Research Article pubs.acs.org/journal/ascecg
Oxygen Transport Membrane for Thermochemical Conversion of Water and Carbon Dioxide into Synthesis Gas Wenyuan Liang,†,§,∇ Zhengwen Cao,‡,#,∇ Guanghu He,† Jürgen Caro,*,‡ and Heqing Jiang*,† †
Qingdao Key Laboratory of Functional Membrane Material and Membrane Technology, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Songling Road No.189, Laoshan District, 266101 Qingdao, China ‡ Institute of Physical Chemistry and Electrochemistry, Leibniz University of Hannover, Callinstrasse 3A, 30167 Hannover, Germany § University of Chinese Academy of Sciences, 100049 Beijing, China S Supporting Information *
ABSTRACT: Conversion of CO2 and H2O into synthesis gas via the solar thermochemical process is usually carried out at a high temperature of above 1500 °C and requires long-term durability of metal oxide catalysts during frequent heating−cooling cycles. Herein, a dual-phase Ce0.9Pr0.1O2−δ-Pr0.6Sr0.4FeO3‑δ oxygen transport membrane made of mixed metal oxides was employed for the one-step thermochemical conversion of CO2 and H2O to synthesis gas with a H2/ CO ratio of 2:1. Benefitting from the in situ removal of the generated oxygen through the highly oxygen-ion permeable membrane, the effective splitting of CO2 and H2O was achieved at the relatively low temperature of 2500 K). To avoid the recombination and the formation of an explosive mixture, the generated gas products have to be separated at such high temperatures.15 To tackle the two issues of (i) ultrahigh temperature and (ii) gas separation at these temperatures, multistep thermochemical cycles - especially twostep thermochemical loop cycles using metal oxide redox reactions - have been put forward and widely studied in the past © 2017 American Chemical Society
Received: April 26, 2017 Revised: August 22, 2017 Published: August 30, 2017 8657
DOI: 10.1021/acssuschemeng.7b01305 ACS Sustainable Chem. Eng. 2017, 5, 8657−8662
Research Article
ACS Sustainable Chemistry & Engineering
glycine-nitrate combustion process (GNP), as described in detail elsewhere.36 The CPO-PSFO powders were calcined in a hightemperature furnace at 950 °C for 10 h. The CPO-PSFO disks obtained under 5 MPa in a steel module were sintered under ambient pressure at 1400 °C for 5 h in air to become gastight membrane disks. To get thin CPO-PSFO membranes with a thickness of 0.6 mm, these membrane disks were polished by carborundum paper. Characterization of membrane materials. XRD study was carried out at room temperature to determine the phase composition of the CPO-PSFO powders, fresh membranes and spent membranes. Bruker-AXS D8 Advance diffractometer was employed using a stepscan mode with intervals of 0.02 in 2θ range of 20−80°. A SEM (JEOL JSM-6700F) was applied to characterize the surface morphology of the fresh and spent CPO-PSFO membranes. The performance measurements of oxygen permeability. The permeation tests were performed in a self-made device as described in our previous work.52,53 The CPO-PSFO dual-phase membrane was fixed to an alundum tube, and sealed with ceramic glue (Huitian rubber industry, China). Herein, high-temperature tube furnace was used as a substitution of solar furnace (see Figure S1). A Ni/Al2O3 catalyst with a particle size of 0.3−0.5 mm was packed on the top of CPO-PSFO membrane surface. The feed side was fed with a mixture of H2O, CO2 and N2, and N2 was used as a diluent gas and an indication of leaking. Ne cofed with He and CH4 on the sweep side was served as an internal standard. The leakage was less than 3%. The effective membrane area is around 0.7 cm2. The gas flow rates were controlled by mass-flow controllers (MFC, Bronkhorst, Germany) and calibrated by a bubble flow meter. The H2O was controlled by a liquid MFC and sent to the reactor after being heated and evaporated at 180 °C. The concentration of the gases at the exit of the reactor was analyzed by an online gas chromatograph (Agilent 7890A). Calculation formulas. No oxygen was detected by GC, thus the oxygen generated from H2O and CO2 splitting was assumed to be removed and the total gas flow rate did not change on feed side. The production rate of hydrogen vH2 and CO vCO, the conversion of H2O X(H2O) and that of CO2 X(CO2) are calculated according to the equations as follows:
mixed conducting membranes has been obtained by Balachandran et al.30,31 Recently, we also demonstrated effective hydrogen production by coupling water splitting with selective oxidation of methane or ethane in a perovskite membrane reactor.32,33 In addition, CO2 decomposition combined with the partial oxidation of methane has been demonstrated in an oxygen transport membrane reactor by Jin et al.34,35 Different from the previous studies on the individual H2O or CO2 decomposition, herein, we experimentally demonstrated the simultaneous thermal decomposition of H2O and CO2 for producing synthesis gas in an oxygen transport membrane reactor. By simultaneously splitting H2O and CO2 in a single process, a unit of adjusting the ratio of H2/CO can be saved leading to the reduction of energy loss and equipment investment in synthesis gas production. As illustrated in Scheme 1, by in situ removing of the generated oxygen from Scheme 1. Synthesis Gas Production by Simultaneous Splitting of H2O and CO2 as Oxygen Generating Reactions on One Side of the Oxygen Transport Membrane; and Partial Oxidation of Methane as Oxygen Consuming Reaction on the Other Side of the Ceramic Membrane
thermal H2O and CO2 splitting through an oxygen transport membrane, the thermodynamic equilibrium limit of the two decomposition reactions can be overcome through continuous oxygen removal. The oxygen generated from the cosplitting of H2O and CO2 in the membrane reactor will be continuously removed as long as an oxygen partial pressure gradient exists between the permeate side and the H2O/CO2 feed side. This very low oxygen partial pressure on the permeate side can be reached by (i) vacuum pumps, (ii) by an inert sweep gas, or (iii) by an oxygen-consuming reaction such as selective methane oxidation, as in our case. Since H2O and CO2 splitting are strongly endothermic, and this heat demand cannot be compensated by the slightly exothermic partial oxidation of methane (Table S1 in Supporting Information), the necessary energy for H2O and CO2 splitting can come from the sun, for instance, by using a normal solar oven as shown in Figure S2. Due to this mechanism, a higher syngas production rate can be expected in the oxygen transport membrane reactor at a lower temperature (
