Chlorine Formation in Thermal Oxidizers
Dan Banks, P.E.
President, Banks Engineering Inc.
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).
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.
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.
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.
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.