[Stoves] Clinker Formation

Sat May 17 00:26:35 CDT 2014

Thanks a lot Tom/Crispin/Jonathan.

Tom, did you use Dolomite or Kaolini Clay at some point? I am wondering how
much % of these should be used. Also are there other natural agro waste
which we can probably mix with bagasse to get the same result (high
potassium I mean).

Regards
Shailendra

---------- Forwarded message ----------
From: Tom Miles <tmiles at trmiles.com>
Date: Fri, May 16, 2014 at 8:29 PM
Subject: Re: [Stoves] Clinker Formation
To: Discussion of biomass cooking stoves <stoves at lists.bioenergylists.org>

Shailendra,

The composition of bagasse varies  considerable. We have gasified and
burned very clean bagasse and very dirty bagasse. The larges variation
comes when the cane tops and leaves, called “trash”, are included with the
cane as it goes through the mill. The tops and leaves contain high
concentrations of sodium, potassium, sulfur, and chlorine and low
concentrations of silica, calcium, and magnesium. The balances of these
minerals determine the melting temperatures of the ash mix and whether
particles stick together as agglomerates or melt and fuse like glass. The
plant takes up salts from alkaline soils which is not always cleaned in the
sugar extraction process. This increases the concentrations of low melting
compounds of chlorine (KCL, NaCl) and sulfur. Straw and other agricultural
residues that contain annual growth have very high levels of potassium. The
proportion of potassium to silica determines if the silica will become
sticky or melt. The more potassium, the lower the melting temperature.

Melting temperature measured I the lab are usually not accurate because
they use methods appropriate for coal in preparing the samples to be
tested, not biomass. For coal the fuel sample is ashed at 850C or 900C.
That vaporizes elements that are important to melting. In Europe and North
American labs the fuel sample is prepared at 550C or 600C which is below
the vaporization temperatures. Then when the ash is heated it will melt at
more typical temperatures.

Pellets are dry so they burn at very high temperatures ( 800-1000 C). To
prevent slagging you need to burn the fuel in stages. The first stage
includes drying, de-volatilization,, pyrolysis (heating) and gasification
(partial oxidation). You must control the air at that stage on the grate so
that temperatures do not exceed 750 C. This is usually done by limiting air
through the fuel pile. When the gases emerge above the fuel pile you can
add air and increase the temperature, usually to 800-900C. Then you must
have a mechanism for removing the dry ash. If it remains exposed to the hot
flame it can stick together (sinter) or melt ()fuse) to ash.

Stages combustion in a pellet burner can be a challenge. Most pellet
burners are designed to burn the fuel and air mixture at the highest
temperature which is usually in the range of 1000C- 1100C range which will
melt most dirty fuels.

Another approach is to add minerals to the fuel ,like dolomite or kaolin
clays, that contain high proportions of calcium or magnesium. These
minerals will raise the melting temperature of the mixture and reduce
sintering or slagging.

Tom Miles

*From:* Stoves [mailto:stoves-bounces at lists.bioenergylists.org] *On Behalf
Of *Crispin Pemberton-Pigott
*Sent:* Friday, May 16, 2014 4:36 AM
*To:* stoves at lists.bioenergylists.org
*Subject:* Re: [Stoves] Clinker Formation

Dear Shailendra

What you describe is what happens when there is a flux present - probably
more than one. A flux reduces the melting temperature and it is not a
surprise that you have a clinker problem. It may well be glass, not ash in
the mineral sense but of course glass is a mineral.

Perhaps Alex English will comment because he has made more glass
accidentally than most of us put together.

Roger Samson showed us that leaving switchgrass outside through the winter
removed Cl and K by leaching. The problem went away.

Another possibility is to change the combustion to try and prevent the
temperature ‎rising so high. That would involve cooling the char burning
zone by not preheating the primary air, just as an example.

Regards

Crispin

BBM 2B567C3

‎

Dear All,

We in India are trying to find a solution to the clinker problem in the
stoves as the temperature reaches above 1000 degrees very soon in a 30KW
stove for example. Now the pellets we are using are mainly bagasse based,
and its ash melting point is above 1000 degrees but still it is not giving
the right result. The pellets are not made of 100% bagasse and the size we
are using is 8mm.

Please give your inputs/suggestions if this problem can be solved by
adjusting the primary secondary air or we need to change the composition of
the pellets.

Regards

Shailendra

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