[Stoves] Heat from combustion of TLUD gases with char remaining

[Paul Anderson]

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:  psanders at ilstu.edu   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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