquid and vice versa.
- where P=pressure, V=volume, n=# of moles, R=gas constant [.08206 (atm∙L)/(mol∙K)], and T=temperature (kelvin)
Methane: One carbon atom bonded to four hydrogens; very light, so exists as a gas
Ethane: Two carbon atoms bonded to six hydrogens; light, gas
Propane: Three carbon atoms bonded to eight hydrogens; light, usually exists as a gas, can exist as a liquid at low temperatures or high pressure
4 carbons: Butane (this is where stuff starts to get liquid-y)
5 carbons: Pentane (getting into gasoline territory)
6 carbons: Hexane
7 carbons: Heptane
8 carbons: Octane (a version of octane called isooctane is used as a benchmark to measure the tendency of hydrocarbon fuels to “knock”/self-ignite – thus the term “octane rating”)
9 carbons: Nonane
10 carbons: Decane
Gasoline is typically a mixture of C4-C12 hydrocarbons. They are not necessarily all straight, single bonded molecules, which means they can be found in rings and other structures (you might have heard of naphthenes and olefins).
Petroleum is a mixture of pretty much everything. When Mother Nature made hydrocarbons, she didn’t care to separate them into groups. So what you drill and suck up from the ground is basically everything, which you have to then separate into different weights so you can use them in different ways.
CNG is compressed methane (gas under high pressure), and LNG is liquefied methane (gas that is super-cooled). LPG is liquefied propane/butane, propylene, and butylene.