R&D on High-Efficiency Hydrogen Production System for Hydrogen Supply Station

Transcription

1 2001.M4.2.5 R&D on High-Efficiency Hydrogen Production System for Hydrogen Supply Station (Hydrogen Production System Group) Osamu Yamase, Yoshiichi Suzuki, Yoshihiko Aihara, Akihiko Matsuoka, Hideto Takahata, Katsumi Yoshida 1. Contents of Empirical Research 1.1 Overview The purpose of this research is to develop a hydrogen production/supply station for supplying hydrogen to the fuel cell vehicle (FCV), which is expected to become the next-generation automobile. A system in which petroleum-based fuels are reformed on-site offers the advantage that it can be realized by modifying the conventional infrastructure of service stations. In the home use and stationary sector, hydrogen production systems from city gas are being established by means of steam reforming reaction. However, if we can process petroleum into hydrogen with making advantage of the fact that petroleum can be stored in underground tanks of service stations, hydrogen will also be able to produced and supplied in areas where city gas is not available. In the present R&D, a hydrogen production system is being developed as hydrogen forecourt for FCV using a catalytic partial oxidation (CPO) reforming technology (owned by Shell Group) that is compact and highly load-responsive. The petroleum-based hydrogen sources are LPG, light naphtha and heavy naphtha for CPO reaction, therefore, these fuel pretreatment technology and reformed gas purification technology should be developed, and these technologies will be integrated and lead to the construction of hydrogen supply station (See Figure 1). Hydrocarbon feed Steam Pretreatment facility Post treatment facility Air Steam Figure 1 Composition of Hydrogen Supply Station System Using CPO Reforming Unit At present there are still some undetermined factors; such as the FCV types and the hydrogen supplying method, therefore, we set the target for high hydrogen quality so that the specifications required by each FCV types can be met. Since CPO alone is already achieved the efficiency of 75% or more, the target efficiency of the total system is set at 70% or more when losses in the separation process are considered. 1

2 In consideration of the current PEFC technological level 1) and the case of vehicle with metal hydride, the CO concentration is targeted at 10 ppm or below. 1.2 Development of Hydrogen Production System Development of Reformer Pretreatment Technology Among the types of petroleum feed assumed for use with reformer for hydrogen supply station are LPG, light naphtha and heavy naphtha, but these feed materials are thought to vary in composition and properties. An investigation of pretreatment technologies required for each feed material prior to the reformer was made Effect of CPO Operating Conditions on Reformed Gas Composition (1) Reactions with CPO The following reactions are believed to occur. Partial oxidation reaction C n H m + n/2 O 2 n CO + m/2 H 2 (exothermic reaction) (a) Reforming reaction C n H m + n H 2 O nco + (m/2+n) H 2 (endothermic reaction) (b) Shift reaction CO + H 2 O H 2 + CO 2 (exothermic reaction) (c) Methane reaction CO +3 H 2 CH 4 + H 2 O (exothermic reaction) (d) In the case of partial oxidation reaction (a) only, the O 2 /C ratio becomes 0.5, the optimum value, and in the case of shift reaction (c) with no reforming reaction (b) the H 2 O/C ratio becomes optimal at 1.0. On the other hand, if reforming reaction (b) and shift reaction (c) occur 100%, the H 2 O/C ratio becomes 2.0. Because the H 2 created in reactions (a) to (c) is consumed in the methane reaction (d), to improve the hydrogen yield, attention must be given to curtailing the production of CH 4 coming as a result of reaction (d). (2) CPO reformer Using the CPO reformer owned by the Shell Global Solutions International (SGSI), experiments were performed to investigate the effect of fuel and other running conditions on CPO reformed gas. An outline of the unit used is depicted in Figure 2. The unit is designed to have a 50kWe output on a fuel cell base and a naphtha supply volume of about 12 kg per hour. In the latest experiments, the naphtha supply volume was roughly halved because of constraints on heater heat supply capacity and on fuel tank capacity. The flow rate of fuel, water and air could each be controlled, therefore O 2 /C ratio and H 2 O/C ratio were established independently. The fuel, water and air mixing temperature (Tmix preheat temperature) can be indirectly regulated by controlling the heater that heats water and air. Catalyst outlet temperature (Tout) is also measured, and the composition of reformed gas is analyzed through gas chromatography measurements when the mixture temperature becomes roughly constant. 2

3 Figure 2 Outline Drawing of CPO Reformer (3) Air and fuel mixture ratio Based on the investigation with (1) described above, the hydrogen production ratio was measured at which conditions are changed within the range of 0.4 to 0.5 for O 2 /C ratio. (4) Mixture ratio of fuel and water (or steam) Similarly, the hydrogen energy efficiency was measured at which conditions are changed within the range of 0 to 1.2 for H 2 O/C ratio. (5) Air-fuel mixture preheating conditions Because of problems with the heat supply capacity of the heater attached to the unit shown in Figure 2, experiments were conducted with 350 C as the upper temperature limit. (6) Effect of feed fuel properties and of differences in composition Table 1 gives the specific gravity, average molecular weight and composition of fuel used in the experiments. In comparison to Japanese naphtha, European naphtha has larger specific gravity, and the specific gravity of light naphtha in Europe is roughly equivalent to that of heavy naphtha in Japan. For this reason, n pentane/n hexane mixed fuel closely resembling light naphtha in Japan was added to the experiments in order to confirm whether Japanese light naphtha can be supplied to the CPO reformer; comparisons were made with three types of fuel. 3

4 Table 1 Fuels Used in Experiments Feed Heavy Naphtha Light Naphtha C5/C6 Tops Density Moleweight Paraffins %w Naphtenes %w Aromatics %w Development of Reformer Purification Technology Reformed gas contains CO, N 2 and other gases besides hydrogen, and it is necessary to remove such gases efficiently. There are various separation/removal technologies including pressure swing adsorption (PSA), membrane separation and metal hydride. Hence an investigation was made for developing a reformed gas purification system that is efficient. 2. Results of Empirical Research and Analysis Thereof 2.1 Development of Reformer Pretreatment Technology As the first step the general properties of heavy naphtha, light naphtha and LPG, scheduled to use as CPO feed, were investigated. Normally, the sulfur concentration in these materials is less than 1.0 ppm, and it was found that this concentration does not have a direct major effect on CPO operation. For the present research, it was decided to mount a Ni guard bed on the CPO unit scheduled for introduction next year before supplying CPO fuel so as to curtail discrepancies in sulfur concentration arising from each lot of feed used. 2.2 Effect of CPO operating conditions on reformed gas composition (1) Fuel and air mixture ratios Using heavy naphtha with the H 2 O/C ratio held constant (1.0 or less), the mixture temperature was varied between 200 C, 250 C, 300 C and 340 C; the O 2 /C ratio was varied within the range of 0.4 to 0.5, and the effect on hydrogen energy efficiency was investigated. The investigation was conducted under three sets of conditions, from high to medium and low, with the O 2 /C ratio in the range of 0.4 to 0.5. Calculation of the hydrogen energy efficiency was defined as follows. ε LHV H2 = LHV H 2 (produced hydrogen mol count + produced carbon monoxide mol count) / LHV feed Following a late stage, CO is reacted with steam in shift reactor and converted to hydrogen at 1 : 1, then added to the volume of hydrogen actually produced and calculated. 4

5 Figure 3 Effect of O 2 /C ratio on Hydrogen Energy Efficiency Figure 3 illustrates the results of an investigation of the effect of O 2 /C ratio on hydrogen energy efficiency. At each mixture temperature there is an optimum O 2 /C ratio, and it was observed that if the temperature becomes too high, the hydrogen energy efficiency tends to drop. Figure 4 Effect of O 2 /C ratio on CO 2 Production Figure 4 illustrates the results of an investigation of the effect of O 2 /C ratio on CO 2 production. It appears that when reaction temperature becomes high due to an increase in the O 2 /C ratio, H 2 and CO 2 declined due to advancement of the shift reaction CO + H 2 O H 2 + CO 2 reverse reaction. From the aforementioned, it is suspected that when the O 2 /C ratio becomes high, the oxidation reaction advances more than necessary, hydrogen is consumed, and the hydrogen energy efficiency tops out. Accordingly, when the H 2 O/C ratio is held constant (1.0 or less), the medium level becomes the optimum level for O 2 /C ratio. 5

6 (2) Fuel and water mixture ratios Using heavy naphtha, the O 2 /C ratio was set to medium level, the mixture temperature was set to 300 C and the H 2 O/C ratio was varied within the range of 0 to 1.2. The results are shown in Figure 5. In the case of insufficient H 2 O, the CO shift reaction might not advance, but because CO is included as hydrogen in the calculation of energy efficiency, it does not have a conspicuous effect. The theoretical line follows a gentle curve, but it is believed that the optimum value obtains when the H 2 O/C ratio is 1.0 or less. Figure 5 Effect of H 2 O/C ratio on Hydrogen Energy Efficiency (3) Air-fuel mixture preheating conditions Using heavy naphtha, the H 2 O/C ratio was first set to 1 or less, considered to be the optimum value, and experiments were performed. Figure 6 depicts the relationship between mixture temperature and CPO catalyst outlet temperature. When the O 2 /C ratio is held constant, the mixture temperature and catalyst outlet temperature are in a roughly proportional relationship. It is believed that the reaction temperature can be controlled by modifying the mixture temperature. Figure 6 Mixture Temperature versus CPO Outlet Temperature 6

7 Figure 7 depicts the relationship between mixture temperature and methane production volume. It shows that methane production dropped as the mixture temperature rose and as the O 2 /C ratio increased. Figure 7 Effect of Mixture Temperature on Methane Production Volume (4) Effect of Feed Properties and Composition Experiments were conducted in which the O 2 /C ratio was set at medium, the H 2 O/C ratio was set to 1.0 or less, and the fuels were changed heavy naphtha, light naphtha and C5/C6 mixed fuel respectively. The results are shown in Table 2. Hydrogen energy efficiency was roughly the same no differences in fuel could be noted. Table 2 Hydrogen Energy Efficiency by Difference in Fuel Feed Heavy Naphtha Light Naphtha C5/C6 Tops T out P bara H2 %mol CO %mol H2+CO/HC ε LHV H2 79% 80% 79% 2.3 Development of Reformer Purification Technology (1) PSA (Pressure Swing Adsorption) Technology An investigation of existing technology was made through the literature 2) 3), PSA specifications were determined and a PSA unit was designed and installed. A two-tower configuration was employed for sufficient capacity (3 liters per tower) for processing some reformed gas for 50kW output CPO (See Figure 8, PSA). Adsorbent selection and combination will be a future topic of investigation. The unit can be used as necessary for the following. 1) Investigation of two-stage processing since the towers can be connected in series. 2) Two types of different adsorbent can be evaluated in each tower. 7

8 3) The two towers can be applied in a normal swing method of use. Figure 8 Outline Drawing of PSA (2) Metal Hydride Technology A survey was made of the literature 4) 5) 6) 7) and a hydrogen purification in reformed gas, using metal hydride (MH), was selected as the unit at the final stage of a hydrogen production system. It was discovered that hydrogen refining technology based on MH has the following features suitable for a hydrogen supply station. The cost of its introduction is relatively low. Energy consumption is low. Operation is simple. The technology is well-suited for use with small-scale facilities. It can be used in a low pressure range of 1MPa or below, and it is applicable to a wide range of hydrogen concentrations. The fact that the technology can be used at low pressure is a potentially great advantage. If the pressure can be 1MPa or below in the entire hydrogen supply system, with separation and refining processes included, the degree of freedom in design of the hydrogen supply system can be enlarged without various legal restrictions. On the other hand, hydrogen separation and refinement by MH poses the following disadvantages. Unless a mixture gas with a certain high level of hydrogen concentration is used in the feed, hydrogen gas of high purity cannot be obtained. The feed gas must undergo pretreatment to avoid alloy poisoning. 8

9 The major types and characteristics of MH were also investigated. It was judged that a AB5-type alloy relatively strong against CO and H 2 O is best suited for a hydrogen station, which can be operated as a single-stage plateau within an operation temperature range of 100 C or below, and a hydrogen separator within reformed gas was designed, fabricated and installed. Cold-warm water High-purity hydrogen (Reformed) gas Impure gas Cold-warm water Figure 9 Schematic Drawing of Purification System Using Metal Hydride 3. Results of Empirical Research The following results were obtained from empirical research conducted this fiscal year. (1) Development of reformer pretreatment technology In the present research, the general properties of petroleum products targeted as CPO feed were investigated, and it was found that these properties do not obstruct the operation of CPO. (2) Effect of CPO operating conditions on reformed gas composition By varying mixing temperature, O 2 /C ratio, H 2 O/C ratio and the effect on hydrogen energy efficiency were measured with gas chromatography. (3) Development of reformed gas purification technology PSA and MH were selected as units for high purification of hydrogen after CPO reformer, and they were designed, fabricated and installed. 9

10 4. Synopsis In the current fiscal year, which is the first fiscal year, an investigation was made for development of reformer pretreatment technology for the catalytic partial oxidation (CPO) method owned by the Shell Group; CPO reformer operating conditions were also investigated and reformed gas purification technology was studied. In FY 2001 the latest findings will be reflected in the design of a CPO unit scheduled to be introduced in Japan, and subsequent items will be carried out. (1) Development of fuel pretreatment technology Steps will be taken to optimize CPO operating conditions in accordance with each fuel and the features of the candidate purification unit. Membrane separation performance will be evaluated and technology for separation of nitrogen from air for processing will be investigated. Oxygen enrichment membrane will be investigated, and load responsiveness will be elevated by lowering the ratio of nitrogen in shift reactor. The effect on performance of minute quantities of ingredients in fuel will be investigated. (2) Development of CPO reformed gas purification technology Investigation of pressure swing adsorption (PSA) Adsorbents will be selected and various combinations of adsorbents will be investigated, followed by an investigation of optimum operating conditions. The objective is to elevate hydrogen concentration to 75% or above and to lower carbon monoxide concentration to 10 ppm or below. Investigation of metal hydride Operating conditions for achieving high purity in hydrogen will be investigated, as will the effects of minute quantities of CPO gas composition on alloy life. (3) Development of optimum system for hydrogen supply station An investigation of methods for effective use of energy as the total system including the reformer unit will be made. The effects of fuel compositions and properties on reformed gas production efficiency, on impurities concentration and on load to purification technology will be investigated. 10

11 Reference bibliography 1) Nobuyuki Kamiya et al, "Development and Application of Solid Macromolecular Fuel Cell", NTS 2) Gu G.W et al. The Progress of PSA Technology for Hydrogen Separation and Purification Proceedings of the 13th World Hydrogen Energy Conference, Beijing, China, June 12-15, ) Takuya Hanada, "Liquid Hydrogen Production, Storage and Transport", Hydrogen Energy System, No.25 edition No.2, December ) Uehara, "Hydrogen Storage by Metal Hydride", Hydrogen Energy System, No.25 edition No.2, December ) Keizo Onishi, "Discussion of Metal Hydride", Japanese Standards Association, June ) Kenji Aihara, "Development of Metal Hydride", WE-NET Project Preliminary Edition, May 24, ) Venkateswara Sarma.V, et al. Investigation on the Synthesis, Characterization and Hydrogen/Dehydrogenation Behavior of Some New AB5 Type MmNi 5-x Fe x (x=0.4) and Mg-ywt% MmNi 5-x Fe x (x=0.4, y=30) Hydrogen Storage Materials Proceedings of the 13 th World Hydrogen Energy Conference, Beijing, China, June 12-15, 2000 Copyright 2001 Petroleum Energy Center all rights reserved. 11