Fluidized bed gasification of raw and torrefied Miscanthus x giganthus

Miscanthus×giganteus (M×G), a typical bioenergy crop, has been widely planted in Ireland. But the virgin miscanthus is considered as a low grade fuel. The aim of this study is to investigate a method to (a) upgrade the fuel properties of raw miscanthus via torrefaction; (b) compare the gasification of raw and torrefied M×G in the allothermal air-blown bubbling fluidized bed gasifier. The M×G was, firstly, torrefied at a series of temperatures from 230 to 290 oC for residence times varying from 10 to 30 min. The torrified biomass showed reduced moisture and hemicellulose content a lower ratio of O/C, a more porous structure with larger specific surface area as well as higher mass ratio of alkali metals. All these features have positive effect on gasification reactivity. The optimal process conditions for torrefaction of M×G are: 250°C and 30 min from which condition the torrefied M×G had a mass yield of 71.6%, energy yield of 85.1% and higher heating value of 21.81 MJ/kg. Its reactivity was higher than other torrefied samples and then that of raw M×G. A preliminary gasification campaign was carried out in an onsite allothermal air-blown bubbling fluidized bed (BFB) gasifier using raw M×G as feedstock and magnesite as bed material. 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 showed that the optimal ER and temperature were approximately 0.262 and 645 °C (in this study). The product gas from this condition had a higher heating value of 6.27 MJ/m3, a gas yield of 1.65 N m3/kgbiomass (including N2), a carbon conversion efficiency of 94.81% and a hot gasification efficiency of 78.76%. The agglomeration was observed at higher temperature tests. The next stage of research involved comparing the gasification of raw and torrefied M×G. The experiments were conducted in an allothermal air-blown BFB gasifier using olivine as bed material at ECN in Netherlands. The optimal ER for the raw and torrefied M×G were 0.22 and 0.21 respectively. Both raw and torrefied materials had the same optimal temperature (800 oC). The risk of agglomeration seems inevitable if the temperature was increased higher than 800 oC. The comparison of raw and torrefied M×G gasification from their optimal conditions revealed that there was no significant difference between the raw and torrefied M×G gasification from the perspective of product gas quality. The gasification performance of raw M×G was slightly better than that of torrefied biomass. It probably could be attributed to the short retention time and high char elutriation rate for the torrefied M×G gasification. However, the value of torrefaction should not be ignored. Torrefaction makes it possible to use low grade biomass in the existing energy producing systems which in turn promotes the use of renewable energy and ease the impact of greenhouse gas emission. Overall, the results reveal that M×G is a promising candidate for energy production via BFB gasification. Torrefaction is a promising thermal pretreating method to improve the fuel properties for the thermo-chemical converting process.