Environmental Engineering Reference
In-Depth Information
the authors' knowledge, other than the published work by Xu and Lancaster (2008), no work
has been reported on treatment of pulp/paper secondary sludge in hot-compressed water or
sub-/supercritical water. As a result, liquefaction as a treatment method is still in the research
stages but could potentially be an effective approach to recover energy from the secondary
sludge waste for production of liquid oil products.
Xu and Lancaster (2008) experimentally treated secondary pulp/paper sludge in hot-
compressed or sub-/supercritical water at a temperature of 250-380
o
C in order to produce
liquid oils for energy recovery from the secondary pulp/paper sludge. An outline of a direct
liquefaction process for the treatment of sludge is pictured in Figure 6, where sludge is fed
into a pressure reactor where liquid products are primarily produced and gas products are
collected. Liquid products are a mixture of water-soluble oil (WSO) and heavy oils (HO) and
solid residue or char. Filtration is performed on the liquid product mixture in order to separate
the liquid oil products from the solid residue, char.
Figure 6. Outline of the direct liquefaction process (adapted from Xu and Lancaster, 2008)
The effects of liquefaction temperature, residence time, initial biomass concentration,
catalysts and liquefaction atmosphere (inert, N
2
or reducing, H
2
) on the liquefaction product
yields were investigated. Treatments of secondary pulp/paper sludge in water at 250-380°C
for 15-120 min in the presence of N
2
atmosphere resulted in yields of water soluble oils
(WSOs) at 20 wt% - 45 wt% and yields of heavy oils (HOs) at 15 wt% - 25 wt%, with HHVs
of 10-15 MJ/kg and >35 MJ/kg, respectively. For temperatures lower than 350
o
C, as
temperature increased the yield of heavy oil (HO) increased at the expense of the water-
soluble oil (WSO) formation, while as temperature increased further to above 350
o
C, the
yield of HO decreased, accompanied by an increase in the yield of WSO. An increased
residence time produced a greater yield of HO (reaching as high as 25wt% for 120min) but a
lower yield of water-soluble and a reduced yield of total oil. A higher initial biomass
