秸秆类生物质成型热黏塑性本构模型构建

Establishment of thermo viscoplastic constitutive model for straw biomass briquetting

针对生物质颗粒生产能耗高、效率低的现状,该文从生物质组成角度,特别是木质素特性出发探讨其成型机理。秸秆成型过程由于内摩擦力的作用产生大量热量,温度的上升会造成木质素的软化,木质素的这一变化为纤维颗粒的团聚提供了黏结力。温度和木质素特性对生物质塑性成型性能产生巨大影响,是热黏塑变形过程。为研究生物质内部特性对塑性成型过程的影响,运用内时理论,以玉米秸秆、小麦秸秆和水稻秸秆为研究对象,构建了秸秆类生物质压缩成型内时本构方程。借助黏土流动理论,推导定义生物质黏塑性强化函数和核函数,运用数值分析和试验得出本构方程的系数。与试验数据相比较,基于内时理论的热黏塑性本构模型较好的模拟了生物质塑性流变过程。结果显示,向秸秆中添加20%的木质素,可有效提高其塑性流动性能,降低其在相同应变下的应力以及生产能耗;当成型温度在100~115℃之间,应变率在1×102~1×103 s-1之间,对于木质素质量分数分别为29%的玉米秸秆、33.5%的小麦秸秆和34.3%的水稻秸秆的固化成型性能最好。

Biomass briquetting is one of the key technologies to solve the problem of its collection, transportation and storage. Meanwhile biomass pellet, as a renewable energy, can be used instead of coal for heating, electricity, etc. Study on the mechanism of biomass briquetting is the basic way to develop new forming methods and equipments, and to improve productivity greatly. The research in this paper is about a thermo viscoplastic constitutive model for the biomass briquetting mechanism based on endochronic theory. Straw mainly consisting of cellulose, hemicellulose and lignin, is a typical example of dissipative material. Biomass briquetting process is a mixed process of extrusion flow and interstitial flow. During the process, a large quantity of heat is generated by internal friction. The increase of temperature causes the lignin to soften which provides cohesive force for fiber particles briquetting. To verify the effect of lignin and temperature on the straw plastic deformation, stress-strain test and briquetting temperature test were run on the straw with different lignin mass fractions. Three kinds of biomass raw materials including corn straw, wheat straw and rice straw were the study objects in this paper. Then, these raw materials were added with additional lignin respectively in accordance with the following mass fractions 10%, 20% and 30%. The results show that the increase of lignin mass fraction can reduce stress and temperature obviously. The maximum stress of straw with additional mass fraction of 20%is reduced by about 10 MPa compared with raw materials′,and the maximum temperature is reduced by about 10%. Consequently, lignin and temperature have a crucial effect on straw biomass briquetting process which is thermo viscoplasticity process. The constitutive model should reflect the internal characteristics of materials, especially the characteristics of lignin. In order to consider the internal variables of straw, endochronic theory applicable to the dissipative materials was adopted to build the thermo viscoplastic constitutive model. First, endochronic time which replaced Newton time was defined to describe thermo viscoplasticity deformation. Strain, strain rate, temperature and lignin mass fraction were the main internal factors to formulate the briquetting mechanism. Second, the endochronic constitutive equation of straw briquetting was derived according to thermodynamic law and clay rheology. Third, tests were planned to analyze the influence of lignin mass fraction and temperature on straw forming process. The optimal lignin mass fractions of corn straw, wheat straw and rice straw are respectively 29%, 33.5% and 34.3%. The optimal temperature is between 100 and 115℃. Strengthening function was deducted based on the test data and Johnson-Cook model. Finally, verification test was designed upon GDS consolidation test system. Coefficients of constitutive equation were got by numerical analysis. Compared with test data, the thermo viscoelasticity constitutive model based on endochronic theory has its extensive applicability. When the temperature is between 100 and 115℃ and the strain rate between 1×102 and 1×103 s-1, corn straw with lignin mass fraction of 29%, wheat straw with lignin mass fraction of 33.5%, and rice straw with lignin mass fraction of 34.3% have the best plastic briquetting performance. This study results provide an operable material model for the finite analysis and an important theoretical basis for equipment structure design.