The mainstream media today is full of allusions to energy
awareness and conservation. Just as visible these days are media
references to astronomical dollar figures that can boggle the mind.
This article does not seek to break out of that mold, but rather to
conform to it - as Oxidizer Stack Heat Recovery offers a tremendous
opportunity for both energy conservation and energy cost
Consider the following:
- At any hour of the day there are likely to be more than 10,000
oxidizer systems in service, using a high temperature reaction
chamber (with or without catalyst) to treat the exhaust gases from
a wide range of industrial processes.
- The final component of nearly all of these oxidizer systems is
an exhaust stack, where the treated exhaust gases are released to
the atmosphere at elevated temperatures.
- Historically, oxidizer systems have been sized to treat exhaust
airflows from 100 SCFM (Standard Cubic Feet per Minute) up to
several hundred thousand SCFM. But conservatively, the average
oxidizer system airflow processing capability (i.e. "size") can be
estimated to be between 15,000 and 20,000 SCFM.
- Now, considering these 10,000 stacks emitting hot, treated
gases to the atmosphere around the clock; if heat recovery
equipment capable of dropping the exhaust stack temperature by 100
˚F could be installed into each one of them, this would lead to an
overall value of over 18 billion BTUs (British Thermal Unit) per
hour of energy conservation!
- Assuming $10/MM BTU and year round operation - this equates to
recovering over $1.5 billion (US Dollars) worth of energy per
Taking this into account, it is no surprise that a wide range of
stack energy recovery options have been developed and marketed to
end-users of oxidizer systems. This article will discuss three
important aspects of energy reclamation from hot oxidizer
- Energy reclamation from oxidizer stacks is one of three
potential areas of optimization for oxidizer systems.
- There are distinct challenges that must be addressed in the
process of evaluating potential energy savings options.
- There are multiple potential equipment options for this
application, each with its own benefits and limitations.
The ABC's of Oxidizer Stack Energy Recovery
Using ABC's in the title of this section
is actually a misnomer. Truthfully, the letters A and
B should be set aside and the caption should read -
The CDE's of Oxidizer Stack Energy Recovery. The reason
for this is twofold:
First of all - any plan for recovering waste heat in the exhaust
stack of an oxidizer system is already a Plan C. For anyone taking
a hard look at optimizing the energy efficiency of an oxidizer
system as a whole, Plan A should consider 'upstream' opportunities.
(For example, retrofits that reduce overall airflow to the oxidizer
system and/or increase the concentration of solvents to be
treated.) Plan B should focus on the internal TER (Thermal Energy
Recovery) of the oxidizer system itself. After airflow reduction,
maximizing the internal energy recovery of an oxidizer system will
almost always lead to the best project payback.
Hence, it follows that energy recovery in the exhaust stack of
the oxidizer is Plan C. Now calling it Plan C is by no means meant
to downplay the opportunities associated with oxidizer stack energy
recovery. The only intent is to fit the concept into the greater
framework of energy usage in the oxidizer system as a whole. There
are many reasons why Plan A and/or Plan B as defined above may not
be attractive or even feasible - making Plan C: Energy Recovery in
the Oxidizer Exhaust Stack - the best overall choice for energy
The second reason that the letters C, D and E
are a better fit for the title of this section is that those three
letters represent the challenges associated with energy recovery
efforts in oxidizer exhaust stacks, namely:
- CAPTURING the energy from the stack itself
- DELIVERING the energy back into the plant cost-effectively
- EMPLOYING the recovered energy effectively inside the
Following is a brief discussion of each of these challenges
along with the different options for recovering oxidizer stack
Challenge #1: Capturing the Energy
Of the three challenges, the first - Capturing the Energy - is
usually the easiest to evaluate and estimate. By simply knowing the
airflow and temperature of the exhaust gases in the oxidizer stack,
suppliers of energy recovery equipment can quickly begin to model
an appropriate device for reclaiming energy effectively. It is
often during this first challenge that the overall opportunity for
yearly savings is also quantified.
The more information that an oxidizer end-user can provide at
this juncture, the more realistic the opportunity analysis can be.
At a minimum, those considering stack energy recovery should gather
the following before beginning this process:
- Expected airflow and average temperature in the oxidizer
- Expected hours of operation per year
- Current energy rates for the plant (gas or oil and
The first two items are often monitored already on a continuous
basis in oxidizer data recorders. If that is not the case for a
particular system, the most recent EPA (Environmental Protection
Agency) stack testing data can be an excellent source for this
Two other issues for consideration during this phase of an
- Constituents in the exhaust gases (and especially their dew
points): Any effort to reclaim energy in the exhaust stack of an
oxidizer will lower the oxidizer exhaust gas temperature, bringing
with it the potential for condensation of acids. Suppliers of
energy recovery equipment will typically take care to ensure that
final stack temperature is above any acid dew points. Given the
typical solvent laden exhaust from printing presses, this is rarely
an issue of concern for oxidizer systems in the flexographic
- Adding energy recovery equipment to an oxidizer exhaust stack
will also come with a system back-pressure penalty. The existing
oxidizer fan will usually be tasked with pushing or pulling air
through the 'hot side' of the added heat recovery component. To
keep overall project payback attractive, the goal is usually to
choose energy recovery equipment that will limit the added system
back-pressure to an amount that the existing oxidizer system fan
can handle without major modification. Therefore, knowing the
additional capacity available in the oxidizer system fan will help
narrow down which cost-effective options for energy recovery are
Challenge #1 for a typical application may look like
Consider a flexographic printer with a ten year old 20,000
SCFM Regenerative Thermal Oxidizer (RTO). The combined exhaust from
all dryers and capture hoods routed to the RTO is 20,000 SCFM at
approximately 150˚F. The average exhaust temperature from the RTO
Plan C - A 50% effective heat exchanger installed in the
oxidizer exhaust stack to transfer the waste heat to air or fluid
would drop the stack temperature by approximately 125˚F - capturing
approximately 2.7 MM BTU/hr. If this energy was 100% useful inside
the plant and the plant operated around the clock, this could lead
to a yearly savings of up to $225,000.00. A payback of one to two
years is certainly possible for a project of this nature.
Plan A - reducing airflow to the RTO by 10% could save
approximately 0.3 MM BTU/hr or up to $25,200.00/year. This could
likely be accomplished with very little capital investment at all.
A payback of less than six months is possible for this
Plan B - for the data presented, this RTO is operating with
an internal thermal energy recovery (TER) of approximately 92%.
Installing additional ceramic heat recovery media to raise the TER
to 95% could save approximately 1.0 MM BTU/hr or up to
$84,000.00/year. A payback of less than one year is possible for
Challenge #2: Delivering the Energy Back into the
Plant Facility Cost Effectively
As seen in Challenge #1, sizing energy recovery equipment and
estimating the overall savings opportunity with oxidizer stack
energy recovery are not difficult tasks. To take an opportunity
analysis and turn it into an actual payback period however, one has
to determine the cost of installing the equipment and providing the
infrastructure for delivering captured energy back to the
For a cursory analysis, some will take the cost of the energy
recovery equipment and double it, calling that the estimated cost
of installation. (i.e. Total Estimated Cost = One Part Equipment
Cost + Two Parts Installation Cost) This can provide for a quick
check of whether a particular idea merits additional investigation.
To obtain true payback numbers then a site visit by different
tradespeople to estimate the overall cost of energy recovery system
installation will be necessary. Fans and/or pumps, control valves,
thermocouples, etc. will all need to be both mechanically installed
and electrically wired to an existing or new control system. This
is often the challenge where the overall project feasibility hangs
in the balance.
Challenge #3: Employing the Recovered Energy
Effectively inside the Plant Facility
The final challenge is also extremely important for optimizing
energy recovery project payback. Ideally, the oxidizer end-user
should look for ways in which recovered stack energy can be used in
the same process that the oxidizer is connected to. This typically
provides the best payback because there are energy demands by that
process at nearly all times that oxidizer waste heat is available.
In contrast, projects focused on recovering oxidizer exhaust stack
energy to help heat a facility, for example, may only be useful for
part of the year.
Oxidizer Stack Energy Recovery Options
Oxidizer stack heat has been recovered to perform a wide variety
of functions in the plant environment.
- Air-to-air heat exchangers have been used to provide pre-warmed
fresh air back to process ovens, dryers and/or plant make up air
- Air-to-fluid heat exchangers have been used to transfer
oxidizer stack heat to boiler feed water, plant makeup water,
process water, glycol and other thermal fluid loops.
- In extreme cases, waste heat boilers have been used with
oxidizer stack heat to create steam.
- And on the horizon, heat-to-power systems are in development
for reclaiming oxidizer stack heat and creating electricity.
One additional option that has been used sparingly is taking hot
oxidizer stack air directly back for use in production processes.
This is sometimes referred to as Direct Heat Recovery,
while the options mentioned above would be termed Indirect Heat
Recovery. Direct Heat Recovery from oxidizer stacks is
generally shied away from due to the risks of introducing products
of incomplete combustion back into a plant environment or the risk
of oxidizer "oven dirt" contaminating product, but there are
limited cases where this form of oxidizer stack energy recovery has
been used effectively.
Each of these options for recovering heat from oxidizer exhaust
stacks can be considered within the framework of the three
challenges discussed previously.
Air-to-Air Heat Recovery
Probably the most common energy recovery product applied to
oxidizer stacks is an air-to-air heat exchanger. Be it a
shell-and-tube or plate type heat exchanger, there is a cold
side air stream (typically fresh air) and a hot side air
stream (typically the oxidizer exhaust) that are used for heat
Air-to-air heat exchangers have been integral to oxidizers
themselves for decades so it is a well-known technology for
oxidizer manufacturers to incorporate into an overall system. The
programs for sizing air-to-air heat exchangers are quick and easy
to use. There are a wide variety of footprints and physical layouts
for ease of installation. There are also many low-backpressure
models that work well with existing oxidizer system fans.
The limiting factor for air-to-air heat recovery in oxidizer
exhaust stacks is Challenge #2: Delivering the Energy Back into the
Plant Facility Cost Effectively. With air-to-air heat recovery,
insulated ductwork is required to transport captured heat back into
the facility. Costs for running ductwork in a plant vary widely and
can also add up very quickly. The best applications are those with
short duct runs for returning heated air.
Air-to-Fluid Heat Recovery
Air-to-fluid heat exchangers are the second most common energy
recovery product for oxidizer stacks. As the name implies, heat is
transferred from the hot oxidizer exhaust air (again the hot side
air stream) to a circulating fluid (the cold side stream). This is
typically accomplished by passing the hot air over a coil
containing the fluid to be heated. As with air-to-air heat recovery
there are a variety of low-backpressure designs that can allow
installation into an oxidizer exhaust stack without adversely
affecting the oxidizer system.
Because piping is less expensive than ducting, air-to-fluid heat
recovery has a definite advantage over air-to-air heat recovery
when considering Challenge #2: Delivering the Energy Back into the
Plant Facility Cost Effectively. However, unless the heated fluid
is used directly back in the process that the oxidizer is connected
to, Challenge #3: Employing the Recovered Energy Effectively inside
the Plant Facility, can be more difficult to address with
air-to-fluid heat recovery. Meeting this challenge requires a
detailed analysis of the demands for energy in the fluid system
verses the availability of waste heat in the oxidizer stack. For
example, in some plants the biggest hot water demands come in
shutdown situations when the oxidizer is not running.
Air-to-Steam Heat Recovery (Waste Heat Recovery
When the solvent laden air sent to an oxidizer system is
sufficiently rich, the oxidizer's internal heat recovery component
may need to be partially bypassed or forgone completely. This leads
to higher than normal oxidizer stack temperatures and allows for
additional options in heat recovery equipment. One such option is a
waste heat recovery boiler to recover oxidizer exhaust stack heat
and produce steam. Waste heat recovery boilers are custom sized for
a particular exhaust gas capacity as well as required steam
pressure. A variety of systems are available in vertical,
horizontal, single or multi pass configurations. Oxidizers on most
applications rarely have the necessary solvent loading and
corresponding exhaust stack temperatures to sustain this
Sometimes referred to as cogeneration, heat-to-power is an
emerging technology that is capable of sending kilowatts directly
back into a facility for electrical power. The concept has been
implemented on different applications throughout the world but is
only now being integrated with combustion devices such as
oxidizers. Heat-to-power systems can currently generate up to 100kw
per hour from a modest heat source. However, the payback is
normally greater than three years, the value most companies use for
acceptable capital investment. As electricity costs increase and
greater efficiencies are achieved with the technology it will be a
very attractive option in the near future. Today, heat-to-power is
not necessarily a cost reduction strategy but rather a green
initiative that could be used to promote a company as a leader in
Oxidizer stacks represent a significant opportunity for the
reclamation of energy. This applies to all oxidizer systems -
including both the aging catalytic oxidizers popular in the
industry years ago as well as the newer, high efficiency
regenerative thermal oxidizers (RTOs) being supplied today.
Achieving a cost-effective installation of energy recovery
equipment with an attractive payback is not without challenges, but
those challenges are being met today in a variety of ways.