Modeling Aspects of Chemical-Looping Combustion for Solid Fuels

Licentiate thesis, 2011

Chemical-looping combustion is a novel technology for combustion of fossil fuel. By using a circulating bed material to transfer oxygen to the fuel, a pure stream of carbon dioxide can be obtained from the flue gas, undiluted by nitrogen. The main advantage of this capture technology is that there is no direct efficiency loss in obtaining the CO2 in a separate stream. The work presented in this thesis is divided into two parts. The first part deals with the behaviour of a 10 kW chemical-looping combustor for solid fuels. This combustor is normally operated in continuous mode but has here been studied with addition of fuel batches. From analysis of gas leaving the air reactor, it was possible to determine the residence-time and residence-time distribution of particles in the fuel reactor. Knowing the solids inventory in the fuel reactor, the circulation mass flow could be directly correlated to measured operational data, i.e. pressure drop, temperature and gas flow in air reactor riser. Using results for carbon-capture efficiency and residence-time distribution, a model was developed which could determine a mass-based reaction-rate constant for char conversion. This reaction-rate constant could also be determined independently from the conversion rates of char during the batch tests. The results showed a good agreement between the two approaches, indicating that the model well describes the behaviour of the unit. The second part of this work presents the design and experimental evaluation of a cold-flow model system, built to simulate a 100 kW chemical-looping combustor for solid fuels. A theoretical background with details on design and calculations on the dynamic similarity is provided. The system has been operated for about 10 hours and shows no indication of imbalances in the bed inventories. Both the internal circulation in the fuel reactor and the circulation between air and fuel reactor could be varied in a large range with only minor impact on the solids inventories. Studies were made on the mass flows, mass fluxes and residence-times in the air and fuel reactors using air as fluidization medium.