Ken, etal, Hi! The discussions nowdays seem to pushing all the theories
ahead, on a theoretical basis, and I can only reiterate as much as I know of
the past. Yes torrification is beneficial for wood fuels! But, so many
people read up the history about steam injection and producing more
hydrogen, without realising that most gasification in the early days reffers
to coal products and not wood. You cooked the coal and got coal gas and
then you got coke as a byproduct. What can you do with pure carbon as coke?
You gasifiy it again, and you either added steam in bursts and got 'water
gas', pure carbon monoxide and hydrogen, or in a less costly plant you added
a small continuous stream of steam in with the air and got 'producer gas' ,
which still contained the nitrogen content of the original air stream and
had a much lower calorific value. Using wood and biomass insteaf of pure
carbon is a totally different story!
A/. Wood is not pure carbon, it is a 'Carbohydrate' and absolutely bone dry
wood. still contains enough water chemically bound into that carbohydrate to
give all the hydrogen that the energy contained in the fuel itself can
provide. Thats a stoichiometrical fact! Even kiln dried wood at 10%
water, or less, will not generate enough heat to get you up into the
temperature range where the most efficient chemistry prevails.
B/. The process of torrification gets rid of some of the chemically bound
water and starts the process of making hydrocarbons. And with
hydrocarbons you are into a different series of reactions. The problem, as
already noted, is that the water one gets rid of is highly polluted and
creates more problems than it solves.
C/. Nevertheless, using exhaust gases to dry wood rather than torrify it
is still a big step in efficiency. I have repeated this story several
times now, but I like it so much that I will drag it out again and repeat
it, because the same questions keep getting asked on this site about every
3-4 years.
During the 2nd Oil Shock in the late 1970s, we had a Mission High
School in the Pacific Islands that could no longer afford the price of
diesel fuel. We wanted to go over to alternative fuels but for social
reasons we could not use normal fire wood or we would have deprived the
local villages of their only source of energy. So we built a fuel hopper
from an old 1000 gallon water tank up on stilts and trained the exhaust from
the generator into the cone we made for a base.
We had a flame trap on the exhaust pipe, and boy did we need it! The
generator was then originally driven by an old Toyota petrol FJ truck engine
converted to gas. Each day a team of students took their machetes to an
old 2nd World war air field that was overrun with leucaena scrub and each
cut 5 stumps as thick as their wrists,40mms. They dragged them home and
ran them through a docking saw that filled the hopper with 50mm lengths of
wood. The hopper held three days supply of fuel going in as straight off
the stump green wet wood, and coming out the bottom as semi torrified wood
that had shrunk so much that it had fractured into pieces that could be
pulled apart with the fingers and so friable that it could be lit with a
single match. There wasn't enough heat to turn the wood black, only a dark
brown, but with no evolution of smoke or other volatiles. Even so, we had
enough troubles with sparks etc out the exhaust and minor conflagrations
that required a hose pipe always on the ready. However, that plant s ran
for 15 years , up until the early nineties, and created all sorts of
records for longevity, exceeding all the experiences of World wars. I
can't quote any fuel efficiencies in terms of figures, but for turning a
renewable sustainable resource, Leucaena scrub, into an educated human
resource, high school graduates, I think it deserves some sort of record!
The actual Gasification unit was a typical downdraft unit with bag filters,
provided by BTG in the Netherlands.
Ken C.
----- Original Message -----
From: "Ken Boak" <[email protected]>
To: "Discussion of biomass pyrolysis and gasification"
<[email protected]>
Sent: Wednesday, October 20, 2010 2:23 AM
Subject: Re: [Gasification] reclaiming exhaust heat
Toby,
The exhaust is coming out of the engine at 400C. This will evaporate any
moisture in the wood, drive off the volatiles and carry this mixture away
with the hot nitrogen and CO2.
This is not like the tailpipe of an auto where the exhaust has cooled to
the
point where the water vapour condenses out.
Suppose the exhaust is connected into the side of the bottom of a tall,
verical cylinder containing a column of woodchips, - like a 6" diameter,
double walled fluepipe, about 48" long. This column is placed just after
the
exhaust manifold where the exhaust is hottest, and preferably insulated to
retain heat
The chips at the bottom of the column will be heated first, begin to dry
and
torrefy and after a certain time period will have achieved minimum
moisture,
and achieved thermal equilibrium with the hot exhaust. Torrefied chips are
hygrophobic and will not attract subsequent moisture.
Once the woodchips have been in residence for sufficiently long for
perhaps
the first 6" to be fully torrefied, a vertical auger is started which
pushes
this first 6" layer through a sprung loaded flap and into the main
gasifier
reactor.
The column of wood chips moves down and the next layer is kept in
residence
long enough for torrefaction.
A thermal gradient will be set up in the column of chips, with the coolest
and dampest at the top.
Ken
On 19 October 2010 14:05, Toby Seiler <[email protected]> wrote:
Ken,
When the hot exhaust hits the wood in a direct heating set-up, the water
vapor will condense on the wood, making it more wet, not less. Yes it
will
raise the temperature some but you will have combined the moisture and
then
have saturated wood, not dry. It will be more problematic.
With indirect heating you may get it done but it better be stainless.
There will still be condensation. Put your hand on your car exhaust and
feel the wet vapors. Do you see water dripping? That condensate will
rust
out a heat exchanger in no time.
Toby Seiler
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