Nitrogen Isotopic Composition of Coal-Fired Power ... - ACS Publications

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Nitrogen Isotopic Composition of Coal-Fired Power Plant NOx: Influence of Emission Controls and Implications for Global Emission Inventories J. David Felix,*,† Emily M. Elliott,† and Stephanie L. Shaw‡ †

Department of Geology and Planetary Science, 4107 O’Hara Street, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States ‡ Electric Power Research Institute, 3420 Hillview Avenue, Palo Alto, California 94304, United States ABSTRACT: Despite the potential use of δ15N as a tracer of NOx source contributions, prior documentation of δ15N of various NOx emission sources is exceedingly limited. This manuscript presents the first measurements of the nitrogen isotopic composition of NOx (δ15NNOx) emitted from coal-fired power plants in the U.S. at typical operating conditions with and without the presence of selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR) technology. To accomplish this, a novel method for collection and isotopic analysis of coal-fired stack NOx emission samples was developed based on modifications of a historic U.S. EPA stack sampling method. At the power plants included in this study, large differences exist in the isotopic composition of NOx emitted with and without SCRs and SNCRs; further the isotopic composition of power plant NOx is higher than that of other measured NOx emission sources confirming its use as an environmental tracer. These findings indicate that gradual implementation of SCRs at power plants will result in an industry-wide increase in δ15N values of NOx and NOy oxidation products from this emission source.

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INTRODUCTION Reducing NOx (NOx = NO + NO2) emissions is of global interest due to adverse effects on the environment and human health.1 NOx emissions can combine with VOCs to form ground-level ozone, particulate matter, and ultimately be oxidized to form nitrate (NO3−). Excess NO3− is a key factor in acidic deposition, degradation of drinking water, ecosystem-N saturation, soil acidification, lacustrine and estuarine eutrophication.1 Although natural NOx sources, including lightning, wildfires, and biogenic soil emissions, account for a portion of global NOx emissions, the magnitude of these contributions is largely uncertain.2 Since the Industrial Revolution, anthropogenic NOx emissions have greatly increased and have now surpassed natural NOx emissions; primarily due to increased fossil fuel combustion via electricity generating units (EGUs) and vehicles.1 Globally, North America contributes approximately 21% to global NOx emissions from natural and anthropogenic sources.1 Recent and ongoing efforts are aimed at further reducing ambient NOx concentrations in the U.S and globally.3−5 Several technologies are available for use in reducing NOx emissions generated from fossil fuel combustion. NOx is produced in EGU boilers either by reaction of nitrogen with oxygen in combustion air (“thermal NOx”) or by reaction of fuel nitrogen (e.g., coal) with combustion oxygen (“fuel NOx”).5 Low NOx burners limit the availability of oxygen to nitrogen in the fuel and have been employed in many EGU boilers. However, low NOx burners do not necessarily reduce NOx emissions © 2012 American Chemical Society

sufficiently to meet stringent emissions standards. To further reduce stack NOx emissions, postcombustion NOx reduction must also be employed. Selective catalytic reduction (SCR) is one such postcombustion technology which can reduce NOx emissions by 80−90%.6 SCRs have been utilized by coal-fired EGUs for decades and are globally recognized as the most efficient NOx emission control technology.6 The SCR process injects ammonia (NH3) into the EGU flue gas stream where the gas is passed over a catalyst (V2O5) in the presence of oxygen. NOx and NH3 react to form N2 and water vapor (eq 1). 4NO + 4NH3 + O2 → 4N2 + 6H2O

(1)

Selective noncatalytic reduction (SNCR) is a similar postcombustion NOx reduction technology that employs NH3 or urea as a reagent and does not use a catalyst because it operates at higher temperatures. The SNCR process is 15−66% less efficient than the SCR process.6 Previous work suggests that the nitrogen isotopic composition of NOx (δ15N-NOx) and its oxidation products may be a robust indicator of regional stationary source NOx contributions to atmospheric nitrogen deposition, and thus may be used to assess effectiveness of NOx reduction technologies.7,8 For Received: Revised: Accepted: Published: 3528

September 23, 2011 January 17, 2012 January 30, 2012 January 30, 2012 dx.doi.org/10.1021/es203355v | Environ. Sci. Technol. 2012, 46, 3528−3535

Environmental Science & Technology

Article

Figure 1. δ15N values of NOx generated from power plants and other NOx sources (left) and estimated δ15N in resulting deposition (right). HNO3 is estimated using δ15N-NO2 = δ15NHNO3 + (1000*(1−0.9971)22. * refers to data from the present study. Acronyms are as follows: FGD - flue-gas desulfurization; LNB - low NOx burner; OFA - overfire air; SNCR - selective noncatalytic reduction; and SCR - selective catalytic reduction.

accomplish this, a novel method for collection and isotopic analysis of coal-fired stack NOx emission samples was developed. This is the first study to document that recently applied emission controls (e.g., selective catalytic reduction) have a significant influence on the isotopic signature of resulting NOx emissions from coal-fired power plants. These results provide definitive confirmation that the isotopic composition of coal-fired power plant NOx emissions is distinct from that of other emission sources (e.g., biogenic soil emissions, lightning,) and thus can be used to trace the fate of NOx emissions. As an example of how additional δ15N-NOx data can be used, we developed two hypothetical models to (1) estimate how increasing implementation of SCRs to meet increasingly stringent air quality regulations can be expected to influence industry wide δ15N-NOx from this sector; and (2) explore, using best available knowledge, how NOx emissions inventories and existing δ15N-NOx data can be coupled to estimate ambient δ15N values in precipitation.

example in the Northeast U.S., significant correlations were observed between δ15N-NO3 in precipitation,7 dry deposition,8 and EGU NOx emissions within a 400 km region. Despite the potential use of δ15N as a tracer of NOx source contributions, prior documentation of δ15N values of various NOx emission sources is exceedingly limited. One prior study measured NOx emitted from four South African coal-fired EGUs and δ15N values ranged from +6 to +13‰;9 this δ15N-NOx range is more positive than the values reported for other NOx sources. For example, NOx resulting from vehicle fossil fuel combustion is reported to have δ15N values ranging from −13 to −+2‰,9 while other studies of vehicle emissions, roadside denuders, roadside vegetation, and roadside gaseous NO2 have reported δ15N values of +3.7, +5.7, +3.8, and +4‰, respectively.10−13 Natural sources, including lightning and biogenic NOx from soils, have been characterized as having δ15N values of 0 to +2‰ and −49 to −20‰, respectively14,15 (Figure 1). Although these prior measurements of source δ15 N-NOx values allow approximation of relative source contributions, further characterization of δ15N-NOx is required to minimize uncertainty, enable quantification of source contributions, and to further understand post-emission transformations of NOx on δ15N values. Further, it is possible that different analytical preparation methods, EGU configurations, emission control technologies, operating conditions, and coal sources could influence δ15N-NOx values from coal-fired EGUs, thus rendering the results documented in South Africa not applicable to other regions. The objective of this research is to report direct measurements of the nitrogen isotopic composition of NOx (δ15NNOx) emitted from coal-fired power plants in the U.S. To

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EXPERIMENTAL SECTION Sample collection was conducted at four separate coal-fired power plants located in the Northeast and Midwest U.S. (hereafter referred to as Plants A−D as described below). All four plants burned regional bituminous coal and were equipped with limestone-based flue gas desulfurization systems. Table 1 describes emission control technologies used at each of the plants. Plant A, a 550 MW gross annual power production facility that employs overfire air (OFA) systems, selective catalytic reduction emission control technology (hereafter referred to as SCR), and low NOx burners (hereafter referred to as ″LNB″) for NOx emissions reduction was sampled on 3529

dx.doi.org/10.1021/es203355v | Environ. Sci. Technol. 2012, 46, 3528−3535


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Plant D: SCR On and SCR Off NOx Collection. Sample collections at Plant D used a dilute H2SO4/H2O2, NOx absorbing solution. Concentration and Isotopic Analysis. Nitrate and nitrite concentrations were analyzed using a Dionex ICS 2000 ion chromatograph. For isotopic analysis, a denitrifying bacteria, Pseudomonas aureofaciens, was used to convert 20 nmoles of NO3− into gaseous N2O prior to isotope analysis.19 Samples were analyzed for δ15N values in duplicate using an Isoprime Trace Gas and Gilson GX-271 autosampler coupled with an Isoprime continuous flow isotope ratio mass spectrometer (CFIRMS) at the University of Pittsburgh. Values are reported in parts per thousand relative to atmospheric N2 as follows:

Table 1. EGU Emission Technologies Used in This Study acronym

technology

FGD LNB

flue-gas desulfurization low NOx burner

OFA

over fire air

SCR

selective catalytic reduction selective noncatalytic reduction

SNCR

purpose reduces SO2 emissions from the flue gas reduces NOx emissions by limiting the availability of oxygen to nitrogen in the fuel reduces NOx emissions by introducing air to produce more complete fuel combustion reduces NOx emissions by reacting NOx with NH3 or Urea over a catalyst to form N2 reduces NOx emissions by reacting NOx with urea or ammonia to form N2

15 14 N/ N) − (15N/14 N) ( sample standard δ15N(‰) = 15 14 ( N/ N)standard

May 6, 2009. After initial sample collection and isotopic analysis, additional experiments were conducted using various absorbing solutions at a second facility (Plant B) on December 8, 2009. Plant B, also a 550 MW gross annual power production facility, employs LNB and OFA systems (i.e., no SCR). A third plant, Plant C, was sampled January 25−27, 2011. Two separate EGUs at Plant C were tested, each producing about 650 MW. One EGU employed LNB, OFA, and SCR, and the other EGU employed LNB, OFA, and SNCR. The latter unit was also tested with the SNCR turned off (e.g., only LNB and OFA were operating). Plant D, sampled April 5 and April 6, 2011, burned low sulfur Powder River Basin coal, produced 660 MW, and also had a limestone FGD system and SCR system. Plant D did not have LNB or OFA. The SCR system at Plant D was also shut off for an additional NOx reduction treatment scenario. Plant A: SCR/OFA/LNB NOx Collection. The sampling method used in this study was modified from U.S. EPA Method 7, “Determination of Nitrogen Oxide Emissions from Stationary Sources”. Briefly, a 25-mL aliquot of absorbing solution (6 mL of hydrogen peroxide (H2O2) in 1 L of ∼0.05 M sulfuric acid (H2SO4)) was transferred to a flask, which was attached to a sampling train, evacuated, and purged before the grab sample was collected.16 The probe of the sampling train was placed into the stack during sampling and the stack emissions collected into the evacuated flask containing the absorbing solution. After a sampling period of approximately 15 s, the flask was removed from the train and sealed. The contents of the flask were shaken for 2 min and allowed to sit for at least 16 h, allowing all NOx gas to oxidize to nitrate. Particulate nitrate was not considered an interference in the NOx collection as it is generally reported to constitute a small percentage (0.000026%) of EGU emissions.17 The contents were then transferred to a 100-mL Teflon bottle and frozen for shipping. Samples were stored frozen until further analysis. Plant B: OFA/LNB NOx Collection and Comparison of Absorbing Solutions. Sample collections at the LNB stack compared multiple NOx absorbing solutions, including dilute H2SO4/H2O2, dilute sodium hydroxide/hydrogen peroxide (NaOH/H2O2), and 1.68 M triethanolamine (TEA). The dilute H2SO4/H2O2 was prepared as described above. The dilute NaOH/H2O2 solution was made by adding 6 mL of 3% hydrogen peroxide to 1 L of ∼0.1 M NaOH. TEA absorbing solution was shown by Nonomura et al.18 to absorb NO2 with both NO2− and NO3− being present in the resulting solution. Plant C: SCR/OFA/LNB, SNCR/OFA/LNB, OFA/LNB NOx Collection. Sample collections at Plant C used a dilute H2SO4/ H2O2, NOx absorbing solution.

× 1000 (2)

International reference standards USGS34 and USGS32 were used for data correction. Reference standards had an average standard deviation (σ) of 0.2‰ for δ15N. Coal samples were obtained prior to combustion from plants A and B. Coal was homogenized by crushing in a ball mill and thoroughly mixed before isotopic analysis. Coal was loaded into tin capsules and analyzed for isotopic composition using an Eurovector elemental analyzer 3028-HT coupled with an Isoprime CF-IRMS at the University of Pittsburgh. Samples were corrected using the international reference standards USGS 40 and USGS41. Reference standards had an average standard deviation (σ) of 0.2‰ for δ15N.

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RESULTS Table 2 details results of individual sample analysis; the text below summarizes results collected from each of the four plants sampled in this study. Coal δ15N Values. The δ15N value of the Illinois basin coal acquired from the SCR-equipped EGU (Plant A) was +2.2‰. The δ15N value of the coal acquired from the LNB EGU (i.e., non SCR-equipped Plant B) was +2.0‰. Coal samples from Plant C and D were not collected. Plant A: Coal-fired EGU with SCR/LNB. NOx concentrations in the SCR stack ranged from 29.2 to 37 ppm (average = 32 ppm, SD ±1 ppm), as measured by the continuous emission monitoring (CEM) system installed at the power plant. Nitrate concentrations of the grab samples collected at the SCR-equipped EGU using the absorbing solution (H2SO4/H2O2) ranged from 4.2 to 16.7 ppm (average = 7 ± 5 ppm). Samples collected using the grab sample method had a mean δ15N-NO3 value of +19.5‰ ± 2.3‰ (n = 5). The relatively large standard deviation is due to one low δ15N measurement of +15.5‰. If this sample is discarded, the standard deviation among remaining samples is 0.8‰ and the mean δ15N-NO3 value is +20.5‰. Based on these results, we determined that the grab sample method and associated modifications were adequately precise for future stack sampling sessions. Plant B: Coal-fired EGU with LNB. NOx concentrations during sampling at the LNB-equipped EGU ranged from 100.5 to 170.8 ppm (average =132 ppm, SD = ± 16 ppm) as measured by the CEM system installed at the power plant. Nitrate concentrations in the H 2SO 4 grab samples ranged from 25.7 to 35.2 ppm (average = 27 ± 6 ppm) and the samples did not contain nitrite (i.e., the HNO2 and NO2 3530



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Table 2. Eluent Concentrations and Isotopic Analysis from SCR-, SNCR-, OFA-, and LNB-Equipped EGU Samplesa sample H2SO4 Grab 1 H2SO4 Grab 2 H2SO4 Grab 3 H2SO4 Grab 4 H2SO4 Grab 5 Mean H2SO4 Grab 1 H2SO4 Grab 2 H2SO4 Grab 3 H2SO4 Grab 4 Mean NaOH Grab 5 NaOH Grab 6 NaOH Grab 7 NaOH Grab 8 Mean TEA Grab 9 TEA Grab 10 TEA Grab 11 TEA Grab 12 Mean H2SO4 Grab 1 H2SO4 Grab 2 H2SO4 Grab 3 Mean H2SO4 Grab 4 H2SO4 Grab 5 H2SO4 Grab 6 Mean H2SO4 Grab 7 H2SO4 Grab 8 H2SO4 Grab 9 Mean H2SO4 Grab 1 H2SO4 Grab 2 H2SO4 Grab 3 H2SO4 Grab 4 H2SO4 Grab 5 H2SO4 Grab 6 H2SO4 Grab 7 H2SO4 Grab 8 Mean H2SO4 Grab 9 H2SO4 Grab 10 H2SO4 Grab 11 H2SO4 Grab 12 H2SO4 Grab 13 H2SO4 Grab 14 H2SO4 Grab 15 H2SO4 Grab 16 Mean

plant and technology A A A A A

SCR/LNB SCR/LNB SCR/LNB SCR/LNB SCR/LNB

B B B B

OFA/LNB OFA/LNB OFA/LNB OFA/LNB

B B B B


B B B B


C SCR/OFA/LNB C SCR/OFA/LNB C SCR/OFA/LNB C SNCR/OFA/LNB C SNCR/OFA/LNB C SNCR/OFA/LNB C OFA/LNB C OFA/LNB C OFA/LNB D D D D D D D D

SCR SCR SCR SCR SCR SCR SCR SCR

D D D D D D D D

SCR SCR SCR SCR SCR SCR SCR SCR

off off off off off off off off

nitrate (ppm)

nitrite (ppm)

mean δ15N-NO3/ NO2 (‰)

SD of N (‰)

N (number of replicates)

8.9 10.8 4.2 8.1 16.7 9.7 35.2 25.1 22.9 25.7 27.2 22.3 14.6 19.6 19.9 19.1 19.0 8.3 10.0 10.3 11.9 5.0 3.4 4.0 4.1 26.4 18.3 29.7 24.8 31.8 31.0 30.3 31.0 12.6 12.7 12.8 12.9 12.8 12.4 13.3 13.7 12.9 23.2 26.5 26.9 33.5 34.6 36.9 37.2 38.1 32.1

0 0 0 0 0 0 0 0 0 0 0 NA NA NA NA NA NA NA NA NA NA 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

20.6 21.0 20.9 15.5 19.3 19.5 9.2 9.0 10.4 10.5 9.8 11.0 11.5 9.6 11.7 11.0 10.2 10.7 10.0 9.5 10.1 15.5 25.6 18.4 19.8 13.6 13.9 15.1 14.2 12.6 12.1 11.8 12.2 18.9 19.1 19.2 19.3 19.2 18.7 18.9 20.4 19.3 10.3 11.5 11.7 10.8 10.2 9.8 9.9 9.6 10.5

0.7 1.0 1.0 1.0 1.0 1.0 0.3 0.1

3 3 4 4 4

0.3 0.2 0.7 0.9 0.5 0.7 0.8 0.4

4 4 2 3 3 2 3 3 3 3 1 2

0.6 1 1 2

0.7

2 2 3

1.0

2 2 3 2 2 2 2 2 2 2 2

0.1 0.2 0.1 0.1 0.3 0.2

2 2 2 3 3 3 3 3

a

Although NaOH and TEA grab samples contained nitrite, sulfate interferences on the IC yielded unreportable nitrite concentrations. Standard deviations (SD) reported are deviations of replicate analysis of individual samples for the number of replicates indicated (N).

absorbed was oxidized). The H2SO4 grab samples had a mean δ15N-NO3 value of +9.8 ± 0.8‰ (n = 4). The NaOH grab sample nitrate concentrations ranged from 14.6 to 22.3 ppm (average =19 ± 3 ppm); these samples contained nitrite (i.e., not all NO2 and HNO2 oxidized), however quantification of nitrite concentrations on the IC was problematic due to

overlapping sulfate peaks. The NaOH samples had a mean δ15N-NO3/ NO2 value of +11.0 ± 0.9‰ (n = 4). The TEA grab sample nitrate concentrations ranged between 8.3 and 19.0 ppm (average =12 ± 5 ppm); these samples also contained nitrite, again determination of nitrite concentrations in these samples using the IC was again problematic due to overlapping 3531



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solution showed the greatest precision of δ15N values within replicate samples (SD = ± 0.2‰) and a standard deviation of ±0.8‰ from sample to sample. As a result, subsequent NOx emission sampling at Plant C and D employed the H2SO4 absorbing solution. Influence of Emission Controls on δ15N-NOx Values. Figure 2 summarizes the ranges of δ15NOx values relative to

sulfate peaks. The TEA samples had a mean δ15N-NO3/ NO2 value of +10.1 ± 0.5‰ (n = 4). The mean δ15N value for grab samples in all three solutions was +10.1 ± 0.5‰ (n = 12). δ15N values were not significantly different among absorbing solution treatments (α = 0.05, p = 0.14) (ANOVA: Single factor) indicating consistency in our results. Plant C: Coal-fired EGU with SCR/OFA/LNB, and Coalfired EGU with SNCR/OFA/LNB (SNCR On) and OFA/LNB (SNCR Off). NOx concentrations during sampling at the SNCR/OFA/LNB equipped EGU ranged from 7.8 to 16.7 ppm (average = 15.2 ppm, SD = ± 1.0 ppm). The SNCR/ OFA/LNB sample nitrate concentrations ranged from 18.3 ppm to 29.7 ppm (average = 24.8 ± 5.8 ppm). The SNCR/OFA/ LNB grab samples had a mean δ15N-NO3 value of +14.2‰ ± 0.8‰ (n = 3). NOx concentrations during sampling at the OFA and LNBequipped EGU ranged from 15.4 to 16.6 ppm (average = 16.0 ppm, SD = ± 0.3 ppm). The OFA/LNB sample nitrate concentrations ranged from 30.3 to 31.8 ppm (average = 31.0 ± 0.8 ppm). The OFA/LNB grab samples had a mean δ15N-NO3 value of +12.2‰ ± 0.4‰ (n = 3). NOx concentrations during sample collection at the SCR/ OFA/LNB-equipped EGU ranged from 8.9 to 18.6 ppm (average = 15.8] ± 0.8 ppm). Nitrate concentrations in the SCR/OFA/LNB H2SO4 grab samples ranged from 3.4 to 5.0 ppm (average = 4.1 ± 0.8 ppm). The SCR/OFA/LNB H2SO4 grab samples had a mean δ15N-NO3 value of +20‰ ± 5‰ (n = 3). Plant D: Coal-fired EGU with SCR System On (SCR On) and Coal-fired EGU with SCR System Off (SCR Off). NOx concentrations during “SCR on” conditions ranged from 26 to 31.6 ppm (average = 29 ppm, SD = ± 2 ppm). The “SCR on” sample nitrate concentrations ranged from 12.8 to 13.7 ppm (average = 12.9 ppm, SD = ± 0.4 ppm). The “SCR on” grab samples had a mean δ15N-NO3 value of +19.3‰ ± 0.5‰ (n = 8). NOx concentrations during “SCR off ” conditions ranged from 134.7 to 155 ppm (average = 149 ppm, SD = ± 5 ppm). The ‘“SCR off” sample nitrate concentrations ranged from 23.2 to 38.1 ppm (average = 32 ppm, SD = ± 6 ppm). The “SCR off” grab samples had a mean δ15N-NO3 value of +10.5‰ ± 0.8‰ (n = 8).

Figure 2. Box and whisker plot summarizing range and mean δ15NOx values observed at plants using each type of emission control technology. Acronyms are as follows: FGD - flue-gas desulfurization; LNB - low NOx burner; OFA - overfire air; SNCR - selective noncatalytic reduction; and SCR - selective catalytic reduction.

power plant technology at each plant. The large difference in values observed between samples from the various SCRequipped (+19.5, +19.8, and +19.3‰ at Plant A, Plant C, and Plant D, respectively) and non-SCR-equipped EGU samples (+9.8, +12.2, and 10.5‰ at Plant B, Plant C, and Plant D, respectively) likely results from the SCR reaction. When NOx reacts with injected NH3 over a catalyst, the resulting N2 forms from nitrogen atoms in each reactant. Higher δ15N values associated with SCR NOx emissions suggests that the isotope with less mass, 14N, preferentially reacts with NH3, whereas the isotope with more mass, 15N, is subsequently released to the atmosphere. This suggests that the N2 product is subject to kinetic fractionation during the reaction between NOx and NH3 at the high temperatures in the power plant stacks. Kinetic fractionation would favor the 14N reacting to form the N2 product. Note that a similar magnitude effect was noted whether the SCR and non-SCR comparison was made across plants or at different units within the same plant. The fact that the SNCR/OFA/LNB δ15N-NOx value (Plant C 14.2‰) falls between the OFA/LNB and SCR/OFA/LNB values indicates that while the SNCR/OFA/LNB may be more efficient than the OFA/LNB technology alone, it is not more efficient than the SCR/OFA/LNB technology. The difference between SNCR/OFA/LNB and SCR/OFA/LNB values could also result from the competing SNCR reaction wherein the SNCR reagent (NH3 or urea) reacts to form NOx.6 The higher standard deviation among samples from SCR operations (2, 5, and 0.5‰ at Plants A, C, and D, respectively) may be due to varying efficiency in the SCR technology or varying NOx concentrations in the stack gas; both of which would lead to varying NH3 to NOx reaction ratios and thus variable nitrogen isotope fractionation. For instance, if the NOx to NH3 reaction

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DISCUSSION Coal δ15N Values. The isotopic composition of coal used at Plant A and B falls within prior published ranges of +1 to +4‰ for δ15N values for international coals, including lignite and Appalachian coal from the U.S.20,21 Although δ15N values of coal from Plants A and B are similar, the resulting postcombustion δ15N-NOx values differed significantly. The δ15N-NOx values were 17.3 and 7.8‰ higher than the δ15N values of coal for Plant A SCR/OFA/LNB and Plant B OFA/ LNB, respectively. This indicates that δ15N-NOx values are a function of fractionation induced by thermal NOx production (affected by fuel to air ratio) and NOx reduction technology, rather than the δ15N values of the coal itself. Comparison of Absorbing Solutions. Similar δ15N-NOx values observed using all three absorbing solutions suggest absorbing solution reactions are not a factor in resulting δ15N-NOx values. All sampling techniques and postsampling treatment among the absorbing solutions were identical. In comparing absorbing solutions during the NOx collection at Plant B, the grab sample method using the H2SO4 absorbing 3532



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quality and thus monitoring changes in δ15N-NOy may be a useful tool for assessing the influence of EGU emissions on regional wet and dry deposition. The influence of this expected change in electricity-sector δ15N-NOx values on the nitrogen isotopic composition of ambient NOx will depend on fluxes and isotopic composition from other major NOx sources (e.g., vehicles, biomass burning) and potential for post-emission fractionations. Whereas this study provides important constraints on the nitrogen isotopic composition of emissions from electricity generation, δ15N values of other NOx sources are more ambiguous. However, to illustrate the potential utility of the type of results reported here, we construct a mixing model using best available information on the isotope distributions reported for major NOx sources (as shown in Figure 3). Using reported ranges of δ15N-NOx values, coupled with 1990 emission inventory data,23,24 we estimate contributions from individual NOx sources to ambient δ15N-NOy in the U.S. for the year 1990 (Table 3, eq 4). The 1990 inventory was used as it includes estimates of NOx emissions from fossil fuel combustion, biomass burning, soil, and lightning, and precedes SCR implementation.

ratio is greater than 1, then NOx does not fully react. This will lead to the less massive 14N atom in NOx reacting first thus resulting in kinetic fractionation. The degree of kinetic fractionation may vary with the varying NOx to NH3 reaction ratio. Comparison with NOy Deposition Products. δ15N values of NOx oxidation products in the atmosphere are generally lower than the values reported here for power plant NOx emissions. For example, δ15N-NO3(p) (particulate nitrate), δ15N-HNO3(g) (nitric acid vapor) (both also commonly referred to as “dry deposition”), and δ15N-NO3 (aq) (nitrate in wet deposition) values have been reported as −9.5 to +14.1‰, −4.9 to +10.8‰, and −8.1 to +3.2‰, respectively7,8 (Figure 1) for sites across the Northeast U.S. This difference between the δ15N values of NOx oxidation products and NOx source emissions may arise from the various reaction pathways associated with subsequent oxidation of NO x in the atmosphere. For example, NOx can be oxidized to HNO3 through two main formation pathways. During the daytime, NOx is oxidized by the OH radical, which is produced photolytically. During the nighttime, NOx is instead oxidized by O3 to form N2O5, which is hydrolyzed to HNO3. Equilibrium and kinetic isotope fractionations with each of these oxidation reactions can alter δ15N values. For instance, Freyer22 suggested a kinetic fractionation of 0.9971 associated with the initial oxidation of NO2 to HNO3(g). This fractionation (eq 3) and the values reported herein can be used to predict δ15N-HNO3(g) values resulting from the oxidation of NOx emitted from SCR and LNB (i.e., non-SCR) power plants. δ15N‐NO2 = δ15N‐HNO3 + (1000*(1 − 0.9971))

Ambient U.S. δ15N‐NOx = fmobile *δmobile + fstationary *δstationary + fbiomass burn *δ biomass burn + fsoil *δsoil + flightning *δ lighting

(3)

(4)

Interestingly, despite significant uncertainty in δ15N-NOx values for other sources, this exercise suggests because SCR implementation reduces EGU NOx emissions (by >80%6), gradual implementation of SCR technology could actually decrease δ15N values of ambient NOx and secondary chemical products; assuming contributions from other NOx sources remain constant (Figure 3B). For example, given the scenario of 100% SCR implementation, overall ambient δ15N-NOx values are expected to decrease by approximately 3‰; this difference would be more pronounced in regions where ambient NOx is primarily derived from coal-fired EGUs. Additionally, given gradual SCR implementation and 1990 emission scenarios, the model predicts an ambient δ15N-NOx value of +4.5‰ for 2000; this equates to an oxidation product δ15N-HNO3 value of +1.6‰. This value falls within the range of δ15N-NO3 in U.S. precipitation for the year 2000 (−11 to +3.5‰, mean −3.1‰, ∼150 sites, n = 883)25 and well within the range of δ15N-HNO3 values reported for 2004−2005 in parts of the Northeastern U.S. (−4.9‰ to +10.8‰).8 In summary, despite the expected industry-wide increase in δ15NNOx values due to gradual SCR implementation, the concomitant large decreases in NOx emissions from the EGU section is expected to decrease U.S. average ambient δ15N-NOx values by approximately 3‰ (Figure 3B). Given the uncertainty in the calculated ambient δ15N-NOx values (Figure 3B), it may require a long time horizon to discern changing NOx source contributors to ambient NOx. However, this uncertainty can be decreased significantly through future studies that produce more precise δ15N-NOx values of NOx sources. One of the largest uncertainties in such exploratory modeling efforts is the isotopic composition of vehicle NOx. The strong influence of emission controls on δ15N-NOx values from power plants reported herein suggests that employment of NOx

Using this approach, the predicted δ15N-HNO3(g) values associated with SCR EGUs, SNCR EGUs, and EGUs equipped with neither SCR nor SNCR are calculated as +16.8, +11.3, and +8.4‰, respectively (Figure 1). Although these values are closer to reported ambient ranges of δ15N-HNO3(g), the remaining differences between ambient NOy products and power plant-derived δ15N-HNO3 values reported here are attributable to: (1) additional fractionations associated with further physical or chemical reactions of power plant NOx; (2) contributions of NOx from other sources characterized by different δ15N-NOx values; or (3) a combination of 1 and 2. Figure 1 summarizes the known isotopic compositions of major NOx sources (fossil fuel combustion, biogenic soil emissions, and lightning), the predicted potential δ15N-NOx of HNO3 derived from our observed ranges of δ15N-NOx SCR-, SNCR-, and LNB-equipped EGUs given the fractionations discussed above, and observed ranges of δ15N values in resulting precipitation and gaseous reactive N.7,8 Predicted Effect of SCR Implementation on Ambient U.S. δ15N-NOx Values. Our data indicate that power plants contribute high δ15N values to atmospheric NOx relative to existing data for other NO x sources. To improve our understanding of how increasing implementation of SCR technology could influence δ15N values of NOx from this individual emission sector, we developed a mixing model that estimates EGU δ 15 N-NO x at various levels of SCR implementation (0−100%). The results of this model suggest that implementation of additional SCR emission controls at power plants will result in an industry-wide increase in average δ15N-NOx emissions and associated power plant NOx oxidation products (Figure 3A). Importantly, increased implementation of SCR technology is expected to significantly improve air 3533



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Figure 3. (A) Percent NOx contribution from coal-fired EGUs to the total U.S. NOx budget given gradual SCR implementation from coal-fired EGUs (squares). For these levels of SCR usage, calculated δ15N-NOx values from coal-fired EGUs are shown (diamonds). For comparison, SCR usage in 2008 is indicated (X). SCR usage is base on MW capacity. Mean δ15N-NOx values of SCR and non-SCR EGUs are used in calculating the δ15N-NOx values of EGU emissions. (B) Estimated ranges of ambient δ15NOx values for U.S. NOx emissions from all sources (eq 4) given varying levels of SCR implementation. For comparison, recent SCR usage by year27 is shown (X).

Table 3. Estimated U.S. δ15NOx Values from Individual Emission Sources and Total Ambient Emissions Based on 1990 Emission Inventory23 a source mobile EGUs, other industrial sources biomass burning biogenic soil lightning

range of literature δ15N (‰)

representative δ15N of literature (‰)

TgN/ yr

% of total U.S. emissions

range of δ15N contribution

δ15N contribution from representative values (‰)

+3.7 to +5.7 +9.0 to +12.6

+4 +11

6 1.3

49.8 36.1

+1.8 to +2.8 +3.2 to +4.5

+2.0 +4.0

+14 −20 to −49 0 to +2

+14 −35 +1

0.3 0.5 0.4

3.5 5.9 4.7 predicted ambient δ15NOx value =

+0.5 −2.9 to −1.2 0 to +0.1

+0.5 −2.1 +0.05

a

+2.6 to +6.7

+4.5 27

EGUs employing SNCR and SCR technology are not included in this table due to minimal use prior to 1999. For this analysis, we assume stationary NOx is comprised of LNB-equipped NOx emissions from EGUs and other industrial operations. Mobile and stationary sources are assumed to constitute 58% and 42% of fossil fuel NOx emissions, respectively, based on U.S. EPA NEI data for 1990.24 The δ15N-NOx value from biomass burning (+14‰) is derived from pre-1850s ice core δ15N-NO3− data.28 Representative δ15N values are the mean or median of literature δ15N values. 3534



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reduction technologies in vehicles could result in variable δ15NNOx emitted by vehicles. SCRs have already been integrated into vehicular diesel engines to meet Euro 4 and Euro 5 emission regulations26 and additionally, the U.S. EPA recently strengthened the health-based National Ambient Air Quality Standard (NAAQS) for nitrogen dioxide. Thus documenting δ15N-NOx values emitted from vehicles with variable emission controls is an important next step in monitoring the progression of NOx reduction technologies on air quality and reactive nitrogen deposition. Monitoring changes in δ15N-NOx and its oxidation products may also be a valuable tool for assessing the effectiveness of SCR or SNCR technology for reducing power plant NOx contributions to reactive nitrogen deposition, understanding the fate and transport of these emissions across landscapes and regions, and ultimately improving our knowledge of how multiple NOx emission sources impact air quality, the environment, and human health. Future work should be aimed at more extensive sampling of coal-fired EGUs, assessing mechanisms for in-stack and postcombustion fractionations, determination of whether fuel type (e.g., biomass, oil) influences δ15N-NOx, and more comprehensively characterizing the δ15N values of other NOx emission sources.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 412-624-8780; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Daniel J. Bain for his assistance with ion chromatography, as well as Katherine Redling and Kathleen Tuite for their assistance with sample preparation and isotopic analysis. We thank Grace Consulting and Greg Sims for their assistance with sample collection. This work benefited from discussions with Carol Kendall (USGS), Ellen Burkhard (NYSERDA), Rick Carlton (EPRI), Eladio Knipping (EPRI), Doug Burns (USGS), and Elizabeth Boyer (Penn State), and comments from three anonymous reviewers.

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