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 56

Table 2 Percent energy
retained in the torre ed
biomass

Biomass Purge gas

Nitrogen Argon Carbon dioxide

Mesquite 88.74 87.73 84.47

Juniper 83.68 83.16 78.86

The energy retention calculations show that torrefaction in a nitrogen environ-
ment resulted in the highest amount of energy being retained. Torrefaction using
argon as the purge gas had a slightly lower amount of energy retention when
compared to nitrogen. However, the energy retention of torrefaction in a carbon
dioxide environment shows that using carbon dioxide as a purge gas results in the
lowest energy retention for all three biomasses tested. The lower amount of energy
being retained after torrefaction using CO 2 is a result of more volatiles being
liberated via the Boudouard reaction.

3.2.5 Grindability Tests

The samples torrefied using the laboratory oven were then ground using an in-house
vibratory grinding mill. From the grindability tests, torre ed samples exhibited

improved grindability in comparison to raw biomass samples. For both mesquite
and juniper, torrefaction using CO 2 resulted in a higher fraction of the ground
biomass passing through an 840- m mesh size. Further grindability tests conducted

μ

on samples torre ed in a bigger batch reactor by Thanapal et al. showed that using

CO 2 as the torrefaction improved the grindability of torre ed samples by 30 %.

More details on those tests are available elsewhere (Eseltine 2011 ).

3.2.6 Torrefaction Model

Properties of the Biomass Components

The ultimate and proximate analyses of mesquite and juniper are presented in
Table 1 . In addition, ultimate and proximate analyses of hemicellulose and cellulose
were obtained from literature (Yoon et al. 2011 ; Van de Weerdhof 2010 ). The
composition of lignin was then determined from the fuel composition and the data
for hemicellulose and cellulose according to Eq. ( 1 ). Composition of cellulose and
hemicellulose is presented in Table 3 .
Based on the pyrolysis tests carried out on the three components, 32, 2.5, and
40 %, were left over as char from hemicellulose, cellulose, and lignin, respectively,
on a dry ash-free basis (Raveendran et al. 1996 ). The estimated bulk char per-
centage on a DAF% based on the percentage of hemicellulose, cellulose, and lignin
(Table 3 ) and the char content left after pyrolysis will be 15.64 and 17.44 % for
hard wood mesquite and softwood juniper samples, respectively. The numbers