Chemical transformations of nitrogen oxides while sampling combustion products
Summary (2 min read)
Types of Transformation
- Chemical transformation of nitrogen oxides in probes and sample lines may be of three general types:.
- An additional transformation path, formation, involves the oxidation of nitrogen containing species such as ammonia (NH3).
- The potential significance of the transformation of nitrogen oxides rests on the use of the emissions data.
- Emission standards for nitrogen oxides are currently proposed or promulgated in terms of nitrogen oxides, NO*. [1] [2] [3] [4] [5].
- Emissions data biased by NO X removal reactions are unacceptable as the basis for any emissions standard, control strategy, or enforcement action.
Assessment of local air quality impact
- The NO/NO2 emission ratio is important in assessing local air quality impact from major sources.
- Ambiguity regarding the NO/NO2 emission ratio from combustion sources presently precludes consideration of the impacts of NO and NO2 emissions on areas in proximity to the source.
2. Assessment of plume visibility impact
- The NO/NO2 emission ratio is important in the prediction of plume visibility from power plants.
- The ambiguity regarding the NO/NO2 emission ratio from power plants presently precludes a full assessment of air quality and plume visibility impact, and compromises attempts to validate visibility impact models.
3. Regional oxidant modeling
- The prediction of local formation and removal rates for oxidant requires spatial and temporal emission inventories for both NO and NO2.
- Ambiguity regarding the NO/NO2 emission ratio from mobile and stationary sources contributes to the limitations of regional oxidant models and compromises efforts to validate these models.
4. Flame studies
- The local concentrations of both NO and NO2 within flames are required to assess the chemical kinetic mechanisms responsible for the formation of NO X .
- Questions attendant to transformation of NO and NO2 in sample probes presently limit the utility of fundamental studies that address NO and NO2 formation in combustion flows.
- The function of the sample probe is to extract and cool the sample to a final temperature.
- (For high and very high temperature probing, rapid expansion of the sample at the probe tip is employed to terminate active reactions.).
- The final temperature is typically controlled (150° to 200°C) to prevent condensation of water and hydrocarbons.
Available Information
- A few general reviews of NO* sampling problems are available to assist in the design of sampling systems.
- 21 -23 Summaries of the transformation reactions that may be active in probes and sample lines are presented in Tables II, III, and IV for homogeneous, heterogeneous, and catalytic reactions, respectively.
- The temperature of the sample at the outlet was near ambient.
- Changes were observed for rich fire with the stainless steel probe.
- Validation of the measurement of transmissivity, and the inlet pressure conditions and sensitivity of the chemiluminescent analyzer), the results reinforce the probability that NO* may be removed within probes and sample lines under conditions encountered in practice.
Experimental
- An adequate accounting of NO* transformations requires that experiments be conducted to identify (1) the conditions for which chemical transformations occur, and (2) the extent to which they occur.
- An experimental study has been initiated to assess NO* transformations that may be encountered when sampling exhaust gas from practical combustion devices that operate air-rich (e.g. boilers, diesel engines, and gas turbine engines) and fuel-rich (e.g. automobile engines).
- Test parameters include carrier gas composition, concentration and composition of the dopant gases, temperature, and probe material.
- Additional species are introduced in an identical manner.
- The gas temperature within the test probe is incrementally varied from 25°t o 400°C.
Results
- The results are presented in Figure 2 for oxidizing mixtures.
- The percent change of NO and NO2 represents the percent change in concentration between sample points 2 and 3 except where otherwise noted.
- Points below the horizontal temperature scale identify those cases for which NO* is not conserved.
- The temperature shown is the gas temperature (maintained uniform) at sample points 2 and 3.
- The results reported are summarized from earlier presentations.
Oxygen
- No significant transformation is observed over the temperature range and the residence time studied.
- At temperatures below the catalytically active temperature, no significant change is observed.
Hydrogen
- Chemical transformations for two H2 concentrations, 0.5% and 3%, are evaluated.
- At 400°C, the changes are dramatic and depend on H2 concentration.
- The formation of CO was observed at 400°C with both test probes and 300°C with the stainless steel test probe.
- The CO concentrations increased with H2 percent, and reached levels approaching 3500 ppm.
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Cites background from "Chemical transformations of nitroge..."
...Stainless steel tubing is found to promote chemical transformation when sampling fuel-rich combustion products [122, 371]....
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...Potential sampling artifacts were minimized by a careful selection of materials for the entire sampling equipment [293, 371]....
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...Three general types of chemical transformation of these nitrogen oxides prevail in probes and sample lines [136, 217, 266, 371]:...
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...For instance, local concentrations of NO and NO2 are required to assess the chemical mechanisms responsible for the formation of NOx [56, 183, 371, 374]....
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...Chromium, nickel, and copper, for instance, are well-known to be active catalysts, and it can be concluded that untreated stainless steel is not an adequate material for gas sampling equipment [122, 136, 145, 179, 247, 371, 387]....
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References
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