The competition between direct gas–solid reduction and oxygen uncoupling of CuO oxygen carrier in chemical looping with oxygen uncoupling: A single particle simulation study

Abstract

In chemical looping with oxygen uncoupling (CLOU), the oxygen carrier (e.g., CuO-based materials) can be reduced either by direct decomposition or by heterogeneous gas–solid reaction. In most cases, the above two reaction pathways take place in parallel and compete with each other, which therefore make it very difficult to quantify the exact proportion of their contributions to the oxygen carrier conversion. The failure to distinguish the individual significance of each reaction route hinders the acquisition of a convincing reaction kinetic model. In this work, the conversion processes of CuO oxygen carrier in free space and in thermogravimetric crucible were simulated by a single particle model, in which the heterogeneous/homogeneous reactions as well as the heat/mass transfer inside and outside a porous particle were incorporated. The competition between the direct gas–solid reduction (using H2 as fuel gas) and oxygen uncoupling of the CuO oxygen carrier was analyzed from the aspects of the relative significance of each reaction route and the interactions between them. The results showed that the significances of each reaction route varied with the temperature and H2 concentration conditions. To quantify the relative significance of each reaction route at different conditions, the controlling regime diagram of CuO conversion in free space was given. The CuO conversion was more likely to be dominated by the oxygen uncoupling process when the particle was in crucible. For the conversion of CuO in free space, the exothermic gas–solid reduction increased the particle temperature and further facilitated the oxygen uncoupling rate, which was considered as the predominant interactions. Differently, for the conversion of CuO in crucible, the key interactions were characterized as: the homogeneous reaction between H2 and O2 decreased the O2 concentration neighboring the CuO particle and thus increased its oxygen uncoupling rate.