Study On Biomass Torrefaction And The Reaction Characteristics Of Its Product In Pyrolysis And Combustion Processes

The development of biomass energy is an effective way to ease energy shortages and environmental pollution.But as a fuel,there are still a lot of defects in the application of biomass,which limits the applying scale of biomass.The biomass torrefaction developed in recent years is a pre-treating technology in which high value product can be obtained,which can effectively improve the energy quality and reduce the transportation and storage cost of biomass materials.However,the domestic research about the technology is still at its early stage.And in particular the research about the thermal conversion process of woody torrefaction product is deficient.Therefore,the characteristics of typical woody biomass after torrefaction in China and the effect of torrefaction on the reactivity of its torrefaction product in the pyrolysis and combustion were studied in this paper.At first,Schima spherical particles taken as experimental samples,the torrefaction experiments with different conditions of different heating temperatures,different dwell times and different particle sizes were carried out.The results showed that the morphological characteristics of the samples after torrefaction would change.The color of the wood particles after torrefaction was deeper,and the volume and quality of samples became smaller.The variation range of particle mass was larger than their volume,so the density of samples increasesed after torrefaction.In terms of elemental content,the relative content of carbon increasesed after torrefaction,however the relative content of oxygen and hydrogen decreasesed,so the higher heating value of the samples also increasesed.But the energy yield of the samples decreasesed due to the volatilization of combustible gas components during the torrefaction process.These changes would be more significant with the deeper degree of torrefaction.All factors being taken into conderation,the torrefaction condition of heating temperature 290 ℃ and dwell time 1 hour were appropriate for Schima woody particles.Then,the pyrolysis characteristics of the woody biomass samples from six typical wood species in China after pretreatment in different torrefaction conditions were investigated by thermogravimetric analysis.The results showed that the pyrolysis process of each sample before and after torrefaction followed the first order bi-component separate-stage model.And the effect of torrefaction on the pyrolysis characteristics of different woody biomass was similar.The pyrolysis process of each sample after torrefaction was shifted to higher temperature,and the volatile releasing interval of each sample was more centralised and violent.But the deeper torrefaction would lead to excessive decomposition of the organic components in the sample,resulting in reduced volatility of the sample.Besides,the activation energies and the pre-exponential factors of the samples after torrefation in different raction stages all increased due to the relative change of the main organic components.Finally,the pyrolytic char yield and char reactivity of the six woody biomass samples before and after torrefaction were analyzed in detail.The results showed that the change range of absolute char yield from before and after torrefaction was inconsisitent.The absolute char yeild of beech,phoenix pine and sapele didn’t change a lot after torrefaction while the absolute char yeild of balsa and Schima would be enhanced after torrefaction.The results of isothermal combustion experiment at 800 ℃ showed that the effects of torrefaction on the char combustion reactivity of different woody biomass were different.For pine and sapele,the combustion reactivity of the char would be enhanced after torrefaction,but the char combustion reactivity of phoenix and Schima would be weakened after torrefaction,while the char combustion reactivity of balsa and beech didn’t change after torrefaction.