Power Plant Plumes Research Articles

Emissions lifetimes and ozone formation in power plant plumes

The concept of ozone production efficiency (OPE) per unit NOx is based on photochemical models and provides a tool with which to assess potential regional tropospheric ozone control strategies involving NOx emissions reductions. An aircraft study provided data from which power plant emissions removal rates and measurement‐based estimates of OPE are estimated. This study was performed as part of the Southern Oxidants Study‐1995 Nashville intensive and focuses on the evolution of NOx, SO2, and ozone concentrations in power plant plumes during transport. Two approaches are examined. A mass balance approach accounts for mixing effects within the boundary layer and is used to calculate effective boundary layer removal rates for NOx and SO2 and to estimate net OPE. Net OPE is more directly comparable to photochemical model results than previous measurement‐based estimates. Derived net production efficiencies from mass balance range from 1 to 3 molecules of ozone produced per molecule of NOx emitted. A concentration ratio approach provides an estimate of removal rates of primary emissions relative to a tracer species. This approach can be combined with emissions ratio information to provide upper limit estimates of OPE that range from 2 to 7. Both approaches illustrate the dependence of ozone production on NOx source strength in these large point source plumes. The dependence of total ozone production, ozone production efficiency, and the rate of ozone production on NOx source strength is examined. These results are interpreted in light of potential ozone control strategies for the region.

Loss rate of NOy from a power plant plume based on aircraft measurements

This study was motivated by the recent work of Buhr et al. [1996] which reported losses of NOy from large power plant plumes as high as 0.25 hour−1, much higher than generally accepted values. If true, conclusions pertaining to the efficiency of ozone and nitrate production in the lower troposphere would need major revisions. The results of Buhr et al. were based on aircraft measurements in four TVA (Tennessee Valley Authority) power plant plumes on July 7, 1995, as part of the Nashville/Middle Tennessee Ozone Study, a measurement program of the Southern Oxidants Study (SOS), whereas the results reported in this paper are also based on measurements made in the same SOS study aboard another instrumented aircraft (the TVA helicopter), in plumes of one of these power plants (the Cumberland Steam Plant in northwestern Tennessee) during five different days in 1994 and 1995. Between the 1994 and 1995 sampling periods, emissions of SO2 at the Cumberland plant were reduced by nearly 95% by installation of scrubbers. Our data from the one 1994 day show that the ratio of excess SO2 to NOy, in the plume core increased significantly with plume age, indicating a potentially high differential loss rate of NOy (excess loss of NOy relative to SO2) of about 0.12 hour−1. However, results based on the larger 1995 data set indicate a low differential NOy loss rate of only 0.00±0.03 hour−1, consistent with accepted low loss rates. Because the SOS‐Nashville/Middle Tennessee Ozone Study was not specifically designed to explore the NOy loss issue, the question of NOy loss rates in plumes is not currently resolved and additional focused field studies are needed.

Ozone formation and transport in southeastern United States: Overview of the SOS Nashville/Middle Tennessee Ozone Study

The Southern Oxidants Study (SOS) is a public‐private partnership collectively engaged, since 1989, in a coordinated program of policy‐relevant research to improve scientific and public understanding of tropospheric ozone pollution. In the summers of 1994 and 1995, SOS implemented the Nashville/Middle Tennessee Ozone Study. This effort, the second SOS urban intensive study, conducted a series of integrated, process‐oriented airborne and surface measurement experiments to better understand the chemistry and meteorology associated with the production, transport, and impact of tropospheric ozone. Specific technical objectives addressed (1) the role of biogenic VOC and NOx emissions on local and regional ozone production, (2) the effect of urban‐rural exchange/interchange on local and regional ozone production, (3) sub‐grid‐scale photochemical and meteorological processes, and (4) the provision of a high‐quality chemical and meteorological data set to test and improve observation‐ and emission‐based air quality forecast models. Some of the more significant findings of the 1994–1995 studies include the following: (1) Ozone production in Nashville was found to be close to the transition between NOx‐sensitive and VOC‐sensitive chemistry. (2) Ozone production efficiency (OPE) in power plant plumes, molecules of ozone formed per molecule of NOx emitted, was found to be inversely proportion to NOx emission rate, with the plants having the greatest NOx emissions exhibiting the lowest OPE. (3) During stagnant conditions, nighttime winds dominated pollutant transport and represent the major mechanism for transporting urban pollutants to rural areas. Ultimately, results provided by this research will allow improved assessment of existing ozone management strategies and provide better scientific tools for the development of future management strategies.

Role of anthropogenic NOx and VOC as ozone precursors: A case study from the SOS Nashville/Middle Tennessee Ozone Study

Using data collected on July 11, 1995, from the Tennessee Valley Authority (TVA) instrumented Bell 205 helicopter during the 1995 SOS Nashville/Middle Tennessee Ozone Study, we examine the relative roles of NOx and volatile organic compound (VOC) emissions on ozone photochemical production in the Nashville area. On this day, instruments onboard the helicopter separately sampled air from within a plume emanating from a nearby coal‐fired power plant and air from the Nashville urban plume. Using SO2 concentrations and latitude to discriminate between the plumes, we compare the effects on O3 production of only adding NOx to the background air (i.e., the power plant plume) to that of adding both NOx and VOC (i.e., the urban plume). Our analysis indicates that NOx was the prime anthropogenic precursor causing the generation of O3 during the experiment, with anthropogenic VOC perhaps playing a secondary but significant role. The addition of VOC emissions from the urban area had no appreciable effect on peak O3 concentrations, clearly demonstrating that anthropogenic NOx emissions by themselves can produce elevated O3 concentrations in the Nashville area. Anthropogenic VOC emissions, however, may have enhanced the ozone production efficiencies and the rate at which O3 reached its peak values. While not conclusive, the data also suggested that the highest O3 concentrations in the Nashville area on July 11, 1995, appeared within embedded plumes that combined the effects of both urban and power plant precursor emissions.

Measurements of peroxides and related species during the 1995 summer intensive of the Southern Oxidants Study in Nashville, Tennessee

Hydroperoxide measurements are presented for 12 flights of the U.S. Department of Energy G‐1 aircraft during the summer 1995 intensive of the Southern Oxidants/Middle Tennessee study. A three‐channel analyzer, utilizing both peroxidase/p‐hydroxy phenylacetic acid (pOHPAA) and ferrous sulfate/benzoic acid (FeBA) reagents permitted continuous measurements of hydrogen peroxide (H2O2), methyl hydroperoxide (CH3OOH or MHP), and hydroxymethyl hydroperoxide (HOCH2OOH or HMHP). The median concentration of total hydroperoxide was 5.2 ppbv, with median concentrations of 2.4, 1.7, and 0.97 ppbv for H2O2, MHP, and HMHP respectively. In the free troposphere, H2O2 concentration correlated with the concentration of its precursors, ozone, and water vapor. H2O2 profiles do not show elevated concentrations in the boundary layer, in contrast to vertical profiles of primary pollutants. The combination of high NOx concentrations that inhibited peroxide formation, and peroxide loss through deposition and reaction with OH, led to markedly lower peroxide concentrations within urban and power plant plumes. Photochemical calculations performed to estimate the magnitude of in‐plume peroxide depression showed reasonable agreement with observations.

A comparison of airborne and surface trace gas measurements during the Southern Oxidants Study (SOS)

The NOAA Twin Otter conducted more than a dozen overflights of ground‐level air quality monitoring stations during the 1995 Southern Oxidants Study (SOS) Nashville/Middle Tennessee Ozone Project Field Intensive. Surface and aircraft observations of ozone and ozone precursors were examined to identify systematic sampling errors, and to assess the degree to which surface measurements may be considered representative of the larger planetary boundary layer (PBL). Overall agreement between surface and aircraft trace gas measurements was excellent in the well developed mixed layer, especially in rural‐regional background air and under stagnant conditions, where surface concentrations change only slowly. On July 2, surface level measurements were representative of the larger mixed layer over distances as far as 70 km in background air, and 30 km in the weakly advected urban plume. Vertical variations in trace gas concentrations were often minimal in the well‐mixed PBL, and measurements at the surface always agreed well with aircraft observations up to 460 m above ground level. Under conditions of rapidly varying surface concentrations (e.g., during episodes of power plant plume fumigation and early morning boundary layer development), agreement between surface and aloft is dependent upon the spatial (aircraft) and temporal (ground) averaging intervals used in the comparison. Under these conditions, surface sites may be representative of the PBL only to within a few kilometers. Under clear skies in the well‐mixed PBL, regression of aircraft trace gas data collected within 5 km of the ground sites against 15–20 min average surface concentrations centered on the times of the overflights yielded the following relationships: [O3]aircraft = ([O3]surface × 0.9374) + 4.86 ppbv (r2 = 0.9642); [CO]aircraft = ([CO]surface × 0.8914) + 16.4 ppbv (r2=0.9673); [SO2]aircraft = ([SO2]surface × 0.9414) − 0.069 ppbv (r2 = 0.9945). Although O3, CO, and SO2 measurements at the surface and aloft generally agreed well, isolated examples of unexplained measurement discrepancies emerged, illustrating the need for side‐by‐side instrument comparisons. NOY measurements agreed poorly between surface and aircraft: [NOY]aircraft = ([NOY]surfacex × 0.9184) + 4.56 ppbv (r2 = 0.9188).

Reduced Gas-Phase Kinetic Mechanism for Atmospheric Plume Chemistry

The subgrid-scale simulation of plume chemistry in three-dimensional air quality models can be computationally demanding. To minimize this computational burden, reduced kinetic mechanism for the gas-phase chemistry of power plant plumes is developed and validated against a full chemical kinetic mechanism. This reduced mechanism simulates plume chemistry according to three stages of the plume evolution: (1) A first stage where plume radical concentrations are negligible and plume chemistry is limited to four major reactions during the day and two at night. (2) A second stage where plume concentrations of OH and NO 3 radicals are sufficiently high to lead to significant formation of HNO 3 and H 2 SO 4 and which can be simulated with 30 reactions (or, alternatively, two different sets of 19 and 18 reactions for daytime and nighttime, respectively). (3) A third and final stage where VOC oxidation and O 3 formation become important, and the full chemical mechanism is required. For 10% accuracy in simulated plume concentrations, the reduced mechanism led to reductions in computational time of up to a factor of 3.

Measurements of reactive nitrogen and ozone to 5‐km altitude in June 1990 over the southeastern United States

Aircraft flights made in June 1990 substantiate the expected variability of ozone and reactive nitrogen constituents in the lower atmosphere over Mississippi and Alabama due to the variety and proximity of sources and the photochemical production and loss of ozone. Urban, biomass burn, electrical power plant, and paper mill plumes are examples of sources encountered during these flights, and some of their characteristics are described. For example, in the urban plume from Mobile, Alabama, an upper limit to the ozone production efficiency was 6–7 parts per billion by volume (ppbv) of O3 per ppbv of NOx oxidized. Emissions from several biomass fires had lower efficiencies of 2–4 ppbv ppbv−1, but the average rate of production of ozone was as high as 58 ppbv hr−1 for one fire where the plume was isolated from mixing to the surface. Close encounters with paper mill and power plant emissions showed chemical consumption of O3, while far‐field encounters with power plant plumes showed enhanced O3 due to photochemical production. Observations made in early morning, below a nocturnal inversion provide indirect evidence of significant nighttime oxidation of freshly emitted NOx to reservoir constituents. Although statistics for the region covered by these flights clearly depend on precisely where the aircraft flew, the mean and median temporal and vertical behaviors of constituent mixing ratios are summarized.

Electrochemical detection of mercury adsorbed from flowing gas samples on porous carbon electrodes

AbstractThe adsorption of mercury from flowing gas streams onto reticulated vitreous carbon (RVC) is combined with postadsorption stripping voltammetric determination. This novel analytical method achieves gas stream detection limits of 1.1 μg/m3 for unmodified RVC electrodes at 52°C and 1.8 μg/m3 for modified RVC electrodes at 25°C. Due to the unique features of this detection technique, it is possible and feasible that it could be applied for mercury analysis in coal fired power plant plumes and waste incinerator stacks.

Microphysical and optical features of polluted cooling tower clouds

In November 1993 an airborne field study was performed in order to investigate the microphysical and radiative properties of cooling tower water clouds initiated by water vapour emissions and polluted by the exhaust from coal-fired power plants. The number-median diameter of the droplet size distributions of these artificial clouds was in the range of 13 μm. The concentration of smaller droplets (diameters d D < 10 μm) increased with height and horizontal distance from the cooling towers. Close to the cooling towers, bimodal spectra were found with a second mode at 19 μm. The liquid water content (LWC) ranged between 2 and 5 g/m 3 and effective droplet radii ( R e) between 6 and 9 μm were measured. LWC and R e decreased with altitude, whereas the droplet concentration ( N D) remained approximately constant (about 2000 cm −3 ). An enrichment of interstitial aerosol particles with particle diameters ( d p) smaller 0.2 μm compared to the power plant plume in the vicinity of the clouds was observed. Particle activation for d m > 0.3 μm. was evident, especially in cooling tower clouds further apart and separated from their sources. Furthermore, radiation measurements were performed, which revealed differences in the vertical profiles of downwelling solar and UV radiation flux densities inside the clouds. The effective droplet radius R e was parameterized in terms of LWC and N D using equations known from literature. The close agreement between measured and parameterized Re indicates a similar coupling of R e, LWC and N D as in natural clouds. By means of Mie calculations, volume scattering coefficients and asymmetry factors are derived for both the cloud droplets and the aerosol particles. For the cloud droplets, the optical parameters were described by parameterizations from the literature. The results show, that the link between radiative and microphysical properties of natural clouds is not changed by the extreme pollution of the artificial clouds.

Sub-grid-scale features of anthropogenic emissions of NO x and VOC in the context of regional eulerian models

Model uncertainty is a major issue concerning regional-scale air quality simulation. One major source of uncertainty in regional Eulerian models is due to sub-grid-scale (SGS) effects related to anthropogenic emissions. Regional models typically have horizontal grid resolution (Δ) of 20–80 km. Since NO x chemistry in plumes is nonlinear and often diffusion-limited, the sudden dilution of plumes over regional grid dimensions, as in current models, can lead to a fundamental distortion of their chemistry, resulting in over-production of ozone, peroxides, sulfates and nitrates in the source region, and a related over-depletion of NO x . The corresponding model uncertainty over the whole regional domain remains unquantified. In this paper, we use high-resolution information from urban and regional emission inventories and plume field studies to examine SGS features of the emissions of anthropogenic NO x and VOC (volatile organic compounds), and of their mesoscale dispersion and chemistry. Such examination provides useful insight into some of the main sources of SGS uncertainty, as well as guidance for reducing it. The mesoscale chemistry of power plant plumes is very diffusion-limited, being controlled by VOC entrainment from the background. The crosswind spread of large point-source plumes typically takes 4–6 h to reach 30 km in convective conditions, and at least a full diurnal cycle to reach 80 km. For Δ much larger than 20–30 km, regional models will not be able to capture the essence of the behavior of rural point-source plumes even with plume-in-grid treatment, or to resolve the NO x emissions from many large power plants in urban peripheries from the urban VOC emissions. Within urban areas, there is progressive improvement in the resolution of the important ratio VOC/NO x as Δ is decreased below 20 km. The nature of these emissions-related SGS features suggests that significant gain in regional model accuracy should result by limiting Δ to 20–30 km in the regional domain, by the use of finer nested gridding in metropolitan sub-domains, and by a reactive plume-in-grid treatment of major point-source emissions.

Airborne streaker sampling for PIXE analysis

Ground based aerosol sampling has inherent limitations for understanding atmospheric transport and dynamics. Sampling from aircraft platforms has limitations on the sampling time and quantity of aerosol mass that can be collected, especially if multiple samples per flight are required to allow for vertical or horizontal resolution. A modification of Florida State University circular streaker sampler has been made to enable aerosol samples to be collected isokinetically from a Lear jet. The samples are matched for analysis by PIXE or scanning electron microprobe. Features of the sampler include: a two stage isokinetic inlet matched to aircraft speeds of 550 km/h; remote computer control of the wing tip mounted sampler; up to twenty samples per flight on a single circular disk. Preliminary tests with the sampler are described, involving sampling of a coal fired power plant plume, and calibration of PIXE elemental concentrations against particle number density measured simultaneously with a forward scattering probe.

The interdependence of aerosol processes and mixing in point source plumes

The basic aerosol phenomena, condensation, coagulation, nucleation and gravitational settling, were studied in plumes generated by industrial point sources. The investigation was concentrated on regions near the source, where the plume is still clearly distinguishable from the background and not vertically mixed. Initially vapors are depleted from the plume more by condensation than by mixing, while later these two processes are often comparable in magnitude. Explicit treatment of condensation is required during most stages of the plume development. Coagulation is normally suppressed by the more rapid dilution, and is probably significant only for nanometer sized nuclei. Gravitational settling depletes largest particles from the plume, but has no effect on other aerosol processes. Binary homogeneous nucleation of sulfuric acid and water is probably a commonplace phenomenon in power plant plumes, can take place only after the plume becomes rather diluted, and is favored by high SO 2(g) to fine particulate mass ratio in the emissions, cool and humid ambient conditions, and strong solar radiation.

A hybrid Lagrangian dispersion model for elevated sources in the convective boundary layer

A hybrid Lagrangian dispersion model is presented for predicting the mean ( C) and root-mean-square fluctuating ( σ c) concentrations from elevated sources in the convective boundary layer (CBL). The model is based on Gifford's (1959, Adv. Geophys. 6, 117–138) meandering plume concept and includes: (1) a Lagrangian stochastic model for treating the plume meander caused by the large-scale convective turbulence, and (2) an entrainment model to determine the plume rise and growth relative to the meander. The hybrid model is currently limited to weakly-to-moderately buoyant plumes as defined by F ∗⪅0.1 , where F ∗ is the dimensions buoyancy flux: F ∗ = F b/U w ∗ 2h , where F b is the stack buoyancy flux, U is the mean wind speed, w ∗ is the convective velocity scale, and h is the CBL depth. The modeled crosswind-integrated concentration ( C y ) and σ c/ C fields showed fair agreement with the laboratory convection tank measurements of Deardorff and Willis (1984, Atmospheric Environment 18, 1297–1309; 1988, Lectures on Air Pollution Modeling, pp. 357–384). The C y fields were modeled both with and without relative dispersion ( σ r ). The C y fields with σ r included showed better agreement with the measurements in the lower half of the CBL than those without σ r , and they demonstrated the importance of including entrainment due to ambient turbulence in the inertial subrange. Plume rise and buoyancy effects were most significant for dimensionsless distances X = w ∗x/Uh⪅2–3 , where the near-surface σ c/ C ranged from 1 to 7; x is the downwind distance. For the same X range, measurements in power plant plumes showed that σ c/ C⪅2 and hence smaller than the values in the laboratory. A possible reason for the difference is the small Reynolds number in the convection tank.

Origins of fine particles in a power-plant plume as determined using individual-particle analysis

Individual particles from the fine fraction of samples of entrained ash from a coal-burning power plant were analyzed using an automated scanning electron microscope with an energydispersive x-ray spectrometer. The particles were of concern as a probable cause of high levels of opacity in the plant emissions. Both the mass loading and the number size distribution of fine particles peak in the size range of 0.4 to 0.8 μm diameter.Most of the fine particles are Na-, S-, P-, or Cl-rich. The coal is likely to be the major source for these particles. Minerals and organically-associated metals in the coal can remain in the solid phase during combustion, forming ash particles of varied sizes, including those in the submicron range. Elements are also vaporized during combustion, and Na- and S-bearing particles may form when gas-phase Na2SO4 condenses homogeneously in the convective pass of the boiler. Phosphate and chloride particles may also condense homogeneously.

Hidden photostationary equilibrium: A case study on the effect of monitor averaging on the calculated oxidation rate of NO to NO 2 in the plume of a power plant

Comparison of measurements and model calculations has shown that NO, NO 2 and O 3 concentrations in power plant plumes are very close to their equilibrium values. Measured deviations from their photostatic equilibrium values are likely to be caused by insufficient spatial resolution of the instruments used, and not (as often suggested) by the slowness of the reactions involved compared to the speed of mixing. Furthermore it is shown that, this being true, probability density functions of plume NO x concentrations can be used to calculate NO 2 concentrations. The spatial resolution of the measurements was 40 m. This resolution was obtained after applying a two-point digital deconvolution filter to the measured concentrations of NO, NO x and O 3.

Operating ranges of meteorological wind tunnels for the simulation of convective boundary layer (CBL) phenomena

The operating ranges of meteorological wind tunnels for convective boundary-layer (CBL) simulation are defined in this paper based on a review of the theoretical and practical limitations of the flow phenomena and the facilities available. Wind-tunnel operating ranges are limited by the dimensions of the simulated circulations and of the tunnel itself, the tunnel flow speed and turbulence processes, and the characteristics of the measurement instrumentation. When it is desired to simulate both the CBL and the behavior of other flows imbedded within the boundary layer, such as power-plant plume rise and dispersion, then additional constraints exist on the fluid modeling process. The capabilities of meteorological wind tunnels can also be extended through the judicious use of boundary and side wall flow controls.

Nitrous oxide from coal‐fired power plants: Experiments in the plumes

We report studies of N2O concentrations in the ambient plumes of power plants. The results show that there is very little excess N2O within 100 m of the stack. There is some evidence that N2O is formed in the plumes farther downwind. The magnitude of the effect cannot be quantitatively evaluated with the present data. Based on our results and published studies, we estimate that the global direct emissions of N2O from power plants are about 0.05 Tg/yr out of a total anthropogenic source of about 7 Tg/yr.

The formation of aerosols from SO 2 and NH 3 in humid air

The formation of ammonium sulphate aerosols from SO2 and NH3 in humid air is a source of atmospheric aerosol production. In particular, the visible aerosol ('blue smoke') formed in the stack plumes of power plants with ammonia scrubbers has been ascribed to this reaction (see e.g. Moore, 1977). In the present study preliminary measurements of the formation of aerosols under conditions similar to those of flue gas ammonia scrubbers are presented. Rather surprisingly, they show that the tendency for homogeneous nucleation of aerosols in this system depends primarily upon an exposure of the gas to light in the near ultra-violet range. Several reactions in the gas may lead to the formation of solids. NH3 and SO2 alone can react to form NH3SO2 and (NH3)2SO2 which have been proposed as the primarily formed seeds of aerosol (Hartley and Matteson, 1975). However, the formation of these compounds is not possible at the conditions of the present study due to the chemical equilibria. In the presence of H20 the following reactions are relevant (Moore, 1977):

Analysis of the dispersion characteristics of the Navajo generating station plume using a Lagrangian Monte-Carlo model

The Monte-Carlo Lagrangian particle model MC-LAGPAR developed by Aero Vironment Inc., been used to analyze the dispersion characteristics of the Navajo Generating Station Power Plant plume over the Grand Canyon area. The study was performed on a 26 × 26 grid covering an area of 67,600 km 2. The simulations were performed for the period January 18–22, 1990, a high SO 2 concentration episode during which the plume from the power plant reached Hopi Point, a receptor site on the south rim of the Grand Canyon. The meteorological data used as input to the MC-LAGPAR model were generated by MM4, the Penn State/NCAR mesocale model. The dispersion characteristics of the NGS plume are presented using plan views (XY) of the particles in the computational domain, for different hours of the day. The four-hour average SO 2 concentrations predicted by the model, assuming no SO 2 oxidation and no deposition, are in good agreement with the measurements at Hopi Point. The time at which the peak SO 2 concentration occurs is predicted accurately. This suggests that the power plant was the major contributor to the levels of SO 2 measured during the episodic hours. However, simulations assuming a one-percent-per-hour SO 2 oxidation give sulfate levels that are different from the measurements both in magnitude and in the time at which maximum sulfate level occurs. This suggests other sources than NGS were the major contributor to sulfate at Hopi Point during the January episode.