Regeneration Game
The use of regenerative thermal oxidizers is expanding into
applications where they wouldn't have been considered years ago,
says Mike Scholz*
Originally printed in the October 2007 issue of The Canmaker
Magazine - Sayers Publishing Group
As the first generation of oxidizer systems in the industry
nears the end of their service life, many canmaking plants face
repair or replacement of their existing air pollution control
systems. Like many others in the industry, a Silgan canmaking plant
in the Midwest had been using a thermal recuperative oxidizer with
direct heat recovery for control of emissions from its sheet
coating lines. After more than a decade of service, the oxidizer
was reaching a point that repairs would be needed in order to
continue to meet strict compliance limits so Silgan began looking
for an effective, efficient solution.
Historically, thermal recuperative oxidizers with direct heat
recovery have been a popular choice in canmaking facilities -
especially those with oven zones operating above 350 deg F (177 deg
C).
In the past, thermal recuperative oxidizers had a capital cost
advantage over regenerative thermal oxidizers (RTOs) and boasted
much more flexible Volatile Organic Compounds (VOC) loading
limitations. Their one drawback has always been in supplemental
fuel usage. Thermal recuperative oxidizers top out at 70 percent
internal heat recovery, whereas RTOs are able to achieve more than
95 percent.
For canmakers, this drawback was minimized with the use of
additional heat recovery. Hot, purified air from the oxidizer is
routed directly back to the oven zones and not lost to the
atmosphere. This has reduced the operating cost 'penalty' of the
thermal recuperative oxidizer and - in the past - has swung the
balance toward specifying that system for VOC loads above ten
percent Lower Explosive Limit (LEL) almost exclusively.
So exclusively that, when hearing that Anguil Environmental
Systems had recommended an RTO for its Midwest coating facility,
Silgan responded almost incredulously: "They recommended what? This
is clearly not an RTO application."
Given the technologies offered when Silgan made its initial
selection of a thermal recuperative oxidizer, this was an
understandable response. It also served as an ideal framework to
study what has changed in oxidizer design over the past decade to
reverse such a drastic initial response:
- Thermal recuperative oxidizers no longer have capital cost
advantage
- With hot gas bypass and feed forward technology, RTOs are now
specified in situations up to 25 percent LEL
- With fuel costs being unstable and still on the rise, every
heat recovery percentage points counts
- New requirements for VOC capture plus destruction have
marginalized direct heat recovery and increased the operating cost
gap between thermal recups and RTOs.
Silgan's existing thermal recuperative oxidizer was designed
based on volume of airflow, organic vapor concentrations and
desired destruction efficiency. During operation, VOC-laden air is
drawn into the system fan and is discharged into a heat
exchanger. The air is preheated through the tube side of the
heat exchanger and then passes the burner, where the contaminated
air is raised to the thermal oxidation temperature (1,200-1,800 deg
F / 650-1,000 deg C). When the VOC-laden air is raised to the
thermal oxidation temperature for the specified residence time
(0.5-2.0 seconds), an exothermic reaction takes place. The VOCs in
the air stream are converted to carbon dioxide and water vapor. The
hot, purified air then passes on the shell side of the heat
exchanger where the energy released by the reaction is used to
preheat the incoming solvent laden air reducing the system's fuel
consumption. Finally, the contaminant-free air is exhausted into
the atmosphere.
A weakness in all thermal recuperative oxidizer designs is that
the steel in the heat exchanger is exposed to high burner chamber
temperatures (typically up to 1600 deg F / 871 deg C). The system
at Silgan had a history of requiring ongoing maintanance in this
area, which had been driving up cost and impacting
throughput. The engineering team at Silgan needed to fix the
aging system, replace it with an equivalent, or look for
alternative equipment.
After evaluating several options, the RTO selection was based on
the capital cost advantage and operating cost savings. It would be
a custom-built abatement system designed specifically for this
application with high loadings and concentrations. Anguil would
design, manufacture and install a 40,000 scfm RTO with heat
recovery, hot gas bypass and oven purge system.
Silgan's new RTO operates as follows:
The solvent laden process gas enters the oxidizer through an inlet
manifold. Flow control, poppet valves direct this gas into one of
two energy recovery chambers where the process gas is preheated.
The process gas and contaminants are progressively heated in the
inlet ceramic bed as they move toward the combustion chamber.
The VOCs are oxidized in the combustion chamber, releasing
thermal energy in the ceramic bed that is in the outlet flow
direction from the combustion chamber. The outlet ceramic bed is
heated and the gas is cooled so that the outlet gas temperature is
only slightly higher than the process inlet temperature. Flow
control, poppet valves routinely alternate the airflow direction
into the ceramic beds to maximize energy recovery within the
oxidizer. The VOC oxidation and high energy recovery within
these oxidizers reduces the auxiliary fuel requirement and saves
operating cost. For example, at 95 percent thermal energy
recovery, the outlet temperature may be only 70 deg F (40 deg F)
higher than the inlet process gas temperature with an RTO. The
oxidizer can reach self-sustaining operation with no auxiliary fuel
usage at typical operating concentrations. The process
emissions at the Silgan facility as well as the temperature of the
oven zones presented some challenges, as well as
opportunities.
With process LEL levels as high as 14 percent there was a
concern over high temperature in the RTO. A hot side bypass valve
was provided to allow excess RTO reaction chamber heat to be vented
directly into the exhaust or the back to the oven inlet manifold
during periods when the inlet VOC loading provides more heat than
is necessary to maintain the set point temperature. This primary
heat recovery saves thousands of dollars in operating costs because
the ovens require much less fuel to reach the desired temperature.
With the Anguil design there is no loss of residence time at
temperature, ensuring destruction and eliminating the concern of
overheating the unit. VOC destruction efficiency is guaranteed
whether the bypass is open or not.
Silgan is also investigating another energy reduction strategy by
using a secondary heat exchanger to recover additional heat from
the RTO exhaust stack. Initial estimates show that an extra
6.5 million btu/hr can be recovered by utilizing a heat exchanger
in the oxidizer stack. Fresh air (at an average outdoor
temperature of 46 deg F / 8 deg C) passes through a single pass 50
percent effective heat exchanger and is heated up to approximately
350 deg F (177 deg C). This recovered heat can be used for
processes or comfort heat during the winter months, which could
translate into significant savings.
The RTO is also equipped with a high temperature bake-out
system, very similar to the self-cleaning option in an oven. This
feature removes organic build-up on the cold face of the heat
exchange media. In the bake-out mode, the RTO is taken off-line
from the process. At a reduced airflow, the outlet temperature is
allowed to reach an elevated temperature before the flow direction
is switched. This hot air vaporizes organic particulate,
essentially clearing the media bed of any obstruction. The flow
direction is then switched and the opposite cold face is cleaned.
Standard bake-out occurs at 650 deg F (343 deg C), stainless steel
media supports and poppet valves were used on the Silgan system
that allowed bake-out temperatures to reach 800 deg F (427 deg C),
ensuring a more complete cleaning. Scheduled RTO bake-outs reduce
the pressure drop across the heat recovery beds. Therefore,
Anguil included the transmitters necessary to monitor media bed
pressure drop and provide both continuous recording of this data as
well as an indication to the operators when a bake-out is
recommended.
Dan Gallo, Silgan's area manager of manufacturing, was pleased
with the outcome. "We selected Anguil because of its technical
excellence and commitment to service," he said. "Not only has the
company been able to troubleshoot its own equipment, but Anguil has
also provided operating solutions for oxidizers made by other
manufacturers. We are pleased with their dedication to excellence
and are happy to have Anguil as a business partner."
* Mike Scholz is a senior application engineer at Anguil
Environmental Systems.