Ind. Eng. Chem. Res. 2005, 44, 4543-4551
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Renewable Production of Methane from Woody Biomass by Catalytic Hydrothermal Gasification Maurice H. Waldner and Fre´ de´ ric Vogel* General Energy Department, Paul Scherrer Institut, Laboratory for Energy and Materials Cycles, CH-5232 Villigen PSI, Switzerland
Production of synthetic natural gas (SNG) from wood by a catalytic hydrothermal process was studied in a laboratory batch reactor suitable for high feed concentrations (10-30 wt %) at 300410 °C and 12-34 MPa with Raney nickel as the catalyst. A maximum methane yield of 0.33 (g of CH4)/(g of wood) was obtained, corresponding to the thermodynamic equilibrium yield. The carbon gasification efficiency was a function of the reaction time, and for reaction times long enough (∼90 min), complete gasification was achieved. At supercritical conditions, the remaining liquid phase always was tar-free, was colorless, and contained less than 2 wt % of the feed carbon. Analysis of the spent catalyst revealed a slight increase of carbonaceous deposits on the surface (15 atom % vs 10 atom % for the fresh catalyst). Introduction Synthetic natural gas (SNG) is a viable option for a renewable energy carrier when produced from biomass. It is also an option for making countries less dependent on imported fossil fuels. Much work is directed toward a conventional route for SNG production by gasification of the biomass to syngas and the subsequent methanation of the syngas to SNG.1-4 These gas-phase processes suffer from a poor efficiency when using wet biomass as the feed. As an alternative, hydrothermal gasification has emerged over the last two decades, initiated by the pioneering work of Modell at the Massachusetts Institute of Technology (MIT).5 The annual energy potential of biomass in Switzerland is ∼181 PJ, out of which 55 PJ arise from wood (currently used: 18 PJ). Manure has a large potential of 85 PJ, but only 0.1 PJ are used to produce energy.6 Hydrothermal gasification is a promising new route for converting biomass into an energy carrier, especially for this kind of wet biomass. “Hydrothermal” designates an aqueous system at elevated pressures and temperatures, especially near the critical point of water (374 °C, 22.1 MPa) or above it. Near-critical and supercritical water provides an interesting environment for carrying out chemical reactions. In particular, hydrolytic reactions and reforming reactions involving water both as reactant and as solvent look promising for converting biomass into liquid and gaseous fuels. The internal energy of water does not exhibit a singularity at the critical point (λ transition) as opposed to the phase change at evaporation. Thus, the energy requirement for bringing water to supercritical conditions is less than that for evaporating and superheating water to the same temperature at subcritical pressures. Thermodynamic equilibrium calculations teach us that methane is preferably formed at lower temperatures, whereas at higher temperatures, hydrogen is the main product, besides CO2. This is also true in a hydro* To whom correspondence should be addressed. Tel.: +41-56-310-2135. Fax: +41-56-310-2199. E-mail:
[email protected].
thermal environment. The behavior is less pronounced for high feed concentrations (see Figure 1). The theoretical maximum methane yield of 0.34 (g of CH4)/(g of wood) (dry ash free, daf, chemical formula CH1.490O0.677) is obtained when no hydrogen is formed according to
CH1.490O0.677 (s) + 0.289H2O (g) f 0.517CH4 (g) + (∆rH° ) -26.1 kJ/moldaf) (1) 0.483CO2 (g) For high feed concentrations, methane yields close to the theoretical maximum yield can be expected in the range of 400-475 °C (compare Figure 1). At these temperatures, a catalyst must be used to overcome the slow gasification rates. The major problem associated with the gasification technology is the formation of tars and char.7 These substances may represent a significant portion of the heating value of the wood and, thus, lower the gasification efficiency, if not used thermally. Tars must be removed from the fuel gas before entering other downstream equipment.8 Modell observed that wood could be gasified in supercritical water without the formation of tars and char, but the conversion to gaseous products was low.9 Modell also studied the effect of adding different catalysts (five different Ni catalysts, one Co/Mo catalyst, and one Pt catalyst). Pioneering work in the field of catalytic hydrothermal biomass gasification has been carried out at the Pacific Northwest National Laboratory and resulted in the TEES process (thermochemical environmental energy system).10,11 Typical TEES conditions are in the subcritical region (350 °C, 20 MPa) with complex chemistry, including pyrolysis, steam reforming, hydrogenation, methanation, and water gas shift. Several catalysts were examined and tested for long-term activity. Ruthenium, rhodium, and nickel proved to be active metal catalysts. Sealock Jr. et al. attained a maximum methane yield of 0.22 (g of CH4)/(g of wood) with 33 vol % CH4 in the product gas using a stirred batch autoclave with a stainless steel liner operated at 450 °C and 34 MPa for 150 min with Harshaw Ni as the catalyst.11 Yoshida et al. gasified wood of unspecified origin in a stainless steel batch reactor at supercritical conditions (400 °C, 25 MPa, residence time 25 min) and attained
10.1021/ie050161h CCC: $30.25 © 2005 American Chemical Society Published on Web 05/26/2005
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Ind. Eng. Chem. Res., Vol. 44, No. 13, 2005
Figure 1. Calculated equilibrium methane and hydrogen concentrations in the product gas as a function of wood feed concentration and reaction temperature (total pressure 30 MPa; Peng-Robinson equation of state used).
a methane yield of 0.112 (g of CH4)/(g of wood), with a methane volume fraction of 0.28 in the product gas.12 Schmieder et al. reported on the results using a tumbling batch autoclave, lined with Inconel 625 and operated at 450 °C, 31.5-35 MPa, and a reaction time of 120 min, with wood as the feedstock at very dilute concentrations (4900 ppm C, corresponding to ∼1 wt % wood).13 They did not state the methane yield but had low concentrations of CH4 (
