煤直接化学链制氢工艺中煤焦与钾修饰铁基氧载体的催化反应机理探索

Compared to the traditional process of coal to hydrogen via gasification, the efficiency of coal direct chemical looping hydrogen is expected to be remarkably increased due to the requirements of air separation, CO shift, CO2-H2 separation, and H2 purification are avoided. The production of FeO/Fe is the precondition of coal direct chemical looping hydrogen with iron based oxygen carrier. However, the reduction rate of Fe2O3 to FeO/Fe by coal char is very slow. This reduction rate can be significantly enhanced by K2CO3 addition. The previous catalytic mechanism studies have found the remarkable potassium migration and autocatalysis. However, the mechanism is not clear. Therefore，this project will detailedly analyze the following aspects: the relationship of reduction rate with the phase of iron oxide, potassium distribution and content; the effects on the potassium migration between iron oxide and coal char; the reaction characteristics of iron oxide and K2CO3; the catalytic effects of iron oxide, K2CO3 and their products on the gasification of coal char; the autocatalysis by iron oxide in the presence of potassium. The implementations of this project could help to understand the catalytic reaction mechanism and the mechanism of activity conserving, and provide a theoretical basis for the research and development of coal direct chemical looping hydrogen process.

与传统煤气化制氢工艺相比，煤直接化学链制氢工艺可省去空分、CO变换、CO2/H2分离和H2精制工序，具有显著提高产氢效率的潜力。FeO/Fe的生成是基于铁基氧载体的煤直接化学链制氢工艺的关键和前提，然而，氧化铁与煤焦直接反应生成FeO/Fe的速率很慢。通过添加碳酸钾可显著增大该反应的速率，前期机理研究发现反应过程中存在明显的钾迁移及自催化现象，但是对其产生机制还缺乏进一步的了解。为此，本项目拟首先详细研究不同碳转化率下反应速率与氧载体物相、钾分布与含量的对应关系；然后进行氧化铁-煤焦之间钾迁移特性的影响因素考察；在考察氧化铁与碳酸钾的反应特性之后，比较氧化铁、碳酸钾及二者反应产物对煤焦气化反应的催化作用；最后总结规律研究钾存在下氧化铁的自催化机制。项目的研究实施有助于揭示钾修饰铁基氧载体与煤焦的催化反应机理和氧载体活性保持机制，为煤直接化学链制氢工艺的研究开发奠定理论基础。

与传统煤气化制氢工艺相比，煤直接化学链制氢工艺可省去空分、CO变换、CO2/H2分离和H2精制工序，具有显著提高产氢效率的潜力。FeO/Fe的生成是基于铁基氧载体的煤直接化学链制氢工艺的关键和前提，然而，氧化铁与煤焦直接反应生成FeO/Fe的速率很慢。通过添加碳酸钾可显著增大该反应的速率，前期机理研究发现反应过程中存在明显的钾迁移及自催化现象，但是对其产生机制还缺乏进一步的了解。本研究首先构建了K-Fe-Al复合氧载体，进一步地研究了粒径等因素对C-Fe2O3反应的影响规律，以及钾的迁移转变。结果表明添加到氧载体的上的钾可迁移到煤焦上，且随着反应的进行部分钾可迁移回氧载体上。还原反应后，氧载体上的钾含量、焦上的钾含量以及挥发性钾含量分别为90.0%、5.2%和4.8%。可能的钾迁移形式为K2CO3。K2CO3在氧载体上可转化为K-Fe-O化合物，因此可在一定程度上避免钾的流失。可能的K-Fe-O化合物为K2Fe22O34和K2Fe10O16 （初始阶段）、KFeO2 （最终产物）。部分KFeO2可被煤焦再生成K2CO3。还原过程中，钾在煤焦上主要以铝硅酸钾、K2CO3和有机钾形式存在。还原过程后，煤焦上3.5%的钾为可回收的水溶性钾，1.7%的钾以很难回收的铝硅酸钾形式存在。