No abstract

Torrefaction is suggested to be an effective method to reduce the cost of biomass provision and improve the fuel properties. In this study, both raw and torrefied Miscanthus × giganteus (M×G) were gasified in an externally heated air-blown bubbling fluidized bed (BFB) gasifier using olivine as the bed material. The effects of equivalence ratio (ER) (0.18–0.32) and bed temperature (660–850 °C) on the gasification performance were investigated. Torrefied M×G has higher energy density primarily due to a higher ratio of lignin to cellulose and hemicellulose; it has lower bulk density, smaller particle diameter and lower reactivity than the original biomass. These properties affect its performance during gasification. The cold gas efficiency was on average 12% lower for torrefied than for raw M×G for the range of operating conditions studied. Within the same temperature range the carbon conversion was about 10% higher for raw than for torrefied biomass. The hydrogen conversion was higher for torrefied M×G since gasification of this feedstock results in higher yields of methane and ethane and lower yields of unreacted process water. The carbon loss with char elutriated from the gasifier for torrefied M×G was significantly higher than that of raw (5% vs 3%) and was driven by physical properties of torrefied M×G. The results obtained suggest that chemical composition expressed as lignin to cellulose and hemicellulose ratio has a pronounced effect on carbon conversion efficiency and tar production.

Gasification of Cynara cardunculus L. was performed in a bubbling fluidized bed (BFB) using air as the gasifying agent and magnesite and olivine as different bed materials. Temperature was varied during the experiments (700-800 ºC) with fixed biomass feeding and air flow rates.The effect of using the magnesite and olivine on the gas and tar composition, carbon and biomass conversion, and cold gas efficiency was investigated. The product gas showed high hydrogen content (13-16 %v/v) for both magnesite and olivine in the temperature range studied.Higher heating value and gas yield were improved with increasing the temperature from 700 to 800 ºC. Biomass and carbon conversion were greater than 75%, giving values higher than 90 % for both 700 and 800 ºC in magnesite and for 800 ºC in olivine. Indane and cresols were the main tar compounds at low temperature while naphthalene was the dominant tar species at the high temperatures. Gasification performance was better with magnesite at 700 ºC while olivine showed better properties at 800 ºC.

Gasification of Miscanthus x giganteus (MxG) was conducted in an air-blown bubbling fluidized bed (BFB) gasifier using magnesite as bed material and a moderate rate of biomass throughput (246.82–155.77 kg/m2h). The effect of equivalence ratio (ER) (0.234–0.372) and bed temperature (645–726 °C) on the performance of gasification was investigated. The results reveal that MxG is a promising candidate for energy production via BFB gasification; of the conditions tested, the optimal ER and temperature are approximately 0.262 and 645 °C, where no sign of agglomeration was found. The product gas from this condition has a higher heating value of 6.27 MJ/m3, a gas yield of 1.65 N m3/kgbiomass (39.5% of CO and 18.25% of H2 on N2 free basis), a carbon conversion efficiency of 94.81% and a hot gasification efficiency of 78.76%. Agglomeration was observed at some higher temperature conditions and believed to be initiated by the formation of fuel-ash derived low melting temperature K-rich (potassium) silicates (amorphous material that cannot be detected by XRD). It is suggested that relatively low temperature (650 °C) needs to be used for the gasification of MxG to avoid potential agglomeration.

Torrefaction is suggested to be an effective method to improve the fuel properties of biomass and gasification of torrefied biomass should provide a higher quality product gas than that from unprocessed biomass. In this study, both raw and torrefied Miscanthus × giganteus (M×G) were gasified in an air-blown bubbling fluidized bed (BFB) gasifier using olivine as the bed material. The effects of equivalence ratio (ER) (0.18-0.32) and bed temperature (660-850°C) on the gasification performance were investigated. The results obtained suggest the optimum gasification conditions for the torrefied M × G are ER 0.21 and 800°C. The product gas from these process conditions had a higher heating value (HHV) of 6.70 MJ/m(3), gas yield 2m(3)/kg biomass (H2 8.6%, CO 16.4% and CH4 4.4%) and cold gas efficiency 62.7%. The comparison between raw and torrefied M × G indicates that the torrefied M × G is more suitable BFB gasification.