Evaluation of a CuO/Al2O3 oxygen carrier for chemical looping combustion

Abstract

The industrial use of hydrocarbon fuel in the energy sector is the major source of carbon dioxide emissions to the atmosphere. Through intense research in the past years, chemical looping combustion (CLC) technology has gained attention as a core component of possible carbon capture and storage (CCS) strategies for power generation and in industry. CCS arrangements imply methods for sequestrating CO2 and preventing it from being released to the atmosphere through long-term storage. CLC reactors are usually constructed as fluidized beds units where the bed material is of major interest. These so-calles oxygen carrier particles circulate between two fluidized bed reactor units. They are periodically brought to reaction with hydrocarbon-rich fuel in one, and regenerated with air in the other at high temperatures. This way, an energetic improvement to carbon capture technologies technology can be achieved, since combustion takes place without mixing fuel and combustion air. Several types of oxygen carriers have been proposed for use in chemical looping such as Fe-, Ni-, Mg-, or Cu-based materials. Compared to Nickel-based materials, which were profoundly examined in the past, Copper seems a quite suitable substitute since it shows high reactivity and has no thermodynamic limitations to completely converting the fuel to CO2 and H2O, as well as for reasons of price and eco-friendliness. At the Vienna University of Technology a fully operational CLC pilot plant of the "Dual Circulating Fluidized Bed" reactor type (DCFB) is used for investigating the performance of different oxygen carrier materials. An oxygen carrier developed at the CSIC Institute of Charbochemistry in Zaragoza, based on an impregnated CuO/Al2O3 compound material called Cu15, was tested in an experimental campaign. Results were gathered and evaluated for this master thesis. Preliminary calculations and tests based on the particle properties showed that the main operating parameters for the 120 kW pilot plant had to be adapted for the Cu15 oxygen carrier. The standard fuel power was set to 70kW and the standard operating temperature in the fuel reactor was set to 850-C. Additionally, the results of the preliminary calculations made clear that the specific active inventories of the 500W and 10kW pilot plants could not be reached in the 120kW pilot plant. The experimental campaign for evaluation of the general process performance showed that temperature, oxygen carrier to fuel ratio and active specific inventory are the parameters influencing fuel conversion the most. These findings correspond very well with results reported from the 500W and 10kW pilot units. However, the general performance in the 120kW unit did not reach the performance observed in the 500W and 10kW pilot units. The oxygen carrier provided fuel conversion between 70% and 80%, reactivity towards higher hydrocarbons as well as sulfur tolerance were satisfying. Detailed process investigation identified the low active solids inventory in the fuel reactor as the limiting parameter. Increasing the active solids inventory to the range of the 500W and 10kW units should increase process performance dramatically. By increasing the active inventory in the fuel reactor, the bed height is increased, leading to longer gas solids interaction times and thus, better fuel conversion. It can be concluded that full conversion in a large-scale plant seems achievable, but further investigation into creating a suitable plant design with high solids inventories is required.