Environmental Engineering Reference
In-Depth Information
P
S
1
S
2
S
3
ML
TMP
MDF
CTMP
RMP
FIGure 15.3
A schematic diagram showing various fiberization mechanisms of wood. (Adapted from
Franzen, R.,
Nordic Pulp Paper Res J,
1, 4, 1986; Salmen,
L.,
Fundamentals of Mechanical Pulping,
in Book
5: Mechanical Pulping, Papermaking Science and Technology, Fapet Oy, Finland, 1999. Used with permission
of Nordic Pulp and Paper Research Journal and Finland Paper Engineers' Association.)
with wood chips fractured through the lumen of wood tracheid. Thermomechanical pulps (TMP)
are produced using low-pressure steam (~2.4 × 10
5
Pa, ~134°C) to soften wood chips before disk
refining. The wood chips are fractured in the S1 and S2 layer of cell wall.
Medium-density fiberboard pulps (MDF) are produced under increased steam pressure of >5 × 10
5
Pa. In the MDF production process, wood chips are fractured in the lignin-rich middle lamella (ML).
This is because the steam temperature reaches the glass transition temperature of lignin (Irvine
1985). Figure 15.3 is a schematic of various fracture mechanisms of wood chips during fiberization.
Energy consumption of different pulping processes varies significantly. Typical energy consumptions
for producing RMP, TMP, and MDF are about 600, 450, 150 Wh/kg oven-dried wood, respectively.
Energy consumption for chemical-thermomechanical pulp (CTMP) is just lower than that for TMP.
The surface chemical compositions of these pulps are very different. RMP exposes mostly cellulose
on fiber surfaces. MDF fibers are lignin-coated on their surfaces. This can be clearly seen from the
color of these pulps, with RMP being the lightest and MDF being brown. The difference in surface
chemical composition certainly affects cellulose enzymatic conversion to glucose, as revealed in our
previous study (Zhu et al. 2009). The significant variations in mechanical energy consumption of
these different pulping processes may provide avenues for potential energy savings in biomass size
reduction. However, attempts have not yet been taken to explore this potential.
15.6.1.3 effect of chemical Pretreatment
The third factor affecting size-reduction energy consumption and enzymatic cellulose saccharifica-
tion is chemical pretreatment. Most of the enzymatic cellulose saccharification research has used
size-reduced substrate for the purpose of reducing substrate recalcitrance to achieve high cellulose
conversion (Nguyen et al. 2000; Allen et al. 2001; Zhu et al. 2005). In fact, to achieve good chemical
penetration and therefore effective pretreatment, size reduction before chemical pretreatment is nec-
essary for the dilute acid process (Lynd 1996), one of the most investigated chemical pretreatment
processes for lignocellulosic ethanol production. Chemical pretreatment alters the chemical com-
position and physical structure of biomass by partly removing some cell-wall components, such as
hemicellulose and lignin. As a result, size reduction after chemical pretreatment can reduce energy
consumption. This energy savings may be insignificant for some agricultural biomass, such as corn
stover or switchgrass, but can be very significant for forest biomass (Zhu et al. 2010). This suggests
