[Gasification] Charcoal Gasifier No 2.

[Daniel Chisholm]

On Tue, 2007-24-07 at 21:43 +1200, doug.williams wrote:
> ...charcoal gasifiers ...

Thanks Doug for starting discussion on charcoal gasifiers.

I have a question for you and the list, and perhaps a suggestion to make
too.  (I've had a number of ideas that I've been meaning to throw out to
the list for comments and suggestions, and you've just forced my hand on
one of them.  A good thing too, or I might never haven gotten a around
to it!)

The question: are charcoal gasifiers largely free of tar production
problems?  (I suspect that the answer is "yes", but I want to make sure
I'm not missing something...)

If so, then here is my suggestion: for engine grade gas applications,
shouldn't we consider using charcoal as the fuel source?  I.e. instead
of solving the tar problem directly, take the easy way out and and solve
it by avoiding it?

Basically what I am suggesting is that for biomass-fuelled engines we
adopt the model successfully used for petroleum-fuelled engines, i.e.
you process your raw feedstock (crude oil, biomass) at a few large
central facilities (refineries, charcoal plants), producing a
high-quality dense convenient and easy to transport fuel (gasoline,
processed charcoal), that can be effectively used on a small scale
end-user engine.

Gasoline and charcoal have several advantages over their raw precursors:

- storage and handling (gasoline is easy to pump; charcoal can be
processed into a variety of consistent handle-able forms, e.g. dust,
pellets, briquettes)

- volumetric energy density (both charcoal and gasoline are on the order
of 1kg per litre

- mass energy density (charcoal is ~14KBTU/lb, gasoline ~18KBTU/lb; both
are much higher than bone-dry raw biomass at ~9KBTU/lb)

- efficient and clean burning.  Burning crude oil or raw biomass cleanly
and completely is difficult on a small scale (i.e. anything smaller than
a chemical plant).  Burning gasoline or charcoal cleanly completely and
effectively is challenging enough, but it is a much more achievable
engineering problem.

- pollutant precursors can be removed at the refining stage (e.g. remove
most of the sulfur and ash from crude oil; remove the tar-forming
volatiles from biomass)


I will suggest that an effective way to manufacture charcoal dust would
be at a handful of relatively large central plants that have a use for
the high quality heat that is available from the volatiles during the
charcoal production process - e.g. steam plants for heating, or power
plants, or sawmill or pulp mill steam plants.  Consider wood waste and
hog fuel for now, because they are some of the kindest biomass fuels
(low ash, sulfur, etc).  It is quite straightforward for a large simple
furnace to burn the volatiles away, and capture the char.  In fact, it
can be a bit of a challenge to retain and fully consume the fixed
carbon.  Might as well turn this into a virtue - consume the volatiles
to produce heat, and capture the char for use as a higher value fuel
outside of the thermal plant.

I think it is appropriate to consider paying a higher amount per MMBTU
for a processed charcoal fuel for engine use, since this permits a
simpler design of an engine gas gasifier.  Gasifying (to engine grade)
an engineered fuel feed is challenging enough; designing an all-in-one
chemical processing plant (which is what a raw biomass--> engine gas
plant it) is perhaps not the best choice to make.  A simpler gasifier
design ought to mean reduced capital and maintenance cost, and smaller
size and weight.  Like many other tradeoffs, this could well be worth
making, even if it means paying (say) $5/MMBTU for a processed charcoal
fuel that is derived from a fuel feed stock costing only $3/MMBTU.





-- 
- Daniel
Fredericton, NB  Canada