Sun flower husk fired furnaces or boilers 7

[marula]

I hope this is the right forum for my question.
We have built furnaces to burn sun flower husk.
The furnace linings have a very short life span of about
6 months.
Temperature probes disintegrate before they even serve their purpose.
The customer has accepted the fact that he has to spend money on new refractory linings every 6 months.
It is still cheaper that way than using conventional fuels such coal, diesel or gas.
Maybe someone out there can help us with some ideas.

 

[StoneCold]

marula
My first guess is that you have flames impinging on the refractory. Have you looked into the furnace when it is running? What did you see? What type of refractory are you using? What does the refractory failure look like? Is it cracking and falling off or melting?
How do you condition the furnace after you replace the refractory? Are you heating it slowly to dry the refractory?

Sorry Lots more questions than answers

StoneCold

 

[Montemayor]

marula:

If you are getting suspicious of something going wrong with the boiler, you are right. Stonecold is absolutely correct in placing his suspicions on a faulty combustion chamber design.

You haven't state how you are burning the solid fuel - traveling grate, etc., so its difficult to visualize the type of flame pattern you might be getting. I've burned soid residue from corn cobs, peanut hulls, oat hulls, and also baggase (sugar cane residue). And they were all very acidic in content since they came from Furfural digesters. I don't believe your sun flower husks are that tough to burn. Nevertheless, I never experienced what you are describing. Our boilers ran for more than a year before a turnaround. That's why I side with Stonecold in suspecting an inherently wrong furnace design.

 

[rmw]

Rice hulls are likewise hard on furnaces and boiler metal parts.

In the case of rice hulls, it is due to (1) the hardness of the rice hull itself, and (2) the high silica content of the rice hull due to it's own chemical make up as well as silica from soil that either gets blown onto the rice hull and grows there or is left from handling.

Could any or all of the above be your problem.

I am not familiar with sunflower husks as a fuel. My only familiarity with them is cracking and spitting them as I enjoy one of my favorite "stay awake" foods for long automobile trips.

rmw

 

[marula]

The refractory melts.The grate is fixed.
We used a fixed step grate, which we have used
successfully on wetter sawmill waste. Admittedly we did
not think further than our noses and thought if it works
on sawmill waste it should work on sunflower husks.

Keep in mind that I am from South Africa. Our country is a mixture of first and third world.
It is difficult to offer customers high tech furnaces.
We have enough literature to most probably design the perfect furnace.
Such a furnace would however only pay itself off after
5-10 years.
Except for some of the larger corporations, very few customers here would budget that far into the future.

The refractory we have used are alumino-silicate bricks
53.6% SiO2 & 42% Al203 & Alumino-silicate castables 42% SiO2 & 50,3% Al2O3 for applications up to 1600 deg C
(2912 deg F)

We use the right slow curing process after installation.

The combustion analysis of Sunflower Husk shows the following:

Inherent moisture: 7.4%
Ash: 2.4%
Calorific Value: 18.82 MJ/kg
Sulphur: 0.1%
Carbon: 59.75%
Hydrogen: 5.97%
Nitrogen: 0.79%
Sinter Range: 960-1290 deg C
Deformation: 1300 deg C (2340 deg F)
Softening: 1340 deg C (2444 deg F)
Hemispherical: 1360 deg C (2480 deg F)
Flow: 1370 deg C (2498 deg F)
Al203: 2.36%
SiO2: 6.37%
CaO: 23.38%
MgO: 15.57%
Fe2O3: 4.26%
TiO2: 0.09%
P2O5: 9.85%
K2O: 26.01%
MnO: 0.13%

 

[macmet]

Could you increase the excess air or recirculate flue gas to bring the temp down? What about mixing it with some wetter material?

I have some info on sunflower husks that is similar to yours. Using it I get a reasonably high combustion temp.

 

[rmw]

I don't recommend recirculation of flue gas since it contains the abrasives or other components that are the culprits in the first place. If you are using recirculation on your sawdust boilers then you are probably seeing accelerated wear in the areas where the recirculated gas enters the furnace.

What I see in the analysis is a very dry fuel especially if compared to a 50% moisture sawdust (when it isn't raining on either one.) I suspect that your temperatures are exceeding the limits of the refractory where as your saw dust has so much moisture in it that it never gets there to give you a similar problem.

I've seen this type of damage in furnaces designed to burn sawdust when the fuel was switched to dry planer shavings or a large percentage of planer shavings with sawdust. Much hotter flame.

rmw

 

[marula]

Thank you all for your responses.
All of them are useful and will most probably help to produce a better design.
Even more suggestions would help.

 

[katmar]

Marula, I have had some dealings with a South African company that is successfully burning sunflower husks. I wouldn't like to post their contact details here, but if you contact me via my web page (see sig below) I will pass on their details to you. I am sure they would not mind discussing the problem with you.

My immediate reaction is that rmw is correct in suspecting that your temperatures are much higher than expected because of the very different moisture levels. At 50% moisture you are a few percentage points away from extinguishing your boiler fire, but at 7.4% you've got a tiger by the tail.

regards
Harvey



Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[macmet]

Are you guys saying he's getting over 1600 deg C? I find that hard to believe, low moisture or not.

I wonder about the fly ash fluxing the refractory. I am not an expert on refractory, but I see high potassium levels and wonder about that.

Still though, lowering the temperature in the chamber would help. Excess air would do that, but I don't fully agree that recirculation of flue gas would necessarily have an negative impact. But assuming you are already getting 100% complete combustion than adding additional excess air would work - as rmw suggested.

 

[macmet]

I just reread my post, and I sounded unnecessarily bitter in that so I apologize.

But, I'm curious what you might say about the ash elements effects on the refractory.

 

[rmw]

macmet,

I wouldn't rule that out either. I have seen it be the cause of the problem before, but am not very conversant regarding ash effects on this type of particular refractory.

marula,

Do you have options for other grades of refractory?

rmw

 

[marula]

We have just done repairs on a dry sawdust fired boiler where we have had similar problems. Before the repair we noticed that castable refractory areas had survived far better than the bricked walls.
Castable refractory also seems to last a little longer in sunflower husk furnaces.
We have tried different grades of refractory ranging from
1300 deg C (2372 deg F)up to 1600 deg C (2912 deg F).
We have used test bricks of even higher grades. These do
not really perform any better.
At one stage we tried to wet the fuel. This created blockages, because the material handling equipment had not
been specifically designed for wet material.
It is definitely something to reconsider.
Excess air will also help. We would have to change the
fuel feed and combustion grate quite a bit compared to
to the current design i.e. control the feed of fuel and
get the balance right between fuel, combustion air & excess air.

 

[macmet]

we have systems using dry sawdust and I am not aware of any problems with the refractory. They were designed before I got here and I can try to find some information about them when I get back to the office in a couple weeks.

I have seen project with refractory problems, and I believe it is due to the refractory not being suitable chemically not thermally but I have not had the time to look into it as of yet.

good luck

 

[JEB66]

The refractory vendor should be able to determine the chemical suitability of the refractory to the ash.

When I have to change the fuel on a coal fired unit, I give the refractory vendor some slag for them to test. They place the slag in a cup made of refractory and heat it up. Then they slice it to see if the slag reacted with the refractory.

 

[EdStainless]

When you look at the refract in a furnace that has only run for a little while does the surface look 'melted'? It may be chemical reactions. The best option may be to add a thin wash coat of a high purity refractory to protect the existing material. This may be a maintaince item since the abrasive nature of the hulls will wear it off.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

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