Study On The Formation Mechanism And Regulation Technology Of Polycyclic Aromatic Hydrocarbons During Straw Pyrolysis

China is rich in crop straw resources,with an annual crop straw output of 8.56×10⁸ t,crop straw is considered as a low-cost and abundant renewable energy.Pyrolysis and carbonization technology of crop straw is an important form of straw energy utilization.However,polycyclic aromatic hydrocarbons（PAHs）are a kind of pollutants produced in the process of straw pyrolysis,which have mutagenic,carcinogenic and teratogenic effects on humans.In view of the above problems,this paper studied the mechanism of PAHs production in the process of straw pyrolysis,the influence of key factors,and the control measures of PAHs in biochar.The formation and evolution mechanism of PAHs in the volatile components of straw pyrolysis were studied by TG-GC/MS;The effects of key factors on PAHs in biochar during straw pyrolysis were studied in a fixed bed tubular furnace;The control mechanism of PAHs in straw pyrolysis biochar was studied by adding biochar,loading K⁺、Ca²⁺、K⁺-Mg²⁺（1∶1）to modify straw,and loading K⁺、Ca²⁺、K⁺-Mg²⁺（1∶1）to modify biochar.The above research on the formation and regulation mechanism of PAHs in the process of straw pyrolysis provides theoretical basis and technical support for the improvement of straw pyrolysis carbonization technology and process optimization,and has important practical significance for the clean and efficient energy utilization of straw.The main research contents and conclusions are as follows:（1）In the process of straw pyrolysis,through on-line monitoring of the instantaneous pyrolysis thermal volatile components produced at 13 temperature points between 200℃and 700℃,the release law and distribution characteristics of PAHs in the volatile components were analyzed,and the physical and chemical characteristics of the corresponding biochar were analyzed.The results show that 8 kinds of PAHs with 2～4 rings can be detected at 9 temperature points in the range of 400℃to 700℃.naphthalene and fluorene begin to form in the volatile components obtained at 400°C,and the relative content of PAHs can reach 40.36%at 560℃.From the physical and chemical properties of biochar produced at 9 temperature points,the straw carbonization stage（400℃～700℃）can be further divided into three evolution stages,including preliminary carbonization stage（380℃～480℃）,amorphous carbon structure stage（480℃～600℃）and dehydrogenation and aromatic ring growth stage（600℃～700℃）.The relationship between the relative content of PAHs in volatile components and the H/C ratio of biochar can be described by power function.According to the DFT calculation results of the possible formation and consumption paths of naphthalene,it is confirmed that the formation and consumption of naphthalene in the released products are in a dynamic equilibrium state.（2）In view of the complexity of the actual working conditions of straw pyrolysis,the effects of key factors（volatile concentration（sample amount）,pyrolysis temperature,carrier gas flow rate and heating rate）on PAHs in straw pyrolysis biochar and residual tar were comprehensively studied.The results showed that the effect of pyrolysis temperature on PAHs in biochar was not clear,and the effect of pyrolysis temperature on PAHs in biochar was significantly different when other pyrolysis conditions were different.Carrier gas purging can significantly reduce the content of PAHs in biochar（16.12 mg/kg,4.69 mg/kg,3.05 mg/kg to 7.68 mg/kg,4.14 mg/kg,1.53 mg/kg）.Through full factor analysis,the key factors that have a significant impact on PAHs in straw pyrolysis biochar are carrier gas flow rate,the interaction between heating rate and carrier gas flow rate,and the interaction between pyrolysis temperature and volatile concentration.（3）In view of the important role of volatile components of straw pyrolysis on PAHs in biochar,cheap and easily available biochar was used as an additive to modify and catalyze the volatile components of straw pyrolysis.Biochar was added to straw in-situ and non in-situ respectively to study the influence of volatile components and PAHs during pyrolysis.The straw was mixed with biochar prepared at different ratios（10%～40%）and temperatures（prepared at 500～900℃）.The effects of biochar on the volatile components of straw pyrolysis were studied by TG-FTIR.The effects of biochar on the production of biochar and PAHs in residual tar were studied by covering the straw with a biochar layer（500～700℃）.The results showed that the addition of biochar at 500℃,700℃and 900℃could reduce the acids,furans,acids and carbonyls in the volatile components of straw pyrolysis.At three temperatures（500℃,600℃and 700℃）,the content of PAHs in biochar produced by straw pyrolysis decreased significantly（7.15 mg/kg,9.14 mg/kg,10.44 mg/kg to 1.25 mg/kg,3.66mg/kg,2.7 mg/kg）and the toxic equivalent decreased significantly.The results showed that the addition of biochar to straw for catalytic modification of volatile components was conducive to the reduction of PAHs in biochar produced by straw pyrolysis.（4）In order to develop simple and easy-to-use measures for reducing PAHs in biochar,the effects of K⁺、Ca²⁺、K⁺-Mg²⁺（1∶1）modification on PAHs in biochar produced by straw pyrolysis and the control effects of loading K⁺、Ca²⁺、K⁺-Mg²⁺（1∶1）modified biochar on PAHs in straw pyrolysis biochar were studied.The results showed that the content of PAHs（PAHs）in biochar produced by straw pyrolysis after modification with K⁺、Ca²⁺、K⁺-Mg²⁺（1∶1）decreased significantly（from 6.04 mg/kg to 0.055 mg/kg～1.273 mg/kg）,and the toxicity decreased significantly.The addition of K⁺、Ca²⁺、K⁺-Mg²⁺（1∶1）modified biochar could reduce the content of PAHs in biochar produced by straw pyrolysis（7.13mg/kg to 0.82 mg/kg～5.58 mg/kg）.The results showed that the kind and content of PAHs in straw pyrolysis biochar and residual tar were controlled by cracking and upgrading the volatile components of straw pyrolysis with K⁺、Ca²⁺、K⁺-Mg²⁺（1∶1）modified biochar.