4. Discussion
The tests demonstrated that an increased amount of kraft lignin in
the pellets seems to increase their mechanical durability and their
length. This is due to the bonding mechanisms of lignin that act as a
binding agent [25]. The results also show that the addition of dry kraft
lignin results in pellets with a higher durability as compared to wet
kraft lignin. According to a problem inventory carried out by Ståhl &
Wikström [2], the amount of fines or the crumble effect on the pellets
in combination with incorrect equipment cause most problems during
usage. It is more important today than it was in the early pioneering
days to minimize these problems as new users are beginning to use
pellets as a source of household heating. The amount of fines or the
crumble effect is defined according to requirements in a standard.
Since 2010 there is a new European standard (SS-EN 15210 [26]) for
the mechanical durability of pellets. According to this standard the
demands on pellet durability must be equal to or above 97.5%. This
study, however, is based on SS 18 71 20 [20], according to which the
requirement pertaining to amount of fines is equal to or below 0.8%.
The method of testing mechanical durability according to SS-EN
15210 [26] is different from the test performed in this study. The result
of this study shows that pellets without kraft lignin would not conform
to the demands of either standard (see Fig. 4). By using 2% of dry kraft
lignin, pellets were produced that conformed to the new European
standards, and by using 4% of dry kraft lignin the results conformed to
both standards when it comes to durability level and amount of fines.
Even if the test methods of the standards differ, the result shows that
adding kraft lignin increases the mechanical durability of the pellets
and decreases the amount of fines. If the aim is to minimize the problems
during usage correlated to amount of fines or pellet crumble effect,
the adding of dry kraft lignin is clearly a useful method.
The increased amount of kraft lignin shows no clear effect on the
bulk density of pellets (see Fig. 3). This, however, could be the case
if kraft lignin and sawdust were of similar density. It is possible, too,
that the varying moisture content of the pellets affects the bulk density.
Fasina [27] showed that increased moisture content decreases
the bulk density. It has to be further examined which parameter
affects the bulk density the most. In conformity with Lehtikangas [28],
no correlation between pellet length and bulk density was found. Due
to the difficulty of carrying out the standard procedure, there is also a
large uncertainty in the measurements, since the standard can be
interpreted in different ways.