Energy & Fuels 2008, 22, 2053–2060
2053
Thermocatalytic Conversion of Landfill Gas and Biogas to Alternative Transportation Fuels Nazim Muradov* and Franklyn Smith Florida Solar Energy Center, UniVersity of Central Florida, 1679 Clearlake Road, Cocoa, Florida 32922-5703 ReceiVed January 23, 2008. ReVised Manuscript ReceiVed March 3, 2008
Landfill gas (LFG) and biogas are important renewable resources for production of alternative transportation fuels. The resources of LFG and biogas are vast and widely available but remain mostly unused. In this paper, the authors assess the technical feasibility of direct catalytic reforming (i.e., without preliminary methane recovery from these gases) of LFG and biogas to synthesis gas that could further be processed to synthetic liquid hydrocarbon fuels via Fischer–Tropsch (FT) synthesis. The catalytic activity and selectivity of a number of noble metal (Ru, Ir, Pt, Rh, Pd) and Ni-based catalysts for reforming of a model CH4-CO2 mixture mimicking a typical LFG into a synthesis gas were evaluated. The issues related to the catalysts stability and process sustainability under the conditions that are favorable for carbon deposition were explored. The experimental data are in a good agreement with AspenPlus simulation results. It was shown that the syngas produced by Ni-catalyzed steam-assisted reforming of a model LFG is suitable for production of liquid hydrocarbons via FT synthesis.
Introduction In view of ever-increasing prices for hydrocarbon fuels and diminishing resources of fossil fuels, it is imperative to find alternative, preferably, renewable resources for production of alternative transportation fuels (ATF), such as hydrogen, methanol, synthetic hydrocarbons, etc. Renewable methanecontaining gases (RMG) such as landfill gas (LFG), biogas, and digester gas are important resources for production of ATF. RMG are generated by anaerobic digestion or fermentation of animal, agricultural, municipal, and other types of biodegradable waste. Although the resources of RMG are vast and widely available throughout the country, they remain mostly unused. For example, in the U.S., only a small portion (330 out of 2100) of landfills utilize LFG to generate heat or electricity, and the majority of landfills burn the gas outright.1 An important advantage to using RMG for ATF production is that it is a local resource that can be obtained at little or no cost. It is noteworthy that if not used RMG may pose serious environmental problems since methane is a more potent greenhouse gas than CO2. RMG are complex gaseous mixtures consisting primarily of methane and CO2 and small amounts of N2 and O2 (typically, less than 10% vol) and trace quantities (in most cases, 1), which may result in catalyst deactivation due to the deposition of carbon (or coke) formed by an undesirable methane decomposition reaction: CH4 f C + 2H2 ∆H ° ) 76.0 kJ/mol
(2)
Methane decomposition is an endothermic reaction favored by high temperatures and low pressure. Thus, from the practical viewpoint, it is preferable to operate CO2-reforming of methane at the moderate temperatures maintaining a CH4:CO2 ratio close to unity, which would require a catalyst that kinetically inhibits the carbon formation under the conditions that are thermody-
10.1021/ef8000532 CCC: $40.75 2008 American Chemical Society Published on Web 04/22/2008
2054 Energy & Fuels, Vol. 22, No. 3, 2008
Figure 1. Production of alternative fuels from RMG. WGS ) water-gas shift, PSA ) pressure-swing adsorption, NG ) natural gas, FT ) Fischer–Tropsch.
namically favorable for carbon deposition.2 Iron-, cobalt-, and nickel-based catalysts are known to be particularly active in the methane decomposition reaction. The reaction productscarbon (or coke)sblocks active sites of the catalyst leading to its rapid deactivation. In principle, the deposition of coke could be prevented by either using highly selective catalysts (i.e., catalysts that promote CO2 reforming of methane, but suppress methane decomposition reaction) or adding an oxidizing agent (e.g., steam, oxygen) to the CH4-CO2 feedstock, which will guard the catalyst against coke formation. Most of the reported research on CO2 reforming of methane relates to noble metal- and Ni-based catalysts. According to Rostrup-Nielsen and Bak Hansen, the amount of carbon deposited on the metal catalysts (at 650 °C) decreases in the following order: Ni > Pd ≈ Rh > Ir > Pt , Ru.3 The catalyst supports and promoters have a significant effect on the rate of carbon deposition; in particular, some authors4 demonstrated the following order of the carbon formation rate for different supports: Pt/Al2O3 , Pt/TiO2 > Pt/ZrO2. The issues related to coke deposition and catalyst deactivation during reforming of an equimolar CH4-CO2 mixture over supported Ni and Pt catalysts were reported by Bitter et al.4 and Wang et al.5 Comprehensive reviews on the topic of CO2 reforming of methane over transition metal catalysts have been reported by Hu and Ruckenstein2 and Bradford and Vannice.6 Another technical challenge facing RMG reforming relates to the presence of potentially harmful impurities (e.g., sulfurous and silicon- and halogen-containing compounds) that could easily deactivate catalysts used in the reforming process. Some of these impurities are not present in natural gas or other industrial gases, so the pretreatment of RMG could substantially differ from that of the conventional feedstocks for reforming processes. In principle, these contaminants could efficiently be removed from RMG before the reforming stage using conventional technologies (e.g., adsorption, absorption, etc.).7–9 (This aspect will be discussed in more detail in the last section of the paper). Other minor components that are present in RMG such as N2 and NMOC are unlikely to adversely affect the catalyst activity during the RMG reforming reaction. Although catalytic reforming of an equimolar CH4-CO2 mixture to a syngas has been actively researched over the years (see the discussion above), there are scarce literature sources (2) Hu, Y.; Ruckenstein, E. AdV. Catal. 2004, 48, 297–345. (3) Rostrup-Nielsen, J.; Bak Hansen, J.-H. J. Catal. 1993, 144, 38–49. (4) Bitter, J.; Seshan, K.; Lercher, J. J. Catal. 1999, 183, 336–343. (5) Wang, S.; Lu, G.; Millar, G. Energy Fuels 1996, 10, 896–904. (6) Bradford, M.; Vannice, M. Catal. ReV. 1999, 41, 1–42. (7) Henz, H. et al. Gas Production. In Ullmann′s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KgaA: New York, 2002. (8) Kohl, A., Nielsen, R. Gas Purification; Gulf Publishing Co.; Houston, TX, 1997. (9) Spiegel, R.; Preston, J. Energy 2003, 28, 397–409.
MuradoV and Smith
with regard to experimental and analytical studies of catalytic reforming of methane-rich CH4-CO2 mixtures (i.e., where CH4: CO2 > 1) to hydrogen or syngas. A few studies relate to reforming of a simulated biogas to hydrogen mostly for fuel cell applications using Ni-based reforming catalysts and conventional water-gas shift catalysts.10–13 No data was found in the open literature with regard to processing of RMG to synthetic hydrocarbons or other alternative fuels. The objectives of this study are to evaluate the catalytic activity and selectivity of a number of catalysts for reforming of methane-rich CH4-CO2 mixtures and to explore the catalyst stability and process sustainability issues under operational conditions favorable to deposition of carbon. A special emphasis was put on the suitability of the produced syngas for Fischer–Tropsch synthesis of liquid hydrocarbons. The technical feasibility of converting the RMG-derived syngas to C5+ liquid hydrocarbons is reported. Due to similarities between LFG, biogas, and digester gas and for the sake of simplification of the data presentation, we limit our discussion to LFG only, noting that all the principle results and conclusions discussed in this paper are also applicable to biogas and digester gas. Experimental Section Reagents and Catalysts. In this work, a premixed gaseous mixture with the composition of CH4 56.9 and CO2 43.1% vol (i.e., a molar ratio of CH4:CO2 ) 1.3:1) obtained from Holox Inc. was used as a feedstock in all experiments (this particular composition mimics that of the LFG produced by the Cocoa landfill, Florida). For the sake of simplicity, N2, O2, H2S, NMOC, and other impurities were not included in the tested feedstock (It was assumed that these minor components could efficiently be removed from LFG prior to the reforming stage by off-the-shelf methods; see the discussion in the Introduction section). Argon (99.999% vol) and oxygen (99.5% vol) were obtained from Air Products and Chemicals. Steam was produced from deionized water and introduced into a reactor using a precision syringe pump (Cole Palmer) and a water evaporator. The catalystssRu (0.5% wt)/Al2O3, Pd (1.0% wt)/ Al2O3, Pt (0.5% wt)/Al2O3swere obtained from Aldrich Chemical Co. Rh (5.0% wt)/Al2O3 catalyst was purchased from Strem Chemicals. Ir (1.0% wt)/Al2O3 and Ni (55–60% wt)/kieselguhr catalysts were obtained from Alfa Aesar. Commercial catalysts, NiO (25–45% wt)-Al2O3 (45–65% wt)-Ca aluminate (3–8% wt) and NiO (1–15% wt)/Al2O3 were obtained from Süd-Chemie Inc. The above catalysts were used in the form of 10–18 mesh granules. The preparation methods for Co- and Fe-based Fischer–Tropsch catalysts were based on the procedures reported in refs 14 and 15, respectively. A proprietary iron-based catalyst was also used in the experiments. The catalysts were used in the form of a powder (
