July 1, 2010

Circulating Fluid Bed Scrubbers Bridge the Gap Between Dry and Wet Scrubbers

By Steven Moss, Nooter/Eriksen Environmental Technologies

Many Advantages to Dry FGD CFB Technology

Although it is relatively new to the U.S. power industry, CFB technology has been broadly applied in Europe and China. A typical 300-MW CFB installation is illustrated in Figure 3. Today, there are more than 60 CFB scrubbers in operation in Europe, with about 34 running on coal-fired units. Seventeen of those units burning coal are rated at 300 MW or higher. Also, 14 projects totaling 6,000 MW have been built in China since 2000.

3. CFB dry scrubbing system schematic. Courtesy: Nooter/Eriksen (NOTE: The flow diagrams for Figures 2 and 3 were inadvertently reversed in the print edition of this story. They have been corrected here.)

In North America, the largest CFB scrubber is under construction at Basin Electric Power Cooperative’s Dry Fork Station located near Gillette, Wyo. The 420-MW plant is expected to enter commercial service in 2011. A video tour of the CFB scrubber under construction is available at (case-sensitive) http://www.basinelectric.com/Projects/Dry_Fork_Station/index.html.

CFB FGD processes are substantially different than processes using SDA absorbers (Figure 4). CFB absorbers are upflow reactors, wherein all reactants (SO₂, SO₃, lime, and water) are introduced at the bottom of the absorber vessel along with a large portion of particulate solids collected from the downstream particulate collection device. They need no external lime preparation system requiring slakers, grit screens, pumps, and associated slurry system piping. Rather, dry hydrated lime, Ca(OH)₂, is injected into the CFB absorber independently of the water and recirculated particulate.

4. Gaining market share. A CFB dry scrubbing system is operating at the 300-MW Theiss Power Station located in Austria. There are approximately 17 CFB dry scrubber installations in Europe operating at coal-fired power plants. Source: Nooter/Eriksen

Through multiple venturis, flue gas is introduced beneath the bed of sorbent and particulate solids and distributed across the full diameter of the CFB absorber vessel. Single-phase mechanical water injection nozzles, located on the perimeter of the absorber and above the introduction point of the recirculated and sorbent solids, spray an atomized cloud of water droplets into the bed of solids fluidized by the incoming flue gas.

CFB technology affords an enhanced heat transfer rate within the bed by spreading the water spray over a much larger surface area of solids. The ratio of solids to spray water is in the range of 20:1, translating into an approximate 20-fold increase in surface area as compared to an SDA absorber. Rather than relying on the filter cake in the downstream fabric filter for added residence time, gases entering the CFB absorber spend about 5 seconds passing through a fluidized bed some 75 feet in depth, churning within the confines of the vessel walls. The added residence time afforded by a tall, narrow CFB absorber vessel translates into improved SO₂ removal efficiencies within a small FGD system footprint.

Unlike SDA absorber designs, the CFB design is not limited by the capacities of rotary atomizers. The only practical limit for a CFB absorber is its ability to disperse the flue gas throughout the vessel diameter to provide adequate suspension of the bed of solids. This limitation is readily addressed using multiple venturis for applications greater than 100 MW–150 MW.

Operating labor and maintenance costs for any FGD system are functions of the system complexity and the number of rotating pieces of equipment within the envelope of the system. Because there are no high-speed rotary atomizers or lime slurry preparation equipment in a CFB system, the annual replacement parts costs for an SDA system are at least a factor of four greater than those for a CFB dry scrubber system. In addition, CFB systems do not mandate the use of fabric filters to achieve greater than 90% SO₂ removal efficiencies, as the CFB design allows using electrostatic precipitators to avoid filter bag and cage replacement costs.

CFB systems have been successfully installed at plants firing high-sulfur (up to 3.5%) coal; however, there is no technical limit on the fuel sulfur content that can be addressed using CFB technology. Demonstrated SO₂ removal efficiencies are in excess of 95% and can be as high as 99% (depending on inlet SO₂ loading) with controlled SO₂ emission rates as low as 0.04 lb SO₂/million Btu. As with SDA absorbers, SO₃removal efficiencies for CFB designs can be designed for 99%, as SO₃has a high affinity for the lime reagent.

Nozzles used to inject water into a CFB absorber must be removed periodically for replacement of wear components, just as with rotary atomizers used in SDA technologies. However, water nozzles are located around the entire perimeter of the CFB absorber vessel, so additional nozzle locations are typically available to allow installation of a spare nozzle prior to removing an operating nozzle for inspection or maintenance.

Bridging the Technology Gap

Though the choice of an SDA, CFB, or wet FGD system ultimately comes down to site-specific parameters and preferences, the application of CFB processes continues to broaden, covering the entire range of SDA processes and a larger portion of wet FGD processes (Figure 5).

5. Comparing stats. This chart shows capacity ranges for dry and wet FGD processes compared to the CFB dry scrubber when using a single absorber vessel. The technology bridge conditions afforded by CFB technologies reflect capacity limits for single absorber vessels, capital costs, labor and maintenance costs, unit availability, and SO2 removal efficiencies. Maximum CFB single scrubber limit will vary by supplier. Source: Nooter/Eriksen

Major considerations beyond the unit capacity and SO₂ removal performance include the system footprint, system availability, particulate collection efficiency, ease of system erection, maintainability, and auxiliary power consumption. For each of these added considerations, CFB dry scrubbing can provide distinct and quantifiable advantages over competitive dry and wet FGD processes.