【作者】 杨中平；

【导师】 郭康权； 杨林青；

【作者基本信息】 西北农林科技大学 ， 农业机械化工程， 2011， 博士

【摘要】 我国年产各类农作物秸秆约7亿吨,其中玉米秸秆约占1/3左右,但其工业化利用比例较低,秸秆各组分理化特性差异大是利用的难点之一。若能将玉米秸秆叶、外皮和髓进行分离,分别作为不同原料加以利用,不仅能显著提高原料的品质与价值,也有利于实现其工业化利用,从而推动玉米秸秆的资源化利用进程。论文在对秸秆主要组分基本特性、分离特性和压缩特性研究的基础上,对玉米秸秆主要组分的气流分离及模压成型流变特性进行了系统深入的研究,主要研究内容及结论如下:(1)对秸秆主要组分的构成及基本特性进行了研究,得到了秸秆主要组分的质量分数和体积分数、外皮厚度、密度、纤维形态等基本特性参数及变化规律。主要组分质量分数为:叶40.2%、茎秆48.9%、其它10.9%;茎秆中外皮和髓的质量比为2.29,体积比为0.33。外皮含量随茎秆直径加大略有增加,髓含量则相应减小。主要组分中,外皮纤维形态较好,其它明显较差。这为主要组分的分离利用提供了基础数据。(2)对茎秆碎料的粉碎特性和压缩特性进行了试验。得到了不同筛孔粉碎原料的粒度分布、组分质量分数、堆积密度及变化规律。提出了以组分质量分布指数评价粉碎物料主要组分的粒度分布状况,该指数反映了粒度分布对组分分离的影响。分析表明粒度较大的碎料有利于组分分离。对髓碎料的压缩试验表明,其压缩特点为初始压缩阶段较长,而中间阶段拐点对应的应力值低(1.2MPa)。该特性适宜进行大深度比的异形面产品模压。(3)对茎秆碎料的空气动力学特性进行了试验研究,制作了不同组分和不同规格的试样,测定并分析了其悬浮速度分布范围及相互之间的关系。在试验范围内,外皮悬浮速度分布范围为1.60～3.22 m/s;髓为0.65～2.75 m/s,叶为0.75～1.25 m/s,不同粒度的悬浮速度的相互交叉是气流分离的主要障碍。据此,提出了临界分离粒度和临界悬浮速度,并分析得出了外皮不同最小粒度对应的轻组分分离动力曲线,为秸秆碎料的气流分离提供了理论依据。(4)分别研制了茎秆碎料水平和垂直气流分离的试验装置,并进行了可行性试验。水平气流分离试验表明,分离物料由外皮碎料区、混合区和髓碎料区组成,但混合区范围较宽,分离率难以达到生产要求。垂直气流分离能大幅度提高原料的分净率和分离率,并实现了髓物料与外皮物料分别排出,表明该方法对茎秆碎料具有较好的适应性。(5)研制了垂直气流分离系统,并采用二次正交旋转设计进行了分离试验研究。优化得到各因素的较佳组合为风速0.85m/s、进料量14kg/min、进料位置0.75m;其分离效果为外皮分净率99.21%,髓分净率99.13%,外皮分离率98.45%,髓分离率98.67%。试验结果表明,该系统设计合理,工作稳定可靠,分离效果优异。这为秸秆主要组分的分离提供了一种稳定可靠的工业化分离方法与设备。(6)采用模拟试验和产品模压试验的方法,对秸秆碎料典型异形面模压流变特性进行了研究,阐明了凹向成型和凸向成型模压流变特性及主要因素的影响。针对异形面模压物料充填不均匀问题,提出了一种可变下平面的模压方法,并对其进行了试验研究,弄清了其模压流变特性。研究表明该方法能显著改善异形面模压时物料充填的均匀性。(7)提出采用物料充填均匀性系数评价物料充填均匀程度,在此基础上对主要模压参数对物料充填均匀性的影响进行了较为深入的分析。建立了充填平面高度和模压倾角与充填均匀性系数之间的关系,发现模压倾角与斜面物料充填均匀性系数的关系符合secα曲线,当模压倾角大于70°时充填状况将迅速恶化。该关系为揭示模压倾角对模压过程的影响提供了重要的理论依据。更多还原

【Abstract】 There are about 0.7 billion tons of various crop straws produced in China every year, and corn straw accounts for 1/3. But corn straw’s industrial utilization is relatively low. The significant difference in physical and chemical characteristics of corn straw’s components is one of the difficulties in utilization. If the leaf, skin and pith of corn straw can be separated and used as different raw materials respectively, not only can the quality and value of raw materials be significantly improved, but also its industrial utilization can be realized. This will promote corn straw’s utilization process. Based on studying basic properties, separation properties and compression characteristics of main components of corn straw, pneumatic separation and rheological characteristic under compression molding on corn straw’s main components were studied deeply. The main research contents and conclusions are as follows:(1) The compositions and their characteristics of main components of corn straw were studied. The basic parameters, such as mass fraction and volume fraction of corn straw’s main components, thickness of skin, density and form of fiber, and their change regularity were obtained. Main components of corn straw are leaf, stem and other components, whose proportion in mass were 40.2%, 48.9% and 10.9%, respectively. In the stem of corn straw, mass ratio of skin to pith was 2.29, and volume ratio was 0.33. The content of skin increased with the increasing of stem diameter slightly, while the content of pith reduced accordingly. In the main components, the form of skin fiber was best, but other components’forms were obviously poorer. This study provides basic data for separating main components.(2) The crush properties and compression characteristics of chopped corn straw were studied. The particle size distributions of crushed materials under different sieves, mass fraction of main components, bulk density and these indexes’change regularity were obtained. A component mass index was put forward to evaluate particle size distribution of main components of crushed material. This index illustrates the effects of particle size distribution on separation of main components. The results shown that larger debris was helpful to main components’separation. Compression test on pith debris showed that the initial compression stage was longer and the stress value (1.2 MPa) was lower at the turn point in middle stage. This feature presented that pith debris was suitable for molding some products with big depth ratio and abnormity plane.(3) The aerodynamic characteristics of corn straw’s main components were studied. The samples with different components and different specifications were made, and their floating speed range and the relationship with each other were measured and analyzed. Within the test range, the suspending velocities of skin, pith and leaf were 1.60~3.22 m/s, 0.65~2.75 m/s, and 0.75~ 1.25 m/s, respectively. The main obstacle in separating main components of corn straw with airflow was suspending velocity’s intercross of different particles. Consequently, critical separation particle size and critical suspending velocity were put forward, and the lighter fractions’separation kinetics curve corresponding to different minimum skin size was obtained. These provides theoretical basis for separating chopped corn straw with airflow.(4) The separation device for stem debris with vertical and horizontal airflow were designed and developed. Its feasibility was demonstrated. Horizontal airflow separation test showed that the separated materials were made up of skin debris, pith debris and mixed debris samples between skin and pith. But the range of mixed samples was relatively broader and separation rate was difficult to reach production requirements. Separation with vertical airflow could greatly increase isolation rate and separation rate. Moreover, skin and pith debris could be blown off separately. This test showed that the vertical airflow separation had excellent adaptability to chopped corn straw.(5) The separation device for stem debris with vertical airflow were designed and developed. The device’s separation rates were tested using design scheme of quadratic-rotation-orthogonality. The optimum separation conditions were 0.85m/s of airflow speed, 14kg/min of feed quantity and 0.75m of feed position. The skin isolation rate was 99.21%, pith isolation rate was 99.13%, skin separation rate was 98.45%, and pith separation rate was 98.67%. The results demonstrated that the device ran well and had a good separation rate. It provides a stable and reliable industrial separation method and equipment for separating corn straw’s main components.(6) The rheological properties of straw debris with typical abnormity shape under compression molding were studied through simulation and practical tests. The rheological properties of concave molding and convex molding and the effects of main factors were clarified. To deal with the problem of non-uniformity in material-filling, a molding method with variable lower plane was put forward and demonstrated with experiments. The results shown that the uniformity in material-filling can be effectively improved by this method.(7) A material-filling uniformity index was put forward to evaluate uniformity in material-filling. Based on this, the effects of main molding parameters on material-filling uniformity were analyzed deeply. The relationships between the height of molding-filling plane, molding-angle and the index of material-filling uniformity were established. It was found that the relationship between molding-angle and the index of material-filling uniformity followed secαcurve. The filling condition became worse rapidly if molding-angle above 70°. This test provides important theoretical basis for revealing the effects of molding-angle on molding process.更多还原