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Combustion of propane with Pt and Rh catalysts in a meso-scale heat recirculating combustor
Applied Energy 130 (2014) 350–356
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Combustion of propane with Pt and Rh catalysts in a meso-scale heat recirculating combustor Teresa A. Wierzbicki a,b, Ivan C. Lee b, Ashwani K. Gupta a,⇑ a b
Department of Mechanical Engineering, University of Maryland, 2181 Martin Hall, Campus Dr., College Park, MD 20742, USA Sensors and Electron Devices Directorate, U.S. Army Research Laboratory, 2800 Powder Mill Rd., Adelphi, MD 20783, USA
h i g h l i g h t s Compared extinction limits of catalytic and non-catalytic combustor operation. Analyzed catalytic conversion, product selectivity/yield, and activation energy. Results used to predict the catalytic combustor performance with liquid fuel. Catalysts offered enhanced stable operation of combustor than without catalyst. Rh is more suitable for liquid fuel due to higher fuel conversion and output energy.
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Article history: Received 9 February 2014 Received in revised form 21 April 2014 Accepted 31 May 2014
Keywords: Catalytic combustion Heat-recirculating combustor Meso-scale combustion Propane oxidation Combustion behavior
a b s t r a c t The results obtained from the combustion behavior of propane over platinum and rhodium catalysts in a meso-scale heat recirculating combustor are presented. The extinction limits, conversion, product selectivity/yield, and activation energy using the two catalysts were compared in an effort to predict their performance using a liquid fuel. The extinction limits were also compared to those of non-catalytic combustion in the same combustor. The results showed that the use of a catalyst greatly expanded the range of stable operating conditions, in terms of both extinction limits and flow rates supported. The Rh catalyst was found to exhibit a higher propane conversion rate, reaching a maximum of 90.4% at stoichiometric conditions (as compared to only 61.4% offered by the Pt catalyst under lean conditions), but the Pt catalyst had superior CO2 selectivity for most of the examined conditions, indicating more of the heat released being used for product formation as opposed to being lost to the environment. However, despite having a higher rate of heat loss, the combustion with the Rh catalyst produced an overall higher amount of enthalpy than the Pt due to its superior fuel conversion. The Pt catalyst also had a significantly smaller activation energy (13.8 kJ/mol) than the Rh catalyst (74.7 kJ/mol), except at equivalence ratios richer than U = 1.75 (corresponding to catalyst temperatures below 500 °C), where it abruptly changed to 211.4 kJ/mol, signifying a transition from diffusion-limited reactions to kinetically limited reactions at this point. The results reveal that Rh would be a more suitable catalyst for use in liquid-fueled meso-scale combustors, as fuel conversion has been found to be a limiting factor for combustion stability in these systems, and as its higher output energy allows for greater flexibility of use. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Much interest in meso-scale combustion systems has been garnered of late due to the benefits they offer in terms of local power production, micro thrust, and micro propulsion generation. Hydrocarbon fuels have much higher specific energies than batteries, ⇑ Corresponding author. Tel.: +1 301 405 5276. E-mail addresses: [email protected], [email protected] (T.A. Wierzbicki), [email protected] (I.C. Lee), [email protected] (A.K. Gupta). http://dx.doi.org/10.1016/j.apenergy.2014.05.069 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.
implying that even devices with very low thermal-to-electric efficiencies can yield higher electrical outputs than batteries, and thus offer significant performance improvements in portable electronic devices, i.e. longer lifetime and reduced weight [1]. Liquid hydrocarbons are ideal for use in these devices, as they are very energy-dense and can be easily stored. Several studies have demonstrated the successful coupling of meso-scale combustion systems with thermoelectric/thermophotovoltaic devices to produce electric power [2–6]. However, there are some problems inherent to meso-scale combustion systems. When the system is scaled
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Nomenclature A0 C/O Ea k [k] NC,k NC,out r
pre-exponential factor carbon to oxygen ratio activation energy reaction rate constant concentration of species k total moles of carbon in species k total moles of carbon in combustion products reaction rate
down, the heat loss to heat generation (or surface area to volume) ratio increases, which can induce flame quenching as the system size is reduced. A prevalent technique to diminish this effect is to utilize a heat recirculating, swiss roll-style combustor, which allows heat to be transferred from the products to the reactants, thus permitting stable combustion in channels with a characteristic dimension smaller than the flame quenching distance. The preheating of reactants also broadens the range of stable operating conditions by extending the lean flammability limits of the fuel used [7]. Much research has been done to investigate the effects of geometry, scaling, operating conditions, and combustor material on the extinction limits and thermal performance of meso- and micro-scale swiss roll combustors [8–14]. By varying the geometry and inlet conditions (i.e. equivalence ratio and Reynolds number), meso-scale swiss roll combustors can be tailored to suit a wide range of both power generation and thrust applications [15,16]. Shirsat and Gupta [9] studied this concept in the context of thrust generation for such applications as nanosatellites, power generation, and small robotics, and reported that the optimal geometry for flame maintenance while minimizing heat loss and maximizing exhaust enthalpy was a single-turn combustor design. Another intrinsic difficulty in meso- and micro-combustion systems is the very short residence time within the combustion chamber. At such short residence times, complete combustion of heavier hydrocarbons, especially in blended fuels, may not always occur, resulting in significant soot formation and flame instabilities [15]. A possible solution to this problem is to utilize a catalyst in the system. Catalysts have been shown to help stabilize liquid fuel combustion at short residence times, even for petroleum-based jet fuels [17–19]. The high surface area to volume ratio that is characteristic of small-scale systems is also ideal for use with a catalyst, as increased surface area favors catalytic combustion by providing more sites upon which the fuel and oxidizer can adsorb. The addition of a catalyst has been shown in many studies to widen the range of stable operating conditions, lower the overall reactor temperatures, and increase the hydrocarbon oxidation rates [20–22]. Spadaccini et al. [23] found that the addition of a porous platinum catalyst to their microscale gas turbine allowed for an increase in operable mass flow rates through the system, leading to an 8.5-fold increase in power density over the maximum achieved for a non-catalytic system of similar geometry. Catalysts also play a key role in thermal management—lower overall temperatures result in smaller temperature gradients throughout the system, reducing the risk of thermal stresses. Also, since chemical reactions theoretically only occur on catalyst surfaces, the heat source location is fixed, unlike in homogenous combustion, where the flame location may change depending on operating conditions [8]. Due to this enhanced stability and improved thermal management, integration of catalytic micro- and meso-scale combustors with such energy-harnessing devices as thermoelectrics/thermophotovoltaics and steam reformers is conjectured to be much simpler than the corresponding non-catalytic burners [24].
R t T V_ k
mk U
universal gas constant time temperature volumetric flow rate of species k stoichiometric coefficient of species k equivalence ratio
The current study examines the impact of adding a porous foam catalyst to a single-turn swiss roll combustor burning a mixture of propane and air. Two different catalysts, rhodium and platinum, were investigated. The objective of this study is to analyze the role of Rh and Pt catalysts in terms of extinction limits (particularly how these limits compare to those of non-catalytic combustion in the same reactor), conversion, selectivity, and activation energy. Total oxidation of propane is used as a probe reaction in this study, since oxidation reactions are essential to maximizing stability and efficiency while minimizing emissions in power generation and thrust production applications [25]. The results will help guide in determining as to which catalyst would be most suitable for implementation in a similar combustor using liquid fuels.
2. Experimental 2.1. Combustor and catalyst A single turn heat recirculating combustor similar to that utilized by Shirsat and Gupta [9] and Wierzbicki et al. [15] was used. The fuel and the oxidizer were introduced to the combustion chamber partially premixed, with the fuel injected downstream from the oxidizer to prevent any flashback into the inlet reactant tubing. A step entrance to the combustion chamber was created to generate a vortical flow structure and local recirculation zone within the chamber, recirculating both heat and active species from the products to the reactants. The aerodynamics of this flow structure allowed for flame stabilization to occur in homogenous gas-phase combustion, a concept used by Shirsat and Gupta [9]. The combustor was fabricated from a 1.5 in (3.8 cm) square monolith alumina silicate block using conventional micro machining tools. The material has a thermal conductivity of 1.98 W/m K in its green state, and has a low porosity relative to other more thermally insulating materials, such as zirconium phosphate, which has a thermal conductivity of 0.8 W/m K [11]. Porosity is an important factor to consider, due to the possibility of altering the ratio of fuel to oxidizer within the combustion chamber via reactants diffusing into the reactor itself. Additionally, the low thermal conductivity of the material ensures large temperature gradients through the channel walls, enabling favorable heat transfer rates from products to reactants, as well as reduced heat loss from the combustion chamber to the surroundings, thus increasing the resistance to thermal quenching [12]. After machining, the combustor was heat treated for 45 min at 1050 °C at a ramp rate of 2 °C per minute. This process increased the hardness and strength of the alumina silicate, thus lessening the possibility of the material cracking due to thermal stresses. The catalysts were fabricated using an incipient wetness impregnation technique. Solutions of Rh(NO3)3 and Pt(NO3)4 were deposited onto yttria-stabilized zirconia (YSZ) felts with a porosity of 95% and thermal conductivity of approximately 0.09 W/m K. This extremely low thermal conductivity allowed for very steep
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Fig. 1. Schematic of experimental facility.
temperature gradients between the front and back faces of the catalyst. The felt was then calcinated at 700 °C for 16 h before insertion into the combustor.
combustion, propane and air were allowed to flow though the combustor and the system was heated until the propane autoignited on the catalyst. For the homogenous combustion experiments, a high voltage spark generator was used to ignite the fuel/air mixture via ignition electrodes located in the combustion chamber. Fig. 2 shows photographs of the heterogeneous and homogeneous combustors. The product gases from the combustor were then fed into a micro-gas chromatograph and a mass spectrometer for composition analysis. The extinction limits were determined by stabilizing a flame within the combustion chamber and varying the flow rates of oxygen, argon, and propane until the reaction could no longer be supported. To determine the effect of equivalence ratio on the combustion behavior, propane flow rate was held constant, and the flow rates of oxygen and argon were varied. For each operating condition examined, the system was allowed to reach steady state, as indicated by constant temperatures and product concentrations. The two primary parameters observed were propane conversion (to determine the effectiveness of the two catalysts) and CO2/CO selectivity, to determine the completeness of combustion. A kinetic analysis was also performed to determine the activation energy of each catalyst.
2.2. Experimental facility and procedure
3. Results and discussion
Fig. 1 shows a schematic diagram of the experimental facility. The experimental facility was designed to allow for efficient data acquisition on the extinction limits, thermal performance, and fuel conversion under both the heterogeneous (catalytic) and homogeneous (non-catalytic) operating conditions. Three thermal conductivity mass flow controllers were used to regulate the flow rates of oxygen, argon, and propane in the combustor. A mixture of 21% O2 and 79% Ar, simulating air, was used as an oxidizer. K-type thermocouples were used to monitor the inlet air temperature, mixture pre-heat temperature, product gas temperature (at the exit of the combustion chamber), and exhaust gas temperature. In addition, for the catalytic combustion experiments, the temperatures at the front and back faces of the catalysts were also monitored. Careful insertion of the thermocouples ensured that the tips did not protrude too far into the channels and disturb the flow. For the heterogeneous combustion experiments, the catalyst was placed directly into the combustion chamber, and blank YSZ felt was placed in the inlet channel to serve as a heat shield. To initiate
3.1. Extinction limits
Fig. 2. Photograph of (a) catalytic combustor and (b) non-catalytic combustor.
The measured extinction limits of the catalytic and non-catalytic combustors are shown in Fig. 3a and b, respectively. These limits were defined as the points at which combustion was no longer sustainable. This occurred when the flame was either blown
Fig. 3. Extinction limits for (a) catalytic combustors and (b) non-catalytic combustor.
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out (at the lean limits) or extinguished (at the rich limits) in the homogeneous experiments, and when the catalyst underwent a steep, steady drop in temperature in the heterogeneous experiments. In this study, the equivalence ratio, U, is defined as the actual carbon to oxygen (C/O) ratio divided by the stoichiometric C/O ratio:
U¼
ðC=OÞactual ðC=OÞstoich
ð1Þ
The catalysts were both able to sustain very lean combustion, down to an equivalence ratio of U = 0.593 for the Rh catalyst and U = 0.322 for Pt. The ability of the Pt catalyst to support such significantly leaner combustion than the Rh catalyst is likely due to its lower activation energy (13.8 kJ/mol for Pt vs. 74.7 kJ/mol for Rh). This will be further discussed in Section 3.3. No absolute rich limits were found for either catalyst, even at equivalence ratios in excess of U = 15, which is consistent with previously reported studies on catalytic microcombustion with propane and air [8]. However, for super-rich combustion (U > 3) the fuel conversion was extremely low and the exhaust gas temperature was low, indicating that the combustor could not be used for any practical purposes at these conditions. Fig. 3a shows the lean blowout limits for the Rh and Pt catalytic combustors. While the results obtained revealed that there is a relationship between total flow rate and blowout limit for the Rh catalyst (higher flow rates can sustain lower equivalence ratios), the same did not appear to be true for the Pt catalyst; for all flow rates studied the blowout limit was approximately constant. This difference in trends is likely due to the activation energies of the catalysts. As less fuel is added to the system (i.e. less input thermal energy), the overall temperature decreases due to the lower heat release from the chemical reactions. At low total flow rates, very low fuel flow rates must be used in order to achieve lean combustion. The Pt catalyst was able to sustain combustion at these low temperatures due to its relatively low activation energy. However, due to its higher activation energy, the Rh required a higher catalyst temperature to propagate the combustion reactions, and at low fuel flow rates the heat release was insufficient to sustain these higher temperatures. The non-catalytic combustor was found to have a much smaller operating range, as shown in Fig. 3b. Although it could sustain combustion down to a minimum equivalence ratio of 0.425, it could not support combustion at total flow rates above approximately 950 SCCM (cm3/min). This is consistent with previous studies, which showed that the use of a porous foam catalyst increases the maximum flow rate supportable by the combustor [23]. Additionally, for all flow rates examined, the non-catalytic combustor had relatively low rich extinction limits, reaching a maximum at an equivalence ratio of only 1.75.
Fig. 4. Propane conversion percentage in catalytic combustors.
exception of equivalence ratios below 0.8, the Rh catalyst delivered a significantly higher conversion than Pt, especially under fuel rich conditions, reaching a maximum of 90.4% at U = 1.0. However, under lean conditions, the conversion was found to have a very steep decline. It is conjectured that this is due to the shorter contact times at the higher flow rates that accompany lean combustion, which result in insufficient time for complete conversion of fuel. This implies that at these conditions mass transfer limitations for the combustion reaction on Rh may exist, as the higher flow rates may prevent the reactants from diffusing from the bulk flow to the catalyst surface. With the Pt catalyst, the conversion increased as equivalence ratio decreased, reaching a maximum of 61.4% at U = 0.66, the leanest condition studied and thus the point at which the most O2 was available to oxidize the propane. The selectivities of CO2 and CO were investigated to evaluate the completeness of combustion, as this is a good representative measure of catalyst performance. In addition to the experimental data, thermodynamic equilibrium product gas compositions were found using CHEMKIN software, and the equilibrium CO2 and CO
3.2. Exhaust gas analysis The fuel conversion, CO2/CO selectivity, and CO2/CO yield were calculated using:
%Conv ersion ¼
V_ C3 H8 ;in V_ C3 H8 ;out 100% V_ C H ;in 3
Selectiv ity of species k ¼
ð2Þ
8
NCout;k 100% NCout
Yield of species k ¼ Conv ersion Selectiv ity
ð3Þ ð4Þ
where V_ C3 H8 represents the volumetric flow rate of propane, and N Cout represents the total moles of carbon present in the combustion products. Fig. 4 shows the total propane conversion under heterogeneous conditions using the Rh and Pt catalysts. It is clear that, with the
Fig. 5. Product selectivity in catalytic combustors for (a) CO2 and (b) CO.
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selectivities calculated from these values. As seen in Fig. 5, the Pt catalyst had a considerably larger CO2 selectivity (and Rh a considerably larger CO selectivity) for all the conditions examined, with the exception of U < 0.8, where there was adequate excess O2 present to completely oxidize the fuel. As CO2 has a higher enthalpy of formation than CO, a higher CO2 selectivity is indicative of a higher percentage of enthalpy being directed into forming products, and thus is not lost to the environment. The general trend for both catalysts is the highest CO2 selectivities under lean conditions, decreasing to a minimum between 1.75 < U < 2, and then increasing again beyond this point. It is expected that the selectivities would be highest at lean operating conditions, as there is excess oxygen available to form CO2, but the reason for the increase above U = 2 is not as obvious. It is conjectured that this may be due to the longer contact times occurring with richer conditions that allow more time for the reaction to overcome the lack of excess oxygen. This requires substantiation and further examination. The equilibrium calculations did not take into account the catalytic effect on product selectivity, as evidenced by the discrepancies between the two data sets. As seen in Fig. 6a, the CO2 and CO selectivities from the Rh mimic the equilibrium trend fairly well up until U = 2, where the experimental trend diverges from the model, with the equilibrium calculations under-predicting the CO (and over-predicting the CO2). This over-production of CO is fairly typical behavior of Rh, as it has been found that, at operating temperatures similar to those in this study, Rh does not catalyze the water gas shift (WGS) reaction (Eq. (5)) very well [26].
H2 O þ CO ! H2 þ CO2
ð5Þ
Fig. 7. Product yield in catalytic combustors for (a) CO2, (b) CO, and (c) H2.
Fig. 6. Experimental (closed symbols) and equilibrium (open symbols) CO2 and CO selectivities for (a) Rh catalyst and (b) Pt catalyst.
The experimental data from the Pt catalyst, on the other hand, although exhibiting a similar trend, does not mimic the shape of the equilibrium curve. As shown in Fig. 6b, lean of U = 1, the CO selectivity is under-predicted and the CO2 selectivity over-predicted, and the opposite is true under fuel-rich conditions. Despite having a significantly lower CO2 selectivity, due to the superior conversion rates of the Rh catalyst, it had a slightly higher CO2 yield than the Pt at all examined conditions, as seen in Fig. 7a. This indicates that, in spite of having a higher rate of heat loss to the environment, there is an overall higher enthalpy in the exhaust from the Rh catalyst, which is reflected in the slightly higher temperatures recorded for virtually every operating condition examined. This was further validated by calculating the enthalpy in the exhaust for both catalysts (see Fig. 8). A higher amount of enthalpy present in the exhaust implies greater potential for use in both thrust production and power generation applications. The Rh catalyst also had significantly higher CO and H2 yields than the Pt (Fig. 7b and c), which suggests that a meso-scale combustion system with an Rh catalyst could be further optimized to produce
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Fig. 8. Exhaust enthalpy in catalytic combustors.
syngas, for such applications as fuel cells. Additionally, at the condition of maximum conversion (90% at U = 1), the Rh system paired with a low-temperature thermoelectric device can achieve a power density up to 70% higher than a lithium ion battery of equivalent mass. The data obtained here suggests that, in terms of conversion, selectivity, and yield, Rh is a more suitable choice than Pt for a meso-scale combustor. Conversion is particularly important when utilizing liquid fuels in a combustor of this scale, as the residence time is often not sufficient to fully combust the heavier hydrocarbons. Fig. 9. Arrhenius plots for (a) Rh catalyst and (b) Pt catalyst.
3.3. Kinetic analysis The rate constant governing steady-state combustion was calculated using:
r¼
1
vC H 3
8
d½C3 H8 dt
r ¼ k ½C3 H8 v C3 H8 ½O2 v O2
ð6Þ ð7Þ
where v C3 H8 and v O2 represent the stoichiometric coefficients of C3H8 and O2, respectively. As argon was a non-reacting species in these experiments, and since its concentration was very large compared to those of propane and oxygen, its contribution to the overall reaction rate was neglected. For non-stoichiometric mixtures of propane and air, the equivalence ratio, U, was used for v C3 H8 . The results were then plotted in an Arrhenius plot (Eq. (8)), shown in Fig. 9, and linear regression was used to determine the activation energies required for steady-state combustion. Ea
k ¼ A0 eRT
ð8Þ
As presented in the extinction limit analysis in Section 3.1, the activation energy of the Rh catalyst was relatively high (74.7 kJ/ mol) and the Pt catalyst relatively low (13.8 kJ/mol) for most of the conditions examined. However, as seen in Fig. 9b, the slope of the Arrhenius plot for Pt changes drastically at low temperatures (T < 500 °C), which corresponds to higher equivalence ratios (U P 1.75). For these low temperatures, the activation energy was calculated to be 211.4 kJ/mol. This drastic change signifies a transition from mass diffusion-limited reactions at lower equivalence ratios to chemical kinetics-limited reactions at higher equivalence ratios. It has been postulated that this transition with respect to equivalence ratio is due to the adsorption of O(s) onto the Pt surface, since at low temperatures O(s) desorption from the catalyst surface is slow, thus reducing the catalytic activity [27]. Additionally, at low temperatures, the high O(s) surface cov-
erage inhibits C3H8 adsorption to the catalyst surface; however, at fuel-rich conditions, the excess C3H8 is able to prevent complete O(s) surface coverage [8,22]. Thus, for these low-temperature, fuel-rich conditions, there is more competitive adsorption on the Pt surface between the C3H8 and the O2, leading to kinetically limited reactions. Furthermore, the calculated value of 211.4 kJ/mol is in excellent agreement with the activation energy of O(s) desorption from Pt at low temperatures (213.0 kJ/mol) provided in an established mechanism [28], further supporting that, at these conditions, the rates of the reactions occurring on the Pt surface are controlled by the chemical kinetics. However, for most operating conditions studied, the activation energy of the Pt catalyst was significantly lower than that of the Rh, indicating greater ease of ignition (i.e. less input energy) for the system with Pt catalyst. Based on the value of the Rh catalyst activation energy one can conjecture that the reactions occurring in the Rh combustor are chemical kinetics-limited. However, the steep drop in propane conversion at equivalence ratios lean of stoichiometry suggests a possible influence of mass transfer limitations. A closer examination of Fig. 9a reveals that it is not entirely clear from the data whether the reactions are entirely kinetics-limited, or if it is a combination of kinetic and diffusion limitations. A future study analyzing a much wider range of operating conditions may strive to address this question thus gaining further insights into the complex phenomena. 4. Conclusions A meso-scale, heat recirculating combustor was utilized to examine the gas phase and catalytic combustion behavior of propane. Rh and Pt catalysts were used. The results showed that the addition of a catalyst to the system greatly increased the stability limits, and completely eliminated the rich extinction limit up to the regions examined. The Pt catalyst was able to support signifi-
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cantly leaner combustion than the Rh catalyst at all the flow rates examined. The two catalysts were further compared to determine their effectiveness, combustion behavior, and activation energies. The results showed that, although having a considerably higher propane conversion rate for virtually all the equivalence ratios examined, the Rh catalyst had an overall poorer CO2 selectivity than Pt, indicating a higher rate of heat loss to the environment. However, due to its superior conversion, the CO2 yield from the Rh was higher than from the Pt, indicating higher overall output enthalpy. The calculated activation energies for each catalyst (74.7 kJ/mol for Rh and 13.8 kJ/mol for Pt) suggested that the combustion reactions on Rh were largely controlled by chemical reaction kinetics and on the Pt by diffusion limitations. However, at high equivalence ratios (U P 1.75), there is an abrupt change in the reaction kinetics on the Pt catalyst, likely due to the low-temperature competitive adsorption between C3H8 and O2 on the Pt surface, which signifies a transition to kinetically limited combustion. The results revealed that higher conversion and CO2 yield with the Rh catalyst make it a better choice for use of this system with a liquid fuel. This is in spite of the higher activation energy of the Rh; conversion is a more important parameter than activation energy to consider for these applications, as the residence time in a meso-scale combustor is often too short to fully combust the heavy hydrocarbons, and the energy required to vaporize the fuel is higher than that needed for catalytic ignition.
Acknowledgments Research support provided to Teresa Wierzbicki by the U.S. Army Research Laboratory is gratefully acknowledged. Thanks are also due to Dr. Vivek Shirsat for all his help and support in various stages of this research and in the design and development of heat recirculating micro-combustors.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Combustion of propane with Pt and Rh catalysts in a meso-scale heat recirculating combustor Teresa A. Wierzbicki a,b, Ivan C. Lee b, Ashwani K. Gupta a,⇑ a b
Department of Mechanical Engineering, University of Maryland, 2181 Martin Hall, Campus Dr., College Park, MD 20742, USA Sensors and Electron Devices Directorate, U.S. Army Research Laboratory, 2800 Powder Mill Rd., Adelphi, MD 20783, USA
h i g h l i g h t s Compared extinction limits of catalytic and non-catalytic combustor operation. Analyzed catalytic conversion, product selectivity/yield, and activation energy. Results used to predict the catalytic combustor performance with liquid fuel. Catalysts offered enhanced stable operation of combustor than without catalyst. Rh is more suitable for liquid fuel due to higher fuel conversion and output energy.
a r t i c l e
i n f o
Article history: Received 9 February 2014 Received in revised form 21 April 2014 Accepted 31 May 2014
Keywords: Catalytic combustion Heat-recirculating combustor Meso-scale combustion Propane oxidation Combustion behavior
a b s t r a c t The results obtained from the combustion behavior of propane over platinum and rhodium catalysts in a meso-scale heat recirculating combustor are presented. The extinction limits, conversion, product selectivity/yield, and activation energy using the two catalysts were compared in an effort to predict their performance using a liquid fuel. The extinction limits were also compared to those of non-catalytic combustion in the same combustor. The results showed that the use of a catalyst greatly expanded the range of stable operating conditions, in terms of both extinction limits and flow rates supported. The Rh catalyst was found to exhibit a higher propane conversion rate, reaching a maximum of 90.4% at stoichiometric conditions (as compared to only 61.4% offered by the Pt catalyst under lean conditions), but the Pt catalyst had superior CO2 selectivity for most of the examined conditions, indicating more of the heat released being used for product formation as opposed to being lost to the environment. However, despite having a higher rate of heat loss, the combustion with the Rh catalyst produced an overall higher amount of enthalpy than the Pt due to its superior fuel conversion. The Pt catalyst also had a significantly smaller activation energy (13.8 kJ/mol) than the Rh catalyst (74.7 kJ/mol), except at equivalence ratios richer than U = 1.75 (corresponding to catalyst temperatures below 500 °C), where it abruptly changed to 211.4 kJ/mol, signifying a transition from diffusion-limited reactions to kinetically limited reactions at this point. The results reveal that Rh would be a more suitable catalyst for use in liquid-fueled meso-scale combustors, as fuel conversion has been found to be a limiting factor for combustion stability in these systems, and as its higher output energy allows for greater flexibility of use. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Much interest in meso-scale combustion systems has been garnered of late due to the benefits they offer in terms of local power production, micro thrust, and micro propulsion generation. Hydrocarbon fuels have much higher specific energies than batteries, ⇑ Corresponding author. Tel.: +1 301 405 5276. E-mail addresses: [email protected], [email protected] (T.A. Wierzbicki), [email protected] (I.C. Lee), [email protected] (A.K. Gupta). http://dx.doi.org/10.1016/j.apenergy.2014.05.069 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.
implying that even devices with very low thermal-to-electric efficiencies can yield higher electrical outputs than batteries, and thus offer significant performance improvements in portable electronic devices, i.e. longer lifetime and reduced weight [1]. Liquid hydrocarbons are ideal for use in these devices, as they are very energy-dense and can be easily stored. Several studies have demonstrated the successful coupling of meso-scale combustion systems with thermoelectric/thermophotovoltaic devices to produce electric power [2–6]. However, there are some problems inherent to meso-scale combustion systems. When the system is scaled
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Nomenclature A0 C/O Ea k [k] NC,k NC,out r
pre-exponential factor carbon to oxygen ratio activation energy reaction rate constant concentration of species k total moles of carbon in species k total moles of carbon in combustion products reaction rate
down, the heat loss to heat generation (or surface area to volume) ratio increases, which can induce flame quenching as the system size is reduced. A prevalent technique to diminish this effect is to utilize a heat recirculating, swiss roll-style combustor, which allows heat to be transferred from the products to the reactants, thus permitting stable combustion in channels with a characteristic dimension smaller than the flame quenching distance. The preheating of reactants also broadens the range of stable operating conditions by extending the lean flammability limits of the fuel used [7]. Much research has been done to investigate the effects of geometry, scaling, operating conditions, and combustor material on the extinction limits and thermal performance of meso- and micro-scale swiss roll combustors [8–14]. By varying the geometry and inlet conditions (i.e. equivalence ratio and Reynolds number), meso-scale swiss roll combustors can be tailored to suit a wide range of both power generation and thrust applications [15,16]. Shirsat and Gupta [9] studied this concept in the context of thrust generation for such applications as nanosatellites, power generation, and small robotics, and reported that the optimal geometry for flame maintenance while minimizing heat loss and maximizing exhaust enthalpy was a single-turn combustor design. Another intrinsic difficulty in meso- and micro-combustion systems is the very short residence time within the combustion chamber. At such short residence times, complete combustion of heavier hydrocarbons, especially in blended fuels, may not always occur, resulting in significant soot formation and flame instabilities [15]. A possible solution to this problem is to utilize a catalyst in the system. Catalysts have been shown to help stabilize liquid fuel combustion at short residence times, even for petroleum-based jet fuels [17–19]. The high surface area to volume ratio that is characteristic of small-scale systems is also ideal for use with a catalyst, as increased surface area favors catalytic combustion by providing more sites upon which the fuel and oxidizer can adsorb. The addition of a catalyst has been shown in many studies to widen the range of stable operating conditions, lower the overall reactor temperatures, and increase the hydrocarbon oxidation rates [20–22]. Spadaccini et al. [23] found that the addition of a porous platinum catalyst to their microscale gas turbine allowed for an increase in operable mass flow rates through the system, leading to an 8.5-fold increase in power density over the maximum achieved for a non-catalytic system of similar geometry. Catalysts also play a key role in thermal management—lower overall temperatures result in smaller temperature gradients throughout the system, reducing the risk of thermal stresses. Also, since chemical reactions theoretically only occur on catalyst surfaces, the heat source location is fixed, unlike in homogenous combustion, where the flame location may change depending on operating conditions [8]. Due to this enhanced stability and improved thermal management, integration of catalytic micro- and meso-scale combustors with such energy-harnessing devices as thermoelectrics/thermophotovoltaics and steam reformers is conjectured to be much simpler than the corresponding non-catalytic burners [24].
R t T V_ k
mk U
universal gas constant time temperature volumetric flow rate of species k stoichiometric coefficient of species k equivalence ratio
The current study examines the impact of adding a porous foam catalyst to a single-turn swiss roll combustor burning a mixture of propane and air. Two different catalysts, rhodium and platinum, were investigated. The objective of this study is to analyze the role of Rh and Pt catalysts in terms of extinction limits (particularly how these limits compare to those of non-catalytic combustion in the same reactor), conversion, selectivity, and activation energy. Total oxidation of propane is used as a probe reaction in this study, since oxidation reactions are essential to maximizing stability and efficiency while minimizing emissions in power generation and thrust production applications [25]. The results will help guide in determining as to which catalyst would be most suitable for implementation in a similar combustor using liquid fuels.
2. Experimental 2.1. Combustor and catalyst A single turn heat recirculating combustor similar to that utilized by Shirsat and Gupta [9] and Wierzbicki et al. [15] was used. The fuel and the oxidizer were introduced to the combustion chamber partially premixed, with the fuel injected downstream from the oxidizer to prevent any flashback into the inlet reactant tubing. A step entrance to the combustion chamber was created to generate a vortical flow structure and local recirculation zone within the chamber, recirculating both heat and active species from the products to the reactants. The aerodynamics of this flow structure allowed for flame stabilization to occur in homogenous gas-phase combustion, a concept used by Shirsat and Gupta [9]. The combustor was fabricated from a 1.5 in (3.8 cm) square monolith alumina silicate block using conventional micro machining tools. The material has a thermal conductivity of 1.98 W/m K in its green state, and has a low porosity relative to other more thermally insulating materials, such as zirconium phosphate, which has a thermal conductivity of 0.8 W/m K [11]. Porosity is an important factor to consider, due to the possibility of altering the ratio of fuel to oxidizer within the combustion chamber via reactants diffusing into the reactor itself. Additionally, the low thermal conductivity of the material ensures large temperature gradients through the channel walls, enabling favorable heat transfer rates from products to reactants, as well as reduced heat loss from the combustion chamber to the surroundings, thus increasing the resistance to thermal quenching [12]. After machining, the combustor was heat treated for 45 min at 1050 °C at a ramp rate of 2 °C per minute. This process increased the hardness and strength of the alumina silicate, thus lessening the possibility of the material cracking due to thermal stresses. The catalysts were fabricated using an incipient wetness impregnation technique. Solutions of Rh(NO3)3 and Pt(NO3)4 were deposited onto yttria-stabilized zirconia (YSZ) felts with a porosity of 95% and thermal conductivity of approximately 0.09 W/m K. This extremely low thermal conductivity allowed for very steep
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Fig. 1. Schematic of experimental facility.
temperature gradients between the front and back faces of the catalyst. The felt was then calcinated at 700 °C for 16 h before insertion into the combustor.
combustion, propane and air were allowed to flow though the combustor and the system was heated until the propane autoignited on the catalyst. For the homogenous combustion experiments, a high voltage spark generator was used to ignite the fuel/air mixture via ignition electrodes located in the combustion chamber. Fig. 2 shows photographs of the heterogeneous and homogeneous combustors. The product gases from the combustor were then fed into a micro-gas chromatograph and a mass spectrometer for composition analysis. The extinction limits were determined by stabilizing a flame within the combustion chamber and varying the flow rates of oxygen, argon, and propane until the reaction could no longer be supported. To determine the effect of equivalence ratio on the combustion behavior, propane flow rate was held constant, and the flow rates of oxygen and argon were varied. For each operating condition examined, the system was allowed to reach steady state, as indicated by constant temperatures and product concentrations. The two primary parameters observed were propane conversion (to determine the effectiveness of the two catalysts) and CO2/CO selectivity, to determine the completeness of combustion. A kinetic analysis was also performed to determine the activation energy of each catalyst.
2.2. Experimental facility and procedure
3. Results and discussion
Fig. 1 shows a schematic diagram of the experimental facility. The experimental facility was designed to allow for efficient data acquisition on the extinction limits, thermal performance, and fuel conversion under both the heterogeneous (catalytic) and homogeneous (non-catalytic) operating conditions. Three thermal conductivity mass flow controllers were used to regulate the flow rates of oxygen, argon, and propane in the combustor. A mixture of 21% O2 and 79% Ar, simulating air, was used as an oxidizer. K-type thermocouples were used to monitor the inlet air temperature, mixture pre-heat temperature, product gas temperature (at the exit of the combustion chamber), and exhaust gas temperature. In addition, for the catalytic combustion experiments, the temperatures at the front and back faces of the catalysts were also monitored. Careful insertion of the thermocouples ensured that the tips did not protrude too far into the channels and disturb the flow. For the heterogeneous combustion experiments, the catalyst was placed directly into the combustion chamber, and blank YSZ felt was placed in the inlet channel to serve as a heat shield. To initiate
3.1. Extinction limits
Fig. 2. Photograph of (a) catalytic combustor and (b) non-catalytic combustor.
The measured extinction limits of the catalytic and non-catalytic combustors are shown in Fig. 3a and b, respectively. These limits were defined as the points at which combustion was no longer sustainable. This occurred when the flame was either blown
Fig. 3. Extinction limits for (a) catalytic combustors and (b) non-catalytic combustor.
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out (at the lean limits) or extinguished (at the rich limits) in the homogeneous experiments, and when the catalyst underwent a steep, steady drop in temperature in the heterogeneous experiments. In this study, the equivalence ratio, U, is defined as the actual carbon to oxygen (C/O) ratio divided by the stoichiometric C/O ratio:
U¼
ðC=OÞactual ðC=OÞstoich
ð1Þ
The catalysts were both able to sustain very lean combustion, down to an equivalence ratio of U = 0.593 for the Rh catalyst and U = 0.322 for Pt. The ability of the Pt catalyst to support such significantly leaner combustion than the Rh catalyst is likely due to its lower activation energy (13.8 kJ/mol for Pt vs. 74.7 kJ/mol for Rh). This will be further discussed in Section 3.3. No absolute rich limits were found for either catalyst, even at equivalence ratios in excess of U = 15, which is consistent with previously reported studies on catalytic microcombustion with propane and air [8]. However, for super-rich combustion (U > 3) the fuel conversion was extremely low and the exhaust gas temperature was low, indicating that the combustor could not be used for any practical purposes at these conditions. Fig. 3a shows the lean blowout limits for the Rh and Pt catalytic combustors. While the results obtained revealed that there is a relationship between total flow rate and blowout limit for the Rh catalyst (higher flow rates can sustain lower equivalence ratios), the same did not appear to be true for the Pt catalyst; for all flow rates studied the blowout limit was approximately constant. This difference in trends is likely due to the activation energies of the catalysts. As less fuel is added to the system (i.e. less input thermal energy), the overall temperature decreases due to the lower heat release from the chemical reactions. At low total flow rates, very low fuel flow rates must be used in order to achieve lean combustion. The Pt catalyst was able to sustain combustion at these low temperatures due to its relatively low activation energy. However, due to its higher activation energy, the Rh required a higher catalyst temperature to propagate the combustion reactions, and at low fuel flow rates the heat release was insufficient to sustain these higher temperatures. The non-catalytic combustor was found to have a much smaller operating range, as shown in Fig. 3b. Although it could sustain combustion down to a minimum equivalence ratio of 0.425, it could not support combustion at total flow rates above approximately 950 SCCM (cm3/min). This is consistent with previous studies, which showed that the use of a porous foam catalyst increases the maximum flow rate supportable by the combustor [23]. Additionally, for all flow rates examined, the non-catalytic combustor had relatively low rich extinction limits, reaching a maximum at an equivalence ratio of only 1.75.
Fig. 4. Propane conversion percentage in catalytic combustors.
exception of equivalence ratios below 0.8, the Rh catalyst delivered a significantly higher conversion than Pt, especially under fuel rich conditions, reaching a maximum of 90.4% at U = 1.0. However, under lean conditions, the conversion was found to have a very steep decline. It is conjectured that this is due to the shorter contact times at the higher flow rates that accompany lean combustion, which result in insufficient time for complete conversion of fuel. This implies that at these conditions mass transfer limitations for the combustion reaction on Rh may exist, as the higher flow rates may prevent the reactants from diffusing from the bulk flow to the catalyst surface. With the Pt catalyst, the conversion increased as equivalence ratio decreased, reaching a maximum of 61.4% at U = 0.66, the leanest condition studied and thus the point at which the most O2 was available to oxidize the propane. The selectivities of CO2 and CO were investigated to evaluate the completeness of combustion, as this is a good representative measure of catalyst performance. In addition to the experimental data, thermodynamic equilibrium product gas compositions were found using CHEMKIN software, and the equilibrium CO2 and CO
3.2. Exhaust gas analysis The fuel conversion, CO2/CO selectivity, and CO2/CO yield were calculated using:
%Conv ersion ¼
V_ C3 H8 ;in V_ C3 H8 ;out 100% V_ C H ;in 3
Selectiv ity of species k ¼
ð2Þ
8
NCout;k 100% NCout
Yield of species k ¼ Conv ersion Selectiv ity
ð3Þ ð4Þ
where V_ C3 H8 represents the volumetric flow rate of propane, and N Cout represents the total moles of carbon present in the combustion products. Fig. 4 shows the total propane conversion under heterogeneous conditions using the Rh and Pt catalysts. It is clear that, with the
Fig. 5. Product selectivity in catalytic combustors for (a) CO2 and (b) CO.
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selectivities calculated from these values. As seen in Fig. 5, the Pt catalyst had a considerably larger CO2 selectivity (and Rh a considerably larger CO selectivity) for all the conditions examined, with the exception of U < 0.8, where there was adequate excess O2 present to completely oxidize the fuel. As CO2 has a higher enthalpy of formation than CO, a higher CO2 selectivity is indicative of a higher percentage of enthalpy being directed into forming products, and thus is not lost to the environment. The general trend for both catalysts is the highest CO2 selectivities under lean conditions, decreasing to a minimum between 1.75 < U < 2, and then increasing again beyond this point. It is expected that the selectivities would be highest at lean operating conditions, as there is excess oxygen available to form CO2, but the reason for the increase above U = 2 is not as obvious. It is conjectured that this may be due to the longer contact times occurring with richer conditions that allow more time for the reaction to overcome the lack of excess oxygen. This requires substantiation and further examination. The equilibrium calculations did not take into account the catalytic effect on product selectivity, as evidenced by the discrepancies between the two data sets. As seen in Fig. 6a, the CO2 and CO selectivities from the Rh mimic the equilibrium trend fairly well up until U = 2, where the experimental trend diverges from the model, with the equilibrium calculations under-predicting the CO (and over-predicting the CO2). This over-production of CO is fairly typical behavior of Rh, as it has been found that, at operating temperatures similar to those in this study, Rh does not catalyze the water gas shift (WGS) reaction (Eq. (5)) very well [26].
H2 O þ CO ! H2 þ CO2
ð5Þ
Fig. 7. Product yield in catalytic combustors for (a) CO2, (b) CO, and (c) H2.
Fig. 6. Experimental (closed symbols) and equilibrium (open symbols) CO2 and CO selectivities for (a) Rh catalyst and (b) Pt catalyst.
The experimental data from the Pt catalyst, on the other hand, although exhibiting a similar trend, does not mimic the shape of the equilibrium curve. As shown in Fig. 6b, lean of U = 1, the CO selectivity is under-predicted and the CO2 selectivity over-predicted, and the opposite is true under fuel-rich conditions. Despite having a significantly lower CO2 selectivity, due to the superior conversion rates of the Rh catalyst, it had a slightly higher CO2 yield than the Pt at all examined conditions, as seen in Fig. 7a. This indicates that, in spite of having a higher rate of heat loss to the environment, there is an overall higher enthalpy in the exhaust from the Rh catalyst, which is reflected in the slightly higher temperatures recorded for virtually every operating condition examined. This was further validated by calculating the enthalpy in the exhaust for both catalysts (see Fig. 8). A higher amount of enthalpy present in the exhaust implies greater potential for use in both thrust production and power generation applications. The Rh catalyst also had significantly higher CO and H2 yields than the Pt (Fig. 7b and c), which suggests that a meso-scale combustion system with an Rh catalyst could be further optimized to produce
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Fig. 8. Exhaust enthalpy in catalytic combustors.
syngas, for such applications as fuel cells. Additionally, at the condition of maximum conversion (90% at U = 1), the Rh system paired with a low-temperature thermoelectric device can achieve a power density up to 70% higher than a lithium ion battery of equivalent mass. The data obtained here suggests that, in terms of conversion, selectivity, and yield, Rh is a more suitable choice than Pt for a meso-scale combustor. Conversion is particularly important when utilizing liquid fuels in a combustor of this scale, as the residence time is often not sufficient to fully combust the heavier hydrocarbons. Fig. 9. Arrhenius plots for (a) Rh catalyst and (b) Pt catalyst.
3.3. Kinetic analysis The rate constant governing steady-state combustion was calculated using:
r¼
1
vC H 3
8
d½C3 H8 dt
r ¼ k ½C3 H8 v C3 H8 ½O2 v O2
ð6Þ ð7Þ
where v C3 H8 and v O2 represent the stoichiometric coefficients of C3H8 and O2, respectively. As argon was a non-reacting species in these experiments, and since its concentration was very large compared to those of propane and oxygen, its contribution to the overall reaction rate was neglected. For non-stoichiometric mixtures of propane and air, the equivalence ratio, U, was used for v C3 H8 . The results were then plotted in an Arrhenius plot (Eq. (8)), shown in Fig. 9, and linear regression was used to determine the activation energies required for steady-state combustion. Ea
k ¼ A0 eRT
ð8Þ
As presented in the extinction limit analysis in Section 3.1, the activation energy of the Rh catalyst was relatively high (74.7 kJ/ mol) and the Pt catalyst relatively low (13.8 kJ/mol) for most of the conditions examined. However, as seen in Fig. 9b, the slope of the Arrhenius plot for Pt changes drastically at low temperatures (T < 500 °C), which corresponds to higher equivalence ratios (U P 1.75). For these low temperatures, the activation energy was calculated to be 211.4 kJ/mol. This drastic change signifies a transition from mass diffusion-limited reactions at lower equivalence ratios to chemical kinetics-limited reactions at higher equivalence ratios. It has been postulated that this transition with respect to equivalence ratio is due to the adsorption of O(s) onto the Pt surface, since at low temperatures O(s) desorption from the catalyst surface is slow, thus reducing the catalytic activity [27]. Additionally, at low temperatures, the high O(s) surface cov-
erage inhibits C3H8 adsorption to the catalyst surface; however, at fuel-rich conditions, the excess C3H8 is able to prevent complete O(s) surface coverage [8,22]. Thus, for these low-temperature, fuel-rich conditions, there is more competitive adsorption on the Pt surface between the C3H8 and the O2, leading to kinetically limited reactions. Furthermore, the calculated value of 211.4 kJ/mol is in excellent agreement with the activation energy of O(s) desorption from Pt at low temperatures (213.0 kJ/mol) provided in an established mechanism [28], further supporting that, at these conditions, the rates of the reactions occurring on the Pt surface are controlled by the chemical kinetics. However, for most operating conditions studied, the activation energy of the Pt catalyst was significantly lower than that of the Rh, indicating greater ease of ignition (i.e. less input energy) for the system with Pt catalyst. Based on the value of the Rh catalyst activation energy one can conjecture that the reactions occurring in the Rh combustor are chemical kinetics-limited. However, the steep drop in propane conversion at equivalence ratios lean of stoichiometry suggests a possible influence of mass transfer limitations. A closer examination of Fig. 9a reveals that it is not entirely clear from the data whether the reactions are entirely kinetics-limited, or if it is a combination of kinetic and diffusion limitations. A future study analyzing a much wider range of operating conditions may strive to address this question thus gaining further insights into the complex phenomena. 4. Conclusions A meso-scale, heat recirculating combustor was utilized to examine the gas phase and catalytic combustion behavior of propane. Rh and Pt catalysts were used. The results showed that the addition of a catalyst to the system greatly increased the stability limits, and completely eliminated the rich extinction limit up to the regions examined. The Pt catalyst was able to support signifi-
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cantly leaner combustion than the Rh catalyst at all the flow rates examined. The two catalysts were further compared to determine their effectiveness, combustion behavior, and activation energies. The results showed that, although having a considerably higher propane conversion rate for virtually all the equivalence ratios examined, the Rh catalyst had an overall poorer CO2 selectivity than Pt, indicating a higher rate of heat loss to the environment. However, due to its superior conversion, the CO2 yield from the Rh was higher than from the Pt, indicating higher overall output enthalpy. The calculated activation energies for each catalyst (74.7 kJ/mol for Rh and 13.8 kJ/mol for Pt) suggested that the combustion reactions on Rh were largely controlled by chemical reaction kinetics and on the Pt by diffusion limitations. However, at high equivalence ratios (U P 1.75), there is an abrupt change in the reaction kinetics on the Pt catalyst, likely due to the low-temperature competitive adsorption between C3H8 and O2 on the Pt surface, which signifies a transition to kinetically limited combustion. The results revealed that higher conversion and CO2 yield with the Rh catalyst make it a better choice for use of this system with a liquid fuel. This is in spite of the higher activation energy of the Rh; conversion is a more important parameter than activation energy to consider for these applications, as the residence time in a meso-scale combustor is often too short to fully combust the heavy hydrocarbons, and the energy required to vaporize the fuel is higher than that needed for catalytic ignition.
Acknowledgments Research support provided to Teresa Wierzbicki by the U.S. Army Research Laboratory is gratefully acknowledged. Thanks are also due to Dr. Vivek Shirsat for all his help and support in various stages of this research and in the design and development of heat recirculating micro-combustors.
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