BTU Question.

[hobbyheater]

Today I weighed the wood for a batch burn on my new Jetstream. Before firing the storage bottom temperature was at 102 F and top temperature was 118 F. Burned 156 lbs old growth Douglas Fir at 15% moisture content. Stack temperature throughout gasification was 335 F. Fire out after a 9.5 hour batch burn. Bottom temp is now 184 F and top is 195 F. For duration of the burn, the draw from storage was about 5,000 BTUs per hour.
What % of potential BTU's ended up in storage?

 

[jebatty]

A fun question to ask. The math is simple, but need to make some assumptions:
1) assume average start tank temperature = 110F
2) assume average end tank temperature = 190F
3) btu available from wood (20%MC, stack temp 400F) = 6,040/lb (might be 6,050, negligible effect)
4) from other postings you have a 1260 gal storage vessel

Btu's in tank = (190-110) x 1260 x 8.34 = 840,672
Btu draw = 5,000 x 9.5 = 47,500
Btu available from wood = 156 x 6,040 = 942,240

% btu in tank = 840,672 / 942,240 = 89%
% btu to draw = 47,500 / 942,240 = 5%
% conversion of available wood btu to tank + draw = 94%

I don't know how much additional btu available from wood increases as MC moves from 20% to 15%, but a lower MC should result in additional btu available, with the result being that your conversion % would decrease.

Since your stack temp was 335F vs 400F, you have about a 2% increase in available btu from wood, with the result being that your conversion % would decrease by about 2%.

Based on your numbers, and before adjustment down for MC and stack temp, 94% (could be closer to 90%) conversion is very good. My recollection is that doing a similar analysis on my Tarm with 1000 gal storage, the conversion was 86%.

 

[hobbyheater]

Another oddity , as the woods moisture content starts to drop below 15% the heat value% to the storage also drops and the flue temperature begins to rise . I have assumed that there is not enough oxygen available too burn extra smoke generated by a more intense fire .
Thanks for the effort you put into the calculations !

After the fire was out we baked some potatoes in the hot burn chamber !

Allan