CRITICAL ISSUES IN HEAT TRANSFER FOR FUEL CELL SYSTEMS

Transcription

1 ECN-RX CRITICAL ISSUES IN HEAT TRANSFER FOR FUEL CELL SYSTEMS.F. van den Oosterkamp resented at the Heat Transfer in Components and Systems for Sustainable Energy Technologies 5-7 April 2005, Grenoble, France ARIL

2 SET 2005 Heat Transfer in Components and Systems for Sustainable Energy Technologies 5-7 April 2005, Grenoble, France Critical issues in heat transfer for fuel cell systems.f. van den Oosterkamp 1 1 Energy research Centre of the Netherlands (ECN) Unit Fuel Cell Technology 1755 ZG etten The Netherlands vandenoosterkamp@ecn.nl ABSTRACT The current state of the art in fuel cell system development will be reviewed with an emphasis of the critical issues on heat transfer. The heat transfer issues for both EM based systems and SOFC based fuel cell systems will be addressed. For systems that are based on hydrocarbon fuels a reforming step is needed and critical heat transfer issues are also present in this fuel processing part of the system where the primary feedstock is converted to reformate. Also, in both the EM and SOFC fuel cell itself, heat transfer is a critical issue. It will be shown what are the implications of the fuel cell heat transfer to the total system architecture for the various fuel cell applications (stationary power, transport). The heat transfer issues in fuel cell system development will be clarified with several examples. target is around 50 $/kwe, for stationary applications this target is $ $/ kwe. Fuel selection and fuel cells One of the advantages of the use of fuel cells is a significant increase in the fuel flexibility. The hydrogen for use as a feed for fuel cells can be produced with a number of different routes (figure 1). From different feed stocks, either centrally or decentrally, directly connected to a fuel cell. High temperature fuel cells can use gaseous fuels like methane (main component in natural gas or produced from biomass) and propane directly. Low temperature fuel cells require relatively pure hydrogen, produced through reforming and subsequent gas treatment from fossil fuels (gaseous or liquid) or from biofuels (for example biodiesel) or through electrolysis of water with solar energy or wind energy. Gaseous Fossil Fuels Liquid Fossil Fuels Liquid Biofuels Liquid Synthetic Fuels INTRODUCTION Hydrogen + carbon monoxide Solar Wind Fuel Cell Technology and applications Fuel cell technology is an important enabling technology for the introduction of a hydrogen economy. The hydrogen economy has the potential to combat climate change and reduce the dependency on fossil fuels, in particular oil. Hydrogen is seen as an important transition fuel for stationary- and transport applications. The commercial introduction of fuel cells for these applications requires robust and integrated systems that meet the cost targets of the market applications. For the transport market the cost High T Fuel Cell Methane Hydrogen Figure 1 Fuel routes for use of fuel cells Biomass Low T Fuel Cell Fossil fuels will continue to play an important role in the next few decades, possibly in combination with biomass. 2

3 3 The introduction of fuel cells and the hydrogen economy The large scale introduction of fuel cells in the large CO 2 reduction Fuel Cells on Hydrogen and with Fuel rocessors for Stationary and mobile niche markets Renewable Hydrogen Hydrogen from Biomass and Fossil Fuels, in combination with CO2 sequestration Large scale introduction of fuel cells Hydrogen from natural gas and biomass Time Figure 2 Introduction of fuel cells in various market segments consumer markets like residential heat and power units, passenger cars, will be preceded by niche applications. In the course of this process, an energy efficient hydrogen infrastructure has to be designed and constructed. THE FUEL CELL SYSTEM ARCHITECTURE The fuel cell system A fuel cell system is defined as an architecture consisting of a reformer, a fuel cell and a power conditioner as main components. Figure 3 presents a schematic block diagram of the fuel cell system architecture. The Fuel rocessor The fuel processor architecture depends on the type of fuel cell that is applied. For a high temperature fuel cell (SOFC or MCFC) the fuel processor consists only of a primary reformer to convert the hydrocarbon fuel to a mixture of methane, hydrogen, CO 2, CO and water. For a low temperature fuel cell system the fuel processor consists of a primary reformer and a gas clean up section, usually consisting of a shift section (high and low temperature) and a preferential oxidation unit, in order to reduce CO levels in the reformate stream to a level < 10 ppm. For the primary reformer different technologies, dependant on the application can be applied. These technologies include steam reforming, autothermal reforming and catalytic partial oxidation. Comparison of the EMFC system and the SOFC system A typical simplified flow schema for the EMFC and the SOFC based system is given in figure 4. rimary reforming WG WG Shift Shift ROX ROX CO C x H y + O 2 ATR C x H y + O + H 2 2 O CO + H 2 SR C x H y + H O 2 CO + H 2 O CO 2 + H 2 EM EM fuel fuel cell cell CO + O CO 2 2 Clean H 2 (<10 ppm CO) SOFC SOFC anode anode exhaust exhaust utilisation utilisation H + CH O 2 H O + CO 2 2 Figure 3 Fuel Cell system, schematic When hydrogen is used as fuel for the system a reformer is not needed; in this case hydrogen will be directly supplied from a hydrogen storage facility (usually a pressurized tank) to the fuel cell. In addition to the main components, balance of plant components (BO) are needed to complete the system. These BO components include an air blower for the fuel cell, an air compressor for the reformer, water pumps, mass flow controllers, a control system and a safety system. Figure 4. Simplified flow scheme EMFC system and SOFC system For the EMFC system a gas treatment section (Shift, rox) is needed to reduce the CO level to a value of <10 ppm. For a SOFC system, the primary reformer product can be applied directly in the SOFC as the temperature levels are very close. For a SOFC system, the heat transfer issue are related to a high (>600 0 C) temperature level. In a EMFC system the heat transfer takes place at different temperature levels, that dictate the need for an efficient heat integration.(figure 5). 3

4 4 EMFC System; General Flow Scheme This means that the required dynamic behaviour determines to a large extent the fuel cell system configuration. Ethanol Off-gas Depleted Water Tap water Reformer HTS LTS rox After Burner Hot tap water Fuel Cell Stack Anode Cathode DC ower Life time For stationary applications the required lifetime of the system is 30,000-40,000 hrs. For transport applications the required lifetime is around 5,000 hrs. The fuel cell is the most critical item in the system with regard to lifetime. The fuel cell lifetime is reflected in the cell voltage decrease over time and is usually expressed as % efficiency drop per 1000 hrs of operation. Cooling Hot cooling air Figure 5 Heat transfer in a EMFC system Size and weight Size and weight are very important criteria for automotive applications and small residential power systems. Size and weight are less important for larder scale stationary power plants. The key heat transfer issues in EMFC systems are a well defined heat integration, using concepts of pinch technology (see below) and concepts of process integration. CRITERIA FOR SYSTEM DESIGN For the design of a fuel cell system a number of general criteria are present that define to a large extent the design basis for a fuel cell system. These general criteria are translated in a basis of design, which combines all relevant design starting points. The design is started with a number of possible conceptual flowsheets which are than judged on the design criteria. The flowsheet which is selected is the basis for the further basic and detailed engineering efforts. Efficiency Efficiency is one of the most important criteria in the design. Net system efficiency is expressed as Electric power produced (AC or DC), minus parasitic power losses, divided by the lower heating value of the feed and fuel that is used in the system. The efficiency level for EMFC system is typically 40 %, whereas the SOFC system can have an efficiency up to 50 %. Dynamic behaviour The dynamic behaviour of a system includes a) the system response to a load change and b) the typical start-up time. Dynamic response time of a running system varies between 1-3 seconds to several minutes. Dependant on the application the system architecture can be designed to meet the required response time. Start up time from cold conditions is more critical as reactors (including the fuel cell) have to be heated up to their operating temperature. This start up time can vary from typically 30 minutes for EMFC systems to 2-3 hours for SOFC systems. CRITICAL HEAT TRANSFER ISSUES Thermodynamic principles The heat transfer required in the fuel cell system is determined by the thermodynamic principles of the chemical and electrochemical conversion of the fuel to a hydrogen rich gas and a subsequent conversion of the hydrogen into electricity in the fuel cell. The inherent advantage of fuel cells is that they do not limited by the Carnot cycle, so that most of the chemical energy in the fuel is available for conversion to electricity (A.J.Appleby et al (1993)) Thermodynamic system analysis A thermodynamic system analysis is usually done with a suitable process simulation tool like Aspenlus or Hysis. Using these simulators the stream compositions and physical properties are calculated. Based on these calculations the heat transfer network between hot streams and cold steams can be designed. A tool like pinch analysis is a very valuable tool for this exercise (Linnhof et a (1983), whereby the combination of the heat exchanger network should be done in combination with a proper application of safety concepts. Exergy analysis In a fuel cell system there are various hot and cold streams as well as streams that represent electrical energy. These streams have a different energy quality, which is reflected in the exergy value of the stream. Next to the above mentioned pinch analysis, exergy analysis is a powerful tool to optimise the heat and power integration of a fuel cell system (vd Oosterkamp et al (1993), A.de Groot (2004). 4

5 5 Heat transfer issues in the primary reformer In a primary reformer the hydrocarbon feed is converted to a hydrogen rich synthesis gas, also indicated as reformate. There are various ways to produce this synthesis gas. For steam reforming the overall reaction can be represented by: C n H m + H 2 O (m/2 + n) H 2 + nco (1) In this case, the endothermic heat of reaction is supplied by hot flue gas that is produced in an external burner, for example burning anode off gas combustion. The heat transfer from the flue gas to the catalyst, through a separating reactor wall (for example a tube wall), is critical (figure 6). Oxidation, total : CH 4 + 2O 2 2H 2 O + CO 2 (4) In the catalytic routes ((1) - (4)) to produce synthesis gas, many reactions occur simultaneously; the composition of the synthesis gas product is determined by the thermodynamics of the equilibrium reactions (1) - (4). In the inlet section of the reactor, part of the feed is oxidised to CO 2 and H 2 O, increasing the temperature of the reacting gas to a level of about C which initiates endothermic steam reforming reactions and reduce the temperature to about C at the reactor exit. Figure 7 shows a measured temperature profile over an ATR reactor for the conversion of diesel to reformate. diesel T wo T wi injector mixer catalyst bed T b Figure 6 Heat transfer through a reactor wall Inside the reactor an axial temperature gradient is present. The heat transfer requires a burner that provides the right heat flux to the reactor where the conversion takes place. The design of this heated reactor tube involves the combination of reaction intrinsic kinetics, catalyst morphology, reactor wall dimension and involves the simultaneous calculation of a set of coupled partial differential equations (ref Froment and Bischoff (1990). The steam reformer temperature levels make it possible to design a reactor that resembles a significant heat recovery inside the reactor. (ref.rostrup Nielsen (1983), van den Oosterkamp (2003). In case the primary reformer is done in an auto thermal reactor (ATR) or Catalytic artial Oxidation (CO) reactor the heat of reaction is supplied by the combustion of a part of the feedstock inside the reactor. The reactions which take place, next to the steam reforming reaction (1) are: air Temperature [0C ] ,0 0,2 0,4 0,6 0,8 1,0 Reactor axis [-] Tdiesel Tgas Figure 7. ATR temperature profile (H.A.J.van Dijk,/ECN,2004) ; O 2 /C=0,45 ; H 2 O/C=1 ; GHSV=20,0000 Tcatalyst Heat transfer issues in Shift conversion In a shift reactor the CO present in the reformate from the primary conversion step is converted to hydrogen an CO 2 through the shift reaction: The shift conversion is mildly exothermic. The shift reactor is designed as an adiabatic reactor and shows a temperature increase over the length of the reactor (figure 8). The shift conversion takes place in two different reactors, a low (LTS)- and a high temperature shift (HTS) conversion. Between the two shift reactors inter-cooling has to be applied. Oxidation, partial : CH 4 + ½ O 2 2H 2 + CO (2) CH 4 + 3/2 O 2 2H 2 O + CO (3) 5

6 6 The shift reactor is usually the rate-determining factor for the start up time of a system. Using more active catalyst materials on a suitable carrier, for example a monolith, can reduce this start up time. CO wet gas [%] % LTS HTS intercooling T [ C] Figure 8.Typical temperature and concentration profile in a shift reactor Heat transfer issues in the Selective Oxidation reactor Objective of the selective oxidation reactor (SELOX) is the conversion of CO to a level that is acceptable by a EMFC fuel cell. This is achieved by a conversion at a temperature level of about C. As the selective oxidation is exothermic the reactor has to be cooled to avoid non-selective oxidation, as well as methanation of CO with H 2 to CH 4. Figure 9 shows the CO exit concentration of a SELOX reactor using ECN's patented rox catalyst that removes CO over a large temperature range with high selectivity and stability with low noble metal loadings. CO (ppm) rox CO at lambda= and 5000 ppm CO inlet 5000 ppm 2500 ppm Temperature ( C) Figure 9. CO exit concentration Selox as a function of the reactor temperature This indicates that the Selox reactor has to be cooled in order to keep the temperature below C, to keep VCO levels below 10 ppm. This is usually achieved by an interbed heat exchanger (figure 10). Figure 10 Selox reactor (25 kw scale, courtesy ECN) Heat transfer issues in the EMFC The EMFC fuel cell today operates at a temperature of about 80 0 C.This temperature is dictated by the polymer membrane. This membrane is of the type of erfluorosulfonic acid (e.g. Nafion). These membranes exhibit high proton conductivity only in the hydrated form. The required presence of water therefore limits the operating temperature to well below C at atmospheric pressure. The EMFC is cooled to remove the fuel cell waste heat. This is normally achieved through water-cooling as primary cooling loop. This primary cooling loop is than giving off heat in a secondary heat exchanger, for example an air cooler. The efficiency of the heat transfer from the fuel cell to the environment is thus strongly dependant on the ambient temperature. In case of a high ambient temperature the temperature difference is relatively small and the heat exchanger surface (for example a radiator in a car) will be very large. (R.K.A.M.Mallant (2003)). New developments in the EMFC area include new polymer materials, for example BI (olybenzimidazole) based polymers (.Li et al, 2004) that would enable the EMFC to operate at a temperature of above C. This would drastically enhance the heat transfer and provide a waste heat at a higher temperature level that could be used more efficiently in a fuel cell system, for example for the preheating of fuel or utility streams. 6

7 7 Heat transfer issues in the SOFC The SOFC fuel cell operates at a temperature level of about C. Like the EMFC, the SOFC produces waste heat, but in this case the heat is produced at a much higher temperature level. In the SOFC this heat is removed by the cathode air stream, that is operated at large excess. The cathode off gas is usually recycled via cathode air heat exchanger and a cathode air recycle compressor (not shown) (figure 11). Anode recycle compressor Reformer Cathode heat recuperator Anode recycle cooler Figure 11 SOFC heat recovery Anode Cathode SOFC After burner As the SOFC fuel cell operates at a hydrogen utilization of about 80 %, non-converted hydrogen is usually recycled to the anode inlet, some hydrogen may also be burned with the cathode air to provide high temperature heat recovery. In order to achieve this hydrogen recycle, a high temperature hydrogen cooler and a high temperature hydrogen recycle compressor are needed. 5. High temperature gas coolers and recycle compressors are critical elements of heat transfer around SOFC based systems. 6. inch analysis and exergy analysis are powerful tools to determine the heat integration options in a fuel cell system, in particular for EMFC based fuel cell systems. REFERENCES 1. A.J.Appleby in : Fuel Cell Systems, ed. J.M.J. Blomen and M.Mugerwa (1993) lenum. 2. B.Linnhoff, E.Hindmarsch, Chem.Eng.Sci.,38 (1983) F van den Oosterkamp A.J.Goorsen, L.J.M.J.Blomen, Journal of ower Sources, 41 (1993) p A.. de Groot, hd thesis, ECN-R (2004) 5. G.F.Froment, K.B.Bisschof, Chemical Reactor analyis and design, Wiley (1990) 6. J.Rostrup Nielsen in: Catalysis, Science and technology ed. J.R. Anderson, M. Boudart (1984) Springer Verlag. 7..F. van den Oosterkamp, Synthesis gas general: Industrial rocesses and Relevant Engineering Issues, Encyclopedia of Catalysis, (2003) (Wiley). 8. H.A.J. van Dijk, private communication (ECN,2005). 9. R.Mallant, J.ower Sources 118 (2003) Li, R.He, J.O.Jensen, N.J.Bjerrum Fuel Cells 4 (3) (2004) 147. CONCLUSIONS The critical heat transfer issues in a fuel cell system include: 1. The design of the heated reactor in case of a steam reformer for the production of primary synthesis gas. 2. The ATR and CO reactor design should prevent excess temperatures as result of partial oxidation reactions that would give runaway reactions. 3. The shift reactors are only sensitive with regard to the start up time. More active catalysts on a suitable carrier (such as a monolith) will reduce the reactor volume and the associated start-up time. 4. Effective cooling of the Selective oxidation reactor, next to a selective oxidation catalyst is essential to avoid non-selective reactions. 7