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Grex Science Item 62: Heating values
Entered by drew on Tue Jun 20 21:29:54 UTC 2000:

From a table of heating values of various fuels, I find:

                                           High-heat   Low-heat
Substance           Overall combustion       Value       Value
                         reaction          (Btu/lb)    (Btu/lb)
------------------------------------------------------------------
Carbon (coal)      C + O2  -->  CO2          14447       14447
Carbon (coal)      C + 0.5 O2 --> CO          4341        4341
Carbon monoxide    CO + 0.5 O2 --> CO2        4344        4344

Somehow this doesn't quite add up. What happens to the extra 5762
Btu/lb - almost 40% of the total energy change - when the reaction
stops at carbon monoxide before going the rest of the way?

Could the table be faulty?

4 responses total.

#1 of 4 by i on Wed Jun 21 01:56:35 2000:

It looks like the first two show Btu per pound *of carbon*; the last one
per pound *of carbon monoxide*.  There's less than half a pound of carbon
in that pound of carbon monoxide.

#2 of 4 by rcurl on Wed Jun 21 06:01:09 2000:

That is precisely the explanation. Doing the calculation with mole weights,
the CO oxidation yields 10136 BTU per pound *carbon*.

#3 of 4 by drew on Sat Jun 24 18:49:21 2000:

I got it now. Per pound of fuel rather than per pound of stuff.

So partially burning carbon, to get it into a form that can be piped, "only"
costs about 30% of its energy content.

#4 of 4 by rcurl on Sat Jun 24 18:56:42 2000:

That was the basis of the old "coal gas" industry, converting coal into
gaseous fuel for distribution for gas heating, cooking, lighting, etc. (It
was also the origin of committing suicide by "putting your head into a gas
oven", when the gas was predominantly CO and H2 (from the
"water-gas-shift"), which doesn't work the same way today with methane.)

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