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Flue Gas Desulfurization for Acid Rain Control
Abstract
This chapter is a reprint of a literature review prepared for the Ohio Coal Development Office by the Ohio Coal Research Consortium in 1990. It represents an assessment of the state of the art in flue gas desulfurization technologies at that time. It also served to motivate the work of the consortium starting in 1991, the results of which are presented in the following chapters.
The discussion begins with a basic review of different technologies which are distinguished first by the amount of moisture in the sorbent feed and product, and second by the temperature range for the sorbent injection. The high temperature range corresponds to furnace injection; the medium temperature range corresponds to economizer injection; and the low temperature range corresponds to any system in place after the air heater. In each case, both process chemistry and contacting technologies are discussed. Low temperature technologies are further divided into duct injection, spray drying, capture in particulate control devices, and other special devices. The chapter concludes with a discussion of the fundamentals of sorbent and sorbent additive chemistry and the fluid mechanics, mass, and heat transfer.
This chapter describes the investigations conducted to improve the understanding of the transport processes related to dry flue gas desulfurization (FGD). It is important to improve the understanding of the various processes involved in FGD; e.g, dry sorbent injection and dispersion, sorbent and flue gas mixing, sor-bent humidification by water injection, so that the sorbent utilization can be enhanced. The objectives of this study were to experimentally obtain a basic understanding of turbulent flow structure of the mixing zone and its influence on particle dispersion; the effect of particle loading on turbulent properties and mixing; the effect of jet entrainment, water spray sorbent cocurrent flow interactions, sorbent wetting and mixing; and to investigate the flow field where certain ratios of jet velocity to flue gas velocity result in regions of negative flow.
In this study, traditional Soxhlet, automatic Soxhlet and ultrasonic extraction techniques were employed to determine the speciation and concentration of polycyclic aromatic hydrocarbons (PAHs) on lime spray dryer (LSD) ash samples collected from the baghouse of a spreader stoker boiler. To test the efficiencies of different extraction methods, LSD ash samples were doped with a mixture of 16 US EPA specified PAHs to measure the matrix spike recoveries. The results showed that the spike recoveries of PAHs were different using these three extraction methods with dichloromethane (DCM) as the solvent. Traditional Soxhlet extraction achieved slightly higher recoveries than automatic Soxhlet and ultrasonic extraction. Different solvents including toluene, DCM:acetone (1:1 V/V) and hexane:acetone (1:1 V/V) were further examined to optimize the recovery using ultrasonic extraction. Toluene achieved the highest spike recoveries of PAHs at a spike level of 10 microg kg(-1). When the spike level was increased to 50 microg kg(-1), the spike recoveries of PAHs also correspondingly increased. Although the type and concentration of PAHs detected on LSD ash samples by different extraction methods varied, the concentration of each detected PAH was consistently low, at microg kg(-1) levels.
A novel reactor, named the circulating fluidized bed absorber (CFBA), has been built and is currently under evaluation as a simple method for control (either retrofit or incorporated with a new design) of SOâ from high sulfur coal sources. The reactor involves contacting a pulverized dry sorbent such as sodium bicarbonate, limestone, lime, or calcium hydroxide with the SOâ laden flue gases in a fluidized bed reactor operated at high velocities. A solid-gas separator, located downstream of the reactor, concentrates unused sorbent for recycle and subsequent reuse for additional SOâ capture. The specific advantages of the system over existing dry flue gas desulfurization systems include ease of retrofitting to existing sources, isothermal operation, applicability to a variety of solid sorbents, high utilization of solid sorbents, and minimization of combustion system disruption. Additionally, the requirement for a fabric filter for sorbent collection and additional SOâ removal (as in typical dry injection techniques) is not necessary in the CFBA because of the high recycle potential. This paper reports on the status of this project, which consists of the design, construction, and evaluation of a pilot scale CFBA.
Atomic transport in solids is a field of growing importance in solid state physics and chemistry, and one which, moreover, has important implications in several areas of materials science. This growth is due first to an increase in the understanding of the fund amentals of transport processes in solids. Of equal importance, however, have been the improvements in the last decade in the experi mental techniques available for the investigation of transport phenomena. The advances in technique have stimulated studies of a wider range of materials; and expansion of the field has been strong ly encouraged by the increasing range of applied areas where transport processes play an essential role. For example, mass transport phenomena play a critical role in the technology of fabrication of components in the electronics industry. Transport processes are involved both during the fabrication and operation of devices and with the growing trend to miniaturisation there are increasing demands on accurate control of diffusion processes. The present book (which is based on a NATO sponsored Advanced Study Institute held in 1981 at Lannion, France) aims to present a general survey of the subject, highlighting those areas where work has been especially active in recent years.
The use of new packings, trays and process technology in distillation processes is reviewed. Equipment from several manufacturers is described. The use of distillation in the fields of biotechnology is considered. (P.M.T.)
Flue gas desulfurization (FGD) systems use low-cost, high-sulfur coal. All FGD systems can be classified either as recovery or throwaway, or as wet or dry. Recovery systems are designed to reclaim a reusable or marketable by-product such as sulfuric acid or elemental sulfur. More widely accepted are throwaway systems, which use a sodium or calcium-based additive such as soda ash (Na//2CO//3), lime (CaO), or limestone (CaCO//3). Wet FGD systems produce a partially dewatered sludge, or a solution for disposal. Dry FGD systems yield a dry product (typically a powder) for disposal.
Coolside desulfurization is a general term describing dry sorbent injection/humidification technology under development at Conoco Coal Research Division, the research arm of Consolidation Coal Company. This technology can provide low cost SO//2 control options attractive for retrofit application in a variety of coal burning utility situations. The capital requirement is particularly low because the duct work downstream of the air preheater and the particulate collection system (ESP or baghouse) provide the reaction space for SO//2 capture. The development work, from laboratory to one MW scale field trials, has been successful in advancing this technology to a level ready for a utility demonstration.
Controlling particulates cost-effectively from industrial and utility powerplants is becoming more complex every day due to existing and potential regulations as well as the state-of-the-art in powerplant engineering and the design of the primary collection devices-electrostatic precipitators (ESP) and fabric filters. This special report describes the progress achieved in fabric filters and electrostatic precipitators and the possibilities of optimizing them in view of the more stringent governmental regulations regarding particulate control and gaseous emissions.
A brief description of the pore plugging model is presented and its differences with that of P. Ramachandran and J. Smith are examined. An analytical calculation of the time required to plug the pore is presented. In addition, a perturbation solution for small times is used to motivate the formulation of semianalytical version of the collocation method for two point boundary value problems with steep concentration profiles.
Resistivity and unsophisticated transference experiments were conducted at temperatures above 200°C. Six commercially produced fly ashes having similar overall chemistry but differing significantly in concentrations of alkali metals and alkaline earths were used. Test specimens were fabricated by pressing and sintering the ash into self supporting discs. An Arrhenius equation was used to interpret the resistivity temperature relationship. From the lack of deviation in experimental activation energy among the ashes, it was concluded that the same conduction mechanism prevailed in each. The transference tests indicated that the quantity of electricity conducted was proportional to a mass transfer and that lithium and sodium ions had migrated. It was determined that resistivity was inversely proportional to the combined lithium and sodium concentration. It was concluded that the volume conduction process was controlled by an ionic mechanism in which lithium and sodium are the principal charge carriers. The overall conduction process involving specimens and electrodes was interpreted using the results of recent fundamental research on silicate glasses.
The impeller fluidizer is an impeller-agitated, unbaffled, cylindrical vessel which concentrates slurry particles at the end of the vessel farthest from the impeller. This paper reports preliminary data on the particle trapping characteristics of glass bead/water and alumina/water systems in fluidizers with diameters of 2. 54 cm (1 inch) and 7. 62 cm (3 inch). Glass beads or alumina particles on the order of 100 mu m are completely separated at impeller speeds of 100 RPM and above. Substantial separation was achieved with particles as small as 25 mu m at impeller speeds in the range of 2000 to 3000 RPM. Higher impeller speeds and larger, denser particles favor separation. Solids loading up to 14%w in the fluidizer affects separation only modestly. Scale-up at constant impeller speed increases separation.
The effectiveness of limestone and dolomite in removing SO2 from flue gas can be estimated by thermochemical calculations. Based on the most likely chemical reactions, both lime and magnesia are shown to be suitable for removing SO2 from hot flue gas, with lime more suitable than magnesia over the temperature range where furnace conditions and reaction kinetics are likely to be favorable.
The injection of dry alkaline compounds into the furnace or post-furnace regions of utility boilers to reduce SO2 is currently under development as a lower cost option to conventional flue gas desulfurization technology. Part I of this paper focused on the science and process development of the various dry sorbent technologies. Part II will address applications of these technologies, including SO2 removals in full-scale boilers, methodologies for designing sorbent injection systems, power plant impacts, process integration issues, and cost.
Because the dry technologies use the furnace and/or ducts as the chemical contactor, potential impacts on power plant operation and reliability are as critical in assessing the commercial applicability of each technology as SO2 removal and sorbent utilization. Consequently, the technical and economic feasibility of the various dry processes is highly site specific.
The utility and industrial sectors continue to come under pressure from both national and local regulatory groups to reduce sulfur dioxide emissions. With a trend in the utility industry for life extension, retrofit technologies are likely to play an important role in any SO2 emission reduction strategy. Potential retrofit technologies include, singly and in combination: coal switching or cleaning, wet or dry FGD, conversion to fluidized bed, and dry sorbent injection. The diversity within the utility industry in terms of unit size, unit age, fuel use, financial base, and geographic location dictates the need for a variety of technologies to address SO2 emission control. Dry injection processes involving the injection of dry powders into either the furnace or post-furnace region offer the potential for low capital cost retrofitable technologies. However, compared to wet FGD processes, the dry calcium based processes will likely have lower SO2 removal efficiencies and may pose more plant-wide integration issues that need to be addressed from both an applications and R&D perspective.
The kinetics of the thermal decomposition of CaCO3 in CO2 were investigated using both dynamic and isothermal techniques. Values of apparent activation energies range from 200–1000 kcal mol−1 depending upon sample size and heating rate. It is concluded that thermal transport rather than mass transport or chemical processes is rate determining. The results are compared with earlier work in O2 and discussed in terms of “the kinetic compensation effect”, i.e., the reported linear relationship between the logarithm of the preexponential term and the activation energy derived from the Arrhenius equation.
Isothermal and dynamic techniques were employed to examine the rate of weight loss of CaCO3. Thermogravimetric studies were conducted in atmospheres of He, N2, Ar, and various percentage of CO2 in Ar. Three methods for deriving kinetic parameters from thermogravimetric data were used and these results were then compared with data obtained from isothermal investigations done on identical samples. It was found that the higher the thermal conductivity of the atmosphere, the more repidly the reaction proceeded. Also, as the percentage of CO2 in Ar increased, the temperature range of the decomposition became higher and narrower, resulting in a higher activation energy.
A modified version of the grain theory was used to analyse the effect of temperature on the sulfation reaction. The theory combines the influence of temperature on the grain size, its effect on the rate of transport of SO2 and the chemical reaction. An efficient numerical procedure is presented for solving the model equations which take into account the radial gradient of the effective diffusivity in a particle.
A simple qualitative model has been developed for flue gas SO2 removed with Ca(OH)2 slurry in a spray drying plant. Through x-ray analysis it has been shown that the total reaction can occur in two phases; a first phase where large numbers of solid Ca(OH)2 particles are freely movable in a liquid phase, and the rate of Ca(OH)2 dissolution diffusion through the formed Ca-sulfite seems to control the rate of reaction; and a second phase where particles form a porous body and the reaction is dependent on pore diffusion for completion. From test work in our laboratory and at our new 20,000 ACFM pilot plant in Colorado Springs, we expect more detailed information which will permit expanding this model to cover other variables.
The sulfation behavior of natural limestones and dolomites has been assessed using a bench scale fluidized bed reactor under conditions which are applicable to bubbling atmospheric fluidized bed condition. By increasing the temperature or reducing the COâ partial pressure in the calcining gas, both precalcined and uncalcined sorbents exhibited relatively similar behavior on sulfation. Under these conditions calcination was relatively fast and therefore did not influence the overall sulfation process. When preparing precalcined sorbents, their reactivities towards SOâ depended on the calcining conditions, in particular on the COâ concentration in the calcining gas and the calcining temperature. The apparent effects of calcining conditions on the subsequent sulfation behavior were directly related to the physical structure of the solid, which changes as a consequence of sintering. For a particular sorbent there is an optimum pore size beyond which the sulfur uptake begins to decrease. The low sulfur uptake at high temperatures was due to early pore blockage which restricted the transport of SOâ into the particle. Higher calcium utilization was achieved using impure limestones. When distributed uniformly within the particles, impurities dilute the CaO concentration and help to delay pore blockage upon reaction with SOâ. In the sulfation of uncalcined sorbents, the presence of impurities will increase the overall rate of calcination. In addition, some impurities will affect the sintering process and ultimately produce a favorable solid structure for sulfation. It can be concluded that the chemical composition of a sorbent should be considered in addition to its physical properties in selecting suitable candidates for reducing SOâ emissions from fluidized bed combustion.
This paper presents a mathematical model of the SOâ removal process in a spray-dryer flue-gas desulfurization system. Simultaneous evaporation of a slurry droplet and absorption/reaction of SOâ in the droplet are described by the corresponding heat- and mass-transfer rate relations. Dissolution kinetics of lime particles within a slurry droplet is included in determining the overall SOâ removal rate. The model identifies several parameters which need to be estimated or determined from experimental data. The model predictions of the effects of major parameters, such as approach to saturation and stoichiometric ratio on the SOâ removal efficiency, follow observed trends. Comparison of the model predictions with one set of pilot-plant data shows very good agreement.
The report gives results of combining reburning with the injection of calcium-based sorbents to investigate the potential for combined NOx and SOx reduction. Reburning, applied to pulverized-coal-fired utility boilers, involves injecting a secondary fuel above the main firing zone to produce a reducing zone where NOx is converted to molecular nitrogen. Overfire air is added above the reburn reducing zone to complete the combustion. Bench-scale evaluations of reburning, carried out in a plug-flow furnace at 23 kWt, have indicated that NOx reductions of up to 70% can be achieved depending on a number of process variables, dominant of which are the initial NOx level that is to be reduced, the reburning fuel type (pulverized coal type or natural gas), and the stoichiometry, residence time, and temperature in the reducing zone. Combining the two processes indicates that the optimum sorbent injection location in a reburning system is with the burnout air because of the increased reactivity of the calcined sorbent. Dolomite consistently produces higher capture than calcitic limestone for a wide spectrum of combustion conditions. It is possible to relate sulfur capture directly to sorbent reactivity as characterized by surface area after calcination. Sulfur capture in a coal-fired system is similar to that obtained with doped propane. Sulfur capture is most dependent on injection temperature.
This report gives results of sulfation behavior measured at 1000 and 1200 C for eight calcium oxide sorbents which were well characterized in terms of particle size, pore structure, and specific surface area. Sulfation results were compared with predictions of a simple mathematical model which applied the measured sorbent characteristics. The comparison, intended to provide direction for model development, suggests need for model improvement in areas such as global kinetics at short times, and accountability for changes in structure due to sintering during sulfation. Subsequently, the effects of high temperatures on surface areas of the sorbents in the absence of sulfation were also determined. Surface areas were marginally higher for the larger sorbents at 1000 C injection; but, in general, no correlation between particle size and surface area loss could be found. Surface-area decay was shown to be very rapid in the first 200 ms, and subsequently very slow.
This paper gives results of tests for the reactivity of calcium oxide (CaO) sorbents derived from calcium hydroxide (Ca(OH)â) and calcium carbonate (CaCOâ) to gaseous sulfur dioxide (SOâ) in a nitrogen atmosphere with a nominal 1100 C, entrained flow reactor and a solids residence time of 0.74 s. The research included direct examination of the combined effect of porosity, particle size, and surface area upon CaO conversion to calcium sulfate (CaSOâ). Conversion was inversely related to the 0.20 to 0.35 power of particle size for diameters ranging from 0.77 to 12.1 micrometers. Increased initial surface area from 7 to 62 sq m/g led to higher conversions at all particle sizes, eliminating the possibility of gas film diffusion control. The conversion for both sorbent types is proportional to initial porosity. Substantial expansion of the sorbents during reaction was shown by achieving conversion levels much greater than theoretically possible, given the initial sorbent porosity, the larger-volume product, and an assumed constant particle size. This is contrary to most modeling assumptions, in which the reaction is limited or cut off by pore filling or pore plugging of the sorbent volume.
This volume presents the estimated costs for the NOXSO, High Sulfur Lime Spray Dryer, Saarberg-Holter/Limestone, SOXAL, Organic Acid Enhanced Limestone and Limestone with Wallboard Gypsum By-Product FGD systems based on December 1982 dollars and 1985 technology. These systems were evaluated for operability, technical merit, and commercial availability. Each FGD system was evaluated for a high sulfur coal application at a new, hypothetical 1000 MW (two 500 MW units) power plant located in Kenosha, WI. This arbitrary reference plant was selected to ensure consistent comparisons, and to increase the relative accuracy of the costs presented. The evaluation methods employed in this study were identical to those used in previous EPRI volumes of the report, CS-3342, ''Economic Evaluation of FGD Systems.'' A flow sheet, material balance, equipment list, system description, and utility consumption list form the basis of each FGD process evaluation. Cost information was obtained from process vendors, the Stearns Catalytic data base, and published reports. Capital costs were estimated by factoring costs of process equipment (i.e., an EPRI Class II estimate). Operating costs were estimated from reagent and utility consumption. The levelized capital and operating costs were developed using EPRI's standard economic premises. Costs have an absolute accuracy of +-30%. However, since methodology, scope, and unit costs are consistent, the relative accuracy between process estimates probably is +-15%. 88 figs., 64 tabs.
The use of different types of lime or limestone for flue gas desulphurisation is discussed, together with the merits of wet and dry FGD processes and the effects of preparation of the material. A table indicates the types of processes favoured in the US in systems which are either operational or under construction.
The report, describing the apparatus and laboratory procedures used to determine resistivity for a number of fly ashes under a variety of test conditions, supports research to develop a technique for predicting fly ash resistivity from chemical analyses of coal and coal ash. This effort requires considerable knowledge regarding the relationship between resistivity and several coal and ash properties. In particular, the report relates the experimental problems encountered when attempts were made to determine the effect of sulfur trioxide on resistivity. Equipment and procedures were developed to solve this problem. The report describes the modified apparatus and technique and illustrates the type of data acquired.
Results of the performance tests on electrostatic precipitators at power plants using pulverized coal are assessed. The relationship between the performance of fly ash collectors and chemistry of ash has been investigated. It has been established that new large cold side precipitators with an collection area of over 400 square feet per thousand acfw can meet the most modern ash emission standards with most of low-sulfur Appalachian coals. 14 refs.
The report describes research to determine the mechanisms that limit the extent of reaction between SOâ and calcium-based sorbents (CaCOâ and Ca(OH)â) by measuring the in-situ physical structure and reactivity of sorbent injected into a combustion environment for residence times as short as 35 ms. Four models of the sulfation reaction were used to guide the research and interpret the data. The extent of sorbent utilization was found to be limited by porosity losses during the sorbent activation process. In situ porosities, a fraction of that theoretically possible, were measured in the absence of SOâ. Calcines from CaCOâ suffered greater losses in porosity than those from Ca(OH)â which, along with the larger CaCOâ particle size, accounts for the substantial differences in SOâ capture between these two sorbents.
The advantages of a vertical stirred mill are presented and compared with other more common lime-slaking equipment. Improved turndown capability and other process, operating, and economic advantages have been proven in a pilot test program, leading to the mill's first commercial application at a US powerplant are reviewed. The stirred mill is illustrated in the article and consists of a stationary vertical cylindrical shell supported on three legs. A spiral agitator or screw, driven by an overhead-mounted motor through a gear reducer, is overhung inside from the drive assembly. This article shows the effect of lime and water quality on stirred-mill performance as measured by particle-size distribution. In these tests, feed rate varied between 10% and 100% of design.
The report gives results of a characterization of the current state of knowledge regarding SOâ capture by dry calcitic sorbent injection. In this project, the experimental data on dry sorbent injection are compiled and critically compared. Sulfation and activation models are developed and used to identify sorbent properties and furnace-environment parameters likely to be of importance. The results of the examination of data and the model evaluation indicated areas where original experimental work could be applied to resolve key issues. The experimental portion of the program developed information on injection temperature, quench rate, and sorbent properties. These results are used to define the overall optimum injection condition for a variety of furnace configurations.
The injection of calcium based sorbents, such as limestone or calcium hydroxide, directly into the furnace of coal fired boilers is currently under development as an option for reducing SOâ emissions from existing power plants. The sorbents react with SOâ to form calcium sulfate. The calcium compounds can increase the particulate loading to a downstream precipitator by a factor of two or three and can increase the resistivity of the particulate by orders of magnitude, leading to significant increases in outlet particulate emissions. Methods which can be used to restore precipitator performance and outlet emissions to presorbent injection levels are evaluated with respect to feasibility, effectiveness and cost for a hypothetical precipitator with a specific collecting area of 200. Methods which are evaluated include increased collection surface area, flue gas cooling by water evaporation, cooling by heat exchange, pulse energization, and SOâ conditioning. 3 refs., 20 figs., 5 tabs.
Studies of the performance of electrostatic precipitators were conducted at six coal-fired power plant sites. Overall collection efficiency and collection efficiency as a function of particle size were measured with the collecting electrode rappers energized and deenergized. Chemical analyses were obtained on samples of coal, fly ash, and flue gas. In situ and/or laboratory measurements of dust resistivity were performed, and secondary voltage-current relationships were obtained from the precipitator transformer-rectifier sets. The measurements of fractional efficiency with and without electrode rapping indicated that efficiency losses due to rapping occurred primarily for particle diameters greater than 2.0 ..mu..m. The performance of the electrostatic precipitators was analyzed using a mathematical model based on the physical principles of the electrostatic precipitation process.
Costs of pressure-hydrated lime injection (PHLI) for the control of SOâ emissions were developed and compared to lime spray dryer absorbers (SDA) operating under identical design conditions. In the PHLI process, dry sorbent is injected into the upper furnace region and the sulfated particulate collected in a downstream baghouse or electrostatic precipitator. In SDA processes, increasingly used for low sulfur coals, flue gas SOâ contacts lime slurries in large reactor vessels with subsequent collection of the dried particulate in a downstream baghouse. Therefore PHLI can have decisive economic advantage over SDA due to the elimination of complex absorption vessels and water chemistry, and reactor pressure drop with overall reductions in labor and water consumption. The evaluations demonstrated PHLI cost-effectiveness over lime SDA over the entire range of operating conditions investigated. Under the worst conditions for PHLI, i.e., operating at the larger new plant, firing a 1% sulfur coal with a 25% sorbent utilization, PHLI affords a 25% savings in levelized revenue over SDA; for the analogous retrofit conditions, PHLI produces a 35% savings. When sorbent utilization rises to 75%, PHLI savings increase to about 65% of SDA costs for both new and retrofit designs. In absolute terms, capital costs for retrofit PHLI are low, ranging closely about 33 to $62, at a 50% utilization rate. 20 refs., 9 figs., 5 tabs.
This paper discusses the fundamental processes in SOâ capture by calcium-based adsorbents for upper furnace, duct, and electrostatic precipitator (ESP) reaction sites. It examines the reactions in light of controlling mechanisms, effect of sorbent physical properties, and important process variables. Upper furnace reactivity is limited to 900-1200 C by rate and equilibrium constraints, respectively. Sulfation is a function of in-situ sorbent characteristics of porosity, particle size, and surface area. Conversion of the sorbent is ultimately limited by the formation of the calcium sulfate (CaSOâ) product layer. The in-duct reaction is accomplished through sorbent scavenging in the flue-gas stream by a water spray. The scavenging efficiency of the sorbent by the water droplets limits the process, while reaction is controlled by the dissolution rate of the sorbent. The E-SOX process in a modified ESP simulates a short time spray dryer through injection of a Ca(OH)â slurry. The Ca(OH)â undergoes aqueous phase reaction to remove SOâ. Evaporation of the droplets prior to the ESP field conditions the flue gas for more-efficient particulate-matter collection by lowering the gas resistivity.
Part I of this work has shown that electrical breakdown in dust layers obeys Paschen's Law, but occurs at applied field values which appear too small to initiate the breakdown. In this paper we will show how an effective dielectric constant characterizing the dust layer can be determined from ac dielectric measurements and the theory of Debye. When combined with an expression for the enhanced local electric field in the void spaces between particles in the layer, field strengths which are large enough to initiate electrical breakdown in the layer are predicted at relatively low values of applied field. The effect of temperature and dust layer thickness on the onset of electrical breakdown within the dust layer can also be explained by the dependence of the effective dielectric constant on these parameters.
The onset of electrical breakdown in dust layers has been studied for hand-deposited dust layers in a parallel plate geometry. It was found that the breakdown was an ordinary electron avalanche process originating in voids within the dust layer and obeying Paschen’s Law. The size of voids where breakdown occurs was in the range of 10 to 20 μm for the layers used. The distribution of particle sizes in a sample Influences its breakdown through changes in the average void dimension where breakdown takes place. Water vapor in the test environment, which affects the electrical conduction mechanism prior to breakdown, lowered the average electric field required to initiate breakdown. Moderate compaction of the sample had little or no effect on its breakdown behavior.
This article is intended for the reader who wishes to perform preliminary sizing and costing for baghouse installations. Descriptive material is provided on the filtration process and procedures are given for estimating baghouse size. These procedures range from simple, venerable methods to a brief discussion of theoretical/empirical equations incorporated in computer models. Information obtained from vendors allows the reader to estimate costs of baghouses, including bags and mounting hardware, based on recent costs. The article is presented in two parts: the first gives a process description, theory, sizing procedures, and a discussion of factors affecting baghouse sizing and operation; and the second gives information for estimating total capital investment and total annual costs. An example problem is given. The material is primarily taken from the EAB Control Cost Manual.
A new concept for enhancing wet-dry scrubbing of SO2 has been tested on a laboratory and pilot scale. By adding small amounts of promotor, e.g. calcium chloride, to a limestone or lime slurry, a considerable increase in the removal efficiency can be obtained. The experimental findings in a 0.5 MW wet-dry pilot indicate promoted limestone to be a feasible option for SO2 removal when burning low sulfur coals.
Discussed are the state-of-the-art of dry scrubbing for sulfur dioxide removal; and a qualitative model, based on more than 300 pilot plant tests, that describes extended technology for optimizing the SOâ absorption reaction as a function of various parameters. The model is used to simulate flue gas SOâ removal with lime as a reagent in a spray drying plant. X-ray analysis of pilot plant operations indicates that the reaction occurs in phases, which are described. 1 table.
Present trends toward larger plants have forced precipitator manufacturers to use collecting electrode plates 30 feet high routinely. After the dust suspended in the gas has been precipitated on these tall plates, it is necessary to transfer it continually to the hoppers underneath, losing less than 1% of it to the gas stream (and hence up the stack) in the process, if dust loss is to be kept below 1%. This is done by striking or vibrating the plates with devices called “rappers,” operated sequentially by controls programmed to any desired sequence, rapping intensity, and repeating time interval.
The Langmuir method for measurement of vapour pressures has been tested for use in studies of decomposition reactions. The isothermal weight loss in vacuum from cleavage (10text-decoration:overline11) planes of calcite (CaCO3) single crystals was measured continuously at temperatures from 934 to 1013 K. The rate was constant until approximately 80% of a 1 mm slice had decomposed. The apparent activation enthalpy was 205 kJ (49 kcal) mol–1. Micrographic examination showed an approximately 30 µm thick layer, probably a metastable form of calcium oxide, separating the calcite from the growing layer of oriented stable calcium oxide. The 30 µm layer yielded a single X-ray diffraction peak which was displaced slightly from the strongest (220) peak of the oriented normal calcium oxide. Lower apparent activation enthalpies measured in previous studies of calcite decomposition in inert atmospheres are suggested to result either from partial diffusion control of the process or from catalysis of the direct formation of normal calcium oxide by carbon dioxide or a component of the system atmosphere. The ratio of the measured decomposition rate in vacuum to the maximum rate, which can be calculated from the Hertz–Knudsen–Langmuir equation, is suggested to be a useful parameter in correlating and predicting decomposition reaction rates.