Study On Pyrolysis Kinetics Of Biomass

Biomass energy is one of most important renewable energies. Pyrolysis is one of most promising methods of biomass utilization in future. Study on biomass pyrolysis kinetics which can obtain pyrolysis kinetic parameters is of great important significance toward judging pyrolysis mechanism and influence factors and optimizing reaction conditions.The thermal characteristics of three biomass samples, pine wood, cotton stalk and fir wood were investigated by thermal analysis technique at different heating rates in N₂ atmosphere. The influences of heating rate, biomass composition and catalysts on the process of biomass pyrolysis were researched. Experiments showed that TG and DTG curves all moved to higher temperature ranges with increasing heating rate. The higher ash content, the lower initial pyrolysis temperature and temperature at the maximum weight loss rates meanwhile the higher volatile content, the higher maximum weight loss rates. NaOH, Na₂CO₃, Na₂SiO₃ and NaCl made initial pyrolysis temperature and temperature at the maximum weight loss rates of three kinds of biomass shift to lower temperature regions, whereas TiC>2 and HZSM-5 made initial pyrolysis temperature and temperature at the maximum weight loss rates of cotton stalk move to higher temperature regions and had no obvious effects on initial pyrolysis temperature and temperature at the maximum weight loss rates of pine wood and fir wood. The basic catalysts NaOH, Na₂CO₃ and Na₂SiO₃ can obviously decrease the maximum weight loss rates while NaCl and HZSM-5 tend to increase the maximum weight loss rates, TiO₂ had few effects on the maximum weight loss rates. The basic catalysts NaOH and Na₂CO₃ can increase the pyrolysis exothermic peak while TiO₂ and HZSM-5 have minor effects on reaction heat.According to TG data, activation energies for pyrolysis of three samples were calculated by Flynn-Wall-Ozawa method. It was found that Zhuralev, Lesokin and Tempelmen (Z-L-T)equation(F(α)=1/(1-α)]^1/3-1}²) was the optimal mechanism function, which indicated that the thermolysis mechanism was three dimensional diffusion in pyrolysis temperature range for untreated samples by doubleextrapolation method, whereas double extrapolation method fail to apply for treated samples. It was found that Z-L-T equation was the optimal mechanism function in pyrolysis temperature range 250°C³50°C, Jander equation(F(a)=[l-(l-a)1/3]2) was the optimal mechanism function.which indicated that the thermolysis mechanism was three dimensional diffusion in pyrolysis temperature range 350oC³70oC for untreated sample and Z-L-T equation was the optimal mechanism function in pyrolysis temperature range 250°C²90°C for treated samples by Popescu method, and then, kinetic parameters of biomass pyrolysis was calculated by the optimal mechanism function of different temperature ranges . It was show that activation energy calculated and the optimal mechanism function obtained by three methods agree well with each other.The thermal characteristics of biomass pyrolysis oil were investigated by thermogravimetric analysis technique at different heating rates in O2 atmosphere. The influences of different atmosphere and heating rate on the pyrolysis or combustion process of biomass pyrolysis oil were researched. According to TG data, activation energy of the biomass pyrolysis oil was calculated by Flynn-Wall-Ozawa method. The optimal mechanism function of combustion in range 400°C⁴30°C was determined by Popescu method. It was found that second order(F(a)=(l-a)"1) , three order(F(a) =(l-a )"2) and one and one-half order(F(a) =(l-a )"1/2) of chemical reaction were fit for describing the combustion process at different temperature scopes, and then, the optimal function of different temperature range was used to calculate the kinetic parameters. Results indicated that the activation energy calculated by the two different methods are approximately the same.