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Autothermal-reformation enhancement using a stratified-catalyst technique
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
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Autothermal-reformation enhancement using a stratified-catalyst technique Nadia Richards, Jacob Needels, Paul Erickson* Department of Mechanical and Aerospace Engineering, University of California, Davis, USA
article info
abstract
Article history:
An investigation of a stratified catalyst arrangement for methanol autothermal reforming
Received 9 May 2017
was conducted. The stratified arrangement consisted of a monolith platinum group metal
Received in revised form
(PGM) catalyst, immediately followed by a pelletized Cu/ZnO/Al2O3 water gas shift (WGS)
2 August 2017
catalyst. The system was experimentally investigated under different test conditions by
Accepted 8 August 2017
varying the flow rate, oxygen-to-carbon ratio, and the packed length of the downstream
Available online xxx
WGS bed. The reactor performance was quantified with several metrics including the fuel conversion, hydrogen yield, system temperatures, and preliminary results are shown for
Keywords:
short-term catalyst degradation. Conversion and hydrogen yield increased significantly at
Methanol
low oxygen-to-carbon ratios as compared to the stand-alone PGM catalyst. Results confirm
Autothermal reformation
the benefits of using a PGM and WGS catalyst in a stratified fashion and the stratified
Reactor design
arrangement increases performance over each of the catalysts used singularly. Further
Stratified catalysts
study is warranted considering potential degradation of the WGS bed caused by high temperatures encountered at the exit of the PGM section. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction
Background
Hydrogen that is chemically bound in organic compounds can be liberated through a process called reformation. Industrially, most hydrogen is generated using natural gas and steam, but other fuels have been studied in research environments [1e3]. The main types of reforming are Steam Reforming (SR), Partial Oxidation (POX), and Autothermal Reforming (ATR). While industrial methods are known and used in large-scale hydrogen production, heat and mass transport limitations make these methods non-ideal for small-scale reforming. Through the use of a stratified bed, it may be possible to increase heat and mass transport resulting in better catalyst performance for small-scale reactors.
Steam reforming is the standard industrial method of producing hydrogen. The reactants for SR are steam, and a fuel source such as hydrocarbons, alcohols or other organics. The reaction of the fuel and steam is endothermic and so a supply of heat is required for the process to occur. The reaction is carried out over a catalyst bed to enable a faster reaction rate and lower temperature. For methanol this general reaction is as listed in Eqs. (1) and (2) and the overall reaction is given in Eq. (3). CH3 OH/2H2 þ CO ðDH ¼ þ90:8kJ=molÞ
(1)
CO þ H2 O #H2 þ CO2
(2)
ðDH ¼ 41:4kJ=molÞ
* Corresponding author. Department of Mechanical and Aerospace Engineering, University of California Davis, Davis, CA, USA. E-mail address: [email protected] (P. Erickson). http://dx.doi.org/10.1016/j.ijhydene.2017.08.050 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Richards N, et al., Autothermal-reformation enhancement using a stratified-catalyst technique, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.050
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CH3 OH þ H2 O/3H2 þ CO2
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ðDH ¼ þ49kJ=molÞ
(3)
Methanol SR requires 49 kJ/mol of heat that is typically supplied from external burners [4]. This external heating requirement can result in long start-up times for the reaction to get underway, as well as causing the system to experience large internal temperature gradients [5]. However, despite these drawbacks, once the catalyst is sufficiently active, SR produces up to 75% molar concentration of hydrogen. This is the highest hydrogen concentration as compared to the other reforming processes. Partial oxidation, as its name suggests, is when the fuel is partially oxidized as opposed to being fully oxidized or combusted. It is susceptible to the formation and deposition of solid carbon within the reactor, also known as coking. The reaction mechanism is complex and produces various intermediate species and radicals [6]. A simplified overall reaction is shown in Eq. (4) for methanol POX. 1 CH3 OH þ O2 /2H2 þ CO2 2
ðDH ¼ 192kJ=molÞ
(4)
The heat release is shown to be 192 kJ/mol for one-half mole of oxygen, and for higher oxygen to fuel ratios more energy is released. Under these idealized conditions (assuming maximum CO2 selectivity) the hydrogen concentration is around 67% and if air is used as the oxidizer, hydrogen concentrations are lower due to nitrogen dilution. POX occurs at high temperatures which can be lowered by using a catalyst. Due to the exothermic reaction which eliminates heat limitations, POX is capable of rapid responses to dynamic loads and benefits from a more compact system, as compared to SR. However, POX is still less efficient than SR since much of the energy is converted to heat rather than hydrogen and carbon monoxide concentrations in actual reactors dominates other products. Autothermal reforming is considered a combination of SR and POX, where the reactions are in close thermal contact within a single reactor. The addition of oxidizer to the SR reactants allows for some of the fuel to partially oxidize thereby liberating some chemical heat to sustain the endothermic SR reaction [7e9]. In ATR the amount of heat generated comes from the oxidation steps and is dictated by the amount of oxygen that is used. The exothermic steps are rapid and the heat released is used to sustain the SR steps, and so ideally the overall reaction is thermally neutral [10e12]. ATR is potentially capable of reforming multiple fuels since it experiences high temperatures from the oxidation reactions. Using the ideal stoichiometry for the reactants the overall methanol ATR reaction is shown in Eq. (5). CH3 OH þ 0:1O2 þ 0:8H2 O/2:8H2 þ CO2
ðDH ¼ 0kJ=molÞ
(5)
ATR is a good candidate for small-scale reformers due to its compactness, its potential ability to handle a variety of fuels, as well as being able to accommodate transient loads. However, ATR does not produce high purity hydrogen from its product gas composition, and on a PGM catalyst has been shown to typically follow equilibrium predictions [1,2,13e15]. Additionally, temperatures generated during the exothermic stage of the ATR reaction has the capability of degrading the catalyst, if not properly controlled [16].
Conversely, SR has the capability to produce high purity hydrogen on a copper-based catalyst [17e20]. A detailed kinetic study has been done showing that copper is highly selective towards hydrogen and carbon dioxide formation [21]. However the overall SR process is large, done in multiple stages, and suffers from thermal limitations. In addition to the temperature gradients, the size of the SR reactor also influences the performance. For a given reactor volume, the change in the overall aspect ratio, as determined by the ratio of the length to the diameter, can produce differences in conversion as much as 30% [22]. These basic problems that plague the reforming processes highlight some of the issues faced when attempting their use in small-scale applications. These issues have presented a challenge for researchers for many decades and remains an active area of research. The literature shows different approaches that have been taken to enhance the reforming process which include introducing internal flow baffles and disturbers [23,24], applying acoustics to the reactor [25], as well as other stratified approaches [26e27]. Typically these stratified systems use equal lengths of platinum and nickel catalysts for methane reforming [26e29]. However it has been shown that the oxidation reactions occur more rapidly than the reforming reactions and so the platinum bed can be much shorter in relation to the nickel bed [28]. These stratified studies kept the overall length of catalyst as a fixed constant, and so explorations into the downstream length effects on the stratified system are yet to be explored. This serves as the basis for this research project. The focus of this research is to combine ATR and SR into a singular stratified unit using a PGM and copper-based WGS catalyst of varying packed lengths in order to enhance the overall reforming process. This enhancement method seeks to improve the hydrogen concentration for ATR, while simultaneously reducing the size of the SR system. If one uses a simplified approach to the reforming reactions as a fuel decomposition step immediately followed by a shift reaction one can gain insight as to where those reactions are occurring and how to enhance the overall process by using the best catalyst at the correct position in close thermal contact. While a solution for all problems with ATR and SR is not addressed, any enhancement scheme for the reforming processes that can lead to reducing the reactor size and weight, increase output hydrogen purity and remove thermal limitations should be investigated. Many studies have attempted to address various aspects of the difficulties in reformation [30e34]. The specific objectives of this research are to study a methanol reforming system using two different formulations and geometries of catalysts arranged in a stratified configuration within a single reactor. In this research, the stratified reactor consisted of a monolith PGM catalyst immediately followed by a pelletized Cu/ZnO/Al2O3 WGS bed. The stratified system performance is observed while varying the oxygen/ methanol ratio, reactant inlet flow rates, and various lengths of the SR packed bed section. From the experimental work, analysis quantified the effects of the variables on the system performance. The reactor performance is quantified with several metrics including the fuel conversion, hydrogen yield, system temperatures, and preliminary results are shown for short-term catalyst degradation.
Please cite this article in press as: Richards N, et al., Autothermal-reformation enhancement using a stratified-catalyst technique, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.050
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Experimental setup The reforming system is located in the Energy Research Laboratory (formerly the Hydrogen Production and Utilization Laboratory) at the University of California, Davis. The existing system was originally configured for methane-steam reforming but has the capability to adapt to liquid fuels and autothermal reforming. The methanol autothermal reforming process starts with flowing a water and methanol premix at 1.5:1 steam-to-carbon ratio. The premix is pumped through a series of vaporizers which change the phase from liquid to gas. At the same time, clean, compressed air is supplied using a mass flow controller and routed to a gas heater. Once the reactants are at a consistent steady temperature they are routed to the reactor through the superheater. The superheater is designed to further promote sufficient mixing and to also maintain a steady temperature (250 C) prior to entering the reactor. A schematic of the system used is shown in Fig. 1. After passing through the reactor, the first stage of the condenser is the shell and tube heat exchanger, which has a back pressure regulator at the outlet to maintain a constant pressure for the various catalyst bed arrangements that were tested. The gaseous products exiting from the reactor, called reformate is then further cooled to 1 C through the coils of the main condenser. Inside the main condenser the reformate flow is split into two streams; one for analysis and the other for exhaust. In the exhaust stream, the liquid water and unreacted methanol are collected in the exhaust condenser trap while the gases are vented to the exhaust. For the analysis stream the condensed methanol and water, called retentate, were collected in the analysis trap and the dry gases
3
were routed to the gas analyzer and then vented. The retentate was measured for its mass and density to determine the methanol conversion. Other features of the system include the dedicated nitrogen line to keep an inert environment on the reduced catalyst during downtime. Additionally, an air purge was used on the condenser analysis line to ensure all the retentate was evacuated into the trap. Inside the reactor, the reforming process is performed over the heated catalyst. In this study, the catalysts were of two different compositions and were placed in a stratified configuration as shown in Fig. 2. Reactor performance can be quantified by several parameters that rely on the specific operating conditions and the output products from the reaction. The results from the experiments are analyzed using these parameters to quantify the performance of the stratified system. Conversion is defined as the reactant consumed divided by the reactant input to the system. It is a common measure of reactor performance since it essentially gives insight into the reaction progression. For methanol the conversion is shown in Eq. (6) below CH3 OHinput CH3 OHoutput $100 (6) Conversionð%Þ ¼ CH3 OHinput Using the constant molar ratio of steam to methanol at 1.5:1, the premix density of 0.9080 g/cm3 at 24 C, and the change in the premix mass during operation, the amount of methanol input to the reactor was obtained. To determine the amount of methanol output from the reaction, the retentate was collected in the analysis trap during the steady state run time. The trap was emptied and the retentate mass was measured. A calibrated table of density with respect to
Fig. 1 e Schematic of experimental setup. Please cite this article in press as: Richards N, et al., Autothermal-reformation enhancement using a stratified-catalyst technique, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.050
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ratio of the hydrogen produced to the methanol input, normalized to 100% maximum using a stoichiometric factor (SF) of 1/3. H2 Yieldð%Þ ¼
H2 produced $SF CH3 OHinput
(8)
Oftentimes the literature reports the hydrogen selectivity, which is similar to yield combined with conversion, which measures the ratio of the hydrogen produced to the fuel that was consumed. For this study the hydrogen yield and conversion are reported separately and are not combined into a selectivity term.
Results
Fig. 2 e Schematic of reactor.
methanol concentration was constructed and retentate density was used to calculate methanol concentration and hence methanol mass in the retentate. The space velocity gives the ratio of the total volumetric flow rate to the catalyst bed volume. The catalyst volume was calculated using the inner reactor diameter and length of the PGM monolithic catalyst as well as the packed length of the WGS pellet bed. The equation for the gas hourly space velocity (GHSV) for the stratified bed is given in Eq. (7). While the GHSV is the standard metric for reporting a flow rate in a fixed bed length, in this analysis gas hourly space velocity is less useful than the raw reactant flow rate as the volume of the catalyst bed changes with increasing WGS catalyst length, thus changing the GHSV without changing the flow rate. V_ reactants GHSV hr1 ¼ Vcatalyst
(7)
Yield is used to give insight into which products are being produced from the reaction. It quantifies the production of a particular specie based on the amount of fuel input to the reactor. It is a useful metric for reactor analysis since it supplements the information given from conversion. For reforming systems, hydrogen is the desired output and so the hydrogen yield is calculated according to Eq. (8), as the
The stratified catalyst bed influences several measurable parameters in the reforming process. The output parameters presented are the reactor temperature profile, fuel conversion, hydrogen yield, and short term degradation. These outputs can translate into potential size reductions and improved capacity for fuel reformers. The stratified catalyst bed was investigated for its influence on the internal reactor temperature. The expected benefit of using this configuration was to generate heat internally to keep the WGS packed bed active for reforming. The improved internal heating should improve the overall conversion and hydrogen yield from the reactor. The reactor was divided into five zones, and at the exit of each zone the temperatures were monitored. The measured internal temperatures are taken at the inlet for the incoming reactants at 250 C, along the centerline for the length of the reactor for each zone exit, and finally the outlet where the products leave the reactor. Additional temperature measurements were taken across the 0.635 cm (0.25 in) monolith placed at the inlet to Zone 1, and the internal wall temperatures at each zone exit. From these measurements the centerline, and an overall interpolated temperature profile were plotted in Figs. 3 and 4. The plot shown in Fig. 3 depicts the internal centerline temperatures for all tested stratified catalyst arrangements. Starting with the baseline at 0 WGS zones (PGM catalyst only), each zone of the reactor was filled with the downstream WGS pellets until the full packed configuration was achieved at 5 WGS zones. The operating conditions used a flow rate of 8.5 ml/min, with a 0.25 O2/C ratio. The vertical dashed lines represent the placement of the PGM catalyst bed and serves to aid the visualization of the arrangement. The plot also shows that at each reactor zone exit, the corresponding WGS packed zone generally has the lowest temperature. For example, at the Z1 reactor exit, the 1 WGS zone has the lowest temperature within the filled catalyst section. Similarly at the Z2 exit the 2 WGS zone gives the lowest reading. This demonstrates that within the WGS packed bed the temperature is in constant decline as a result of the endothermic reactions taking place. In addition to exploring the effects of stratification on the centerline temperatures, the overall internal profile for the reactor was investigated using the centerline and the radial temperatures.
Please cite this article in press as: Richards N, et al., Autothermal-reformation enhancement using a stratified-catalyst technique, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.050
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Fig. 3 e Reactor centerline temperature values.
Fig. 4 e Interpolated experimental reactor temperature profiles.
The experimental results of three configurations are shown in the contour plots of Fig. 4. The reactors were tested using a constant flow rate of 8.5 ml/min, and an O2/C ratio of 0.15. Each plot shows the half-radial reactor profile, and are
not drawn to scale. The contour plots were achieved using a linear interpolation of the measured temperatures at the center and wall for each zone exit and across each zone resulting in curved lines. It is important to note that local
Please cite this article in press as: Richards N, et al., Autothermal-reformation enhancement using a stratified-catalyst technique, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.050
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inaccuracies in the radial gradients exist from the interpolation, but information of the overall thermal distribution is still observable and useful in the analysis. The left-most plot is the baseline PGM-only configuration (0 WGS length), the center plot is a stratified bed having the PGM monolith in addition to 2 WGS zones (0.09 m packed length), and the right-most plot is a fully packed with the PGM plus all 5 zones filled with the WGS pellets (0.25 m). The dashed horizontal lines represent the packing of the PGM and the WGS pellets within the reactor, and serve as a visual aid for their placement. The 0 WGS bed length, or the PGM baseline, establishes the thermal distribution within the reactor and shows that significant heat that is available downstream of the PGM catalyst. Operating at an O2/C ratio of 0.15 raises the center temperatures to over 325 C just after the monolith. This hotter region extends axially down through the reactor to around the Zone 4 exit. The oxidation reactions that are occurring on the PGM catalyst causes the temperature spike, and appears to keep it axially located. Going from the Zone 4 exit to the reactor outlet the gases gradually cool down until they leave the reactor in the 275e300 C range. The reactor walls were also raised slightly above their 250 C set-point temperature, in the range of 275e300 C. Since the heat bands were controlled to maintain the wall set-point temperature, they are turned off and the heating of the wall is only due to the hot center region. This indicates that the heat released from the upstream reactions on the monolith is capable of heating the reactor, but is being exhausted as waste heat which could be potentially utilized by a downstream WGS packed bed. Adding two sections or zones of WGS pellets to the reactor and placing them immediately downstream of the PGM monolith creates the temperature profile shown in the center plot in Fig. 4. The plot shows that the hot center temperatures in excess of 325 C are now confined to the Zone 1 region. Within the first zone the active WGS pellets are no longer limited by heat transfer, and work to carry out the
endothermic reforming reaction. This shortens the heat progression down the reactor into the second zone. In effect, the WGS packed beds facilitate heat transfer radially, as opposed to the axial transport due to the fluid velocity, allowing more of the catalyst to remain active. A similar trend is seen with four packed bed zones. The increase in radial heat transfer results in a more uniform temperature distribution. As shown in previous studies, methanol ATR on a PGM catalyst alone can achieve maximum conversions exceeding 99% once the catalyst is sufficiently active [14e16]; although lower maximums of around 72% are possible depending on the particular formulation used and reactor pressure [17,18]. In the baseline tests, shown in Fig. 5, maximum conversion values of 72% and 90% were attained for PGM placement in zones 1 and 4 respectively. Subsequent tests only used the PGM at Zone 1. The profile for the conversion on the PGM catalyst, along with the PGM with 5 WGS Zones were established at different O2/C ratios. These results were plotted in Fig. 6. The points represent the measured values from the data collection, while best efit polynomial and exponential curves are used to show the overall trends. Both configurations were investigated at flow rates of 8.5 and 20 ml/min. The PGM-only tests (0 WGS zone) show an overall increasing trend in conversion as the O2/C ratio increases. Low conversions were generally achieved, the highest being 72% at the 0.25 O2/C ratio. At the lower O2/C ratios the exothermic heat release is insufficient to sustain the reaction resulting in low conversions under 50%. Increasing the flow rates from 8.5 to 20 ml/min shows a drop in conversion. This is likely due to the rapid depletion of the reactant oxygen causing the remainder of the methanol and steam to pass through unreacted, since the PGM catalyst does not support the SR reaction. This establishes another possible necessity for the downstream WGS pellets to continue reforming the unreacted fuel from the PGM section. Adding the downstream WGS pellets in the stratified arrangement allows the reactor to increase the conversion at
Fig. 5 e Baseline conversion values with single PGM. Please cite this article in press as: Richards N, et al., Autothermal-reformation enhancement using a stratified-catalyst technique, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.050
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Fig. 6 e Plot of conversion for different flow rates, WGS zones, and O2/C.
all O2/C ratios. At 0.05 O2/C the conversions were around 92e94%. This raises the conversion by approximately 20% compared to the best possible test conditions from the PGMonly catalyst at the higher O2/C ratio. The improvement is expected since the WGS pellet bed remains active from the upstream heat, and the fuel that was unconverted from the PGM catalyst is able to undergo SR. A strong dependence on flow rate at low O2/C ratio was observed which is expected for SR. The hydrogen yield shown in Fig. 7 was explored to assess the stratified reactor performance. The improvements shown for the conversion were strongly attributed to the internal heating of the WGS pellets. Therefore, similar improvements for the hydrogen yield were expected since the copper-based WGS pellets are highly selective toward the production of
hydrogen. This combined stratified method of reforming allows previously unobtainable results both for ATR and for SR reforming. For SR only the flow rates typically lower conversion and yield and for ATR only the Hydrogen quality suffers and does not exhibit light off at low O2/C ratios. Fig. 7 shows the hydrogen yields that were calculated for the baseline PGM-only setup, and the 5 WGS stratified configuration tested at 8.5 ml/min and 20 ml/min for O2/C ratios ranging from 0 to 0.25. These values are the corresponding yields for the conversions that were shown in Fig. 6. The tests on the PGM catalyst (0 WGS) show that the yields are low for all the O2/C ratios that were investigated. This is expected since the PGM does not promote high selectivity towards producing hydrogen. However, by increasing the O2/C
Fig. 7 e Plot of yield for different flow rates, WGS zones, and O2/C. Please cite this article in press as: Richards N, et al., Autothermal-reformation enhancement using a stratified-catalyst technique, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.050
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ratio more of the fuel was partially oxidized to produce more hydrogen. The gradual increase in the hydrogen concentration makes the yield rise, but even at the highest O2/C ratio of 0.25 the yield reached only 35%. At the higher flow rate the yields are lower for each corresponding O2/C ratio. This occurs since the yield is a ratio of the hydrogen produced to the fuel being fed to the reactor. Therefore it is expected that increasing the flow rate introduces more fuel to the reactor, and for the same quantities of hydrogen produced, the yield would decrease. Adding the WGS zone downstream of the PGM monolith greatly increases the hydrogen yield for all O2/C ratios. This increase is possible since the WGS pellets are being internally heated, and remain active while reforming the methanol that is escaping the upstream PGM section. This additional reforming of the fuel with the WGS section not only increased the conversions, but allows larger concentrations of the desired product to be produced. Operating the stratified reactor under the various O2/C ratios caused large temperature spikes that were measured within the reactor. The highest temperature was consistently measured at the PGM monolith exit, which was the inlet to the downstream WGS pellet bed. The temperatures were typically around 550 C, exceeding the maximum recommended operating temperature of 300 C. While this is beneficial for internal heating, the high temperatures were potentially problematic for the WGS catalyst. At the higher temperatures, degradation becomes an issue that potentially inhibits the reactor performance. Preliminary results for a short term degradation test as performed are shown below. For the 2 WGS zone stratified reactor, short term degradation was investigated at different O2/C ratios. 30 h of tests were conducted on the same stratified PGM and WGS catalyst, with the O2/C ratio varying between 0, 0.10, and 0.25, for an 8.5 ml/min flow rate. The conversions and hydrogen yields over time are plotted in Fig. 8. Each data point was taken consecutively according to the elapsed time. The higher
temperatures experienced at the 0.25 O2/C ratio appear to cause significant degradation of the WGS catalyst observed by the drop in conversions for the 0 and 0.10 O2/C test ratios. This suggests that the reactants have access to fewer active sites on the WGS catalyst causing a decrease in the activity. However when the O2/C ratio is raised back to 0.25, the system holds steady in its fuel conversion at 98%. This result suggests that at the high O2/C ratio the majority of the fuel is being converted by the PGM catalyst, while the WGS catalyst is able to remain active and selective toward hydrogen despite being increasingly degraded toward fuel decomposition. This is an important result with far reaching implications for design of reformers and the expected degradation mechanisms therein. Hydrogen yield allows more insight into the reaction mechanisms as the degradation influence is observed. For the incoming reactants the oxidation, fuel breakdown, and the shift reactions can occur on the different catalysts depending on the O2/C ratio. At the 0 O2/C ratio, the absence of air causes the fuel and steam to react only on the WGS catalyst. The methanol is broken down by a decomposition step, and in conjunction with the shift reaction the yield is approximately 18%. After intermittently experiencing the high temperatures at the 0.1 and 0.25 O2/C ratios, returning to the 0 O2/C ratio setting shows the drop in performance due to the catalyst degradation. At the 0.25 O2/C ratio the yield holds fairly constant around 76%, dropping by 1% after 20 h of testing. The high temperatures do not appear to greatly diminish the WGS selectivity at the high O2/C ratio. This suggests that since the majority of the methanol is broken apart by the oxidation reactions on the PGM, the role of the downstream section is primarily used to increase the amount of hydrogen produced from the shift reaction. Hence the fuel decomposition step is degraded significantly in the WGS section by the high temperatures but the selectivity is still active. While these results are only a preliminary finding, this result warrants further study into the degradation mechanisms in the stratified reactor.
Fig. 8 e Plot of conversion showing evidence of short-term degradation. Please cite this article in press as: Richards N, et al., Autothermal-reformation enhancement using a stratified-catalyst technique, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.050
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Conclusion A first exploration into stratified reforming of methanol was done by combining two catalysts within a single reactor. The upstream catalyst was a PGM monolith, followed by the downstream Cu/ZnO/Al2O3 WGS catalyst. From the literature and theory, the benefits of having the two catalysts within the reactor were shown. Specific mechanisms and the benefits from internal heating were reviewed. Experimentally, different stratified catalyst arrangements were tested. The upstream reactions on the PGM were used to promote fuel decomposition and convect heat and high temperatures into the downstream WGS packed bed. This allowed the WGS catalyst to remain active for reforming. The downstream bed was used for its selectivity in producing higher concentrations of hydrogen. In doing this, stratified reformation promoted the overall two-step reaction mechanism: methanol oxidation followed by steam reforming. The benefit of using the PGM for the oxidation reactions was to break apart the fuel, allowing the reformation to occur on the WGS catalyst. The influence of the internal heat transfer allowed the system to be run autothermally since the external power demand was removed. The degradation over a 30 h period was significant for lower O2/C ratios of 0.1, and for SR at a 0 O2/C ratio. While operating at the 0.25 O2/C ratio the catalysts showed a minimal drop in performance for the yield, while the conversion remained unchanged. This study has shown that the use of a stratified system is effective in promoting a simplified two-step reaction mechanism for methanol ATR.
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[10] Tong GCM, Flynn J, Leclerc CA. A dual catalyst bed for the autothermal partial oxidation of methane to synthesis gas. Catal Lett 2005;102(3e4):131e7. [11] Nguyen BNT, Leclerc CA. Metal oxides as combustion catalysts for a stratified, dual bed partial oxidation catalyst. J Power Sources 2007;163(2):623e9. [12] Smith Mark W, Shekhawat Dushyant, Berry David A, Haynes Daniel J, Floyd Donald L, Spivey James J, et al. Effect of the catalyst bed configuration on the partial oxidation of liquid hydrocarbons. Energy Fuels 2013;27(8):4363e70. http:// dx.doi.org/10.1021/ef3021975. [13] Choi K, Kim H, Dorr J, Yoon H, Erickson P. Equilibrium model validation through the experiments of methanol autothermal reformation. Int J Hydrogen Energy 2008;33(23):7039e47. [14] Traxel BE, Hohn KL. Partial oxidation of methanol at millisecond contact times. Appl Catal A Gen 2003;244(1):129e40. [15] Semelsberger Troy A, Brown Lee F, Borup Rodney L, Inbody Michael A. Equilibrium products from autothermal processes for generating hydrogen-rich fuel-cell feeds. Int J Hydrogen Energy 2004;29(10):1047e64. [16] Lassi U, Polvinen R, Suhonen S, Kallinen K, Savimaki A, Harkonen M, et al. Effect of ageing atmosphere on the deactivation of Pd/Rh automotive exhaust gas catalysts: catalytic activity and XPS studies. Appl Catal A Gen 2004;263(2):241e8. € ffer DG, McDermott SD, Renn CN. Activity and durability of [17] Lo water-gas shift catalysts used for the steam reforming of methanol. J Power Sources 2003;114(1):15e20. [18] ShingWu Ho, Lesmana Donny. Short review: Cu catalyst for autothermal reforming methanol for hydrogen production. Bull Chem React Eng Catal 2012;7(1):27e42. [19] Pettersson Lars, Lindstrom Bard. Hydrogen generation by steam reforming of methanol over copper-based catalysts for fuel cell applications. Int J Hydrogen Energy 2001;26(9):923e33. [20] Idem Raphael O, Bakhshi Narendra N. Production of hydrogen from methanol 2. Experimental studies. Ind Eng Chem Res 1994;33(9):2056e65. [21] Amphlett John C, Peppley Brant A, Kearns Lyn M, Mann Ronald F. Methanol steam reforming on Cu/ZnO/Al2O3 catalysts. Part 2. A comprehensive kinetic model. Appl Catal A Gen 1999;179:31e49. [22] Davieau D, Erickson P. The effect of geometry on reactor performance in the steam-reformation process. Int J Hydrogen Energy Jun 2007;32(9):1192e200. [23] Erickson Paul Anders, Liao Chang-Hsien. Heat transfer enhancement of steam reformation by passive flow disturbance inside the catalyst bed. J Heat Transf Oct 2006;129(8):995e1003. [24] Uy Kevin. An exploration of flow detection and heated length for pelletized and structured catalysts in steamreforming reactors. Masters thesis. Davis: University of California; 2014. [25] Erickson Paul Anders. Enhancing the steam-reforming process with acoustics: an investigation for fuel cell vehicle applications. Doctoral dissertation. University of Florida; 2002. [26] Tong GCM, Flynn J, Leclerc CA. A dual catalyst bed for the autothermal partial oxidation of methane to synthesis gas. Catal Lett 2005;102(3e4):131e7. [27] Nguyen BNT, Leclerc CA. Metal oxides as combustion catalysts for a stratified, dual bed partial oxidation catalyst. J Power Sources 2007;163(2):623e9. [28] Bell Christa J, Leclerc Corey A. Effect of relative bed lengths on a platinum- nickel stratified dual bed catalyst for the catalytic partial oxidation of methane at millisecond contact times. Energy Fuels 2007;21:3548e54.
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[29] Arpornwichanop Amornchai, Wasuleewan Manatsanan, Patcharavorachot Yaneeporn, Assabumrungrat Suttichai. Investigation of a dual-bed autothermal reforming of methane for hydrogen production. Chem Eng Trans 2011;25:929e34. [30] Yoon H, Erickson P, Kim H. Hydrogen from coal-derived methanol via autothermal reforming processes. Int J Hydrogen Energy 2008;33(22):6619e26. [31] Ghenciu Anca Faur. Review of fuel processing catalysts for hydrogen production in PEM fuel cell systems. Curr Opin Solid State Mater Sci 2002;6(5):389e0399. [32] Choi Kap-Seung, Choi In-Jae, Hwang Se-joon, Kim HyungMan, Dorr J Lars, Erickson Paul A. An experimental study of
methanol autothermal reformation as a method of producing hydrogen for transportation applications. Int J Hydrogen Energy Jun 2010;35(12):6210e7. [33] Tang Hong-Yue, Greenwood Jason, Erickson Paul. Modeling of a fixed-bed copper-based catalyst for reforming methanol: steam and autothermal reformation. Int J Hydrogen Energy July 2015;40(25):8034e50. [34] Liao C, Erickson Paul A. Characteristic time as a descriptive parameter in steam reformation hydrogen production processes. Int J Hydrogen Energy Mar. 2008;33(6):1652e60.
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Autothermal-reformation enhancement using a stratified-catalyst technique Nadia Richards, Jacob Needels, Paul Erickson* Department of Mechanical and Aerospace Engineering, University of California, Davis, USA
article info
abstract
Article history:
An investigation of a stratified catalyst arrangement for methanol autothermal reforming
Received 9 May 2017
was conducted. The stratified arrangement consisted of a monolith platinum group metal
Received in revised form
(PGM) catalyst, immediately followed by a pelletized Cu/ZnO/Al2O3 water gas shift (WGS)
2 August 2017
catalyst. The system was experimentally investigated under different test conditions by
Accepted 8 August 2017
varying the flow rate, oxygen-to-carbon ratio, and the packed length of the downstream
Available online xxx
WGS bed. The reactor performance was quantified with several metrics including the fuel conversion, hydrogen yield, system temperatures, and preliminary results are shown for
Keywords:
short-term catalyst degradation. Conversion and hydrogen yield increased significantly at
Methanol
low oxygen-to-carbon ratios as compared to the stand-alone PGM catalyst. Results confirm
Autothermal reformation
the benefits of using a PGM and WGS catalyst in a stratified fashion and the stratified
Reactor design
arrangement increases performance over each of the catalysts used singularly. Further
Stratified catalysts
study is warranted considering potential degradation of the WGS bed caused by high temperatures encountered at the exit of the PGM section. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction
Background
Hydrogen that is chemically bound in organic compounds can be liberated through a process called reformation. Industrially, most hydrogen is generated using natural gas and steam, but other fuels have been studied in research environments [1e3]. The main types of reforming are Steam Reforming (SR), Partial Oxidation (POX), and Autothermal Reforming (ATR). While industrial methods are known and used in large-scale hydrogen production, heat and mass transport limitations make these methods non-ideal for small-scale reforming. Through the use of a stratified bed, it may be possible to increase heat and mass transport resulting in better catalyst performance for small-scale reactors.
Steam reforming is the standard industrial method of producing hydrogen. The reactants for SR are steam, and a fuel source such as hydrocarbons, alcohols or other organics. The reaction of the fuel and steam is endothermic and so a supply of heat is required for the process to occur. The reaction is carried out over a catalyst bed to enable a faster reaction rate and lower temperature. For methanol this general reaction is as listed in Eqs. (1) and (2) and the overall reaction is given in Eq. (3). CH3 OH/2H2 þ CO ðDH ¼ þ90:8kJ=molÞ
(1)
CO þ H2 O #H2 þ CO2
(2)
ðDH ¼ 41:4kJ=molÞ
* Corresponding author. Department of Mechanical and Aerospace Engineering, University of California Davis, Davis, CA, USA. E-mail address: [email protected] (P. Erickson). http://dx.doi.org/10.1016/j.ijhydene.2017.08.050 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Richards N, et al., Autothermal-reformation enhancement using a stratified-catalyst technique, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.050
2
CH3 OH þ H2 O/3H2 þ CO2
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ðDH ¼ þ49kJ=molÞ
(3)
Methanol SR requires 49 kJ/mol of heat that is typically supplied from external burners [4]. This external heating requirement can result in long start-up times for the reaction to get underway, as well as causing the system to experience large internal temperature gradients [5]. However, despite these drawbacks, once the catalyst is sufficiently active, SR produces up to 75% molar concentration of hydrogen. This is the highest hydrogen concentration as compared to the other reforming processes. Partial oxidation, as its name suggests, is when the fuel is partially oxidized as opposed to being fully oxidized or combusted. It is susceptible to the formation and deposition of solid carbon within the reactor, also known as coking. The reaction mechanism is complex and produces various intermediate species and radicals [6]. A simplified overall reaction is shown in Eq. (4) for methanol POX. 1 CH3 OH þ O2 /2H2 þ CO2 2
ðDH ¼ 192kJ=molÞ
(4)
The heat release is shown to be 192 kJ/mol for one-half mole of oxygen, and for higher oxygen to fuel ratios more energy is released. Under these idealized conditions (assuming maximum CO2 selectivity) the hydrogen concentration is around 67% and if air is used as the oxidizer, hydrogen concentrations are lower due to nitrogen dilution. POX occurs at high temperatures which can be lowered by using a catalyst. Due to the exothermic reaction which eliminates heat limitations, POX is capable of rapid responses to dynamic loads and benefits from a more compact system, as compared to SR. However, POX is still less efficient than SR since much of the energy is converted to heat rather than hydrogen and carbon monoxide concentrations in actual reactors dominates other products. Autothermal reforming is considered a combination of SR and POX, where the reactions are in close thermal contact within a single reactor. The addition of oxidizer to the SR reactants allows for some of the fuel to partially oxidize thereby liberating some chemical heat to sustain the endothermic SR reaction [7e9]. In ATR the amount of heat generated comes from the oxidation steps and is dictated by the amount of oxygen that is used. The exothermic steps are rapid and the heat released is used to sustain the SR steps, and so ideally the overall reaction is thermally neutral [10e12]. ATR is potentially capable of reforming multiple fuels since it experiences high temperatures from the oxidation reactions. Using the ideal stoichiometry for the reactants the overall methanol ATR reaction is shown in Eq. (5). CH3 OH þ 0:1O2 þ 0:8H2 O/2:8H2 þ CO2
ðDH ¼ 0kJ=molÞ
(5)
ATR is a good candidate for small-scale reformers due to its compactness, its potential ability to handle a variety of fuels, as well as being able to accommodate transient loads. However, ATR does not produce high purity hydrogen from its product gas composition, and on a PGM catalyst has been shown to typically follow equilibrium predictions [1,2,13e15]. Additionally, temperatures generated during the exothermic stage of the ATR reaction has the capability of degrading the catalyst, if not properly controlled [16].
Conversely, SR has the capability to produce high purity hydrogen on a copper-based catalyst [17e20]. A detailed kinetic study has been done showing that copper is highly selective towards hydrogen and carbon dioxide formation [21]. However the overall SR process is large, done in multiple stages, and suffers from thermal limitations. In addition to the temperature gradients, the size of the SR reactor also influences the performance. For a given reactor volume, the change in the overall aspect ratio, as determined by the ratio of the length to the diameter, can produce differences in conversion as much as 30% [22]. These basic problems that plague the reforming processes highlight some of the issues faced when attempting their use in small-scale applications. These issues have presented a challenge for researchers for many decades and remains an active area of research. The literature shows different approaches that have been taken to enhance the reforming process which include introducing internal flow baffles and disturbers [23,24], applying acoustics to the reactor [25], as well as other stratified approaches [26e27]. Typically these stratified systems use equal lengths of platinum and nickel catalysts for methane reforming [26e29]. However it has been shown that the oxidation reactions occur more rapidly than the reforming reactions and so the platinum bed can be much shorter in relation to the nickel bed [28]. These stratified studies kept the overall length of catalyst as a fixed constant, and so explorations into the downstream length effects on the stratified system are yet to be explored. This serves as the basis for this research project. The focus of this research is to combine ATR and SR into a singular stratified unit using a PGM and copper-based WGS catalyst of varying packed lengths in order to enhance the overall reforming process. This enhancement method seeks to improve the hydrogen concentration for ATR, while simultaneously reducing the size of the SR system. If one uses a simplified approach to the reforming reactions as a fuel decomposition step immediately followed by a shift reaction one can gain insight as to where those reactions are occurring and how to enhance the overall process by using the best catalyst at the correct position in close thermal contact. While a solution for all problems with ATR and SR is not addressed, any enhancement scheme for the reforming processes that can lead to reducing the reactor size and weight, increase output hydrogen purity and remove thermal limitations should be investigated. Many studies have attempted to address various aspects of the difficulties in reformation [30e34]. The specific objectives of this research are to study a methanol reforming system using two different formulations and geometries of catalysts arranged in a stratified configuration within a single reactor. In this research, the stratified reactor consisted of a monolith PGM catalyst immediately followed by a pelletized Cu/ZnO/Al2O3 WGS bed. The stratified system performance is observed while varying the oxygen/ methanol ratio, reactant inlet flow rates, and various lengths of the SR packed bed section. From the experimental work, analysis quantified the effects of the variables on the system performance. The reactor performance is quantified with several metrics including the fuel conversion, hydrogen yield, system temperatures, and preliminary results are shown for short-term catalyst degradation.
Please cite this article in press as: Richards N, et al., Autothermal-reformation enhancement using a stratified-catalyst technique, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.050
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Experimental setup The reforming system is located in the Energy Research Laboratory (formerly the Hydrogen Production and Utilization Laboratory) at the University of California, Davis. The existing system was originally configured for methane-steam reforming but has the capability to adapt to liquid fuels and autothermal reforming. The methanol autothermal reforming process starts with flowing a water and methanol premix at 1.5:1 steam-to-carbon ratio. The premix is pumped through a series of vaporizers which change the phase from liquid to gas. At the same time, clean, compressed air is supplied using a mass flow controller and routed to a gas heater. Once the reactants are at a consistent steady temperature they are routed to the reactor through the superheater. The superheater is designed to further promote sufficient mixing and to also maintain a steady temperature (250 C) prior to entering the reactor. A schematic of the system used is shown in Fig. 1. After passing through the reactor, the first stage of the condenser is the shell and tube heat exchanger, which has a back pressure regulator at the outlet to maintain a constant pressure for the various catalyst bed arrangements that were tested. The gaseous products exiting from the reactor, called reformate is then further cooled to 1 C through the coils of the main condenser. Inside the main condenser the reformate flow is split into two streams; one for analysis and the other for exhaust. In the exhaust stream, the liquid water and unreacted methanol are collected in the exhaust condenser trap while the gases are vented to the exhaust. For the analysis stream the condensed methanol and water, called retentate, were collected in the analysis trap and the dry gases
3
were routed to the gas analyzer and then vented. The retentate was measured for its mass and density to determine the methanol conversion. Other features of the system include the dedicated nitrogen line to keep an inert environment on the reduced catalyst during downtime. Additionally, an air purge was used on the condenser analysis line to ensure all the retentate was evacuated into the trap. Inside the reactor, the reforming process is performed over the heated catalyst. In this study, the catalysts were of two different compositions and were placed in a stratified configuration as shown in Fig. 2. Reactor performance can be quantified by several parameters that rely on the specific operating conditions and the output products from the reaction. The results from the experiments are analyzed using these parameters to quantify the performance of the stratified system. Conversion is defined as the reactant consumed divided by the reactant input to the system. It is a common measure of reactor performance since it essentially gives insight into the reaction progression. For methanol the conversion is shown in Eq. (6) below CH3 OHinput CH3 OHoutput $100 (6) Conversionð%Þ ¼ CH3 OHinput Using the constant molar ratio of steam to methanol at 1.5:1, the premix density of 0.9080 g/cm3 at 24 C, and the change in the premix mass during operation, the amount of methanol input to the reactor was obtained. To determine the amount of methanol output from the reaction, the retentate was collected in the analysis trap during the steady state run time. The trap was emptied and the retentate mass was measured. A calibrated table of density with respect to
Fig. 1 e Schematic of experimental setup. Please cite this article in press as: Richards N, et al., Autothermal-reformation enhancement using a stratified-catalyst technique, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.050
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ratio of the hydrogen produced to the methanol input, normalized to 100% maximum using a stoichiometric factor (SF) of 1/3. H2 Yieldð%Þ ¼
H2 produced $SF CH3 OHinput
(8)
Oftentimes the literature reports the hydrogen selectivity, which is similar to yield combined with conversion, which measures the ratio of the hydrogen produced to the fuel that was consumed. For this study the hydrogen yield and conversion are reported separately and are not combined into a selectivity term.
Results
Fig. 2 e Schematic of reactor.
methanol concentration was constructed and retentate density was used to calculate methanol concentration and hence methanol mass in the retentate. The space velocity gives the ratio of the total volumetric flow rate to the catalyst bed volume. The catalyst volume was calculated using the inner reactor diameter and length of the PGM monolithic catalyst as well as the packed length of the WGS pellet bed. The equation for the gas hourly space velocity (GHSV) for the stratified bed is given in Eq. (7). While the GHSV is the standard metric for reporting a flow rate in a fixed bed length, in this analysis gas hourly space velocity is less useful than the raw reactant flow rate as the volume of the catalyst bed changes with increasing WGS catalyst length, thus changing the GHSV without changing the flow rate. V_ reactants GHSV hr1 ¼ Vcatalyst
(7)
Yield is used to give insight into which products are being produced from the reaction. It quantifies the production of a particular specie based on the amount of fuel input to the reactor. It is a useful metric for reactor analysis since it supplements the information given from conversion. For reforming systems, hydrogen is the desired output and so the hydrogen yield is calculated according to Eq. (8), as the
The stratified catalyst bed influences several measurable parameters in the reforming process. The output parameters presented are the reactor temperature profile, fuel conversion, hydrogen yield, and short term degradation. These outputs can translate into potential size reductions and improved capacity for fuel reformers. The stratified catalyst bed was investigated for its influence on the internal reactor temperature. The expected benefit of using this configuration was to generate heat internally to keep the WGS packed bed active for reforming. The improved internal heating should improve the overall conversion and hydrogen yield from the reactor. The reactor was divided into five zones, and at the exit of each zone the temperatures were monitored. The measured internal temperatures are taken at the inlet for the incoming reactants at 250 C, along the centerline for the length of the reactor for each zone exit, and finally the outlet where the products leave the reactor. Additional temperature measurements were taken across the 0.635 cm (0.25 in) monolith placed at the inlet to Zone 1, and the internal wall temperatures at each zone exit. From these measurements the centerline, and an overall interpolated temperature profile were plotted in Figs. 3 and 4. The plot shown in Fig. 3 depicts the internal centerline temperatures for all tested stratified catalyst arrangements. Starting with the baseline at 0 WGS zones (PGM catalyst only), each zone of the reactor was filled with the downstream WGS pellets until the full packed configuration was achieved at 5 WGS zones. The operating conditions used a flow rate of 8.5 ml/min, with a 0.25 O2/C ratio. The vertical dashed lines represent the placement of the PGM catalyst bed and serves to aid the visualization of the arrangement. The plot also shows that at each reactor zone exit, the corresponding WGS packed zone generally has the lowest temperature. For example, at the Z1 reactor exit, the 1 WGS zone has the lowest temperature within the filled catalyst section. Similarly at the Z2 exit the 2 WGS zone gives the lowest reading. This demonstrates that within the WGS packed bed the temperature is in constant decline as a result of the endothermic reactions taking place. In addition to exploring the effects of stratification on the centerline temperatures, the overall internal profile for the reactor was investigated using the centerline and the radial temperatures.
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Fig. 3 e Reactor centerline temperature values.
Fig. 4 e Interpolated experimental reactor temperature profiles.
The experimental results of three configurations are shown in the contour plots of Fig. 4. The reactors were tested using a constant flow rate of 8.5 ml/min, and an O2/C ratio of 0.15. Each plot shows the half-radial reactor profile, and are
not drawn to scale. The contour plots were achieved using a linear interpolation of the measured temperatures at the center and wall for each zone exit and across each zone resulting in curved lines. It is important to note that local
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inaccuracies in the radial gradients exist from the interpolation, but information of the overall thermal distribution is still observable and useful in the analysis. The left-most plot is the baseline PGM-only configuration (0 WGS length), the center plot is a stratified bed having the PGM monolith in addition to 2 WGS zones (0.09 m packed length), and the right-most plot is a fully packed with the PGM plus all 5 zones filled with the WGS pellets (0.25 m). The dashed horizontal lines represent the packing of the PGM and the WGS pellets within the reactor, and serve as a visual aid for their placement. The 0 WGS bed length, or the PGM baseline, establishes the thermal distribution within the reactor and shows that significant heat that is available downstream of the PGM catalyst. Operating at an O2/C ratio of 0.15 raises the center temperatures to over 325 C just after the monolith. This hotter region extends axially down through the reactor to around the Zone 4 exit. The oxidation reactions that are occurring on the PGM catalyst causes the temperature spike, and appears to keep it axially located. Going from the Zone 4 exit to the reactor outlet the gases gradually cool down until they leave the reactor in the 275e300 C range. The reactor walls were also raised slightly above their 250 C set-point temperature, in the range of 275e300 C. Since the heat bands were controlled to maintain the wall set-point temperature, they are turned off and the heating of the wall is only due to the hot center region. This indicates that the heat released from the upstream reactions on the monolith is capable of heating the reactor, but is being exhausted as waste heat which could be potentially utilized by a downstream WGS packed bed. Adding two sections or zones of WGS pellets to the reactor and placing them immediately downstream of the PGM monolith creates the temperature profile shown in the center plot in Fig. 4. The plot shows that the hot center temperatures in excess of 325 C are now confined to the Zone 1 region. Within the first zone the active WGS pellets are no longer limited by heat transfer, and work to carry out the
endothermic reforming reaction. This shortens the heat progression down the reactor into the second zone. In effect, the WGS packed beds facilitate heat transfer radially, as opposed to the axial transport due to the fluid velocity, allowing more of the catalyst to remain active. A similar trend is seen with four packed bed zones. The increase in radial heat transfer results in a more uniform temperature distribution. As shown in previous studies, methanol ATR on a PGM catalyst alone can achieve maximum conversions exceeding 99% once the catalyst is sufficiently active [14e16]; although lower maximums of around 72% are possible depending on the particular formulation used and reactor pressure [17,18]. In the baseline tests, shown in Fig. 5, maximum conversion values of 72% and 90% were attained for PGM placement in zones 1 and 4 respectively. Subsequent tests only used the PGM at Zone 1. The profile for the conversion on the PGM catalyst, along with the PGM with 5 WGS Zones were established at different O2/C ratios. These results were plotted in Fig. 6. The points represent the measured values from the data collection, while best efit polynomial and exponential curves are used to show the overall trends. Both configurations were investigated at flow rates of 8.5 and 20 ml/min. The PGM-only tests (0 WGS zone) show an overall increasing trend in conversion as the O2/C ratio increases. Low conversions were generally achieved, the highest being 72% at the 0.25 O2/C ratio. At the lower O2/C ratios the exothermic heat release is insufficient to sustain the reaction resulting in low conversions under 50%. Increasing the flow rates from 8.5 to 20 ml/min shows a drop in conversion. This is likely due to the rapid depletion of the reactant oxygen causing the remainder of the methanol and steam to pass through unreacted, since the PGM catalyst does not support the SR reaction. This establishes another possible necessity for the downstream WGS pellets to continue reforming the unreacted fuel from the PGM section. Adding the downstream WGS pellets in the stratified arrangement allows the reactor to increase the conversion at
Fig. 5 e Baseline conversion values with single PGM. Please cite this article in press as: Richards N, et al., Autothermal-reformation enhancement using a stratified-catalyst technique, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.050
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Fig. 6 e Plot of conversion for different flow rates, WGS zones, and O2/C.
all O2/C ratios. At 0.05 O2/C the conversions were around 92e94%. This raises the conversion by approximately 20% compared to the best possible test conditions from the PGMonly catalyst at the higher O2/C ratio. The improvement is expected since the WGS pellet bed remains active from the upstream heat, and the fuel that was unconverted from the PGM catalyst is able to undergo SR. A strong dependence on flow rate at low O2/C ratio was observed which is expected for SR. The hydrogen yield shown in Fig. 7 was explored to assess the stratified reactor performance. The improvements shown for the conversion were strongly attributed to the internal heating of the WGS pellets. Therefore, similar improvements for the hydrogen yield were expected since the copper-based WGS pellets are highly selective toward the production of
hydrogen. This combined stratified method of reforming allows previously unobtainable results both for ATR and for SR reforming. For SR only the flow rates typically lower conversion and yield and for ATR only the Hydrogen quality suffers and does not exhibit light off at low O2/C ratios. Fig. 7 shows the hydrogen yields that were calculated for the baseline PGM-only setup, and the 5 WGS stratified configuration tested at 8.5 ml/min and 20 ml/min for O2/C ratios ranging from 0 to 0.25. These values are the corresponding yields for the conversions that were shown in Fig. 6. The tests on the PGM catalyst (0 WGS) show that the yields are low for all the O2/C ratios that were investigated. This is expected since the PGM does not promote high selectivity towards producing hydrogen. However, by increasing the O2/C
Fig. 7 e Plot of yield for different flow rates, WGS zones, and O2/C. Please cite this article in press as: Richards N, et al., Autothermal-reformation enhancement using a stratified-catalyst technique, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.050
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ratio more of the fuel was partially oxidized to produce more hydrogen. The gradual increase in the hydrogen concentration makes the yield rise, but even at the highest O2/C ratio of 0.25 the yield reached only 35%. At the higher flow rate the yields are lower for each corresponding O2/C ratio. This occurs since the yield is a ratio of the hydrogen produced to the fuel being fed to the reactor. Therefore it is expected that increasing the flow rate introduces more fuel to the reactor, and for the same quantities of hydrogen produced, the yield would decrease. Adding the WGS zone downstream of the PGM monolith greatly increases the hydrogen yield for all O2/C ratios. This increase is possible since the WGS pellets are being internally heated, and remain active while reforming the methanol that is escaping the upstream PGM section. This additional reforming of the fuel with the WGS section not only increased the conversions, but allows larger concentrations of the desired product to be produced. Operating the stratified reactor under the various O2/C ratios caused large temperature spikes that were measured within the reactor. The highest temperature was consistently measured at the PGM monolith exit, which was the inlet to the downstream WGS pellet bed. The temperatures were typically around 550 C, exceeding the maximum recommended operating temperature of 300 C. While this is beneficial for internal heating, the high temperatures were potentially problematic for the WGS catalyst. At the higher temperatures, degradation becomes an issue that potentially inhibits the reactor performance. Preliminary results for a short term degradation test as performed are shown below. For the 2 WGS zone stratified reactor, short term degradation was investigated at different O2/C ratios. 30 h of tests were conducted on the same stratified PGM and WGS catalyst, with the O2/C ratio varying between 0, 0.10, and 0.25, for an 8.5 ml/min flow rate. The conversions and hydrogen yields over time are plotted in Fig. 8. Each data point was taken consecutively according to the elapsed time. The higher
temperatures experienced at the 0.25 O2/C ratio appear to cause significant degradation of the WGS catalyst observed by the drop in conversions for the 0 and 0.10 O2/C test ratios. This suggests that the reactants have access to fewer active sites on the WGS catalyst causing a decrease in the activity. However when the O2/C ratio is raised back to 0.25, the system holds steady in its fuel conversion at 98%. This result suggests that at the high O2/C ratio the majority of the fuel is being converted by the PGM catalyst, while the WGS catalyst is able to remain active and selective toward hydrogen despite being increasingly degraded toward fuel decomposition. This is an important result with far reaching implications for design of reformers and the expected degradation mechanisms therein. Hydrogen yield allows more insight into the reaction mechanisms as the degradation influence is observed. For the incoming reactants the oxidation, fuel breakdown, and the shift reactions can occur on the different catalysts depending on the O2/C ratio. At the 0 O2/C ratio, the absence of air causes the fuel and steam to react only on the WGS catalyst. The methanol is broken down by a decomposition step, and in conjunction with the shift reaction the yield is approximately 18%. After intermittently experiencing the high temperatures at the 0.1 and 0.25 O2/C ratios, returning to the 0 O2/C ratio setting shows the drop in performance due to the catalyst degradation. At the 0.25 O2/C ratio the yield holds fairly constant around 76%, dropping by 1% after 20 h of testing. The high temperatures do not appear to greatly diminish the WGS selectivity at the high O2/C ratio. This suggests that since the majority of the methanol is broken apart by the oxidation reactions on the PGM, the role of the downstream section is primarily used to increase the amount of hydrogen produced from the shift reaction. Hence the fuel decomposition step is degraded significantly in the WGS section by the high temperatures but the selectivity is still active. While these results are only a preliminary finding, this result warrants further study into the degradation mechanisms in the stratified reactor.
Fig. 8 e Plot of conversion showing evidence of short-term degradation. Please cite this article in press as: Richards N, et al., Autothermal-reformation enhancement using a stratified-catalyst technique, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.050
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Conclusion A first exploration into stratified reforming of methanol was done by combining two catalysts within a single reactor. The upstream catalyst was a PGM monolith, followed by the downstream Cu/ZnO/Al2O3 WGS catalyst. From the literature and theory, the benefits of having the two catalysts within the reactor were shown. Specific mechanisms and the benefits from internal heating were reviewed. Experimentally, different stratified catalyst arrangements were tested. The upstream reactions on the PGM were used to promote fuel decomposition and convect heat and high temperatures into the downstream WGS packed bed. This allowed the WGS catalyst to remain active for reforming. The downstream bed was used for its selectivity in producing higher concentrations of hydrogen. In doing this, stratified reformation promoted the overall two-step reaction mechanism: methanol oxidation followed by steam reforming. The benefit of using the PGM for the oxidation reactions was to break apart the fuel, allowing the reformation to occur on the WGS catalyst. The influence of the internal heat transfer allowed the system to be run autothermally since the external power demand was removed. The degradation over a 30 h period was significant for lower O2/C ratios of 0.1, and for SR at a 0 O2/C ratio. While operating at the 0.25 O2/C ratio the catalysts showed a minimal drop in performance for the yield, while the conversion remained unchanged. This study has shown that the use of a stratified system is effective in promoting a simplified two-step reaction mechanism for methanol ATR.
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