Gang_2014_fluidized.pdf (5.1 MB)
Fluidized bed gasification of raw and torrefied Miscanthus x giganthus
thesis
posted on 2022-09-19, 14:30 authored by Gang XueMiscanthus×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.
History
Degree
- Doctoral