Energy Fuels 2009, 23, 5042–5048 Published on Web 08/24/2009
: DOI:10.1021/ef900426g
Impact of Steam on the Interaction between Biomass Gasification Tars and Nickel-Based Solid Oxide Fuel Cell Anode Materials Joshua Mermelstein,† Nigel Brandon,‡ and Marcos Millan*,† †
Department of Chemical Engineering, ‡Department of Earth Science Engineering, Imperial College London, U.K., SW7 2AZ Received May 8, 2009. Revised Manuscript Received August 3, 2009
The combination of solid oxide fuel cells (SOFCs) and biomass gasification is a potentially attractive technology for the production of clean and renewable energy. However the impact of tars, formed during biomass gasification, on the performance and durability of SOFC anodes has not been well established experimentally. This paper reports on the comparison between thermodynamic predictions and experimental measurements of carbon formation arising from the steam reforming of 15 g/Nm3 benzene as a model biomass gasification tar over two commercially available nickel-based SOFC anode materials, Ni/YSZ (yttria-stabilized zirconia) and Ni/CGO (gadolinium-doped ceria). Parallel experiments were performed using 60:40 wt % NiO/YSZ and 50:50 wt % NiO/CGO powders as catalyst material, and the degree of carbon formation was examined by temperature-programmed oxidation. The addition of steam reduced carbon formation on both materials, with Ni/CGO showing slightly more carbon formation compared to Ni/YSZ. This could reflect the differing nickel content in both materials, the activity of the anode material toward tar reforming, and/or the difference in surface area of each material. Carbon formation was excessive for both materials at steam to carbon ratios below 1. However, carbon formation was also present above the thermodynamically stable region for carbon formation with both materials. Tar conversion, and thus CO concentration, was higher for the Ni/YSZ material, whereas the oxidative behavior of ceria in the Ni/CGO anode material resulted in higher CO2 concentrations.
experimentally. Previous results7 showed that benzene could be effectively used as a model tar to study the degradation of SOFC anodes arising from associated carbon formation. This work aims to study the mitigation of carbon formation from benzene model gasification tars in a synthetically generated biomass gasification syngas through the use of steam over two commercially available SOFC anode materials Ni/YSZ (yttria stabilized zirconia) and Ni/CGO (gadolinium-doped ceria), comparing the results with thermodynamic predictions. 1.1. Biomass Gasification Tars. Biomass gasification involves the partial oxidation of biomass materials at temperatures of 600-1000 °C (downdraft gasification) resulting in the production of a syngas consisting of hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and water (H2O), which can be used as a fuel gas to power combustion engines, heaters, burners, turbines, and fuel cells to produce electric power. The gas composition resulting from biomass gasification depends on both the temperature and pressure at which gasification takes place and the gasifier type.8 Table 1 describes the typical properties of a syngas from downdraft biomass gasification.9-14
1. Introduction The combination of biomass gasification with solid oxide fuel cells (SOFCs) is of increasing interest as a high-efficiency and environmentally friendly method of producing electricity and heat.1-4 The use of biomass plays an important role in the mitigation of the environmental impacts of energy production, such as global warming. It has been well documented in the literature that the gasification of biomass has the most favorable thermo-chemical conversion to renewable energy, while at the same time producing lower levels of sulfur and nitrogen compounds, and most importantly reduced carbon emissions, than those associated with processes such as coal gasification.5,6 SOFCs operating at high temperatures are able to internally reform a wide range of fuels, making them one of the most flexible of all fuel cells. Combined heat and power processes based on SOFCs and biomass gasification have the potential to achieve efficiencies of >85%. Until recently,7 the impact of tars from the gasification process has not been well established *To whom correspondence should be addressed. Fax: þ44(0)20 7594 5638. Phone: þ44 (0) 20 7594 1633. E-mail:
[email protected]. uk. (1) Fryda, L.; Panopoulos, K. D.; Kakaras, E. Energy Convers. Manage. 2008, 49, 281–290. (2) Omosun, A. O.; Bauen, A.; Brandon, N. P.; Adjiman, C. S.; Hart, D. J. Power Sources 2004, 131, 96–106. (3) Seitarides, T.; Athanasiou, C.; Zabaniotou, A. Renew. Sustain. Energy Rev. 2008, 12, 1251–1276. (4) Vasileiadis, S.; Ziaka-Vasileiadou, Z. Chem. Eng. Sci. 2004, 59, 4853–4859. (5) Chaudhari, S. T.; Dalai, A. K.; Bakhshi, N. N. Energy Fuels 2003, 17, 1062–1067. (6) Ni, M.; Leung, D. Y. C.; Leung, M. K. H.; Sumathy, K. Fuel Process. Technol. 2006, 87, 461–472. (7) Mermelstein, J.; Millan, M.; Brandon, N. P. Chem. Eng. Sci. 2009, 64, 492–500. r 2009 American Chemical Society
(8) Stevens, D. NREL/SR-510-29952; National Renewable Energy Laboratory: 2001. (9) Giltrap, D. L.; McKibbin, R.; Barnes, G. R. G. Solar Energy 2003, 74, 85–91. (10) McKendry, P. Bioresour. Technol. 2002, 83, 55–63. (11) Reed, T., The Biomass Energy; Foundation Press: Golden, CO, 1988, 1-99. (12) Vervaeke, P.; Tack, F. M. G.; Navez, F.; Martin, J.; Verloo, M. G.; Lust, N. Biomass Bioenergy 2006, 30, 58–65. (13) Zainal, Z. A.; Rifau, A.; Quadir, G. A.; Seetharamu, K. N. Biomass Bioenergy 2002, 23, 283–289. (14) Baron, S.; Brandon, N.; Atkinson, A.; Steele, B.; Rudkin, R. J. Power Sources 2004, 126, 58–66.
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Solid char, plus tars that are in the gas phase but would normally condense at ambient temperatures, are also formed from the gasification process. The amount of tars in the gas stream can be as high as several hundred g/Nm3 for updraft gasifiers, 2.18,19 The significance of this work shows that under high steam and current density operation, the fuel cell is able to utilize readily available liquefied hydrocarbons for home heating and hot water. Higher fuel utilization (>50%) has an effect on the SOFCs ability to operate at low steam to carbon ratios with reduced carbon formation, as H2O and CO2 molecules, produced in the fuel stream by electrochemical reaction during power generation, will be expected to suppress carbon deposition and/or remove deposited carbon.17,18 Yamaji et al.18 found that operating a Ni/ScSZ SOFC on ethane in 1.2 mol % steam at a fuel utilization of 55% resulted in minor carbon formation, but that cell operation still remained stable over a one week period. Partial oxidation (POX) can be used directly at the anode to reform the incoming hydrocarbon fuel and suppress carbon formation. Cheekatamarla et al.20 demonstrated the use of a four-layer internal reforming SOFC containing a porous catalyst support membrane coated on the anode surface to internally reform natural gas, propane, butane, LPG, and biomass gas using POX in air, leading to high power densities with stable power output. In most cases SOFCs are able to run on the heavier hydrocarbon-based fuels. However, carbon deposition is still a major problem for catalyst and anode deactivation when using these fuels. Additionally, while a body of work has been done to develop catalysts for the conversion of gasification tars, the effects of polyaromatic tar-structured hydrocarbons on SOFC anodes has not been well established. Previous studies7 found that benzene was a sensitive indicator of the tendency for carbon formation on SOFC anodes from biomass gasification tars. This work studies the impact of steam on carbon formation from benzene as a biomass gasification tar analog, over two commercially available SOFC anode materials, Ni/YSZ and Ni/CGO. 2. Experimental Section
Kishimoto et al.17 investigated the use of C12H26 as a model for kerosene fuel with Ni/ScSZ SOFC anodes operating at a steam to carbon ratio (S/C) of 2. Over one week of operation they obtained an average current density of 140 mA/cm2, operating at a fuel utilization of 55% without significant degradation. Their thermodynamic predictions
2.1. Experimental Setup. A SOFC test station has been developed to test the carbon deposition characteristics of synthetically generated biomass gasification tars over nickelbased catalysts, and the effect of tars on SOFC performance, as shown in Figure 1. The test station has the capability of synthetically generating a typical biomass gasification syngas using pure N2, H2, CO, CO2, and methane, mixed to the desired partial pressures using a Fideris FCTS GMET mass flow control unit. CO, CO2, and methane were not used in this study as these components may contribute to additional carbon formation and/or additional oxidation behavior via CO2 reforming and steam production from the reverse water-gas shift (RWGS) reaction, which is the subject of present and future research work. Therefore, to identify the effect of steam on carbon formation from biomass gasification tar, only the fraction of
(15) Cao, Y.; Wang, Y.; Riley, J. T.; Pan, W.-P. Fuel Process. Technol. 2006, 87, 343–353. (16) Zheng, J.; Strohm, J. J.; Song, C. Fuel Process. Technol. 2008, 89, 440–448. (17) Kishimoto, H.; Yamaji, K.; Horita, T.; Xiong, Y.; Sakai, N.; Brito, M. E.; Yokokawa, H. J. Power Sources 2007, 172, 67–71.
(18) Yamaji, K.; Kishimoto, H.; Xiong, Y.; Horita, T.; Sakai, N.; Brito, M. E.; Yokokawa, H. J. Power Sources 2006, 159, 885–890. (19) Yi, Y.; Rao, A. D.; Brouwer, J.; Samuelsen, G. S. J. Power Sources 2005, 144, 67–76. (20) Cheekatamarla, P. K.; Finnerty, C. M.; Cai, J. Int. J. Hydrogen Energy 2008, 33, 1853–1858.
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Figure 1. Schematic illustration of the SOFC test station. Table 1. Typical Properties of Biomass Gasification Syngas compound H2 CO CO2 H2O CH4 N2
11
gas (vol %) 14.5 21 9.7 4.8 1.6 48.4
A 40 mg portion of unreduced catalyst material was lightly compressed between two pieces of quartz wool in the 6 mm OD quartz tube reactor. The material was heated to a typical SOFC operating temperature of 765 °C at a rate of 10 °C/min in dry nitrogen. Reduction took place by exposing the catalyst at temperature in 2.5% humidified steam and 5% H2 for 30 min, then increasing to 25% H2 over a period of 30 min at a flow rate of 50 mL/min. The sample was then held in 25% H2 for 30 min. After reduction, flow was increased to 100 mL/min, and the hydrogen concentration changed to the experimental operating conditions of 15% H2 and the experiment’s appropriate steam content. Samples were exposed to 15 g/Nm3 benzene model tar for 1 h. Due to the excessive pressure drop occurring from high carbon deposition, 1% humidified steam experiments were only run for 30 min. The benzene concentration was monitored continuously via an online mass spectrometer (Thermo Fisher Prolab). The downstream gas composition (CO2, CH4, and CO) was analyzed by gas chromatography (Varian 3600) after 20 and 55 min. At the end of the experiment, the reactor was cooled to room temperature in dry 15% H2 balance N2. 2.3. Temperature Programmed Oxidation. Temperature-programmed oxidation (TPO) of the sample was done in situ to determine the quantity of carbon deposited on the anode material, assuming total oxidation of carbon to CO2. Oxidation of the carbon was driven by flowing 80 mL/min of a 2% O2 balance argon gas (purity (2%, BOC gases) to the reactor with a heating rate of 10 °C/min from room temperature to 900 °C. A Thermo Fisher Pro Lab mass spectrometer was used to monitor CO2, CO, O2, and other minor constituents in the effluent gas stream. A calibration curve was made by diluting a mixture of 1% CO2 balance argon (purity ( 2%, BOC gases) with argon (purity 99.999%, BOC gases) from 0.05 to 0.5% CO2 and relating this to the mass intensity as measured by the mass spectrometer.
dry gas (avg. vol %) 15 24 11 2 48
hydrogen represented in Table 1 was used, using 15 mL/min H2 mixed with 85 mL/min N2 (purity >99.999%, BOC gases). The synthetic mixture is passed through a temperature-controlled water bath for gas humidification, or fed directly to a heated line to study the effects that dry gas has on the system. The gas enters a heated and insulated line that contains a port to inject synthetic model tars via a syringe pump (KD Scientific) at a rate of 102 μL/h, producing a tar concentration of 15 g/Nm3. A portion of the tar injection line is heated to a temperature slightly above the boiling point of the model tar compound to allow for vaporization of the tar species into the gas phase, and subsequent mixing with the incoming syngas. Downstream, the tar-containing syngas is fed to a heated furnace containing a quartz tube reactor with an OD of 6 mm. The discharge from the reactor is sent to a mass spectrometer and gas chromatograph for gas stream analysis. Inlet and outlet baseline concentrations of benzene as model tar were measured using the mass spectrometer while gas flowed through a blank quartz tube reactor. The results showed insignificant changes in baseline concentration indicating thermal decomposition of benzene was not occurring. 2.2. SOFC Anode Catalyst Preparation. Two types of catalysts were used in these experiments. A 60:40 NiO/YSZ (yttria-stabilized zirconia) powder was sourced from Fuel Cell Materials having an average particle size (d50) of 0.48 μm and a surface area of 2.45 m2/g. The material was calcined at 1300 °C in air for 1 h, a typical sintering temperature for SOFC anodes. The calcined powder was then sieved to a particle size of 250 μm. NiO/CGO (gadolinium-doped ceria) powder was derived from an existing screen-printable ink sourced from Fuel Cell Materials containing 50% nickel, an average particle size (d50) of 0.34 μm, and a surface area of 7.59 m2/g. The ink was first dried at 250 °C for 2 h and milled to form a fine powder. Then powder was then calcined in air at 1300 °C for 3 h and sieved to a particle size of 250 μm.
3. Results and Discussion 3.1. Thermodynamic Predictions. 3.1.1. Phase Diagram of Ni and NiO. It is important to understand the effects of each syngas component arising from the gasification of biomass material on the interaction of tars with SOFC anodes. In this phase of the research, hydrogen at a concentration of 15 vol%, typical of the output from biomass 5044
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Figure 3. Thermodynamic equilibrium diagram of the main species present as a function of steam/carbon ratio for a mixture of 15 g/Nm3 benzene tar in 15% H2/N2 at 765 °C.
Figure 2. Combined phase equilibrium diagram of Ni and NiO system from 100 to 1100 °C across a pO2 range of 10-40 to 100 bar. “2” represents the pO2 within the fuel cell for the fuel stream of 15% H2 and 7.5% humidified steam at 765 °C used in this study.
as the model tar with a concentration of 15 g/Nm3. Steam was varied from 0 to 7.5% (S/C = 3) at the experimental operating temperature of 765 °C. The equilibrium mole fraction is shown in Figure 3 with increasing steam to carbon ratios. In Figure 3, the major species in the gas phase are H2, CH4, H2O, CO, CO2, and N2 (not shown) with carbon existing as a pure condensed phase. The thermodynamic calculations show that in dry conditions carbon formation is severe, which has been previously shown experimentally.7 At a temperature of 765 °C, carbon is not thermodynamically favored to form at a S/C ratio of greater than 1 (2.5% steam). Although, as noted by Yi et al.,19 systems should be designed to operate under average conditions that are significantly away from the point where carbon formation and deposition is favored. Additionally, thermodynamic calculations show that as the S/C ratio is increased up to a value of 1, CO and H2 concentrations increase with the level of CH4 and CO2 remaining stable. As the S/C ratio continues to increases above S/C=1, CO is consumed and CO2 and H2O are produced. These trends are similar to thermodynamic results discussed in references 22-26, describing the thermodynamic analysis of steam reforming of hydrocarbon-based fuels, and have similar trends to the thermodynamic effects of current density on an operating fuel cell as discussed in Koh et al. and Singh et al.27,28 3.2. Steam Reforming of Benzene over SOFC Anode Materials. The conversion of tar compounds, arising from gasification, over commercially available nickel-based catalysts
gasification described in Table 1, along with increasing concentrations of steam, were used to explore the reforming capability of the anode materials and the degree of carbon deposition arising from tars present in such fuels. However, due to the low hydrogen concentration and moderate steam content fed to the system, it is first important to understand if the commonly used nickel-based anodes are likely to oxidize from the increase in pO2 arising from equilibrium with H2 and H2O via the hydrogen oxidation reaction: 2H2 þ O2 T2H2 O The phase equilibrium diagram of Ni and NiO shown in Figure 2 was constructed using HSC Chemistry software (version 5.11, Outokumpu Research Oy, Finland) over a temperature range of 100-1100 °C with increasing pO2. Figure 2 shows that, at the operating temperature of 765 °C used in these experiments, the thermodynamic boundary for Ni/NiO lies at a pO2 of 1 10-14.5. At the operating conditions of 765 °C, 15% H2, and a maximum steam content of 7.5% humidified steam (S/C ∼ 3), the pO2 is 1.62 10-20, well below the thermodynamic boundary where the nickel will begin to reoxidize, such that nickel will remain in the metallic phase during the experimental studies reported here. 3.1.2. Thermodynamic Calculations;Effects of Steam on Carbon Formation. Thermodynamic calculations were carried out using HSC Chemistry software (version 5.11, Outokumpu Research Oy, Finland) via the Gibbs free energy minimization method as described by Sasaki and Teraoka.21 Thermodynamic calculations provide an insight into the expected amount of carbon that will deposit under equilibrium conditions. However, such calculations do not account for the kinetics of the reactions. Calculations were conducted for a dry feed of 15% hydrogen, balance nitrogen, with a total flow rate of 100 mL/min and benzene
(22) Aktas, S.; Karakaya, M.; AvcI, A. K., Int. J. Hydrogen Energy 2009, 34, 1752–1759. (23) Douvartzides, S. L.; Coutelieris, F. A.; Demin, A. K.; Tsiakaras, P. E. AIChE J. 2003, 49, 248–257. (24) Mahishi, M. R.; Goswami, D. Y. Int. J. Hydrogen Energy 2007, 32, 3831–3840. (25) Sasaki, K.; Teraoka, Y. J. Electrochem. Soc. 2003, 150, A878– A884. (26) Vasudeva, K.; Mitra, N.; Umasankar, P.; Dhingra, S. C. Int. J. Hydrogen Energy 1996, 21, 13–18. (27) Koh, J. H.; Kang, B. S.; Lim, H. C.; Yoo, Y.-S., Electrochem. Solid-State Lett. 2001, 4 (2), A12-A15. (28) Singh, D.; Hernandez-Pacheco, E.; Hutton, P. N.; Patel, N.; Mann, M. D. J. Power Sources 2005, 142, 194–199.
(21) Sasaki, K.; Teraoka, Y. J. Electrochem. Soc. 2003, 150, A885– A888.
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Figure 5. Formation of carbon as measured by TPO over 39 mg of Ni/YSZ and 36 mg of Ni/CGO catalysts (dp =250 μm), 40 mg as oxides, in humidified steam exposed to 15% H2 and 15 g/Nm3 benzene model tar for 1 h (1% steam shown for 30 min) at 765 °C.
Ni/CGO anode could lead to a more active catalyst for steam reforming as shown by Timmerman et al.,39 who reported that at 800 °C methane conversion was higher over a Ni/ CGO anode than a Ni/YSZ anode. In some cases, materials such as Ni/ScSz (scandia-stabilized zirconia) or Ni/CGO are infiltrated into the anode to increase ionic conductivity and improve hydrogen oxidation kinetics.40,41 This could alter the internal reforming capability of the SOFC anode. Additionally, ceria-based anodes have been widely recognized to be effective in suppressing carbon deposition42 due to the redox behavior of ceria.43 It is therefore beneficial to understand the carbon formation and reforming characteristics of both types of SOFC anode cermet by using TPO to study the behavior of catalysts made from these materials. The pretreatment of both the NiO/YSZ and NiO/CGO powders were similar. However, as the NiO/CGO material was sourced as a screen printable ink, additional steps, as previously described, were required to extract the NiO/CGO powder. Additionally the NiO/CGO had been further processed from its original powder form to make the ink. Thus, the physical structure of the NiO/YSZ and NiO/CGO powder differed. The NiO/YSZ exhibited a uniform spherical shape (Figure 4a) while the NiO/CGO powder was rough and jagged (Figure 4b). The anode materials were each exposed to 15 g/Nm3 (1.92 10-5 mol/min) benzene model tar in a gas stream of 15% H2 balance N2 with a total flow rate of 100 mL/min for 1 h and steam concentrations from 1 to 7.5%. TPO was used to measure the amount of carbon formed on the catalyst after each experiment. Figure 5 shows the amount of carbon formed for Ni/YSZ and Ni/CGO under humidified steam concentrations of 1-7.5% (S/C of 0.4-3). Data taken at 1% humidified steam is shown in this plot at a 30 min duration, as the experimental error greatly increased due to excessive
Figure 4. SEM micrographs of NiO/YSZ (a) and NiO/CGO (b) powder calcined at 1300 °C.
has been widely studied outside of the SOFC context.15,29-38 The support structure of the catalyst, formulation, and manufacturing process play an important role in determining catalyst reforming activity and ability to inhibit carbon formation. However, the effect of tar compounds on SOFCrelevant anode materials has not been studied in great detail. High-temperature SOFCs utilize a Ni/YSZ-based cermet, while intermediate-temperature SOFCs often utilize a Ni/ CGO-based cermet. The mixed ionic-electronic conductivity of CGO that will be evident when it is present in the anode compartment of an SOFC is expected to increase the reaction surface area by facilitating electron and oxygen ion transport in the anode.14 This may indicate that in a fuel cell the use of a (29) Bergman, P.; Paasen, S. v.; Boerrigter, H., 2002. (30) Coll, R.; Salvado, J.; Farriol, X.; Montane, D. Fuel Process. Technol. 2001, 74, 19–31. (31) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Fuel Process. Technol. 2005, 86, 707–730. (32) Engelen, K.; Zhang, Y.; Draelants, D. J.; Baron, G. V. Chem. Eng. Sci. 2003, 58, 665–670. (33) Han, J.; Kim, H. Renew. Sustain. Energy Rev. 2008, 12, 397–416. (34) Kimura, T.; Miyazawa, T.; Nishikawa, J.; Kado, S.; Okumura, K.; Miyao, T.; Naito, S.; Kunimori, K.; Tomishige, K. Appl. Catal., B 2006, 68, 160–170. (35) Milne, T. A.; Evans, R. J. NREL/TP-570-25357; National Renewable Energy Lab: 1998. (36) Sutton, D.; Kelleher, B.; Ross, J. R. H. Fuel Process. Technol. 2001, 73, 155–173. (37) Wang, T.; Chang, J.; Cui, X.; Zhang, Q.; Fu, Y. Fuel Process. Technol. 2006, 87, 421–428. (38) Zhang, R.; Brown, R. C.; Suby, A.; Cummer, K. Energy Convers. Manage. 2004, 45, 995–1014. (39) Timmermann, H.; Fouquet, D.; Weber, A.; Ivers-Tiffee, E.; Hennings, U.; Reimert, R. Fuel Cells 2006, 6, 307–313.
(40) Blennow, P.; Hansen, K. K.; Wallenberg, L. R.; Mogensen, M. ECS Trans. 2008, 13, 181–194. (41) Wang, Z. R.; Qian, J. Q.; Wang, S. R.; Cao, J. D.; Wen, T. L. Solid State Ionics 2008, 179, 1593–1596. (42) Zhu, W. Z.; Deevi, S. C. Mater. Sci. Eng. A 2003, 362, 228–239. (43) Laosiripojana, N.; Sutthisripok, W.; Assabumrungrat, S. Chem. Eng. J. 2005, 112, 13–22.
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carbon build up, plugging of the reactor, and reduced flow arising from increased pressure drop for times >30 min. It should be noted that each of these experiments, except those at 1% humidified steam, were run above the thermodynamic region in which carbon formation is favored, shown in Figure 3. On average, neither anode material appeared to show a significant trend in the amount of carbon formed for steam concentrations between 2.5 and 7.5%. However, a sharp decrease in carbon formation was observed as steam was increased from 1 to 2.5%, which is consistent with thermodynamic predictions. There was an apparent decline in the amount of carbon formed on the Ni/YSZ material under 5% steam, which is not currently understood, although this could reflect experimental error as the total conversion of benzene model tar to carbon over a period of 1 h amounted to
