Research On Biomass Oxidative Torrefaction And Combustion Properties

Biomass is the fourth biggest energy resource in the world, which can be used as arenewable and environmental friendly resource, but there are still some shortcomingsrestrict its large-scale utilization such as low energy density，difficult to collect and crush.Torrefaction is an efficient pretreatment method which can improve the physical andchemical characteristics of biomass and lay the foundation for its large-scale industrialutilization.This thesis focuses on the key issues in the process of large-scale industrial utilizationof torrefaction including the impact of oxidative torrefaction on the physical and chemicalcharacteristics of biomass, the feasibility of rotary kiln for large-scale torrefaction, and theeffect of torrefaction on biomass combustion.Firstly, a fixed bed bench-scale reactor was used for the torrefaction experiment underdifferent temperatures and oxygen concentrations. The effects of different conditions on theyields were discussed. The results showed that the optimal oxygen concentration fortorrefaction of agriculture and forestry samples is6%and the optimal temperature is260oC.Fourier Infrared Raman Spectroscopy, X-ray Diffraction and Scanning Electron Microscopewere used for the analysis of biomass physical and chemical properties after torrefaction.The oxidative Torrefaction can be divided into3stages. The process can be described bythe parallel twin-response model. Torrefaction reaction and oxidation reaction wasindependent and parallel.Then, a rotary kiln was used for the large-scale continuous feeding torrefactionexperiment. After torrefaction, the pulverization indexes of all the samples increasedsignificantly, which indicated that flue gas torrefaction was conducive to enhancing thecrushing performance. With the increase of torrefaction temperature, the balance rate ofwater absorption decreased, which was lower in flue gas than in nitrogen. Hence, rotarykiln is feasible for large-scale continuous feeding torrefaction. Flue gas slightly affected theenergy yield but enhanced the hydrophobic and grindability.Finally, a fixed-bed bench-scale reactor and a TG-DTG analyzer were used for combustion experiment. It was found that the volatile combustion peak and fixed carboncombustion peak separated after the addition of oxygen during torrefaction. The ignitiontemperature changed slightly, while the burnout temperature and activation energydecreased significantly, which were both lower than those of lignite and bituminous coal.With the increase of torrefaction temperature, the combustion efficiency was increasedfirstly and then decreased, with the maximum locating at260oC. When the temperature fortorrefaction was higher than260oC, the amount of NOXincreased significantly while thatof SO2increased slightly.