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

[Crispin Pemberton-Pigott]

Thanks for the spread. When conducting a test on a scale, you can get the power and efficiency in real time. Obviously it is done post facto with a char producing stove unless you have real time gas calculations to work with. The true heat value is still needed. If the wood heat is used, the test under-reports the thermal efficiency significantly. 

It is important to note that this not related to the overall efficiency as described in the message about the char remaining. The efficiency here is the sub-system that includes the flame, the pot and the water in it.  

Regards
Crispin
-----Original Message-----
From: Paul Anderson <psanders at ilstu.edu>
Date: Sun, 04 Nov 2012 08:27:11 
To: Discussion of biomass cooking stoves<stoves at lists.bioenergylists.org>
Cc: Crispin Pemberton-Pigott<crispinpigott at gmail.com>; Hugh McLaughlin<wastemin1 at verizon.net>; biochar at yahoogroups.com<biochar at yahoogroups.com>; Doc Anderson<psanders at ilstu.edu>
Subject: Heat from combustion of TLUD gases with char remaining

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
>
>
>
> _______________________________________________
> Stoves mailing list
>
> to Send a Message to the list, use the email address
> stoves at lists.bioenergylists.org
>
> to UNSUBSCRIBE or Change your List Settings use the web page
> http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
>
> for more Biomass Cooking Stoves,  News and Information see our web site:
> http://www.bioenergylists.org/
>