Elsevier

Fuel

Volume 216, 15 March 2018, Pages 71-82
Fuel

Full Length Article
Performance and degradation mechanisms of CLC particles produced by industrial methods

https://doi.org/10.1016/j.fuel.2017.11.115Get rights and content

Highlights

  • Ageing of CLC particles at 900°C in IFPEN’s 10 kWth circulating fluidized bed using methane as fuel.

  • Characterization of the oxygen carriers’ morphological, textural and crystallographic evolutions.

  • Insight in particles’ degradation mechanisms at 900 °C.

Abstract

A promising copper based oxygen carrier produced using industrially-relevant manufacturing equipment has been aged in IFPEN’s 10 kWth Chemical Looping Combustion pilot plant. Methane combustion was performed at 900 °C for 160 h with an OC/fuel ratio of 1.2. While full methane conversion to CO2 and water was achieved in the early stage of the experiment, fast deactivation was observed. The oxygen carrying particles were thoroughly characterized at various stages of the ageing experiment, from which a pathway leading to deactivation is proposed.

Introduction

In the context of global warming, Carbon Capture and Storage (CCS) has to be implemented to limit CO2 emissions from fossil fuel power plants in order to minimize the accumulation of this greenhouse gas in the atmosphere [1]. Among the various CO2 capture technologies being developed, chemical looping combustion (CLC) is one of the most promising since it allows for CO2 separation with minimal energy penalty. The process relies on circulating metal oxides which act as oxygen carriers (OC) between an oxidation and a reduction reactor [2]. Numerous synthetic materials, natural ores, and industrial waste materials have been reported as suitable oxygen carriers for the CLC process both in terms of oxygen transfer capacity and reaction rates [2]. However, chemical reactivity and particle attrition has been investigated in most cases for a limited number of cycles, and long term ageing has rarely been investigated over hundreds of redox cycles, with the noticeable exception of Ni based particles [3].

The ageing mechanism of oxygen carriers is not fully understood yet, but in the past few years, reports on the drastic morphological evolution of oxygen carrying particles due to successive redox cycles started to appear, [4] pointing out that phase segregation phenomena and the increase in particles’ porosity may stem from sintering and ionic diffusion within the particles [5]. In order to properly evaluate different oxygen carriers lifetime, it is therefore necessary to test them for a long enough period of time (i.e. a large enough number of redox cycles) and to thoroughly characterize the aged particles. Cu based particles prepared at the lab scale have already been tested in different circulating fluidized bed CLC pilot plants at the Instituto de Carboquimica (CSIC Saragossa) [6], [7] and at the Vienna University of Technology [8]. Up to 100 h test runs at 800 °C were performed, of which 60 h corresponded to combustion [7]. Methane conversion was complete throughout the test runs, providing the OC/fuel ratio was maintained above 1.5. The particles were characterized by different techniques, which indicated that the copper content decreased over the first half of the test, the pore size distribution increased slightly, while the BET surface area decreased, and the γ-Al2O3 support tended to form α-Al2O3. The attrition rate was rather high at the beginning of the test, and stabilized to a low value of 0.04 wt%/h.

As part of the FP7 SUCCESS project, a promising copper based OC formulation selected during the FP7 INNOCUOUS project, has been produced by the Johnson Matthey Company using industrial scale manufacturing tools and commercially available alumina particles, in sufficient amounts for it to be tested in various CLC pilot plants by the different partners of the project. Cabello et al. have tested this OC in their 500 Wth CLC pilot plant [9]. Depending on the combustion temperature, different behaviors were observed: at 800 °C, the OC performed well in terms of combustion efficiency and mechanical stability at OC/fuel ratios over 1.5. The run was performed for 60 h, without fluidization/agglomeration problems and with little attrition. When the combustion temperature was increased to 900 °C, the combustion efficiency was better than at 800 °C, but the mechanical stability was greatly degraded, and significant attrition was observed after ∼50 h of combustion. Characterization of the OCs was performed before and after test, with an emphasis on the particles tested at 800 °C.

The results presented in this paper were obtained with the same OC in IFPEN’s 10 kWth CLC pilot plant. The manufactured particles were fluidized for 212 h between 800 and 900 °C, of which 160 h corresponded to methane combustion at 900 °C, and the OC particles were thoroughly characterized at various stages of the ageing campaign. No fresh OC was added during the test to make sure that all the particles had the same average ageing history, which limited the total ageing time due to solid losses. The ageing temperature of 900 °C was chosen to allow the comparison of the oxygen carrier’s performance with that of CaxMnyTizO3-δ perovskite particles also studied within the SUCCESS project, for which the reduction temperature should be at least 900 °C.

Section snippets

Pilot plant

A schematic view of the 10 kWth CLC pilot unit used in this study is shown in Fig. 1. The pilot is composed of three interconnected bubbling fluidized bed reactors. Two fluidized bed reactors (AR1 and AR2) are dedicated to the oxygen carrier oxidation and the last one (FR) is dedicated to fuel combustion, therefore performing OC reduction. More details on the design and dimensions of this unit can be found in Ref. [10] Recently, the unit has been equipped with a carbon stripper for combustion

Performance of the OC during ageing test

The experimental results obtained during this test are summarized in Figs. 2and 3. The oxygen carrier has circulated for 10 days and combustion of methane has been carried out during almost 160 h, which corresponds to 276 cycles of reduction and oxidation on average.

The first two days, the solid was circulated with air injection in AR1 and AR2 at 800 °C, and nitrogen in FR at 850 °C, after which the temperature was increased to 900 °C in all reactors and methane combustion was started. Some

Discussion

As mentioned earlier, the particles were first circulated with air injection in AR1 and AR2 at 800 °C, and with nitrogen in FR at 850 °C. Under such conditions, oxygen might be released in the FR, but the gas composition was not monitored, and no sampling of the particles was performed prior to methane injection. In a later test run with the same fresh OC, circulation was performed under the same conditions for 7 h and the particles from the FR were sampled for analysis prior to combustion: SEM

Conclusions

A copper based oxygen carrier prepared by impregnation of commercially available alumina particles using industrially-relevant manufacturing equipment has been aged in IFPEN’s 10 kWth CLC pilot unit. Methane combustion was performed at 900 °C for 160 h with an OC/fuel ratio of 1.2, until loss of solids made it impossible to run the test.

Full methane conversion to CO2 and water was achieved in the early stage of the experiment, but the combustion efficiency started to decrease after about 10 h

Acknowledgements

This work was supported by the European Commission Seventh Framework Program under the grant agreement n° 608571 (Project acronym SUCCESS).

The Johnson Matthey Company is gratefully acknowledged for supplying the oxygen carrier within the SUCCESS project.

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