Got it, Ken! But I'd like to point out that without a pressure
differential
(as in a multiple-effect evaporation system) that the daisy-chain that
you
describe is impossible. Saturated steam exiting a nozzle, for instance,
will
create a local zone where the steam is superheated, but once it is fully
expanded it becomes saturated. One can't generate a pound of steam,
which,
in turn, creates another pound of steam ad infinitum. I think that's a
Second Law violation.
Water is virtually the only condensable gas in an exhaust stream, as far
as
I am aware. Certainly there is no restriction on the exhaust gas
transferring its sensible heat to the wood and to the water vapor, but
there's plenty of room to 'expand' in a hopper that is not a pressure
vessel. The example that you give of a teakettle is not apt. Water vapor
becomes visible at the spout of a teakettle because the vapor is cooled
by
the surrounding air and condenses into visible droplets. Once it is mixed
with the surrounding air, the vapor disappears, as you will observe, but
only if the surrounding air is has capacity for more water. Entropy
increases and the combination of water vapor and air that results has
much
less utility.
I am not asserting that woodgas IC exhaust will not dry wood. I am
claiming
that it will do so only until the point where the partial pressure of the
water in the exhaust gas does not exceed that at the surface of the wet
fuel. If one had a very, very deep hopper, the moisture from lower levels
of
fuel would condense in upper levels and migrate back down toward the
drying
zone, essentially putting the dryer in reflux mode (operating much the
same
as a packed column evaporator). Saturated, but cooled, exhaust gas and
some
evaporant would exit the hopper but it would not be a good design because
the upper layers of fuel would experience very little heat transfer. The
stratum where actual drying would be occurring would slowly move upwards
in
the fuel bed.
We are all aware of the sensitivity of woodgas quality to moisture
content
of fuel. When setting out to design the 'perfect' woodgas IC system,
recuperating exhaust heat for use in fuel drying is obviously very
desirable. I am just trying to sort-out the heat exchange mechanisms
involved in order to make suitable design decisions. Most decisions hinge
on
the characteristics of the exhaust gas itself, as it exits the IC engine
under normal operating conditions.
Thank you for your insights.
Best, Mark
-----Original Message-----
From: Ken Calvert [mailto:[email protected]]
Sent: Sunday, December 19, 2010 1:28 AM
To: [email protected]
Subject: Re: [Gasification] Drying fuel with IC exhaust and other
pleasures...
No Mark I am saying that running an engine on producer gas has a lot less
moisture than on petrol. And I am saying that hot exhaust gas has so much
excess heat that it will boil off water as steam and and as long as
that
steam is moving through the wood it will boil off more steam until the
temperature drops to the point where psychrometrics apply, and
generally,
if you have ever seen the steam condensing above a hopper of wet wood, or
watched the spout of a boiling kettle, that only happens after it has
left
the confined drying zone and had room to expand. Ken C.
--- Original Message -----
From: "Mark Ludlow" <[email protected]>
To: "'Ken Calvert'" <[email protected]>; "'Discussion of biomass
pyrolysis and gasification'" <[email protected]>
Sent: Sunday, December 19, 2010 9:48 PM
Subject: RE: [Gasification] Drying fuel with IC exhaust and other
pleasures...
Hi Ken,
Not to flog this horse(power) to death, but are you saying that the
combustion chemistry (and composition of the exhaust gas) is
substantially
different at idle than when an engine is running higher up its
horsepower
curve? I fully realize that the heat flux would be greater. But
intuitively,
it seems as if when a load were placed on a normally aspirated, running
engine, disproportionately more fuel would be required and thus there
would
be more products of combustion--including water--by design. Carbureted
engines used to run pretty rich at idle, but ECMs have addressed that
issue
for the most part on fuel injected powerplants. I realize that this may
have
little to do with fueling with biogas!
Modern industrial tunnel dryers are often counter-flow; the driest
product
comes in contact with the driest (lowest RH) air and this air, in turn,
'preheats' the new product coming into the dryer, principally through
surface condensation, i.e. latent heat transfer. Efficiency is at its
highest when the exhaust is saturated. This is the principle behind 93%+
eff. Furnaces and water heaters. This is a quite different scenario from
a
packed-bed arrangement where the feedstock closest to the engine dries
much
sooner than higher layers. Once dry, it serves no useful function in the
feed bed and is responsible for much of the pressure drop through the
dryer.
A live-bottom hopper/dryer would address this, I suppose.
If anyone has any dewpoint measurements of IC exhaust gases under
various
load conditions in a woodgas-fueled engine, I would be very interested
in
having these data. I am too lazy to crank the numbers on this one!
Very best regards, Mark
-----Original Message-----
From: Ken Calvert [mailto:[email protected]]
Sent: Saturday, December 18, 2010 11:29 PM
To: [email protected]; Discussion of biomass pyrolysis and gasification
Subject: Re: [Gasification] Drying fuel with IC exhause and
otherpleasures...
Mark, sure you can watch the condensation from an engine ticking over,
and
which you have probably just started, but I challenge you to bore a hole
in
the floor of your car and point a IR beam thermometer down on the
exhaust
pipe when you are doing >80mph. You wouldn't be worrying about
relative
humidity then.
Its apples with apples, not iceblocks out of the frig!
Ken C.
----- Original Message -----
From: "Mark Ludlow" <[email protected]>
To: "'Guag Meister'" <[email protected]>; "'Discussion of biomass
pyrolysis and gasification'" <[email protected]>
Sent: Sunday, December 19, 2010 3:26 PM
Subject: Re: [Gasification] Drying fuel with IC exhause and
otherpleasures...
You are right in a way, Peter. But the water has to leave the wood,
yes?
Where will it go if the surrounding atmosphere is saturated with water?
Let me illustrate with an extreme example: Try drying wood at, say,
110C
in
a pressure cooker, with the wood covered with water.
If the exhaust gas has room for the vapor phase of water, the water
will
be
carried away; if it does not (if it is already near its dewpoint), the
gas
will become supersaturated and nucleate condensation will occur. There
will
be rain in the forecast.
I held my hand to the exhaust pipe of my 3.6-l automobile while it was
idling. It was not scorchingly hot but it was palpably humid. Not
exactly
Science, I admit.
Best, Mark
-----Original Message-----
From: Guag Meister [mailto:[email protected]]
Sent: Saturday, December 18, 2010 4:48 PM
To: 'doug.williams'; 'Discussion of biomass pyrolysis and
gasification';
[email protected]
Subject: Re: [Gasification] Drying fuel with IC exhause and other
pleasures...
Hi Mark ;
This is true, but 100C is a special case because the water can exist as
both
a liquid and gas. If we discuss a slightly higher temperature, it may
be
clearer. If you heat wet wood to 105C by any means, all the water
present
in the wood will be a gas. Yes it will be in equilibrium, but since
water
as a gas occupies 100x more volume than water as a liquid, you will
have
succeeded in driving off 99% of the water. You could even do the
heating
with saturated steam and it would still work.
Please correct me if there are any errors in the logic.
Best Regards,
Peter G. (armchair gasifierist)
Thailand
www.gac-seeds.com
--- On Sat, 12/18/10, Mark Ludlow <[email protected]> wrote:
If the IC exhaust is saturated at
100C, the best the wood can do is reach an
equilibrium moisture content with respect to this
environment.
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