[Stoves] Particles and particle types (was Charcoal MakingStove)

[AJH]

On Thu, 26 Oct 2006 21:16:09 -0500, Paul S. Anderson wrote:

>Quoting Tom Miles <tmiles at trmiles.com>:
>
>> The biggest influence on temperature is likely to be fuel moisture. That's
>> going to determine your peak flame temperature and heat load in combustion.
>> Moisture in wood also regulates the rate of combustion as it diffuses to the
>> wood surface and evaporates.

This is my view also.

>In a T-LUD, the lower (gasifier) section must fight the moisture content
>(MC)issue, with need for more air and more char consumption to sustain the
>pyrolysis process. 

As with my observation of the difference in chucking a dry log and a
green log on a fire, the presence of the water determines how the burn
can proceed, with the tlud the heat for raising the wood to pyrolysis
temperature  provided by combustion of some of the char, before this
temperature can be reached the wood in the pyrolysis zone has to boil
off any moisture present, so this heat is also provided by the char.
With the weak thermal feedback of a tlud this seems to limit
combustion to wood of <30mc wwb. I've never been able to measure my
experiments accurately but I suspect this attribute of a tlud has a
corollary with a simple contained fire (updraught stove) in that wood
wetter than this is quite difficult to burn cleanly with a natural
draught. 

> The moisture present (up to MC 30%) is turned into vapor,
>that is, all of the latent heat for evaporation is provided down in the
>gasifier.

I think so.
>
>But when the pyrolysis gases plus the water vapor rise up to the combustion
>zone, no more latent heat is needed.  Therefore, will the combustion of the
>gases from high MC wood (30%) yield temperatures about the same as the gases
>from moderate (20%) or low (10%) MC wood?

No because the mass flow is greater because of the water, so the heat
energy has to be distributed over a greater mass, also the excess air
tends to be greater to counter the dilution from the water in the
secondary flame.
>
>(I acknowledge that some heat is needed to take the water vapor molecules from
>initial temperature (perhaps 500 deg C after pyrolysis) up to the temperature
>of the final flame at perhaps 900 to 1100 deg C).

OK you realise the massflow increase needs heat.
>
>The question:  In regard to higher moisture content in the fuel, is there any
>advantage in the FINAL flame temperature attained when using a T-LUD versus
>using the traditional combustion devices?  In theory, yes or maybe or no.  But
>in practice, I doubt if it has ever been tested.


The theory is simple, all the energy from combustion has to heat up
all the products of combustion, as what enters the fire is the same as
what leaves it then you can understand a fuel with a low mass and high
energy needing just a stoichiometric amount of air will reach a higher
temperature than a bulky wet fuel with modest calorific value that
requires 150% excess air to burn cleanly. That's why you need to turn
wood to charcoal before you can melt iron, you need a more
concentrated form of energy to attain the required temperatures.

AJH