Modeling of Cu oxidation in an adiabatic fixed-bed reactor with N2 recycling

Abstract

A dynamic model was developed to describe the oxidation of Cu in a large-scale Cu/CuO chemical looping process performed in adiabatic fixed-bed reactors at high pressure. An ideal plug flow pattern without axial dispersion or radial gradients and with negligible intra-particle concentrations and temperature gradients on the scale of millimeters were assumed. Cu oxidation is favoured at high pressure and therefore fast reaction rates and total oxygen conversion were achieved, even with low contents of oxygen in the feed (around 4–6%). Short breakthrough periods were achieved, which is highly favorable in operations carried out in alternative fixed-bed reactors. In order to maximize energy efficiency, the oxidation needs to be carried out at the highest allowable temperature, but CuO tends to decompose and agglomerate at relatively low temperatures (over 1223K). Also the high exothermicity of Cu oxidation can generate hot spots in the reaction front. The use of a large recycle of nitrogen (previously cooled down) so that it mixes with regulates the temperature in the reaction front. At these conditions, the gas–solid heat exchange front advances faster than the reaction front and the oxidized bed is finally left at a lower temperature (as the cooled N2 recycle), which is insufficient to initiate the subsequent reduction of CuO to Cu. Therefore, an additional stage is introduced to carry out a gas–solid heat exchange between the hot N2 rich recycled gas and the oxidized bed. The bed is then ready for the next reaction step that involves the exothermic reduction of CuO. Operating parameters, such as the recirculation ratio (content of O2 in the feed) and the proportion of Cu in the solid bed, which have a substantial effect on Cu oxidation and CO2 capture efficiency, were also evaluated. Recirculation ratios higher than 0.75 and inlet gas temperatures of around 423K limit the maximum temperature to reasonable values (generally below 1200K). A trade-off between the O2 content in the feed (4–6%) and the amount of Cu in the bed (20–33%) leads to high energy efficiencies in CLC processes and minimal CaCO3 calcination in the case of the Ca–Cu looping process.