In many chemical looping processes calcium sorbents are used to separate CO2 from the other gases. These reactions generally involve absorption/calcination processes, based on the following

CO? and SO}

Figure 5.28. The Alstom process (Rizeq et al., 2002; Andrus et al., 2006).

chemical reactions:

CaO + CO2 ^ CaCO3

+heat (5.36)

CaCO3 —CaO + CO2

These reactions can be shifted to the left or to the right by changing the temperature and pressure.

The combustion of carbonaceous fuels can be assumed as follows:

(lx + y/2 — z) MO + CxHyOz ^ (lx + y/2 — z)M + xCO2 + y/2H2O

M + Air ^ MO + N2 + unreacted O2 (5.37)

where the looping medium is a metal oxide. The first reaction only produces CO2 and steam; therefore carbon dioxide may be easily separated from steam by condensing the latter. The second reactor oxidizes the metal again and the flue gases are nitrogen and oxygen. The second reaction provides heat to the first reactor. The overall combustion process is thus divided into two sub processes which separate inherently the flue gas components.

If the fuel consists of carbon only, then the chemical looping reactions can be as follows:

2MO + C ^ 2M + CO2 M + H2O ^ MO + H2

The looping medium is the same as in the previous reactions, but the flue gas of the second reactors is now pure hydrogen. The process allows separating CO2 for capture and producing hydrogen as a byproduct.

The second reactor produces hydrogen instead of heat and steam is the oxidant of the overall combustion process. The carbon capture is greatly simplified and requires much less energy than in any other process producing hydrogen.

Hydrogen can also be produced including a calcination process in a chemical looping combustion:

CO (g) + H2O (g) ^ CO2 (g) + H2 (g) CO2 (g) + CaO (s) ^ CaCO3 (s) CaCOs (s) ^ CO2 (g) + CaO (s)

One positive effect of capturing CO2 in the process by means of CaO, is to enhance the hydrogen production from the first reaction because the chemical equilibrium favors its formation. The calciner in the second reactor allows an efficient separation of CO2 from the flue gas which can be ready for capture.