3. Results and discussion
3.1. The thermo-t process
Fig. 3 shows the schematic of the overall process. Initially, the
biomass is converted to bio-oil, a char containing the ash and a
gaseous stream. Typically, the calorific value of the gas and the
char is sufficient to drive the overall pyrolysis process. Also, virtually
all the inorganic material stays with the char at this stage.
The details of the pyrolysis are published elsewhere indicating
that the bio-oil yield are around 50–60 wt.% [30,31]. The ash-free
bio-oil is then entering the Thermo-t process where it is split into
a light bio-crude, an ash-free bio-coke (BC) and a bio-gas. The
bio-gas has some calorific value and can to a large degree offset
the heating costs of the Thermo-t process. The Thermo-t conversions
are published elsewhere [32,33] and the coke yields are
typically about 30–40 wt.%. Hence, about 15–25 wt.% of the original
dry biomass can be converted into an ash-free bio-coke for
co-combustion.
3.2. Proximate and ultimate analysis
The elemental analyses and high heating values of the raw
materials, starting bio-oils and their bio-coke products are compared
with the bituminous coal and lignite in Table 1. As expected
from a biomass, both RSM and WSG have very high oxygen contents
of 44 and 46 wt.% with correspondingly low HHV of 19 and
18 MJ/kg, respectively. The bio-oils originated by pyrolysis show
oxygen content partially decreased (35 wt.% averagely), low ash
content, moisture content ranging from 9 to 16 wt.% and calorific
values ranging from 23 to 27 MJ/kg.
On the contrary, all the bio-cokes show very low oxygen
contents ranging from 10 to 14 wt.% with HHV in the range from
29 to 37 MJ/kg. Taking into account the overall conversion rate of
biomass into bio-coke of 15–25 wt.% this result indicate that about
20–40% of the heating value of the original biomass is retained in
the coke. The nitrogen content of the original biomass samples
appears to be retained in the bio-coke especially for RSM where
it increased from 6.5% to 10–11%. This might be of great concern
for co-firing due to increase in fuel NOx. Compared to the bituminous
coal in Table 1 it is clear that the bio-cokes have very similar
physical properties to that of the bituminous coal and far exceeds