Co2 Absorption

Ind. Eng. Chem. Res. 2010, 49, 12269–12275

12269

Formation of a Ca12Al14O33 Nanolayer and Its Effect on the Attrition Behavior of
CO2-Adsorbent Microspheres Composed of CaO Nanoparticles
Su F. Wu* and Ming Z. Jiang
Department of Chemical and Biological Engineering, Zhejiang UniVersity, Hangzhou 310027, China

The attrition behavior of microspheres composed of CaO nanoparticles thatwere used as high-temperature
CO2-reactive adsorbents was investigated. The nano-CaO/Al2O3 microsphere adsorbents were prepared by a
spray technique from a slurry containing a precursor of CaCO3 nanoparticles and an aluminum oxide gel. A
mechanism of formation of the layer of nano-Ca12Al14O33 was proposed and optimized, with calcination
temperatures ranging from 900 to 1000 °C and Ca/Al molarratios between 2.3 and 3.5. The attrition behavior
of the adsorbent was investigated in detail using the air jet method to measure fine loss, as well as with
scanning electron microscopy and a particle size analyzer to examine the changes in surface morphology and
particle size distribution. The attrition studies showed that complete formation of a Ca12Al14O33 nanoscale
framework under calcinationtemperatures ranging from 900 to 1000 °C and Ca/Al molar ratios between 2.3
and 3.5 resulted in improved resistance to attrition and increased durability of the reactive sorption capacity
of the adsorbent.
1. Introduction
CaO-based CO2 adsorbents with high reactive sorption
capacities have been widely studied in the capture of CO2 from
flue gas1-4 and the reactive sorption-enhanced reforming(ReSER) process.5-7 A continuous operation of CaO carbonations and CaCO3 calcinations needs a cycling system, and a
cycling fluidized bed reactor system with high efficiency is
desirable. However, under the fluidized bed operation conditions,
the severe mechanical degradation caused by the motion and
collisions of the sorbent particles must be considered.
Recently, a number of studiesconcerning the attrition of
limestone-based CO2 adsorbents in a fluidized bed have been
reported.8-10 The study concluded that natural sorbents such
as limestone and dolomite have severe attrition problems.
Johnsen et al.11 compared the attrition resistance of two natural
CaO-based sorbents (limestone and dolomite). Their studies
concluded that dolomite was an inferior attrition-resistant
adsorbentcompared to limestone because of its loose structure
and low density.
To improve the attrition resistance of the adsorbent, a
synthetic method was introduced to prepare CaO-based CO2
adsorbent. Binders, such as SiO2, Al2O3, and SiO2- and Al2O3containing natural materials (e.g., kaolin and diatomite), were
introduced. SiO2 is commonly used as a binder in FischerTropsch catalysts to improvetheir strength.12,13 However, few
studies have been conducted on CaO-based CO2 adsorbents
because the mobile phase leads to the adhesion of sorbent
particles at high temperatures (greater than 800 °C). Alternatively, a number of studies have shown that the CaO/Al2O3
adsorbent has better attrition resistance properties as a CO2
adsorbent than nonaluminum CaO-based particles.14,15 Pacciani
etal.16 studied the synthetic CaO/Al2O3 binary sorbent system
and its performance in a fluidized bed. The CaO/Al2O3 sorbent
system was found to provide relatively high strength due to the
formation of mayenite (Ca12Al14O33). The CaO/Al2O3 adsorbent
system is known to have the ability to form a variety of
compounds under different conditions with distinct properties.17
* To whom correspondenceshould be addressed. E-mail: wsf@
zju.edu.cn. Tel.: +86-571-87953138. Fax: +86-571-87953735.

The effect of the composition on the attrition resistance of
adsorbents is not clear; similarly, attrition mechanisms are not
well understood. A number of studies have shown that particle
attrition in a fluidized bed is caused by two mechanisms:
abrasion and fragmentation.18 Results indicate that...