Performance of Rice Straw Ash As an Alternative, Low Cost Oxygen Carrier in Chemical Looping Combustion Technology

Direct coal fuelled chemical looping combustion (CLC) is one of the efficient, advanced carbon capture techniques to generate CO₂ enriched flue gas for sequestration. This technology separates oxygen by the chemical reaction between metal particles and air and the resultant metal oxides can be used for fuel oxidation. Biomass ash contains a significant quantity of Fe, Ca etc. and this would be a promising source of metal oxides. In this context, rice straw based ash content (RSA) is used as the oxygen carrier for the CLC of low ash Indian coal (LAC) in a fixed bed reactor. Further, thermo-gravimetric analyses was carried out using a mixture of RSA and LAC under CO₂ and N₂ atmosphere from 25°C to 1000°C. The instantaneous reactivity of 3×10^-4 s^-1 and 6×10^-4 s^-1 was estimated in two distinct regions at the temperature intervals of 400-600°C and 800-1000°C, respectively, under N₂ atmosphere. This confirms the reactivity of metal oxide particles in the RSA with the coal. Also, a higher instantaneous reactivity of 1×10^-3 s^-1 was found under CO₂ atmosphere between 800 to 1000°C. It is found that the RSA exhibited 3% higher mass loss, compared to the reactivity of pure Fe₂O₃ with coal char. Hence, the performance of RSA is found as equivalent to that of Fe₂O₃ oxygen carriers. X-ray diffraction analysis of RSA was conducted. It was found that Ca-ferrites (CaFe₂O₄ and Ca₂Fe₂O₅) and SiO₂ were present in the RSA. These components could be responsible for the CLC reactivity of coal as per the following reactions (Ca₂Fe₂O₅→2Fe+2CaO+1.5O₂, 5CaFe₂O₄→0.5Ca₂Fe₂O₅+Ca₄Fe₉O₁₇+0.25O₂) (Ismail et al. 2016). Hence, the results of the present study shows that RSA ash could be a potential oxygen carrier. However, further modifications are required to improve its durability such that it can be efficiently used over multiple cycles for commercial purpose.