Experimental Study On Drying Behavior Of Biomass During Infrared Radiation

Biomass energy is recognized as the alternative source for fossil energy,and the drying pretreatment is an essential step towards its further application.In present work,infrared radiation drying behaviors of typical biomass were investigated based on a thermogravometry analyzer and a lab-scale far-IR dryer.The effects of temperature,biomass types and thickness on the drying kinectics,heat transfer and microcosmic pore structure of the samples were determined based on heat and mass transfer theroy and fractal geometry.The influence of temperature on the drying kinetics for biomass basic chemical components,cotton stalk and eucalyptus bark was revealed based on a thermal analysis technique and kinetic theroy.During the isothermal drying,the average drying rates of the cotton stalk,eucalyptus bark,pure cellulose,pure xylan and pure lignin increased by about 2 times with the temperature rising from 60? to 120 ?(100 ? for xylan).The average drying rates of eucalyptus bark were higher than that of the cotton stalk,and the average drying rates for the pure xylan were less than that of another two components.Between 60 ? and 70 ?,the isothermal drying of five samples presented a combination of a short warm-up stage,a constant rate stage,and two distinguished falling rate stages.The activation energy of the five samples in the first falling rate stage was higher than that of the second falling rate stage.The activation energy in for the cotton stalk was larger than that of the eucalyptus bark,and activation energy for the pure lignin was lower than that of the pure cellulose and pure xylan.Infrared radiation drying behaviors of four typical biomass briquettes(populus tomentosa leaves,cotton stalk,spent coffee grounds and eucalyptus bark)were investigated based on a lab-scale far-IR setup.The effect of radiation source temperature on the drying kinetics,heat transfer and microstructure of the samples was addressed.As the temperature went up from 100? to 200?,the average drying rates of the four biomass briquettes grew about 1.4-1.9 times,and the average radiation heat transfer fluxes increased by about 3.3-3.7 times.In addition to a warm-up stage,and two falling rate stages,the far-IR drying of the samples also presented a constant rate stage(below 180?).Without obvious size reductions for the IR drying manners are observed,and the average values of the effective moisture diffusivity for the four biomass briquettes in the first falling rate stage were between 1.015E-09 and 3.951E-09 m2·s-1,while that in the second falling rate stage were from 1.473E-09 to 6.484E-09 m2·-1.Among four biomass briquettes,the values of the average effective moisture diffusivity for the eucalyptus bark briquette in two falling rate stages were the highest ones,while that for the spent coffee grounds briquette was the smallest ones.The surface fractal dimensions of the eucalyptus bark briquette under far infrared irradiation were highlighted through mercury porosimetry and fractal theory.The fractal dimension based on thermodynamic analysis was more suitble to quantitatively describe the pore structure characteristics of the eucalyptus bark briquette.And the fractal dimension decreased with increasing the proportion of macroporous pore volume.The influences of radiation source temperature,thin layer thickness and initial mositure content on the far-IR drying kinetics of the loose structure of eucalyptus bark thin layer were investigated.The results indicated that the effective moisture diffusivities were significantly affected by radiation source temperature,thickness and initial mositure content,respectively.Moreover,the effective moisture diffusivity increased with the increase of the thin layer thickness and the reduction of the initial mositure content.With the radiation temperature growing from 100? to 200?,the effective moisture diffusivities for thin layers in the first falling rate stage were between 8.127E-10 and 5.056E-09 m ·s-1,while that in the second falling rate stage were from 1.269E-09 to 7.630E-09 m2·s-1.The results developed in the work can be used for designing new dryer and biomass thermal utilization.