Stackless Power Plant

When the 230-MW Volklingen power plant entered service in August 1982, it became the first power plant in Europe to desulfurize 100 percent of the flue gas without bypass. The FGD system forms an integral part of the cooling tower, and the flue gas is discharged through the tower. There is no stack. Mathematical models for the dispersal of the mixed emissions from the cooling tower have now been confirmed by readings of ground level concentrations. The dispersion pattern was recorded as part of exhaustive emission readings, which included measurements from powered gliders. It was shown that the dispersal of the desulfurized flue gas encounters more favorable atmospheric conditions than would be the case if it were discharged separately from a stack. One major element is the thermal thrust of the exhaust from the cooling tower. This approach to flue gas discharge reduces costs considerably, eliminating the need for reheating the gases to the 72° C required in West Germany for conventional FGD systems discharging through a stack.     

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.     

Use of Seawater in Flue Gas Desulfurization

For seasite power plants using seawater for condenser cooling, Bechtel developed a new process to remove better than 90 percent of flue gas SO₂ using seawater and lime. Tests demonstrate that marine life is not affected by the effluent from this unique scrubbing process; therefore, the aqueous effluent containing reacted products (gypsum) in solution at low concentration is suitable for discharge to the sea.     

Formation of secondary inorganic aerosols by power plant emissions exhausted through cooling towers in Saxony

Background, aim, and scope  The fraction of ambient PM₁₀ that is due to the formation of secondary inorganic particulate sulfate and nitrate from the emissions of two large, brown-coal-fired power stations in Saxony (East Germany) is examined. The power stations are equipped with natural-draft cooling towers. The flue gases are directly piped into the cooling towers, thereby receiving an additionally intensified uplift. The exhausted gas-steam mixture contains the gases CO, CO₂, NO, NO₂, and SO₂, the directly emitted primary particles, and additionally, an excess of ‘free’ sulfate ions in water solution, which, after the desulfurization steps, remain non-neutralized by cations. The precursor gases NO₂ and SO₂ are capable of forming nitric and sulfuric acid by several pathways. The acids can be neutralized by ammonia and generate secondary particulate matter by heterogeneous condensation on preexisting particles. Materials and methods  The simulations are performed by a nested and multi-scale application of the online-coupled model system LM-MUSCAT. The Local Model (LM; recently renamed as COSMO) of the German Weather Service performs the meteorological processes, while the Multi-scale Atmospheric Transport Model (MUSCAT) includes the transport, the gas phase chemistry, as well as the aerosol chemistry (thermodynamic ammonium–sulfate–nitrate–water system). The highest horizontal resolution in the inner region of Saxony is 0.7 km. One summer and one winter episode, each realizing 5 weeks of the year 2002, are simulated twice, with the cooling tower emissions switched on and off, respectively. This procedure serves to identify the direct and indirect influences of the single plumes on the formation and distribution of the secondary inorganic aerosols. Results and conclusions  Surface traces of the individual tower plumes can be located and distinguished, especially in the well-mixed boundary layer in daytime. At night, the plumes are decoupled from the surface. In no case does the resulting contribution of the cooling tower emissions to PM₁₀ significantly exceed 15 μgm⁻³ at the surface. These extreme values are obtained in narrow plumes on intensive summer conditions, whereas different situations with lower turbulence (night, winter) remain below this value. About 90% of the PM₁₀ concentrations in the plumes are secondarily formed sulfate, mainly ammonium sulfate, and about 10% originate from the primarily emitted particles. Under the assumptions made, ammonium nitrate plays a rather marginal role. Recommendations and perspectives  The analyzed results depend on the specific emission data of power plants with flue gas emissions piped through the cooling towers. The emitted fraction of ‘free’ sulfate ions remaining in excess after the desulfurization steps plays an important role at the formation of secondary aerosols and therefore has to be measured carefully.     

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.     

Opacity Reduction Using Dry Hydrated Lime Injection

Abstract

This investigation studied the effects of injecting dry hydrated lime into flue gas to reduce sulfur trioxide, (SO₃) concentrations and consequently stack opacity at the University of Missouri-Columbia power plant. The opacity was due to sulf uric acid mist forming at the stack from high SO₃ concentrations. As a result of light scattering by the mist, a visible plume leaves the stack. Therefore, reducing high concentrations of SO₃ reduces the sulfuric acid mist and consequently the opacity. To reduce SO₃ concentrations, dry hydrated lime is periodically injected into the flue gas upstream of a baghouse and downstream of an induced draft fan. The hydrated lime is transported downstream by the flue gas and deposited on the filter bags in the baghouse forming a filter cake. The reaction between the SO₃ and the hydrated lime takes place on the filter bags. The hydrated lime injection system has resulted in at least 95% reduction in the SO₃ concentration and has reduced the opacity to acceptable limits. Low capital equipment requirements, low operating cost, and increased bag life make the system very attractive to industries with similar problems.     

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.     

Asbestos: Rationale Behind A Proposed Air Quality Standard

A computerized simulation model has been developed to compute energy requirements of a limestone slurry flue gas desulfurization (FGD) system as a function of FGD system design parameters, power plant characteristics, coal properties, and sulfur dioxide emission regulation. Results are illustrated for a "base case" plant of 500 MW, burning 3.5% sulfur coal, meeting the federal new source performance standard of 1.2 lb SO₂/10⁶ Btu. The flue gas is cleaned by an electrostatic precipitator followed by a limestone FGD system with a TCA scrubbing vessel and an optimized in-line steam reheater. The total FGD system energy requirement for this case was found to be 3.4% of the total energy input to the boiler. Sensitivity analyses were then performed in which the nominal values of ten system parameters were individually varied. This caused the total FGD system energy requirement to vary between 2.5 % and 6.1 % of the gross plant output for the range of parameters tested. The most sensitive parameters were found to be scrubbing slurry pH, which affects pumping requirements, and stack gas exit temperature, which affects reheat requirements. In all cases, FGD energy requirements were minimized when the SO₂ emission standard was met by partially bypassing the scrubber. In light of the recent Clean Air Act Amendments this option may not be feasible in the future.     

Dependence of the Wind Profile Power Law on Stability for Various Locations

Recent environmental regulations have increased the need for construction of meteorological towers at power generation facilities. Due to practical and economic considerations, tower heights are usually lower than effluent release heights. At heights where wind speed data are not available, the wind speed is usually estimated from the measured wind speed using the %th wind profile power law and assuming neutral stability conditions. This study examines published data for many locations and shows that the %th wind profile power law is often unrepresentative of actual conditions because the degree of variation of wind speed with height depends greatly on atmospheric stability. The frequency of neutral stability conditions also varies appreciably by site. These two considerations are especially important in dispersion models which extrapolate wind speed at stack height from low level wind speed data.     

The Use of Nahcolite Ore and Bag Filters For Sulfur Dioxide Emission Control

This paper describes some technical and economic aspects of the nahcolite ore injection process for the simultaneous removal of fly ash and sulfur oxides from stack gases. The process is capable of removing greater than 99% of the particulate matter and greater than 70% of the sulfur oxides present in such gases. In the process, nahcolite ore, a naturally occurring material containing 70 to 90% sodium bicarbonate, is ground to 90% passing through —200 mesh screens. Approximately 20% of the ground ore is used to precoat the filter bags in a baghouse filter while the remainder of the material is fed into the flue gas Just ahead of the baghouse. The flue gas is drawn through the baghouse by induced draft fans and sent up the stack. Most of the SO₂ and practically all of the fly ash in the flue gas can be removed as the gas passes through the filter bags. The spent nahcolite ore and fly ash are collected and conveyed to waste disposal as landfill, or alternatively processed for insolubilization by coprecipitation prior to landfilling. The technical feasibility of the process has been demonstrated in both bench scale and pilot scale engineering studies. Economic analyses performed for the cases of plants located in the midwest and southwest indicate lower capital costs for the nahcolite injection process when compared to wet scrubbing. On an annual cost basis, the nahcolite ore Injection process is comparable in cost to wet scrubbing for the case of the southwestern power plant, and somewhat more expensive for the case of the midwestern plant.     

湿式烟气脱硫系统同时脱汞研究

研究表明,湿法烟气脱硫装置(WFGD)可去除烟气中绝大部分Hg²⁺,但对单质汞的吸收效果不明显,因此研究提高湿法烟气脱硫系统中单质汞的氧化率的方法对控制汞的排放具有重要意义。综述了WFGD及在此系统中各种添加剂的脱汞性能,认为在添加剂中,气态的臭氧、液态的次氯酸和氯化钠稀溶液、 黄磷乳浊液、氢硫化钠溶液及EDTA的汞去除效果较好,且不会被SO₂大量消耗,可在WFGD系统实现同时脱硫脱汞;而气态的氯气,液态的K₂S₂O₈溶液虽然也有较好的汞去除效果,但因易被SO₂或亚硫酸盐溶液消耗,当在WFGD系统中用其氧化单质汞时,需要对脱硫塔进行分层或其他改造,使烟气中的SO₂被吸收后再控制汞,提高经济性。     

Maximum Ground-Level Concentrations with Downwash

Equations are derived from the Gaussian plume mode! and prescribe the critical downwind distance, wind speed, and plume rise values that result in maximum ground-level concentrations (MGLC) under downwash conditions. The derivations apply to bent-over plumes and encompass the Schulman-Scire and Huber-Snyder building downwash treatments.     

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.     

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.     

A Study of Primary Sulfate Emissions From a Coal-Fired Boiler with FGD

A study was carried out to investigate the emissions of SO₂ and primary sulfate materials (H₂SO₄ and inorganic particulate matter) from a boiler burning fossil fuel and using a wet-limestone scrubber for SO₂ removal. Experiments were designed to assess the scrubbing efficiency for SO₂ and sulfate, as well as the potential for scrubber liquor reentrainment. The boiler studied was an 820 MW cyclone-fired unit equipped with a wet, limestone scrubber, consisting of eight two-stage venturi-absorber modules designed to treat a flue gas flow rate of 2,760,000 acfm. The boiler fuel was a low-grade sub-bituminous coal with ash and sulfur contents of 25 and 5%, respectively. Multiple-sampling methods were employed concurrently on the inlet and outlet of a candidate absorber module to measure SO₂, total water-soluble sulfate, and free H₂SO₄. Samples were collected during three field experiments from September 1977 through April 1978. The average SO₂ scrubbing efficiency was 76% and was observed to decrease over the 5 day operation/maintenance cycle of the module. The total water-soluble sulfate input to the scrubber amounted to approximately 1% of the total sulfur oxides and was composed of a 5:1 ratio of H₂SO₄ to particulate sulfate. The total sulfate scrubbing efficiency, averaging about 29%, was invariant with respect to SO₂ removal. The sulfate emissions measured in the scrubber exit gas consisted of about 85 % H₂SO₄ as a fine aerosol. Mass emissions of acid and particulate sulfate were calculated as 1730 Ib/hr and 305 Ib/hr, respectively.     

Electric Utility Applications Of Fabric Filters

Encouraged by the successes attained with fly ash control by fabric filters in Pennsylvania Power & Light and Colorado Ute, other utilities are installing, planning, and/or considering baghouses as a practical and economical means for controlling emissions from the burning of low sulfur coals. Where deposits of alkaline reagents (i.e. nahcolite) are available, some power plants are also considering a process for dry scrubbing SO₂ from the flue gas. By introducing such reagents with the emission ahead of the fabric collector, both partlculates and SO₂ are removed.     

Simulation of Stack Plume Opacity

ABSTRACT

The visual impact of primary particles emitted from stacks is regulated according to stack opacity criteria. In-stack monitoring of the flue gas opacity allows plant operators to ensure that the plant meets U.S. Environmental Protection Agency opacity regulations. However, the emission of condensable gases such as SO₃ (that hydrolyzes to H₂SO₄), HCl, and NH₃, which may lead to particle formation after their release from the stack, makes the prediction of stack plume opacity more difficult.

We present here a computer simulation model that calculates the opacity due to both primary particles emitted from the stack and secondary particles formed in the atmosphere after the release of condensable gases from the stack. A comprehensive treatment of the plume rise due to buoyancy and momentum is used to calculate the location at which the condensed water plume has evaporated (i.e., where opacity regulations apply).

Conversion of H₂SO₄ to particulate sulfate occurs through nucleation and condensation on primary particles. A thermodynamic aerosol equilibrium model is used to calculate the amount of ammonium, chloride, and water present in the particulate phase with the condensed sulfate. The model calculates the stack plume opacity due to both primary and secondary particles. Examples of model simulations are presented for three scenarios that differ by the emission control equipment installed at the power plant: (1) electrostatic precipitators (ESP), (2) ESP and flue gas desulfurization, and (3) ESP and selective catalytic reduction. The calculated opacity is most sensitive to the primary particulate emissions. For the conditions considered here, SO₃ emissions showed only a small effect, except if one assumes that most H₂SO₄ condenses on primary particles. Condensation of NH₄Cl occurs only at high NH₃ emission rates (about 25 ppm stack concentration).     

Removal of SO2 from Simulated Flue Gases Using Non-Thermal Plasma-Based Microgap Discharge

Abstract

The removal of sulfur dioxide (SO₂) from simulated flue gases streams (N₂/O₂/H₂O/SO₂) was experimentally investigated using microgap discharge. In the experiment, the thinner dielectric layers of aluminum oxide (Al₂O₃) were used to form the microgap discharge. With this physical method, a high concentration of hydroxyl (OH·) radicals were produced using the ionization of O₂ and H₂O to further the conversion of SO₂ into sulfuric acid (H₂SO₄) at 120° C in the absence of any catalysts and absorbents, which were captured with the electrostatic precipitator (ESP). As a result, the increase of discharge power and concentrations of O₂ and H₂O increased the production of OH· radicals resulting in enhanced removal of SO₂ from gas streams. With the test and analysis, a number of H₂SO₄ droplets were produced in experiment. Therefore, a new method for removal of SO₂ in semidry method without ammonia (NH₃) additive was found.     

The Effect of Coal Combustion Flue Gas Components on Low-Level Chlorine Speciation Using EPA Method 26A

ABSTRACT

U.S. Environmental Protection Agency (EPA) Method 26A is the recommended procedure for capturing and speci-ating halogen (X₂) and hydrogen halide (HX) stack emissions from combustion sources. Previous evaluation studies of Method 26A have focused primarily on hydrogen chloride (HCl) speciation. Capture efficiency, bias, and the potential interference of Cl₂ at high levels (>20 ppm [u,g/m³]) and NH₄Cl in the flue gas stream have been investigated. It has been suggested that precise Cl₂ measurement and accuracy in quantifying HX or X₂ using Method 26A are difficult to achieve at Cl₂ concentrations <5 ppm; however, no performance data exist to support this. Coal contains low levels of Cl, in the range of 5-2000 ppmw, which results in the presence of HCl and Cl₂ in the products of combustion. HCl is the predominant Cl compound formed in the high-temperature combustion process, and it persists in the gas as the products of combustion cool. Concentrations of Cl₂ in coal combustion flue gas at stack temperatures typically do not exceed 5 ppm. For this research, bench-scale experiments using simulated combustion flue gas were designed to validate the ability of Method 26A to speci-ate low levels of Cl₂ accurately. This paper presents the results of the bench-scale tests. The effect of various flue gas components is discussed. The results indicate that SO₂ is the only component in coal combustion flue gas that has an appreciable effect on Cl₂ distribution in Method 26A impingers, and that Method 26A cannot accurately speciate HCl and Cl₂ in coal combustion flue gas without modification.