Rt

Transcription

1 711E AMERICAN SOCIETY OF MECHANICAL ENGINEERS Three Park Avenue, New York, N.Y GT-419 The Society shall not be responsible for statements or opinions advanced In papets or ilscussion at meetings of the Society or of its DiviSIOAS or Sections, or printed in its publications. Diexussion is printed only if the paper is published in an ASME Journal. Authorization to photocopy for internal or personal use is granted to libraries and other users registered with the Copyright Clearance Center (CCC) provided $3/article is paid to CCC, 222 Rosewood Dr., Danvers, MA Requests for special permission or bulk reproduction should be addressed to the ASME Technical Publishing Department. Copyright by ASME M Rights Reserved Printed In USA HYBRID GAS TURBINE AND FUEL CELL SYSTEMS IN PERSPECTIVE REVIEW Rt David J. White Collaborating Scientist University of California at Irvine ABSTRACT The concept of hybrids combining fuel cell and gas turbine systems is without question neoteric, and probably is less than eight years old. However, this concept is in a sense a logical development derived from the many early systems that embodied the key features of rotating machinery to compress air. It was the introduction of high temperature fuel cells such as the solid oxide fuel cell (SOFC) that allowed the concept of hybrid gas turbine fuel cell systems to take root. The SOFC with an operating temperature circa 1000 C matched well with small industrial gas turbines that had fixing temperatures on the same order. The recognition that the SOFC could be substituted for the gas turbine combustor was the first step into the realm of fuel cell topping systems. Fuel cells in general were recognized as having higher efficiencies at elevated pressures. Thus the hybrid topping system where the gas turbine pressurized the fuel cell and the fuel cell supplied the hot gases for expansion over the turbine promised to provide a high level of synergy between the two systems. Bottoming systems using the exhaust of a gas turbine as the working fluid of a fuel cell such as the molten carbonate fuel cell (MCFC) have been identified and are potential future power generation hybrid systems. The MCFC is especially well suited to the bottoming role because of the need to have carbon dioxide present in the inlet air stream. The carbon dioxide in the gas turbine exhaust allows the high temperature blower, normally used to recirculate and inject exhaust products into the inlet air, to be eliminated. Hybrid systems have the potential of achieving fossil fuel to electricity conversion efficiencies on the order of 70% and higher. The costs of hybrid systems in dollars per kilowatt are generally higher than say an advanced gas turbine that is available today but not by muck The net energy output over the life of a hybrid topping system is similar to that of a recuperated gas turbine but possibly lower than a high-efficiency simple-cycle machine, depending on the efficiency of the hybrid. Methodologies to aid in the selection of the hybrid system for future development have to be developed and used consistently. Life cycle analyses (LFA) provide a framework for such selection processes. In particular the concept of net energy output provides a mechanism to assign relative worth to competing concepts. INTRODUCTION The concept of a hybrid that combines a solid oxide fuel cell with a gas turbine system is without question neaten, and probably is less than eight years old. However, it did not burst upon the energy conversion scene "fully-formed", the general hybrid concept has been reinvented each time a new type of fuel cell has been introduced. Early hybrid patents (see References 1 through 10) describe systems that feature rotating turbomachinery to compress air for use in phosphoric acid fuel cells (PAFC). In some cases the air blower was driven by a steam turbine and in others by a turbine driven by a mixture of steam and exhaust gases. In these latter cases, the steam raised in the exhaust of the fuel cell would be used for both fuel reforming and for expansion over the turbine. The main problem encountered by these early hybrid concepts was that the temperature of the fuel cell exhaust was too low for efficient expansion over a turbine. Temperatures leaving the fuel cell exhaust were usually between 150 and 200 C. Such temperatures are Presented at the International Gas Turbine & Aeroengine Congress Downloaded From: Indianapolis, on 10/14/2018 Indiana Terms of Use: June 7 June 10, 1999

2 usually not high enough to provide a turbine efficiency adequate for operation. To offset these low temperatures other arrangements were proposed that used modified automotive "turbo-chargers" at similar devices to supply the high-pressure air driven by expanding hot gases from an auxiliary combustor. When the molten carbonate fuel cells began development in the 1980s a number of hybrid system patents (References 11 through 15) using gas-turbine-like machinery to turbo-charge the system were awarded to various companies. A number of mechanical problems plagued these early turbo-blower developments and in most cases the programs were stopped. This was due primarily to the fact that many of fuel cell companies involved did not take advantage of the body of knowledge in the gas turbine arena instead they relied on turbocharger and industrial expander technologies. These expanders primarily employed rugged but low efficiency radial machinery. This machinery was often difficult to integrate with a blower or compressor because of dissimilar operating speeds and characteristics. In addition these expanders usually had low efficiencies at the typical operating temperatures (650 C) of the MCFC. It was the introduction of high temperature fuel cells (once considered undesirable) that paved the way for advanced hybrid gas turbine fuel cell systems. In particular the solid oxide fuel cell (SOFC) with an operating temperature circa 1000 C was matched well with small industrial gas turbines that had firing temperatures on the same order. The recognition that the SOFC could be directly substituted for the gas turbine combustor was the first step into the realm of pressurized fuel cell hybrid systems. Fuel cells in general were acknowledged as having higher efficiencies at elevated pressures through reductions in internal losses. Thus the hybrid topping system where the gas turbine pressurized the fuel cell and the fuel cell supplied the hot gases for expansion over the turbine promised. to provide a high level of synergy between the two systems. As with the PAFC and the MCFC hybrid cycles the SOFC gas turbine hybrid cycle was the subject of a number of patents, see References 16 through 23. In describing the various hybrid cycles the gas turbine has been arbitrarily selected as the primary system and the fuel cells as the secondary systems. Thus fuel cells are categorized here, as either bottoming or topping systems. In the bottoming mode the fuel cell is placed in the exhaust of the gas turbine in much the same manner as a heat recovery steam generator (HRSG) in a combined cycle. In the topping mode the fuel cell effectively replaces the gas turbine combustor acting as 2 a "solid state" electricity generator and high temperature heat source. It should be noted that the fuel cell producers have used different terms to describe the various hybrid cycles. GAS TURBINE COMPANY PERSPECTIVE In the late 1980s and early 1990s when it was becoming apparent that the solid oxide fuel cell (SOFC) was a possible contender for future commercial power generation, gas turbine manufacturers saw the system both as a threat and a potential venue for diversification. The use of modified gas turbine systems that used the SOFC in a topping mode was considered by gas turbine manufacturers as an effective means of avoiding competition and potentially controlling the date of SOFC introduction to the marketplace. Investigations into the use of the SOFC in a topping mode, which essentially meant that the fuel cell "replaced" the combustor for normal operation, were driven by the realization that the SOFC exhaust gas temperatures (circa 1000 C) closely matched the turbine inlet temperatures of small industrial gas turbines. This approach has also been referred to as the high-pressure hybrid. A drawing of an early topping mode hybrid concept (produced by a gas turbine manufacturer) together with a brief discussion of the system was first published in 1992 as part of a paper on a "Vision of the Future" reference 24. This description was based on unpublished work that had been performed between 1990 and 1991 at Solar Turbines Incorporated and at its parent company Caterpillar, Inc. under the aegis of the Caterpillar Fuel Cell Committee. This early work concentrated primarily on the SOFC although molten carbonate fuel cells (MCFC) were also considered. Because the MCFC exhaust temperatures were lower than those of the SOFC, on the order of 650 C, they were less suitable for integration with gas turbines in the topping mode than the SOFC systems. In addition it was feared that there would be fugitive alkali carbonate vapors in the exhaust which could damage the protective oxide coating of the turbine blades. Instead the MCFC systems were considered primarily as a bottoming cycle system. In this arrangement the exhaust of the gas turbine provided both the oxygen and the required carbon dioxide to the cathode of the MCFC. This eliminated the expensive and often unreliable high temperature blower that had previously been used to recirculate carbon dioxide fiom the exhaust and inject it into the inlet air stream It also minimized or eliminated modifications to the gas turbine proper. In the bottoming cycle arrangement the gas turbine was a standard fired simple cycle machine with the exhaust either completely or partially fed into the cathode section

3 of the MCFC. The pressure drop over the fuel cell was sufficiently low that it did not materially reduce the performance of the gas turbine. This bottoming arrangement is also known as the low-pressure hybrid and could utilize an SOFC with some small efficiency penalties instead of a MCFC. Thus there were two hybrid cycles considered in the early stages of system development a low-pressure version and a high-pressure cycle. It was also considered feasible to use both types of hybrid cycle with one gas turbine; the SOFC in the topping mode and the MCFC as a bottoming cycle. This was a very large and complex system with a high efficiency (over 70%) but with a number of potential control and safety problems. Today, the concept of hybrid fuel cell gas turbine systems has largely devolved to the use of a SOFC in some type of topping arrangement with a gas turbine. Further investigation into such high-pressure hybrids has revealed that there will probably be at least two types of topping or high pressure hybrid system. A recuperated gas turbine will probably be used with SOFC systems that can separate the exhaust anode gas flow from the cathode flow and recycle part of it to the fuel reformer. The exhaust anode gases are sufficiently rich in high temperature steam to provide all of the needed water to reform the fuel SOFC systems that have mixed exhaust flows have to use an exhaust steam generator to produce the required water at temperature for fuel reformation. In this latter case it is convenient to generate steam in excess of that needed for fuel reformation and use it as a form of recuperation. Thus a simple cycle gas turbine combined with a beat recovery steam generator (HRSG) matches better the needs of mixed exhaust flow SOFC systems. FUEL CELL COMPANY PERSPECTIVE It appears in retrospect (based on patent dates) that the SOFC manufacturers did not explore the use of gas. turbine hybrid systems until some time after the gas turbine companies had completed their studies. It would also appear in retrospect that the gas turbine companies considered that the high-pressure hybrid system was not a patentable arrangement. This was due to the fact that the general concept had been publicly described in patents and the literature before patents were issued to the SOFC companies. The high pressure SOFC topping arrangement may well have been placed in the public domain. All of the SOFC and the MCFC manufacturers have adopted the hybrid system in some form as part of their future high efficiency system offerings. The SOFC companies appear to have undertaken far 3 more complex analytical studies of the hybrid approaches than the gas turbine manufacturers and have taken the lead in the understanding of these systems. This is based on comparisons between the presentations given at the Workshop on Very High Efficiency Fuel Cell/Gas Turbine Power Cycles, DOE 1995 and those at the Advanced Turbine Systems Annual Review, DOE, November A number of high efficiency arrangements using fuel cells combined with various gas turbine cycles have been evaluated by the fuel cell companies. These cycles have included simple cycle machines, recuperated gas turbines and intercooled recuperated (ICA) systems. Some of the published configurations and much of the balance of plant proposed by the fuel cell manufacturers is not acceptable to the gas turbine companies and vice versa. Thus a level of dichotomy exists. This dichotomy has produced a gap separating the concepts generated by the two groups. The proliferation of hybrid concepts in both camps has lead to some confusion over what are acceptable and practical configurations. As a group all of the SOFC and some of the MCFC companies believe that the hybrid fuel cell gas turbine system is one of the fuel cell products of the future. Most of these companies have programs in place to develop some type of hybrid system. The US Department of Energy (DOE) has also recognized the promise of hybrid systems and is supporting the fuel cell community in their developments. These evolving developments are presently in their early stages and are comprised mostly of configuration and performance studies together with analyses of system dynamic behavior. The complex nature of the topping mode hybrid systems has lead to concerns about system control, safety, and turn-down capability. It is these potential problems that are being addressed by DOE and the fuel cell companies in both private and government funded studies. HYBRID GAS TURBINE FUEL CELL SYSTEMS Bottoming Cycles The simplest of the hybrid systems are the bottoming cycles or low pressure hybrid cycles. There are two basic versions of this arrangement, one uses a fired gas turbine and the other an indirectly heated gas turbine where the required thermal energy is extracted by heat exchange from the fuel cell exhaust. The fired version simply uses the fuel cell as a true bottoming cycle with the gas turbine exhaust supplying the air to the fuel cell as shown in Figure 1. For the sake of completeness a possible fuel system and an inert gas generator to safely blanket the fuel cell with carbon

4 dioxide and water vapor in case of complete load loss is also shown in Figure 1. The inert gas safety system is a possible solution to the problem of hydrogen accumulation in the fuel cell following rapid load loss. When the load is lost the fuel cell ceases to react hydrogen allowing a certain level to accumulate before the shut-off valves are activated. This level can be in the ir PIM Figure 1 Hybrid Directly Fired gas Turbine Low Pressure MCFC explosive range. The time lag between sensing the load loss and shutting-off the fuel is on the order of fractions of a second sufficient to allow a significant amount of hydrogen to accumulate. A steam/fuel reformer employing anode exhaust as the steam source is shown along with the fuel system. Because the gas turbine eliminates the fan or blower that is normally used to supply the air equipment requirements are simplified and costs are reduced and efficiency improved. Both the gas turbine and the fuel cell generate electrical power in this arrangement. Although both the SOFC and the MCFC can be employed in this form of bottoming cycle the MCFC is much better suited because as mentioned above it can use the carbon dioxide and the oxygen in the exhaust without efficiency penalties. In fact the MCFC requires carbon dioxide to be added to the inlet air stream to allow the production of the carbonate ion. Thus the MCFC accrues the advantage of the elimination of the high temperature blower used to take high temperature carbon dioxide rich exhaust and add it to the inlet air stream. This reduces costs and improves overall efficiency. The SOFC proper suffers a penalty in efficiency when used in this mode. Typical system efficiencies achieved using this approach lie between 45 and 48%3. Efficiency as used in this paper is the alternating current (AC) energy out compared to the energy at the lower heating value (LHV) of the fuel. A variation on this approach is to locate the bottoming 4 fuel cell between the gas producer turbines and the power turbine (two-shaft engine). This pressurizes the fuel cell slightly increasing its efficiency. The additional energy provided by this second fuel cell acts in the same manner as a reheat combustor increasing the power produced by the gas turbine portion of the system. It may be possible to add a true reheat combust downstream of the low pressure fuel cell to produce more power if there is sufficient oxygen left in the exhaust of the fuel cell. One version of the indirectly heated gas turbine and fuel cell combination is shown in Figure 2. This particular arrangement is an atmospheric pressure fuel cell integrated with a two-shaft gas turbine. Although this cycle has been proposed many times it is at present impractical for both the SOFC and the MCFC. The high efficiencies provided by this arrangement (between 50 and 55%) depend on not firing the gas turbine. To eliminate firing, the hot SOFC exhaust is passed through a heat exchanger where it heats the air leaving the gas turbine compressor to close to normal firing temperatures. Figure 2 Hybrid Indirectly Heated Gas Turbine Low Pressure SOFC The problem with this approach is that today there are no practical heat exchangers that can accept the 1000 C SOFC exhaust temperatures. The highest inlet temperatures that a metallic recuperator can withstand are on the order of 650 C. This latter tunp.rature matches better with the exhaust of the MCFC. However, if the MCFC were to be used the temperatures entering the turbine section of the gas turbine would be too low to be effective. The turbine efficiencies would be so low that auxiliary firing would

5 be required. This reduces the cycle efficiency. However, for the MCFC because there is carbon dioxide present in the vitiated air during firing, the high temperature blower is eliminated thus reducing the losses incurred through firing the gas turbine. If a low temperature SOFC were to be operated at 650 C and used in this mode there would be an added efficiency penalty for operation on vitiated air. Thus of the low pressure hybrid systems the only one that makes practical sense today, is the simple fired bottoming cycle system. Until new high temperature materials are developed for use in recuperators, the unfired concept will be impractical. The preferred fuel cell for such operation is the MCFC although the SOFC could be used if the added penalties could be accepted. Topping Cycles High-pressure hybrid systems or topping arrangements are the most likely of the hybrids to be placed into production in the near future. The simplest of these is the system shown in Figure 3, which is a topping cycle SOFC integrated with a recuperated gas turbine. This particular arrangement is for those SOFC systems that can capture and recirculate steam laden anode exhaust gases to an internally integrated fuel reformer to produce h) drogen and carbon monoxide. approaches have the potential of achieving efficiencies between 65 and 70%. Because of cost considerations the efficiencies for practical systems will probably be on the order of 60%. As the cost of cells decrease higher efficiency systems with a larger number of cells for a given power output will be introduced. This "unloading" of cells provides increased efficiency. A typical efficiency versus power relationship for a pressurized SOFC integrated with a small gas turbine is shown in Figure 5. This particular curve is based on a recuperated gas turbine with a power output of approximately 350-kW. The SOFC cells were added to the gas turbine as modules each nominally producing 500-kW. Power production per cell (for a fixed air mass flow) decreases as the number of cells increases. The cell efficiency on the other hand increases as the cells are "unloaded." The cost per kilowatt increases as the number of cells and the efficiency increases. Because the cells produce less power (at higher efficiency) more cells are required per kilowatt produced. Combination Topping and Bottoming Cycles The above described topping and bottoming cycles can both be integrated with a single gas turbine. An ideal approach would be to use a SOFC as the topping unit and a MCFC as the bottoming system. This is unlikely to happen in practice because of the competitive positions taken by the SOFC manufacturers and the MCFC producers. Two SOFC systems could Figure 3 Hybrid Recuperated Gas Turbine SOFC In Topping Mode Those SOFC systems that have mixed cathode and anode exhaust flows will integrate better with a simple cycle gas turbine coupled to a heat recovery steam generator. Such SOFC systems require steam production from an external generator to supply the reformer with the needed water reactant. With an exhaust boiler required it is more cost effective to generate and use excess steam as a method of recuperation than to add an air to air recuperator. This latter configuration is shown in Figure 4. Both 5 Figure 4 Hybrid Simple Cycle Gas Turbine SOFC In Topping Mode be employed as an alternative approach although the unit used as the bottoming system will suffer some penalties due to the use of vitiated air. The advantages of this

6 approach are the potentially high efficiencies that can be achieved. Efficiencies on the order of 70% or higher are projected for this type of system. One version of this approach employing both SOFC and MCFC systems is shown in Figure 6. This schematic shown in has been simplified for the sake of clarity. The fuel line to the start combustor has not been included and similarly a line from the inert gas generator to the bottoming fuel cell is also not shown. There are some potential control and safety problems with this large and complex hybrid. In particular sudden load loss can create possible dangerous hydrogen accumulations in both fuel cell sets. These will have to be solved either by mechanical dilution or displacement of the hydrogen or by other means. Control of the individual fuel cells during turn-down and during rapid power increases so that one does not dominate the other will be difficult because the response times for the two fuel cell systems will be very different. If the fuel cells react at different time rates one fuel cell may be starved of fuel and the other have too much. This overaccumulation of hydrogen in one of the cells could lead Figure 5 Typical Efficiency Power Characteristics Hybrid Recuperated Gas Turbine SOFC In Topping Mode to destructive bum-out or even an explosion. SYSTEM SELECTION METHODOLOGY The general approach in selecting one or more of the many possible hybrid gas turbine - fuel cell systems for future product development is to utilize Life Cycle Analyses (LFA) to determine a relative worth between the various available options. LFA has a number of subsets that includes life-cycle-cost analyses, net energy output calculations, and fuel efficiency effects. These latter effects include the potentially significant Figure 6 Hybrid Recuperated Gas Turbine SOFC Topping MCFC Bottoming reductions in pollutant emissions that high efficiencies can provide. Efficiency is important in the system selection processes but the net energy output of the system calculated over its predicted life is usually pivotal in determining relative worth. Life cycle financial analyses will usually select the same systems that the net energy output analyses indicate as having the highest week Differences occur when tax incentives or other financial inducements favor one system over others. There are many financial subsidies and tax incentives in place in the US, some of these are overt and some are well hidden. These subsidies often mask the true costs of energy and thus skew the decision processes. If decisions are to be universal and independent of financial systems the net energy output analyses should be employed in system development selection. This would be essential in those cases when a system is to be manufactured in more than one county. If a complex hybrid of high efficiency but low net energy output were to be compared to a slightly lower efficiency system with a much higher net energy output level, the latter system would probably prove to have a higher "worth." The hybrid employing both topping and bottoming fuel cell systems may well fall in to the category of high efficiency but low net energy output systems. A simpler topping system can reach efficiencies that are very close to those of the combined topping and bottoming hybrid but with a higher net energy output level, thus making it the system more likely to be chosen. More detailed analyses are needed before generalizations such as this can be used in the system 6

7 selection process. Note was made of these potential problems in the system selection processes because a number of existing and proposed programs concentrate exclusively on devising systems capable of attaining high efficiencies with no regard for the net energy output of the total system or the life-cycle-cost. Net energy output and its effect on the relative worth of systems has not been used much to date either as a goal or as part of the criteria in the hybrid selection process. If the "best" system is to be selected for development whether by government or private industry LFA analyses and especially the net energy output part of the analysis should be employed both as program goals and as system selection criteria. Such analyses will reflect both market and manufacturer needs and if used will allow the prosecution of successful development programs. 16 United States Patent 4,522,894; 6/11/85; Hwang et al 17 United States Patent 5,413,879; 5/9/95; Domeracki et al 18 United States Patent 5,541,014; 7/30/96; Ivlicheli et al 19 United States Patent 5,482,791; 1/9196; Shingai et al 20 United States Patent 5,501,781; 3/26/96; Hsu et al 21 United States Patent 5,678,647; 10/21/97; Wolfe et al 22 United States Patent 5.693,201; 12/2/97: Hsu et al 23 United States Patent 5,811,201; 9/22/98; Skowronski 24 Solar Turbines Incorp orated; TTS85/492; "Energy Conversion: A Vision of the Future;" White REFERENCES United States Patent 3.973,993; 8/10/76; Bloomfield et al 2 United States Patent 3,976,506; 8124/76; Landau 3 United States Patent 3.976,507; 8/24176; Bloomfield 4 United States Patent 4,004,947; 1/25/77; Bloomfield 5 United States Patent 4,001,041; 1/4177; Menard 6 United States Patent 4,128,700; 12/5178; Sederquist 7 United States Patent 4,678,723; 717/87; Wertheim 8 United States Patent 4,738,903; 4/19/88; GUM et al 9 United States Patent 4,743,517; 5/10/88; Cohen et al 10 United States Patent 4,865,926; 9/12/89; Levy et al 11 United States Patent 4,743,516; 5/10/88; Noguchi et al 12 United States Patent 4,820,594; 4/11/89; Sugita et al 13 United States Patent 5,319,925; 1/14/94; Hendnis et al 14 United States Patent 5,314,761; 5/24/94; Pietrogrande et al 15 United States Patent 5,449,568; 9/12/95; Mkheli et al 7