Oxygen Carriers Materials for Chemical-Looping Technologies
Doctoral thesis, 2007
Carbon dioxide is the gas which contributes most to the greenhouse effect. It is released in large quantities from fossil fuel-based power plants around the world. It is generally accepted that a rapid decrease in the emissions of carbon dioxide is needed. One method to achieve rapid reductions in the emissions and still use fossil fuels is to capture and store the carbon dioxide. However, the separation of carbon dioxide from a flue gas stream of a power plant is an expensive and energy-intensive process resulting in a large decrease in efficiency. Thus there is a need to find cheaper and more efficient methods to perform the separation. Chemical-looping combustion (CLC) and chemical-looping reforming (CLR) are innovative technologies for power and hydrogen production from natural gas with the capture of carbon dioxide. In CLC, CO2 is inherently separated from other flue gas components i.e. N2 and O2 with minor energy losses. With some modifications CLC can be modified for production of hydrogen, i.e. CLR. Both processes involve the use of an oxygen carrier that transfers oxygen from combustion air to the fuel. Two inter-connected fluidized beds, a fuel and an air reactor, are used in the process. Whereas the natural gas is fully oxidized to CO2 and H2O in the fuel reactor for CLC, it is only partially oxidized by the metal oxide in the fuel reactor for CLR, resulting in a mixture of H2, CO2, CO and H2O. The exit stream from the fuel reactor can be sent to a water gas shift reactor to get an undiluted stream of CO2 and H2. The reduced metal oxide is sent into the air reactor where it is oxidized by air. The oxidation reaction is exothermic resulting in heat production in the air reactor. This heat is used to maintain the oxygen carrier particles at the high temperature necessary for the endothermic reaction in the fuel reactor. The hot gases exiting from the air reactor can be used for power production.
For the chemical-looping technologies to become successful it is important to find suitable oxygen carriers. This thesis focuses on the development and reactivity testing of such oxygen carriers. For CLR, metal oxides based on Ni, Cu, Mn and Fe were prepared by impregnation on SiO2 and MgAl2O4 and tested in a laboratory fluidized bed reactor as well as a thermo gravimetric analyzer (TGA). The particles were exposed to alternating reducing (50% CH4/ 50% H2O) and oxidizing (5% O2) conditions. With respect to the metal oxides on the SiO2 support, the particles based on NiO and CuO showed the highest reactivity, whereas Fe2O3 and Mn2O3 showed signs of deactivation as function of cycle number, likely due to the formation of metallic silicates. Only NiO showed high selectivity toward H2. All the MgAl2O4-supported metal oxides exhibited high reactivity under reducing and oxidizing conditions. In contrast to the SiO2 based particles, no deactivation as a function of cycle number was seen for any of these oxygen carriers.
Reduction and oxidation kinetics of oxygen carriers of NiO/MgAl2O4 and Mn3O4/Mg-ZrO2 for CLC were investigated using methane and air in a TGA. At high temperature both oxygen carriers reacted rapidly under both reducing and oxidizing conditions and the reaction rate was a function of the temperature and concentration of reacting gas. However, it was found that NiO/MgAl2O4 may not be feasible to be used below 900 °C due to low reactivity. The reactions were modelled using the shrinking-core model for spherical grains assuming chemical reaction control and the kinetic parameters were calculated for both oxygen carriers. From the kinetic parameters the solid inventories in a real CLC system were calculated. The minimum solid inventories needed were 22 kg/MWf for NiO/MgAl2O4 and 135 kg/MWf for Mn3O4/Mg-ZrO2. These masses are very low compared to other oxygen carriers investigated previously, and thus both type of particles are very promising for a real CLC system.