Assessment of the carbon looping (CarboLoop) concept in a novel twin fluidized bed reactor

CarboLoop is an innovative method, developed and patented by Salatino and Senneca (2009, 2010) and Salatino et al. (2010), for combustion of coal with inherent separation of CO2 which represents an alternative to chemical looping combustion (CLC) for solid carbon. Unlike other CLC processes, in CarboLoop there is no need for oxygen carriers because the property of carbons to uptake oxygen at low temperatures, forming oxygenated surface complexes, and to release them as CO and CO2 at higher temperatures is exploited (Haynes, 2001). The CarboLoop concept requires the utilization of two reactors (in particular two interconnected fluidized beds): an Oxidizer, where the coal is kept in contact with air at relatively mild temperature (4400°C) to foster oxygen chemisorption, and a Desorber, operating at higher temperature (7700°C) where the oxygenated C-O compounds are desorbed in an almost pure CO2 stream. The first proof-of-concept of CarboLoop has been given by discontinuous experiments in a thermogravimetric analyzer (Salatino and Senneca, 2009, Salatino et al., 2010, Senneca et al.,2013). Experiments have been carried out using different solid carbon materials and aimed at assessing the extent and rate of oxygen uptake at different temperatures. In this study the CarboLoop concept is tested in a looping apparatus. The "Twin Bed 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 real looping plant. It consists of two lab-scale bubbling beds of silica sand, acting as thermal ballast, operated batchwise, connected 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 400-1000?m. 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 was monitored following the profiles of CO and CO2 concentration in the exhaust. Moreover, the effect of multiple cycles on char oxidation/desorption propensity was investigated. Results pave the way for validation of the Carboloop concept and optimization of the process conditions.