(598v) Optimal Experimental Design of Chemical-Looping Combustion of Syngas in Fixed Bed Reactors

Optimal experimental design of
chemical-looping combustion of syngas in fixed bed reactors

Lu
Han,
Ari Fischer, George M. Bollas

Department of Chemical, Materials & Biomolecular
Engineering, University of Connecticut, Storrs, CT

Abstract

The
objective of this work is to evaluate and explore current processes for
chemical-looping combustion of synthesis gas and propose optimal experimental
design (OED) algorithms to assist in reactor design. Chemical-looping
combustion (CLC) is a novel technology for efficient and low cost CO₂
separation. The basic concept of the process involves two reactors, a Reducer
and an Oxidizer with an oxygen carrier (a metal oxide) circulating between the
two (Figure 1).  In the Reducer, synthesis gas (H₂, CO) reacts with
metal oxide, according to the Reactions 1 and 2. The relevant catalytic
reactions are methanation (Reaction 3), water gas shift (Reaction 4), carbon
gasification by CO₂ (Reaction 5) or steam (Reaction 6), and dry
reforming (Reaction 7). The reduced metal oxide, M_yO_x-1,
is transferred to the Oxidizer, where it is oxidized back to M_yO_x
with air, Reaction 8. The metal oxide is then returned to the Reducer and
begins a new cycle of reactions. The flue gas leaving the Oxidizer contains N₂
and unreacted O₂. The exit gases from the Reducer contain CO₂
and H₂O, which are inherently separated from the rest of the flue
gas. After condensation of the water almost pure CO₂ is obtained,
without significant energy penalties for separation.

Figure 1: Chemical-looping
combustion.

Application of CLC on the oxidation of
various fuels is being researched in studies of the effect of experimental
parameters including pressure, temperature, and oxygen carrier reactivity,
selectivity and stability. CLC with coal-derived synthesis gas is of interest
due to the high CO₂ emissions from coal combustion. The effect of
reactor pressure in syngas CLC is of interest due to the elevated pressure at
which coal is typically gasified and the planned pressure for CO₂
sequestration. Therefore, the effect of pressure on reaction kinetics is a
concern to the development of CLC technology. A literature review of this
effect on CLC reactions in fixed bed reactors using natural gas, solid coal,
and coal derived syngas reveals inconsistencies in experimental observations.
Positive effects of increased pressure are noted by Xiao et al. [1], who
observed an increase in reactor efficiency with pressures up to 0.5MPa and
Zhang et al. [2] who observed intensified reactions of coal gasification
products with the oxygen carrier at 0.5MPa. On the other hand, negative effects
are reported by Jin et al. [3] who observed lower reduction rates at elevated
pressures due to pressure restrictions on the reforming reactions and Tian et
al. [4] who noted reduced reduction rates due to slower bulk diffusion.
Utilizing previous experimental studies and current modeling of CLC reaction
kinetics at the University of Connecticut, we are designing and constructing a
bench-scale fixed bed reactor for oxidation of syngas with metal oxides. This
will enable a study for measuring metal oxidation and reduction kinetics at
high pressures, furthering the research of pressure effects on CLC technology.
Results will be translated into reaction mechanisms and kinetic constants to be
used in modeling and scale-up studies. Overall, the effects of pressure on CLC
reactors are undetermined and require further investigation to understand the
reaction kinetics. This presentation will focus on the peculiarities of
high-pressure chemical-looping combustion and showcase a novel method for
optimal design of a bench-scale experimental apparatus to be used for the
identification of the effect of operating conditions (e.g., reactor pressure)
on the accuracy and consistency of experimental measurements.

References

1.
Rui Xiao, Qilei Song, Shuai Zhang, Wenguang Zheng, Yichao Yang Energy &
Fuels 2010 24 (2), 1449-1463

2.
Shuai Zhang, Chiranjib Saha, Yichao Yang, Sankar Bhattacharya, Rui Xiao Energy
& Fuels 2011 25 (10), 4357-4366

3.
Hongguang Jin and Masaru Ishida Industrial & Engineering Chemistry
Research 2002 41 (16), 4004-4007

4.
Hanjing Tian, Karuna Chaudhari, Thomas Simonyi, James Poston, Tengfei Liu, Tom
Sanders, Götz Veser, and Ranjani Siriwardane Energy & Fuels 2008 22 (6),
3744-3755