Hydrogen production in ultrarich combustion of hydrocarbon fuels in porous media

international journal of hydrogen energy 34 (2009) 1818–1827

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Hydrogen production in ultrarich combustion of hydrocarbon fuels in porous media Mario Toledoa, Valeri Bubnovichb, Alexei Savelievc,*, Lawrence Kennedyd a

Department of Mechanical Engineering, Universidad Tecnica Federico Santa Maria, Av. Espan˜a 1680, Valparaiso, Chile Department of Chemical Engineering, Universidad de Santiago de Chile, B.O’Higgins 3363, Santiago, Chile c Department of Mechanical and Aerospace Engineering, North Carolina State University, 2601 Stinson Drive, Raleigh, NC 27695, USA d Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 West Taylor Street, Chicago, IL 60607, USA b

article info

abstract

Article history:

Rich and ultrarich combustion of methane, ethane, and propane inside inert porous media

Received 16 August 2007

is studied experimentally and numerically to examine the suitability of the concept for

Received in revised form

hydrogen production. Temperature, velocities, and chemical products of the combustion

11 November 2008

waves were recorded experimentally at a range of equivalence ratios from stoichiometry

Accepted 1 December 2008

(4 ¼ 1.0) to 4 ¼ 2.5, for a filtration velocity of 12 cm/s. Two-temperature numerical model

Available online 7 January 2009

based on comprehensive heat transfer and chemical mechanisms is found to be in a good qualitative agreement with experimental data. Partial oxidation products of methane,

Keywords:

ethane, and propane (H2, CO, and C2 hydrocarbons) are dominant for ultrarich super-

Hydrogen production

adiabatic combustion. The maximum hydrogen yield is close to 50% for all fuels, and

Partial oxidation

carbon monoxide yield is close to 80%.

Filtration combustion

ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

Porous media

1.

Introduction

Hydrogen is considered to be the fuel of the future. It contains more energy per unit mass than any other fuel. Hydrogen also generates minimum emissions when burned and essentially no emissions when electro-chemically converted to electricity in a fuel cell. For the projected demand in hydrogen, simpler and efficient processes of hydrogen generation from various feedstocks have to be engineered. One such solution is a superadiabatic partial oxidation of light hydrocarbons in porous media. Numerous studies describing combustion of gaseous fuels in porous media have been published during the past two decades [1–7]. Within this context, stationary and transient systems are the two major design approaches commonly

reserved.

employed in porous combustion. The first approach is widely used in radiant burners and surface combustor-heaters where the combustion zone is stabilized within the finite element of the porous matrix. The second (transient) approach involves a traveling wave representing unsteady combustion zone freely propagating in either downstream or upstream direction in the porous media. As a result of intensive interfacial heat transfer between the gas and the solid, observed combustion temperatures can differ significantly from the adiabatic values and are controlled mainly by the reaction chemistry and heat transfer mechanism. Low-velocity regime (LVR), high-velocity regime (HVR), sound velocity regime and other regimes are distinguished in literature [1,4] based on the velocity of the combustion front relative to solid phase and peculiarities of

* Corresponding author. Tel.: þ1 919 5155675; fax: þ1 919 5157968. E-mail address: [email protected] (A. Saveliev). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.12.001

international journal of hydrogen energy 34 (2009) 1818–1827

Nomenclature c d Dax F hk k _ M T v u Vk Wk Yk x

specific heat particle diameter axial dispersion coefficient radiation exchange factor specific enthalpy of species k thermal conductivity mass flow rate temperature interstitial velocity combustion wave propagation velocity diffusion velocity of species k molecular weight of species k mass fraction of species k axial coordinate

the heat and mass transfer in the system. Most experimental and modeling studies have been performed when flame propagation speeds are of the order of 104 m/s. These conditions correspond to the LVR of filtration gas combustion, according to classification given by Babkin [1]. On the other hand, when the heat exchange is rather weak, LVR converts into HVR where characteristic flame speed is w10 m/s. Compared to LVR, the more important factor in HVR is not thermal but aerodynamic interfacial interaction and its consequences. Superadiabatic filtration combustion of rich and ultrarich mixtures creates a situation in which partial oxidation and/or thermal cracking of hydrocarbons takes place [2]. For example, multiple studies have shown that conversion of methane to hydrogen occurs in these conditions and involves two stages where initial partial oxidation is followed by steam reforming. Partial oxidation of methane generates hydrogen, and heat generated in the partial oxidation can supply the energy for steam reforming, generating more hydrogen [5–7]. The vast majority of work in fuel reforming via filtration combustion has been done using the packed bed design. The modern applications of transient porous burners include combustion of low-calorific fuels [8], VOC emissions’ control with the possibility of heat recovery [9], and fuel reforming in the ultrarich superadiabatic flames [5,7,10]. The group at the University of Illinois at Chicago has conducted extensive research in transient filtration combustion and the production of hydrogen in packed bed reactors [11]. Recent work by the group utilizes a reciprocal flow burner (RFB) which periodically reverses the flow direction to restrict the propagating wave front to a fixed volume of aluminum oxide beads [5]. RFBs extend the ultrarich limit attainable by superadiabatic combustion to practically interesting range of equivalence ratios from 3.0 to 4.0. However, the operation of RBF is designspecific and does not bear the same generality as unrestricted combustion waves freely propagating in linear reactors. Thus, the studies of linear propagating combustion waves is of great interest and are comparable in significance with the studies of laminar flames in homogeneous combustion. Detailed studies were performed on hydrogen production in ultrarich filtration combustion of methane and hydrogen sulfide [10,12] and filtration combustion in oxygen-enriched

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Greek symbols 4 equivalence ratio b effective coefficient for heat exchange with the surroundings 3 bed void fraction (porosity) l effective thermal conductivity of the porous media r density u_ molar rate of reaction of species k Subscripts e effective g, s gas, solid k species index R radiant 0 initial

and oxygen-depleted environments [13]. The particularly interesting area of ultrarich filtration combustion of complex fuels remains unexplored. In this paper, the results on hydrogen production in rich and ultrarich filtration combustion of methane–air, ethane– air and propane–air mixtures are presented. Temperature profiles and combustion wave velocities were measured experimentally and predicted numerically, as were the chemical product compositions. Particular interest is to use the reactor to convert rich hydrocarbon mixtures to a syngas and various other compounds utilizing the concept of superadiabatic partial oxidation.

2.

Experimental apparatus

Experiments on filtration combustion were conducted using the setup schematically shown in Fig. 1. The apparatus consisted of a combustion tube filled with a porous medium, fuel and air supply system, temperature measurement system, and gas emission analyzers. Combustion tube with internal diameter of 38 mm, wall thickness of 2 mm, and length of 40 cm was made of quartz. The inner surface of the combustion tube was covered with a 2 mm layer of Fiberfrax insulation. To prevent heat losses and achieve quasi-uniform temperature profiles, additional 30 mm thick high-temperature insulation was applied on the external diameter of the reactor. The packed bed of 5.6 mm solid Al2O3 spheres was used as a porous medium having a porosity of w40%. The combustible mixtures of methane, ethane, and propane with air were prepared by a continuous flow method where the fuel and air flows were metered using a set of MKS mass flow controllers (Model 1179A, MKS Inc.). To ensure uniform gas composition the reactants were premixed in a mixing chamber. They were introduced in the reactor through the distribution grid at the reactor bottom. The exit of the reactor was open to atmosphere. During the experiments, the upstream propagating combustion wave was initiated at the reactor exit. The upstream propagation was recorded. As the wave reached the reactor bottom, the experimental conditions were altered and downstream propagating wave was recorded on the reverse run.

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the combustion wave [2]. The waves are also free of inclination instabilities typically observed in the large diameter reactors [14]. The volume-averaged model [15] is used to mathematically describe the filtration combustion. The flame zone is considered to be isobaric, steady, and one-dimensional. The velocity of the wave, u, is assumed to be at least three orders of magnitude less than interstitial velocity of the gas mixture, v. A two-temperature approximation was formulated to describe a fully developed steady combustion wave in a system of coordinates moving with the reaction zone [2]. The numerical model [2,16,17] describes gas and solid as two phases interacting via fluid dynamics and heat transfer. It should be noted that model is limited to describing LVR combustion. The governing equations, describing fully developed steady combustion wave in a system of coordinates moving with the reaction zone are (e.g., see [2,16,17]): Continuity equation: _ vðMÞ vð3rvÞ ¼ ¼0 vx vx

(1)

Gas phase energy equation:    ivT  X vTg v h g u_ k hk Wk ¼3 3 3cp rg v kg þ cp rg Dax vx vx vx k X   vTg  hv Tg  Ts 3 rg Yk Vk cpk vx k Fig. 1 – Schematic of the experimental setup.

Solid phase energy equation: ð1  3Þcs rs u

Running axially in the centre of the quartz tube was a ceramic shell of 0.32 cm in diameter containing six 0.08cm-diameter holes with S-type (platinum/rhodium) thermocouples. Voltage signals from these thermocouples were transferred to a PC where they were digitized using an Analog Instrument’s RTI-800 board. The thermocouple junctions were equally spaced along the length of the shell, with the bottom one being 13 cm away from the gas inlet. The junctions completely covered by ceramic material recorded temperatures very close to the temperatures of the solid phase. Axial positioning of thermocouples allowed minimal disturbances of gas flow and heat fluxes in the reaction zone. The experimental error in the temperature measurements was estimated as 50 K; the error in the wave velocity measurements performed based on displacement of thermal profile along the reactor length was w10%. Combustion products were quenched as they exit the reactor and, then, directed for chemical analysis to a gas chromatograph (Varian 3600 GC). A ceramic tube (alumina) connected to a vacuum line was used as a sample probe. To avoid exterior air vortices, the probe was inserted by w2 cm into the alumina bed. The accuracy of chemical sampling was close to 10%.

3.

Numerical model

Experimentally recorded negligible radial temperature gradients allow for a credible one-dimensional representation of

(2)

    vTs v vTs ¼  bðTs  T0 Þ  hv Tg  Ts ½ke þ kR  vx vx vx

(3)

Species conservation equation:  dYk d þ rg v r Yk Vk ¼ u_ k Wk dx dx g

(4)

Assuming thermal equilibrium between gas and solid at the limits of long computational domain, the following boundary conditions are imposed: At the inlet ðx ¼ 0Þ : Tg ¼ Ts ¼ T0 ;

Y ¼ Yk;0

At the exit ðx ¼ LÞ : vYk =vx ¼ 0; Tg ¼ Ts ¼ T; vT=vx   ¼ bðT  T0 Þ= 3cp rg v  ð1  3Þcs rs u

(5)

(6)

Downstream boundary condition for the wave temperature is imposed using an analytical solution of Eqs. (1)–(4). It is assumed that vYk =vx ¼ 0; Tg ¼ Ts ¼ T; u_ k ¼ 0 and radiation is negligible at the cold interface far from the reaction zone. This allows temperature and species profiles calculations for a combustion wave enclosed within a finite spatial domain (L ¼ 40 cm). The volumetric convective heat transfer coefficient, hv , is found as: hv ¼ ð63=d2 ÞNu kg . The correlation for Nu number is given by Wakao and Kaguei [18] as: Nu ¼ 2 þ 1:1Pr1=3 Re0:6 . Radiation is treated with radiant conductivity model [15], kR is represented as kR ¼ 4FsT3s , where F is the radiation exchange factor. In the considered conditions, F depends upon the value of the conductivity of the solid state (alumina). The values of F from 0.3 to 0.6 are used. The contribution of flow irregularities to the effective mass diffusion is described by the axial gas

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international journal of hydrogen energy 34 (2009) 1818–1827

A

2200

1800

1400

1000

B

2200

Temperature (K)

dispersion coefficient calculated as Dax ¼ 0:5dv [18]. According to a heat–mass transfer analogy the dispersive thermal diffusivity is the same as the mass diffusivity and the respective dispersion coefficients are equal. Similar to Henneke and Ellzey [16], the effective diffusion is represented as a sum of molecular diffusion and dispersion. Finally, the thermophysical properties, i.e. alumina conductivity, heat capacity, and radiative properties were obtained from published data [19] and verified against data of ceramic manufacturer (Coors, Inc.). Effective conductivity of the packed bed and its porosity were estimated as ke ¼ 0:005ks , 3 ¼ 0:4. The GRI 3.0 [20], an extensively validated chemical kinetic mechanism with inclusive NOx chemistry, was used along with the Chemkin [21] package of corresponding subroutines and databases. While this mechanism is not directly applicable to ultralean and ultrarich mixtures, it serves as a good first approximation for combustion mechanisms in these areas. It was also found that it adequately reproduces the combustion in ultralean region [2]. The calculations were performed for a given value of the interstitial velocity and the implemented numerical algorithm in the modified PREMIX [22] code was used to find the wave propagation velocity.

1800

1400

1000

4.

Results and discussion

Experimental data were collected for a filtration velocity of 12 cm/s in the range of equivalence ratios from stoichiometry (4 ¼ 1.0) to 4 ¼ 2.5. In the employed linear reactor configuration, the waves were extinguished at equivalence ratios higher than 2.5. The experiments were similar for three hydrocarbon fuels mixtures in procedure and apparatus. Solid phase temperatures as well as product compositions were measured, and the propagation rates were obtained from thermocouple traces.

4.1. Combustion wave temperature and propagation rate Downstream, upstream, and standing waves were observed for tested fuel–air mixtures, depending on experimental parameters, mainly the equivalence ratio. Fig. 2 shows the experimentally recorded and numerically predicted combustion temperatures of the solid. The calculated adiabatic temperatures are provided for reference. For methane–air mixtures (Fig. 2A) at 4 ¼ 1 experimental combustion temperature is w1350 K while adiabatic temperature is close to 2240 K. The combustion temperature rises to 1620 K at 4 ¼ 1.4 and remains practically constant for 4 > 1.4. The results of the numerical model show good qualitative agreement with the experimental data. Equivalence ratio 1.9 marks the crossing point where the combustion temperature begins to exceed the adiabatic temperature and combustion proceeds in the superadiabatic regime. For ethane–air combustion, experimental measurements and numerical predictions suggest the demarcation point for under and superadiabatic regions around 4 z 2.3 (Fig. 2B). Experimental data near the stoichiometry show large discrepancies (w300 K) between numerically predicted and

C

2200

1800

1400

1000

1

1.4

1.8

2.2

2.6

Equivalence Ratio Fig. 2 – Experimental data (symbols) and numerical predictions (lines) for peak solid temperatures in filtration combustion waves: (A) methane–air, (B) ethane–air, (C) propane–air. The calculated adiabatic temperatures are plotted for reference as dotted line.

experimentally measured combustion temperatures. The recorded experimental temperature is 1067 K at 4 ¼ 1.0 (w1200 K less than the adiabatic temperature). The temperature decreases to 1023 K at 4 ¼ 1.1. At the same time the predicted temperature of the solid is w1350 K. A practically constant maximum temperature value of 1470 K is observed at equivalence ratios above 4 ¼ 1.7. The temperature difference could be explained by inadequate chemical kinetic model used for ethane combustion. Filtration combustion and especially upstream wave propagation is very sensitive to the ignition temperature of the mixture. It is shown in a number of publications [23–26] that ethane ignition can occur through low-temperature

international journal of hydrogen energy 34 (2009) 1818–1827

mechanism preceding conventional high-temperature ignition. This signifies importance of low-temperature chemistry that is a presently omitted from GRI 3.0 Mech. Several detailed chemical kinetic models have been developed for ethane oxidation that is applicable over a wide range of temperatures and pressures [23–26]. The related low-temperature ignition effects in filtration combustion of ethane merit further experimental and modeling studies and are out of scope of the present work. Combustion temperatures of propane display behavior similar to methane (Fig. 2C). The demarcation point for under and superadiabatic regions is close to 4 z 2.2. The combustion temperature increases from w1280 K at 4 ¼ 1.0 (w1000 K less than the adiabatic temperature) to 1530 K near equivalence ratio of 1.6. From there on, combustion temperature remains practically independent of the propane content. The results of the numerical model for combustion temperatures show good agreement with the experimental data. For three fuel–air mixtures the regimes of under and superadibatic combustion also manifest themselves in the directions of the wave propagation. Fig. 3, separated in two parts by the heavy zero velocity line, shows the velocity of wave propagation as a function of equivalence ratio. For methane–air mixtures (Fig. 3A), the maximum experimental absolute velocity value of 0.0063 cm/s is observed for 4 ¼ 1.0. Upstream underadiabatic regime of propagation is observed experimentally for a range of equivalence ratios from 1.0 to 1.6 and a rich standing wave is realized at 4 z 1.6. Numerical simulations predict transition point close to 4 z 1.9. The downstream superadiabatic propagation occurs in the ultrarich region, where the propagation rate increases with methane content. The numerical deviation observed in the region of ultrarich mixtures is probably related to limited applicability of GRI 3.0 mechanism in this range. For ethane–air mixtures (Fig. 3B), high propagation velocity is observed experimentally in the range of equivalence ratios from 1.0 to 1.15 where the maximum absolute velocity value reaches 0.015 cm/s. Upstream underadiabatic regime of propagation is recorded in the range of equivalence ratios from 1.0 to 2.0, and a standing wave is observed at 4 z 2.0. The downstream superadiabatic propagation occurs in the ultrarich region, where the speed increases slowly with ethane content. The satisfactory agreement in predicting combustion temperatures and wave velocities is obtained for ethane in the ultrarich regions. The significant difference in the numerical and experimental results is observed near the stoichiometry. Here the measured combustion temperatures are lower by w300 K, and, as expected from the energy balance equations, the experimental propagation velocities are essentially higher (by w3 times). The observed discrepancy can be attributed to the limited applicability of the GRI 3.0 mechanism to combustion of ethane–air mixtures as discussed above. The experimental wave propagation rate, for propane–air mixtures, shows a maximum absolute value of 0.0098 cm/s in the direction opposite to the gas flow at 4 ¼ 1.0 (Fig. 3C). For comparison, the maximum concurrent value of 0.0019 cm/s is recorded at 4 ¼ 2.5. A rich standing wave is observed at 4 z 2.0 and downstream propagation occurs in the ultrarich region, where the propagation rate increases with propane content. The satisfactory agreement in predicting wave velocities is

A 0

-0.01

-0.02

Wave Propagation Rate (cm/s)

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B 0

-0.01

-0.02

C 0

-0.01

-0.02

1

1.4

1.8

2.2

2.6

Equivalence Ratio Fig. 3 – Experimental data (symbols) and numerical predictions (lines) for propagation rates of fuel–air filtration combustion waves: (A) methane, (B) ethane, (C) propane.

obtained in this region. The difference in the numerical and experimental results is observed near the stoichiometry (Fig. 3C) where the experimental propagation velocities are essentially higher (by w2 times). As in the case of ethane, the observed discrepancy could be explained by the limitations of the employed kinetic model.

4.2.

Combustion products

Starting from equivalence ratios above 1.0, complete combustion cannot be achieved due to the insufficient oxygen content of the mixtures. The numerical and experimental results for concentrations of major combustion products (CO, H2, CO2, CH4) are presented in Fig. 4. The numerically predicted trends show good agreement with the experimental results for methane–air mixtures (Fig. 4A). In particular, the concentrations of hydrogen, carbon monoxide, and methane are increased with equivalence ratio and concentration of CO2 is reduced. The concentration of H2 and CO are underpredicted by the model especially on the boundary of the rich flammability limit (4 z 1.6). The predicted concentrations of CO2 agree well with the experiment. The leftover CH4 is overpredicted in comparison with the experimental data.

international journal of hydrogen energy 34 (2009) 1818–1827

20 H2

A

CO CO2

15

CH4

10

5

0 20

Major Product Mole Fraction, %

B 15

10

5

0 20

C 15

10

5

0

1

1.4

1.8

2.2

2.6

Equivalence Ratio Fig. 4 – Major combustion products of rich and ultrarich mixtures of (A) methane, (B) ethane, and (C) propane with air: lines – model, symbols – experiment.

The improvement of the model accuracy for prediction of chemical products is closely related to improvement of chemical kinetic mechanism describing combustion in these rich and ultrarich conditions. The major combustion products for ethane–air mixtures display a behavior similar to methane combustion both for

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model and experiment (Fig. 4B). The results show increase of H2 and CO and decrease of CO2 with increase of the equivalence ratio. The methane concentration is predicted with a good accuracy. The concentration of H2 and CO are underpredicted by the model especially at the upper range of the rich flammability limit (4 z 1.6). For propane–air mixtures the concentrations of CO2 and H2O decrease and partial oxidation products such as H2, CO (Fig. 4C) appear in the exhaust gases. H2 and CO become dominant products for superadiabatic combustion waves at 4 > 2.0. The numerical results for concentrations of major combustion products (CO, H2, CO2, CH4) show good agreement with the experimental results. While the concentration of hydrogen, carbon monoxide and methane are increased with equivalence ratio, the concentration of CO2 is reduced. The concentrations of H2 and CO are underpredicted as in the previous cases. The predicted concentrations of CO2 and methane agree well with the experiment. Essential concentrations of C2 hydrocarbons appear in the exhaust gases (Fig. 5) at high equivalence ratios for all studied fuels. Ethylene and acetylene appear in the exhaust gases at equivalence ratio above 1.7 for methane–air mixtures (Fig. 5A). For ethane–air mixtures, CH4 mole fraction increased from w0.2% to over w1.7%, C2H2 from w0.4% to over 2% and C2H4 from w0.06% to over 0.4%, at 4 ¼ 1.3 and 4 ¼ 2.3, respectively (Fig. 5B). Ethane concentration in the products is small. At 4 ¼ 1.9, the mole fraction of C2H6 detected is w0.05%. Unreacted C3H8 is measured in the products of propane–air mixtures starting from 4 ¼ 1.5 (Fig. 5C) where recorded temperatures are greater than 1500 K. The measured concentrations of C2H4 and C2H6 could be as high as 0.2%. In fact, the ultrarich superadiabatic wave is capable of inducing chemical transformation of methane in the mixtures with very small oxygen concentrations. Thus, the process could be characterized as a fuel reformation or modification rather than combustion. For example, Fig. 6A shows that the degree of methane conversion to hydrogen could be as high as 40% according to the experimental data and as high as 30% according to numerical model. The numerical model predicts the overall trend of hydrogen yield quite well, but gives hydrogen conversion values that are lower by w10%. Very close hydrogen yield values are predicted for ethane and propane (Fig. 6B and C). These values are slightly lower than hydrogen yields predicted for supeadiabatic methane flames (Fig. 6A). The experimental data indicate abrupt drop of hydrogen conversion near the wave extinction limits. These drops are observed for methane (Fig. 6A) and propane (Fig. 6C) near the equivalence ratio 2.35. Experimental and numerical data on carbon monoxide yield are shown in Fig. 7. In practical systems carbon monoxide can be further converted to hydrogen using watergas-shift reaction. Experimental carbon monoxide yield for methane reaches w80% at 4 z 1.5 and then reduces to 60% at higher equivalence ratios. The similar experimental trends are observed for propane and ethane. This reduction is due mainly to reduced oxygen content available for carbon monoxide formation at higher equivalence ratios. Numerical model underpredicts carbon monoxide yield for all studied fuels by w20%. However, it qualitatively predicts maximum

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1

50

A

A C2H2 C2H4 C2H6

0.8

40

[H2]/2[CH4]0,

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B 40

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[H2]/3[C2H6]0,

Minor Product Mole Fraction, %

30

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0

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C

C2H2 C2H4 C2H6 C3H8

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[H2]/4[C3H8]0,

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10

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1.4

1.8

2.2

2.6

Equivalence Ratio

0

1

1.4

1.8

2.2

2.6

Equivalence Ratio Fig. 5 – Experimentally measured minor combustion products for (A) methane, (B) ethane, and (C) propane mixtures with air.

yield of CO in the range of equivalence ratios from 1.4 to 1.6 and its further reduction at higher equivalence ratios (Fig. 7). The numerical results on combustion temperatures and hydrogen yields for a range of filtration velocities from 12 to 100 cm/s are presented in Fig. 8 for equivalence ratio 2.2. The increase of reactant velocity leads to increase of combustion

Fig. 6 – Numerical (solid line) and experimental (dashed lines and symbols) results on degree of conversion to hydrogen for rich and ultrarich waves for (A) methane, (B) ethane, and (C) propane mixtures with air.

temperatures. The highest temperatures are reached during methane combustion (Fig. 8A). Combustion temperature of w1750 K corresponds to a filtration velocity of 12 cm/s of methane–air mixture; this temperature is increased to 1950 K as filtration velocity increases to 100 cm/s. The combustion

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100

2000

A

A CH4 1900

Temperature, K

[CO]/[CH4]0,

80

60

40

1800 C2H6 C3H8

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45

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B

80 CH4 60

C3H8

H2 Yield,

[CO]/2[C2H6]0,

40

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35

C2H6

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0

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70

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C 80

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CO Yield,

[CO]/3[C3H8]0

CH4

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40 20

0

1

1.4

1.8

2.2

2.6

Equivalence Ratio

30

0

20

40

60

80

100

Filtration Velocity, cm/s

Fig. 7 – Numerical (solid lines) and experimental (dashed lines and symbols) results on degree of conversion to carbon monoxide for rich and ultrarich waves for (A) methane, (B) ethane, and (C) propane mixtures with air.

Fig. 8 – Numerical predictions for combustion temperatures, hydrogen and carbon monoxide yields obtained with variation of filtration velocity for methane, ethane, and propane mixtures with air. Equivalence ratio – 2.2.

temperature of propane is lower by w80 K. The lowest combustion temperature corresponds to ethane. The similar trend of temperature increase by w200 K is observed for all fuels in the studied velocity range.

The hydrogen yield increases with an increase of filtration velocity (Fig. 8B). For methane, the hydrogen yield rises from 36 to 44% when velocity changes from 12 to 100 cm/s. The yield increase can be attributed to higher combustion temperatures

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and accelerated kinetic of secondary equilibration reactions. This effect is counterbalanced by lower residence times corresponding to higher reactant velocities. The predicted hydrogen yields for propane and ethane are close. If compared to methane they are lower by more than 5%. Similar temperature dependences are observed as hydrogen yields grow by w4% in the range of filtration velocities studied. The model also predicts the slight increase of carbon monoxide yield with filtration velocity for all studied fuels, as shown in Fig. 8C. A variety of hydrocarbon fuels can be presently used in practical combustion applications. For example, natural gas compositions typically contain various concentrations of C1– C3 hydrocarbons. This suggests that further experimental and numerical studies of porous combustion of C1–C3 hydrocarbons and their mixtures are practically important. The possible extension of this work will be experimental studies on porous combustion of characteristic hydrocarbon mixtures typical for various sources such as gasification, biogasification, natural gas suppliers and refineries. The numerical model describing the porous combustion of C1–C3 hydrocarbons and their mixtures have to adopt comprehensive chemical mechanisms that are validated in comparison with experimental data.

conversion to H2 reaches 50%, and carbon monoxide yield reaches 80% for all fuels studied. The hydrogen yield increases with equivalence ratio up to the limit corresponding to flame extinction at 4 z 2.4. The carbon monoxide yield peaks at 4 z 1.5 and then slightly reduces at higher equivalence ratios. According to numerical model the conversion to hydrogen and CO can be further increased with an increase in filtration velocity. Considerable amounts of CH4, C2H2 and C2H4 were measured in the combustion products of methane, ethane and propane at higher equivalence ratios. The results show that ultrarich combustion inside a porous medium can be used to reform C1–C3 gaseous fuels into hydrogen and syngas.

Acknowledgments The authors wish to acknowledge the support by the CONICYT-Chile (FONDECYT 1050241 and FONDECYT 11080106) and by Academia Polite´cnica Aerona´utica de FACH, Chile.

references

5.

Conclusions

The hydrogen production using filtration combustion waves in an inert porous media are studied experimentally and numerically for methane–air, ethane–air and propane–air mixtures at the range of equivalence ratios from stoichiometry (4 ¼ 1.0) to 4 ¼ 2.5. The studied range covers the superadiabatic and underadiabatic combustion waves formed in the rich and ultrarich region. Downstream (superadiabatic) wave propagation was observed for ultrarich (4 > 1.6) methane–air mixtures. Upstream (underadiabatic) propagation corresponds to the range of equivalence ratios from 1.0 to 1.6. For ethane–air and propane–air mixtures, downstream (superadiabatic) wave propagation was observed for ultrarich (4 > 2.0) mixtures. Upstream (underadiabatic) propagation corresponds to the range of equivalence ratios from 1.0 to w2.0. For ethane–air mixtures, the essential drop of combustion temperature and increase of the propagation rate is observed in the range of equivalence ratios from 1.0 to 1.15. This variation is attributed to the peculiarities of low-temperature kinetic of ethane combustion. The maximum combustion temperature is observed for the ultrarich mixtures (4  1.7) where it is practically independent of equivalence ratio. For propane–air mixtures, a maximum absolute velocity value of w0.01 m/s is observed at 4 ¼ 1.0. The maximum combustion temperature is reached at the ultrarich mixtures (4  1.6). In rich and ultrarich mixtures complete combustion could not be achieved due to the low oxygen content in the mixture. As a result, the partial oxidation products, such as H2, CO, and C2 hydrocarbons, are formed. The maximum generated hydrogen concentrations are 15%, 14%, and 13% for methane, ethane, and propane flames, respectively. The corresponding

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