(212e) Super-Dry Reforming of CH4

Super-dry reforming of CH₄

L.C. Buelens¹_, V.V. Galvita^1,*, H. Poelman¹_,
G.B. Marin¹

¹Laboratory for Chemical Technology, Ghent University, Technologiepark 914, 9052 Ghent, Belgium

*Corresponding author: vladimir.galvita@UGent.be

1. Introduction

During the
past decade, environmental research targeting the mitigation of CO₂
emissions has been shifting from CO₂ storage towards CO₂
utilization. The conversion of CO₂ to CO could provide a cheap C₁
building block for the chemical industry. Development of processes for CO₂
conversion are necessary to fully exploit these properties. In this respect, the
conversion of diluted CO₂ streams into CO by means of chemical
looping is particularly interesting [1]. Among the chemical looping processes,
super-dry reforming of CH₄ takes up a prominent position as it allows
for intensified CO₂ utilization and can be driven by renewable
energy sources such as biogas [2].

The super-dry
reforming process (Figure 1) relies on the combination of a reforming catalyst,
typically nickel based, with an oxygen carrier and CO₂ sorbent
material, which are typically iron oxide and calcium oxide based. In the CH₄
oxidation step, a CO₂ rich mixture of CH₄ and CO₂
is dry reformed into syngas over the reforming catalyst. This syngas is fed to
a mixed bed of oxygen carrier and CO₂ sorbent, where CO and H₂
are converted into CO₂ and H₂O upon reduction of the
oxygen carrier, while CO₂ is fixated by the CO₂ sorbent. In
the CO₂ reduction step, the oxygen carrier and CO₂
sorbent materials are regenerated either by increasing temperature or
isothermally by means of a sweep gas. A major advantage of this process is its
potential to achieve a three times higher capacity for CO₂
conversion than conventional dry reforming of CH₄.

The oxygen
carrier is the heart of the super-dry reforming process, periodically cycling
between reduction by syngas and reoxidation by CO₂,
thus allowing the conversion of CO₂ into CO. Indeed, the choice of
oxygen carrier material is governed by the degree of reducibility of the oxygen
carrier and its capacity to convert CO₂ into CO. The reforming
catalyst aids in achieving deepest possible reduction of the oxygen carrier by
first converting the fuel to syngas. The role of the CO₂ sorbent in
super-dry reforming is threefold: (i) A tremendous
decrease in coke formation on the nickel catalyst can be achieved because a
higher CO₂:CH₄ feed ratio can be used without
compromising the product purity. (ii) The CO₂ sorbent also
enhances the reducibility of the oxygen carrier by a continuous in situ
removal of CO₂. Hence, H₂O is the main product during the
first step of super-dry reforming, even though 4 different processes (dry
reforming, WGS reaction, oxygen carrier reduction, CO₂ removal)
occur. (iii) In the CO₂ reduction step of the super-dry reforming
process, decomposition of the CO₂ sorbent provides CO₂
for the oxygen carrier reoxidation, hence producing CO.

The aim of
this work is to study the stability and activity of functional materials
applied in super_dry reforming.

2. Experimental

As dry reforming
catalyst, a nickel based material was prepared [3]. As oxygen carrier and CO₂
sorbent, an iron oxide based [4] and calcium oxide [2] or lithium orthosilicate based material were prepared. Nanomorphology and chemical information were obtained by
materials characterization via STEM-EDX. For studying solid phase transitions, time-resolved
in situ XRD experiments were performed in a Bruker-AXS D8 Discover apparatus
with Cu Kƒ¿ radiation and a Vantec linear detector. For studying the gas phase product
yields, a quartz tube microreactor (inner diameter of
10mm) was used. The reactor, equipped with K_type thermocouples contacting the
reactor walls at the position of the catalyst bed, was heated by an electric
furnace. EkviCalc [5], a software package containing
an extensive database of chemical species, was used for performing
thermodynamic calculations.

3. Results and discussion

By
combining a Ni/MgAl₂O₄ dry-reforming catalyst, Fe₂O₃/MgAl₂O₄
oxygen carrier and CaO/Al₂O₃ CO₂
sorbent in the isothermal super-dry reforming process, a higher CO production
yield was obtained than through conventional dry reforming. The increased yield
of CO can be explained through the inherent separation of H₂O from
CO, hence avoiding equilibrium limitations in CO yield caused by the water-gas
shift reaction. In fact, this two-step process owes its success to the
application of Le Chatelierfs principle in both
steps. In the CH₄ oxidation step, the reduction of iron oxide is
improved by continuous removal of CO₂ in the form of CaCO₃.
In the CO₂ reduction step, the introduction of an inert sweep gas
initiates the decomposition of CaCO₃ upon which the oxygen carrier
is reoxidized by CO₂. Here, the formation
of CO provides a driving force towards the complete decomposition of CaCO₃.
Overall, the theoretical yield of super-dry reforming goes up to four molecules
of CO per CH₄ with a space-time yield of 7.5 mmol_CO
s^-1 kg_Fe^-1 at 1023 K. [2]

The effect of the choice of materials (reforming catalyst, oxygen carrier
and CO₂ sorbent) and their cyclic stability are investigated. A
major challenge in the design of the reforming catalyst is the mitigation of
catalyst deactivation through carbon deposits. One promising strategy is the
addition of iron to obtain a Ni-Fe/MgAl₂O₄ catalyst, in
which iron assists in the oxidation and removal of carbon deposits from the
catalyst surface [3]. The long-term application of an oxygen carrier is mostly
limited by sintering and the undesired formation of inactive solid phases. To
this end, 50Fe₂O₃/MgAl₂O₄ seems to
provide a good candidate in terms of material stability and activity based on
cyclic redox experiments over 4 days
time on stream. As for the CO₂ sorbents, CaO
based and Li₄SiO₄ based materials are compared.
One possible strategy for increasing the cyclic stability of the CO₂
sorbent is the application of a coating, such as ZrO₂ (Figure 2). The
aim of doing so is to reduce the effect of crystallite growth due to sintering
by providing a physical barrier between the different CO₂ sorbent
particles. However, it needs to be addressed whether or not this coating has any
adverse effect on the properties of the CO₂ sorbent material, such
as a reduced accessibility of the core material for CO₂. Hereto,
both experimental evidence and thermodynamic considerations are taken into
account.

4. Summary

This work discusses the material
properties, important for the real-life implementation of the super-dry
reforming process. On the one hand, the properties of a structured CO₂
sorbent material, based on Li₄SiO₄ and ZrO₂, are
investigated in view of its application. On the other hand, the effect of redox
cycling on different Fe₂O₃/MgAl₂O₄
oxygen carrier materials is elaborated on.  

Acknowledgment

This work was supported by the Long Term
Structural Methusalem Funding of the Flemish
Government, the Interuniversity Attraction Poles Programme,
IAP7/5, Belgian State _ Belgian Science Policy and the Fund for Scientific
Research Flanders (FWO; project G004613N). L.C. Buelens acknowledges financial
support from the Institute for the Promotion of Innovation through Science and
Technology in Flanders (IWT Vlaanderen). The authors
also thank Prof. Christophe Detavernier (Department of Solid State Sciences,
Ghent University) for access to in situ XRD equipment.

References

[1] V.V. Galvita, H. Poelman, C.
Detavernier, G.B. Marin, Appl. Catal. B-Environ. 164
(2015) 184-191.

[2] L.C. Buelens, V.V. Galvita, H. Poelman,
C. Detavernier, G.B. Marin, Science 354 (2016) 449-452.

[3] S.A. Theofanidis, V.V. Galvita, H. Poelman,
G.B. Marin, ACS Catal. 5 (2015) 3028_3039.

[4] N.V.R.A.
Dharanipragada, L.C. Buelens, H. Poelman, E. De Grave, V.V. Galvita, G.B. Marin,
J. Mater. Chem. A. 3 (2015) 16251_16262.

[5] B. Nolang, EkviCalc and Ekvibase, version
4.30 (2013) Svensk Energi
Data: Balinge, Sweden.

Figure  SEQ Figure \* ARABIC 1 Schematics of the
super-dry reforming process

Figure  SEQ Figure \* ARABIC 2 STEM-EDX characterization of Li₄SiO₄-ZrO₂
CO₂ sorbent material. (A) EDX elemental mapping of Zr and Si, clearly showing the presence of a silica based
particle, coated by zirconia. (B) Corresponding HAADF-STEM image. Scale bar
represents 100 nm.