CO2 Utilization Via Chemical Looping Process

Introduction

Chemical looping is one of several emerging technology options capable to reduce the carbon dioxide emissions and helping in a diverse range of applications for production of fuels , chemicals and electricity [1-3]. In a general chemical looping process , a metal oxide (MeO) is circulated between two reactors - the reducer and oxidizer. For a combustion process , MeO reacts in the reducer with the fuel to produce CO₂ , H₂O and leaving metal (Me) or metal oxide in reduced state [1]. In the oxidizer it is re-oxidized to its initial state with air. Thus , fuel and air never mix and CO₂ is not diluted by nitrogen.

Other chemical looping schemes can produce hydrogen or synthesis gas. In chemical looping steam (CLSR) or dry reforming (CLDR) processes the fuel is again completely oxidized over the metal oxides (Eq. 1). The H₂O or CO₂ product is used as oxidant instead of air. The outlet from the looping reactor then consists of H₂ or CO.

A key issue in CLDR processes is the selection of an appropriate oxygen carrier material. The reactivity of the oxygen storage material , its cost and thermal stability are critical selection criteria. A broad range of metals was analyzed to determine the thermodynamic equilibrium limitations for redox cyclic processes [4-7]. While a number of metals/metal oxides (Mo , Cr , Zn , Co , Nb , Ce) give reasonable CO₂ reduction capacity , it is apparent that only iron yields high oxygen storage capacity from CO₂ (0.7 mol CO₂/ mol Fe) over a wide range of operating temperatures (600-1800^°C).

Thus , the chemical looping conversion of CO₂ into CO or CO₂ and H₂O mixture to syngas over iron oxide as oxygen storage material has been one of the possible target technologies for CO₂ utilization. The ability of the oxygen storage material to maintain its high activity in repeated reduction/re-oxidation cycles is the most critical issue for the overall economic viability of the CO₂ splitting process.

The objective of the present study is to investigate the CO₂ re-oxidation efficiency of a series of CeO₂-Fe₂O₃ mixed oxides prepared by co-precipitation for the utilization of CO₂ in a chemical looping process. Experimental performance data are reported as well as structural characterization of the oxygen storage materials during reduction and CO₂ re-oxidation. In addition , the stability of performance was tested through prolonged use of the material.

Results

Mixed CeO₂-Fe₂O₃ materials with 0 , 20 , 50 ,70 , 90 and 100 wt% CeO₂ were prepared for CO₂ transformation to CO by means of chemical looping. The samples were characterized with HRTEM , EDX , SAED , EEL and in situ XRD and tested in cycled reactions for activity and stability. The generation of CO from CO₂ through H₂-CO₂ redox cycles was investigated at 600^oC to evaluate the effect of CeO₂ upon Fe₂O₃. Two different phases were observed in the CeO₂-Fe₂O₃ samples. For a CeO₂ content below 70wt% , a distinct Fe₂O₃ phase was always present. The second phase appeared in all mixed CeO₂-Fe₂O₃ samples and was identified as a nano-crystalline solid solution of iron in ceria with 20 nm crystallite size and an atomic ratio of Fe:Ce up to 1:5. In all mixed CeO₂-Fe₂O₃ samples , the solid solution crystallite size was considerably smaller compared to pure ceria , while a size decrease for Fe₂O₃ was noticed with increasing CeO₂ loading. The crystallographic structure of all CeO₂-Fe₂O₃ was studied under H₂ reduction and CO₂ oxidation , using time-resolved in situ X-ray diffraction. In a 90wt% CeO₂ sample , no iron related diffractions were present at the start , but the H₂-TPR treatment made Fe appear as phase segregating from ceria. For lower CeO₂ loadings , H₂-TPR reduced the separate Fe₂O₃ phase to Fe₃O₄ , FeO and Fe. The solid solution phase was always partially reduced. In CO₂-TPO , re-oxidation of iron followed a one-step pathway to Fe₃O₄ around 500^oC , while for the solid solution re-oxidation occurred at 450°C. The addition of CeO₂ to Fe₂O₃ had a beneficial effect upon the activity and stability of the material. Reduction-re-oxidation cycles could be repeated many times without significant loss of CO₂ conversion efficiency.

The activity and stability of the mixed CeO₂-Fe₂O₃ samples as compared to pure Fe₂O₃ and CeO₂ a test reaction with H₂ reduction and CO₂ re-oxidation was set up. The samples are reduced in 5%H₂ in Ar during 1 min. By contacting the reduced sample with CO₂ , CO is produced. As an example , the observed space time yield of CO as a function of time for 20wt% CeO₂- Fe₂O₃ during the 2nd , 5th and 10th cycle is displayed in Fig. 1a.

Figure 1. (a) Space time yield of CO during the re-oxidation phase with CO₂/He for 20wt% CeO₂-Fe₂O₃ after 1 , 5 and 10 redox cycles; (b) CO produced after 1^st (dark bar) and 10^th cycle (light bar) over the xCe-Fe samples.

Fig. 1b shows the amount of produced CO after the 1^st and 10^th redox cycle for samples 0 to 100wt%CeO₂-Fe₂O₃. In both cycles , the product yield is higher for mixed materials than for pure ones. Especially the samples 20 to 70wt% CeO₂-Fe₂O₃ produce over 5 times more CO than Fe₂O₃ , with the highest yield for 20 wt%CeO₂-Fe₂O₃. The amount of CO produced over CeO₂ was 10 times lower than for Fe₂O₃. Highest CO yield was obtained for 20wt% CeO₂-Fe₂O₃ , while 70 and 50wt% CeO₂-Fe₂O₃ were the most stable combinations. The addition of ceria to Fe₂O₃ thus formed a solid solution phase , yielding improved activity. Moreover , the combination of both oxides efficiently suppressed the sintering effect , resulting in improved stability in chemical looping processes.

Acknowledgment

This work was supported by the ‘Long Term Structural Methusalem Funding by the Flemish Government’ , the Fund for Scientific Research Flanders (FWO , project 3G004613) , the Interuniversity Attraction Poles Programme , IAP7/5 ,- Belgian State – Belgian Science Policy.

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