Article pubs.acs.org/EF
Ultrarich Filtration Combustion of Ethane Mario Toledo,*,† Khriscia Utria,‡ and Alexei V. Saveliev§ †
Department of Mechanical Engineering, Universidad Tecnica Federico Santa Maria, Avenida España 1680, Valparaiso 2340000, Chile ‡ Department of Mechanical Engineering, Universidad Autónoma del Caribe, Calle 90 No. 46-112, Barranquilla, Colombia § Department of Mechanical and Aerospace Engineering, North Carolina State University, 911 Oval Drive, Raleigh, North Carolina 27695, United States ABSTRACT: Ultrarich filtration combustion of ethane is studied in a porous medium composed of alumina spheres with the aim to achieve optimized conversion to hydrogen and syngas. Temperature, velocities, and chemical products of the combustion waves are recorded experimentally in a range of equivalence ratios ϕ from stoichiometry (ϕ = 1) to ϕ = 2.5. Experimental and numerical results are reported for 3.5 mm diameter alumina spheres; an oxygen content of the oxidizer is varied from 21 to 30%. Two temperature models based on GRI-MECH 3.0 and San Diego-MECH demonstrated good agreement with the experimental results. The maximum hydrogen concentration of 19.3% was recorded at ϕ = 2.5 and a 30% oxygen content in the oxidizer.
1. INTRODUCTION World reliance on oil and petroleum products is coming to an end. New clean and renewable energy sources, such as biofuels and hydrogen, are extensively explored. Hydrogen is considered to be a clean fuel, because it generates the lowest pollutant emissions when burnt and no emissions when converted into electricity in a fuel cell. Furthermore, it has more energy per unit mass than any other fuel. One way to generate hydrogen involves a partial oxidation of a hydrocarbon fuel in an inert porous medium (IPM).1−3 In particular, ethane is an intermediate product of the oxidation of many other hydrocarbons, including propene, propane, 1-butene, and n-butane. It is also an important intermediate in combustion of lighter hydrocarbons. Combustion in IPM has been studied focusing on stationary and transient systems.3−10 The stationary systems have been used principally for radiant burners and combustion heaters. Transient combustion in IPM is the process of propagation of the chemical exothermic reaction zone in a gas filtered through a solid chemically inert. The inert porous medium effectively participates in the process. Three characteristic zones can be identified. In the first zone, the reactant gases are mixed naturally and preheated by the heat transfer from the heated porous medium. This zone is located before the flame front. The second zone represents the chemical reaction zone, which can move in the direction of the gas flow or against it. Here, an enthalpy is released during the exothermic reaction of fuel oxidation in the gas phase. The third zone behind of the flame front contains the combustion products actively exchanging the heat with the porous medium through convection. In this zone, the energy is absorbed by the solid phase. The stored energy can be carried back to the reaction zone by the wave movement. As a result, the heat regeneration in the system creates an excess enthalpy in the reaction front and can increase the combustion temperature above the adiabatic temperature of the mixture. Toledo et al.3 performed experimental and numerical studies of rich and ultrarich combustion of methane, ethane, and © 2014 American Chemical Society
propane inside inert porous media to examine the suitability of the concept for hydrogen production. Temperature, velocities, and chemical products of the combustion waves were recorded experimentally at a range of equivalence ratios from stoichiometry (ϕ =1.0) to ϕ = 2.5, for 5.6 mm diameter alumina spheres and a filtration velocity of 12 cm/s. Discrepancies were found between numerical and experimental results, especially for ethane filtration combustion. Therefore, the principal objective of this research is to study the combustion of ethane−oxygen-enriched air mixtures in IPM at a low-velocity regime stabilized in a packed bed formed by 3.5 mm diameter spheres of alumina ceramic. To maximize the hydrogen production, the oxygen content in the oxidizer was varied from 21 to 30%. Temperature profiles and combustion wave velocities were measured experimentally and predicted numerically as well as the chemical product compositions.
2. EXPERIMENTAL SECTION Experiments on ethane combustion in IPM were conducted using a setup schematically shown in Figure 1. The setup consisted of a combustion tube filled with a porous medium, ethane, air, and oxygen supply system, flow control system, temperature measurement system, and gas chromatograph. The quartz combustion tube had an internal diameter of 41 mm, a wall thickness of 6 mm, and a length of 42 cm. To protect the quartz tube, the inner surface of the combustion tube was covered with a 4 mm layer of Fiberfrax insulation. To avoid heat losses and achieve quasi-uniform temperature profiles, a 30 mm thick layer of high-temperature insulation was applied on the external diameter of the reactor. The porous medium was packed with 3.5 mm solid Al2O3 spheres, resulting in a porosity of ∼40%. Combustible mixtures of ethane with oxygen-enriched air were set up by a continuous flow method, where the fuel, oxygen, and air flows were metered using a set of Aalborg mass flow controllers. Before entering the combustion tube, the reactants were premixed in a mixing chamber to ensure uniform gas composition. Then, the mixture was introduced Received: November 16, 2013 Revised: January 28, 2014 Published: January 28, 2014 1536
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Hunters et al.13 developed a reaction mechanism to describe the experimental data at relatively low temperatures of ∼950 K and elevated pressures (3−10 atm). The mechanism included 277 elementary reactions and 47 species and was based on the GRI-MECH 1.1 and an additional set of reactions for C2 species. The resultant reaction mechanism properly predicted the fuel consumption rate and the evolution of the intermediate species as a function of the reaction time. A detailed kinetic model of ethane oxidation was developed by Naik et al.14 The mechanism incorporated the results of recent studies of the important low-temperature pathways of the C2H5 + O2 reaction as well as several C2H6 + RO2 abstraction reactions. These results significantly alter the nature of the chain-branching reactions during ethane oxidation. The mechanism was validated against experimental data for various fuel compositions, temperatures, pressures, and equivalence ratios. Numerical modeling of ethane oxidation with separate cool flame ignition stages (with chain branching via ethyl hydroperoxide, methyl hydroperoxide, and partial hydrogen peroxide decomposition) preceding the hot flame (with chain branching of hydrogen peroxide decomposition alone) was performed.15 GRI-MECH 3.011 and San Diego-MECH12 were used in this work. GRI-MECH 3.0 is developed to model natural gas combustion and NOx formation pathways. This mechanism includes a set of reactions responsible for ethane oxidation. However, GRI-MECH 3.0 is not optimized for ethane combustion and do not consider all pathways discussed in the other studies.13−15 Prince and Williams12 developed San Diego-MECH, where the original detailed mechanism of 235 elementary reactions was revised and augmented slightly to reproduce the two-stage ignition and negative temperature coefficient (NTC) behavior seen experimentally for propane and ethane below 1000 K.
Figure 1. Schematic of the experimental setup. Dimensions are given in centimeters. through a distribution grid at the reactor bottom. The exit of the reactor was open to the atmosphere. Up- and downstream wave propagation runs were recorded during the experiments. Upstream runs were initiated at the reactor exit. As the wave reached the reactor bottom, the experimental conditions were changed to obtain a downstream propagating wave that was recorded on the reverse run. A ceramic shell of 0.5 cm in diameter was positioned axially in the combustion tube. It contained 0.08 cm diameter holes with five S-type (platinum/rhodium) thermocouples fabricated by OMEGA. The voltages measured by the thermocouples were recorded by an OMB-DAQ-54 acquisition module and converted by the Personal DaqView software. The thermocouple junctions were spaced 4 cm apart along the length of the shell. The first thermocouple junction was located at 16 cm below the reactor exit. The thermocouples were completely covered by the ceramic shell, allowing for the recording of temperatures very close to solid-phase temperatures. The axial position of the thermocouples allowed for minimal disturbances of the gas flow and heat fluxes in the reaction zone. The experimental error in the temperature measurements was estimated as 10 K; the error in the wave velocity measurements performed on the basis of displacement of the thermal profile along the reactor length was ∼10%. To measure the exhaust gas composition, a PerkinElmer gas chromatograph, model CLARUS 500, was used. The gas sampling was made through a Pyrex tube inserted 3 cm deep in the porous media at the reactor exit. The other end of the tube was connected to the chromatograph via a nylon tube with an inner diameter of 5 mm and a length of 2 m. The suction sampling was performed through a vacuum pump connected to the exhaust port of the gas chromatograph. The sampling was always performed when the flame front was at the third thermocouple; each sample was collected for 3 min.
4. RESULTS AND DISCUSSION Experimental studies were performed for the porous medium formed by 3.5 mm diameter alumina spheres and the oxygen content in the oxidizer equal to 21, 25, and 30%. The equivalence ratio was varied from 1.0 to 2.5, keeping the flow rate of the mixture constant at 6 L/min. Combustion temperatures (averaged maximum temperatures recorded by each thermocouple) and flame propagation rates were measured as well as the product compositions. 4.1. Temperatures, Velocities, and Products of Ethane−Air Combustion Waves. Dependent upon experimental parameters, mostly the equivalence ratio, downstream, upstream, and standing waves were observed for tested ethane−air mixtures. Up- or downstream displacements of a combustion wave relative to the inert solid lead to positive or negative enthalpy fluxes to the unburned mixture. As a result, the combustion temperature is no longer defined by the adiabatic combustion temperature. It is affected by the free displacement of the wave and controlled mainly by the kinetic and heat-transfer mechanisms. An upstream wave propagation results in the negative enthalpy flux and underadiabatic combustion temperature. Conversely, a downstream propagating wave generates the positive enthalpy flux and superadiabatic combustion temperature. The superadiabatic effect allows for self-sustained burning of mixtures with compositions very far from stoichiometry in downstream propagating waves. Figure 2 shows the experimentally recorded combustion temperatures of the solid and wave velocities. Temperatures for 3.5 mm pellets
3. MODEL AND REACTION MECHANISM The numerical model by Toledo et al.3 was used to describe gas and solid as two phases interacting via fluid dynamics and heat transfer. The calculations were performed for a given value of the interstitial velocity, and the numerical algorithm implemented in the modified PREMIX code was used to find the wave propagation velocity. Numerical data at equivalence ratios from 1 to 2.5 were obtained using two reaction mechanisms: GRI-MECH 3.011 and San Diego-MECH.12 Previous studies on ethane combustion13−15 have been performed with the objectives: (i) to examine the effect of the pressure and (ii) to develop detailed reaction mechanisms. 1537
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the bed of 3.5 mm spheres using both mechanisms predicted the wave velocities with sufficient accuracy. This result is attributed to the larger heat-transfer area and, hence, the higher combustion temperatures recorded at the 3.5 mm bed. Hydrogen concentrations above 10% were measured at the reactor exit (Figure 3). Maximum H2 and CO concentrations
Figure 2. Experimental data (□) and numerical predictions (lines) for (A) temperatures and (B) velocities of ethane−air filtration combustion waves propagating in a bed of 3.5 mm solid alumina spheres. Experimental data (●) for a packed bed of 5.6 mm solid alumina spheres.3 Adiabatic combustion temperature Tad as a function of the equivalence ratio is shown for reference.
Figure 3. Experimental results (symbols) and numerical predictions of combustion products using GRI-MECH 3.0 (dashed lines) and San Diego-MECH (dotted lines).
(14.3 and 18%, respectively) were recorded at ϕ = 2.5. CH4 generation maximized near ϕ = 2.5 at 2.7%. Experimental and numerical data show that the concentrations of hydrogen, carbon monoxide, and methane increased with the equivalence ratio and the concentration of CO2 was reduced. Numerically predicted trends agree well for GRI-MECH 3.0 and San DiegoMECH. However, numerical results underpredicted H2 and CO concentrations measured experimentally. 4.2. Effect of the Oxygen Enrichment. Oxygen enrichment showed almost no effect on combustion temperatures, as shown in Figure 4A. The highest temperature was 1407 K at ϕ = 2.3 with 30% O2. The lowest temperature was 1260 K at ϕ = 1.0 with 25% O2. Temperature differences recorded for any given equivalence ratio were lower than ∼20 K. The oxygen enrichment resulted in an essential increase of the adiabatic temperatures, as shown in Figure 4A. An energy balance of a filtration combustion wave requires a variation of the wave velocity (u) according to the following equation:5
were higher than the temperatures recorded for 5.6 mm pellets.3 The most significant differences can be found in a range of equivalence ratios from ϕ = 1.0 to 1.4. The maximum temperature difference was ∼380 K at ϕ = 1.0. The temperature discrepancies between the model and experiment were becoming smaller for higher equivalence ratios, where the low-temperature ignition of ethane is suppressed because of the low oxygen content. The differences were negligible at equivalence ratios above ϕ = 1.6. Maximum and minimum temperatures were 1381 and 1290 K at ϕ = 2.5 and 1.0, respectively. Numerically predicted temperatures for both mechanisms (Figure 2A) demonstrated good agreement with the experimental data in the range of equivalence ratios studied. Filtration combustion is very sensitive to ignition temperature, especially during upstream wave propagation.5 Ethane ignition can happen through a low-temperature mechanism preceding conventional high-temperature ignition.2,4,8,9 This low-temperature chemistry is omitted in GRI-MECH 3.0. Essential disagreement between model and experiment was observed for the 5.6 mm packed bed.3 However, the packed bed of 3.5 mm alumina spheres has the larger heat-transfer area, and hence, the higher temperatures were obtained for ethane−air mixtures. As a result, a satisfactory agreement (ΔT ≤ 50 K) was observed between the experimental data and the numerical results based on GRI-MECH 3.0. San Diego-MECH is optimized for conditions relevant to free flames in the temperature range of 650−1000 K and also shows good agreement with the experimental data. Figure 2B shows the wave velocities recorded during the experiments with 3.5 mm spheres. The fastest velocity of the upstream wave was −0.005 cm/s at ϕ = 1.0. The standing wave was observed at ϕ = 2.1. Waves propagated downstream for higher equivalence ratios. Numerical modeling performed for
u/vt=1 − ΔTad /ΔTc
(1)
where vt is the velocity of the thermal wave, ΔTad is the adiabatic temperature rise, and ΔTc is the combustion temperature rise. Equation 1 is not directly applicable to rich mixtures; however, it could be used for qualitative description of velocity dependence from the equivalence ratio. With a constant combustion temperature, a velocity of the superadiabatic wave will be first reduced and then changed to the downstream velocity with an increase of the adiabatic temperature. For example, the superadiabatic waves propagating upstream were observed for the case of 21% oxygen content in the oxidizer at the equivalence ratios higher than 2.3. Oxygen enrichment resulted in the increase of the adiabatic temperatures and transition from superadiabatic upstream waves to 1538
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Figure 4. (A) Experimental combustion temperatures and (B) velocities recorded with oxygen contents of 21, 25, and 30%.
underadiabatic downstream waves at this range of equivalence ratios. The propagation velocities changed significantly with the oxygen enrichment (Figure 4B). The highest velocities were −0.0102 cm/s at ϕ = 1.0 with 30% O2, −0.0076 cm/s with 25% O2, and −0.005 cm/s with 21% O2. Standing waves were observed at ϕ = 2.1 with 21% O2 and at ϕ = 2.5 with 25% O2. The standing wave was not observed at 30% O2 in the range of equivalence ratios tested. Experimental results on major combustion products (H2, CO, CH4, and CO2) are shown in Figure 5. Major product concentrations changed significantly with the O2 enrichment. The H2 concentration in the exhaust gases increases with the O2 increase (Figure 5A). Maximum hydrogen concentrations recorded at ϕ = 2.5 were 19.3, 16.9, and 14.3% at oxygen contents of 30, 25, and 21%, respectively. It means that the hydrogen concentration in the exhaust gases increased at a rate of ∼0.6% H2/O2. CO generation (Figure 5B) reveals the same trend of increasing with oxygen enrichment. The maximum CO concentration was 24.3% with 30% O2, 21% with 25% O2, and 18% with 21% O2 at ϕ = 2.5. CH4 generation behaved similarly, with the maximum near an equivalence ratio of 2.5 at 2.7% for 21% O2, 2.8% for 25% O2, and 2.7% for 30% O2. The resulting differences were insignificant, as shown in Figure 5C. CO2 decreased to 0 for 21 and 25% O2 and to 2.6% for 30% O2 (Figure 5D). For 21 and 25% O2, CO2 concentrations were practically the same at high equivalence ratios. A difference of ∼2% was observed for 30% O2. 4.3. Hydrogen and Carbon Monoxide (CO) Yields. Conversions of C2H6 to H2 and CO are shown in Figure 6. From ϕ = 1.0, an increase in the equivalence ratio results in an increase in H2 and CO yields up to a maximum at ϕ ≈ 1.6 and then a decrease for higher equivalence ratios. The initial increase can be attributed to the increase in the ethane content and direct formation of H2 and CO through combustion mechanisms. The drop in the CO yield at high equivalence ratios is associated with the reduction in the oxygen content. At
Figure 5. Experimentally measured combustion products for oxygen contents of 21, 25, and 30%: (A) H2, (B) CO, (C) CH4, and (D) CO2.
the same time, the H2 yield remains practically constant. A further increase in H2 and CO production in ultrarich waves is limited by slow secondary reactions of formed water vapors with unburned hydrocarbons. Yields of H2 and CO did not change significantly with O2 enrichment. For H2, the maximum yields were 40% at ϕ = 2.3 for 21% O2, 37% at ϕ = 2.3 for 25% O2, and 35% at ϕ = 2.5 for 30% O2. For CO, the yields were 81% at ϕ = 1.9 and 78.8 and 66.4% at ϕ = 2.5. It means that the best hydrogen yields were achieved with 21% O2.
5. CONCLUSION Ultrarich ethane−air combustion in IPM with a pellet diameter of 3.5 mm was studied numerically and experimentally in the range of equivalence ratios from stoichiometry (ϕ = 1.0) to ϕ = 2.5. The studied range covers the super- and underadiabatic combustion waves formed in the rich and ultrarich regions. For 1539
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REFERENCES
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Figure 6. Yields of hydrogen and carbon monoxide at oxygen contents of 21, 25, and 30%.
air−ethane mixtures, downstream (superadiabatic) wave propagation was observed for ultrarich combustion waves at equivalence ratios higher than ϕ > 2.1. Upstream (underadiabatic) propagation was observed at the range of equivalence ratios from 1.0 to 2.1. The highest absolute velocity of wave propagation in IPM was 0.005 cm/s at ϕ = 1.0. The oxygen enrichment and corresponding increase of adiabatic combustion temperatures shift the areas of up- and downstream wave propagation to the higher equivalence ratios. The upstream wave propagation range was observed at equivalence ratios from 1.0 to 2.5 with 25% O2. No downstream wave propagation was observed with 30% O2. The maximum absolute velocity was 0.01 cm/s at ϕ = 1.0 with 3.5 mm bed and 30% O2. The maximum temperature of 1360 K was reached at an equivalence ratio of 2.5. An increase of the oxygen content to 30% resulted in a decrease of the maximum temperature to 1400 K at ϕ = 2.3. In rich and ultrarich mixtures, complete combustion could not be achieved because of the low oxygen content in the mixture. As a result, the partial oxidation products, such as H2, CO, and C 2 hydrocarbons, are formed. The highest concentration of H2 in the products was 19.3% with 30% O2. The highest corresponding conversion of ethane to H2 reaches 55%, and the carbon monoxide yield reaches 40%, with 21% O2. It was found that the hydrogen concentration increases with the equivalence ratio. The same trends were observed for carbon monoxide and methane.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the support by the CONICYT-Chile (FONDECYT 1121188) and DGIP-UTFSM. 1540
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