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    湍流气流中生物质颗粒热解过程碱金属析出规律

    Release characteristics of alkali metal in biomass pyrolysis under turbulent condition

    • 摘要: 生物质作为一种洁净零碳可再生能源,在优化能源结构、减少碳排放方面具有重大意义。我国作为农业大国,具有丰富的生物质资源,但其相对于煤炭具有碱金属含量高、易结渣的特性,容易造成锅炉设备的腐蚀与损坏。为研究湍流热解条件下生物质颗粒的碱金属析出特性,利用四风扇对冲电加热试验装置在200~900 ℃的湍流条件下开展了6 mm球形荷木颗粒的热解试验。采用SEM−EDS、ICP−OES、IC等检测方法探究了不同热解条件及热解阶段下碱金属的赋存、释放和转化机制。结果表明:荷木中Na元素占碱金属物质的量分数的86%,以水溶性Na为主,其次是Ca、K、Mg。在热解过程中,NaNO3最先释放,并且在400 ℃时达到最大释放量。随温度进一步升高,NaCl与Na2CO3相继释放,各释放峰值与其熔点正相关,并且在水溶性钠释放时氨溶性Na会向水溶性Na转化。在600、800和900 ℃时Na的总释放率分别达到58.25%、65.15%和71.13%。通过对比在自然对流与湍流热解环境下Na的释放率发现,生物质颗粒升温过程中,湍流热解条件下的Na释放率始终高于自然对流环境下的Na的释放率,这是由于湍流热解条件下颗粒升温更快且孔隙率明显增加。但相同环境温度下,湍流和对流热解条件下Na的最终释放率差异不显著,表明温度仍然是碱金属最终释放率的决定性因素。

       

      Abstract: As a clean and renewable energy, biomass has great significance in optimizing energy structure and reducing carbon emissions. As a large agricultural country, China has abundant biomass resources. These biomass materials have high alkali metal content relative to coal, causing the problem of slagging, corrosion and damage to boiler equipment. To investigate the alkali metal release characteristics, the pyrolysis experiments of 6 mm spherical lotus wood particles were carried out. Research was conducted in a turbulent environment from 200 ℃ to 900 ℃ using a four-fan opposed furnace. SEM−EDS, ICP−OES, IC and other detection methods were adopted to investigate the alkali metal accumulation, release and transformation mechanisms. The results showed that Na accounted for 86% of the alkali metal content in the wood, with water-soluble Na dominating, followed by Ca, K, and Mg. During the pyrolysis process, NaNO3 was released firstly and reached the maximum release at 400 ℃. With the temperature further increasing, NaCl and Na2CO3 were released, and the release peaks were positively correlated with their melting points. The ammonia-soluble Na would be converted to water-soluble Na during the release of water-soluble Na. The total Na release proportion were 58.25%, 65.15% and 71.13% at 600 ℃, 800 ℃ and 900 ℃, respectively. Comparing the Na release under natural convection and turbulent pyrolysis, it was found that the Na release rates under turbulent pyrolysis were always higher than those under natural convection. It was attributed to the fact that the faster particle heating and significant increase in porosity under turbulent pyrolysis conditions. However, the final release rate of Na in both turbulent and natural convective environments at the same temperature exhibited no significant difference, indicating reaction temperature is the determining factor for the final release rate of alkali metals.

       

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