Fluid Bed Scrubbing TECHNOLOGY

Transcription

1 CIRCULATING Fluid Bed Scrubbing TECHNOLOGY Circulating fluid bed scrubbing technology is a flexible multi-pollutant technology quickly gaining recognition. >> BY BOB GIGLIO, VICE PRESIDENT OF STRATEGIC BUSINESS DEVELOPMENT, FOSTER WHEELER GLOBAL POWER GROUP Emission limits for conventional pollutants, notably SO X, NO X and particulate matter, continue to tighten for power plants and industrial facilities around the world. The CSAPR (Cross State Air Pollution Rule), while vacated by the courts in August 2012, will likely be re-proposed and adopted. CSAPR will likely mandate further 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 versus shutting down units otherwise out of compliance. This has triggered a countrywide debate on how low states should regulate their SO X, NO X and particulate emission limits, causing a concern of power reliability from fear of excessive coal plants retirements. For power plants, Europe s new IED (Industrial Emission Directive) has tightened SO X and NO X emission limits by 50 mg/nm 3 and particulate limits by 10 mg/nm 3 compared to its prior LCPD (large combustion plant directive). Last year, China lowered its SO X, NO X and particulates limits for power plants to levels even less than those in Europe s IED. A new trend is on the horizon, with regulation expanding to compounds previously unregulated. The United States is leading this trend by including metals, acid gases and organic compounds to the list of pollutants that must be controlled under their new Mercury and Air Toxics Standards rules. Europe is not far behind with its IED requiring best available technology standards for the additional compounds as well. The compounds have always been regulated for certain waste fuel applications, such as waste-to-energy plants and incinerators, but now regulators are applying them to all boilers, including large utility coal boilers. The trend is clear with environmental concerns continuing to drive stricter limits for an expanding range of pollutants. To address these concerns, cleaning flue gases from boilers in plants producing vital commodities and products including power, heat, transportation 24 Pollution Engineering OCTOBER 2013

2 fuels, chemicals, steel and many others is the most economical and effective solution. SYSTEM CAPABILITIES Wet FGD SDA FGD CFB FGD A Better Way to Clean Flue Gases In the past, due to its proven large scale and high-capture ability of sulfur dioxide (SO 2 ) over a wide range of fuel sulfur levels, wet flue gas desulfurization (WFGD) scrubbing technology has been the more popular choice for removing sulfur from boiler flue gases in large power plants and industrial facilities. WFGD technology has a low-operating cost as it utilizes low-cost limestone as the reagent and can produce gypsum for sale to wallboard manufacturers. However, on the downside, WFGDs are expensive to build, use the most water, occupy the largest amount of real estate and can keep a full crew busy maintaining its large number of pumps, pipes, valves and vessels. More importantly, due to its chemistry and process, WFGDs are 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 Hg, 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 a simple injection of a sorbent into the boiler flue gas (direct sorbent injection or DSI), to the more established spray dryer absorber (SDA) technology that sprays a fine dry mist of lime into the flue gas, to newer circulating fluidized bed technology (CFB), 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 mid- to large-scale power and industrial facilities, CFB scrubbers are growing in popularity. This is evident given the pipeline of U.S. retrofit scrubber projects where more projects are selecting CFB technology over others. In the past, these alternative scrubbing technologies were typically chosen over wet FGD technology for their lower capital cost and water usage, provided the boiler size was not too large and the fuel sulfur level was not too high. Today, CFB scrubber technology has broken through these limitations with single unit designs up to 700 megawatt electrical (MW e ), backed by operating references on coal power plants of over 500 MW e and on fuels with sulfur levels above 4 percent. 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; Large scale; and Wide fuel sulfur flexibility. SO 2 Capture to Meet Low Permit Limits* Low Water Consumption* Fuel Flexibility (Fuel Sulfur Variability)* Fine Particulate Capture* High SO 3 Capture* Compact System Footprint* Minimal Maintenance Requirements* Mercury Capture CO 2 Emissions Waste Water Treatment Use of Low Quality Water Use of Limestone Reagent Large Scale Single Unit Size (>350 MWe) Necessary for Retrofit ESP Improvements Stack Improvements Flue Gas Reheater OVERALL Advantage Neutral Disadvantage Table 1: Wet FGD, SDA, and CFB system capabilities comparison chart. As summarized in Table 1, CFB scrubbers also offer other benefits, including low maintenance cost, compact footprint and the flexibility to use low-quality lime and water. One key process advantage of a CFB scrubber is that, unlike SDA technology, the amount of lime injection is not limited by the flue gas temperature, allowing significantly greater acid gas scrubbing performance. CFB Scrubbing Technology Proven for Multiple Applications Circulating fluidized bed (CFB) scrubbing technology provides a viable pathway for addressing multi-pollutant control in a cost effective manner. The technology utilizes advanced CFB scrubber technology, which efficiently and economically captures a wide array of pollutants, including SOX, PM, acid gases, and organic compounds while using the least amount of water a vital resource. The multi-pollutant CFB scrubber is a flexible and economical technology capable of removing an array of pollutants from flue gases from 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 OCTOBER

3 Absorber Self-cleaning CFB process minimizes maintenance Carbon steel design avoids expensive liners and alloy metals Multiple venturi design allows wide range of capacities with no scale-up risk Turbulator wall surface ensures high mixing and capture efficiency of multiple pollutants Plug-free water spray nozzles can be replaced while unit is on-line Long gas and solid mixing time for high pollutant capture and maximum lime utilization Flexible design allows unit operation even when one water nozzle is out of service Fabric Filter Low gas velocity and minimum baffles result in gas pressure drop several times lower than conventional baghouse Pulse frequency across filter sectors allows efficient solid recirculation, dust capture and long bag life Optimized low pulse pressure for effective solid removal and long bag life Flexible design allows unit operation even with one compartment is out of service Carbon steel materials of construction avoids expensive liners and alloy metals Can utilize bags from many qualified suppliers Clean Gas to the Stack Guiding and Distribution Plate Water Shut Down Valve Rotary Valve Air Baghouse Hopper Hydrated Lime Air Process Control Valve Solids Ash storage is integrated into baghouse, avoiding additional solid handling equipment and for maintaining buffer during load changes Flue Gas and Fly Ash Absorber Bottom Flue gas inlet chamber drops out large particles before entering scrubber Automated drains for easy maintenance Rotary Valve Solid Recirculation Reliable non-mechanical air slide proven by years of use in the power industry Control valve for precise solids flow into the absorber Fast acting shut down valve to allow purge of absorber solids during a boiler trip Figure 1: CFB scrubber process flow schematic (courtesy Foster Wheeler). 26 Pollution Engineering OCTOBER 2013

4 Figure 2: Basin Electric Dry Fork Unit 1 (Courtesy Basin Electric Co-Op and Wyoming Municipal Power Agency) The trend is clear with environmental concerns continuing to drive stricter limits for an expanding range of pollutants. 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 a high-capture efficiency of the vapor phase acid gases and metals contained within the flue gas. High Reliability by Design CFB scrubbing technology incorporates a number of built-in features to maximize reliability. 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 nozzle absorbers 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 and additional nozzle locations. It is typically available to allow installation of a spare nozzle prior to removing an operating nozzle for inspection or maintenance. Operators can thus switch to a spare nozzle on the fly without shutting down the unit. The multi-compartment baghouse lends itself to online replacement of filter bags with one compartment off-line. Multi-compartment fabric filter baghouses are located above the absorber vessel to allow recirculation of particulate solids. Separate compartments are each lockable on the flue gas side for maintenance purposes, thus it is possible to shut down one compartment for maintenance while running the remaining compartments with 100 percent boiler flue gas flow. The baghouse hoppers serve as temporary storage bins for the large portion of the 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 520 MW e coal power plant at Basin Electric s Dry Fork station (Figure 2) went online in Gillette, Wyo. Due to its 4,430 foot site elevation, the actual net plant output of the Dry Fork power plant is rated at 420 MWe. Behind its pulverized-coal boiler sits the largest CFB scrubber operating in the world today. OCTOBER

5 During the project planning phase, Basin Electric hired Sargent & Lundy LLC as consulting engineers to evaluate and recommend the FGD technology based on the criteria of achieving the strict emission limits while delivering the best economics and reliability. After months of study and evaluation, the consultants recommended CFB scrubber technology ultimately selected. Since the system went online, the CFB scrubber has provided 98 percent availability while meeting all strict emission requirements set by the EPA 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, Hg and other heavy metals. The CFB scrubber has exceeded its design performance reducing SO X by 95 to 98 percent to levels below 0.06 lb/ MMBtu (50 to 60mg/Nm 3 ). It also passed a 30 day Hg removal compliance test by meeting the permitted emission limit of 20 lb/twh (2.35µg/ m 3 ) while demonstrating a Hg removal rate in excess of 95 percent. The CFB scrubber provided other key benefits for the project, including reducing the scrubber s water requirement by 30 percent and real estate by 80 percent compared to WFGD technology. In addition, the scrubber ash is being used to fill and stabilize a nearby open pit coal mine. Conclusion Multi-pollutant CFB scrubbing is a flexible and economical technology capable of removing an array of pollutants such as SO X, particulate matter, 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. The technology has been applied widely in power plants; steel mills; refineries; waste-toenergy 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. Bob Giglio is vice president of strategic business development for Foster Wheeler Global Power Group. Send questions to robert_giglio@fwc.com. 28 Pollution Engineering OCTOBER 2013