Ilmenite with addition of NiO as oxygen carrier for chemical-looping combustion

Abstract

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 h 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 2O 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.