[Stoves] Mixing char/coke with raw fuels

[AJH]

On Tue, 10 Apr 2007 08:46:14 -0500, Paul S. Anderson wrote:

>>> [Paul said:] ...there are no sources of pyrolysis gases in that
>char [created in the TLUD].
>>
>> [Andrew]:  There will still be some ...
>
>Yes, but very little, and probably not enough to keep reworking 
>(re-heating) the
>char to try to extract the final pyrolysis gases.

Agreed this would not be a worthwhile method to cook out the remaining
tars, I doubt the temperature is high enough, it is a way of reburning
the char so as to get the energy out of it if there are no other uses
for the char.
>
>> [Andrew]:  ...but the char will still burn [in the TLUD], have
>> a try, from memory the downward rate of spread of the fire is much
>> faster as the fire front only has to heat dry char to its ignition
>> temperature.
>
>Because the goal is to ultimately combust pyrolysis gases in the burner, the >mixing of TLUD-char with the new raw fuel is ESSENTIALLY adding a 
>"filler" that
>must be heated TO THE TEMPERATURE OF THE PYROLYSIS, not to the temperature of
>the ignition of the char.  

In fact I think they are very similar temperatures, around 300C, the
fire front in a wood tlud seems to be around 500-700C.


>The NEW char that is being created in the pyrolysis
>process will reach the char-ignition temperature (in order to sustain the heat
>generation to release the pyrolysis gases), and there seems to be little
>justification to have additional (the old char) char also come to the ignition
>temperature (unless but unlikely that a case can be made that sufficient
>pyrolysis gases could be extracted by re-heating the old char).

Yes but see above, it's a way of using up the char in the same stove.


>
>And the reason for the faster downward rate of the pyrolysis front is 
>that there
>is less biomass there that needs to be heated for releasing pyrolyisis 
>gases. Fast is not good when duration of the fire is important to the 
>cook.

Agreed, one of the merits of tlud is that the constant cross  section
of the fuel canister and the rate of descent of the fire front gives a
fairly constant enthalpy of offgas, and hence thermal power output of
the secondary flame.

>I believe that John Davies was not mixing the coke/char in with the new 
>batch of
>coal, so what Andrew is describing is extremely interesting.  Please provide
>more details about that "coal in TLUD" experiment. 

I posted about it at the time, again it required the coal to be broken
into small pieces to work at all, this all comes back to the rate at
which the particle can be heated, I see in Dale's report which you
cited he found just his when he put a larger piece of wood in the
"champion" stove. John Bertl also alluded to it, I believe this is
related to the Biot number which is a ratio of the resistance of heat
transfer at the surface to the resistance of heat conductance within
the object, but with biomass the latent heat of water vapourisation
modifies this greatly.


> Is there a theoretical
>explanation as to why "coal with coke/char" would burn more cleanly than just
>the coal alone in a Davies' style TLUD?

As I posted earlier it can be a number of things, anything that makes
a flame hold will release a lot of energy currently lost as smoke,
which even in a tlud stove will feedback some radiant heat to the
combustion area. The smoke consists of nitrogen and CO2 diluting any
offgas and probably at fairly low temperatures in the beginning, as
the coal has a high thermal inertia, so the offgas has a low cv and
low temperature, both these things need addressing. The coke is
probably able to reach higher surface temperatures, if this puts up
the ratio of CO:CO2 in the offgas and a spark is present then the
relatively high flammability of CO could provide the means by which a
flame can hold. I probably used a fan, which would have increased the
air velocity through the coals, and so a lot more CO would have been
produced than under natural draught, the cost of this on damage to a
simple grate is high though.

AJH