Chlorine Formation in Thermal Oxidizers

By

Dan Banks, P.E.

President, Banks Engineering Inc.

www.banksengineering.com

Thermal Oxidizers (TOX units) are often used to dispose of chlorinated hydrocarbons (hydrocarbon molecules which contain at least one atom of chlorine).

The simplest chlorinated hydrocarbon is Methyl Chloride, which consists of one atom of carbon, three of hydrogen and one of chlorine. The formula for Methyl Chloride is CH₃Cl. The next simplest chlorinated hydrocarbon is Methylene Chloride, which contains two atoms of chlorine (formula CH₂Cl₂).

Carbon Reactions

When Methylene Chloride is destroyed in a TOX unit, it is broken down by the heat, as oxygen from the combustion air is added. The carbon atom combines with oxygen to form carbon dioxide (CO₂) as well as a small amount of carbon monoxide (CO). There is always some CO present in the TOX flue gas because CO₂ and CO exist in a "chemical equilibrium", which can be described by the equation:

CO + ½ O2 <--> CO2

This equation says that CO₂ can be converted to a mixture of CO and O2, while CO plus O2 can convert to CO₂. In nature CO₂ and CO always exist in equilibrium with each other, and that equilibrium favors CO₂. Thus, at high temperature and with the passage of time, any mixture of CO₂ and CO will end up with mostly CO₂ and very little CO left. Laboratory testing is used to determine the "equilibrium constant" for this reaction so chemists can calculate how much CO to expect in any particular situation. The equilibrium constant for this reaction at 1800°F is 15,848,932 - such a large number indicates that the final mixture will contain much more CO₂ than CO. The equilibrium constant is not infinite, which means there will always be at least a trace of CO remaining in the TOX flue gas. Note that the source of the carbon and oxygen atoms does not affect the equilibrium reaction, so burning natural gas and burning fuel oil both result in the same CO₂/CO ratio, everything else being equal. The ratio can be changed by changing the reaction temperature or by changing the amount of O2 present.

Effect of Temperature

At 3000°F the equilibrium constant is 1000, indicating that more CO (and less CO₂) will exist at higher temperature.

Effect of Oxygen

If oxygen is added to the reaction, less CO will remain. In the equilibrium equation, balance is always maintained if the temperature does not change. So if you increase the amount of O2, you must decrease the amount of CO a corresponding amount so that the left side of the equation will continue to equal the right side of the equation.

Hydrogen Reactions

The hydrogen atoms in Methylene Chloride react according to this equation:

H2 + ½ O2 <--> H2O

The equilibrium constant for this reaction is also a very large number, indicating that almost all of the hydrogen present will exist as H₂O in the TOX flue gas.

Chlorine Reactions

The chlorine atoms in Methylene Chloride react according to this equation:

Cl2 + H2O <--> 2HCl + ½ O2

At 1700°F, the equilibrium constant for this reaction is 9.5. This means that most of the chlorine atoms will leave the TOX furnace as HCl (hydrogen chloride gas molecules), but a considerable part will be in the form of Cl₂ (chlorine gas molecules). As above, the source of the chlorine atoms does not matter - the TOX flue gas will always rearrange itself to agree with the equilibrium equation. Also as above, adding water vapor (H₂O) will reduce the amount of Cl₂ present. And adding 0₂ will shift the balance back towards Cl₂.

Example:

In one incinerator, 1.0 gpm of methylene chloride is vaporized and fed to the furnace for destruction. The other feeds to the TOX unit are fuel oil, air, steam and nitrogen. The methylene chloride is oxidized to CO₂, CO, H₂, H₂O, Cl₂ and HCl. In addition, traces of methylene chloride also remain, giving a hydrocarbon destruction efficiency of almost 100%. If all of the chlorine atoms were converted to HCl, we would measure 573.3 lb/hr of HCl in the TOX flue gas. Due to the chemical equilibrium (constant = 9.5 at 1700 °F) we actually will measure 5.77 lb/hr of Cl₂ gas, with the rest of the chlorine existing as HCl. This calculation can be made for any particular waste gas composition, waste gas flow, air flow, fuel oil flow and furnace temperature. In this way we can confirm direct measurements of the chlorine content of any TOX flue gas stream.

The HCl / Cl2 equilibrium graph is widely published in text books and papers. For a copy, contact us at (918) 740 5657.