Tuesday, December 22, 2009

air fuel ratio for firewood

you're burning wood to heat your home. but the inside air gets dry ... you're pulling in cold dry outside air to replace the air used in combustion. you need to add water to the air. but how much ?

first, how much air is needed to burn a kg of firewood ?

there isn't as much info on firewood on the intrawebs as i'd have thought.


stoichiometry for dry firewood ends up being about: c6h9.5o4.45 + 6o2 --> 6co2 + 4.75h2o, and air is 21% o2. resulting in an air/fuel ratio for firewood of about 6.

all the numbers are very imprecise, wood is heterogenous, moisture varies a lot, combustion probably isn't perfect, hemicellulose is a mixture and lignin isn't well defined, so the result is only approximate (and that assumes i haven't made any errors :). i'm guessing the true mean is plus or minus .5

assuming that you're drawing in cold outside air with very little humidity, and you have to add all the water to it to achieve 50% relative humidity at 25oC (which is 10g water per kg air), you'll need to add 60g water per kg of firewood. realistically, the incoming air has some water, and we don't need to go to 50%, so maybe 30g-40g water per kg of firewood will work

other stuff:
firewood: 15MJ/kg HHV (ie including the energy that's lost with the combustion gases)

found a great pdf talking about some of this stuff

mostly agrees with my calculations
ideal air fuel ratio is 6, but to get decent combustion, stoves run 10-12

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