The combustion of two different sewage sludges and a semianthracite coal was studied and compared by thermogravimetric analysis. Non-isothermal thermogravimetric data were used to evaluate the Arrhenius parameters (activation energy and the pre-exponential factor) of the combustion of these carbonaceous materials. The paper reports on the application of model-free isoconversional methods for evaluating and comparing the corresponding activation energy of the combustion process.