Experimental Study of Model Biogas Catalytic Steam Reforming: 1

Sep 10, 2008 - short-term objective of the present investigative work; the results will be applied to optimize reformer operation at the anaerobic bio...
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Energy & Fuels 2008, 22, 4182–4189

Experimental Study of Model Biogas Catalytic Steam Reforming: 1. Thermodynamic Optimization Mojdeh Ashrafi,* Tobias Pro¨ll, Christoph Pfeifer, and Hermann Hofbauer Institute of Chemical Engineering, Vienna UniVersity of Technology, Getreidemarkt 9/166, 1060 Vienna, Austria ReceiVed February 5, 2008. ReVised Manuscript ReceiVed June 13, 2008

The general objective of this investigation is the development of a biogas steam reforming concept and to bring it to a state of readiness for industrial demonstration. Through biogas steam reforming, H2-rich synthesis gas will be produced from which gas engines benefit in terms of higher efficiency and lower NOx emissions compared to direct combustion of raw biogas. Furthermore, pure hydrogen can be produced out of a renewable source using this route. The aim of this experimental study is to determine the operational envelope of biogas steam reforming by optimizing the performance of an externally heated reformer in terms of CH4 conversion, H2 yield, and catalyst efficiency. Therefore, a clean model biogas, using a constant molar ratio of CH4/CO2 ) 1.5, is contacted to different supported nickel catalysts in a fixed bed reactor. The influence of temperature, water vapor portion, and contact time is analyzed. The resulted reformate composition is plausible with respect to thermodynamic equilibrium calculations. On the basis of the results, the steam/carbon molar ratio in the range of 3-4 and operating temperature of 700 °C are found as the optimal operating conditions. In addition, the catalyst activity, thermal stability, and resistance to carbon formation have been observed as critical parameters on the application of different kinds of catalysts.

1. Introduction Limited sources of fossil fuels and also global climate changes caused by CO2 emissions are currently discussed around the world. Renewable and also green sources of energy are being sought as alternatives to replace fossil fuels. Hence, research activities on this topic are gaining more and more importance.1,2 Biomass is an attractive renewable energy source. Sustainable use of biomass for energy production does not contribute to CO2 emissions production but has a high CO2 abatement potential.3 Biogas, with a typical composition of 50-75 mol % CH4, 25-45 mol % CO2, 2-7 mol % H2O, 0-2 mol % N2, 0-1 mol % H2, 0-2 mol % O2, and 0-2 mol % H2S, produced from anaerobic digestion of biomass has, up to now, been used mainly for combined heat and power production.4–6 Today, biogas has also received increased attention for upgrading to * To whom correspondence should be addressed. Telephone: (43) 1-58801-159-74. Fax: (43) 1-58-801-159-99. E-mail: mashrafi@ mail.zserv.tuwien.ac.at. (1) International Energy Agency. Renewables in global energy supply. An IEA fact sheet: http://www.iea.org/textbase/papers/2006/renewable_ factsheet.pdf, 2007. (2) Maney, J. N. Y. U. Carbon dioxide emissions, climate change, and the clean air act: An analysis of whether carbon dioxide should be listed as a criteria pollutant. EnViron. Law J. 2005, 13, 298–378. (3) Mo¨llersten, K.; Yan, J.; Moreira, J. R. Potential market niches for biomass energy with CO2 capture and storage opportunities for energy supply with negative CO2 emissions. Biomass Bioenergy 2005, 25, 273– 285. (4) Biogas Handbook BaVaria; Bavarian State Ministry of the Environment: Rosenkavalierplatz 2, 8 1925 Mu¨nchen, 2004; Chapters 1-3. (5) Kaltwasser, B. RegeneratiVe Energieerzeugung durch anaerobe Fermentation organischer Abfaelle in Biogasanlagen, 1st ed.; Bauverlag: Wiesbaden and Berlin, Germany, 1980. (6) Kaltschmitt, M.; Hartmann, H. Energie aus Biomasse; Springer Publishing: Berlin, Germany, 2001.

high-quality fuels as an important intermediate in the industrial syntheses of a wide range of chemicals.7–10 This work contributes to the current discussion about H2 as a future energy carrier, where the available sustainable sources for the hydrogen required are still raising questions.11,12 Currently, hydrogen is, because of economics, produced from fossil fuels through steam reforming.13–16 In analogy, steam reforming of biogas can also be considered as a possible process for the production of H2-rich synthesis gas.7,8 Further steps can either be shift conversion to obtain a H2/CO2 gas mixture and, after selective CO2 removal, pure H2 from renewable sources or, on the other hand, the combustion in gas engines obtaining higher engine efficiencies and lower NOx emissions. The latter is the (7) Effendi, A.; Zhang, Z. G.; Hellgardt, K.; Honda, K.; Yoshida, T. Steam reforming of a clean model biogas over Ni/Al2O3 in fluidized- and fixed-bed reactors. Catal. Today 2002, 77, 181–189. (8) Effendi, A.; Hellgardt, K.; Zhang, Z. G.; Yoshida, T. Optimising H2 production from model biogas via combined steam reforming and CO shift reactions. Fuel 2005, 84, 869–874. (9) Zhang, Z. G.; Xu, G.; Chen, X.; Honda, K.; Yoshida, T. Process development of hydrogenous gas production for PEFC from biogas. Fuel Process. Technol. 2004, 85, 1213–1229. (10) Xu, G.; Chen, X.; Honda, K.; Zhang, Z. G. Producing H2-rich gas from simulated biogas and applying the gas to a 50 W PEFC stack. AIChE J. 2004, 50 (10), 2467–2480. (11) Rostrup-Nielsen, J. R.; Rostrup-Nielsen, T. Large-scale hydrogen production. CATTECH 2002, 6 (4), 150–159. (12) Zittel, W.; Wurster, R. Hydrogen in the energy sector. LudwigBoelkow-Systemtechnik GmbH, http://www.hyweb.de/Knowledge/w-ienergiew-eng.html, 1996. (13) Twigg, M. V. Catalyst Handbook, 2nd ed.; Manson Publishing: London, U.K., 1996. (14) Rostrup-Nielsen, J. R.; Sehested, J. Hydrogen and synthesis gas by steam and CO2 reforming. AdV. Catal. 2002, 47, 65–139. (15) Cromarty, B. Effective steam reforming of mixed and heavy hydrocarbon feedstocks for production of hydrogen. Presented at NPRA Annual Meeting, San Francisco, CA, 1995. (16) Rostrup-Nielsen, J. R. Catalytic Steam Reforming; Springer Publishing: New York, 1984.

10.1021/ef800081j CCC: $40.75  2008 American Chemical Society Published on Web 09/10/2008

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short-term objective of the present investigative work; the results will be applied to optimize reformer operation at the anaerobic biomass fermentation plant in Strem, Austria, where a test facility for autothermal biogas steam reforming with subsequent combustion of the reformate in a 500 kWel gas engine is currently investigated. The efficiency benefits of H2- and COrich fuels for gas engines is discussed in detail by Herdin et al. and Gruber et al.17,18 On the basis of these investigations, also, CO contributes to performance improvements of gas engines, however, in a smaller extent compared to H2. In a previous study, the high efficiency of nickel was observed also for biogas steam reforming.19 Therefore, different nickelbased catalysts are tested in this work. Among them, two commercially available catalysts are selected according to several criteria (e.g., particle size, reactivity, and carbon formation) for further thermodynamic investigations. Through the experiments, the steam/carbon ratio in the reformer, operating temperature, and space velocity are optimized in terms of parameters, such as CH4 conversion, H2 yield, and H2/CO ratio. The CH4/CO2 molar ratio in the raw biogas is normally determined by the biomass feedstock and operating parameters of the fermenter. If a substantial increase in efficiency for higher CH4/CO2 ratios (i.e., lower CO2 contents) were obtained for a plant concept, bulk CO2 removal prior to steam reforming can be considered.20 2. Theory 2.1. Main Reactions. The global steam reforming mechanism of biogas consists of four reversible reactions. The steam methane reforming reactions 1 and 3 are linked by the water-gas shift reaction 2 and the methane carbon dioxide reforming reaction 4: ◦ ( ) (1) CH4 + H2O T CO + 3H2 ∆rH298 K ) +206 kJ/mol ◦ ( ) CO + H2O T CO2 + H2 ∆rH298 K ) -41 kJ/mol

(2)

CH4 + 2H2O T CO2 + 4H2

◦ ( ) ∆rH298 K ) +165 kJ/mol

(3)

CH4 + CO2 T 2CO2 + 2H2

◦ ( ) ∆rH298 K ) +247 kJ/mol

(4)

Because of the high CO2 portion in the biogas during CH4 reforming with water vapor, dry reforming of methane (reaction 4) will also proceed. Dependent upon the steam/methane ratio and CO2 content in the feed gas, CH4 is more or less reformed with H2O or CO2. Thereby, the equilibrium of the water-gas shift reaction and portion of H2 and CO in the synthesis gas are effected. 2.2. Definitions and Calculation Method. The parameters, concerning the description of reactions, reaction conditions, and also evaluation of the results, are defined as follows: Steam/carbon molar ratio in the reformer (S/C): S/C )

YH2O,in YCH4,in

(mol/mol)

(5)

(17) Herdin, G. R.; Gruber, F.; Klausner, J.; Robitschko, R.; Plohberger, D. Use of hydrogen and hydrogen mixture in gas engines and potentials of NOx emissions. Presented in ARES-ARICE Symposium on Gas Fired Reciprocating Engines, Canada, 2005. (18) Gruber, F.; Herdin, G. R. The use of H2-content process gas in gas engines. Presented in ASME International Combustion Engine Division, Spring Technical Conference, Chicago, IL, 1997. (19) Kolbitsch, P.; Pfeifer, C.; Hofbauer, H. Catalytic steam reforming of model biogas. Fuel 2008, 87, 701–706. (20) Ashrafi, M.; Pro¨ll, T.; Hofbauer, H. Biogas upgrading to hydrogen rich gas by steam reforming: Comparison and optimization of plant configurations. Proceeding of the World Bioenergy Conference, Sweden, 2006.

The carbon, which is considered in this definition, is the fraction of organic carbon that takes part in carbon-forming reactions. Here, a general definition of organic carbon is considered. In biogas, organic carbons are presented in CH4 and CO2. Because this parameter is considered for coking tendency and CO2 does not have any influence on carbon formation, CO2 is not considered as a carbon carrier. Therefore, CH4 is the only compound that is relevant for the S/C ratio. Space velocity (SV): SV )

V˙in (m3 h-1 kg-1) mcat

(6)

The conversions of substance x: conversion (x) )

N˙x,BG - N˙x,SG N˙x,BG

(%)

(7)

H2 yield: H2 yield )

N˙H2,SG 1 N˙CH ,BG 4

(8)

4

The H2 yield is defined as a factor showing the molar ratio of the produced H2 to CH4 in the raw biogas provided to the system. The factor 1/4 is determined by the fact that maximal 4 mol of H2 can be produced from 1 mol of CH4. CO selectivity: CO selectivity )

YCO,SG YCO,SG + YCO2,SG

(9)

During the calculations, it is assumed that the reaction of inlet gases produces CO, CO2, H2, and solid C and unreacted CH4 and H2O remain in the gas. The calculations of all outlet fluxes are based on the carbon, oxygen, and hydrogen mass balance. The deposited solid carbon amount can be calculated using the mass balance equations. Because no carbon deposition and catalyst deactivation is observed during these experiments, solid carbon formed is assumed to be zero throughout the calculations. It is important to estimate the amount of outlet effluent flux of H2O. In this experimental system, it is impossible to measure the unreacted H2O amount at the same time with other outlet gases, because H2O gas liquefies easily below 100 °C. Therefore, it should be calculated using the oxygen and hydrogen mass balances or measured gravimetrically. The unreacted H2O amount is also separated in water traps and weighted during the experiments. Because it can be dried only to a saturation temperature of approximately 15 °C, a small amount of unreacted water remains in the synthesis gas. After the determination of synthesis gas composition, this small amount of water can be determined through iterative calculations. m ˙ H2O,out )

m ˙ H2O,water traps tE - tS

+m ˙ H2O,dried synthesis gas

˙ H O,dried synthesis gas ) m ˙ SGxs m 2

(kg/h)

(kg/h)

(10) (11)

where xs is the mass-based saturation degree in (kgH2O /kgSG). To check the plausibility of the experimental results, the thermodynamic equilibrium of reactions 1 and 2 (reactions 3 and 4 are dependent) is calculated for each experiment based on the given input stream and the prevailing temperature in the reformer. The equilibrium concentrations are calculated by the IPSEpro steady-state simulation program using the minimization

4184 Energy & Fuels, Vol. 22, No. 6, 2008 Scheme 1. Flow Chart of the Experimental Setup

of Gibbs free-energy method. The thermodynamic property data has been taken from Burcat and McBride.21 3. Experimental Section 3.1. Experimental Setup. A schematic diagram of the experimental equipment used is given in Scheme 1. It consists of three main sections: feed section, reaction section, and analysis section. The feed section supplies the components of interest, such as CH4, CO2, H2O, and N2. Nitrogen is used as the carrier gas, needed for heating and cooling the reactor. After pressure reduction from the gas cylinders, the flow rate of each gas is controlled by a mass flow controller at the desired value. After mixing, the model biogas flows into the preheater. The necessary water is deionized before being added to the biogas and delivered by a peristaltic pump to the evaporator, where it is vaporized and simultaneously mixed with other gases. The model biogas/water mixture reaches the desired temperature in the preheater/evaporator section. Subsequently, the gas mixture flows into the reactor and over the catalyst bed. A straight section serves as the reactor, while the upper section is used as the combined preheater/evaporator. The fixed bed reactor and preheater/evaporator section used in the present work are made from a stainless-steel (1.4841, X15CrNiSi2520) tube enclosed by three electric resistance heating coils. The catalytic bed, supported on a metallic mesh, is 47 mm in diameter and can reach a maximum length of 700 mm. Because of the short length of the catalyst bed in the present experiments (less than 100 mm), the reactor is operated at nearly atmospheric pressure and any pressure profile inside the reactor can be neglected. Because of simplicity, a fixed-bed reactor is used here. In traditional steam reforming configurations, a low-temperature shift reactor follows the high-temperature steam reformer, to convert more CO to CO2 through the exothermic shift reaction (reaction 2) and improve H2 production.13,15 However, because CO is a fuel for gas engines, therefore and for means of process simplicity, the shift reactor following the reforming step is avoided here. The reactor is also electrically heated to apply the necessary heat of the endothermic reforming process. A thermocouple is placed near the heating coil, surrounding the evaporator, and two other thermocouples are located along the axis of the reactor, one before and one after the reactor section. The three thermocouples are connected to the temperature indicator and controller. The heaters are regulated in such a way that all temperature measuring points (TMP1, TMP2, and TMP3) have the same constant temperature. In the consequent section of the catalyst bed, the synthesis gas is cooled to ambient temperature. A cold trap at the outlet of the reactor is used to condense any water from the product gas stream. (21) Burcat, A.; McBride, B. Ideal gas thermodynamic data for combustion and air pollution use. Technion Israel Institute of Technology, Aerospace Engineering Report, TAE 804, garfield.chem.elte.hu/Burcat/ burcat.html, 1997.

Ashrafi et al. Table 1. Parameters Used for the Empty Pipe Experiment parameter

unit

value

inlet CH4 inlet CO2 inlet H2O inlet CH4 (wf) inlet CO2 (wf) CH4/CO2 ratio S/C ratio reactor temperature test duration

mol % mol % mol % mol % mol % mol/mol mol/mol °C min

23.15 15.50 61.35 60 40 1.5 2.72 700 180

After condensation of the steam and drying of the gas mixture, the effluent is sent to the analysis section. 3.2. Model Biogas and the Used Catalysts. Because no natural biogas is available at the equipment location, model biogas (synthetic biogas), containing 60% CH4 and 40% CO2, is used throughout this investigation. H2S is usually present in biogas, and even very small concentrations of it can lead to significant catalyst deactivation.13,14,16 A smaller effect on catalytic activity has been reported for NH3.8 The experiments in the present work are executed in the absence of sulfur and ammonia. Thus, different catalyst activity, stability, and resistance against coking are studied. Further investigations on catalyst poisoning will be presented in the subsequent part of this paper. However, in traditional commercial applications, sulfur is removed prior to the steam reforming step. The most commonly encountered catalysts for methane steam reforming are nickel-based.13,14,16,22 In this work, two commercially available catalysts, both with the description “G-90” from the company Sued-Chemie, are used in the thermodynamic investigations. The catalysts are in spherical form and termed in the following: catalyst A (diameter ) 5-7 mm) and catalyst B (diameter ) 2-4 mm). 3.3. Execution of the Experiments. In each experiment, the catalyst bed is initially heated to the desired temperature at a heating rate of approximately 6-7 °C/min with the help of a nitrogen flow of 300 L/h. In the second step, steam is introduced into the reactor for about 15 min and then both reactants, CH4 and CO2, are supplied and nitrogen is terminated. If methane alone is supplied into the reactor, there will be serious coke formation on the metallic Ni sites. The start-up process for each experiment lasts up to 3 h until the reactor reaches a steady-state condition and the measuring is started. The amount of water in the beaker is gravimetrically determined at the beginning and end of each test run to calculate the actual steam/carbon ratio. The water content of the water traps is also measured approximately once per hour, and they are emptied thereby. The experiments are terminated by introducing a nitrogen flow of 300 L/h and switching off the gas valves and water pump. The heating coils are then switched off, and the reactor is cooled below 300 °C under N2 flow. The N2 flow is then switched off, and the reactor is cooled to room temperature automatically. The shut down process lasts for about 2 h.

4. Results and Discussion 4.1. Empty Pipe Experiment. Because the entire reactor is made of stainless steel and this material can exhibit a catalytic effect, the reactor is operated in the first test without a catalyst to quantify the auto-catalytic effect of the reactor itself. The parameters are shown in Table 1, and the experimental results are shown in Table 2. As it can be seen, methane conversion in the absence of catalyst is low at the experimental conditions used in this work. Thus, it can be concluded that the auto-catalytic effect of the reactor material can be neglected. (22) Beurden, P. V. On the catalytic aspect of steam reforming. ECN, http://www.ecn.nl/docs/library/report/2004/i04003.pdf, 2004.

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Table 2. Results of the Empty Pipe Experiment parameter

unit mol mol mol mol %

outlet CH4 (wf) outlet H2 (wf) outlet CO2 (wf) outlet CO (wf) CH4 conversion H2 yield CO2 conversion CO selectivity H2/CO ratio

value

% % % %

% mol/mol

56.51 3.24 39.81 0.45 2.50 0.09 -2.57 0.01 7.23

Table 3. Parameters of the Experiments for Catalyst Activity parameter

unit

CH4/CO2 ratio S/C ratio space velocity reactor temperature catalyst amount catalyst bed height residence time test duration

mol/mol mol/mol m3 kg-1 h-1 °C g mm s h

catalyst A

catalyst B

1.5 3.19 18.77 700 110 62 0.20 8

1.5 3.50 30.07 700 70 45 0.13 8×5

Table 4. Results of the Experiments for Catalyst Activity, Catalyst A parameter outlet CH4 (wf) outlet H2 (wf) outlet CO2 (wf) outlet CO (wf) CH4 conversion H2 yield CO2 conversion CO selectivity H2/CO ratio

unit mol mol mol mol %

% % % %

% mol/mol

value 1.74 64.02 16.15 18.09 91.93 2.84 -11.90 0.53 3.54

equilibrium at S/C ) 3.19 0.44 65.14 17.65 16.77 97.91 3.11 -26.57 0.49 3.88

4.2. Catalyst Activity and Resistance toward Carbon Formation. To obtain acceptable reaction rates, a catalyst is required to accelerate the process. Even more, the catalyst should be stable under the rather extreme conditions under which high CH4 conversion can be reached (i.e., high temperatures and high probabilities of unwanted side reactions involving carbon deposition).22 Because nickel is economically and highly active in steam methane reforming, it has proven to be very efficient in commercial scale for more than 40 years.13,14 The correct choice of a catalyst can overcome the problems raised by carbon formation. Therefore, six commercially available catalysts are tested to determine their activity, stability, and resistance to carbon formation. Two of them (catalysts A and B, as mentioned before), which have shown the best results, are selected for further thermodynamics investigations, and the results of the primary experiments are also presented here. According to the laboratory scale of the reformer, the proper size and shape of these two catalysts were also important factors. A reference temperature of 700 °C is selected to test their activity and resistance toward carbon formation. According to the lower active surface of catalyst A, a lower space velocity is selected for the experiment with this catalyst. Other experimental parameters are listed in Table 3. Tables 4 and 5 summarize the results. Both catalysts maintain constant conversion and product distribution after about half an hour after the feed inlet throughout the duration of the experiments. The H2/CO ratio in the run with catalysts A is lower compared to catalyst B, while the methane conversion is higher. This indicates that the water-gas shift reaction is more favorable when CH4 conversion is relatively low.

Table 5. Results of the Experiments for Catalyst Activity, Catalyst B parameter outlet CH4 (wf) outlet H2 (wf) outlet CO2 (wf) outlet CO (wf) CH4 conversion H2 yield CO2 conversion CO selectivity H2/CO ratio

unit mol mol mol mol %

value

% % % %

3.88 62.05 18.69 15.38 82.87 2.63 -23.34 0.45 4.04

% mol/mol

equilibrium at S/C ) 3.50 0.35 65.55 18.36 15.74 98.30 3.17 -33.23 0.46 4.16

Table 6. Parameters of the Experiments for Optimizing Reactor Temperature parameter

unit

value

CH4/CO2 ratio S/C ratio space velocity reactor temperature catalyst type catalyst amount catalyst bed high residence time test duration

mol/mol mol/mol m3 kg-1 h-1 °C

1.5 2.71 ( 0.20 16.94 ( 0.74 600-900 A 110 62 0.20 180

g mm s min

The experiment is repeated for catalyst B for totally more than 40 h. During these experiments, the catalyst remained stable in activity and the product distribution was close to equilibrium. In this condition, there was no deactivation over more than 40 h. As it can be seen in the results, despite a lower active surface of catalyst A and also the influence of channeling (because of the larger dimensions), the result obtained with this catalyst is better than that of catalyst B. This can be related to the lower space velocity selected in the experiment with catalyst A. However, in the following experiments to test the influence of the S/C ratio (presented in section 4.4), the higher activity of catalyst B in comparison to catalyst A in the same operating conditions and also space velocity is proven. 4.3. Variation of Reaction Temperature. According to the acceptable results for both catalysts A and B, catalyst A is selected for further investigations because of its availability. In this part, the effect of different operating temperatures is studied under steam reforming conditions. The experimental parameters are listed in Table 6. The reaction conditions are employed considering the commercial steam methane reforming process; thus, excess steam is used (S/C ) 2.5-3). For the experiment for catalyst A at similar conditions and again in the absence of H2S, no carbon formation and therefore no catalyst deactivation is observed for about 8 h. The product gas composition remained also constant when all experimental parameters were in the steady-state condition. Therefore, 3 h of experiment in the steady-state condition seems to be enough for each test in this part. The experiments at 600, 650, and 750 °C are performed consequently, while the temperature is changed stepwise during the test, and at each temperature, the steady-state condition is reached for at least 180 min. The experiments for 800, 850, and 900 °C are also executed in the same way. The space velocity depends upon the catalyst quantity, the adjusted S/C value, and biogas volume flow rate, which are dependent upon the temperature. Therefore, to have a constant SV value, the volume flow rate of the biogas is adapted for each experiment to obtain respective reactor parameters. During each experiment, it is observed that the used peristaltic pump promotes a constant flow rate. However, the flow rate varied for each experiment. Thus, S/C values and therefore also

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Table 7. Actual S/C and SV Values for the Experiments to Optimize Reactor Temperature reactor temperature (°C)

S/C (mol/mol)

SV (m3 kg-1 h-1)

600 650 700 750 800 850 900

2.58 2.67 2.91 2.72 2.81 2.77 2.51

16.41 16.75 17.68 17.00 17.33 17.19 16.20

Figure 4. CO selectivity versus equilibrium temperature.

Figure 1. CH4 conversion versus equilibrium temperature.

Figure 5. H2/CO ratio in synthesis gas versus equilibrium temperature.

Figure 2. H2 yield versus equilibrium temperature.

Figure 3. CO2 conversion versus equilibrium temperature.

the SV values are not exactly the same in all experiments. Table 7 shows the actual S/C and SV values for each experiment. The results are presented in Figures 1–5. In addition to the experimental measured values, the diagrams contain equilibrium values for S/C ) 2-5 (mol/mol) and also S/C ) 2.71 (mol/ mol) as the actual mean S/C value of the experiments. From the diagrams, it can be observed that high temperatures would favor the steam reforming reactions, reactions 1 and 3 according to their strong endothermic nature, lead to higher conversion of methane in the reactor. According to the equilibrium calculations, at higher reformer temperature, an increasing steam concentration in the feed results in a slight increase of the CH4 conversion. At outlet temperatures higher that 750 °C, CH4 conversion is observed to remain practically constant and independent of S/C and also shows a

high value. Therefore, the reactor temperature should be within the range of 700-750 °C but not below 700 °C because the CH4 conversion decreases significantly below 700 °C. It can be seen that high exit temperatures enhance H2 production through higher conversion of methane up to a certain point, where the H2 yield reaches a maximum value in the temperature range of 650-700 °C. At higher exit temperatures, there is a slight decrease in H2 yield, because the prevented exothermic water-gas shift reaction at high temperatures results in a decrease of the hydrogen production. The increase of CO2 conversion with temperature shows that the methane dry reforming reaction (reaction 4) takes place. At higher temperatures, this endothermic reaction is favored and therefore CO2 is consumed. It is recognized from the diagrams that the theoretical characteristic of the calculations are well-observed in the experimental results. Anyway, the difference between the experimental and theoretical results can be related to the high space velocity, which results in a short contact time between the active catalyst surface and particles. On the other hand, as a result of irregular flow, the individual feed gas molecules can pass the reactor without sufficient catalyst contact and not be reformed. The distinction between them is anyway not easily possible. At lower SV values, equilibrium condition can be reached, which make the comparison of experimental results especially at higher temperatures impossible; therefore, intentionally an approximate high SV value is selected and seems to be appropriate for such investigation. 4.4. Variation of Steam/Carbon Ratio. From reactions 1 and 2, it can be seen that the overall stoichiometric requirement for steam per carbon atom is 2. However, carbon forming reactions are promoted in the presence of active catalysts. This results in rapid deactivation of catalysts.7,8,13–16,23–25 Therefore, (23) Xu, J.; Froment, G. F. Methane steam reforming, methanation and water-gas shift: 1. Intrinsic kinetics. AIChE J. 1989, 35 (1), 88–96. (24) Rostrup-Nielsen, J. R.; Bak Hansen, J. H. CO2 reforming of methane over transition metals. J. Catal. 1993, 144, 38–49.

Biogas Steam Reforming

Figure 6. CH4 conversion versus S/C ratio.

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Figure 10. H2/CO ratio in synthesis gas versus S/C ratio. Table 8. Parameters of the Experiments for Optimizing S/C Ratio

Figure 7. H2 yield versus S/C ratio.

Figure 8. CO2 conversion versus S/C ratio.

Figure 9. CO selectivity versus S/C ratio.

excess steam is practically provided (S/C > 2). There are several reaction mechanisms that can cause carbon to be formed depending upon the conditions in the steam reformer (i.e., gas composition, operating temperature, total pressure, catalyst loading and selectivity, and also reformer design), and deposited carbon can be gasified again by steam or oxygen.15,23,25 Thus, there is a dynamic equilibrium between the carbon formation (25) Groote, A. M. D.; Froment, G. F. Simulation of the catalytic partial oxidation of methane to synthesis gas. Appl. Catal., A 1996, 138, 245– 264.

parameter

unit

value

CH4/CO2 ratio S/C ratio space velocity reactor temperature catalyst amount catalyst bed high residence time test duration

mol/mol mol/mol m3 kg-1 h-1 °C g mm s min

1.5 1.52-4.69 12.52 ( 0.73 700 158 89 0.28 120

Table 9. Parameters of the Experiments for Different S/C Values parameter

unit

value

CH4/CO2 ratio S/C ratio space velocity reactor temperature test duration

mol/mol mol/mol m3 kg-1 h-1 °C min

1.5 1-5 7-18 700 120

and carbon removal reactions. Overall, the steam reformer must be operated in a carbon-removing regime. While a very low value of S/C may cause significant carbon formation, a high S/C ratio enhances CH4 conversion but requires an additional amount of energy to produce the steam and also increase the mass flow through the plant and thus the size and cost of the equipment. Moreover, it involves the heating of the excess steam up to the reforming outlet temperature and subsequent condensation of the water downstream. Hence, an optimum S/C ratio should be optimized according to these considerations. For an experimental determination of the S/C effect on catalyst A and also reactor performance, a further series of tests are carried out at a reactor temperature of 700 °C and variable S/C values. The parameters are listed in Table 8. The results are shown in Figures 6–10. In addition to the experimentally measured values, the diagrams also contain equilibrium values for temperatures from 650 to 850 °C. An increase in the steam concentration of the feed results in a very strong increase of the CH4 conversion and a steep decrease of the CO2 conversion. A maximum CH4 conversion of about 99%, corresponding to an S/C ratio of about 4 is reached. However, a slighter effect of the S/C ratio is observed at higher temperatures. More steam in the feed also results in reaction 3 dominating over reaction 1, producing more CO2 and less CO. Consistently, a higher CO2 consumption is observed for lower steam/carbon ratios, indicating a higher rate of dry reforming. The CO2 conversion equal to zero shows the balance of the CO2 consuming reaction (reaction 4) with CO2 producing reactions (reactions 2 and 3). The increased S/C value leads to a smaller CO concentration and a higher selectivity toward H2 in the product stream. The H2 yield was observed to increase for increasing steam/methane ratios.

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Figure 11. CH4 conversion versus S/C ratio (mol/mol) and space velocity (m3 kg-1 h-1).

Figure 13. CO2 conversion versus S/C ratio (mol/mol) and space velocity (m3 kg-1 h-1).

Figure 12. H2 yield versus S/C ratio (mol/mol) and space velocity (m3 kg-1 h-1).

Figure 14. CO selectivity versus S/C ratio (mol/mol) and space velocity (m3 kg-1 h-1).

The experimental results follow closely those predicted by equilibrium calculations, and the difference as mentioned before can be related to the high space velocity or irregular gas flow. It can be finally recognized that, for S/C ratios in the range of 3-4 (mol/mol), an acceptable CH4 conversion and H2 yield can be obtained. Anyway, the available waste heat has an important effect on the optimization of this parameter in a plant, which is investigated for two plant concepts in Ashrafi et al.20 4.5. Variation of Space Velocity. The space velocity (or feed gas contact time with active sites of the catalyst) has a high impact on the investment costs of the installation. High space velocities induce small reactor dimensions and little catalyst content. At the same time, reactor performance is reduced.19 Therefore, the space velocity has a high potential for optimization. The following diagrams show steam reforming dependency on contact time over catalysts A and B under atmospheric pressure condition. In these experiments, the S/C ratio was changed stepwise from higher to lower values at a constant space velocity. Other parameters are listed in Table 9. The results are shown in Figures 11–15. As observed clearly at low space velocities, methane conversion increases. In the case of catalyst A, a space velocity of 7.78 m3 kg-1 h-1 led almost to equilibrium methane conversion. However, at higher SV values, methane conversion decreased drastically. The steep decrease of methane conversion with a decreasing space velocity can be reflected in the reforming ability of the catalysts. On the other hand, the H2/CO profiles remained unchanged for both catalysts for different space velocities, indicating much higher reaction rates of the water-gas shift reaction (reaction 2) in comparison to other reactions. The relative yields of CO and CO2 are in good agreement with a fully equilibrated thermodynamic model of the system even at high SV values. This suggests that, under the present operating conditions, the

Figure 15. H2/CO ratio in synthesis gas versus S/C ratio (mol/mol) and space velocity (m3 kg-1 h-1).

water-gas shift reaction is controlled by thermodynamic parameters rather than kinetic limitations. It should be noted that catalyst A showed lower conversion than catalyst B at the same space velocity equal to about 17.85 m3 kg-1 h-1, according to its higher active contact surface. 5. Conclusion Six different Ni-based commercial catalysts have been investigated in terms of their activity, stability, resistance to carbon formation, size, and shape. Among these catalysts, two catalysts of Su¨d Chemie are employed for further investigations on the purpose of testing model biogas steam reforming thermodynamics, which are presented in this paper. On the basis of the above results, the following conclusions are drawn. It is confirmed that both employed catalysts are very active and stable under the typical steam methane reforming conditions. No catalyst deactivation and carbon formation is observed during the above experiments. Anyway deactivation and coking are of great importance in long-term experiments, which will be performed at the pilot plant in Strem, Austria. The investigation of other Ni-based catalysts with different compositions and supports and also iron-based catalysts could be suggested.

Biogas Steam Reforming

Steam reforming of biogas (60% CH4 and 40% CO2) must therefore ideally be carried out at a high temperature and high steam/carbon ratio to achieve maximum conversion. A high S/C ratio always improves the reformer performance, which should be optimized according to economics and entire plant heat integration. However, at S/C ratios lower than 3, the impact of the dry reforming reaction of methane with CO2 is significant. A high temperature always improves methane conversion in the reformer, while it remains approximately constant at temperatures higher than 750 °C. On the other hand, operating at temperatures lower than 700 °C is not suggested because of the significant decrease of methane conversion, higher danger of carbon formation, and high required catalyst load. In contrast, as a result of the exothermic nature of the water-gas shift reaction, H2 yield shows a maximum value within the range of 650-700 °C, which can be different depending upon the entire plant configuration. According to the above results, 700 °C is found to be the optimal choice. Finally, it should be examined how these parameters affect the entire plant and how the efficiency will be changed consequently; thus, the optimum operating area will be limited. Basic investigations regarding this topic have been performed for two different plant configurations and presented by Ashrafi et al.20

Energy & Fuels, Vol. 22, No. 6, 2008 4189 Acknowledgment. The authors gratefully acknowledge the financial support by the Renewable Energy Network Austria (ReNet Austria), Energy from Biogas (Austrian funds program KNET/KIND), as well as the company Su¨d-Chemie for providing the catalysts.

Nomenclature BG ) biogas cat ) catalyst E ) end in ) inlet stream of the reformer m ) mass, kg N˙x ) molar flow rate of component x, mol/h m ˙ x ) mass flow rate of component x, kg/h out ) outlet stream of the reformer S ) start SG ) synthesis gas S/C ) steam/carbon molar ratio, mol/mol SV ) space velocity, m3 kg-1 h-1 t ) time, h V˙x ) operating volume flow rate of component x, m3/h wf ) water free xs ) mass-based saturation degree, kgH2O/kgSG YA ) concentration of component A in a mixture, mol % ∆rH ) enthalpy change of a reaction, kJ/mol EF800081J