I am choosing between natural gas and propane for a boiler. I see a discussion here where the accepted response primarily deals with the CO2 released during actual burning of the gas.
I am curious to know if there is any good information about the life cycle impact in the production and transportation phases -- getting the gas from the ground through to combustion -- and if one or the other has a significant environmental advantage from that perspective.
According to Wikipedia:
Propane is produced as a by-product of two other processes, natural gas processing and petroleum refining.
The amount from petroleum refining is minimal. The U.S. Energy Information Agency provides detail about how much propane (a hydrocarbon gas liquid) is produced by each source:
Continuing from Wikipedia, if propane wasn't removed from natural gas, it would cause problems:
The processing of natural gas involves removal of butane, propane, and large amounts of ethane from the raw gas, in order to prevent condensation of these volatiles in natural gas pipelines.
Since most propane is an unavoidable by-product of natural gas production, they have essentially identical life-cycle impacts in the production stage.
In the transport stage, propane does have one key advantage:
Unlike natural gas, propane is denser than air.
This means that it does not escape into the atmosphere as a greenhouse gas. However since there's no way to produce more propane without also producing more natural gas, switching from propane to natural gas would have no net benefit.
Ultimately, if a consumer needs to use one of these fuels for heating, the analysis should focus on which will be most cost-effective, which will ultimately be a function of the local market. The money saved can be used for a more efficient appliance, better insulation, or perhaps some warm sweaters!
In addition to the excellent answer by @LShaver, other things to consider are:
My experience is that there is only one reticulated gas main in residential areas and that such mains supply methane. Similarly, the use of propane requires the use of on site pressurized gas containers which either have to be replaced with full containers as one becomes empty or there is a larger single pressure vessel that is periodically refilled by a truck.
This means that with methane the supply is already at your premises, or just outside it and your premises just need to be connected and every two months or so a person reads the gas meter. With propane a person has to walk around your premises every so often to resupply the premises with gas. For multiple gas containers this requires you to monitor gas levels in the containers and to call your supplier as need arises. For a single larger gas container a refilling schedule between you and your supplier can be arranged.
Per unit volume, cubic foot or cubic meter or liter, whichever units apply in your situation, propane generates more heat than methane.
To heat anything to the same temperature will use nearly 2.5 times the amount of methane than propane. This is why the burners for propane heaters are different to those for methane heaters.
The other implication is the cost of each gas is usually sold on a dollar per Btu (or $/MJ) basis. In effect, your gas supplier isn't selling you gas, they are selling you potential heat.
