Oxidation has proven to be an exceedingly capable method of removing Volatile Organic Compounds (VOC), Hazardous Air Pollutants (HAP), and odors from a process exhaust stream. The effectiveness of the oxidation system’s sustainability depends on several interrelated elements, of which the combustion process is often the most consequential. That process is itself controlled by three main factors, the three “T’s”:
Oxygen content and equipment design features that enhance the combustion process are also highly relevant.
When a hydrocarbon or organic vapor is being burned, it must be held at a specific temperature for a sufficient time to ensure that the organic compounds burn completely. 100% combustion means that the hydrocarbons are fully oxidized (i.e. complete oxidation of the carbon, hydrogen, and other combustible elements has taken place).
If the retention time of the hydrocarbons in the purification combustion zone is insufficient for full combustion, carbon monoxide may be generated and unburnt hydrocarbons will be present in the exhaust stream, resulting in a reduction of hydrocarbon removal efficiency (DRE). On the other hand, excessive retention time and the use of a larger than necessary purification combustion chamber will result in the consumption of wasted auxiliary fuel, adding both capital and operating costs to the process. Ideally, hydrocarbons should be heated only as long as it takes to completely oxidize them before they are exhausted. Thus, timing plays a very important role in determining (DRE).
DRE is directly related to retention time (i.e. increasing retention time will increase DRE linearly).
Uniformity of air flow and temperature in the combustion zone is also a key factor. NESTEC incorporates specialized design features to ensure steady, maximum air flow and consistent temperatures in the purification combustion zone.
Oxygen is an essential part of the process of combustion. The higher the oxygen content, the greater the combustion of the hydrocarbons. In addition to being in sufficient supply, oxygen must also be thoroughly mixed with the hydrocarbons in the system for optimal combustion. Adequate turbulence ensures a thorough blending of the two and improves temperature uniformity. If turbulence is not maintained, certain parts of the fuel will have excess oxygen available for combustion and the remainder will have too little. This will result in the incomplete combustion of carbon, forming carbon monoxide instead of carbon dioxide. If proper turbulence is not maintained, unoxidized hydrocarbons will be exhausted, decreasing DRE.
The DRE will vary based on the turbulence factor (i.e. increasing turbulence will increase the DRE to the second power).
During the combustion process, if the purification temperature is not sufficiently high, hydrocarbons will have a longer ignition lag time, thus increasing the retention time for maximum DRE. Hence, it is very important to maintain a uniform and optimal temperature for complete combustion of the hydrocarbons.
Increasing the purification temperature offers the greatest method for improving DRE, typically increasing it to the fourth power.
If your oxidation system is coming up short in achieving the required DRE for your permit requirements, and you want to find the best corrective solution, contact three reputable equipment suppliers, making sure that one of them is NESTEC.
For assistance from our extensive experience, call or e-mail NESTEC Inc. for a free evaluation of your VOC control equipment needs.
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