Pyrolysis is the thermal decomposition of organic matter under inert atmospheric conditions or in the presence of a limited supply of air, leading to the release of volatiles and formation of char. In the proposed kinetic model of this study, the kinetic scheme of biomass decomposition by two competing reactions giving gaseous volatiles and solid charcoal is used. Two different models are proposed based on different relation of activity of biomass with normalized conversion: (1) the activity is taken as unity, (2) rate of change of activity decreases with normalized conversion. The corresponding kinetic parameters of the above two models are estimated by minimizing the square of the error between the reported experimental data of thermogravimetry of hazelnut shell and simulated model predicted values of residual weight fraction. As the above objective function is highly nonlinear, complex in nature, and traditional optimization techniques failed to yield the global optimum set of kinetic parameters, Differential Evolution (DE), a population based search algorithm, is employed. Model -2, which considers the variation of activity of biomass with the normalized conversion, gave the best agreement with the experimental data.