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Demonstration of hot gas filtration in advanced coal gasification system
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Powder Technology 180 (2008) 122 – 128 www.elsevier.com/locate/powtec
Demonstration of hot gas filtration in advanced coal gasification system Xiaofeng Guan a,⁎, Ben Gardner a , Ruth Ann Martin a , Jack Spain b a
Power Systems Development Facility, Southern Company Services 31800 Highway 25 North, P.O. Box 1069 Wilsonville, Alabama, 35186, USA b Southern Research Institute 757 Tom Martin Drive Birmingham, Alabama, 35211, USA Received 3 February 2006; accepted 1 March 2007 Available online 12 March 2007
Abstract The Power Systems Development Facility (PSDF) is an engineering scale demonstration of advanced coal–fired power systems and high– temperature, high pressure gas filtration systems. The PSDF was designed at sufficient scale so that advanced power systems and components can be tested in an integrated fashion to provide data for commercial scale–up. The PSDF is funded by the U.S. Department of Energy (DOE), the Electric Power Research Institute (EPRI), Southern Company, Kellogg Brown and Root, Inc. (KBR), Siemens Westinghouse Power Corporation (SWPC), Peabody Energy, Burlington Northern and Santa Fe Railroad, and the Lignite Energy Council. Coal gasification at the PSDF is achieved with the KBR Transport Gasifier, and a Siemens Westinghouse particulate control device (PCD) is used for filtration of gasification ash from synthesis gas (syngas). As a critical process in the gasification system, hot gas filtration in the PCD removes the particulate so that the syngas can be utilized in a downstream gas turbine or a fuel cell. Testing at the PSDF has provided valuable experience with and enhanced understanding of PCD equipment and operation, filter element and failsafe material properties, and gasification ash characteristics. This testing and demonstration of the PCD system have significantly increased readiness towards commercialization of the hot gas filtration technology. This paper summarizes the testing and demonstration of the hot gas filtration system at the PSDF and addresses issues associated with the commercialization of this technology. © 2007 Elsevier B.V. All rights reserved. Keywords: Synthesis gas filtration; Filter element testing; Failsafe testing; Particulate collection efficiency; PSDF PCD system
1. Introduction The PSDF is a joint project of the U.S. DOE, Southern Company, and other industrial participants. As a premier advanced coal power generation research and development facility, the PSDF has played a key role in demonstrating and developing critical components of advanced coal–fired power systems in an engineering scale to provide data for commercial scale–up. The demonstration has been focused on a Transport Reactor–based clean coal power generation system. As a major component, a high temperature and high pressure PCD is used to remove particles from the gas stream before the process gas is fed to the downstream system such as turbine, fuel cell, or gas cleanup process. The Transport Reactor and the PCD were designed to operate in both combustion and gasification modes.
⁎ Corresponding author. E-mail address: [email protected] (X. Guan). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.03.005
The system was commissioned in 1996 followed by three years of demonstration in combustion mode with 5000 on–coal hours. In 1999, the Transport Reactor was converted to operate as a gasifier. By May 2005, the system has accumulated 6400 on–coal hours in gasification operation. Over the years, significant effort has been made at the PSDF to improve the system reliability and lower capital cost. The data collected and experience gained have greatly increased the readiness of the commercialization of the Transport Reactor–based coal gasification technology. Hot gas filtration, as the initial focus of the project, posed difficulties in the first few years primarily due to filter element failures under developmental process conditions. The PCD operation in combustion is summarized in a previous paper [1]. After considerable process modifications were made and better knowledge was gained on filter element material performance under these severe operating conditions, the failure rate has been significantly reduced in normal operation. Since the process was transitioned to gasification, the emphasis of the PCD
X. Guan et al. / Powder Technology 180 (2008) 122–128
test program has been on improving particulate collection efficiency, long–term filter element material testing, and failsafe testing. Characterization of the particulate matter and filter cake formation has also been performed to aid in evaluating PCD performance and provide a basis for commercial design [2]. These efforts have led to significant improvement in overall PCD reliability, operability and maintainability. Further information about PCD operations and filter element testing in recent years can be found in References [3] and [4]. 2. PCD system description The current Transport Reactor–based gasification process at the PSDF is shown in Fig. 1. A detailed description of the process and test run reports can be found on the PSDF website [3]. The PCD in the Transport Reactor train is a Siemens Westinghouse–designed filtration system (see Fig. 2). It has two tiers of plenums that combined can hold up to 91 barrier type filter elements. A tangential inlet and a cylindrical shroud inside the vessel distribute the particulate laden synthesis gas (syngas) in a swirling pattern to the filter elements inside the shroud. Gasification ash particles deposit on the outside surfaces of the filter elements and clean syngas passes through the plenums and exits the PCD from the top of the vessel. The filter elements are periodically cleaned online with back–pulses using nitrogen to
123
remove gasification ash from the surfaces. The gasification ash is then collected at the PCD cone and conveyed to an ash removal system below the PCD. The filter elements are “candle” type with one end capped and the other end flanged. They are individually installed onto the plenum tubesheets using holders. Support bars are installed at the bottom to connect all the filter elements to prevent damage due to vibration. The vibration behavior of the flexible PCD internals is discussed in another paper [5]. Above each filter element, there is a failsafe designed to prevent particulate leaking in the case of a filter element failure. If the filter element breaks, cracks, or leaks, the failsafe will quickly plug (or close) to effectively protect the downstream equipment from being damaged by particles in the gas stream. Back–pulse cleaning is achieved with a quick release of high pressure nitrogen from an accumulator tank for each plenum. Müller co–ax valves and Venturi devices are used to inject nitrogen into the plenums. The reversed flows and the pressure rises in the plenums dislodge the particle deposit on the filter element surfaces. The back–pulsing is initiated by a timer or predetermined differential pressure (DP) limit across the tubesheet. The work on the back–pulse valve testing and modification is described in the run reports (1997I and TC02) [3]. Particulate concentrations at the inlet and outlet of the PCD are monitored by Southern Research Institute (SRI) using batch
Fig. 1. PSDF gasification process diagram.
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Fig. 2. Schematic of the PCD.
sampling systems. Several types of online particulate detection instruments have also been tested to continuously monitor the particulate concentration in the outlet gas stream [3]. Since 1999, there have been 16 test runs in gasification, with each run typically lasting 200 to 1000 hours. A variety of coals have been tested under different operating conditions, though Powder River Basin coal has been the most extensively tested. Typical PCD operating conditions and syngas composition are listed in Tables 1 and 2, respectively. 3. PCD test program update Testing and demonstration of the PCD system and its components in the past several years are summarized below. More detailed information is published on the PSDF website [3]. 3.1. Filter element testing More than twenty types of filter elements have been tested at the PSDF [4]. In combustion runs, commercially available monolithic oxide and silicon carbide ceramic elements were the primary focus at that time because of the high temperature Table 1 Typical PCD operating conditions in gasification Temperature, °F (°C) Pressure, psig (kPa) Face Velocity, ft/min (cm/s) Baseline DP, inH2O (kPa) DP Rise Rate, inH2O/min (kPa/s) Back–Pulse Timer, min Back–Pulse Duration, s Back–Pulse Pressure, psi (kPa) Inlet Particulate Loading, ppmw Outlet Particulate Loading, ppmw Particle Size, micron MMD
750−950 (400−510) 140−240 (965−1655) 3.5−5 (1.8−2.5) 80 (19.9) 10 (0.042) 5−20 0.2 250−400 (1724−2758) above system pressure 13000−20000 Below detection limit of 0.1 ppmw 14
environment (1400 °F or 760 °C). Several developmental types of ceramic fiber composite elements and Pall iron aluminide (FEAL) metal elements were also tested. Unstable process conditions in these early runs and inadequate material properties and fabrication problems with some types of elements led to unacceptable filter element failure rates. The major failure modes included breaking due to thermal shock, membrane spalling, and leaking through media. However, FEAL filter elements showed promising results without failure in 2780 hours of exposure. Testing of several types of composite elements was continued in the first gasification run. However, the majority of the elements failed in the reducing environment under unstable operating conditions. Many manufacturers terminated R and D programs for the composite elements in this application. As a result, the PSDF stopped testing elements in this category. The lower PCD operating temperature (750 °F or 400 °C) in gasification runs allowed the introduction of more metal elements. A major advantage of the metal materials is higher durability than ceramic materials, primarily due to their higher thermal shock resistance and higher ductility. In the first gasification run, several types of sintered metal powder and sintered metal fiber elements were installed. However, many metal fiber elements were screened out after the first run, not due to poor material performance, but due to their high flow Table 2 Typical syngas composition
H2, % CO, % CO2, % CH4, % H2O, % N2, % H2S, ppm NH3, ppm
Air Blown
Oxygen Blown
2−8 1−14 7−14 1−2 6−14 54−72 100−1300 500−2400
8−16 4−14 12−14 2−5 17−40 30−55 250−1600 2000−3000
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resistance caused by their structures. A coarse protection layer outside of the media tended to retain a thick gasification ash cake, which could not be removed by back–pulsing. In the following two runs, monolithic ceramic elements and later a mixture of ceramic and FEAL metal elements were installed. After a ceramic element failed during an off–coal period, the decision was made to stop testing the ceramic elements and focus on testing metal elements. The primary concern was the potential thermal shock due to system upsets during the early stages of development of the gasification process. Since 2000, the FEAL elements have been extensively tested in gasification operation. About 5780 exposure hours have been achieved for some longest exposed individual elements. The FEAL elements showed excellent particulate collection efficiency. In stable operations with only FEAL elements, the PCD outlet particulate concentration was less than 0.1 ppmw, which is the current SRI particulate sampling system lower detection limit. Testing in a cold flow unit at the PSDF has shown that the FEAL elements are among the elements that have the highest collection efficiencies. In general, the FEAL elements show promising results. However, there are concerns about their structural integrity and long–term corrosion resistance. The FEAL elements are constructed with multi–sections of sintered metal powder media, which are welded to solid metal parts. There have been several element failures when cracks occurred at the welds or in the heat affected zone, indicating a need to improve the weld structure. FEAL elements are made of iron aluminide material. Like other metal materials, iron aluminide is subject to corrosion under certain conditions. The extended exposure testing at PSDF showed that corrosion products do form over time. The severity of the corrosion increases with the exposure time, as shown in Fig. 3. Corrosion of FEAL elements has also been reported elsewhere [6]. In some cases, the corrosion products have partially plugged the pores, causing increased flow resistance with extended exposure time. The filter DP measured at
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Fig. 4. Strength of FEAL elements.
room temperature after washing is also noted in Fig. 3 for each element shown. The rising trend of DP is evident. Metallurgical analysis has been initiated to identify the mechanism. Despite the presence of corrosion, the mechanical strength of the media has not decreased in over 4000 hours of exposure. Fig. 4 shows strength of the FEAL element tested at room temperature. It should be noted that the testing at the PSDF is not continuous as in a commercial plant. The multiple startups and shutdowns may have exacerbated the corrosion condition as the thermal, mechanical, and chemical cycling may cause extra damage for the FEAL material. For use in a commercial plant, the elements must last for at least 8000 exposure hours without significant loss of strength or increase in flow resistance. The testing of the FEAL elements at PSDF will continue to evaluate their long–term performance. Testing of other types of metal elements was also conducted at the PSDF both in the PCD and the cold flow unit. Two types with significant exposure were Pall Hastelloy (sintered metal powder) and Pall/Fluid Dynamics HR– 160 (sintered metal fiber)
Fig. 3. FEAL corrosion with exposure time.
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elements. These elements have better structural integrity than the FEAL elements due to their higher ductility or stronger support core. After 1500–2500 hour exposure tests, no noticeable corrosion was found in these elements. However, their particulate collection efficiencies have not been satisfactory in this application especially during the initial period of the operation. The efficiency did improve as the porous media became “conditioned” after days of operation. Testing in the cold flow unit showed unacceptable particle penetration during back–pulsing. Collaboration with Pall Corporation is underway to improve collection efficiency of these types of elements as the materials demonstrated promising results in this application. Currently, the PSDF is also working with several other filter element manufacturers to test their newly–developed elements. The major requirements of a reliable filter element are (1) good particulate collection efficiency to meet particle concentration criteria for downstream equipment, (2) adequate structural integrity, (3) corrosion resistance for at least one year of continuous operation, (4) good material compatibility with the reducing environment, and (5) low and stable flow resistance during long–term operation. Of course, low cost is also an important factor for a commercial PCD. To facilitate the filter element testing, the aforementioned cold flow unit was built at the PSDF to conduct screening tests for newly– developed filter elements and failsafes. The collection efficiency can be evaluated in the cold flow unit in a cost–effective way. If the elements pass this test, they will be tested in the PCD under real operating conditions. Recently, titanium filter elements made by Graver Technologies have successfully passed the cold flow unit testing with an acceptable collection efficiency. As a result, three titanium elements have been installed in the PCD for a run scheduled in the summer of 2005. Prototypes of metal fiber elements from Pall, Purolator, and Microfiltrex will be tested in the cold flow unit in the near future. 3.2. PCD Particulate Collection Efficiency The overall particulate collection efficiency of the PCD system depends not only on the filter elements, but also on other components such as failsafes and gaskets. The failsafe issue will be discussed in the next subsection. To install the filter elements
and failsafes onto the plenum tubesheets, different types of high temperature gaskets have to be used. One type of the originally designed gasket did not survive in the reducing environment although it had reasonable sealing in the combustion runs. As a result, the particle leaking exceeded acceptable levels in the first gasification run. To solve the leaking problem, PSDF staff redesigned the holders, and Siemens Westinghouse supplied a different type of gasket to replace the problematic gasket. Starting from the second gasification run in 2000, the particle leaking has been significantly reduced to less than 1 ppmw. During normal operation, the overall outlet particle concentration is controlled below 0.1 ppmw. Fig. 5 shows PCD outlet particulate concentrations in recent test runs from TC09 (2002) to TC17 (2004). The slightly higher concentration during a startup may indicate particle penetration through gaskets and filter media while they were under “conditioning” with particulate. Further investigation is underway. The modifications of the holders and gaskets and improvement on the filter element and failsafe performance have made the overall PCD collection efficiency acceptable for gas turbine applications. Although there is no comprehensive data about particle requirements for a wide range of commercial turbines, it has been generally considered that the particulate should be controlled below about 1–2 ppmw in the syngas generated from air–blown coal gasification. A typical example of the particle requirements for gas turbines can be found in Reference [7]. The overall collection efficiency of the PCD system at the PSDF has been demonstrated to meet these criteria for particles. 3.3. Failsafe testing and development A commercial PCD design should allow for a certain filter element failure rate. Should a limited number of filter elements fail during operation, the PCD should function normally without the need for a system shutdown. A practical way to achieve this goal is to install failsafes on the clean side of the filter elements. The failsafe is a device that is designed to block particles in case of a filter element failure. Although the blocking mechanism may be different depending on the design (i.e., particle plugging, mechanical closing, etc.), a reliable failsafe should have the following features: (1) quick response to the filter element failure, (2) tight and stable long–term
Fig. 5. PCD outlet particulate concentration in ppmw.
X. Guan et al. / Powder Technology 180 (2008) 122–128
blocking that is not influenced by back–pulsing, (3) insignificant flow resistance in normal operation without filter failure, and (4) adequate structural integrity and corrosion resistance. Easy installation and low cost are also required for the failsafes in a commercial PCD. As an important component in the hot gas filtration system, the failsafe has long been considered a critical part in the commercialization of the technology. However, extensive R and D and testing for the failsafes in syngas application has not been established in the past. A typical work for the failsafes in the hot gas filtration can be found in Reference [8]. The first failsafes used in the Siemens Westinghouse PCD at the PSDF were based on a passive action mechanism (plugging). These failsafes did not perform adequately, particularly with gasification ash, so system shutdowns were necessary when filter failures occurred. To advance the failsafe technology, starting in 2001, the PSDF has initiated a program to identify, develop and test failsafes [9]. The program led to the development of a new type of failsafe that uses a combined filtration and plugging mechanism. The PSDF–designed failsafe and other commercially available failsafes have been systematically tested. So far, the failsafes tested include: • PSDF–designed failsafe (sintered metal fiber media with a solids holding ring) • Pall fuse (sintered metal powder media with iron aluminide material) • CeraMem failsafe (re–crystallized SiC ceramic honeycomb structure) • Specific Surface failsafe (re–crystallized SiC ceramic honeycomb structure) The test approach is detailed in [9]. The tests are generally in a sequence: (1) testing in the cold flow unit for screening purpose, (2) material exposure in the PCD, (3) solids injection with a low particulate loading to simulate a small crack in the filter element, (4) solids injection with a moderate loading to simulate an opening in the filter element, (5) solids injection with a high loading to simulate a catastrophic filter element failure, and (6) long–term testing to evaluate its durability. The PCD system was modified to inject gasification ash into the failsafe. SRI particulate sampling system is used to evaluate failsafe particulate collection efficiency. Test results are summarized in Table 3. The total number of filter elements installed in the PCD including the test element was 85 in TC08–TC12, 72 in TC14, and 73 in TC16–TC17. Except for the cases where media damage or particle leakage occurred, the PSDF–designed, Pall fuse, and CeraMem failsafes showed good collection efficiencies, which can meet gas turbine limits for particles with a certain filter element failure rate depending on the detailed PCD design. In recent operation, filter element failures were not detectable from outlet loading samples because of good failsafe performance. So far, extended syngas exposure time ranging from 2000 to over 5000 hours has been achieved for the PSDF–designed, Pall fuse, and CeraMem failsafes. Each type has its own advantages and disadvantages. Several areas of improvements have been
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Table 3 PCD outlet loading in ppmw during failsafe testing Test Run (Loading)
PSDF– Designed
TC08 (Low) b0.10 (S) TC09 (Low) TC11 (Moderate) 0.16 (S) TC12 (Moderate) 0.10–0.18 (L) TC14 (Moderate) TC16 (High) TC17 (High)
Pall Fuse
CeraMem
Specific Surface
0.33 (S)
0.46⁎(S) 0.45⁎(S) 0.15 (S) b0.10 (L) b0.10 (S) 3.34⁎⁎(S),b0.10 (L) b0.10 (S) b0.10 (S)
Note: (S) denotes sampling shortly after the injection (varying from 1 to 7 hours). (L) denotes sampling after a longer time injection (varying from 0.6 to 2 days). ⁎Failsafe media were damaged due to insufficient support in the old design. ⁎⁎ This number is out of norm and probably indicates particle leaking through the packing material between the media and the casing.
identified for these types of failsafes. The PSDF–designed failsafe uses sintered metal fiber porous media with different alloys. Several alloys, particularly HR–160, have shown good corrosion resistance. However, the initial media is coarser than desired causing particle penetration during back–pulsing. This was also confirmed in the cold flow unit testing. Finer media or tighter pore structure is needed to minimize the particle penetration. The Pall fuse, simple in structure, is welded inside the filter element which requires no gasket between the failsafe and filter. There have been concerns about Pall fuse's weld structure and long–term media corrosion resistance. Failures have been observed in certain fuses with cracks near the weld and breaking due to corrosion. For the CeraMem failsafe, its honeycomb configuration allows the syngas to flow through a surface area four times greater than that in other types of failsafes. The flow resistance is, therefore, significantly lower. However, modifications are needed to simplify the installation of the CeraMem failsafe with an optimized configuration. While long–term testing continues, more solids injection tests for individual types of failsafes will also be conducted to further characterize their performance such as particle loading versus time, peak loading during back–pulsing, particle size, etc. These tests will be greatly enhanced by a planned installation of a laser–based online particulate monitor (Process Particle Counter) from Process Metrix. Testing of newly– developed failsafes is also planned including titanium failsafes (sintered metal powder media) from Graver Technologies. 4. Summary Extensive testing and demonstration of the PCD system and its components in syngas environment have been achieved at the PSDF. Significant improvement has been made to increase the commercial readiness of hot gas filtration technology in the Transport Gasifier based gasification process. More than twenty types of filter elements have been tested with the longest exposure time of 5780 hours. Several types of failsafes have been tested with promising results. Filter element evaluation and successful failsafe program have led to high PCD particulate collection efficiency acceptable for gas turbine applications.
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The outlet particulate loading has been consistently below 0.1 ppmw during normal operation. Acknowledgements The authors wish to acknowledge the contributions and support provided by various project managers: Ron Breault (DOE), Neville Holt (EPRI), Nicola Salazar (KBR), and Ben Wiant (SWPC). The authors would also like to thank Bob Dahlin and Carl Landham of SRI, and everyone who has been involved in supporting the PCD test program. This project is supported by the DOE under contract DE–FC21–90MC25140. References [1] M.D. Davidson, E.N. Galloway, X. Guan, H.L. Hendrix, P.T. Scarborough, R.S. Dahlin, E.C. Landham, J.D. Spain, Power Systems Development Facility: High Temperature, High Pressure Filter System Operations in a Combustion Gas,, Proceedings of the 15th International Conference on Fluidized–Bed Combustion, American Society of Mechanical Engineers, New York, New York, 1999. [2] R.S. Dahlin, E.C. Landham, Factors Affecting Dustcake Drag in a Hot–Gas Filter System Collecting Coal Gasification Ash, 6th International
[3] [4]
[5]
[6]
[7] [8]
[9]
Symposium on Gas Cleaning at High Temperatures, Osaka, Japan, October 20–22, 2005. PSDF Website: http://psdf.southernco.com/. R.A. Martin, J.D. Spain, B. Gardner, X. Guan, Characterization of Candle Filter Elements in Coal Gasification Operation, American Filtration and Separation Society 2005 Annual Conference, Atlanta, Georgia, April 10– 13, 2005. H.L. Chen, X. Guan, Vibration Response in the Filtration System at PSDF, 6th International Symposium on Gas Cleaning at High Temperatures, Osaka, Japan, October 20–22, 2005. C.G. McKamey, D. McCleary, P.F. Tortorelli, J. Sawyer, E. Lara–Curzio, R.R. Judkins, Characterization of Field–Exposed Iron Aluminide Hot Gas Filters, Proceedings of the 5th International Symposium on Gas Cleaning at High Temperature, Morgantown, West Virginia, September 18–20, 2002. Specification for Fuel Gases for Combustion in Heavy–Duty Gas Turbines, GEI 41040G, GE Power Systems, Gas Turbine, Revised, January 2002. Z. Sanjana, G. Bruck, E. Smeltzer, M.A. Alvin, R. Newby, J. Foote, Development of Candle Filter Failure Safeguard Device, Proceedings of the 5th International Symposium on Gas Cleaning at High Temperature, Morgantown, West Virginia, September 18–20, 2002. B. Gardner, X. Guan, R.A. Martin, J. Spain, Hot Gas Filtration Meeting Turbine Requirements for Particulate Matter, Proceedings of GT2005 ASME Turbo Expo 2005: Power for Land, Sea and Air, Reno–Tahoe, Nevada, June 6–9, 2005.
Powder Technology 180 (2008) 122 – 128 www.elsevier.com/locate/powtec
Demonstration of hot gas filtration in advanced coal gasification system Xiaofeng Guan a,⁎, Ben Gardner a , Ruth Ann Martin a , Jack Spain b a
Power Systems Development Facility, Southern Company Services 31800 Highway 25 North, P.O. Box 1069 Wilsonville, Alabama, 35186, USA b Southern Research Institute 757 Tom Martin Drive Birmingham, Alabama, 35211, USA Received 3 February 2006; accepted 1 March 2007 Available online 12 March 2007
Abstract The Power Systems Development Facility (PSDF) is an engineering scale demonstration of advanced coal–fired power systems and high– temperature, high pressure gas filtration systems. The PSDF was designed at sufficient scale so that advanced power systems and components can be tested in an integrated fashion to provide data for commercial scale–up. The PSDF is funded by the U.S. Department of Energy (DOE), the Electric Power Research Institute (EPRI), Southern Company, Kellogg Brown and Root, Inc. (KBR), Siemens Westinghouse Power Corporation (SWPC), Peabody Energy, Burlington Northern and Santa Fe Railroad, and the Lignite Energy Council. Coal gasification at the PSDF is achieved with the KBR Transport Gasifier, and a Siemens Westinghouse particulate control device (PCD) is used for filtration of gasification ash from synthesis gas (syngas). As a critical process in the gasification system, hot gas filtration in the PCD removes the particulate so that the syngas can be utilized in a downstream gas turbine or a fuel cell. Testing at the PSDF has provided valuable experience with and enhanced understanding of PCD equipment and operation, filter element and failsafe material properties, and gasification ash characteristics. This testing and demonstration of the PCD system have significantly increased readiness towards commercialization of the hot gas filtration technology. This paper summarizes the testing and demonstration of the hot gas filtration system at the PSDF and addresses issues associated with the commercialization of this technology. © 2007 Elsevier B.V. All rights reserved. Keywords: Synthesis gas filtration; Filter element testing; Failsafe testing; Particulate collection efficiency; PSDF PCD system
1. Introduction The PSDF is a joint project of the U.S. DOE, Southern Company, and other industrial participants. As a premier advanced coal power generation research and development facility, the PSDF has played a key role in demonstrating and developing critical components of advanced coal–fired power systems in an engineering scale to provide data for commercial scale–up. The demonstration has been focused on a Transport Reactor–based clean coal power generation system. As a major component, a high temperature and high pressure PCD is used to remove particles from the gas stream before the process gas is fed to the downstream system such as turbine, fuel cell, or gas cleanup process. The Transport Reactor and the PCD were designed to operate in both combustion and gasification modes.
⁎ Corresponding author. E-mail address: [email protected] (X. Guan). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.03.005
The system was commissioned in 1996 followed by three years of demonstration in combustion mode with 5000 on–coal hours. In 1999, the Transport Reactor was converted to operate as a gasifier. By May 2005, the system has accumulated 6400 on–coal hours in gasification operation. Over the years, significant effort has been made at the PSDF to improve the system reliability and lower capital cost. The data collected and experience gained have greatly increased the readiness of the commercialization of the Transport Reactor–based coal gasification technology. Hot gas filtration, as the initial focus of the project, posed difficulties in the first few years primarily due to filter element failures under developmental process conditions. The PCD operation in combustion is summarized in a previous paper [1]. After considerable process modifications were made and better knowledge was gained on filter element material performance under these severe operating conditions, the failure rate has been significantly reduced in normal operation. Since the process was transitioned to gasification, the emphasis of the PCD
X. Guan et al. / Powder Technology 180 (2008) 122–128
test program has been on improving particulate collection efficiency, long–term filter element material testing, and failsafe testing. Characterization of the particulate matter and filter cake formation has also been performed to aid in evaluating PCD performance and provide a basis for commercial design [2]. These efforts have led to significant improvement in overall PCD reliability, operability and maintainability. Further information about PCD operations and filter element testing in recent years can be found in References [3] and [4]. 2. PCD system description The current Transport Reactor–based gasification process at the PSDF is shown in Fig. 1. A detailed description of the process and test run reports can be found on the PSDF website [3]. The PCD in the Transport Reactor train is a Siemens Westinghouse–designed filtration system (see Fig. 2). It has two tiers of plenums that combined can hold up to 91 barrier type filter elements. A tangential inlet and a cylindrical shroud inside the vessel distribute the particulate laden synthesis gas (syngas) in a swirling pattern to the filter elements inside the shroud. Gasification ash particles deposit on the outside surfaces of the filter elements and clean syngas passes through the plenums and exits the PCD from the top of the vessel. The filter elements are periodically cleaned online with back–pulses using nitrogen to
123
remove gasification ash from the surfaces. The gasification ash is then collected at the PCD cone and conveyed to an ash removal system below the PCD. The filter elements are “candle” type with one end capped and the other end flanged. They are individually installed onto the plenum tubesheets using holders. Support bars are installed at the bottom to connect all the filter elements to prevent damage due to vibration. The vibration behavior of the flexible PCD internals is discussed in another paper [5]. Above each filter element, there is a failsafe designed to prevent particulate leaking in the case of a filter element failure. If the filter element breaks, cracks, or leaks, the failsafe will quickly plug (or close) to effectively protect the downstream equipment from being damaged by particles in the gas stream. Back–pulse cleaning is achieved with a quick release of high pressure nitrogen from an accumulator tank for each plenum. Müller co–ax valves and Venturi devices are used to inject nitrogen into the plenums. The reversed flows and the pressure rises in the plenums dislodge the particle deposit on the filter element surfaces. The back–pulsing is initiated by a timer or predetermined differential pressure (DP) limit across the tubesheet. The work on the back–pulse valve testing and modification is described in the run reports (1997I and TC02) [3]. Particulate concentrations at the inlet and outlet of the PCD are monitored by Southern Research Institute (SRI) using batch
Fig. 1. PSDF gasification process diagram.
124
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Fig. 2. Schematic of the PCD.
sampling systems. Several types of online particulate detection instruments have also been tested to continuously monitor the particulate concentration in the outlet gas stream [3]. Since 1999, there have been 16 test runs in gasification, with each run typically lasting 200 to 1000 hours. A variety of coals have been tested under different operating conditions, though Powder River Basin coal has been the most extensively tested. Typical PCD operating conditions and syngas composition are listed in Tables 1 and 2, respectively. 3. PCD test program update Testing and demonstration of the PCD system and its components in the past several years are summarized below. More detailed information is published on the PSDF website [3]. 3.1. Filter element testing More than twenty types of filter elements have been tested at the PSDF [4]. In combustion runs, commercially available monolithic oxide and silicon carbide ceramic elements were the primary focus at that time because of the high temperature Table 1 Typical PCD operating conditions in gasification Temperature, °F (°C) Pressure, psig (kPa) Face Velocity, ft/min (cm/s) Baseline DP, inH2O (kPa) DP Rise Rate, inH2O/min (kPa/s) Back–Pulse Timer, min Back–Pulse Duration, s Back–Pulse Pressure, psi (kPa) Inlet Particulate Loading, ppmw Outlet Particulate Loading, ppmw Particle Size, micron MMD
750−950 (400−510) 140−240 (965−1655) 3.5−5 (1.8−2.5) 80 (19.9) 10 (0.042) 5−20 0.2 250−400 (1724−2758) above system pressure 13000−20000 Below detection limit of 0.1 ppmw 14
environment (1400 °F or 760 °C). Several developmental types of ceramic fiber composite elements and Pall iron aluminide (FEAL) metal elements were also tested. Unstable process conditions in these early runs and inadequate material properties and fabrication problems with some types of elements led to unacceptable filter element failure rates. The major failure modes included breaking due to thermal shock, membrane spalling, and leaking through media. However, FEAL filter elements showed promising results without failure in 2780 hours of exposure. Testing of several types of composite elements was continued in the first gasification run. However, the majority of the elements failed in the reducing environment under unstable operating conditions. Many manufacturers terminated R and D programs for the composite elements in this application. As a result, the PSDF stopped testing elements in this category. The lower PCD operating temperature (750 °F or 400 °C) in gasification runs allowed the introduction of more metal elements. A major advantage of the metal materials is higher durability than ceramic materials, primarily due to their higher thermal shock resistance and higher ductility. In the first gasification run, several types of sintered metal powder and sintered metal fiber elements were installed. However, many metal fiber elements were screened out after the first run, not due to poor material performance, but due to their high flow Table 2 Typical syngas composition
H2, % CO, % CO2, % CH4, % H2O, % N2, % H2S, ppm NH3, ppm
Air Blown
Oxygen Blown
2−8 1−14 7−14 1−2 6−14 54−72 100−1300 500−2400
8−16 4−14 12−14 2−5 17−40 30−55 250−1600 2000−3000
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resistance caused by their structures. A coarse protection layer outside of the media tended to retain a thick gasification ash cake, which could not be removed by back–pulsing. In the following two runs, monolithic ceramic elements and later a mixture of ceramic and FEAL metal elements were installed. After a ceramic element failed during an off–coal period, the decision was made to stop testing the ceramic elements and focus on testing metal elements. The primary concern was the potential thermal shock due to system upsets during the early stages of development of the gasification process. Since 2000, the FEAL elements have been extensively tested in gasification operation. About 5780 exposure hours have been achieved for some longest exposed individual elements. The FEAL elements showed excellent particulate collection efficiency. In stable operations with only FEAL elements, the PCD outlet particulate concentration was less than 0.1 ppmw, which is the current SRI particulate sampling system lower detection limit. Testing in a cold flow unit at the PSDF has shown that the FEAL elements are among the elements that have the highest collection efficiencies. In general, the FEAL elements show promising results. However, there are concerns about their structural integrity and long–term corrosion resistance. The FEAL elements are constructed with multi–sections of sintered metal powder media, which are welded to solid metal parts. There have been several element failures when cracks occurred at the welds or in the heat affected zone, indicating a need to improve the weld structure. FEAL elements are made of iron aluminide material. Like other metal materials, iron aluminide is subject to corrosion under certain conditions. The extended exposure testing at PSDF showed that corrosion products do form over time. The severity of the corrosion increases with the exposure time, as shown in Fig. 3. Corrosion of FEAL elements has also been reported elsewhere [6]. In some cases, the corrosion products have partially plugged the pores, causing increased flow resistance with extended exposure time. The filter DP measured at
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Fig. 4. Strength of FEAL elements.
room temperature after washing is also noted in Fig. 3 for each element shown. The rising trend of DP is evident. Metallurgical analysis has been initiated to identify the mechanism. Despite the presence of corrosion, the mechanical strength of the media has not decreased in over 4000 hours of exposure. Fig. 4 shows strength of the FEAL element tested at room temperature. It should be noted that the testing at the PSDF is not continuous as in a commercial plant. The multiple startups and shutdowns may have exacerbated the corrosion condition as the thermal, mechanical, and chemical cycling may cause extra damage for the FEAL material. For use in a commercial plant, the elements must last for at least 8000 exposure hours without significant loss of strength or increase in flow resistance. The testing of the FEAL elements at PSDF will continue to evaluate their long–term performance. Testing of other types of metal elements was also conducted at the PSDF both in the PCD and the cold flow unit. Two types with significant exposure were Pall Hastelloy (sintered metal powder) and Pall/Fluid Dynamics HR– 160 (sintered metal fiber)
Fig. 3. FEAL corrosion with exposure time.
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elements. These elements have better structural integrity than the FEAL elements due to their higher ductility or stronger support core. After 1500–2500 hour exposure tests, no noticeable corrosion was found in these elements. However, their particulate collection efficiencies have not been satisfactory in this application especially during the initial period of the operation. The efficiency did improve as the porous media became “conditioned” after days of operation. Testing in the cold flow unit showed unacceptable particle penetration during back–pulsing. Collaboration with Pall Corporation is underway to improve collection efficiency of these types of elements as the materials demonstrated promising results in this application. Currently, the PSDF is also working with several other filter element manufacturers to test their newly–developed elements. The major requirements of a reliable filter element are (1) good particulate collection efficiency to meet particle concentration criteria for downstream equipment, (2) adequate structural integrity, (3) corrosion resistance for at least one year of continuous operation, (4) good material compatibility with the reducing environment, and (5) low and stable flow resistance during long–term operation. Of course, low cost is also an important factor for a commercial PCD. To facilitate the filter element testing, the aforementioned cold flow unit was built at the PSDF to conduct screening tests for newly– developed filter elements and failsafes. The collection efficiency can be evaluated in the cold flow unit in a cost–effective way. If the elements pass this test, they will be tested in the PCD under real operating conditions. Recently, titanium filter elements made by Graver Technologies have successfully passed the cold flow unit testing with an acceptable collection efficiency. As a result, three titanium elements have been installed in the PCD for a run scheduled in the summer of 2005. Prototypes of metal fiber elements from Pall, Purolator, and Microfiltrex will be tested in the cold flow unit in the near future. 3.2. PCD Particulate Collection Efficiency The overall particulate collection efficiency of the PCD system depends not only on the filter elements, but also on other components such as failsafes and gaskets. The failsafe issue will be discussed in the next subsection. To install the filter elements
and failsafes onto the plenum tubesheets, different types of high temperature gaskets have to be used. One type of the originally designed gasket did not survive in the reducing environment although it had reasonable sealing in the combustion runs. As a result, the particle leaking exceeded acceptable levels in the first gasification run. To solve the leaking problem, PSDF staff redesigned the holders, and Siemens Westinghouse supplied a different type of gasket to replace the problematic gasket. Starting from the second gasification run in 2000, the particle leaking has been significantly reduced to less than 1 ppmw. During normal operation, the overall outlet particle concentration is controlled below 0.1 ppmw. Fig. 5 shows PCD outlet particulate concentrations in recent test runs from TC09 (2002) to TC17 (2004). The slightly higher concentration during a startup may indicate particle penetration through gaskets and filter media while they were under “conditioning” with particulate. Further investigation is underway. The modifications of the holders and gaskets and improvement on the filter element and failsafe performance have made the overall PCD collection efficiency acceptable for gas turbine applications. Although there is no comprehensive data about particle requirements for a wide range of commercial turbines, it has been generally considered that the particulate should be controlled below about 1–2 ppmw in the syngas generated from air–blown coal gasification. A typical example of the particle requirements for gas turbines can be found in Reference [7]. The overall collection efficiency of the PCD system at the PSDF has been demonstrated to meet these criteria for particles. 3.3. Failsafe testing and development A commercial PCD design should allow for a certain filter element failure rate. Should a limited number of filter elements fail during operation, the PCD should function normally without the need for a system shutdown. A practical way to achieve this goal is to install failsafes on the clean side of the filter elements. The failsafe is a device that is designed to block particles in case of a filter element failure. Although the blocking mechanism may be different depending on the design (i.e., particle plugging, mechanical closing, etc.), a reliable failsafe should have the following features: (1) quick response to the filter element failure, (2) tight and stable long–term
Fig. 5. PCD outlet particulate concentration in ppmw.
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blocking that is not influenced by back–pulsing, (3) insignificant flow resistance in normal operation without filter failure, and (4) adequate structural integrity and corrosion resistance. Easy installation and low cost are also required for the failsafes in a commercial PCD. As an important component in the hot gas filtration system, the failsafe has long been considered a critical part in the commercialization of the technology. However, extensive R and D and testing for the failsafes in syngas application has not been established in the past. A typical work for the failsafes in the hot gas filtration can be found in Reference [8]. The first failsafes used in the Siemens Westinghouse PCD at the PSDF were based on a passive action mechanism (plugging). These failsafes did not perform adequately, particularly with gasification ash, so system shutdowns were necessary when filter failures occurred. To advance the failsafe technology, starting in 2001, the PSDF has initiated a program to identify, develop and test failsafes [9]. The program led to the development of a new type of failsafe that uses a combined filtration and plugging mechanism. The PSDF–designed failsafe and other commercially available failsafes have been systematically tested. So far, the failsafes tested include: • PSDF–designed failsafe (sintered metal fiber media with a solids holding ring) • Pall fuse (sintered metal powder media with iron aluminide material) • CeraMem failsafe (re–crystallized SiC ceramic honeycomb structure) • Specific Surface failsafe (re–crystallized SiC ceramic honeycomb structure) The test approach is detailed in [9]. The tests are generally in a sequence: (1) testing in the cold flow unit for screening purpose, (2) material exposure in the PCD, (3) solids injection with a low particulate loading to simulate a small crack in the filter element, (4) solids injection with a moderate loading to simulate an opening in the filter element, (5) solids injection with a high loading to simulate a catastrophic filter element failure, and (6) long–term testing to evaluate its durability. The PCD system was modified to inject gasification ash into the failsafe. SRI particulate sampling system is used to evaluate failsafe particulate collection efficiency. Test results are summarized in Table 3. The total number of filter elements installed in the PCD including the test element was 85 in TC08–TC12, 72 in TC14, and 73 in TC16–TC17. Except for the cases where media damage or particle leakage occurred, the PSDF–designed, Pall fuse, and CeraMem failsafes showed good collection efficiencies, which can meet gas turbine limits for particles with a certain filter element failure rate depending on the detailed PCD design. In recent operation, filter element failures were not detectable from outlet loading samples because of good failsafe performance. So far, extended syngas exposure time ranging from 2000 to over 5000 hours has been achieved for the PSDF–designed, Pall fuse, and CeraMem failsafes. Each type has its own advantages and disadvantages. Several areas of improvements have been
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Table 3 PCD outlet loading in ppmw during failsafe testing Test Run (Loading)
PSDF– Designed
TC08 (Low) b0.10 (S) TC09 (Low) TC11 (Moderate) 0.16 (S) TC12 (Moderate) 0.10–0.18 (L) TC14 (Moderate) TC16 (High) TC17 (High)
Pall Fuse
CeraMem
Specific Surface
0.33 (S)
0.46⁎(S) 0.45⁎(S) 0.15 (S) b0.10 (L) b0.10 (S) 3.34⁎⁎(S),b0.10 (L) b0.10 (S) b0.10 (S)
Note: (S) denotes sampling shortly after the injection (varying from 1 to 7 hours). (L) denotes sampling after a longer time injection (varying from 0.6 to 2 days). ⁎Failsafe media were damaged due to insufficient support in the old design. ⁎⁎ This number is out of norm and probably indicates particle leaking through the packing material between the media and the casing.
identified for these types of failsafes. The PSDF–designed failsafe uses sintered metal fiber porous media with different alloys. Several alloys, particularly HR–160, have shown good corrosion resistance. However, the initial media is coarser than desired causing particle penetration during back–pulsing. This was also confirmed in the cold flow unit testing. Finer media or tighter pore structure is needed to minimize the particle penetration. The Pall fuse, simple in structure, is welded inside the filter element which requires no gasket between the failsafe and filter. There have been concerns about Pall fuse's weld structure and long–term media corrosion resistance. Failures have been observed in certain fuses with cracks near the weld and breaking due to corrosion. For the CeraMem failsafe, its honeycomb configuration allows the syngas to flow through a surface area four times greater than that in other types of failsafes. The flow resistance is, therefore, significantly lower. However, modifications are needed to simplify the installation of the CeraMem failsafe with an optimized configuration. While long–term testing continues, more solids injection tests for individual types of failsafes will also be conducted to further characterize their performance such as particle loading versus time, peak loading during back–pulsing, particle size, etc. These tests will be greatly enhanced by a planned installation of a laser–based online particulate monitor (Process Particle Counter) from Process Metrix. Testing of newly– developed failsafes is also planned including titanium failsafes (sintered metal powder media) from Graver Technologies. 4. Summary Extensive testing and demonstration of the PCD system and its components in syngas environment have been achieved at the PSDF. Significant improvement has been made to increase the commercial readiness of hot gas filtration technology in the Transport Gasifier based gasification process. More than twenty types of filter elements have been tested with the longest exposure time of 5780 hours. Several types of failsafes have been tested with promising results. Filter element evaluation and successful failsafe program have led to high PCD particulate collection efficiency acceptable for gas turbine applications.
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The outlet particulate loading has been consistently below 0.1 ppmw during normal operation. Acknowledgements The authors wish to acknowledge the contributions and support provided by various project managers: Ron Breault (DOE), Neville Holt (EPRI), Nicola Salazar (KBR), and Ben Wiant (SWPC). The authors would also like to thank Bob Dahlin and Carl Landham of SRI, and everyone who has been involved in supporting the PCD test program. This project is supported by the DOE under contract DE–FC21–90MC25140. References [1] M.D. Davidson, E.N. Galloway, X. Guan, H.L. Hendrix, P.T. Scarborough, R.S. Dahlin, E.C. Landham, J.D. Spain, Power Systems Development Facility: High Temperature, High Pressure Filter System Operations in a Combustion Gas,, Proceedings of the 15th International Conference on Fluidized–Bed Combustion, American Society of Mechanical Engineers, New York, New York, 1999. [2] R.S. Dahlin, E.C. Landham, Factors Affecting Dustcake Drag in a Hot–Gas Filter System Collecting Coal Gasification Ash, 6th International
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Symposium on Gas Cleaning at High Temperatures, Osaka, Japan, October 20–22, 2005. PSDF Website: http://psdf.southernco.com/. R.A. Martin, J.D. Spain, B. Gardner, X. Guan, Characterization of Candle Filter Elements in Coal Gasification Operation, American Filtration and Separation Society 2005 Annual Conference, Atlanta, Georgia, April 10– 13, 2005. H.L. Chen, X. Guan, Vibration Response in the Filtration System at PSDF, 6th International Symposium on Gas Cleaning at High Temperatures, Osaka, Japan, October 20–22, 2005. C.G. McKamey, D. McCleary, P.F. Tortorelli, J. Sawyer, E. Lara–Curzio, R.R. Judkins, Characterization of Field–Exposed Iron Aluminide Hot Gas Filters, Proceedings of the 5th International Symposium on Gas Cleaning at High Temperature, Morgantown, West Virginia, September 18–20, 2002. Specification for Fuel Gases for Combustion in Heavy–Duty Gas Turbines, GEI 41040G, GE Power Systems, Gas Turbine, Revised, January 2002. Z. Sanjana, G. Bruck, E. Smeltzer, M.A. Alvin, R. Newby, J. Foote, Development of Candle Filter Failure Safeguard Device, Proceedings of the 5th International Symposium on Gas Cleaning at High Temperature, Morgantown, West Virginia, September 18–20, 2002. B. Gardner, X. Guan, R.A. Martin, J. Spain, Hot Gas Filtration Meeting Turbine Requirements for Particulate Matter, Proceedings of GT2005 ASME Turbo Expo 2005: Power for Land, Sea and Air, Reno–Tahoe, Nevada, June 6–9, 2005.