Basic Study On Heat Transfer Of Biomass And External Thermal Flue Gas In Pyrolysis Carbonization Process

Biomass has been attracted high attention around the world as renewable energy. Biomass briquettes with high energy density, easy to transport and store, are in favor of large-scale development of biomass energy. The technology of carbon production from biomass pyrolysis is an effective energy unilization. A lot of high-temperature flue gas production from industry provides energy for biomass pyrolysis, to achieve the waste of resources utilization. In this thesis, the research is based on heat transfer between the flue gas and biomass briquettes. For developing this research, investigations were conducted.Firstly, the experimental temperature in the range of25～900℃, the study of char formation and energy required from pyrolysis reaction of cold forming and thermoforming of biomass were carried on in a thermogravimetric analyzer at different heating rates (10℃/min.15℃/min.20℃/min.30℃/min) with N2The experimental results show that the pyrolysis of cold forming and thermoforming of biomass were very similar with increasing pyrolysis temperature, maximum weight loss rate presented first increase and then decrease, then increase, the final decreasing trend,and there were two peaks. The DEAM mode was used to calculate activation energy and pre-exponential factors of cold forming and thermoforming of biomass at different conversion rates. The activation energy value of cold forming of biomass was mainly in range of38～137kJ/mol, presented first increase and then decrease. The activation energy value of thermoforming of biomass was mainly in range of28-190kJ/mol, presented increase-slow increase-decrease-increase. In addition, the final pyrolysis temperature of900℃, with the heating rate raised, the energy required was230.19.316.65.404.54.600.47kJ/kg of cold forming of biomass, and the thermoforming of biomass was165.41.365.09.367.50.419.67kJ/kg. The energy required of thermoforming of biomass was lower than cold forming of biomass.Secondly, using different heating rates (5℃/min.7℃/min.10℃/min.15℃/min) and isothermal (400℃、500℃、600℃、700℃) to study of char formation mechanism of large-thermoforming of biomass pyrolysis in a self-build experimental apparatus. The results show that with the increase of heating rate, the peak of weight loss rates corresponding to temperature increase while peak decrease, the fixed carbon content of the pyrolysis products was reduced. The Zhuralev Equation for Three Diffusion Model is the most probable mechanism function, and the differential expression of the mechanism function is f(a)=(2/3)(l-a)5/3/1-(1-a)1/3,the integral expression is g(a)=[(1-a)-1/3-1]2.The thermoforming biomass had minimum energy activation value of195.52kJ/mol at10℃/min.Thirdly, the temperature field of biomass briquettes pyrolysis process, and precipitation characteristics during pyrolysis gas products were studied The results show that the process of biomass briquettes pyrolysis progressive layers. in different positions presented the different stages of the pyrolysis at the same time. Low heating rate was conducive to forming a balanced overall temperature of biomass briquettes, char of biomass pyrolysis was compact with high quality, and the time of biomass pyrolysis became shorter at high heating rates, the high temperature difference between inside and outside, due to the large and rapidly escaped of gas to non-uniform thermal stress generated by biomass charcoal porous structure, porosity with low quality. With the pyrolysis temperature increased, the pyrolysis gas of CO2was increases and then decrease, the content of CO showing a rapid increase and then flat. CH4and H2were similar, increased with increasing temperature.Finally, the simulation of transfer of heat from high-temperature flue gas to biomass which combination with different experimental results of biomass pyrolysis at different size The simulation results, the mathematical model used is reasonable. Since the model assumes a uniform temperature distribution in the axial direction of the heat exchanger, the accuracy of the model was reduced, but it may provide the basis for the design of the heat exchanger. The outer diameter of the heat exchanger is1.6m, the biomass in the reactor forming1m diameter when the1000℃of flue gas flow rate was0.076kg/s and forming biomass feed rate was100kg/h, the length of the heat exchanger required to achieve complete carbonization of biomass is2.4m.