Natural gas, which consists largely of methane, and propane are similar in some respects. They are both hydrocarbon gases and they are both odorless and colorless. The distinctive smell of propane and natural gas that we know is due to the odorant distributors are required to add to the gas for safety reasons. The odorant is normally a smelly sulfide compound, like ethanethiol in the case of propane.
Methane consists of a single carbon and four hydrogen atoms and propane has three carbons and 8 hydrogen atoms. The chemical structures of the various hydrocarbon gases one might find in natural gas and store-bought propane are shown below.
Both gases can be compressed for storage purposes but a particularly attractive feature of propane is that it can be readily converted to a liquid form by compressing the gas at moderate pressures. It is this easy conversion of propane gas into liquid form, enabling useful amounts to be stored on-site in steel storage tanks of various sizes, that makes it a versatile fuel. At 80oF the pressure in a propane storage tank is about 150 pounds per square inch (psi) which is not much higher than the pressures in my road bike tires which I typically inflate to 110 psi with a bicycle pump. Natural gas can also be liquefied, but very low temperatures and higher pressures are involved.
Most of us are familiar with propane in its liquefied form in those 5 gallon propane tanks that many of us have attached to our backyard barbeques (unless you are a charcoal purist - which I used to be). Once condensed into a liquid, propane weighs quite a bit. In fact, a full 5 gallon tank of propane can contain almost 20 lbs of propane - which is why those little cylinders are so heavy once they are filled. The weight of the 5 gallon empty tank is about 20 lbs so a full tank weighs about 40 lbs. Liquid propane is readily converted back into a gaseous form simply by turning open the valve on the tanks and releasing the pressure.
Propane, like methane, is a clean-burning hydrocarbon gas with fewer harmful combustion products than oil or coal. The main emission products are carbon dioxide and water, but on a per energy unit basis, propane does release ~20% more carbon dioxide than natural gas. Out of all the carbon-based fuels, methane has the lowest amount of carbon released, per unit of energy released, which is the reason that carbon emissions in the US have dropped as we have moved from coal-fired to natural gas-fired electricity generation. The table below shows the carbon dioxide emissions per million BTUs produced by the combustion of different fossil fuels.
Other than backyard barbequing, propane has a host of other uses including petrochemical production, home heating and cooking, a fuel for industrial forklifts and extensive use in powering farm-based irrigation and refrigeration systems. It also has a growing importance, due to its portability and easy storage, as a back-up fuel for renewable energy systems such as solar power.
Some of the attractive features of propane include the following:
- High energy density once liquefied and available in many different storage sizes.
- Highly portable fuel.
- Bulk transportation by pipeline, rail car or tanker truck.
- Useful alternative to natural gas where natural gas pipelines are not available. It is often the fuel of choice in remote areas.
- Versatile home-based fuel that can be used for heating, hot water and cooking applications.
- Easy onsite storage and, if leaks occur, they do not contaminate the ground like oil.
To get an understanding of the propane business, it is helpful to know where propane comes from. Propane is a byproduct of the natural gas and oil business and it is not produced for its own sake. The byproduct nature of propane means that propane supply, and thus pricing, are highly dependent on oil refining output and natural gas supply. When natural gas is recovered from conventional or shale gas deposits, it is often accompanied by other hydrocarbon gases, such as ethane, propane, propylene and butanes. Natural gas that contains a lot of these other hydrocarbons is referred to as "wet" gas. These other gases are removed during the processing of natural gas, which serves to remove water, sulfur and other byproducts as well. The hydrocarbon gases are also separated into separate fractions - ethane, propane, butane, etc., - each of which has its own specific use. Propane is also a byproduct of the crude oil refining process, during which longer chain hydrocarbon molecules are cracked into shorter chain molecules such as propane, butane, pentane, etc.
Propane was first harvested and liquefied as a useful byproduct of oil refining which is why it is also sometimes called Liquid Petroleum Gas, or LPG. Because the propane we get is a byproduct of various gas separation processes, it does contain other components. The consumer grade we purchase is known as HD-5 (Heavy Duty – no more than 5% propylene) and it is required to contain over 90% propane, a maximum of 5% propylene and 5% ethane and butanes. It can also contain trace amounts of water and sulfur.
As with other energy forms, propane usage in New Hampshire has increased over time. Recent data indicate that over the 1960 to 2011 period, usage has increased 3.8% on a compounded annual basis, outstripping total NH energy use which grew by 2.4% over the same period. Even though growth in propane usage has been greater than that of general energy consumption, propane is a very small percentage of our total New Hampshire energy use: in 2011 it represented only 3.6% of the total consumption of energy in NH. So, in the larger scheme of things, some might view propane as unimportant, but for folks out in remote areas, without access to natural gas, it is very critical. The consumption figures for 2011 were 3.7 million barrels of propane, which is equivalent to 152 billion gallons (at 42 gallon/barrel) or 13.9 trillion BTU. The figure below shows the growth in NH propane consumption since 1960.
The following chart shows the 2011 annual consumption of propane in the New England States and it shows that my original hunch, that propane usage in NH was high, was correct.
However, if the numbers are adjusted to a per capita basis as I have done in the table below, it is Vermont and then New Hampshire that lead the pack on a per person basis. The state that uses the most propane overall is Texas, which is responsible for 60% of the US propane consumption. The reason for this high consumption is the large petrochemical industry in Texas and the bulk of propane consumption in Texas is for the production of petrochemicals used to produce plastics and other organic compounds.
Propane is a useful fuel but one of the biggest concerns associated with propane is its cost. In the table below, I show a listing of the costs of the various home energy sources we use in NH along with their recent energy prices. This is an update of one previously published in Closer to Home. Included in the table are the energy content per BTU/unit, the cost in $ per million BTU ($/MMBTU) and then, using energy conversion efficiency concepts for each fuel, I have calculated the cost of the useful energy produced from each type of energy, assuming the energy source is used for heating only.
It is easier to examine this information in graphical form and, to this end, I have generated the chart below which allows us to directly compare the costs of the input and useful heating output values for each of these fuel sources on a common basis, $ per million BTU. The chart tells us a lot but if we focus on propane which is right at the top of the chart, it is clear that at this time, propane is the most expensive fuel in the State on energy output basis. Presently, natural gas is by far the cheapest energy source in NH.
Like other energy sources, propane prices have risen over time as shown in the figure below and, for the most part, propane prices have moved in lock step with oil prices. The figure also clearly shows the decrease in natural gas prices since the large-scale advent of fracking technology in 2008 which is used to harvest natural gas from shale deposits. The tight relationship between propane and oil prices is somewhat explained by the fact that propane is a byproduct of oil production but propane is also a byproduct of natural gas drilling and there is presently a surfeit of propane due to all the natural gas we are harvesting. What's more, there is now so much propane being produced that we are now exporting propane from the US. New Englanders do not appear to have benefitted much from the increased supply of propane: that will be the topic of Part 2 of this blog where I will be looking at the supply, demand and pricing issues pertinent to propane usage in New England.
Many of us use propane at some time or another so a few safety comments about propane are appropriate. In terms of home usage, whether using a gas grill or for home heating, it is important to understand that propane is a highly combustible gas under pressure* and it is crucial to make sure that all the gas line fittings are tightly fastened and that there are no leaks. You can easily check for leaks using a soapy water solution and for those of you using propane for home heating and cooking, I would strongly recommend the installation of a combustible gas monitor in your home which can detect dangerous levels of methane and propane. If there is a propane leak you might be able to smell it, but sometimes, because propane is heavier than air, it can accumulate to dangerous levels in basements and trenches in or around your home where you might not be able to smell it. My advice is to back up your nose with technology. A home combustible gas detector unit only costs about $50 and is a wise investment. It will also work if you have natural gas in your home.
To wrap up this week's post, I thought I would cover a topic that is of great interest to all us home grillers. One of the great mysteries of gas grilling is how to determine how much propane is left in the propane cylinder and whether you will run out before all the hamburgers are grilled. Now, if you are like me, you have run out of propane when grilling on a Sunday evening when no refilling stations are open and you have had to endure dirty looks from your significant other and beer-fueled jibes from friends. Well, those days are over - there is an easy way to determine how much propane you have left. Simply weigh the cylinder on a regular bathroom scale and subtract the tare weight which you can find stamped on the top ring of the cylinder. The pictures below are of my propane cylinder just a few days ago. As you can see the weight of the cylinder is 28.5 lbs and the tare weight is 18lbs so my tank contained 10.5 lbs of propane – it was about half full.
The next thing to figure out is how much propane a grill will consume. Typically a home barbeque with all the burners running has a rating of about 40,000 BTU/hr. The BTU content of propane is 91,333 BTU/gal and, at 4.23 lbs propane per gallon, this is equivalent to 21,550 BTU/lb. This means that you should be able to grill for about 1 hour for every 2 lbs of propane you have in the propane tank. So, based on the photos above, I have enough in my tank to grill for about five hours. By the way, those pressure gauges that you can buy for propane tanks are pretty useless. Because propane is a liquefied gas, the vapor pressure is constant as long as there is propane in the tank. The pressure will only begin to drop when there is no longer any liquid in the tank and by then it might be too late and you are likely to run out of propane while grilling.
Until next time, don't run out of propane and remember to turn the lights off when you leave the room.
Mike Mooiman
Franklin Pierce University
mooimanm@franklinpierce.edu
7/14/13
(*Under Pressure – A big 1980s hit for Queen and David Bowie who put this song together while improvising in a recording studio in Montreux, Switzerland. It retains some of its improvisational roots in its "Um, boom, ba, bay.." type lyrics and its distinctive bass riff is something every bass player fools around with one time or another. It is easy to find this song on Youtube but here is an interesting version featuring Annie Lennox and David Bowie practicing for the Freddie Mercury tribute concert. David Bowie could not be more relaxed, singing and smoking at the same time.)
