Chemical-looping with oxygen uncoupling (CLOU) is a developing technology, which allows for inherent separation of carbon dioxide during combustion of gaseous and solid fuels. In contrast to the related chemical-looping combustion technology (CLC), where gaseous or gasified fuels react directly with oxygen carrier materials, CLOU utilizes oxygen carriers capable of releasing gaseous oxygen, provided appropriate thermal conditions exist in the fuel reactor, whereupon fuel reacts with the released oxygen. The oxygen carriers are thereafter regenerated by oxidation in the air reactor. In this work, a set of oxygen carrier materials synthesized from manganese oxides mixed with iron, nickel, and silicon oxides are evaluated in terms of their ability to release oxygen and to be regenerated with oxygen and in terms of demonstrated methane conversion in a quartz fluidized bed batch reactor at 810, 850, and 900 °C. The results demonstrate that two Mn/Fe oxygen carriers and one Mn/Ni oxygen carrier have both oxygen release characteristics and high reactivities with methane. The Mn/Fe oxygen carrier synthesized at 1100 °C was additionally shown to have an increasing methane conversion on a long-term basis, which opens for a possibility to use Mn/Fe oxygen carriers in time-extended large-scale chemical-looping operations.

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The naturally occurring mineral ilmenite, FeTiO 3 , has been examined as oxygen carrier for chemical-looping combustion. NiO-based particles have been used as an additive, in order to examine if it is possible to utilize the catalytic properties of metallic Ni to facilitate decomposition of hydrocarbons into more reactive combustion intermediates such as CO and H 2. Firstly, ilmenite was examined by oxidation and reduction experiments in a batch fluidized-bed reactor. These experiments indicated moderate reactivity between ilmenite and CH 4 , which was used as reducing gas. However, adding 5 wt% of NiO-based particles to the ilmenite improved the conversion of CH 4 greatly, resulting in an increase in combustion efficiency with a factor of 3. Secondly, 83 hours of chemical-looping combustion experiments were conducted in a small circulating fluidized-bed reactor, using ilmenite as oxygen carrier and natural gas as fuel. A wide range of process parameters and different levels of NiO addition were examined. Occasionally, there were problems with the circulation of solids between the air reactor and fuel reactor, but most of the time the experiments worked well. The products were mostly CO 2 , H 2 O and unconverted CH 4. Adding small amounts of NiO-based particles to the reactor increased the conversion of the fuel considerably. For the base case conducted at 900°, the combustion efficiency was 76% for pure ilmenite and 90% for the corresponding experiments with 1 wt% NiO-based particles added to the reactor. The properties of ilmenite were found to change considerably during operation. Used particles had lower density, were more reactive and more porous than fresh particles. These changes appear to have been physical, and no unexpected chemical phases could be identified.

Fe 2 O 3 -containing waste materials from the steel industry are proposed as oxygen carrier for chemical-looping combustion. Three such materials, red iron oxide, brown iron oxide and iron oxide scales, have been examined by oxidation and reduction experiments in a batch fluidized-bed reactor at temperatures between 800 and 950°C. NiO-based particles have been used as additive, in order to examine if it is possible to utilize the catalytic properties of metallic Ni to facilitate decomposition of hydrocarbons into more reactive combustion intermediates such as CO and H 2 .The experiments indicated modest reactivity between the waste materials and CH 4 , which was used as reducing gas. Adding small amounts of NiO-based particles to the sample increased the yield of CO 2 in a standard experiment, typically by a factor of 1.5-3.5. The fraction of unconverted fuel typically was reduced by 70-90%. The conversion of CH 4 to CO 2 was 94% at best, corresponding to a combustion efficiency of 96%. This was achieved using a bed mass corresponding to 57 kg oxygen carrier per MW fuel, of which only 5 wt% was NiO-based synthetic particles. The different materials fared differently well during the experiments. Red iron oxide was fairly stable, while brown iron oxide was soft and subject to considerable erosion. Iron oxide scales experienced increased reactivity and porosity as function of the numbers of reduction cycles.

Oxygen-carrier particles for chemical-looping combustion have been manufactured by freeze granulation. The particles consisted of 60 wt % Fe 2 O 3 as active phase and 40 wt % stabilized ZrO 2 as support material. Ce, Ca, or Mg was used to stabilize the ZrO 2 . The hardness and porosity of the particles were altered by varying the sintering temperature. The oxygen carriers were examined by redox experiments in a batch fluidized-bed reactor at 800-950 C, using CH 4 as fuel. The experiments showed good reactivity between the particles and CH 4 . NiO was used as an additive and was found to reduce the fraction of unconverted CH 4 with up to 80%. The combustion efficiency was 95.9% at best and was achieved using 57 kg oxygen carrier per MW fuel. Most produced oxygen carriers appear to have been decently stable, but using Ca as stabilizer resulting in uneven results. Further, particles sintered at high temperatures had a tendency to defluidize.