As shown in Fig. , the intermediate species or products with carbon numbers in the range of C10~C16 and C17~C21 increase rapidly at the beginning of the simulation because they can be directly polymerized by reactants. For larger PAHs with carbon numbers in the range of C22~C 30 and C>30, the speed of production is relatively slow because they are mainly generated from secondary reactions by the initial reactants and C11~C21 species.