626 / 2019-01-20 19:40:38

The influences of torrefied parameters on hygroscopicity of torrefied biomass

Biomass;torrefaction;hygroscopicity

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Biomass as one of promising renewable energy sources has been attracting interest worldwide due to their abundant reserve and availability. Biomass utilization is limited due to untreated agriculture and forestry residues' characteristics of higher moisture, lower heating value and energy density, higher quality decay by biodegradation, and higher energy consumption during collection. The high water content of biomass causes ignition difficulty in furnace. When gasification reacts, water evaporation will absorb a large amount of heat. Meanwhile, a large amount of water vapor contains heavy metal ions precipitated by biomass, which is extremely bad for boiler itself. A thermo-chemical method (called torrefaction) that is earmarked by a lower operating temperature at 200-300ºC under an inert atmosphere and less than 50ºC/min of heating rate was found to be effective for improving energy density and shelf life of the biomass. The equilibrium moisture content (EMC) and the hygroscopicity reduction extent (HRE) are important parameters for evaluating biomass performance.
The objective of this work is to study the effect of torrefaction parameters on hygroscopicity of torrefied biomass. The raw biomass including herbage (corn straw (CS), wheat straw (WS)), hardwood (poplar wood (PW)) and softwood (cedar wood (CW)) was selected as experimental materials. The torrefaction pretreatment was carried in a fixed-bed reactor. The effects of biomass species, torrefaction temperature (T), and holding time (τ) on biomass torrefaction were analyzed based on physiochemical characteristics of torrefied solid product (biochar). The obtained results indicate that the equilibrium moisture content of herbaceous biomass is significantly higher than that of woody biomass, and hardwood's is smaller than softwood's. The effect of torrefaction temperature on the equilibrium moisture content of biomass is similar. The hemicellulose content of biochar decreases continuously with the increasing of torrefaction temperature. The water content is constantly smaller, that is, the hydrophobicity is also stronger. However, when the torrefaction temperature reached 300℃, the cellulose and hemicellulose in the herbaceous biomass completely decomposed, and the molecular structure in biochar was completely destroyed by the complete decomposition of the cellulose, which led to the destruction. The increase in equilibrium moisture content, that is, the hydrophilicity of biochar is becoming stronger. When the torrefaction temperature is constant, the holding time has a significant effect on the equilibrium moisture content of the biochar. When the hemicellulose is not completely decomposed, the equilibrium moisture content of the biochar increases with the increase of the holding time while it decreases with the increase of holding time when the hemicellulose is completely decomposed.