MadSci Network: Physics

Re: How many btu are there in a dollar bill? (Or a piece of paper?)

Date: Thu Nov 3 10:51:28 2005
Posted By: Todd Whitcombe, Associate Professor, Chemistry
Area of science: Physics
ID: 1127719494.Ph

Interesting question. And actually, much more difficult than it would 
first appear!

I would like to point out, initially, that I am from Canada and our 
dollar bill is now a coin. Hence, I couldn't simply go to the lab and 
check this one out! (Nor was I willing to burn up a five dollar bill for 
the sake of science.)

So, what is the heat content of a dollar bill? At first blush, one might 
think to simply look up the heat of combustion for paper (13.21 kJ/g - 
give or take - or 12.44 Btu/gram) and then look up the mass of a dollar 
bill (3.46 ounces per 100 or 1 gram per bill). Using these two numbers, 
the answer would then be 12.44 Btu per dollar bill.

Except that dollar bills aren't made from paper. Paper money is actually 
made from "linen" and the rag content of the money is supposed to be a 
secret to protect the money supply from counterfeiters. That said, it 
would appear that the paper money is typically about 25% cloth - and 
since linen is a cellulosic material, it should have pretty close to the 
same heat of combustion. However, its burning characteristics are 
significantly different from paper.

Also, paper money is printed with inks and such. They will reduce the 
heat content available from a dollar bill as the inks tend to be 
inorganic and not particularly combustible. Some of the heat content 
available from burning the cellulose would need to go into volatilizing 
the ink. Hence, it would be slightly less than 12.44 BTU per bill. But 
probably not a lot.

To further complicate things, though, if you were to actually burn money 
to generate electricity, you would have to deal with the issue of 
combustion of the dollar bills. They don't burn very well when piled 
together. There is no way for oxygen to get to the core of the stack and 
hence, they tend to burn on the surface. Try burning a telephone book to 
see this effect.

Consequently, if you truly want to compare the burning of money with say 
a fuel like methane (natural gas), you need to account for the efficiency 
of the combustion process. Natural gas burners operate at virtually 100% 
combustion as it is easy to mix oxygen and gas. Burning money is going to 
be significantly less efficient - probably closer to 30% - unless the 
money is either introduced one bill at a time or burnt in an incinerator 
where there is some other source of fuel for the flame (i.e. natural gas).

The "straight up" recoverable energy is probably closer to 4 BTU per 
bill. And then there are the inefficiencies associated with converting 
heat to steam and using the steam to drive the turbines and the turbines 
to driven the generators that produce the electricity. I have seen this 
number quoted as 23% but I suspect that is a little low. One the other 
hand, it is not even close to 100%.

So, is burning "money" energy efficient? Probably not. At 13,210 joules 
per gram and with one watt being one joule per second, a paper bill could 
run (at 100% energy conversion) a 1500 Watt hair dryer for about 9 
seconds. Not enough time to really dry your hair. And realistically, if 
you take all of the inefficiencies and losses due to conversion into 
account, that time is probably closer to 1 second.

On the other hand, with soaring fuel prices, sometimes it does feel like 
we are burning money! 

Hope that answers your question.

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