Kinetic modeling of CO2 gasification of biochars prepared from Brazilian agro-industrial residues: effect of biomass indigenous mineral content

The gasification of agro-industrial residues has been identified as a viable alternative for energy generation, acting simultaneously as a solution for the final disposal of these wastes. Knowing the kinetics of the gasification reaction is crucial to the comprehension of the mechanisms and the phenomena involved in the process, and the development and optimization of industrial gasifiers. Accordingly, this study aims to examine, based on four kinetic models, the CO2 gasification behavior of three biomass chars prepared from apple pomace, spent coffee grounds and sawdust, and to associate the results with differences in the ash composition of the samples. The biochars were obtained from pyrolysis under N2, in a fixed-bed tubular quartz reactor, with an average heating rate of 12 K min−1 and a residence time of 60 min at the final temperature of 873 K. Biochars were isothermally gasified in a thermogravimetric analyzer with CO2, at atmospheric pressure, at temperatures of 1033, 1083 and 1133 K, in the kinetically controlled regime. From the characterization of raw biomasses, it was found that the apple pomace had the highest amount of potassium, and its biochar was the most reactive. Also, it was observed an increase in the gasification rates in the higher conversion region for all biochars. Concerning these reactivity profiles, the inorganic components were more important to the gasification behavior than the biomass lignocellulosic matter. For all biochars, the semi-empirical modified random pore model (MRPM) was the best-fitted model, indicating its suitability to CO2 gasification and confirming the catalytic influence of the inorganic matter in the studied samples. After demineralization, biomass chars presented higher values of activation energy and pre-exponential factor. According to the MRPM model, activation energy values within the range of 161.5–187.2 kJ mol−1 and 232.1–240.2 kJ mol−1, respectively, were obtained for the CO2 gasification of regular biochars and demineralized biochars.