[Gasification] Re: [Stoves] improving charcoal stoves

[psanders at ilstu.edu]

Tom,

I always appreciate your scientific completeness, but I still have a 
question or
two, especially about how much of the air is primary and how much is 
secondary. To All:  Tom is very concise, and my comments below are my 
efforts to understand
and restate those chemical events.  Please correct my errors.

1.  You say that wood is    CH1.4 + O .6  with molecular weight of 23.  
Where do
you get the 23 out of values of 12, 1, 16?   1.4 x 13 = 18.2   and 0.6 
of 16 is
9.6.    I am happy with it being 23 if you say it is, but I do not know where
you got that value.       (more questions after this repeat of what you wrote:

Quoting Thomas Reed <tombreed at comcast.net>:  (cleaned up because my email copy
had highlight marks, etc.)

> *Dear Paul and All:
>
> Charcoal (C plus small amounts of volatiles) is VERY different in its 
> combustion characteristics from wood, CH1.4O0.6.  Complete 
> (stoichiometric) combustion of charcoal requires an air/fuel ratio of 
> ~12, while wood has an air/fuel ratio of 6.

Charcoal:  C + (O2 + 3.76N2) ==> CO2 + 3.76 N2
A/F = 137/12= 11.4 = ~12

Wood:  CH1.4O0.6 + 1.05 (O2 + 3.76N2) ==> CO2 + 0.7 H2O + 4 N2
A/F = 137.3/23 = 5.96 = ~ 6

> (To find A/F ratio, convert formulas to molecular wt, (C = 12, H=1, O 
> = 16, N=14, so wood = 23))
> A/F = 6
>
> Pyrolytic gasification of wood typically has an A/f ratio of <1.5, 
> while gasification of charcoal to CO has an A/F ratio of ~6.  So 
> after the WoodGas campstove finishes the pyrolytic gasification of 
> the wood, it requires 4 times as much air to gasify the charcoal.
***************  end of quote *********

Question 2:  After the pyrolytic gasification, (but before any 
combustion of the
resultant gases), we have some char (almost pure carbon) and some hydrocarbon
gases plus the inert N2 from the air.  BY WEIGHT, can we say that the char is
20% or 25% or 30% or 33% of the total initial weight?  Using 25% has some
advantages for ease of calculations (below).

3.  If you need 6 units of air for each unit of wood for _complete_
stoichiometric combustion, and if 25% of the weight remains behind as charcoal
after pyrolysis (resulting in 25% LESS units of air for the wood part, 
which is
therefore 1.5 units of air not used), AND that the charcoal will take TWICE as
many units of air for the stociometric combustion of the charcoal (meaning 2 x
1.5 = 3 units), then the stoiciometric combustion of JUST the pyrolytic 
part of
the wood fuel has the remaining 3 units to accomplish both the 
pyrolysis AND the
combustion of those gases.  Tom (and all), did I say this correctly?

4.  For the 0.75 unit of wood weight that is consumed (made gaseous) via
pyrolysis, 1.5 is the air/fuel ratio for the pyrolysis part.  1.5 x 0.75 =
1.125 = ~1.1 of the 3 units of air, leaving 1.9 units for the 
combustion of the
pyrolysis gases.  That is almost 1 unit primary to 2 units secondary 
air.   Tom,
I thought you told me it was about 1 to 4.  Any errors in my numbers?  
Even if 1
to 2, that has an important impact on stove development.

Note:  This makes much sense with gasifiers like the T-LUD units where primary
and secondary air are controlled separately.  With traditional fires, the
secondary air must either in from the sides or survive (have oxygen) the rise
through the burning fuel, therefore impacting the the primary combustion in
less controlled conditions.

5.  For the remaining 0.25 units of char, there are two senarios (and probably
many variations between them.)

    a.  If the burning of the char is only at the level of the char (as 
in a low
fire), the results are CO2 plus H2O plus some of the bad CO that 
escapes because
the temperature above the char is too low for CO combustion once the necessary
air gets to it.  That means that the full allotment of 3 units of air were
basically used as a mixture of primary air and secondary air coming 
through the
char from below.
Tom wrote above:   A/F ratio of 12 units for complete combustion: 
therefore 0.25
of fuel requires 0.25 x 12 = 3 of air for complete combustion.

    b.  If the char has a good draft of air (by fan or sufficient pull of
natural draft) focused on the multilayers of char, the "gasification of char"
will use half of the air (being primary air) to create the CO, and the other
half (that is, 1.5 units) is needed for the secondary combustion.

Paul



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