Analysis of mixed cattle manure: Kinetics and thermodynamic comparison of pyrolysis and combustion processes

The study investigates the non-isothermal thermogravimetric analysis of cattle manure at varied heating rates to compare the kinetic and thermodynamic behaviors of cattle manure in N2 and air atmospheres. The Thermogravimetric (TG) curves indicated that the decomposition of the manure can be divided into three different regions. Exclusive of stage I (dehydration), the activation energy (Ea) values of stage II (devolatilization) for all model-free methods varied between 144 - 270 and 32–227 kJ/mol in pyrolysis and combustion respectively. While in stage III, Ea values varied in ranges of 49–336 kJ/mol and 71–136 kJ/mol for pyrolysis (carbonation) and combustion (char burning), respectively. The overall average Ea for the pyrolysis (≈146 kJ/mol) is higher than the combustion process (≈127 kJ/mol). The variation in the values of frequency factor “A” also indicated the complexity of both processes. Pyrolysis followed overall 2-Diffusion (D2) reaction models, while combustion followed 3-Power law (P3) and reaction Order-1 (O1) in stage II and III respectively. The thermodynamic study was also conducted to evaluate the important parameters, change in Enthalpy change (ΔH), Gibbs free energy (ΔG), and Entropy (ΔS).