Innovative Oxygen Carriers for Chemical-looping Combustion

Doktorsavhandling, 2015

Chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) are combustion technologies where carbon dioxide is inherently obtained in pure form without any gas separation step. In the processes, fuel is introduced to the fuel reactor and combustion air is introduced to the air reactor. Circulating metal oxide particles transport oxygen from the air to the fuel reactor, and high purity CO2 can be obtained after steam condensation. Some metal oxides have the ability to release oxygen to the gas phase, or so-called uncoupling properties, which may facilitate fuel conversion. The metal oxide, named oxygen carrier, is the cornerstone of the CLC process. Prior to this work, NiO was the benchmark oxygen carrier for gaseous fuels, like natural gas. However, the high cost, toxicity and thermodynamic limitation of Ni would likely make it difficult to up-scale a process using this type of oxygen carrier. Thus, the focus in this work is on oxygen carriers based on cheaper and more environmentally benign materials, i.e. combined manganese oxides and CuO-based oxygen carriers. Both of these types of oxygen carriers have the propensity to release gas phase oxygen in the fuel reactor, something which was deemed highly beneficial. The main focus is on the combined materials, and this work presents the first major screening of these types of oxygen carriers. All oxygen carriers were manufactured by the commercial spray-drying method and examined in a batch fluidized reactor system with respect to parameters important for chemical-looping. Several combined manganese oxide systems were investigated in this work, with the main focus on three rather promising systems: i) calcium manganites, ii) manganese-silica and iii) manganese with magnesium. For the first system, Ca-Mn-X-O (X= Fe, Ti and Mg), all materials had perovskite structure and performed very well. Clear oxygen uncoupling ability and full conversion of CH4 were achieved in the batch testing. Adjusting the production parameters, i.e. calcination temperature, calcination time and milling time, the physical properties of the oxygen carrier can be enhanced. The oxygen carrier with molar composition CaMn0.775Mg0.1Ti0.125O3-δ was produced by a wide range of Mn- and Ti-sources available commercially at tonnage scales. All materials showed similar oxygen uncoupling behaviour and had the perovskite structure. This shows that this type of oxygen carrier not only can be produced with cheap raw materials, but is simple to produce independent of the material source. Although the oxygen carriers based on Mn-Si-O had limited oxygen release at lower temperatures, there was a remarkable increase in release at temperatures above 950C for particles with less than 45 wt% SiO2. Similarly, the ability to convert CH4 for these particles increased with temperature, and over 90% combustion could be achieved at temperatures at and above 950°C. The third promising system investigated was a combination of Mn and Mg oxides. In this system, the uncoupling reactions were more pronounced at 900°C for the material with a molar ratio of Mn/Mg of one. Also, the methane conversion for some samples studied was high, making this material yet another interesting alternative. CuO-based materials with different support materials have a seemingly fast release rate of oxygen, approaching equilibrium at 900°C. Most investigated materials had the ability to fully convert CH4 at 925°C at the experimental conditions. Some CuO-support combinations did not perform so well, for instance the Cu36FAl24 sample due to formation of Cu0.95Fe1.05AlO4. Several very promising oxygen carriers have been developed in this work. Some of them have been successfully tested in continuous operation at 120 kW scale. Further, the work has led to the development of calcium manganites ready to be up-scaled to large scale application. In addition to this, several other promising systems have been developed, which may not be ready for upscaling, but have great potential when optimized further.