With the increase in population and industrial growth, the need for power has increased manifold. A major share of India’s power generation is coal-based generation. Thermal power generation through coal combustion produces minute particles of ash that causes serious environmental problems. Most of the thermal power stations in India use bituminous coal and produce large quantities of fly ash. Fly ash is produced primarily by thermal power plants and, to a lesser extent, by cement and steel plants and railways. This poses problems in the form of land use, health hazards, and environmental dangers. Today the fly ash from thermal power plants is utilized to make cement. Cement industry needs fly ash with more fine particles. If these fine particles (of diameter in micro-meter range) are left out to atmosphere, it will be easily breathed into lungs, creating health hazard to human as well as animal life. If properly collected, it forms a valuable by product. Hence collecting fly ash is important to reduce pollution and also to increase revenue. Even though there are many devices like cyclones, fabric filters etc. the Electrostatic Precipitator (ESP) is the most efficient device to capture the fly ash. It is at this juncture a need for such a simple ESP model was felt to facilitate prediction of the V-I characteristics of dust loaded precipitators, be it cement dust or fly ash. Given the fact that 99% of Indian ESPs are operated under DC energization and most of them are running inefficiently due to lack of proper diagnostic tools and also due to lack of interest to invest on an ESP. In such circumstances, the free availability of a simple model that combines the V-I curves with collection efficiency serves to improve the ESP performance in our Indian industries. In an ESP Voltage-Current (V-I) characteristics are used to diagnose any electrical problems occurring in it. Mathematical model of V-I characteristics under clean air and dust laden conditions will be helpful in diagnosing the ESP problems as well as in designing the ESP. The model will also indirectly reflect upon the collection efficiency of the ESP. The collection efficiency should be as high as possible not only to prevent pollution but also to collect maximum fly ash which is a valuable by-product. The modeling of collection efficiency will help the industries to design a new precipitator as well as to improve the performance the collection efficiency to meet the changing restrictions set by the government to reduce pollution. In this thesis a mathematical model of ESP based on Finite Difference Method is developed. The modeling is done in three sections. 1. Simulation of clean air V-I characteristics. 2. Simulation of dust laden V-I characteristics. 3. Simulation of collection efficiency. Simulation of clean air V-I characteristics is done by iteratively solving the Poisson’s equation and current continuity equation, using FDM in one quarter region of the ESP. Just by introducing the effect of particle charge into this solution the dust laden V-I characteristics are simulated. Finally, the collection efficiency is calculated using average charge density at the plates obtained from the above solution. The developed model is validated at first against published experimental and simulated data and then, with the data obtained through conduction of experiments, by the author, on commercial precipitators situated at a thermal power station and a cement plant, in India. The thesis discusses in detail these theoretical and experimental studies.

There are many U.S. health-related standards for ozone that aim to limit exposure to ozone. The potential for ozone emission from electrically connected air cleaners is well- known and has led to standards and regulations for portable indoor air cleaning devices, which emit ozone at measured rates of 0.056 – 13.4 mg/hr. However, there is evidence that some in-duct air cleaners may actually emit more ozone than portable air cleaners, despite being exempt from most regulations due to the lack of a suitable test method for measuring ozone generation. To explore if in-duct cleaners actually do emit ozone, I investigated seven commercially available residential in-duct air cleaning devices. These devices used one of two broad technologies as means of air cleaning: UV light or electrical corona. The lowest measured emission rates came from two air cleaners that utilized UV light technology and were 0.309 ± 1.7 mg/hr, which was likely below the detection limit of the apparatus and method, and 4.29± 1.5 mg/hr. Three of the air cleaners tested, also with UV lamps, were of the same brand and model yet exhibited differing emission rates, ranging from 7.44± 1.6 mg/hr to 15.8± 2.6 mg/hr. These three air cleaners were classified as medium emitters and also utilized UV light technology. The high median measured emission rates were measured from both an air cleaner utilizing electrical corona technology, 30.2 ± 4.0 mg/hr, and UV light technology, 29.4 ± 3.9 mg/hr. These experimental results confirm that some in-duct air cleaners are able to generate more ozone than some portable air cleaners and also suggest potential health risks to the indoor environment. / text

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Even minimal improvements in particle collection efficiency of electrostatic precipitators significantly reduce dust emission from fossil-fuelled power stations and reduce pollution. Yet current designs rely on the Deutsch collection theory, which was developed for tubular precipitators and has been applied to wire-plate precipitators on the assumption that the inter-electrode electric fields at the same discharge distance in both were similar. Differences in geometry and associated collector electric fields and current density non-uniformity have not been taken into account, although the collector electric field and current density of the wire-plate precipitator are not uniform. And observations show that precipitated dust patterns and the distribution of collector current density are interrelated. Investigations revealed a simple square law relationship between the collector electric field and the collector current density in the space charge dominated coronas. Applying this relationship to the Deutsch collection theory led to a current-density-based collection formula that takes into account the non-uniform collector current density distribution. The current-density-based collection formula is then used to assess the impact of collector current density on collection efficiency, the results closely following published measurements. Applying the current-density-based collection formula to estimate the dust accumulation shows that most of the dust accumulates at collector locations facing the corona wires. The effect of the non-uniform precipitated dust layer on collection performance is assessed using the distributed corona impedance - the ratio of the inter-electrode voltage and the non-uniform collector current. Re-distribution of the collector current profile as dust builds up is also compatible with published measurements. Finally this is applied to optimize the wire-plate precipitator collection performance. This shows that optimal collection performance is obtained with the wire-wire spacing less than the wire-plate distance, once again confirming published experimental results. This is the first analytical approach to show better collection performance can be achieved at the ratio of wire-wire spacing/wire-plate distance not equal to unity, which has been the standard industry practice since 1960.

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Thesis (M. Tech. Mechanical engineering -- Vaal University of Technology / Strict emission legislation has forced industry in general to seriously consider the negative impact it has on the environment, specifically concerning emissions from burning fossil fuel into the atmosphere. In cases where emission levels exceed the allowable limit, companies are forced to operate at lower operating conditions and these load losses can result in a significant loss of revenue. This has forced companies to improve their ash filtering capabilities by optimising electrostatic precipitation systems. One of the main factors impacting on the efficiency of such a system is the distribution of the flow across the collection plates. The design of the inlet diffuser plays a major role in the ultimate distribution of the flow through the precipitator. Porous screens are positioned in the diffuser in order to distribute the flow across the total flow area with the aim to achieve a uniform distribution of the flow. CFD is widely used in industry to simulate the flow through precipitators in order to optimise the flow distribution and thus increase the efficiency of the system. It was found however that the current methods used to simulate these screens in CFD models were not well researched and employed fixed resistance values that could not reliably compensate for changes in the resistance coefficient due to a change in the angle of incidence. This study investigates advanced numerical methods for the simulation of porous screens in applications where the angle of incidence changes continuously across the face of the screen. New methods are introduced where the resistance of the screen is calculated as a function of the changing angle of incidence. The methods currently used are also investigated and compared with results from the new methods. Extensive experimental work was required to supply empirical data for the validation of the numerical methods that are proposed. For this reason, the first part of this study focused on the design construction and commissioning of a low speed wind tunnel. Results are presented and discussed for flow profiles through wide-angle diffusers at different angles and also for a number of different screens positioned in the centre of the diffuser. This study also investigates the sensitivity of a CFD simulation code to factors such as numerical discretisation schemes, turbulence models and solution relaxation specifically for wide-angle diffusers. These factors were tested for diffusers at different angles and included tests on open diffusers and also with screens positioned inside the diffuser. It was concluded that the current methods used are not adequate to capture the true flow profiles for a range of different screen geometries. Although the proposed models did improve on the limitations of the current methods, it was found that the applicability of these models is still limited and that further research would be required to develop numerical methods that are valid for a wide range of applications. / Vaal University of Technology, The National Research Foundation (NRF) and Eskom TSI

La détection d'agents biologiques dans l'air ambiant est devenue un enjeu majeur notamment en environnement hospitalier et dans la protection contre le bioterrorisme. Dans ce contexte, la miniaturisation des dispositifs d'analyse permet d'envisager leur utilisation directement sur la zone d'étude. Afin d'obtenir un échantillon concentré et représentatif, la filtration de l'air reste cependant un point délicat. Parmi les différents principes exploitables pour la collecte de particules aéroportées, l'emploi des forces électriques semble être prometteur pour améliorer les performances des dispositifs qui se trouvent généralement fondés sur des forces inertielles. Dans cette étude, une modélisation fine des collecteurs électrostatiques a été conduite pour une géométrie fil / cylindre. Elle décrit tout d'abord les champs hydrodynamiques d'un écoulement charriant des inclusions dans lequel est imposée une décharge couronne. Une injection éventuelle de vapeur dans la chambre de collecte a nécessité ensuite la détermination des champs de température et concentration de la vapeur. Une analyse dimensionnelle inspectionnelle a montré que ces champs possèdent deux termes de couplage fort dont on a justifié l'omission dans cette étude ; les phénomènes physiques mis en jeu ont alors pu être classés selon une cascade d'influences non réciproques et la résolution numérique du modèle s'en est trouvée facilitée. Quatre configurations d'écoulement différentes, caractérisées par des recirculations d'origine électro hydrodynamiques, ont été identifiées et leurs impacts sur les rendements de collecte quantifiés. De plus, une procédure de dimensionnement des filtres électrostatiques fondée sur un nombre de Deutsch représentatif des rendements a été mise en place. Son exploitation a montré l'intérêt de la mise en parallèle de petits collecteurs pour filtrer des débits d'air importants. Cette étude s'est achevée par l'analyse des effets engendrés par l'injection de vapeur dans la chambre de collecte. Elle a jeté les bases d'une explication pour l'augmentation des rendements de collecte résultant de cette injection. / Detection of airborne biological agents has become a major challenge especially in hospitals and the protection against bioterrorism. In this context, the miniaturization of analytical devices allows to consider their direct use in the field. To obtain a representative and concentrated sample, air filtration remains a delicate point. Among the various principles used to collect airborne particles, the use of electrical forces seems to be promising to improve performance beyond these of devices that are based on inertial forces.In this study, a detailed model of electrostatic collectors was developed in the wire/cylinder geometry. It first describes the hydrodynamic flow fields carring inclusions in which a corona discharge is imposed. Afterwards, the possible injection of steam into the collection chamber required the determination of the temperature and vapor concentration fields. An inspectionnal dimensional analysis justified the omission of two strong coupling terms. Therefore, in this study, the involved physical phenomena could be classified according to a non-reciprocal influences cascade and the numerical model is become simpler. Four different flow patterns, characterized by their electrohydrodynamic secondary flows, were identified and their impact on the collection efficiencies was quantified. In addition a design procedure of electrostatic filters, based on a representative efficiency Deutsch number, has been developed. This procedure shows the interest of parallelizing small collectors to filter important airflows. This study was completed by the analysis of the effects of steam into the collection chamber. It provides the basis for an explanation of the collection efficiencies increase related to this injection.