Transitions - Sustainable Design: The Science of Sustainability and Green Engineering - page 139

Pyrolysis
Pyrolysis is the process of chemical decomposition induced in organic materials by
heat in the absence of oxygen. It is practicably impossible to achieve a completely
oxygen-free atmosphere, so pyrolytic systems run with less than stoichiometric
quantities of oxygen. Because some oxygen will be present in any pyrolytic
system, there will always be a small amount of oxidation. Also, desorption will
occur when volatile or semivolatile compounds are present in the feed.
During pyrolysis 18 organic compounds are converted to gaseous components,
along with some liquids, as coke (i.e., the solid residue of fixed carbon and
ash). CO, H 2 ,CH 4 , and other hydrocarbons are produced. If these gases cool
and condense, liquids will form and leave oily tar residues and water with high
concentrations of total organic carbon. Pyrolysis generally takes place well above
atmospheric pressure at temperatures exceeding 430 ◦ C. The secondary gases need
their own treatment, such as by a secondary combustion chamber, by flaring, and
by partial condensation. Particulates must be removed by additional air pollution
controls (e.g., fabric filters or wet scrubbers).
Conventional thermal treatment methods, such as a rotary kiln, a rotary hearth
furnace, or a fluidized-bed furnace, are used for waste pyrolysis. Kilns or furnaces
used for pyrolysis may be of the same design as those used for combustion (i.e.,
incineration) discussed earlier, but operate at lower temperatures and with less air
than in combustion.
The target contaminant groups for pyrolysis include semivolatile organic com-
pounds, including pesticides, PCBs, dioxins, and polynuclear aromatic hydro-
carbons (PAHs). It allows for separating organic contaminants from various
wastes, including those from refineries, coal tar, wood preservatives, creosote
and hydrocarbon-contaminated soils, mixed radioactive and hazardous wastes,
synthetic rubber processing, and paint and coating processes. Pyrolysis systems
may be used to treat a variety of organic contaminants that chemically decompose
when heated (i.e., “cracking”). Pyrolysis is not effective in either destroying or
physically separating inorganic compounds that coexist with the organics in the
contaminated medium. Volatile metals may be removed and transformed, but of
course the mass balance will not be changed.
Emerging Thermal Technologies
Other promising thermal processes include high-pressure oxidation and
vitrification. 19 High-pressure oxidation combines two related technologies, wet
air oxidation and supercritical water oxidation, which combine high tempera-
ture and pressure to destroy organics. Wet air oxidation can operate at pressures
of about 10% of those used during supercritical water oxidation, an emerging