Kinetic Model of Biomass Pyrolysis Based on Three-component Independent Parallel First-order Reactions

The pyrolysis behavior of two kinds of typical biomass (pine wood and cotton stalk) was studied in nitrogen atmosphere at various heating rates by thermogravimetric analysis (TGA). The pyrolysis process can be divided into three stages: evolution of moisture (200 ℃), devolatilization (200～400℃) and carbonization (400 ℃). The comparison of DTG curves of two biomass materials show that the higher the hemicellulose content of biomass, the more evident the shoulder peak of DTG curve. The weight loss process of two materials was simulated by the kinetic model assuming cellulose, hemicellulose and lignin pyrolyzing independently and in parallel, obeying first-order reactions. The pyrolysis kinetic parameters corresponding to the three components were estimated by the nonlinear least square algorithm. The results show that their fitting curves are in good agreement with the experimental data. Their activation energy values for pine wood and cotton stalk are in the range of 188～215, 90～102, 29～49 and 187～214, 95～101, 30～38 kJ/mol, respectively. The corresponding pre-exponential factors are in the range of 1.8×1015～2.0×1016 , 1.6×107～7.1×108 , 9.3×101～1.5×103 and 1.2×1015～6.7×1017 , 1.2×108～1.4×109 , 1.4×102～4.6×102 min-1 , respectively. In addition, the activation energy of cellulose and lignin increased and their contributions to volatile tended to fall, whereas the activation energy of hemicellulose decreased and its contribution to volatile tended to rise with increasing of heating rate.