Thermal oxidizers offer a safe, effective and reliable solution to the need for the elimination of objectionable waste streams, odors and volatile organic compounds (VOCs) while complying with environmental regulations. With proper selection of the thermal oxidizer and the employment of lightweight refractory linings and natural-draft operation, the technology can act as an immediate backup to other pollution-control methods in the event of an equipment or power failure. Sizes can vary from large stationary units to micro units for laboratories, pilot plants and research. Modular oxidizers that bolt together in the field can provide a further range of sizes for both stationary and portable applications.
Waste streams can range from gases and vapors to liquid, two-phase flows to slurries. Typical thermal oxidizer applications can be found in chemical and process plants, food processing, pulp and paper, wastewater treatment, soil remediation, pilot plants and some fields of research, including alternate energy, biomass, fuel cells and hydrogen.
Thermal oxidizers consist of the following components:
- Waste feed system. This includes the inlet piping with isolation and control valves, flame arrester, disentrainment or water liquid-seal drums.
- Fuel piping trains to burner and pilot, including the inlet piping with isolation and control valves.
- Combustion chamber with high temperature refractory lining.
- Air inlets (air dampers or blowers).
- Waste burner, assist fuel burner and pilot.
- Flame safeguard, temperature, overfire, fuel and air controls.
- Sample ports for performance monitoring of the pollution-control device.
Oxidizers also might include secondary recovery for toxic components in the oxidizer combustion exhaust (chlorine, sulfur, particulate) and heat recovery technology.
Waste Feed System for Industrial Oxidizers
Considerations in the selection of a thermal oxidizer start with plant and process needs. The first step is to determine the flow rate of each waste stream (maximum, normal, minimum). If there are multiple waste streams, review to see if they can be combined or must be kept separate with individual waste trains and burners. Most thermal oxidizers have one waste train while some have two or more.
Different types of waste streams might include high and low pressure streams or wastes that react if mixed together. It is important to provide a detailed waste composition for each stream along with waste temperatures and available pressure. If the pressure is low, then a booster blower or, for liquid, a pump will be required. Pressure is critical to deliver the waste through the feed piping, equipment (arresters, drums, valves) and burner.
In addition, it is important to know the composition of the waste. This information is used to select the material of the piping and other equipment. Waste composition is also important in the selection of the stack material and refractory lining. For example, does the inside of the stack need to have a corrosion-prevention coating if the thermal oxidizer is not continuously running due to cool down and condensate formation? Waste composition and temperature are important for streams that will condense inside the feed piping system.
The necessity of heat tracing also is a consideration. Heat tracing might be needed to prevent plugging and fouling. If large amounts of water vapor are present in the waste, then a knock-out pot or disentrainment drum is required. Special care should be taken with flame arresters because they have small passages to stop flame travel. Plugging and cleaning can require constant maintenance. Sometimes, a prefilter before the flame arrester will solve the problem. Also, heat tracing and keeping the waste and flame arrester warm will eliminate plugging.
At the same time, care should be taken that the flame arrester is not overheated because this will allow the flame to pass. For dirty waste streams that require a flame arrester, a liquid- or water-seal drum will provide protection against flame flashback. Seal drums have a bubbling flow through them to prevent flame passage. For streams that are highly reactive such as hydrogen, special detonation seal drums are used. Water is the most common media in the seal drum, but for some chemically reactive streams, other seal liquids are required.
Assist fuel might be required for the thermal oxidizer if the heat content of the waste is low. With medium-to-high waste heating values, the waste is the fuel. However, pilot gas will be needed for ignition of the waste stream or assist gas burner. Normally, natural gas is used, but propane and other fuel gases can be used. For locations where fuel gas is not available, oil-fired pilots or even high energy electric ignition is possible; however, both are more complex and costly than normal gas pilots.
Assist and pilot fuels are controlled in separate trains. The assist line will have pressure gauges, a pressure switch and solenoid valves (normally double block and bleed). The valves turn on or off the fuel, and a modulating control valve regulates the fuel flow for the combustion process. A pilot gas line normally is equipped with a pressure gauge, pressure switch and a single solenoid valve for on/off operation. All fuel lines will have manual isolation valves. Many times, strainers or pressure regulators are included as well.
The combustion chamber can be vertical, horizontal or down-fired. The most common configuration is vertical, where the combustion chamber also acts as the stack. Horizontal combustion chambers are used when secondary pollutant or heat recovery is required. The down-fired model is used where particulate is present. Combustion chambers can be in any shape — circular, rectangular or multi-sided (with six to 20 sides). Rectangular chambers with their sharp corners can be a problem for proper burner mixing and operation. Multi-sided units are used for large thermal oxidizers that require final assembly at the jobsite. The multi-side units have an open corner that is similar to the circular and does not cause problems.
The configuration and location of the combustion chamber also is controlled by available plant space. Most of the time, the thermal oxidizer is in a location away from buildings or other equipment. However, in some plants, space is not available for such placement. In these cases, the unit might need to be positioned on the roof or an elevated platform to provide safe elevation for the oxidizer exhaust.
Small units are sometimes located inside the plant building with the stack going through the roof. These are rare cases and need safety review for possible leaks and sources of combustible materials. The exhaust temperatures of 1400 to 1800°F (760 to 982°C) will need to disperse, so the safety of nearby people and equipment must be evaluated. The combustion chamber is lined with refractory to control the high temperatures. Because the outside shell of the chamber can be in the range of 250 to 350°F (121 to 177°C), personnel protection shielding is included to protect operators and anyone working in the area.
Combustion air is supplied through inlets on the thermal oxidizer. For vertical units, the inlets are normally at the base of the combustion chamber to provide the greatest working volume for the oxidization process. For horizontal units, the inlets are located on the front end. However, some oxidizers have secondary air inlets later in the process that are used for NOX control. If the oxidizer is handling a simple case of near-constant flow and composition, then a simple inlet is all that is needed. For most oxidizers, however, these inlets will have automatic control dampers that are regulated by the control system for temperature and oxygen levels. If the flow range and composition vary widely, then special tight dampers must be used to control the air. Normal air dampers leak and do not provide air control in their low range.
The moving or motive force for the combustion air can be natural draft. Negative pressure is created by the high temperature, low density products of combustion rising in the combustion chamber and stack. Forced draft (positive pressure) is created by a blower. Induced draft (negative pressure) is created by a high temperature blower in the discharge of the oxidizer to pull the flue gases through secondary treatment such as a quench tower and scrubber.
The inlets to the thermal oxidizer need to be considered as a source of ignition if the area around them could accumulate any flammable vapors from a leak, release or spill. If such a condition can occur, even if it would be unlikely, some means of safety protection is needed. Commonly used approaches include flame arresters on all inlets, elevated air-intake snorkels and gas detectors with flame-shutdown control.
The waste and assist burners are the most important components in the system. They provide proper distribution, mixing and turbulence to complete the oxidization process.
When the heat content of the waste is sufficient, only a waste burner is needed. For an application where the flow and composition range is limited, a simple drilled-pipe burner can be used.
If the flows and composition vary, however, problems of internal burning, carbonization, pulsations and poor combustion will result. Burner tips and mixing nozzles normally are used to prevent these problems and provide the required waste-to-air distribution, mixing and turbulence. Their configurations and positioning are critical for good combustion with low CO and NOX.
Some burners have small secondary combustion chambers and recirculation to enhance operation. If the waste stream can have air or oxygen present, then flame-arrester burners are used to prevent flashback of the flame into the piping and upstream process.
For liquid waste, air or steam atomizing burners are used. Some cases of “light” liquid waste (low viscosity and density) will allow the use of a simple mechanical atomizer, which uses high pressure.
In conclusion, if you have considered all of the conditions under which the oxidizer will operate, you are in a good position to specify an air-pollution-control system. A properly specified thermal oxidizer should provide many years of reliable service.