FGD and DeNOx
NEWSLETTER       

January 2010
No. 381

Why Was a CDS Chosen for Dry Forks?

Basin Electric Power Cooperative is currently in the construction phase of the new 385-MW mine-mouth, greenfield, Powder River Basin (PRB) coal-fired Dry Fork generating station near Gillette, WY. The station will operate in an arid climate with limited water availability. Water supply for the station will be taken from deep wells and pumped a significant distance to the plant site. Therefore, water consumption was a major consideration when selecting the optimal SO₂ control technology. Additionally, due to the close proximity to various Class I areas, very strict emission limits were chosen for SO₂ control at the station. The most recent advances in wet FGD and dry FGD (spray-dry absorbers, SDA) technologies were investigated along with advanced circulating dry scrubber (CDS) technology.

In a CDS system flue gas passes through parallel venturies mixing with hydrated lime, water and recycled solids to create a fluidized bed where the reaction of the calcium with the SO₂ and SO₃ to form calcium sulfite and calcium sulfate takes place. Fresh reagent is added by itself as a dry powder into the absorber to allow for adequate mixing with the flue gas. Water is injected independently into the absorber, cooling the flue gas to a specified temperature range above the saturation temperature. The flue gas, containing calcium sulfite, calcium sulfate and flyash, then travels to the particulate collection device, typically a fabric filter in new applications, where solids are captured and removed from the system. Large quantities of solids are recycled back to the absorber for use in the fluidized bed.

For Dry Fork Station, three site specific parameters played a significant role in selecting the optimal FGD for the station:  water availability, SO₂ collection efficiency and flexibility in fuel fired. The capital cost of the FGD system and the cost to operate and maintain the system were also considered, although they were not as influential as the three parameters mentioned above in the final decision. Approximately 60 percent less water is required for the dry technologies.

The CDS utilizes neither a rotary atomizer, like SDA, nor fresh reagent feed in the form of wet slurry, like WFGD. Instead, hydrated lime and water are injected independently. Other than cost considerations, there are few limitations in the amount of fresh hydrated lime that can be fed into the absorber. As such, the CDS should be capable of achieving SO₂ collection efficiencies beyond that of an SDA system and similar to those achieved with WFGD technology, while providing the necessary margin to operate within the permit under all operating scenarios.

Dry Fork Station is located adjacent to the Dry Fork Mine, which mines PRB coal. The coal delivered from the mine will come from multiple coal seams and can vary in sulfur content by as much as 20 percent. The FGD system was designed for 1.20 lb SO₂/MMBtu coal but should be capable of handling spikes in sulfur up to 1.43 lb SO₂/MMBtu. This has a significant impact on the sizing of the FGD system and the ability to meet the permit limits. In particular, care was taken in selecting the optimal SO₂ mitigation technology to allow for quick response to sudden increases in coal sulfur content consistent with the variability shown in the coal analyses from the multiple coal seams. The CDS is capable of making more rapid adjustments to account for a spike in coal sulfur content because the hydrated lime reagent is injected independently of the water.

The ability of the selected FGD system to capture sulfur trioxide (SO₃), particulates and mercury (Hg) was considered. The coal to be fired at the station is low in chlorine and, as a result, it is anticipated that a large percentage of the mercury present in the system will be in the elemental form. The CDS technology is also expected to achieve greater than 90 percent capture of SO₃, based upon a downstream fabric filter. There is significantly greater reaction surface area available in the CDS system due to the solid recycle system so a greater degree of co-benefit mercury capture is expected compared to the SDA (with a fabric filter) or WFGD systems. This is likely due to adsorption on the surface of the solids in the reactor and fabric filter.

The auxiliary power consumption is approximately the same for the CDS and the SDA. Additional horsepower is required for the atomizer in the SDA system, which is not required for a CDS. However, the pressure drop through the CDS system is greater and therefore the induced draft (ID) fans are larger, requiring higher horsepower motors. The CDS and SDA systems each consume approximately half of the power consumed by a wet FGD system.

The CDS system is a very compact system, with nearly all equipment installed in one building. The central location for all equipment, along with indoor installation, facilitates maintenance activities. The Dry Fork CDS requires approximately 20 percent of a WFGD footprint. The difference is less severe between a SDA and a CDS footprint, with a CDS requiring about 70 percent of the footprint of a comparable SDA system.

Ease of erection of the system components was another appealing attribute of the CDS system. The absorber, baghouse and storage silos lend themselves to pre-assembly (ground fabrication). These large, pre-fabricated sections can be lifted into place during the erection, reducing construction costs.

Nearly the entire CDS system is fabricated of welded or bolted carbon steel components. A WFGD requires alloy steel materials, lined carbon steel or acid-resistant materials, such as fiberglass reinforced plastic, be used for system components including ductwork and the chimney liner. The use of these special materials results in an increase in capital cost and also increases the cost of future maintenance to the system.

The CDS system is comprised of equipment that is easier to maintain than that of a WFGD or SDA. The CDS system requires routine testing of the water spray nozzles to ensure appropriate spray patterns are achieved. The nozzles are small assemblies that can be removed and replaced with relative ease, reported Ajay Jayaprakash and Lauren Barbin of Sargent & Lundy and Tom Stalcup of Basin Electric Power Cooperative at Coal-Gen 2009.

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