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Investigation Of The Spatiotemporal Evolution Of Chemical Structure And The Combustion Characteristics Of Pyrolytic Char From Sawdust Pellet

Posted on:2023-05-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y J ChenFull Text:PDF
GTID:1522307172952339Subject:Thermal Engineering
Abstract/Summary:PDF Full Text Request
Biomass pellets have a great potential as a fuel alternative to coal.Bio-char,as one of the main pyrolytic products,is the react of gasification and combustion,and its structure will affect the subsequent reaction process.Therefore,further understanding the chemical structure of pellet char,is crucial to thermochemical untilisation of biomass pellet.Pyrolysis of biomass pellet is a continuous process,and the temperature gradient inside the pellet would lead to heterogeneous pyrolysis.Thus,clarifying the spatial and temporal evolution of chemical structure is the key to fully describe the mechanism of pyrolysis of biomass pellet.Firstly,in this thesis,the overall chemical structure of pellet char was further understood through solvent extraction.The char yield of pellet is at most 3.93%higher than that of powder,and the extraction ratio of pellet char is at most 1.19%higher than that of powder char.The aromatization degree of pellet char is lower than that of powder char.There are more stable radicals and small aromatics in the extract of pellet char.The average molecular weight of the extract of powder char is higher,while the extract of pellet char has more lipid compounds.Furtherly,the changes in chemical structure of the extract and residue at various temperature and pellet sizes were studied.It was found that the pellet with a diameter of 16mm has the highest char yield and extraction ratio at low temperature,while the pellet with a diameter of 8 mm has the lowest char yield and extraction ratio.The difference in char yield and extraction ratio could reach to 7.03%and 21.23 mg/g,respectively.At higher temperature and surface area-to-volume ratio,the polymerization degree of char is higher,and the aromatics in the extract is less.Besides,the ratio of large molecules and high oxygen compounds in the extract increase.In order to intuitively obtain the chemical structure of different spatial positions in pellet char.The trends of chemical structure in axial and radial directions were obtained through an innovative Raman characterization method.The relative concentration of large aromatics(AD/AG)and ratio of large aromatics to small aromatics(AD/A(Gr+Vr+Vl))showed an increase from the center to the edge of pellet char.The spatial variance in chemical structure of pellet char became gentler with temperature increasing.,and the slopes for different pellets were different.Therefore,the heterogeneity index(HI)was proposed to quantitatively express the heterogeneity in chemical structure of pellet char.It was found that the char prepared by pellet with a diameter of 16 mm at 600 oC had the more homogeneous structure.HI(AD/AG)and HI(AD/A(Gr+Vr+Vl))for the char is 0.947×10-5 and0.101×10-5,respectively.Subsequently,the temporal and spatial evolution of chemical structure of pellet char was investigated.It was found that HI(AD/AG)first increased and then decreased with time,and HI(AD/A(Gr+Vr+Vl))first decreased and then increased.The char with pyrolysis period of90 s has the smallest HI(AD/AG)of 4.354×10-5,and the char with pyrolysis period of 120 s has the smallest HI(AD/A(Gr+Vr+Vl))of 0.219×10-5.Then,through correlating the pyrolysis conditions(temperature,pyrolysis period,pellet size)with heterogeneity index of chemical structure of pellet char,the equation for calculation of heterogeneity index was obtained,which could well reflect the influence of pyrolysis conditions on the heterogeneity in chemical structure of pellet char.Finally,the combustion characteristics of pellet char were studied on a photothermal thermogravimetric.With the pyrolysis period increasing,the ignition temperature first decreased and then decreased,and the average combustion rate decreased.The flammability index and comprehensive combustion index both increased first and then decreased.The pellet char with pyrolysis period of 180 s has the highest flammability index and comprehensive combustion index of 1.771×10-3 and 5.07×10-5,respectively.Based on the in-depth understanding on the chemical structure of pellet char,the correlation model between the chemical structure and combustion characteristics of pellet char was established and verified,realizing the goal of predicting the combustion characteristics of pellet char from its chemical structure.
Keywords/Search Tags:sawdust, pellet, pyrolysis, pyrolytic char, extraction, char structure, heterogeneity, combustion
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