Chemical looping processes are still under development to reach full scale and commercial opera­tion and availability. This is due to the fact that conventional processes, such as those described in the previous chapters, are efficient from an energy conversion point of view, are well known and are based on simple technical components that have been constantly improved over the centuries.

Now that the focus is shifted to environmental performance of combustion processes and greenhouse gases mitigation, the conventional technologies are showing their limits in allowing an efficient and cost effective separation of pollutants and greenhouse gases from the flue gases.

The irreversibility of the chemical reactions processes is so high that inverting the process requires a large amount of energy and complex technologies.

Most chemical looping processes currently studied and under development use a solid medium to perform the reactions.

The first chemical looping processes were developed in the early 20th century to produce hydrogen from carbonaceous fuels, using steam: the so-called steam-iron process. The global reaction is the following:

CO + H2O ^ CO2 + H2 (5.34)

The following reactions take place in two different reactors, using as looping medium iron and iron oxide:

Fe3O4 + 4CO ^ 3Fe + 4CO2

(5.35)

3Fe + 4H2O ^ 4H2 + Fe3O4

In the second reaction iron is oxidized by steam producing hydrogen and in the first reaction the iron oxide is reduced by CO to regenerate the oxide to Fe and produce CO2. This process was abandoned when more efficient systems based on natural gas and oil reforming was being developed to produce hydrogen. However, a similar system was later developed to produce CO2 from solid carbonaceous fuels. This process was based on two fluidized bed reactors using a metal oxide as medium. The process is depicted in Figure 5.24 and either iron or copper oxides may be used. The development of these early chemical looping processes to produce technical gases was prompted by the need of separating gases at a time when no other separation technologies were available.

More recently a renewed interest was raised by the need of separating gases from mixtures in much more efficient systems. In a process developed at Ohio State University (Fig. 5.25) Fe2O3 particles are introduced into the reducer with pulverized coal, which is gasified producing CO and H2. Since the syngas has reducing properties, Fe2O3 is converted to Fe and FeO, while producing CO2 and H2O. Steam can be easily condensed and CO2 can be therefore removed from the flue

Recycle CO.-

Figure 5.24. Chemical looping process for CO2 production (Fan, 2010; Lewis and Gilliland, 1954).

Steam

Makeup

^Jvenzed

Reducer

Combustor

Steam

Fe C

Figure 5.25. Coal-Direct Chemical Looping Process (Rizeq et al., 2002; Gupta et al., 2006).

gases. The Fe and FeO particles are introduced into the oxidizer where they react with steam to produce H2 and Fe3O4. While being conveyed to the reducer Fe3O4 will be oxidized to the original Fe2O3.

The use of chemical looping processes is possible with any carbonaceous fuel, including biomass, and can include gasification processes in the whole power plant.

A second process which is quite interesting for hydrogen production and CO2 separation is the HyPr-Ring Process which was developed in Japan in the 1960s and 1970s (Fig. 5.26). This process includes a gasifier, fed with coal, CaO, steam and oxygen, where the excess steam increases the

formation of H2, whereas CaO reacts with carbon dioxide generated in the water-gas shift reactor. The solid medium consists of CaCO3 and carbon which is burnt in the regenerator calcinating the calcium components to CaO and allowing the extraction of CO2.

General Electric has developed a chemical looping process where coal or biomass may be used to produce hydrogen and power. The technology is quite similar to the HyPr-Ring Process, but three fluidized beds are necessary to complete the reactions. In the first reactor, coal is partially gasified with steam producing a syngas rich in H2, CO and CO2. The latter reacts with calcium sorbents. The solids in the first reactor are calcium carbonate and sulfate and unburned carbon and are introduced in the second reactor where they are reduced by Fe2O3 entering from the third reactor. The flue gas from the second reactor is mainly composed by CO2 and SO2. The third reactor is used to regenerate the iron oxide with air. The heat of all reactors is used to generate steam and the hot air exiting the third reactor may be used in a gas turbine for power generation. In the end the GE CLC process produced power, hydrogen and allows separating CO2 for capture or any industrial use; the scheme of this process is depicted in Figure 5.27.

Alstom has developed a combustion-gasification process based on chemical looping where three different configurations are possible: (i) coal combustion; (ii) coal gasification for syngas production and (iii) coal gasification to produce hydrogen (Fig. 5.27).

When used as a coal combustion process, the calcium sulfate is reduced to calcium sulfide, which is then burnt in the second reactor with air. Heat is transferred from the combustor to the gasifier and to a lesser extent to a steam generator to produce high temperature steam. In the second configuration more coal and more sorbents are used and hydrogen and carbon monoxide, without any CO2. In the third configuration, pure hydrogen is produced by adding a third reactor where a calcination reaction occurs. Calcium oxide captures CO2 in the first reactor, separating H2. Calcium carbonate is calcinated by burning calcium sulfate, producing CO2 in the third reactor. The process is shown in Figure 5.28.