Flame Safeguarding using Ionization Detection

Principles of Ionization Flame Monitoring

An ion is a charged atom that has either gained an electron to become negatively charged (anion) or has lost an electron to become positively charged (cation). The energy released during a combustion process will cause electrons to be knocked loose from an atom, resulting in a positively charged particle and a free electron. This ionization, if monitored properly, can be used to generate a safe and reliable indication of a flame.

During normal combustion: e.g., burning methane in the air and getting carbon dioxide and water vapor, the following chemical reaction occurs:

CH₄ + 2O₂----->CO₂ + 2H₂O

However, there is sometimes an intermediate step in which a uniform proportion of the molecules in this reaction do the following:

CH₄ + 2O₂----->C⁺⁺ O₂ + 2H₂O + e^- ----->CO₂ + 2H₂O

The number of ions produced is greatest where the chemical reaction is the strongest. If the air-fuel ratio is optimal, the reaction will be the strongest, and more free ions and electrons will be produced. Since the electrons are so much lighter than the ions, the electrons travel much faster and move away from the burner mouth toward the tip of the flame much more quickly than the heavier ions. This leaves a greater concentration of positively charged ions in the area near the burner mouth than free electrons.

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Flame Detection Using Flame Conductivity:

If two electrodes are placed in or near the flame and a voltage is applied to the electrodes, a current will flow between the electrodes using the ions and the electrons as charge carriers. A current will flow between the electrodes when a flame is present, but there will be no current when the flame is not present.

This type of system has one serious and potentially dangerous problem. If another current path between the electrodes develops, such as a coating of soot on the electrodes and insulators, this leakage current would give the indication that a flame
is present at all times. To overcome this dangerous failure mode, Hegwein pilots
and burners uses a flame detection system that uses the rectification characteristic
of the flame.

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Flame Detection Using Flame Rectification:

In a rectifying flame ionization detection system, an alternating potential (AC) is supplied to the two electrodes. In addition, one of the electrodes must have a large surface area compared to the other electrode. To increase the size of one of the electrodes and to simplify construction, one of the electrodes is the burner tube. The small electrode is then electrically isolated from the burner tube (the other electrode) with ceramic insulators.

 
Since the majority of the charge carriers near the ionization detector electrodes are positively charged ions, as described above and shown in figure A, the negatively charged electrons can be ignored for the purpose of this description. It is important to remember for this discussion the basic principle that oppositely charged particles attract and like-charged particles repel one another. When the small electrode is at a positive potential compared to the burner tube, the positive ions are repelled from the small electrode and attracted to the lower potential burner tube. When the small electrode is at a negative potential compared to the burner tube, positive ions are attracted to the negative small electrode and repelled from the burner tube. When the ions contact the metal surface, whether it is the small electrode or burner tube (large electrode), an electron from the metal combines with the positive ion. This neutralizes the charge of the ion and the particle is no longer an ion--it is back to being a normal carbon atom. The metal surfaces giving up an electron is what generates the current flow that is measured. This neutralization of the ions occurs more frequently (and results in a larger current) when the burner tube is at the negative potential rather than when the small electrode is at the negative potential. This occurs because the surface area of the burner tube is much larger than that of the small electrode and it is easier for the ion to run into the larger target. Figure D shows the potential on the electrodes and the associated current through the ionized atoms.

If the current meter reading is averaged, the result would be a direct current (DC) value. Once the flame is lit, the ionization of the gases occurs, and the DC value of the ionization current hits a predetermined threshold, the "flame on" signal is generated. If the small electrode (sometimes referred to as the flame rod) should short to the burner tube, the AC signal would be large but it would be the same magnitude in both the positive and negative directions so the resulting DC value would be zero. If soot or other contaminants covered the ceramic insulators and allowed current to leak between the small and large electrodes, it would be the same for both polarities, and the resulting current would still have a DC value of zero. Figure D illustrates the fact that there is a small current present when the small electrode is at a negative potential compared to the burner tube. This is because there are some free electrons in this vicinity (but at a much lower concentration than the positive ions) that allow a very small current to flow during this portion of the sine wave.

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Application Summary

This flame ionization detector utilizing the rectifying nature of the flame allows Hegwein to build an extremely reliable and safe flame safeguarding system. In addition to the flame detection system, a spark rod is built in to ignite the flame. In this fashion, the burner or pilot can integrate the spark igniter, and flame detector into a single housing. This greatly simplifies installation and reliability when compared to installing all three of these components as separate systems.

DURAG, Inc.
St. Paul, MN
Oct.13,1999

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