Stovers and Biochar-ites,
Crispin has made a very useful comment that should not be buried under a
Subject/Thread called Re: [Stoves] Smoke-free biomass pellet fueled stove
So I have started a new thread and invite comments from those who know
much more about this than I do. Can Aprovecho or CSU or ISU (Iowa) or
U of Dayton or others confirm, adjust or reject the calculations?
Even if the numbers change a little, I believe this could be
important. We do want to see how it is useful.
Paul
Paul S. Anderson, PhD aka "Dr TLUD"
Email: [email protected] Skype: paultlud Phone: +1-309-452-7072
Website: www.drtlud.com
On 11/4/2012 4:59 AM, Crispin Pemberton-Pigott wrote:
Dear Paul
Have you decided on a favourite value for the heat of combustion from the
gases developed from a wood fire in the TLUD's? If you have a wood at 18.4
MJ/kg dry and you have (say) 20% char left at the end, what would you
consider the heat value per missing kg? Obviously it is not the same as the
initial wood.
So I wanted to explore this with you. Let's set the moisture at 15% to be
typical of air-dried hot-country fuel wood.
Starting with an LHV for the dry wood, the net heat LHV is 15.25 MJ/kg. If
there is 200 g of char remaining, and we treat it as being the same as
regular hardwood charcoal, we can assign a heat value of 29.5 * 0.2 = 5.9 MJ
for the remaining char.
What remains as heat available is 9.35 MJ. This is from 800 g of missing
fuel so the energy average per missing gram is 9.35/0.8 = 11.69 MK/kg or
11.69 Joules per gram.
Do you agree with this approach?
Paul Anderson inserts: I like the approach. But at best we are dealing
with 2 digit accuracy. And it is easier to remember 11 to 12 MJ/kg =
the pyrolytic heat from TLUD making biochar.
Crispin continues:
What it means is that if you put a char-making stove on a scale and run it
you have a value of heat per g missing from the scale. The missing mass is
moisture, wood gas and some of the carbon (about 1/2).
Using this approach you can determine the net thermal efficiency of the
flame-to-pot+water. As the amount of char remaining is variable and not
known until after the test is completed, it is hard to know what the
performance is during the test but that is a detail.
You can determine if the thermal efficiency has been improved should you
make changes to the stove body.
For those who like details, the heat value of the char is usually unknown.
It would have to be homogenised and a sample tested in a bomb calorimeter to
know what it is exactly. As Penn Taylor pointed out, the value can range
from about 12 to 33 MJ/kg so the real value is going to vary from test to
test.
Regards
Crispin
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