Chemical Looping

A variant of gasification is the concept of Chemical Looping. Chemical looping, a series of continuous oxidation-reduction steps that converts fuel to useful energy at a very high efficiency, is a promising design approaches in fossil energy combustion (an idealized concept is shown in Fig. 10.7). That is, if it could ever be achieved at commercial scale. Most of the research has been limited to paper studies, or half-reactions in the laboratory using milligram quantities of metal/metal-oxide reactants. In a simple form, chemical looping takes fuel in one reactor vessel (on the right in figure), and air in another, while exchanging the reactive medium (typically a metal/metal oxide) between the two. The oxidation step of the reactor extracts the oxygen from the air supply, leaving a stream of enriched nitrogen. The metal (nickel for example) is reduced by the fuel, producing CO2 and H2O in the other reactor. The metal in both reduced and oxidized states are cycled (or looped) between each half-reactor. Energy/work is extracted between the two. The oxidation/reduction mechanism has a built in air-separation component,

Fig. 10.7 Chemical looping conceptual design

isolating the nitrogen and argon and eliminating the need for cryogenic air separation. Conceptually, chemical looping could, in several steps, substantially reduce the largest parasitic losses found in the current approaches targeting carbon capture/ reduction. It might do so at efficiencies comparable to those found in the most efficient combined cycle plants available today [25-30].

As noted by the McGlashan, et. al. a current weakness of chemical looping is its inability to handle fuels with a significant ash component [31]. For this reason, most researchers have focused their efforts on oxidation of relatively clean fuel gases, such as natural gas. The ultimate prize with chemical looping is efficiency levels that are beyond the reach of combustion driven vapor power cycles.

Work by Xian Wenguo and Chen Yingying (in Nanjing, China) suggests that it might be possible to achieve three objectives simultaneously with chemical looping [32]. They simulated the performance of a system using ASPEN software to understand the key boundary conditions limiting the process. Using a mixture of iron oxides, their work suggests that they could produce high purity hydrogen in one stream, CO2 in another, with some excess power, and an overall net efficiency of 57%. Since hydrogen is so often cited as one of the best solutions to a reduced carbon footprint, this approach offers some very promising insight, if in fact it can be achieved. In Fig. 10.5, it was noted that a modest increase of a single percentage point would yield some 60 million tonnes reduction in CO2 (with a vapor power cycle system). Reaching performance as high as 50% would cut most CO2 emissions from fossil coal plants in half.

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