[Stoves] burning particulates

[Daniel Pidgeon]

For the Admin,

I don't know why some messages get marked as "SPAM", but they show up in my Inbox anyway, and I simply look for the full message subject line before I believe that word.

But then I need to check the Junk mail box regularly, as for some reason, every message from Crispin every time gets sent to Junk, no matter how many times I tell it that it's a trusted sender!

I don't get all these automatic things, just try to work around them...

Daniel
________________________________
From: Stoves <stoves-bounces at lists.bioenergylists.org> on behalf of ajheggie at gmail.com <ajheggie at gmail.com>
Sent: Monday, 25 January 2021 9:15 AM
To: Discussion of biomass cooking stoves <stoves at lists.bioenergylists.org>
Subject: [Stoves] burning particulates

This is a repost to see if it gets propagated without the spam flag
the list server appears to insert on certain posts

We seem to have a discussion going on between stoves and the biochar
horde but it has got a bit disjointed. I picked up on this snippet
below  from Daniel in Australia who hopes to use a TLUD to heat his
house by heating water. I am an advocate of underfloor heating in the
UK and it fits well with batch burning, as like a masonry heater, you
can burn hot and fast and the time constant of the floor slab  evens
out the peaks from the stove. The other good thing about underfloor is
to do with our perception of comfort, if your feet are colder than
than your head you only feel comfortable at a higher overall
temperature than when your feet are warmer than your head, so a warm
floor and rising heat tends to feel more comfortable at a lower
average temperature, i.e. you maintain a lower temperature and use
less energy with underfloor heating.

Read below for my take on Daniel's point about smoke particles.

On Mon, 28 Dec 2020 at 12:22, Daniel Pidgeon <daniel.pidgeon at hotmail.com> wrote:

>
> I assume or figure with smoke it gets to a point when the volatile particles are burned off, that there are particles that no longer burn up, and so with further combustion, instead of burning up, they are disintegrated into smaller pieces, which as James mentioned, can only be removed by filtering rather than further combustion.
> Kinda like if you pulverise an egg shell. There is still the same volume of shell, just a larger amount of smaller pieces.
> That's my logic anyway. I will read further.

Smoke is a sol of either a liquid or solid particles in air, it can be
unburned volatiles that have condensed out in the flue but we are
mostly concerned with those black sooty particles that are the result
of incomplete secondary combustion. The mechanism for this, asI
understand it, is the heat of the flame splits an oxygen molecule into
two radicals, these then strip hydrogen atoms away from the fuel gas.
As oxygen necessary to completely burn the carbon rich fuel remaining
in the flame has to diffuse into the flame  it takes a while for this
to happen and the carbon glows yellow in the flame. If the flame were
premixed, as in a gasifier where the CO and H2 have been pre cooled
before the air is added to make a homogenous mixture of fuel and gas,
the carbon and hydrogen would react simultaneously and the flame would
be blue. In our burners we cannot premix the fuel gas and air as the
fuel gas is hot and already above its auto ignition point.

If conditions in this diffuse flame are such that the flame can be
sustained long enough for sufficient air to react with the glowing
carbon then the flame is clean. If the flame is quenched before this
happens, and it is dependent on chemicals in the flame as well as
temperature[1], then the carbon does not completely burn and sooty
particles are emitted. These consist of both black carbon and other
partially burned species including polycyclic aromatic hydrocarbons
(these are the one implicated in causing cancers).

Now a bit of surmising on my part; once formed these sooty particles
seem difficult to burn, some of them will have formed from chain like
carbohydrates, like cellulose, and some from phenol like carbon rings
in resins and lignin, we also know that as you heat carbon above about
450C it likes to form rings and these can grow into graphene like
structures, graphene being a single layer of graphite. Graphite is
highly resistant to oxidation so my thought is that if the carbon
fails to burn out in the flame  it can form graphene like structures
on which other Products of Incomplete Combustion can aggregate and
once formed these are then difficult to burn out even at quite high
temperatures, i.e. to burn cleanly you only have one shot at it.

[1] Tom Reed explained that liquid fuels could be characterised by
burning then in a wick lamp, increasing the wick length  volatises
more fuel and creates a longer flame, the length of the flame before
it gives off soot is related to the fuel make up, high octane fuels
give a longer sootless fuel than low octane ones and natural resins,
like turpenes, from pine have a very short flame before they create
soot. I see something similar with my woodburning stove, most woods
burn very cleanly as long as they are dry but holly and birch barch
will produce black smoke if burned at a high rate.

Any comments from the more erudite combustion scientists here?


Andrew

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