A study on torrefaction of various biomass materials and its impact on lignocellulosic structure simulated by a thermogravimetry

• Main Authors: Po-chih Kuo, 郭柏志
• Other Authors: Wei-Hsin Chen
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/34302058521215996600

碩士 === 國立臺南大學 === 綠色能源科技研究所碩士班 === 99 === Torrefaction is a novel thermal treatment process for biomass where raw biomass is heated in the temperatures of 200-300°C under an inert or nitrogen atmosphere. The purposes are to produce a fuel with increased energy density by decomposing the reactive hemicellulose fraction, and grindability of the fuel is improved to a great extent. In order to understand further the characteristics of torrefied biomass, four kinds of biomass and five basic constituents, including hemicellulose, cellulose, lignin, xylan and dextran, were individually torrefied in a thermogravimetry. Two pure materials, xylose and glucose, were torrefied as well for comparison. Therefore, the thesis can be classified into three parts. Firstly, torrefaction processes of four kinds of biomass materials, including bamboo, willow, coconut shell and wood (Ficus benjamina L.), were investigated using the thermogravimetric analysis (TGA). Two different torrefaction processes, consisting of a light torrefaction process at 240°C and a severe torrefaction process at 275°C, were considered. When the light torrefaction was performed, the results indicated that the hemicellulose contained in the biomass was destroyed in a significant way, whereas cellulose and lignin were affected only slightly. Once the severe torrefaction was carried out, it further had a noticeable effect on cellulose. From the viewpoint of torrefaction application, the investigated biomass torrefied in less than one hour with light torrefaction is an appropriate operation for producing a fuel with with greater energy density. The main constituents contained in biomass include hemicellulose, cellulose and lignin; therefore, the thermal decomposition characteristics of these constituents play a crucial role in determining the performance of torrefaction of lignocellulosic materials. Secondly, to gain a fundamental insight into biomass torrefaction, aforementioned five basic constituents and two pure materials, were individually torrefied in a thermogravimetry. Three torrefaction temperatures of 230, 260 and 290°C, corresponding to light, mild and severe torrefactions, were taken into account. The experiments suggested the weight losses of the tested samples could be classified into three groups; they consisted of a weakly active reaction, a moderately active reaction and a strongly active reaction, depending on the natures of the tested materials. Co-torrefactions of the blend of hemicellulose, cellulose and lignin at the three torrefaction temperatures were also examined. The weight losses of the blend were very close to those from the linear superposition of the individual samples, suggesting that no synergistic effect from the co-torrefactions was exhibited. The last part of thesis develops isothermal kinetics to predict the thermal decomposition characteristics of hemicellulose, cellulose, lignin and xylan in order to provide a useful insight into biomass torrefaction. A thermogravimetry is used to perform torrefaction and five torrefaction temperatures of 200, 225, 250, 275 and 300°C with 1 h heating duration are taken into account. From the established kinetics, the recommended values of order of reactions of hemicellulose, cellulose, lignin and xylan occur at 3, 1, 1 and 9, respectively, whereas their activation energies are 187.06, 124.42, 37.58 and 252.28 kJ mol-1, respectively. A comparison between predictions and experiments suggests that the developed model can provide a good description on the thermal degradation of the constituents, expect for cellulose at 300°C and hemicellulose at 275°C. Eventually, co-torrefaction of hemicellulose, cellulose and lignin based on the model is predicted and compared to the thermogravimetric analysis. . Keywords: Torrefaction; Pyrolysis; Thermogravimetric analysis (TGA);Thermal decomposition; Lignocellulosic structure; Biomass; Isothermal kinetics; Synergistic effect