| To solve the problem of PCDD/Fs emission caused by the incineration of waste,especially plastic waste(which was widely contained in municipal solid waste and medical waste),the in-situ gasification chemical-looping combustion(iG-CLC)pattern of plastic waste using adsorbent-decorated Fe-based oxygen carrier(OC)was first proposed in this work to control the emission of PCDD/Fs.The research involved the confirmatory study of controlling the emission of PCDD/Fs through CLC pattern,key reaction kinetics during iG-CLC processes,syngas-CLC experiments in a batch fluidized-bed reactor,iG-CLC experiments of plastic waste in a batch fluidized-bed reactor,as well as the design and iG-CLC experiments of a 150W semicontinuous fluidizcd-bed reactor.The confirmatory study of controlling the emission of PCDD/Fs through CLC pattern was investigated.First,the choice of OC and adsorbent as well as the feasibility of dechlorination had been analyzed through calculating the Gibbs free energy for some single reactions by using a thermodynamic software(HSC).The simulation results indicated that HCl hardly spontaneously interacts with Fe2O3 and Fe3O4(which can avoid the consumption of OC);however,when decorating OCs with three adsorbents(CaO,K2O and Na2O),HCl can spontaneously convert into the corresponding chloride in the FR,and these chlorides cannot react with O2 in the AR.Hence,it can be speculated that three adsorbents(CaO,K2O and Na2O)can react with HCl and produce stable chlorides in the reactors.In addition,the combustion experiments of plastic waste under air atmosphere were conducted in a two-step reaction analyzer and subsequently the catalytic actions of Fe-based OC and three adsorbents on the organic matters in exhaust gases were tested through detecting the toluene absorption solutions with a high resolution fourier transform mass spectrometer(FT-MS).Results demonstrated that the catalytic actions from Fe-based OC and three adsorbents on the organic matters in exhaust gases are insignificant,which indicated that the introduction of Fe-based OC and three adsorbents hardly can bring about adverse effects.Next,an infusion tube was selected as a representative type of plastic waste and its pyrolysis experiments were respectively conducted in a TGA with different heating rates(5,10,20 and 40 ?/min)and in a batch fluidized-bed reactor at different constant temperatures(850,875,900 and 925 ?).The TGA results can clearly present the temperature range of plastic waste pyrolysis,while the batch fluidized-bed experiments can reduce the limitation form the external transfer of mass and heat.The pyrolysis kinetics of plastic waste were calculated respectively for the TGA results and the batch fluidized-bed results,which can provide useful information for knowing the pyrolysis mechanism of plastic waste.Besides,the redox reaction kinetics of four OC particles(raw OC and those respectively with CaO,K2O and Na2O decoration)with a HCl-containing synthesis as reducing gas and with air as oxidizing atmosphere were investigated through isothermal TGA experiments and these temperatures included 850,875.900 and 925 ?.It was found that the translation Fe2O3 into Fe3O4 can be described by the 3-D nuclei growth model,while the translation FeO into Fe2O3 can be described by the phase boundary-controlled(contracting cylinder).Subsequently,the CLC experiments of HCl-containing synthesis were conducted in the batch fluidized bed reactor that simulated a real chemical looping process,and the dechloridation efficiencies and combustion efficiencies were calculated according to the chlorine content and gas components in exhaust gases,respectively.For synthetic OCs,the effects of the adsorbent species,the CaO decoration method(wet impregnation,coprecipitation and physical mixture),CaO loading and reaction temperature on the efficiencies of dechloridation and combustion were investigated,and results indicated that CaO can be used as the applicable adsorbent,the wet impregnation method demonstrates the best adsorbent decoration for synthetic OCs and 5 wt.%CaO loading reaches an optimized level of dechloridation as well as that 900 ? was considered as the suitable temperature.Next,the dechloridation efficiency gradually descends,and correspondingly,the mole ratio between Ca and Fe on the OC particle surface exhibited a declining trend with an increase in the cycle number for 60 redox cycles.Last,the reduced samples after thirty cycles were washed using a dilute hydrochloric acid solution,and the ESEM-EDX result demonstrated the removal of the Ca and Cl elements from the surface of these OC particles and implied the regeneration of these OC particles.Considering that natural iron ore has relatively pyknotic surface compared with synthetic OC,the decoration method for using iron ore as OC was developed and eventually the ultrasonic impregnation method was considered as a applicative method for iron ore according to the dechlorination and combustion efficiencies.Then,the iG-CLC experiments of plastic waste were conducted in the same batch fluidized bed reactor and the fluidizing gas involved N2 as well as the mixtures of N2 and vapour.For synthetic OC,operation conditions,including reaction atmosphere,supply oxygen ratio and reaction temperature,were investigated to explore their influence on carbon conversion,maximum instantaneous rate of fuel conversion and CO2 yield.Ultimately,the optimal operating condition was that the mixture of 40 vol.%vapour and 60 vol.%N2 as the fluidizing gas,2.5 as the supply oxygen ratio and 900 ? as reaction temperature.Next,the iG-CLC experiments of plastic waste for two synthetic OCs(raw synthetic OCs and CaO-decorated synthetic OCs)under the optimal operation conditions as well as the conventional incineration experiments of plastic waste under air atmosphere were conducted with 10 cycles to compare the combustion efficiencies.Meanwhile,17 toxic PCDD/Fs during the 10 cycles process were measured by a HRGC/HRMS,it can be found that the amount and toxic equivalency quantity(TEQ)of PCDD/Fs can be reduced by 88%and 75%respectively when using CaO-decorated synthetic OCs(the amount and TEQ of PCDD/Fs can be reduced by 42%and 20%respectively when using CaO-decorated synthetic OCs),which demonstrated that the CLC technique can effectively inhibit the formation of PCDD/Fs.Besides,Cl2 yield(which were measured through methyl orange spectrophotometric method)decreased by 73%when raw Fe2O3/Al2O3 was used as OCs compared with the conventional incineration mode.CaO-decoration on Fe-based OCs further lowered the Cl2 yield(decreased by 82%),which confirmed that lowering Cl2(which is an important chlorine source for the formation of PCDD/Fs)yield is an effective to inhibit the formation of PCDD/Fs.Two analogous chlorine substitution models were respectively applied to know the inhibition mechanisms of PCDDs and PCDFs through calculating the chlorine substitution probability and similarity.Results indicated that the formation of PCDD/Fs is mainly dominated by the chlorine substituent process and the function on the formation of PCDDs is greater than that on the formation of PCDFs.Thus,it can be conjectured that the inhibition mechanisms through CLC pattern are the absence of O2,the lower Cl2 yield under reducing atmosphere and the effective dechlorination using CaO adsorbent.Last,a semicontinuous fluidized-bed reactor(which can simulate the interconnected fluidized-bed reactor)using plastic waste as fuel was designed to test the CLC performance of long-term operation.Note that the feed of plastic waste was continuous and the continuous feed of OC was realized through controlling the input and output mass rate of OC.The iG-CLC experiments of plastic waste were conducted at 900 ? in the semicontinuous fluidized bed reactor under 11 experimental conditions(each experiment sustaining more than 90 min)to evaluate the effect of bed inventory,reaction atmosphere.thermal power and mass rate of OC on average carbon conversion and CO2 yields.It was found that CaO decoration cannot cause obvious adverse influence in terms of stable operation CLC performances.Besides,the organic compounds including chlorobenzene of exhaust gases for two experimental processes were absorbed by toluene solutions and subsequently measured by a gas chromatograph-mass spectrometer(GC-MS),which demonstrated that CaO decoration can obviously inhibit the emission of chlorobenzene. |
PDF Full Text Request