Chemistry: analytical and immunological testing – Rate of reaction determination
Reexamination Certificate
1998-10-26
2001-06-26
Snay, Jeffrey (Department: 1743)
Chemistry: analytical and immunological testing
Rate of reaction determination
C436S116000, C436S172000
Reexamination Certificate
active
06251679
ABSTRACT:
BACKGROUND OF THE INVENTION
Nitric oxide (NO) is a serious atmospheric pollutant readily formed when fuels are combusted with any oxidant containing oxygen and nitrogen. Minimization of NO production has become a key goal of current combustor development.
To date, such efforts have relied on measurements of the total quantity of NO exiting a combustor or on measurements of local NO concentrations within the combustion volume. The gross measurement has the advantage of being relatively simple but provides no direct data on the spatial distribution of NO formation in the combustor. On the other hand, sampling as a function of position reveals the spatial distribution of NO concentration but the available implementations—intrusive sampling probes or laser-based optical sampling—are less-than-ideal for combustor development work. Intrusive probes perturb flow through the combustion volume and are subject to problems related to premature degradation of the sample, and furthermore each probe provides data for only one location at a time. Laser-based optical sampling requires a sophisticated and complex optical system, as well as open optical access to the combustion volume.
Their other strengths and weaknesses aside, both the gross and local types of measurement show only the instant quantity of NO present, which is a result of the entire previous combustion history of the gas sample examined. Neither approach is capable of indicating the local rate of NO formation, a parameter critical to intelligent refinement of combustor design.
SUMMARY OF THE INVENTION
The present invention provides diagnostic methods for determining an instantaneous rate of pollutant formation due to a combustion reaction of reactants in a combustion system, based on measurement of chemiluminescence intensity generated simultaneously with the formation of the pollutant. The method of the invention measures the chemiluminescent signal due to an analog reaction which occurs in parallel with a key step in the formation of a specific pollutant of interest. The connection between the analog reaction and the pollution reaction is such that the chemiluminescent signal indicates the local, instantaneous formation rate of the pollutant of interest.
The analog reaction may involve species normally present under the conditions giving rise to the formation of the pollutant, as in the case of naturally occurring combustion radicals which react to emit light. For example in hydrocarbon flames, OH*, which undergoes chemiluminescent decay, is produced naturally, primarily in accordance with
CH+O
2
→CO+OH* (equation 1).
Having similar reactants to the initiating reaction for generation of NO by the so-called “prompt” mechanism,
CH+N
2
→HCN+N,
makes equation 1 a possible analog reaction. Such a correlation would allow NO formation by this route to be monitored by observation of chemiluminescence from OH* occurring naturally in combustion systems. In other cases, the analog reaction is undergone by one or more species provided by an additive which is extraneous to the principal combustion process.
In a preferred embodiment, the technique includes introducing a boron-containing additive into the combustion system and identifies formation of nitric oxide based on light emitted by species provided by the additive. Under combustion conditions typically used for fossil fuels, notably in the presence of excess oxidant, the dominant mode of NO generation is the “thermal” mechanism reported by Zel'dovich:
N
2
+O→NO+N (reaction 1)
N+O
2
→NO+O.
The first reaction, which typically controls the overall process, is controlled by the concentration of oxygen radicals and the local gas temperature. The boron-containing additive enables formation of BO radical, which makes possible the analog reaction. The parallel reaction to reaction 1, also governed by oxygen radical concentration and temperature, forms excited BO
2
*
in the reaction volume:
BO+O+M→BO
2
*
+M.
The transition of the excited BO
2
*
to the ground state,
BO
2
*
→BO
2
+h&ngr;,
follows essentially immediately from the parallel reaction, resulting in the familiar “green” chemiluminescent emission, for which the main emission bands are at 518, 548 and 580 nm. The invention exploits the strong correlation between the rate-controlling step of the thermal NO mechanism and the coupled parallel reaction and chemiluminescent step comprising the analog reaction in order to directly determine the NO formation rate from BO
2
*
chemiluminescence intensity.
Boron compounds appropriate as additives for this embodiment include diborane (B
2
H
6
) and trialkyl borates such as trimethyl borate (B(OCH
3
)
3
) and triethyl borate (B(OC
2
H
5
)
3
). Diborane has a high vapor pressure and is attractive as a seed compound for introduction into gaseous fuels or with air or some other gaseous oxidant. Trimethyl borate is a liquid and is especially attractive for adding to liquid fuels. In general, it is desirable for such an additive to have a vaporization temperature similar to the vaporization temperature of the fuel with which it is mixed.
As used herein, the phrase “due to a combustion reaction of reactants” as applied to pollutant generation encompasses formation during or as a byproduct of the combustion reaction or an ensuing post-combustion reaction. The phrase “reaction volume” refers to the space in which the reaction generating the pollutant occurs, whether it is the combustion volume or a post-combustion volume. Also, an additive's “providing a species” for generating a chemiluminescent signal is intended to encompass cases in which the additive itself is the species and cases in which the additive somehow reacts or decomposes so as to generate the species. An “analog reaction” denotes a reaction that occurs in parallel with the reaction giving rise to the pollutant, or a series of reactions including one such step. The analog reaction may further encompass additional steps, for example, decay with photon emission, of an excited species created during the parallel reaction.
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patent: 4822564 (1989-04-01), Howard
patent: 4897548 (1990-01-01), Dome et al.
patent: 5185268 (1993-02-01), Bonometti et al.
patent: 5300441 (1994-04-01), Fujinari et al.
A. Leipertz, R. Obertacke, F. Wintrich;Industrial Combustion Control Using UV Emission Tomography, 1996.
Annen Kurt
Stickler David B.
Aerodyne Research Inc.
Cesari & McKenna
Snay Jeffrey
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