The
major drawback of biomass for direct combustion applications
is its lower heating values and poor grindability when compared to
conventional fossil fuels. Torrefaction is one of the thermochemical
pretreatment techniques used to improve the properties of biomass.
An inert environment is maintained to prevent oxidation of biomass
during torrefaction. A novel method for utilization of carbon dioxide
as the pretreatment medium for woody biomass has been investigated
in the current study. Previous studies on smaller samples using thermogravimetric
analysis (TGA) showed an increased mass loss in the CO2 environment, which was attributed to possible structural changes
in the biomass and potential effect of ash constituents in the biomass.
However, those claims were not validated. The current study on bigger
batches of samples also resulted in increased mass loss when using
CO2 compared to using N2 as the torrefaction
medium. Scanning electron microscopy (SEM) studies and Brunauer–Emmett–Teller
(BET) surface area investigation on the torrefied samples from different
environments showed increased internal surface area indicating a mild
effect of the CO2 reacting with the samples at temperatures
normally employed for torrefaction conditions (200–300 °C).
Further, grindability studies were performed on the samples pretreated
in CO2 and torrefied in N2. The results on grindability
showed improved grindability on using CO2 as the pretreatment
medium. Proximate and heating value analysis on the pretreated samples
showed an increasing trend in the heating value of the samples with
increased temperature. Comparable mass loss at lower temparatures
improved grindability and improved fuel properties, makes utilization
of carbon dioxide as a torrefaction medium for pretreating biomass
for combustion applications an attractive technology.
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