[Stoves] What about carbonization? Re: Composting at 70 deg C ?

[ajheggie at gmail.com]

On Monday 30 May 2011 17:36:29 Kevin wrote:
> Dear Andrew and Paul
>
> Certainly, one intuitively thinks of "char" and "fixed carbon" as the
> result of a "pyro-process.
>
> What about the fact that the ignition temperature of wood has been
> demonstrated by the National Fire Protection Association (NFPA) as
> being as low as about 180 degrees F?

Hi Kevin, I was beginning to wonder where you'd got to. From an 
investigation into a fire in a drier we had I found out that the 
temperature for ignition for wood does drop with frequent heating cycles. 
The auto ignition point of fresh charcoal is similarly around 200C which 
is why re adsorption of water can cause enough temperature rise to ignite 
it. So if the volatiles can be removed by some sort of low temperature 
process leaving free carbon at the surface I can see autoignition could 
happen at lower than pyrolysis temperatures ( generally taken to start at 
330C. Once one bit gets hot then the chain reaction follows in dry wood.
>
> What about "carbonization" of biomass in a pond over the winter? What
> about carbonization of waste material in a septic tank? Does not
> anaerobic digestion of biomass result in an end product with more
> "fixed carbon?"

No I don't think we should confuse blackened with carbonised but as you 
say defining fixed carbon is difficult because even when cooked to 900C 
some oxygen and hydrogen molecules remain attached to the carbon and the 
edges of sheets of graphite have oxygen attached to them. IIRC the 
sparkly surface of a diamond is a molecule thick layer of hydrogen.

The fuel industry uses the 900C residual mass less the ash as the fixed 
carbon, e.g. pyrolyse in an inert atmosphere to 900C, weigh, maintain 
temperature and add air/oxygen to incinerate, weigh the ash. That's 
actually not rigorously scientific. I expect Nuclear Magnetic Resonance 
would establish the amount of carbon atoms that are only attached to 
other carbon atoms as well as identify the amount of bonds between carbon 
and other groups.
>
> How fixed must fixed carbon be fixed to be fixed?

For biochar purposes, and this is not the biochar list, that fraction that 
persists in soils for millenia I'd suggest.
>
> We have the "High Temperature Carbonization" (HTC) process, where
> biomass is reacted with water and temperature, to "fix" the carbon. At
> lower temperatures and pressures, the process would proceed, but at a
> somewhat slower rate.

Isn't this hydrothermal carbonisation, done with and without catalysts? 
There was some talk of this and the characteristics are quite different 
from pyrolysed char.
>
> I did a quick Google on "Temperature of Formation of Coal", and other
> than frequent repetition of "Coal is formed when biomass is subjected
> to temperature and pressure for a considerable period of time", there
> is nothing specific about the minimum temperature, pressure and time
> required to form coal. Coal formation seems to be a loosey goosey
> collection of intuitives.

Probably but we do know from the known solubility of the heavy metal salts 
retained within the coal that it was probably in a superheated liquid 
water environment, that is why fly ash from coal plants can contain high 
levels of radio isotopes of uranium because the superheated water thay 
were dissovel in has either boiled away or cooled and drained away. Coal 
varies from near pure carbon like anthracite to the brown lignites that 
are just one step further than peat. I bet they didn't try to make town 
gas from anthracite apart for the fact it was too valuable as a steam 
coal it did not have enough volatiles. 

AJH