[Stoves] question about char

[ajheggie at gmail.com]

On Fri, 24 Aug 2012 17:16:07 -0700, Frank Shields wrote:

>I've dried lots of biomass and char with no problem. But today I upped the
>temperature to 200c but before it got to 130c one char sample started
>smoldering like a fine Macanudo. 

There are plenty of examples of hydrocarbons having a spontaneous
ignition point around 200C and I have in the past cited the example of
damp gunpowder catching fire at moderate temperatures. In the past Tom
Reed has pointed out that the length of a flame before it becomes
sooty is inversely proportional to its cetane rating so my guess is
derivatives of resins will have auto ignition temperatures as low as
diesel, 230C or even less.



> Sometimes in the past I've had biomass
>with oils on a paper plate smolder at temps less than 105c. Just wondered if
>others have seen this and why this would happen. 


Again, and Frans has commented on this, turpentine as it dries
undergoes an oxidation which is exothermic, this can build up to a
high temperature even in ambient conditions and there are many cases
of turpentine soaked rags being left in a bucket and as they have
dried the rags have ignited.

I think the auto ignition temperature is just the threshold at which
there is enough energy for an oxygen molecule to dissociate on the
fuel and would expect this temperature to decrease as the oxygen
content of the air increased. Similarly a catalyst is a chemical that
in some way enters into the process and causes this dissociation to
occur at a lower temperature.

AJH