Catalytic Microreactors for Power Generation and Hetero-/Homogeneous Combustion of Hydrogen/Air over Platinum

The combustion and heat transfer characteristics of catalytic microreactors fueled by hydrogen and hydrogen-rich mixtures are investigated for power generation applications. Moreover, the fundamental hetero-/homogeneous combustion properties of such reactive mixtures are studied both experimentally and numerically. In particular, the combustion stability limits of hydrogen in platinum-coated microchannels, the onsite production of hydrogen fuel via catalytic partial oxidation (CPO) of methane for microreactors, and the hetero-/homogeneous kinetic interactions in the combustion of hydrogen over platinum are addressed. To examine the combustion and heat transfer characteristics of microreactors for power generation, a catalytic microreactor made of SiC with dimensions 30×30×4 mm^3 was designed, constructed and tested. Surface temperatures were measured with an infrared (IR) camera, exhaust compositions were analyzed with a micro gas chromatograph, while simulations were carried out with a newly built 3-D code that included conjugate heat transfer, external heat losses, and detailed hetero-/homogeneous chemistry. Fundamental kinetic studies were subsequently performed experimentally in the high-pressure optically-accessible channel flow catalytic reactor at the Combustion Fundamentals Group of Paul Scherrer Institute and numerically using a 2-D reactive CFD code with detailed hetero-/homogeneous chemical reaction schemes and transport. Raman measurements of major gas phase species concentrations assessed the heterogeneous combustion processes, while planar laser induced fluorescence (LIF) of radical species determined the onset of homogeneous ignition. Combustion of fuel lean H2/air mixtures in the catalytic microreactor was studied at equivalence ratios 0.25-0.50 and inlet velocities 15-50 m/s. Higher mass throughputs reduced the surface temperature spatial non-uniformities, while the onset of gaseous combustion lowered the catalyst surface temperatures. Different channel configurations were tested for optimum temperature uniformity. Counterflow configurations were shown superior to the coflow configuration in attaining better surface temperature spatial uniformities. Comparisons of measurements and predictions were very favorable in terms of temperature probability density function (PDF) shapes and higher distribution moments. Radiation efficiencies increased with increasing inlet velocity and equivalence ratio. Application of the microreactor to power generation systems, in conjunction with thermoelectric devices, appeared quite promising given the high values of surface temperatures (up to 1311 K, the highest reported in the literature for catalytic microreactors with sizes of a few cm) and the attained good spatial uniformity (less than 20 K standard deviations). The catalytic combustion in the microreactor was further studied using fuel lean H2/CO/air and H2/CH4/air mixtures. The diverse transport, kinetic and thermodynamic properties of the H2, CO and CH4 fuels gave rise to rich combustion phenomena. H2/CO mixtures yielded lower wall temperatures compared to undiluted H2. Although CO had a high catalytic reactivity when combusting in H2/CO mixtures, its larger than unity Lewis number did not allow for the attainment of high surface temperatures. Mixtures of H2/CH4 were the least attractive due to the substantially lower catalytic reactivity of CH4. To demarcate stable combustion operating regimes for the microreactor, the hetero-/homogeneous combustion and stability limits of fuel-lean H2/air mixtures (φ = 0.40) were investigated numerically in a platinum coated planar microchannel. Two pressures and two solid thermal conductivities were examined, while stability maps were constructed in terms of the critical extinction heat transfer coefficient h_cr versus inlet velocity U_IN (or mass throughput). For a given solid thermal conductivity, there existed a crossover mass throughput above (below) which the stability envelope was broader at 5 bar (1 bar), originating from a shift in the pressure dependence of the catalytic reactivity of H2. The stability limits of H2 were solely determined by the catalytic chemistry, as it sustained combustion at temperatures down to 320-380 K. Critical extinction heat transfer coefficients for H2 were three to four orders of magnitude higher than those reported for CH4 and C3H8 fuels. Fundamentals of H2/air combustion over platinum have been studied. The hetero-/homogeneous combustion of fuel rich H2/O2/N2 mixtures (φ = 2.5-6.5) was investigated at pressures 1-14 bar. Planar laser induced fluorescence (LIF) of OH at pressures below ~5 bar and of hot O2 at pressures above ~5 bar, was applied to determine the onset of homogeneous ignition. The agreement between measured and predicted homogeneous ignition distances validated the aptness of the employed hetero-/homogeneous chemical reaction mechanisms. Analytical homogeneous ignition criteria revealed that the catalytic reaction pathway introduced a scaling factor 1/p to the homogeneous ignition distances. Moreover, the hetero-/homogeneous combustion of fuel-lean H2/O2/N2 mixtures (φ = 0.20-0.28) was also studied in the narrow pressure range 1 to 3.5 bar where microreactors generally operate. It was found that pressures above 2 bar led to increased safety against homogeneous ignition. This pressure dependence is the opposite to the foregoing dependence at fuel-rich stoichiometries, whereby rising pressures promote the onset of homogeneous ignition. To better understand the onsite hydrogen production for microreactors via CPO of CH4, the heterogeneous and homogeneous combustion of fuel rich CH4/O2/N2/CO2 mixtures (φ = 1.8-3.5) over rhodium and platinum was investigated at 5 bar. Rh was shown superior to Pt in syngas production. The higher syngas production on Rh, and in particular of the highly reactive H2, had a drastic impact on the ensuing gas phase combustion characteristics. While vigorous homogeneous combustion was always established on Rh, it was altogether suppressed on Pt. The strong gaseous combustion on Rh considerably reduced the length of the oxidation zone in CPO reactors such that the catalytic reforming zone could be initiated farther upstream. It was also shown that homogeneous combustion did not affect the reactor thermal management and that it promoted the syngas yields at the reactor outlet. Finally, the complete oxidation of methane over rhodium (a topic not falling in the general framework of the microreactor work) was also investigated (φ = 0.3-0.4, p = 2-12 bar and catalyst temperatures 700-1250 K). Performance of detailed surface reaction mechanisms was evaluated and a one step catalytic reaction was constructed. For the investigated range, the catalytic reactivity of CH4 over Rh exhibited an overall positive pressure dependence ~p^0.30, which was weaker compared to an earlier reported pressure dependence of methane over platinum (~p^0.47). An elementary gaseous reaction mechanism was shown to reproduce the measured homogeneous ignition distances at all pressures, particularly when used in conjunction with the herein proposed one step catalytic reaction.