7-17-08 am

Gypsum Dewatering options include Centrifuges and Drum Filters - Hot Topic Hour July 17 Morning

There is more gypsum being produced than wallboard companies presently want to purchase. This creates more interest in dewatering options which produce off-specification calcium sulfate.

The five speakers with a combined 100 years of experience in FGD were:

Carl Weilert, Burns & McDonnell

Mike Meadows, Black & Veatch

James McGillicuddy, KMPT

Folke Friesen, Evonik

Bob Boller, Sefar

Prior to the meeting one of the major utilities posed these questions:

We are considering mobile dewatering and ponding rather than permanent belt filters.
We are also considering permanent belt filters without gypsum washing.
We would wet stack the gypsum.

What would be the chlorine content of the sludge be under the three scenarios?

Also prior to the meeting we received and posted this summary of Commercial-grade Gypsum Dewatering Issues provided by United States Gypsum Company - John Halm, Synthetic Gypsum Technical Manager.

Quality Requirements
1. Target Moisture Content: 6% - 10%
2. Washing Soluble Salts:

i. Total salt limits: 600 ppm

ii. Chloride salt limits: 120 ppm

iii. Some dewatering equipment is wash water limited.

Some gypsum process equipment can handle higher levels of moisture with corresponding productivity and energy losses.

Transportation Issues
1. Lower moisture contents may lead to dusting issues during loading, transit, and unloading.
Economic Impact
1. Transportation costs

i. Offset rising fuel costs by 5 – 10% by reducing free moisture.

Process Drying costs

i. Free moisture will increase gypsum processing fuel usage for synthetic vs. natural rock.

Utility Equipment
1. Type of dewatering equipment generally does not impact gypsum usability other than moisture content differences and wash water efficiencies.
2. Increased fines blow down typically improves dewatering ability

i. Combustion trace metals/contaminants (mercury, selenium, etc) appear to concentrate in fines fractions.

ii. Increasing blow down can reduce moisture content and generate a “cleaner” end product for consumer use while reducing re-emission potential.

Carl Weilert, Burns & McDonnell, told the participants that rotary drum filters (RDVF) are an alternative with some advantages over belt filters (HBVF).

Rotary drum vacuum filters have been shown to be capable of producing FGD gypsum filter cake with moisture contents below 10 percent by weight.
Proper attention to RDVF design, including use of heated cake wash, can provide consistent results that rival or better those of HBVFs.
RDVFs have inherent advantages in terms of cost, plot space required and power consumption.
Evaluations of alternatives for dewatering of FGD gypsum should include appropriate consideration of the RDVF alternative.
RDVFs may be favored for all disposal-grade cases.

Mike Meadows, Black & Veatch, addressed operating issues. He advised utilities that there will come a time when off-spec gypsum is produced. The utility should have a plan to dispose of it as agricultural gypsum, to blend it to produce an acceptable quality, or choose several other options. In any case the plan should be formulated in advance. The plant should also plan for off-site transport disruptions. If the customer cannot take shipment there should be a plan to identify temporary stock out areas and provide for storage without contaminating the gypsum. Contractual issues also need to be addressed. Multiple and alternative end–users reduce the risk of disruptions.

James McGillicuddy of KMPT listed the advantages that basket centrifuges offer. The smaller footprint, lower maintenance and dryer end product more than offset the higher capital cost for smaller power plants.

Folke Friesen of Evonik was able to draw on the decades of experience operating both basket centrifuges and belt filters. The long term comparison shows that the total cost over 20 years is about the same for each. This is based on cloth changes of once per year for the basket centrifuges and once every 3-5 years for the belt filters. The capital investment is twice as high for centrifuges but the maintenance cost is only 69 percent.

Bob Boller, Sefar, covered the belts and fabrics for all the dewatering options. The Sefar Double Layer Weave has proven to be a good selection for horizontal belt filters. The Sefar CLICKSEAL™ filter press cloths have eased installation for operators of filter presses used in the FGD wastewater treatment.

Biographies and pictures of the authors are displayed at http://www.mcilvainecompany.com/utilityalert/subscriber/UtilityE-Alert/2008/bios_and_photos_July_17,_2008.htm

The individual presentations can be accessed in the FGD Decision Tree as follows:

Robert Boller - Sefar

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