Selecting the Correct Combustion System for Your Thermal Oxidizer
At the heart of any oxidizer is the burner used to destroy harmful emissions such as hazardous air pollutants and volatile organic compounds.
Thermal oxidizers are used to destroy hazardous air pollutants (HAPs) and volatile organic compounds (VOCs) that are the end result of an industrial manufacturing process. The basic principles for thermal oxidization are easily understood:
Inject the hazardous gases into a high temperature chamber.
Inside the chamber, the toxic gases are burned off and converted into carbon dioxide and water vapor.
The nontoxic gases are released into the atmosphere.
Variables such as temperature, time, turbulence, availability of oxygen and overall system efficiency are all important parameters for thermal oxidizer systems used for industrial heating processes.
The use of thermal oxidizers is most prevalent in the petroleum, refining, chemical process, metal finishing, printing and glass industries as well as electricity generating power plants. In these applications, most of the emissions are carbon based and contain toxins such as benzene, toluene, methyl ethyl ketone (MEK) and carbon monoxide.
Many different types of thermal oxidizers exist to burn off these hazardous gases effectively. Each type has its own specific applications in industry. They include direct thermal, recuperative thermal, catalytic, recuperative catalytic, regenerative thermal and regenerative catalytic.
Direct Thermal. With this technology, the process stream is heated up to thermally oxidize the VOCs in the process stream. This is usually a temperature of 1400°F (760°C) or higher, depending upon the VOCs to be destroyed.
Recuperative Thermal. This technology is similar to direct thermal oxidizers, except recuperative thermal units use a heat exchanger to recover heat from the oxidizer exhaust and preheat the incoming process air to conserve fuel.
Catalytic. This type of oxidizer uses a catalyst to allow the VOCs to oxidize at lower temperatures.
Recuperative Catalytic. This technology is similar to catalytic oxidizers but uses a heat exchanger to recover heat from the oxidizer exhaust and preheat the incoming process air to conserve fuel.
Regenerative Thermal Oxidizer. Frequently known as an RTO, this technology uses multiple beds of ceramic media as a highly efficient heat exchanger. Valves are used to alternate the flow pattern through the ceramic media bed from inlet to outlet. The ceramic absorbs heat from the process gas on the outlet cycle and then desorbs the heat to the process on the inlet cycle. The combustion chamber typically runs at 1500°F (815°C) or higher.
Regenerative Catalytic Oxidizer. A layer of catalyst is added to the media in a regenerative oxidizer, allowing the combustion chamber to run at a lower temperature.
At the heart of any oxidizer system is the burner. The burner generates sufficient heat to reach temperatures — usually in excess of 1400°F (760°C) — where the toxins will self-ignite and burn.
The proper selection, sizing, control and maintenance of the burner have a direct impact on the oxidizer system’s ability to efficiently burn upwards of 99 percent of the toxic vapors. Whether you are designing a new oxidizer or refurbishing an old unit, proper selection of the burner and combustion controls is important for optimizing performance and trouble-free operation.
Two main types of burners used in oxidizers: raw-gas-airflow burners and sealed-nozzle burners. Raw-gas-airflow burners use oxygen from the process stream mixed with natural gas to create a clean, stable flame. These are the most efficient type of burners to use because they do not require any outside combustion air to be introduced into the system. This is more thermally efficient because the use of outside air requires additional fuel to heat the cooler combustion air up to the combustion chamber temperatures.
By contrast, sealed-nozzle burners use an external source of air for combustion of the fuel. The air is introduced through the burner and mixed with the fuel in the burner. No oxygen from the process stream is required for combustion. Additional fuel is needed to heat the added combustion air, making this burner less fuel efficient. However, this type of burner provides more control over how the air and fuel are mixed and provides greater turndown, which provides more versatility in operation.
Factors to Consider When Selecting the Burner for an Industrial Oxidizer
Many factors should be considered when selecting the correct burner style for your oxidizer. (Also, remember it is important that any selection be thoroughly designed, tested and certified by a qualified vendor prior to installation.) Among the factors to evaluate are the oxidizer type and construction, its operating temperature and emissions permit requirements.
Oxidizer Type and Construction. Specific flow patterns through the oxidizers allow the use of some styles of burners and limit the use of the others. For catalytic, direct thermal and recuperative thermal oxidizers, the design and flow patterns allow for the use of an airflow-gas burner because the air flows continuously in one direction. Because of the reversing of the process flow in regenerative oxidizers, an airflow burner cannot be used. A sealed burner with external combustion is the best option.
Operating Temperature. The burner design and materials of construction must be selected based upon the internal operating temperature of the oxidizer. For catalytic systems, the temperature is usually lower, allowing the use of most industrial burners that operate in the sub-1000°F (537°C) range.
For thermal oxidizers running at 1400°F (760°C) or higher, these require the use of a high temperature burner that is specifically designed to operate at these sustained temperatures. High temperature airflow burners that are made of high temperature grades of stainless steel, or Hastelloy mixing plates can be used — in some cases, at temperatures up to 1700°F (926°C). The destruction of some difficult VOCs may require temperatures at or above 1700°F (926°C), which requires the use of a sealed-nozzle burner with a ceramic refractory block. Failure to properly specify the right burner can result in higher maintenance costs as well as burner head melt-down and burner failure.
Burner Emissions. Common burner emissions of concern are NOX and CO. In order to better understand what burner and system fit your needs, your permit should be reviewed to determine the allowable limits of emissions for the facility. Thermal oxidizers will convert the CO emitted from the burner to CO2 before it reaches the stack. This is only a concern in low temperature oxidizers such as catalytic oxidizers. NOX, however, is a concern for any oxidizer burner.
One method of reducing NOX emissions is using excess combustion air, which means you are supplying more air than what is required to effectively burn the fuel. The excess air cools the flame temperature, reducing the NOX formation.
The use of excess combustion air also increases fuel usage because this excess air must be brought up to temperature for proper combustion. Specific models of burners are designed for the purpose of meeting low emissions requirements and typically are sealed-nozzle-style burners. Sealed-nozzle burners are more efficient because they enable better control of the air/fuel ratio, tightly controlling how the fuel and air are mixed. The burner that meets the lower emission requirements with the lowest amount of excess air is the most fuel-efficient choice.
Air/Fuel Ratio Selection for Industrial Burners
Once the proper burner is selected, you must choose the correct air/fuel ratio control to optimize the burner performance. A raw-gas-airflow burner is controlled by modulating the fuel using a single control actuator while the airflow is not modulated. For a sealed-nozzle burner, both the air and fuel must be modulated to optimize the ideal mix of air/fuel to minimize NOX emissions.
Several methods can be used to manage and control the air/fuel ratios.
Ratio Regulator. The combustion air is controlled by a single control valve. An impulse line from the combustion air loads a regulator in the fuel line. As the combustion pressure to the burner increases, so does the gas pressure to the burner. It is a simple and reliable approach, but it does not allow the air/fuel ratio to be adjusted specifically for different firing rates. Also, it can have limited turndown.
Parallel Positioning. A gas control valve and a combustion-air control valve are either mechanically or electronically linked. The air and gas valves are synchronized and move together, opening and closing at the same rate to control the air/fuel ratio.
The valves can be characterized to set the air/fuel ratio curves from low fire to high fire. This allows for greater adjustment of the air/fuel ratio throughout the firing range. This characterization method allows for differential air/fuel ratios to be set at different firing rates to optimize fuel usage and emissions.
While this method is the most prevalent in the industry and offers control of your air/gas mix, it does not compensate for changes in air- and gas-system pressures, meaning it does not always permit the tightest control of emissions.
Mass-Flow Control. This system uses mass flowmeters in the combustion air and gas lines to measure exact fuel and airflow rates. Pressure fluctuations in the system are monitored and adjustments are made in order to maintain optimal air/fuel ratio. This is usually done on-the-fly via independently controlled valves driven by a PLC or smart combustion management program.
The air/fuel ratio can be set to optimize emissions and fuel usage and will maintain the proper mix, despite variations in pressure. This is the most precise option to minimize NOX because it compensates for changes in system pressure and changes in combustion air temperature to maintain the specific air/fuel ratio.
In conclusion, processors are interested in tight control of the process. As global concerns for greenhouse emissions increase, it is more important than ever to ensure that operations meet and maintain emission targets, and that equipment is sufficiently rugged to enable you to maintain these targets long term. While the burner and the control of the air/fuel ratio are only a portion of the entire oxidizer system, they are essentially the heart of the system. As such, they must be kept healthy over the life of the system.