CFB Scrubber at Basin Electric Dry Fork

Nooter/Eriksen, in partnership with Graf-Wulff, designed, supplied and installed a circulating fluidized bed (CFB) scrubber for Basin Electric Power Cooperative at Dry Fork. This 420-MW PRB coal-fired boiler went on line in June 2011 and has 20 low NO_x burners, overfire air and an SCR. This is the largest CFB scrubber system ever built as it is located at an elevation of 4250 feet and with an equivalent volumetric gas flow of a 500-MW boiler located at sea level. This scrubber is designed to reduce SO₂ by 95 percent down to 0.06 lb/MMBtu. It also includes the largest of its kind Graf-Wulff lime dry hydration (LDH) system, which is designed to maximize lime reactivity thereby minimizing lime consumption.

The FGD technology selection was decided based on three site-specific parameters — water availability, SO₂ collection efficiency, and fuel flexibility. For water consumption, the dry technologies (SDA and CFB) afford distinct advantages over wet FGD as they consume approximately 60 percent less water. For plants like Dry Fork firing low sulfur sub-bituminous PRB coal, SDA is often selected as capital costs are lower than wet FGD and fuel sulfur levels are lower than those for bituminous coals. With SDA, however, SO₂ capture is limited by the amount of slaked lime reagent slurry that can be introduced into the absorber vessel through atomizers located at the top of the absorber. Given the stringent SO₂ permits limits for Dry Fork and the fact that the coal will come from multiple seams and vary in sulfur content, SDA was deemed insufficient.

CFB FGD technology was deemed to provide an optimum balance of water consumption, SO₂ removal efficiency and fuel sulfur content flexibility. Basin Electric thus selected CFB technology offered by Nooter/Eriksen Environmental Technologies.

The CFB absorber vessel is an upflow reactor wherein all reactants are introduced at the bottom of the vessel along with a large portion of particulate solids collected from a downstream fabric filter. SO₂ and SO₃ enter with the boiler flue gas through multiple venturis, and the lime reagent (first converted to calcium hydroxide) is then introduced to the absorber above the entry point of the flue gas. As the reactants move in turbulent flow to the top of the absorber, the gas is cooled by evaporation of a fine spray of water injected into the absorber. Without peripheral equipment such as rotary atomizers, spray spargers, or mist eliminators, the CFB absorber vessel functions as a “highly-engineered duct” containing only the flue gas, water and fluidized solids.

Residence time for gases entering the tall and narrow CFB absorber is about five seconds providing improved SO₂ removal efficiencies within a small absorber footprint. Two lime hydration units are located near the absorber vessel. Lime, low pressure steam and water are injected into the hydration reactor for conversion to calcium hydroxide.

Two fabric filter baghouses, each with three compartments, are located above the absorber vessel to allow recirculation of particulate solids. Nearly all of the components are contained in a single building with a footprint of 190 ft x 100 ft.

During startup, only two major CFB scrubber adjustments were made. Both adjustments focused on optimizing the fluid dynamics and response time within the CFB absorber for the final boiler outlet conditions. Stable operation was maintained through a flue gas flow turndown to 33 percent of full flow. All contracted and permitted emissions have been demonstrated to be below the required permit levels, i.e.:

· SO₂ 00.7 lb/MMBtu – 12-month rolling average

· HCl 2.05 lb/hr – 12-month rolling average

· PM 0.012 lb/MMBtu – 30-day rolling average

All EPA required emission levels were achieved at lime consumption rates well below (10 percent) contractually required values with no impacts or restrictions on boiler operation, reported Glen Bostick and Steve Moss of Nooter Eriksen Environmental Technologies at Air Quality VIII, fall 2011. Auxiliary power consumption was well below guaranteed values. The fixed rate Lime Dry Hydration (LDH) Unit has been placed in operation, has demonstrated greater than 96 percent conversion of CaO to Ca(OH)₂ and has been operated at elevated make rates. Reduced valve trim will be implemented to allow operation at lower demand rates.