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Recently the use of algae for CO 2 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 were 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 H 2 yield increased and CO 2 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/CO 2 /H 2 yield as compared to coal alone. Samples of fly ash, bed ash, and bed material agglomerates were analysed 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 co-gasification 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 of 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 of agglomeration. Debersia was suggested to be a potential alternative fuel which can be co-gasified 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 co-gasification process.

In this work, the emission of particulate matter (PM) from combustion of agricultural biomass was investigated in comparison to woody biomass. The mechanism of PM emission was studied by means of mass-based particle size distributions (PSDs), inorganic elemental component analysis and morphology at variant combustion temperatures, and different biomass feedstocks. The mass-based PSDs of PM 10 of cotton stalk, rice husk, and camphor wood exhibit a bimodal distribution, while that of corn stalk exhibits a unimodal distribution. The emission of PM 10 of agricultural biomass is much higher than that of woody biomass, and it is mainly composed of PM 1 , in which Na and K are enriched as alkali metal chloride and sulfide. On the other hand, Mg and Ca are enriched as the main inorganic compounds in PM 1−10 for woody biomass. A higher combustion temperature is favorable for the formation of fine PM particles against a reduction of PM 10 . PM 1 and PM 1−10 formation mechanisms are different for different biomass feedstocks, and their formation pathways are hereby proposed for each biomass resource.

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Algae utilization in energy production has gained increasing attention as a result of its characteristics, such as high productivity, rapid growth rate, and flexible cultivation environment. In this paper, three species of algae, including a fresh water macroalgae, Oedogonium sp., a saltwater macroalgae, Derbersia tenuissima, and a microalgae species, Scenedesmus sp., were studied to explore the potential of using smaller amounts of algae fuels in blends with traditional woody biomasses in the gasification processes. Co-gasification of 10 wt % algae and 90 wt % Swedish wood pellets was performed in a fluidized bed reactor. The effects of algae addition on the syngas yield and carbon conversion rate were investigated. The addition of 10 wt % algae in wood increased the CO, H 2 , and CH 4 yields by 3−20, 6−31, and 9−20%, respectively. At the same time, it decreased the CO 2 yield by 3−18%. The carbon conversion rates were slightly increased with the addition of 10 wt % macroalgae in wood, but the microalgae addition resulted in a decrease of the carbon conversion rate by 8%. Meanwhile, the collected fly ash and bed material samples were analyzed using scanning electron microscopy combined with an energy-dispersive X-ray detector (SEM−EDX) and X-ray diffraction (XRD) technique. The fly ashes of wood/marcoalgae tests showed a higher Na content with lower Si and Ca contents compared to the wood test. The gasification tests were scheduled to last 4 h; however, only wood and wood/Derbersia gasification experiments were carried out without significant operational problems. The gasification of 10 wt % Oedogonium N+ and Oedogonium N− led to defluidization of the bed in less than 1 h, and the wood/Scenedesmus (WD/SA) test was stopped after 1.8 h as a result of severe agglomeration. It was found that the algae addition had a remarkable influence on the characteristics and compositions of the coating layer. The coating layer formation and bed agglomeration mechanism of wood/macroalgae was initiated by the reaction of alkali compounds with the bed particles to form low-temperature melting silicates (inner layer). For the WD/SA test, the agglomeration was influenced by both the composition of the original algae fuel as well as the external mineral contaminations. In summary, the operational problems experienced during the co-gasification tests of different algae− wood mixtures were assigned to the specific ash compositions of the different fuel mixtures. This showed the need for countermeasures, specifically to balance the high alkali content, to reach stable operation in a fluidized bed gasifier.

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In this work, the effects of operating conditions of hydrothermal carbonization on the hydrochar pelletization and combustion characteristics were investigated. Corn stalk was carbonized under different conditions and then pelletized to obtain the hydrochar pellets. It was found that hydrothermal temperature and residence time greatly affect the pellet quality. When the temperature was raised up to 240 °C with the residence time longer than 60 min, the heating values of hydrochar were close to or even higher than those of lignite. After hydrothermal treatment, 73.71−94.71% K and 91.81−94.32% Cl contained in the feedstock were removed, indicating a low fouling and slagging tendency when the pellets are used in combustion. The compressive strength and durability increased first with increasing temperature and then decreased with further increasing the temperature from 240 to 300 °C. The influence of residence time showed a similar trend, and the compressive strength and durability reached its maximum value at the temperature of 240 °C and residence time of 60 min. The hydrophobicity of the hydrochar pellets increased with increasing the temperature and residence time. Hydrochar pellets obtained at the temperature of 240 °C with residence time of 60 min gives the best performance, which can meet the requirement of industrial fuel pellets. Finally, the combustion characteristics were investigated by thermogravimetric analysis, and the results indicated that hydrochar pellets were combusted in a comparatively mild way with a high thermal efficiency. As a general conclusion, the hydrochar pellets have much better qualities than the raw corn stalk, facilitating the transportation, longterm storage, and combustion application.