稻壳与稻秆灼烧灰颗粒的微观形态特征及其能谱分析

Microstructure characteristic and X-ray energy dispersive microanalysis of combustion ash of rice husk and rice straw

应用扫描电镜(scanning electron microscopy,SEM)和能谱(energy disperse X-ray microanalysis,EDX)结合联用技术对稻壳和稻秆在600和815℃下灼烧产生的灰渣的微观形态特征及其元素组成进行了全面地研究,并考察了不同灰化温度对生物质灰的粒度分布、微观形态、颗粒表面、内部结构及元素组成等方面产生的影响。结果表明:灰化温度升高,灰粒的粒度减小,且分布较均匀,稻壳灰中多为团状颗粒,而稻秆灰以棒状颗粒居多;对于600℃稻壳灰,部分灰粒仍保留着稻壳的原始纤维结构,且灰中存在许多松散状的密实小颗粒,但并未出现粘结团聚,而600℃稻秆灰表面存在大量粘连着小颗粒的絮状物,表明此时灰中已经出现低熔点成分熔融而产生的弱粘结;815℃时2种生物质灰粒表面都出现熔融态的碱金属物质和以玻璃体突起形式存在的石英结构,而且都存在明显的团聚结渣现象;稻壳灰与稻秆灰的主要组成元素是C、O、Si、K和Ca,较少的Mg、Al、Fe、P等也被检出,而S只在稻秆灰中检出,稻壳灰中未检出S元素;灰化温度升高,稻壳灰的K含量明显下降,而稻秆灰中K、Na、Ca的含量变化较小,但是Cl、Fe、Al的含量均明显下降;稻秆灰的K、Na、Ca和Cl含量都远高于稻壳灰,稻秆灰比稻壳灰更易造成设备腐蚀、结渣等危害。

In the past decades, sustainable development and increasing fuel demand have necessitated the identification of possible energy sources. The utilization of biomass will reach up to 50 million tons in China after ten years. Nowadays, many countries are putting great emphasis on the exploration of biomass energy. The techniques used are various such as combustion, gasification, pyrolysis, hydrogen production. The use of the biomass as fuel generates a large amount of residual ash which causes serious environmental problems, as the biomass ash is easy to melt and volatilize. During the thermo-chemical conversion process, the residual inorganic materials also form the slag inside the boiler and the fly ash deposits on the tail heating surface, which retards heat transfer, deteriorates burning, and causes high temperature corrosion and super heat explosion. As one kind of clean and renewable energy, rice husk（RH） and rice straw（RS） are the main by-products during the process of rice processing. Especially, in comparison with other agricultural wastes, the ash content of RH is much higher. After thermo-chemical conversion, the inorganic mineral matters left are ashes, which can easily bond with tar and lead to the phenomenon of slagging, fouling and corrosion for the thermo-chemical conversion equipments. Therefore, in this study, the analysis of microstructure characteristics and elemental composition of these ashes from the combustion of RH and RS at 600℃ and 815℃ respectively was comprehensively conducted by using the scanning electron microscopy（SEM） and the energy dispersive X-ray microanalysis（EDX）. The influence of ashing temperature on the particle size distribution, micromorphology, particle surface, interior structure and elemental composition of biomass ash was investigated. The results showed that the higher the ashing temperature was, the smaller the size of the ash would be. With the increase of ashing temperature, the distribution of ash was more homogeneous. A majority of rounded particles were found in the rice husk ash（RHA）, while most of rod-like particles were found in the rice straw ash（RSA）. For the 600℃ RHA, part of the ash particles still retained the original fiber structure of rice husk, and some dense granules were observed which had no sticking phenomenon. Whereas, plenty of floccule that was adhered to by many small particles existed in the 600℃ RSA, which indicated that the weak caking occurred which was caused by the melting of the constituents with low melting point. Molten alkali metal material and quartz structure in the form of vitreous bumps were found on the surface of 815℃ ash particles, which presented obvious reunion slagging phenomenon. The main composition elements of RHA and RSA were carbon（C）, oxygen（O）, silicon（Si）, potassium（K） and calcium（Ca）. Magnesium（Mg）, aluminum（Al）, iron（Fe） and phosphorus（P） which had only a small quantity were also detected. The element surphur（S） was only detected in the RSA, which was not detected in the RHA. With the increase of ashing temperature, the K content of RHA obviously decreased. But for RSA, the content of K, sodium（Na） and Ca changed slightly while the content of chlorine（Cl）, Fe and Al decreased obviously. The K, Na, Ca and Cl contents of RSA were much higher than those of RHA, which indicated that the RSA was much easier to cause equipment corrosion than the RHA. This work reveals the effects of ashing conditions on the microstructure characteristics and elemental composition of RHA and RSA comprehensively, and can provide the guidance not only for preventing slagging, fouling and corrosion in the thermo-chemical conversion equipments, but also for the clean burning of biomass and comprehensive utilization of both RHA and RSA.