Kinetics of CuO/SiO2 and CuO/Al2O3 oxygen carriers for chemical looping combustion

M.A San Pio,M Martini,F Gallucci,I Roghair,M Van Sint Annaland

doi:10.1016/j.ces.2017.09.044

M.A San Pio, M Martini + Show 3 more

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https://doi.org/10.1016/j.ces.2017.09.044

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Abstract

Copper oxide supported on silica and supported on alumina are often used as oxygen carriers for chemical looping combustion owing to their very high reduction rates at lower temperatures and their very good mechanical and chemical stability at temperatures below 1000°C compared to other oxygen carriers. In this work, a comprehensive experimental study has been carried out to better understand the reaction mechanism and quantitatively describe the reaction kinetics of the oxygen uncoupling reaction and the reduction and oxidation reactions under different reaction conditions.First, the oxygen uncoupling and reduction reaction kinetics of the CuO/SiO2 oxygen carrier was studied. A shrinking core type model (SCM) was developed that can well describe the oxygen uncoupling reaction rate and final conversion. Subsequently, a SCM and a simplified pseudo-homogeneous model was developed to describe the reduction kinetics of CuO/SiO2. Subsequently, the study was extended to investigate the reduction kinetics of CuO/Al2O3, where it was observed that the formation of tenorite spinel (CuAl2O4) and cuprite spinel (CuAlO2) strongly affects the overall reduction kinetics. Assuming that the reduction of CuO to Cu is independent of the support, the pseudo-homogeneous model was extended to include the reduction and oxidation kinetics of the spinel compounds, with which the experimentally determined redox kinetics could be well described.Regarding the CuO on Al2O3, The maximum temperature reached was 1000°C in thermogravimetric analysis (TGA) without observing any sintering or melting effects. It can be due to the good stability with the Al2O3 support and the relatively small amount of CuO (13%). However, for CuO/SiO2 (70% CuO), the maximum temperature tested in the TGA was 900°C, since at higher temperatures the sample was melting, due to the lower melting point of Cu.The main results of the study can be summarized as: (i) the oxygen uncoupling and reduction/oxidation of CuO/SiO2 and CuO/Al2O3 has been elucidated; (ii) a grain model that can describe the oxygen uncoupling and reduction reactions of CuO for different operating conditions has been developed; (iii) a pseudo-homogeneous grain model that can describe the redox kinetics of CuO/SiO2 and CuO/Al2O3 has been developed.

Highlights

Chemical-looping combustion (CLC) has been shown to be a promising combustion technology for power production with integrated CO2 capture (Adánez et al, 2013; Eyring et al, 2011; Hossain and de Lasa, 2008; Mattisson et al, 2009)
For CuO/SiO2 it is known that the copper oxide does not interact with the support, achieving full conversion in the redox cycles at each temperature. This was confirmed with X-Ray diffractometer (XRD) analysis for fresh CuO/SiO2, and the fully oxidized and fully reduced sample
To 100%) has to be due to the reduction of CuAl2O4 and CuAlO2 to Cu, so that the amount of CuAl2O4 + CuAlO2 present is 6 wt%. These results have been corroborated and they are in good agreement with the results found in an X-ray tomography study (San Pio et al, 2017c), where it was elucidated that for the same CuO/Al2O3 oxygen carrier, 30 wt% was the active material in the CuAl2O4 form, corresponding to 10 wt% of active Cu, as found from the thermogravimetric analysers (TGAs) experiments

Summary

Introduction

Chemical-looping combustion (CLC) has been shown to be a promising combustion technology for power production with integrated CO2 capture (Adánez et al, 2013; Eyring et al, 2011; Hossain and de Lasa, 2008; Mattisson et al, 2009). In CLC, a metal oxide (Me/MeO) that acts as an oxygen carrier, is alternately reduced by a fuel (syngas or natural gas) in a fuel reactor and oxidized by air in an air reactor, facilitating the intrinsic separation of CO2 from a nitrogen-free CO2 exhaust stream. This is made possible because direct contact between fuel and combustion air is avoided. To be competitive with other state-of-the-art technologies, CLC should be operated at very high temperatures (1000–1200 °C) and high pressures (15 bar or higher) (Noorman, n.d.; Wolf et al, 2005).

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