(212d) Assessment of Looping Combustion and Gasification of Carbon (CarboLoop) in a Twin Fluidized Bed Reactor

CarboLoop is a process for looping combustion of carbon with inherent separation of CO₂, developed by Salatino and Senneca (2009, 2010), which represents an alternative to chemical looping combustion (CLC) of solid carbon. Unlike other CLC processes, CarboLoop makes no use of oxygen carriers, but takes advantage of the inherent propensity of carbons to uptake oxygen at low temperatures, forming oxygenated surface complexes. Surface oxides are eventually released as CO and CO₂ by exposure to higher temperatures (Brown et al., 1992; Haynes, 2001). The CarboLoop concept requires the utilization of two reactors (most typically two interconnected fluidized beds): one acting as Oxidizer, where the carbon is kept in contact with air at relatively mild temperature (200-400°C) to foster oxygen chemisorption; the other acting as Desorber, operating at higher temperature (700-800°C) where desorption of the oxygenated C-O surface compounds is promoted. The ratio between CO and CO₂as desorption products depends on the nature of the carbon and is influenced by the desorption temperature. The original CarboLoop concept, aimed at chemical looping combustion of solid carbon, can be easily implemented into a chemical looping gasification (CLG) version, by proper definition of operational conditions of the desorption step.

An experimental campaign based on discontinuous experiments in a thermogravimetric analyzer (Salatino and Senneca, 2009, Salatino et al., 2010) has been directed to provide the proof-of-concept of the process. An extensive experimental campaign based on the combined use of TGA and DSC (Senneca et al., 2013) complemented by microstructural characterization by XPS (Levi et al., 2015) contributed to shed light on important and non trivial mechanistic and thermochemical features of the cyclic oxidation-desorption reactions, and on the chemical nature of surface oxides involved.

In this study the CarboLoop concept is tested under conditions that approach those of a realistic looping setup, consisting of a dual interconnected fluidized bed reactor. The “Twin Bed” test reactor (Coppola et al., 2016) has been purposely developed for the characterization of looping processes at the bench scale while preserving the time-temperature history that particles experience in a realistic looping conditions. It consists of two lab-scale bubbling beds of an inert bed, acting as thermal ballast, operated batchwise, connected to each other by a rapid solids transfer line. Carbon samples are fed to the system and undergo sequential steps of Oxidization and Desorption of pre-set duration by rapid transfer from one reactor to the other. The fuel tested is a bituminous coal char with size range of 400-1000µm. The Oxidizer was operated in air at different temperatures in the range 200-300°C with a holding time of 20 min. The desorption stage was carried out at 700-800°C with the same holding time of 20 min in N₂. The progress of char oxidation has been monitored following the profiles of CO and CO₂concentration at the exhaust of the Desorber over iterated cycles. The effect of multiple cycles on char oxidation/desorption propensity has been investigated. A systematic characterization of the influence of the operating conditions holding in the oxidation and in the desorption steps has been performed, and analyzed in the light of a simple semi-lumped kinetic scheme. Results are relevant to the development of a tentative process layout for the implementation of the CarboLoop concept.

References

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P. Salatino, O. Senneca, Int. Patent application WO2010/026259A2 (2010).

P. Salatino, O. Senneca, L. Cortese, CarboLoop: a novel concept of looping combustion of carbon, Proc. of the 1st International Conference on Chemical Looping 2010, Lyon (FR).

T.C. Brown, A.E. Lear, B.S. Haynes, Oxygen chemisorption on carbon, Proc. Combust. Inst. 24 (1992) 1199-1206.

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O. Senneca, P. Salatino, L. Cortese, Assessment of the thermochemistry of oxygen chemisorption and surface oxide desorption during looping combustion of coal char, Proc. Comb. Inst. 34 (2013) 2787-2793.

G. Levi, O. Senneca, M. Causà, P. Salatino, P. Lacovig, S. Lizzit, Probing the chemical nature of surface oxides during coal char oxidation by high-resolution XPS, Carbon, 90 (2015) 181-196

A. Coppola, F. Scala, L. Gargiulo, P. Salatino, A twin-bed test reactor for characterization of calcium looping sorbents, Powder Technol. 2016 (Article in press) http://dx.doi.org/10.1016/j.powtec.2016.11.067.