DE102007012585A1 - Procedure for improving operational behavior of reformer system to adjust the natural gas treatment to hydrogen by using a catalyst on the basis of cellular metallic material, comprises coating the cellular metallic material - Google Patents

Abstract

The procedure for improving operational behavior of reformer system to adjust the natural gas treatment to hydrogen in the power requirement of polymer membrane fuel cell useful in domestic energy supply by using a catalyst on the basis of cellular metallic materials, comprises electrolytically or electrochemically coating the cellular metallic materials with a commercial nickel sulfamate electrolyte without additives at 55-60[deg] C for 15-17 minutes so that a high number of defective point and fabric anomalies consists of the cellular metallic material. The procedure for improving operational behavior of reformer system to adjust the natural gas treatment to hydrogen in the power requirement of polymer membrane fuel cell useful in domestic energy supply by using a catalyst on the basis of cellular metallic materials, comprises electrolytically or electrochemically coating the cellular metallic materials with a commercial nickel sulfamate electrolyte without additives at 55-60[deg] C for 15-17 minutes so that a high number of defective point and fabric anomalies consists of the cellular metallic material. The coating is carried out under potentiostatic condition and the cell is imprinted between working electrode and reference electrode with a constant potential difference of 3 Volt. An independent claim is included for a catalyst for improving operational behavior of reformer system.

Description

The The invention relates to a method for improving the performance reforming systems to adapt natural gas processing to hydrogen on the power requirement of polymer membrane fuel cells (PEMFC), especially during start-up and load changes of the system and a catalyst for a reformer for use with such a method. This requires the dynamics of Reformers regarding the provision of the fuel gas for the PEMFC are increased, which through the use of novel catalysts with substrates of cellular metallic materials (also called Metal foams or metal sponges called), the design in sectional reactor design and integration an innovative, compact burner system (pore burner) shall be.

Of the Reformer is to a thermal hydrogen power of about 2.5 kW, which is sufficient sizing for the hydrogen supply of a fuel cell for supplying energy to a Represents a family house.

The Development work on the novel catalyst system started with the analysis of metal sponges, which were used for substitution the conventional used in conventional reformers Catalyst support materials to be used. The characterization took place in the first trial period of metal foams of different composition and different pore size in terms of for the planned use relevant parameters thermal conductivity, Material dispersion and pressure loss. These investigations led to an initial selection for the intended use potentially suitable carrier materials which are as follows shortly have been described to catalysts and subsequently in one specially prepared experimental apparatus a catalyst screening were subjected.

Due to the positive experience with other catalyst systems developed by IUTA for industrial applications ( DE 198 15 004 C1 ) based on metallic substrates, which manifest themselves in particular with regard to their robustness and their rigidity with regard to a catalytic activation, was provided as a method for the coating of the metal foam substrates, the electrodeposition. In addition, for comparison purposes, some metal foams have been produced by conventional wet-chemical processes. The process steps of both production methods are briefly presented below.

Out The selected metal foam panels were initially By means of high-pressure water jet cutting circular Samples with a diameter of 37.2 mm (corresponding to the inner diameter of the test reactor). Metal foam panels were used with thicknesses of 5 and 10 mm. The metal foam samples produced in this way were subsequently organically cleaned by taking about 15 minutes degreased in an ultrasonic bath with acetone at room temperature were.

In the electrolytic metal foam samples to be built up, the metal surface was activated by pickling in 10% sulfuric acid for 5 minutes. The surface of the wet-chemically prepared samples was roughened to increase the adhesiveness by treatment with 0.5 M nitric acid (HNO ₃ ) for ₃ hours. All samples were then thoroughly rinsed with demineralised water (fully demineralized water).

The Applicants of the present application have based on the the metal sponges characterizing data a concept for the design and parameterization of a for the production Metal-sponge-supported catalysts suitable electrochemical Coating process developed. The objective was the Generation of the highest possible number of defects and structural anomalies that act as catalytically active centers Act.

For the electrolytic coating became a commercial Ni sulfamate electrolyte used without additives. The electrolyte temperature was set to 55-60 ° C, also became the electrolyte subjected to ultrasound during the deposition process.

The Coating was done under potentiostatic conditions. Of the Cell between the working electrode and the reference electrode was a constant potential difference of 3 V impressed. To a treatment duration of approx. 15-17 minutes was the Separation process completed.

This was followed by rinsing the coated samples with deionized water. The predrying of the samples was carried out by blowing off with nitrogen gas. After predrying, the metal foam sheets were dried in a drying oven at a temperature of 105 to 110 ° C to constant weight net and then reweighed to determine the amount of catalyst applied.

With The process described was a total of 16 catalyst samples produced in four production batches. After testing the catalysts the respective batch in the test reactor, the optimization, the metal foam catalysts to be produced in the following batch, by adapting and optimizing the relevant production parameters. As shown in Table 1, the nickel content was from <0.5 wt .-% to 10.0 wt .-% varies. The listed in some samples in Tab. 1 Addition "+ Ru" indicates that in the samples of a production batch made co-deposition of 0.1-0.2 wt .-% ruthenium through the soot formation observed in the first experiments and counteracts the associated catalyst deactivation become.

When Basis for the wet-chemically produced catalyst patterns were - as well as the electrochemically manufactured catalysts - commercially available Metal foam substrates used. Specifically, these had the in Table 1.

With Exception of the substrate NC 2733, in which the wet-chemical coating was done directly on the metal surface, pretreatment was performed on all other metal foam substrates, when enlarging the surface a so-called washcoat was applied to the carrier.

Application of the washcoat

For washcoat preparation, the starting alumina hydrate (Al ₂ O ₃ .xH ₂ O) was converted into γ-Al ₂ O ₃ by calcination at a temperature of 550 ° C. in air over a period of 6 hours. The γ-Al ₂ O ₃ was then treated by grinding for three hours in a ball mill and subsequent screening of particles> 45 μm.

The produced γ-Al ₂ O ₃ powder was mixed with distilled water (Al ₂ O ₃ / H ₂ O ratio = 1/3) to a suspension in which the metal sponges were soaked. Overhead shaking for a period of 5 hours ensured uniform particle distribution within the metal sponge framework. The drying of the applied suspension takes place in several steps (50 ° C., 100 ° C., 150 ° C. for one hour each). Subsequently, a thermal treatment was carried out in which the samples were heated at 1.5 K / min up to 550 ° C and held for a further 4 h at this temperature. The process was repeated several times until an Al 2 O 3 loading of about 25 wt .-% was achieved.

Apply the catalytic active substances

The Loading with the catalytically active substance nickel is carried out by several hours impregnation of metal sponges in a 0.5 molar nickel (II) acetylacetonate solution.

The Drying was carried out at 60 ° C for a period of 6 h made. Subsequently, a temperature treatment, in which the catalyst patterns in a period of 4 h to a Temperature of 350 ° C were heated, causing a finely dispersed Distribution of nickel particles can be achieved on the substrates should. Before the use of the catalyst pattern was another Sintering process (550 ° C, 12 h in air) for stabilization the catalysts.

Activation of the catalysts

In order to achieve the maximum activity of the catalysts, it is necessary to convert the nickel which is present in oxide form into the metallic form in the case of the precipitated catalysts. For this purpose, the catalyst samples were reduced over several hours, in which they were subjected to a water vapor stream, which was increased from 2 vol .-% hydrogen at the beginning of the treatment with progressive reduction time up to about 20 vol .-% (space velocity (R. ≅ 5000 I _{N, gas} · I _cat ^-1 · h ^-1 ).

catalyst screening

With regard to the gas metering, the evaporator unit, the product gas cooling, the volumetric flow measurement and the gas analysis, the experimental plant for the catalyst screening is largely identical to the pilot plant with sectioned reactor used in the further course of the investigations for the optimal design of the operating parameters. Thus, both test parts provided in the context of the project work can be carried out with the same system and, accordingly, under the same conditions. For the investigation of the optimal design of the operating parameters of the section reactor only the reactor must be exchanged and the three additional steam lines, which are not used in the screening, be connected.

The process flow diagram of the pilot plant for the catalyst screening is in 1 shown. Below, the structure and operation of the pilot plant will be explained by subassemblies.

gas metering

The in the lower part of 1 Gas metering unit shown consists of gas strands for methane, nitrogen and hydrogen. The adjustment of the educt volume flow (methane) for the reforming reaction and the hydrogen metering which was used for the activation of the wet-chemically produced catalyst patterns is effected by means of electronic mass flow controllers. For Methandosierung a commercially available mass flow controller is used with a working range of 0-500 l / h of methane. The operating range of the flow meter for the hydrogen metering is in the range 0-250 l / h H ₂ . The metering of the nitrogen flow, which is only required for purging the gas analyzers, is done manually with the aid of a variable volume meter.

Evaporator

The Water dosage takes place in the liquid state of aggregation Water with an electronic mass flow controller. Initially was an MFC with a control range 0-1000 g / h of water used. Even at low volumes of catalyst samples (half plate thickness of 0.5 cm for some samples) small To be able to adjust room speeds and not through the minimum flow (about 12 g / h) of the first used MFC limited, took place in the further course of the experiments an exchange against an MFC of the same type with a workspace 0-250 g / h of water.

It VE water is used that with a peristaltic pump from the Master tank pumped into a pressure vessel becomes. The pump is operated at intervals and controlled so that a form corresponding to the design of the MFC results (4-5 MPa). Based on initial operating experience was a compressed air Retrofitted compensator container. This one became connected in parallel to the peristaltic pump to which occurred when starting the peristaltic pump, to avoid and to ensure a uniform water dosage.

the Downstream mass flow controller is an aluminum block (B, H, T: 400 x 300 x 120 mm), which is divided into four central Holes are located in the stainless steel piping (⌀12.5 mm), which lead to the reactor. For the use of the plant as a test stand for the catalyst screening is - like already described - initially only one of the four steam pipes used.

The heating of the aluminum block is done electrically. For this purpose, ten heating cartridges (length 150 mm, ⌀12 mm) are installed in the longitudinal direction of the introduced steam pipes from both end faces of the block. In total, this results in an installed heating capacity of about 10 kW at 230 V. This power is sufficient both for evaporation and overheating of the required amounts of water during Katalysatorcreening and the subsequent test operation of designed for a power of 1 kW _el test reactor sufficient. In plant operation, the aluminum block is adjusted to a temperature of 450 ° C.

Around an unwanted lowering of the reforming temperature in the reactor too avoid, the water vapor must be close to the respective Working temperature of the reactor are injected. It takes place therefore further overheating of the water vapor within the supply line to the reactor. This pipe section is with a high temperature resistant Heating cord wrapped with a nominal temperature of 900 ° C (370 W). In the pipe sections between the aluminum block and the Reactor is a further overheating of the water vapor.

reactor

Immediately before the reactor methane is added to the water vapor. After passing through a heated inlet section, the mixture is injected from below into the reactor. The stainless steel reactor, schematically in 2 has an inner diameter of 37.2 mm, a wall thickness of 2 mm and a length of 300 mm. The heating is done by a 6 m long heating cord (1000 W) bifilarly wrapped around the outer shell of the reactor. To accommodate the metal foam samples cut in a circular manner by means of a water jet cutting process, an inner tube inserted into the reactor serves to provide a bearing surface for the foam.

The Internal reactor temperature is immediately below the catalyst sample detected with a thermocouple (k-type). This temperature is used as a reference variable for the temperature control. To prevent overheating of the heating cord is a additional thermocouple between the reactor outer wall and the heating cord installed.

One At the reactor inlet installed pressure sensor detects the pressure in Interior of the reactor. The executed steam reformer system should be operated atmospherically. The pressure monitoring serves on the one hand safety aspects and on the other hand as an indicator of the deposition of soot particles serve on the catalyst by reducing the free flow area of the metal foam lead to an increase in pressure.

by virtue of evidence of the occurrence of water vapor during the course of the project with the methane in the metal foam catalyst upstream Mixing chamber is insufficiently mixed, were for optimization the reaction guide additionally perforated plates and an approximately 50 mm high bed of cylindrical ceramic hollow body introduced into the lower part of the reactor.

gas analysis

For the orienting investigations at the beginning of the term of the project For the quantitative determination of the product gas, a micro-gas chromatograph was used (Varian, CP-2003P). This analyzer has a molecular sieve column, which u. a. the constituents hydrogen, Methane, carbon monoxide, oxygen and nitrogen are detected, as well as over a HayeSep A column, with the u. a. Detected carbon dioxide can be. For additional online data logging came an infrared gas analyzer (Emerson Process Management, Binos 100) used with which the methane content in the range 0-80 Vol .-% and the carbon monoxide content in the range 0-20 vol .-% detected could be.

The The above-described meter configuration consisted of a Multi-range analyzer, using three non-dispersive infrared spectrometer measuring cells (NDIR) and a heat conductivity detector (WLD). An overview of the Measuring ranges of this device are given in Table 3.

The measuring instruments are connected in the bypass behind an installed gas cooler (see below). With a variable area flowmeter (0-100 l / h), the bypass gas flow is set to 60-80 l / h. After the two partial streams have been combined (bypass and main gas stream), the gas volume flow is measured using a wet-type laboratory drum gas meter with a measuring range of 1-200 dm3 / h. To avoid condensation of the product gas in the GC columns, NDIR cuvettes or WLD measuring cells, the product gas is fed downstream of the reactor to a condenser, which is supplied with cooling water (θ _{cooling water} = 2 ° C) from a cryostat. The resulting condensate is collected in the cooler. It can be tapped after the trial and also enters the mass balance.

Regulation and control

When Process control system and to control the plant is a Fieldpoint I / O system (NI-FP-1000) from National Instruments. The process control system and the controller have on the one hand the task of the automated To ensure operation of the plant, on the other hand they are used for safety-related protection of the system, so that a controlled, accidental shutdown of Pilot plant is possible. The Fieldpoint module connects The connected I / O modules (see Table 4), which control the drive take over the sensors and control systems attached to the system, connected to a process computer via an RS-232 interface is.

from Process computers become the control and regulation tasks with the help of a "labview" development environment based software program.

As part of the project work, a total of 16 electrochemically and 4 wet-chemically produced metal foam catalysts were investigated with respect to their reactivity, S / C ratio, space velocity and gas distribution in terms of their activity and selectivity in the reforming reaction. As already mentioned in chapter 2.1.2.1, at the first, orienting scree For the quantitative determination of the product gas composition, a gas chromatograph and an NDIR gas analyzer are used for the online measurement of the components methane and CO ₂ . Qualitative statements with regard to the dynamic behavior of the activity of the catalyst systems were therefore initially based on the residual methane content in the product gas and were checked discontinuously with respect to their significance for the hydrogen content by means of GC measurements. As expected, the control measurements showed that the methane content in the product gas is largely inversely proportional to its hydrogen content.

exemplary Diagram 1 shows a comparison of the residual methane contents, involved in the reforming of methane with various metal foam catalysts at a temperature of 700 ° C and an S / C ratio of 3.

The Space velocity as a measure of the catalyst load was in these experiments - because of the small volume the metal foam samples and due to device technology conditional restrictions on dosing very small Flow rates - set high. Most investigated metal foam samples have a height of 1 cm, resulting in a catalyst loading of about R = 20,000 1 / h calculated. In the case of metal foam samples having a height of 0.5 cm, made to both the space velocity as well as to limit the scope of the study series, at first a joint survey of two similar samples. Provided that in these experiments a sufficiently high catalytic Activity, the metal foam samples were subsequently measured separately again. Represent in diagram 1 the dashed curves (yellow, green and purple) Metal foam specimens of half height (0.5 cm). The catalyst load in the screening experiment shown above was about 40,000 1 / h about twice as high as in the other samples, from which For these samples, a relatively high activity results. The orange curve shows the residual methane content which sets if the two samples, when screening the second production batch the highest catalytic activity in terms of Reformierreaktion, were measured together to comparable Conditions regarding the catalyst load (about 20,000) too create.

to Determination of suitable parameters for the production of suitable metal foam catalysts for methane or natural gas reforming were screenings at temperatures of 500 to 750 ° C, S / C ratios from 1.5 to 6 and space velocities in the range of 20,000 up to 80,000 1 / h carried out. The production parameters for electrolytic coating of the metal foams were on the basis of the respective examination results successively optimized.

The Activity of the electrochemically prepared catalyst systems exceeded the wet-chemically produced by far. In addition, proved the handling of the electrolytically coated metal foams as very simple, because of the deposited in metallic form Nickels the need for a pre-commissioned Activation in a reducing atmosphere is eliminated. Even a cleaning of the catalyst system of adherent carbon particles by "burning off" by air supply during operation is possible without detectable damage to the catalyst.

From The tested metal foams proved in the experiments carried out sample NC3743 (nickel-chromium foam, Pore density 37-43 ppi) as the most suitable substrate, followed by the sample designated Ni2733 (nickel foam, Pore density 27-33 dpi). The evaluation of the activity and selectivity, with variation of nickel content as well as other manufacturing parameters produced metal foam catalysts, gave the best results for the samples electrochemically (under potentiostatic conditions and ultrasound effect) were coated with about 4 wt .-% nickel.

Especially in the initial stages of the investigations, problems with the Long-term stability of the catalytic activity the metal foam catalysts. The decrease of activity the catalysts with the operating time was - depending the set experimental parameters - in all tested Catalyst samples (electrochemically and wet-chemically produced as well uncoated). The catalyst deactivation proved turns out to be reversible. So could by cleaning the metal foam surface by "burning off" adhering carbon by air injection restored the original activity become. This enabled the deactivation mechanisms of sintering "Agglomeration of nickel particles" and a discharge of nickel particles be excluded. For the increasing with the operating time Deactivation of the catalyst seems rather an occupancy of active centers for the reforming and shift reaction with To be responsible for carbon.

sowie die Boudouard-Reaktion

zu betrachten. Während eine Kohlenstoffbildung durch die Methan-Spaltungsrektion vorwiegend bei höheren Temperaturen (☐ > 600°C) auftritt, ist eine Bildung aufgrund des Boudouard-Gleichgewichtes eher bei Temperaturen unter 600°C anzutreffen.As a possible reaction mechanisms for the formation of soot are in particular the methane Spaltungsre action

as well as the Boudouard reaction

consider. While carbon formation by the methane cleavage reaction occurs predominantly at higher temperatures (□> 600 ° C), formation due to the Boudouard equilibrium is more likely to occur at temperatures below 600 ° C.

There the deactivation by carbon formation in the performed Try at higher temperatures (up to 750 ° C) occurred, it is to be presumed that prefers the methane cleavage reaction contributes to carbon formation.

According to the in the literature, data relating to the water vapor methane Ratio should be one at S / C = 2 and above Carbon formation over the entire temperature range be excluded. In the experiments performed showed the variation of the water vapor / methane ratio in the range from S / C = 1.5 to 5 (S / C up to 5) but only a small one Influence on the occurrence of carbon deposits, from which rather an insufficient mixing of the reactants or one too short residence time was derived.

After this also in addition to the nickel coating made electrolytic deposition on the metal foams of 0.1 to 0.2 wt .-% ruthenium not the desired effect a noticeable suppression of soot deposits modifications were made to the gas dosage, made the flow guide and the reactor assembly. Specifically, an exchange of the MFC for the water metering took place against a model with a lower working range, the installation of perforated plates to equalize the flow and the introduction of a bed of ceramic hollow cylinder to ensure adequate gas mixing simultaneous assurance of sufficient heat transfer on the gas.

For the subsequent attempts were made from the pool of the previously made Catalyst systems four copies selected in the Previous tests have similar performance the reforming of methane. This is it by two Recemat foams with pore densities of 27-33 dpi and 37-43 dpi. The foams had an electrolytic Coating of 4.1-3.9-3.7 and 3.0 wt% nickel on. Two metal foams were additionally 0.1 to 0.2 wt .-% ruthenium occupied, but not in the preliminary tests to a significant influence on the product composition the reform had led. By the summary the metal foam catalysts could - in conjunction with the other o. g. Measures - the adjustable Residence time can be significantly increased.

in the Diagram 2 is the result of investigations using the four metal foams combined into a sample - after Implementation of the described modifications to the pilot plant - shown. The reforming temperature in the experiments was in the range of 400 up to 700 ° C and the space velocity in the range of approx. 4,000 to 65,000 1 / h varies.

The at the different temperatures occurring deviations of Values for the space velocities arise through the influence of temperature on the density of methane or water. Based at ambient temperature correspond to the illustrated space velocities the value series R = 4,000, 8,000, 16,000, 32,000 and 48,000 1 / h.

In The diagram clearly shows the increase in hydrogen contents with increasing temperature (curve color: blue = 400 ° C, brown = 500 ° C, yellow = 600 ° C and magenta = 700 ° C) and decreasing space velocity, according to an increase the residence time. In addition, lets out read off the graph that the curves are increasing with temperature and decreasing space velocity take a flatter course, which is a decrease in the previously described tendency for (reversible) deactivation by formation and deposition of carbon black.

As can be seen from the graph, the hydrogen contents in the experiments with a Space velocity of 5,000 1 / h and temperatures of 600 and 700 ° C almost identical. However, it must be taken into account that the reforming reaction does not proceed in an equimolar manner and therefore the total volume of the product gas stream must also be included in the assessment.

Clear This is done by comparing all in the product gas occurring components. As is apparent from the diagram 3, are at approximately the same hydrogen contents at the reforming temperatures 600 and 700 ° C at the methane and carbon monoxide levels 700 ° C due to the more complete reforming reaction significantly lower, while the carbon dioxide content is higher.

in the Diagram 4 are for the last performed Experiments presented on the reactant methane related sales. In this presentation, performance differences are off the different parameter settings, especially clearly visible. At 700 ° C are therefore sales of approx. 95% at a space velocity R = 5,300 1 / h to approx. 80% at R = 10,700 achievable (in each case measured after approx. 30 minutes Test period). At comparably high space velocities the sales at 600 ° C each about 10% lower.

Around in view of increasing soot formation with the catalyst loading and the associated power loss of the catalyst Estimation of the maximum for the practice operation to be able to carry out an adjustable catalyst loading, Long-term tests were performed at 700 ° C and room velocities of R = 5,300, 10,070, 13,400 and 21,500 1 / h. Diagram 5 shows the gas composition during the short run six-hour trial period at a set space velocity from 5,300 1 / h. There was no loss of activity during this experiment the catalyst recognizable. (The short-term peaks are the consequence of pressure surges when switching on Peristaltic pump, and have no significance regarding the problem.)

at the further attempts at which higher space velocities were adjusted, the hydrogen content in the product gas fell starting from an initial concentration of about 80% by volume within each two-hour test period by about 10 vol .-% (R = 10,700 1 / h), 15% by volume (R = 13,400 1 / h) and 20% by volume (R = 21,500 1 / h).

• – im ausgeführten Reformersystem mit Porenbrenner die Reformiertemperatur über die beim Testreaktor realisierbare Temperatur von 700°C hinaus gesteigert werden kann,
• – durch das Verlöten der Metallschäume mit der Reaktorwand eine wesentlich bessere Wärmeübertragung erreicht wird, als dies beim Testreaktor der Fall ist,
• – für die weiteren Untersuchungen mit dem Abschnittsreaktor und dem Prototyp des Reformersystems der ausschließliche Einsatz der bestgeeignetsten Katalysatoren erfolgt (und die Versuche nicht – wie bei den zuletzt durchgeführten Tests – hilfsweise mit einem Mix aus unterschiedlichen Katalysatoren durchgeführt werden).

Additional tests to determine the maximum long-term catalyst load will be carried out later in the project. It can be assumed that a substantial increase in the space velocity, at which a stable operation of the catalyst system can be ensured, is possible. This assumption arises in particular from the fact that

• In the executed reformer system with pore burner, the reforming temperature can be increased beyond the temperature of 700 ° C. that can be realized at the test reactor,
• - By soldering the metal foams with the reactor wall a much better heat transfer is achieved than is the case with the test reactor,
• - For the further investigations with the section reactor and the prototype of the reformer system the exclusive employment of the most suitable catalysts takes place (and the attempts are not carried out - as with the last carried out tests - alternatively with a mix from different catalysts).

The subject of this patent application is also the structure and the testing and optimization of a heat-integrated, innovative reformer system in the power range 2.5 kW _th for domestic energy supply, which in particular by a high efficiency, a compact design with resulting high load cycle dynamics (including due to small thermal Mass), simple operation management and high power modulation is distinguished and compared to conventional models advantages in terms of startup and Abfahrverhaltens. As planned, according to the proof of the fundamental functionality of the innovative technologies (pore burner, metal foam based reformer catalyst and reforming reactor based on it), further development and introduction to practical maturity will take place in this follow-up project. For this purpose, the developed reformer stage is supplemented by a likewise metal foam-based shift stage with passive temperature control to achieve low CO contents in the product gas and thus built up a functional pattern of a heat-integrated, compact reformer overall system. By connecting a SelOx fine cleaning stage and a desulphurisation unit, the overall reformer system is to be coupled to a PEM fuel cell and tested under practical conditions.

Fuel cell systems (BZ systems) with reformers that use hydrocarbons as fuel gas are still not commercially available. The reason is that it is still around very complex and expensive systems. This leads to very high investment costs with still unsatisfactory system availability.

Worldwide are being developed by some groups of reformer prototypes. The case Achieved performance data (efficiencies, product gas qualities) lie in comparable orders of magnitude (Osaka Gas, Tokyo gas, ZBT etc.) and barely become significant improve as they encounter thermodynamic limits. about The market success of a reformer's concept therefore determines the differences especially concerning the economics of the systems. Successful will be reformers, on the one hand cost-effective let produce (as little material, inexpensive Materials, compact construction, as simple as possible construction, in mass production processes, etc.), on the other hand significant have lower operating costs (low maintenance, if possible Low active regulatory requirement, less active components with accordingly low parasitic energy demand and high efficiency). It therefore exists u. a. a considerable need for optimization regarding System Simplification (a component that is not needed will, reduces the investment costs, does not require any regulation or monitoring and can not fail) and the reduction the production costs.

It was a novel steam reformer stage for the production of hydrogen-rich Synthesis gas for stationary fuel cell applications developed, in particular through the use of a catalyst based on electrolytically coated cellular metallic Materials (metal foams) and the heating by means of a Pore burner distinguishes.

Even though at the time of this application final tests with the partial systems to be integrated in a reformer prototype (pore burners, Catalyst etc.) still outstanding, can be determined on the basis of Experimental results with the newly developed components in Test stands have been achieved, already at this time foresee that the expectations set in these new technologies were not only fulfilled, but sometimes even exceeded can be. In addition to the successful development of an extremely compact, multi-fuel combustion system based on the pore burner technology, for the test stand the suitability for the operation with anode residual gas as well as very short thermal response and heating times are detected particular, this affects performance based on the catalyst developed in the framework of the project an electrolytically coated, open-pored metal foam. Already at the current state of development are with the catalyst in the reforming of methane conversion rates in the field of thermodynamic Balance achieved. Leave in stable operation At the same time, space velocities can be realized, many times over set goal. In addition, the metal foam catalyst is characterized by a robustness that is unusual for nickel-based catalysts in particular, in a pronounced insensitivity expresses against oxygen-containing atmospheres. about the achieved development successes, the metal foam catalyst offers a very high potential regarding further performance improvements as well as with the development of further Fields of application in the field of heterogeneous catalysis, possibly also include the mobile application sector.

At the Steam reformer for stationary fuel cell applications are due to the use of open-pored metal foams as well in the shift stage advantages in particular with regard to Efficiency, compactness and system simplification. Therefore, in the context of the further development of the reformer, an innovative Shift level with passive temperature control on the base metal foam supported catalysts and built in the system will be integrated. The good thermal conductivity Metal foams should be used to exotherme Reaction heat of the shift reaction controls so on Dissipate environment (for example, to the surrounding flue gas), that - taking into account the thermodynamic equilibrium position the shift reaction - with passive operational management a favorable for CO conversion temperature profile results in very low CO levels at the output of the shift stage leads. The low CO content reduces the air requirement for the subsequent fine cleaning in a SelOx stage and thus leads to an increase in efficiency. moreover resulting from the reduction of the exothermic heat of reaction essential Simplifications in operational management.

In addition to increasing the efficiency of the steam reformer by further developing the metal foam catalysts of the reforming stage and the described development of an innovative single-shift reactor, another primary development objective is the efficient heat integration of the plant components, which is a prerequisite for achieving a high system efficiency. The aim is to produce a functional model with a capacity of 2.5 kW _th , which contains the reaction stages pre-reformer, main reformer, CO conversion (single-shift), steam generator and heat integration in a compact, well-insulated metal cylinder. To ensure a universal use of the new reformer system (eg the An binding to the promising, but still in development high-temperature PEM) further possibly necessary peripheral plant parts are connected outside the reformer shell. The function model is to be equipped with a desulphurisation and fine-cleaning stage (SelOx) and coupled to a PEM fuel cell with an electric power of 1 kW adapted to the thermal reformer performance in order to be able to perform performance tests under practical conditions.

A known polymer membrane fuel cell system for use in domestic energy supply consists essentially of the following system components:

following The prior art becomes the core components of a steam reformer system described. In addition, a representation of the Current state of development of fuel cell heaters.

reformer

The research carried out relates to the in 1 dashed framed reforming unit of the system. The selected process of steam reforming is established on a large scale in the industry and technically mature, the downscaling of this technique, however, in the power range relevant for fuel cell heaters (about 2.5-15 kW _th ) not yet succeeded satisfactorily.

In the case of catalytic reforming by steam reforming, it is preferable to use relatively inexpensive nickel catalysts in addition to the usually expensive noble metal catalysts. The reaction is endothermic, ie, the catalyst must be kept at reaction temperature for a sufficiently high conversion. From an industrial point of view, it is customary to split natural gas in the tube furnace at temperatures of 750-950 ° C. and pressures of 15-40 bar with the addition of steam to the catalyst (eg Ni or Pt). Steam reforming essentially follows the two independent reaction equations CH ₄ + H ₂ O → CO + 3H ₂ Δ _R H ⁰ = +206 kJ / mol (Eq. 2.1) CO + H ₂ O → CO ₂ + H ₂ Δ _R H ⁰ = -41 kJ / mol (Eq. 2.2) from where the second-mentioned reaction is referred to as shift conversion. From the gross reaction, which results from the formal summation of the above-mentioned reaction equations, CH ₄ + 2H ₂ O → CO ₂ + 4H ₂ Δ _R H ⁰ = +165 kJ / mol (Eq. 2.3) Immediately a water requirement of at least two water molecules per methane molecule can be read off. In practice, however, it has prevailed for reaction-technical reasons to supply the reactor usually three times the amount of H ₂ O per mole of CH ₄ . The ratio between the amount of water vapor and the amount of carbon is referred to as the S / C ratio (steam-to-carbon ratio).

Knowledge of the position of the thermodynamic equilibrium is of fundamental importance for the optimized design of reactors and processes. It represents an idealized borderline case, which however can be achieved in practice in the field of homogeneous gas reactions at correspondingly high temperatures and the use of suitable catalysts with a good approximation. Diagram 6 graphically shows the temperature-dependent equilibrium for the methane steam reforming for an S / C ratio of 3 at ambient pressure and the subsequent shift conversion. It becomes clear that in the reforming the equilibrium for low temperatures is almost completely on the educt side, ie with methane and water vapor, and only with increasing temperature on the product side to what serstoff, carbon monoxide and carbon dioxide is shifted.

Probably the most disturbing side reaction is carbon deposition, which can occur in steam reforming, especially in the case of water shortage in certain temperature ranges. Various carbon formation mechanisms, the complicated kinetics of the reactions, and the influence of the catalyst on carbon formation make it difficult to unambiguously predict the limits of carbon capture. Soot formation in the reforming of hydrocarbons is preferably produced by cracking methane at temperatures above about 300 ° C or by the Boudouard reaction (2CO <=> CO ₂ + C) even at lower temperatures. The Boudouard reaction is favored at low levels of water vapor by the presence of metals of the eighth subgroup of the periodic table, such. As nickel or iron. At higher water vapor concentrations, these metals - especially nickel - catalyze the water gas reaction, thereby counteracting soot formation.

With some safety can exclude the formation of soot only with a sufficient excess of water vapor and at temperatures that face the reforming reactions promote carbon deposition. That's how it starts Temperature of about 600 ° C the tendency to soot ever smaller, since the oxidative conversion of the carbon black precursors faster than their formation takes place. In an individual case and in particular however, metallic catalyst substrates are experimental Investigations needed to be made by proper choice of litigation as well as by modification of the catalyst properties (eg by Doping with suitable substances such. As MgO or cerium) the Shift carbon formation limits appropriately.

When Catalyst material in the steam reforming is conventional Steam reformers usually a bed of pellets or to the pressure loss over the catalyst bed reduce from rings (Raschig rings) or hollow balls in typical Fixed bed reactors used. For newer catalyst developments Also, precious metals such as Pt, Rh and Pd are described above Substrates or on ceramic or metal monoliths used. The higher costs are usually better with these systems Resistance to temperature fluctuations as well as a higher activity, through the higher space velocities and thus one to realize a more compact design. In addition, noble metal catalysts behave in contrast to conventional nickel-based catalysts not pyrophoric, d. h., They remain active even when in contact with air, which leads to advantages when starting and stopping the reforming unit.

Of the Sales of methane and thus the yield of hydrogen depend for a given catalyst activity in the first place from how efficiently heat from outside into the reactor room introduced and distributed for the endothermic reaction can be. That means the thermal conductivity the catalyst system (consisting of catalyst material, catalyst support, geometric arrangement) of crucial importance for the performance (for example, characterized through the achievable space velocity) of a reactor stage. The thermal inertia of classical fixed bed reactors Catalyst bed leads to an unsatisfactory Behavior when driving through load change situations, what is in a limited dynamics and quality losses of the product gas (eg CO increase in the reformate). moreover usually need cold start times in the double-digit minutes or even in the hourly range.

Shift stage

Behind the reforming reactor, the hot reformate is cooled and fed to the shift stage. There is practically only the desired shift conversion to Eq. 2.2 instead, since the process of Rückmethanisierung is suppressed by the selectivity of the catalyst used. Water vapor and carbon monoxide decrease in equal parts, while at the same time hydrogen and carbon dioxide increase to the same extent. The equimolar, exothermic reaction is in technical systems up to about 70 bar purely temperature-dependent and the equilibrium composition is with decreasing temperature on the product side. At about 100 ° C it is almost completely on the side of H ₂ and CO ₂ .

Since, depending on the reaction parameters of pressure, temperature and water vapor excess, after the reforming stage, CO concentrations reach well above 10% by volume, the shift conversion following reforming is often carried out in two reaction stages, the high-temperature shift (HT shift). z. B. on a Fe / Cr catalyst at temperatures between 330 and 500 ° C, and the low-temperature shift (NT shift), z. B. on a Cu / Zn catalyst at 190 to 280 ° C. The temperature ranges of the two shift stages are limited by the catalysts used. In the HT shift, the sintering temperature of the catalyst forms the upper limit. Below is the temperature range through which to achieve acceptable Revenues to be set low space velocities (space velocity = related to the catalyst volume educt volume flow) limited. In particular, at low steam-carbon ratios (S / C), a reduction of iron oxide (catalyst) can occur in the HT-shift, which is promoted by the Fischer-Tropsch hydrocarbon synthesis. In the NT shift, the temperature operating range is limited upwards by the recrystallization temperature of copper and down by temperature-related loss of activity.

Due to the higher reaction rate in the upper temperature range, the main conversion of carbon monoxide takes place in the HT shift. Here, after the reforming still very high CO content of depending on the chosen reforming temperature sometimes well above 10 vol .-% to z. B. be reduced about 3 vol .-%. After cooling of the product gas, this is fed to the NT shift, in which, at a favorable thermodynamic equilibrium position of the CO content is reduced to about 1 vol .-%, depending on the design even below. For the operation of HT shift stages, space velocities of approximately R = 1000-3000 h ^-1 are customary. NT-shift stages based on Cu / Zn catalysts are operated at R = 2000-5000 h ^-1 . However, these relatively low space velocities, with which conventional shift catalysts are operated, also mean that a relatively large amount of catalyst mass is needed, which is at the expense of the compactness of the system. Recent HT-shift steps, using noble metal catalysts, can achieve space velocities of 10,000 h ^-1 and above, but are also significantly more expensive and typically do not reach the activity of the Cu / Zn catalyst at low temperatures, leading directly to higher temperatures CO levels in the product gas leads.

at the developments of small reformers to house energy supply for reasons of system simplification and achievement a more manageable system behavior at part load often worked with only one shift level. After that it contains hydrogen-rich gas mixture usually still a carbon monoxide content of about 1% by volume in dry product gas, possible values of about 0.5 Vol .-% you usually buy with a lavish multi-stage NT shift stage and / or active temperature control.

CO fine cleaning

Especially PEM fuel cells react at operating temperatures of 60-80 ° C sensitive to the presence of carbon monoxide (only 10 up to 20 ppm i. A. permissible). This is based on a Poisoning of the platinum occupancy of the anode, causing cell voltage and Reaction speed and thus the efficiency of the fuel cell decline. By adding ruthenium, the CO compatibility the PEM-BZ be improved. Relocated with increasing temperature the equilibrium of the adsorption or desorption process of CO in the desorption direction. This is also the reason why the working at about 130-200 ° C high-temperature PEM fuel cells (HT-PEM) Carbon monoxide levels in the fuel gas on the order of magnitude of about 1 vol.%. The basis of this type of fuel cell forms a newly developed, temperature-resistant plastic membrane made of polybenzimidazole. These new high temperature membranes for Fuel cells are considered inexpensive and insensitive Alternative to classic membrane fuel cells are considered currently, however, still in the test phase.

For the use in classical low-temperature membrane fuel cells (NT-PEM) is the CO content after the low-temperature shift conversion However, with about 1% still significantly too high, so a CO fine cleaning stage necessary is. Basically, there are different procedures the CO fine cleaning known and available. For The use in membrane fuel cell systems are predominantly the pressure swing adsorption, the high-temperature diffusion through metal membranes as well as the selective catalytic methanization (SelMeth) and the Selective catalytic oxidation (SelOx) of carbon monoxide used. The latter method, in which a small amount of air the Gas flow is metered by most system manufacturers favored.

Selective oxidation is a gas-fine purification process in which carbon monoxide is added to carbon dioxide after the reaction with the addition of atmospheric oxygen in the presence of a suitable catalyst CO + 0.5O ₂ → CO ₂ Δ _R H ⁰ = -284 kJ / mol is implemented as selectively as possible. The unwanted side reaction, in which hydrogen reacts with oxygen to form water, should be suppressed as much as possible by the catalyst: H ₂ + 0.5O ₂ → H ₂ O, Δ _R H ⁰ = -241 kJ / mol.

Both Reactions are highly exothermic, so that with adiabatic reaction a significant increase in temperature and in isothermal reaction to consider a corresponding heat loss is. Most of the SelOx catalysts used have a temperature window from about 80 to 160 ° C, therefore, is usually to strive for a possible isothermal temperature control and to provide a cooling device. The stoichiometric Addition of oxygen (□ = 1) is sufficient for reformate gases However, by far not enough, because real catalysts are not perfect Have selectivity in favor of CO oxidation. Therefore is the lambda value to be set (ratio of the supplied Amount of oxygen to the minimum required amount of oxygen) each after catalyst and operating conditions mostly in the range between two and four. Another important finding is that the thermodynamic Equilibrium with respect to the carbon monoxide concentration in the temperature window from 80 to 160 ° C far above the normally required upper limit from 10 to 20 ppm, since under the given reaction conditions the reverse shift conversion is analogous to leading to carbon monoxide formation. In a selective oxidation state the required gas composition can only be achieved if the oxidation reaction of CO kinetically faster than the reverse-going shift conversion, which is not allowed to react to balance. Due to the undesirable Side reactions usually have very restrictive operating conditions be complied with, so although a stationary process is possible, however, load change, partial load or fluctuating entry conditions great difficulty represent. This requires a careful one in each case designed reaction stage.

burner

The burner has the task of heating the steam reformer in stationary operation. For this purpose, the exhaust gases of the fuel cell and of the gas generating system, which still contain considerable amounts of combustible constituents such as H ₂ , CO and CH ₄ , are burned to increase the system efficiency and to reduce pollutant emissions. However, the amount of heat contained is significantly less than the energy required by the endothermic steam reforming process, so the system fuel must be added to natural gas. In addition, the burner has the task of heating the system to operating temperature during a cold start. For this purpose, the system fuel natural gas is burned. An essential goal here is to keep the time duration as short as possible until a stationary operating state of the gas generation system is attained.

in the Unlike traditional combustion processes the combustion reactions in pore burner technology within the cavities of a porous matrix. Since that Stabilization concept in pore burner technology essentially is one-dimensional, the entire room can be used as a reaction zone become available through the porous medium is provided. For this reason, can be with the pore burner technology realize almost arbitrarily shaped combustion chambers that the geometric requirements of the process can be adjusted exactly can. This flexibility in terms of geometric design as well as the high power density of the pore burner were used, to make a particularly compact reformer unit.

Previous studies have confirmed the suitability of the burner system for use in reformer systems for use in domestic energy supply. In addition to the high power modulation capability required for economical operation in this field of application, the pore burner in the previous test operation was able to convince in particular by a pronounced multi-fuel capability (which also includes operation with low-calorie gas mixtures) and by low pollutant emissions. A detailed illustration of a pore burner is in 4 shown.

at a joint project involving the development of a new steam reformer system for the use in the domestic energy supply has the goal, is the Focus on the research work of the project initially the development of a new, metal foam-supported catalyst system, the construction of one for use in a steam reformer system suitable porous burner and the design of the reactor in sectional construction. The project concludes with the construction and testing of a Reformer demonstration model, consisting of a reformer-burner combination with integrated steam generator, with which the general suitability the new technologies used for the first time in this application and the scientific-technical bases for the Operation of the novel system can be provided.

The new catalyst was developed on the basis of detailed laboratory investigations of the substrate properties, experimental investigations to determine suitable coating parameters for achieving a high catalytic activity and extensive catalyst screening. The matrix of the samples to be examined was made up of four different metal foams (Ni, CrNi, CrNiAl foams and a reinforced CrNi foam), each with three different pore size ranges (17-23, 27-33 and 37-43 ppi pores per inch), which were electrochemically coated with 0.5 to 10 wt .-% nickel (dissolution about 1 wt .-%). In selected cases, additional doping with ruthenium was also carried out. In addition, for comparison purposes, further catalyst patterns were prepared by wet-chemical methods and measured.

Of the Prototype of the newly developed catalyst system consists of a Nickel-chromium metal foam substrate with an average pore density from 37-43 ppi, on which with an electrochemical coating process, a defined amount (about 4% by weight) of nickel is deposited, that the highest possible number of defects and structural anomalies arises on the substrate, which act as catalytically active centers. According to investigations, the uncoated Ni-Cr metal foams have been carried, in addition to the dispersed applied, metallic nickel particles and the nickel-containing metal foam matrix even to the high activity of the catalyst, which itself in a fast kinetics and accordingly high, realizable Space speeds expresses. This will be hydrogen yields achieved in the field of thermodynamic equilibrium maximum achievable values.

at Space velocities of about 6000 1 / h (for comparison: conventional Catalysts containing nickel as the active substance are operated at space velocities of about 3000 1 / h) are sales of about 90-95% achieved. The concentration of hydrogen in dry product gas amounts to about 75-78 Vol .-% and the residual methane content in the product gas is maximum about 3% by volume.

at Performance tests over the period of several hours could under the o. g. Conditions no degradation phenomena of Catalyst can be detected. The hydrogen production per Time unit proved in the previously feasible test periods of up to 8 hours as constant, partly as it progressed Duration of use even a slight increase in the hydrogen content observed in the product gas.

Next the high activity is the new catalyst system by a very low pressure loss and - due to the metallic carrier - a high thermal conductivity which, in particular during startup and shutdown processes as well as In case of load changes advantages due to the fast introduction into the system Give heat energy. Furthermore, the catalyst is extreme robust and insensitive to an oxygen-containing atmosphere, which also has a positive effect on startup and shutdown processes and by saving system components (such as a Nitrogen supply) to reduce plant complexity can contribute.

about the already good performance data in addition is particular the potential remarkable, which the catalyst system with appropriate Development seems to offer. The previous investigations have shown that the space velocity without significant losses in terms of recoverable and hydrogen yields can be increased to ranges of over 30,000 1 / h. Indeed the catalyst system tends to form soot when overloaded, what an occupancy of the active sites of the catalyst by deposition resulting in soot particles and resulting to a decrease in the activity of the catalyst with increasing Operating time leads. Although these deactivation effects reversible and the precipitated soot at any time (also during operation) by short-term air injection can be burned down, are very high space velocities in the current development stage of the metal foam catalyst without additional technical effort for one at intervals successful cleaning not yet feasible. If it succeeds, Soot formation even at very high space velocities could suppress the specific catalyst volume be greatly reduced again, which makes the perspective extremely compactly based reformer systems.

The development work on the pore burner to be integrated in the reforming reactor has been completed so far that the second prototype has already been designed, manufactured and tested and measured in the test stand. Currently, work is being carried out to integrate and test the pore burner in the reformer system. The concept favored in the application of a centrally located cylindrical burner, which is surrounded by the reaction space of the reformer ( 5 ), could be realized meanwhile. This arrangement allows a high heat input by radiation and conduction from the combustion zone into the reforming zone. In addition, with this arrangement it is possible to adapt the heat input to the axial heat requirement profile of the steam reformer by means of different insulating thicknesses and possibly insulating materials in the region of the combustion zone. The burner design elaborated for the above concept is in 6 shown. The porous combustion zone consists of a 10 ppi SiC foam (pores per inch) and a porosity of about 90%. This porous combustion zone has a height of 100 mm and a diameter of 40 mm, which at nominal load results in a surface load of about 1100 kW / m ² .

In the laboratory tests, the burner was in a test stand with the relevant for real load and operating conditions Fuel mixture compositions applied and in terms characterized the relevant operating parameters. The new burner system is characterized by a compact design, a stable operation at high power density and modulation capability as well low pollutant emissions. Currently the testing of the Pore burner in the demonstration model of the reformer unit.

Also has already developed a steam reformer system, which is a simple Construction with efficient heat integration or Heat connection unites. The system, its heat connection and basic structure as a basis for the reformer consists of the reaction stages pre-reformer, main reformer, the CO conversion, the steam generator, the burner and the heat integration within a well-insulated metal cylinder as a shell and the selective oxidation step as a CO fine purification process outside the reformer shell.

The Gas processors are characterized in test stands, wherein because of comparability first methane as reactant gas is used before then the survey with the real feed gas, z. As natural gas, takes place. As experimental Results are the hydrogen, methane and carbon monoxide levels and the hydrogen production efficiencies of the reformer the shift level for a realized reformer with a Hydrogen power of 6 kW, in diagram 8 as a function of the load played. The gas processor can operate in a wide performance range from approx. 30% to full load stable operation. The illustrated Hydrogen generation efficiency of the reformer system is here defined as the ratio of the lower heating value of produced hydrogen to the lower calorific value of the reforming process and methane supplied to the burner. This efficiency Measured behind the shift level at full load reaches approximately 77%. Of the Measured efficiency hardly decreases in the partial load range, although the increasing relative to the output of the gas processor Heat losses through the flue gas of the burner and the container wall (outer insulation) increase.

The hydrogen content in the dry product gas (measured after the shift conversion) is approximately 77 to 78% by volume over the entire operating range. In contrast, the CH ₄ (residual methane) not converted in the reforming process decreases with increasing load, as the reforming temperature rises. Due to the increasingly turbulent flue gas flow increasingly efficient heat input from the burner in the reformer stage of the methane conversion is improved. The content of carbon monoxide behind the shift conversion increases due to the increasing catalyst load (increasing space velocity with increasing gas flow rate) of 0.5% at 30% partial load and reached at full load about 1.5%. Behind the SelOx stage for gas fine cleaning, which is required for the operation of a NT-PEM, a 2-5% lower efficiency is achieved due to the virtually unavoidable parasitic hydrogen conversion (undesirable side reaction) in the gas fine purification stage depending on the CO content.

A Increasing the efficiency would be possible if it would be possible to control the CO content of the substance introduced into the SelOx stage Keep fuel gas low over the entire load range. One such approach is the development of a single-shift stage with metal foam-supported catalysts. The good thermal conductivity of metal foams should allow it, too with passive temperature control one for the thermodynamic Equilibrium position favorable temperature course over to adjust the shift level, resulting in low CO levels at the output would lead to the shift stage or at the entrance of the gas fine cleaning stage.

task Accordingly, the present invention is the structure, the testing and the optimization of a heat integrated, innovative Reformer system with a thermal hydrogen power of 2.5 kW for the supply of single-family homes. The reformer uses metal foam-based catalysts in reforming and shift level with passive temperature control to achieve low CO levels in the product gas is characterized in particular a compact design, in principle low production costs and a high load change dynamics as well as a high power modulation out. He points to conventional models Advantages regarding the approach and departure behavior on.

• (1) Der metallschaumbasierte Reformer-Katalysator wird weiterentwickelt. Im momentanen Entwicklungsstand wird seine Leistungsfähigkeit noch durch die begrenzte Standzeit (Lebensdauer) und die bei hohen Raumgeschwindigkeiten einsetzende Rußbildung beeinträchtigt. Die weiterentwickelten Reformer-Katalysatorsysteme werden hinsichtlich ihrer Deaktivierung und Degradation auch in Gegenwart des Katalysatorgiftes Schwefel untersucht und der bisher verwendete Reformer-Reaktor an den weiterentwickelten Reformer-Katalysator angepasst.
• (2) Aufgrund der bei Metallschäumen gegebenen hohen Wärmeleitfähigkeit, die aus der Kombination großer Berührungsfläche und geringem Wärmewiderstand entsteht, ist zu erwarten, dass metallschaumbasierte Katalysatoren auch bei der Shift-Reaktion vorteilhaft eingesetzt werden können. Dementsprechend sollen die Metallschäume -analog zur bisherigen Reformer-Katalysatorentwicklung-elektrochemisch mit Cu bzw. Cu/Zn als den für die Shift-Reaktion im angestrebten Temperaturbereich katalytisch aktiven Substanzen beschichtet werden. Der so entwickelte Shift-Katalysator wird als innovative Weiterentwicklung ebenfalls auf einem Langzeitprüfstand hinsichtlich seiner Standzeit untersucht.
• (3) Mithilfe des metallschaumbasierten Shift-Katalysators wird ein kompakter, einstufiger Shift-Reaktor mit passiver Temperaturführung aufgebaut. Im Einströmungsbereich wird der Katalysator durch die exotherme Shift-Reaktion auf höhere Temperaturen aufgeheizt, um dort die dadurch bedingte gute Kinetik auszunutzen. Im weiteren Strömungsverlauf wird dann die günstige Gleichgewichtslage durch Absenkung der Temperatur aufgrund der guten Wärmeleitfähigkeit des Metallschaums (nicht adiabater Reaktor, Wärmeabgabe an die Umgebung über die Reaktorwand) ausgenutzt. Damit ist zu erwarten, dass ein sehr niedriger CO-Gehalt von weniger als 0,5 Vol.-% im trockenen Produktgas ohne aktive Kühlung erreicht werden kann.
• (4) Die bisher entwickelte Reformerstufe-Brenner-Kombination mit Katalysator auf Metallschaumbasis und Porenbrenner wird zu einem wärmeintegrierten Reformer-Gesamtsystem ausgebaut. Dazu werden als Basis die von ZBT entwickelte Wärmeverschaltung und der oben beschriebene einstufige Shift-Reaktor auf Metallschaumbasis mit passiver Temperaturführung und einer vorgeschalteten Entschwefelungs-Stufe verwendet. Das Ziel ist ein wärmeintegrierter, effizienter, kompakter und prinzipiell preisgünstiger Reformer.
• (5) Das Reformer-Gesamtsystem wird in Kopplung mit einer Membran-Brennstoffzelle betrieben, um die Funktionskette eines Gesamtsystems bestehend aus Reformer, CO-Feinreinigung (SelOx) und Brennstoffzellen-Stack (NT-PEM) darzustellen. Durch die Kopplung mit einer Brennstoffzelle kann die Leistungsfähigkeit des Reformers in Bezug auf Gasqualität, Wirkungsgrad und Dynamik getestet werden. Nach ersten orientierenden Versuchen mit dem Brenngas Methan soll das System anschließend mit realem, schwefelhaltigem Erdgas als Einsatzstoff betrieben und vermessen werden.

The performance data of the reformer stage were determined experimentally under simulated load conditions. The tested reformer-burner combination must now be integrated into a heat-switched overall reformer system whose characteristics are finally to be checked by a coupling with a fuel cell stack first with pure methane as the fuel gas. Furthermore, there are currently no Knowledge of the operating behavior of the new reformer in the use of real, sulfur-containing natural gas known. The most important central aspects are therefore:

• (1) The metal foam-based reformer catalyst is being further developed. At the current stage of development, its performance is still impaired by the limited service life (service life) and the soot formation that occurs at high space velocities. With regard to their deactivation and degradation, the advanced reformer catalyst systems are also investigated in the presence of the catalyst poison sulfur and the previously used reformer reactor is adapted to the advanced reformer catalyst.
• (2) Due to the high thermal conductivity of metal foams, which results from the combination of a large contact surface and low thermal resistance, it is to be expected that metal foam-based catalysts can also be advantageously used in the shift reaction. Accordingly, the metal foams-analogous to the previous reformer catalyst development-are to be electrochemically coated with Cu or Cu / Zn as the substances catalytically active for the shift reaction in the desired temperature range. The shift catalyst developed in this way is also being investigated as an innovative further development on a long-term test bench with regard to its service life.
• (3) The metal foam-based shift catalyst is used to build a compact, single-stage shift reactor with passive temperature control. In the inflow region of the catalyst is heated by the exothermic shift reaction to higher temperatures in order to exploit the resulting good kinetics there. In the further course of the flow then the favorable equilibrium position is exploited by lowering the temperature due to the good thermal conductivity of the metal foam (not adiabatic reactor, heat transfer to the environment via the reactor wall). It is therefore to be expected that a very low CO content of less than 0.5% by volume in the dry product gas can be achieved without active cooling.
• (4) The heretofore developed reformer-burner combination with metal foam-based catalyst and pore burner is being expanded into a total heat-integrated reformer system. For this purpose, the basis of the ZBT developed heat connection and the above-described single-stage shift reactor based on metal foam with passive temperature control and an upstream desulfurization stage. The goal is a thermally integrated, efficient, compact and in principle low-cost reformer.
• (5) The overall reformer system is operated in conjunction with a membrane fuel cell to represent the functional chain of an overall system consisting of reformer, CO fine cleaning (SelOx) and fuel cell stack (NT-PEM). By coupling with a fuel cell, the performance of the reformer can be tested for gas quality, efficiency and dynamics. After first preliminary experiments with the fuel gas methane, the system is then operated with real, sulfur-containing natural gas as a feedstock and measured.

By the provision of an innovative concept for efficient fuel gas production for PEM fuel cells will be a significant contribution supplied to optimize currently used reformer systems, the - mainly because of their structural design substantiated unsatisfactory heat and mass transfer properties - currently still have weaknesses in the dynamics of fuel production and also because of their system complexity with disadvantages in terms of their profitability.

Of the to be developed reformer is characterized by low production costs to distinguish, d. h., He is basically inexpensive to produce be and have a simple and compact design. An essential Contribution to increase the compactness of the reforming stage, with the immediately a material and cost saving is connected, should the further development of the new catalyst system on the Base electrolytically coated metal foams provide. It is foreseeable that regarding the realizable space velocities There is considerable potential for further performance enhancement. whereby a reduction of the catalyst volume and thus a Reducing the size of the system achieved can be.

Besides should open-cell metal foams as a catalyst support can also be used in the shift stage. In this regard, is expected to have the very good thermal conductivity The metal foams can be exploited to be more passive Operation management for the thermodynamic equilibrium reach the shift reaction favorable temperature level to be able to. This should be very low CO levels on Output of the shift stage can be achieved, which is favorable would affect the subsequent fine cleaning stage. at the currently used SelOx stages are reduced a low Co-content the air requirement and leads to so an increase in efficiency. In addition, result from the reduction the exothermic heat of reaction essential simplifications in the management. Furthermore, the reduction contributes of the active regulation needs by realization of the desired passive management to reduce operating costs at.

• 1. Optimierung des Reformer-Katalysators und des Reformer-Reaktors,
• 2. Entwicklung eines metallschaumbasierten Shift-Katalysators,
• 3. Entwicklung eines Shift-Reaktors mit passiver Temperaturführung,
• 4. Aufbau eines wärmeintegrierten Reformer-Gesamtsystems und
• 5. Kopplung des Reformers mit einer Membran-Brennstoffzelle.

Primary objectives in the further development of the steam reformer are its performance enhancement by further development of the metal foam-supported catalyst system, the transfer of the advantageous properties of the metal foam catalyst on the conditions at a shift stage and the adaptation of the reformer prototype produced in the ongoing project to the requirements of a combined operation a steam reformer with a fuel cell brings with it. According to the desired results, the course of the work will be subdivided into five basic steps:

• 1. Optimization of the reformer catalyst and the reformer reactor,
• 2. development of a metal foam-based shift catalyst,
• 3. Development of a shift reactor with passive temperature control,
• 4. Construction of a heat integrated overall reformer system and
• 5. Coupling of the reformer with a membrane fuel cell.

following become the further development and testing of the overall reformer system planned work steps based on the investigation and development needs the individual system components explained.

Reformer catalyst and reformer reactor

It a metal foam-supported nickel catalyst was developed, its performance already at its current state of development the original Goals surpasses. At the same time have further investigations a considerable potential for further increasing the performance of the catalyst system shown what to achieve a high compactness of the system and in the consequence by material saving also to the cost reduction would contribute.

Together with the metal foam used so far, this results in five Samples using the developed electrolytic coating process built up to the catalyst and then by means of catalyst screening in terms of activity, selectivity and their Degradationsverhaltens measured and compared should. Accompanying are investigations of the surface structure (XPS, IR, TEM) before and after coating, and before and after the initial activation process performed to knowledge of morphological surface changes Information about the occurring catalytic mechanisms win and from this strategies for further development steps, especially for the selection of suitable promoters derived to be able to. The effectiveness of the modifications made will be with the help of catalyst screenings on the existing test bench performed.

At the three catalyst systems with the best performance Subsequently, further optimization by the catalysts systematically stabilized against soot formation. For this purpose, the Catalyst support by wet-chemical addition of potassium carbonate and / or alkaline earth compounds as well as by electrochemical Doping modified with suitable appearing additives (eg cerium) become. The catalyst performance is determined by means of Catalyst screening in the existing catalyst test stand. in this connection the sales of the main and side reactions become over a temporal temperature profile for different residence times determined. These investigations serve to determine the catalyst performance, to compare the catalysts with each other and thus to choose from.

Added These investigations are to be based on kinetic measurements ZBT existing kinetics test stands and simple model calculations to optimize the catalyst and reactor design. About that In addition, the advanced catalyst systems should be based on existing, fully automated long-term test benches with regard to their lifetime and their deactivation behavior in the presence of catalyst poisons (especially sulfur) for periods of time be examined by about 100 h. Based on the obtained in this way Performance data, the selection of the optimized catalyst system, that should be used in the overall reformer system.

Of Furthermore, on the basis of the measured data, an adaptation of the already built in the precursor project reformer reactor to the possibly changed, kinetic conditions. On the basis of kinetic measurements and a CFD simulation of the existing reformer reactor geometry may be the reactor design adapted to the modified catalyst system.

Shift catalyst

The advantages that metal foams offer, in particular with regard to the heat transport properties and the low pressure loss, should be exploited in order to enable the construction of an efficient single-shift stage realize. The thermodynamic equilibrium of the non-pressure-dependent, equimolar, exothermic shift reaction shifts with decreasing temperature to the product side. In addition to the reaction temperature, the amount of available water is another important reaction parameter. The extent of the development steps that will be required to build the shift catalyst system, based on the work carried out to develop the reforming catalyst. Based on the experience gained so far in the characterization of the metal foams in the course of the project so far, it is estimated that a scope of five different metal foam samples will suffice for the development work. The material is initially used as in the development of the reformer catalyst on Ni or NiCr foams.

The Metal foams are - analogous to the previous reformer catalyst development - electrochemical with Cu or CuZn as the one for the shift reaction in the desired Temperature range coated catalytically active substances.

Shift reactor

aim the shift-reactor development is the construction of a single-stage, metal foam-based compact shift stage with passive temperature control, d. H. automatic release of the exothermic shift reaction heat via the reactor wall to the environment (eg the flue gas of the reformer burner) using the good thermal conductivity of the metal foam carrier. This results in a very low CO content of less than 0.5% by volume. aimed at in the dry product gas. Due to the metal foams given high thermal conductivity, resulting from the Combination of large contact surface and low thermal resistance arises, it is expected that Metal foam catalysts also advantageous in the shift reaction can be used to very compact, single-stage To realize conversion stages.

As already mentioned, results of catalyst screenings are suitable limited only for an exact reactor design, since the transmission only under the same operating conditions of laboratory and system reactor is reliable. That's why for catalytic and kinetically sensitive processes, such. As for the shift conversion, in addition to the catalyst screening applied a second methodology of catalyst investigations. It This will be kinetic experiments on very small catalyst samples performed in the isothermal differential laboratory reactor. reactant concentrations and reaction temperatures are in turn with in many areas Help of simulations set. In the investigated catalyst form In doing so, the ZBT uses a new method, not like otherwise usual catalyst powder, but later used catalyst carrier system (eg., The metal foam carrier with applied catalyst material) as a very small volume is used. On this basis, the determination of the model parameters takes place by ZBT for z. B. the power approach with Arrhenius temperature dependence for the description of the reaction rate in dependence from the temperature and partial pressures of each Species for the corresponding reactor pressure by evaluation the measured data obtained in step 2.3.

The actual reactor design is composed of a catalyst design considering z. B. inflow and outflow areas and heat transfer surfaces (z. B. reactor outer wall) and a CFD simulation, which in addition to the simulation of the flow-mechanical Conditions also for the implementation of the reaction kinetics and the thermo-technical observation is suitable. The simulation can again with the experimental results of the kinetics measurement be validated. With such models could already good matches achieved by reactor design and operation. At the following Reactor construction and fabrication must be in addition to the the kinetic data based reactor design further criteria, such as B. Requirements for the material, semi-finished products, insulation and those for the experiments or the later operation necessary sensors (eg temperature measuring points) are taken into account become.

To Reactor production and assembly is the single reactor test in a special single reactor test stand among the corresponding ones Conditions. Thereby investigations become stationary Operation at full and part load, dynamic operation and cold start and standby behavior. After these investigations an iterative optimization may be required, i. H. dependent the outlined development process is repeated from the results run through. After the successful basic tests will be further investigations (for example, process stability) as life tests carried out.

Reformer overall system

The previous demonstration model (reformer-burner combination) with catalyst on metal foam base and burner designed as pore burner is upgraded to a heat integrated reformer overall system. The aim of the work step is an efficient, small-scale reformer, the compactness being achieved substantially by the use of the metal foam catalysts. The smaller size causes a cost reduction, it is less catalyst material needed because of the high achievable space velocities in the shift and reformer reactor.

For the determination of the concrete boundary conditions are first Thermodynamic simulation calculations to determine the substance and energy balances performed. As a basis for the heat connection becomes the thermal integration developed by ZBT used, so only one adjustment for this step must be done by simulation calculations. Furthermore already can the heat exchanger to the internal Heat utilization be designed roughly. As a result, the boundary conditions z. For example the reactors or the heat exchangers as mass flow rates, Concentrations, temperatures and pressures set. For every reactor and every heat exchanger then performed further calculations during development in detail. In this design work the developed reformer and shift reactors are used and interconnected to a gas processor.

The Investigations of the hydrogen production system are carried out after the same scheme as the individual reactor tests. This is especially the behavior of the individual reactors and heat exchangers evaluated within the hydrogen generator. Often it comes despite exact interface definitions between the reactors to deviations in the integrated operation, since z. B. process and heat flows of differ from the simulations. This first stage of development of a Gas processor is thus often used as the basis for a redesign and for optimizations or altered Detailed solutions.

Coupling with a membrane fuel cell

Following the successful operation of the overall reformer system, the test facility will be expanded to include a membrane fuel cell stack. Due to the availability of NT-PEM stacks in the power range of 1 kW _el and a corresponding gas fine cleaning stage (SelOx), the test mode can be carried out by coupling to such a membrane fuel cell. By using a fuel cell with an adapted to the reformer electrical power and the use of a real anode residual gas (Anodic Off Gas, AOG) can be tested in the pore burner. The process chain is first characterized with methane in order to obtain reference values for the following series of measurements with real natural gas.

The Previous investigations have been to avoid peripheral by waiving Aggregate the focus of development initially on the innovative To focus on the core components of the new reformer, with pure methane carried out as a raw material for the reforming. In contrast to the work carried out so far the reformer of the now following development stage with practice Natural gas instead of methane operated and to a desulfurization stage be extended.

by virtue of the previously determined high potential of metal foams for use in the catalysis but also also the further development and optimization of metal foam supported Catalysts a high priority in the context of the connection project granted. In the medium term leads in particular the development a passively guided single-shift stage with metal foam-based Catalysts for reducing the development costs of fuel cell heaters, because of system complexity and the need for control would be reduced by such a system. The expected, low CO levels in the product gas stream behind the Metal foam-based shift level facilitate first the coupling with an NT membrane fuel cell via a SelOx stage. In addition, the low CO levels let the reformer system for the future Coupling with HT-PEM fuel cells currently still in the test stage appear particularly suitable. After obtaining the technical maturity Both systems will additionally benefit from the savings give a fine cleaning stage.

Of the largest part of the development work and equipment needed for the necessary examinations are part of laboratory and pilot plant equipment and can be used. This includes in particular a special manufactured test stand for the characterization of the metal foam catalysts, kinetics and long-term test stands and test stands for examination the effects of catalyst poisons.

The development of the pore burner was largely completed in the initial application, so that the development effort in this regard is limited to adaptation work. The new construction of the reforming reactor, which is required as part of the further development of the burner-reformer combination to form a heat-integrated, passively managed overall reformer system, will also make a contribution to the new Geo Metronome adapted pore burner necessary. This pore burner is provided by the developer of the system at cost price.

Ni

= Nickel

NC

= Nickel-Chrom

NCA

= Nickel-Chrom-Aluminium

NCX

= Nickel-Chrom, verstärkte Matrixstruktur

Tabelle 2: Spezifikationen der für die nasschemisch hergestellten Katalysatormuster verwendeten Substrate Bezeichnung NCX NCA NC NC Mittlerer Porendurchmesser [mm] 0610 1723 1723 2733 Mittlerer Porendurchmesser [mm] 2,3 0,9 0,9 0,6 Spez. Oberfläche [m²/m³] 1000 1500 1500 2300

NC

= Nickel-Chrom

NCA

= Nickel-Chrom-Aluminium

NCX

= Nickel-Chrom

Tabelle 3: Messbereiche des für die Untersuchungen eingesetzten Mehrbereichs-NDIR-Analysators mit WLD-Zelle Gaskomponente Messprinzip Messbereiche CH₄ NDIR MB 1:0–5 Vol.-% MB 2:0–100 Vol.-% CO₂ NDIR MB 1:0–2,5 Vol.-% MB 2:0–50 Vol.-% CO NDIR MB 1:0–5 Vol.-% MB 2:0–50 Vol.-% H₂ WLD MB 1:0–10 Vol.-% MB 2:0–100 Vol.-% Tabelle 4: Art und Funktion der zur Anlagenregelung eingesetzten Fieldpoint-Module Bezeichnung Modulart Kanäle Funktion FP-AO-200-1 Analogausgangsmodul 8 Sollwertvorgabe Regelventile (Wasser + Methan) FP-AO-200-2 Analogausgangsmodul 8 Sollwerte zur Ansteuerung der Thyristor-Leistungssteller (Temperaturregelkreise) FP-DO-401 Diskretes Ausgangsmodul 16 Ansteuerung Schlauchpumpe Steuerung Sicherheitsventil (Reaktorüberdruck) FP-AI-110 Analogeingangsmodul 8 Istwerte Regelventile (Wasser + Methan) Istwerte Drucksensoren FP-TC-120 Thermoelementeeingangsmodul 8 Temperaturerfassung des Heizblockes, der Heizschnüre, der Reaktortemperatur und der Produktgastemperatur The catalyst screening is to be carried out on the test stand specially prepared for the investigation of the metal foam catalysts. Although this has proved to be well suited for the given problem, it came - due to the high thermal surface load that occurred in the heating of the relatively small surface area of the test reactor with high temperature resistant heating cords - during the experiments often to a failure of these heating elements. In addition to relatively high costs for the procurement of spare parts, this caused additional repair work and associated longer downtime. For further extensive screening experiments, therefore, the test reactor heated with heating cords is to be replaced by a vertical hinged-wall furnace with working tubes made of quartz glass, whereby additional advantages arise with regard to the realization of higher temperatures or higher space velocities at constant temperature. In addition, the use of a quartz glass tube as a reactor in combination with a Klapphrofen the possibility of visual control of the state of the catalyst used and the reactor itself. The hinged tube is integrated into the existing control system of the test stand, so that can be dispensed with a separate, furnace-side regulator , Table 1: Specifications of electrolytically produced catalyst samples

Ni

= Nickel

NC

= Nickel-chromium

NCA

= Nickel-chromium-aluminum

NCX

= Nickel-chromium, reinforced matrix structure

Table 2: Specifications of the substrates used for the wet-chemically prepared catalyst patterns description NCX NCA NC NC Mean pore diameter [mm] 0610 1723 1723 2733 Mean pore diameter [mm] 2.3 0.9 0.9 0.6 Specific surface area [m ² / m ³ ] 1000 1500 1500 2300

NC

= Nickel-chromium

NCA

= Nickel-chromium-aluminum

NCX

= Nickel-chromium

Table 3: Measuring ranges of the multi-range NDIR analyzer with WLD cell used for the tests gas component measuring principle measuring ranges CH ₄ NDIR MB 1: 0-5 Vol .-% MB 2: 0-100 Vol .-% CO ₂ NDIR MB 1: 0-2.5 vol.% MB 2: 0-50 Vol .-% CO NDIR MB 1: 0-5 Vol .-% MB 2: 0-50 Vol .-% H ₂ WLD MB 1: 0-10% by volume MB 2: 0-100 Vol .-% Table 4: Type and function of the Fieldpoint modules used for system control description Module channels function FP-AO-200-1 Analog output module 8th Setpoint specification control valves (water + methane) FP-AO-200-2 Analog output module 8th Setpoints for controlling the thyristor power controllers (temperature control circuits) FP-DO-401 Discrete output module 16 Control hose pump Control safety valve (reactor overpressure) FP-AI-110 Analog Input Module 8th Actual Valve Valves (Water + Methane) Actual values Pressure sensors FP-TC-120 Thermocouple Input Module 8th Temperature detection of the heating block, the heating cords, the reactor temperature and the product gas temperature

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• - DE 19815004 C1 [0004]

Claims (16)

Method for improving the operating behavior reforming systems to adapt natural gas processing to hydrogen on the power requirement of polymer membrane fuel cells (PEMFC), in particular for use in domestic energy supply, characterized by the use of a catalyst on the base cellular metallic materials coated in such a way that the highest possible number of defects and structural anomalies arises on the cellular metallic material. Method according to claim 1, characterized in that the coating takes place electrolytically. Method according to claim 1, characterized in that that the coating is electrochemical. Method according to one of claims 1 to 3, characterized in that the coating with a commercial Ni-sulfamate electrolyte is carried out without additives. Process according to Claims 1 to 4, characterized characterized in that the coating at a temperature of 55 up to 60 ° C takes place. Process according to Claims 1 to 5, characterized characterized in that the coating is under potentiostatic conditions takes place and the cell between the working electrode and the reference electrode imprinted a constant potential difference of 3 volts becomes. Process according to Claims 1 to 6, characterized characterized in that the coating time is 10 to 20 minutes, preferably 15 to 17 minutes, is. Catalyst for use with the method of one of claims 1 to 7, characterized in that as a catalyst carrier a substrate of cellular metallic Materials (metal foam) is used and that the metal foam substrate degreased before coating, pickled (roughened) and then is rinsed. Catalyst for use with the method of Claim 8, characterized in that the metal foam substrate made of nickel. Catalyst for use with the method of Claim 8, characterized in that the metal foam substrate made of nickel-chromium. Catalyst for use with the method of Claim 8, characterized in that the metal foam substrate made of nickel-chromium-aluminum. Catalyst according to one of claims 8 to 11, characterized in that the density of the metal foam substrate is 0.5 to 0.9 g / cm ³ . Catalyst according to one of claims 8 to 12, characterized in that the amount of the to be coated Metal between 0.5 and 10, preferably between 2 and 6 wt .-% is. Catalyst according to one of claims 8 to 13, characterized in that the amount of to be coated Metal is 4 wt .-% is. Catalyst according to one of claims 8 to 14, characterized in that as the catalytically active substance Nickel is used. Catalyst according to one of claims 8 to 14, characterized in that as the catalytically active substance Ruthenium is used.