Effect of the presence of siloxanes in biogas chemical looping combustion

Siloxanes are a group of Si-based impurities, common in biogas. Although normally present in relatively small amounts, their presence could be highly problematic, as the generated Si could be a precursor to abrasion and wear in downstream components. In this work, the siloxane effect in biogas chemical looping combustion (CLC) was evaluated. CLC of biogas could be an efficient way of achieving CO2 negative emissions. Two oxygen carriers (CuO-based and Fe2O3-based residue) were used in the combustion of a simulated biogas stream in a batch fluidized bed reactor. Here one of the most common siloxane compounds (hexamethyldisiloxane, L2) was utilized together with methane, the most common combustible component in biogas. The siloxane decomposed in the batch reactor, forming gaseous compounds and Si-based particles. The gaseous compounds formed (mainly CO, H2, CH4, and C2) were able to react with the oxygen carrier, whereas the Si originating from the siloxane interacted with the oxygen carrier particles. Elemental analyses of samples obtained from the reactor bed after combustion experiments revealed that significant amounts of Si from siloxane could be found in the oxygen carrier. The X-ray photoelectron spectroscopy (XPS) analyses of the same samples showed that silica in the oxygen carrier was mainly found at the surface of the particles and in the form of SiO2 or silicates/aluminosilicates. The XPS results were in agreement with a theoretical thermodynamic analysis performed to determine the possible stable Si-based species that may be formed. Although the deposition/formation of silica compounds in the oxygen carrier may lead to the agglomeration of particles, such a phenomenon was not observed in the present work.