[Stoves] Comparable heat outputs of stoves

[AJH]

On Thu, 01 Jun 2006 22:37:25 -0500, psanders at ilstu.edu wrote:

>So, if we allow that most people are using 15% to 20% moisture wood, 
>>> the 18.6
>>> MegaJoules per kilo must come down by that amount, so the actual MG would be
>>> about 16 or 15 MJ per kilo of common moisture (seasoned or air-dried? or
>>> "regular") woody biomass.
>>>
>>> Do we need to also allow that some of the fuel must be used to boil away the
>>> moisture, resulting in less combustibles available for making high
>>> temperatures?

Yes we do but I don't think we should get overly complicated. Whilst
we should bear in mind that wetter wood tends to need higher excess
air, which greatly affects heat transfer efficiency, I use a ball park
figure.

I derive this from the fact that the moisture in the wood is boiled
off and rejected as steam at flue temperature. The latent heat of
steam is about 2.3MJ/kg and steam mass has to be raised from ambient,
through boiling, to the flue temperature. I think subtracting 2.7MJ
for every kg of water in the wood is close enough but stand open to
argument on that.

So oven dry wood has no moisture and yields ~18.6MJ/kg
Denver dry wood has 15% mc wwb and yields .85*18.6 minus .15*2.7 MJ/kg
=15.4MJ/kg
Freshly felled willow has 60% mc wwb and yields .4*18.6 minus .6*2.7
=5.82MJ/kg and is next to impossible to burn cleanly without
sophisticated burning.
>>>
>>> My original objective was to get some ball-park comparisons.  Here 
>>> is a start:
>>>
>>> 4 BTU =~ 1 kcal
>>> 1 BTU =~ 0.25 kcal
>>>
>>> 50,000 kcal =~ 200,000 BTU
>>>

I've posted my feeling on this, I don't think any system is
fundamentally better or more accurate but this is an international
list and, as was pointed out, even the US preference for imperialism
is contrary to their agreed position to prefer SI units.

My preference would be to go with Joules, Watts and kWhr rather than
calories, even though they are the original heat units. To help people
more familiar with imperial measure these could be parenthesised after
the SI units.

>>> 1 kg reg-wood =~ 16 MJ =~ 3800 kcal =~ 15,000 BTU =~ 4.5 kWhr(t),
>>> which means that a nicely sized residential cookstove that takes one hour to
>>> fully consume 1 kg of regular woody biomass is operating in the 4 to 5 kWhr
>>> range.  Because all of this energy is expressed as being in one hour of
>>> constant burning, we can say that the power is a 4 to 5 kW stove.

You've got it.

AJH