The Value Proposition of Circulating Fluidized Bed Scrubbing Technology. Robert Giglio Foster Wheeler Global Power Group

Transcription

1 The Value Proposition of Circulating Fluidized Bed Scrubbing Technology Robert Giglio Foster Wheeler Global Power Group Written for Reinhold APC Conference St. Louis, Missouri July 9-10, 2013

2 A New Direction for Environmental Regulations Emission limits for conventional pollutants like SOx, NOx and particulate matter (PM) continue to tighten for power plants and industrial facilities around the world. Europe s new IED (Industrial Emission Directive) has tightened both SOx and NOx emission limits by 50 mg/nm3 and particulate limits by 10 mg/mn3 compared to its prior LCPD (large combustion plant directive). Last year, China lowered its SOx, NOx and particulates limits for power plants to levels even less than those in Europe s IED. In the US, the dismissed CSAPR (Cross State Air Pollution Rule) has triggered a country wide debate on how low states should go with their SOx, NOx, and particulate emission limits, causing a concern of power reliability from fear of excessive coal plants retirements. But now a new trend is on the horizon with regulations expanding to new compounds never before regulated. The US is leading this trend by including metals, acid gases and organic compounds to the list of pollutants that must be controlled under the new MATS rules (Mercury and Air Toxin Standard). Europe is not too far behind with its IED requiring a BAT (best available technology) standard for these additional compounds as well. These compounds have always been regulated for certain waste fuel applications like waste-to-energy plants and incinerators, but now regulators are applying them to all boilers; right up to large utility coal boilers. The trend is clear: Environmental concern will continue to drive stricter limits for an expanding range of pollutants. But taking a step back, products and commodities like power, heat, transportation fuels, chemicals, steel and many others are vital for both economic growth and improving our standard of living. So cleaning flue gases from boilers in these plants in the most economical and effective way is very important and necessary for both our quality of life and our environment. A Better Way to Clean Flue Gases In the past, due to its proven ability to capture a high level of SO 2 over a wide range of fuel sulfur levels in large power plants, wet flue gas desulfurization (WFGD) scrubbing technology has been the most popular choice for removing sulfur from boiler flue gases in 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, WFGD s don t do a good job capturing metals, including mercury, or acid gases such as SO 3, HCl, or HF. Now with regulations expanding toward the capture of additional pollutants coupled with growing restrictions on water use for power plants, other FGD technologies are becoming more popular. These technologies range from the simple injection of a sorbent into the boiler flue gas (called direct sorbent injection or DSI), to the more established SDA (spray dryer absorbers) technology, which 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. 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 sulfur level was not too high. Today, Foster Wheeler s (FW s) 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

3 MWe and on fuels with sulfur levels above 4%. FW s 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 FW CFB scrubbers also offer other benefits, including compact footprint, and the flexibility to use lowquality lime and water. One key process advantage of a FW 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. FW s 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 multiple venturies (shown in Figure 2) and 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 and wide operational turn-down flexibility while minimizing scaleup risk. Water injection nozzles provide an atomized spray cloud of water droplets over the large surface area of solids churning some 23 meters within the CFB absorber to cool the flue gas and reactivate the boiler ash to improve its inherent ability to capture pollutants. 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. Figure 2. CFB Absorber Venturies A high efficiency baghouse captures not only the solid particles but also the vapor phase metals and acid gases, as well as ammonia slip from upstream SCR systems if present in the boiler. 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 solids that are returned to the CFB absorber via reliable fluidized ash slide technology. A material transport system continuously withdraws and stores a small portion of the circulating scrubber solids in a by-product silo for use in land restoration, road construction or disposal.

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

5 Optional 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 fuels requiring larger quantities of lime or in locations where hydrated lime suppliers are limited, plant owners can purchase less costly quicklime (CaO) and hydrate it on site with Foster Wheeler s dry lime hydration system. The system is located near the CFB absorber vessel to avoid the need for a dedicated fabric filter for the cyclone exhaust of the hydrator since this stream can be sent directly to the CFB scrubber absorber. As shown in Figure 3, lime and low pressure steam or water are injected into the hydration reactor for conversion to calcium hydroxide. Hydrated lime from the hydrator is separated from the hydrator exhaust vapors in a downstream cyclone and then collected in a hydrated lime hopper. The hydrated lime can then be sent directly to the CFB absorber or to a hydrated lime storage silo. The hydration system is operationally flexible with the capability of following the boiler down to a 25% load and is reliable with low maintenance since it has no rotating equipment except for a screw conveyor to meter lime to the hydrator. Figure 3. Schematic of CFB lime hydration system High Reliability by Design Foster Wheeler s CFB scrubbing technology incorporates a number of built-in features to maximize system reliability. The solids circulation within the CFB absorber vessel provides a self-cleaning function preventing the build-up of deposits on the CFB absorber walls. Further, spare water injection nozzles located on the perimeter of the absorber can be placed in service while others are taken out of service for inspection, cleaning and replacement without bringing the scrubber down for service.

6 The multi-compartment baghouse lends itself to online replacement of filter bags with one compartment off-line. Separate compartments 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 built-in baghouse hoppers which serve as temporary storage bins for the recycled solids reduce equipment cost and improving overall system reliability. Air slides containing no moving mechanical components reliably recycle the solids back to the CFB absorber vessel. The Largest CFB Scrubber in the World In June 2011, a large coal power plant at Basin Electric s Dry Fork station (Figure 4) went online in Gillette, Wyoming. At sea level, the plant would produce about 520 MWe of electric power based on its 3 million cubic meters per hour of boiler flue gas flow, but due to the nearly 1400 meter elevation of the Basin site, the actual power plant rating is much less at 440 MWe. Behind its pulverized coal boiler sits the largest CFB scrubber operating in the world today. Figure 4. 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. Table 2 qualitatively shows results of the Sargent and Lundy evaluation.

7 Table 2. Qualitative Results from S&L Study CFB Scrubber Start-up and Commissioning Experience at Dry Forks Since it went on line, the Foster Wheeler CFB scrubber has demonstrated close to 100% availability while meeting all its consumption and emission guarantees. Stable operation was maintained down to at a flue gas flow turndown of 33% of full flow while meeting all performance goals. In the first half of October 2011, performance tests showed that electricity, water and lime, consumptions were 10-20% below their guarantee values. The emission regulations set by the U.S. Environmental Protection Agency and the state of Wyoming 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. As shown in Table 3, the CFB scrubber has exceeded its design performance, reducing SO x by 95% to 98%, to levels below 15 to 50 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. Further testing showed total mercury removal rates above 95% when using activated carbon. Flue Gas Data Inlet Design Outlet Design Outlet Measured Flue Gas Flow* [m³/h] 3,045,000 2,630,000 2,700,000 SO 2 [mg/m³] 800-2, SO 3 [mg/m³] *** HCI [mg/m³] *** Dust [mg/m³] 4,000 6, Temperature [ C] Hg-removal rate µg/m3, % removal µg/m %** * % of flue gas flow without recirculation * % of flue gas flow with recirculation ** without AC injection *** below detection limit

8 Table 3. Design vs. measured performance of FW CFB scrubber at Dry Forks plant In addition to being about 30% less costly than a WFGD scrubber, 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. Further, the scrubber by-product is being used to fill and stabilize the nearby open pit coal mine from where the coal is mined for the power plant. 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 and capital. 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 With the operational success of the FW CFB scrubber at the Basin Dry Folk station in Gillette, Wyoming, single train CFB scrubbers for large scale power plants have been proven at the 500 MWe power plant scale. Single train designs up to 700 MWe are available and can be offered with low scale up risk due to the technologies modular CFB absorber venturi and compartmental baghouse design concept. 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.