Thermal Oxidizers (and their pollution preventing powers)

Heavy industries transform material from the earth. The material are usually combinations of organic and inorganic materials.

As we know chemistry is sub divided into ‘organic” and “inorganic”. Organic chemistry is defined as “A discipline within chemistry which involves the scientific study of the structure, properties, composition, reactions, and preparation (by synthesis or by other means) of chemical compounds consisting primarily of carbon and hydrogen…” [source]

The word “organic” implies those materials that are seen to be originally derived from plants.

Whenever we work with materials from the earth and recombine using heat and pressure, we drive off gases — fumes and smoke — that are both organic and inorganic. These by-products of transformation are those things that when carried in the air become acid rain, smog, or noxious odors — what we call generally term air pollution.

To avoid the harmful effects of these discharges industries are mandated and regulated to capture and control these releases.

For those processes where the discharge is primarily particulate-based process guys use electrostatic precipitators (ESPs).

For those processes where the discharge is primarily organic and gaseous based we typically use thermal oxidizers.

We will talk about ESPs and scrubbers at some time in the future but for now I want to concentrate on air-borne organic compounds controlled using thermal oxidizers.

The organic gaseous air stream are labeled “Volatile-Organic-Compounds” or “VOCs” because they are volatile ie possess a tendency to vaporize and are organic (contain carbon and hydrogen). Examples of VOCs are the fumes from oil based paint used in a manufacturing plant’s paint shop, to odors coming from a sewage treatment plant to a food processing company such as a distillery. In each case as these organic compounds are airborne they can enter the lungs and bloodstream of people and may cause harm.

Sometimes they are pleasant such as a bakery. Other times not so pleasant like a factory converting hydrocarbons into plastic products.

But in all cases because they are organic materials they will have a heat value. In essence they are a fuel and can be treated by being burned. A thermal oxidizer accomplishes this by using high heat in a controlled environment to destroy the VOCs. The destroyed VOCs are turned them into water vapor and carbon dioxide, which are much safer, before being released into the atmosphere.

If there is a fair amount of VOCs with higher heat value — hence a higher fuel value — they can self combust in units call “regenerative thermal oxidizers”. These units may use a small amount of natural gas to get the process started and from there on the organic material off-gas sustains the reaction into water and CO2.

Sometimes however the VOCs are so dilute, or the fuel value within the organic material is so low, that the controlled burn must happen with extra heat added – typically natural gas as a supplement fuel. In these cases the type of thermal oxidizer is termed a “recuperative thermal oxidizer”.

However in this last case using catalyst technology we can often enhance the heat value of the air pollution stream to be a self-sustainable burn. In these types of “concentrator thermal oxidizers” a catalyst is used to accumulate the organic material from the very diffuse VOC-laden air stream. The catalyst is then heated to liberate the collected, organic compounds into a secondary air flow stream that has a much lower air flow.

Because the air flow is much lower than the higher original air flow — but with the same amount of organic VOC content — the concentration of the organic compounds is higher i.e. the parts per million (or ppm) of the organic material is higher.

As we get the ppm higher it will approach the lower explosive limit (LEL). As we get closer to the LEL we get closer to the point at which it will burn in a self-sustaining manner. And if we can get the concentrated air stream above the LEL it will be self-sustaining thereby negating the need for supplemental natural gas.

In this manner, the cost of operating the thermal oxidizer is decreased radically as fuel use in a recuperative unit can be significant and therefore expensive.

In fact, retrofitting recuperatives into regenerative units or catalyst-enhanced concentrator regeneratives can save a ton of operational dollars and is well worth exploring.

Particularly as burning natural gas — although considered a clean fuel — also impacts the environment.


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