A chemical looping process that uses biomass as the feedstock could prove to be a highly economical, environmental and sustainable means for electricity and/or chemical production with net negative carbon emissions. A chemical looping system converts carbonaceous fuels, such as biomass, using metal oxide based oxygen carriers circulating between three reactors - a reducer, an oxidizer and a combustor – to produce heat and high purity hydrogen with simultaneous carbon dioxide (CO2) sequestration. In the reducer, the biomass fuel is converted to CO2 and H2O by oxygen transfer from the oxygen carriers. The reduced oxygen carriers are then partially oxidized by steam in the oxidizer producing high purity hydrogen and subsequently fully oxidized by air in the combustor. The energy generated due to the exothermic reactions in the combustor is sufficient to compensate for the heat required to perform the endothermic reactions in the reducer and to produce electricity via steam generation.
Tar evolved during biomass conversion in the reducer mainly consists of oxygen containing heavy hydrocarbons and complex polycyclic aromatics. It is a major concern as it can hinder the continuous operation and performance of a biomass chemical looping (BCL) system. Hence, it is important to design oxygen carriers that have the ability to eliminate the tars in the reducer. While Fe2O3 is an ideal candidate for oxygen carrier material for chemical looping processes, in this study, different tar cracking catalysts (Olivine, Dolomite, K2CO3 and NiO) have been used in conjunction with Fe2O3 to enhance the carriers’ ability to crack tar. Thermogravimetric Analysis (TGA) has been used to study the ability of the oxygen carriers to undergo multiple redox cycles. A series of fixed bed experiments have been used to crack biomass-derived tar with the mixed oxygen carriers. The product gaseous species have been identified using Gas Chromatography–Mass Spectrometry (GC-MS) and fresh and reacted solids have been characterized using X-Ray Diffraction (XRD). Addition of NiO to the Fe2O3 based oxygen carrier was observed to greatly improved its tar cracking ability and maintained its redox reactivity over multiple cycles. The results from this study will help in the development of oxygen carriers towards designing an optimal BCL system.
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