| Summary: | 博士 === 國立中興大學 === 機械工程學系所 === 105 === In this study, rice husk pellet (RHP) and Crymptomeria Japonica (CJ) have been used as feedstock in gasification process. The feedstock has been torrefied at 250 °C and 350 °C in 1 hour. before performing gasification in bubbling fluidized bed (BFB) gasifier and thermogravimetric analyzer to determine exergy efficiency and kinetic characteristics respectively.
To evaluate exergy efficiency of combined torrefaction and gasification, the exergy analysis was performed in the BFB gasification process using raw and torrefied RHP. The gasification was conducted in a 30 kWth bubbling fluidized bed. The effects of air equivalence ratio (ER) and torrefaction temperature on overall exergy efficiency were examined. Results of experiments showed that torrefaction process may enhance chemical energy (exergy) of RHP due to lower values of O/C and H/C. However, the overall energy (exergy) efficiency decreases due to the energy loss in volatile gas and electric energy input during the combined torrefaction-gasification process. The efficiency decrease was more severe in the case of torrefaction at 350 °C. The overall exergy efficiencies of the torrefied RHP at 250 °C and 350 °C are 30% and 21%, respectively.
To study the gasification reactivity of the raw and torrefied biomass, the CJ was used in the kinetic analysis of non-isothermal) and isothermal gasification in CO2 environment. For non-isothermal process, thermal decomposition occurred in three different stages, including dehydration, hemicellulose-cellulose and lignin decomposition on 30.2-102.6 oC, 222.6-422.1 oC and 426-847.2 oC respectively. A linear model was proposed in this paper, and this model fits the experimental data quite well. In main decomposition stage, the activation energy of raw CJ, CJ-250 and CJ-350 are 77, 114.3 and 49.9 kJ/mol respectively. The highest activation energy of CJ-250 is found due to the higher quantity of cellulose in the sample compared with other two samples. The activation energy of CJ-350 shows the lowest value because most of volatile in hemicellulose-cellulose zone are expelled during the torrefaction process. Rising heating rate leads to shift the mass loss curve towards higher temperature, increase activation energy and pre-exponential factor.
For isothermal process, three gasification temperature of 750°C, 800°C, and 850°C were conducted to evaluate the kinetic parameters of Arrhenius form with proposed models. Homogeneous model (HM), shrinking core model (SCM) and linear model (LM) were used and the predicted results obtained from these models were compared with experimental data. The reaction rate of gasification was enhanced as the temperature was raised, and a correlation of kinetic parameters with temperature was obtained. The simulated results of linear model (LM) fit well with experimental data.
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