Cogasification of Australian brown coal with algae in a fluidized bed reactor

Abstract

Recently, the use of algae for CO₂ abatement, wastewater treatment, and energy production has increasingly gained attention worldwide. In order to explore the potential of using algae as an alternative fuel as well as the possible challenges related to the algae gasification process, two species of macroalgae, Derbesia tenuissima and Oedogonium sp., and one type of microalgae, Scenedesmus sp. were studied in this research. In this work, Oedogonium sp. was cultivated with two protocols: producing biomass with both high and low levels of nitrogen content. Cogasification of 10 wt % algae with an Australian brown coal was performed in a fluidized bed reactor, and the effects of algae addition on syngas yield, ash composition, and bed agglomeration were investigated. It was found that CO and H₂ yield increased and CO₂ yield decreased after adding three types of macroalgae in the coal, with a slight increase of carbon conversion rate, compared to the coal alone experiment. In the case of coal/Scenedesmus sp, the carbon conversion rate decreased with lower CO/CO₂/H₂ yield as compared to coal alone. Samples of fly ash, bed ash, and bed material agglomerates were analyzed using scanning electron microscopy combined with an energy dispersive X-ray detector (SEM-EDX) and X-ray diffraction (XRD). It was observed that both the fly ash and bed ash samples from all coal/macroalgae tests contained more Na and K as compared to the coal test. High Ca and Fe contents were also found in the fly ash and bed ash from the coal/Scenedesmus sp. test. Significant differences in the characteristics and compositions of the ash layer on the bed particles were observed from the different tests. Agglomerates were found in the bed material samples after the cogasification tests of coal/Oedogonium N+ and coal/Oedogonium N−. The formation of liquid alkali−silicates on the sand particles was considered to be the main reason for agglomeration for the coal/Oedogonium N+ and coal/Oedogonium N− tests. Agglomerates of fused ash and tiny silica sand particles were also found in the coal/Scenedesmus sp. test. In this case, however, the formation of a Fe−Al silicate eutectic mixture was proposed to be the main reason for agglomeration. Debersia was suggested to be a potential alternative fuel, which can be cogasified with brown coal without any significant operating problems under the current experimental conditions. However, for the other algae types, appropriate countermeasures are needed to avoid agglomeration and defluidization in the cogasification process.