[Stoves] combustion of char

[Alex English]

Art,

It is an interesting problem. One only has to be wrong once.

Your experience may be the only real advise.

If coal is a corollary then particle size is an issue. Yes powdered 
char, or fines would be less able to convect away heat than  lump or 
chip char with the voids to aid gas flow. It could hinder gas movement 
inward as well. However liquid water could move in and create a damp 
zone. The coal fire paper mentioned previously suggested that it was at 
the margins between wet and dry where fire might get started. The logic 
that works for me is that a puddle won't catch fire (tough sell) but it 
provides humidity to react in neighboring dry regions of char which 
begin to heat up. That paper graphed lab tests and in each case the 
temperature rise was over hours not days.

With coal the risk was higher for low rank coal with higher volatile 
content. This could suggest that relatively high temperature TLUD chars 
are at lower than average risk.

The spontaneous combustion page which has a short piece on Charcoal
"Charcoal, that has been exposed to air for a period of eight days, is 
not considered to be hazardous."

http://en.wikipedia.org/wiki/Spontaneous_combustion
lists the following reference.
Bowes, P. C. (1984)/Self-heating: Evaluating and Controlling the 
Hazards/, London: Department of the Environment, Building Research 
Establishment. ISBN 011671364

I have been unable to obtain it. Perhaps some one out there in stoveland 
can help.


We have been making char and dry quenching in leaky, or unsealed, drums. 
After a few days I decant into 200 liter plastic bags and leave them 
outside. Later, before use, we add water 1 to 1 by mass. No trouble yet 
but the ambient temperatures are low.

I am starting to ponder data logged tests to explore this further. I 
don't like the worst case scenarios.
We need to establish what a critical mass or time or miss treatment is, 
and then avoid it with a wide margin.

Alex
PS. Crispin, I know, I know ......no problem........... just make ash:)





On 23/02/2013 9:28 PM, Art Donnelly wrote:
> Frank,
> Glad to hear it was a near miss and not a disaster, but thanks for 
> using this as an opportunity to flag this important safety issue. I 
> want to make sure we take away a clear message in terms of best 
> practices for handling our inventories of biochar.
>
>   * Is auto-ignition only likely to occur in char that has been wetted?
>   * Is there a low risk of auto-ignition in char that has been dry
>     quenched?
>   * Is particle size a risk factor: IE is powdered char more likely to
>     ignite?
>   * When you say: "So before storage better give it time to take up
>     oxygen." What is the safest way to do this?
>
> We (the Estufa Finca project) have picked over 3-tons of cook-stove 
> biochar from just 32 households over the past 7 months. I currently 
> have an inventory of approx. a ton. It's piled in woven-plastic feed 
> sacks, under a shed roof. It was all quenched dry in the steel pails 
> we provide all participants and we only buy dry biochar. However this 
> is a tropical humid environment. So we assume the char is taking on 
> moisture. As we are now starting to develop demand, I expect to see 
> the volume of material on hand go up substantially in the coming year. 
> We are trying to figure out how to avoid the problem you just had. As 
> someone pointed out it's not a matter of whether it is a matter of when.
>
> chao
>
> Art
>
>

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