COMPACT METHANOL REFORMER TEST

Transcription

1 COMPACT METHANOL REFORMER TEST Design, Construction and Operation of a 25 kw Unit J. Bøgild Hansen, B. Fastrup, S.L. Jørgensen, M. Boe Haldor Topsøe A/S, Denmark B. Emonts, T. Grube, B. Höhlein, R. Peters, A. Tschauder Forschungszentrum Jülich GmbH, Germany H. Schmidt, W. Preidel Siemens AG, Germany Contract Number: JOE3-CT PUBLISHABLE REPORT Research funded in part by THE EUROPEAN COMMISSION In the framework of the Non Nuclear Energy Programme JOULE III Publishable Final Report 1-16 January 00 JOE3-CT

2 1. Abstract In the future, drive systems for vehicles with polymer electrolyte membrane fuel cells (PEMFC) may be the environmentally most acceptable alternative to conventional drives with internal combustion engines. The energy carrier is neither gasoline nor diesel as in combustion engines today, but methanol, which is converted on-board into a hydrogen-rich synthesis gas in a reforming reaction with water. After removal of carbon monoxide in a gas-cleaning step, the conditioned synthesis gas is converted into electricity in the fuel cell using air as the oxidant. The electric energy thus generated serves to supply the electric drive system of the vehicle. On the basis of the overall process design, a compact methanol reformer (CMR) was designed and constructed by Haldor Topsøe A/S. A test facility was prepared and assembled at Forschungszentrum Jülich. Siemens constructed and supplied a 1 kw PEMFC. Final function tests with the PEMFC and the integrated CMR were carried out to determine the performance and the dynamic behaviour. With regard to the 50-kW(H 2 )-compact methanol reformer, a special technical design of a catalytic burner was made. The burner units with a total power output of 16 kw were constructed and tested under different stages of constant and alternating load. If selecting a specific catalyst loading of 40 g Pt/m 2, the burner emissions are below the SULEV standard. The stationary performance test of the CMR shows a specific hydrogen production of 6.7 m 3 N/(kg cat hour) for a methanol conversion rate of 95% at 280 C. Measurements of the transient behaviour of the CMR clearly show a response time of about 20 s reaching 99% of the demanded hydrogen flow due to the limited performance of the test facility control system. Thin palladium membranes deposited on ceramic tubes have been prepared. The measured hydrogen permeability through the Pd-layer has been as high as 150 Nm 3 /m 2 /hr/bar 0.5. The best membranes were only 3-4 microns thick. The adhesion of the Pd-film to the ceramic support is very good, even after 1000 hours on stream. Pinholes in the membranes lead to a small amount of CO in the permeate, but the complete elimination of this has been demonstrated by selective CO methanantion employing space velocities above 1,000,000 h -1 at 200ºC. JOE3-CT-038 2

3 2. Partnership Haldor Topsøe A/S, Nymøllevej 55, DK-2800 Lyngby, Denmark John Bøgild Hansen Forschungszentrum Jülich GmbH, D-52425, Jülich, Germany Bernd Emonts Siemens AG, ZTEN1, D Erlangen, Germany Harald Schmidt 3. Objectives The overall objective of this JOULE III project was the design, construction and operation of a compact methanol reformer, including gas cleaning in combination with a PEMFC and the interfaces as part of a passenger car drive system based on experience and results from the partners. The technical objective of HTAS was to design a compact reformer (50 kw hydrogen capacity) integrated with a catalytic converter, which provides heat for the endothermic reforming reaction. The design should fulfil the criteria for vehicle application, e.g. low weight and volume, and should be capable of rapid variation in load as well as start-up. The design was based on overall heat and mass balances for the total system, including the gascleaning step and PEMFC. A prototype using a catalytic burner from FZJ should be constructed and test runs carried out. The methanol decomposition catalyst for the unit should be manufactured by HTAS. The unit should be delivered to FZJ for detailed tests. HTAS also has the technical goal of developing a membrane selective for hydrogen permeation based on thin Pd-membranes supported on a ceramic carrier. The aim was to manufacture a prototype and have it tested at FZJ. FZJ was to design and construct a catalytic combuster for the CMR. Its performance would be pre-screened before it was delivered to HTAS for installation as part of the CMR. An energy management strategy for the total system had to be evaluated, ultimately selecting the optimum. The energy storage system had to be integrated into the total drive system. On the basis of the process design of the test-drive system, FZJ had to prepare a test facility which included units delivered by the partners, energy storage system, auxiliary units, supply of methanol and process materials, control engineering and power electronics, gas analysis, data collection, and management. After delivering the test stand, FZJ had to operate the test drive system in long-term test runs, starting with the final function tests. The main goal was to operate the CMR in cyclic tests based on modified driving cycles. FZJ had to collect and to document the test results for the evaluation, which had to be worked out together with the partners. Based on the experimental results, specific energy consumption, emissions and costs for full fuel cycles (FFC) for road traffic (passenger cars) and fuel production as well as fuel transportation had to be calculated for the methanol/ PEMFC powered cars. Comparison with other alternatives would be made. Siemens had to construct a Solid Polymer Fuel Cell stack generating a power of 1 kw and had to conduct endurance tests under H 2 /air operation in order to establish its performance JOE3-CT-038 3

4 characteristics. This stack had to be delivered to FZJ, where it should be combined with the methanol reformer and gas-cleaning step and should be operated with cleaned-up reformate. The fuel-cell stack should be examined after use. 4. Technical Description The partners have collaborated on specifying the foreseen performance of the different units delivered to the project. This has resulted in a flowsheet giving all the material and energy streams in the system. A simplified version is shown as Figure 1. Exhaust Gas Catalytic Burner Air Q Reformer Pd/Ag- Membrane Storage Anode Fuel Cell (PEM) Cathode Cooler Methanol Water Driving Cycle Gearbox E-Motor Electronic Subsystem Air Figure 1 Realistic efficiencies have been assumed for the air blowers and pumps needed for the total system. The system has been calculated for a total maximum electricity production of 25 kw. This requires a methanol flow of 10.4 kg/h resulting in a hydrogen production of 20.4 Nm 3 /h in the compact methanol reformer. As the membrane system developed was not foreseen to be 100% selective due to small cracks in the system, it is foreseen to apply a methanation step downstream the membrane. Based on the maximum tolerable amount fuel from the membrane to the catalytic burner in order not to have a fuel surplus, a hydrogen recovery of 82% from the membrane was calculated. The hydrogen utilization in the fuel cell was predicted to be 92.5%. The detailed design of the compact methanol reformer integrates a catalytic reformer, a catalytic combustion section, consisting of 6 individual burners, and 4 heat exchangers into one single unit. It is the demand to a compact unit that has led to the integration of the different sections instead of arranging them separately connected by piping, which would be the conventional approach. Heat is consumed by the catalytic reforming and is also needed to evaporate the methanol/water feed to the reformer. The heat is supplied by a heat transport system. Throughout the heat transport system, the temperatures are rather uniform. JOE3-CT-038 4

5 The sensible heat in the hot flue gas from the catalytic burner is utilized by means of heat exchange with the incoming streams, methanol/water feed and combustion air. Furthermore, the unit is designed to have a low heat loss to its surroundings by placing the hotter parts of the unit at the centre encapsulated by the colder parts. A picture of the CMR is shown on Figure 2. Figure 2 CMR With respect to the gas clean-up unit, thin Pd-membranes supported on ceramics were selected as the most attractive. Massive, non-supported Pd-membranes are commercially available but have a thickness of microns, which for this project means that the needed amount of Pd would be 1100 to 1740 g, which is clearly unacceptable from a cost point of view. With 3 4 microns thick membranes, the cost can be brought down to acceptable levels as seen in Table 1, which shows the initially estimated Pd amount and the actual amount found in the project. JOE3-CT-038 5

6 Table 1 Use of Pd-membranes as gas clean-up Operating Conditions Inlet gas 66.8% H 2 2.6% CO 20.5% CO 2 9.9% H 2 O 0.2% MeOH Pressure 20 bar g (feed-side) 0.5 bar g (sweep side) Temperature 350 Demands H 2 delivery: 17 Nm 3 /hr H 2 yield: 85% Membrane Properties First Estimate Updated Area 552 cm cm 2 No. of tubes* Weight of Ceramic 330 g 1380 g Mass of Pd 2.51 g 10.5 g Price of Pd 9 ECU 100 ECU * Tubes: Dv Di L = 10 mm 7 mm 250 mm Pd prices increased by a factor 2.6 during Due to small cracks in the thin membranes, a leakage flow from the feed side to the permeate side of the membrane will occur leading to small amounts of carbon monoxide and carbon dioxide in the permeate. In order to remove the carbon monoxide from the permeate, a second gas-cleaning step must be foreseen. In this project, a methanation step has been chosen. Carbon monoxide is converted together with hydrogen into methane and water according to: CO + 3 H 2 CH 4 + H 2 O (1) As an undesired side reaction, the methanation of carbon dioxide occurs: CO H 2 CH H 2 O (2) A catalyst had to be found that was more selective for the methanation of carbon monoxide than of carbon dioxide. The catalytic burner in the compact reformer has the function to provide the thermal energy required for the reforming reaction, and to quantitatively convert all the gases from the system, which still contains flammable fractions, into carbon dioxide and water. This requires the following of the catalytic burner: The emissions of CO, nitrogen oxides and hydrocarbons must be below the SULEV standard through appropriate burner design. The operating range is to be selected so as to prevent flashbacks since these leads to a disruption of the fuel supply and thus interruption of operation. JOE3-CT-038 6

7 Different fuels of various compositions are supplied to the burner. Moreover, inert gases also leave the drive system through the burner. Attention must be paid to the influence of the emission balance. Furthermore, the burner must not change its properties throughout the lifetime of the drive system, i.e. it must have adequate long-term stability. The basic burner structure is a ceramic, hollow cylinder to which fuel gas premixed with air is supplied internally. The outer cylinder surface is provided with a wire mesh coated with noble metal on which the catalytic combustion reaction takes place. In order to control the combustion process, the temperature is measured both on the outer side, i.e. on the catalyst, and on the inner side of the hollow cylinder by use of thermocouples. The burner has the following properties: a density of 311 kg/m 3 and the amount of catalyst on the wire mesh is 20 g/m 2 with an active layer thickness of 0.26 mm. A picture of the burner is shown on Figure 3 below. Figure 3 The PEM fuel-cell system is defined as a stack containing 8 cells and an additional humidification system of the fuel gas and the air. The fuel-cell stack will produce an open clamp voltage of 8.4 V. The rated power will be 930 W at 5.6 V and 165 A. The current density of the MEAs is 300 ma/cm² and the cell voltage 0.7 V. Therefore, the power density is 210 mw/cm². The active size of a cell is 552 cm². The system is designed for a pressure of 1.5 bar abs on the anode and the cathode side, respectively. The Siemens PEMFC is designed to operate at 70ºC. JOE3-CT-038 7

8 Figure 4 Complete test stand 5. Results and Conclusion The completed CMR was delivered to FZJ and built into the completed test stand as shown on Figure 4. The Siemens PEM fuel cell was investigated in a first set of experiments. The I/U characteristics were measured varying stack temperature (T), hydrogen and air pressure (pf), nitrogen pressure (P(N 2 )), purge rate (PR), and air ratio (X) in the maximum ranges possible with the test stand. The I/U characteristics with the highest and the lowest voltage at a certain current are plotted in Figure 5. The highest stack voltages were observed at a hydrogen and air pressure of 0.8 bar (g), a temperature of 60 C, an air ratio of 3.0, and a purge rate of 2.0. Current densities above 120 ma/cm 2 could not be investigated because of the flow operation range of the flow controllers that were chosen for a maximum power of 1 kw at an air ratio of 2.0 and a purge rate of The lowest stack voltages were observed at a pressure of 0.49 bar g, a temperature of 35.1 C, an air ratio of 2.0, and a purge rate of 1.5. The difference between both characteristics is lower at low power densities than at high current densities. The difference increases from 200 mv at 8 ma/cm 2 up to 350 mv at 120 ma/cm 2. It can be assumed that the influence of the parameters will increase further with increasing current density. JOE3-CT-038 8

9 8.0 Stack Voltage / [V] Worst Case: p F = 0.49 bar(g) T = 35.1 C λ = 2.0 PR = 1.5 Best Case: p F = 0.8 bar(g) T = 60 C λ = 3.0 PR = Current Density / [ma / cm 2 ] Figure 5 The dynamic operation of the fuel cell has also been investigated. For this purpose, the performance characteristic of the New European Driving Cycle (NEDC) has been scaled to the power range of the fuel cell of 1 kwe. For the sake of controllability of the mass flow controller, however, a minimum power of 50 W was defined. This assumption is moreover in compliance with the electric power requirements of a real vehicle since this permanently requires electrical energy for operating the ancillary components. Essential parameters of the fuel cell like the hydrogen and air volume flow, current intensity, and overall voltage were investigated. The mass flow controller thus supplies as much hydrogen to the fuel cell as is necessary. It can be shown, however, that the mass flow controller does not assume a much lower new value rapidly enough. When plotting the hydrogen volume flow and the power over time conformance is less pronounced. This is above all due to the fact that the overall voltage drops with increasing current (see Figure 6). In conclusion, however, it can be stated that it is basically possible to simulate a load profile as could arise in a real traffic situation in the test rig. On the basis of investigations concerning long-term stability, conversion of different fuels and flashback behaviour, a burner unit was designed, constructed and manufactured for the compact reformer. The power density for the nominal load case was fixed at 50 kw/m 2. This value is a compromise. Higher power densities are favourable for the emission behaviour of the burner since the temperature, and thus the conversion of the fuel gases, increases with rising power density. However, high power densities lead to a degeneration of the catalyst due to long-term behaviour indicated by high emission values. Lower power densities have a positive effect on flashback behaviour in addition to increasing long-term stability. In the case of a burner diameter of 80 mm, it is possible to accommodate six catalytic burners in an annular arrangement in the bottom section of the compact reformer. Since a total power output of 16 kw must be provided in the nominal load case, the height of the individual burners determined from the optimum power density determined is 212 mm. JOE3-CT-038 9

10 overall voltage/v current intensity/a overall voltage/v time/s current intensity/a Figure 6 - Time characteristics of overall voltage and current intensity The catalytic burner was also operated dynamically simulating the full NEDC and the different emissions measured. The results have been integrated and compared to the SULEV and EURO IV standards as shown in Table 2. Table 2 Emissions Experimental results SULEV standard EURO IV standard mg/km mg/km mg/km CO NO x < HC It can be seen that the burners (the only source of emissions from a PEMFC car) fulfil even the super ultra-low emission standard by large margins. The compact methanol reformer has been operated in a test mode at 260 C and 280 C up to hydrogen production rates of 5 Nm 3 /(kg cat h) and 7 Nm 3 /(kg cat h), respectively. In order to accommodate the Pd-membrane, the system pressure was chosen to be 21 bar. The compact methanol reformer performance is compared in terms of methanol conversion and dry CO concentration with the experiments of a single tube filled with a catalyst loading of 35.5 g in a tube of 25 cm length and a diameter of 12.2 mm. These dimensions correspond to the construction data of the CMR. In Figure 8, the methanol conversion rate is sketched as a function of the specific hydrogen for the CMR and for a single reactor tube at two temperatures, i.e. 260 C and 280 C. As shown, the methanol conversion rates of the CMR are somewhat lower than those for the single tube JOE3-CT

11 considering a comparable load, i.e. an equal specific hydrogen production. At 280 C, a methanol conversion of 95% is reached at a production rate of 6.7 Nm 3 /(kg cat h) for the CMR and 7.87 Nm 3 /(kg cat h) for the single tube. The high pressure of 21 bar for the CMR experiments leads to a lower equilibrium conversion. The equilibrium methanol conversion can be determined to 99.2% at 280 C and 98.1% at 260 C. The experiments clearly shows a limitation of the methanol conversion at low loads and higher pressures. The methanol conversion rate of the CMR provides 86% of the design value derived from experiments with single tubes. Taking into account that the CMR consists of a bundle of tubes, the efficiency of such a scale-up is very good Methanol conversion rate / % C, 3.8 bar, single tube 280 C, 21 bar, CMR 260 C, 3.8 bar, single tube 260 C, 21 bar, CMR Theoretical specific hydrogen production / m³ N /(h kg Cat. ) Figure 7 In the following section, the CMR performance with regard to transients will be discussed. The experiments were performed by load changes, for example from 5% up to 10%, from 10% up to 20%, from 20% up to 40%, and from 40% up to 70%. These partial loads are related to the maximum flow of the methanol liquid flow controller. The partial loads related to the design value of the CMR are by a factor of 1.15 higher, for example the load change of the liquid flow controller from 40 up to 70% leads to a load change of the CMR from 46.1% up to 80.7%. Figure 8 shows the signal of the control system for the methanol liquid flow controller, the resulting methanol flow passing into the reformer and the response functions concerning the hydrogen and the carbon dioxide flow out of the CMR. As shown, the signal to the liquid flow controller for methanol changes during a time interval of 10 seconds. The change in the input flow of methanol is finished after a period of 20 seconds. The hydrogen and the carbon dioxide flow at the reformer exit follow the input flow of methanol immediately without any delay. The hydrogen flow response is in fact determined by the transients of the delivery system for the methanol/water feed. JOE3-CT

12 % % % V / Nl/h % 30% Load % V_H2 Load (set point) V_CO2 Load (current value) t / s 10% 0% Figure 8 In the next section, the energy balance of the CMR will be discussed. Figure 9 shows the efficiency of the CMR as a function of the partial load of the CMR at 260 C and 280 C. This efficiency is defined as the ratio of the enthalpy flow (Lower Heating Value, LHV) of hydrogen outlet the reformer to the input enthalpy flow (LHV) of methanol and the heating values of the burner fuel. During the first tests of the CMR, the catalytic burner is fed with methanol and partly hydrogen, while for the balance of plant calculations done by HTAS, it is assumed that the catalytic burner will be fed with hydrogen, methanol, methane, and carbon monoxide coming from the gas-cleaning system and the fuel cell. The efficiencies from the BoP-calculations amount to 80.7% at full load and 90% at 25% partial load. Additional calculations with a dynamic simulation tool considering the fuel processing system in the NEDC result in an average efficiency of 87%. As shown in Figure 9, the efficiency calculated by the measured flows from the test facility clearly shows a dependence on load. Starting at a low partial load of 6%, the efficiency of the CMR is 60%. At 25% load the efficiency increases up to 78% which is near to the value of 80% for partial loads between 50% and full load. Especially at a temperature of 260 C, the efficiency slightly decreases with increasing load due to a decreasing methanol conversion rate. The efficiency of the CMR is with a maximum of 82% at 280 C for a load between 70%-80% somewhat higher than the design value. The lower efficiency at lower loads is mainly due to the burner operation mode at low loads. JOE3-CT

13 Average CMR efficiency during New European Driving Cycle Efficiency / % Partial load CMR / % 260 C 280 C Design Values Figure 9 Thin palladium membrane deposited on the inner side of ceramic tubes has been prepared. The measured hydrogen permeability through the Pd-layer has been as high as 150 Nm 3 /m 2 /hr/bar 0.5. The best membranes were 3-4 microns thick, so the requirements for diminishing the cost of noble Pd-metal have been fulfilled. The adhesion of the Pd-film to the alumina support is very good, even after 1000 hours on stream. The developed membranes are not 100% selective with respect to hydrogen due to pinholes. The number of pinholes and thus the rate of leakage of CO and CO 2 have been gradually reduced throughout the project. However, the CO content in the permeate is still not low enough to be tolerated by the fuel cell. Measurements with high pressure reformate gas indicated that the hydrogen permeability is significantly smaller than expected from the results obtained in pure hydrogen. In addition, it depends on the hydrogen yield, which is defined as H2 permeated through the membrane[nl/hr] 100 H2Yield = H in reformed gas 2 to feed - side of membrane[ Nl/hr] (%) The measured diffusion capacities of three different Pd-membranes are shown on Figure 10. The diffusion capacity of these membranes in pure hydrogen varied by more than a factor of two; yet they are not very different in reformate gas. JOE3-CT

14 0,3 0,25 0,2 0,15 0,1 0, Hydrogen Yield [%] Figure 10 We have verified that the main reason for this discrepancy is CO poisoning of the membrane. Figure 11 shows the effect of adding CO to a stream of pure hydrogen. 1,0 0,8 350 C 0,6 0,4 0,2 0, % CO inlet Figure 11. The relative effect of CO in the hydrogen feed (20 bar g) JOE3-CT

15 The CO poisoning is temperature dependent. This gives rise to increased activation energy for the diffusion capacity in reformate gas (Fig. 12). Figure 12 Due to the small number of pinholes in even the best membranes, experiments were carried out at FZJ using a special methanation catalyst MC-4 delivered by HTAS. Different leak rates were simulated at various temperatures. The results are shown in Figure CO 2 Conversion Rate / % C 250 C 20 Leak Rate 200 C 1 % 2 % 4 % Space Velocity / (1/h) Figure C JOE3-CT

16 For all the experiments, the conversion of CO to CH 4 was quantitative. It can be seen that CO 2 methanation can be completely avoided at 200ºC with space velocities above 1,000,000 h Exploitation Plans and Anticipated Benefits Presently, there are different views concerning the start of a mass market for PEMFC vehicles. Some car manufacturers (e.g. Daimler Chrysler and GM/Opel) still announce that a mass market will start in 2004 whereas other car manufacturers expect a mass market not before In Japan, NEDO revealed that the costs of PEMFC cars in 2004 would be three times the costs of a conventional car at that time. Five of the biggest Japanese car manufacturers expect that there will be at least another twenty years before PEMFC vehicles come into a mass market. Considering these statements, it is not possible at the moment to make an exploitation and marketing plan for the PEMFC system for the near future. Ultimately, the customer plays an important role in future fuel-cell scenarios. The start of a mass market for PEMFC vehicles has to be prepared by car manufacturers, fuel providers (e.g. infrastructure for methanol) and government (tax and other regulations). The partners have, however, gained valuable insight into the different units supplied to the project and it is anticipated that some of the units can be used in the shorter term for other applications, f.inst. stationary or marine PEMFC plants where the requirements to cost and dynamic behaviour are less stringent. All in all, the anticipated benefits to society will be less pollution, higher energy conversion efficiency and less dependence on crude oil derived enrage. JOE3-CT