Case study - Carbon fiber: Oven emission control strategies

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Courtesy of Anguil Environmental Systems, Inc.

OVERVIEW

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 proprietary design has over-sized 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.

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