OPERATING EXPERIENCE OF CIRCULATING FLUIDIZED BED SCRUBBING TECHNOLOGY IN UTILITY SIZE POWER PLANTS AND REFINERIES

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1 OPERATING EXPERIENCE OF CIRCULATING FLUIDIZED BED SCRUBBING TECHNOLOGY IN UTILITY SIZE POWER PLANTS AND REFINERIES AUTHORS Tobias Bönsel, Dr.Rolf Graf, Boguslaw Krzton ABSTRACT Tightening environmental regulations are lowering the requirements for emissions from power plant and industrial facilities around the world. Newer, stricter standards are being required by more governments for pollutants that are already being regulated SOx, NOx and particulate matter. In addition, metals, acid gases, and organic compounds are setting requirements for flue gas cleaning systems. USA s EPA Mercury and Air Toxics Standards (MATS) rule enacted in December 2011 and the upcoming IED requirements become completly in force by January 2016 in Europe. Due to the new requirements for flue gas acids like SOx,HCl, HF, dust and many other multi-pollutants, owners of coal fired,oil fired and waste fired power plants are continuously evaluating the merits of adding back-end air quality control systems (AQCS). This paper takes a special focus to the first operation experiences of Basin Dry Fork station project, which entered in commercial operation 2011, featuring best available dry flue gas scrubbing technology (BAT) for the first time worldwide in single unit size for a MWe coal fired boiler plant. It will be shown how the technology is able to meet the strict emission requirements and even more. Furthermore, experiences from an oil-fired refinery boiler in Europe taken into operation in Jan.2012 are highlighted. The dry technology of Circulating Fluidized Bed Scrubbing (CFBS) is 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, all CFBS system equipment can be installed in one building or outdoor. 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 technology provides high pollutant removal efficiencies up to 99% for SO 2, SO 3, HCl and HF. Further on the absorber/fabric filter arrangement is highly adaptable for sorbent injection for removal of heavy metals including mercury. INTRODUCTION - THE LARGEST DRY ABSORBER The 420 MW Dry Fork Station of Basin Eletric power cooperative (BEPC) near Gillette Wyoming was planned in 2006 and 2007 and main contractors were Mitsubishi for the turbine and steam cycle, Babcock/Wilcox for the boiler, coal handling and the DeNO x (SCR) and Graf-Wulff in cooperation with Nooter/Eriksen for the FGD and dust removal. Corporately with Sargent & Lundy (S&L) the FGD plant was designed and supervised until completion. During the planning phase, S&L together with BEPC have made a decision for a dry circulating fluidized bed (CFB) FGD-process. FGD (WFGD) the main advantages of this technology are up to 30% less water consumption, compared to a wet FGD (WFGD), the high removal efficiencies in particular SO 2, SO 3 and H 2 SO 4, the assured product utilization for landfill at the nearby opencast coal mine and significantly lower investment costs (up to 50%). Commercial operation of the FWGW Dry CFB began in June

2 As presented in Figure 1, the absorber vessel is a self cleaning CFB 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. SO 2 and SO 3 enter with the boiler flue gas, and the hydrated lime reagent is then introduced to the absorber above the entry point of the flue gas. The turbulator absorber wall surfaces provide high mixing and pollutant capture efficiency as reactants move to the top of the absorber. The gas is cooled by evaporation of a spray of low quality water injected into the absorber. The ratio of hydrated lime, recycled particulate, and, if necessary, fly ash solids to spray water is approximately 20:1 translating into extremely high surface area for conversion of SO 2 and SO 3 to calcium sulfate and calcium sulfite. The process does not require peripheral equipment such as rotary atomizers, spray spargers, or mist eliminators utilized in conventional dry or wet scrubbers. From the fabric filter clean flue gas is directed to the stack with almost no emissions left. Fig 1: CFB Scrubber Process Flow schematic, Dry Flow Station Major Components CFB Absorber The heart of the CFB scrubbing process is the CFB absorber. Hydrated lime sorbent and solids recirculated from the downstream fabric filter comprise an expanded bed of sorbent and particulate solids that is fluidized as flue gas is introduced through multiple venturis beneath the bed. The flue gas venturies, shown in Figure 2, 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 with no scale-up risk. Water injection nozzles, located on the perimeter of the absorber above the introduction points for the lime and recirculated solids, provide an atomized spray cloud of water droplets. Residence time for gases entering the tall and narrow CFB absorber is in excess of two up to about six seconds and the residence time for lime solids is between 2

3 ten and sixty seconds (without solids recirculation from the bag house) due to the high slip velocity. Both are providing improved one to the SO 2 removal efficiencies within a small absorber footprint. The CFB absorber maintenance costs are 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 in carbon steel avoiding costs for expensive liners or alloy metals. Fig. 2: Venturi Flue Gas Inlet at Bottom of Absorber Fabric Filter Multi-compartment fabric filter baghouses are located downstream from the absorber vessel to allow recirculation of particulate solids. The separate compartments are each lockable on the flue gas side for maintenance purposes. It is 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 baffle-free design result in pressure drops several times lower than conventional baghouses. Thus the baghouse itself is free of wetted solids and the housing is very clean. The penthouse area is shown in Figure 3. 3

4 Fig. 3: Baghouse penthouse above pulse-jet fabric filters The baghouse is equipped with a Pulse Jet type cleaning system with differential pressure and flow rate controlled online cleaning, using intermittent compressed air bursts. 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. 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. Lime hydration units are located near the CFB absorber vessel. As shown in Figure 4, lime and water 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 directed directly to the CFB absorber or to a hydrated lime storage silo. 4

5 Fig. 4: Dry Lime Hydration System Schematic The dry lime hydration system does not require a dedicated fabric filter to handle the cyclone overflow as this stream is directed 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. SINGLE ABSORBER CFB DRY SCRUBBING FOR A COOL FIRED 420MW BOILER The Basin Dry Fork FGD system is world s largest single absorber vessel CFB scrubbing system. This CFB scrubber is installed on a nominal 420 MW PRB-fired boiler located at elevation of 4,250 feet, corresponding to an equivalent volumetric flue gas flow for a 500 MW boiler at sea level. This represents a 40% scale-up from the previous largest CFB scrubber unit ever built. This CFB scrubber design can be extended to accommodate single absorber installation to 750 MW. Design, Installation and Commissioning Shortly after the contract was awarded in March 2007 the system basic design has been prepared by Graf-Wulff. 5

6 The major components for the Basin Dry Fork CFB scrubbing system are the absorber vessel, a 6- compartment pulse-jet fabric filter, two lime hydration units, and storage silos for pebble lime, hydrated lime, and product ash. The total system is very compact with a footprint requirement only about 20% - 30% of that for a comparable WFGD and 70% - 80% of that for a comparable SDA. While the fabric filters must be elevated for recirculation of particulates and reagents, the structure required to do this provides space at grade and middle elevations sufficient for installing and maintaining the auxiliary equipment. Nearly all of the components are contained in a single building (Figure 5) with a footprint of about 60mx30m. Prefabrication of the absorber and baghouse components began in 2008 and system erection commenced in February, Erection was completed in December And the plant was commissioned particularly with all components until May Fig. 5: CFB Scrubbing System Building Shown at Left The first tests under full operation 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 through a flue gas flow turndown to 33% of full flow, and the CFB scrubber system exceeded all contracted and permitted emission levels with more than 98% SO 2 removal, opacity <1%, less than 3 ppm PM emissions and up to 70% native mercury removal (without injection of activated carbon). Flue gas properties and emission reduction levels are presented in Table 1. 6

7 Units Inlet Design Outlet Design Outlet Measured Flue Gas Flow m 3 /h 3,045,000 2,630, Temperature ⁰C SO 2 mg/m SO 3 mg/m HCl mg/m Dust mg/m 3 4,000-6, Hg removal % * Reliability % Opacity % *without AC Injection Table 1: Flue gas properties and emission reductions (Design Data) FGD Product utilisation During the whole one year operation period the FGD-Product has been utilized in the nearby coal mine for recultivation. This has been done by mixing a fraction of FGD-Product with fraction of fly ash and water as follows: The fly ash is continuously introduced with the flue gas from the Boiler in to the CFB-Scrubber and there homogeneous mixed. The mixing of approximate 60-70% fly ash and 30-40% FGD-Product takes place within the CFB-Scrubber with simultaneous desulfurization. The mixed dry Product is collected in the hoppers of the baghouse and pneumatically transported into the Product Silo. The outlet of the Product Silo is equipped with a screw-mixer and approximate 30-35% water of low quality is mixed into the FGD-product to start the stabilization of the final product. The final product is loaded into an open truck and taken for stabilized landfill in the close by area of an open pit coal mine (see Fig. 6). The approximate product fractions are: 20-30% FGD-Product 40-50% fly ash 30-35% water After a few days (3-7 days) at the open mine this product is stabilized to a lean concrete with fixation of any species due to a low permeability. 7

8 Fig. 6: Loading of FGD-Product to be used for stabilized landfill in the open pit coal mine CFB SCRUBBERS FOR PETROLEUM REFINERIES Petroleum refineries worldwide are being pressured with tightening environmental regulations and increasing sulfur content of the crude oils they process. Refineries are processing heavier and higher sulfur crude oils in refinery residues used for steam and power production. In Europe, sulfur emission limits for boilers in refineries have been slashed to 200 mg/nm³ to include both SO 2 and SO 3. Flue gases from refinery power and steam boilers firing residues such as heavy fuel oil, liquid residues and tail gases from vacuum units, solvent deasphalting and hydrotreating units have historically relied on wet FGD scrubbers for sulfur removal. Residues can exceed 6% sulfur. However, wet FGD technology is struggling to meet emissions with high sulfur fuels due to limited capability to capture SO 3. Elevated SO 3 levels have resulted in high levels of corrosion and unsightly acid plumes from stacks. Both of these problems are difficult and expensive to fix as they require costly material changes, installation of non-metallic liners and/or retrofit of a wet ESP. 8

9 Historically, wet FGD scrubbers have been the preferred choice for cleaning the flue gas from power and steam boilers within oil refineries due to their high capture ability of sulfur dioxide (SO 2 ) over a wide range of fuel sulfur levels typically found in refinery boiler fuels (can be as high as 6%), such as heavy fuel oil, liquid residues and tail gases from vacuum units, solvent deasphalting units, hydrotreaters. Until now, dry scrubbers were unable to handle fuels with sulfur levels above 2% and were seldom chosen for refinery applications. After several months of study and evaluation, in 2009 a large refinery in western Germany selected CFB scrubbing technology for two 175 MWth boilers firing a mix of heavy fuel oil residue, methanolwater mixture, and refinery gas with up to 5% sulfur. The principle process arrangements for the FWGW Dry FGD Technology for two refinery units are presented in the schematic figure 7. Fig.7: Refinery in West Germany Block diagram 2-line plant concept Design flue gas data and emission levels are presented in Table 2. 9

10 Parameters Boiler and FGD Configuration 1 Heavy Fuel Oil Fired Boiler + 1 SCR train+ 1 FGD train Firing-capacity 175 (MWth) Fuel Sulfur Content 1 5 (% daf) FGD-scrubber type CFB Scrubber with single venturi Particulate-Collector Baghouse with 2 compartments - online cleaning Air to cloth ratio: 0,8 1,0 m/min FGD Reagent Hydrated lime produced from lime in a Foster Wheeler/ Graf Wulff lime hydrator CFB Scrubber Design Reliability 98 % within 12 months FGD Inlet FGD Outlet Flue gas flow (Nm³/h) Flue gas temperature ( C) SO2-content, 6% O 2 (mg/nm³) SO3-content, 6% O 2 (mg/nm³) Particulate content 6% O 2 (mg/nm³) Table 2: Refinery Boiler and FGS Design Data for one unit Operation and Performance In May 2012 the FWGW scrubbing unit for one of the two boilers successfully began commercial operation. During the commercial operation the FGD plant has demonstrated high reliability and achieved all emission levels below those specified while meeting all consumptions values such as power, lime and water. The second FWGW scrubbing unit is ready for start up. After completion of the second boiler this unit is going to operate as the first FWGW scrubbing unit. The final installation of both units is shown in figure 8. 10

11 Fig. 8: CFB Scrubber Building (center) with Silos Conclusion CFB dry scrubbing affords a number of distinct advantages over conventional dry and wet FGD technologies. CFB scrubbers offer a Best-in-Class multi-pollutant scrubbing technology combining low operating and maintenance costs. As shown in Table 3, this highly flexible multi-pollutant control technology is well suited for the power, heat, transportation fuel, chemicals, steel, waste-to energy and other industries subject to increasing global regulatory pressures. 11

12 Table 3: Wet FGD, SDA, and CFB System Capabilities Comparison 12