灰化温度及热解气氛对生物质灰灼烧失重特性的影响

Influence of ashing temperature and pyrolysis atmosphere on weight loss properties of biomass ash obtained by combustion

应用热重-差热分析法研究了空气和氮气气氛中2种灰化温度(600和815℃)下制得的稻壳和稻秆等生物质灰的灼烧失重特性。结果表明:不同飞灰在空气中灼烧的质量损失均可分为剩余水分的蒸发、有机成分的快速热裂解以及无机成分的缓慢挥发等阶段,而氮气中飞灰的质量损失并未出现明显的分段热解规律;空气中600℃生物质灰的主要质量损失发生在300~550℃之间,而815℃稻壳灰的主要质量损失区间为200~600℃,815℃稻秆灰在300~550℃之间的质量损失只有1.76%;灰化温度升高,飞灰的总质量损失明显降低。空气中不同灰化温度下的稻壳灰在950~1 050℃之间都出现放热峰,但该温度区间并无明显质量损失;815℃的稻秆灰在750℃附近则存在由于KCl熔融吸热引起的吸热峰,而600℃稻秆灰的热解过程则未出现明显的吸热峰;氮气中生物质灰的热解存在2个分隔明显的质量损失速率峰,这是由2种质量损失机制单独作用而成:600~800℃之间的质量损失主要是由不稳定的碳酸盐(Ca CO3、Ma CO3等)受热分解造成,而在800~1 200℃之间的质量损失则主要是由灰中活泼的碱金属氯化物(KCl、Na Cl等)受热进入气相所致;相同灰化温度下的稻壳灰或稻秆灰在空气中热解的总质量损失明显高于氮气。

Nowadays, sustainable development and increasing fuel demand necessitate the identification of possible energy resources. Biomass resource is regarded as a green renewable energy and will be more important in the future, which attracts the worldwide attention regarding their renewable nature, carbon dioxide-neutral characteristic, and world-wide availability. Consequently, many countries are putting great emphasis on the exploration of bio-energy. However, the use of biomass as fuel generates a large amount of residual ash, which causes serious environmental problems and has great passive influence on the chemical conversion of biomass. The biomass ash is easy to melt and volatilize, and it can not only reduce the utilization efficiency of equipment but also shorten their service life. Moreover, the inorganic species existing in biomass such as alkali oxides and salts can aggravate agglomeration, deposition, and corrosion problems on boiler’s heat transfer surfaces. So during the combustion or gasification processing, the ash with complex composition and high volatility often leads to slugging and erosion/corrosion in thermal conversion processing systems. Rice husk (RH) and rice straw (RS) are the main by-products during the process of rice processing, and they are the clean and renewable energy. Especially, in comparison to other agricultural wastes, the ash content of RH is much higher. So far, a series of studies have been carried out to investigate the characteristics of biomass ash through experiment. But the studies on the influence of ashing temperature and pyrolysis atmosphere on the properties of biomass ash are limited. In this paper, in order to investigate the weight loss regularities of biomass ash at different ashing temperature and pyrolysis atmosphere, thermogravimetric analysis was conducted to comparatively study the pyrolysis weight loss mechanism of rice husk ash (RHA) and rice straw ash (RSA) ashing at 600 and 815℃ in air and nitrogen. The results showed that the weight loss of fly ash in air could be divided into 3 stages while no obvious subsection of weight loss was observed in nitrogen. The weight loss of 600℃ fly ash pyrolyzing in air mainly occurred in the range of 300-550℃, while the main weightlessness temperature widened to 200-600℃ in the case of 815℃ RHA. The weight loss of 815℃ RSA in 300-550℃ was only 1.76%. The total weight loss decreased with the ashing temperature increasing. For HRA under different ashing temperature in air, an evident exothermic peak was observed in the range of 950-1 050℃, which was due to the fusion of amorphous phase or phase change of quartz. On the contrary, for the 815℃ RSA, an endothermic peak caused by the melting of KCl was found near 750℃. But there was no obvious endothermic peaks during the pyrolysis process of 600℃ RSA. There were 2 peaks for different biomass ash in nitrogen, which were separated clearly and caused by 2 kinds of weightlessness mechanisms separately. The weight loss of fly ash in the range of 600-800℃ was considered to be caused by the decomposition of carbonate (CaCO3, MaCO3, etc), while that in the range of 800-1 200℃ may be caused by the vaporization of active alkali chloride (KCl, NaCl, etc.) at high temperature. At the same ashing temperature, the total weight loss of RHA or RSA in air was much higher than that in nitrogen. The results are of great significance for studying the release and transition of alkali/alkaline earth metal during the thermal conversion process and analyzing these fundamental factors affecting the fusion properties.