Abstract

Chemical-looping combustion with oxygen uncoupling (CLOU) is a method for combustion of solid and gaseous fossil fuels, which enables easy separation of carbon dioxide from the gaseous product mixture. In contrast to the related chemical-looping combustion (CLC) technology where gaseous or gasified fuels react directly with oxygen carriers, CLOU processes require oxygen carrier materials to be able to release oxygen in the fuel reactor and to regenerate by re-oxidation in oxygen-rich atmosphere in the air reactor at elevated temperature. Oxygen uncoupling properties and reactivities for methane combustion of 12 oxygen carrier particles, produced from mixtures of manganese and magnesium oxides with optional addition of titanium dioxide or calcium hydroxide, are investigated in a quartz batch reactor at 810 °C, 850 °C, 900 °C and 950 °C. All investigated oxygen carriers have oxygen release characteristics. The addition of calcium hydroxide facilitates oxygen release and combustion of methane, whereas addition of titanium dioxide does not have a pronounced effect on either oxygen uncoupling or reactivity of the oxygen carrier. In general, particles with greater extent of oxygen release have superior methane combustion properties.

Abstract

Particles of the perovskite material CaMn_0.875Ti_0.125O₃ has been examined as oxygen carrier for chemical-looping with oxygen uncoupling, and for chemical-looping combustion of natural gas, by 70 h of experiments in a circulating fluidized-bed reactor system. For the oxygen uncoupling experiments, it was found that the particles released O₂ in gas phase at temperatures above 720 °C when the fuel reactor was fluidized with CO₂. The effect increased with increased temperature, and with the O₂ partial pressure in the air reactor. At 950 °C, the O₂ concentration in the outlet from the fuel reactor was in the order of 4.0 vol%, if the particles were oxidized in air. For the chemical-looping combustion experiments the combustion efficiency with standard process parameters was in the order of 95% at 950 °C, using 1000 kg oxygen carrier per MW natural gas, of which about 30% was located in the fuel reactor. Reducing the fuel flow so that 1900 kg oxygen carrier per MW natural gas was used improved the combustion efficiency to roughly 99.8%. The particles retained their physical properties, reactivity with CH₄ and ability to release gas-phase O₂ reasonably well throughout the testing period and there were no problems with the fluidization or formation of solid carbon in the reactor. X-ray diffraction showed that the particles underwent changes in their phase composition though.

Research highlights

? The perovskite CaMn_0.875Ti_0.125O_3−δ is examined as oxygen carrier for chemical-looping combustion with O₂ uncoupling. ? CaMn_0.875Ti_0.125O_3−δ release gas phase O₂ in inert atmosphere at temperatures greater than 700 °C. ? CaMn_0.875Ti_0.125O_3−δ has high reactivity with natural gas and appears to have good phase stability.

Abstract

Norwegian industrial tailings and by-products, as well as naturally occurring minerals and ores have been surveyed with the purpose of identifying candidate oxygen carrier materials for use in a chemical looping combustion process. Nine materials, based on manganese and/or iron oxide, were selected for an initial screening test; six were deemed promising and were hence investigated further. Thermogravimetric experiments were performed to investigate the oxygen capacity, the reaction kinetics and reversibility of the oxygen absorption reaction. A manganese ore with a reversible capacity of 4.9 wt% oxygen at 1000 °C was selected as the most promising for chemical looping combustion applications. This material was modified by addition of calcia to explore the possibility of enhancing the kinetic, catalytic and mechanical properties. The addition of excess calcium relative to manganese resulted in formation of calcium manganite and related phases. The oxygen capacity of the modified material was 4.5 wt% at 1000 °C, but it has potential advantages in terms of kinetics and chemical and mechanical stability relative to the pure ore.

Abstract

Chemical-looping combustion (CLC) has been suggested as an energy-efficient method for the capture of the greenhouse gas carbon dioxide from combustion. The reactivity of using Fe₂O₃ as an oxygen carrier during CLC of coal has been investigated experimentally at 800–950°C. The experiments were carried out in a fluidized bed, where the steam acted as the gasification-fluidization medium. The reactivity of Fe₂O₃ as a function of the reactor temperature, reaction time, and cyclic reduction number was discussed. The reactivity of Fe₂O₃ oxygen carriers was enhanced as temperature increased at 800–950°C. Moreover, the time of chemical reaction control between the oxygen carrier and coal gasification products decreased with increased reaction temperature. When the reaction temperature was above 900°C, the rate of carbon to form CO₂ was higher than 90%; however, it was lower than 75% below 850°C. At 900°C, the dry basis concentration of CO₂ decreased with increased cyclic reduction period, while that of CO and CH₄ increased. Moreover, the value of the CO concentration was less than that of CH₄. The performance of the reacted Fe₂O₃-based oxygen carriers was also evaluated using an X-ray diffractometer and a scanning electron microscope to characterize the solid residues of oxygen carrier. The results show that Fe₂O₃-based oxygen carriers are only reduced to Fe₃O₄. With the increase of cyclic reduction period, the oxygen carrier sinters gradually.