Study On Pyrolysis Characteristics And Thermal Analysis Kinetics Of Wheat Straw

The development and utilization of biomass energy have received more and moreattention due to the energy and environment problems. Pyrolysis technology is the basisof energy development and utilization for biomass. Wheat straw pyrolysis were studiedby using Thermogravimetric Analysis (TG or TGA). The effects of experimentalconditions (particle size and heating rate), pretreatment methods (washing, acid washing,alkali treatment, microwave, ultrasonic treatment) and catalyst (alkali metal salts,alkali-earth metal salts, transition metal salts, Al2O3, Al2O3-Na2CO3) on pyrolysis wereinvestigated. Thermogravimetric and differential thermogravimetrie (TG-DTG) datawere analyzed by using kinetics analysis method and pyrolysis kinetic triplet (Pyrolysismechanism function, Eaand lnA) were obtained. Co-pyrolysis characteristics of wheatstraw and two coals (soft coal and anthracite) were also studied.Analytic results showed that the optimal particle size of wheat straw for pyrolysiswas at the range of0.1~0.25mm and optimum heating rate was10℃/min. Themechanism function of pyrolysis for wheat straw was No.6function, whose integraland differential mechanism functions were g(α)=[1-(1-α)1/3]2andf(α)=1.5(1-α)2/3[1-(1-α)1/3]-1, and the reaction order n was2. The pyrolysis of wheatstraw was controlled by three-dimensional diffusion mechanism (cylindricallysymmetric) and followed Jander Equation. The apparent activation energy of pyrolysiswas between120and170kJ/mol. With the increase of the conversion rate (α), changesof Eaand lnA were similar. It showed that the kinetic compensation effect existed in thepyrolysis processes. Pyrolysis process of wheat straw was divided into four stages:drying, pre-heating period, the main pyrolysis stage and carbonization stage.The contents of volatile compound in pyrolytic products increased and Ealoweredafter wheat-straws were treated with water and acid washing. TG-DTG curves of wheatstraws soaked by alkali were moved toward low temperature region. Eaof wheat strawssoaked by NaOH reduced from151.44kJ/mol to84.09kJ/mol. Pyrolytic rate ofwheat-straw treated by microwave and ultrasonic increased, while Eadecreased.TG-DTG data were analyzed by using single rate methods. Pyrolysis mechanismfunction of wheat straws treated with water, HCl, H3PO4, microwave and ultrasonicaccorded with No.7function. Pyrolysis mechanism function of wheat straws soaked byH2SO4accorded with No.17function. Pyrolysis mechanism function of wheat strawssoaked by KOH and Na2CO3accorded with No.9function. Pyrolysis mechanismfunction of wheat straws treated by NaOH accorded with No.36function.The disappearances of shoulder peaks located at the lower-temperature region onDTG curve showed that the decomposition of hemicellulose was catalyzed by alkalimetal salts and CaCO3. With inhibitive effects of FeSO4and ZnSO4, pyrolysis rate was significantly reduced. Initial temperatures of pyrolysis for wheat straw mixed withAl2O3and Al2O3-Na2CO3moved to high temperature and pyrolytic rate of wheat strawmixed with Al2O3was significantly increased. With catalytic effects of inorganic metalsalts, the pyrolysis of wheat straw was controlled by three-dimensional diffusionmechanism (cylindrically symmetric) and followed G.-B. equation, whose integral anddifferential mechanism functions were g(α)=1-2α/3-(1-α)2/3and f(α)=3/2[(1-α)1/3-1]-1.The apparent activation energy of pyrolysis for wheat straw reduced with the catalysisof FeSO4, ZnSO4, CaCl2and MgSO4, especially FeSO4which could reduce apparentactivation energy from135.66kJ/mol to113.66kJ/mol. Pyrolysis of wheat strawsmixed Al2O3was controlled by two-dimensional diffusion mechanism and followedValensi equation, whose Eaincreased.Synergistic effect existed in co-pyrolysis process of wheat straw and soft coal(anthracite). When mass ratio of wheat straw in mixture was low, wheat straw wasbeneficial to pyrolysis of coal. When mass ratio was high, wheat straw was unfavorableto pyrolysis of coal. Mechanism function of co-pyrolysis of wheat straw and soft coalaccorded with No.7function. The activation energies of pyrolysis were between165and175kJ/mol. When mixing ratios of anthracite to wheat straw was8:2, pyrolysismechanism function accorded with No.9function and Eawas203.86kJ/mol. Whenmixing ratios of anthracite and wheat straw were5:5and3:7, pyrolysis mechanismfunctions accorded with No.2function and Eawere at the range of160and170kJ/mol.