【作者】 沈建锋；

【导师】 谭俊杰； 朱曙光； 刘心志；

【作者基本信息】 南京理工大学 ， 工程热物理， 2011， 博士

【摘要】 稻壳是重要的农业废弃物之一,仅中国每年就有约4000万吨的产量。作为一种可再生的生物质资源,稻壳通常用作燃料直接燃烧以提供热量,但利用品位相对较低。热解是目前生物质研究的热点技术之一,通过热解,可以将稻壳转化为高品位的可燃气和易存储、运输且能量密度高的焦油,以及工业上需求量很大的活性炭和具有高附加值的无定形二氧化硅。虽然,稻壳中无定形二氧化硅含量较高,但对热解温度较为敏感,因此,对稻壳进行低温慢速热解以获得更多的固相产物,并将气、液相产物作为副产品加以利用是一种合理的技术途径。目前针对稻壳低温慢速热解的试验和机理研究相对较少,热解反应过程和机理尚不明确,本学位论文围绕稻壳低温慢速热解展开了以下几个方面的工作：(1)原始稻壳基本特性研究从能源利用角度,建立了基于工业分析结果的生物质碳、氢和氧含量三元预测关系式；讨论了稻壳用量、氧弹氧压和苯甲酸添加量对稻壳热值测试结果的影响,提出了稻壳热值测试的参考方法。从低温慢速热解角度,考虑到稻壳粒径和堆积方式不同会对床层孔隙率和传热等特性产生影响,从而影响稻壳固定床低温慢速热解过程,对稻壳破碎产物——稻壳粉的堆积密度进行了测试,比较了不同粒径和不同堆积方式下稻壳粉堆积密度测试结果的差异,并分析了差异成因。(2)浸泡稻壳基本特性研究水浸泡预处理会对稻壳基本特性产生影响,从而影响稻壳固定床低温慢速热解过程以及固相产物特性。对浸泡前后稻壳的燃烧特性、收缩和卷曲特性以及稻壳灰中二氧化硅品质对灰化参数的敏感性进行深入研究,确定了经济的浸泡水量和浸泡时间,提出了稻壳热利用过程中的收缩和卷曲机理,指出了稻壳无定形二氧化硅合理的制备工况。(3)稻壳低温慢速热解动力学模型建立与产物预测研究出于工况优化的目的,建立合适的动力学模型对指导稻壳低温慢速热解具有十分重要的意义。综合考虑气、固相产物以及二次反应沉积系数,建立了改进的热解动力学模型,并以木块为对象验证了模型的准确性。最后,将该模型应用于稻壳低温慢速热解产物分布的预测,得到了稻壳热解产物随时间以及升温速率的变化规律。(4)稻壳固定床低温慢速热解试验研究设计并搭建了稻壳固定床低温慢速热解试验系统,在此基础上,深入研究了反应室压力、原料含水量、热解时间、氮气升温速率、氮气预热终温和预处理对稻壳固定床低温慢速热解过程的影响。通过工业分析、元素分析、比表面积测试、灰形貌分析和二氧化硅相态分析等手段讨论了不同热解条件对炭化稻壳特性的影响。(5)稻壳固定床低温慢速热解机理研究基于大量试验结果,提出了稻壳固定床低温慢速热解机理：在稻壳固定床低温慢速热解过程中,下层稻壳脱水过程蒸发的水分以及主要热解过程析出的可凝性气体均会在上层稻壳中冷凝,使得上层稻壳中的水分和焦油含量增加,脱水和主要热解过程所需时间延长。下层稻壳主要热解阶段释放的热量会沿层高传递给上层稻壳,从而缩短上层稻壳脱水和主要热解过程所需时间。同一层高稻壳完成热解所需时间主要受这两个因素的影响,在稻壳固定床低温慢速热解过程中,水分和焦油的凝结是决定性影响因素。在主要热解阶段,由于稻壳内外表皮二氧化硅的不均匀分布,随着挥发分的大量析出,稻壳发生收缩和卷曲。这会对床料的堆积特性产生显著影响,从而影响床层阻力和传热等特性,而这些特性的变化又会作用于稻壳的低温慢速热解过程。本文工作丰富了对稻壳基本特性以及稻壳固定床低温慢速热解反应机理的认识,所得研究结果可为稻壳固定床低温慢速热解装置的设计和应用提供指导和依据。更多还原

【Abstract】 Rice husk which is one of the most important agricultural wastes has the annual output about 40 million tons in China. As a renewable bio-resource, rice husk is used as fuel for the direct combustion to provide heat. However, the use is of relatively low quality. Pyrolysis is one of the important technologies in the field of biomass research. Biomass can be effectively converted into gas with high quality, tar which is easy to storage, transport and with high energy density, as well as char and amorphous silica with great demand in industry by means of pyrolysis. Although rice husk has high content of amorphous silica, it is sensitive to the pyrolysis temperature. Therefore, it is more appropriate to prepare more solid product, as well as gaseous and liquid product as by-products with low temperature and slow heating rate pyrolysis. However, research efforts to the basic characteristics, as well as the mechanism of low temperature and slow heating rate pyrolysis of rice husk in fixed bed, are of relatively sparse, and there are still some problems to be resolved. This dissertation is focused on the following aspects:(1) The basic characteristics of original rice huskThe new ternary prediction correlations based on the simple proximate analysis were developed to compute the elemental composition of biomass. The effects of sample mass, oxygen gauge pressure and mass ratio of rice husk and benzoic acid were discussed. Moreover, the suitable testing conditions for determining the heating value of rice husk were formulated. As the particle size and the accumulation of rice husk would affect the porosity and the heat transfer characteristics of bed materials, so as to affect the process of low temperature and slow heating rate pyrolysis of rice husk, the bulk density of different sizes of rice husk powders were measured. In order to explain the rule that the bulk density changed with particle size, the structures of rice husk powders were observed.(2) The basic characteristics of leached rice huskLeaching will affect the characteristics of rice husk, which indirectly affect the process of pyrolysis of rice husk. In this section, the effects of leaching procedure on the combustion characteristics, the shrinkage and crispation characteristics, as well as the phase of silica in rice husk ash were studied. The economical water application rate and leaching duration were proposed. The mechanisms of the shrinkage and crispation or rice husk were developed. The reasonable preparation conditions for amorphous silica were proposed.(3) Modeling and product prediction of low temperature and slow heating rate pyrolysis of rice huskFor the purpose of operating conditions optimization, it is of significance to develop an appropriate kinetic model to guide the process of the low temperature and slow heating rate pyrolysis of rice husk. Therefore, the new kinetic model which considering gaseous products and deposition coefficient was developed. The accuracy of model was verified by comparing the simulated results with the experimental and the simulated results reported in literatures for wood. The model was applied to predict the products distribution for three kinds of rice husk. As a result, the variation of products yields, including primary and secondary solid and gaseous products, changes with pyrolysis temperature and heating rate was obtained.(4) Experimental study of low temperature and slow heating rate pyrolysis of rice husk in fixed bedThe fixed bed reactor for performing the low temperature and the slow heating rate pyrolysis of rice husk was designed and then installed. Subsequently, a series of experiments were carried out in order to discuss the effects of inlet and outlet pressure of reactor, water content of rice husk, pyrolysis time, heating rate of nitrogen, preheated final temperature of nitrogen and leaching treatment on the pyrolysis characteristics of rice husk. Furthermore, the effects of pyrolysis conditions on properties of carbonized rice husk were discussed by means of proximate and ultimate analysis, specific surface area analysis, ash morphology analysis, as well as silica phase analysis.(5) Mechanism of low temperature and slow heating rate pyrolysis of rice husk in fixed bedThe mechanism of low temperature and slow heating rate pyrolysis of rice husk in fixed bed was proposed based on a series of experimental results. In the process of low temperature and slow heating rate pyrolysis of rice husk in fixed bed, the evaporated free water and the escaped condensable gas from the lower rice husk layer will condense in the upper rice husk layer. Therefore, the contents of water and tar increase in the upper rice husk layer, which leads to the major extension of time required for the pyrolysis process. Moreover, the heat released in the main pyrolysis stage will passed along to the upper rice husk layer, which reduces the dehydration and the primary pyrolysis time. In the low temperature and the slow heating rate pyrolysis of rice husk in fixed bed, water and tar condensation is the decisive factor which affect the pyrolysis time. In the main pyrolysis process, as the silica is uneven inside and outside the epidermis of rice husk, the rice husk shrinks and crisps with a large number of volatile substances in the rice husk escape. It will affect the resistance and the heat transfer characteristics of the rice husk in fixed bed. Moreover, these changes will act on the properties of low temperature and slow heating rate pyrolysis of rice husk.This work enriches the understanding of the basic characteristics and the mechanism of low temperature and slow heating rate pyrolysis of rice husk. The results provide guidance for designing the equipment for the low temperature and slow heating rate pyrolysis of rice husk.更多还原