Experimental study and modeling of a high-temperature solar chemical reactor for hydrogen production from methane cracking
Stéphane Abanades ∗ , Gilles Flamant
Processes, Materials, and Solar Energy Laboratory, CNRS (PROMES-CNRS, UPR 8521), 7 Rue du Four Solaire, 66120 Odeillo Font-Romeu, FranceAvailable online 22 November 2006
Abstract A high-temperature ﬂuid-wall solar reactor was developed for the production of hydrogen from methane cracking. This laboratory-scale reactor features a graphite tubular cavity directly heated by concentrated solar energy, in which the reactive ﬂowing gas dissociates to form hydrogen and carbon black. The solar reactor characterization was achieved with:(a) a thorough experimental study on the reactor performance versus operating conditions and (b) solar reactor modeling. The results showed that the conversion of CH4 and yield of H2 can exceed 97% and 90%, respectively, and these depend strongly on temperature and on ﬂuid-wall heat transfer and reaction surface area. In addition to the experimental study, a 2D computational model couplingtransport phenomena was developed to predict the mapping of reactor temperature and of species concentration, and the reaction extent at the outlet. The model was validated and kinetics of methane decomposition were identiﬁed from simulations and comparison to experimental results. 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Keywords: Hydrogen;Solar thermal energy; Methane; Cracking; Solar reactor; CFD model
1. Introduction This study addresses an unconventional route for potentially cost effective hydrogen production with concentrated solar energy as heat source. The process thermally decomposes natural gas (NG) in a high-temperature solar chemical reactor. When feeding the reactor with NG, the reaction is equivalent to: CH4 → 2H2 +C(solid) . The solar process is schemed in Fig. 1. This process results in two products: a H2 -rich gas and a high-value nano-material carbon black (CB). The energy carrier H2 and marketable CB are thus produced with renewable energy. Fossil fuels are saved and solar energy is stored as a transportable fuel: the solar thermal dissociation of one mole of methane contributes to the storage of solarenergy into two moles of hydrogen that can be transported over long distances. The fuel has zero CO2 emission since carbon as opposed to CO2 can be sequestered or marketed. A key point of the process economics is the added value of the produced CB. The selling price depends on the product nano-structure and applications in the ﬁelds of polymer composites and batteries are
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targeted. Potential impacts on CO2 emission reduction and energy saving are, respectively: 14 kg CO2 avoided and 277 MJ per kg H2 produced, with respect to conventional NG steam reforming and CB processing . Projected cost of H2 for largescale solar plants depends on the price of CB: 14¥/GJ for the lowest CB grade sold at 0.66 ¥/kg and decreasing to 10¥/GJ (i.e. the reference price of H2 from steam reforming with CO2 sequestration) for CB at 0.8 ¥/kg. This study aims at designing, testing, and simulating laboratory-scale solar reactor (1 kW) operating at 1500–2000 K and 1 bar. A ﬂuid-wall solar chemical reactor was designed and tested for the production of hydrogen from methanecracking. The effect of temperature on the reactor performance (CH4 conversion and H2 yield) was studied experimentally. This experimental work was highly combined with advanced reactor modeling. Heat transfer mechanisms are complex and they determine the gas temperature that controls directly the extent of reaction. The gas temperature in the reaction zone cannot be measured experimentally with...