[Gasification] Biofiber drying quantification

Thu Jan 31 10:10:42 CST 2008

 Greetings Norma, and list members.

 I have a very similar problem, but related this way (hot gases, warming
water) I guess it's kind of the same :).

 Time is relative to temperature coefficient of the said exchanger material
(eg: brass is a better heat conductor than cast iron, and aluminum is better
than brass, however, they all are correspondingly lower in their own melting
point, when they conduct heat better).

 Flow of hot medium vs. Flow of medium requiring water removal, (heater /
heated) if your biofiber requires a reduction rate of 4 gallons a minute ?
What is the flow rate of the said biomass fiber  within the transport auger
??

 Another factor to consider is condensate within the exhaust air causing
acids in the "air side" of the heat exchanger (caking or etching) this then
has to be factored into the thickness of the transfer medium (eg: auger
"middle" shell is say, 20ga. gav. tin, exhaust gases run around the outer
area of this shell, while biofiber is being augured within the center of
this middle shell (this is the transfer wall or exchanger wall) you would
get fantastic heat transfer to the said biofiber, but I bet at the outlet
end of this exchanger, the condensed acids would rot it out within a month,
if not a week.

 (caking action is like the buildup of creosote in a chimney),  (etching is
like the effect of water and air on steel (rust)). Both of these factors
MUST be quantified into the said exchanger design, or the exchanger will
only work well for the first little bit, until these items reduce, or negate
the heat transfer from medium to medium. (this can happen very quickly (like
within a week) if not factored into the design).

 Instead of trying "one long" exchanger on the auger, you can try multiple
"segments" all seeing the original heat value of the waste heat this way the
outlet side of the hot air does not go below condensing temperature, and
therefore you would not have to deal with the acids of condensate (this is a
common design in forced air furnaces). Another effect this does is
exponentially raise the biofiber's temperature, and at some point the water
evaporation will drastically increase (dew point).

 Fire tube and water tube boilers have way-different transfer rates, BUT
they also have way-different life rates as well, guess which has the better
transfer rate, also has the shorter life ???? (yes it does).

 If one looks to the air handler industry (fan coil) much like a cars
radiator, I can quote some numbers:

  (remember this is hot water to air but working the reverse should be at
least 75% the same  (air to water temp.)

 Based at 6 gallons a minute hot water @ 180 f,  and an incoming air temp of
70 f with 1800 cu ft/min,  a 4 circuit triple pass exchanger with  "75 sq/
ft" of surface area will transfer roughly 133,000 btu/hr of heat. (or about
40 degrees of heat rise on your air.)

 Knowing this, now lets look at your situation.

 75sq. ft. of transfer area, if the incoming "exhaust air" is moving at 1800
cu.ft/min. (converting this to engine size we would be roughly a 11.2L
(700cid) engine with about 80% volumetric efficiency, would be 560 cu in,
per revolution, so say running at 1800 rpm (std. generator speed) this
engine would ONLY produce 583 Cu ft/min. of exhaust so you would have to
triple the surface area to equal the original air flow...)

 OK so now 225 sq. ft. of transfer surface, from a 11.2 L (700cid) engine
running at 1800 rpm would produce a temperature rise of 40 degrees on your
said biofiber AT a exhaust temperature of 180 f. BUT, your exhaust is (or
should be) in the area of 500f or a ratio of almost 3 to 1 so in theory your
biofiber should have a temperature rise of 160 f.

So NOW, assume that you do a dual pass (or segmented lateral exchanger) this
would up the biofiber's root temp (assuming it started at 70 f) from 70 f
to 300f (now reduce the efficiency of the exchanger to real time transfer of
say 1.0 to 0.7 (30% losses) and you wind up with biofiber coming out of the
auger at about 270 f

Next, one would have to factor the temperature coefficient of the transfer
medium into the calculation, steel (normal cold formed sheets have a factor
of roughly 0.8 per 1/8" of thickness, so we now need to adjust that
biofiber's output temp down to about 216 f (80% of the original) your water
is still becoming steam, but not supersteam.

End result...  one pass through TWO exchangers of 225 sq ft. EACH, that are
1/8" in sheet thickness, that have DIRECT contact with 90+% of the biofiber
will take biofiber that was at 70 f and raise it to 216 f, BUT at what flow
rate ???  (well I would have to say, exchanger surface area "tri-ed") to
account for surface to volume conversion, 130 sq. ft. of surface would equal
1 cubic feet of biofiber per minute, so 225 x 2 (dual) = 450 sq. ft. this
would be 450 / 130 = 3.46 cu ft /min flow rate on the biofiber will yield
the stuff coming out at 216 f

 A 10" diameter tube has a "per foot length" surface area of 377 sq. in, (or
2.6 sq. ft.) so your auger would have to be 173 ft. long.

As you move to a larger diameter, the 90% contact factor goes way down, so
you might have to run a 3 pass exchanger to get the dimensions to something
useable. (eg:  a 20" diameter tube has a surface area of  5.23 sq. ft. per
foot of tube, therefore your dual exchanger would only have to be 86 ft
long, BUT.. your outlet temp would fall to about 170 f.

 Final Things.... caking or etching.... I've made no account for these,
Also, if your flow rate is much lower, you can drastically reduce the length
of the exchanger tube..

  If anyone see's a flaw (or flaws) in my calculations, please assist in
correction. (I could very well be WAY out to lunch).

 Regards,
Greg Manning

Brandon, Manitoba, Canada

-----Original Message-----
From: gasification-bounces at listserv.repp.org
[mailto:gasification-bounces at listserv.repp.org]On Behalf Of Norma McDonald
Sent: Thursday, January 31, 2008 5:58 AM
To: gasification at listserv.repp.org
Subject: [Gasification] Biofiber drying quantification

We are trying to reduce the moisture level in the biofibers that are
mechanically (screw press) separated after anaerobic digestion of manure.?
The moisture content is 65-70%, all tightly bound to the lignocellulosic
fibers (no free moisture).? We want to lower this using waste heat (250F -
500F air) from the generator down to ~20% moisture so that we can pelletize
for fuel pellets.? We are also thinking of directly gasifying these
materials.?

Dynamic air flow produces too much dust and fibers all over the process
area, so we are thinking about radiant heating in an auger.? For our first
installation, the flow rate requires removal/evaporation of 4 gallons per
minute.? How do I calculate the time and surface area required in order to
design the heat exchanger?

Norma McDonald

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