生物质化学链燃烧过程中钾素的迁移及保持机理研究

Our country is badly deficient in resources of potassium which, however, is one of the indispensable nutrient elements for human beings, plants and animals. The recycling of the potassium in biomass helps to build a sustainable circulating system of potassium and therefore, compensating the exhaustion of potassium resources all over the word. Since air is directly used as oxidant for biomass, the conventional biomass combustion is an intensive exothermic process and it results in a larger amount of gaseous potassium and molten vitreous potassium. It leads to both the loss of potassium in biomass ash and the security issue of boiler. Chemical looping combustion (CLC) of biomass could overcome the limitations of combustion mode and heat release form of traditional biomass combustion, and it is supposed to reduce the loss of potassium in biomass. Since the reaction atmosphere in the CLC process of biomass is completely different from that in the conventional combustion, the conversion characteristics of biomass would also be different. In the present project, suitable oxygen carriers of high performance and the reaction conditions for the biomass CLC process are prior selected and the biomass CLC mechanism is elucidated. Afterwards, the kinetic effect of potassium on the catalyzed pyrolysis and gasification of biomass and the reduction of oxygen carrier is investigated. This project focuses on revealing the migration regularity of potassium in the biomass, i.e., to flue gas, ash and oxygen carrier, the competitive reaction mechanism between potassium and oxygen carrier, the migration path of potassium from biomass to the oxygen carrier and its effect on the reactivity of oxygen carrier. The results from the present project would provide theoretical basis and experimental data for the efficient utilization of biomass.

钾是人类和各种动植物生命活动必需的营养元素之一，但我国的钾资源十分匮乏。构建钾素的持续循环体系，回收和利用生物质中的钾，补偿全球钾资源的衰竭。常规的生物质燃烧直接以空气为氧化剂，强烈的放热过程加剧了钾蒸汽的析出和熔融态玻璃体钾的生成，造成生物质灰中钾肥的流失，并引发锅炉的安全问题。生物质化学链燃烧技术不仅可以克服传统生物质燃烧方式和热量释放形式的局限，而且有望抑制生物质中钾素的流失。生物质化学链燃烧的气氛与常规的燃烧完全不同，生物质的转化特性也有着本质的区别。研究选择适合于生物质化学链燃烧的高反应性能的载氧体，阐明生物质化学链燃烧反应活性，揭示生物质中钾与载氧体协同催化生物质热解气化－载氧体还原反应动力学、生物质中的钾向烟气、灰渣以及载氧体迁移的规律、钾与载氧体的竞争反应机理和生物质中的钾向载氧体迁移的途径及对载氧体反应活性的影响，为今后生物质能源的高效利用提供理论基础和实验数据。

化学链燃烧是一种具有CO2内分离特性的燃烧方式。生物质化学链燃烧技术不仅可以克服传统生物质燃烧方式和热量释放形式的局限，能防止锅炉的安全问题，而且有望抑制生物质中钾素的流失。生物质化学链燃烧的气氛与常规的燃烧完全不同，生物质的转化特性也有着本质的区别。因此，本项目开展了以下3个方面的研究：（1）基于化学链燃烧生物质转化特性研究：选取典型的载氧体和生物质进行化学链燃烧实验，建立载氧体-生物质反应活性与物理参数的对应关系，筛选高性能载氧体；（2）生物质中碱金属钾的迁移规律的研究：揭示生物质灰中钾的向载氧体的迁移规律及在载氧体中的含量及赋存形态，探讨氯与钾的竞争反应机理及对物理结构和化学特性的影响；（3）碱金属钾向载氧体迁移对载氧体活性的影响：阐明草木灰与载氧体的竞争反应机理，揭示载氧体对钾的捕集能力，揭示钾向载氧体迁移对其反应活性的影响，揭示基于草木灰修饰铁基载氧体的反应活性。取得了如下成果：（1）揭示了生物质化学链燃烧和气化过程中，在Cu，Fe，Ni载氧体的作用下，生物质的转化特性，Cu基载氧体的活性最高，Fe活性最低；（2）化学链燃烧过程中，草木灰中K、SiO2迁移到载氧体的表面，对载氧体的微观结构造成显著的影响；揭示了HCl与载氧体的竞争反应机理，燃料中少量的HCl并不会引起载氧体孔隙结构的严重变化，高岭土在防止钾流失方面有一定的优势，但其孔隙结构不够发达；（3）草木灰中钾通过蒸汽与灰的形式与Fe2O3发生反应，生成复杂的Si-K-Fe-O物质，对载氧体的活性影响显著，一般来讲，高钾低硅灰促进载氧体反应活性，而高硅低钾灰会引起载氧体表面严重烧结，造成反应活性严重下降；采用草木灰对铁基载氧体进行修饰提高载氧体活性是可行的，改性的效果与生物质灰中K、Si、Ca的含量关系密切。