Page 74 - Materials Chemistry, Second Edition

Life Cycle Assessment of Beneficial Reuse of Waste Streams 55

4.3.2 pyrolysis anD oTHer THerMal cHeMical processes To TreaT sluDge
Besides incineration, several thermal chemical processes, including pyrolysis, gas-
ification, and liquefaction, have been proposed as an energy source using dewatered
or digested biosolids in wastewater treatment plants. Although they have still only
been developed at pilot scale, these technologies have attracted increasing attention,
as they can produce liquid fuels such as petroleum and diesel, which can be directly
used in vehicles and replace fossil fuels.
Pyrolysis is a process in which the decomposition of biosolids is facilitated by
high temperatures in anaerobic conditions. For pyrolysis to have maximum yields
and efficiency, the biosolids must first be pretreated to the desired moisture content
(<10%) and particle size. If the feedstock has an overabundance of moisture, it must
be dried first. If the particle size is too large, it will need to be reduced in size before
continuing the process. The pretreated biosolids are added to the pyrolysis reactor,
where the conditions are anaerobic. In general, the pyrolysis of organic substances
produces gas and solid residue, char, which is high in carbon content. Next, the raw
gases and char are separated. The gases are then quenched with cold water. In this
step, the cold water will quickly cool the gases, and oil vapor will be condensed
into bio-oil. The non-condensable gases are collected and recycled back into the
pyrolysis reactor (Speight, 2008). The operation temperature of pyrolysis is around
300–600°C, and the pressure is 0.1–0.5 MPa.
The yield of bio-oil, gas, and char depends on many factors, including operating
temperature, pressure, time, and heating speed as well as the composition of feed-
stocks. Generally, bio-oil yields are high in conditions of fast heating, high tempera-
ture (500–1300°C), and low pressure (50–150 bars). For example, using a fluidized
bed, Fonts et al. (2008) conducted that pyrolysis of sewage sludge at the tempera-
ture of 540°C could obtain the maximum liquid yield of about 33 wt.%. By using
a pyrolysis centrifugal reactor (PCR), Trinh et al. (2013) obtained the maximum
liquid yield of about 41 wt.% on a dry ash free feedstock basis (daf) and a sludge oil
energy recovery of 50% at the temperature of 575°C. Chang et al. (2016) completed
a pilot-scale pyrolysis experiment on municipal sludge and proceeded to operational
effectiveness evaluation, and the result showed that the optimal operating condi-
tions were a pyrolysis temperature of 450–500°C and a pyrolysis time of 30–40 min
(Chang et al., 2016).
The bio-oil collected in pyrolysis has to go through the thermal upgrading pro-
cesses to be converted to commercialized transportation fuels. The upgrading usu-
ally includes two steps. First, the hydro treating removes impurities that could affect
downstream equipment. H is imported and added at a rate of 5 wt.% of bio-oil.
2
The hydrocracking process then breaks down heavy molecules into shorter-chain
fuels, diesel and gasoline. The yields of biofuels depend on the composition of the
bio-oil. The total fuel yield varies from 40 to 80 wt.% of raw bio-oil sent to upgrad-
ing (NREL, 2010). As sludge-derived bio-oil upgrading has not been well studied,
the exact biofuel yields and quality are still unclear. Many studies proposed to send
bio-oil to a local oil refinery where bio-oil can be combined with regular petroleum
raw oil for upgrading. In this situation, the impact of bio-oil on the final products
will be mitigated.