Warren Kent J, Hill Caroline M, Carrillo Richard J, Scheffe Jonathan R
Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, 32611, USA.
Phys Chem Chem Phys. 2020 Apr 29;22(16):8545-8556. doi: 10.1039/d0cp00924e.
A novel chemical-looping combustion scheme is proposed, where facile gas separation via steam condensation enables the production of sequestrable CO2 from alkanes, such as CH4, and pure H2 from H2O. This cycle consists of two steps, namely, (1) the endothermic reduction of a ceria-based solid solution via the complete oxidation of CH4, followed by (2) the exothermic oxidation of the reduced metal oxide via H2O splitting. Relative to iron oxide-based materials and undoped ceria, ceria-zirconia solid solutions possess favorable partial molar enthalpic and entropic properties; this promotes selective production of complete combustion products, H2O and CO2, during the reforming reaction. Thermodynamic predictions suggest that the complete oxidation of CH4 is possible by increasing the Zr content to 20 mol%, operating below 600 °C, increasing total pressure, or reducing the amount of delivered reactant. Furthermore, any H2, CO, or unreacted CH4 that may persist is thermodynamically favored to oxidize if exposed to unreacted oxide downstream, as is typical for a packed-bed or downer reactor configuration. Experiments were performed to validate the thermodynamic trends using isothermal thermogravimetry coupled with residual gas analysis, which confirmed that high selectivity towards H2O and CO2 is attainable for methane-driven reduction of Ce0.9Zr0.1O2; selectivities greater than 0.70 were observed at initial reaction extents. Importantly, metal oxide oxidation via H2O splitting and selective production of H2 (or CO if CO2 is the delivered oxidant) is also thermodynamically favored at the operating conditions considered for the first step. This work ultimately presents a viable avenue for the carbon-neutral conversion of CH4 (or other alkanes) to H2 if a renewable energy resource, such as solar energy, is leveraged to supply process heat.
提出了一种新型化学链燃烧方案，其中通过蒸汽冷凝实现的简便气体分离能够从烷烃（如CH₄）中生产可封存的CO₂，并从H₂O中生产纯H₂。该循环包括两个步骤，即：(1) 通过CH₄的完全氧化对基于二氧化铈的固溶体进行吸热还原，随后 (2) 通过H₂O分解对还原的金属氧化物进行放热氧化。相对于基于氧化铁的材料和未掺杂的二氧化铈，二氧化铈 - 氧化锆固溶体具有良好的偏摩尔焓和熵性质；这促进了重整反应过程中完全燃烧产物H₂O和CO₂的选择性生成。热力学预测表明，通过将Zr含量增加到20 mol%、在600 °C以下运行、增加总压或减少输送反应物的量，可以实现CH₄的完全氧化。此外，任何可能残留的H₂、CO或未反应的CH₄，如果暴露于下游未反应的氧化物中，在热力学上都有利于氧化，这对于填充床或下行床反应器配置来说是典型的。进行了实验，使用等温热重分析结合残余气体分析来验证热力学趋势，结果证实，对于甲烷驱动的Ce0.9Zr0.1O₂还原，对H₂O和CO₂具有高选择性；在初始反应程度下观察到选择性大于0.70。重要的是，在所考虑的第一步操作条件下，通过H₂O分解进行金属氧化物氧化以及选择性生成H₂（如果输送的氧化剂是CO₂，则生成CO）在热力学上也是有利的。如果利用太阳能等可再生能源来提供过程热量，这项工作最终为将CH₄（或其他烷烃）碳中性转化为H₂提供了一条可行途径。
