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Module 6: Air Pollutants and Control Techniques - Incompletely Oxidized Compounds - Formation Mechanisms
Lesson Material
Practice
Problems
Objective
- Describe the basic mechanisms that contribute to the formation of incompletely oxidized organic compounds and carbon monoxide.
Many sources of partially oxidized organic compounds and carbon monoxide have the potential to be major sources of nitrogen oxides. Combustion sources, such as boilers and incinerators, and automobiles are potential generators of both these categories of pollutants. However, the conditions that favor nitrogen oxides generation are opposite of those that contribute to the formation of incompletely oxidized organic compounds and carbon monoxide. In other words, when combustion conditions are modified to suppress nitrogen oxide formation, conditions are not favorable for the complete oxidation of organic compounds.
In combustion processes, partially oxidized organic compounds and carbon monoxide are emitted from localized areas of the combustion zone where the gas temperatures are too cold or where there is insufficient oxygen. The organic vapors are initially released from the fuel or waste during the early stages of combustion. Once in the vapor phase, they are available to participate in the oxidation reactions. These reactions persist as the combustion gases pass through the high gas temperature zones of the system.
The oxidation reactions do not go to completion if the gas stream becomes too cold or if there is insufficient oxygen. These conditions can occur due to a variety of combustion problems.
- Poor fuel-air distribution in the boiler or incinerator
- Cold air infiltration into portions of the boiler or incinerator
- Insufficient excess air levels
- Excessive moisture levels in the fuel or waste being fired
The partially oxidized organic vapors, which remain as the oxidation reactions stop prematurely, include some nonvolatile compounds, which nucleate heterogeneously and some volatile compounds, which remain in the vapor state throughout the air pollution control system. These volatile organic compounds can then form particles in the plume due to the cooling action of the ambient air mixing with the gas stream (condensable particulate matter).
The easily measured carbon monoxide concentration is a good indicator species for the difficult-to-monitor concentrations of the partially oxidized organic compounds. As indicated in Figure 30, carbon monoxide formation is more sensitive to low oxygen concentrations (and reduced gas temperatures) than partially oxidized organic concentrations are; however, as the CO levels increase, the concentrations of the partially oxidized organic compounds also increase.
When the fuel-to-air ratio is in the correct range, the emissions of CO and the partially oxidized organic compounds are at a minimum. When there is insufficient oxygen, CO and partially oxidized organic emissions increase rapidly as indicated by the left side of the curve shown in Figure 2. When too much air is used, the gas temperatures are reduced due to the heat taken up in heating the excess oxygen and nitrogen in the combustion air stream. These reduced gas temperatures contribute to CO and partially oxidized organic emissions as indicated by the right side of the curve shown in Figure 2.
Practice Problems
Incompletely Oxidized Compounds - Formation Mechanisms
- Instructions:
- Complete the Practice Problems before proceeding to the next section. Click on the button below.