[Stoves] Fwd: Biomass stoves v. PV-induction cooking (re: Frank)

[Andrew Heggie]

I replied to Ronal meaning to send to the whole list.

I would point out that S Varunkumar the author of the thesis I cited
is still a subscriber to [stoves] but we have not hear from him
recently.

AJH


---------- Forwarded message ----------
From: Andrew Heggie <aj.heggie at gmail.com>
Date: 2 February 2017 at 11:29
Subject: Re: [Stoves] Biomass stoves v. PV-induction cooking (re: Frank)
To: "Ronal W. Larson" <rongretlarson at comcast.net>


On 1 February 2017 at 18:34, Ronal W. Larson <rongretlarson at comcast.net> wrote:
> Andrew:  cc List and Norm

> Interesting last question, I would like to know if one could see these
> pyrolysis products passing back through the front as it descends past
> the pyrex.

This is exactly what I found when I similarly tried an old pyrex dish.

> Others may not understand the complex nature of this front.  A minimum
> amount of CO2 is normally produced at the front.  Andrew is here saying that
> these unusual smoke (combustible) particles when they progress inward may be
> producing CO2 - which we don’t want.   Such produced CO2 is probably largely
> converted back to CO when it meets the hot char - but then you are losing
> char (and energy).

No I was meaning that as the offgas/smoke, having descended below the
pyrolysis front, rises through it again  it should ignite, as long as
the front is hot enough  to initiate auto ignition or a flame is
present. It is bound to be hot enough at ~500C as this is way above
the auto ignition temperature of most tars but still below that of
methane and carbon monoxide.

I don't believe the front is hot enough for the reduction of CO2 to
CO, this is an equilibrium reaction  and perceived wisdom is that it
favours CO2 below about 800C, if it did occur it would drive the front
temperature down as it is endothermic.



> Andrew - I don’t understand your first phrase (“As velocity increases”).
> Are you referring to the presumed normal situation, or this “new” case in
> Norm’s geometry.

I was referring to the normal case superficial velocity. Noman seems
to be observing some sort of chaotic  phenomena which is likely  a
combined effect of gas diffusion and convection. Given he acknowledges
a temperature differential of 200C it is a small  "heat engine". Of
course not having seen  the burn I'm largely guessing.

Also his front appears to be deep with elements of a standard
updraught burn caused by particles falling through to lower levels to
start combustion below the pyrolysis front.

>
> [RWL:  It has been decades since you described that (if ever) on this list.
> (I learned about your work mostly while visiting you in the UK.)   Can you
> repeat that description - and anything that might help in drying wet wood -
> which was a big topic at the recent ETHOS meeting.


Yes my basic premise for charcoal making was that  the actual
pyrolysis in the 300-450C region was mildly exothermic, Tom Reed
mentioned this, now above 450C char undergoes changes  (from linear
aliphatic to phenolic ring like structures), these may be endothermic
and thus limit further temperature rise in the absence of heat input.

So the energy required was simply that needed to raise  the
temperature of the wood to 300C. The specific heat of dry wood is
about half that of water, however to get past 100C you have to
evapourate all the sensible water in the wood so not only do you have
to  raise the wood to this temperature but also all the water and the
latent heat of evaporation which is an order of magnitude higher.

To minimise this heat requirement  to initiate pyrolysis  we aimed to
dry the wood prior to charring and built a high temperature dryer to
do so quickly.

> I’m not sure if this is pertinent, but Nat Mulcahy has a very different
> approach to char making in his World Stove.

I met Nat with you at the Newcastle biochar meeting, saw his flat pack
worldstove on display but have never seen it working and he went quiet
on me some time after.


> Another question - were you ever in those days with your large systems,
> attempting to also get useful energy output - or just char?

Our intention was always to utilise the energy in the offgas, so I
deprecate burns  just for making char without utilising the heat,  In
our case another justification of the pre drying was to maximise the
energy in the offgas. My colleagues successfully ran a pressurised
hybrid retort/kiln for a short period powering a gas turbine and then
funds ran out so we liquidated the company. We were targeting the
lumpwood char market .



> Anything to add about any of this for small cookstove design?

Not really because any of the techniques we used required some sort of
forced draught and most developers seem to have moved away from fan
powered stoves, for good reason.

Andrew