(243c) Solar Thermochemical Hydrogen/Syngas Production from Methane and/or Biogas in the Presence of Non-stoichiometric Solid Oxide Carriers
Equilibrium thermodynamic calculations were performed using HSC 5.0 Outokumpu software. Such thermodynamic calculations evidenced that more important deviations from stoichiometry (Î´) can be obtained at the same temperature under carbothermal conditions, vis-Ã -vis CeO2 reduction in inert atmosphere. For example, at 900Â°C, Î´ equals 0.239 for the carbothermal reduction in the presence of methane, whereas the Î´ reaches only 0.008 for the reduction of CeO2 under inert atmosphere. Using 70% CH4 â 30% CO2 (simulated biogas) results in forecasted Î´ values amounting to 0.212 at 900Â°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 â 30% CO2. The overall syngas quality can be further adjusted when performing the second, CeO2-Î´ 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 â 30% CO2, assuming that the oxidation of CeO2-Î´ 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 500Â°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 5Â°C/min until 900Â°C, followed by isothermal carbothermal reduction during 1 hour at 900Â°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, Î´ 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 900Â°C, H2 and CO evolution decrease and stabilize at 1.3 and 0.5%. Upon 1-hour carbothermal reduction Î´ 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, Î´ constantly increases while heating, reaching values around 0.55 at 900Â°C. CO concentration reaches 3%, corresponding to H2/CO ratios around 1. In the further isothermal step, Î´ 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.
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