Fluidized-Bed Reactor Systems for Chemical-Looping Combustion with Inherent Separation of CO2

Doktorsavhandling, 2005

In chemical-looping combustion (CLC) a gaseous fuel can be burnt with inherent separation of carbon dioxide. The system consists of two reactors, a fuel reactor and an air reactor and an oxygen-carrier in the form of metal oxide particles that transports oxygen from the air to the fuel. The gas leaving the air reactor consists of nitrogen and unreacted oxygen, while the gas from the fuel reactor consists of water and carbon dioxide. The water can easily be condensed, and the remaining carbon dioxide can be liquefied for subsequent storage. A CLC reactor system can be designed with two interconnected fluidized beds. Two types of cold-flow models of CLC systems and one 300 Wth chemical-looping combustor have been designed and constructed, and tested with respect to parameters which are important for CLC. The first design is similar to a circulating fluidized bed, with an extra bubbling bed added on the return side. This bubbling bed is the fuel reactor and the riser is the air reactor. The main purpose of testing the reactor was to establish suitable operating conditions as well as testing the influence of bed mass and fluidization velocities on solids circulation flux and gas leakage. The second design is a two-compartment reactor, where a downcomer transports particles from the air reactor to the fuel reactor, and a slot in the bottom of the wall between the reactors leads the particles back to the air reactor. The second design was used both for a cold-flow model and the 300 Wth chemical-looping combustor. Both cold-flow models indicated that the solids circulation rate between air and fuel reactor was high enough for CLC, and that leakage was small and could be lowered even more with simple countermeasures. The second design was also successfully tested with combustion of natural gas and syngas in the temperature range 800-950˚C, using two different nickel based oxygen-carriers. One oxygen-carrier was tested with syngas, and the conversion of the fuel was high, often exceeding 99%, with hydrogen and carbon monoxide concentrations close to thermodynamic equilibrium. For natural gas, high conversion of the fuel was found for both oxygen-carriers, also around 99%. One of the particles tested was in operation for 150 h, of which 30 h with combustion, the other particle for 18 h, of which 8 h with combustion. No loss in reactivity was observed during testing and virtually no attrition of the two particles was detected.