(664d) 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-
Fe₂O₃during
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.

References

1.     J.
Adanez, A. Abad, F. Garcia-Labiano, P. Gayan, and L.F. de Diego, Progress in
Energy and Combustion Science 38 (2012) 215

2.     L.-S.
Fan, and F. Li, Ind. Eng. Chem. Res. 49 (2010) 10200.

3.     C.
Luo, Y. Zheng, N. Ding, Q. Wu, G. Bian, and C. Zheng, Ind. Eng. Chem. Res. 49
(2010) 11778.

4.     M.
Najera, R. Solunke, T. Gardner, and G. Veser, Chemical Engineering Research and
Design 89 (2011) 1533.

5.     A.
Sim, N.W. Cant, and D.L. Trimm, Int. J. Hydrog. Energy 35 (2010) 8953-8961.

6.     S.
Abanades, Int. J. Hydrog. Energy 37 (2012) 8223.

7.     J.R.
Scheffe, M.D. Allendorf, E.N. Coker, B.W. Jacobs, A.H. McDaniel, and A.W.
Weimer, Chemistry of Materials 23 (2011) 2030.