Problem
Carbon fiber (fibre) and graphite reinforced polymers are
materials that have, and will continue to revolutionize the
products we use everyday by making them stronger, lighter and more
durable. However, the manufacturing process can have serious
environmental ramifications and immediate danger to human health if
careful consideration is not given to emission control at the
production phase of these materials.
Action
A carbon fiber company in the Peoples Republic of China was
faced with this challenge while designing a new facility and
process line for their specialty fiber products. Company officials
knew they would need a pollution control device that not only met
the local regulations but also protected their employees and
heavily populated neighborhood. The new line would include a
furnace and oven with the potential to discharge significant levels
of Carbon Monoxide (CO), Ammonia (NH3) and lethal amounts of
Hydrogen Cyanide (HCN).
There are two primary pollution control technologies applied
downstream of the ovens and furnaces at carbon fiber processing
plants. The industry has historically used dual stage, DFTOs
(Direct-Fired Thermal Oxidizers) for emission control on the
furnaces and RTOs (Regenerative Thermal Oxidizers) for oven exhaust
treatment. Both technologies are capable of destruction
efficiencies over 99% but the advantage to installing an RTO is the
very low operating costs.
Emission laden process gas enters the RTO through an inlet
manifold to flow control, poppet valves that direct this gas into
energy recovery chambers where it is preheated. The process gas and
contaminants are progressively heated in the ceramic media beds as
they move toward the combustion chamber.
Once oxidized in the combustion chamber, the hot purified air
releases thermal energy as it passes through the media bed in the
outlet flow direction. The outlet bed is heated and the gas is
cooled so that the stack temperature is only slightly higher than
the process inlet temperature. Poppet valves alternate the airflow
direction into the media beds to maximize energy recovery within
the oxidizer, up to 97% heat recovery is possible. The high energy
recovery within these oxidizers reduces the auxiliary fuel
requirement and operating costs. The Anguil oxidizer achieves high
destruction efficiency and self-sustaining operation with no
auxiliary fuel usage at low concentrations.
Solution
When searching for an air pollution control partner, the
carbon fiber processor looked for a vendor that not only had the
necessary experience but also a local presence. Each
producer's fiber differs from those of its competitors, and the
processing details that give each brand its signature
characteristics should be considered when selecting the emission
control device. The Anguil Asia team located in both Taiwan
and China demonstrated their understanding of the capture, control
and compliance hurdles that the processing plants face. Prior
to equipment selection, Anguil ran an energy analysis at the
facility which helped in selecting the proper technology based on
destruction requirements, efficiency needs and process
parameters.
Anguil recommended a model 25,000 SCFM RTO with several
features that improved reliability, performance and
efficiency.
- The system has oversized valves, fan and stack to handle the
elevated temperatures coming from the process and allowing for
future expansion.
- On most applications, airflow is generally pushed through an
RTO but this application was designed for an induced draft
configuration. This ensures that all of the Hydrogen Cyanide
emissions would be drawn into the oxidizer for destruction,
protecting the company's employees and neighborhood from a
potentially lethal situation.
- A Supplemental Fuel Injection (SFI) system was included on the
RTO for increased fuel efficiency and ultra low NOX
emissions.
- The poppet valve design on the RTO operates without process
interference at the oven.
Once fabricated, the Anguil RTO was installed and running in
less than four weeks. It is currently achieving greater than
98% destruction removal efficiency with over 95% thermal heat
recovery. The system is extremely efficient, self-sustaining
at low emission loading and requires very little supplemental fuel
for destruction.
Anguil's involvement didn't stop at the oxidizer; they saw
this emission control project as an opportunity to reduce operating
costs for their customer. Ovens on a carbon fiber process can
require a significant amount of natural gas to maintain
temperatures from 392°F to 572°F (200°C to 300°C). A secondary
heat exchanger made of 304-stainless steel was installed after the
oxidizer to pre-heat the oxidation oven. The plate-type heat
exchanger recovers 75% of the RTO exhaust, using that preheated air
in lieu of ambient air for the oven. Initial estimates
indicated a 1 year payback on the added capital equipment cost but
it actually took only 5 months.
The project resulted in an overall reduction of emissions and
operating expenses for the carbon fiber company and they are
currently considering future green initiatives with Anguil.