Silica and Alumina leaching from furnace refractory?

[SREisme]

Has anyone experienced silica, alumina, or sodium leaching or volitizing out of newly installed hot face refractory in a fired furnace? We fouled a downstream waste heat boiler with a very fine, low density, thin ~1/32" coating that analyzes as Fe, Si, Al, Mn, Na, and other trace minerals. We just replaced all the refractory, (hot face and insulating brick)in the reaction furnace prior to restarting this unit. Within 4-6 weeks of startup, we lost approximately 10% duty across the waste heat boiler. The fouling stabilized after about 6 weeks. The reaction furnace is your typical Claus process, burning H2S, NH3, and trace impurities with atmospheric air and 23% O2 enrichment. Normal operating temperatures of the refractory inside the furnace is 2200-2400 deg F. Has anyone else experienced this effect? What can be done to prevent it? Thanks.

 

[unclesyd]

There are several names for the process you describe. We just call it dusting and it is a problem with monolithic refractories especially if gunited.
Things that can affect dusting are temperature swings. gas velocities, and changes in the oxidation state in the firebox.

I would checkout all the parameters on your end like the drying out process, the initial heatup, etc. I would then get with your refractory supplier and discuss it with him as there maybe a better product to use in your case.
Make sure your supplier is well versed in the art and not just cook-booking the whole process.

The best way to mediate the problem is a different refractory material. You can also apply a refractory coating or wash that will also help mediate the problem, but they are very expensive per sq.ft.

 

[SREisme]

Thanks, Unclesyd. Our refractory is installed in blocks, no guniting. We'll defintely be talking with our supplier. You call it dusting. I called it leaching as the deposit analysis was heavily weighted to Si, while the hotface brick is primarily Alumina. Do you see similar compositions? Is silica more susceptible to dusting than alumina? Also, we can't "see" about 50% of our deposit material on XRF, so I am wondering what else may be in the deposit.

 

[SREisme]

Unclesyd- your reply makes me think you have seen this issue numerous times. Can you tell me what types of refractory you've seen it in, what services, what compositions, etc? Is this a start up issue due to whatever machanism is at work or can it continue to occur over the life of the refractory? Looking a little deeper into our operation, we found a similar deposit in another unit, although not as severe, that developed within two years of replacing the refractory (same material as the current issue) but very shortly after implementing oxygen enrichment in the furnace. We also have a third furnace we rebricked with the same material but do not run O2 enrichment in that was put back in service the same time as the problem unit. So far we have no indications of fouling in the third waste heat boiler.
I appreciate the information. Once again I am reminded of how little I really know

 

[unclesyd]

Apparently your refractory was rammed instead of gunned as I surmised. I had a talk with a friend that pretty well has the refractory angles covered. He stated that he hasn't see or heard of the particular problems you are experiencing in the sulphur burner. Within the few months he has inspected burners with 10+ yr old refractory that had presented no problems during their service life.
All his work has been with air bond materials and very little with the chemically bonded materials, specifically the one linked below.

We use a little of all types of air bond but mainly the type linked below. We push it to limit both on temperature and environment. I checked with the people at a onetime sister company that have numerous Sulphur burners and all use an air bond refractory like the 60 Alumina. There has been incidences of dusting but nothing to clog the heat exchanger.

The silica component is a much smaller animal when it get air borne and travels further and sticks to things better.
AS for the O2 enrichment he would have to defer that to the engineering department. Is the O2 raised to 23% or 1% enrichment, or is it 23& O2 added by volume of air?

How does the refractory proper look especially around the burner can? Problems in the front end can create problems down stream due to erosion.
Note any slick area where they might have been some incipient melting of the refractory. This is in reference to the NH3 in the system.
Get a copy of the bake out procedure and the temperature charts.

Sorry I can't get the link from Engineering.com to work, it's me not them. I'll keep trying.
The cut sheet was for a 60% air bond Alumina Silica material
used a lot in sulphur burners.

 

[unclesyd]

I finally got someone to pull up some analysis of the duet we get on our HRSG, thermal oxidizer. The refractory component of the dust as in your case is silica and alumina. The material in this analysis is loaded with CU and V from our process catalyst, > 25% total, There is also a something listed as phosphates, a mystery.

Here is the material that compromises better than 95 of the fireside insulation in the unit. Nearly all of it is rammed. We push this material to the limit as during an upset we make sapphires.
According to Dale Johnson at Plibrico this is the material they use on most of in furnaces such as yours.

http://my.engineering.com/action/file/download?file_guid=251360