Re: role of forest fires in CO2 emissions

[RuthWeiner@AOL.COM]

First of all, CO2 is CO2.  Certainly CO2 is taken up by photosynthesizing 

plants, including trees, but it takes decades to grow a tree and a day or 

several days to burn one in a forest fire.  The CO2 taken up in 

photosynthesis can of course come from coal burning as well as wood burning.  

 So forest fires can add to the CO2 burden of the atmosphere.



Here is a shot at how much:  Cellulose is a polymer whose units are 

(C)6(H)12(O)6: 180 gm.per mole, so when burned, every unit mole would produce 

six moles of CO2 (6 x 44 = 264 grams).  A similar unit of a hydrocarbon 

C6H14: 86 gm/mole, would similarly produce 6 moles, or 264 gm of CO2.  Thus, 

when completely burned to CO2, one kg of coal will produce about twice as 

much CO2 as one kg of wood (cellulose).  This is because the wood itself 

contains oxygen.  



However, this is a relatively meaningless comparison.  One kg of coal when 

burned produces approximately twice as much heat as a kg of well-dried wood 

(for you sticklers out there, this is VERY back-of-the-envelope: -- I just 

assumed the difference due to the number of CH bonds broken).  So, in terms 

of the ratio of heat produced to CO2 produced, this simple example is 

something of a wash.



For your forest fire example, you would have to look at the number of 

board-feet of lumber per acre burned and the approximate mass of a 

board-foot.  Coal-burning plants are about 42% thermally efficient, so for 

100 MW of electricity you have to put in about 238 MW of heat.  High-grade 

bituminous coal has a heat content of about 32,600 kilojoules/kg    (14000 

BTU/lb) so the coal input for 100 MW of electricity would be about 7.3 kg/sec 

or about 690 tons  per 24-hour day.  This would produce about 2080 tons of 

CO2 if burned completely.  Given that coal is at least 25% ash and other 

elements than carbon and hydrogen, this would actually be closer to 1560 

tons. By the above approximate calculation, 1360 tons of wood burning for 24 

hours would produce about the same amount of CO2.  I have no idea how many 

trees that would be.



My numbers for coal come from K. Weston, Energy Conversion, West Publishing, 

San Fancisco, 1992.

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