CO2 Gasification of a Lignite Char in Microfluidized Bed Thermogravimetric Analysis for Chemical Looping Combustion and Chemical Looping with Oxygen Uncoupling

Chemical looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU) are novel combustion technologies with low energy penalty CO2 capture. In a CLC/CLOU unit, the gasification of char is the rate controlling step as a result of the slow gasification kinetics of char under a CO2 and H2O atmosphere. Reliable measurements of char gasification kinetics are absolutely necessary for CLC/CLOU reactor design, operation, and optimization and are the focus of this paper. To solve the problems of measuring gasification kinetics using thermogravimetric analysis (TGA) and fluidized bed (FB) reactors with gas measuring, a microfluidized bed thermogravimetric analysis (MFB-TGA) method was developed and reported in this paper to investigate the CO2 gasification of a lignite char. The feasibility of the novel MFB-TGA approach was tested, and its stability and accuracy were validated. MFB-TGA was then used to study the CO2 gasification of a lignite char. First, the CO2 gasification kinetics of the lignite char in MFB-TGA were compared to those obtained using a TGA-Q500 instrument. The char gasification kinetics in MFB-TGA were found to be much faster than those in the TGA-Q500. Second, the effect of the particle size on the gasification kinetics was investigated, and it was found that 900-1250 mu m was the threshold size range, below which there was no obvious particle size effect on the kinetics. However, for particle sizes larger than 900-1250 mu m, the effect of the particle size on the kinetics was significant. Then, using 900-1250 mu m particles, the influence of the temperature, CO2 concentration, and O-2 concentration was studied, and the experimental results obtained from MFB-TGA showed that the lignite char could be fully converted within 300 s at 900 degrees C with 75 vol % CO2. The time for the full conversion of the char could be reduced to 190 s when the gasification temperature was increased to 950 degrees C or when 3 vol % O-2 was present in the atmosphere. Finally, a predictive kinetic model was developed from the experimental results, which could predict both the conversion of carbon versus time and the reaction rate versus carbon conversion well.