EVALUATING HYDROGEN PRODUCTION IN BIOGAS REFORMING IN A MEMBRANE REACTOR

Abstract

Syngas and hydrogen production by methane reforming of a biogas (CH₄/CO₂ = 2.85) using carbon dioxide was evaluated in a fixed bed reactor with a Pd-Ag membrane in the presence of a nickel catalyst (Ni 3.31% weight)/γ-Al₂O₃) at 773 K, 823 K, and 873 K and 1.01×10⁵ Pa. Operation with hydrogen permeation at 873 K increased the methane conversion to approximately 83% and doubled the hydrogen yield relative to operation without hydrogen permeation. A mathematical model was formulated to predict the evolution of the effluent concentrations. Predictions based on the model showed similar evolutions for yields of hydrogen and carbon monoxide at temperatures below 823 K for operations with and without the hydrogen permeation. The hydrogen yield reached approximately 21% at 823 K and 47% at 873 K under hydrogen permeation conditions.

Membrane reactor; Biogas; Methane; Syngas; Hydrogen

INTRODUCTION

The increasing availability of methane has generated substantial interest in alternative methods for its conversion into synthesis gas (syngas) and/or hydrogen. Research has shown that the catalytic reforming of methane with carbon dioxide (dry reforming) may be employed with natural gas and carbon dioxide emissions rather than traditional methane steam reforming (MSR) for syngas production (Amoro, 1999; Topalidis, 2007; Abreu et al., 2008; Silva et al., 2012). Major anthropogenic sources of global carbon dioxide emissions include flue gases from coal, oilfired power stations, thermoelectric plants, FCC refining units, alcoholic fermentation, and several heavy industries, such as those that produce iron, lime, and cement. Biogas, a gaseous mixture of methane and carbon dioxide, can be directly processed by catalytic reforming (Kolbitsch et al., 2008; Lau et al., 2011). Biogas is produced from organic household waste, industrial waste, and animal dung. Landfill gas is the most important source of biogas because of its relatively high carbon dioxide content (CO₂: 36-41%, CH₄: 48-65%, N₂: 1-17%, and high methane emissions. Qian et al. (2002) reported a gas generation rate of 2.50 Nm³/landfill ton/year for old landfills (> 10 years old). Themelis and Ulloa (2007) estimated a global methane production rate of 75×10⁹ Nm³/year from 1.5×10⁹ tons/year of solid landfill, of which only approximately 10% was collected and used.

The expertise developed from the methane dry reforming process (Abreu et al., 2008) can be used to increase the production of syngas and hydrogen from biogas. However, thermodynamic constraints and low catalytic performance have been identified as limitations to achieving high methane conversions into hydrogen and carbon monoxide. Operational initiatives of selective product permeation (Garcia-Garcia et al., 2013; Faroldi et al., 2013; Gallucci et al., 2013) can be used to overcome these restrictions when the process is in chemical equilibrium by shifting the composition of the media to increase reactant conversions.

In general, under the same operating conditions, a higher reactant conversion is obtained for reactor operation with permeation using a selective membrane than for a fixed-bed reactor (Kumar et al., 2008). Membranes of palladium alloys exhibit a high permeability to hydrogen. Gallucci et al. (2008), Shu et al. (1991) and Dittmeyer et al. (2001) have written reviews on palladium membranes that analyze the effects of permeation on reactor performance.

Kikuchi (1995) demonstrated that the catalytic activity of various metals that were used in a membrane reactor for reforming processes decreased in the following order: Ni>Rh>Pt>Pd>Ru>Ir (alumina support). He used a membrane reactor with a nickel catalyst in methane reforming with carbon dioxide at 773 K and 1.01 MPa to obtain 47% methane conversion versus the 52% conversion predicted by thermodynamic equilibrium calculations. Thus, the high activity of nickel and its low cost make it the best choice for a reforming catalyst, although there is no evidence that it is susceptible to coke formation (Pompeo et al., 2007).

This study investigated the activity of a nickel catalyst to convert a biogas via methane reforming with carbon dioxide in order to produce synthesis gas and hydrogen. The process was evaluated in a fixed bed reactor containing a Pd-Ag membrane selective to the permeation of hydrogen and in the presence of the catalyst, in which operations with and without the permeation of hydrogen through the membrane were carried out.

EXPERIMENTAL SECTION

Catalyst Preparation and Characterization

The catalyst was prepared using nickel nitrate (Ni(NO₃)₂.6H₂O, Sigma-Aldrich, Germany) and gamma-alumina (γ-Al₂O₃, Degussa, Brazil) via impregnation of alumina with a nickel nitrate solution. First, the impregnated solution was evaporated to dryness. The solid was then dried at 393 K for 12 h and calcinated at 873 K in an argon flow for 5 h. Finally, the catalyst was activated in a hydrogen atmosphere at 973 K for 2 h.

The nickel catalyst was characterized by atomic absorption spectrophotometry (AAS), textural analysis (by the B.E.T. method), and X-ray diffraction (XRD, using CuK-alpha radiation and a Siemens D5000 diffractometer).

Experimental Evaluation

The reforming experiments were performed in a fixed-bed membrane reactor (Figure 1; useful height, H_R = 45.72×10^-2 m; outside diameter, D_R = 1.07×10^-2 m; Pd-Ag, H₂ selective membrane, height, H_m = 0.19 m; inner diameter, dm = 0.32×10^-2m; thickness, δm =7.62×10^-5 m; REB Research & Consulting, USA) with a nickel catalyst (<d_p> = 412 µm, m_cat = 0.02×10^-1 kg) at 773 K, 823 K, and 873 K and 1.01×10⁵ Pa. The reactions were investigated with and without hydrogen permeation. A pressure reduction was applied to the internal zone of the membrane under permeation conditions to facilitate hydrogen transfer from the external reaction zone (1.01×10⁵ Pa) to the internal zone (0.20 Pa).

Figure 1
(a) Scheme of the processing unit of biogas. (A) gas chromatograph, (C1, C2, C3) mass flow meter, (F) Electrical furnace, (R) membrane reactor, (S1) PC computer-mass flow meter control,(S2) PC computergas chromatograph, (V1, V2) valves, (M1) U manometer, (E1) vacuum pump, (b) Membrane reactor.

The reactants were fed into the reactor with a biogas gaseous mixture of CH₄:CO₂:Ar = 0.89:0.31:1.00 vv at flow rates ranging from 150 to 400×10^-6 m³/min (STP).

At the top of the reactor, three streams of CH₄, CO₂ and Ar were fed. The residual reagents and products were analyzed using on-line gas chromatography (with a Saturn 2000, Varian, Carbosphere/ Porapak-Q, TCD) of the reactor effluent flow.

After having reached a steady-state regime of processing, the feed stream of methane was stopped while the flows of argon and carbon dioxide were maintained. The residual carbon in the reactor was removed using carbon dioxide via the Boudouard reverse reaction; the effluent gas was analyzed to determine the amount of carbon monoxide produced.

Permeation tests were performed using gaseous mixtures of H₂ and Ar (H₂:Ar:5:50,10:50, 15:50, 20:50, 25:50 v/v) at 723 K, 773 K, and 823 K. The results were fitted to the Sieverts equation (J_H2 = J_H20.exp(-E_D/RT)[(P_rH2)^1/2- (P_pH2)^1/2], Rival et al., 2001); to determine the parameters J_H20 and E_D. Sieverts tests were performed after each reaction experiment to evaluate the state of the membrane.

RESULTS AND DISCUSSION

Catalyst Characterization

The nickel content and the surface areas of the support (pre-treated Al₂O₃) and catalyst (Ni/Al₂O₃) were 3.31% by weight, 226 m²/g, and 145 m²/g, respectively, as characterized by AAS and B.E.T.-N₂.

The solid phases of the catalyst used in the reforming reactions were detected by XRD. The γ-Al₂O₃ support was identified at 2θ = 37.4, 45.3, 65.8, and 66.6, and the nickel metallic phase was identified at 2θ = 44.1, 52.0, 77.5, and 93.4. Carbon was found in catalyst samples that were analyzed after the reaction evaluations. Elementary carbon analysis of the used catalyst indicated a carbon content ranging from 0.21% to 0.26% in weight.

Hydrogen Permeation Tests

Experiments on permeation through the selective membrane were conducted in terms of the variables of the Sieverts equation (Figure 2). The permeation rate was investigated as a function of the pressure difference in the membrane at three operating temperatures (723 K, 773 K, and 823 K). The hydrogen permeation rate increased with the H₂/Ar ratio and temperature. Hydrogen permeation experiments (Figure 1(b)) were also performed after the reactions while the system was being cleaned and the catalyst was being regenerated with carbon dioxide (i.e., corresponding to the Boudouard reverse reaction: CO₂ + C → 2CO).

Figure 2
Permeation rate as a function of the differences between the square roots of pressure in the membrane: (a) effect of temperature and (b) permeation rate after regeneration at an external reaction zone pressure of 1.01 × 10⁵ Pa^0.5 and an internal zone pressure of 0.20 Pa^0.5.

Figure 2(a) presents the fits to the experimental data using the linear form of the Sieverts equation. These linear fits were used to estimate the following orders of magnitude of the parameters: J_H20 = (2.21 ± 0.41)×10^-5 mol/m²s kPa^0.5 and E_D = (3.37 ± 0.13)×10³ J/mol.

Figure 2(b) presents the results of the hydrogen permeation tests that were obtained after regeneration of the catalyst in terms of the variables of the Sieverts equation. For comparison, were also included in this figure the results of the initial tests of permeation at 823 K.

Figure 3
Carbon removal from the membrane reactor. Catalyst and membrane cleaning/regeneration. Conditions: mcat= 0.02×10^-1 kg, feed molar ratio Ar/CO₂ = 1.0, flow rate = 120 cm3/min, temperature = 823 K, and pressure = 1.01×10⁵ Pa.

The operational performance of cleaning/ regeneration of the membrane reactor, including carbon removal from the catalyst and/or membrane, was determined from the evolution of the reactant and product concentrations in the Boudouard reverse reaction (see Figure 3). After 250 min of operation under a CO₂ stream, the CO level was reduced. In addition, the results of the hydrogen permeation tests (Figure 2(b)) indicated similar system performance before and after the reaction.

Process Evaluation

Experimental evaluations of the reforming process of the biogas were performed in the fixed-bed membrane reactor at three different temperatures (773 K, 823 K, and 873 K) at 1.01 × 10⁵ Pa for a feed gas composition of CH₄:CO₂:Ar = 0.89:0.31:1.00 vv and a spatial time of τ =1,204.8 kg.s/m³.

The operations were performed in two steps, without and with hydrogen permeation, followed by measuring the component concentrations in the reactor effluents. These experimentally determined concentrations (i.e., the CH₄ and CO₂ conversions and the CO, H₂, and H₂O production) were evaluated over a 4.5-h period. Figure 4 presents the reactant conversions (i.e., X_i = [Ci₀-C_i]10²/Ci₀, where i = CH₄ and CO₂ and where Ci₀ denotes the initial concentrations) as a function of the time for operation with and without hydrogen permeation at the three temperatures shown. Operations at 823 K and 873 K with H₂ permeation resulted in higher methane conversions than operation without H₂ permeation under the same conditions. The same trend was not observed for the carbon dioxide conversions, which exhibited only a slight increase at higher-temperature operation. The permeation effect occurred during the reaction step in the methane cracking process, shifting the reaction equilibrium such that the conversion of methane to hydrogen was increased. The carbon dioxide concentrations remained largely unchanged.

Figure 4
Reactant conversions for (a) CH₄ and (b) CO₂ as a function of time for operations in a membrane reactor illustrating the effects of temperature under the following conditions: catalyst Ni (3.31% weight)/γ-Al₂O₃, m_cat = 0.02 × 10_-1 kg, feed molar ratio CH₄/CO₂ = 2.85, and τ = 1,204.8 kg.s/m³ at a pressure of 1.01 × 10⁵ Pa.

Figure 4 illustrates that initiating hydrogen removal by permeation significantly increased the methane conversion at the two highest temperatures. In the stages following the permeation step, higher steady-state methane conversions were attained than for operation without permeation. Figure 5 presents the hydrogen and carbon monoxide yields (Y_j = [C_j/ΣC_i0] x 10²), j = CO and H₂) for the process. The hydrogen production increased for operation with H₂ permeation at 823 K and 873 K, whereas the carbon monoxide level remained steady.

Tables 1 and 2 provide the experimental conversions and yields obtained under steady-state conditions at the three temperatures considered.

Table 1
Component conversions of the biogas reforming process in a membrane reactor at the following steady-state conditions: Ni (3.31% weight)/γ-Al₂O₃, m_cat = 0.02 × 10^-1 kg, feed molar ratio CH₄/CO₂ = 2.85, and τ = 1,204.8 kg.s/m³ at a pressure of 1.01 × 10⁵ Pa.

Temperature (K)	Conversion (%) without permeation		Conversion (%) with permeation
	CH4	CO2	CH4	CO2
773	4.61	35.11	7.79	34.09
823	11.53	42.56	14.54	48.16
873	19.25	92.61	35.33	92.87

Table 2
Product yields for the biogas reforming process in a membrane reactor under the following steady-state conditions: catalyst Ni (3.31% weight)/γ-Al₂O₃, m_cat = 0.02 × 10^-1 kg, feed molar ratio CH₄/CO₂ = 2.85, and τ = 1,204.8 kg.s/m³ at a pressure of 1.01 × 10⁵ Pa.

Temperature (K)	Yield (%) Without permeation		Yield (%) with permeation
	H2	CO	H2	CO
773	7.52	7.14	9.23	15.41
823	15.49	30.06	21.14	25.22
873	22.80	53.21	47.00	46.82

Under hydrogen permeation at 873 K, the methane conversion increased by 83% and the hydrogen yield was approximately 113% higher compared to operation without permeation. Galuszka et al. (1998) obtained a methane conversion of 48.6% and a hydrogen yield of 46.5% using a nickel catalyst in a membrane reactor under the same conditions as this study.

Although the effects of the operating conditions (i.e., the feed flow rate, feed ratio, temperature, pressure, and catalyst weight) on the CH₄ and CO₂ conversions should be considered, the biogas conversion in a membrane reactor is generally expected to be greater than that in a fixed-bed reactor.

Munera et al. (2003) evaluated the methane dry reforming process in a membrane reactor (with a Pd/Ag membrane) using a 0.6% weight Rh/Al₂O₃ catalyst at 823 K and atmospheric pressure. The study obtained methane and carbon dioxide conversions of 33.9% and 41%, respectively, for a feed molar ratio of CH₄/CO₂ = 1.0. The methane and carbon dioxide conversions obtained in the present study were 14.5% and 48%, respectively, under the same conditions. The lower methane conversion may be attributed to the applied feed molar ratio (CH₄/CO₂ = 2.85) of the biogas.

Figure 5
Product yield for (a) H₂ and (b) C_O as a function of time for operation in a membrane reactor illustrating the effects of temperature under the following conditions: catalyst, Ni (3.31% weight)/γ- Al₂O₃, m_cat = 0.02 × 10^-1 kg, feed molar ratio CH4/CO2 = 2.85, and τ = 1,204.8 kg.s/m³ at a pressure of 1.01 × 105 Pa.

The experimental results for biogas reforming in this comparative evaluation were explained using a set of reaction mechanisms for the reforming process developed by Abreu et al. (2008) for the same catalyst, which consisted of the following major reaction steps: methane catalytic cracking, with carbon deposition and hydrogen production (I: CH₄ ↔C + 2H₂); hydrogen consumption via a water gas-shift reverse reaction (II: CO₂ + H₂↔ CO + H₂O); and carbon consumption by carbon dioxide (III: CO₂ + C ↔ 2CO, i.e., the Boudouard reverse reaction). Thus, in operation with hydrogen permeation, the methane conversion increased to maintain the methane catalytic cracking equilibrium (step I). Accordingly, the removal of hydrogen from the reaction medium delayed the reverse water gas-shift reaction (step II), decreasing the carbon dioxide consumption. Therefore, the available carbon dioxide partially cleaned the carbon deposited on the catalyst (step III), maintaining the activity level of the nickel catalyst during the process.

Kinetic and Reactor Modeling

The relations r_ji (j = I, II, III), which correspond to the rate laws of the reaction steps of the biogas reforming process, were expressed as follows:

The global reaction rates of each component (r_i; i = CH₄, CO₂, CO, and H₂O) were written as follows:

The evolution of the effluent concentrations for the membrane reactor was obtained from mass balances incorporating component reaction rates based on the three aforementioned reaction steps. In the mass balance equation a constant flow rate along the reactor was considered based on the experimental conditions employed (biogas mixture diluted in argon, with low content in carbon dioxide, and operations with low methane conversions).

The resulting differential equations were expressed as dC_i/dτ + r_i= 0, where τ (kg.s/m³) was the modified spatial time. The differential equation for the mass balance of hydrogen for the permeation operation was as follows:

where S_m = A_m/m_cat = (4d_m.[(D_R²- d_m²)ρcat(1-ε)]^-1) is the ratio between the membrane surface area and the mass of the catalyst and ε denotes the bed porosity. The corresponding initial conditions (τ = τ₀) were as follows:

The isothermal conditions of the operations were guaranteed by the feed, the convective heat discharge, the heat released with hydrogen permeation, the reaction enthalpies (ΔH_J.r_Ji), and the heat transferred from the oven through the reactor wall. The thermal behavior of the reactive operations was modeled by a steady-state energy balance incorporating the aforementioned effects. The following differential equation (Equation (7)) describes the temperature evolution as a function of the modified spatial time:

where ρg = 15.28 mol/m³, U (the overall heat transfer coefficient) = 2.41 J/m².s.K, DR = 1.07x 10^-2 m, ρ_cat = 1,200 kg/m³, and ρ_H2 = 71 kg/m³. The reformer and sweep gas enthalpy are denoted by HrH2 and HmH2, respectively. The temperature TRE = T0 = 750 K and -62 -93 Cp_M =29.30 +0.023T -8.96×10 T -1.40×10 T J/mol.K, where CpM denotes the heat capacity of the mixture of CH₄, CO₂, and Ar. The enthalpies of re-action can be expressed in the following form:

The mass balance equations and Equation 7 were solved for the effluent concentrations and the temperature of the reaction medium using the fourthorder Runge-Kutta method. The values of the kinetic and adsorption parameters were estimated from our previous work (^{Abreu et al., 2008}). The Arrhenius and van't Hoff correlations were expressed as follows:

- methane cracking reaction: k₁ = 3.58×10⁹exp (-248.55/RT) mol/kg.s and K_CH4 = 31.39×10^-11exp (167.32/RT) m³/mol;

- water gas-shift reverse reaction: k₂ = 1.07×10¹³exp (-350.08/RT) (m³)²/mol/kg.s; and

- carbon dioxide-carbon interaction (i.e., the Boudouard reverse reaction): k3 = 1.16×10⁵exp(-115.86/ RT) m³/kg.s.

The equilibrium constant of the water gas-shift reaction was given as follows:

The component concentrations at the reactor exit were calculated as a function of the temperature over the 750-895 K range for operations with and without hydrogen permeation under different steady-state conditions. The experimental results obtained under steady-state conditions at 773 K, 823 K, and 873 K (see Figures 6 and 7) are shown on the same graph for comparison with the predictions.

Figures 6 and 7 illustrate that the experimental component concentrations exhibited the same trends predicted by the model equations, except for hydrogen concentrations at temperatures higher than 823 K (Figure 7(a)). The concentrations of carbon dioxide and carbon monoxide in the output gas were similar with and without the permeation of hydrogen. However, the concentrations of methane and hydrogen were approximately 23% and 51% lower, respectively, when the system operated with permeation. Methane was further consumed to maintain the equilibrium, which was temporarily displaced by hydrogen permeation. The small amount of available hydrogen in the reaction medium could be processed by the water gas shift reverse reaction (step II). Thus, residual carbon dioxide, not consumed by the reaction with hydrogen, was used to process the carbon into carbon monoxide by the Boudouard reverse reaction (step III).

Figure 6
Model predictions and experimental concentrations as a function of temperature for steadystate operation with and without hydrogen permeation for methane-carbon dioxide biogas reforming in a membrane reactor for reactants (a) CH₄ and (b) CO₂, under the following operating conditions: m_cat = 0.02 × 10^-1 kg, P = 1.01 × 10⁵Pa, CH₄/ CO₂ = 2.85, and τ = 1,204.8 kg.s/m³.

Figure 7
Model predictions and experimental concentrations as a function of temperature for steadystate operation with and without hydrogen permeation for methane-carbon dioxide biogas reforming in a membrane reactor, showing products (a) H₂ and (b) CO under the following operating conditions: m_cat = 0.02 × 10^-1 kg, P = 1.01 × 10⁵ Pa, CH₄/CO₂ = 2.85, and τ = 1,204.8 kg.s/m³.

Figure 8
Model predictions and experimental product yields for (a) H₂ and (b) CO for operations with and without hydrogen permeation for methane-carbon dioxide reforming in a membrane reactor under the following operating conditions: m_cat = 0.02 × 10^-1 kg, P = 10.1 × 10⁵ Pa, CO₂/CH₄ = 0.35, and τ = 1,204.8 kg.s/m³.

The predicted component concentrations had approximate values at temperatures lower than 823 K for operations with and without hydrogen permeation. When hydrogen permeation was used, lower methane, carbon monoxide and hydrogen concentrations were predicted in the reactor effluent gas for temperatures above 823 K.

Figure 8 presents the model predictions and the evolution of the carbon monoxide and hydrogen yields (Yj = [Cj/ ΣCi₀] x 10², where j = CO and H₂) with and without hydrogen permeation (C_H2 = C_H2permeated + C_{H2reactor exit}). The hydrogen yield was predicted to increase strongly with the temperature and more so for operation with permeation. A higher carbon monoxide yield was predicted for operation without permeation than with permeation at 873 K.

An increased sensitivity to thermal effects was predicted using Sieverts equation for hydrogen mass transfer by permeation for operation at temperatures above 840 K. Thus, hydrogen permeation increased rapidly for operations at temperatures above 840 K, and the hydrogen production (Figure 8(a)) increased via methane cracking.

CONCLUSIONS

A fixed-bed reactor with a Pd-Ag/H₂ selective membrane was used to convert biogas into syngas by a reforming process. The performance of a nickel catalyst (3.31% weight)/γ-Al₂O₃) was evaluated at 773 K, 823 K, and 873 K and 1.01 x 10⁵ Pa with and without hydrogen permeation. Operation with permeation at 873 K increased the biogas methane conversion to approximately 83%, and the hydrogen yield was 113% higher than that for operation without hydrogen permeation.

A mathematical model was formulated to predict the evolution of the effluent concentrations of the membrane reactor as a function of the operating temperature. At temperatures lower than 823 K, similar evolution profiles were predicted for the hydrogen and carbon monoxide yields for operations with and without hydrogen permeation. The hydrogen yield reached approximately 21% at 823 K and 47% at 873 K under hydrogen permeation conditions.

NOTATIONS

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ACKNOWLEDGMENTS

The authors acknowledge the financial support provided by CAPES, FINEP, and PETROBRAS, Brazil for this study.

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Publication Dates

• Publication in this collection
  Jan-Mar 2015

History

• Received
  1 July 2013
• Reviewed
  25 Feb 2014
• Accepted
  29 Apr 2014