Page 382 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

12.4 Carbon Capture Processes 361

The reduced oxygen-carrier Me x O y 1 is regenerated in the air reactor according
to the reverse reaction in Eq. (12.16), where the oxygen is from the air. Effluent
gases from the air-reactor containing air with reduced amount of oxygen can be
discharged without negative environmental impact, and the regenerated solid
oxygen carrier is transported to the fuel reactor, and another cycle starts.
The key of this technology is the special oxygen carrier! About one thousand
oxygen carrier materials have been studied in laboratory [2]. The first-generation
oxygen carriers focused on mainly oxides of four metals: Ni, Fe, Mn, and Cu.
Recent advances focused on low cost materials by combing different metal oxides
and oxygen releasing materials aiming at solid fuel combustion. Among many
combined materials tested in laboratory, very few have been tested in continuous
operation. Most researchers have used ilmenite (FeTiO 3 )[8, 15] because of its low
cost and reasonably high reactivity towards syngas.
This process must enable the release of oxygen from the oxygen-carriers.
Thereby, only those metal oxides that have a suitable equilibrium partial pressure of
oxygen at temperatures of interest for combustion (800–1,200 °C) can be used as
oxygen-carriers. These materials include, but are not limited to CuO/Cu 2 O, Mn 2 O 3 /
Mn 3 O 4 , and Co 3 O 4 /CoO [34]. Operating conditions in the air-reactor and fuel-
reactor would be determined for specific oxygen-carrier [50].
Three example metal oxides are as follows:

4CuO $ 2Cu 2 O þ O 2 ð12:17Þ
6Mn 2 O 3 $ 4Mn 3 O 4 þ O 2 ð12:18Þ

2Co 3 O 4 $ 6CoO þ O 2 ð12:19Þ

Then the oxygen reacts with the gas fuel or char and volatiles (mainly
H 2 ; CO, CH 4 ) from the solid fuels following the combustion principles.

Gases þ Volatiles þ Char þ O 2 ! CO 2 þ H 2 O ð12:20Þ

If we take CH 4 ,H 2 and CO as examples, the overall chemical reactions are

CH 4 þ 2O 2 ! CO 2 þ 2H 2 O ð12:21Þ
1
H 2 þ O 2 ! H 2 O ð12:22Þ
2
1
CO þ O 2 ! CO 2 ð12:23Þ
2
As described in Eq. (12.20), the ultimate goal of chemical-looping combustion is
to convert fuel into carbon dioxide and water without NO x . The degree of fuel