Calcium and iron oxide reactivity studies for chemical looping applications of clean energy conversion

The following study entails independent investigations carried out on the reactivity of metal oxides involved in the calcium and chemical looping applications. Chapters 2 through 5 involve studies on the various applications and aspects of the calcium looping process, and Chapter 6 and 7 discuss two independent investigations of chemical looping oxygen carrier particles.;The hydration of calcium oxide (CaO) sorbent has been investigated as a reactivation method in the three step calcium looping process for pre and post combustion carbon dioxide (CO2) capture. The feasibility of the process concept was established using lab scale fixed bed reactor setup, and reactivation of sorbent was achieved with high temperature steam at 500°C over multiple cycles. Further development of the design and operation of a fluidized bed hydrator is reported upon, and fast fluidization regime was identified as the most suitable for a scalable steam hydrator design. Further, a screening study was conducted on multiple egg and sea shells as a renewable source of the CaO sorbent, and excellent reactivity towards CO2 is reported. A novel method for the simultaneous cleanup of CO2, SOx and NOx impurities from coal combustion flue gas is proposed based on the calcium looping process. Proof of concept experiments were performed and 90% CO2 and NO and 100% SO2 removal was demonstrated at 1 atm, 650°C fixed bed experiments, using the calcium sorbent and coal char. For pre-combustion application of the calcium looping process (CLP), the fate of sulfurous species is explored, which are formed as a byproduct of the coal to H2 plant with the CLP. The CaS formed in the carbonator at the operating conditions of about 600°C and 23 bar is found to be oxidized to CaSO4 at the calciner operating conditions of the CLP. Treatment options for the purge stream are discussed for the oxidation of unreacted CaS for the safe disposal and integration with the cement industry.;In the latter half of the present study, the iron-based metal oxide oxygen carriers are investigated for the chemical looping partial oxidation (CLPO) of CH4 for the production of syngas at elevated pressures. The favorable impact of increased pressure on the redox reaction rates is illustrated through experiments conducted on the iron-titanium complex metal oxide (ITCMO) particles between 1 and 10 atm at 900-950°C in a thermogravimetric setup. The observed change in morphology through SEM and BET analysis at increased pressures is related to the change in reactivity obtained. Lastly, an application of chemical looping gasification (CLG) for the coproduction of H2 and electricity is explored. Specifically, the recyclability of iron based oxygen carriers is investigated in steam redox environments using a specialized thermogravimetric setup. Isothermal tests are conducted for 20 redox cycles using steam as the oxidizing agent for iron and cobalt based metal oxide oxygen carriers at 900°C. MgAl2O4 is used as an inert support. While cobalt-based samples exhibited a loss in reactivity, the excellent recyclability of iron-based oxygen carriers has thus been established.