CaMnO3-δ Made from Low Cost Material Examined as Oxygen Carrier in Chemical-looping Combustion

Abstract

Carbon Capture and Storage is a promising method to limit the increasing amount of greenhouse gases in the atmosphere. In this method high purity carbon dioxide is captured at large emission sources, e. g. fossil fuelled power plants. The carbon dioxide can then be transported to a long term storage site, rather than being emitted to the atmosphere. Among the different alternatives for obtaining high purity carbon dioxide during combustion of fossil fuels, Chemical-looping Combustion (CLC) is one of the most promising. Here, the oxygen needed to oxidize a fuel is provided by a solid oxygen carrier. The oxygen carrier is subsequently circulated to another reactor where it is reoxidized with air. By separating these two operations mixing of the combustion products and the nitrogen in the air is avoided. An energy demanding gas separation is thus not necessary.The most crucial part of Chemical-looping Combustion is the solid oxygen carrier. The oxygen carrier should have high reactivity with fuel and oxygen, sufficient oxygen carrying capacity and preferably also low cost. Furthermore it is important that it is able to withstand the tough conditions it is exposed to in a hot fluidizing environment, both with respect to physical attrition and chemical degradation. The most commonly suggested setup of Chemical- looping Combustion is a dual fluidized bed system where gas velocities and mechanical abrasion can be high. When the technology was first demonstrated, nickel oxide based oxygen carriers were typically used. But as nickel is quite costly as well as potentially harmful, alternatives have been sought after.In 2009 Leion et al. [1] investigated an oxygen carrier based on calcium manganite of perovskite structure CaMnO3-δ for chemical looping combustion. The results were very promising and similar materials have since then been successfully tested in pilot rigs up to 120 kWth, including extended operation in continuously operating 10 kWth reactor with very positive results, see Källén et al. [2]. A key feature of these materials is that they are able to release gas phase oxygen at relevant conditions, so called Chemical-looping with Oxygen Uncoupling, see Rydén et al. [3]. Having gas phase oxygen available for fuel oxidation makes gas-solid mixing less critical and thus makes it easier to reach complete fuel conversion.2Most studies in which CaMnO3-δ based oxygen carriers have been examined have been using particles manufactured from high quality chemicals. While that is reasonable in the early stages of development, cheaper raw materials would be favourable for industrial applications. Promising oxygen carriers based on manganese ores have been manufactured and characterized by Fossdal et al. [4] and Mohammad Pour et al. [5].This study aims to further examine CaMnO3-δ based oxygen carriers made from low cost, commercial raw materials available in large quantities such as manganese ore. The materials are examined during continuous Chemical- looping Combustion and Oxygen Uncoupling in an experimental reactor with the nominal fuel power 300 Wth. The reactor has previously been used in numerous studies which make comparisons with materials made from high purity chemicals straightforward. During operation several gas concentrations as well as temperatures and pressure drops are measured which allows monitoring of the chemical reactions and fluidization behaviour in the reactor.Fines (particles <45μm) are collected from exiting gas streams which gives an indication of the degree of attrition of the particles. Attrition was also studied in separate jet cup attrition test.The study show that it is possible to manufacture CaMnO3-δ based oxygen carrier with appropriate properties for continuous operation from low-cost materials. The ability to release gas phase oxygen to an inert atmosphere and high conversion of natural gas is demonstrated. The results indicate that this approach should be feasible also for large-scale applications.