[Stoves] What about carbonization? Re: Composting at 70 deg C ?
ajheggie at gmail.com
ajheggie at gmail.com
Mon May 30 13:00:26 CDT 2011
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
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