(243c) Solar Thermochemical Hydrogen/Syngas Production from Methane and/or Biogas in the Presence of Non-stoichiometric Solid Oxide Carriers

Solar energy provides by far the largest of all carbon neutral-energy sources. Solar irradiation is, however, dilute, intermittent and non-equally distributed. These facts represent an enormous challenge for the development of solar processes. The conversion of solar energy into chemical energy carriers, i.e. solar fuels, which can be long term stored and/or transported, can alleviate some of these drawbacks. Solar fuels can be produced through thermochemical cycles based on redox metal pairs [1]. Non-stoichiometric oxides, such as CeO₂, doped CeO₂ and perovskites [2,3], involve oxygen release from their crystalline network. One of the main issues is the high temperature needed for the reduction step. Over 1400°C, lower overall efficiencies and important material problems may impede long-term cyclic operation. Carbothermal reduction, in the presence of carbon or methane [4], can bring down the temperatures needed for the 1^st step. In the present work, the methanothermal reduction of CeO₂ and CeO₂-ZrO₂ has been considered. Their oxygen exchange capacity was tested at 700-1000°C, in order to produce syngas from methane and/or CH₄/CO₂ mixtures (simulating a typical biogas composition).

Equilibrium thermodynamic calculations were performed using HSC 5.0 Outokumpu software. Such thermodynamic calculations evidenced that more important deviations from stoichiometry (δ) can be obtained at the same temperature under carbothermal conditions, vis-à-vis CeO₂ reduction in inert atmosphere. For example, at 900°C, δ equals 0.239 for the carbothermal reduction in the presence of methane, whereas the δ reaches only 0.008 for the reduction of CeO₂ under inert atmosphere. Using 70% CH₄ – 30% CO₂ (simulated biogas) results in forecasted δ values amounting to 0.212 at 900°C, slightly lower than in the presence of pure CH₄. Moreover, a syngas of a certain quality is obtained since this first carbothermal reduction step. H₂/CO ratios range from 5.8 in the presence of CH₄, to 1.7 under 70% CH₄ – 30% CO₂. The overall syngas quality can be further adjusted when performing the second, CeO_2-δ oxidation step in the presence of steam, CO₂ or H₂O-CO₂. The theoretical solar-to-fuel efficiencies range from 0.47 to 0.57, just for the first carbothermal step in the presence of either CH₄ or 70% CH₄ – 30% CO₂, assuming that the oxidation of CeO_2-δ for the regeneration of CeO₂ can be simply performed using air. Taking into account the second H₂O and/or CO₂ splitting steps the cycle efficiencies increase respectively to 0.54 and 0.64.

Different CeO₂ and CeO₂-ZrO₂ oxides were used as oxygen carriers in carbothermal reduction experiments, performed in a fixed bed reactor heated by an electric furnace. The solid materials were oxidized at 500°C for 30 minutes under 20%O₂-Ar flow. CH₄-Ar or CH₄/CO₂-Ar were then fed to the reactor while the temperature was increased at 5°C/min until 900°C, followed by isothermal carbothermal reduction during 1 hour at 900°C. A mixture CO₂-Ar was finally fed to simulate the second (exothermal) step of the cycle, leading to the re-oxidation of the solid oxygen carrier. The results obtained confirm that H₂ and CO can be obtained in important amounts and at moderate temperature over CeO₂ and CeO₂-ZrO₂ solid oxygen carriers. Important differences were however observed when feeding CH₄-Ar and CH₄/CO₂-Ar. Under CH₄-Ar, δ reaches 1.47 at the end of heating ramp, corresponding to peak H₂ and CO concentrations of 3 and 1.25%, respectively. During subsequent isothermal carbothermal reduction at 900°C, H₂ and CO evolution decrease and stabilize at 1.3 and 0.5%. Upon 1-hour carbothermal reduction δ finally reaches a value of 2.18. This total amount of oxygen released from the solid oxygen carrier can be completely restored by feeding CO₂-Ar. CO evolution and the resulting oxygen balance point indeed to a complete regeneration of the O-stoichiometry in the original CeO₂. In the presence of CH₄/CO₂-Ar, δ constantly increases while heating, reaching values around 0.55 at 900°C. CO concentration reaches 3%, corresponding to H₂/CO ratios around 1. In the further isothermal step, δ does no longer increase and stabilizes at 0.57. When feeding CO₂-Ar, only a very small amount of the oxygen released from the non-stoichiometric oxide is restored into the solid, pointing to oxygen exchange occurring between the gas phase, i.e. CO₂, and the solid oxygen carrier, and leading to an almost continuous regeneration of its non-stoichiometry. It must be also noted here that no carbon formation was observed during these experiments. The experimental efficiencies calculated from the syngas yields remain still relatively low. Further improvements are needed in order to boost the oxygen release from the non-stoichiometric oxide.

[1] M.E. Galvez, P.G. Loutzenhiser, I. Hischier, A. Steinfeld, Energ. Fuels 22 (2008) 3544.

[2] J.R. Scheffe, D. Weibel, A. Steinfeld, Energ. Fuels 27 (2013) 4250.

[3] T. Cooper, J.R. Scheffe, M.E. Galvez, R. Jacot, G. Patzke, A. Steinfeld, Energ. Technol. 3 (2015) 1130.

[4] M.E. Galvez, A. Frei, F. Meier, A. Steinfeld, Ind. Eng. Chem. Res. 48 (2009) 528.