Study On The Torrefaction,Pelletization And Particulate Matter Emission Behaviors During The Combustion Of Agricultural Biomass

The disadvantages of biomass,such as low energy density,high moisture content and high particulate matter（PM）emission,restrict its promotion and application.In view of this,torrefaction process with pelletization technology were combined to improve the fuel properties,pelletization qualities and combustion behavior of agricultural biomass.The PM formation mechanisms during the combustion of torrefied biomass pellet（TOP pellet）were also revealed.By means of additive introduction or biomass mixing（municipal sewage sludge,MSS）,the PM emission behavior was effectively controlled.The main research contents are as follows:Firstly,the torrefaction behavior and the pelletization quality of cornstalk were studied using a horizontal fixed bed and a universal material testing machine,respectively.It was found that with the increased torrefaction temperature and oxygen concentration,the H/C and O/C ratios decreased while the heating value increased significantly.However,at a high oxygen concentration（21%）,a large number of combustible components were consumed,which led to lower fuel qualities.Appropriate torrefaction conditions could improve the pelletization qualities of TOP pellet.However,the excessive torrefaction conditions（300°C and 21%O₂ concentration）could completely destroy the internal structure of biomass.The surface of material particle became rough and the viscoelasticity of pellet decreased,resulting in a higher compressing energy.Secondly,the combustion characteristics of TOP pellet were studied by a thermal analyzer.It was found that after non-oxidative torrefaction,the ignition temperature of decreased while the burnout temperature increased.The temperature corresponding to the maximum weight loss peak gradually decreased at the same time.With regard to the oxidative torrefaction,the oxidation process would consume combustible components and produce much ash.The higher ash content could hinder the release of volatile matter and the contact between char and oxygen,which eventually led to higher ignition and burnout temperatures.The combustion kinetic of raw cornstalk pellet was mainly dominated by the diffusion mechanism model.After torrefaction,the initial combustion stage belonged to the chemical reaction series model while the later stage was transformed into the diffusion mechanism model.Subsequently,the PM emission characteristics during the combustion of TOP pellet were investigated in a vertical fixed bed reactor.The results indicated that the torrefaction contributed to the formation of both PM₁ and PM_1-10,of which the PM₁ was dominant.Due to the release of Cl to the gas phase in form of HCl and CH₃Cl,the main formation way of PM₁ was the evaporation and subsequently condensation of K₂SO₄ and a small amount of KCl.The chemical compositions of PM_1-10 remained unchanged and the alkali and alkali earth metals（AAEMs）silicates would form PM_1-10 via melting and coalescence processes.Afterwards,additives were introduced to control the emission of above-mentioned PM.It was found that kaolin could obviously reduce the emission of PM_0.2while calcium hydroxide and ammonium dihydrogen phosphate promoted the total PM₁₀ emission.After phosphoric acid modification,the specific surface area and the active sites of kaolin increased,which enhanced the physical adsorption and the chemical reaction for PM_0.2.The residual phosphoric acid on the surface or in the internal structure of modified kaolin also improved the capture efficiency of alkali metal.The maximum reduction ratio of PM_0.2could reach to 64.5%.Finally,the MSS was selected to mix with biomass for pellet preparing.The pelletization qualities and the PM emission bahaviors of the resultant pellets were studied.The results demonstrated that with the MSS mixing ratio increasing,the relaxation density,compressive strength,durability and hydrophobicity of pellet were enhanced while the compressing energy decreased to some extent.During the co-firing,a large number of mineral elements provided by MSS had good capture effects on alkali metal,which mitigated the transformation of it into fine model particles and contributed to the reduction of PM₁.The interaction between MSS and cornstalk was the most obvious,followed by camphor wood and rice husk was the worst.