Power Plants

I was reading the comments following a column in a Very Famous Newspaper and came across a good comment about how conservation is preferable to fossil fuel extraction.  However, the commenter attempted to figure out how much natural gas would be needed to supplant the coal we currently use for electrical generation, and things kind of off track, but got me wondering what the answer was and what the ramifications were.

Some background:

Using rough numbers, power plants are approximately 33% efficient.  In other words, to deliver 1 kilowatt-hour of electricity to your house, the plant must burn 3 kilowatt-hour’s worth of fuel.  Basically, 2/3 of the heat just goes up the stack and vanishes, while 1/3 of the energy actual does the work to turn a generator and produce electricity.

You might think that capturing some of that stack heat would make sense, and you would be right.  Stack heat economizers allow plants to pre-heat the incoming combustion air and boiler make-up water, which can  give them a percent or two higher efficiency.

Far more effective, and far more common overseas, is to take the stack heat and use it to provide district heating to nearby homes and businesses, generally by heating water and pumping around the district with below-grade piping.  However, in the U.S., power plants are often at a good remove from facilities that might be able to use their waste heat, so district heating, while not exactly rare, is not exactly common, either.

Anyway, I did a bit of Googling and it looks like we generate about 2 trillion kilowatt-hours of electricity with coal every year.  That’s a lot of kilowatt-hours: 2,000,000,000,000 if you like to look at big numbers.

Actually, if you consider that there are about 350,000,000 people in the U.S. that’s 5,700 kWh per per year for every man, woman and child in the country.  Which is amazing.

According to the Energy Information Agency (a GREAT resource, by the way), the average energy content of coal is about 19,583,000 Btu per ton, or 9,792 Btu per pound.  I was surprised it was this low, but our average coal quality may not be that good.  In any event, we can approximate how much coal was burned pretty easily:

(2,000,000,000,000 kWh * 3,412 Btu-per-kWh) / (33% efficient * 19,583,000 Btu-per-Ton) = 1,056,000,000 tons of coal.

Man, that’s a lot of coal.  A billion tons!  And remember from our earlier posts, coal is mostly carbon by weight, and CO2 weights 3.7 times more than carbon.  So we’re talking of carbon emissions in the neighborhood of 3+ billions of tons of CO2 that enters the atmosphere each year from electrical generation.

I see that coal prices vary quite a bit from region to region, but if we use $50/ton as an average price, that means fuel for coal fired generation costs about $50 billion dollars annually.

Natural gas is generally measured in units of volume (normally 100’s of cubic feet [or CCF]) or in units of energy (Therms at 100,000 Btu, or Decatherms at 1,000,000 Btu.  Gas is purchased in bulk in unit of Decatherms, but most customer bills are in therms or CCF.)  Fortunately for us,  the energy content of a CCF of natural gas is about the same as a Therm, so they can  be used more or less interchangeably without introducing undue error.  Also, from the above we see that a cubic foot of gas is about 1,000 Btu and that a decatherm is equivalent to about 1,000 cubic feet.

We can now readily calculate how much natural gas would be required to supplant coal at our power plants:

(19,583,000 Btu-per-ton-coal * 1,056,000,000 tons/year) / 1,000,000 Btu-per-Decatherm = 20,679,648,000 DTh.

That’s a hell of a lot of natural gas.  By the way, delivered gas might cost $5 a DTh.  So the cost for this much natural gas would be about $100 billion dollars.  Each year!  Note that this is roughly twice the cost of coal for the same electrical generation – there’s one reason why coal is a hard habit to quit.

Now, the Energy Information Agency (EIA) reports natural gas production in millions of cubic feet, so I am going to do a rough conversion by multiplying our Dth number by 1,000 – recall that a Dth is about 1,000 cubic feet.  We’re now talking roughly 20 trillion cubic feet of natural gas (or as EIA would state it, 20,000,000 million cubic feet)

US Natural Gas Consumption

via: http://www.eia.gov/dnav/ng/hist/n9140us2a.htm

Given that the U.S. consumes about 25,000,0000 million cubic feet for all end uses today, we would need to almost double our production capability to convert our coal burning power plants to natural gas.

You may recall from an earlier post that natural gas puts almost 50% less CO2 in the atmosphere than coal.  So converting to natural gas would reduce our annual carbon dioxide emissions by somewhere in the neighborhood of 1.5 billion tons.

Interesting stuff.


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