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air mixtures in catalytic microburners
Proceedings of the
Proceedings of the Combustion Institute 30 (2005) 2473–2480
Combustion Institute www.elsevier.com/locate/proci
Hydrogen assisted self-ignition of propane/air mixtures in catalytic microburners D.G. Norton, D.G. Vlachos* Department of Chemical Engineering, Center for Catalytic Science and Technology (CCST) Center for Composite Materials (CCM), University of Delaware, Newark, DE 19716-3110, USA
Abstract A catalytic Pt-based microdevice is evaluated for the combustion of hydrogen and/or propane. It is found that in confined ceramic microchannels hydrogen/air mixtures self-ignite over a wide range of compositions. This discovery is capitalized to self-ignite propane/air mixtures with the assistance of hydrogen addition. It is shown that propane kinetically inhibits hydrogen catalytic combustion at low hydrogen fractions. The minimum hydrogen composition for self-ignition of propane/air mixture compositions is found to be relatively constant, irrespective of propane composition. The transient and steady state behavior of these systems is described, and the minimization of hydrogen usage and startup time are discussed. 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Microburners; Propane; Hydrogen; Catalytic combustion; Self-ignition
1. Introduction Microburners are increasingly studied for the catalytic and non-catalytic portable production of heat and/or energy [1–6]. The energy produced can be utilized by various means. Examples include heat generation for remote use, such as for soldiers and space flights, thermoelectrics to produce electricity [7], and heat supply to microscale reactors carrying out endothermic reactions, such as ammonia decomposition or steam reforming [8–11], to produce hydrogen for portable fuel cells. Because hydrocarbons possess a significantly greater energy density than traditional metal acid batteries [4], hydrocarbon-based microburners are
*
Corresponding author. Fax: +1 302 831 1048. E-mail address: [email protected] (D.G. Vlachos).
an enticing prospective energy source for portable power applications, such as cell phones, laptops, portable electronics, and personal heaters. Most current prototypes depend on external heating to generate energy for ignition. The additional apparatus necessary to power such external heaters can negate mass advantages of microburners. One concept for designing a microburner is to downscale conventional scale gaseous burners. However, homogeneous flames are typically quenched when confined in spaces with dimensions below 1–2 mm because of thermal and radical quenching at burner walls [12–14]. As the characteristic length scales of a reactor decrease, the surface area to volume ratio increases, leading to enhanced heat and mass transfer rates between the surface and fluid. As a result, thermal losses to the wall and radical adsorption followed by recombination into stable molecules rise. Despite these challenges, devices capable of self-sustaining
1540-7489/$ - see front matter 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2004.08.188
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homogeneous flames in channels with gaps smaller than 1 mm have been developed [1,2]. To complement the aforementioned gas-phase microburner experimental work, computational fluid dynamics simulations in our group were performed to analyze the stability of gaseous microburners [5,6,15,16]. One of the most important findings is that the thermal properties of materials of construction play a vital role in the overall thermal stability of microburners. The reactor walls not only account for heat losses through transverse conduction, but they are often responsible for the majority of the upstream heat transfer, which is necessary to preheat the feed to the ignition temperature. To compound this problem, large wall temperatures and oscillatory instabilities can occur under certain conditions eventually leading to mechanical failure. Even with optimum wall materials, homogeneous combustion allows external heat losses just in the range of free convection for methane/air mixtures and in the low end of forced convection for propane/air mixtures. It becomes clear that while gaseous microburners are feasible to operate, their operation is restrictive. Combustion at the mesoscale has also emerged including the ‘‘swiss roll’’ heat-recirculating burner with gap sizes of the order of 3 mm, for homogeneous and heterogeneous combustion of propane with air [3,17]. A mesoscale electrospray device was also developed to atomize liquid fuels by flowing the fuel through a capillary tube and applying a large electrical potential, leading to small droplets that were then mixed with preheated air and fed to a catalytic screen to be combusted [18–20]. We have recently discussed a fabrication protocol of catalytic micro-channel burners along with their performance [21]. Two reactor sizes were discussed, one with a 250 lm gap and another with a 1000 lm gap. These devices were fabricated from alumina, with multiple integrated thermocouples, to measure axial and transversal temperature profiles. Additionally, it was found that propane/air mixtures can be self-combusted in both reactors. The 250 lm gap size reactor exhibited reduced ignition temperatures compared to the 1000 lm reactor (e.g., 150 C versus 235 C). Finally, the 250 lm gap size reactor achieved nearly complete conversion of propane under most ignited states, whereas the 1000 lm gap size reactor converted less of the propane (60–80%). Overall, confinement (decreased microburner size) leads to an increased performance due mainly to an enhanced mass transfer coefficient. In this work, we first provide evidence of selfignition of hydrogen/air mixtures at the small scale in ceramic microchannels with Pt catalyst over a wide range of equivalence ratios. Capitalizing on the self-ignition of hydrogen, a novel idea of igniting hydrocarbons by hydrogen-assisted
catalytic combustion at the small scale is explored. Finally, the synergism of mixture combustion is exploited. 2. Experimental overview In this work, microburners consisting of a rectangular channel within a cylindrical alumina support are used. A schematic of such a microburner is shown in Fig. 1. The channels are 1 cm wide by 5 cm long with a channel gap of 250 lm giving a reactor volume of 125 lL. A cross-section of one of the reactors is shown in Fig. 1. The dark area in the center is the platinum catalyst. Posts are placed at the inlet in an offset pattern to ensure uniform flow through the channel. Thermocouples are also embedded in the support with the tips flush to the channel wall to obtain axial and transverse temperature profiles. Details of the fabrication are discussed elsewhere [21]. The flow rates of the gases are controlled using mass flow controllers supplied by MKS Instruments. To determine conversions and selectivities, the exhaust gasses are sampled with a probe, which is then sent to a gas chromatograph (GC). A Hayesep column, in conjunction with a molecular sieve with a thermal conductivity detector (TCD), and an HP-PLOT column with a flame ionization detector (FID) are used. Grade 2.0 compressed air, Grade 5.0 compressed nitrogen, and compressed hydrogen are used (Keen Compressed Gases). The propane is >99% pure (Scott Specialty Gases). In all experiments, the total volumetric flow rate at the entrance is kept constant at 2 SLPM, resulting in an average inlet velocity of approximately 13.3 m/s. The temperatures, conversions, and selectivities are all reproducible to within a few percent over 10–15 h of operation. Deviations in the maximum reactor temperature in propane combustion
Fig. 1. Schematic showing the placement of the embedded reactor components. The inset is a crosssection of half of a reactor showing the orientation of the inlet posts.
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up to 10% have been observed over the course of >100 h of operation. To understand the system, order of magnitude estimates of dimensionless groups were computed. The aspect ratio (width to gap distance) is 40, indicating that one could focus along the flow. The Reynolds number is 226, i.e., the flow is laminar. In addition, the axial Peclet number, i.e., the ratio of the convective to the diffusion rates, is >3 · 104, indicating that the axial diffusion of heat and species is nearly negligible. Finally, the transverse diffusion time scale between the bulk and the surface is 1 ms. Fast transverse transport rates are expected to result in small transverse species and temperature gradients. It is possible that this system behaves effectively like a 1D one where the gap distance dictates the transverse microscale transport rates.
3. Ignition and self-sustained catalytic combustion of hydrogen/air and propane/air mixtures at the microscale We start with hydrogen combustion and then discuss propane combustion. Hydrogen/air mixtures are known to be self-igniting over Pt foils and wires under very fuel-lean conditions [22–24] but fuel-richer mixtures exhibit ignition temperature above room temperature. Here, we report that H2/air mixtures with equivalence ratios as high as 0.8 that we tested self-ignite. We have verified that this effect is not due to large dilution, which was used in literature experiments due to safety. This effect may stem from the microchannel confinement (see Section 1 for the effect of confinement on propane ignition). Boundary layers are often of the order of a few millimeters for typical laboratory experiments in stagnation flows associated with catalyst foils. Thus, the microchannel confinement accelerates the rate of transport of reagents to the surface, possibly rendering hydrogen/air mixtures self-igniting (see [25] for modeling analysis). Another possible cause is the insulating nature of the ceramic that retains the heat localized, as compared to the very non uniform temperature across foils attached to heat sinks (leads). Additionally, the surface of the catalyst is geometrically different from those of polycrystalline systems. Using scanning electron microscopy (SEM), the catalyst was found to form particles of 10 lm in diameter embedded in or on the substrate (data not shown). In addition, the possibility of the support enhancing hydrogen reactivity and leading to selfignition needs to be thoroughly studied. Figure 2 shows the maximum autothermal temperature measured within the reactor versus the equivalence ratio, /, of H2/air mixtures. Hydrogen/air mixtures are self-burning down to the lowest compositions that our mass flow con-
Fig. 2. Maximum autothermal temperature versus equivalence ratio of propane/air (solid line, circles) and hydrogen/air (dashed line, squares) mixtures. Propane/ air mixtures extinguish at / < 0.6 whereas hydrogen/air mixtures are self-sustained down to the leanest detectable limit.
trollers can handle. Furthermore, GC measurements indicate that there is significant conversion, even of the leanest detectable hydrogen feed concentrations (for specific values of conversion, see the next section). The conversion of hydrogen for all fuel-lean cases is >99%. The ignition of propane/air mixtures exhibits the expected behavior. To ignite propane/air mixtures, external heating is necessary. Figure 3 shows the maximum reactor temperature measured as a
Fig. 3. Maximum reactor temperature versus resistive power supplied for a propane/air mixture of / = 0.77. Approximately 30 W of power is necessary to achieve ignition. Autothermal behavior, where combustion is self-sustained, is observed.
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function of the power supplied via resistive heating for a propane/air mixture of / = 0.77. As the power increases from zero, the temperature ramps linearly until the ignition temperature is reached at 425 K with 30 W of power supplied. At this point, the system ignites, and the temperature increases abruptly. As the power supplied decreases from high values to zero, the temperature ramps down linearly to the autothermal temperature. Thus, typical hysteresis behavior is found to be consistent with previous work employing foils and wires [26,27]. In contrast to hydrogen, propane is a much heavier molecule, and its adsorption on Pt is activated [28], leading to a more difficult ignition. However, the measured ignition temperature in the 250 lm microreactor is 80 K lower than that measured using foils. On the other hand, measurements in a 1 mm gap microchannel show that the ignition temperature is comparable to that over Pt foils. Propane/air mixtures also exhibit autothermal behavior, i.e., they can be self-sustained, as shown in Fig. 2. Propane/air mixtures exhibit highest temperatures near stoichiometric conditions, and the temperature decreases with varying equivalence ratio away from the stoichiometric point. While the autothermal regime in microchannels is quite large, it is narrower than that of hydrogen. Extinction is observed for / < 0.6. Overall, microcatalytic systems exhibit much lower temperatures and a wider autothermal window of operation than their gas-phase counterparts of the same size [5]. At the same time, they are robust to thermal cycling in start up and shut down. The latter issue highlights that these systems will not fail mechanically. GC measurements along the locus of autothermal points indicate that propane conversion and the selectivity to water and carbon dioxide are >99%. Thus, despite the small length of the microchannel and the well-known mass transfer limitations of catalytic combustion, nearly complete combustion is achieved in our system, a requirement for high efficiency microscale systems. This important result stems from the mere fact that the mass transfer coefficient is high in narrow channels. On the other hand, the relatively open structure of the reactor leads to a maximum pressure drop of approximately 5 kPa.
applications, one can envision storage of small amounts of hydrogen during device operation from reforming of hydrocarbons that is subsequently used for startup. Here, we explore the feasibility of this idea, measure the necessary hydrogen content needed to self-ignite propane/ air mixtures, and finally study the synergism of binary fuels. Ignition of binary mixtures is not a new concept. For example, Deutschmann et al. experimentally and computationally studied the use of hydrogen combustion to ignite methane/air mixtures in a catalytic monolith. They found that the thermal release from the combustion of sufficiently fuel-rich hydrogen/air mixtures causes methane ignition. However, high temperatures limited their experimental study to initial transients only [29]. Here, we capitalize on the self-ignition of hydrogen at the small scale and focus on the self-ignition of propane by adding hydrogen. There are different ways to carry out such experiments. One is to first start the hydrogen/air mixture followed by introduction of the hydrocarbon. Another is to simultaneously start flowing all reagents. Figure 4 shows the maximum reactor temperature versus the hydrogen mole fraction for propane/air mixtures of various equivalence ratios. In this experiment, the propane/air ratio is held constant. As the hydrogen mole fraction increases starting from a zero value, the flow rates of propane and air are reduced, as the hydrogen flow rate increases to maintain the total volumetric flow rate fixed at 2 SLPM. The equivalence ratio
4. Bifurcation of hydrogen assisted self-ignition of propane/air mixtures The self-ignition nature of hydrogen/air mixtures at the small scale of ceramic burners offers an opportunity to self-ignite hydrocarbons. This concept may be a way toward elimination of ignition sources from microscale devices leading to further reduction of system size. Furthermore, since hydrogen is a main target for fuel cell
Fig. 4. Maximum reactor temperature of three propane/ air mixtures versus hydrogen mole fraction. Propane ignition occurs at a hydrogen mole fraction of 3.5– 3.6%. Different ignition modes are observed depending on the propane/air equivalence ratio.
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depicted in Fig. 4 denotes the propane/air one. Without hydrogen, the system is at room temperature because the reactor is not preheated. As the hydrogen composition increases, the reactor temperature increases slowly until approximately the hydrogen molar content is 3%. The subsequent behavior exhibits different ignition modes depending on the equivalence ratio of propane/air mixtures. In particular, for fuel-richer mixtures, e.g., / = 0.9, the temperature jumps immediately from approximately room temperature to the ignited temperature at 3.6% hydrogen in the mixture. On the other hand, fuel-leaner mixtures, e.g., / = 0.77, show a slight increase in temperature before ignition occurs, again at 3.6% hydrogen. Prior to ignition, nearly complete combustion of hydrogen but no combustion of propane is measured. For a propane/air mixture of / = 0.60, there is an initial jump in temperature at 3.2% hydrogen. This temperature increase is associated with a ÔdelayedÕ hydrogen ignition, and it is followed by a second ignition at 3.6% hydrogen. This second ignition is associated with the onset of propane combustion. Prior to the second ignition, >98% H2 conversion is measured with very small conversion of propane (selective oxidation of hydrogen). It is clear that the reactivity of hydrogen is delayed by propane due to kinetics, and the reactivity prior to overall ignition increases with reduction of the propane content. Upon overall ignition, as the hydrogen content is decreased from high values, the temperature of the system drops, as shown in Fig. 4. For sufficient fuel-rich mixtures, / P 0.77, when the hydrogen is eliminated from the feed, self-sustained (autothermal) combustion is observed. On the other hand, for fuel-leaner mixtures (/ 6 0.60) extinction occurs when the hydrogen content in the feed is reduced. This behavior is consistent with the propane/air autothermal data discussed above. The results of Fig. 4 clearly indicate that hydrogen-assisted self-ignition of the propane/air mixtures is feasible and the fraction of hydrogen needed to achieve this task is relatively small and fairly constant, i.e., independent of the propane/air equivalence ratio. At the same time, the two-stage ignition discovered for some compositions and the delayed hydrogen reactivity suggest that complex kinetic interactions occur in binary fuels under certain conditions. To better understand the ignition process, mixtures identical to those used above are fed to the microburner, except that propane is now replaced with nitrogen. Since nitrogen is inert, this experiment delineates the effect of propane on hydrogen combustion. The fraction of propane that is replaced in the stream is small (4 wt%); therefore the thermo-physical properties of the fluid change slightly. Similarly, the nitrogen fraction remains nearly constant. GC data provide further insights into the observed behavior. Figure 5 shows the
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Fig. 5. Maximum reactor temperature of a propane/air mixture at / = 0.77 (squares) versus the hydrogen mole fraction, and a mixture where propane has been replaced with nitrogen (circles). The straight line is an overall energy balance fit to the data. The catalytic ignition temperature of propane/air of / = 0.77 is shown as a horizontal line. Propane inhibits kinetically hydrogen combustion.
maximum reactor temperature versus hydrogen mole fraction for a propane/air mixture of / = 0.77 (squares, redrawn from Fig. 4 at higher magnification), and a mixture where propane is replaced by nitrogen. In the latter case, there is nearly complete conversion of hydrogen in all cases as measured by GC, resulting in a linear ramping of temperature with increasing hydrogen mole fraction (circles). The fit for this solid curve was calculated using an overall energy balance, considering complete conversion of hydrogen and external heat losses proportional to reactor temperature with an effective convective heat loss coefficient of 0.7 W/m2/K. Despite the low value of the heat loss coefficient, the measured temperature rise is approximately half of the adiabatic rise because of the high surface area to volume ratio. When propane is included in the mixture, the system behaves differently. For low hydrogen content in the feed, very little hydrogen conversion is observed via GC, and the temperature is low. As the hydrogen concentration increases past 2%, the temperature starts increasing due to partial catalytic hydrogen combustion. At 3.6% hydrogen, propane ignites, and the temperature increases out of the scale of Fig. 5 (see Fig. 4). These results more clearly elucidate that propane has an inhibitory kinetic effect on the catalytic combustion of hydrogen. Once the hydrogen concentration in the gas-phase is sufficiently high, hydrogen competes with propane more effectively for active catalyst sites, causing partial hydrogen combustion that leads to a temperature rise. This
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temperature increase eventually leads to the onset of propane combustion. The temperatures observed are lower than the adiabatic flame temperatures. Adiabatic operation would most likely expand the lean combustion limit but not the ignition process as much, as ignition occurs at low temperatures, where heat losses to the ambient are small. Improvements over a single channel used here are expected, for example, by heat recirculation, but are beyond the scope of this work where the focus is on selfignition. 5. Transient self-starting ignition Aside from steady state operation, it is important to understand the transient light-off behavior. We first study the effect of catalyst history on the transient behavior of microburners combusting hydrogen. Figure 6 shows the temperature at 1.6 cm from the inlet of the reactor as a function of time for hydrogen/air/nitrogen mixtures for three catalyst histories (states). The molar hydrogen composition in the feed is low (mixture composition is 3.6% H2/93.1% air/3.3% N2). The small fraction of N2 is used for consistency with the propane ignition studies reported below. The first state refers to the reactor being reduced at 600 C for 1 h in the presence of flowing 5% hydrogen in nitrogen (denoted as a ‘‘reduced surface’’). The second state is after exposure to 5% propane in nitrogen at room temperature for
Fig. 6. Temperature at 1.6 cm from the reactor inlet versus time for 3.6% H2 in air diluted with additional N2 for various catalyst pretreatments, a reduced surface (solid line), exposure to C3H8 in N2 at room temperature for 0.5 h (dotted line), and fuel-lean C3H8 combustion in air (dashed line). Exposure to C3H8 slows the ignition of hydrogen in air, but the steady state is the same in all cases.
0.5 h, which was then flushed with nitrogen. The third state is after exposure to fuel-lean propane combustion for 1 h. The reduced surface exhibits the quickest response of all cases and reaches 400 K in 1–2 min, as shown in Fig. 6. On the other hand, a surface exposed to room temperature propane reaches 400 K more slowly (in 3– 4 min). Longer exposure to propane delays the response time further (data not shown). For example, an exposure to propane for >50 h causes a hydrogen ignition delay of 40 min. However, when the system is left in air for several hours, the transient response returns to that of the surface that has been exposed for only 0.5 h (not shown). A surface exposed to propane combustion also exhibits delayed ignition, as shown in Fig. 6. These transient studies indicate that propane inhibits hydrogen ignition but since propane adsorption is activated and thus slow, the surface coverage of propane (and possible coke) depends on the time on-stream. However, when all modified surfaces are reduced (by baking at 600 C in the presence of 5% hydrogen in nitrogen for 1 h), the transient response returns to that of the reduced surface shown in Fig. 6. It is important to note that the steady state values of temperature discussed in the above sections are independent of the initial catalyst state. To ensure that transient measurements start with the same catalyst state, the surface was reduced prior to each experiment discussed below. Next we examine the effect of hydrogen composition on startup time in H2/air/N2 mixtures. Figure 7 shows the reactor temperature at 1.6 cm from the inlet as a function of time for
Fig. 7. Temperature at 1.6 cm from the reactor inlet versus time for H2 in air diluted with additional N2 for various H2/air/N2 compositions indicated. Fuel-richer mixtures lead to higher temperatures faster due to increased heat liberation. The dashed lines indicate the steady state values.
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three compositions indicated. The dotted horizontal lines indicate steady state values for each composition (note that a small overshoot is seen for the richer mixture as the maximum temperature moves past the specific thermocouple). The temperature initially increases rapidly and then asymptotically approaches a steady state value. While the initial transient is similar for sufficiently fuel-rich mixtures, a certain high temperature is reached faster for fuel-richer mixtures due to the increased heat generation. For example, a surface temperature of 500 K is reached in 20 s in 25% H2 containing mixtures, in 3 min in 10% H2 containing mixtures, and in 45 min in 4% H2 containing mixtures. The results from these transients provide insights into strategies needed for selfstarting of propane discussed next. Figure 8 shows the temperature at 1.6 cm from the inlet as a function of time for two startup methods of propane in mixtures containing 10% hydrogen. The first method entails simultaneously turning on the flow of hydrogen, propane, and air until the propane ignites (co-feed mode), and then turning off the hydrogen flow and increasing the flow rates of propane and air at the point indicated by an arrow so that the total flow rate remains constant throughout the experiment. In the second method, one preheats the reactor using H2/air/N2 combustion, then switches the propane on and the N2 off at an instance indicated by an arrow (when the ignition temperature of propane has been reached), and finally turns off the H2 flow while increasing the propane and air flow rates to
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maintain a total flow rate of 2 SLPM (swapping mode). In the co-feed mode, although propane inhibits the hydrogen ignition at short times, propane ignites as soon as the ignition temperature is reached, leading to a faster heatup of the system as compared to the unoptimized switchon of propane employed in the swapping mode. Furthermore, it appears that the heat release from the co-fed propane is beneficial during startup. Through optimization and external controls, the timing for introducing propane in the second method may be improved. However, the simultaneous starting of flows (co-feed method) is simple and has the additional benefit of heat release from both fuels, so it appears to be preferred. The inset of Fig. 8 depicts the same experiment performed with a mixture containing only 4% H2. The ignition method is not as critical since the temperature becomes roughly the same after a short transient. The time required to reach the ignition of propane is 45 min, 15–20 times longer than those of mixtures containing 10% hydrogen. This is obviously a slow startup and requires 6–8 times more hydrogen. Therefore, flows with higher H2 fraction are desirable from both the ignition time viewpoint and minimum utilization of H2. The inherent safety of hydrogen combustion at the microscale allows one to use mixtures well within the flammability regime to ensure fast startup. Finally, the reported times may be a worst-case scenario, given the large thermal mass of the ceramic support.
6. Conclusions
Fig. 8. Temperature at 1.6 cm from the reactor inlet versus time for H2/air/N2/C3 H8 mixtures using different ignition procedures. Solid lines correspond to the cofeed mode, and dashed lines correspond to the swapping mode. The H2 flow rate is fixed to 10% of 2 SLPM. The inset shows the same experiment for a 4% H2 containing mixture.
In this paper, we have shown that both hydrogen/air and propane/air mixtures can be self-sustained in Pt-based ceramic microchannels and exhibit very robust behavior. Furthermore, hydrogen self-ignites over a wide composition range and can successfully cause self-ignition of propane/air mixtures in these microburners, eliminating the need for startup devices. A minimum hydrogen concentration of 3.6% (on a molar basis) is needed, irrespective of the propane/air composition. A two-stage ignition can occur under certain conditions where hydrogen first ignites followed by ignition of propane. Kinetic inhibition of hydrogen by propane has been observed. While small fractions of hydrogen are adequate to cause self-ignition of propane, higher hydrogen compositions lead to a relatively fast startup of the system and minimum H2 utilization while being safe. Co-feeding hydrogen with propane appears as a good startup strategy. The low operation temperatures, the nearly complete conversions and selectivities to complete combustion products (CO2 and H2O), and the low pressure drops point to the high efficiency and low pollutant emissions features of the proposed microburners.
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Acknowledgments This work has been funded through the Army Research Laboratory Composite Materials Research program through the Center for Composite Materials at the University of Delaware. The authors acknowledge fruitful discussions with Dr. E. D. Wetzel of the Army Research Lab.
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[11] C.N. Satterfield, Heterogeneous Catalysis in Industrial Practice. McGraw-Hill, New York, 1991. [12] H. Davy, Trans. Roy. Soc. London 107 (1817) 45–76. [13] B. Lewis, G. von Elbe, Combustion, Flames and Explosions of Gases. Academic Press, Orlando, 1987. [14] A. Linan, F.A. Williams, Fundamental Aspects of Combustion. Oxford University Press, New York, 1993. [15] S. Raimondeau, D. Norton, D.G. Vlachos, R.I. Masel, Proc. Combust. Inst. 29 (2003) 901–907. [16] D.G. Norton, D.G. Vlachos, in: Proceedings of the Third Joint Meeting of the US Sections of the Combustion Institute. Chicago, IL, 2003. [17] P.D. Ronney, Combust. Flame 135 (4) (2003) 421–439. [18] S. Kaiser, D.C. Kyritsis, P. Dobrowolski, M.B. Long, A. Gomez, J. Mass Spectrom. Soc. Jpn. 51 (1) (2003) 42–49. [19] D.C. Kyritsis, I. Guerrero-Arias, S. Roychoudhury, A. Gomez, Proc. Combust. Inst. 29 (2003) 965–972. [20] D.C. Kyritsis, B. Coroton, F. Faure, S. Roychoudhury, A. Gomez, Combust. Flame (2004) (in press). [21] D.G. Norton, E.D. Wetzel, D.G. Vlachos, Ind. Eng. Chem. Res. 43 (16) (2004) 4833–4840. [22] M. Fassihi, V.P. Zhdanov, M. Rinnemo, K.-E. Keck, B. Kasemo, J. Catal. 141 (1993) 438–452. [23] M. Rinnemo, M. Fassihi, B. Kasemo, Chem. Phys. Lett. 211 (1993) 60–64. [24] N. Fernandes, Y.K. Park, D.G. Vlachos, Combust. Flame 118 (1–2) (1999) 164–178. [25] P.A. Bui, D.G. Vlachos, P.R. Westmoreland, Ind. Eng. Chem. Res. 36 (7) (1997) 2558–2567. [26] G. Veser, L.D. Schmidt, AIChE J. 42 (4) (1996) 1077–1087. [27] G. Veser, M. Ziauddin, L.D. Schmidt, Catal. Today 47 (1999) 219–228. [28] P. Aghalayam, D.G. Vlachos, in: Eastern States Combustion Symposium. Raleigh, NC, 1999. [29] O. Deutschmann, L. Maier, U. Riedel, A.H. Stroemman, R.W. Dibble, Catal. Today 59 (1–2) (2000) 141–150.
Comment Jacqueline H. Chen, Sandia National Laboratories, USA. How far downstream of the inlet do you observe self-ignition of hydrogen? How does the location of self-ignition depend upon the gap distance and flow rate? Is ignition limited by transport to the catalytic surface? Reply. Hydrogen ignition itself occurs downstream near the exit of the reactor, around 5 cm, or 50–200 gap distances depending on gap size, from the inlet. Once ignition occurs, the reaction zone moves upstream and stabilizes near the beginning of the catalytic zone. This is observed as a temperature rise near the end of the reactor upon starting the reactant feed. Upon ignition, this temperature peak moves upstream to the catalyst zone inlet. The effects of gap distance, transport, and flow rate on the self-ignition and location of reaction zone of hydrogen are difficult to quantify due to the
self-ignition nature and high reactivity of hydrogen. It is only for very fuel-lean mixtures that the narrower gaps give better performance in terms of complete conversion of hydrogen [1]. However, in the case of propane, where heating is needed and fuel adsorption on Pt is slower, it is observed that the gap distance plays a clear role in both ignition temperature and the location where the reaction zone is stabilized [2].
References [1] D.G. Norton, E.R. Wetzel, D.G. Vlachos, Ind. Eng. Chem. Res. 43 (2003) 4833–4840. [2] D.G. Norton, S.R. Deshmukh, E.D. Wetzel, D.G. Vlachos, in: Y. Wang, J.D. Holladay (Eds.), Microreactor Technology and Process Intensification. ACS book chapter (submitted).
Proceedings of the Combustion Institute 30 (2005) 2473–2480
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Hydrogen assisted self-ignition of propane/air mixtures in catalytic microburners D.G. Norton, D.G. Vlachos* Department of Chemical Engineering, Center for Catalytic Science and Technology (CCST) Center for Composite Materials (CCM), University of Delaware, Newark, DE 19716-3110, USA
Abstract A catalytic Pt-based microdevice is evaluated for the combustion of hydrogen and/or propane. It is found that in confined ceramic microchannels hydrogen/air mixtures self-ignite over a wide range of compositions. This discovery is capitalized to self-ignite propane/air mixtures with the assistance of hydrogen addition. It is shown that propane kinetically inhibits hydrogen catalytic combustion at low hydrogen fractions. The minimum hydrogen composition for self-ignition of propane/air mixture compositions is found to be relatively constant, irrespective of propane composition. The transient and steady state behavior of these systems is described, and the minimization of hydrogen usage and startup time are discussed. 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Microburners; Propane; Hydrogen; Catalytic combustion; Self-ignition
1. Introduction Microburners are increasingly studied for the catalytic and non-catalytic portable production of heat and/or energy [1–6]. The energy produced can be utilized by various means. Examples include heat generation for remote use, such as for soldiers and space flights, thermoelectrics to produce electricity [7], and heat supply to microscale reactors carrying out endothermic reactions, such as ammonia decomposition or steam reforming [8–11], to produce hydrogen for portable fuel cells. Because hydrocarbons possess a significantly greater energy density than traditional metal acid batteries [4], hydrocarbon-based microburners are
*
Corresponding author. Fax: +1 302 831 1048. E-mail address: [email protected] (D.G. Vlachos).
an enticing prospective energy source for portable power applications, such as cell phones, laptops, portable electronics, and personal heaters. Most current prototypes depend on external heating to generate energy for ignition. The additional apparatus necessary to power such external heaters can negate mass advantages of microburners. One concept for designing a microburner is to downscale conventional scale gaseous burners. However, homogeneous flames are typically quenched when confined in spaces with dimensions below 1–2 mm because of thermal and radical quenching at burner walls [12–14]. As the characteristic length scales of a reactor decrease, the surface area to volume ratio increases, leading to enhanced heat and mass transfer rates between the surface and fluid. As a result, thermal losses to the wall and radical adsorption followed by recombination into stable molecules rise. Despite these challenges, devices capable of self-sustaining
1540-7489/$ - see front matter 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2004.08.188
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homogeneous flames in channels with gaps smaller than 1 mm have been developed [1,2]. To complement the aforementioned gas-phase microburner experimental work, computational fluid dynamics simulations in our group were performed to analyze the stability of gaseous microburners [5,6,15,16]. One of the most important findings is that the thermal properties of materials of construction play a vital role in the overall thermal stability of microburners. The reactor walls not only account for heat losses through transverse conduction, but they are often responsible for the majority of the upstream heat transfer, which is necessary to preheat the feed to the ignition temperature. To compound this problem, large wall temperatures and oscillatory instabilities can occur under certain conditions eventually leading to mechanical failure. Even with optimum wall materials, homogeneous combustion allows external heat losses just in the range of free convection for methane/air mixtures and in the low end of forced convection for propane/air mixtures. It becomes clear that while gaseous microburners are feasible to operate, their operation is restrictive. Combustion at the mesoscale has also emerged including the ‘‘swiss roll’’ heat-recirculating burner with gap sizes of the order of 3 mm, for homogeneous and heterogeneous combustion of propane with air [3,17]. A mesoscale electrospray device was also developed to atomize liquid fuels by flowing the fuel through a capillary tube and applying a large electrical potential, leading to small droplets that were then mixed with preheated air and fed to a catalytic screen to be combusted [18–20]. We have recently discussed a fabrication protocol of catalytic micro-channel burners along with their performance [21]. Two reactor sizes were discussed, one with a 250 lm gap and another with a 1000 lm gap. These devices were fabricated from alumina, with multiple integrated thermocouples, to measure axial and transversal temperature profiles. Additionally, it was found that propane/air mixtures can be self-combusted in both reactors. The 250 lm gap size reactor exhibited reduced ignition temperatures compared to the 1000 lm reactor (e.g., 150 C versus 235 C). Finally, the 250 lm gap size reactor achieved nearly complete conversion of propane under most ignited states, whereas the 1000 lm gap size reactor converted less of the propane (60–80%). Overall, confinement (decreased microburner size) leads to an increased performance due mainly to an enhanced mass transfer coefficient. In this work, we first provide evidence of selfignition of hydrogen/air mixtures at the small scale in ceramic microchannels with Pt catalyst over a wide range of equivalence ratios. Capitalizing on the self-ignition of hydrogen, a novel idea of igniting hydrocarbons by hydrogen-assisted
catalytic combustion at the small scale is explored. Finally, the synergism of mixture combustion is exploited. 2. Experimental overview In this work, microburners consisting of a rectangular channel within a cylindrical alumina support are used. A schematic of such a microburner is shown in Fig. 1. The channels are 1 cm wide by 5 cm long with a channel gap of 250 lm giving a reactor volume of 125 lL. A cross-section of one of the reactors is shown in Fig. 1. The dark area in the center is the platinum catalyst. Posts are placed at the inlet in an offset pattern to ensure uniform flow through the channel. Thermocouples are also embedded in the support with the tips flush to the channel wall to obtain axial and transverse temperature profiles. Details of the fabrication are discussed elsewhere [21]. The flow rates of the gases are controlled using mass flow controllers supplied by MKS Instruments. To determine conversions and selectivities, the exhaust gasses are sampled with a probe, which is then sent to a gas chromatograph (GC). A Hayesep column, in conjunction with a molecular sieve with a thermal conductivity detector (TCD), and an HP-PLOT column with a flame ionization detector (FID) are used. Grade 2.0 compressed air, Grade 5.0 compressed nitrogen, and compressed hydrogen are used (Keen Compressed Gases). The propane is >99% pure (Scott Specialty Gases). In all experiments, the total volumetric flow rate at the entrance is kept constant at 2 SLPM, resulting in an average inlet velocity of approximately 13.3 m/s. The temperatures, conversions, and selectivities are all reproducible to within a few percent over 10–15 h of operation. Deviations in the maximum reactor temperature in propane combustion
Fig. 1. Schematic showing the placement of the embedded reactor components. The inset is a crosssection of half of a reactor showing the orientation of the inlet posts.
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up to 10% have been observed over the course of >100 h of operation. To understand the system, order of magnitude estimates of dimensionless groups were computed. The aspect ratio (width to gap distance) is 40, indicating that one could focus along the flow. The Reynolds number is 226, i.e., the flow is laminar. In addition, the axial Peclet number, i.e., the ratio of the convective to the diffusion rates, is >3 · 104, indicating that the axial diffusion of heat and species is nearly negligible. Finally, the transverse diffusion time scale between the bulk and the surface is 1 ms. Fast transverse transport rates are expected to result in small transverse species and temperature gradients. It is possible that this system behaves effectively like a 1D one where the gap distance dictates the transverse microscale transport rates.
3. Ignition and self-sustained catalytic combustion of hydrogen/air and propane/air mixtures at the microscale We start with hydrogen combustion and then discuss propane combustion. Hydrogen/air mixtures are known to be self-igniting over Pt foils and wires under very fuel-lean conditions [22–24] but fuel-richer mixtures exhibit ignition temperature above room temperature. Here, we report that H2/air mixtures with equivalence ratios as high as 0.8 that we tested self-ignite. We have verified that this effect is not due to large dilution, which was used in literature experiments due to safety. This effect may stem from the microchannel confinement (see Section 1 for the effect of confinement on propane ignition). Boundary layers are often of the order of a few millimeters for typical laboratory experiments in stagnation flows associated with catalyst foils. Thus, the microchannel confinement accelerates the rate of transport of reagents to the surface, possibly rendering hydrogen/air mixtures self-igniting (see [25] for modeling analysis). Another possible cause is the insulating nature of the ceramic that retains the heat localized, as compared to the very non uniform temperature across foils attached to heat sinks (leads). Additionally, the surface of the catalyst is geometrically different from those of polycrystalline systems. Using scanning electron microscopy (SEM), the catalyst was found to form particles of 10 lm in diameter embedded in or on the substrate (data not shown). In addition, the possibility of the support enhancing hydrogen reactivity and leading to selfignition needs to be thoroughly studied. Figure 2 shows the maximum autothermal temperature measured within the reactor versus the equivalence ratio, /, of H2/air mixtures. Hydrogen/air mixtures are self-burning down to the lowest compositions that our mass flow con-
Fig. 2. Maximum autothermal temperature versus equivalence ratio of propane/air (solid line, circles) and hydrogen/air (dashed line, squares) mixtures. Propane/ air mixtures extinguish at / < 0.6 whereas hydrogen/air mixtures are self-sustained down to the leanest detectable limit.
trollers can handle. Furthermore, GC measurements indicate that there is significant conversion, even of the leanest detectable hydrogen feed concentrations (for specific values of conversion, see the next section). The conversion of hydrogen for all fuel-lean cases is >99%. The ignition of propane/air mixtures exhibits the expected behavior. To ignite propane/air mixtures, external heating is necessary. Figure 3 shows the maximum reactor temperature measured as a
Fig. 3. Maximum reactor temperature versus resistive power supplied for a propane/air mixture of / = 0.77. Approximately 30 W of power is necessary to achieve ignition. Autothermal behavior, where combustion is self-sustained, is observed.
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function of the power supplied via resistive heating for a propane/air mixture of / = 0.77. As the power increases from zero, the temperature ramps linearly until the ignition temperature is reached at 425 K with 30 W of power supplied. At this point, the system ignites, and the temperature increases abruptly. As the power supplied decreases from high values to zero, the temperature ramps down linearly to the autothermal temperature. Thus, typical hysteresis behavior is found to be consistent with previous work employing foils and wires [26,27]. In contrast to hydrogen, propane is a much heavier molecule, and its adsorption on Pt is activated [28], leading to a more difficult ignition. However, the measured ignition temperature in the 250 lm microreactor is 80 K lower than that measured using foils. On the other hand, measurements in a 1 mm gap microchannel show that the ignition temperature is comparable to that over Pt foils. Propane/air mixtures also exhibit autothermal behavior, i.e., they can be self-sustained, as shown in Fig. 2. Propane/air mixtures exhibit highest temperatures near stoichiometric conditions, and the temperature decreases with varying equivalence ratio away from the stoichiometric point. While the autothermal regime in microchannels is quite large, it is narrower than that of hydrogen. Extinction is observed for / < 0.6. Overall, microcatalytic systems exhibit much lower temperatures and a wider autothermal window of operation than their gas-phase counterparts of the same size [5]. At the same time, they are robust to thermal cycling in start up and shut down. The latter issue highlights that these systems will not fail mechanically. GC measurements along the locus of autothermal points indicate that propane conversion and the selectivity to water and carbon dioxide are >99%. Thus, despite the small length of the microchannel and the well-known mass transfer limitations of catalytic combustion, nearly complete combustion is achieved in our system, a requirement for high efficiency microscale systems. This important result stems from the mere fact that the mass transfer coefficient is high in narrow channels. On the other hand, the relatively open structure of the reactor leads to a maximum pressure drop of approximately 5 kPa.
applications, one can envision storage of small amounts of hydrogen during device operation from reforming of hydrocarbons that is subsequently used for startup. Here, we explore the feasibility of this idea, measure the necessary hydrogen content needed to self-ignite propane/ air mixtures, and finally study the synergism of binary fuels. Ignition of binary mixtures is not a new concept. For example, Deutschmann et al. experimentally and computationally studied the use of hydrogen combustion to ignite methane/air mixtures in a catalytic monolith. They found that the thermal release from the combustion of sufficiently fuel-rich hydrogen/air mixtures causes methane ignition. However, high temperatures limited their experimental study to initial transients only [29]. Here, we capitalize on the self-ignition of hydrogen at the small scale and focus on the self-ignition of propane by adding hydrogen. There are different ways to carry out such experiments. One is to first start the hydrogen/air mixture followed by introduction of the hydrocarbon. Another is to simultaneously start flowing all reagents. Figure 4 shows the maximum reactor temperature versus the hydrogen mole fraction for propane/air mixtures of various equivalence ratios. In this experiment, the propane/air ratio is held constant. As the hydrogen mole fraction increases starting from a zero value, the flow rates of propane and air are reduced, as the hydrogen flow rate increases to maintain the total volumetric flow rate fixed at 2 SLPM. The equivalence ratio
4. Bifurcation of hydrogen assisted self-ignition of propane/air mixtures The self-ignition nature of hydrogen/air mixtures at the small scale of ceramic burners offers an opportunity to self-ignite hydrocarbons. This concept may be a way toward elimination of ignition sources from microscale devices leading to further reduction of system size. Furthermore, since hydrogen is a main target for fuel cell
Fig. 4. Maximum reactor temperature of three propane/ air mixtures versus hydrogen mole fraction. Propane ignition occurs at a hydrogen mole fraction of 3.5– 3.6%. Different ignition modes are observed depending on the propane/air equivalence ratio.
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depicted in Fig. 4 denotes the propane/air one. Without hydrogen, the system is at room temperature because the reactor is not preheated. As the hydrogen composition increases, the reactor temperature increases slowly until approximately the hydrogen molar content is 3%. The subsequent behavior exhibits different ignition modes depending on the equivalence ratio of propane/air mixtures. In particular, for fuel-richer mixtures, e.g., / = 0.9, the temperature jumps immediately from approximately room temperature to the ignited temperature at 3.6% hydrogen in the mixture. On the other hand, fuel-leaner mixtures, e.g., / = 0.77, show a slight increase in temperature before ignition occurs, again at 3.6% hydrogen. Prior to ignition, nearly complete combustion of hydrogen but no combustion of propane is measured. For a propane/air mixture of / = 0.60, there is an initial jump in temperature at 3.2% hydrogen. This temperature increase is associated with a ÔdelayedÕ hydrogen ignition, and it is followed by a second ignition at 3.6% hydrogen. This second ignition is associated with the onset of propane combustion. Prior to the second ignition, >98% H2 conversion is measured with very small conversion of propane (selective oxidation of hydrogen). It is clear that the reactivity of hydrogen is delayed by propane due to kinetics, and the reactivity prior to overall ignition increases with reduction of the propane content. Upon overall ignition, as the hydrogen content is decreased from high values, the temperature of the system drops, as shown in Fig. 4. For sufficient fuel-rich mixtures, / P 0.77, when the hydrogen is eliminated from the feed, self-sustained (autothermal) combustion is observed. On the other hand, for fuel-leaner mixtures (/ 6 0.60) extinction occurs when the hydrogen content in the feed is reduced. This behavior is consistent with the propane/air autothermal data discussed above. The results of Fig. 4 clearly indicate that hydrogen-assisted self-ignition of the propane/air mixtures is feasible and the fraction of hydrogen needed to achieve this task is relatively small and fairly constant, i.e., independent of the propane/air equivalence ratio. At the same time, the two-stage ignition discovered for some compositions and the delayed hydrogen reactivity suggest that complex kinetic interactions occur in binary fuels under certain conditions. To better understand the ignition process, mixtures identical to those used above are fed to the microburner, except that propane is now replaced with nitrogen. Since nitrogen is inert, this experiment delineates the effect of propane on hydrogen combustion. The fraction of propane that is replaced in the stream is small (4 wt%); therefore the thermo-physical properties of the fluid change slightly. Similarly, the nitrogen fraction remains nearly constant. GC data provide further insights into the observed behavior. Figure 5 shows the
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Fig. 5. Maximum reactor temperature of a propane/air mixture at / = 0.77 (squares) versus the hydrogen mole fraction, and a mixture where propane has been replaced with nitrogen (circles). The straight line is an overall energy balance fit to the data. The catalytic ignition temperature of propane/air of / = 0.77 is shown as a horizontal line. Propane inhibits kinetically hydrogen combustion.
maximum reactor temperature versus hydrogen mole fraction for a propane/air mixture of / = 0.77 (squares, redrawn from Fig. 4 at higher magnification), and a mixture where propane is replaced by nitrogen. In the latter case, there is nearly complete conversion of hydrogen in all cases as measured by GC, resulting in a linear ramping of temperature with increasing hydrogen mole fraction (circles). The fit for this solid curve was calculated using an overall energy balance, considering complete conversion of hydrogen and external heat losses proportional to reactor temperature with an effective convective heat loss coefficient of 0.7 W/m2/K. Despite the low value of the heat loss coefficient, the measured temperature rise is approximately half of the adiabatic rise because of the high surface area to volume ratio. When propane is included in the mixture, the system behaves differently. For low hydrogen content in the feed, very little hydrogen conversion is observed via GC, and the temperature is low. As the hydrogen concentration increases past 2%, the temperature starts increasing due to partial catalytic hydrogen combustion. At 3.6% hydrogen, propane ignites, and the temperature increases out of the scale of Fig. 5 (see Fig. 4). These results more clearly elucidate that propane has an inhibitory kinetic effect on the catalytic combustion of hydrogen. Once the hydrogen concentration in the gas-phase is sufficiently high, hydrogen competes with propane more effectively for active catalyst sites, causing partial hydrogen combustion that leads to a temperature rise. This
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temperature increase eventually leads to the onset of propane combustion. The temperatures observed are lower than the adiabatic flame temperatures. Adiabatic operation would most likely expand the lean combustion limit but not the ignition process as much, as ignition occurs at low temperatures, where heat losses to the ambient are small. Improvements over a single channel used here are expected, for example, by heat recirculation, but are beyond the scope of this work where the focus is on selfignition. 5. Transient self-starting ignition Aside from steady state operation, it is important to understand the transient light-off behavior. We first study the effect of catalyst history on the transient behavior of microburners combusting hydrogen. Figure 6 shows the temperature at 1.6 cm from the inlet of the reactor as a function of time for hydrogen/air/nitrogen mixtures for three catalyst histories (states). The molar hydrogen composition in the feed is low (mixture composition is 3.6% H2/93.1% air/3.3% N2). The small fraction of N2 is used for consistency with the propane ignition studies reported below. The first state refers to the reactor being reduced at 600 C for 1 h in the presence of flowing 5% hydrogen in nitrogen (denoted as a ‘‘reduced surface’’). The second state is after exposure to 5% propane in nitrogen at room temperature for
Fig. 6. Temperature at 1.6 cm from the reactor inlet versus time for 3.6% H2 in air diluted with additional N2 for various catalyst pretreatments, a reduced surface (solid line), exposure to C3H8 in N2 at room temperature for 0.5 h (dotted line), and fuel-lean C3H8 combustion in air (dashed line). Exposure to C3H8 slows the ignition of hydrogen in air, but the steady state is the same in all cases.
0.5 h, which was then flushed with nitrogen. The third state is after exposure to fuel-lean propane combustion for 1 h. The reduced surface exhibits the quickest response of all cases and reaches 400 K in 1–2 min, as shown in Fig. 6. On the other hand, a surface exposed to room temperature propane reaches 400 K more slowly (in 3– 4 min). Longer exposure to propane delays the response time further (data not shown). For example, an exposure to propane for >50 h causes a hydrogen ignition delay of 40 min. However, when the system is left in air for several hours, the transient response returns to that of the surface that has been exposed for only 0.5 h (not shown). A surface exposed to propane combustion also exhibits delayed ignition, as shown in Fig. 6. These transient studies indicate that propane inhibits hydrogen ignition but since propane adsorption is activated and thus slow, the surface coverage of propane (and possible coke) depends on the time on-stream. However, when all modified surfaces are reduced (by baking at 600 C in the presence of 5% hydrogen in nitrogen for 1 h), the transient response returns to that of the reduced surface shown in Fig. 6. It is important to note that the steady state values of temperature discussed in the above sections are independent of the initial catalyst state. To ensure that transient measurements start with the same catalyst state, the surface was reduced prior to each experiment discussed below. Next we examine the effect of hydrogen composition on startup time in H2/air/N2 mixtures. Figure 7 shows the reactor temperature at 1.6 cm from the inlet as a function of time for
Fig. 7. Temperature at 1.6 cm from the reactor inlet versus time for H2 in air diluted with additional N2 for various H2/air/N2 compositions indicated. Fuel-richer mixtures lead to higher temperatures faster due to increased heat liberation. The dashed lines indicate the steady state values.
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three compositions indicated. The dotted horizontal lines indicate steady state values for each composition (note that a small overshoot is seen for the richer mixture as the maximum temperature moves past the specific thermocouple). The temperature initially increases rapidly and then asymptotically approaches a steady state value. While the initial transient is similar for sufficiently fuel-rich mixtures, a certain high temperature is reached faster for fuel-richer mixtures due to the increased heat generation. For example, a surface temperature of 500 K is reached in 20 s in 25% H2 containing mixtures, in 3 min in 10% H2 containing mixtures, and in 45 min in 4% H2 containing mixtures. The results from these transients provide insights into strategies needed for selfstarting of propane discussed next. Figure 8 shows the temperature at 1.6 cm from the inlet as a function of time for two startup methods of propane in mixtures containing 10% hydrogen. The first method entails simultaneously turning on the flow of hydrogen, propane, and air until the propane ignites (co-feed mode), and then turning off the hydrogen flow and increasing the flow rates of propane and air at the point indicated by an arrow so that the total flow rate remains constant throughout the experiment. In the second method, one preheats the reactor using H2/air/N2 combustion, then switches the propane on and the N2 off at an instance indicated by an arrow (when the ignition temperature of propane has been reached), and finally turns off the H2 flow while increasing the propane and air flow rates to
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maintain a total flow rate of 2 SLPM (swapping mode). In the co-feed mode, although propane inhibits the hydrogen ignition at short times, propane ignites as soon as the ignition temperature is reached, leading to a faster heatup of the system as compared to the unoptimized switchon of propane employed in the swapping mode. Furthermore, it appears that the heat release from the co-fed propane is beneficial during startup. Through optimization and external controls, the timing for introducing propane in the second method may be improved. However, the simultaneous starting of flows (co-feed method) is simple and has the additional benefit of heat release from both fuels, so it appears to be preferred. The inset of Fig. 8 depicts the same experiment performed with a mixture containing only 4% H2. The ignition method is not as critical since the temperature becomes roughly the same after a short transient. The time required to reach the ignition of propane is 45 min, 15–20 times longer than those of mixtures containing 10% hydrogen. This is obviously a slow startup and requires 6–8 times more hydrogen. Therefore, flows with higher H2 fraction are desirable from both the ignition time viewpoint and minimum utilization of H2. The inherent safety of hydrogen combustion at the microscale allows one to use mixtures well within the flammability regime to ensure fast startup. Finally, the reported times may be a worst-case scenario, given the large thermal mass of the ceramic support.
6. Conclusions
Fig. 8. Temperature at 1.6 cm from the reactor inlet versus time for H2/air/N2/C3 H8 mixtures using different ignition procedures. Solid lines correspond to the cofeed mode, and dashed lines correspond to the swapping mode. The H2 flow rate is fixed to 10% of 2 SLPM. The inset shows the same experiment for a 4% H2 containing mixture.
In this paper, we have shown that both hydrogen/air and propane/air mixtures can be self-sustained in Pt-based ceramic microchannels and exhibit very robust behavior. Furthermore, hydrogen self-ignites over a wide composition range and can successfully cause self-ignition of propane/air mixtures in these microburners, eliminating the need for startup devices. A minimum hydrogen concentration of 3.6% (on a molar basis) is needed, irrespective of the propane/air composition. A two-stage ignition can occur under certain conditions where hydrogen first ignites followed by ignition of propane. Kinetic inhibition of hydrogen by propane has been observed. While small fractions of hydrogen are adequate to cause self-ignition of propane, higher hydrogen compositions lead to a relatively fast startup of the system and minimum H2 utilization while being safe. Co-feeding hydrogen with propane appears as a good startup strategy. The low operation temperatures, the nearly complete conversions and selectivities to complete combustion products (CO2 and H2O), and the low pressure drops point to the high efficiency and low pollutant emissions features of the proposed microburners.
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Acknowledgments This work has been funded through the Army Research Laboratory Composite Materials Research program through the Center for Composite Materials at the University of Delaware. The authors acknowledge fruitful discussions with Dr. E. D. Wetzel of the Army Research Lab.
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Comment Jacqueline H. Chen, Sandia National Laboratories, USA. How far downstream of the inlet do you observe self-ignition of hydrogen? How does the location of self-ignition depend upon the gap distance and flow rate? Is ignition limited by transport to the catalytic surface? Reply. Hydrogen ignition itself occurs downstream near the exit of the reactor, around 5 cm, or 50–200 gap distances depending on gap size, from the inlet. Once ignition occurs, the reaction zone moves upstream and stabilizes near the beginning of the catalytic zone. This is observed as a temperature rise near the end of the reactor upon starting the reactant feed. Upon ignition, this temperature peak moves upstream to the catalyst zone inlet. The effects of gap distance, transport, and flow rate on the self-ignition and location of reaction zone of hydrogen are difficult to quantify due to the
self-ignition nature and high reactivity of hydrogen. It is only for very fuel-lean mixtures that the narrower gaps give better performance in terms of complete conversion of hydrogen [1]. However, in the case of propane, where heating is needed and fuel adsorption on Pt is slower, it is observed that the gap distance plays a clear role in both ignition temperature and the location where the reaction zone is stabilized [2].
References [1] D.G. Norton, E.R. Wetzel, D.G. Vlachos, Ind. Eng. Chem. Res. 43 (2003) 4833–4840. [2] D.G. Norton, S.R. Deshmukh, E.D. Wetzel, D.G. Vlachos, in: Y. Wang, J.D. Holladay (Eds.), Microreactor Technology and Process Intensification. ACS book chapter (submitted).