Thermal behaviour of biomass ashes in air and inert atmosphere with respect to their decarbonation

Abstract

The carbonation/decarbonation of biomass ashes (BAs) is responsible for the CO2 capture and storage (CCS) potential of these waste products. Therefore, the thermal behaviour of eight BAs treated from ambient temperature up to 900 °C in air and inert atmosphere was studied to evaluate the decomposition of carbonates and associated with them minerals. A combination of thermal, mineralogical and chemical analyses was used for that purpose. The chemical composition of BAs is highly variable and their phase-mineral composition consists of diverse inorganic amorphous material, carbonates, sulphates, and silicates, plus some phosphates, chlorides, oxyhydroxides, and char. It was found that there are significant differences in the minerals transformation of BAs thermally treated under oxidizing and pyrolyzing conditions. Most BAs have lower mass loss in air than in inert atmosphere up to 900 °C due to the decomposition of minerals and char with subsequent oxygen uptake during oxidation of their decomposition products. The carbonates in BAs consists of calcite and kalicinite, and to a lesser extent, fairchildite and butschliite. The most important temperature for the decarbonation of carbonates is 600–900 °C and all BAs have higher mass loss under pyrolyzing conditions than under oxidizing environment in this temperature range. The inert atmosphere is more favorable for the decomposition of calcite, K2CO3, fairchildite, and butschliite, and melting of chlorides than air atmosphere. In contrast, the air atmosphere is more favorable for the dehydration, dexydroxylation, and oxidation of minerals and char than inert atmosphere. Hence, the determination of decarbonation for BAs (respectively CCS) is more representative in inert atmosphere because it avoids the oxidation reactions.