(684d) Holistic Kinetic Study of the Reduction of CH4 with NiO in Chemical-Looping Combustion

The objective of this work is to compare
reaction schemes and kinetic mechanisms that accurately describe the dynamic behavior
in chemical-looping reducers, operating with CH₄ and NiO/support (oxygen carrier).

The reaction scheme proposed is composed
of catalytic gaseous reactions and heterogeneous reactions with the oxygen
carrier include reactions of CH₄, CO and H_2. Ni-catalyzed
gaseous reactions considered in this scheme include steam reforming, overall
steam reforming, water gas shift, dry reforming, methane decomposition and
carbon gasification by CO₂ and H₂O.

Kinetic expressions reported in the
literature are compared and their parameters are estimated on the basis of
experimental data of Ni-based oxygen carriers for chemical-looping combustion
of CH₄. Rates of heterogeneous reactions based on shrinking core
model and modified volumetric model with reported data from literature are
compared.

Dynamic parameter estimation is
performed to estimate kinetics of Ni-based oxygen carriers for chemical-looping
reduction. Pre-exponential factors and activation energies for the various
mechanisms reported reactions of the scheme proposed are fitted to dynamic data
of chemical-looping reduction of NiO using CH₄.
CLC data from the literature vary in terms of reactor volume, space velocity,
and temperature, generating a sufficient span of conditions to explore the
kinetic mechanisms most accurate for CLC. The consistency and discrepancy between
the best fit kinetic parameters of different units will be presented.

The capability of the model to predict
the gas composition at various operating conditions and reactor configurations
using the same reaction mechanisms will be shown, allowing for confidence in
the kinetics used in other types of reactor models (e.g., bubbling beds,
risers, circulating fluidized beds) and a comprehensive analysis of the
scale-up capabilities of CLC reducers.

Acknowledgement: This material is based
upon work supported by the National Science Foundation under Grant No. 1054718.