[Gasification] Temperature of Ash Slagging (Clinker) formation

[Tom Miles]

Greg,

While temperature control will avoid some clinkering it is more due to the
ash content and chemistry of the fuel than to the operating temperature.
Over time the compounds that decompose at gasification and combustion
temperatures will accumulate and react with other compounds to form
eutectics that melt at low temperatures. The principal component is
potassium, which is abundant in all annual growth.

A working temperature for fusion of a potassium-silica mix is 750C
(1400F).Unfortunately in a downdraft gasifier you need the gas temperature
in oxidation to be well above 1400F, more like 2000 F, to be able to have
enough energy to reduce the carbon. So you are constantly in a balancing act
between enough temperature to maintain gas quality and low enough
temperature to keep the ash moving. Ash removal is clearly important to
making anything work when you have some hardwoods, or any branches, twigs,
or leafy materials.

There are many publications on sintering, slagging and fouling but you can
find the basics in our 1995 report, "Alkali Deposits in Biomass Power
Plants." http://www.trmiles.com/alkali/alkali.htm
Download the 4 MB file.

You can also see an interesting report on staged gasification for
bio-solids, which is a very high alkali and high nitrogen, at:
http://gasifiers.bioenergylists.org/lewisstagegas
Mike Lewis reports how he used staged gasification in a fluidized bed with a
very difficult fuel. The plant, with 3 gasifiers, each capable of about
50,000 pph steam, operated for 10 years between 1986 and 1996.

Tom      

 

-----Original Message-----
From: gasification-bounces at listserv.repp.org
[mailto:gasification-bounces at listserv.repp.org] On Behalf Of Greg Manning
Sent: Sunday, January 21, 2007 10:23 PM
To: A WoodGas; A Gasification
Subject: [Gasification] Temperature of Ash Slagging (Clinker) formation


 Greetings Everyone.

 I'm wondering about ash slagging (Clinker) formation in downdraft
gasifiers.

 I've found a type K thermocouple (Good to 2750f) that can be permanently
mounted in an overhead mount (in the bunker, pointing down into the core
between Tuyeres and Reduction Throat), and I'm wondering if I could
configure it on a simple "your over maximum" type cutback unit as to limit
internal temperature to inhibit slag formation (worst case scenario).

 This would be done by closing/limiting a master air inlet valve, with an
electrically modulating valve controller head, commonly used in hydronic
heating systems.

 My question is, for most wood, 20%MC and down, what is the rough
temperature of slag formation (ballpark area)?

I have lots of clinker and catbox clumps as samples of this over-temperature
condition, but have no idea of the temperature needed to create these nice
little "presents".....

P.S. Catbox clumps are simply lightly-fused ash clumps, but not yet clinker.

 Greg Manning,

 Brandon, Manitoba, Canada



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