(280b) Oxygen Uptake and Release Kinetics of Cu/Cu2O/CuO On Al2O3 and ZrO2-SiO2 As Materials for Chemical Looping Combustion | AIChE

(280b) Oxygen Uptake and Release Kinetics of Cu/Cu2O/CuO On Al2O3 and ZrO2-SiO2 As Materials for Chemical Looping Combustion

Authors 

Brandt, B. - Presenter, Columbia University in the City of New York
Rosowski, F. - Presenter, BasCat – Unicat BASF JointLab


One key parameter in Chemical
Looping Combustion (CLC) is the oxygen transport capacity of the CLC materials
between the two process reactors; in practice, it may be limited by a
combination of many factors, including the metal content of the inert binder,
the oxygen uptake and release kinetics and the thermodynamic equilibrium under
the process conditions in the two reactors, the respective residence times, the
rate of contaminant deposition and removal (such as coke, sulfur, or ash
deposits), and deterioration of the oxygen carrier. Such factors can have a significant
impact on the economics and viability of a CLC power plant, because it is
expected that many tons of CLC materials would be required in a practical CLC
power plant (the required inventory of CLC material can be estimated to be in
the range of 0.5 to 6 tons/MW power for typical CLC materials and process
operation conditions).

With our ongoing program, we
aim for a systematic study of those interconnected aspects, and for obtaining a
more quantitative description of CLC from the perspective of gas-surface
kinetics. As CLC materials, we have chosen copper oxide supported on two inert
binders: 1. ZrO2-SiO2
(a high-performance material used in the field of catalysis), and 2. γ-Al2O3
for comparison (a more economic, generic binder previously described in
literature); those materials are prepared by incipient wetness impregnation
followed by a quantification of the prepared formulation by calibrated X-Ray fluorescence.
The study is centered on TGA/DTA/DSC experiments, which allow for testing the
oxygen uptake and release kinetics as a function of important process
parameters; they will be amended with morphological characterizations (BET, Hg-porosimetry, electron microscopy, EDX elemental
dispersion).

We will present experiments
obtained in the temperature range from 450 to 1000 °C, using CLC materials
with Cu loadings ranging from ~ 5 to 50 wt.-%. Our first results
indicate that the ZrO2-SiO2 material (17 wt.-% Cu)
can be cycled many times (more than 30 reduction/oxidation cycles
 have been studied) with less
than 2 % change of its oxygen uptake and release capacity. The experiments
show that the material can be looped quantitatively between the fully oxidized
state (CuO), and the fully reduced (metallic) state
using methane as fuel at 750 °C; at higher temperatures (starting around
800 °C), the material begins to spontaneously decompose to Cu2O
when the air is removed, likely due to thermodynamic driving forces; thus, this
temperature constitutes a significant upper boundary in order to maximize the
oxygen transport capacity of the material in CLC. The observed oxygen uptake
rates are 2.5x10-8 mole O2 sec-1 (for 1
mg of this CLC
material), and varied by less than 4 % over a
temperature rate from 550 to 750 °C; this apparent lack of variation
of the rate is probably due to extrinsic limitations of the oxygen uptake step
e.g. due to bulk or pore
diffusion processes. On the other hand, oxygen release rates under methane
exposure displays a strong temperature dependence; the reduction rate varies
between 1.8x10-8 mole O2 sec-1 at 750 °C
,
and 3.1x10-9 mole O2 sec-1 at 550 °C (for
1 mg of the material, respectively). Moreover, reduction rates are lower than
the oxygen uptake rates at the same temperatures (26 % slower at
750 °C, 79 % slower at 550 °C). These temperature findings
indicate that oxygen diffusion in the lattice is not the limiting factor for
the reduction rates. It will be discussed whether the observation of different
reduction rates is linked to the kinetics of the fuel activation on the CLC
materials. Finally, to begin to understand the impact of a more realistic fuel
looping cycle, tests done with simulated gasified coal including steam and
ppm-levels of H2S will be shown.