Study On Mechanism Of Pyrolysis And Elemental Mercury Adsorption Of Biomass Char Under Multi-parameter Coupled Conditions

There are two major sources of mercury?Hg?in the environment,namely natural and man-made.Mercury has strong carcinogenic,teratogenic,mutagenic and other toxicities and is extremely harmful to human health.People have paid considerable attention to the pollutant mercury.When coal is burned,the mercury contained in the coal is released into the environment,accounting for 50%of the total mercury emissions in China and becoming the main source of mercury in atmosphere.Given the current situation,the method of adsorbent injection in combination with typical existing air pollution control equipment,such as electrostatic precipitators?ESPs?and fabric filters?FFs?,is a promising technology for controlling mercury emissions in coal-fired flue gas.Therefore,the development of efficient,low cost and substitutable adsorbents has important practical application value.Meanwhile,biomass char?biochar?,a solid product obtained from the pyrolysis of biomass,exhibits complex pore structures and good surface characteristics.Studies on applying biochar to the removal of combustion pollutants have been widely reported.The flue gas generated in coal-fired boilers can form high-temperature pyrolysis conditions,providing the energy necessary for the pyrolysis of biomass.Hg⁰ is then efficiently adsorbed by biochar.With this characteristic,the preparation process of the adsorbent?biochar?can be completed.This method can not only remove the mercury pollution without special preparation process,but also utilize renewable energy.The mercury abatement cost of this method is relatively low.It has great potential for development and has not been widely reported domestically or abroad.In this thesis,walnut shell was selected as the raw material,the mercury adsorption process of biochars prepared was studied in various pyrolysis modes,particle sizes,pyrolysis atmospheres and modification conditions.Through multiple characterization and analysis methods,based on the construction of molecular structure of biochar,and by using the density functional theory combined with the adsorption kinetics process,the mercury adsorption characteristics and mechanism of biochar were obtained.?1?Effects of the pyrolysis mode and particle size on the elemental mercury adsorption characteristics and mechanism of biocharThe pyrolysis process of walnut shell biomass could be divided into three stages.The biomass with the smaller particle size had the larger reaction area,weakening the influence of mass transfer and heat transfer on the pyrolysis process,which was beneficial to the progress of the pyrolysis and the devolatilization of the volatiles.The heating rate played a more important role in the pyrolysis of the biomass than the particle size.With an increase in the stationary pyrolysis temperature in the isothermal mode and the final pyrolysis temperature in the non-isothermal mode,the mercury adsorption capacities of the biochars initially increased and then decreased.The temperature that corresponded to the highest mercury adsorption capacity was 600°C,followed by800°C,1000°C,and 400°C.With decreasing particle size in 58?m-270?m,the mercury adsorption capacity of the biochar sorbents showed an overall gradually increasing trend.For the biochars prepared under the non-isothermal mode,the mercury adsorption capacities of the biochars initially increased and then decreased with an increase in the heating rate.The Hg⁰ adsorption capacities of the biochar sorbents obtained with a heating rate of 10°C/min were better.With the increase in the pyrolysis temperature,the effects of the external mass transfer and surface chemical adsorption on the mercury adsorption process became equivalent.With the decrease in the particle size,the rate-limiting step of the mercury adsorption process transitioned to chemical adsorption from physical adsorption.The rate constants of the pseudo-first-order and pseudo-second-order models gradually increased with decreasing particle size.?2?Effects of the pyrolysis atmosphere on the elemental mercury adsorption characteristics and mechanism of biocharVarious pyrolysis atmospheres could affect the pyrolysis process of biomass,resulting in differences in the microscopic properties of biochar,which thus affected the mercury adsorption characteristics.The processes of walnut shell biomass pyrolysis in the O₂ atmosphere were complicated,as three reaction pathways were possible.A critical concentration occurred between O₂concentrations of 5%and 7%.Below this concentration,the heterogeneous reaction of biomass oxidation was controlled by O₂ diffusion.When the concentration exceeded the critical concentration,the reaction was accelerated and controlled by kinetics.Above 750°C,CO₂ could react directly with the biochar through the Boudouard reaction.The Hg⁰ adsorption capacities of the biochars obtained in O₂ atmosphere were worse than those obtained in N₂pyrolysis atmosphere.With an increase in the O₂ concentration,the mercury adsorption capacities of the biochars initially increased and then decreased.CO₂significantly enhanced the adsorption capacity.With the increase in the CO₂concentration,the mercury adsorption capacity of the biochar sorbents showed an overall gradually increasing trend.Among the examined biochars,the biochar sorbent prepared in the 20%CO₂ pyrolysis atmosphere had the highest adsorption capacity.In addition,the mercury adsorption was related to the adsorption sites of the biochar rather than monolayer adsorption.Mercury adsorbed to the surface of the biochar in various ways during the mercury adsorption process and thus was attached to the surface of the biochar in several forms.The products from chemical adsorption were Hg-OM and HgO.?3?Elemental mercury adsorption characteristics and mechanism of iron-based modified biocharThe mercury adsorption performances of the modified biochar sorbents were significantly improved.Among them,the Fe-2%Mn/BC sorbent had the strongest mercury adsorption capacity.For the modified biochar sorbents obtained by modification with FeCl₃,Mn?CH₃COO?₂ and CuSO₄,the mercury adsorption performances first increased and then decreased with increasing loading amount.Among them,the mercury adsorption performances of the 10%Fe/BC,Fe-4%Cu/BC and Fe-2%Mn/BC sorbents were higher than those of the sorbents with other loading amounts in the corresponding modification methods.However,as the loading amount of KMnO₄ increased,the mercury adsorption performance decreased gradually.The modification decreased the graphitization degree.The crystallite structure of the Fe-2%KMnO₄/BC sorbent varied the most greatly.Elemental metals and the corresponding various forms of metal oxides appeared on modified biochar.The crystal sizes of the Fe-4%Cu/BC and Fe-2%Mn/BC samples were significantly smaller than those of non-modified biochar.In addition,spinel structure solid solutions of CuFe₂O₄?copper iron oxide?and MnFe₂O₄?galaxite?were formed,resulting in many cation vacancies on the surface of biochar,which facilitated mercury adsorption.Compared to the non-modified biomass,the pyrolysis of the modified biomass was more violent and complete.The BET specific surface area,cumulative pore volume and pore richness of the modified biochar sorbents were improved to different extents.The content of carbonyl and carboxyl groups on the modified biochar surface increased.Moreover,after modification,metal-coordinated hydroxyl functional groups?M-OH?appeared on the sorbent surface.For the modified sorbents,HgO and Hg-OM were the main forms,indicating that the modification process significantly promoted the chemical adsorption of Hg⁰.The removal of Hg⁰ was the result of a combination of adsorption and oxidation.During the further oxidation of the adsorbed Hg⁰,the functional groups,lattice oxygen,chemisorbed oxygen,halogen components,and metal oxides or ions on the sorbent surface all played roles.Fe₂O₃ and CuO?MnO₂ and Fe₂O₃?double metal oxides had a synergistic effect on the removal of Hg⁰.However,the strong oxidative ability of Mn⁷⁺and Mn⁶⁺allowed them to oxidize Hg⁰ competitively with Fe³⁺.After the adsorption and oxidation of Hg⁰,the formation of a monolayer or multilayer"secondary adsorption layer"occurred,and additional Hg⁰ content could adsorb to this"secondary adsorption layer",allowing more adsorbed Hg⁰ to be oxidized.?4?Molecular structure and elemental mercury adsorption mechanism of biocharWalnut shell biochar consisted mainly of C,H,O and N elements.Aromatic carbon was the main component of the biochar molecular structure,while aliphatic carbon was linked to aromatic structural units.The biochar contained graphite microcrystalline structures,and the fringe spacing was 0.3624nm.The molecular structure model was mainly constituted by aromatic structure,including a methyl,4 hydroxyls and 8 carbonyls.The molecular formula was C₅₅H₃₇NO₁₄,and the molecular weight was 935.After the optimization,the total potential energy was the maximum in the UFF force field,and the potential energy was the minimum in the MM2 force field.The SE method PM6 was also applied to obtain the formation heat of the biochar molecular in the three force fields.And the molecular was the most stable in the Dreiding force field.The Hg⁰ adsorption process on the surface of biochar mainly depended on the charge of the adsorption site.If the adsorption site carried negative charge and the number of charges was large,it could facilitate the elemental mercury adsorption on the surface of the biochar.Moreover,the charge of the ortho atom adjacent to the adsorption site also had a great influence on the adsorption activity of the adsorption site.