[Gasification] Drying

[Greg Manning]

Adam,  (and list)

You have brought a very good point to the table.

The system I have been working on is in reverse, low (jacket) grade heat, is
used as a heating system, somewhat a liken to an outdoor boiler with a small
district heating approach, where as, the high (exhaust and cooling) grade
heat is used for drying.

The key component here is "how wet is to wet", as in, at what level will the
drying equipment be over-burden with too high a percentage of water content.

Let me rephrase that,

The drying equipment will handle up to what percentage of water, with the
heat available, and still produce the that same "oven dry" or "bone dry"
product for the gasifier....

So far, the magic number at my end, is 35% "free" water content, anything
higher and the dryer simply outputs the difference (1 - n % MC over oven
dry).

So far, the gasifier design I'm working with, will handle up to 3-4% over
oven dry, after that, the gas quality drastically changes, and anything
above that MC, the entire burnable portion of gas, falls off.

The true point here, for 30kg/hr of 35%MC input feedstock, I can produce
enough "dryer" heat, to continue the process, at 37%, the entire thing goes
into a downward progression that will stop at about the 3 1/2 hour
mark...........

Side note: I would say that the amount of heat I'm capturing is still only
about 70% of what is available, I have left the last 30% as "headroom" for
non-condensing final output of heat. (remember, I live where it gets VERY
cold.....).

Greg Manning,
Brandon, Manitoba, Canada





-----Original Message-----
From: gasification-bounces at listserv.repp.org
[mailto:gasification-bounces at listserv.repp.org]On Behalf Of Adam Carr,
Renergy
Sent: Thursday, December 14, 2006 4:37 AM
To: gasification at listserv.repp.org
Subject: [Gasification] Drying


Taking into account that there are probably 3 sources of recoverable heat on
most commercial gasifiers: i.e.1) Gas cooling circuit, 2) Engine exhaust, 3)
Engine water jacket, there is more than enough heat to dry - say wood - down
from ~50% to <20%. It is merely a question of the trade off-between time
taken to dry (quicker = less expensive biomass storage space needed) and the
capital cost of the dryer. Very generally speaking drying things faster
involves greater technical sophistication/capital costs.

However, for me this is not the central issue. In the UK relatively small
scale gasification(< 5MWe) is probably not commercially viable unless CHP,
i.e. a market can be found for "waste heat", and so it follows that it is
important to use as little energy for drying as possible so as to maximise
revenue.

As a rule of thumb in most heat applications higher grade heat (i.e. from
gas cooling and engine exhaust) has a greater commercial value than
low-grade (water jacket) heat. This is certainly true of the project we are
currently working on, so for me the key drying issue is can we dry ~one
tonne an hour continuously using ~500KW of hot water at 90C thereby enabling
us to derive revenue from the higher grade heat.

I have to say that opinions differ. The gasifier technology supplier we are
currently contemplating has a commercially operated gasifier running (in
Germany - I will shortly be visiting) for which it is claimed that a walking
floor the last third of which is perforated through which warm air derived
from water jacket is blown is capable of drying sufficient "chunks" (
~50mmX50X50) to run the gasifier continuously. However specialist UK dryer
manufacturers tell me this is simply not possible as at least twice as much
heat would be required.

Persons interested in this topic might want to look at 2 research notes
(NC-308 & NC-293) by Joseph B. Sturos (Google him) produced by Forest
Services USDA. N.B. difference between drying "Chunks" as opposed to
"Chips". I would be grateful if some of the more scientifically competent
than me list members  (I am merely a bloated capitalist) could let me know
whether they think some of the data presented in these papers might or might
not have a bearing on this subject. There is also an ETSU paper produced by
the DTI/Border Biofuels, but this relates to a monstrous machine capable of
drying > 500K tonnes/yr.

Adam Carr
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