bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Heat Treatment on the Reactivity and Crystallinity of Coal-Char
Authors: Zhang Shouyu , Lu Junfu , Zhang Jianmin , Liu Qing , Yue Guangxi
Published: October 2008

The effect of heat treatment on the reactivity and crystallinity of char prepared from the vitrinite of two coals (YX, JJ) was investigated by using XRD and TGA in this paper. The results from TGA show that the reactivity of the chars from YXV and JJV decreases with the increase of heat treatment temperature. The reactivity of YXV char decreases quickly and significantly as heat treatment time increases. However, after heat treatment time of 60 min, it decreases slowly. The effect of heat treatment time on the reactivity of JJV char is small. The results from XRD show that the crystallinity of coal-char is determined by the intensity of heat treatment. When heat treatment time is more than 60 minutes, the turbostratic crystallite of YXV char prepared under 900°C changes remarkably and becomes more orderly. The aromatic layer stacking heights (Lc) of YXV Char when heat treated above 900°C increased with the increase of heat treatment time. The effect of heat treat time on Lc of JJV char is small, but under heat treatment temperature of 1200°C, the crystalline of JJV char grows distinctly. There is a good parallel relationship between the crystalline growth and deactivation of the chars. It can be concluded that the growth of the crystalline is the main reason for the deactivation of coal-char.
Operational Performance and Optimization of a 465t/h CFB Boiler in China
Authors: Zhao Xinmu , Lu Junfu , Yang Jianhua , Zhang Qingguo , Dong Fengliang , Yu Long , Yang Zhongming , Yue Guangxi
Published: October 2008

In the last three years in China, more than 80 units of 135MWe circulating fluidized bed (CFB) boilers were ordered, and about two dozens of them have been put into operation. So far, the experience and performance evaluation of the boilers with such large capacity are very limited. A series of cold and hot tests were carried out on the boiler in order to optimize the operation and provide more information to the future design. The influence of coal properties, bed material fluidization, air distribution, bed temperature and pressure drop on the boiler performance such as carbon content in fly ash was assessed and discussed. Some problems of the boiler, including the bottom ash system, milling system, abrasion of the heating surface in the furnace, refractory stability, and exhaust fuel gas temperature are reported and suggestions are given for the future improvement and design.
Combustion Behavior and SO2, NOx Emissions of an Anthracite Coal in a Circulating Fluidized Bed
Authors: Cheng Leming , Luo Zhongyang , Shi Zhenglun , Zheng Haixiao , Wang Qinghui , Gao Xiang , Fang Mengxiang , Yu Chunjiang
Published: October 2008

Combustion behavior and SO2, NOx emissions of anthracite coal in a circulating fluidized bed are reported in this paper. Experimental researches were done on a 1 MWt circulating fluidized bed facility with a 0.31 m × 0.31 m cross section and 11.2 m height combustor. The anthracite coal with 6.28% volatile and 3.76% sulfur content burns steadily during the test. The bed was operated under different temperature, Ca/S ratio and excess air. A limestone containing 75% CaCO3 and 15% MgCO3 was used as the sulfur sorbent. Results show that the SO2 emission varies with operating bed temperature and more than 90% sulfur capture efficiency can be reached while Ca/S is about 3. With Rosemount Analytical NGA2000, N2O, NO and NO2 were also measured in the test. It was found the majority content of NOx was NO and the least was NO2. Those NOx emissions change highly with the excess air number.
Partitioning of Trace Elements During Fluidized Bed Combustion of High Ash Content Lignite
Authors: Selc¸uk Nevin , Gogebakan Yusuf , Gogebakan Zuhal
Published: October 2008

The behavior of 20 trace elements (As, B, Ba, Cd, Co, Cr, Cu, Hg, Li, Mn, Mo, Ni, P, Pb, Sb, Se, Sn, Tl, V, Zn) and 8 major and minor elements (Al, Ca, Fe, K, Mg, Na, Si, Ti) during the combustion of high ash content lignite with and without limestone addition have been investigated in the 0.3 MWt Middle East Technical University (METU) Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) Test Rig. Experiments were performed without fines recycle. Inert bed material utilized in the experiments was bed ash obtained previously from the combustion of the same lignite without limestone addition in the same test rig. Concentrations of trace elements in coal, limestone, bottom ash, cyclone ash and filter ash were determined by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Measurements show that the distribution of major and minor elements follows the ash split between the bottom ash and fly ash and that the major proportion of most of the trace elements (As, Ba, Cr, Hg, Li, Mo, Ni, Sn, V, Zn) are recovered in fly ash. Comparisons between the trace element partitioning of the runs with and without limestone addition reveal that addition of limestone shifts the partitioning of Ba, Cr, Hg, Mo, Ni, Sn, V, Zn from bottom ash to fly ash.
Effect of Recycle on Radiative Heat Transfer in the Freeboard of a Fluidized Bed Combustor
Authors: Selc¸uk Nevin , Ayranci Isil , Gogebakan Yusuf
Published: October 2008

Effect of recycle on radiative heat transfer in the freeboard of a fluidized bed combustor is investigated by applying a previously developed 3-D radiation model to the prediction of incident radiative heat fluxes along the freeboard walls of lignite-fired 0.3 MWt Middle East Technical University (METU) Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) Test Rig and comparing its predictions with measurements. Freeboard is treated as a rectangular enclosure containing gray, absorbing, emitting and isotropically scattering medium bounded by gray and diffuse walls. Radiative properties of the medium are calculated by using Leckner’s correlations for gas and Mie theory for polydisperse particle cloud. Radiative transfer equation for this system is solved by using Method of Lines (MOL) solution of Discrete Ordinates Method (DOM). Experimental data required for application and validation are generated from two runs in which parameters other than recycle ratio was held as nearly constant as possible. Comparisons between predicted incident radiative heat fluxes and measurements with and without recycle reveal that the agreement is excellent and that the effect of recycle on incident radiative heat fluxes is significant. A parametric study is also carried out to investigate the effect of particle load on fluxes. Predictions are found to be relatively insensitive to the particle load but strongly affected by the temperature profile.
The Rise and Combustion of Particles of Sewage Sludge and Petroleum Coke in a Slugging Fluidized Bed
Authors: Rees A. C. , Davidson J. F. , Dennis J. S. , Hayhurst A. N.
Published: October 2008

The rise-times, from just above the base to the top of the bed, of single particles of dried sewage sludge and of two types of petroleum coke were measured in a slugging bed of sand fluidized by air. The sewage sludge and the petroleum cokes contained, on an as received basis, 49.5, 9.0 and 0.5 wt.% volatile matter, respectively. The bed (i.d. 25.5 mm; height 1300 mm) was electrically heated to a fixed temperature between 700 and 900°C. Two sizes of sand were fluidized: +212, −300 μm and +710, −850 μm. The expansion of the bed was held constant by keeping the excess gas velocity at (U − Umf) = 190 mm/s. In a bed of the coarser sand, it was found that the time of rise of single particles of sewage sludge (< 4 mm diam.) was large enough to allow devolatilization and char burn-out to be complete before the particle reached the upper surface of the bed. The measured times of rise are significantly longer than those found in bubbling beds [12, 13]. Thus, a slugging bed, of say 2 m in depth, would have significantly better heat retention and contact of the volatile material with the particulate phase than a bubbling bed of the same depth. With the finer sand, particles of sewage sludge and of petroleum coke containing volatile material rose rapidly to the top of the bed, well before combustion was complete. Published theory [6] was used to predict the rise-times of inert particles in a hot slugging bed. The predictions compare favourably with measurements using petroleum coke containing negligible amounts of volatile material and moisture. The use of a modified form of Stokes’s law to predict the velocity of rise of inert particles is shown to compare well with experiments. The experimental results suggest that once a minimum rate of gas evolution (from volatiles or moisture) is achieved a bubble forms which is large enough to lift a fuel particle to the surface. For particles meeting this criterion, the rise-time appears to be independent of the size of the fuel particle.
Interaction of the Behavior of Lead and Zinc With Alkalis in Fluidized Bed Combustion or Gasification of Waste Derived Fuels
Authors: Backman Rainer , Hupa Mikko , Hiltunen Matti , Peltola Kari
Published: October 2008

Combustion of waste derived fuels in fluidized bed boilers may result in fly ashes containing increased amounts of lead and zinc, besides the common alkali and alkaline earth metal compounds. Although the absolute concentrations of lead and zinc may be relatively low, around 1%, in the bulk ash, they may induce unwanted effects in combustors, partly due to their significant enrichment in the fly ash. First, lead and zinc in fly ashes may lead to unwanted heavy metal emissions. Further, they can also alter the behavior of the fly ash and cause it to become sticky and possibly corrosive. This paper discusses the mechanism of volatilization of lead and zinc and stickiness properties of their fly ash compounds under different conditions, based on data from a FBC gasifier using waste fuels with significant amounts of lead and zinc. Advanced thermochemical calculations using the data bases developed at A˚bo Akademi show that both lead and zinc can form volatile compounds and thus be strongly enriched in the fly ash. They can be volatilized as elemental gases, Pb(g) and Zn(g), or they can form gaseous chlorides, PbCl2(g) and ZnCl2(g). But they can also form non-volatile oxides. Thus their behavior is very dependent on the combustion conditions, particularly on the availability of chlorine. This way there is also a direct coupling of the volatilization behavior of lead and zinc with the chemistry of the alkali metals and calcium, all of which govern the availability of chlorine. Simplified thermochemical diagrams are shown to explain the complex interaction of the lead and zinc chemistry with the rest of the flue gas and fly ash chemistry. The thermochemical data can be used to explain the practical results from full scale boilers.
Combustion Characteristics of Fuels: Experiment Scale-Up From Bench Scale Reactors to Commercial Scale CFB Boiler
Authors: Ha¨sa¨ Heidi , Kirkinen Ari-Pekka , Tourunen Antti , Hyppa¨nen Timo , Saastamoinen Jaakko , Kettunen Ari
Published: October 2008

The equipment scale-up towards larger CFB units requires accurate knowledge of the process and combustion behavior of fuels. Unit sizes of 300 MWe are in operation and plans for larger units have been made. Shift from natural circulation to once through steam cycle requires more precise knowledge of the dynamic behavior of the fuel since there is no steam drum. The combustion of inhomogeneous fuels, as well as, special demands for dynamic process behavior poses new challenges to boiler manufactures. Nowadays, dynamic models are used to develop and analyze the dynamic behavior of the combustion process. Testing all the dynamic changes in the full-scale reactor would be both expensive and risky. Therefore, bench and pilot scale experiments, combined with dynamic models of the combustion processes, give a good basis to study behavior of larger scale units. At the same time models also increase knowledge of different process relations. The main objective of this paper is to present results of scale-up experiments from the bench scale, via pilot scale, to full-scale boilers. Further, how the combustion and reactivity of fuels in the full-scale boilers can be studied with the aid of small-scale experiments and simulations. Dynamic experiments were carried out with three reactors of different scale. Calculation and simulation models have been developed to illustrate the combustion in the reactors; e.g. heat release profiles, fuel reactivity and particle size distribution. Results from the dynamic experiments are used to adjust the computer models.
Advancement in CFB Technology: A Combination of Excellent Environmental Performance and High Efficiency
Authors: Stamatelopoulos George N. , Seeber Joachim , Skowyra Richard S.
Published: October 2008

The CFB technology achieved its commercial breakthrough in the field of power generation in the late eighties as a result of: • very low emissions levels, without secondary measures, that allows the fulfillment of the more stringent environmental legislation; • and the capability to burn a wide range of fuels, that allows the utilization of opportunity fuels.
Fuel Gas Clean-Up in a Transport Reactor: Model Development and Analysis
Authors: Monazam Esmail R. , Shadle Lawrence J.
Published: October 2008

Integrated Gasification Combined Cycle (IGCC) is the leading coal-fired system being developed by the U. S. Department of Energy to produce clean electricity from coal. One of the key components of this IGCC is fuel gas desulfurization (HGD) which is used to remove sulfurous compounds from coal gases with efficient, regenerable, mixed-metal oxide sorbents. Previous commercial desulfurization processes are based on wet scrubbing at or below ambient temperatures, resulting in considerable thermal efficiency loss as well as costly wastewater treatment. This has led to the development of gas-solid processes using transport reactors above the dew point of water. These systems offer the advantages of high throughput, continuous operation, and efficiency gains over low-temperature scrubbing of H2S. The National Energy Technology Laboratory (NETL) is developing a simplified transport reactor mathematical model to provide a quick estimation of pressure drops and conversions as a function of riser dimensions, sorbent properties and gas velocity. Hydrodynamics and a solid conversion model, together with a transport reactor mass balance, are used to predict the performance of a fuel gas desulfurization reactor. Experimental data collected at NETL’s Gas Process Development Unit (GPDU) are used to provide an initial assessment of the model.
The Effects of Heterogeneous Reactions on the Reduction of NO in Petroleum-Coke-Fired Fluidized Beds
Authors: Zhang Chun-Lin , Liu De-Chang , Chen Han-Ping
Published: October 2008

Because of high heating value, low volatile, high nitrogen content and high sulfur content, some kinds of petroleum coke are only suitable for use as fuel, especially combusted in fluidized beds. Based on experiments in a 1t/h fluidized bed, we found that lots of NO and N2O were emitted, and they reached to 780ppm and 150ppm respectively. By analyzing the contributions of char-N and volatile-N to the formation of NO and N2O, we also found it was more important to control the combustion of char to reduce the emission of NO and N2O. This paper tried to find a denitrification agent that could work as desulfuration agent in fluidized beds. We chose Fe as the denitrification agent. The influence of iron on the reduction of NO was studied on. The effects of petroleum-coke char, CO and limestone on the reaction of iron and NO were investigated in a bench scale fluidized bed. Quantitative Fe and petroleum coke char were added into a quartz sand bed respectively, the conversions of NO between these conditions were compared with. The results showed that the ability of Fe to reduce NO was much stronger than the char, and the conversion of NO almost reached to 100%. After minutes that depended on the amount of Fe, Fe was oxidized to oxide of Fe, and the conversion of NO decreased. Petroleum coke char could deoxidize the oxide of Fe. Fe that mixed with petroleum coke char could apparently increase the reaction time of Fe and NO. CO was also a reductive agent of the oxide of Fe, but the effect was not strong as char. Limestone little increased the conversion of NO. From the experiments, we suggested that iron or its oxides would be a possible denitrification agent to reduce NO in fluidized beds in situ.
Analysis on the Bed Temperature Characteristics of 450t/h Circulating Fluidized Bed Boiler
Authors: Gao Jian-Qiang , Du Chao-Bo , Wang Yin-Song
Published: October 2008

Normal and steady bed temperature provides a basis for the stabilization and safe operation of the circulating fluidized bed boiler (CFBB) unit. Because the bed temperature is affected by many operating factors, it often fluctuates unexpectedly. The bed temperature auto-control system seldom could be put into service during the operation of large capacity CFBB unit. On the basis of operation data of a commercial 450t/h CFBB, which is the first biggest CFBB made in China, the authors systematically analyze the main factors which influence the changes of bed temperature, such as fuel supply rate, air supply rate, oxygen concentration, dense zone height and the unit load et al. The record of the operation data show that the lag of the bed temperature is common phenomenon while fuel supply is changed, and the lag time varies with the fuel supply time, fuel supply operating mode and fuel supply rate; Coal quality and its grain diameter spectrum also have notable influence on bed temperature; Primary air rate has different influences on bed temperature at different operating conditions, and its adjustment range on bed temperature is limited while it is used to bed temperature control; The change of oxygen concentration can reflect combustion conditions in the furnace; The change of dense zone height will result in the change of bed temperature, so the adjustment of fluidization air rate is necessary; Bed temperature is one of the main factors which lead to the change of boiler load. At last, regulating rules on bed temperature control are analyzed and summarized. The conclusions can provide references for the design of control system and safe operation of CFBB unit.
Hydrodynamics of a Transport Reactor Operating in Dense Suspension Upflow Conditions for Coal Combustion Applications
Authors: Mei Joseph S. , Shadle Lawrence J. , Yue Paul , Monazam Esmail R.
Published: October 2008

A series of experiments were conducted in the 0.3-meter diameter, 15.45-m high cold flow circulating fluid bed (CFB) test facility at the National Energy Technology Laboratory (NETL) of the U. S. Department of Energy. Operation of the CFB demonstrated that high density conditions can be achieved throughout the entire riser with sufficiently high solid fluxes in a riser taller than what has been previously reported in the literature. Tests were conducted on Geldart type B, 60 μm diameter, glass beads at two different gas velocities (5.1 and 7.8 m/s). The riser’s axial solids fraction profile provided distinct characteristics that enabled us to differentiate between dense suspension upflow (DSU) and core annular flow regimes. The apparent solids holdup in the riser exceeded 7% when operating in DSU. A fiber optic probe was used to measure particle velocities near the wall 8.5 m above the solids entry. These measurements did not always record upward particle velocities when in DSU conditions. A number of possible reasons are identified and discussed. Solid fluxes greater than 250 kg/m2-s for 5.1 m/s and 350 kg/m2-s at 7.8 m/s appeared to be sufficient to achieve DSU conditions. The trend in the measured particle velocities near the wall was also consistent with these transitions. The transition from core annular conditions to DSU operations depended upon both gas velocity and solids flux and was in good agreement with an existing correlation found in the literature.
The A˚bo Akademi Database: Fuel Characterization
Authors: Zevenhoven Maria , Yrjas Patrik , Backman Rainer , Skrifvars Bengt-Johan , Hupa Mikko
Published: October 2008

Standard fuel analyses have shown to provide insufficient information for proper prediction of ash deposition. Therefore, chemical fractionation, i.e. a stepwise leaching procedure, has been developed in the past. It has been used extensively for determination of the distribution of ash forming matter both in coal, peat, waste and biomass fuels. With this method fuels are leached in three steps, i.e. leached with water, ammonium acetate and hydro chloric acid respectively. This paper presents the fuel database developed at A˚bo Akademi University. The database contains over 100 fuels, i.e. coal, peat, bark, wood derived fuels, agricultural crops and waste fuels. Standard fuel analyses, stepwise leaching results, thermodynamic melting calculations and SEM pictures are available for the majority of these fuels. For 11 fuels also trace metals were analyzed.
Chlorine in Deposits During Co-Firing of Biomass, Peat, and Coal in a Full-Scale CFBC Boiler
Authors: Yrjas Patrik , Skrifvars Bengt-Johan , Hupa Mikko , Roppo Juha , Nylund Marko , Vainikka Pasi
Published: October 2008

Co-combustion of coal with biomass or firing biomass alone is increasingly used as a first step to meet the Finnish commitments under the Kyoto agreement. Fluidized bed combustors are commonly used when co-firing, however, even if FBC’s have a wide tolerance for different fuel qualities, co-combustion of biomass or firing biomass alone may lead to unwanted ash-related problems. A deposit measurement campaign was done, at the 550 MWth biofuelled CFB in Jakobstad, Finland. During the campaign a total of 16 different fuel blends were burned. The deposits were sampled with air-cooled probes with detachable rings. The deposits were sampled at two different locations, one where the flue gas temperature was about 730°C (probe surface temp. 540°C) and the second where the flue gas temperature was about 530°C (probe surface temp. 350°C). From every deposit sample three elemental analyses were done — one from the wind side, one from the lee side, and one from an angle of about 50° from the wind side. The analyses were done with a SEM/EDX analyzer. The fuels used during the measurement campaigns were sampled and analyzed. In addition to proximate and ultimate fuel analysis so called fuel fractionation was applied. The fractionation method is based on selective leaching by water, ammonium acetate, and hydrochloric acid, consecutively. After each leaching step the solutions are analyzed for the most important elements. The method can be used to determine how the elements are bound in the fuel and how they may behave during combustion. The analysis results from the measurement campaign and from the advanced fuel analysis were combined and are reported in this paper, with emphasis on the fate of chlorine.
Factors Affecting Primary Fragmentation During Combustion of Serbian Coals
Authors: Paprika Milijana J. , Komatina Mirko M. , Winter Franz , Dakic Dragoljub V.
Published: October 2008

A two-phase experimental method to establish primary fragmentation behavior of Serbian lignites is presented. The first phase was fragmentation quantification, and second was measuring temperature response of center of coal particle during devolatilization. A dynamic simulation model gave thermal properties of coal used. The influence of various factors such as original size of coal, bed temperature, and coal rank on fragmentation is presented and discussed. A new factor, ratio of coal volatile content and coal thermal diffusivity, as representative of heat and mass transfer processes inside of coal particle, has been introduced and its influence on fragmentation index was analyzed.
Effect of Dilute and Dense Phase Conditions on Bed-to-Wall Heat Transfer Mechanism in the Riser Column of a Circulating Fluidized Bed Combustor
Authors: Nirmal Vijay G. , Reddy B. V.
Published: October 2008

The bed-to-wall heat transfer in the riser column of a circulating fluidized bed (CFB) combustor depends on the contributions of particle convection, gas convection and radiation heat transfer. The percentage contribution of each of these components depends on the operating conditions i.e., dilute and dense phase bed conditions and bed temperature in the riser column. In the present paper, the contribution of particle convection, gas convection and radiation heat transfer components with operating conditions is estimated using the cluster renewal mechanistic model. The present results contribute some fundamental information on bed-to-wall heat transfer mechanism under dilute and dense phase conditions with bed temperature. The results will further aid for better understanding of heat transfer mechanism to water-wall surfaces in the upper region of the riser column.
Co-Combustion of Coal With RDF and Biomass: Prevention of Chlorine Deposition by Using Coal Ash Alkali Absorption Ability
Authors: Silvennoinen Jaani , Roppo Juha , Nurminen Riku-Ville , Aho Martti , Vainikka Pasi , Ferrer Eduardo
Published: October 2008

The co-combustion of fossil fuels with CO2-neutral fuels is an attractive way both to decrease CO2 emissions in energy production and to use fuel synergies which decrease each other’s undesirable properties. This paper presents a new approach to understand and predict the chlorine deposition tendency in the co-combustion of coal with biomass and RDF. This novel approach combines the results from deposit analysis with flue gas emission measurement and advanced fuel characterization methods. The experiments were carried out in a 0.1 MW circulating fluidized bed reactor. Two different types of bituminous coal (South African and Polish) were co-fired with RDF, demolition wood and bark. The traditional way to predict risk for chlorine deposition, the fuel S/Cl molar ratio, and the safe limit molar ratio > 4 for biofuels were shown to be inadequate. The mineral kaolinite in coal ash was found to be able to capture alkalis and, in most cases, more effectively than sulphur compounds. The alkali capture capability of coal sulphur is quickly consumed due to reactions with calcium compounds. Furthermore, the ability of SO2 to sulfate alkali chlorides were found to be weaker than presented in the literature. Thus in many cases it is only kaolinite that keeps Cl away from the deposit. New index to predict chlorine deposition tendency were introduced: the reactive (Al+Si)/fuel Cl molar ratio. The results showed a good correlation between the chlorine concentration in the deposit and the new index. The reactive (Al+Si)/fuel Cl molar ratio higher than 8–10 was found to prevent chlorine to deposit. The new approach for better understanding and preventing of chlorine deposition promotes the co-combustion of coal with biomass and RDF by introducing new synergy benefits.
Reactivation of Fluidized Bed Combustor Ashes: Economic Evaluation and Implementation
Authors: Trass O. , Gandolfi E. A. J. , Anthony E. J. , Maryamchik M.
Published: October 2008

When high-sulfur-content coal or coke is used as fuel in fluidized bed combustors, a large excess of limestone or dolomite must be added for good SOx capture. All of the limestone is calcined but only 30–40% is actually sulfated. The resultant ashes are difficult to dispose of because of the free calcium oxide. These ashes can be reactivated for further SOx capture. A proposed, economic process involves wet grinding of the ashes with sufficient excess water to allow both complete hydration and good grinding conditions. To prevent cementitious solidification of the wet product, it is then mixed with selected dry materials, for example fine coal, to absorb the excess water. Wet waste coal fines or sludges may also be used, then both to provide the water and prevent solidification. The product is then granulated with the cementitious reactions providing a binder for the granules. Good results with large additional SOx capture have been observed both in a small pilot-sized CFBC and during a 54-hour utility boiler test in a 35 MWt boiler. Calcium utilization was nearly doubled, with significant reduction of CO2 emissions. Based on the test results, quick equity payback is expected with savings from reduced limestone purchase and ash disposal costs. In collaboration with The Babcock and Wilcox Company (B&W), a long test program at the Southern Illinois University in Carbondale, IL is planned.
Partitioning of Mercury and Other Trace Elements From Coal and Waste-Derived Fuels During Fluidised Bed Pyrolysis
Authors: Zevenhoven Ron , Savolahti Jaakko , Verhoeven Liselotte , Saeed Loay
Published: October 2008

The potential releases of toxic trace elements such as mercury, lead and arsenic call for emission control during fluidised bed (FB) combustion, pyrolysis or gasification of waste-derived fuels and fossil fuels. Control measures for sulphur oxides, nitrogen oxides and particulates effectively remove many other pollutants from the exhaust gases as well, but mercury and several other trace elements are already problematic and this situation will only worsen with time. Besides the effect of temperature, gas atmosphere and halogens, the presence of other species, for example metal oxides, have an effect on under which conditions and in what form trace elements are released from fuels. Understanding the events of trace elements release from solid fuels during the pyrolysis or char combustion stage will provide a key to manipulating their partitioning and controlling their emissions. Pyrolysis experiments were made with coal, sewage sludge and automotive shredder residue (ASR) in a two-stage fluidised bed combustion (FBC) facility. An Ontario Hydro measurement train plus an additional sampling system were used to measure mercury and around fifteen other trace elements in the gases, and also char samples were taken and analysed. Results from these experiments are presented. An issue that is addressed explicitely is the bed material, which may be contaminated with significant amounts of toxic trace elements.
Simultaneous CO2 and SO2 Capture at Fluidized Bed Combustion Temperatures
Authors: Sun P. , Grace J. R. , Lim C. J. , Anthony E. J.
Published: October 2008

Simultaneous carbonation and sulphation have been investigated to simulate non-calcining conditions at FBC temperatures (750–850°C) using an atmospheric-pressure thermogravimetric reactor (TGR) with up to 80% CO2 in the gas stream to extend the pressure range of applicability of the results. This investigation was undertaken to provide insight on simultaneous carbonation and sulphation and to provide knowledge relevant to FBC, including PFBC, operations. Two calcium-based sorbents (one limestone and one dolomite) were tested with particles of diameter 212–250 μm and 500–600 to determine the effects of operating conditions such as temperature, CO2 and SO2 concentrations, particle size and reaction time on the sorbent performance. SO2 was found to impede CO2 capture; on the other hand, CO2 enhanced the capture of SO2. The calcination rate was also observed to decrease as a result of the presence of the sulphate layer.
Design Theory of Circulating Fluidized Bed Boilers
Authors: Yue Guangxi , Lu Junfu , Zhang Hai , Yang Hairui , Zhang Jiansheng , Liu Qing , Li Zheng , Joos Eric , Jaud Philippe
Published: October 2008

Studies on circulating fluidized bed (CFB) boilers have being conducted at the Tsinghua University (TH) for about two decades and much of works are done to link the fundamentals with practical application. A full set of design theory was developed and some key elements of this theory are presented in this paper. First, a classification of state of the solid-gas two-phase flow in CFB boiler is given. TH’s studies validated that a CFB boiler can be generally described as the superposition of a fast bed in the upper part with a bubbling bed or turbulent bed in the bottom part. A concept model of material balance for the open system of CFB boiler was developed and later improved as a more comprehensive 1-D model taking ash formation, particle attrition and segregation in bed into account. Some results of the models are discussed. Then the concept of State Specification of a CFB boiler is defined and discussed. The State Specification is regarded as the first step to design a CFB and a base to classify different style of CFB boiler technologies for various CFB boiler manufacturers. The State Specification adopted by major CFB boiler makers is summarized and associated importance issues are addressed. The heat transfer model originally developed by Leckner and his coworkers is adopted and improved. It is further calibrated with experimental data obtained on the commercial CFB boiler measurements. The principle, improvements and application of the model are introduced. Some special tools developed for heat transfer field test are also given. Also, combustion behaviors of char and volatile content are studied, and the combustion difference between a CFB boiler and a bubbling bed is analyzed. The influence of volatile content and size distribution is discussed. The concept of vertical distribution of combustion and heat in CFB boiler furnace is introduced and discussed as well. In the last, the suggested design theory of CFB boiler is summarized.
Numerical Simulation of the Combustion Behavior of Different Biomasses in a Bubbling Fluidized Bed Boiler
Authors: Mueller Christian , Brink Anders , Hupa Mikko
Published: October 2008

Solid fuels currently used for energy production in thermal power plants are characterized by a large variety ranging from different coals to biomasses and wastes. This manifold of fuels offers opportunities to the energy producers and nowadays many power plants do not fire single fuels but fuel mixtures. While this procedure may lead to overall economic and environmental advantages it is very demanding for the boiler operators to maintain boiler performance and availability and to meet emission limits. The development of mathematical models that are capable of predicting the combustion behavior of fuel mixtures and provide guidelines for operators and manufacturers has been a challenge over the last years. Since bubbling fluidized beds are frequently used for firing fuel mixtures and especially biomass mixtures, current CFD based BFB models, such as the A˚bo Akademi Furnace Model, have been used widely over the last years to predict emission tendencies and ash deposition behavior. However, due to the complexity of the processes during combustion of fuel mixtures and the combustion process in the bubbling fluidised bed itself, the models are characterized by strong simplifications. This is especially true for the description of the lower part of the furnace, the region of fuel intake and bubbling bed. Recently, the A˚bo Akademi Furnace Model has been extended by a more detailed description of the fuel conversion by considering the combustion of individual biomass particles and a first simplified approach describing heat and mass transfer processes between the bubbling bed and the freeboard. Both submodels guarantee a closed mass and energy balance over the bed-freeboard region. In the current study the new submodels have been used to investigate the combustion conditions in a 290 MW bubbling fluidized bed boiler firing peat and forest residue. Clear differences in the simulation results for the both fuels can be found with regard to the specific combustion characteristics, the location of the main combustion zone and the total heat generated during combustion.
Numerical Simulation of 3D Cavitating Turbulent Flow in Francis Turbine
Authors: Liu Shuhong , Wu Yulin , Luo Xianwu
Published: October 2008

Generally, cavitation occurs very easily when liquid local pressure in the Francis turbine falls below the saturated vapor pressure. It will injure the hydropower plant seriously by some problems, and has become one of the main concerns in the field of hydraulic machinery. In this paper, we present steady simulation on cavitation performed through the whole passage of a prototype Francis turbine with mixture model for multiphase flow, and full cavitation model for cavitation effect, from the commercial code of FLUENT 6.1. The simulation is conducted at several operating conditions, and the internal flow conditions and the critical cavitation number were obtained for each case. The simulation results indicate that the internal flow condition in the flow parts before the runner is not affected by the cavitating flow. The tendency of the cavitating flow in the runner and draft tube agrees well with the turbine empirical results, and the critical cavitation number has been quantitatively predicted.
Wake Flow Behind a Cylinder With a Detached Splitter Plate
Authors: Cheng Minter
Published: October 2008

Fluid flow across a bluff body can induce a series alternating vortices in the downstream flow field. The vortex flow can produce adverse effects on many engineering applications. A number of studies have shown that the wake splitter plate is one of the means to stabilize the vortex formation process. However, most of the previous studies are confined to cylinders with attached splitter plates. Very few studies investigate the effects of the spacing between the cylinder and the splitter plate on the formation of wake vortices. In the present study, the effects of the splitter plate length as well as the gap distance between the splitter plate and the cylinder on the wake flow behind a cylinder have been studied experimentally for low Reynolds number of 400. Both circular and square cylinders are studied in this research. Four splitter plates with different length, 1 ≤ L/D ≤ 4, have been used and a range of cylinder and splitter plate gap distance, 0 < G/D < 6, have been studied. By using flow visualization technique and hot-film anemometer measurement, detailed measurements of the velocity distribution, the vortex shedding frequency, the wake width, and the wake formation length are carried out in order to get a clear understanding of the flow interference behavior. The experimental results indicate that splitter plates alter the vortex formation process in the wake causing a decrease in vortex shedding frequency. The Strouhal number decreases with increasing the splitter plate length as well as the gap distance between the cylinder and the splitter plate. It is shown that a jump in Strouhal number occurs at G/D of 3 to 6. The jump is splitter plate length dependent, and generally the gap distance at which jump takes place increases as the splitter plate length increases.
Numerical Solutions of Single and Multiple Laminar Jets
Authors: Prasanna D. , Aung K.
Published: October 2008

Modern power plants discharge approximately 1.5 to 2kWhr of waste heat for every kWhr of electrical energy produced. Modern power plants discharge approximately 1.5 to 2kWhr of waste heat for every kWhr of electrical energy produced. Usually this heat is discharged to an adjacent water body which increases the water temperature near the outfall. In order to assess the ecological consequences of waste heat discharge one must first know the physical changes (temperature, velocity, salinity) induced by these discharges. It is with this later aspect, prediction of physical properties, that the current work is primarily concerned. Existing theoretical work on axisymmetric buoyant jets is confined to integral techniques developed by Morton in the early 1950’s. From these techniques only centerline velocities and temperatures can be calculated. Experimental data for this type of flow are essentially confined to centerline temperature measurements except for pure jet or plume data which constitute the extremes for a buoyant jet. The present work addresses the problem of developing a theoretical model for an axisymmetric laminar buoyant jet. The governing equations for an axisymmetric buoyant jet in rectangular co-ordinates are transformed into an orthogonal curvilinear co-ordinate system which moves along the length of the jet axis. The complete partial differential equations governing steady, incompressible laminar flow are solved in the new curvilinear co-ordinates using finite-difference techniques. This method is applicable to a much wider range of jet flows issued at arbitrary angles into quiescent or flowing ambience. This method is also applied to the case for multiple jets spaced by a finite distance apart. Results for the momentum jet, axial and radial distribution of velocity and temperature, show good agreement with published data.
Stability of a Pressure Compensated Circuit for an Inlet Throttle Pump
Authors: de Ojeda William , Herna´ndez Arturo
Published: October 2008

The stability of a pressure compensating circuit used to regulate the output pressure in a variable displacement radial piston pump is considered here. A throttle spool placed in the pump inlet controls the flow into the pump to match the flow requirement of the system. A pilot, solenoid operated, with a hydraulic main stage, pressure-regulating valve in the pump outlet regulates the system pressure. The discharge flow of the pressure regulator controls the inlet throttle spool position. Too large of a flow spill through the pressure regulator denoting excess pump flow results in flow and pressure build up in the inlet throttle control cavity so as to throttle the inlet flow. Hydraulic instability may arise if the control pressures applied to the inlet throttle device over restrict the inlet flow. Stability is jeopardized when the inlet and outlet flows are too closely matched. Excessive pressure build-up in the actuating volume acting over the inlet throttle piston yields a phase-shift between the piston motion and the system pressure response. This phase-shift clearly affects the stable operation of the control system Tightening the reaction between components in the system requires limiting the pressure acting over the inlet throttle control piston. Lower pressures are obtained by opening up the bleed orifice in the control circuit. Other design elements including the inlet throttle flow area vs. stroke schedule, main stage area gain and pilot geometry are considered. The advantages of an overland over a poppet design in the main stage are illustrated in the correction of a rolling instability. Experimental tests monitored torque, pressures in both the pilot and main stages of the valve, pressure at the pump outlet and in the system accumulator.
Confinement Effects on Air Cross-Flow Over an Elliptical Cylinder
Authors: Faruquee Zakir , Ting David S.-K. , Fartaj Amir
Published: October 2008

Cylinders in cross-flow encountered in engineering applications and in nature may or may not have confinement influence due to the presence and proximity of neighboring structures. On the other hand, fluid-structure experiments conducted in wind and water tunnels are typically restricted and affected by confinement. The confinement is usually expressed in terms of the blockage ratio, which is the cylinder/tunnel cross sectional area ratio. Therefore, a good understanding of the role of blockage is critical in further advancing the knowledge of fluid-structure interactions. The effect of blockage ratio on the fluid flow over a circular cylinder has been addressed by some authors, but, literature on elliptical cylinders is limited. In this study, fluid flow over an elliptical cylinder with axis ratios (minor to major axis ratio) of 1 (circular cylinder), 0.5 and 0.4 were investigated numerically, using control volume approach in FLUENT, at a Reynolds number (based on the hydraulic diameter) of 40. The flow was considered to be uniform at the entrance (40d and 15d upstream from the center of the cylinder for unbounded and bounded flow respectively) of the two dimensional computational domain, steady and parallel to the major axis of the cylinder. Blockage ratios of 0 (un-confined scenario), 0.07 and 0.12 were studied because of the availability of experimental data at these blockage ratios for the circular cylinder case. From the simulation output, the velocity distributions were analyzed in detail. The increase of blockage ratio increases the drag coefficient and reduces the wake size of the circular cylinder. On the other hand, the increase in blockage ratio increases the wake length of elliptical cylinders with (minor/major) axis ratios of 0.5 and 0.4. These results agree well with the available data in the literature.
Comparison of Hydrofoil Drag Reduction by Natural and Ventilated Partial Cavitation
Authors: Kopriva Jim , Arndt Roger E. A. , Wosnik Martin , Amromin Eduard
Published: October 2008

The theory of cavitation in an ideal fluid is utilized to design hydrofoils that have a significant increase of lift to drag ratio for a regime of partially cavitating flows. Our recently reported experiments with natural cavitation have confirmed the existence of such an increase within a certain range of cavitation number and angle of attack for the specially designed hydrofoil designated as OK-2003. For applications of such a design to engineering, it would be necessary to keep the cavitation number within this favorable range and ventilation looks to be the most promising tool for control of cavitating flows. Therefore, comparative water tunnel tests have been carried out for both natural and ventilated cavitation of the OK-2003. The general similarity between the two kinds of partial cavitation for the developed low-drag hydrofoil is proven. When validating theory with the aid of water tunnel experiments, a general issue of how to make a comparison between natural cavitation and ventilated cavitation was encountered. This issue is the difficulty to determine the pressure within partial cavities. During natural cavitation the cavity pressure can deviate from vapor pressure due to the effects of dissolved gas and possibly other water quality effects. Direct pressure measurements within the partial cavity have proved to be unstable due to the unsteadiness of the cavity. The unsteadiness effect becomes more dominant as cavitation number is increased and the cavity becomes smaller. There is a point where the measured cavity pressure becomes unusable. In the case of ventilated cavitation, the interaction of the airflow with the surface of relatively thin cavities can be significant. Finally, it was experimentally determined that different dynamics of cavity pulsation are inherent to natural and ventilated cavitation.
Diameter and Size Distributions of Poly (Lactide-Co-Glycolide) [PLG] Based Microspheres
Authors: Berchane Nader , Jebrail Farzaneh F. , Carson Kenneth H. , Rice-Ficht Allison C. , Andrews Malcolm J.
Published: October 2008

A quantitative study of Poly (lactide-co-glycolide) [PLG] microsphere size distribution is presented. A fluid mechanics based correlation for the average microsphere diameter is formulated based on the theory of emulsification in turbulent flow under non-coalescing conditions. Measured average diameters of PLG microspheres prepared at different stirring speeds are used to construct the correlation formula. This correlation is particularly useful to the pharmaceutical industry for predicting the size of the encapsulated microspheres used in controlled drug delivery prior to microsphere preparation.