Biomass Fuel Quality Enhancement and Respiratory Quotient (RQ) for Ranking Fossil and Biomass Fuels Based on CO2 Emissions - Novel Combustion Concepts for Sustainable Energy Development - page 54

previous experiments with mesquite. The slight differences are attributed to the
variations in the samples tested. Comparison of the thermograms using nitrogen and
carbon dioxide as the purge gas shows that using carbon dioxide results in a higher
amount of weight loss.

3.2.3 Optimum Torrefaction Temperature

Based upon the TGA thermograms, an optimal torrefaction temperature can be
determined. From the TGA thermograms presented for each biomass and purge gas
at 240 C, more than 70 % of the initial mass of the biomass remains after torre-

faction. This is independent of both biomass and purge gas. Tests conducted at a
torrefaction temperature of 260

°

65 % of the biomass
remaining. Furthermore, an estimated percent of energy retained after torrefaction
can be calculated based upon the raw biomass fuel properties and the TGA weight
traces. The energy retention estimates were calculated based upon the HHV of the
fuel on a dry ash-free (DAF) basis and the amount of

° C resulted in roughly 58

-

fixed carbon (FC) and volatile
matter (VM) in the biomass. If we use the DAF HHV of the biomass, the energy
content comes entirely from the VM and FC and the following relation can be used.

¼

kJ
kg

HHV DAF HHV VM VM þ HHV FC FC ð 13 Þ

If we assume that the mass lost during torrefaction is entirely due to liberation of
volatiles then the HHV of the torre ed biomass can be estimated as

¼

kJ
kg

HHV DAF ; TB ð wt % VM HHV VM Þ þ ð HHV FC FC Þ ð 14 Þ

where wt% represents the weight remaining after torrefaction. Using the estimation
proposed above to determine the HHV of the torre ed biomass on a DAF basis, it is

possible to estimate the amount of energy retained after torrefaction using the
following relation (Eseltine 2011 ).

M TB HHV TB

Energy Retained ð%Þ¼ 100 ð 15 Þ

M RB HHV RB

Using data from the heat value analysis of the raw biomass, which gives the
HHV of the VM, with the proximate analysis results an estimated HHV of the
torre ed biomass can be calculated and therefore an estimated energy retained %.

Based upon the mass loss observed from TGA thermograms as well as the esti-
mated energy retention calculations,

the optimal torrefaction temperature was

determined to be 240 C. For all of the biomass tested in each purge gas envi-

ronment, torrefaction at 240

°

C results in over 85 % energy retention as well as
70 % of the initial mass remaining.

°