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Murphy on SO2 treatment in a Circulating Fluidized Bed

By: Murphy [21] 4 Stars
Date: Tuesday, November 07, 2017
Time: 2:52:25 PM

The unqualified answer is yes CaO can be used in a circulating fluidized bed boiler instead of Limestone CaCO3 and MgCO3 or Dolomite, for the material balance will work when CaO is used instead of CaCO3 and MgCO3 as a raw feed for treating SO2. By using CaO the extra CO2 that would have otherwise been in place would now be removed from the material balance and material inventory that are recirculating.

From an economic perspective the CaCO3 and MgCO3 has an economic advantage for it is abundant on Earth like South America. Where as Lime or CaO or Calcia is not naturally abundant and is generally manufactured with reasonably valuable commercial uses other than a FCB Boiler. When Lime is made it still has to be calcined at 1600F, which still produces CO2, and generally in a lime kiln where as in a FCB boiler it is done in the bed zone. An example is- should there have been a Pulp Kraft Mill that has excess lime then the application of Lime can be added in the raw feed for treating SOx. Dry desulfurization using Lime or CaO occurs at the same temperatures so in the bed zone desulfurization occurs too. So, retention time of materials is very important.

In the FCB boiler or fluidized circulating bed boiler the lime would best be sized to keep the solids in the bed zone, for even though it is a circulating bed the furnace height has a density profile. For instance, coal is milled or crushed to around 0.25 inches before being feed in the bed. So, the heavy solids are kept in the bed zone to do their business and above this zone is where the solids either drop into the bed or go to the transition zone. The point is the air flow through the zones is about 20 ft per second but varies and has to be right to keep the solids to particulates in a fluidized flow to there point of designation for recycling without carrying over. So, air is controlled at the windbox for primary air and the overfire air that is above the bed.

Not to worry, for the any unused CaO or solids that gets past the furnace freeboard zone should be captured at the dust collecting separator -designation point- and before the stack or chimney and recycled back to the bed. What is important to me is keeping material balance of lime to sulfur, where sulfur is in the fuel, proportional for excess has to be dumped and shortages have to be made up in adding raw sorbant Limestone or in your case lime.

What make CFB boiler so good is the carbon is nearly 100 percent converted. A very eco friendly unit. An I believe that to get pass a reduction of 90 percent Sulfur SOx you will need to use straight CaO for limestone will not do it. I never got that far.

Another good point about CFB boilers is the bed temperatures are low at around say 1600F and that means no thermal NOx production which starts to occur at 2700F.

I know I skipped over a lot, but to answer your question in my opinion and I believe yours as well is the Circulating Fluidized Bed steam boiler can use lime as a supplement to limestone to treat sulfur oxides since a portion of CaO is not consumed since unused portions are recycled when limestone is used.

I am sure there are smarter people out there to answer your question another way or maybe with a differing opinion than mine.

Murphy

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