Wednesday, February 25, 2009

Past, Present and Future Ghosts

There are three fossil fuels, one solid, one liquid and one gas. In the United States we have very little oil left and it is pretty much universally recognized that we are going to have to use less oil. The means to do this are not settled. Some want to see more alternative liquid fuels while others look to substitution with electricity or with natural gas. Sources for alternative liquid fuels are possibly plants including algae or coal converted to a liquid or natural gas converted to a liquid. If given a choice between electricity generated with natural gas or natural gas burned directly in a vehicle, the electricity route is more efficient since gas turbines are 60% efficient and electric cars are 80% efficient so the overall efficiency is about 48% while average performance for car engines is about 20%. So, we'd use more than twice as much gas using it directly for transportation than if we converted it to electricity first. One thing that we are settled on is that when we do use oil in transportation, we will do it with better fuel efficiency though we could do more on this.

The US has oil reserves of about 21 billion barrels and consumes about 21 million barrels a day so that is three years of oil at our current rate of use. If we were to use only our own oil, it would not make sense to buy a car because we would be out of fuel before the warranty is done. However, oil is shipped all over the world so we mainly rely on imported oil. Oil is on its way out and so should be thought of as a ghost of the past.

Usually we think of coal as the most abundant fossil fuel in the United States with something like 200 years of supply at our current rate of use. However, new work suggests that only 20 years of economically viable coal remains. How does this compare to natural gas? One set of estimates gives a range of between 88 and 118 years of supply. In terms of primary energy we use about as much gas as coal so that we seem to have 4 to 6 times more usable gas than usable coal. So, we probably need to reverse the usual assumption that coal is the most abundant fossil fuel in the US and declare coal the ghost of the present and natural gas the ghost of the future.

When we consider oil, buying a car makes no sense without oil imports but for coal and natural gas, imports don't make too much sense. With coal, shipping the stuff takes about as much energy as mining the stuff as things stand so getting coal from overseas seems counter productive. Gas travels in pipelines fairly well but it needs to be liquefied to be shipped over oceans and this cost quite a lot of energy so importing gas does not make a lot of sense. And that is pretty much how the world markets work. Most coal and gas are used on a continent scale but don't cross oceans.

So, what we have on hand is all we are going to get for coal and natural gas to a pretty good approximation. What does this mean?

If there are only 20 years of coal left, it makes absolutely no sense to build a power plant that is meant to last 40 years to burn coal. Thus, all attempts to work out carbon capture and storage methods for coal are wasted efforts. First, most plans are to build new plants with this technology but it makes no sense to build a new plant. Second, we should be using less and less coal in order to save some for future steel making so there will be limited application for existing plants.

For natural gas, we only need about two thirds of the primary energy use of coal to replace coal since gas generation is more efficient so that completely replacing coal leads to between 53 and 70 years of natural gas available. A natural gas plant built today with a design lifetime of 40 years will not run out of fuel before this time. Thus, if one is going to attempt to capture and store carbon dioxide, doing it at a natural gas plant makes much more sense. It is also a lot easier to do since the exhaust is a lot cleaner to start with.

Oil use is on its way out already. Apparently we have been mistaken about coal and we need to start it in the same direction now. Only natural gas has the potential for expanded use and so if we are going to put effort into trying to use fossil fuels without emissions, this is where we need to concentrate what we do.

Clean coal is a dirty lie but it is also a pointless one.

8 comments:

popmonkey said...

excellent post! i've been a big fan of NG as a transitional fuel as there's a lot of it left, enough to mitigate the oil shortage and last us long enough where, hopefully, new energy sources are developed.

lets hope the oil controlled status quo is finally broken. it's getting tiresome.

Chris Dudley said...

Hi popmonkey,

I saw your comment a while back. Sorry for not answering sooner. I don't think in makes a lot of sense to try to use natural gas as a substitute for oil unless it is first converted to electricity. A natural gas fueled internal combustion engine is going to see a average conversion efficiency to mechanical energy of about 20%. With a combined cycle turbine, conversion to electricity has an efficiency of about 60% and an electric car has an efficiency of about 80% so that the overall efficiency is about 50%. That is more than a factor of two reduction in resource use. Further, building a natural gas generation infrastructure is useful as a future backup to renewable energy plus storage.

popmonkey said...

hey Chris, you're absolutely right and i wasn't implying using NG to power vehicles directly.

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