Study On The Mechanism Of Biochar Densification And The Properties Of Gasification/Reduction Of Biochar Pellet

Densification of bio-based material helps to improve its volume density and energy density,thus benefit the transportation,storage and the following conversion process.Bio-based material densification is of great significance for the large-scale application of biomass energy.Biochar,obtained from pyrolysis of biomass,has different physical properties,component content,fuel character and chemical structure,thus leads to a different densification process mechanism and conversion rule.This study mainly focused on the densification of carbonized biomass and the gasification/reduction application of biochar pellet.The effects of densification parameters,binders on the densification of biochar were studied to reveal the densification mechanism.And then the reaction properties,char structure evolution and conversion mechanism were investigated during the pyrolysis and gasification of biochar pellet,and the reduction conversion of iron-bearing pellet.The results will then guide the high-efficient utilization of biochar pellet.The research work in this paper includes the following aspects:Biochar obtained from pyrolysis of wood shavings at different temperature,and then densification at different pressures and water contents.Results found that with the increase of densification pressure and water addition content,the compressive strength was increased first and then decreased,and the volume density was also changed at different trend.With the increase of pyrolysis temperature,the compressive strength,volume density were also decreased first and then increased.The quality of biochar pellet made from high temperature biochar was better than that from low temperature biochar.Biochar pellets formed at pyrolysis temperature of 550–650oC,compressive pressure of 128 MPa,and water content of 30-35%,exhibited superior properties to be handled and used in the following application process.The main factors contributing to the cohesion of biochars at the different temperatures were hydrophilic functional groups,particle size and binder,due to they related to the hydrogen bond,pore structure,and adhesive force.Two organic binders?alkaline lignin and starch?and two inorganic binders?Ca?OH?₂and NaOH?were selected to compress with biochar.And the effect of binder type and addition content on the rheological properties,mechanical strength,energy consumption,fuel characters and combustion behaviors were studied.Results showed that with the addition of binders,the energy consumption during densification was decreased,the strength of pellet was increased.And with the increase of binders content,the asymptotic modulus was gradually reduced which increased the cohere of biochar particles.The mechanical strength and volume density were very low with the addition of starch as the binder.The moisture uptake of the NaOH pellet was very high which would be harm to the following handling and storage.With the addition of lignin or Ca?OH?₂ content of 10%w/w,the pellets showed lower compressive energy consumption,acceptable moisture uptake,enhanced mechanical strength,and promoted combustion performance,which showed better potential for utilization as biofuels.To study the effect of densification on the pyrolysis/gasification conversion of biochar pellet in-depth,this thesis firstly investigated the pyrolysis and subsequent in-situ gasification of the low-temperature pyrolysis?torrefied?pellet without binder addition.Results showed that with the increase of torrefaction temperature,the decreased surface area,enhanced pore size and decreased K content in ash of the pellet pyrolysis char,along with the promoted graphitized crystal structure of the pellet pyrolysis char,all resulted in the decrease of gasification reactivity,the increase of activation energy and the increase of reaction resistance.In addition,the gasification of high-temperature pyrolyzed biochar pellet formed with KOH as the binder was studied.Results showed that with the increase of KOH content,the melting and permeating of K during char gasification was enhanced,which enlarged the pore structure of char.And the catalytic function of K was promoted by the higher KOH content in pellet.Thus the gasification reactivity and reaction rate were increased with the increase of KOH content.As for the reduction of iron ore by biochar,the reduction reaction properties of iron ore-biochar pellet was studied.It was found that with the decrease of carbon to iron ratio in iron ore-biochar pellet,the carbon conversion was increased and the iron reduction extent was decreased.With the increase of reaction temperature,both the carbon conversion and iron reduction extent were increased.The carbon conversion was controlled by one-dimensional diffusion.And during reaction,the further pyrolysis of biochar firstly kept it reacted with a high reactivity,and then the char structure evolution with small rings turned into large rings,then changed into crystalline graphite finally with relative low reactivity.The iron phase reduction reaction was controlled by three dimensional diffusion,and the increased activation energy as the reaction proceeded restricted the iron reduction process.The reaction of iron ore-biochar pellet in N₂,CO₂ or O₂ atmosphere was studied to analyze the effect of reaction atmosphere on the reduction of pellet.Results showed that the reaction rate of iron ore-biochar pellet in different atmosphere was in the order N₂<CO₂<O₂.And with the increase of the concentration of the active gas,the reaction rate increased.When the pellet reacted in CO₂/N₂ atmosphere,with the increase of CO₂ concentration,the iron phase was reduced firstly and then oxidized into Fe₃O₄ again by superfluous CO₂,thus resulted in the decrease of Fe and FeO.When the pellet reacted in O₂/N₂ atmosphere,the hypoxic environment promoted the production of CO firstly and reduced the iron ore into Fe and FeO quickly.While with the continue injection of O₂,the reduced iron was oxidized into Fe₃O₄/Fe₂O₃ again.When the mole ratio of O₂ to C higher then 1.2,the main solid residual after reaction was Fe₂O₃.The research was a good exploration for the technology development of pyrolytic biochar compression and the binder selection during biochar densification.And provide a scientific reference for the high-efficient utilization of the biochar pellet.