The concerns over climate change due to CO2 emitted from the combustion of fossil fuels have been growing for decades. There is CCS (Carbon dioxide Capture and Storage) technology that can reduce CO2 emissions efficiently, but the disadvantage of this technology are low power generation efficiency and high CO2 capture cost. Because of its low loss of efficiency in power generation and low cost of capturing CO2, the gas combustion in
Chemical Looping-combustion (CLC), which is an indirect combustion technique using OC (oxygen carrier) particles, has been world-widely studied. The key element of the gas combustion research for CLC is development of gas combustion process with high efficiency and OC
particles.
NiO/Al2O3, known as one of the most efficient oxygen carriers, has been fabricated by spray-dryer method and calcined at 1100 ℃ and 1300 ℃, and the structural characteristics are investigated using ICP, XRD, SEM, TEM and XPS. For the characterization of surface and bulk microstructure of the fabricated NiO/Al2O3 OC particles,
were investigated as-fabricated powders, internal structure of the crumbled particles, and cross-section specimens. The results showed that the fabricated OC particles well distributed NiAl2O4 with NiO particles of 100∼500 nm via reaction keeping on the mole ratio. The oxygen carrier particles developed in this study showed pertinent characteristics for CLC and good effect on the strength, indicating a potential for wide application in the future.
Our research staffs have been able to perform long term operation of a 200 kW gas-circulating combustion system successfully for over 100 h with NiO/Al2O3 OC particles that were produced in large quantities using a pilot-plant spray-dryer. Following such results, we are planning to scale-up the process to a 1~10 MW system. Iron oxide, which is relatively cheaper than NiO, has been considered for this step, and 9 different types of NiO/Fe2O3/Al2O3 mixed metal oxides were produced by adding iron oxide to NiO with 22.5~47.5 wt% ratio, adding 30 wt% of Al2O3 as support and finally forming material using the spray-dryer method. Results from both physical property test and chemical property test suggest that mixed metal oxide of N1F2 (NiO 22.5 wt%, Fe2O3 47.5 wt%, Al2O3 30 wt%)calcined at 1100 ℃ is suitable for the fluidized bed process with excellent Oxygen Transfer Capability (OTC). OTC was higher than theoretical value of N1F2 particles, and it can be explained as spinel Ni(Al,Fe)2O4 solid solution through XRD analysis. It is believed that mixed metal oxides produced are good candidates for media circulating combustion of fluidized bed process for large capacity of
a 1~10 MW system for the future.
While looking for cheaper iron oxide from wastes or industrial products, considerable amounts of scale and sludge were found from power plants. The main components of the sludge and scale collected from power plants were more than 95% of iron oxides and the residues were oxides, such as MnO, SiO2, Cr2O3 and CaO, which
can be an active material or a binder for OC particles. The particle size of the sludge and scale is also suitable for the OC particles fabrication, eliminating the need of pre-treatment. Four different oxygen carrier particles were fabricated by the spray-dryer method, calcined at 1100 ℃, 1200 ℃ and 1300 ℃, and the physical/chemical properties were measured. It was found that mixed NiO/Fe2O3/Al2O3
OC particles are good candidates for a 1∼10 MW class circulating combustion of fluidized bed. The contribution of this research is that sludge and scale collected from power plants can be used to capture CO2 emitted from power plants.