Pyrolysis kinetics of new bioenergy feedstock from anaerobic digestate of agro-waste by thermogravimetric analysis

Abstract

In this study, thermogravimetric and kinetic analysis of digestate from anaerobic co-digestion of dairy manure and dry biomass of Amaranthus retroflexus L. is presented. Thermal decomposition of digestate was studied in the temperature range 40–1000 °C in an inert atmosphere using thermogravimetric and differential scanning calorimetry (TG-DSC) at the heating rates of 5, 10, 15 and 20 °C/min. According to the results of TG/DTG analysis, the thermal decomposition of digestate occurs in three stages: dehydration, devolatilization and decomposition of carbonaceous residue. The devolatilization stage in pyrolysis is a key stage; therefore, it was divided into two steps: I-st step and II-nd step and analyzed in detail. The kinetic parameters were estimated by isoconversion Friedman, OFW (Ozawa-Flynn-Wall) and KAS (Kissinger-Akahira-Sunose) methods and using model-based methods, such as CnB (reaction of n-th order with autocatalysis by product), Bna (expanded Prout-Tompkins equation) and Fn (the reaction of n-th order). For the model-free methods, the average Eα values for the I-st step were 136.3 kJ/mol and for the II-nd step 143.1 kJ/mol. The average Eα values obtained using the CnB, Bna, and Fn models were 105.8 kJ/mol for the I-st step and 145.6 kJ/mol for the II-nd step, i.e. Eα increased by 37.6%. The results of the evaluation of the thermal behavior of digestate from agro-waste indicate a good prospect for the use of biomass as a new bioenergy feedstock. The obtained kinetic parameters are of great importance for the design of pyrolysis reactors.