A Circulating fluidized bed combustor system with inherent CO2 separation - application of chemical looping combustion

− Possible operational conditions of a conceptual design of a circulating fluidized bed for chemical looping combustion (CLC) were investigated. CLC is a method where a gaseous fuel is combusted with inherent separation of the greenhouse gas CO2 from the flue gas. Oxygen is transferred from the combustion air to a gaseous fuel by means of metal oxide particles acting as oxygen carriers. A bubbling bed below the downcomer acts as a fuel reactor where oxygen is transferred from the metal oxide (MexOy) to the fuel and the riser acts as the air reactor, where the previously reduced metal oxide (MexOy-1) is oxidized by the oxygen in the air. Hence, the fuel and combustion air are not in direct contact. A cold model of such a system was designed and operated according to scaling laws corresponding to an original 30 MWth combustor design operating at 0.9 MPa with natural gas as the fuel and iron oxide as the oxygen carrier. A mapping of the range of operational conditions with respect to combinations of fluidization velocity in the air reactor, bed mass, and the net solids flux (Gs), from the air to the fuel reactor is presented. Introduction It is generally accepted that the effect of global warming caused by emissions of greenhouse gases requires substantial measures in the near future in order to stabilize the atmospheric concentration of CO2 at an acceptable level. The world reserve of fossil fuels is large and introduction of renewable energy sources are most likely not enough to achieve such a level of CO2 during the next decades. One method to achieve CO2-free combustion and still use fossil fuels as an energy source is to separate and dispose of the CO2 from combustion (Lyngfelt and Leckner, 1999). Disposal costs are generally low, but a drawback with most methods is the decrease in efficiency and costly equipment necessary (Göttlicher et al., 1998; Undrum et al., 2001). In chemical looping combustion (CLC) the separation of CO2 is inherent and both reduced efficiency and separation costs can be avoided. Chemical-Looping Combustion In chemical looping combustion metal oxide particles are used to transfer oxygen from air to a gaseous fuel. The system consists of two separate reactors, as shown in Figure 1. In the fuel reactor the particles react with the fuel: (2n+m)MexOy + CnH2m → (2n+m)MexOy-1 + mH2O + nCO2 (1) The reduced metal oxide is then transported to the air reactor where oxygen from the air is transferred to the particles: MexOy-1 + 1⁄2O2 → MexOy (2) *Corresponding author, e-mail address evjo@entek.chalmers.se 7th Int. Conf. on Circulating Fluidized Beds, Niagara Falls, Ontario, May 5-7, 2002, p. 717-724 Thus, the reduced metal oxide is oxidized back to the original metal oxide and can be returned to the fuel reactor for a new cycle. Possible metal oxides are some oxides of common transition-state metals, such as iron, nickel, copper and manganese (Mattisson et al., 2001a,b). For these oxides, reaction (2) is exothermic with subsequent heat release, while reaction (1) is most often endothermic. However, the total heat produced in the oxidation and the reduction is the same as in normal combustion where oxygen and fuel are in direct contact. The advantage with performing the combustion in two reactors/steps compared to conventional combustion is that the carbon dioxide is not diluted with nitrogen gas, but is received almost pure, without any extra energy demand and costly external equipment for CO2 separation. Figure 1. Chemical looping combustion (CLC). MexOy and MexOy-1 denote oxidized and reduced oxygen carriers. Figure 2. Layout of the model.1) air reactor, 2) cyclone, 3)fuel reactor, 4) downcomer 5) particle loop-seal Different types of interconnected fluidized beds have been investigated by several authors, (e.g. Janse et al., 1999 and Snip et al., 1996). The CLC-system in Figure 2 is a circulating fluidized-bed system in which the air reactor (1), is the riser. The riser is connected by a cyclone (2) to the fuel reactor (3) in the form of a bubbling fluidized bed. Particle loop-seals, one in position (5) and one in the bottom of the downcomer from the cyclone, prevent gas mixing between the two reactors. In the process of developing a CLC-system, possible combinations of mass of oxygen carrier in the reactors, flow of oxygen carrier (external solids flux), air and fuel flows are crucial. First estimates of required external solids flux, Gs, under atmospheric conditions (Lyngfelt et al., 2001) yield values of Gs in the order of 50 kg/m s, somewhat higher than for a typical large-scale conventional CFB combustor, (e.g. Zhang et al., 1995). The tentative design by Lyngfelt et al. uses natural gas, iron oxide as oxygen carrier and has a thermal power of 10 MW. In the present work a similar design is used, but with the important difference that the system is pressurized to 0.9 MPa. The thermal power is 30 MW, the air ratio is 2.6 and the fluidization velocity is 3.5 m/s in the riser. Important aspects on the layout of the system are: N2 O2