Effects Of Torrefaction Pretreatment On Steam Activation Characteristics Of Rice Husk Pyrolysis Char

Polygeneration of biomass pyrolysis is one of the effective means for the utilization of biomass resources.However,products produced by pyrolysis are of low quality and difficult to be directly utilized.Therefore,pretreatment is an essential process.Torrefaction pretreatment technology has attracted more and more attention because it can improve biomass energy density and change the composition.Besides improving the quality of bio-oil,torrefaction pretreatment can increase the yield of pyrolysis char and change its microstructure in a certain extent.It is a good way to use torrefied pyrolysis char to prepare biomass-based activated carbon.Therefore,in this paper,rice husk was used as raw material to study the characteristics of torrefaction pyrolysis process and the physicochemical properties of the solid products.The characteristics of steam activation with pyrolysis char and the effects of torrefaction and desilication on the activated carbon derived from steam activation were investigated from the point of view of fixed bed experiment.The influence of torrefaction pretreatment with different temperature on torrefied biomass and substantial pyrolysis product were obvious.With the increase of torrefaction pretreatment temperature,the volatile content of torrefied rice husk decreased gradually,the fixed carbon and ash content increased gradually,the O/C and H/C ratio decreased,the high heating value increased gradually,and gradually approached the fuel characteristics of coal.Torrefaction can increase the yield of pyrolysis char and carbon element,but the specific surface area and pore volume of torrefied pyrolysis char decreased,and the degree of graphitization decreased with the increase of torrefaction temperature.At the same time,the content of amorphous carbon and carbon active sites of pyrolysis char with torrefaction pretreatment were higher.The single factor experiment and steam activation experiment of rice husk char showed that,the optimum activation conditions of steam activation experiment were particle size with 30 to 60 mesh,activation temperature at 800 C,steam flow rate at 6 mL/h and activation time at 60 minutes.During the activation process,steam preferentially reacts with amorphous carbon on the surface of pyrolysis char.At this stage,the mass yield of activated carbon decreases rapidly,the proportion of amorphous carbon decreases,and the degree of graphitization increases rapidly.Water vapor produces more micropores on the basis of existing pore in raw materials,and the specific surface area and micropore volume increase rapidly.After that,water vapor continues to diffuse into the interior of carbon materials,and graphite microcrystalline is deficient.When the activation time continues to increase to 90 minutes,the water vapor is relatively‘excessive',the etching reaction is serious,and some surface pore collapse occurs.At this stage,the pore volume increases,but the specific surface area and micropore volume decrease.The effects of torrefaction and desilication on activated carbon obtained from steam activation were as follows.Compared with activated carbon derived from rice husk char,the yield and adsorption performance of activated carbon obtained by steam activation of torrefied-pyrolysis char are improved.The specific surface area and pore volume of activated carbon obtained by torrefaction at 280 C are increased by about 20%compared with that of the rice husk char,and the micro-pore structure is more abundant.The specific surface area and pore volume of activated carbon prepared by steam activation of rice husk char with desilication almost doubled compared with that of pyrolysis char without desilication,and the proportion of mesoporous volume and the average pore size increased.Compared with technology of activation after desilication,the activated carbon derived from activation before desilication has larger specific surface area and pore volume.The combined treatment of torrefaction and desilication showed that the effect of torrefaction and desilication was mutually beneficial.The activated carbon obtained had larger specific surface area and pore volume.The specific surface area and pore volume of LAPT28060 reached 1124.3 m~2/g and 0.9804 cm~3/g respectively,with a mesoporous ratio of 63.2%and an average pore size of 3.488 nm.In addition,the final yield of activated carbon increased by 8%-16%and ash content decreased by 9%-32%after torrefaction treatment.Activated carbon with activation followed by desilication has higher specific surface area and lower ash content than activated carbon with activation after desilication.