Study On Microwave Pyrolytic Characteristics Of Straw Bale

The biomass resources,especially agricultural straw stalks,are abundant in China. But the utilization ratio of biomass transformed into energy is low in China.At present,nearly all the straw stalk biomass conversion technologies need a pretreatment of straw crushing,which increases the complexity,initial investment and energy consumption of system.Therefore,it is necessary to seek for a new straw conversion method without crushing pretreatment.A novel straw biomass conversion method without crushing---microwave pyrolysis of straw stalk bale is introduced in this thesis.This method need no straw crushing,but to pyrolyse compressed straw bale to use it integratedly.Studies on straw bale microwave pyrolysis are carded out theoretically and experimentally, compared with conventional pyrolysis by electric heating.Firstly,the pyrolysis process and heat conduction mechanism of straw bale under microwave radiation are investigated on the microwave pyrolysis bench.The pyrolysis process is divided into four stages:drying stage,preheating stage,pyrolysis stage with massive volatile release and carbonization stage.The corresponding temperature zone and heating mechanism of each stage are different.Heat and mass transfer mechanism of large size material(straw bale)under microwave radiation is different from that of small size material,heat transfers from surface to inner part, while the mass transfer is reverse in this experiment.Uniformity of temperature field in straw bale under microwave radiation is fine,and bigger microwave power results in evener temperature field.Compared with the microwave pyrolysis,preheating stage of electric heating pyrolysis with no volatile matter releasing platform is not obvious. Temperature in heat conduction direction inside the straw bale is monotone decreasing from the heat source to distant place.Secondly,microwave thermogravimetric characteristics of straw bale are studied by thermogravimetric analytical method,pyrolysis products are analyzed and energy consumption is monitored.The microwave thermogravimetric rule of straw biomass is in accordance with conventional thermogravimetric rule basically.Most volatile matters are released in the temperature zone between 180℃and 450℃.Above 450℃,weight loss rate is low.When microwave power increases,every stage appears ahead of time,the maximum decomposition rate increases,the final pyrolysis time reduces,but microwave power has little effect on final pyrolysis temperature.Of three-phase products,char is most,liquid next,gas least according to the weight ratio under microwave power of 668(W/kg straw).The main gas constituents are CO,CO₂, H₂,CH₄ and few light hydrocarbons.The pure pyrolysis gas has high concentration of H₂(up to 35 vol.%)and synthetic gas(H₂+CO,up to 50 vol.%).Varying tendency of heat value and its components' concentration of pyrolysis gas are same,they first increase from zero to the maximum value and then reduce.Concentration peaks of CO and CO₂ appear ahead of other components'.The specific surface area of microwave pyrolytic char is 30～40 m²/g,which is bigger than that of conventional pyrolysis by electric heating.Under the conditions in this paper,electric power consumption of microwave pyrolysis is between 0.58 and 0.88 kW·h/(kg straw stalk), and along with the increasing of microwave power,power consumption per kg straw stalk increases.Under the similar power,the microwave heats up more quickly than electric heating and the calorific value of microwave pyrolysis gas is greater than that of electric heating pyrolysis.Tar concentration in pyrolysis gas by microwave is higher than that of electric heating due to the different heating mechanism.At last,reaction kinetics for microwave pyrolysis of straw bale is analysed theoretically.A one-step comprehensive model is chosen and established to describe the microwave pyrolytic reaction in this experiment.Apparent activation energy E, preexponential factor A and reaction order n are computed according to the experimental data.The activation energy under microwave power of 334(W/kg straw) and 668(W/kg straw)are 24～27 kJ·mol^-1and 67～71 kJ·mol^-1respectively,the order of pyrolytic reaction is one.Activation energy E and preexponential factor A increase with the increasing of microwave power.Microwave pyrolysis of straw bale needs no crushing of straw stalks,and its process and products have shown some advantages.It provides a new method for straw biomass utility for the purpose of energy.