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Study On Pyrolysis Kinetics Of Biomass Residue

Posted on:2014-01-17Degree:MasterType:Thesis
Country:ChinaCandidate:Y R HaoFull Text:PDF
GTID:2232330395983089Subject:Thermal Engineering
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As a clean and renewable energy with zero carbon dioxide emission, low sulfide and low NOx emissions, biomass energy is playing a more and more important role in energy utilization structure. Biomass pyrolysis is not only an efficient biomass conversion pathway, but also an initial and associated reaction of gasification, liquefaction, and combustion process. Kinetic analysis on weight loss characteristics of biomass wastes can help understand biomass pyrolysis process and reaction mechanism in depth, which provides theoretical basis and basic data for design and development of efficient biomass conversion equipments.Firstly, a hot air drying device is designed and built, which is used to dry a large amount of fresh and humid materials to generate analysis samples in very short time. Thus biomass materials can be stored reliably and well prepared for experimental work in future.Some biomass wastes (e.g., JDB dregs, sawdust, sugarcane bagasse and fermentation mattress material) are studied on pyrolysis kinetics through thermal analyzers. Thermal gravimetric analysis (TGA) experiments are performed for four biomass wastes with particle size less than65mesh and heating rate β=10、20、30、50℃/min respectively. TGA experiments are performed for JDB dregs with four size ranges, i.e.<0.15mm,0.15mm~0.23mm,0.23mm-0.5mm and0.6mm-the0.9mm and the same heating rate P=20℃/mim. Multi-heating rate methods developed by Ozawa, Staring and Doyle respectively are adopted to obtain activation energy E value to determine the "three kinetic factors" of biomass wastes. The Coats-Redfern integral method is used to deduce pyrolysis mechanism and the "three kinetic factors" comparison method is used to acquire the optimal mechanism function for specified biomass. Finally, on the basis of the optimal mechanism functions, the activation energy E and frequency factor A of JDB dregs, sawdust and sugarcane bagasse are determined with the introduction of the kinetic compensation effect. Then, pyrolysis kinetic expressions of three samples are established. The results are summarized as follows.1. Experimental study of pyrolysis characteristics of biomass wastes(1) The pyrolysis process of JDB dregs is divided into three stages, i.e. preheated drying stage, main pyrolysis stage and carbonization stage. Different from JDB dregs, pyrolysis process of sawdust and sugarcane bagasse can be divided into four stages, with pre-pyrolysis stage different.(2) With the increasing of the heating rate, the maximum weight loss rate and the corresponding temperature increases significantly and the TG/DTG curve moves to the high-temperature side.(3) The carbonization degree of larger particle samples is higher, but the maximum weight loss and maximum weight loss rate are lower than that of smaller particle samples.(4) There is an evident phenomenon of two-peak separation with the DTG curve of sugarcane bagasse because the content of hemicellulose is higher than cellulose.(5) Wood biomass with higher content of lignin in the negative pyrolysis region is more active, and high heating rate is advantageous to the volatile release.2. Study of pyrolysis kinetics of biomass wastes(1) Through the three multi-heating rate method with the particle size less than65mesh, the activation energy values of JDB dregs, sawdust, sugarcane bagasse are respectively in the range70.5~76.8kJ/mol,126.4~167.5kJ/mol, and154.3~193.9kJ/mol.(2) With the increase of the conversion rate a, the activation energy values of sawdust and sugarcane bagasse increase at first and then decreased. When α=40%and35%respectively, the activation energy values of sawdust and sugarcane bagasse reach the maximum respectively.(3) Based on the mechanism functions, D1model best describes the pyrolysis reaction process of JDB dregs and D3model best describes the pyrolysis reaction process of sawdust and sugarcane bagasse.(4) By introducing the kinetic compensation effect, E and A are determined as follows: E=74.1kJ/mol、A=1.58×103s-1for JDB dregs, E=136.1kJ/mol、A=8.21×107s-1for sawdust, and E=123.9kJ/mol、A=4.99×106s-1for sugarcane bagasse based on the selected optimal mechanism functions. Three expressions of pyrolysis kinetics are then built.
Keywords/Search Tags:Biomass, Pyrolysis characteristics, Pyrolysis model, Reaction kinetics
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