基于CO2氧化的生物质化学链气化中NiFe2O4双功能载氧体的构筑及反应机理研究

Biomass Chemical Looping Gasification (BCLG) is an innovation gasification technique, which can directly convert biomass into synthesis gas via the lattice oxygen of oxygen carrier instead of gaseous oxygen in the fuel reactor, and then the reduced oxygen carrier is re-oxidized to recover lattice oxygen for its initial state by air in the air reactor. It presents several advantages, such as, inexpensive oxygen source, realizing auto-thermal, low tar content and high calorific value. In this proposal, a series of spinel structure NiFe2O4 oxygen carrier, which possess dual function of oxygenation-catalysis, are synthesized by several preparation methods. The oxidation-reduction performance of oxygen carrier is firstly investigated through a TGA reactor, which reveals the rule of release-recover of lattice oxygen. Meanwhile, the control steps of oxygen carrier reduction and reduced oxygen carrier after CO2 oxidation are obtained. The tar model compounds cracked by NiFe2O4 oxygen carrier are studied in a fix bed reactor, and the mechanism of tar cracking is analyzed. Based on the synergistic effects, the highest reactivity oxygen carrier is obtained. The CLG system of biomass reduction – CO2-air oxidation of NiFe2O4 oxygen carrier is established. The performance of NiFe2O4 oxygen carrier in BCLG is detailed discussed through a bubbling fluidized bed reactor. Biomass is gasified to produce synthesis gas, at the same time, the NiFe2O4 oxygen carrier is reduced to iron suboxide and metallic iron/nickel, which can act as catalysis to crack tar for evaluating syngas quality. The reduced oxygen carrier is restored lattice oxygen to its initial state under CO2-air atmosphere, which realizes the circulation of oxygen and heat source. Thus, high quality syngas is obtained and greenhouse gas CO2 is diminished in this investigation. Therefore, the project shows an important significance to explore novel biomass conversion technology.

生物质化学链气化是利用载氧体中的晶格氧替代气相氧使生物质部分氧化制取合成气的过程，具有廉氧、自热、合成气低焦油和高热值等优点。本项目拟合成一系列具有原位氧化-催化双功能特性的尖晶石结构NiFe2O4载氧体；首先在TGA反应器上研究载氧体的氧化-还原性能，揭示晶格氧的释放-恢复规律，获得载氧体还原、CO2氧化过程的控制步骤；并在固定床反应器上研究载氧体对焦油模型化合物的裂解行为及反应机理。然后筛选出高活性的载氧体，建立基于鼓泡流化床反应器的生物质还原—CO2-空气氧化化学链循环反应体系，探讨载氧体在生物质化学链气化过程中的反应性能。载氧体在生物质气化的同时被还原成低价铁氧化物或金属，后者能原位催化裂解焦油提高合成气品质；还原后的载氧体依次在CO2、空气气氛中恢复晶格氧。本项目实现了生物质转化为高品质合成气的同时又利用了温室气体CO2，对于探索新颖的生物质转化技术具有重要意义。

将化学链气化与CO2活化相结合，既能将生物质转化为高品质合成气，又能实现温室气体CO2的资源化利用，具有明显的经济与环境效益。本项目首先从四种不同方法合成的载氧体中筛选出具有高反应活性的载氧体，即通过溶胶-凝胶法合成的尖晶石结构NiFe2O4双金属氧化物；然后分别研究了还原态载氧体活化CO2的反应性能、载氧体与生物质半焦的反应性能、载氧体裂解生物质焦油模型化合物-甲苯的反应性能以及载氧体与真实生物质的化学链气化特性；最后对化学链转化过程中晶格氧的释放/恢复规律进行了详细的研究并对载氧体的循环稳定性能进行了探讨。研究发现：由于Fe-Ni协同效应，还原态载氧体具有良好的活化CO2能力；载氧体即使在惰性气氛下也具有良好的氧化生物质半焦性能，气化过程中最大碳转化率和有效气产率分别为88.12%和2.79 Nm3/kg (H2O气氛)，载氧体与半焦的反应动力学模型符合随机成核和随后增长模型，平均活化能约为228.63 kJ·mol-1；载氧体呈现了优异的裂解甲苯性能，具有原位氧化-催化双功能特性，水蒸气在抑制积炭生成的同时降低了甲苯裂解率，积炭率为4.97%时甲苯裂解率为86.93%，连续实验过程中，载氧体活性先在较长时间内保持稳定，然后存在一个短暂的活化，最后呈现出轻微的降低，载氧体分散度越高，催化活性就越高；载氧体的存在显著地促进了生物质的转化，最大碳转化率和产气率分别为92.53%和1.71Nm3/kg (H2O气氛)；载氧体的还原-氧化过程即为晶格氧的逐级释放-恢复过程，化学反应界面固定在载氧体颗粒表面，晶格氧在体相→表面/表面→体相氧浓度梯度驱动下，由体相外迁至表面不断失去晶格氧/由表面内迁至体相不断恢复晶格氧，直至形成最终还原/氧化态；由于循环过程中部分Fe元素脱离Fe-Ni尖晶石结构成为游离的Fe2O3物相，同时颗粒表面存在部分烧结，从而导致载氧体的反应活性随循环次数的增加而逐渐降低，但ZrO2惰性组分的加入能显著地改善载氧体的循环稳定性。在全球气候问题越来越严重、国际社会普遍倡导低碳经济的时代，研究开发这种新颖的生物质气化、CO2活化技术具有重要的现实意义。本项目共发表论文21篇，授权(申请)专利4项，以本项目研究工作为基础，项目负责人获得国家自然科学基金面上项目资助(51776210)，同时，依托团队获得国家重点研发计划-政府间国际科技创新合作重点专项资助。