Abstract
Chemical looping combustion (CLC) is one of the most promising methods for carbon capture and storage (CCS). An oxygen carrier, i.e., a mineral that can be oxidized and reduced, is used to convert the fuel in the process. The produced CO2 is inherently separated from the air components that enables easier CCS. The use of biomass-based fuels is desirable since it can lead to negative CO2 emissions. On the other hand, alkali compounds from the biomass may interact with the oxygen carrier causing problems, such as deactivation of the oxygen carrier. The most common oxygen carriers contain iron, since iron-based ores and industrial waste materials are readily available and cost-efficient. Therefore, the interaction between the iron oxygen carriers and the biomass ash-forming compounds needs to be investigated. Since Ca/Mg are abundant in biomass, it is important to clarify how their compounds interact with the oxygen carrier. In this study, the effect of Ca/Mg carbonates, chlorides, nitrates, sulfates, and phosphates along with synthetic biomass-derived ash on iron oxides was investigated. Redox reactions were investigated at 950 °C during 5 h under both oxidizing and reducing atmospheres. The results showed that the effect of Ca/Mg salts on the oxygen carrier varied depending on the anion of the salt. Generally, the nitrate- and phosphate-based salts of both Ca and Mg showed the harshest effect regarding agglomeration of the oxygen carriers. It was shown that the Ca/Mg-based compounds interacted differently with iron oxides, which was an unexpected result.
Highlights
Chemical looping combustion (CLC) is one of the most promising methods to reduce the cost of CO2 capture to tackle the climate change.[1]
When Ca(OH)[2] was used as an alkaline earth compound representative in the mixture, the formation of new compounds, such as CaFe2O4 and Ca2Fe2O5, was observed under both reduction and oxidation conditions most likely via reactions 1 and 2. These reactions are only solid-state reactions and do not involve reduction of the iron. As both new compounds formed have been reported as potential oxygen carriers for energy-related applications, their formation as such was not considered as a problem.[34,35]
It is vital to understand the nature of interaction between the ash-forming matters and oxygen carriers, since ash-forming matters may result in the deactivation of the oxygen carriers
Summary
Chemical looping combustion (CLC) is one of the most promising methods to reduce the cost of CO2 capture to tackle the climate change.[1] CLC enables us to separate CO2 from other combustion products; there is no energy consumed for the gas separation.[2] A CLC process consists of two interconnected fluidized beds: a fuel reactor and an air reactor.[3] In the fuel reactor, the fuel is oxidized by a solid oxygen carrier, which gets reduced. The oxygen carrier is oxidized by air. The oxygen carriers are chosen from oxides of Cu, Fe, Mn, and their oxide-based combinations or ores.[4−7] Even though gaseous fuels are most commonly used in CLC systems, solid fuels can be favorable since they are less expensive and more abundant.[8,9] Recently, the use of biomass in CLC has attracted great attention since it gives a possibility to achieve “negative CO2 emission” goals.[10,11] There is no doubt that the use of biomass in CLC systems brings a lot of advantages.[12,13] biomassderived ash consists of highly reactive species such as alkali metal compounds and compounds of alkaline earth elements,[13−15] which may cause partial sintering or agglomeration of the oxygen carriers.[16]
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