Article pubs.acs.org/EF
Elementary Reaction Modeling and Experimental Characterization on Methane Partial Oxidation within a Catalyst-Enhanced Porous Media Combustor Yuqing Wang,†,‡ Hongyu Zeng,† Aayan Banerjee,‡ Yixiang Shi,*,† Olaf Deutchmann,*,‡ and Ningsheng Cai† †
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China ‡ Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstraße 20, Karlsruhe 76131, Germany S Supporting Information *
ABSTRACT: In this study, the fuel-rich combustion of methane in a two-layer porous media burner consisting of dense alumina pellets of different diameters was investigated experimentally and numerically. For a fixed inlet gas velocity of 0.15 m/s, methanerich flames were stabilized near the interface of two layers for equivalence ratios from 1.4 to 1.6. It was found that 40% of the methane was converted to syngas at the equivalence ratio of 1.6 using a reforming efficiency based on low heating values. To further increase the hydrogen yield and make the burner more suitable for applications in fuel cells, a portion of the downstream layer was coated with 0.08 wt % Ni catalyst. The reforming efficiency of methane to hydrogen increased from 18.2% to 23.9% after the catalytic enhancement. A combined homogeneous and heterogeneous elementary reaction mechanism was developed for methane partial oxidation in the porous media burner with catalytic enhancement. A one-dimensional model was explored by coupling the combined mechanism with heat-transport and mass-transport processes within the burner. The modeled temperature profiles and gas compositions showed good agreement with the experimental results. The model is demonstrated to be a useful tool for understanding the reaction processes within the burner and for burner design optimization. The nickel catalyst mainly promoted the water−gas shift reaction, and the heterogeneous reactions were dominant in the region where the catalyst was loaded. The burner design was optimized by studying the effects of the pellet diameter, layer length, and catalyst loading on the reforming efficiencies.
1. INTRODUCTION Hydrogen-based technologies, such as fuel cells, have been developed rapidly in recent years because of their high efficiencies and low pollution levels compared to fossil-fueldependent and coal-dependent power generation. At present, the most widely used method of hydrogen production is the steam reforming of methane.1 However, steam reforming is a highly endothermic reaction, requiring an efficient external heat source and additional feeds such as steam, and it is therefore not suitable for small-scale applications. In contrast to steam reforming, the exothermic partial oxidation (POX) process has a good dynamic response time and does not need external heat or water. Consequently, interest in small-scale fuel cells applying partial oxidation processes has increased.2−4 One such partial oxidation process is the fuel-rich combustion of methane in porous media burners. The combustion process transfers energy through porous media to preheat the incoming reactants and increase the flame stability under fuel-rich conditions. Research in this area can be classified into two categories: (1) filtration combustion5−7 and (2) stationary combustion.8,9 In the filtration combustion technique, transient combustion zones usually propagate within the porous media, although under certain operating conditions, stationary combustion can also be achieved. In the stationary technique, a two-layer porous media burner can support a © XXXX American Chemical Society
stationary combustion zone near the interface of the two layers. A disadvantage of transient combustion compared to stationary combustion is that, because of the propagating reaction zone, the porous media are subjected to a higher number of thermal cycles that and periodic restarting or reversal of the flow is required when the reaction zone ultimately reaches the end of the reactor.10 In the stationary technique, an upstream layer with a smaller pore size can act as a stabilizing holder for the flame and prevent flashback.11 Pedersen-Mjaanes et al.9 studied the steady fuel-rich combustion of methanol, methane, isooctane, and petrol in a two-layer porous media burner. In their study, fuel-rich flames of various fuels were stabilized between two differently sized pored porous media layers over a range of equivalence ratios. Both ceramic foams and Al2O3 beads were used as porous media with methanol, and the conversion efficiency of methanol using the bead burner was higher than that using the foam burner. For the case of methane, the reforming efficiency reached 45% at an equivalence ratio of 1.85. However, the contents of CH4 and CO increased as the equivalence ratio increased because of finite-rate kinetic effects. As mentioned above, because of its Received: July 6, 2016 Revised: August 23, 2016
A
DOI: 10.1021/acs.energyfuels.6b01624 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 1. Schematic of the experimental setup. (a) Overview of the main parts of the experimental system. (b) Detailed dimensions of the porous media burner. and they remained spherical after a few firing cycles without any signs of deterioration. The internal diameter of the burner was 54 mm, and the length was 200 mm. A 30-mm-thick ceramic insulation layer surrounded the outside of the burner for added insulation. Seven Stype thermocouples were mounted at 10-mm intervals for temperature measurements. All temperatures were corrected for radiation effects.14 A sampling probe was used to sample the combustion products at the burner outlet. The combustion species were measured using a gas chromatograph (GC) with a thermal conductivity detector (TCD). 2.2. Parameter Definitions and Uncertainty Analysis. The equivalence ratio ϕ is defined as
quick startup and compact size, one of the main practical applications of the porous media burner is to act as the fuel partial oxidation reformer in a fuel cell system. Recently, the porous media burner has been applied in a flame fuel cell in which the fuel-rich flame provided heat and fuels (H2 and CO) for the solid oxide fuel cell.12 However, the contents of CH4 and CO in the rich exhausts can lead to carbon deposition in fuel cells as a result of the cracking of CH4 and the Boudouard reaction: CH4 ⇄ C + H2, 2CO ⇄ CO2 + C. The deposited carbon will further degrade the fuel cell performance. To further increase the H2 yield for application in fuel cells, a portion of the downstream Al2O3 pellets were coated with Ni catalyst in this study. Although a numerical study has been carried out for methane partial oxidation in two-layer porous media burners,13 the catalytic effects on this process and the interactions between the homogeneous and heterogeneous reactions have not been studied before. To that end, a combined approach of experiment and modeling was undertaken to investigate the reforming of methane to syngas through a fuel-rich flame stabilized two-layer packed-bed burner with catalyst enhancement. An elementary reaction model was developed that couples the homogeneous and heterogeneous reactions with the mass-transport and heattransport processes within the burner. The numerical results were validated against the experimental results for various fuel/ air equivalence ratios. The effects of the catalyst on the flame temperature and species compositions, as well as the interactions between the homogeneous and heterogeneous reactions, were studied. Finally, the validated model was used for burner design optimization by studying the effects of the pellet diameter, the length of each porous layer, and the catalyst loading on the flame temperature and species composition.
ϕ=
̇ /Vair ̇ VCH 4 ̇ stoich /Vair ̇ stoich VCH
= 9.52
4
̇ VCH 4 ̇ Vair
(1)
where V̇ CH4 and V̇ air are the volumetric flow rates of the fuel and air, respectively, which were obtained from the mass flow controllers. The inlet gas velocity is calculated as
u in =
̇ + Vair ̇ VCH Viṅ 4 = A A
(2)
where A is the cross-sectional area of the flow path. The uncertainty of the equivalence ratio15 can be calculated as
Eϕ =
⎛ ∂ϕ ⎞2 ̇ ⎟ + ⎜ δ V CH ̇ 4⎠ ⎝ ∂VCH4
(
∂ϕ ̇ ∂Vair
2
)
̇ δVair
ϕ
(3)
The uncertainty of the volume flow rate was less than ±1%, and the calculated value of Eϕ was less than ±1.3% for the experimental conditions in this study. The uncertainty of the inlet velocity can be calculated as
Euin =
2. EXPERIMENTS
(
∂u in ∂Viṅ
2
) +(
δViṅ
∂u in ∂A
2
δA
)
u in
(4)
The calculated value of Euin was is less than ±1.1% for the experimental conditions in this study. It should be noted that only gas temperatures were measured because bare thermocouple junctions were installed in the voids of the porous media. All gas temperatures were corrected for radiation effects as follows
2.1. Experimental Setup and Measurement Techniques. Figure 1 shows a schematic of the experimental setup. Methane and air were regulated by mass flow controllers and flowed into a premix chamber. Then, the premixed mixture flowed into a chamber filled with 1−2-mm quartz sand for flashback protection. Finally, the premixed gases flowed through two layers of porous media. The two layers of porous media were surrounded by an inner insulation tube of aluminum silicate. The inner diameter of the tube was 30 mm, which means that the diameter of the flow path was 30 mm. The upstream layer consisted of a 20-mm-long packed bed of 2−3-mm Al2O3 pellets, and the downstream layer consisted of a 60-mm-long packed bed of 5mm Al2O3 pellets. All pellets were solid spheres without micropores,
Tg = Tj + ΔT = Tj +
σεj(Tj 4 − Tw 4) hc
(5)
where Tg is the corrected temperature, σ is the Stefan−Boltzmann constant, εj is the emissivity of the probe emissivity, Tj is the thermocouple junction temperature, Tw is the wall temperature, and hc B
DOI: 10.1021/acs.energyfuels.6b01624 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels is the convective heat-transfer coefficient. The uncertainty of the temperature is ET =
Continuity equation ∂(ερg )
(δTj)2 + (∂ΔT )2 T
∂t
(6)
∂(ερg u)
+
∂x
Ng
= a v ∑ si̇Wi
(7)
i=1
where ρg is the gas density. ε is the porosity of the porous media and was calculated as ε = 0.375 + 0.34d/D,16 for particle diameter d and reactor diameter D. av is the ratio of the active catalytic area to the volume, Ng is the number of gas-phase species, ṡi is the molar rate of production of species i by surface reaction, and Wi is the molecular mass of species i.
Because the temperature uncertainty was analyzed in a previous work,12 the details are not included here. The calculated uncertainty in the reported temperature was less than ±2%. The uncertainty in GC measurements were less than ±5%, except for CO because of the overlap between the peaks of CO and N2, as discussed in detail in section 4.2. 2.3. Experimental Procedure. A methane/air mixture with an equivalence ratio of 0.8 and an inlet gas velocity of 0.15 m/s was ignited by a spark torch igniter at the burner’s exit to allow the flame to propagate into the porous media. After the initial ignition, the flow rates were set to the desired values for the experiment. After several minutes, the system reached thermal equilibrium. Then, the outlet products were sampled and injected into GC.
Momentum equation ∂(ερg u) ∂t
+
∂(ερg uu) ∂x
= −ε
∂(ετ ) ∂P + +F ∂x ∂x
(8)
∂u
where τ = μg ∂x is the viscosity stress tensor17 and F = −[150(1 − ε)2μg/(d2ε2) + 1.75ρg(1 − ε)|u|/(dε)]u is the pressure loss term given by the Ergun equation. Gas energy equation
3. MODEL DEVELOPMENT 3.1. Model Assumptions and Geometry. A onedimensional model was developed to better understand the experimental results. Figure 2 shows the calculation domains and boundaries. The computational domain was discretized into 160 uniform grid nodes to ensure that the problem was accurately resolved.
ε
∂(cgρg Tg) ∂t
+ε
∂(cgρg uTg) ∂x
Nr,g
=ε
∂ ⎛ ∂Tg ⎞ ⎜λ g ⎟ ∂x ⎝ ∂x ⎠
Nr,s
+ ε ∑ ωihiWi + a v ∑ sihiWi − hv (Tg − Ts) i=1
i=1
(9)
where cg is the specific heat of the gas mixture, λg is the thermal N conductivity of the gas mixture, ∑i =r,g1 ωihiW is the heat release N
of the gas-phase chemical reactions, and a v ∑i =r,s1 sihiWi is the heat release of the surface reactions. hv is the volumetric convective heat-transfer coefficient and can be determined by hv = (6ε/d2)Nuλg.18 Nu is the Nusselt number, and the correlation for Nu is given by Nu = 2 + 1.1Pr1/3Re0.6.19 Solid energy equation (1 − ε)
∂(csρs Ts) ∂t
=
∂T ⎞ ∂ ⎛ ⎜λeff‐s s ⎟ + hv (Tg − Ts) ∂x ⎝ ∂x ⎠ − β(Ts − T0)
(10)
where cs is the specific heat of Al2O3, which was taken as 920 J/ (kg K), and ρs is the density of Al2O3, which was taken as 3707 kg/m3.20 λeff‑s is the effective thermal conductivity of the porous media and can be written as λeff‑s = (1−ε)λs + λrad. λrad is the radiative conductivity of alumina pellets and is described by the Rosseland approximation. 18 The heat losses to the surroundings were described as −β(Ts − T0),6 where β = 3100 W/(m3 K) was estimated by considering the heat conduction through the insulation layers and natural convection to the surrounding air.
Figure 2. Model domains and boundaries of the two-layer porous media burner.
The following assumptions were made in the model: (1) (2) (3) (4)
Gases were assumed to be incompressible ideal gases. The porous media were homogeneous and isotropic. Gas radiation was neglected. The porous media were optically thick (the optimal thickness was τopt ≥ 13.5 for the packed bed in this study), and solid-phase radiation was taken into account using the Rosseland approximation. 3.2. Governing Equations. The following governing equations for mass, momentum, gas energy, solid energy, and species were solved:
Species transport equation ε
∂(ρg Yi ) ∂t
+ε
∂(ρg uYi ) ∂x
=ε
∂ (ρ YV i i ) + Wi (εωi + a v si) ∂x g (11) 21
where Vi is the diffusion velocity for species i. This system of differential equations was closed with the ideal-gas equation of state,
P = ρg RTg C
(12) DOI: 10.1021/acs.energyfuels.6b01624 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels 3.3. Boundary Conditions. The following boundary conditions are considered in the model: Inlet, gas phase u = u in ,
Yi = Yi ,in ,
Tg = 300 K
Inlet, solid phase λs,eff
∂Ts = −ξσ(Ts,in 4 − T0 4) ∂x
Outlet, gas phase ∂Tg ∂Y ∂u =0 = i = ∂x ∂x ∂x
Outlet, solid phase λs,eff
Figure 3. Temperature distribution and heat budget in porous media combustion.
∂Ts = −ξσ(Ts,out 4 − T0 4) ∂x
convection. The difference between the gas and solid temperatures was less than 100 K except in the flame zone, highlighting the strong impact of porous media on heat transfer. 4.2. Combustion Products and Reforming Efficiency. The main products of methane fuel-rich combustion are shown in Figure 4. It can be seen that more H2 and CO were present
3.4. Chemistry. A combined homogeneous and heterogeneous reaction mechanism was developed based on the gasphase methane partial oxidation mechanism22 and the heterogeneous reactions on the Ni catalytic surface23 for modeling the chemistry of methane partial oxidation in the porous media burner with catalytic enhancement (see Table S1, Supporting Information). The combined mechanism involves 17 gas-phase species, 14 surface species, 58 elementary homogeneous reactions, and 52 elementary heterogeneous reactions. The gas-phase and surface chemistries are coupled by adsorption and desorption reactions with the net molar production rate of species given by mass action kinetics.24 3.5. Solution Method. The model was solved using ANSYS Fluent, with chemical reaction mechanisms imported from CHEMKIN.24 Transient simulations were carried out with the time interval Δt = 1 ms. Steady solutions were acquired by running the computations with enough time steps to reach a time-independent solution. Initially, a hightemperature zone of 1500 K was patched for ignition.25
4. RESULTS AND DISCUSSION 4.1. Temperature Distribution. The modeled gas and solid temperature distributions inside the burner were compared with the experimental results obtained at an equivalence ratio of 1.6 and an inlet gas velocity of 0.15 m/s, as shown in Figure 3. It can be seen from this figure that the simulated profiles match reasonably well with the experimental values. Figure 3 also shows the heat budget in the porous media burner. It should be noted that the energy was divided by the maximum heat release to obtain the dimensionless energy shown in Figure 3. The region of highest temperature and the location of the maximum heat release by combustion were both near the interface, which shows that the flame was stabilized near the interface of the two layers of the porous media, which was located at 0.02 m. In the upstream layer, the solid temperature was higher than the gas temperature because of the solid conduction and radiation from the downstream layer. As a result, the premixed gases were convectively preheated by the solid in the upstream layer. In the upstream region, the gas− solid convection and the solid radiation were both important, with radiation becoming dominant at high temperatures. The gas temperature increased rapidly near the reaction region and exceeded the solid temperature in the downstream layer. Consequently, heat was transferred from gas to solid through
Figure 4. Mole fractions of major species for various equivalence ratios.
in the combustion products when the equivalence ratio was increased from 1.4 to 1.6. It should be noted that, because the peak of CO overlapped with that of N2 in the GC measurements, there was a relatively large error in the experimental results on the CO composition. The overlap of the peaks led to a positive error in the measurement of the CO composition, which is related to the ratio between the peak area of CO and the peak area of N2.26 The positive error could reach 13% for the CO composition measured in this study. To address this issue, the measured CO composition was revised following a check of the mass balance. Both the modeling and experimental results showed that more CO than H2 was produced in the range of equivalence ratios studied. The differences between the exact values of the experimental and simulation results are likely due to both uncertainties in the measurements and inaccuracies in the model caused by the model assumptions. The measured reforming efficiency of methane to syngas and hydrogen were calculated as follows D
DOI: 10.1021/acs.energyfuels.6b01624 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels ηc = =
chemical energy of H2 and CO input chemical energy of CH4 YH2,out LHVH2 + YCO,out LHVCO
ηc‐h =
YCH4,in LHVCH4
chemical energy of H2 input chemical energy of CH4
(13)
=
YH2,out LHVH2 YCH4,in LHVCH4 (14)
where LHVCH4 = 50184.5 kJ/kg, LHVCO = 10125.1 kJ/kg, and LHVH2 = 120.1 MJ/kg. Figure 5 shows the reforming Figure 6. Illustration of the porous media burner with catalytic enhancement.
Although not shown here, the modeled temperature distribution varied little, with only a small increase in the gasphase temperature after the loading of Ni, which is in agreement with the experimental results. The reason for this behavior is that the heat source of the heterogeneous reactions is negligible compared to that of the homogeneous reactions, as shown in Figure 7. The modeled species compositions matched
Figure 5. Reforming efficiencies of methane for various equivalence ratios.
efficiencies for various equivalence ratios tested in the porous media burner. The reforming efficiency to hydrogen increased linearly from 15.0% to 18.2% as the equivalence ratio was increased from 1.4 to 1.6. The reforming efficiency of methane to syngas reached 40.0% at the equivalence ratio of 1.6, which is comparable to the value obtained at an equivalence ratio of 1.85 in a previous study.8 4.3. Effects of Catalytic Enhancement. As discussed in section 4.2, as the equivalence ratio was increased, the amount of unreacted CH4 also increased. Moreover, more CO than H2 was present in the products. Both the unreacted CH4 and the produced CO can lead to carbon deposition in fuel cells, which will further degrade the performance. To further increase the H2 yield for applications in fuel cells, a portion of the downstream Al2O3 pellets were coated with Ni catalyst, as shown in Figure 6. The pellets were dip-coated with nickel by wet impregnation with an acidic aqueous solution of Ni2(NO3)2·5H2O. Subsequently, the catalyst was dried at 400 K for 3 h, calcinated in oxygen at 1073 K for 3 h, and reduced by H2 at 723 K for 17 h. The loading of Ni was approximately 0.08 wt %. The ratio of the active catalytic area to the volume, av, was calculated as m 1 1 a v = D Ni Ni WNi Γ Vbed (15)
Figure 7. Heat release of reactions.
well with the experimental results for the equivalence ratio of 1.6 and inlet gas velocity of 0.15 m/s, as shown in Figure 8. Both the predicted and experimental results showed that the
where DNi is the catalyst dispersion, which was determined experimentally by chemisorption measurements; mNi is the catalyst loading (g); WNi is the molar mass of nickel; Vbed is the volume of the catalytic bed; and Γ = 2.66 × 10−5 mol/m2 is the surface-site density.23 A value of av = 1.5 × 104 m−1 was calculated for the catalyst used in this study.
Figure 8. Mole fractions of major species without and with catalytic enhancement. For experimants labeled With Catalyst_mod1, only heterogeneous reactions were included where the catalyst was loaded. E
DOI: 10.1021/acs.energyfuels.6b01624 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Effects of Pellet Diameter (ϕ = 1.6, ν = 0.15 m/s) case
upstream
downstream
Tmax (K)
CH4
CO
H2
reference A B C D
d1 = 2.5 mm d1 d1 1.5d1 d1/2
d2 = 5 mm 1.5d2 d2/1.25 d2 d2
1749.7 1817.7 1713.7 − 1764.5
0.011 0.009 0.012 − 0.010
0.080 0.081 0.079 − 0.081
0.074 0.081 0.072 − 0.077
pellet diameter upstream was scaled by 1.5, the flame propagated downstream rather than stabilizing at the interface. An increase in pellet diameter adversely affects convection because of a reduced surface-area-to-volume ratio, whereas it improves radiative heat transfer through the solid because of a lower extinction coefficient. Because both solid radiation and solid−gas convection are important in the upstream region, the net result is a less steep temperature distribution upstream and a lower flame temperature, which, in turn, results in a lower flame velocity, causing the flame to propagate downstream. On the other hand, a decrease in the upstream pellet diameter leads to an increase in the maximum temperature and, thereby, better reforming of CH4 to H2 and CO. Thus, to increase the reforming efficiency of methane, smaller-diameter pellets upstream and larger-diameter pellets downstream are preferred. However, it should be noted that the volume-average model used in this study has some limits because the detailed porous structure is neglected, which will influence the accuracy of the predictions of the flow and heat transfer inside the porous media. Further, when the tube-to-pellet ratio is rather low, the volume-average design concept can fail where local phenomena dominate, which will lead to inaccuracies in the model prediction. To address these issues, the consideration of a complete three-dimensional packing structure will be necessary to resolve the local inhomogeneities.27 4.4.2. Effects of Porous Media Layer Length. In this section, the effects of the ratio of the upstream layer length to the downstream layer length on the burner performance were analyzed. This ratio was changed as specified in Table 2.
CH4 composition barely decreased because of the relatively small catalyst loading. However, H2 and CO2 increased, whereas CO decreased, meaning that the catalyst mainly promoted the water−gas shift reaction CO + H2O ↔ CO2 + H2. The reforming efficiency to syngas increased from 40.0% to 41.4%, and the reforming efficiency to hydrogen increased from 18.2% to 23.9% after catalytic enhancement. The Ni catalyst also increased the ratio between the H2 yield and the CO yield from 0.9 to 1.6, indicating that the exhaust gases are more suitable for use in fuel cells when the traditional porous media burner is integrated with a Ni catalyst. A comparison between the experimental results and the equilibrium composition calculated by the National Aeronautics and Space Administration (NASA) computer program Chemical Equilibrium with Applications (CEA) at the outlet temperature (960 K) shows that the reactions did not reach equilibrium at the outlet. The loading of catalyst increased the reaction rates and drove the reactions toward equilibrium. The addition of catalyst introduced heterogeneous reactions into the system. Consequently, there was an interaction between the homogeneous reactions that occur in traditional noncatalytic porous media combustions and the heterogeneous reactions that were introduced upon addition of the Ni catalyst. The homogeneous reactions in the flame released large amounts of heat and governed the temperature distribution inside the reactor. The high-temperature environment provided by the flame initialized the downstream heterogeneous reactions. Simulation results with and without the homogeneous reactions in the region with catalyst are compared in Figure 8. The results show that the mole fractions of the main products varied little with and without homogeneous reactions, meaning that the heterogeneous reactions dominated in the region loaded with catalyst. 4.4. Optimization of Burner Design. In this section, the model developed in this study was used for burner design optimization. First, the conversion process without catalytic enhancement was optimized by changing the Al2O3 pellet diameter and the layer length of the porous media. Then, the effects of the catalyst loading were studied to achieve the optimum reforming efficiency. All of the simulations in this section were carried out at a fixed equivalence ratio of 1.6 and an inlet gas velocity of 0.15 m/s. 4.4.1. Effects of Pellet Diameter. Table 1 lists the modeling results of species composition and maximum temperature for different pellet diameters. When the pellet diameter of the downstream region was increased, the mole fractions of H2 and CO both increased, whereas that of CH4 decreased. As discussed in section 4.1, heat release of combustion is dominant in the downstream region. Increasing the pellet diameter increases the porosity, further increasing the heat release of combustion because of the additional area for gas-phase reactions. As a result, the maximum temperature increases with increasing downstream pellet diameter, leading to a more complete reforming of methane to syngas. However, when the
Table 2. Layer Lengths of Each Case case
L1 (mm)
L2 (mm)
L2/L1
reference E F G H I J
20 5 10 30 40 50 60
60 75 70 50 40 30 20
3 15 7 5/3 1 3/5 1/3
The maximum temperatures and reforming efficiencies of methane to hydrogen for various length ratios are shown in Figure 9. It can be seen that, for L2/L1 > 3, the maximum temperature decreased because of the decrease of the preheating region (upstream layer length L1) and the flame propagated downstream when L2/L1 was increased to 15. However, for L2/L1 < 3, the increase of the upstream layer length (L1) did not lead to an increase in the maximum temperature because the effective preheating length was about 20 mm, as shown in Figure 3. Moreover, the decrease of the reaction region decreased the heat recovered from this region, which decreased Tmax even further. The flame also propagated downstream when L2/L1 was decreased to less than 1/3. An F
DOI: 10.1021/acs.energyfuels.6b01624 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
equivalence ratio was increased from 1.4 to 1.6, and the reforming efficiency of methane to syngas reached 40% at the equivalence ratio of 1.6. The hydrogen yield was further increased by integrating the traditional porous media burner with a Ni catalyst. The interaction between the homogeneous and heterogeneous reactions in this reforming process was studied by developing a one-dimensional model that couples the detailed chemistry and complex transport phenomena. Numerical predictions of the temperature and product compositions showed good agreement with experimental measurements for various equivalence ratios. The catalyst mainly promoted the water−gas shift reactions under the conditions considered in this study. The homogeneous reactions provided a high-temperature environment in the burner to initialize the heterogeneous reactions. In the region where the Ni was loaded, the heterogeneous reactions were dominant. However, the coupling of the homogeneous reactions and the heterogeneous reactions governed the overall reactor performance. Finally, the effects of pellet diameter, layer length, and catalyst loading were studied for burner design optimization. Smaller upstream pellets and larger downstream pellets resulted in higher temperatures and more complete reforming of methane. A layer length ratio in the range from 3 to 7 was found to be optimal for a total burner length kept fixed at 80 mm. The reforming of methane was predicted to be almost complete when the Ni loading was increased to 1.6 wt %.
Figure 9. Effects of the layer length ratio on the maximum temperature and reforming efficiency of methane to hydrogen.
increase of L2/L1 will affect the reforming efficiency of CH4 in two ways: On one hand, the reforming efficiency of CH4 will increase because of the increase of the reaction region. On the other hand, the temperature change will also affect the reforming of methane, in that a higher temperature will lead to a more complete reforming. As a result, the reforming efficiency of methane to hydrogen showed an increasing tendency when L2/L1 was increased from 3/5 to 7. Considering the effects on both the temperature and the reforming efficiency, the ratio of L2/L1 should be set in the range of 3−7. 4.4.3. Effects of Catalyst Loading. As discussed in section 4.3, both the experimental and simulation results showed that more H2 was produced when part of the downstream region was coated with Ni. However, the unreacted CH4 did not show a significant decrease with a Ni loading of 0.08 wt %. In this section, the effects of the Ni loading are analyzed. The loading was increased by factors of 5, 10, and 20 in the model, and the calculated compositions are shown in Figure 10. The mole
■
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01624. Combined homogeneous and heterogeneous reaction mechanism for methane partial oxidation in a porous media burner with catalytic enhancement (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■ ■
ACKNOWLEDGMENTS The authors acknowledge Projects 51576112 and 51476092 supported by National Natural Science Foundation of China.
Figure 10. Effects of the catalyst loading on the mole fractions of major species.
fractions of H2 and CO2 increased whereas that of CH4 decreased with increasing Ni loading because of the promotion by the catalyst of both the water−gas shift reaction and the steam reforming reaction. When the loading of nickel reached 1.6 wt % (20m0), the reforming of methane was almost complete, with the mole fraction of CH4 being less than 0.1%.
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5. CONCLUSIONS In this study, a two-layer porous media burner was modeled, designed, fabricated, and tested. Methane fuel-rich flames were stabilized near the interface of the two layers at equivalence ratios between 1.4 and 1.6 for a fixed inlet velocity of 0.15 m/s. The mole fractions of H2 and CO increased when the G
DOI: 10.1021/acs.energyfuels.6b01624 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.6b01624 Energy Fuels XXXX, XXX, XXX−XXX
