Treating contaminants by thermal oxidation

The use of different processes for treating fumes and atmospheric compounds is an important public health issue. These pollutants include emissions from diesel engines¹ (ships, locomotives and trucks), carbon monoxide (CO), and volatile organic compounds (VOCs) from numerous industries. VOCs are one of the main ingredients of ground-level ozone, which in turn contributes to the formation of smog². In this article, we will explore ways to effectively deal with these emissions through oxidation using natural gas.

Recover or destroy?

In Quebec, air pollutant emissions are covered by the Regulation respecting the quality of the atmosphere, and certain municipalities have their own, sometimes more stringent, rules. A number of manufacturing companies are required to reduce these emissions below the levels prescribed by these regulations. Reusing or destroying these pollutants are the two main strategies for controlling them. It is preferable to reuse compounds if their calorific values are sufficiently important and the process is cost-effective. This is the case for some large steel mills, which recover the CO produced in the smelting process and recycle it via other related processes in the plant, using natural gas as a backup.

Otherwise, it is better to destroy them, particularly in the case of CO and VOCs. This can be done through thermal oxidation, again with the help of natural gas, a combustion-clean and efficient energy source. But as discussed below, even if the decision is made to destroy the compound, it is possible and desirable to recover lost energy through heat recovery.

Thermal oxidation (or incineration)

Thermal oxidation refers to the incineration or oxidation of odorous, hydrocarbon-based substances or VOCs, which are usually a mixture of several organic compounds. Because of the complexity of these mixtures, it would be tedious to describe in detail all the chemical reactions involved in this process. The important thing to remember is that the caloric values of some VOC mixtures are high, but often not high enough for the mixtures to burn or self-ignite. So they need to be heated using auxiliary burners that are usually powered by natural gas. When the mixture reaches a temperature between 800°C and 1200°C and enough time is allowed for the reaction to occur, combustion is intense enough to destroy the compound. As an added benefit, no contaminants are released. Most thermal oxidation systems also have an integrated heat recovery system (see diagram opposite).

The waste gases treated through thermal oxidation are mainly composed of water vapour, nitrogen, carbon dioxyde and oxygen. Other pollutants may be present in treated waste gases if the thermal oxidizer is not working properly or if the upstream pollutant content varies in time. Further upstream or downstream treatment of the gases emitted by the process may be required. This is especially the case when treating small amount contaminant levels diluted in high air flow, like painting exhaust process: upstream of the thermal oxidizer, the usage of a regenerative adsorption/desorption emission concentrator is required.

Regenerative thermal oxidation

Regeneration is another way to recover energy during the thermal oxidation process. This method, known as regenerative thermal oxidation (RTO), is used in most thermal oxidation processes. It employs heat exchangers (usually made of ceramic) located in two combustion chambers used in rotation. The heat exchangers store heat from the air flowing out of the device and then re-inject it into the incoming air. The heat recovery rate is normally on the order of 95%, and very little external heat is needed. Transfer valves are used to reverse the heat flows.

Catalytic oxidation

The advantage of catalytic oxidation is that it occurs at temperatures between 200 and 700°C, well below the 800 to 1200°C required for thermal oxidation. This reduces auxiliary power requirements and thermal nitrogen oxides produced through any high temperature combustion. Catalytic oxidation involves passing the preheated gases through a catalytic bed that enables oxidation. Catalysts are generally made from noble metals such as palladium and platinum, or copper oxides.
The gases to be treated first pass through a preheating chamber, which raises their temperature to that required for catalytic oxidation. The heat is supplied by a burner, usually fueled by natural gas.

The major drawback of catalytic oxidation systems is the possible deactivation of the catalyst by excessive temperature, particles, soot, or polymeric materials. Particulate-rich emissions must be pretreated using separator filters. In addition, most catalysts are contaminated by substances such as phosphorus, arsenic, and mercury.

Energy recovery from system effluents

Whether or not regenerative thermal oxidation is used, the possibility of recovering energy from gases flowing out of the system should be considered.
First and foremost, it is however important to:

Make sure that the combustion is optimized by automated controls
Make sure that the natural gas burner is functioning properly
Minimize uncontrolled air infiltration
Properly maintain the equipment

These aspects should be checked by experts, who can also suggest how to identify and recover available energy from system outflow, for example by:

Preheating make up air entering the system, building, or other combustion equipment in the plant
Preheating a heat transfer fluid used for any process
Using a recovery boiler that produces auxiliary steam or hot water to an existing grid
etc

Grant applications regarding these measures and analysis, including optimizing existing controls, may be submitted to Énergir’s energy efficiency program. Like RTO, ways to recover energy from the output of a thermal oxidation system can be considered such as a surplus cost of a referential system, whether a destruction or recovery strategy is chosen. If the installation of new equipment is being considered to handle existing contaminant emissions, Énergir’s program provides the opportunity to simulate energy use for a referential incinerator and compare the data to a more expensive recovery scenario.

Advantages of natural gas and RNG

Using a natural gas burner has several advantages, such as available capacity and rapid temperature rise to better control the destruction process. It is also more reliable because no contact with a heat exchanger is required (direct flame). The fouling risk is therefore reduced.

In that context, using renewable natural gas (RNG) can be an interesting path to follow. Combined with an energy recovery process, its usage reduces the overall environmental footprint of pollutants disposal.

Expertise and sound advice to help you choose the right technology

The preferred technology for treating fumes or contaminant emissions depends on the problem to be resolved. Constant gas flow is critical to the system’s cost effectiveness. Our Datech team can advise you if you have questions about how to incinerate contaminants from a wide range of processing operations. Please feel free to contact your sales representative to discuss how to save energy and reduce your carbon footprint.