Page 138 - New Trends In Coal Conversion

Coal and biomass cofiring: CFD modeling 101

shows the straw particle samples collected before injection into the boiler. The straw
particles are large and highly nonspherical: a mean length of about 16 mm and a
maximum length of 180 mm, as detailed in (Yin et al., 2004; Rosendahl et al., 2007).
In the latest retrofits for cofiring biomass in suspension-fired boilers, biomass pel-
lets, rather than raw biomass, are transported to power plants and then milled and fired
in the plants. For example, for biomass suspension cofiring in Amager unit 1 in
Denmark (Mandø et al., 2010), the commercially available wood pellets from different
suppliers are mixed in the silo and are ground in the traditional coal roller mills without
changing any setting in the mills (as illustrated in Fig. 4.4). Some roller-milled samples
that are collected from the transport pipe to a burner in the plant are shown in Fig. 4.4.
The same wood pellets are also ground in hammer milling trials, as illustrated in
Fig. 4.5, in which some of the milled samples are shown.
Fig. 4.6 shows an example of the cumulative particle size distribution measured by
the conventional sieving analysis for the roller-milled and hammer-milled wood pellet
samples. Milling of biomass pellets is found to result in much smaller particles than
milling of raw biomass, and the hammer mill produces smaller particles than the con-
ventional coal roller mill does. However, the biomass particles are still large in size and
nonspherical in shape, which induces differences in modeling of biomass particle mo-
tion and conversion in a suspension-fired furnace.

Wood pellets
(commercially =2 mm =3 mm
d p d p
available)

Roller-milled Roller-milled
Roller-milled wood pellet samples, collected from
a transport pipe to a burner in a co-firing plant
Milled samples
Figure 4.4 Roller milling of wood pellets and the milled samples.

Wood pellets
d =2 mm d p =3 mm
p

Hammer-milled Hammer-milled
Hammer-milled wood pellet samples,
collected from hammer-mill trails
Milled samples
Figure 4.5 Hammer milling of wood pellets and the milled samples.