[Stoves] Stoves Digest, Vol 14, Issue 17

ajheggie at gmail.com ajheggie at gmail.com
Sat Oct 15 06:31:07 CDT 2011


On Saturday 15 October 2011 06:56:03 Ron Larson wrote:
> Trevor, Paul, and lists
>
>     See Table 1 on p 12 of the draft IBI guidelines, which states that
> the ash content shall be less than 50%.  If you think this is the wrong
> level, now is the time to speak up.
>
>    See
> http://www.biochar-international.org/sites/default/files/IBI_Guidelines
>_for_Specifications_of_Biochars_for_October_2011_Public_Review.pdf
>

Ronal, does this refer to total ash after the sample has been incinerated 
and include ash in the char matrix as well as "free" ash in a partly 
incinerated air starved fire? I can see the desire to not have loose ash 
in a product sold as biochar.

> > If we gasified all volatile matter,
> > the fixed carbon content of the char would rise to about 43%,
> > and the ash content would rise to about 57%.
> > But typically rice hull biochar has an ash content of about 40%.
> > This means that there is still a lot of volatile matter that remains
> > in the biochar.
> >


I've still not seen rice husks so take my comments with a pinch of salt.

I'd be careful about using the term "gasified" in the first sentence, it 
could be taken to mean using sufficient oxygen to deliberately gasify the 
fixed carbon in the husks. If this is done completely then the 
incinerated sample would be 100% ash.

A pyrolysed sample would keep all the ash locked in a char matrix. The 
amount of char would depend on the rate of temperature rise ( which we 
could expect to be high in the thin, low mass hulls) and the final 
temperature attained in the treatment

A typical gasifier would still have some fixed carbon in the ash because 
the air supply is restricted to below that necessary to react out all the 
carbon, because things are not perfect and it is better to discard char 
than have any free oxygen downstream of the gasification process. This is 
the opposite of the internal combustion process where a slight excess air 
is necessary to prevent soot and CO being carried into the exhaust. 

> > In the case of rice hulls,
> > the amount of volatile matter that remains in the biochar is
> > determined by the rate of gasification. At times the yield in rice
> > hull biochar by weight is as low as 30%. At times the yield is as
> > high as 50%.

Yes, this I would expect, biochar will consist of two parts, the fixed 
carbon ( which usually  is usually dependant largely on lignin content of 
the feedstock and soots redeposited from secondary reactions) and the 
higher temperature tars. These latter are volatilised as temperature 
increases.


> > If the rate of gasification is high, high temperatures within the
> > reactor are reached. With high temperatures, more volatile matter is
> > gasified.

Yes, the heat of burning the carbon raises the temperature and this 
volatilises any surviving tars.
> >
> > We need operating temperatures well beyond 1000 C before fixed carbon
> > gets gasified. 

Well the fixed carbon will remain static above 900C in an airless 
environment but if any oxygen is present it will continue to react with 
any carbon it meets but some carbon will have changed from an amorphous 
form to less reactive graphene like clumps. I'd like to learn more about 
how carbon graphitises with increased temperatures, we know diamond will 
do this but I see little about how biomass derived carbon might undergo 
this change.

 http://www.bris.ac.uk/Depts/Chemistry/MOTM/diamond/cphased.gif

shows a phase diagram that suggests at low pressures and high temperatures 
graphite is favoured.


> > In my opinion it makes little sense to gasify fixed 
> > carbon.

Then keep air away and quench it fast.

My question is: why do you want fixed carbon? It is after all a small 
portion of normal charcoal.

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 ... and please try not to repost whole digests to the list.

AJH




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