Chemistry in the Coastal Environment”,ACS Symposium Series, No. 18,T. M. Church, Ed., ACS, Washington, D.C., 1975. (15) Federal Water Pollution Control Administration,“The National Estuarine Pollution Study”,Vol 11,U S . Department of the Interior, Washington, D.C., 1969. (16) U S . Coast Guard, “Polluting Incidents In and Around U S . Waters, 1974”, (G-WEP),USCG, Washington, D.C., 1975. (17) U S . Coast Guard, “Polluting Incidents In and Around U.S. Waters, 1975”,CG-487, USCG, Washington, D.C., 1976. (18) Farrinzton. J. W.., Quinn. J. G.. J . Water Pollut. Control Fed.. ‘ 45 (4),70i-12(1973). (19) Blumer. M.. Sass, J., Mar. Pollut. Bull., 3 (6). 92-4 (1972). (20) Schnitzer, M., “The Chemistry of Humic Substances”, in “En-
vironmental Biogeochemistry”,Vol 1,pp 89-107, J. 0. Nriagu, Ed., Ann Arbor Science, Ann Arbor, Mich., 1976. (21) Kremer, J. N., PhD thesis,University of Rhode Island,Kingston, R.I., 1975. (22) Zafiriou, 0. C., Estuarine Coastal Mar. Sci., 1 (l), 81-7 (1973). (23) National Academy of Sciences, “Water Quality Criteria for 1972”, Nat. Acad. Sci., Washington, D.C., 1972.
I
Received for review January 21, 1977. Accepted May 31,1977. Research supported by a grant (04-6-158-44002)from the National Sea Grant Program.
Interferences in Chemiluminescent Measurement of NO and NOp Emissions from Combustion Systems Ronald D. Matthews, Robert F. Sawyer*, and Robert W. Schefer’ Department of Mechanical Engineering, University of California, Berkeley, Calif. 94720
Two factors that may affect the quantification of NO and NO2 concentrations when using chemiluminescent analysis are investigated. The first is the dependence of the chemiluminescent intensity on competing third body quenching reactions. Relative quenching efficiencies are determined as a function of third body concentration for six different species that are common products of combustion. A mathematical expression is derived that allows calculation of the actual concentration of NO, given the indicated concentration of NO and the concentrations of the important third bodies. An example calculation is presented. The second factor investigated involves the conversion to NO of low molecular weight nitrogen-containing species, other than NOz, in commercial NO, converters. Conversion efficiencies for six species are determined using a commercial stainless steel catalyst a t 920 K. In recent years, chemiluminescent analysis has become a widely used method for determination of low concentrations of gas-phase nitric oxide and nitrogen dioxide. This popularity is generally attributable to the ease of use, sensitivity, and linearity normally associated with commercially available chemiluminescent analyzers. However, several problems may be encountered during the use of these instruments. Difficulties related t o measurements of NO, in fuel-rich combustion systems have been previously reported in the literature (1-4). This report addresses two additional factors that may affect the quantification of NO and NO2 in combustion products when using chemiluminescent analysis. The first is the dependence of the chemiluminescent intensity on competing third body quenching reactions. The possible conversion of various low molecular weight nitrogen-containing compounds (in addition to N O 4 to NO in the commercial NO, converters is also explored. T h i r d B o d y Quenching Efficiencies
Chemiluminescent analyzers are usually calibrated with a mixture of NO in an Nz balance. Additional species are introduced when sampling from combustion systems. These species typically include H20, COZ, CO, 0 2 , and H2. Under certain circumstances, the composition of the gas mixture can affect the indicated measurement of NO ( 5 )simply because Present address, Lawrence Berkeley Laboratory,70-143, Berkeley, Calif. 94720. 1092
Environmental Science & Technology
of the difference in third bodies between the calibration gas and the sample gas. The reactions of importance in the chemiluminescent detector are: ki
NO -k &+NO;
-t 0 2
(1)
In Reaction 1,ozone reacts with nitric oxide to produce electronically excited nitrogen dioxide. This NO; can reach equilibrium either through photoemission (Reaction 2) or through collisional energy transfer (Reaction 3). The intensity of the photoemission is given by (6): (4)
where k3M is a function of the specific third body. Because chemiliminescent analyzers measure this intensity, it is advantageous to decrease the probability of collisional de-excitation. This is conventionally accomplished by operating a t subatmospheric pressures, typically less than 10 mm of Hg. Since k3 is a function of the third body, M, the chemical composition of the carrier gas also influences the measured intensity. Several investigators have studied the relative quenching efficiencies of various third bodies for Reaction 3. We have added to those data by dynamically mixing NO with a carrier gas comprised of various concentrations of Ar, H2, 02, CO, Con, or H2O with a n N2 balance. In all cases, the nitric oxide concentration, [NO], was maintained at 830 ppm while N2 was replaced in varying proportions as the carrier gas. If the relative quenching efficiency, RM?,is defined as the photointensity with N2 as the carrier gas, Z N ~ divided , by the photointensity in the presence of a specific third body, ZM,then the results of these experiments may be graphically displayed as in Figure 1. Ar and CO are less efficient than N2 in quenching the reaction while 0 2 , Hz, COZ, and H20 are more efficient. The diatomic molecules are significantly different from Nz only at high concentrations. On the other hand, even at relatively low concentrations, Ar, COz,and H20 can alter the in-
dicated [NO]. Note that, even though the reaction pressure does affect the photointensity, the relative quenching efficiency is approximately independent of pressure over the range considered in these investigations. For a concentration of COz in the sample of 11%,increasing the reaction chamber pressure from 3 to 14 torr increased the relative quenching efficiency by 9%. Our measurements were taken a t 3, 8, and 14 mm Hg. The measurements of Niki et al. (7) were taken a t 7 mm Hg, Clyne et al. (8)worked a t 1.3 mm Hg, and Maahs (9) used a pressure of 5 mm Hg. J. H. Tuttle and A. M. Mellor (unpublished data, 1976) took their measurements using a pressure of 10 mm Hg. The least-squares curve fits in Figures 1 and 2 average out the pressure effect. I t is important to note that the quenching effect is a strong function of the particular type of analyzer used. This fact is most obvious when the HzO and COz data for the various researchers are compared. The major reason for lack of agreement is that different chemiluminescent analyzers have different ( 0 3 Oz)/sample ratios in the reaction chamber. For example, in the analyzer used by Clyne et al. ( 8 ) ,98% of the molecules in the reaction chamber were introduced with the sample, and there was very little dilution by the ozone. In the TECO 12A used in the present investigation, only 45% of the content of the reaction chamber is attributable to the sample gas. (Our instrument had an 8-mil sample capillary: TECO also offers a 5 mil in the 12A.) Figure 2 shows the data of Figure 1plotted as a function of the third body concentration in the reaction chamber. The agreement among the data of different investigators is much better. This graph is much more generally applicable than Figure 1. Table I gives the slopes of the first order least-squares curve fits to the data.
+
The use of curves such as those presented in Figure 2 can allow calculation of the actual [NO] if the concentrations of the important third bodies are known. The following relations provide the basis for this procedure (see the Appendix for the derivation of Equation 5 ) : J -“OIACT - 1 + E ( R M - 1)
[NOIIND
M= 1
where
R M = -IN2
The relative quenching efficiency, R M ,may be calculated from the relation
R M = 1.0
+ md[M\
^
^
I
i
I
I
l
l
I
0 U
i V
1.4
w
I
I O
H20
17.0% by volume CO2 0 2
LL
W
I
(3
zI V
z
W 3
I
0 W
E O
‘ a i
0.61
t
1
L
0
20
40
60
80
100
M O L E PERCENT OF M IN T H E S A M P L E STREAM Figure 1. Relative intensity of chemiluminescent emission for six different third bodies as a function of third body concentration in sample mixture
9.2% 1.8%
-
0
HzO: R H ~ = OIN’ = 1.0 IH20
+ m(d){HaO}
m is obtained from Table I and cl is taken as 0.4545 (Le., TECO 12A with an 8-mil sample capillary). Therefore
+ 0.03430(0.4545)(17.0)= 1.2651 Con: Rco2 = IN2 = 1.0 + 0.01047(0.4545)(9.2)= 1.0438 IC02 RHlO =
1.0
IN2 0 2 : Roz = = 1.0 + 0.001149(0.4545)(1.8) = 1.0011
102
=1
LL
(7)
where m is found in Table I, d is the dilution constant (a function of the analyzer being used), and (MI is the third body concentration in mole percent. The following example demonstrates how these equations are to be used. I t also indicates the importance of third body quenching on the quantitative determination of nitric oxide concentrations in combustion systems. Example Calculation: The concentrations of the major species emitted from a power plant operating on natural gas and 10% excess air are:
HP,CO
Z.V
(6)
IM
+ 0.2651 + 0.0438 + 0.0011 = 1.31
therefore, [NOIACT= 1.31 [NOIIND. In other words, the [NO] indicated by the chemiluminescent analyzer is more than 30% too low, primarily because of the high concentration of H20 emitted from the combustion process. Removing the water before analysis or calibrating out the water effect through saturating both the calibration gas and sample with water could help. Such procedures are sometimes, but not always, employed in power plant pollutant monitoring. These approaches may introduce new errors because NO, can be adsorbed on desiccants and absorbed by liquid phase H20.In some systems CO2 is a more important interference than is H20. Table I1 presents the results of similar calculations for the exhaust products of other combustion systems. Potential errors greater than 20% are not uncommon, even for overall fuel lean conditions. Only for the extremely lean conditions associated with jet engines is dilution great enough to compensate for the third body effect. As a general rule, the error increases with increasing H/C ratio of the fuel and with increasing equivalence ratio, 9 (defined as the actual fuel-air ratio diVolume 11, Number 12, November 1977
1093
vided by the stoichiometric fuel-air ratio). Measurements taken in very fuel-rich systems or in the fuel-rich regions of diffusion flames may be subject to even greater errors since there is the possibility of encountering high concentrations of polyatomic fuel molecules which might be expected to have high relative quenching efficiencies. These examples demonstrate that the third body quenching effect may significantly affect the determination of NO concentrations in most combustion systems. Systems which operate under very fuel lean conditions may not be severely influenced. Also, the effect may be partially compensated for in those research systems that have high concentrations of Ar or He.
using chemiluminescent analyzers ( 1 4 , 1 5 ) .The value of 86% conversion is in agreement with data of Zolner (16),and the value of 68% for the conversion of HCN is in agreement with the value reported by Halgren et al. ( I 7). Table 111also reveals that monomethylamine can be converted to NO in this system, and it can be assumed that other nitrogenous hydrocarbons can be oxidized to NO under these conditions. These conversion efficiencies may be functions of pressure, concentration, and the catalytic surface conditions, in addition to temperature. Furthermore, Winer et al. (18) have demonstrated that commercially available carbon and molybdenum converters also respond quantitatively to other nitrogencontaining species.
Conversion of Various N-Species to NO Modern chemiluminescent analyzers are equipped with catalytic converters so that NO2 may also be analyzed. These converters function by catalytically reducing NO2 to NO. Other low molecular weight nitrogen-containing compounds have been shown to be the products of some combustion systems [Sawyer et al. (10) (fuel nitrogen); Schuchmann and Laidler ( 1 1 ) (auto exhaust); Haynes (12) (prompt NO); Stumpf and Blazowski (13) (aircraft gas turbines)]. Even partial conversion of these species to NO on commercial catalysts can lead to substantial errors in NO, measurements. To investigate this possibility, low concentrations of several N-species (with an air balance) were passed through the stainless steel catalyst supplied with the TECO 12A. All measurements were made a t the recommended operating temperature for this converter, 650 O C (920 K). The results are shown in Table 111. The results for NO2 and Nz are as expected. The fact that NH3 can be converted to NO in this catalyst has lead to the observation that NH3 can be measured
Summary This investigation was intended to explore two phenomena that may interfere with the accurate analysis of NO and NO2 with chemiluminescent detectors. The third body quenching effect is most important for samples that have large concentrations of nondiatomic constitutents. If unaccounted for, it may be a significant source of error in the chemiluminescent analysis of combustion effluents, generally leading to an indicated [NO] that is too low. In combustion systems which may contain N-species other than NO, NO2, and Nz, care should be taken to ensure that the catalytic converters normally used in the measurement of NO2 ~~
~
Table 1. Slopes of First Order Curve Fits to Reduced Data Where Equation of Line Is Given by: RM = 1.0 md{M/
+
Slope of first order fit, rn
Third body species, M
Ar
I
2.0
E
$1 w
1.4
-0.00480 -0.001 18 0.00115 0.00337 0.01047 0.03430
co 0 2
H2
con H20
Table II. Typical Errors Associated with Third Body Quenching When Sampling from Combustion Systems Fuel
#
Oil, CH1.65
0.87
Coal
0.83
C8Hi8,
Combustion system
Oil-fired power plant Coal-fired power plant Auto exhaust, undiluted sample Auto exhaust, diluted, as in CVS method Gas turbine, 100% afterburning Gas turbine, cruise
% Excess air
[NO]ACT
WIIND
1.o
15 20 0
1.22 1.17 1.28
1.o
0
1.04
CHi.94, JP4
1.0
0
1.27
JP4
0.3
233
1.02
isooctane C8H18,
isooctane
CHi.94,
-
Table 111. Conversion Efficiencies of Various N-Species Over Stainless Steel at 920 K Species
NO2
t 0
i 4rllrrllllf 20
40
60
80
"3
CH3NH2 HCNa
100
MOLE PERCENT OF M I N THE REACTION CHAMBER
Relative chemiluminescent intensity as a function of third body concentration in reaction chamber Figure 2.
1094
Environmental Science 8, Technology
N20 N2 a
Ar/Op balance.
Concn, ppm
Conversion efflciency,
993 978 976 297 3300 931
100 86 02 68 1
He balance.
Yo
0
are not also converting these other species to NO. Investigators involved in the fields of fuel nitrogen, automotive NO, emissions, NO, emission from gas turbines, NO, measurements in fuel-rich combustion systems or the fuel-rich regions of diffusion flames, and NO, measurements in systems that emit high concentrations of unburned hydrocarbons should give a cautious interpretation to total NO, measurements obtained with catalytic converters.
therefore,
Invoking these definitions and approximations:
Appendix The following mathematical derivation provides a formula which indicates how the data for the individual third bodies may be used to correct the indicated [NO] when a mixture of several third bodies is present. The important reactions are: NO
+03
NO;
+
+
NO;
NO2
+02
+ hv
NO;+M+NOz+M NO;
+ N2
+0 2 NO; + 0 3
NO;
+
4
4
NO2
(1)
When a calibration gas is introduced, the actual nitric oxide concentration is proportional to:
If only one third body, M, partially replaces N2:
(2) M#N2
+ N2
+0 2 NOp + 0 3
NO2
(34 (3b) (3c) (34
The [O,] and [ 0 3 ] used in Reactions 3c and 3d refer only to the ozonated oxygen stream. Any 0 2 and O3 introduced with the sample are identified with the general M of Reaction 3a. Since the dark current (caused by thermal emission plus any NO; resulting from reverse of Reactions 2-3d) is very low, most of the NO; must be formed via Reaction 1. Therefore, the reverse of Reactions 2-3d can be ignored. Since measurements on the chemiluminescent analyzer are made only after a steady state reading is attained, the equation d [NO;] = 0 dt -
is exact. I t is assumed that equilibration is dominated by collisional energy transfer (as in refs. 6 and 8) and not photoemission. That is: k2
