02-09-2013, 03:29 PM
Synthesis of CaO-based sorbents through incorporation of alumina/aluminate and their CO2 capture performance
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ABSTRACT
CaO-based CO2 sorbents derived from various calcium and aluminum precursors were prepared by a
wet mixing method and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM),
differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and N2 adsorption–
desorption techniques. The as-prepared sorbents consisted of active CaO and inert support materials
that could be Al2O3, Ca12Al14O33 or Ca9Al6O18, depending on calcium and aluminum precursors used
during the preparation process. A formation mechanism for the inert support materials was proposed.
Compared to pure CaO, most of the synthetic CaO-based sorbents showed much higher CO2 capture
capability and stability over multiple carbonation/calcination cycles, which was ascribed to the
relatively high specific surface area of the sorbents, the bimodal pore-size distribution with a fair
number of small pores, and the inert support material that can effectively prevent or delay sintering of
CaO particles. Among these synthetic sorbents the CaO–Ca9Al6O18 sorbent with a CaO content of 80 wt%
(weight fraction) derived from calcium citrate and aluminum nitrate exhibited the best performance for
CO2 capture.
Introduction
The utilization of CaO-based sorbents to capture CO2 has
become of special interest in recent years since it is one of the
most promising methods to reduce CO2 emissions from flue gases
(Gupta and Fan, 2002; Feng et al., 2007; Blamey et al., 2010;
Anthony, 2011). In addition, through CaO-carbonation/CaCO3-
calcination cycles or the so-called Ca looping, CaO-based sorbents
have found many applications such as in sorption-enhanced
hydrogen production from hydrocarbons (Harrison, 2008; Bhat
and Sadhukhan, 2009; Martavaltzi et al., 2010) or biomass (Chen
and He, 2011), and in chemical heat pumps (Kato et al., 1999).
However, bulk CaO usually suffers from a major problem of loss-
in-capacity, i.e., a sharp decay in CO2 capture capacity with the
number of carbonation/calcination cycles (Abanades and Alvarez,
2003). The degradation is mainly ascribed to the loss of suitable
surface area and pore volume (useful for the growth of the CaCO3
product layer), resulting from sintering of CaO during calcination
at high temperature that induces growth and aggregation of
crystals (Liu et al., 2010a; Wang et al., 2011)
Experimental section
Preparation of sorbents
The wet mixing method developed by Liu et al. (2010b) is used
to prepare CaO-based sorbents. First, a calcium salt solution and
an aluminum salt solution (each with a predetermined concen-
tration) were simultaneously added into a flat bottom beaker, and
the mixture was vigorously agitated with a magnetic stir bar at
75 1C for 1 h, followed by static state at room temperature for
12 h. Second, the above solution was dried in an oven at 120 1C
overnight and dry white powders were obtained. Finally, the fine
powder was calcined in air in a muffle furnace at 900 1C for 1.5 h
(at a heating rate of 2 1C/min). Different from the aforementioned
method for the synthetic sorbents, a pure CaO serving as a
reference was prepared by calcination of CaCO3 powders at
900 1C for 1.5 h (at a heating rate of 2 1C/min).
Performance test of sorbents
The cyclic carbonation and calcination experiments were
performed with a WRT-3P thermogravimetric analyzer (TGA)
(Shanghai Precision & Scientific Instrument Co., Ltd.). For each
run about 10 mg of sorbent with the diameter of 50–75 mm was
loaded in a platinum basket (i.d. 8 mm and height 2 mm).
Temperature and sample weight were continuously recorded in
a computer. N2/CO2 flow rates were controlled by mass flow
meters. During carbonation/calcination cycles the total gas flow
rate was maintained at 50 cm3/min. A complete cycle consisted of
carbonation and calcination steps at 650 and 800 1C, respectively.
In a typical run, the sorbent sample was heated to 650 1C at a rate
of 10 1C/min under an atmosphere of 100% N2. When the tem-
perature was reached and stabilized, the pure N2 was replaced
with a gas mixture of 15% CO2 and 85% N2. After carbonation for
30 min, the gas mixture was switched to the pure N2, and mean-
while, the TGA was heated to 800 1C at a rate of 10 1C/min and
kept at 800 1C for 10 min. After complete calcination, the tem-
perature was decreased at 10 1C/min to 650 1C for next cycle.
Conclusions
A series of CaO-based CO2 sorbents were prepared with a wet
mixing method using different calcium and aluminum precursors.
These synthetic sorbents were composed of active CaO and inert
support materials that could be Al2O3, Ca12Al14O33 or Ca9Al6O18,
depending on calcium and aluminum precursors used. A mechan-
ism based on the solid-state reactions between CaO and Al2O3,
which were controlled by diffusion of Ca2 þ through the reaction
interface, was proposed to explain the formation of the inert
support.