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Technical Cement Forum / Excess O2 & Boulder Formation

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Nael 26/07/2011 12:07:34

Excess O2 & Boulder Formation

Hey;

We have 3 preheater kilns with PC and bypass system, kiln feed approximately the same for all the kilns, gas fuel source is also the same. Although the three kilns are exactly the same excepts capacity (2000, 4000, and 5000 tpd) the phenomena of boulders formation inside each kiln is different.

Kiln-1: high rate of boulder formation (some times 3-4 boulder/ days)

Kiln-2: from time to time boulder forming ( 1 boulder/ 2 mounth)

Kiln-3: No formation of boulder.

My question will not be about the reason of boulder formation, beacuse i have read alot about reasons of boulder formation here in this forum and in other documents and I havn't found any reason we can accept it or matching our case.

My question is about one point which I have observed and would like to share it with you to know if it may be the reason of boulders in our kiln-1 and 2or not.

This point is the excess O2 in the kiln. Before three mounths we have started to increse our feeding rate in the kilns but with this increasing, excess O2 in kiln-1 dropped to < 1.0% (always), in kiln-2 dropped to 1.5% (sometime) while in kiln-3 excess O2 remain > 2.5%.

My questions: Do you think (according to my observation) low excess O2 in the kiln be reason of the boulders?? If yes, Why? Is there any relation between excess O2 and reactions of salt formation??

I belive that there is a relation between the boulder and excess O2 but i don't know how to joint between them, any help!!!!

Best Regards

Ted Krapkat 27/07/2011 05:11:39

Hello Nael,

Yes, there is a relationship between low kiln inlet O2 and the appearance of boulders.

Low O2 in the kiln inlet is known to increase the volatilisation of sulphur in that area. This subsequently increases the chances of buildups, which often fall into the kiln inlet as solid slabs.

If these slabs are large enough, or hard enough, they may not fully break up as they travel down the kiln. Instead they can have their sharp edges rounded down by frictional wear and become more and more ball-like in shape.

If there is a ring in its path, this may stop the ball and it will roll around behind the ring. If the conditions at this point are right, the ball may slowly pick up 'sticky' material and start to grow in size until it is quite a large boulder.

Unfortunately, that's when the real problems start.

Regards,

Ted.

Nael 27/07/2011 08:21:37

Hi Ted;

Thank you for your answer. I am agree with you 100%, Sulfur/ Alkali Ratio is very high in kiln-1. We have ring and heavy coating at kiln backend, and we also facing the problem of continuous falling of coating.

Ted; do you agree with me that less excess O2 makes the flame longer (lazy flame), shifts the burning zone back, and as a result forms liquid phase earlier which may help in boulder formation!!!!

Thanks again

Best Regards

Nael

Ted Krapkat 29/07/2011 00:30:18

Hi Nael,

I don't think that it is the low O2 at the kiln inlet that makes the flame longer but rather, the other way round.

If there is some problem with the burner itself or it's settings, or there are fluctuations in coal feed rate or the fine coal is ground too coarse, then this could cause a long, lazy flame, which in turn would undoubtedly have a detrimental effect on the O2 level at the back end.

However, there is another mechanism associated with the high sulphur recirculation that can cause heat to be transported up the kiln.

High sulphur recirculation is invariably associated with increased dust loads in the kiln and the generation of 'dusty' clinker. This dust comes back from the cooler in the secondary air flow and if the dust load is bad enough it can partly obscure the flame and absorb some of its radiant heat, transporting that heat back up the kiln... at the same time this dust is likely to pick up molten coal ash droplets from the flame.

Over time, this stream of 'sticky' dust can form buildups and/or rings and cause any balls rolling around behind them to slowly grow in size and form quite large boulders.

Hope this helps...

Regards,

Ted.

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