Below you will find an interesting discussion, concerning the drying of fuel,
that was on the gasification list.
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Greg Manning wrote:
The Drying Process I use is simple, the sun and an inclined sheet of heavy
wire screen mounted in a large tiltable frame, chips are loaded onto the
screen, screen is inclined, and about 3 hours later, the chips start to
slide, or tumble down (moisture released, they are now lighter in mass, and
slide down),
screen is tipped up, and chips are gathered into the back of a
tilting box, one ton truck, then the whole process is started again.
I know this is "simple", but very cost effective, even though somewhat time
consuming, (Generally I'm at the same location, splitting and bundling cord
wood for resale, while the chips are drying).
Oh, the inclined screen.... it takes 3 bucket fulls of my tractors front end
loader, to fill the screen.....
Greg Manning.
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Tom Miles wrote:
There is considerable experience in fuel drying among list members. It's
just a topic that hasn't been discussed much. People on this list who are
developing or providing small scale gasifiers for commercial use will
probably show you a picture of their dryer but not discuss the details.
I can design one for you. That's what I do for a living. We have designed,
installed and fixed all kinds of fuel drying and processing systems since
the 1970s. (See: www.trmiles.com)
We have built feeders and dryers for
gasifiers and pyrolyzers from 250 g/hr to 40 tph. Just this week I was in
Montana with the engineer for whom we built our first gasifier fuel system
in 1976. Others on the list have similar experience.
You will find there are a variety of small scale dryer solutions. At a
hunting and fishing resort called Snowshoe in Northern Ontario the owner
used to generate heat and power using an Imbert gasifier. He claimed that
split cordwood simply dried in the extremely cold winter if he left it out
long enough.
For your application a batch bin dryer or a small continuous belt dryer
would both be suitable. Small scale chip systems usually use batch dryers.
Or they use continuous dryers based augers, woven belts, moving floors or
vibrating conveyors with perforated plates. They are often custom built.
Rotary drum dryers like the Barr-Rosin have been the most reliable for
industrial systems. They are usually very expensive at the small scale. Drum
dryer design and operation has changed significantly over the years. A major
cost in drying these days is emission control so exhaust gas is increasingly
re-circulated and indirectly heated systems are used.
Indirect and direct steam systems where exhaust vapor is recovered are
thermally the most efficient systems. The most innovative dryers have been
developed in Europe in recent years. These are very expensive for North
America. You will find a few systems like the GEA/Niro Exergy dryers in
Denmark and Sweden.
Flash or suspension drying is most suited to finely divided materials that
are handled in suspension. They are used for fiber drying and for peat or
other finely divided feedstocks that will typically be used in suspension
pyrolysis or gasification, or in fluidized beds. The morphology of fine
materials permits rapid heating and short residence times which are the
defining characteristics of flash drying. Some semi-mechancal flash drying
systems are now being promoted. One of my clients has a system I think would
be very suited to easily milled fuels at a small scale.
Reports are available for several small scale drying studies for biomass
fuels that have been funded by the EU and in the US by US DOE and US
Department of Agriculture. If you develop a bibliography of these we can put
it on the web.
Fuel selection, preparation, handling, sizing, drying and feeding are all
essential topics for gasification. A FUEL topic is something that is on the
list of things to add to the Gasification website but the site is only being
developed as time permits.
There is information about dryers on the message archives and on the
website.
You will find some messages referring to fuel drying by using the (Google)
search on the Bioenergy Lists: Gasification.
The general site search utility has been temporarily disabled due to server
problems. We expect to put it back sometime next week.
Regards,
Tom Miles
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John Flottvik wrote:
Hi Tom and list.
A rotary drum dryer does not have to be expensive. We built one from a 3ft
by 20ft long pipe. We welded 6 inch flanges inside to move the material
along, adding small baffles that would lift the material up to drop through
the hot air. We found an old cement mixer and welded the mechanical drive
from it on one end and used the steel rollers to turn the pipe on. Hot air
and wet material was loaded in one end and dry material would come out the
other. Moisture would also escape out the exit end.
Scrounge around like we did for parts and your cost will be mostly your own
labor
and since we all love what we do, almost free
John Flottvik
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William Carr wrote:
I've been wondering about this subject.
I could just build a shed for fuel storage with a south side and
south facing roof of transparent plastic.
There's a form of translucent building material, corrugated like
steel siding but it's fairly light-conductive.
It's pretty good for allowing natural light into a garage and I think
would make a good roofing material.
In the Spring and Summer such a shed would get insufferably hot
inside. Daily peak temps of 130 degrees Fahrenheit could be
expected in June.
Whether storing bags of corn or split firewood, whatever you store in
a solar shed would get the moisture baked out of it.
You should add a gutter to the inside to catch nightly condensation
and route it outside, to maximize the drying effect.
Come to think of it, you should probably seal your 2 x 4's well to
keep them from warping as they dry out.
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Mark Ludlow wrote:
Erik,
Purely from my perspective, a counter-current configuration makes the most
sense for a small scale wood chip dryer. The incoming wet feed is exposed to
moist air which has increased enthalpy and is much more effective at
transferring its latent and sensible heat to the cooler in-feed material.
While not much moisture is removed in the first contact zone, rapid heating
occurs because of the superior thermal characteristics of high RH warm air
versus its low RH equivalent. A second advantage is the lessening of a
tendency to case-harden the surface of the chips which subsequently retards
water vapor transmission.
Efficiencies improve when a portion of the exhaust is recycled to the input,
to the limit of having a saturated or condensing exhaust stream. Dissipative
heat losses from the dryer shell itself are a major factor in determining
overall system efficiency.
Mark
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