Regenerative, catalytic and thermal: all heat the effluent to the point of destruction of 95 percent or more of VOC content in your exhaust stream. But which design is right for your process?

FIGURE 1. Thermal oxidizers can use a heat exchanger for primary heat recovery. The heat exchanger raises the temperature of the incoming exhaust air using the air already heated during the incineration process, thus causing the burner to require less fuel.

Sometimes, there's no way around it. You have a process that includes volatile organic compounds (VOCs) and you can't reformulate the coating. The law says you need pollution control. An effective type of pollution control is incineration, but what equipment do you use?

Obviously, the least expensive, initially and operationally. However, this is an area where some can go wrong by choosing the wrong type for the process. There are three common types of incinerators: regenerative, catalytic and thermal. The basic concept to all: heating the effluent to the point of destruction of 95 percent or more of VOC content. How this occurs varies among the three designs.

Regenerative oxidizers usually are much higher in initial cost than thermal or catalytic systems. They can be rather complex in design, but the complexity can be worth the initial and maintenance costs due to low fuel requirements. They use far less fuel because of the manner of heat storage that is a part of the inherent design. Effluent enters one of two or three heat storage chambers. Ceramic components in each chamber retain the heat from the burner to preheat the incoming effluent to 1,000oF (538oC) or more. Then, the air switches over using large, mechanically or hydraulically actuated dampers to another chamber heated from the exhaust and uses that heat to reach the destruction temperature. This all occurs in a continuous cycle. Regenerative incinerators usually are the better choice for systems with exhaust flows of 10,000 cfm or more.

Catalytic oxidizers also use less fuel for operation because they run at low temperatures. This type of oxidizer uses a catalyst that reacts with the solvent to generate heat of its own, helping to bring the effluent to the destruction temperature. Catalytic oxidizers can be a good choice for the right application; one example is printing, which is suited to catalytic oxidizers because it has only solvent emissions. If there are any silicones, oils or solids released from the process, however, this system will fail quickly, and cleaning or replacing the catalyst will become a time-consuming and costly process in itself.

Thermal oxidizers are a good choice for processes that have silicones, solids and oils as well as VOCs. Because of the higher operating temperatures of 1,500 to 1,600oF (816 to 871oC), this type of incinerator can be far more expensive to operate. But, careful selection of the size and operating parameters can reduce this cost. Thermal oxidizers can be cost effective up to about 10,000 scfm, depending on overall system design.

Thermal: A Closer Look

When designing a thermal oxidizer for efficiency, there are a number of factors that need to be considered, including:

  • The volume of the exhaust.
  • The temperature of the exhaust.
  • Secondary heat recovery.

One way of achieving efficiency is a seemingly simple reduction of the volume of exhaust. The lower the volume, the smaller the incinerator and burner required to achieve the necessary temperatures. In some cases, this is a feasible method. However, one must be careful not to exceed more than 25 percent of the lower explosive limit (LEL) of the exhaust mixture at any point during the process, and that this exhaust reduction does not adversely affect the product being processed. An exception to the percentage is if a continuous LEL monitor is installed on the equipment. LEL levels of up to 50 percent are allowed when a monitor is interlocked with the process heat source to shut off the burner and, on some systems, increase the exhaust airflow to help dilute the concentration. Another benefit to providing high solvent concentrations is that the solvent will become a greater fuel source, contributing to less gas usage by the burner.

Another fuel-saving design feature is preheating the process air to the incinerator, which is also referred to as primary heat recovery. This entails installing a heat exchanger in the exhaust airstream (figure 1). The heat exchanger raises the temperature of the incoming exhaust air using the air already raised to 1,500oF, thus causing the burner to require less fuel. When designing a system such as this, one must know the autoignition temperature of the solvent being used to keep the exhaust air below that threshold prior to reaching the burner. For example, in designing a system for a process using MEK, which has an autoignition temperature of 960oF (515oC), suppose the process air reaches the incinerator at 120oF (49oC). In this example, the solution would be to integrate a heat exchanger to raise the process air from 120 to 860oF (49 to 460oC). The heat exchanger on a 5,000 scfm system can reduce the fuel consumption from 7.5 million BTU/hr to 3.5 million BTU/hr, saving more than half of the operating costs.

A third way of achieving fuel efficiency is to install secondary heat recovery. A typical thermal oxidizer with a primary heat exchanger has a final exhaust temperature between 800 and 1,000oF (427 to 538oC). This heat is exhausted to atmosphere. Some processes can use this waste heat, diluted with outside air, to become the process heat source. Because incinerator exhaust contains the products of combustion, some particulate and water vapor, the direct exhaust may not be usable for many processes. A second heat exchanger can provide clean, heated air back to the process. This can often be high in initial cost. The heat exchanger, necessary insulated ductwork and controls for the temperature and flow can add up, but if the volume is high enough and the proximity of the process equipment to the incinerator is close, the savings in operating cost can be significant, with paybacks of one year or less.

Pollution control can be a costly venture. There are many pitfalls a buyer can fall into when going strictly for price and low operating costs. Choosing the correct equipment for your application will allow proper destruction of VOCs without destroying your profitability. PH