[Stoves] How to make smokeless coal?

[Andrew Heggie]

On 22 May 2017 at 18:53, Darpan Das <darpandasiitb at gmail.com> wrote:
> Dear Mr Frank
>
> Carbonised coal is a broad term for coal heated under inert atmosphere, the
> end product of which is called coke. The temperature is probably specific to
> user needs and coal properties.
>
> I have come across studies reporting heating of coal at 1000 C for
> metallurgical applications and 600 C for domestic applications.
>


Darpan

You plainly know something about coal pyrolysis and I don't know much
more. I do know it was a large scale technology here in UK as it
formed part of the industrial process that linked coal mining with
town gas with steel making.

We don't major in coal burning on this list but there are similarities
between charcoal making and coking. Wood is a mixture of basically
carbohydrates, as you heat it up in the absence of air you gradually
split it into vapours and gases but the early fractions are those
compounds which have a lot of oxygen associated with them, like
vinegar, as you get hotter the gases given off have less oxygen
compounds and higher hydrogen component. So pyrolysis can be see as a
process which steadily removes the oxygen and hydrogen components to
leave a residue of carbon. In the case of wood by 900C the char is
mostly pure carbon and associated mineral ash I suspect there is a
similar case with metallurgical coke. Of course coal is already highly
carbonised biomass with most of the oxygen removed, in the process of
being heated and squashed in a superheated water environment it picks
up  other minerals and metallic compounds which make its products of
combustion more of a problem. I imagine there are two reasons
metallurgical coke is made at high temperature, one will be the mass
flow consideration that means pure carbon will burn at a higher
temperature than the associated tars, the other might be that any
residual hydrogen is not good for the steel.

In my childhood my home was heated by coal but it was used in an open
fire, these apart from being extremely inefficient do not cope with
burning the volatiles released from the coal, leading to much smoke
and soot. This became a big problem in major cities and after some
nasty smogs in the 1950s coal burning was banned. What should have
been banned was open fires.

As I said the coal-coke-steel cycle was well integrated[1] and was the
reason the railways were successful transporting these products on a
triangular journey. Such that coke wasn't readily available for
domestic use so smokeless coal was used to supply this market after
raw coal was banned in "smokeless" zones. At home we used two
varieties, a reconstituted one, phurnacite, which was made in a pillow
press and coalite which was a semi coke made at 650C.

After town gas was abandoned in the 1960s the cycle was broken so coal
was coked at the steel works, coalite remained available for the
domestic market till about 2000. Coalite was more difficult to ignite
than coal but easier than coke, burned with a short flame as it still
contained volatiles.

In a private e-mail exchange off list Crispin made this point with
which I concur:

"In Ulaanbaatar we need to package the coal in a 10g briquette but I
don’t want people removing the volatiles. Most coal people think the
volatiles are ‘the problem’ because they burn coal in stupid boxes.
But in a proper combustor the lignite burns extremely cleanly – below
the limit of detection for PM2.5 many times."

The particulates problem from burning coal is better addressed by
better stoves than an industrial process in those places where it
remains the only viable means of heating. After all we have seen how
TLUD burning of dry wood produces a pyrolysis offgas which can be
burnt cleanly compared with burning whole wood.

[1] therein lies a good story in itself of how gradually town gas,
whilst still being a mixture of CO and H2 made from pyrolysis but was
augmented by producer gas and water gas reactions operated like a see
saw, this produced more gas but less coke.