930 PCBs AND ASSOCIATED AROMATICS
been extensively tested on 900,000 Kg of dioxin contami-
nated soil and 80,000 Kg of liquid waste and found to have
a destruction and removal efficiency which meets the EPA
criterion of 99.99999% while operating at a feed rate of over
2000 Kg per hour.
ENSCO Corporation has developed a mobile rotary kiln
incinerator which has been used to treat up to 100 tons per
day of contaminated soil. The unit operates in the range of
500° to 1000°C. The off-gas from the rotary kiln has a resi-
dence time of over two seconds in a secondary combustion
chamber operating at 1000° to 1300°C in an atmosphere
with excess oxygen. Acid gases produced in the combustion
air are removed with a countercurrent flow of water which is
passed through activated carbon filters and then neutralized.
The unit operates at a feed rate of about 6 tons per hour and
has been demonstrated to achieve 99.9999% destruction and
removal efficiency in an 11,000 ton filed trial.
The J.M. Huber Corp. in Borger, Texas began work-
ing with Thagard Research Corporation on high tempera-
ture fluid wall (HTFW) reactor technology in 1976 and
acquired the patents from Thagard. The general operating
principle of the process is to rapidly heat materials to tem-
peratures in excess of 4,000°C with a residence time of
tenths of a second. The energy input to the waste measured
as the integral over temperature and time is then sufficient
to obtain the DREs required by RCRA. The technical
problem posed in the application of the principle is that
the reaction vessel must be able to withstand the operat-
ing conditions without becoming chemically or physically
degraded. The core is a porous refractory cylinder heated
externally by electric elements so that wastes fed into it
are heated by radiant energy. The core material allows the
permeation of a gas such as nitrogen through the core wall
into the interior. The gas is admitted radially to produce an
annular envelope which blankets the walls and reduces the
contact of the reactants with the reactor. This device tends
to avoid the degradation of the reactor. Nitrogen is used
partly for the reason that it is transparent to the radiant
infrared energy and also because the production of par-
tially oxygenated pyrolysis products, such as PCDFs and
PCDDs, is avoided.
In Huber’s “Advanced Electric Reactor” a graphite core
is maintained at 2200°C with a fluid wall of gaseous nitro-
gen to avoid oxidation of the carbon. The burned material
emerges into a sealed steel container and the offgases are
vented through activated carbon cylinders. Fixed site and
mobile reactors based on the HTFW technology have been
found effective on 2,3,7,8-TCDD contaminated soils. The
capacity of the pilot plant is 5,000–15,000 tons/yr with costs
in the range $77 to $385/ton.
Plasma pyrolysis of PCBs has been investigated (Barton
et al.^92 ) by both Lockheed Aerospace and the Royal Military
College in Kingston, Ontario. The principle is applied on
a large scale for the recovery of metals from waste dust
generated in steel mills. A plasma consists of both charged
and neutral particles exhibiting collective behaviour with
an overall charge of close to zero. A common method of
plasma generation is electrical discharge through a gas.In the case of a plasma torch, the plasma is stabilized by
collimation and resembles the heat source used for atomic
emission spectroscopy. Arc temperatures can occur of up
to 50,000°K and the material entering the plasma tends to
become atomized and ionized.
The overall efficiency of the process depends upon the
conversion of the plasma electrical energy into thermal energy.
Large scale generators convert about 85–90% of the electrical
energy into usable heat and are therefore useful for the rapid
decomposition of materials which would otherwise require
a long residence time for complete pyrolysis. Nevertheless,
extensive work has been done to determine the conditions
necessary to avoid the release of chlorinated compounds with
molecular weights larger than tetrachlorobenzene.
Pyrolysis Systems Inc. in Welland, Ontario has built the
prototype mobile unit for the Department of Environmental
Conservation of New York State, which is based on the work
done at the Royal Military College. The unit has a through-
out rate of one gallon per minute and has shown destruc-
tion efficiencies for askarel of the order of 99.9999999%.
Atomization of fluids has been estimated to take place in
less than one third of a millisecond. The estimated cost
is about $300,000 per unit with electrical consumption
of about 1.2 kWh/kg of feed. This is about one-tenth the
energy cost of incineration. The system is not designed to
destroy solid material.
The SKF Steel engineering plant in Hofors, Sweden is
constructing a 6 MW plasma generator designed to treat
70,000 tons/yr of waste dust from the steel mill. The cost of
the facility is estimated at $23 million dollars. Thus, there
is no doubt that the plasma technology can be scaled up but
the size of the associated generators and equipment imposes
a limitation on its mobility. In the case of the treatment of
PCB wastes the siting of a fixed facility is often a difficult,
if not impossible issue to resolve so that the limitation of the
method in terms of throughput rate is governed by political
factors rather than technical considerations.
The circulating bed combustor manufactured by GA
Technologies Inc., is an application of fluidized bed technol-
ogy to the disposal of hazardous wastes. In a fluidized bed
unit the temperature is limited to the melting point of the inert
support material. In the case of sand this is about 1100°C.
The bed acts as a large radiative surface to produce high
destruction efficiencies as well as several operating advan-
tages over other incinerator designs. Ogden Environmental
Services uses a design in which a high velocity air flow sus-
pends the fluidized bed to give a turbulent combustion zone
at about 800° to 1100°C. Solids have a very long residence
time of 30 minutes and are discharged with the bed materials
into a cyclone and recirculated through the furnace. Gases
have a 2 second residence time and are vented through a con-
vective gas cooler and flue gas filter. Ogden has received a
National TSCA PCB permit for the use of its circulating bed
combustors.
Waste-Tech Services use fluidized bed systems for the
destruction of both solids and liquids. Aluminum silicate
firebrick particles are used as the bed material in a conical
combustion chamber. Air is used to fluidize the bed.C016_003_r03.indd 930C016_003_r03.indd 930 11/18/2005 1:12:43 PM11/18/2005 1:12:43 PM