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.     

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.     

Spray-dry desulfurization of flue gas from heavy oil combustion

An experimental investigation on sulfur dioxide removal in a pilot-scale spray dryer from the flue gas generated by combustion of low-sulfur (S) heavy oil is reported. A limewater slurry was sprayed through an ultrasonic two-fluid atomizer in the spray-dry chamber, and the spent sorbent was collected downstream in a pulse-jet baghouse together with fly ash. Flue gas was sampled at different points to measure the desulfurization efficiency after both the spray-dry chamber and the baghouse. Parametric tests were performed to study the effect of the following variables: gas inlet temperature, difference between gas outlet temperature and adiabatic saturation temperature, lime-to-S ratio, and average size of lime particles in the slurry. Results indicated that spray drying is an effective technology for the desulfurization of low-S fuel oil flue gas, provided operating conditions are chosen carefully. In particular, the lowest gas inlet and outlet temperatures compatible with baghouse operation should be selected, as should a sufficiently high lime-to-S ratio. The attainment of a small lime particle size in the slurry is critical for obtaining a high desulfurization efficiency. A previously presented spray-dry flue gas desulfurization model was used to simulate the pilot-scale desulfurization tests, to check the ability of the model to predict the S capture data and its usefulness as a design tool, minimizing the need for pilot-scale experimentation. Comparison between model and experimental results was fairly good for the whole range of calcium/S ratios considered.     

Dioxins/furans emissions from fluidized bed combustion of salt-laden hog fuel

Polychlorinated dibenzodioxins/furans (PCDD/F) were formed in substantial quantities in a pilot-scale fluidized bed combustor burning salt-laden waste wood, a common fuel for Canadian coastal pulp and paper mills. Formation of PCDD/F increased with increasing chloride content in the feed, and appeared to correlate with the chlorine content in the fly ash. It took a very long time for the ash chlorine content to stabilize, suggesting that chlorine transferred slowly from the flue gas to the ash. The baghouse may contribute largely to formation of the PCDD/F, owing to its temperature range and the potentially long residence time for ash particles. Controlling the baghouse temperature to reduce the PCDD/F formation in the baghouse should be effective in reducing the total emission level. While sulphur addition was found to reduce the emission level by as much as 90%, the emission level was still above the regulated level for the mills burning salt-laden wood under the conditions of the present study. No relation between the emission level and CO concentration in the flue gas was observed.     

Joint US/USSR Test Program for Reducing Fly Ash Resistivity

An electrostatic precipitator preceded by a wet scrubber was tested at the Reftinskaya Power Station. The unit collects a high resistivity fly ash from the combustion of low sulfur Ekibastuz coal. The operating parameters of the precipitator were measured as well as the mass emissions and the in-situ electrical resistivity of the fly ash. Density, particle size distribution, electrical resistivity, and chemical composition were determined for collected samples of the fly ash. The fly ash was also characterized by x-ray diffraction and scanning electron microscopy. When a centrifugal wet wall scrubber was installed ahead of the electrostatic precipitator, the temperature of the flue gas entering the precipitator was decreased and the moisture content increased. The electrical resistivity of the fly ash was attenuated a decade, but not enough to overcome the adverse effects of back corona in the precipitator. Lowering the flue gas temperature to about 85°C by the addition of a venturi scrubber ahead of the centrifugal scrubber reduced the electrical resistivity of the fly ash another decade and allowed the operation of the precipitator without back corona.     

Natural desulfurization in coal-fired units using Greek lignite

This paper analyzes the natural desulfurization process taking place in coal-fired units using Greek lignite. The dry scrubbing capability of Greek lignite appears to be extremely high under special conditions, which can make it possible for the units to operate within the legislative limits of sulfur dioxide (SO2) emissions. According to this study on several lignite-fired power stations in northern Greece, it was found that sulfur oxide emissions depend on coal rank, sulfur content, and calorific value. On the other hand, SO2 emission is inversely proportional to the parameter gammaCO2(max), which is equal to the maximum carbon dioxide (CO2) content by volume of dry flue gas under stoichiometric combustion. The desulfurization efficiency is positively correlated to the molar ratio of decomposed calcium carbonate to sulfur and negatively correlated to the free calcium oxide content of fly ash.     

Filtration CharacteristicsOf Fly Ash from a Pulverized Coal-Fired Power Plant

The operating characteristics of a pilot baghouse and the filtering characteristics of fly ash filtered from the flue gas of a pulverized coal-fired power plant were studied by techniques developed in the engineering research laboratories of the National Center for Air Pollution Control in Cincinnati. The permeability of the dust cake varied with the operating conditions of the baghouse in a way that significantly affects the pressure drop and power requirements of the system.     

Mercury Balance on a Large Pulverized Coal-Fired Furnace

This paper presents results of a survey of mercury concentrations in coal, ash, water, fly ash, and flue gas discharges from a 5.5 × 10⁶ Ib/hr steam generator serving a 775 MW (net) turbine-generator set. Representative composite or grab samples were obtained for inlet coal and outlet ash and water. Stack samples were obtained for fly ash and mercury vapor emissions while the unit was operated at 660 MW (net) (85% of full load). Samples were analyzed by anodic stripping voltammetry, plasma emission spectroscopy, and neutron activation analysis to determine mercury concentration entering the furnace in the coal and leaving the furnace in the flue gas, fly ash, bottom and hopper ash, and water. Method inter-comparisons are discussed. A material balance for mercury has been calculated from fuel, ash, and stack gas flow rates. About 90% of the mercury in the coal is released and appears as vapor discharged in the stack gas while 10% remains in the residual ash. For a 700 MW (net) unit, about 5 lb/day of mercury vapor is released to the atmosphere.     

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.     

A pilot study of mercury liberation and capture from coal-fired power plant fly ash

The coal-fired electric utility generation industry has been identified as the largest anthropogenic source of mercury (Hg) emissions in the United States. One of the promising techniques for Hg removal from flue gas is activated carbon injection (ACI). The aim of this project was to liberate Hg bound to fly ash and activated carbon after ACI and provide high-quality coal combustion products for use in construction materials. Both bench- and pilot-scale tests were conducted to liberate Hg using a thermal desorption process. The results indicated that up to 90% of the Hg could be liberated from the fly ash or fly-ash-and-activated-carbon mixture using a pilot-scale apparatus (air slide) at 538 degrees C with a very short retention time (less than 1 min). Scanning electron microscope (SEM) evaluation indicated no significant change in fly ash carbon particle morphology following the thermal treatment. Fly ash particles collected in the baghouse of the pilot-scale apparatus were smaller in size than those collected at the exit of the air slide. A similar trend was observed in carbon particles separated from the fly ash using froth flotation. The results of this study suggest a means for power plants to reduce the level of Hg in coal-combustion products and potentially recycle activated carbon while maintaining the resale value of fly ash. This technology is in the process of being patented.     

滤料负载粉尘层对气态汞脱除性能的实验研究

通过不同性能纤维滤料负载燃煤飞灰粉尘层,来模拟袋式除尘器滤袋表面粉尘附着层,进而研究袋滤器用不同性能纤维滤料和粉尘附着层对燃煤烟气中Hg0的联合脱除性能。在固定床实验系统上分别进行了不同纤维滤料和燃煤飞灰粉尘层,以及经实验优选得到的华博特滤料负载燃煤飞灰粉尘层脱除燃煤烟气中Hg0的实验研究。结果表明,燃煤飞灰粉尘层和华博特滤料对Hg0分别有一定的脱除作用,脱除效率可达35%和42.5%,它们对Hg0的脱除是物理吸附和化学吸附共同作用的结果;同时,华博特滤料负载燃煤飞灰粉尘层对Hg0的联合脱除效率受到吸附反应温度、入口汞浓度和烟气停留时间等因素的影响,最佳脱汞率可达64.4%;吸附反应温度越高,脱除效率越低;烟气停留时间越大,脱除效率越高;入口汞浓度的提高并不一定提高华博特滤料负载飞灰粉尘层的脱汞效果。     

The Effective Collection of Fly Ash At Pulverized Coal-Fired Plants

The relationship between sulfur in coal, boiler exit gas temperature, and the carbon portion of fly ash have a major effect on the electrical properties of fly ash. Whether effective collection of fly ash is obtained by the electrostatic precipitator installation alone or the precipitator—mechanical combination depends primarily on a knowledge of this relationship. Fly ash electrical properties can range from a highly "resistive" to a highly "conductive" state which can appreciably alter the precipitator collection performance. A correlation of coal sulfur and boiler exit flue gas temperature is given to indicate the probability of expecting an optimum voltage—current relationship with different combinations of these factors. Carbon affects the electrical conditioning of fly ash by providing parallel paths of current leakage through the deposited dust layer. Therefore, removal of the carbon particles in a mechanical collector placed before the precipitator can alter the precipitator electrical characteristics.     

Ca/S比小于1条件下半干半湿法烟气脱硫技术的研究

通过对煤中灰分、粉煤灰中碱性物质成分、含量的分析 ,证明在Ca/S小于 1的条件下进行半干半湿法烟气脱硫是可行的。阐述了粉煤灰中碱性物质被有效利用的条件为高温、高湿和高压 ,该条件可以通过蒸汽输送粉煤灰来实现。     

An alternative to additional SO3 injection for fly ash conditioning

Small concentrations, approximately 2-10 parts per million (ppm), of injected sulfur trioxide (SO3) have improved particulate collection efficiencies of electrostatic precipitators burning lower-sulfur coal. However, the addition of extra SO3 not only incurs costs but also presents negative environmental effects. This work explored a method that could be applied to existing coal-fired power plants to convert the sulfur dioxide (SO2) already present in the flue gas to sufficient levels of SO3 for fly ash conditioning as an alternative to adding SO3 by burning elemental sulfur. During this research, a pre-mixed natural gas flame was used to promote the conversion of SO2 to SO3 in a drop-tube furnace with average non-flame, free stream gas temperatures of 450 and 1000 K. SO3 concentrations measured by wet chemistry and confirmed using elemental balances of other sulfur species measured by gas chromatography revealed that as much as 7% of SO2 was homogeneously transformed to SO3. The results also showed that at low temperatures, the rate at which SO3 is converted back to SO2 decreased, thus extending the time period during which SO3 concentrations would be sufficient for ash conditioning. An additional benefit of this technique is speculated to result from increased flue gas humidity.     

Sorption of SO2 on Ground Nahcolite Ore

Large quantities of nahcolite ore, a naturally occurring mineral containing between 70 and 90% sodium bicarbonate are known to occur in the Green River Formation of the Piceance Creek Basin of northwestern Colorado.¹ Deposits occur in two major forms, as either bedded or disseminated ore. In the latter case, the host rock is oil shale, a dolomitic marlstone containing kerogen material. Bench and pilot scale studies have shown finely divided nahcolite ore to be an effective SO₂ sorbent in baghouse filters.² These studies form the basis of the nahcolite ore injection process as a means for controlling SO₂ emissions to the environment.³ Although there have been published investigations of reactions of SO₂ with Na₂CO₃and NaHCO_3, ⁴ no reference could be found to a systematic study of nahcolite ore reacting with SO₂.     

The fate and behavior of mercury in coal-fired power plants

For the past 22 years in the Netherlands, the behavior of Hg in coal-fired power plants has been studied extensively. Coal from all over the world is fired in Dutch power stations. First, the Hg concentrations in these coals were measured. Second, the fate of the Hg during combustion was established by performing mass balance studies. On average, 43 +/- 30% of the Hg was present in the flue gases downstream of the electrostatic precipitator (ESP; dust collector). In individual cases, this figure can vary between 1 and 100%. Important parameters are the Cl content of the fuel and the flue gas temperature in the ESP. On average, 54 +/- 24% of the gaseous Hg was removed in the wet flue-gas desulfurization (FGD) systems, which are present at all Dutch coal-power stations. In individual cases, this removal can vary between 8% (outlier) and 72%. On average, the fate of Hg entering the power station in the coal was as follows: <1% in the bottom ash, 49% in the pulverized fuel ash (ash collected in the ESP), 16.6% in the FGD gypsum, 9% in the sludge of the wastewater treatment plant, 0.04% in the effluent of the wastewater treatment plant, 0.07% in fly dust (leaving the stack), and 25% as gaseous Hg in the flue gases and emitted into the air. The distribution of Hg over the streams leaving the FGD depends strongly on the installation. On average, 75% of the Hg was removed, and the final concentration of Hg in the emitted flue gases of the Dutch power stations was only -3 microg/m3(STP) at 6% O2. During co-combustion with biomass, the removal of Hg was similar to that during 100% coal firing. Speciation of Hg is a very important factor. An oxidized form (HgCl2) favors a high degree of removal. The conversion from Hg0 to HgCl2 is positively correlated with the Cl content of the fuel. A catalytic DENOX (SCR) favors the formation of oxidized Hg, and, in combination with a wet FGD, the total removal can be as high as 90%.     

Distribution of mercury in the combustion products from coal-fired power plants in Guizhou,southwest China

Method 30B and the Ontario Hydro Method (OHM) were used to sample the mercury in the flue gas discharged from the seven power plants in Guizhou Province, southwest China. In order to investigate the mercury migration and transformation during coal combustion and pollution control process, the contents of mercury in coal samples, bottom ash, fly ash, and gypsum were measured. The mercury in the flue gas released into the atmosphere mainly existed in the form of Hg°. The precipitator shows a superior ability to remove Hg^p (particulate mercury) from flue gas. The removal efficiency of Hg²⁺ by wet flue gas desulfurization (WFGD) was significantly higher than that for the other two forms of mercury. The synergistic removal efficiency of mercury by the air pollution control devices (APCDs) installed in the studied power plants is 66.69–97.56%. The Hg mass balance for the tested seven coal-fired power plants varied from 72.87% to 109.67% during the sampling time. After flue gas flowing through APCDs, most of the mercury in coal was enriched in fly ash and gypsum, with only a small portion released into the atmosphere with the flue gas. The maximum discharge source of Hg for power plants was fly ash and gypsum instead of Hg emitted with flue gas through the chimney into the atmosphere. With the continuous upgrading of APCDs, more and more mercury will be enriched in fly ash and gypsum. Extra attention should be paid to the re-release of mercury from the reutilization of by-products from APCDs.

Implications: Method 30B and the Ontario Hydro Method (OHM) were used to test the mercury concentration in the flue gas discharged from seven power plants in Guizhou Province, China. The concentrations of mercury in coal samples, bottom ash, fly ash, and gypsum were also measured. By comparison of the mercury content of different products, we found that the maximum discharge source of Hg for power plants was fly ash and gypsum, instead of Hg emitted with flue gas through the chimney into the atmosphere. With the continuous upgrading of APCDs, more and more mercury will be enriched in fly ash and gypsum. Extra attention should be paid to the re-release of mercury from the reutilization of by-products from APCDs.     

Inhibition of PCDD/F by adding sulphur compounds to the feed of a hazardous waste incinerator

Sulphur compounds, including (NH₄)₂SO₄ and pyrite, were tested as suppressants in a hazardous waste incineration facility. The test results suggested that adding sulphur compounds only slightly reduced PCDD/F stack emissions; this restricted effect was attributed to the release of fly ash in large amounts during the sulphur adding experiments, i.e., it was due to a malfunctioning of the baghouse filter. Nevertheless, for the combined flow of flue gas + fly ash a reduction of more than 50% was achieved for the total PCDD/F concentrations and the total toxic concentrations, and an even higher inhibition capability was observed for PCDD. Also, a simulation of the thermodynamic equilibrium conditions by sulphur dioxide was conducted in the domain of experimental interest. Deactivation of catalysts, which promote PCDD/F formation, was found to be the dominant inhibition mechanism in low temperature PCDD/F formation. SO₂ could also inhibit the formation of molecular Cl₂ via the Deacon reaction, but that was not the main reason for inhibition.     

From Air Repair to EM: A&WMA’s Publications through a Century of Change

Abstract

This paper analyzes the natural desulfurization process taking place in coal-fired units using Greek lignite. The dry scrubbing capability of Greek lignite appears to be extremely high under special conditions, which can make it possible for the units to operate within the legislative limits of sulfur dioxide (SO₂) emissions. According to this study on several lignite-fired power stations in northern Greece, it was found that sulfur oxide emissions depend on coal rank, sulfur content, and calorific value. On the other hand, SO₂ emission is inversely proportional to the parameter y _CO2max, which is equal to the maximum carbon dioxide (CO₂) content by vol ume of dry flue gas under stoichiometric combustion. The desulfurization efficiency is positively correlated to the molar ratio of decomposed calcium carbonate to sulfur and negatively correlated to the free calcium oxide content of fly ash.     

Estimation and characterization of unintentionally produced persistent organic pollutant emission from converter steelmaking processes

Unintentionally produced persistent organic pollutants (UP-POPs) including polychlorinated dibenzo-p-dioxins, and dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs), and polychlorinated naphthalenes (PCNs) were characterized and quantified in stack gas and fly ash from the second ventilation systems in five typical converters in five different steelmaking plants. The 2378-substituted PCDD/Fs (2378-PCDD/Fs) and dioxin-like PCB (dl-PCBs) toxic equivalents (TEQs) were 1.84–10.3 pg WHO-TEQ Nm^?3 in the stack gas and 5.59–87.6 pg WHO-TEQ g^?1 in the fly ash, and the PCN TEQs were 0.06–0.56 pg TEQ Nm^?3 in the stack gas and 0.03–0.08 pg TEQ g^?1 in the fly ash. The concentrations of UP-POPs in the present study were generally lower than those in other metallurgical processes, such as electric arc furnaces, iron ore sintering, and secondary metallurgical processes. Adding scrap metal might increase UP-POP emissions, indicating that raw material composition was a key influence on emissions. HxCDF, HpCDF, OCDF, HpCDD, and OCDD were the dominant PCDD/Fs in the stack gas and fly ash. TeCB and PeCB were dominant in the stack gas, but HxCB provided more to the total PCB concentrations in the fly ash. The lower chlorinated PCNs were dominant in all of the samples. The 2378-PCDD/F, dl-PCB, and PCN emission factors in stack gases from the steelmaking converter processes (per ton of steel produced) were 1.88–2.89, 0.14–0.76_, and 229–759 μg t^?1, respectively.