Evaluation of Downwash Modifications to the Industrial Source Complex Model

The Industrial Source Complex (ISC) model has been modified to account for Improved understanding of plume rise and downwash around buildings. Two sets of observations are used to compare the modified and the original ISC models, showing that the new model better simulates the observed concentrations during high-wind conditions when downwash usually occurs.     

Maximum Ground-Level Concentrations with Downwash: The Urban Stability Mode

Abstract

Gaussian model-based equations for critical downwind distance, wind speed, and plume height that result in maximum ground-level concentrations (MGLC) under downwash conditions for the rural stability mode were presented in a previous paper. This paper presents general equations for the critical downwind distance xc for the urban stability mode. Specific examples are presented for Schulman-Scire and Huber-Snyder downwash treatments for building-enhanced and regular sigmas.     

Development and evaluation of the PRIME plume rise and building downwash model

A new Gaussian dispersion model, the Plume Rise Model Enhancements (PRIME), has been developed for plume rise and building downwash. PRIME considers the position of the stack relative to the building, streamline deflection near the building, and vertical wind speed shear and velocity deficit effects on plume rise. Within the wake created by a sharp-edged, rectangular building, PRIME explicitly calculates fields of turbulence intensity, wind speed, and streamline slope, which gradually decay to ambient values downwind of the building. The plume trajectory within these modified fields is estimated using a numerical plume rise model. A probability density function and an eddy diffusivity scheme are used for dispersion in the wake. A cavity module calculates the fraction of plume mass captured by and recirculated within the near wake. The captured plume is re-emitted to the far wake as a volume source and added to the uncaptured primary plume contribution to obtain the far wake concentrations. The modeling procedures currently recommended by the U.S. Environmental Protection Agency (EPA), using SCREEN and the Industrial Source Complex model (ISC), do not include these features. PRIME also avoids the discontinuities resulting from the different downwash modules within the current models and the reported overpredictions during light-wind speed, stable conditions. PRIME is intended for use in regulatory models. It was evaluated using data from a power plant measurement program, a tracer field study for a combustion turbine, and several wind-tunnel studies. PRIME performed as well as or better than ISC/SCREEN for nearly all of the comparisons.     

Cooling Towers and the Environment

Measurements of natural draft cooling tower plume behavior, as well as meteorological variables, were obtained from aircraft flights near major power plants of the American Electric Power System. Persistence of the visible plume to great distances depends essentially on ambient humidity. Atmospheric stability at plume elevation was also important. Cooling tower-induced fog at ground-level was never observed in any of the tests, and aerodynamic downwash of the visible plume was absent also. The cooling towers did cause modification of natural clouds and they occasionally shadowed some local areas from the sun. Merging of the stack and cooling tower plumes was a common occurrence.     

Development and Evaluation of the PRIME Plume Rise and Building Downwash Model

ABSTRACT

A new Gaussian dispersion model, the Plume Rise Model Enhancements (PRIME), has been developed for plume rise and building downwash. PRIME considers the position of the stack relative to the building, streamline deflection near the building, and vertical wind speed shear and velocity deficit effects on plume rise. Within the wake created by a sharp-edged, rectangular building, PRIME explicitly calculates fields of turbulence intensity, wind speed, and streamline slope, which gradually decay to ambient values downwind of the building. The plume trajectory within these modified fields is estimated using a numerical plume rise model. A probability density function and an eddy diffusivity scheme are used for dispersion in the wake. A cavity module calculates the fraction of plume mass captured by and recirculated within the near wake. The captured plume is re-emitted to the far wake as a volume source and added to the uncaptured primary plume contribution to obtain the far wake concentrations.

The modeling procedures currently recommended by the U.S. Environmental Protection Agency (EPA), using SCREEN and the Industrial Source Complex model (ISC), do not include these features. PRIME also avoids the discontinuities resulting from the different downwash modules within the current models and the reported overpredictions during light-wind speed, stable conditions. PRIME is intended for use in regulatory models. It was evaluated using data from a power plant measurement program, a tracer field study for a combustion turbine, and several wind-tunnel studies. PRIME performed as well as or better than ISC/SCREEN for nearly all of the comparisons.     

Panel Discussion

A stack design procedure is developed which accounts for the effect of plume interception by downwind buildings, and which provides information on effluent concentrations in a form useful to planning authorities. The information presented in this paper is directed to engineers carrying out stack designs for locations where downwind buildings are of comparable height to the stack. A wind tunnel investigation using tracer gas techniques indicates that, for a plume at building height, downwash on the upwind face of a building causes the high concentrations observed near the roof to be transported to ground level. The effect of a plume on elevated points is determined by the concept of the minimum descent height of the maximum allowable ambient concentration isopleth. This minimum descent height, computed using Gaussian plume dispersion theory, defines a building height below which pollutant concentrations will always lie within safe limits. A case study is presented for the use of the design procedure for a small thermal power plant in an urban area.     

Maximum ground-level concentrations with downwash--analysis

Equations derived previously for critical downwind distance xc' wind speed uc' and plume rise zc' the values that produce maximum ground-level concentrations (MGLC) chi c under downwash conditions, have been solved. Tables of chi c' xc' uc' and zc' and graphs of the relationships among uc and zc, for a range of stack heights hs' and building heights hb' are presented. Results for two types of sources--a turbine and a reciprocating engine--are discussed. Some comparisons are made to the U.S. Environmental Protection Agency's (EPA) SCREEN3 model.     

Pseudo-source parameters for flares: Derivation,implementation, and comparison

The United States Environmental Protection Agency (US EPA) flare pseudo-source parameters are over 30 years old and few dispersion modellers understand their basis and underlying assumptions. The calculation of plume rise from the user inputs of pseudo-stack diameter, temperature and velocity have the most influence on air dispersion model predictions of ground-level concentrations. Regulatory jurisdictions across Canada, the United States and around the world have adopted their own approach to pseudo-source parameters for flares; all relate buoyancy flux to the heat release rate, none consider momentum flux and flare tip downwash as adopted by the Alberta Energy Regulator (AER). This paper derives the plume buoyancy flux for flares burning a gas in terms of combustion variables readily known or calculated without simplifying assumptions. Dispersion model prediction sensitivity to flared gas composition, temperature and velocity, and ambient conditions are now correctly handled by the AER approach. The AER flare pseudo-source parameters are based on both the buoyancy and momentum flux, thus conserving energy and momentum. The AER approach to calculate the effective source height for flares during varying wind speeds is compared to the US EPA approach. Instead of a constant source for all meteorological conditions, multiple co-located sources with varying effective stack height and diameter are used. AERMOD is run with the no stack tip downwash option as flare stack tip downwash is accounted for in the effective stack height rather than the AERMOD model calculating the downwash incorrectly using the pseudo-source parameters. The modelling approaches are compared for an example flare. Maximum ground level predictions change, generally increasing near the source and decreasing further away, with the AER flare pseudo-source parameters. It's time to update how we model flares.

Implications: What are the implications of continuing to model flare source parameters using the overly simplified US EPA approach? First, the regulators perpetuate the myths that the flare source height, temperature, diameter and velocity are constant for all wind speeds and ambient temperatures. Second, that it is acceptable to make simplifying assumptions that violate the conservation of momentum and energy principles for the sake of convenience. Finally, regulatory decisions based on simplified source modelling result in predictions that are not conservative (or realistic). The AER regulatory approach for flare source parameters overcomes all of these shortcomings. AERflare is a publicly available spreadsheet that provides the “correct” inputs to AERMOD.     

A review of the conditions leading to downwash in physical modeling experiments

Although stack downwash is not a widespread problem at modern fossil-fuel power plants and larger industrial stacks, it is a very important problem in simulating buoyant plumes in wind tunnels and towing tanks. Scaling criteria for avoiding downwash in ten subcritical model experiments have been reviewed. A comparison was made between data on the occurrence of downwash from 10 modeling studies to the theory proposed by Tatom (1986). In general, there was good agreement of Tatom's theory with the occurrence of downwash.     

Flue gas discharge from cooling towers. Wind tunnel investigation of building downwash effects on ground-level concentrations

German power plants are required to meet new emission standards which limit the maximum sulfur dioxide (SOs) concentration in flue gas discharges to 400 mg m⁻³. To achieve this level of reduction in SO₂ concentration, wet scrubbing is necessary for large plants using lignite or hard coal.Wet scrubbing results in a significant reduction in the flue gas temperature leading to low effective stack heights. Instead of using stack gas reheating to achieve the plume rise necessary to satisfy local environmental standards, it was proposed to discharge the scrubbed flue gas from the existing natural-draft cooling towers (NDCT). This method should be effective in reducing local ground-level concentrations since NDCT-plumes are typically very buoyant (densimetric Froude number below 1 ) and normally reach considerable heights of rise. Only under strong wind conditions does the situation reverse itself. For such strong winds, the NDCT-plume is subject to tower and building downwash with the possibility of unacceptably high ground-level concentrations.For a 2700 MW_e lignite-fired power plant near Cologne, a wind tunnel study was carried out to investigate the effects of tower and building downwash effects on the ground-level concentrations of SO₂ produced by discharging the scrubbed flue gas from the natural-draft cooling towers. Also, a comparison was made between the ground-level concentrations produced by the cooling tower discharge method and those produced by a traditional stack. It was found that for low and intermediate wind speeds, the groundlevel concentrations are lower for the case of the cooling tower discharge. Only for strong winds, which occur only very rarely at most German sites, did the conventional stack discharge appear to be superior.     

Different parameterisation of a Gaussian scheme: intercomparison study

The aim of this work is to define the most efficient parameterisation of the PBL (planetary boundary layer) by using the simulation with a simple Gaussian dispersion model of the plume type and comparing it with the improved OML model. A different type of σ_y and σ_z parameterisation in a Gaussian dispersion scheme was introduced. The old Pasquill–Gifford scheme and the new boundary layer parameterisation based on Monin–Obukhov length and other convective scaling parameters were compared, using a classical Gaussian model. For this comparison, the hourly average concentration and the long-term concentrations generated by these models, using same input data, were computed. These computed values and the field observations clearly show the importance of the parameterisations used by the models and the necessity of improving them. With this new parameterisation, we can easily add in new phenomena, such as dry and wet deposition, building effects and stack tip downwash penetration.     

Comparison of the industrial source complex and AERMOD dispersion models: case study for human health risk assessment

Air quality models are typically used to predict the fate and transport of air emissions from industrial sources to comply with federal and state regulatory requirements and environmental standards, as well as to determine pollution control requirements. For many years, the U.S. Environmental Protection Agency (EPA) widely used the Industrial Source Complex (ISC) model because of its broad applicability to multiple source types. Recently, EPA adopted a new rule that replaces ISC with AERMOD, a state-of-the-practice air dispersion model, in many air quality impact assessments. This study compared the two models as well as their enhanced versions that incorporate the Plume Rise Model Enhancements (PRIME) algorithm. PRIME takes into account the effects of building downwash on plume dispersion. The comparison used actual point, area, and volume sources located on two separate facilities in conjunction with site-specific terrain and meteorological data. The modeled maximum total period average ground-level air concentrations were used to calculate potential health effects for human receptors. The results show that the switch from ISC to AERMOD and the incorporation of the PRIME algorithm tend to generate lower concentration estimates at the point of maximum ground-level concentration. However, the magnitude of difference varies from insignificant to significant depending on the types of the sources and the site-specific conditions. The differences in human health effects, predicted using results from the two models, mirror the concentrations predicted by the models.     

Maximum Ground-Level Concentrations with Downwash—Analysis

ABSTRACT

Equations derived previously for critical downwind distance x , wind speed u , and plume rise z , the values that produce maximum ground-level concentrations (MGLC) X_c under downwash conditions, have been solved. Tables of %_c, x_c, u_c, and z_c, and graphs of the relationships among u_c and z_c for a range of stack heights h_s, and building heights h_b, are presented. Results for two types of sources— a turbine and a reciprocating engine—are discussed. Some comparisons are made to the U.S. Environmental Protection Agency's (EPA) SCREEN3 model.     

Critical review of the building downwash algorithms in AERMOD

The only documentation on the building downwash algorithm in AERMOD (American Meteorological Society/U.S. Environmental Protection Agency Regulatory Model), referred to as PRIME (Plume Rise Model Enhancements), is found in the 2000 A&WMA journal article by Schulman, Strimaitis and Scire. Recent field and wind tunnel studies have shown that AERMOD can overpredict concentrations by factors of 2 to 8 for certain building configurations. While a wind tunnel equivalent building dimension study (EBD) can be conducted to approximately correct the overprediction bias, past field and wind tunnel studies indicate that there are notable flaws in the PRIME building downwash theory. A detailed review of the theory supported by CFD (Computational Fluid Dynamics) and wind tunnel simulations of flow over simple rectangular buildings revealed the following serious theoretical flaws: enhanced turbulence in the building wake starting at the wrong longitudinal location; constant enhanced turbulence extending up to the wake height; constant initial enhanced turbulence in the building wake (does not vary with roughness or stability); discontinuities in the streamline calculations; and no method to account for streamlined or porous structures.

Implications: This paper documents theoretical and other problems in PRIME along with CFD simulations and wind tunnel observations that support these findings. Although AERMOD/PRIME may provide accurate and unbiased estimates (within a factor of 2) for some building configurations, a major review and update is needed so that accurate estimates can be obtained for other building configurations where significant overpredictions or underpredictions are common due to downwash effects. This will ensure that regulatory evaluations subject to dispersion modeling requirements can be based on an accurate model. Thus, it is imperative that the downwash theory in PRIME is corrected to improve model performance and ensure that the model better represents reality.     

Evaluation of the reactive and optics model of emissions (ROME)

The reactive and optics model of emissions (ROME) is a reactive plume visibility model that simulates the potential atmospheric impacts of stack emissions. We present here an evaluation of the ability of ROME to simulate several plume physical and chemical variables, using an experimental data base that consists of a total of 40 case studies from four field programs. The evaluation variables include plume height, horizontal width, NO_x and SO₂ maximum concentrations, NO₂/NO_x concentration ratio at the plume centerline, and plume-to-sky radiance ratios. Three algorithms used to simulate plume dispersion in ROME were compared: (1) the empirical Pasquill–Gifford–Turner (PGT) scheme, (2) a first-order closure (FOC) algorithm and (3) a second-order closure (SOC) algorithm that simulates the instantaneous plume dimensions.The plume height results show a correlation of 0.82 between simulated and measured values and a gross error that is 13% of the mean measured value. For plume horizontal dispersion, the second-order closure algorithm produces a moderate correlation (0.54) and a small bias (5% of the mean measured value) in comparison with the field data. Although the PGT scheme also demonstrates moderate correlation with the measurements, it produces a negative bias by significantly underestimating plume horizontal dispersion. The first-order closure algorithm overestimates plume width and shows the least correlation (with the measurements) of the three dispersion algorithms.For the NYSEG data set where coordinated measurements of stack emissions, meteorology at plume height and plume characteristics were available, the SOC algorithm provides better correlations for NO_x concentrations, NO₂/NO_x ratios and plume visibility than the FOC and PGT algorithms. For plume visibility, the SOC algorithm shows a correlation of 0.96 at 405 nm, the wavelength where the plume was visible, and it simulates no visible plume at the other wavelengths (550 and 700 nm).A comparison of ROME simulations with those of the plume visibility model PLUVUE II shows that ROME, with the SOC algorithm, performs better for all variables.     

Water tank measurements of buoyant plume rise and structure in neutral crossflows

In this paper, an experimental study of the rise and development of a single buoyant plume and a pair of in-line buoyant plumes is presented. The investigations were carried out at small scale in a water filled towing tank using both quantitative flow visualisation and local concentration measurements. The measured plume trajectories for a single plume were compared with the Briggs plume rise equation and predictions from a numerical integral model. Plume trajectories were studied for twin in-line plumes, with particular attention to changes in the plume trajectory, especially any additional rise that resulted from the interaction between the two plumes. Concentration field distributions in cross-sections through both single and interacting twin plumes were obtained from the local concentration measurement system. These showed how the interaction affected the plume structure, notably the double vortex system that occurs in a fully developed plume.     

Expanding box models for the long-range transport of chemically reacting airborne material

If a power station plume significantly perturbs the levels of chemically active species in the atmosphere, then the rates of chemical reactions become non-uniform across the plume. This results in different effective plume widths for the different chemical species, which in turn influence the reaction rates. Here coupled equations are derived which for a reaction involving a single oxidant accurately model the total amount of a species in a plume and the associated plume width. The conventional box model slightly underestimates the amount of oxidized material produced. It is therefore suggested that the sensitivity of plume models to assumptions regarding lateral mixing should be tested, using the system of coupled equations derived in the paper.     

The evolution of photochemical smog in a power plant plume

The evolution of photochemical smog in a plant plume was investigated with the aid of an instrumented helicopter. Air samples were taken in the plume of the Cumberland Power Plant, located in central Tennessee, during the afternoon of 16 July 1995 as part of the Southern Oxidants Study – Nashville Middle Tennessee Ozone Study. Twelve cross-wind air sampling traverses were made at six distance groups from 35 to 116 km from the source. During the sampling period the winds were from the west–northwest and the plume drifted towards the city of Nashville TN. Ten of the traverses were made upwind of the city, where the power plant plume was isolated, and two traverses downwind of the city when the plumes were possibly mixed. The results revealed that even six hours after the release, excess ozone production was limited to the edges of the plume. Only when the plume was sufficiently dispersed, but still upwind of Nashville, was excess ozone (up to 109 ppbv, 50–60 ppbv above background levels) produced in the center of the plume. The concentrations image of the plume and a Lagrangian particle model suggests that portions of the power plant plume mixed with the urban plume. The mixed urban power plant plume began to regenerate O₃ that peaked at 120 ppbv at a short distance (15–25 km) downwind of Nashville. Ozone productivity (the ratio of excess O₃ to NO_y and NO_z) in the isolated plume was significantly lower compared with that found in the city plume. The production of nitrate, a chain termination product, was significantly higher in the power plant plume compared to the mixed plume, indicating shorter chain length of the photochemical smog chain reaction mechanism.     

On plume meandering in unstable stratification

Vertical plume meandering of gaseous pollutant is commonly experienced in the daytime atmospheric boundary layer (also know as convective boundary layer, CBL) that arose from the complicated interaction between buoyancy-generated turbulence and gravitational force. It leads to rapid pollutant mixing that cannot be accurately modeled by conventional Gaussian plume model. In the light of explaining the mechanism of plume rises and descents in CBLs, this study employs a direct numerical simulation (DNS) technique to compute the plume behaviors for pollutant emitted from line sources placed parallel to the spanwise direction in an unstably stratified turbulent open channel flow. The DNS results show that the plume meandering is due to the domination of uni-directional mean vertical pollutant fluxes above and below the mean plume height.     

Stable plume rise in a shear layer

Solutions are given for plume rise assuming a power-law wind speed profile in a stably stratified layer for point and finite sources with initial vertical momentum and buoyancy. For a constant wind speed, these solutions simplify to the conventional plume rise equations in a stable atmosphere. In a shear layer, the point of maximum rise occurs further downwind and is slightly lower compared with the plume rise with a constant wind speed equal to the wind speed at the top of the stack. If the predictions with shear are compared with predictions for an equivalent average wind speed over the depth of the plume, the plume rise with shear is higher than plume rise with an equivalent average wind speed.