Given the choice to adjust a new furnace to either propane or natural gas (?), is one less unsustainable than the other?
There is probably not a lot in the choice but some considerations are listed below. A significant consideration is that while Propane is superior environmentally, individual purchasing decisions are not likely to alter the overall Propane to natural gas mix overall. If users encourage Propane separation because it is popular they effectively degrade the quality of the remaining "natural gas" so that overall user experience is worse and more of the inferior product may have to be consumed to meet user needs.
Hydrocarbon gases are slightly more energetic the shorter the carbon chain. So in order of decreasing energy per volume the gases are
Methane, Ethane, Propane, Butane, ... .
Natural gas is a mix of hydrocarbons - but usually principally Propane + Butane.
Butane has a lower calorific output per volume than Propane, takes about 30% more air to burn and releases about 33% more CO2 per volume burnt.
As Butane and thus natural gas have a lower calorific value you'll burn more to get a given amount of heat - so the CO2 released is perhaps 40%+ higher for a given heat output.
When burnt with Oxygen hydrocarbons produce CO2 and H2O (water) as end products.
If you want to reduce CO2 per volume of gas burnt you want a shorter chain hydrocarbon and Propane is shorter than Butane.
Hydrocarbons have formula CnHn+2 as there are two Hydrogens per carbon + 1 extra at each end of the chain.
So in increasing carbon chain length:
- Methane = CH4 = H-CH2-H
- Ethane = C2H6 = H-CH2-CH2-H
- Propane = C3H8 = H-CH2-CH2-CH2-H
- Butane = C4H10 = H-CH2-CH2-CH2-CH2-H
- Methane has 4:1 H:C,
- Ethane has 3: H:C
- Propane has 8:3 = 2.666:1
- Butane has 10:4 = 2.5:1.
Burning with Oxygen:
Propane: 2.C3H8 + 10.O2 = 6.CO2 + 8.H2O
Butane: 2.C4H10 + 13.O2 = 8.CO2 + 10.H2O
So, per volume of gas, Butane makes 33% more CO2 and 25% more water vapor.
If lowest CO2 production was your major concern then Propane is significantly superior.
Natural gas usually consists of a mix of gases. As Propane is easily liquefied and has a higher calorific value when burnt it is often extracted and sold separately.
This means that the remaining product has higher Butane content, lower heating value and makes more CO2. Removing some Propane downgrades the gas generally.
When mixed NG is burnt from a tank the more volatile Propane tends to exit preferentially leaving the bottle Butane enhanced, Propane reduced and generally less nice to use. If the tank is not fully emptied before refilling it tends to always be Butane rich and increasingly so. Emptying the tank completely sometimes "resets" the process.
In some centres Propane can be bought specifically.
So - Overall:
If all the Propane and all the Butane available are consumed by burning then choices re which to burn do not influence its overall impact.
Conversely - if excess Butane is used as feedstock for a manufacturing process (plastic etc) so that more Propane and less Butane are burned, then its adverse aspects are reduced.
Propane should cost more than NG - how much so can depend on marketing issues rather than just energy content.
So, overall, it's hard to say. Propane is best or you and the environment if its use means Butane rich NG is not used instead. But as Propane and Butane may exist in fixed ratios, using Propane may cause a worse net overall environmental effect.
Note: If Propane is made by "cracking" longer length hydrocarbons then the ratio of available Propane to Butane will vary on demand, but odds are the Propane you get has been separated from naturally occurring Propane+Butane + other mix.
Depending on where you are, the natural gas may be mostly propane ;-)
The answer from Russel McMahon gives an excellent summary of the chemistry. However, I suspect that the differences there will easily be outweighed by differences in distribution. Since you refer to "natural gas" rather than its specific contents, I assume that you have the option of "mains" piped gas to your property, and are comparing this to using bottled propane. In this case I would expect the use of natural gas to have substantially lower emissions since it is not being compressed into bottles, trucked to your home, etc - as well as being much cheaper!