| he box are a burner related hydrogen flow and air flow both controlled by a MKS Alta series mass flow controller. Also incoming is a flow of premixed ethanol/water flow controlled by a Teledyne ISCO Syringe pump model 500D. Outgoing is a steam of ultra pure hydrogen entering into a Pfeiffer vacuum pump model XtraDry 150-2 or into atmospheric pressure after passing through a MKS Alta mass flow meter. The other outgoing streams are the burner exhaust and the raffinate that enters the analysis section to determine its composition and flow rate. Apart from the above, electricity is entered into the box for the electric band heater on the membrane separator. Some more details of the various sections are discussed hereunder:
- Steam Reformer & Burner The burner is an integral part of the steam reformer where flames surround the outside wall of each of the parallel micro channel reactor tubes. The reformer reaction gas flows on the other side of the reactor wall confined to their micro-channel. In this case, hydrogen and air are used for the flame while these associated flows enter each through a mass flow controller after exchanging heat with the remainder of the burner exhaust flow. The bulk of the heat of the burner exhaust is passed on to the incoming fuel mix stream in a separate heat exchanger. Prior to this last heat exchanger, heat from the exiting reformate stream is also passed on to the stream of fuel/water mixture.
- Hydrogen Separation The separator removes part of the hydrogen from the incoming reformate stream. The heat exchanger shown in figure 1 passes the in- and outgoing separator streams which are mass balanced. This makes that, once started up, the auxiliary electric heater of the separator only has to provide a relatively small amount of energy to compensate for thermal losses such as through insulation, piping and radiation. Separated hydrogen can either exit through a vacuum pump or directly into atmospheric pressure after going through a mass flow meter. A vacuum pump was used in this test to remove essentially all of the H2 produced for ease of interpretation.
- Raffinate Analysis Raffinate exiting the box passes through a cooled heat exchanger to remove condensable vapors, water, from the stream. Cooling is obtained with glycol and a circulating bath chiller. Hereafter, the dry raffinate flow passes through a Tescom backpressure regulator maintaining the upstream pressure and is either send to a GC, HP 5890 series II, for analysis or to a flow rate measurement provision. The GC determines the concentrations of CO, CO2 and CH4 in the stream and is calibrated using a traceable standard mixture provided by GTS, Morrisville, PA.
Individual component testing
- Wall-catalyzed steam reformer:
Figure 2 shows the separate setup used to measure the performance of the micro-channel wall-catalyzed reformer design. A premixed ethanol/water mixture with a steam to carbon ratio of 10 is fed at a liquid flow rate of 0.25 cc/minute into the reactor through a syringe pump.
Instead of using the integrated burner, electric band heaters were used in this test. Prior to the reactor the mixture is preheated to 600 °C while the external electric, one inch OD, band heater of the wall-catalyzed reformer was operated at temperature of 950 °C and 750 °C. The pressure of the reformate stream was set at 7 atm. The volume created by the large mismatch between the diameter of the band heater and that of the reactor was filled with hydrogen to assure some thermal conductance. The given flow rate corresponds to a calculated 85.6 cc/min of maximum extractable H2. The micro-channel wall-catalyzed reactor has a volume of 90 micro-liter with a geometric surface area per reactor volume of 100 cm2/cm3.
As can be seen in figure 2, no hydrogen separation membrane was used in this experiment. Though a hydrogen peak appears in the chromatograms, its calibration through a calibrated gas mixture was deemed unreliable as hydrogen might not mix well in the cylinder. The hydrogen content in the reformate stream was calculated from the concentrations of the other peaks (CO, CO2 and CH4) applying conservation of the number of C, O and H atoms. This hydrogen calibration issue is avoided when using the vacuum pump on the permeate side a shown in the setup of figure 1 where the size of the hydrogen flow is measured at the exit of the pump, knowing that in this mode nearly 100% of the H2 in the reformate gas stream is removed.
- Burner: A small test setup was build featuring flow control for the hydrogen and air flow and a Chromel-Alumel thermocouple roughly positioned at the center of the flame. Ignition was accomplished with a piezo device guiding the conductive electrode wire into the all welded burner assembly through a ceramic tube.
Results
Reported here are results of the individual component testing, while additional data on the integrated system of figure 1 will be shown at the presentation.
Wall-catalyzed steam reformer: The 0.25 sccm ethanol/water mixture produced 110 +/- 20 cc/min. dry gas mixture at a band heater temperature of 950 °C corresponding with a 0.5 m/sec. average gas speed in the micro-channel. At that temperature, the dry gas composition was 66% H2, 13% CO, 6% CH4 and 15% CO2 corresponding with a flow rate of hydrogen of 73 +/- 15 cc/min. With the band heater operating at 750 °C, the dry gas composition was 58% H2, 18% CO, 12% CH4 and 12% CO2. Absence of other species in the chromatogram and checking the mass balance indicate that the fuel cracking is basically complete under the conditions of the experiment.
The dry gas composition corresponds to a much lower equilibrium temperature of about 600 °C where the large temperature gradient (350 °C) falls in large part over the mentioned hydrogen filled volume. Calculation of the heat transfer through the hydrogen and the Hastelloy wall showed that the temperature drop over the hastelloy was less than 30 degrees C.
Burner: Figure 3 shows a wide range of ignitable hydrogen-air mixtures consistent with 4 – 74% flammability limits reported in the literature, reference 8. This wide range characteristic remains at higher than atmospheric pressures. Applied spark ignition showed to be easy and reliable consistent with the order of magnitude lower ignition energy of 0.02 [mJ] compared to gasoline vapor and natural gas.
Given a fixed hydrogen flow rate, increasing the airflow in figure 3 results in burning more of the fuel up to reaching the stochiometric ratio for complete combustion where after more air only cools the flame.
Discussion Thermal management is critical to the overall fuel processor efficiency and corresponds to the degree at which heat can be recovered and used at various points in the process as well as the equipment’s size and insulation. Regular tube in tube heat exchangers were used in the current setup and higher efficiency micro-channel heat exchangers should be considered for a product version. The calculated very modest temperature drop over the reactor wall is encouraging as it means that the burner’s flame temperature only has to be slightly higher than the desired reactor temperature. This will also help to kept thermal radiation losses low together with minimizing the radiating surface to volume ratio which improves with large systems.
As for cost efficiency of the integrated fuel processor, the most expensive component is the palladium based membrane separator. The vacuum pump is used only in the experiment and is avoided in practical applications. Increasing the partial hydrogen pressure, that drives the separation process, can be done by condensing the water out of the reformate stream. Where this, together with water produced by a subsequent fuel cell, provides a basis for a water autonomous process, recovering and making useful use of some of the associated latent heat would further increase the overall energy efficiency. Recovering this last energy might require the use of an intermediate fluid heat exchanger while the efficiency of such effort will set limits on the desirability of higher steam to carbon ratios. The degree by which such water removal can be accomplished will be dictated by the need to avoid coking conditions in the separator.
The hydrogen/air burner provides some very user friendly features such as ease of control and ignition, a very clean carbon free burn, no smokestack, no clogging, easy maintenance robustness and reliability. A microchannel structure at the burner side of the reactor wall allows a high degree of control over the flame related gas streams which minimizes channeling of combustion gases in a parallel reactor tube arrangement.
Conclusion
Results on individual system component testing show that a significant improvement in generation of H2 in the reformate gas can be achieved by wall-catalyzed micro-channel reactor. The production of H2 with this initial system (73 sccm hydrogen out of 86 sccm theoretic max.) is in line with the literature provided argument regarding the effectiveness factor of steam reformer catalyst as becoming roughly proportional to the external surface area of the catalyst. The hydrogen production capacity per reactor volume seems very promising for a reformer with integrated raffinate burner where the heat transport bottleneck and been greatly reduced.
A complete fuel processor system based on a wall-catalyzed micro-channel steam reformer and a palladium membrane hydrogen separator is demonstrated. Data is provided on the effectiveness by which the heat transfer bottleneck, encountered in conventional steam reformers, is addressed. Testing so far shows a cracking of the ethanol fuel that is largely complete. The wall-catalyzed reactor concept offers an outlook that avoids the multitude of problems associated with the use of conventional catalysts such as hot spots, dead volume, coking and degrading efficiency. The concept also offers a good outlook for sulfur immunity and a high potential for fuel flexibility. The micro-channel design is highly scalable and manufacturable.
Further developments toward a corresponding fuel processor product include operating with different fuels, heat exchanger optimization, reformate water vapor removal and raffinate burning.
Acknowledgements
P&E acknowledges support for this project under a BAA contract from the Office of Naval Research, Ships and Engineering Systems Division, Code 331, Arlington, VA and guidance from NAVSEA, Energy Conversion Section, Code 9823, Philadelphia, PA.
United Technologies, UTC, providing independent membrane performance testing, comparison with DOE goals as well as chairing of coking condition information.
References
1 H. Chul Yoon et al., Reactor design limitations for the steam reforming of methanol, Applied Catalysis B: Environmental, 26 Sept. 2007, pp 264-271.
2 R. Killmeyer et al., Water-Gas Shift Membrane Reactor Studies, NETL FY 2003 Progress Report, available on the DOE/NETL website.
3 P&E presentation at conferences: NHA 2008, Sacramento CA; AICHE 2007, Salt Lake City, UT; Fuel Cell Conference 2008, Long Beach, CA.
4 P. Marty and D. Crouset, High temperature hybrid steam-reforming for hydrogen generation without catalyst, Journal of Power Sources, vol. 118, number 1, 25 May 2003, pp. 66-70.
5 Rostrup-Nielsen J. R. and Sehested J., Hydrogen and Synthesis Gas by Steam- and CO2 Reforming, Advances in Catalysis 47, pp. 65-139, 2002.
6 Rostrup-Nielsen J. R., Catalytic Steam Reforming. In Catalysis Science and Technology Vol. 5, pp 1-117, Springer Verlag, Berlin 1984.
8 V. Schroeder and K. Holtappels, Explosion Characteristics of Hydrogen-Air and Hydrogen-Oxygen Mixtures at Elevated Pressures, Proceedings International Conference on Hydrogen Safety, September, 2005 Congress Palace, Pisa, Italy, http://conference.ing.unipi.it/ichs2005/Papers/120001.pdf.
9 H. Funke et al., Optimization of palladium cell for reliable purification of hydrogen in MOCVD, Journal of crystal growth (J. cryst. growth), vol. 248, Feb. 2003, pp 72-76(5), ISSN 0022-0248 CODEN JCRGAE.
Figure 1 Steam reforming & burner, H2 separation and raffinate analysis sections.
Figure 2 Additional test setup for wall-catalyzed reformer component
Figure 3 test of flame temperature vs air supply rate |
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