| In view of mercury pollution emitted by coal-fired power plants,activated carbon mercury sorbent injection method has better effect.However,the high cost of activated carbon makes it impossible for the technology to be widely used.Biochar has become a promising alternative to activated carbon due to its good pore structure and abundant surface chemical characteristics.Because of the high temperature and oxygen-poor environment in flue of coal-fired power plant,the necessary energy can be provided for biomass pyrolysis.Bio-char was prepared by injecting biomass into the flue gas duct and adsorbing mercury in flue gas at suitable temperature range.Because the preparation time of bio-char was very short,which coincides with the characteristics of rapid pyrolysis.It is necessary to study the adsorption of mercury on bio-char prepared under fast pyrolysis conditions.In this dissertation,walnut shell biomass was used as raw material to produce bio-char by rapid pyrolysis on a self-designed dropper furnace test bench.Thermogravimetric analyzer,nitrogen adsorption and desorption analyzer,FT-IR analyzer and VM-3000 mercury analyzer were used to characterize and analyze the micro-characteristics and adsorption characteristics of bio-char.In this dissertation,a rapid pyrolysis device for drop-tube furnace was designed,which could realize gradual connection between three different temperature zones.The residence time of biomass particles in the device could reach 4-6s.The study of biomass with different particle size ranges showed that the adsorption performance of bio-char prepared from biomass with 106-150μm is the best.This is due to the agglomeration of biomass particles in the sedimentation process.The smaller the particle size,the stronger the agglomeration degree was,which hindered the process of mass transfer and heat transfer,and was not conducive to the development of pore structure.In addition,with the increasing pyrolysis temperature,the mercury adsorption performance and pore structure parameters of bio-char increased synchronously,while the content of surface functional groups decreased.Among them,the best adsorption capacity was bio-char prepared from biomass in the particle size range of 106-150μm at 800-1000℃.On this basis,three oxygen concentrations(3%,5%,7%)and three carbon dioxide concentrations(10%,15%,20%)were selected to further investigate the effect of pyrolysis atmosphere on its microscopic properties.The results show that the presence of oxygen decreases the content of functional groups on the surface of bio-char,that was,oxygen promotes the pyrolysis of biochar.The pore structure parameters showed an inflection point at 5% oxygen concentration,which showed a trend of decreasing first and then increasing,and the best pore parameter was 7% oxygen concentration.Carbon dioxide can also get a similar rule,but the effect was small.From the characteristics of mercury adsorption of bio-char,it could be seen that the mercury adsorption capacity of bio-char increases synchronously with the increase of pyrolysis temperature.Bio-char prepared at 800-1000℃ has the strongest adsorption capacity.Meanwhile,characterization of adsorption kinetics showed that the adsorption of mercury by bio-char prepared at 500-700℃ and 600-800 ℃ is mainly chemical adsorption.With the increased of pyrolysis temperature,the content of functional groups on the surface of bio-char began to decrease,and the pore development tended to be complete.Physical adsorption became the main control step.Based on the above,the adsorption conditions were changed to further explore the adsorption mechanism of bio-char to elemental mercury.Increasing the concentration of mercury at the entrance of bio-char could properly increase the cumulative mercury adsorption capacity of bio-char.With the further increased of the concentration of mercury at the entrance,the cumulative mercury adsorption capacity of bio-char gradually tends to a certain value.This was because the adsorption of mercury by bio-char follows the adsorption isotherm equation.The adsorption temperature was negatively correlated with the adsorption performance of bio-char,that was,with the increased of adsorption temperature,the accumulated mercury adsorption capacity of bio-char decreases significantly.Compared with pure nitrogen atmosphere,the cumulative mercury adsorption increased in 7% oxygen concentration by 26.32% in 10000 s adsorption time.Carbon dioxide could significantly improve the mercury adsorption capacity of bio-char.Compared with pure nitrogen atmosphere,20% carbon dioxide concentration increased the cumulative mercury adsorption capacity by 90%. |
