JEC COMPOSITES MAGAZINE - Issue #81 - May/June 2013 - 78

carbon
thermal efficiencies over 97%, the RTO
is capable of operating with little to no
supplemental fuel use.
In most applications, the airflow is typically pushed through an RTO in a forced
draft configuration. This reduces the
main system fan size and the electrical
consumption required for it to push
process air into the oxidizer. Due to the
HCN found in carbon fibre processing,
many RTOs in this application are supplied in an induced draft configuration.
The main system fan must be oversized
to pull air through the oxidizer but it
will ensure that all HCN emissions are
drawn into the oxidizer for destruction,
protecting the company’s employees and
neighbourhood from a potentially lethal
situation.
Whenever oxidizing siloxane
vapours in an RTO, considerations must be made to
minimize the impact of silica
dust build-up in the heat recovery beds. The formation of
the inorganic silica particulate
starts at elevated temperatures and tends to accumulate within the media beds.
Heat recovery beds should
be designed to prevent silica
dust from collecting and
clogging in the oxidizer
without compromising
the high thermal efficiency. If build-up does
occur, the RTO media
should accommodate
periodic cleaning and
potential replacement,
if required.

Multi-stage direct-fired thermal oxidizer (MS-DFTO)
The low- and high-temperature (LT/
HT) furnaces used in carbon fibre processes generate emission streams heavily
concentrated with pollutants. This makes
it an ideal application for the DFTO;
however this abatement technology
may require a significant amount of
supplemental fuel to achieve destruction
temperatures. Subsequently, they can be
a large source of carbon dioxide (CO2)
and NOX which are greenhouse gases
and contribute to global warming.
However GHGs are not necessarily a
by-product of these air pollution control
devices. In recent years, the technology
has evolved, incorporating multiple
zones or stages that that minimize the
effects of combustion. The MS-DFTO
is being uniquely applied in the carbon
fibre industry to ensure the production
of these materials is environmentally
sound.
During operation, furnace exhaust is pulled through the system with an induced
draft fan configuration. This increases
safety by ensuring that all of the hydrogen cyanide emissions are drawn into
the oxidizer for destruction, protecting
the company’s employees and neighbourhood from the potentially lethal gas
leaking out of flanges, instruments, etc.
This also ensures that oxygen does not
migrate back to the furnaces.
The furnace (LT & HT) exhausts are
injected into the first stage of the DFTO
where there is insufficient oxygen for
complete combustion of the hydrocarbons. In this reducing environment, the
substoichiometric operation causes the
hydrocarbons, hydrogen cyanide and
ammonia to dissociate, resulting in free
nitrogen. The free nitrogen and
combustibles gases
are competing
for the limited
oxygen avai-

lable from the oxygenated hydrocarbons
and combustion air from the burner. This
keeps the nitrogen from being oxidized
to NOX.
This first stage of combustion takes place
in a chamber capable of continuous operation at very high temperatures for the
required residence time. The second zone
of the system takes the N2, H2O and
residual hydrocarbons leaving the first
zone and cools these gases. This chamber
uses a venturi-type design to maximize
the mixing of the hot gases and cooling
medium. The gases leaving the first zone
are cooled to the point where auto-ignition of the remaining combustibles will
still occur in the last zone. The proper
cooling medium is required to adsorb a
great deal of energy without substantially
increasing the outlet gas volume.
The third and final zone reintroduces air
into the stream so that oxidation of the
residual hydrocarbons, carbon monoxide
and hydrogen can occur. Fresh air is
added to this section so that the outlet
oxygen concentration is maintained at
the discharge of the system, prior to the
induced draft fan. This proper oxygen
concentration and residence time at temperature ensures extremely high levels
of destruction. Since NOX formation in
this zone is to be prevented, the outlet
temperature from this stage is limited by
a predetermined temperature.

Process integration
Today’s processors are taking a much
more holistic approach when designing
and optimizing their plants, from polymerization and spinning to oxidation,
carbonization, surface treatment and pollution control. Physical connection techniques as well as operational interfaces
between the process equipment and abatement devices are critical to a successful
installation. Optimizing energy recovery
from thermal oxidizers will improve
overall plant efficiency by integrating the
process heating equipment. n

Emissions from carbon fiber
furnaces are uniquely handled in
this Anguil Multi-Stage DFTO to
lower Greenhouse Gas output

78 jec composites magazine / No81 May - June 2013

More information:
www.anguil.com
rich.grzanka@anguil.com

http://www.anguil.com

JEC COMPOSITES MAGAZINE - Issue #81 - May/June 2013

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