Panel Discussion

Catalysis
of calcium on Fe₂O₃/Al₂O₃ oxygen
carriers in chemical looping combustion

ZhiFeng
Zhang, Yifei Wang*, Guangsuo Yu, Fuchen
Wang

Key
Laboratory of Coal Gasification and Energy Chemical Engineering of the Ministry
of Education, East China University of Science and Technology, Shanghai 200237,
PR China

Chemical
looping combustion technology has the function of capturing CO₂ with
near zero energy consumption.[1,2] Oxidation and reduction of oxygen carriers
(the substances that transfer oxygen during chemical looping combustion) are
carried out in two different reactions, namely air reactor and fuel reactor. The
oxygen carrier absorbs oxygen in air reactor, and then transfers the lattice
oxygen to fuel in fuel reactor, avoiding the direct contact between the air and
fuel, so the concentration of CO₂ in the flue gas is very high. In addition,
the process belongs to flameless combustion and the reaction temperature is so low
that no NO_x pollutants form. Unlike the chemical looping combustion
of gaseous fuels, a certain amount of ash and oxygen carriers will participate
in the cycle after the combustion of solid fuels, especially coal. Bao et al. [3]
studied the interaction between 4 types coal ash and iron base oxygen carrier.
The study showed that most ash will reduce the activity of the oxygen carrier
and appear sintering and slagging except the ash mainly containing CaO. Zhang et
al. [4] studied the chemical looping combustion process of Fe₂O₃/CaSO₄
composite oxygen carrier. It was found that the reactive activity of the
compound oxygen carrier was stronger than that of single Fe₂O₃
or CaSO₄ oxygen carrier. It indicated that there was synergistic
effect between Fe₂O₃ and CaSO₄ in chemical looping
combustion.

Based
on coal based chemical looping combustion, the Catalysis of Ca, a typical
active mineral in coal, to iron based oxygen carriers is studied in a tubular
fluidized bed reactor. The effects of Ca content in oxygen carriers on the
reactivity and combustion product release characteristics are compared. It is
found that Ca can significantly increase the reactive activity of oxygen
carrier. The oxygen carrier with 5% Ca content has the best reactivity, and the
further increase of Ca content is not good for CO transformation. Through
repeated redox cycles, it is found that the activity of oxygen carriers
containing CaSO₄ rapidly declined after third cycles. CaSO₄
decomposition is the main reason for the decrease of oxygen carrier activity.
The oxygen carrier added to CaCO₃ shows good stability. The oxygen
carrier composition and apparent morphology are characterized by scanning
electron microscope (SEM) and X ray diffractometer (XRD). It was found that
CaAl₂O₄ and CaAl₄O₇ are formed
after the Ca content is over 5%, which result in oxygen carriers slagging and
sintering.

Figure 1. The change of the
average conversion rate of CO with the amount of Ca added

Figure 2.
The instantaneous
conversion of CO varies with time

Figure 3
The change of
average conversion rate of CO with the number of cycles

（a）Add CaCO₃                   
（b）Add
CaSO₄

Figure 4 XRD Atlas of oxygen
carriers after 10 cycles

Figure 5 SEM analysis of oxygen
carriers

Keywords: chemical
looping combustion, Fe₂O₃/Al₂O₃
oxygen carriers, calcium additive, catalysis

[1]   
Lyngfelt A. 2nd Nordic Minisymposium on Carbon Dioxide Capture and
Storage. Göteborg, October 2001.

[2]   
Moghtaderi B. Review of the recent chemical looping process
developments for novel energy and fuel applications. Energy Fuels 2012, 26 (1),
15-40.

[3]   
Bao J.; Li Z.;
Cai N. Interaction between iron-based oxygen carrier and four coal ashes during
chemical looping combustion. Applied Energy 2014, 115, 549-558.

[4]   
Zhang
S, Xiao R, Liu J, Bhattacharya S. Performance of Fe₂O₃/CaSO₄
composite oxygen carrier on inhibition of sulfur release in calcium-based
chemical looping combustion. International Journal of Greenhouse Gas Control,
2013, 17, 1-12.