What is Carbon Monoxide?

Carbon monoxide is a toxic gas that can occur in homes and buildings where combustion by-products are generated and allowed to disperse.  It is colorless, odorless, tasteless and it is an asphyxiant.  As a poison, it is deadly at high levels.

At low concentrations, CO can go undetected and contribute to nagging illnesses.  It can compound pre-existing health problems and can go unblamed in premature deaths.

How is carbon monoxide formed?  How is it measured?

Carbon monoxide is a result of unburned fuel.  Fossil fuels require specific ranges of oxygen and temperature to allow for complete combustion.

CO production is commonly associated with insufficient combustion air.  However, excess introduction of combustion air (or insufficient fuel supply) can reduce flame temperatures to the point where CO is produced.  When any portion of a flame is reduced below 1128°, CO will be produced.  Flame impingement on heat exchanger surfaces, for example, can also result in lowering flame temperature and CO production.

If we completely burn all of the fuel and properly exhaust the by-products of combustion, we should not have any measurable CO.

Certain requirements must be met for combustion to occur.  The quality of combustion is dependent upon and rated against the quality of the fuel and its potential to burn completely under ideal or perfect conditions.

Fuel that has the potential to burn, like carbon fuels (C), must be surrounded by air or oxygen (O₂) but not flooded with oxygen.  Ignition or flash point heat must be enacted and maintained.  Fuel, air and heat must all be present or combustion will not occur.  In controlled combustion systems, fuel is forced into a combustion zone with limited time constraints because more fuel is being forced into the zone.

TIME TEMPERATURE TURBULENCE

Combustion is a very violent action.  Turbulence that is controlled helps ensure a complete burning of the fuel and the maintenance of flame temperature. 

As an example, if the flame temperature is cooled and the turbulence is affected, all the fuel may not have the time to burn because it is being forced out of the system by the force of fuel entering. This fragile window of time that fuel has to burn has now been flooded with fuel that spills where it can or it follows the strongest drawing forces, perhaps out of and below the ignition temperature zone.

The following diagrams of gas burners help illustrate how carbon monoxide is formed in some very basic and easy to see ways.  It must be remembered that all gas burners are designed to work with controlled fuel mechanisms and in environments that supply sufficient air and oxygen to the fuel at combustion.  These burners are designed to burn all the fuel.

The ignition temperature of natural gas is between 1100 and 1200°F and the flame temperature is around 3,000°F.  The burner with nothing on it has conditions that allow for the complete burning of the fuel and no CO is produced.  When a cold pan of water is set over that flame, a dramatic cooling of the flame occurs and the violent turbulence pushes or spills the unburned fuel out of the flame before it could fully burn and CO is produced.

As the pan surface heats up and the water begins to boil, CO generation may cease due to increased heat. (It is recommended that every gas range top burner be tested this way in front of the consumer to help educate them about intermittent carbon monoxide production.)  Additionally, it could also be demonstrated that a cooler flame also results in a longer time period for the water to boil.  This may be a fraction of fuel savings, but savings none-the-less and obviously less CO generation and dispersion into their living space. 

Carbon monoxide is typically measured in parts per million (PPM); the number of CO molecules present in a million molecules of air. 

CO is also measured and referenced as CO Air Free. This is a unit of measurement designed to compensate for the excess air to the burner and is only used to express CO levels in a flue gas sample as opposed to ambient air testing.

CO Air Free readings can only be calculated with a test instrument that measures both CO and O₂.  Most modern combustion analyzers will make CO air free calculations and provide a CO air free reading in the display.

Depending on the particular piece of test equipment, this CO Air Free reading may be expressed as “CF”, “Air Free” or “0% Corrected CO”.

This represents the CO levels with no excess air in the sample or with ‘perfect’ combustion (an unrealistic situation).

An analogy would be comparing two cups of coffee, one of which has had cream added.  You still have the same number of coffee ‘molecules’, just one has been diluted down with the cream.  A CO Air Free reading would be like performing a calculation on the coffee to remove the cream.

Also be aware that some ‘authorities of jurisdiction’ require CO readings corrected to a 3% O₂ reading.

Should a test instrument not make the calculations electronically, the CO and O₂ readings can be plugged into the following formula to derive the CO Air Free reading.

Another way to determine air free readings is to use the following chart.  The top horizontal column references the ‘diluted’ CO reading, the left vertical column references the flue gas O₂ reading.

For example, a burner operating with a 3.0% O2 and a 100 ppm ‘diluted’ CO reading equates to a 117 ppm CO Air Free reading.

Check with local codes to determine the allowable flue gas CO levels in your community.  Should local codes not address this issue, national ‘authorities of jurisdiction’ such as the American Gas Association (AGA) require flue gas CO levels be under 400 ppm Air Free.                                  

However, years of test experiences have found that CO levels in excess of 50 ppm Air Free (once the burner has reached steady state conditions) are indicative of a combustion problem and that CO Air Free levels in excess of 400 ppm Air Free on light of represents a ‘rough’ ignition.