Due to the large carbon footprint of existing fossil fuel based powered plants, it is necessary to find potential alternatives to existing gas scrubber technology. One suggestion is by using high temperature, dry sorbent technologies like chemical looping combustion (CLC). CLC provides increased power efficiencies while additionally having lower energy penalties towards carbon capture than traditional combustion and gasification technologies that include carbon capture. CLC uses metal oxide particles termed “oxygen carriers†in a cyclic redox mode typically using two reactors to separate the combustion of carbonaceous fuel and power generation/ oxygen regeneration step. CLC is typically operated in a circulating fluidized bed (CFB) reactor where the oxygen carriers are first reacted with the carbonaceous fuel than are transported to the air reactor to regenerate the oxygen in the reduced carrier. Chemical looping combustion faces some uncertainty due to economic feasibility. This arises partially from the solids makeup rate as a result of particle attrition encountered during operation. Particle attrition is a concern for chemical looping combustion due to the mechanical, thermal, and chemical stresses that an oxygen carrier experiences. Mechanical attrition typically occurs in two primary mechanisms: wear and impact. Thermal stresses resulting from the high temperature and potential temperature swing experienced in the separate fuel and air reactors can lead to oxygen carrier degradation by increasing the amount of internal stresses and affect material properties such as hardness and fracture toughness. Finally, chemical stress due to the redox process in chemical looping can lead to degradation of the oxygen carrier directly or indirectly by increasing the likelihood of mechanical attrition through increased internal stresses. While some studies have observed degradation due to undesired chemical reactions such as carbon formation from methane decomposition, the effect of chemical reactions have not been effectively studied for the particle attrition involved in chemical looping combustion.
This study aims to investigate and explain the effect of chemical reactions on a hematite ore oxygen carrier. This is done by utilizing a conical jet cup that can operate similar to a fluidized bed reactor by allowing for high temperature and under redox conditions. To focus on the chemical reactions involved in chemical looping, the bed is operated in a fixed bed mode targeting the transition from hematite (Fe2O3) to magnetite (Fe3O4). Effects of fuel (H2 and CH4), temperature (800-900°C), and cycles (25-50 cycles) are investigated. A second set of experiments will utilize the tangential jet of the jet cup to determine the impact on mechanical attrition during the in-situ redox cycling of the oxygen carrier. Preliminary results indicate that for the hematite ore undergoing the redox process cause minor fines to be elutriated during the reduction process. The jet cup indicates that the material becomes weakened due to the chemical reaction as the oxygen carrier undergoing a 25 redox cycle experiences nearly 3 times more mechanical attrition than the unreacted oxygen carrier.
