生物质中碱金属（钾和钠）在气化过程中易释放至气相，导致气化炉腐蚀、积灰、结渣等问题，严重影响气化炉的长周期运行。采用热力学模拟，分别从碱金属释放和分布规律、物相演化机理、熔渣结构单元和熔渣固定碱金属吉布斯自由能，阐明添加高岭土对秸秆气化过程中碱金属释放的影响机制。结果表明，800 ℃秸秆灰渣中碱金属钾主要存在于钾镁硅酸盐和长石，而钠存在于长石，温度升高促进灰渣中碱金属释放至气相；此外，矿物质种类和熔渣结构分别是决定碱金属释放行为的关键因素；800 ℃时添加高岭土促进灰渣中固钾矿物质由钾镁硅酸盐转变为白榴石，固钠矿物质由钠钙硅酸盐转化为霞石，减弱碱金属释放行为。高温下添加高岭土，熔渣解聚参数G下降，熔渣中Si-Si-O-O结构单元转变为Al-Si-O-O结构单元，促进碱金属以电荷补偿形式与四配位Al3+结合，Si-KAl-O-O结构单元和Si-NaAl-O-O结构单元含量升高，抑制碱金属释放。此外，1 400～1 600 ℃时，添加高岭土导致熔渣固定碱金属反应的吉布斯自由能降低，熔渣固定碱金属能力增强，碱金属释放减弱；熔渣固定碱金属钾反应的吉布斯自由能高于碱金属钠，因此碱金属钾更易释放至气相。

Alkali metals (potassium and sodium) in biomass are easy to be released into the gas phase during gasification, causing corrosion, ash deposition, slagging and other problems in the gasifier, seriously affecting the long-term operation of the gasifier. In this study, the effect of kaolin addition on the release behavior of alkali elements during straw gasification was explored in the aspects of the distribution of alkali elements, phase evolution behavior, slag structure, and the Gibbs free energy of alkali element capture by slag, via thermodynamic modelling. The results indicate that K and Na are mainly in the form of the K-Mg silicate and feldspar, respectively, while sodium exists in feldspar, and the increase of temperature promotes the release of alkali metal in ash to gas phase. The released content of alkali elements increases with the gasification temperature. In addition, the mineral species and slag structure are the key factors governing the release behavior of alkali elements at low temperature and high temperature, respectively. The addition of kaolin at 800 ℃ promotes the transformation of potassium fixing minerals from potassium magnesium silicate to leucite in the ash, and the transformation of sodium fixing minerals from sodium calcium silicate to nepheline, reducing the release behavior of alkali metals. At high temperature, the addition of kaolin decreases the depolymerization parameter G value of slag, leading to the conversion of the structure unit Si-Si-O-O to Si-Al-O-O. The resulted Si-Al-O-O is the key structure unit of capturing alkali elements in the slag by forming the structure unit Si-KAl-O-O and Si-NaAl-O-O, and this process is driven by the charge compensation effect. Furthermore, the kaolin addition decreases the ΔG of the reaction of slag capture alkali elements at 1 400-1 600 ℃, which enhances the ability of alkali metal fixation of slag and inhibits the alkali element release. In contrast to Na, the ΔG of the K capture reaction by slag is higher, indicating that the K is easier to be released than Na during biomass gasification.

0 引言

1 试验

   1.1 样品及表征

   1.2 玉米秸秆灰的制备及表征

   1.3 热力学模拟

2 结果与讨论

   2.1 秸秆气化条件下碱金属释放机理

   2.2 高岭土对碱金属钾释放的影响

   2.3 高岭土对碱金属钠释放的影响

   2.4 高岭土对秸秆灰渣物相组成的影响

   2.5 熔渣中碱金属释放机理

   2.6 高岭土对熔渣固定碱金属吉布斯自由能的影响

3 结论