Experimental Study On Torrefaction And Pyrolysis Of Biomass Pellets

With increasing environmental problems caused by fossil fuels usage,alternative energy sources become more and more attractive.Among all renewable energy sources,biomass energy is the only one having potential to replace fossil fuels in the future and has been paid more attention around the world in the past decades.In this paper,the typical industrial biomass waste(i.e.furfural residue,FR)and forestry waste(i.e.sawdust)were used as raw materials,and the experiments on pelletization,torrefaction(low temperature pyrolysis)and pyrolysis(medium temperature pyrolysis)were carried out successively.The results were as follows:The physical and mechanical properties and specific energy consumption of furfural residue pellets(FRPs)and sawdust pellets(SPs)were significantly correlated with temperature and pressure(p<0.05).Under the same pelletization conditions,FRPs had higher particle density,strength(tensile strength),hardness and lower porosity.At the same pressure,the true densities of both pellets decreased with increase of temperature.Nonlinear fitting models were well simulated the trend of particle density and porosity with pelletization temperature and pressure.Considering the specific energy consumption and the properties of pellets,the optimum operating conditions of the two types of pellets were 130? and 130 MPa.Under the optimum operating conditions,the particle density,tensile strength and hardness of SPs were 1.22±0.01 g/cm3,4.08±0.53 N/mm2 and 61.45±5.25 N/mm2,respectively.The particle density,strength and hardness of FRPs were 1.42±0.00 g/cm3,6.70±0.39 N/mm2 and 426.11±12.73 N/mm2,respectively.With temperature increasing from 200 to 300? or residence time increasing from 15 to 30 min,the qualities of torrefied pellets were both significantly improved,while residence time of ?45 min didn't have considerable effects.Torrefied sawdust pellets(TSPs)achieved the highest lower heating value(i.e.26.76 MJ/kg)and energy density ratio(i.e.1.46)at 300? and 120 min,higher than torrefied furfural residue pellets(TFRPs)(i.e.23.78 MJ/kg and 1.27),respectively.With increasing temperature and residence time,the volumetric energy densities of TSPs decreased from 20.81(200 0C and 15 min)to 16.96 kJ/cm3(300 0C and 120 min),while TFRPs correspondingly increased from 25.69 to 27.59 kJ/cm3.The highest true densities(i.e.1.85 and 2.40 g/cm3)and porosities(i.e.65 and 52 v%)of TSPs and TFRPs were achieved at 300? and 120 min,much higher than untorrefied pellets.Atomic ratio H/C of TSPs and TFRPs at 300? were very near to that of coal(i.e.0.067).The activation energies of both pellets were?31 MJ/mol,which were in accordance with the non-isothermal torrefaction kinetics model with the reaction order of 1.In hydrophobicity test,TSPs and TFRPs conducted at 300? only absorbed water of 2.0-2.8 wt.%and 1.4 wt.%,showing strong water-resistant ability.Particle densities of furfural residue pyrolytic pellets(FRPPs)were mainly in the range of 1.15-1.28 g/cm3,and it was 6.96-16.52%lower than FRP.The moisture uptake of FRP was 17.98 wt.%.The range of moisture uptake of FRPPs were 1.95-12.32 wt.%.The strength of FRPPs after and before absorbing water were very close.FRPPs had similar iodine adsorption numbers and methylene blue adsorption values with pyrolysis char of furfural residue powders(FRPCs).At 450?,the methylene blue adsorption value of FRPP(i.e.96.40 mg/g)was much higher than FRPC(i.e.63.16 mg/g).Considering the differences between north and the south regions in China,the main performance indexes of FRPs at different temperatures during torrefaction and pyrolysis were optimized.It was found that 250-300 0C was the best temperature range in terms of comprehensive index of pellets in both south and north regions in China.