Page 75 - Materials Chemistry, Second Edition

56 Life Cycle Assessment of Wastewater Treatment

Char is co-produced in the pyrolysis process. Elements such as phosphorus and
magnesium are concentrated in the pyrolysis process; the char containing these ele-
ments can be used as a soil amendment or fertilizer (Smith et al., 2009). The pyroly-
sis and sludge drying processes consume energy during operation. Char has to be
burned to generate the amount of heat required for the drying and pyrolysis pro-
cesses. The energy required for drying depends on the water content of the sludge.
If the sludge has been dewatered or digested, the water content is around 70%–80%.
That sludge requires at least 2600 MJ of heat per cubic meter of wet sludge to reduce
the water content to 10%. In some cases, the heat from char is not enough for sludge
drying, so some bio-oil will need to be burned for heat as well. Although the oil yield
is reduced, the process can keep energy self-contained, and therefore, no fossil fuel
will be consumed.
Another thermal chemical conversion technology proposed is sludge gasification,
in which the sludge breaks into gaseous fuels called syngas in an oxygen-deficient
condition. The gasification has to operate at a higher temperature (800–1300°C) and
a higher pressure (>0.1 MPa) than pyrolysis. The syngas contains CO (15%–20%),
CO (10%–12%), H (20%–24%), CH (0%–4%), and N (48%–52%). As the gas
4
2
2
2
exits the gasifier at 300–400°C, it contains tar and particulates; therefore, it must
be cooled and cleaned. The syngas can be used directly for gas engines, or it can
be converted into biofuels. A very famous process that converts clean syngas into
transportation fuel is called the Fischer-Tropsch (F-T) process, in which H and CO
2
react to form liquid fuels (diesel/gasoline) with a catalyst. The gasification process
requires more operation energy than pyrolysis; however, the diesel produced through
gasification and F-T has higher quality; it is free of sulfur and nitrogen and low in
aromatics, which makes the process attractive.
Both pyrolysis and gasification need dry biosolids before the processes can begin,
which consumes a significant amount of energy. In the past 2 years, hydrothermal
liquefaction technology (HTL) has been introduced to treat sludge due to its ability
to treat biomass with moisture content up to 80–95 wt.% (PNNL, 2016). The energy-
intensive thermal drying process, therefore, could be eliminated in the production
pathway. In HTL, sludge is directly converted to liquid oil at a reaction temperature
of less than 400°C. Char, gas, and aqueous phases are co-produced in HTL. The
product yields depend on multiple factors, including operation temperature, pres-
sure, water content of sludge, and the catalyst. A study by the PNNL (2016) showed
that when primary sludge was treated, the product yields from daf sludge were 40
wt.% for oil, 35 wt.% for aqueous, 22 wt.% for gas, and 3 wt.% for char. The bio-oil
produced needed to be further updated with the hydrotreating and hydrocracking
processes. The energy in biochar and biogas from HTL was assumed to be recycled
into the heat and power system (HPS) as heat for in-plant use. The aqueous portion of
material from the HTL process containing C, N, and P was assumed to be recycled
to grow algae in the open pond.
Many LCAs have been conducted for the thermal technologies that produce bio-
fuels and bioenergy. Hospido et al. (2005) compared the environmental performance
of sludge treatment with anaerobic digestion (AD) followed by land application ver-
sus pyrolysis. The results showed that the AD process was better for eutrophica-
tion, global warming, and acidification, whereas pyrolysis was better for human and