PROBLEM
A carbon fiber company in the Peoples Republic of 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).
ACTION
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 Asia
team 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.
SOLUTION
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 efficiencies are 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 exchangers 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. Following the first two heat exchangers the process
exhaust 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 efficiencies 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 exchanger 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.