Study On Low Temperature And Slow Heating Rate Pyrolysis Mechanism Of Rice Husk

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.