Since the early 1970s, air pollution control regulations have required many businesses to install emission control systems to destroy volatile organic compounds (VOCs) and/or hazardous air pollutants (HAPs), or face fines. While general guidelines regarding the destruction of air pollutants in process exhaust air streams are somewhat consistent within each industry, a company’s individual requirements and desires can greatly vary.

Developing an optimal design for each operation depends on many variables, including the type and quantity of air pollutant, the volume and temperature of the air being exhausted, etc. Future manufacturing growth expectations and even a facility’s geographical location also should be considered.

Review of destruction technologies

For companies with emission controls that are outdated or incorporate a low heat recovery design, upgrading to a modern, energy efficient RTO could significantly reduce natural gas usage.

The basic design concept of both thermal and catalytic oxidizers is to promote a chemical reaction of the air pollutant with oxygen at elevated temperatures. This reaction destroys the pollutant in the air stream by converting it to CO₂, water and heat. The rate of reaction is controlled by three interdependent, critical factors: time, temperature and turbulence. The temperature at which the air pollutant is destroyed and the methods used to recover the heat are what distinguishes one technology from another.

Rotary concentrator system

In applications involving large airflow volumes containing low concentrations of solvents, a Rotary Concentrator System can be used to increase the concentration of pollutants in the air stream before the highly concentrated air is directed to a Regenerative Thermal Oxidizer for destruction.

A rotary concentrator system is a hybrid air pollution control system designed to efficiently remove and destroy air pollutants from a process exhaust air stream. This technology is limited to exhaust air streams at or near ambient temperature. The system requires a multi-step process:

1. Removal of the air pollutant from the air stream using a hydrophobic zeolite rotating wheel.
2. Desorbing or removing the pollutant from the zeolite with a reduced air stream
3. Destruction of the concentrated air pollutant using an RTO.

If a greater level of efficiency is desired, secondary recovery units can be incorporated into a new control system or retrofitted to an existing system. Secondary recovery units capture the 250˚F to 1,500˚F of heat energy (depending on the control system currently in use) that normally would be vented to the atmosphere. The unit can be designed for minimal pressure drop so as not to affect the operation of the oxidizer, while returning temperature-controlled fresh air for a variety of uses such as building comfort heating, process make-up air (ovens/dryers, kilns, curing zones, etc). In some cases, the process can completely replace the need for natural gas-fired burners in the manufacturing process itself.

Using the same idea of capturing heat from the exhaust stream, a hot water or thermal oil heat transfer coil can be installed in the exhaust stack.

Thermal oil is used as a main process heat source where direct flame heating is not desired. Adding a coil in the exhaust stream can reduce or even remove the heat load required from the thermal oil heating system. Depending on the stack temperature, the exhaust from the oxidizer could be routed directly to a low-pressure steam generator. If the plant uses steam for any reason (carbon bed regeneration, humidity control, etc.), this system could supplement steam production capacity any time the oxidizer is running. In an ideal situation, the steam produced from the oxidizer exhaust would allow the main steam generator to function as a backup system.

Technology selection/design criteria

No matter what emission control technology is finally selected, modern air pollution control systems typically exceed EPA, state and local regulations by destroying in excess of 99 percent of a facility’s air pollutants. When a decision is made to install or upgrade an air pollution control system, be prepared to provide the following information to assure the proper technology is applied:

Describe the type of production process to be controlled. If possible, include a rough sketch of the building floor plan showing the location of all pertinent production equipment.

Provide the geographical location (and elevation level if known) where the system will be installed. Both the outdoor climate (surface finishes/types of dampers, etc.) and the elevation (fan sizing) could have an effect on system design.

Estimate the hours per day that the system will be operated. The heat exchanger efficiency, chamber design, etc., could possibly change depending upon the operation hours required.

List the total number of emission points (exhaust stacks) that are to be controlled by the control system. A process control/bypass tee-damper may be required at each stack.

List the exhaust rates and temperatures for each individual emission point. The exhaust rates are important for sizing the unit, ductwork and dampers. The temperature is used to calculate estimated operating costs and to determine the necessity for ductwork insulation.

Describe the type of heat source used for any dryers/ovens that are to be controlled. If the heat sources for process dryers or ovens are gas-fired burners, there are National Fire Protection Association regulations that determine the method of purging and damper control. If the heat source is by steam or hot oil, process control/bypass tee-dampers may not be required at each stack.

List the air pollutant types and quantity being used. In addition to affecting the choice of catalyst used in catalytic units, solvent type and quantity also will affect the destruction efficiencies, the heat exchanger efficiency, the internal materials of construction and the estimated operating costs.

Determine possible need for a permanent total enclosure (PTE). Permitting a facility with a properly designed PTE will assure 100-percent capture of all air pollutants present within the production area.

Provide the electrical voltage available and power cost. The available voltage determines the type of electronics that are required and the cost is used to estimate operating costs.

Provide the type, cost and line pressure of supplemental fuel available. The fuel type (natural gas, propane, etc.) and the line pressure are used to determine the burner and fuel train design. The fuel cost is used to calculate the estimated operating costs.

Describe the physical location of the air pollution control system installation. The actual location determines whether a concrete equipment pad or steel support structure is required.

Indicate the percent of pollutant destruction efficiency required. This will determine the amount of catalyst needed (in catalytic models) as well as the operating temperature and residence time in either technology.

List any catalyst masking or poisoning agents that could potentially be present in the air stream. Compounds such as silicones, phosphorus, heavy metals, halogen, sulfur and any particulates could be of concern and should be identified. An air pollution control system can be designed to handle various levels of most compounds if the user can quantify them in advance.

Plan ahead. When selecting or sizing an air pollution control system, the facility’s growth expectations for the next two to five years should be considered. It is typically less costly to install a system designed to handle additional capacity now rather than to install a second system in the future. PE