Removing VOCs and HAPs from Exhaust Streams
Thermal oxidation can effectively remove airborne contaminants from process exhaust streams. Oxidizers can incorporate heat recovery for process use to maximize return on investment.
Thermal oxidation is the most commonly used method to control air pollution emissions. Oxidation systems also are referred to as incineration or combustion-type systems because they rely on the process of combustion to oxidize volatile organic compound (VOC), carbon monoxide (CO) and volatile hazardous air pollutant (HAP) emissions down into carbon dioxide (CO2) and water.
Oxidation, or combustion, is a chemical process arising from the rapid combination of oxygen with various elements or chemical compounds, resulting in the release of heat. An oxidation reaction follows the general equation:
CxHy + aO2 → bCO2 + cH2O
CH4 + 2O2 → CO2 + 2H2O
The principle of oxidation or combustion on which this process is based is simple. The oxidation of VOCs needs certain activation energy such as thermal energy to initiate the reaction. The amount of activation energy — or necessary temperature — depends on how strong the chemical bonds are between hydrogen (H2), carbon (C) and other possible atoms. Therefore, the required reaction temperatures vary quite a lot based upon the specific compounds found in the process stream. Oxidation systems have a high VOC destruction efficiency because at sufficiently high temperatures, essentially any hydrocarbon can be oxidized down to carbon dioxide and water.
Another unique property of these oxidization systems is their ability to destroy small particulate matter as well as airborne VOCs. Generally, these processes will be able to destroy more than 95 percent of the VOCs in the process gas. These systems also can reduce acid gas, NOX and particulate matter when combined with other methods of VOC abatement such as scrubbers, baghouse filters and selective catalytic reduction (SCR). Using such technologies, industrial process thermal oxidizer makers can offer more than 99 percent destruction and removal (DRE) efficiency guarantees.
Air-pollution-control systems based upon oxidation are useful because of their overall design simplicity. The three most important components of a thermal oxidizer are:
- Combustion chamber.
The combustion chamber is the reaction zone of the unit and provides a place for the process gas to mix with oxygen at elevated temperatures to complete the combustion process. The burner is used to ignite fuel and supplements the process gas flow while maintaining the appropriate combustion temperature. The blower is used to draw air through the incinerator and maintain the system’s pressure levels.
To achieve complete oxidization of VOCs once the process air waste and fuel have been brought into contact, the following conditions, referred to as the three Ts of combustion, must be provided:
- Temperature high enough to ignite the waste-fuel mixture.
- Turbulent mixing of the air and waste-fuel mixture.
- Sufficient residence time for the reaction to occur.
The rate at which a combustible product is oxidized is greatly affected by temperature. The higher the temperature, the faster the oxidation reaction will proceed. Thermal destruction of most organic compounds occurs between 1200 and 1600°F (590 and 650°C). However, with VOC oxidation, the maximum temperature must be limited because of formation of thermal nitrogen oxidizes (NOX). The higher the temperature, the more the nitrogen and oxygen contained in the air will start to react together, forming NOX.
As oxidation systems are designed to operate under specific temperature windows, careful consideration of material and insulation are key factors. Materials must be able withstand high heat. Proper insulation regulates the combustion chamber temperatures and ensures that the equipment exterior surfaces remain within safe handling temperature limits.
The time for which the pollutants stay in the incinerator is called residence time (time).It is during this period that the oxidation process occurs, the compounds interact and the VOCs break down. The greater the residence time, the lower the temperature can be within the combustion chamber. A longer residence time provides more of an opportunity for these compounds to interact and be oxidized.
In addition to temperature, the residence time of the system is dictated by the air velocity, which can range between 10 and 50 ft/sec within the chamber. However, in general, most VOC oxidation requires between 0.5 and 2.0 sec.
The residence time of gases in the combustion chamber may be calculated by:
t = V / Q
t is the residence time, measured in seconds.
V is the chamber volume, measured in cubic feet.
Q is the gas volumetric flow rate combustion, measured in cubic feet per second.
For complete combustion to occur, every particle of waste and fuel must come in contact with air (oxygen). If this does not happen, unreacted waste and fuel will be exhausted from the stack.Incomplete combustion of the pollutant gases can result in increasingly toxic byproducts. Therefore, engineers must follow good design practices to avoid these problems. This is where air movement or turbulence is introduced to the system (turbulence).
A number of methods are used to improve mixing the air and waste streams, including the use of refractory baffles, swirl-fired burners and baffle plates. Unless properly designed, many of these mixing devices may create dead spots and reduce operating efficiency. Controlling turbulence — the process of mixing flame and waste stream — to obtain a uniform temperature is the most difficult part in the design of an oxidizer.
Some process streams benefit from the addition of a catalyst to affect the rate of reaction, temperature, pressure and required reactor volume. A catalytic oxidizer typically has lower operating requirements and can greatly reduce the combustion fuel requirements. Despite these benefits, catalytic oxidizers remain less common due to the high cost of the catalyst material and the restrictions for handling and disposing of the eroded catalyst. (The spent materials can be toxic to both handlers and the environment.) In addition, catalytic oxidizer efficiencies are typically between 95 and 98 percent.
Another way to reduce fuel usage required for oxidation is to incorporate some form of heat recovery into the system. The percentage of heat recovery in the design of thermal oxidizers generally increases with decreasing inlet VOC/HAP concentration. One of the biggest challenges with oxidation is managing low VOC content. In order to oxidize a small concentration of VOC, an enormous amount of air must be heated to a temperature where the reactions can happen. In many cases, the energy contained in the VOCs is not high enough to maintain continuous oxidation. Operating an oxidation system with low VOC concentrations without heat recovery methods in place is inefficient and has a high associated operating cost. Therefore, the type of oxidizer and heat recovery methods are largely determined by the VOC/HAP levels found in the pollution stream.
Heat-exchange methods are the defining feature of a thermal oxidizer and are designed to achieve a predetermined amount of effectiveness. Their design parameters must be meticulously calculated to ensure maximum process efficiency.
Both primary and secondary heat-exchange methods are used. A primary heat-exchange method is the preheating of the process gas as it enters the thermal oxidizer. The heat is transferred from the hot gas leaving the combustion process to the cold air entering it. A secondary heat-exchange system uses the waste heat from the combustion process to run other parallel production processes throughout the manufacturing facility such as ovens, furnaces, hot water generators and general comfort heating.
Evaluating the appropriate oxidation system for a specific process stream is challenging and requires in-depth analysis of the specific process gas and operating parameters. Oxidation systems are designed based on the following parameters of the process gas:
- Concentration of combustibles in the waste stream.
- The amount of oxygen in the waste stream.
- The amount and type of particulates in the waste stream.
- Inlet temperature of the waste stream.
- Rate of heat loss from the combustion chamber.
- Residence time and flow pattern of the waste stream.
- Combustion chamber geometry and materials of construction.
While oxidation systems can be used over a fairly wide range of organic vapor concentrations, there are potential hazards with these systems. They include overheating, flashbacks, fires or explosions.To mitigate these risks, flame-safety detection devices, flame arrestors and control valves are added in pollutant lines, and the system must always be maintained under negative pressure. In addition, the concentrations of the organics in the air must be substantially below the lower flammable level to enhance the safety of the system. Typically, this means operating at 25 percent or below of the lower explosive limit (LEL). With additional process monitoring, systems can be operated at up to 49 percent LEL.
Because there are many different types of oxidation systems and configurations, when considering the appropriate oxidation system, it is critical to have engineering expertise involved from the beginning.Oxidation specialists can ensure proper operating safety and controls and maximize the overall design efficiencies and cost optimization.