(727a) Effect of Pore and Particle Size of Oxygen Carriers On Chemical-Looping Combustion and Reforming

Effect of pore and particle size of
oxygen carriers on chemical-looping combustion and reforming

Lu Han,
Zhiquan Zhou, George M. Bollas

Abstract

Chemical-looping
combustion (CLC) addresses the current CO₂ problem caused by fossil
fuel utilization for power production. The main advantage of CLC over
conventional power production technologies is that direct contact between air
and fuel is circumvented, so that a pure stream of CO₂ is obtained
without nitrogen dilution. An intermediate oxygen carrier (metal/metal oxide)
is alternatively oxidized and reduced by the fuel and oxygen. Two successive
reactions take place between two reactors shown in Figure 1. In this
manner, CO₂ can be captured from the flue gases of the Fuel reactor
without additional separation and high energy penalty. Chemical-looping
reforming (CLR) is similar to CLC however a mixture of steam and fuel is sent
to the Fuel Reactor to induce reforming reactions and produce syngas.

Research in the
chemical-looping (CL) process has been focused on the development and testing
of oxygen carrier particles. Favorable characteristics of high performance
oxygen carriers include: high reactivity toward oxidation and reduction cycles,
high melting temperature, low tendency of attrition and fragmentation, low
toxicity, low cost and high combustion efficiency. Most
reported work on CL has been accomplished using oxygen carrier particles where
the active component (Cu, Fe, Ni, Mn)
is formulated on an inert material [1].
This work focuses on reported studies utilizing Ni-based oxygen carriers for
its high reactivity in the combustion of methane and catalytic properties.

A particle model is
developed to study the effect of internal transport on the reaction rates. Intraparticle
diffusion limitations need to be accounted for particles that are millimeters
in size [2].
Due to the importance of Knudsen diffusion, internal transport resistance of
different gases gives rise of concentration gradients inside the pores. Reaction
rates are observed highest at the particle surface and decrease along the pore
length. This knowledge can aid in oxygen carrier design to avoid
over-estimation of the system performance when no mass transfer effects are
considered. Incorporated into a reactor model, optimum particle dimensions are
proposed to conduct CLR in a fixed-bed reactor that makes use of the
intraparticle effects to maximize H₂ selectivity.

It is also worth to
explore the change in particle morphology over many reduction and oxygen cycles
in CLC and CLR. Investigators have reported increasing reactivity with usage
associated with micro-cracking of the particle surface and migration of the Ni
particles to the surface. With the particle model, the real changes to particle
porosity, surface area, and pore size are accounted for, which influence the
role of transport resistance on the performance. Observations made from
experiments on the particle morphology are utilized to predict reactivity
changes over cyclic tests.

Lastly the interaction
between NiO and Al₂O₃ support is addressed because when
present, the spinel can significantly lower the reactivity of the oxygen
carrier. For instance, the reactivity of two supported NiO-oxygen carriers
differing only by the calcination temperature of the support was measured in a
thermogravimetric analyzer at 800°C in a H₂-environment (Figure
2). The Al₂O₃ support existed in the γ-phase in
the low temperature pre-treatment and exhibited a phase transformation into
α upon high temperature calcination. More instance of spinel formation
occurred in the γ-Al₂O₃ support which resulted in
comparably lower reactivity of the oxygen carrier. A kinetic scheme that
combines the simultaneous reduction of NiO and NiAl₂O₄ is
used in the particle model. Reduction data of Ni-based oxygen carriers with
varying spinel phases are studied for the purpose of assessing the impact of
spinel on kinetic rates. Kinetic parameters derived from the particle model are
presented and model-experiment deviations are discussed.

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

References:

[1]         M. Johansson, T. Mattisson, M. Rydén, and
A. Lyngfelt, ?Carbon Capture via Chemical-Looping Combustion and Reforming
Chemical-Looping Combustion,? in International Seminar on Carbon Sequestration
and Climate change, 2006, no. October, pp. 24?27.

[2]         S.
Noorman, F. Gallucci, M. van Sint Annaland, and J. a. M. Kuipers, ?A
theoretical investigation of CLC in packed beds. Part 1: Particle model,? Chemical
Engineering Journal, vol. 167, no. 1, pp. 297?307, Feb. 2011.