Multi-cyclone installation

[macmet]

Hello everyone, we work on wood fired boilers and use multicyclones for flyash separation. These seem to work fairly well and we are generally satisfied with the performance we get out of them.

However, we are looking at a system with sludge as fuel and the ash content is much higher. I am wondering if anyone has tried using two multicyclones in series to further reduce particulate emissions and what sort of results you may have got from this setup.

I will be talking to our supplier about this soon, but would like to get some outside opinions as well.

Cheers

 

[rmw]

I have successfully used multiple multi-cyclone dust collectors on several occasions. I assume that you have the fan to pull through both. You may want to consider a smaller tube size in the second dust collector and depending upon whose you buy if they offer a disc type discharge boot on the bottom of the tube and a conical type, use the disc type on the first collector and the conical on the second.

The primary reason it was done where I did it was for fan protection and it worked well. The fan exihibited very little wear over a lot of years while units with single multiclones had horrific fan wear problems.

Keep the hopper access doors shut and sealed and use a good brand of rotary valve for the dust extraction. A leaking rotary valve can cause fires that can trash dust collector hoppers.

rmw

 

[macmet]

We have very abrasive ash as well as it is very high in silica. We were looking at cyclones in series simply to get the particulate emissions down as the fan is outside of our scope for this project.

We currently use 9" tubes and I do not see us going to anything smaller. We would likely use 9" tubes on both.

What fuel were you burning?

 

[davefitz]

I would have thought the better solution for a wood fired boiler would be : 1st stage U-shaped impact plate seperator , 2nd stage multiclone.

If the objective is to reduce partilce erosion in downstream equipment, I seem to recall tha only particles larger than ( 30 um ?) and of mineral matter harder than steel will contribute to erosion. ( as per published Mitsui Babcock papers) . So the cut size of 2nd stge multi clone does not have to consider particles smaller than 30 um.

 

[davefitz]

correction: I just re-read the Mitsui report. Particle size greater than 25 um only affect erosion. Refer to report Coal R241, DTI/pub 04/701 March 2004.

 

[macmet]

We generally do not have an issue with erosion in our wood systems. This system we are looking at is not burning wood but a sludge like material which has a much higher ash content.

The main reason for looking at multicyclones in series was to reduce overall emissions, not reduce erosion. It was not until that was pointed out by RMW that erosion came up.

 

[rmw]

What are the constituents of your ash? The fuel I experience with was mixed pine and hardwood bark with some sawdust and planer shavings mixed in in varying proportions.

In some applications there were jokers in the deck because of phenolics from product scraps also burned in the furnace.

I can't picture any jurisdiction in this fine country of ours where multiclones, even in series (unless you had more than a few) would meet current pollution laws. You need a scrubber or a baghouse to meet environmental laws of today.

IMHO.

Also, do you have a particle size distribution efficiency curve from your prospective multiclone vendor?

The fine ash that multiclones don't get out is what makes the plume that is objectionable.

You may want to use a 7 inch tube for your second stage, and if you are compelled to stay with 9", then at least use the conical discharge boots on the bottom of the tubes. They are much better for the fine stuff.

rmw