2.2. Kraft lignin
Kraft lignin from the LignoBoost process (Metso)was used as a pellet
additive. This way of extracting lignin by pulping black liquor, producing
amaterialwith high energy density and low ash content, has been developed
by Innventia AB and Chalmers University of Technology in Sweden.
The process is based on gradually lowering the black liquor pH. Other
steps included in this process are filtration, washing and dewatering.
About 50–70% of the lignin in the treated black liquor can be extracted
in this way. For typical properties of kraft lignin fromthe LignoBoost process,
see Table 1. The pellet additives used for this work are dried lignin
(b10% moisture content [wb]), and lignin received directly from the
LignoBoost process (30–40% moisture content [wb]).
2.3. Production equipment
The pellets were produced in a production unit located at the
Department of Energy, Environmental and Building Technology at
Karlstad University, Sweden (see Fig. 1). It consists of: (1) a diagonal
mixer, (2) a conveyor screw, (3) an inlet feeder where conditioning
takes place – if needed, (4) an Amandus Kahl C33-390 pelletizing press
with a flat die and a maximum output of 300 kg/h – a description of the
machine can be downloaded from www.akahl.de/akahl/en/products/
biomass_pelleting/, and (5) a volumetric feeder for additives, and a
cooling tower.
In the pelletizing press, the sawdust is pressed through the flat die
by pan grinder rollers. The die has a nine hole radius with 52 holes
on each row, i.e., a total of 468 holes. It has a working width of
75 mm, a hole diameter of 8 mm, a total thickness of 50 mm, a relief
depth of 20 mm, and a hole inlet diameter of 10.2 mm. The inlet tapers
17° and it has no cutting blade. The open area of the die is 64%. The
effective compression length was chosen to be 30 mm for this study.
The additive was supplied to the inlet screw feeder (3) using a
volumetric feeder (5). The volumetric feeder consists of a hopper