Oxidizer Stack Heat Recovery for Energy Conservation and Cost Reduction

Today the mainstream media is full of allusions to energy conservation and awareness. This article details the topic of oxidizer stack heat recovery and offers an opportunity for both energy cost reduction and energy conservation.

Action Taken

There are likely to be over 10,000 oxidizer systems in service at any hour of the day using a high temperature reaction chamber to treat exhaust gases from a broad range of industrial processes.

The final component of nearly all oxidizer systems is an exhaust stack, where treated exhaust gases are released into the atmosphere at elevated temperatures. Historically, oxidizer systems are scaled to treat exhaust airflows from 100 SCFM up to several hundred thousand SCFM. The average oxidizer system is estimated between 15,000 and 20,000 SCFM.

These 10,000 stacks emit hot treated gases to the atmosphere around the clock. If it is possible to incorporate heat recovery equipment that can drop the exhaust stack temperature by 100 °F, an overall value of over 18 billion BTUs per hour of energy conservation is observed.

Figure 1.

Three important aspects of energy reclamation from hot oxidizer stacks are discussed in this article:

  • Energy reclamation from oxidizer stacks is a key area 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 CDE’s of Oxidizer Stack Energy Recovery

C,D and E represent the challenges related to energy recovery efforts in oxidizer exhaust stacks that include the following:

  • Capturing energy from the stack
  • Delivering the energy to the plant cost-effectively
  • Employing the recovered energy inside the plant

Figure 2.

Capturing the Energy

This is the easiest to evaluate and estimate. By knowing the temperature of the exhaust gases in the oxidizer stack and the air flow, suppliers of energy recovery equipment can model an appropriate device for reclaiming energy effectively.

While considering stack energy recovery the following must be considered:

  • Expected airflow and average temperature in the oxidizer stack
  • Expected hours of operation per year
  • Current energy rates for the plant (gas or oil and electric)

At the evaluation phase, these two issues must be considered:

  • Constituents in exhaust gases
  • Adding energy recovery equipment to an oxidizer exhaust stack will also come with a system back-pressure penalty.

A typical application for this is:

  • 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 is 275 °F.

The solution is to install a 50% effective heat exchanger in the oxidizer exhaust stack to transfer the waste heat to air or fluid and bring down the stack temperature by approximately 125 °F, capturing approximately 2.7 MM BTU/hr. A payback of one or two years is possible for a project of this nature.

Delivering the Energy Back into the Plant Facility Cost Effectively

For cursory analysis, the cost of the energy recovery equipment may be taken and doubled calling it the estimated cost of installation. This can result in a quick check whether a specific idea merits additional investigation. To obtain true payback numbers, a site visit by different tradespeople to estimate the overall cost of energy recovery system installation will be required. It will be required to install fans and/or pumps, control valves, thermocouples mechanically and wire the same electrically to an existing or new control system.

Employing the Recovered Energy Effectively inside the Plant Facility

The oxidizer end-user needs to seek ways in which recovered stack energy can be used in the same process that the oxidizer is connected to. This offers the best payback as there are energy demands by that process. In contrast, projects focused on recovering oxidizer exhaust stack energy to help heat a facility, for example, may only be useful for a part of the year.

Oxidizer Stack Energy Recovery Options

Oxidizer stack heat has been recovered to perform a wide range 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 units.
  • Air-to-fluid heat exchangers have been used for transferring 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.
  • Direct heat recovery is also sparingly used that involves 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.

Conclusion

Oxidizer stacks represent a significant opportunity for energy reclamation. This applies to all oxidizer systems - including both the aging catalytic oxidizers popular in the industry years ago as well as the more recent, high efficiency regenerative thermal oxidizers (RTOs) being supplied today. A cost-effective installation of energy recovery equipment is possible with an attractive payback.

This information has been sourced, reviewed and adapted from materials provided by Anguil Environmental Systems.

For more information on this source, please visit Anguil Environmental Systems.

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Comments

  1. David Dunn David Dunn United Kingdom says:

    Stack energy losses are huge, as described above there are solutions, mainly making direct use of the waste heat directly somewhere else, however is there not technology that works by producing electricity directly from the heat differential between the inside and outside of the stack, thus making the stack a generator  directly producing electricity that has many more uses than heat transfers.?

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