Study On The Formation Characteristics Of Particulate Matter During Biomass Pellets Combustion

As a renewable resource with huge reserves and great potentials,biomass is an excellent choice to cope with the energy crisis and the greenhouse effect.Although the use of biomass resources can effectively reduce greenhouse gas emissions,a lot of fine particulate matters?PM?will be generated during combustion,thus polluting the air environment.Therefore,depend on The National Natural Science Foundation of China"Research on the formation mechanism of particulate matters form biomass pellets combustion",the generation mechanism of particulate matter during the combustion of biomass was studied.In addition,the control mechanism on PM formation was explored from two perspectives of composite additives and mixed biomass pellets.Based on this,control measures for particulate matter are proposed,which are of great significance for efficient combustion and clean utilization of biomass.First,the combustion characteristics and kinetic mechanism of biomass particles were studied.The weight loss characteristics of biomass powders and pellets were studied by a microscale thermobalance.Subsequently,the effect of densification on the the dynamic process of biomass combustion was analyzed by the non isothermal and Malek discriminant methods in thermal analysis kinetics.The release of volatile matter was slowed down due to the densification,thus leading to the delay of the ignition of the volatile.The combustion of volatiles overlapped with the fixed carbon,which increased the rate of weight loss during the first stage of the combustion process,and contributed to significant weightlessness peaks at this stage.After the combustion of volatile matter,the second weightless peaks during the combustion process formed due to the combustion of the residual fixed carbon which was relatively weak.The mechanism of combustion kinetics was different due to the difference of biomass types.In general,the combustion of the cotton stalk pellets was mainly dominated by the diffusion mechanism models,while the cornstalk pellets was controlled by the reaction series models.The initial stage of rice husk pellets combustion belonged to the diffusion mechanism models,but later transformed into the reaction series models.For camphorwood pellets,the initial stage of combustion was dominated by the diffusion mechanism models,and then transformed into the reaction series models,ultimately,was controlled by the phase interface models.The influence of biomass types and temperature on the formation of particulate matter from bulk biomass combustion were studied in a drop tube furnace.The particulate samples were collected by a low-pressure impactor?DLPI?,and then the elemental content was analyzed using inductively coupled plasma mass spectrometry?ICP-MS?and ion chromatography?IC?.Finally,the morphology of the fine particles was observed by environmental scanning electron microscopy?ESEM?.Particulate matter generated from agricultural biomass combustion was mainly composed of submicron particles mainly formed by the evaporation-condensation of KCl and K₂SO₄.However,for camphorwood,particulate matter was mainly composed of PM_1-10?particles of aerodynamic diameters between 1 and 10?m?,which was produced by the direct transformation of alkaline earth metals and Si.In addition,the fragmentation and coalescence of silicates and phosphates also promoted the formation of PM_1-10.The influence mechanism of densification and binders used in the densification process on particulate emission was investigated in a fixed bed.The elemental composition in PM was analyzed using environmental scanning electron microscopy and energy dispersive spectroscopy?ESEM-EDS?.While the elemental composition and crystal structure of bottom ash were analyzed by X-ray fluorescence spectrometry?XRF?and X-ray diffractometer?XRD?.Particulate matter generated from four kinds of biomass pellets combustion are mainly composed of submicron particles.The yield of PM₁increased significantly when CMC was applied to biomass pellets.For cotton stalk pellets,the use of three binders?CMC,bentonite and calcium lignosulphonate?leads to an increase in PM emission,especially CMC and calcium lignosulphonate.Subsequently,in order to reduce the emission of PM during the combustion of biomass pellets,three inorganic mineral additives were introduced,and the effects of the inorganic minerals on the emission characteristics of PM were studied.Among the three inorganic minerals,the effect of diatomite on reducing submicron particulate emission was the most obvious,with a reduction of 18.7%.While kaolin had moderate inhibitory effect on submicron particulate emission.However,the addition of calcium hydroxide significantly increased the emission of particulate matter.In order to control the emission of particulate matters,diatomite and CMC were selected to prepare composite additives,and the inhibitory effect of the composite additives on the formation of PM was studied.Composite additives?CMC+diatomite?effectively reduce particulate matter emission.Under the current experimental conditions,when the ratio of CMC to diatomite is 1/4,the emission reduction effect of particulate matter is the best.Finally,rice husk rich in silicon was selected and mixed with cotton stalk,cornstalk and camphorwood at different proportions,respectively.The particulate matter emission characteristics from the combustion of the mixed biomass pellets with binders were analyzed in detail.The interaction between cotton stalk and rice husk during combustion was not obvious.The synergies between cornstalk and rice husk mainly inhibited the formation of PM_0.1-1?particles of aerodynamic diameters between 0.1 and 1?m?.The synergistic effect between camphorwood and rice husk is the most obvious on the reduction of particulate matter emission among the three kinds of mixed biomass pellets.The research results played an important role in understanding the formation mechanism of particulate matter in the process of biomass combustion.At the same time,it provided effective methods and scientific guidance for the control of particulate matters emission.