A versatile, steam reforming based small-scale hydrogen production ...

WHEC 16 / 13-16 June 2006 – Lyon France

A versatile, steam reforming based small-scale hydrogen production process P.C. Hulteberga, B. Porterb, F.A. Silversandc, R. Woodsd a

Catator AB, Scheelevägen 19 F, 223 70 Lund, Sweden, [email protected] b Intelligent Energy, 2955 Redondo avenue, Long Beach, 90806 CA, USA, [email protected] c Catator AB, Scheelevägen 19 F, 223 70 Lund, Sweden, [email protected] d Intelligent Energy, 2955 Redondo avenue, Long Beach, 90806 CA, USA, [email protected]

ABSTRACT: In this paper, a new design methodology and process is proposed for small scale pure hydrogen production capable of serving energy markets ranging from distributed generation to vehicular refuelling. The system was designed for producing 7 Nm3/hr pure hydrogen (purity of < 1ppm CO dry), yielding 10 kWe net power from a fuel cell system with an overall parasitic power loss < 10 %. The discussion of this process includes a detailed description of the design methodology and operational results of the catalytic converter, the hydrogen purification system and the fuel cell system. This paper will discuss the design methodology of the overall system, as well as the specific design of the catalytic converter, the catalysts used within, and the hydrogen purification system. It will also report the system performance including gas purity, recovery rate, overall hydrogen production efficiencies, and electrical efficiencies during fuel cell operation.

KEYWORDS: Hydrogen production, Steam reforming, Pressure swing adsorption, Fuel cell

1

INTRODUCTION

Increasing oil prices coupled with increasing environmental awareness has increased the pace in the development of alternative fuel sources and improved fuel economy. As a part of this development hydrogen is being carefully investigated as a possible energy carrier of the future. There are two main routes to manufacturing hydrogen; the first one being the electrolysis of water and the second one being steam reforming of hydrocarbons. In both cases the product, hydrogen, can be made carbon dioxide neutral if the source of the electricity or hydrocarbon is carbon dioxide neutral e.g. wind/solar power or biomass. Further, the use of electrochemical conversion devices, such as fuel cells, in place of internal combustion engines significantly reduces resource consumption, be it electricity or hydrocarbon fuel, for an equivalent amount of work. Each of these methods has advantages and disadvantages but any method that involves the production of an intermediate energy carrier, electricity in the case of electrolysis, prior to the production of hydrogen will have significant efficiency disadvantages due to the energy required for the conversions [1]. There are three ways to produce hydrogen within the fuel cycle when scale is considered [2]. First there is the possibility of complete centralization with distribution of the hydrogen via pipelines and trucks to refuelling stations. The second possibility is distributed hydrogen generation (DHG), where the hydrogen is produced locally at the refuelling stations. The third possibility is mobile hydrogen generation with on-board hydrogen production from hydrocarbons in every vehicle. A number of studies have however shown that distributed hydrogen production at refuelling stations will supply hydrogen at the lowest cost by 2020 [3-7]. The hydrocarbon used for producing hydrogen in a distributed hydrogen generation system should be such that it is easy to store and distribute, while also having a high hydrogen density. Liquid fuels such as gasoline and diesel currently provide high storage densities for hydrogen and use existing infrastructure for distribution and storage [8]. Similarly, one could envision biomass based liquid fuels, such as bioethanol or biodiesel, easily assimilating into this existing infrastructure and having a similar characteristic of high storage density for hydrogen. Natural gas (methane) is abundant, particularly in

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WHEC 16 / 13-16 June 2006 – Lyon France remote locations, and is a good source of hydrogen, but is not as easily distributed or easily stored on a vehicle [9]. It is within the context of this basic energy primer that a hydrogen production process should be developed. It should be small-scale to suit the distributed hydrogen generation model and use a high hydrogen density fuel, preferably with an existing distribution system, to process into hydrogen. Because fuel cells are considered a revolutionary technology for emissions reductions and sustainability, the design must also consider a pathway towards renewable or “green” hydrogen production. The intended use of the produced hydrogen is yet another criterion that determines the configuration of the fuel processing system e.g. a hydrogen internal combustion engine requires less purity than a proton exchange membrane fuel cell (PEMFC). The hydrogen production process usually consists of a fuel processor comprised of a reforming stage; the most common are steam reforming (SREF), auto-thermal reforming (ATR), and partial oxidation (POX) [10]. The reforming stage is usually followed by a water gas shift (WGS) (unless the synthesis gas is to be used in a solid oxide fuel cell (SOFC)) where the composition is further shifted towards hydrogen and carbon dioxide. To supply PEMFC grade hydrogen further purification must be performed on the hydrogen rich mixture, called synthesis gas and there are at least four major routes, two physical and two chemical. The first physical route is pressure-swing adsorption (PSA), where the non-hydrogen constituents (e.g. carbon dioxide, carbon monoxide, nitrogen, methane, hydrogen sulfide and water) are adsorbed in a porous bed at one pressure and released when the pressure is lowered [11]. The second physical route is membrane separation, where the hydrogen is separated, e.g. by using a palladium membrane through which only the hydrogen can pass [12]. These two methods of gas cleaning have the advantage of delivering pure hydrogen. The third route, a catalytic approach, is the preferential oxidation of carbon monoxide by oxygen (PROX) [13], yielding hydrogen diluted with carbon dioxide, methane, (and nitrogen if air is used in the reformer or in the preferential oxidation) and very low carbon monoxide concentrations. The fourth purification method is selective methanisation [14] whereby the carbon monoxide is reacted with hydrogen to methane, yielding a synthesis gas with high hydrogen content but still diluted by carbon dioxide, water and methane. The hydrogen is then stored or fed directly to the source of consumption. This paper investigated a process design methodology for small scale pure hydrogen production capable of serving energy markets ranging from distributed generation to vehicular refuelling. While typical small scale hydrogen generation systems are designed primarily for power generation via tight thermal integration with PEMFCs, this proposed approach allows for improved technical and economic characteristics. By purifying the hydrogen product, the hydrogen energy can be efficiently stored in pressurized cylinders or metal hydrides for instant start-up, rapid transient electrical load following, and for vehicular refuelling. This increased flexibility from generating pure hydrogen can result in decreases in overall system cost through creative hybridization with conventional energy technologies. To compete with traditional energy and heat sources, for example, a 5kW system must cost less than $6000 USD[15]. The purification is done utilising a PSA approach, which in combination with a pressurized steam reformer comprises the hydrogen production process. The produced hydrogen is fed to a metal hydride storage tank and, for demonstration purpose, three PEMFC are used to produce electricity from the hydrogen produced. The feedstock is a Fischer-Tropsch synthesised diesel fuel, with low sulphur and aromatic hydrocarbon content. To further increase the usefulness the catalyst has been developed to handle different types of fuels like e.g. methane, natural gas, propane, alcohols, kerosene etc. (shown elsewhere [16] ) at various levels of sulphur, further improving the unit versatility. Within this study the only fuel used has been the synthesised diesel type. This paper gives a thorough discussion on the design methodology and engineering of the hydrogen production system and further gives experimental data from both stand alone operation of the fuel processor system and PSA and the fuel processor integrated with PEMFCs from the first 280 hours of operation. The paper concludes with a discussion section and finally the major conclusions are outlined. There are a few other researchers that have built complete systems with both fuel processor systems and fuel cells. A recent study [17] compares different reformer configurations with each other in combination with a fuel cell. One showcased methanol used as the feedstock and reported an overall efficiency of about 50 % using PROX purification and slightly below 50 % when a Pd membrane was used, the membrane however results in a much more compact reactor construction. A system similar to the one presented here with a steam reformer, WGS, PSA and PEMFC has been simulated [18]. The results show that the overall operating efficiency is somewhere about 44 %, accounting for

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WHEC 16 / 13-16 June 2006 – Lyon France parasitic power losses as well. On the production of PEMFC grade hydrogen alone there are several other reports [19-22]. Most of these works have been performed on methanol as the fuel and some type of purification e.g Pd membrane, but some are computer simulations as well [23]. Trends within reforming seem to be the reverse flow reforming bed [24-29] and the circulation fast fluidized bed membrane reformer [30-34]. Experimental results of the system discussed in this paper show that efficiencies in almost in the same regions can be achieved with this system as well.

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EXPERIMENTAL

The experimental setup consists of three major components, the catalytic converter, the PSA and the fuel cell. Each of these major components will be discussed individually and then the system integration will be described.

2.1

CATALYTIC CONVERTER

The catalytic converter consists of three integrated, catalysis based processes. The first process is the SREF where the synthetic diesel is reacted with superheated steam to yield synthesis gas. The synthesis gas composition is made up of hydrogen, carbon monoxide, carbon dioxide and steam. The process is run at a pressure of 4 bar(g) and at a temperature ranging from 650°C in the inlet to 800°C in the outlet. The catalyst used is spinel based utilising a mixed noble metal active phase. The catalyst is supported on a patented thermally sprayed woven wire mesh system [35], enabling high heat and mass transfer compared to monoliths and lower pressure drop compared to a pellet based catalyst system. In comparison to micro-channel reactor configurations, the wire mesh system provides most of the advantages of high heat and mass transfer without the high costs of chemical etching and diffusion bonding. The second catalytic process is the WGS where the concentration of the carbon monoxide is reduced to 1,5 % (dry gas analysis) by reacting the carbon monoxide with water. This is an equilibrium reaction and the outlet concentration is determined by the operation temperature, at least in this region of operation where the kinetics are not rate limiting. The catalyst used is a promoted noble metal with a doped ceria based material that is used as the active phase carrier; the same woven wire mesh system described above is used as support. The operating temperature in this reaction ranges from 400°C in the inlet to 350°C in the outlet and the operating pressure is the same as the SREF, 4 bar(g). The reaction is exothermic with a reaction enthalpy of -40.6 kJ/mol when CO is converted into CO2 with water [36]. The problem of the excess heat is solved using 4 WGS catalyst sections with cooling in between sections ensuring that the overall reactor temperature profile is optimized for the reaction taking place. The heat is adsorbed by the water used for cooling and transported to the SREF utilising it in this part of the reactor instead. Both catalysts used are sulphur resistant which allow fuels containing up to 50 ppm sulphur. This enables the unit to be run on a wide array of fuels without any upstream sulphur removal without compromising with the fuel cell operation. The PSA adsorbent used will remove any sulphur impurities before reaching the fuel cells. The third catalytic process is the catalytic combustion of hydrogen or PSA off-gas that supplies the reaction heat to the endothermic SREF reaction. The combustion section and the SREF section are fully integrated to ensure a compact design with as few heat losses as possible. The steam-reformerreactor part of the STUR can best be described as a heat exchanger with combustion taking place on one side of the heat-exchanger surface, supplying energy, and the steam reforming reaction taking place on the other side, Figure 1.

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Figure 1 the catalytic converter outlined with the combustion/steam reforming blocks visible and the outline of the pressure vessel in the top. The same patented coating technique used to thermally spray the wire mesh has been used to coat the plates on the combustor side with combustion catalyst (alumina supported noble metal catalyst). The direct coating of the surface, enabling the combustion to take place in direct contact with it, further increases the heat transfer between the combustion and the SREF. This heat transfer is the factor determining the size of this part of the reactor. The integrated SREF and combustor part of the reactor is the first part of the reactor and it includes the steam superheating and the vaporisation of the liquid fuel as well. The steam that is used in this reactor is the same water that enters the reactor in the WGS part of the reactor for cooling purposes. Some of the water ending up in the SREF is used for cooling the synthesis gas leaving the SREF part of the reactor before it enters the WGS part of the reactor. The process side (SREF and WGS) is pressurized to 4 bar(g) but, to avoid unnecessary parasitic system losses due to the use of an air compressor instead of an air fan and the low pressure of the PSA off-gas, the combustor side is run at ambient pressure. The overall catalytic converter system consists of one single reactor with a square cross section of 200 by 200 mm with a flange to separate the SREF and WGS parts of the system. This flange will however be removed when adapting the system to any future mass production and is only used for research purposes, to give access to catalysts etc. This reactor is inserted into a pressure vessel with connections for the combustor and fuel processor in the lid and the bottom of the vessel. The catalytic converter system is schematically depicted in Figure 2.

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WHEC 16 / 13-16 June 2006 – Lyon France H2 O

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Figure 2 a schematic outline of the catalytic converter with the in and outlet flows and the dashed line representing the border line of the pressure vessel.

2.2

PRESSURE SWING ADSORPTION UNIT

The hydrogen is purified in the system with a Pressure Swing Adsorption (PSA) device consisting of nine beds, each filled with a proprietary adsorbent. The beds are circumferentially positioned around two rotary valve assemblies which are driven by a single variable speed drive motor. The hydrogen purification cycle includes eight stages that manage the reformate inlet stream, off-gas outlet stream, high purity hydrogen product stream and pressure equalization gas flow rates within the multiple tubes. Timing and duration of each stage is a function of valve design, internal purge set points and rotational speed. The assembly is packaged within a vessel that functions as a product hydrogen reservoir tank which simplifies process gas sealing. The off-gas from the PSA unit is released at close to atmospheric pressure, typically peaking at 250 mbar(g). The high purity hydrogen gas is generated at elevated pressures (approx. 4 bar(g)) and can therefore be effectively stored in low pressure hydrides or tank storage.

2.3

FUEL CELL

The fuel cell systems consist of three units. The first two are Intelligent Energy’s 2.0 kWe Combined Heat & Power (CHP) units and the third is an advanced 1.3 kWe generator. The CPH units are designed to be fuelled by pure hydrogen and deliver a continuous 2kW of regulated 24 volt DC electricity. An additional 2kW of thermal energy can be recovered for use in water or space heating applications or rejected to ambient. During the testing the units were operated as electricity generators in combined heat and power mode, providing excess thermal energy to heat the metal hydride hydrogen storage systems and the ambient air of the laboratory. The CHP system is based on IE’s closed cathode stack technology which is also used in IE automotive and other high power technology platforms. The advanced generator system produces a continuous output of 1.3 kWe at a regulated 48 volt DC. The generator is designed to be hybridized with a battery bank to meet peak load requirements. The 1.3 kWe generator design is based on IE’s open cathode stack technology, which uses low cost and low pressure air blowers to provide cathode air and cooling. This eliminates the need for water cooling and humidification loops. The system is controlled by an internal microcontroller that handles all internal system control including battery system charge and discharge.

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WHEC 16 / 13-16 June 2006 – Lyon France 2.4

SYSTEM INTEGRATION

The system design has been based on the production of pure hydrogen allowing for market flexibility, hybridization and lower cost fuel cells. Purification of hydrogen requires pressurization, either if a PSA or a membrane is used, or if the pressurization can be performed either before or after the catalytic converter. Autothermal reforming, as an alternative, would require compression of air to high pressures, a costly and energy intensive proposition. Pressurization before the steam reformer system appears to be the easiest and most efficient way as liquid pumps can be used instead of a reformate gas compressor. The combustion side of the catalytic converter is operated at atmospheric pressure due to the low pressure of the PSA off-gas and to avoid the use of an air compressor. The combination of high temperatures and pressure differentials give rise to difficulties in choosing construction materials etc. The use of a PSA will, besides supplying medium pressure H2 (versus low pressure hydrogen from a membrane purification system), remove any sulphur impurities potentially harmful to the fuel cell. The integration of the reformer with the PSA off-gas provides an appropriate and well matched thermal balance for the steam reformer. Through the unique implementation of catalytic combustion, pulsations of the PSA off-gas actually serve to benefit heat transfer characteristics by periodically breaking boundary layer formation. This close integration between PSA and combustor also eliminates the large buffer tanks typically employed in PSA-off gas combustion systems [37]. The system output is 7 nm3 of H2 per hour witch enables a total power output of 10 kW of electricity from the produced hydrogen in the employed fuel cell system. The system schematic is found in Figure 3. 8

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INTELLIGENT ENERGY Inc. B

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THIS DOCUMENT CONTAINS PROPRIETARY INFORMATION OF INTELLIGENT ENERGY Inc. AND IS SUBJECT TO BEING RETURNED UPON REQUEST. IT SHALL NOT BE REPRODUCED IN WHOLE OR IN PART WITHOUT PRIOR WRITTEN PERMISSION OF INTELLIGENT ENERGY Inc . ANY AUTHORIZED REPRODUCTIONS MUST BEAR THIS ENTIRE LEGEND.

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Figure 3 the system schematic including catalytic converter, PSA and heat and water management. The design target for the parasitic power consumption of the system, pumps, fans, PSA, etc. was set to 500 W. While the parasitic power target was low compared to the produced system energy, the actual parasitic power consumption was approximately 550W. The total parasitic power consumption was slightly above target but not more than can be solved by a more thorough selection of components. The primary test area consists of a 10 by 10 foot isolated, hooded, and ventilated space equipped with automated test facility designed for the operation of liquid-fuelled hydrogen generation systems. The control system will continuously monitor temperatures and flow rates within the system with four alarm

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WHEC 16 / 13-16 June 2006 – Lyon France levels over fifty parameters. This ensures that all process parameters are maintained within tolerances to ensure optimal system efficiency and product purity. Gas composition is monitored by four continuous instruments via Non-Destructive Infra Red and Thermal Conductivity detection. Gas composition data is continuously recorded in a consolidated data acquisition system, which also records temperatures, pressures, flow rates, and process control information. Three gas sample loops are established to monitor the six gas sample locations. Loop one provides hydrogen concentrations 90-100% (high purity with +/-0.01%), CO concentrations 0250ppm, and CH4 concentration 0-2500ppm and is used for product hydrogen to storage, product hydrogen from storage, and PSA outlet hydrogen. Loop two provides hydrogen concentrations 0100% (mid range with +/- 0.5% accuracy), CO2 0-25%, CO 0-25%, and CH4 0-10% and is used for synthesis gas, inlet PSA gases, and PSA outlet or off-gas. The third loop monitors combustion exhaust O2 concentrations 0-25%. The data acquisition system monitors and stores all measured variables every 100ms. An exportable analysis record is also stored at a defined recording interval which is adjustable and typically set to 1 minute. Data can be stored for up to one year within the system. The system allows for real time data trending and graphic export with customizable data formats. The results have all been treated the same way, with every data point in the figures as a mean of 10 actual data points. This was done to improve the clarity of the figures and to remove small variations in the data, focusing on the data trends.

3

RESULTS

The results were collected during a normal run with the unit, run intermittently during daytime. In this case the run time was 900 min or 15 h, between 265 and 280 h of reactor hot time. The unit has been started and stopped approximately 50 times up to date. The following figure, Figure 4, depicts how the hydrogen flow varies with the fuel feed as well as the hydrogen concentrations in the product hydrogen, the PSA off-gas and the synthesis gas entering the PSA respectively. 120,00

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Figure 4 the concentrations of the product hydrogen, synthesis gas entering the PSA and the PSA off-gas respectively, as well as the fuel feed. It is clear that the hydrogen concentration of the product gas almost immediately reach a very high purity, and the synthesis gas and PSA off-gas level out at 73 % and 48 % respectively at this fuel feed. The fuel feed in this case, 56 cc/min is equivalent to full capacity.

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Figure 5 the fuel conversion and the outlet CO level compared to the fuel feed. The fuel conversion is about 88-90 % during the time of operation which is considered good since the fuel conversion is limited by thermodynamics at 94 %. The outlet CO levels are good indeed, the average never reaching above 1 ppm and the highest single value of CO is 1,5 ppm. The CO level is even below 0,3 ppm during the major part of the run, further emphasised by Figure 6 where the hydrogen product was feed to a fuel cell. 3000

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Figure 6 Switching from bottled hydrogen to product hydrogen at 150 minutes.

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It is clear that the produced hydrogen has the same quality, when used in the fuel cells, as bottled hydrogen. This empirically suggests that both the sulphur and CO removal is sufficient for long term operation. In Figure 7 the hydrogen product flow and the overall unit efficiency, omitting the FC system, is depicted together with the fuel feed. 120,0

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Figure 7 the hydrogen product flow and the overall efficiency of the unit compared to the fuel feed. The hydrogen production is about 110 sl/min which is slightly below the target of 117 sl H2/min or 7 nm3 H2/h and the reason for this is found in the efficiency. The efficiency of the system is slightly below 60%, averaging 59%, without the fuel cell and 30 % including it. This is slightly lower than the ultimate design target of 70% and is mainly associated with operating the PSA at a lower overall recovery, delivering less H2 than first anticipated and operating at a slightly higher steam to carbon ratio than is ultimately feasible. These conditions will be further tested and improved upon during future operation and optimization of the integrated system.

4

DISCUSSION

After evaluation of the integrated system it is obvious that the design methodology with tight temperature integration and gas purification fulfils most of the targets set on forehand for the unit. The production rate of hydrogen, 6,6 nm3 H2/h instead of 7 nm3 H2/h, is very close to the design target. The results show that when the product hydrogen is used in the fuel cell, no power output decrease can be noted which indicate that no fuel cell poisons, neither CO nor sulphur, reach the fuel cell in any damaging degree. The overall efficiency is about 59%, 30 % including the fuel cell system, when the parasitic power consumption has been taken into consideration, mainly due to the PSA recovery rate and steam to carbon ratio, which is slightly short of the design target but allows room for improvement. Increased net efficiency can be obtained by eliminating heavy components which contribute to heat loss, optimising the PSA cycle time and recovery rate, and further improvements to overall thermal management.

5

CONCLUSIONS

The hydrogen production and fuel cell technology utilized with the design approach presented in this paper represent an alternative approach to hydrogen economy infrastructure development. This approach represents a cost effective means to low cost distributed hydrogen generation systems capable of serving multiple functions, including vehicular refuelling and power generation when coupled to a fuel cell. Further, this approach achieves flexibility of feedstock including liquid biofuels and hydrocarbons containing low levels of sulphur. Through the use of such feedstock in combination

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WHEC 16 / 13-16 June 2006 – Lyon France with fuel cell power generation, drastic reductions in air emissions and petroleum dependency can be achieved while simultaneously dramatically reducing lifecycle carbon dioxide emissions.

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