There is renewed interest worldwide in the production of sustainable energy from renewable sources, such as biomass, aimed mainly at the decrease of fossil fuels use. Except for its potential to be CO2 “neutral” during combustion, biomass has a low sulfur content and a high volatile concentration:  these characteristics favor clean combustion conditions. The knowledge of the kinetics of biomass pyrolysis and combustion is important for the control of such thermochemical processes. The main objective of this work was to determine the kinetic parameters of thermal decomposition of two biomass materials, olive kernel and straw, and of their mixtures with a high- and a low-rank coal. The study was carried out using a thermogravimetric analyzer (TGA) in nitrogen atmosphere, at a heating rate of 10 °C/min. A kinetic model, involving first-order independent parallel reactions, was used. Activation energies and frequency factors were determined for two different particle sizes. The analysis indicated that the pyrolysis of the coals and the biomass samples could be modeled successfully via the independent reactions models, the pyrolysis of biomass being described by reactions corresponding to hemicellulose, cellulose, and lignin decomposition. The results showed that the chemical composition of each biomass type plays a fundamental role in the kinetics determination. Smaller conversion times and increased devolatilization rates were obtained, when biomass was added in the fuel blend with coal. The additive properties of coal and biomass, pyrolyzed in blends, were examined. It was proven that the mass loss vs time during thermal conversion of coal/biomass blends is well-described by the sum of each individual coal and biomass decomposition.