A carbon fiber company in China was faced with the challenge of selecting an emission control system for a new pilot line at their specialty fiber products facility. The line would include furnaces and three ovens, both of which would emit Carbon Monoxide (CO), Ammonia (NH3) and lethal amounts of Hydrogen Cyanide (HCN).
The carbon fiber processor selected Anguil Asia because of their local presence in the region and specific design for this application. Before beginning the project, the Anguil team in Asia ran an energy analysis at the facility which ensured that the proper technology would be applied based on the destruction requirements, efficiency needs and process parameters. A Direct Fired Thermal Oxidizer (DFTO) was selected to process the furnace exhausts while a Regenerative Thermal Oxidizer (RTO) was chosen to process the oven exhausts.
To treat the higher concentration exhaust stream coming from the carbon fiber furnaces, Anguil designed a specialized multi-zone DFTO whereby the nitrogen compounds are disassociated at high temperatures in an oxygen depleted chamber. The remaining gases are quenched before moving into a secondary zone where total emission destruction efficiency is over 99% with minimal NOX generation.
The furnace exhausts typically contain tar which often causes plugging in a standard emission control device. Special design considerations were taken to reduce these maintenance concerns and improve reliability. The Anguil system introduces furnace exhaust into the DFTO with a unique inlet manifold that eliminates tar build up and plugging concerns. Anguil also provided an induced draft system for increased safety. 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 the potentially lethal gas leaking out of flanges, instruments, etc.
Because the customer's three oxidation ovens were electrically heated, reducing the electrical consumption was a critical objective on this project. As part of the complete energy analysis done at this facility, Anguil understood that the oxidation ovens can require a significant amount of supplemental energy to maintain temperatures from 392°F to 572°F (200°C to 300°C). The customer wanted to recover as much energy as possible from the oxidizer systems to save on the electrical power used in the ovens. Keeping this in mind Anguil proposed several secondary heat exchangers to provide the necessary preheated makeup air back to the electrically heated ovens.
The DFTO would be exhausting at 1600°F (870°C) so Anguil incorporated three shell and tube heat recovery bundles in series, following the oxidizer. The first two stainless steel heat exchangers would be providing preheated makeup air back to Oven #2 and Oven #3. The process exhaust downstream of that system still contained usable heat so a third shell and tube heat exchanger was incorporated to preheat the combustion air used in the DFTO. Preheating the DFTO combustion air made the destruction device itself more energy efficient and reduced the amount of supplemental natural gas required.
A summary of this energy recovery project is listed below:
- The shell and tube heat exchangers recover approximately 1.0 MMBTU/h (293 kW/h) to be returned as preheated air back to Oven #2 and Oven #3
- The estimated payback on the heat exchangers is less than 3 months
- The shell and tube heat exchanger to preheat combustion air to the DFTO will recover approximately 0.18 MMBTU/h (53 kW/h)
- The estimated payback on the combustion air heat exchanger is less than 7 months (based on a natural gas cost of $10.00/MMBTU and assuming 24 hour/day operation)
To treat the higher flow, lower concentration exhaust from the ovens Anguil selected an RTO. This type of oxidizer is capable of 98-99%+ destruction efficiency with very low operating costs compared to other emission abatement technologies. With achievable thermal efficiency over 96% the RTO is capable of operating with little to no supplemental fuel use.
During operation the 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.
In keeping with the overall goal of the oxidation system to provide all of the required preheated makeup air back to the ovens, Anguil installed a secondary heat exchanger following the RTO. The plate-type heat exchanger recovers 70+% of the RTO exhaust energy. That preheated air is used in lieu of ambient air for the oven.
- The plate heat exchanger will recover approximately 0.42 MMBTU/h (123 kw/h)
- The estimated payback on the heat recovery system is less than 3 months
The project resulted in an overall reduction of emissions and operating expenses for the carbon fiber company. Even on this small pilot line operation Anguil was able to show a substantial reduction in the overall energy requirement. Due to the success of this project Anguil will be installing air pollution control equipment on the customer's full scale production line. The new system will also be energy efficient keeping with Anguil's goal of providing air pollution control equipment today to keep our customers profitable tomorrow.