(212e) Super-Dry Reforming of CH4
AIChE Annual Meeting
2017
2017 Annual Meeting
Topical Conference: Innovations of Green Process Engineering for Sustainable Energy and Environment
Chemical Looping Processes II
Monday, October 30, 2017 - 4:40pm to 5:01pm
Super-dry reforming of CH4
L.C. Buelens1, V.V. Galvita1,*, 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 K¿ 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 Chatelierfs 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 mmolCO
s-1 kgFe-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.