Abstract
Ice-templating (freeze-casting) technique was applied to a novel class of geopolymer composites containing Fe/Mn oxides, previously tested and reported in others works as synthetic oxygen carriers for chemical looping combustion (CLC), in order to obtain composite monoliths with lamellar macro-porosities by unidirectional freezing of water-based sol-gel systems. Geopolymer-Fe/Mn oxides composites carriers were also produced as beads, suitable for fixed bed reactors, by an injection-solidification method in liquid nitrogen. After conditioning at 900 °C, the temperature needed for CLC applications, the composite beads and monoliths possess similar total porosity % and total pore volume, being ≈65% and 570 mm3 g−1, respectively, as well as a specific surface area of around 2.4–2.9 m2/g.
Highlights
Geopolymers are commonly obtained at low temperatures by the mixing of an aluminosilicate source material with an alkali hydroxide or alkali silicate solution [1]
The aim of the present work is to extend the use of this technique to the novel geopolymer-Fe/Mn oxides composites, in order to obtain oxygen carriers with specific geometries and porosity for chemical looping combustion (CLC)
The geopolymer matrix was prepared mixing metakaolin grade M1200S purchased from Imerys with a solution of potassium di-silicate used as alkali activator, having a molar ratio of SiO2 :K2 O = 2 and H2 O:K2 O = 13.5
Summary
Geopolymers are commonly obtained at low temperatures (under 100 ◦ C) by the mixing of an aluminosilicate source material with an alkali hydroxide or alkali silicate solution [1]. The production of composites is of considerable importance to further increase the potentialities of geopolymers In this respect, geopolymer composites have been already formulated to obtain oxygen carriers suitable for chemical looping combustion (CLC) processes [4,5] or catalysts for tar decomposition [6]. CLC is currently considered one of the best alternatives to reduce the economic cost of CO2 capture. This technology is based on indirect fuel combustion by use of a solid oxygen carrier, generally a metal oxide having the capability of transporting the oxygen needed for the combustion from an air reactor to a fuel reactor, usually designed as two coupled fluidized beds [7]
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