[Stoves] Charcoal from waste - home cooking or other markets? (Re: Crispin, Anand Karve)

[ajheggie at gmail.com]

[Default] On Thu, 13 Oct 2016 15:07:35 -0700,Tom Miles
<tmiles at trmiles.com> wrote:

>A cyclone sawdust gasifier was developed and tested by Jim (JW) cousins in New Zealand in the early 1980s. Google “cyclone gasifier” http://pubs.acs.org/doi/abs/10.1021/i200031a063
>
>There have been various research versions since but nothing commercial. 
>
> 
>
>Jaques Lede worked on a fast pyrolysis cyclone reactor. I suspect that the char yields were pretty low. 
>
>https://www.researchgate.net/publication/231393861_The_Cyclone_A_Multifunctional_Reactor_for_the_Fast_Pyrolysis_of_Biomass
>
> 
>
>Cyclonic pyrolysis and gasification of lighter ag residues in the 1970s and 1980s required recirculation of pyrolysis gases to maintain high mas flow in the reactor while limiting air or reactive gases. 


A few points:

pyrolysis of individual particles from sawdust will inevitably lead to
a lower yield of char than the pyrolysis of the same mass solid piece
of wood, this is because secondary reactions  which deposit solid
carbon in the matrix of char cannot occur and lighter pyrolysis
products are carried away as gases and vapours.

The processes cited by Tom don't appear to be aimed at making char and
yields as low as 3% are mentioned. But these are probably concurrent
or co current devices with through flow rather than one ended?

My experiments with a device, which Ken will remember seeing, (it was
intended for burning fairly high moisture content wood chip with no
smoke and  low CO) showed that a flame curtain developed all the time
volatiles were evolved and the nascent char could be seen below the
flame, adding small quantities of wood chip resulted in the fresh wood
falling onto the char surface and pyrolysing before being burned out
once primary air made it past the flame curtain.

By changing the air supply it is possible to change the characteristic
in the pot. The flame then becoming the feedback path for the
pyrolysis.

As to the limiting reactive gases, yes there will always be some
complete combustion taking place but I speculate with  increasing air
pressure and smaller nozzles to deliver the sawdust into the burner
air mass could be minimised at the cost of more power consumption in
the fans. Also more dry sawdust could be metered in separately from
the air:sawdust supply

I'd made bulk char many years earlier by burning "lop and top" in a
charcoal ring kiln and I believe I posted pictures of the flame
curtain to the list in the 90s. It was the fastest way of producing
char from otherwise waste material but only 50% made the size class
required for lump wood charcoal in this country and it was difficult
to preserve the char without quenching with water.



If the requirement is to maximise the char yield then briquetting then
pyrolysis would enable the secondary reactions to take place within
the briquette.

One thing we learned very early on is that in a kiln the yield is
drastically affected by moisture content. This is because the enthalpy
of vaporisation of the water content is so much higher than raising
the particle temperature above 100C to the pyrolysis temperature and
all this energy is supplied by the feedstock. We see this in TLUD
burning but that is self limiting to an extent as high mc material
will not support the pyrolysis front.

So I can envisage a simple continuous feed retort  for fresh sawdust
which still uses combustion of the pyrolysis offgas to drive the
drying and initial heating to pyrolysis temperature but keeps the
processes separate so that steam does not dilute the offgas.

I have an old colleague from many years back when we used to harvest
timber together who now runs a sawmill by Billingshurst which is about
30 miles from where you lived Ken, should you wish to progress
anything on the sawdust front. I also still have a collection of parts
from wood chip and pellet burners to play with.

My worry would be finding a use for the fine char and the realisation
that fine carbon dust would not be good for the environment,
especially if it got into the waterways.

Andrew