Fabric filters versus electrostatic precipitators

Control of particulate emissions from pulverized coal fired steam generators is becoming a significant factor in the siting and public acceptability of large coal burning power plants. The particulate emission limit established by the EPA for new coal fired boilers is 0.03 lb/10⁶ Btu (13 ng/J) Possibly more restrictive than this is the state of New Mexico's particulate regulation which calls for no more than 0.05 lb/10⁶ Btu (22 ng/J) total, and no more than 0.02 lb/10⁶ Btu (9 ng/J) less than 2 microns in diameter. This paper will evaluate the effect of these stringent limitations on the technical feasibility and economics of dry particulate removal. Electrostatic precipitators have been the dominant particulate collection device in the electric utility industry for many years because of their low capital and operating cost. However, increasingly stringent emission standards have led to substantially higher costs for precipitators. These costs have increased sufficiently for fabric filtration to become a competitive alternative in achieving cost effective control. This paper will compare the economics and performance of fabric filtration with respect to conventional electrostatic precipitators. The paper will also address the preliminary evaluation procedures that should be followed in order to select the appropriate device for new or existing coal-fired boilers.     

Criteria for designing electrostatic precipitators

This paper discusses the concepts of effective migration velocity and specific collector area as derived from the Deutsch equation. These two factors are commonly used as a means of specifying plant size and comparing proposals by different suppliers.It is demonstrated that effective migration velocity is not a constant dependent only on the fuel, being subject to large variations according to various features of the design of the precipitator. It is also shown that specific collector area cannot be used without at the same time considering the contact time of the gases in the electrostatic field.The removal of dust by a precipitator is carried out in two stages. The dust is first deposited on the collecting electrode, and a layer of significant thickness must be allowed to form so that when, in the second stage, it is dislodged by rapping, the layer breaks into agglomerated masses sufficiently large to fall into the hoppers below the collecting electrodes before being carried by the moving gas stream into the outlet flue. This requires that the frequency of rapping of each field of the precipitator shall be set at the correct rate according to the concentration of dust entering the field.Currently much stress is being placed on the intensity of the rapping blow and there is a tendency to specify increasing values of this blow. Due to the necessity of allowing a layer of dust to form, no increase in rapping intensity can overcome the effect of highly resistive dust. While the rapping force must be enough to shear the dust layer, any increase above this level is likely to result in the breakdown of agglomerates with an increase in dust emission and may also cause metal fatigue problems.     

Tubular electrostatic precipitators of two-stage design

Although the “two stage” electrostatic precipitator concept was first developed in 1910, until recently most of their use was confined to in-plant air cleaning. In the last decade, plate type designs have been modified to make them suitable for industrial applications involving organic emissions, most notably in asphalt saturating, plastic curing, food processing, printing, textile finishing, and heat treating industries. However, plate type designs are inadequate in applications where very high loading and/or high particulate content are involved.Tubular design with wider spacing and higher voltages incorporates the best features of both the single stage and two stage type precipitators. These units have been used in molybdenum roasting, zirconium calcining, ammonia scrubbing of oxides of sulfur, meat broiling, foundry exhaust, etc.The paper discusses design parameters, field test data, and operating data on these units. Comparisons with single-stage type precipitators are made wherever applicable. Advantages and disadvantages of two stage precipitators are also discussed.     

Interaction between electrostatics and fluid dynamics in electrostatic precipitators

Significant effort has been devoted by past researchers in electrostatic precipitators to electrostatics and particle chemistry. Relatively little, however, has been published on the effect of turbulent flow and its interaction with electrostatics and particle dynamics. This paper describes an experiment and a fundamental model which is focused on the coupled effects of the fluid dynamics and electrostatics.A laboratory precipitator flow facility with laser beam illumination was used to visualize smoke flow. Detailed photographs of flow patterns with and without electric field were recorded in a horizontal plane through the precipitator.A theoretical model was used to numerically simulate the observed flow patterns in the experiment. In the model the flow was described by time-averaged mass and momentum equations with a two-equation turbulence model. The electric field was described by Maxwell electrostatic equation and the continuity of the current equation. The system of the coupled partial differential equations was solved by a finite difference scheme.The interaction between fluid dynamics and electrostatics is shown to have a significant role in altering the flow in the precipitator. The practical implication is that the interaction will aid in particle migration in some regions, but it will increase reentrainment and reduce particle migration in others. Comparison between the results from the experiment and numerical computations shows good qualitative agreement between the two. Since both the electrostatic and the fluid dynamic flow fields depend on the electrode geometry, further investigation may lead to improved design of precipitators.     

Theoretical methods for computing electrical conditions in wire-plate electrostatic precipitators

A new semi-empirical, approximate theory for predicting electrical conditions is described. In the approximate theory, analytical expressions are derived for calculating voltage-current characteristics and electric potential, electric field, and space charge density distributions. Comparisons of numerical and approximate solutions over a wide range of possible precipitator geometries and electrical operating points indicates that for practical purposes the approximate theory can be used in lieu of the more rigorous numerical theory. This saves large amounts of computer time and makes possible hand calculator usage. Recent in situ gaseous ion mobility data which are needed in the models are presented. For coal fired power plants, the reduced effective ion mobility in positive corona is found to be 1.6 times that for negative corona. Approaches for describing particulate space charge effects in the gas and electrical conditions in the collected particulate layer are briefly discussed.     

Optimization of collection efficiency by varying plate spacing within an electrostatic precipitator

Use of the Deutsch equation as an evaluation tool of electrostatic precipitators has resulted in the restriction design to a single plate spacing and has not enabled optimal use of the space charge arising from the presence of charged dust particles. Space charge is dependent on dust loading and plate spacing and also influences the electric field, hence the migration velocity and, therefore, collection efficiency. Thus a trade-off is indicated between plate spacing and collection efficiency as a function of dust loading. A first step examination of this trade-off indicates that varied spacing can be used in precipitators while maintaining high levels of efficiency and decreasing the required specific collection area by 15% to 25%.     

Ion current densities produced by energetic electrons in electrostatic precipitator geometries

A new laboratory test system for electron beam ionization in electrostatic precipitator geometries has been constructed to measure ion current density as a function of voltage difference for clean plate (no dust contamination) conditions. The new system incorporates improved electrodes which withstand an applied voltage of ± 55 kV, a factor of 5 increase over the previous test system. A 3 MeV Van de Graaff accelerator produced ionizing electron beams of 1.2 and 2 MeV energy with beam currents of 10.5 and 21 μA in place of corona wire ionization. Ion current densities of up to 130 mA/m² were measured before breakdown between the plates, and no ion current saturation was observed. A comparison of I-V curves and sparkover voltages for various beam energies, currents, and types of collimation is discussed and the need for measurements with good beam geometry is addressed.     

Evaluation of the George Neal electrostatic precipitator

The Electric Power Research Institute (EPRI) is currently supporting a major research program characterizing the performance of high efficiency electrostatic precipitators (ESP). One such effort evaluating the George Neal ESP of Iowa Public Service Company is described in this paper.Results show that under well tuned conditions the ESP overall collection efficiency was 99.7% at a specific collecting area of 745 ft²/kacfm (147 m²/m³/s) (520 MW) with associated mass concentrations of 0.025 lb/10⁶ Btu and stack opacity of 4.6%. The boiler outlet size distribution was found to be bimodal with submicron and large particle peaks at 0.2 and 5 microns diameter, respectively. Consequently, an apparent bimodal fractional efficiency curve results with efficiencies of 99.6%, 98%, and 90% measured for 20, 2 and 0.2 micron diameter particles, respectively. Rapping reentrainment losses were found to be insignificant except during episodes of high ash hopper levels resulting from a malfunctioning ash removal system, when large rapping puffs were observed. In addition, outlet emissions increased dramatically to 0.08 lb/10⁶ Btu (34 ng/J) during these periods, suggesting that an emission level more representative of daily operation lies somewhere between 0.025–0.08 lb/10⁶ Btu (10.8–34 ng/J).     

Towards a microscopic theory of electrostatic precipitation

The statistical mechanics of particles at the surface of a dust deposit are presented as a step towards a microscopic theory of electrostatic precipitation. The paper consists of three major divisions: (1) field induced entrainment, (2) particle condensation at the surface, and (3) hydrodynamic limits on precipitation.In the first part of this paper, the entrainment of dust from surfaces by electrostatic forces was considered as a simple Markov process. A choice of x⁻¹ for the dependence of the adhesive potential (van der Waals particle attraction) led to an exponential emission rate in the Kramers-Chandrasekhar approximation. This result suggested a simple experimental procedure for determining the adhesive properties of the dust. In addition it was found that both thermal Brownian and particle-deposit collisions contribute to the entrainment.In the second part, the precipitation problem was discussed in terms of a two-dimensional Ising model. The Markovian assumption demanded that the efficiency of the electrostatic precipitator be divided into a product of factors depending on particle migration and condensation in the two-dimensional surface. From this perspective a new collection criterion was established and the importance of particle collisions at the surface of the deposit was demonstrated.Finally, in the third part the role of bulk fluid flows in particle condensation and migration was addressed. The superficial gas flow and secondary flow were found to introduce anisotropy into the Ising lattice and also to enhance the mean particle energy at the surface. The Navier Stokes Equation was numerically integrated for the case of a wire-plate type corona. Plots of the stream function and gas velocity profile were presented and these suggested the existence of a lower limit on the shear stress that acts on surface particles.     

Radon exhalation from phosphogypsum building boards: symmetry constraints, impermeable boundary conditions and numerical simulation of a test case

Comprehensive understanding of (222)Rn exhalation from phosphogypsum-bearing building material and its accumulation in indoor air is likely to rely on numerical simulation, particularly if transient effects, three-dimensional domains and convection are to be included and investigated. Yet, experimental data and analytical results are helpful (if not crucial) as far as validation is concerned. Having in mind computational code simplicity and in the light of a recent experimental and theoretical report on (222)Rn release from phosphogypsum boards for housing panels, this paper presents and discusses an alternative testing set-up and the corresponding boundary conditions, namely one side of the panel bounded by impermeable wall. Although this is a new facility to be tested, the resultant steady-state one-dimensional diffusion-dominant analytical solution is shown to match the counterpart deduced in the aforementioned previous report, despite it relaxes the constraint of symmetry about the phosphogypsum board centerline, which is inferred in that prior experimental scenario. In addition, numerical results are conducted for a diffusion-dominant two-dimensional time-varying test case concerning (222)Rn accumulation in a closed chamber having an exhaling phosphogypsum board tightly placed at one wall.     

Control of fine particle emissions with wet electrostatic precipitation

This paper presents the results, both pilot and full-scale, of experience with the application of wet electrostatic precipitation technology for the control of fine particle emissions from industrial processes. Performance data involving the collection of such difficult-to-clean emissions as a sulfuric acid mist and recovery boiler salt fume are presented. The measured wet precipitator performance parameters (e.g., SCA, effective migration velocity) are compared to those reported in literature for dry precipitators operating on equivalent processes. Particular emphasis is directed toward examining wet electrostatic precipitator performance levels in controlling very high concentrations of submicron fume with the accompanying problem of space charge corona quenching. Relationships between these performance data and specific wet precipitator operating parameters such as operating voltage and current density are also examined.     

Design of high intensity ionizer—electrostatic precipitator systems

This paper presents the results of a comprehensive set of analytical and experimental studies concerning the application of high intensity ionization technology to retrofit a flyash electrostatic prepitator.Described in detail are the computer-aided methods of performing functional design for ionizer-precipitator systems, selected results of ionizer-precipitator flow model tests, and the chosen physical retrofit. Also presented is the projected performance of an ionizer-precipitator system, derived from two independent design procedures which take into account the effect of space charge and gas velocity distribution on system performance.Results of these studies confirm that high intensity ionizer technology can be applied to significantly improve the collection efficiency of conventional wire-plate electrostatic precipitators.     

Improving the efficiency of free-jet scrubbers

A free-jet scrubber is an innovative gas cleaner in which the primary liquid/gas contacting takes place in the turbulent mixing zone emanating from a jet nozzle. The first such devices, powered by supersonic steam or air ejectors, were exceptionally effective cleaners but were criticized as being high-energy users. A great reduction in the energy consumption of free-jet scrubbers has been accomplished in the last four years, a feat which may seem in contradiction to the “contacting power rule.” An explanation of what is happening in these devices must take into account the effectiveness with which energy and water are utilized. thereby leading one to a somewhat circular definition of “effective contacting power.” The theoretical implication of this analysis is primarily to remind us that there are many paths between two states and some are more energy efficient than others. The practical value is that such analysis generates information which aids in choosing between several design options, a direct benefit of the various free-jet configurations.     

市场分割对能源效率的影响研究

探究能源效率的影响因素并有针对性地提出对策建议是实现"十三五"节能目标的前提条件。本文利用1986—2014年的省际面板数据,基于随机前沿分析和反事实计量方法,重点考察了市场分割对能源效率的影响。结果显示:1市场分割显著地抑制了能源效率的提升;2在考虑市场分割的情形下,能源效率从1986年的0.413提升到了2014年的0.739,年均增速为2.1%,但能源效率依然有较大的改善空间,如果能够消除市场分割的不利影响,能源效率平均每年将会获得1.5%的额外提升;3在样本考察期内,由于市场分割导致的直接能源损失平均每年约为1 200万t标准煤,若考虑市场分割产生的其他间接影响及其动态效应,这一数字估计值会更高;4产权结构改革、对外开放、产业结构升级以及节能法的颁布促进了能源效率的改善,而金融发展规模、政府干预以及以煤炭为主的能源消费结构不利于能源效率的提升。以上研究结论蕴含的政策含义是:为提高能源效率,需要加强市场一体化建设,打破省际壁垒,整合国内市场。鉴于市场分割可能是一些地方政府的占优策略,因此,需要中央政府来进行推动。一方面,中央政府可以通过加强监管和查处力度直接惩罚市场分割行为,另一方面,中央政府还可以通过转移支付的方式鼓励落后地区积极主动融入国内整体市场。当然,提高能源效率,也需要诸如推进产权结构改革,提高对外开放水平,优化产业结构与能源消费结构,改善金融发展效率,减少政府干预以及完善能源政策法规等其他措施的共同跟进。     

北京市城市生活垃圾收运系统研究

对北京市城市生活垃圾收运系统现状进行了全面的分析,对北京城八区生活垃圾收集、分类收集、转运、生活垃圾处理设施和垃圾物流现状进行了分析,同时对北京市城市生活垃圾收运系统进行了分析,指出了北京市城市生活垃圾收运系统存在的主要问题和今后改进的建议及处理处置设施的规划,为北京城市生活垃圾收运系统的建设和完善起到了积极地推动作用。     

The impact of energy awareness on energy efficiency

The increasing levels of global warming, depleting sources of fossil fuels and increasing energy costs are all having a large detrimental effect on today's society. Many efforts are being made to try and increase energy efficiency all over the world. One of the major problems is unnecessary and excessive energy utilisation. This problem has been identified by Loughborough University and they are making efforts to try and reduce energy wastage. One of their strategies, used up until recently, involved increasing energy awareness within students in halls of residence by means of a reward system called the Residential Halls Energy Efficiency League (Imago Services, Loughborough Students Union, Estates Services 2006). This scheme managed to reduce energy consumption by up to 10%, saving an estimated £9000 in only 52 days. Can those results be improved by targeting students more specifically? How much energy can be saved by increasing energy awareness?     

Experimental and numerical study of biogas,methane and carbon dioxide produced by pre-treated wheat straw and pre-digested cow dung

ABSTRACT

The biogas constituting majorly CH₄ and CO₂ has been produced by Ca(OH)₂ pre-treated wheat straw with pre-digested cow dung. Some of the key thermodynamic parameters like specific heat capacity, density and heating capacity of the biogas produced have also been calculated per day as well as throughout the hydraulic retention time. The governing equations of biogas with appropriate phase and interfacial conditions describing the physics of the biogas have been derived. The control volume approach has been used to predict the total volume (ml) of biogas, CH₄ and CO₂ throughout the experiment and on the daily basis. The effects of feedstock, temperature and pressure on the production of biogas, CH₄ and CO₂ in anaerobic digestion have also been studied. The average number of molar fraction and conversion ratio of CH₄ and CO₂ are correlated with number of carbon atoms available in feedstock. Numerical calculations by using developed model and Modified Gompertz model have shown proficient agreement with the experimental observations.     

A case study of radon-222 transport from continental North-East Asia to the Japanese islands in winter by numerical analysis

A case study of the regional transport ( approximately 3000 km) of radon-222 ((222)Rn) from continental North-East Asia to the Japanese islands was performed by numerical analysis using five separate source areas (South, Middle and North China, Russia and Korea), while a seasonal northwest wind blew over the Japan Sea. The results for three periods (Term I: 16-18, Term II: 22-25 and Term III: 27-28 in December 1990) were compared with concentrations measured at the Kanazawa site (near the coast of the Japan Sea facing the seasonal wind) and the Nagoya site (overland and downwind on the shores of the Pacific Ocean). Most of the (222)Rn at the Kanazawa site was calculated to come from North China and Korea in Term I, Middle China, North China, and Korea in Term II, and Russia and Korea in Term III. The considerable differences in the origins of (222)Rn emanated from the continent were estimated between Terms I, II and III, even though the similar northwest wind was dominant over the Japan Sea. A contour line analysis indicated movement of (222)Rn emanated from Middle China in a northerly direction first and then a southeasterly direction, resulting from low pressure. The results suggest that the low-pressure systems play an important role in the transport of (222)Rn in North-East Asia.     

Using radon-222 as indicator for the evaluation of the efficiency of groundwater remediation by in situ air sparging

A common approach for remediation of groundwater contamination with volatile organic compounds (VOCs) is contaminant stripping by means of in situ air sparging (IAS). For VOC stripping, pressurized air is injected into the contaminated groundwater volume, followed by the extraction of the contaminant-loaded exhaust gas from the vadose soil zone and its immediate on-site treatment. Progress assessment of such remediation measure necessitates information (i) on the spatial range of the IAS influence and (ii) on temporal variations of the IAS efficiency. In the present study it was shown that the naturally occurring noble gas radon can be used as suitable environmental tracer for achieving the related spatial and temporal information. Due to the distinct water/air partitioning behaviour of radon and due to its straightforward on-site detectability, the radon distribution pattern in the groundwater can be used as appropriate measure for assessing the progression of an IAS measure as a function of space and time. The presented paper discusses both the theoretical background of the approach and the results of an IAS treatment accomplished at a VOC contaminated site lasting six months, during which radon was applied as efficiency indicator.     

Evaluation of Brasilia wastewater sludge as a biomass resource for the production of energy by gasification simulation

This work evaluated the sludge potential of the Wastewater Treatment Plant (ETA) in the city of Brasília to be used as a fuel by gasification. It is known that ETA sludge is a significant environmental liability, since current legislation restricts its final disposal. For this, the chemical characterisation of ETA sludge was performed by immediate and elemental analysis. No traces of heavy metals were observed, and the moisture (ω) and ash contents were 31.17 and 51.77%, respectively, different from those already reported in the literature because the composition depends on the water treatment technology employed. The gasification process was numerically simulated; once dry, it constitutes a residue with an energy content (HHV) of 22.4498 MJ kg^?1, comparable with other types of biomass currently used for large-scale energy generation by thermochemical processes (e.g. agricultural residues, wood and sugar cane bagasse). For the numerical simulation with an equivalence ratio (Φ) near 3, higher concentrations of CO and H₂ can only be achieved with ω lower than 15%. The results showed that gasification can be an attractive option for the disposal and use of a renewable waste resource, such as ETA sludge, in an environmentally safe way, and it is allowed by local legislation.