Application of Circulating Fluidized Bed Scrubbing Technology for Multi Pollutant Removal

Transcription

1 Application of Circulating Fluidized Bed Scrubbing Technology for Multi Pollutant Removal Horst Hack Foster Wheeler North America Corp. Robert Giglio Foster Wheeler Global Power Group Rolf Graf Foster Wheeler Energie GmbH Presented at The 38th International Technical Conference on Clean Coal & Fuel Systems Clearwater, Florida, USA June 2 to 6, 2013

2 Application of Circulating Fluidized Bed Scrubbing Technology For Multi-Pollutant Removal Horst Hack a (horst_hack@fwc.com), Robert Giglio a (robert_giglio@fwc.com), Rolf Graf b (rolf.graf@fwde.fwc.com) a Foster Wheeler North America Corp., Perryville Corporate Park, Hampton, N.J., 08827, USA b Foster Wheeler Energie GmbH, Am Zollstock 1, Friedrichsdorf, Germany Abstract Emissions reduction of heavy metals including mercury, arsenic, chromium, nickel and acid gases such as hydrochloric (HCl) and hydrofluoric (HF) acids is now within the primary focus of the nation s coal fired power plants by virtue of the EPA Mercury and Air Toxics Standards (MATS) rule enacted in December The EPA s 2011 Cross State Air Pollution Rule (CSAPR), while vacated in August 2012, will likely be re-proposed and adopted, further mandating SO 2 reductions on owners of coal fired power plants, who will continue to evaluate the merits of adding back-end air quality control systems (AQCS), versus shutting down units that would otherwise be out-of-compliance. Circulating Fluidized Bed (CFB) Scrubbing technology, offered by Foster Wheeler, offers a viable pathway for addressing multi-pollutant control in a cost effective manner. Combining lime hydration and storage equipment, a circulating fluidized bed upflow reactor/absorber, and downstream fabric filter or ESP, all CFB scrubber system equipment can be installed in one building, or outdoors. Construction costs can be reduced as the major system components can be pre-assembled on the ground and lifted into place during system erection. The Foster Wheeler multi-pollutant CFB scrubber is a flexible and economical technology capable of removing a wide array of pollutants such as SOx, particulate matter, metals, acid gases and organic compounds from flue gases from almost any combustion and industrial process. It does all this while using the least amount of water, a vital resource. Results for a CFB scrubber installation at a coal-fired utility boiler in the United States will be discussed.

3 A New Direction for Environmental Regulations Emissions limits for conventional pollutants emitted from power plants particularly SO 2, NO x, and particulate matter continue to tighten around the globe. In the U.S., the Cross-State Air Pollution Rule, although vacated in August 2012, will likely be reworked and eventually promulgated, mandating additional SO 2 reductions. Consequently, plant owners must continue to evaluate the costs and benefits of adding a back-end air quality control system (AQCS) against shutting down noncompliant units. The timing of rule changes makes this a very difficult evaluation. The North American Electric Reliability Corp., responsible for the bulk power system reliability, has spoken often about the impact of increasingly restrictive emissions regulations on the reliability of the U.S. power delivery infrastructure. In effect, the debate centers on balancing lower emissions limits with the effect of excessive coal plant retirements on system reliability. This discussion is not limited to the United States. Europe s new Industrial Emission Directive (IED) has tightened both SO x and NO x emission limits by 50 mg/nm3 and particulate limits by 10 mg/nm3 compared to the prior large combustion plant directive. Recently, China lowered its SO x, NO x, and particulate limits for power plants to levels even lower than those in Europe s IED. New constituents are also being added to the U.S. regulatory framework, such as metals, acid gases, and organic compounds included under Mercury and Air Toxics Standards rules. Europe is not far behind, with the IED now requiring a best available technology standard for these constituents as well. These constituents have always been regulated for certain waste fuel applications, such as waste-to-energy plants and incinerators, but now regulators have set limits for all boilers, including large utility coal boilers. Plant owners are being asked to make billion-dollar AQCS purchase decisions when the regulations are in flux and court oversight is uncertain. In this era of regulatory ambiguity, selecting the most flexible AQCS design is surely the prudent decision. A Better Way to Clean Flue Gases In the past, due to its proven large scale and ability to capture a high percentage of SO 2 over a wide range of fuel sulfur levels, wet flue gas desulfurization (WFGD) scrubbing technology was the most popular choice for removing sulfur from boiler flue gases in large power plants and industrial facilities. WFGD technology has a low operating cost, because it utilizes low-cost limestone as the reagent and can produce gypsum for sale to wallboard manufacturers. However, on the downside, a WFGD system is expensive to build, uses the most water, occupies the largest amount of real estate, and can keep a full crew busy maintaining its large number of pumps, pipes, valves, and vessels. But more importantly, due to its chemistry and process, a WFGD system is only marginal for capturing metals, including mercury, or acid gases such as SO 3, HCl, or HF. Now with U.S. regulations requiring capture of mercury, acid gases, dioxins, and furans, in addition to SO 2 and particulates, other FGD technologies are becoming more popular due to their ability to capture this expanded set of pollutants. There are different types of technologies, ranging from simple injection of a sorbent into the boiler flue gas (direct sorbent injection) to the more established spray dryer absorber (SDA) technology (which sprays a fine dry mist of lime into the flue gas), to newer circulating fluidized bed (CFB) technology, which circulates the boiler ash and lime between an absorber reactor and fabric filter. These different FGD technologies have their pros and cons, but for many power and industrial facilities, CFB scrubbers are growing in popularity. This is most evident in the pipeline of U.S. retrofit scrubber projects, where more and more projects are selecting CFB technology. In the past, these alternative scrubbing technologies were typically chosen over wet FGD technology for their much lower capital cost and water usage, provided that the boiler size was not too large and the fuel

4 sulfur level was not too high. Today, CFB scrubber technology has broken through these limitations with single-unit designs up to 700 MWe backed by operating references on coal power plants of over 500 MWe and on fuels with sulfur levels above 4%. CFB scrubber technology has now stepped out in front of other technologies due to five key advantages: High multi-pollutant capture capability Low installed cost Low water use Low maintenance cost Wide fuel sulfur flexibility CFB scrubbers also offer other benefits, including compact footprint, and the flexibility to use low-quality lime and water. One key process advantage of a CFB scrubber, unlike SDA technology, is that the flue gas temperature does not limit the amount of lime injection. This feature allows a significant increase in acid gas scrubbing performance. The multi-pollutant CFB scrubber is a flexible and economical technology capable of removing a wide array of pollutants from flue gases of nearly any combustion or industrial process. As shown in Figure 1, boiler flue gas enters at the bottom of the CFB scrubber s up-flow absorber vessel. The gas mixes with hydrated lime and water injected into the absorber, as well as recirculated solids from the downstream fabric filter. The turbulator wall surface of the absorber causes high turbulent mixing of the flue gas, solids, and water to achieve high capture efficiency of the vapor phase acid gases and metals contained within the flue gas.

5 Figure 1. Advantages of circulating fluidized bed (CFB) scrubbers

6 Figure 2. Schematic of CFB lime hydration system

7 Major Components CFB Absorber Multiple flue gas venturies, shown in Figure 3, provide the required fluidizing gas dispersion and adequate suspension of the solids across the full diameter of the absorber vessel. The multi-venturi design allows a wide capacity range while minimizing scale-up risk. Water injection nozzles provide an atomized spray cloud of water droplets enhancing heat and mass transfer rates over the large surface area of solids churning some 75 feet within the confines of the vessel walls. Residence time for gases entering the tall and narrow reflux CFB absorber can be as high as five seconds providing excellent capture efficiency for multiple gas pollutants, while maintaining a small absorber footprint. The CFB absorber maintenance is minimal as the vessel is self-cleaning. Water spray nozzles can be replaced, if necessary, while the unit is on-line. The absorber is fabricated from carbon steel, avoiding expensive liners or alloy metals. Figure 3. Venturi flue gas inlet at bottom of absorber Fabric Filter A multi-compartment baghouse is located downstream of the absorber vessel for high efficiency capture and recirculation of the solid particles. In certain installations, such as the Basin Electric Dry Fork Unit 1 example described below, there are separate compartments, each lockable on the flue gas side for

8 maintenance purposes, making it possible to shut down one compartment for maintenance while running the remaining compartments with 100% boiler flue gas flow. Solids from the absorber entering the baghouse are completely dry, given the small amount of water added, and the long flue gas and solids residence time in the absorber. Low gas velocity and the bafflefree design result in pressure drops several times lower than conventional baghouses. Thus the baghouse itself is free of wetted solids and the housing remains very clean. The penthouse area is shown in Figure 4. The baghouse is equipped with a Pulse Jet cleaning system which delivers intermittent compressed air bursts to the separate compartments based on either baghouse differential pressure or flow rate. Optimized pulse pressure and frequency across filter sections ensures efficient ash collection, dust capture and long bag life. The baghouse hoppers serve as temporary storage bins for the large portion of the material that is fed into the solids recycling system. This is accomplished by means of a control valve via air-slides back into the CFB absorber. A small percentage of the scrubber by-product is continuously discharged from the insulated filter hoppers by means of a control valve and material transport system to the product silo for further utilization. Figure 4. Baghouse penthouse above pulse-jet fabric filters Dry Lime Hydration System Hydrated lime [Ca(OH) 2 ] used in the CFB scrubbing process can be purchased directly from suppliers. However, for high sulfur fuel applications requiring larger quantities of reagent, or in locations where hydrated lime suppliers are limited, owners can purchase less costly quicklime (CaO) and hydrate it on site.

9 A dry lime hydration system can be located near the CFB absorber vessel. As shown in Figure 2, lime and low pressure steam are injected into the hydration reactor for conversion to calcium hydroxide. Hydrated lime product from the hydrator is separated from the hydrator exhaust vapors in a downstream cyclone and then collected in an ash hopper. From the product hopper, the hydrated lime can be sent directly to the CFB absorber or to a hydrated lime storage silo. The dry lime hydration system does not require a dedicated fabric filter to handle the cyclone overflow as this stream is sent directly to the CFB scrubber. The hydration system is low maintenance with no rotating equipment except for a screw conveyor to meter lime to the hydrator. The hydration system has 25% turndown capability for following the boiler load. High Reliability by Design CFB scrubbing technology incorporates a number of built-in features to maximize reliability. The absorber vessel is a self-cleaning up-flow reactor. Water injection nozzles, located on the perimeter of the absorber above the introduction points for the recirculated and sorbent solids, provide an atomized spray cloud of water droplets. These nozzles must be removed periodically for replacement of wear components. However, the entire perimeter of the CFB absorber vessel is used to locate the water nozzles, thus additional nozzle locations are typically available to allow installation of a spare nozzle prior to removing an operating nozzle for inspection or maintenance. The multi-compartment baghouse lends itself to online replacement of filter bags with one compartment off-line. Separate compartments, such as those installed at Basin Electric Dry fork station, are each lockable on the flue gas side for maintenance purposes, making it possible to shut down one compartment for maintenance while running the remaining compartments with 100% boiler flue gas flow. The baghouse hoppers serve as temporary storage bins for the large portion of material that is fed into the solids-recycling system reducing equipment and improving overall system reliability and cost. The Largest CFB Scrubber in the World In June 2011, a 440-MWe rated coal power plant at Basin Electric s Dry Fork station (Figure 5) went online in Gillette, Wyoming. (Due to its 4,430-foot site elevation, the Dry Fork plant has a gas volumetric flow rate equivalent to a 520 MWe plant at sea level.) Behind its pulverized coal boiler sits the largest CFB scrubber operating in the world today.

10 Figure 5. The world s largest CFB scrubber is found at Basin Electric Dry Fork Unit 1. Courtesy: Basin Electric Co-Op and Wyoming Municipal Power Agency During the project planning phase, Basin Electric hired Sargent and Lundy to evaluate and recommend a FGD technology based on the criteria of achieving strict emission limits while delivering the best economics and reliability. After months of study and evaluation, Sargent and Lundy recommended the CFB scrubber technology ultimately selected by Basin Electric. Since it has gone online, the CFB scrubber has demonstrated a very high, 98% availability while meeting all the strict emission requirements set by the U.S. Environmental Protection Agency and the state of Wyoming. The emission regulations are designed to directly or indirectly limit a broad array of compounds designated as pollutants such as SO 2, SO 3, HCl, H 2 SO 4, HF, PM 10, PM 2.5, mercury, and other heavy metals. The CFB scrubber has exceeded its design performance, reducing SO x by 95% to 98%, to levels below 0.06 lb/mmbtu (50 to 60 mg/nm³). It also passed a 30-day mercury removal compliance test by meeting the permitted emission limit of 20 lb/twh (2.35µg/m3) while demonstrating a mercury removal rate of up to 70% without activated carbon injection. The CFB scrubber provided other key benefits to the Basin Electric Dry Fork project such as reducing the scrubber s water requirement by 30% and real estate by 80% compared to WFGD technology. In addition, the scrubber ash is being used to fill and stabilize a nearby open pit coal mine

11 CFB Scrubber Start-up and Commissioning Experience The first tests under full operation at the Basin Electric Dry Fork project were successfully demonstrated in June During startup, only two major CFB scrubber adjustments were made. One adjustment focused on optimizing the fluid dynamics, and the other adjustment addressed response time within the CFB absorber for the final boiler outlet conditions. Stable operation was maintained down to at a flue gas flow turndown of 33% of full flow, and the CFB scrubber system maintained emissions below all contracted and permitted levels, with more than 98% SO 2 removal, opacity <1%, less than 3 ppm PM emissions and up to 70% mercury removal (without injection of activated carbon). Flue gas properties and emission reduction levels are presented in Table 1. Table 1. Flue gas properties and emission reductions (Design Data) Units Inlet Outlet Typical Efficiency Flue Gas Flow ACFM 1,792,000 1,550,000 m 3 /h 3,045,000 2,630,000 Temperature ⁰F ⁰C SO 2 lb/mmbtu ~85-99% mg/m ppmv SO 3 mg/m ~99% ppmv HCl mg/m ~60-85% ppmv Dust lb/mmbtu ~99%+ mg/m 3 4,000-6, Conclusion The patented Foster Wheeler multi-pollutant CFB scrubber is a flexible and economical technology capable of removing a wide array of pollutants such as SO x, particulate matter, metals, acid gases and organic compounds from flue gases from almost any combustion and industrial process. It does all this while using the least amount of water, a vital resource. The CFB scrubber maintains advantages over other technologies in these five key areas: High multi-pollutant capture capability Low installed cost Low water use Low maintenance cost Wide fuel sulfur flexibility The technology has been applied widely in power plants, steel mills, refineries, waste-to-energy plants, combined heat and power plants, and plants in many other industries. It has been demonstrated over a range of flue gas flow rates from small industrial boilers to large coal power plants with capacities over 500 MW e.