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What are Flue Gases?

Mary McMahon
Updated May 21, 2024
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Flue gases are byproducts of combustion which are classically vented through long pipes known as flues. These gases are treated as pollutants. Flues can be referred to as “stacks,” and they may be found in the form of chimneys, ducts, or simple pipes. Large amounts of flue gases are generated around the world on a daily basis, with heavy industry and the power industry in particular being responsible for a huge percentage of the total generated. These gases are also created whenever people light a wood fire or drive a car.

The contents of flue gases are quite variable. The medium being burned can contribute a number of different compounds, and the conditions under which combustion is occurring can also generate more or less emissions. Incomplete combustion at low temperatures or in poorly managed facilities, for example, tends to generate more pollution.

Some things commonly found in flue gases include: water vapor, carbon dioxide, nitrogen, particulates, oxygen, carbon monoxide, nitrogen oxide, hydrocarbons, and sulfur oxide. Some of these compounds are potentially harmful for the environment, making these gases an issue of concern among environmental advocates. Flue gases can also be hazardous for human health, as might occur if they were trapped in an air inversion which pinned them close to the ground for several days, forcing people in the area to inhale hazardous pollutants which could damage their lungs.

There are a number of ways in which flue gases can be controlled, and processes which produce them are often heavily regulated to force emissions levels down. One of the best methods for control is to avoid generating them at all, either by using alternative technology, improving efficiency levels at a plant, or studying ways in which operating conditions could be improved to reduce the production of combustion byproducts. Gases which cannot be prevented can be trapped using filters and scrubbers which clean the air coming out of flues so that when it is released into the environment, it contains primarily harmless components.

In fact, scrubbing flue gases can even be profitable for a savvy company. For example, the food industry has a use for carbon dioxide, and is willing to pay for purified carbon dioxide extracted from flue gases. This use also resolves the problem of what to do with the pollutant once it has been removed from the flue. Emissions at flues are routinely tested to determine whether or not the gases are being adequately scrubbed before they are vented into the environment.

All The Science is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a All The Science researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments

By FrameMaker — On Oct 26, 2011

@highlighter- Your assumption that the new EPA rules are an infringement on state's rights is voided by the fact that the federal government is constitutionally allowed to intervene when the effects of the issue at question cause interstate consequences. The utilities that create the pollution cannot stop the wind from taking pollution across state land, and until they can the issue will remain a federal issue.

Additionally, before the rules were even implemented, numerous studies were conducted that concluded that rate raises form compliance would be well within the range of normal price fluctuations in the electricity market. None of the scare tactics over soaring energy prices hold any truth.

Furthermore, any utility that would shut down before compliance is already very outdated, and would have been struggling to stay afloat before the rules were implemented. This has nothing to do with destroying the free market. No one investing in these poorly managed firms did his or her homework, and that is part of the risk. I assume you are against bailouts, so why should other Americans bail these investors out for betting on high risk investments? Installing a gas scrubber would afford a utility the same tax treatments (depreciation) as any capital cost for investing in equipment and machinery.

By highlighter — On Oct 25, 2011

@Aplenty & Babalaas- While I understand your opinions and situations, I would have to disagree. The EPA has no right to mandate something like this that is supposed to be regulated by the individual public utility commissions (PUC) in each state. The EPA ruling gives the federal government too much power, and infringes on state’s rights.

The EPA is essentially enacting federal implementation plans for states to follow. The correct course of action would have been to create pollution targets that the states could then choose to comply with in ways that they see fit. The EPA plan runs incredibly close to interfering with the free market workings of our capitalist society. The plans that they have may not be the most cost effective plans for reducing pollution, and compliance could mean that utilities are breaking their obligation to shareholders. These utilities should be able to comply, or choose to take fines if compliance is too costly.

Gas scrubbing is expensive, and an older utility forced to comply might be forced to shut down. While it is sad that your daughter has allergies, is it fair to force all of the employees that work at that power point into unemployment? Is it fair to force shareholders to take a loss resulting from this forced compliance?

By Babalaas — On Oct 24, 2011

@Aplenty- I agree with you completely. The United States is the most developed country in the world, but if the states and utilities opposed to this new air quality rule win their lawsuits against the federal government, the air will soon look more and more like China's. The fact that this has turned into a political dogfight between republicans and democrats is ridiculous. The environmental regulations that republicans are trying to defeat were originally enacted under a republican, Richard Nixon, so the issue should not be one of politics. My daughter is allergic to pollutants, and has severe allergy attacks on bad pollution days.

Why should I be responsible for paying her doctor's bills and missing work and school to take care of her when other companies are dirtying the air for their own profit? It shouldn't be too hard to install a few flue gas scrubber systems on older, dirtier power plants. My daughter should not be held hostage so people in other states do not have to pay a few cents extra per month for their electricity bills. Energy demand is only increasing so if we do not start to enact heavier pollution regulations, health problems and deaths associated with pollutants and air borne carcinogens will only increase.

By aplenty — On Oct 23, 2011

I do not understand what the big fuss is about over the EPA's new rules on power plant pollutants. I think that the new rules governing cross border air pollution standards are a great idea because I am personally tired of dealing with air pollution in my clean state that crosses over from dirty states.

When I lived in Vermont, we would often have bad pollution days because of pollutants drifting over from Ohio's coal fired plants. Now I live in Arizona, a state that gets most of its energy from clean nuclear and hydropower, and we get added air pollution from states like Texas with very little environmental controls. You would think that a country as advanced as ours would work to make sure the air we breathe is clean.

I really hope that the EPA succeeds in regulating air interstate air quality. I would gladly pay a little more on my monthly utility bill so that the utilities could install gas scrubbers on dirty power plants. Maybe this will push us towards using intrinsically cleaner and more efficient fuel sources like natural gas, nuclear, and renewables.

Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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