Environmental Engineering Reference
In-Depth Information
bond will occur at a high temperature, leading to formation of gas products (mainly CO, CO
2
,
H
2
and CH
4
). Accordingly, pyrolysis process can be tuned to produce char, oil and/or gas by
properly selection of the operating conditions of temperature, heating rate and reaction time.
If the purpose is to maximize the oil yield then a high heating rate and short gas residence
time would be required, while for high char production, a low temperature and low heating
rate would be preferred. High yield of bio-oil up to about 70-75% (Agblevor et al., 1995) can
be produced in fast pyrolysis processes (with very short residence time and elevated reactor
temperature of ~500°C or higher). The commonly used reactors for fast pyrolysis include
bubbling fluidized bed, circulating fluidized bed, ablative, entrained flow, rotating cone
reactors, and vacuum reactors (Mohan et al., 2006). A significant amount of char and equal
amounts of oil and gas products can be obtained in slow pyrolysis processes (operating at a
low temperature for a long residence time). The char produced typically has a higher heating
value of ~ 30 MJ/kg, which can be used as a valuable fuel for generating heat and electricity,
or can be turned into activated carbon by activation. Pyrolysis oils are normally composed of
a variety of organic oxygenates and polymeric carbohydrate and lignin fragments derived
from the thermal cracking of cellulose, hemi-cellulose and lignin components of the biomass
(Mohan et al., 2006; Tsai et al., 2007). The physical properties of wood fast-pyrolysis oil are
compared with those of a petroleum-based heavy fuel oil in Table 2 (Czernik and
Bridgewater, 2004). As shown in this Table, pyrolysis oil contains a high concentration of
water (15-30%), and is highly acidic (corrosive) and unstable liquid with a lower caloric
value of 16-19 MJ/kg compared with 40 MJ/kg for the petroleum-based heavy oil. Pyrolysis
oil is a potential liquid fuel for turbines and boilers, or it can also be applied to produce
chemicals directly, or be upgraded to high-quality fuels by hydro-cracking or catalytic
cracking (Vitolo et al., 1999).
Figure 4. Chemistry of biomass pyrolysis (Lange, 2007)
Pyrolysis has long been recognised more advantageous over conventional incineration
processes for the treatment of sewage-sludge with respect to fuel economy, energy recovery,
and the control of heavy-metal emissions (Lewis, 1975). However, process efficiency is
affected by sludge moisture content, such that co-pyrolysis with other wastes has been
recommended in order to increase the dry-solids content of the sludge (Olexseyr, 1975). The
process normally needs additional treatment to remove excess water. Higher water content
feedstocks cause increases in the production of hydrogen and methane, but these do not
compensate for the losses of carbon monoxide and thermal efficiency (Carre, et al., 1989).
Water reduction is accomplished by dewatering to about 25% DS followed by thermal drying
to 95% DS.
