Chemical-looping combustion (CLC) has emerged as a promising technology for fossil fuel combustion which produces a sequestration ready concentrated CO2 stream in power production. A CLC system is composed with two reactors, an air and a fuel reactor. An oxygen carrier such as hematite (94%Fe2O3) circulates between the reactors, which transfers the oxygen necessary for the fuel combustion from the air to the fuel. An important issue for the CLC process is the selection of metal oxide as oxygen carrier, since it must retain its reactivity through many cycles. The primary objective of this work is to develop a global mechanism with respective kinetics rate parameters such that CFD simulations can be performed for large systems. In this study, thermogravimetric analysis (TGA) of the reduction of hematite (Fe2O3) in a continuous stream of CH4 (15%, 20%, and 35%) was conducted at temperatures ranging from 700 to 825°C over ten reduction cycles. The mass spectroscopy analysis of product gas indicated the presence of CO2 and H2O at the early stage of reaction and H2 and CO at the final stage of reactions. A kinetic model based on two parallel reactions, (1) first-order irreversible rate kinetics and (2) Avrami equation describing nucleation and growth processes, was applied to the reduction data. It was found, that the reaction rates for both reactions increase with, both, temperature and the methane concentration in inlet gas.

The presented work is part of the Industrial Carbon Management Initiative (ICMI) on the development of metal oxide oxygen carriers, for use in the chemical looping combustion process. An oxygen carrier, CuO/bentonite. (60:40%), was reacted with methane gas and then oxidized in air. The change in weight and reaction gas concentrations Were measured using a thermogravimetric analyzer (TGA) equipped with a real time gas analyzer: The reduction-oxidation cycle was conducted within the temperature range of 750-900 degrees C for 10 cycles, using 20, 50, and 100% CH4 concentrationa in N-2 for the reduction segment and dry air for the oxidation segment Several analysis Methods were evaluated to fit the Oxidation of reduced CuO (Le., CO) data over the complete conversion range with suitable rate expression derived from existing. models for Oxidation, including the shrinking core model (diffusion and reaction control), first- and second order reaction rates, parallel and series reaction mechanisms, and Johnson-Mehl-Avrami (JMA) rate. The best agreement between the experimental data and the models of the Cu oxidation was accomplished using the JMA Model. The reactivity of the oxygen carrier during the oxidation reactions was affected by the CH4 concentration as well as the temperature. The rate of fractional uptake of oxygen onto the carrier decreased as the temperature increased, contrary to expectations and indicative that the mechanism is changing during the test. Analysis of the exit gas provided evidence of carbon deposition on the reduced sorbent particle and resulted in the CO2 product upon oxidation. The oxidation of this carbon releases significant heat that is capable of changing the particle morphology (Zhu, Y.; Mimura, K.; Isshiki, M. Oxid. Met 2004, 62, 207-222). On the basis of experimental results, the overall reaction process in the fuel reactor may be considered to consist of the decomposition of CH4 into C and H-2 and reduction of CuO/bentonite by the resulting H-2 and the parallel reaction of CH4 with CuO/bentonite. The extent of carbon deposition in the carrier particle increased with an increasing temperature and CH4 concentration. This deposited carbon not only leads to CO2 release from the oxidation reactor but, more importantly, causes degredation: of the carrier capacity and its reactivity.

Chemical looping combustion (CLC) is a process that uses an oxygen-carrier metal, instead of air or pure oxygen, to provide oxygen for combustion. The products of CLC of methane are CO2 and H2O. After condensation of H2O, a concentrated CO2 gas stream is produced and ready for sequestration. An important issue for the CLC process is the selection of metal oxide as an oxygen carrier, because it must retain its reactivity through many cycles. In this study, isothermal thermogravimetric analysis is used to evaluate the rates of reduction of CuO impregnated in bentonite with methane (CH4) over the range 1023-1173 K for 20%, 50%, and 100% CH4 over 10 reduction cycles. The mechanism and reactivity of the CuO oxygen carrier were evaluated by 10 different rate models. The results indicate that the transformation kinetics described by the Johnson-Mehl-Avrami (JMA) model was the best fit. The Avrami exponent n ranges from 1.55 to 2.16. The average value of 1.77 indicates that the crystallization mechanism is mainly two-dimensional diffusion-controlled. The activation energy was estimated to be 37.3 +/- 1.3 kJ/mol. No deactivation was observed over 10 cycles at any CH4 concentration. In the first 10 reaction cycles, the reaction rates increased slightly with the increasing number of cycles. Moreover, the rate-time and rate-conversion curves for all the temperatures show that the maximum rate occurred at t > 0. This was confirmed by the outlet gas measurements. The experimental results suggested that the CuO/bentonite oxygen carrier is a promising candidate for the CLC system burning methane.

The water gas shift reaction was evaluated in the presence of novel carbon dioxide (CO2) capture sorbents, both alone and with catalyst, at moderate reaction conditions (i.e., 300–600 °C and 1–11.2 atm). Experimental results showed significant improvements to carbon monoxide (CO) conversions and production of hydrogen (H2) when CO2 sorbents are incorporated into the water gas shift reaction. Results suggested that the performance of the sorbent is linked to the presence of a Ca(OH)2 phase within the sorbent. Promoting calcium oxide (CaO) sorbents with sodium hydroxide (NaOH) as well as pre-treating the CaO sorbent with steam appeared to lead to formation of Ca(OH)2, which improved CO2 sorption capacity and WGS performance. Results suggest that an optimum amount of NaOH exists as too much leads to a lower capture capacity of the resultant sorbent. During capture, the NaOH-promoted sorbents displayed a high capture efficiency (nearly 100%) at temperatures of 300–600 °C. Results also suggest that the CaO sorbents possess catalytic properties which may augment the WGS reactivity even post-breakthrough. Furthermore, promotion of CaO by NaOH significantly reduces the regeneration temperature of the former.