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1、Fluidized-Bed Methanation: Interaction between Kinetics and Mass TransferJan Kopyscinski, Tilman J. Schildhauer,* and Serge M. A. BiollazGeneral Energy Research Department, Paul Scherrer Institute, 5232 Villigen PSI, SwitzerlandIn the present work, the influence of reaction and mass transfer on the
2、fluidized-bed methanation have been investigated by both experiments and modeling. By applying spatially resolved gas concentration andtemperature measurements in a bench-scale fluidized-bed reactor, it was shown that most of the reactionproceeds in the first 20 mm while the temperature increases by
3、 74 K in the first 2 mm of the bed. A CO conversion of practically 100% is achieved. The experimental data indicate that the measured gas composition represents mainly the dense phase and that mass transfer between bubble and dense phase is the dominanteffect in the upper part of the bed. A fluidize
4、d-bed model is proposed based on the two-phase model approach, hydrodynamic correlations from the literature and kinetic parameters previously determined. Although it wasnot possible to reproduce all measured phenomena within the methanation reactor, this first attempt to modelthe fluidized bed prov
5、ides a better understanding of the behavior of the reactor and the reactions.1. IntroductionThe conversion of coal and dry biomass to a clean fuel such as methane, the so-called substitute or synthetic natural gas (SNG), draws much interest because of the longer availability of these resources. SNG
6、is produced from coal and dry biomassvia thermochemical processes: gasification followed by gas cleaning, gas conditioning, methanation of the producer gas, and subsequent gas upgrading. This process allows for an easy and cost-effective carbon dioxide (CO2) removal as the separa- tion of a highly c
7、oncentrated CO2stream is inherent for the production of SNG in pipeline quality. SNG is a versatile energy carrier that is interchangeable with natural gas (95% methane, high heating value). The advantages of producing SNG are thehigh conversion efficiency (65%), the already existing gas distributio
8、n infrastructure such as pipelines and the well-established and efficient end-use technologies, e.g., compressed natural gas (CNG) cars, heating, combined heat and power (CHP), power stations. In the methanation of carbon oxides to methane, three independent reactions are important (reactions I-III)
9、.If the stoichiometric ratio of the reactants H2/CO is at least three or more, carbon monoxide (CO) reacts with hydrogen (H2) to produce methane (CH4) and water (H2O), according to reactionI. However, producer gases from biomass and coal gasifiers usually have a H2/CO ratio of 0.3-2, which is too lo
10、w for a good CO conversion and long catalyst lifetime. By means of the water gas shift reaction (WGS, reaction II), the H2/CO ratio can be adjusted by converting CO with H2O to CO2and additional H2. The Boudouard reaction (reaction III) is also important in systems where the H2/CO ratio is low. On o
11、nehand, carbon on the catalyst surface can be considered as a necessary intermediate during the methanation reaction. On the other hand, it may lead to catalyst deactivation if C atoms are not hydrogenated fast enough and form polymeric or graphitic carbon deposits.1The reaction mechanisms and kinet
12、ics have been studied intensively,2-15since Sabatier and Senderens16 found in 1902, that nickel and other metals catalyze this reaction. In the past 50 years, a few dozen papers have been published regarding the kinetics of CO and CO2methanation over different nickel catalysts. In all of these studi
13、es, only the gas composition at the reactor outlet was measured for different experimental conditions. The rate of the methanation was directly calculated from the conversion of CO or the exit gas concentration of CH4. Recently, by applying spatially resolved gas concentration measurements in a cata
14、lytic plate reactor, the effects of reactants (H2, CO) and products (CH4, H2O, CO2) on the rates were analyzed for similar conditions and for the same catalyst as that used in this work. Furthermore, the kinetic parameters of the proposed Langmuir-Hinshelwood rate expressions of the methanation and
15、WGS reaction were determined.17There is no consensus in the literature on the elemental steps for the methanation of CO on a nickel surface. Two mechanisms have been proposed with different rate expressions. The common assumption is that the methanation proceeds via an intermediate carbon species fo
16、rmed either by direct CO dissociation (leading to a adsorbed C species)7-10,18-20or by dissociation of an oxygenated carbon compound (COHxcomplex).4,5,13,21 The methanation reaction is exothermic and, because of the high inlet CO partial pressures in the methanation reactors for SNG production, an enormous amount of heat must be removed.In this case, gas-solid fluidized-bed reactors have been suc- cessfully applied for the production of SNG from biomass and coal-derived synthesis gas in several
