Comparison of White Dwarf Mass Determinations

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_______________________________________________________________________________________________ Part 1. 天文热点1. 我国首个自主发射的火星探测器“天问一号”计划于何时入轨火星？ BA. 2020年12月底左右B. 2021年2月底左右C. 2021年5月中左右D. 2022年7月底左右2. 我国嫦娥五号探测器在今年何时发射升空？ CA. 12月24日B. 12月7日C. 11月24日D. 11月28日3. 2020年的诺贝尔物理学奖分别颁发给罗杰·彭罗斯、莱因哈特·根策尔和安德烈娅·盖兹。

其中彭罗斯获奖的理由是“发现黑洞形成是广义相对论的一个预言”，那根策尔和盖兹获奖的理由是？ AA. 发现银河系中心的超大质量致密天体B. 发现第一颗环绕类太阳恒星运动的系外行星C. 验证了引力波的存在D. 发现了一种测量宇宙大尺度结构的探针4. 以下哪所高校在今年正式成立天文系？ DA. 香港科技大学B. 贵州大学C. 中山大学D. 广州大学5. 今年7月，一颗“黑马”彗星亮度达到2等以上，吸引了很多爱好者拍摄。

这颗彗星是？CA. C/2019 Y4 (ATLAS)B. C/2020 F8 (SWAN)C. C/2020 F3（NEOWISE）D. C/2019 U6 (Lemmon)6. 关于今年6月21日的日环食，下列哪个地点位于环食带之外？ BA. 纳木错B. 武汉C. 厦门D. 泸州7. 今年4月24日是我国首个人造卫星“东方红一号”成功发射_______周年。

a r X i v :a s t r o -p h /0309287v 1 10 S e p 2003Astronomy &Astrophysics manuscript no.INTEGRAL32February 2,2008(DOI:will be inserted by hand later)JEM-X background models ⋆J.Huovelin,1S.Maisala,1J.Schultz,1N.J.Westergaard,2C.A.Oxborrow,2P.Kretschmar,3,4and N.Lund 21Observatory,P.O.B.14(Kopernikuksentie 1),FIN-00014University of Helsinki,Finland 2Danish Space Reasearch Institute,Juliane Maries Vej 30,DK 2100Copenhagen,Denmark 3Max-Planck-Institut f¨u r Extraterrestrische Physik,Giessenbachstrasse,85748Garching,Germany 4INTEGRAL Science Data Center,Chemin d’Ecogia 16,Versoix,Switzerland Received July 15,2003;accepted Abstract.Background and determination of its components for the JEM-X X-ray telescope on INTEGRAL are discussed.A part of the first background observations by JEM-X are analysed and results are compared to predictions.The observations are based on extensive imaging of background near the Crab Nebula on revolution 41of INTEGRAL.Total observing time used for the analysis was 216502s,with the average of 25cps of background for each of the two JEM-X telescopes.JEM-X1showed slightly higher average background intensity than JEM-X2.The detectors were stable during the long exposures,and weak orbital phase dependence in the background outside radiation belts was observed.The analysis yielded an average of 5cps for the di ffuse background,and 20cps for the instrument background.The instrument background was found highly dependent on position,both for spectral shape and intensity.Di ffuse background was enhanced in the central area of a detector,and it decreased radially towards the edge,with a clear vignetting e ffect for both JEM-X units.The instrument background was weakest in the central area of a detector and showed a steep increase at the very edges of both JEM-X detectors,with significant di fference in spatial signatures between JEM-X units.According to our modelling,instrument background dominates over di ffuse background in all positions and for all energies of JEM-X.Key words.X-ray background –X-ray data analysis –INTEGRAL satellite 1.Introduction Background of X-ray and γ-ray detectors for astronomy can generally be divided in two components,di ffuse sky back-ground and instrument background .JEM-X consists of two mechanically identical telescope units (JEM-X 1and 2),with a position sensitive xenon-filledMicrostrip Gas Chamber,a collimator,and a coded mask as anoptical element in each unit (see Lund et al.2003for moredetails).There are four internal radioactive sources for spectralcalibration in the collimator of each unit.The sky image is thus a result of mathematical processingof the mask shadow pattern on the position sensitive detectors(e.g.in’t Zand 1992;in’t Zand,Heise &Jager 1994).Di ffuse sky background enters the detectors via the apertureand is a ffected by the mask and the collimator,similarly to thedistinguishable sources in the sky.Instrument background in-cludes detector signal due to interactions between cosmic radi-2Huovelin et al.:JEM-X background modelsFig.1.Spatial distribution of the background.Upper panels:JEM-X1,lower panels,JEM-X2.Left panels:Instrument back-ground,Right panels:Diffuse background.The white rectan-gles denote the positions of the calibration sources,which havebeen excluded from our analysis.The collimator signature canbe seen as weak vertical and horizontal line structures in the shadowgrams.The broad vertical lines are due to dead anodes.Also some photon leak from the calibration sources is evident.The sharp and very narrow lines are graphical artifacts causedby the plotting routine.Total intensity of each shadowgram isnormalized to1.Table1.INTEGRAL background pointings during cycle41.45324.0+210850.17000053645.0+122415.14082553645.2+122546.44277553645.3+122543.564468Huovelin et al.:JEM-X background models3Table2.The normalization factors of the background con-tinuum components(10−3photons keV−1cm−2s−1at1keV). Mean and standard deviation of values derived from the six extraction regions at each radius are given.The energy range used in thefitting is4-33keV.Diffuse denotes diffuse sky back-ground,Flat denotes theflat continuum of the instrument back-ground.Note that the normalization is determined on the basis of source spectra from1R in R out JEM-X1JEM-X2pix pix Diffuse Flat Diffuse FlatThe diffuse background decreases towards the edges of the detector,as expected.The instrument background is stronger than expected,dominating the spectrum at all radii.The ten expected K-shell lines(from the109Cd and55Fe calibration sources,collimator(Mo),and detector gas(Xe))were detected close to their nominal positions.This implies that the energy scale is correctly determined.The previously unknown weak line near13keV turned out to be the uranium L-shell line.It most likely originates in the detector beryllium window.Near the edges of the detector,the background is highly nonuniform. Additional nonuniformity in the outer parts was introduced by photon leak from calibration sources,which could not be com-pletely eliminated.We also searched for possible dependence of background on the orbital phase of the observations.The spectrum varied with a range of approximately5%between three separate or-bital sections well outside radiation belts.The variation is sta-tistically significant but small.Also dependence on solar aspect angle and particle radiation level can be utilised in the JEM-X ISSW background modelling.Significant variations were not found.The variation in the solar aspect due to different point-ings was20◦,which is probably not sufficiently large for stud-ies of an effect on instrument background.Also,there was no proper indicator of particle radiation level on INTEGRAL available during our observations to search for a correlation.4.ConclusionsWe have analyzed a part of thefirst INTEGRAL background observations with JEM-X.Estimates of the spatial and spec-tral distributions are obtained for diffuse sky background and instrument background.The total background observed for JEM-X1was28cps,for JEM-X223cps,and25cps on the average.A part(∼1/5)of the excessively large background may be due to residual Crab Nebula emission in JEM-X data.According to XSPECfitting,the diffuse background was at maximum in the centre of the detector and it decreasedra-Fig.2.Four sample background spectra extracted from differ-ent parts of JEM-X2.At the sides of the detector,a blend of K-shell lines from the spacecraft structure is seen.Note also the prominent lines in spectrum extracted from the surround-ings of the calibrationsources.Fig.3.The background extraction regions.Units in both axes are pixels.All regions cover an equal area of the detector. Exclusion of calibration sources(not shown)reduces the ac-tual area of some regions.dially towards the edge,which is due to vignetting.There is also slight asymmmetry in the spatial distribution of the diffuse background,which is caused by a small angular misalignment of the detector plane.The count rate for diffuse background was approximately20%of the total background.The instrument background intensity and spectrum are highly position dependent,with a steep increase near the edges at all radial directions.Leakage of the radiative calibration sources causes residual line emission in the neighbourhood of4Huovelin et al.:JEM-X background modelsTable3.The lines detected from the background.Line ID is the element and transitions producing the line,E is the line en-ergy in keV(Thompson et al.,2001).Subscripts1and2denote the detectors JEM-X1and2,F is the largest line strength de-tected,¯F(N)mean of detected line strengths where N is the number of regions from which the line is detected(maximum is48regions/detector).The Mn/Fe line at6.45keV is a blend of Mn Kβ(6.49keV)and Fe Kα(6.40keV).Line strengths are given in10−3photons cm−2s−1.Note that the line strengths are determined on the basis of source spectra from1Line E F1¯F1(N)F2¯F2(N)Origin the source positions.The count rate for the instrument back-ground was approximately80%of the total background.The total background level varied with a range of approx-imately5%between different orbital sections.the variation is significant,but small.Also,it is impossible to say,what fraction of this,if any,is caused by the simultaneous variation of the so-lar aspect angle of the satellite,and the unknown variations of particle radiation level.We plan to separate these effects by the support of future background observations.Although our modelling is simple,and does not provide accurate absolute estimates of physical backgroundfluxes,it yields information which can be applied to the JEM-X analysis software to properly account for background contribution in spatially resolved spectral data.A thorough analysis of JEM-X background will be presented in a future paper. Acknowledgements.Authors from the Observatory,University of Helsinki acknowledge the Academy of Finland,TEKES,and the Finnish space research programme ANTARES forfinancial support in this research.J.Schultz is grateful for thefinancial support of the Wihuri Foundation.The Danish Space Research Institute acknowl-edges support given to the development of the JEM-X instrument from the PRODEX programme.ReferencesArnaud,K.A.,1996,Astronomical Data Analysis Software and Systems V,eds.Jacoby G.and Barnes J.,ASP Conf.Series V ol.101.Covault,C.E.,Grindlay,J.E.,Manandhar,R.P.,and Braga,J.,1991, IEEE Transact.Nucl.Sci.,V ol.38,No.2.Ferguson,C.,Barlow,E.J.,Bird,A.J.,et al.,2003A&A,this volume in’t Zand,J.,1992,Ph.D.thesis,SRON.in’t Zand,J.,Heise,J.,Jager,R.,1994,A&A288,665.Lund,.N.,Brandt,S.,Budtz-Joergensen,C.,et al.,2003,A&A,this volumeMarshall,F.E.,Boldt,E.A.,Holt,S.S.,et al.,1980,ApJ235,4 Oxborrow C.A.,Kretschmar,P.,Maisala,S.,Westergaard,N.J., Larsson,S.,2002,Instrument Specific Software for JEM-X: Architectural Design Document,DSRI homepage:www.dsri.dk Thompson, A.C.,Attwood, D.T.,Gullikson, E.M.,et al.,2001,“The X-ray data booklet”,2nd ed.,Lawrence Berkley National Laboratory,Univ.of California,available at / Westergaard,N.J.,Kretschmar,P.,Oxborrow,C.A.,et al.,2003,A& A,this volumeWillmore,A.P.,Bertram,D.,Watt,M.P.,et al.,1992,MNRAS258, 621。

Combustion and NO x emission characteristics of a retrofitted down-fired 660MW e utility boiler at different loadsZhengqi Li ⇑,Guangkui Liu,Qunyi Zhu,Zhichao Chen,Feng RenSchool of Energy Science and Engineering,Harbin Institute of Technology,92,West Dazhi Street,Harbin 150001,Chinaa r t i c l e i n f o Article history:Received 7September 2010Received in revised form 23November 2010Accepted 20January 2011Available online 12February 2011Keywords:Down-fired boiler RetrofitCarbon content in fly ash Thermal efficiencya b s t r a c tIndustrial experiments were performed for a retrofitted 660MW e full-scale down-fired boiler.Measure-ments of ignition of the primary air/fuel mixture flow,the gas temperature distribution of the furnace and the gas components in the furnace were conducted at loads of 660,550and 330MW e .With decreasing load,the gas temperature decreases and the ignition position of the primary coal/air flow becomes farther along the axis of the fuel-rich pipe in the burner region under the arches.The furnace temperature also decreases with decreasing load,as does the difference between the temperatures in the burning region and the lower position of the burnout region.With decreasing load,the exhaust gas temperature decreases from 129.8°C to 114.3°C,while NO x emissions decrease from 2448to 1610mg/m 3.All three loads result in low carbon content in fly ash and great boiler thermal efficiency higher than 92%.Com-pared with the case of 660MW e before retrofit,the exhaust gas temperature decreased from 136to 129.8°C,the carbon content in the fly ash decreased from 9.55%to 2.43%and the boiler efficiency increased from 84.54%to 93.66%.Ó2011Elsevier Ltd.All rights reserved.1.IntroductionReserves of anthracite and lean coal are abundant and globally distributed.The use of anthracite and lean coal,which have low-volatility contents,presents difficulties in ignition and burnout.More and more research has been conducted around the world to solve these problems.At present,down-fired combustion is widely applied in the power industry to consume low-volatility coals,and its main merit is that it achieves a high degree of burn-out by prolonging the residence time of pulverized-coal in the fur-nace.However,a practical down-fired boiler operation still suffers from the problems of high carbon content in the fly ash and poor flame stabilization at low load without oil support firing.Precise understanding of the behavior of char particles in pul-verized-coal combustion systems is critical in determining funda-mental processes that occur during heterogeneous bustion data obtained for full-scale equipment can give the combustion and NO x emission characteristics of real combustors,in particular the turbulent flow of industrial coal flames.As a re-sult,studies using full-scale equipment are highly desirable and a necessity.In the surrounds of wall-fired boilers,measurements are made of the local mean concentrations of O 2,CO,CO 2,and NO x ,gas temperatures,and char burnout through several observ-ing doors in the utility boilers [1–5].Experiments have also been performed for tangentially fired boilers [6–9].However,for down-fired boilers adopting Foster Wheeler technology,there has only been the work of Li et al.,who measured the gas temperature,gas species concentrations in the furnace and carbon content in the fly ash in a 300MW e boiler before and after retrofit [10–14].In the present study,in situ experiments were carried out for a 660MW e down-fired pulverized-coal boiler after retrofit;this unit has the maximum capacity of any currently used worldwide [15].Measurements of ignition of the primary air/fuel mixture flow,the gas temperature distribution of the furnace and the gas compo-nents in the furnace were made for this full-scale boiler at the rated,middle and half loads.The collected data were used to deter-mine the combustion and NO x emissions characteristics of the boi-ler at different loads.The results obtained from these experiments help solve similar problems and benefit the design and operation of 600MW e and 1000MW e down-fired boilers,and the data can be used to support theoretical and numerical calculations.2.The utility boilerThe investigated 660MW e boiler,having the largest capacity of any down-fired boiler in the world,was made by Foster Wheeler (FW)Corp.Fig.1a is a schematic diagram of the furnace.Arches di-vide the furnace into two:the lower furnace below the arches and the upper furnace above the arches.Originally,36cyclone burners were arranged on the arches to produce a W-shaped flame.The fuel-rich flow streaming from the cyclone nozzle is near the0306-2619/$-see front matter Ó2011Elsevier Ltd.All rights reserved.doi:10.1016/j.apenergy.2011.01.048Corresponding author.Tel.:+8645186418854;fax:+8645186412528.E-mail address:green@ (Z.Li).water-cooled wall while the fuel-leanflow jetting from the fuel-lean pipe faces the furnace center.Under the arches,there are three tiers of secondary air denoted D,E and F,all of which are fed into the furnace horizontally.Details of the specific structure of this kind of boiler can be found in the literature[10–13].During practical operation,the boiler suffers from great resis-tance and serious abrasion of the cyclone and high carbon content infly ash up to7–10%.Fuel-richflows form after the separation of the coal/airflows in the cyclones.The swirl intensities of fuel-rich flows decrease at the exits under the effect of the adjustable vane, which provides great resistance.The fuel-richflows with residual swirl have little rigidity and abrade the nozzles of the cyclones [16].The secondary air under the arches enters the furnace horizon-tally and mixes prematurely with the fuel-richflow.This reduces the down-forwardflow and space for combustion and decreases the temperature in the lower furnace,which causes a series of problems including the delayed ignition of pulverized-coal and high carbon content in thefly ash[13,17].There is fuel-leanflow with low momentum and shallow penetration depth near the furnace center, which readily shortens theflame and reduces upflow to the burnout region quickly.This is also a reason for the poor burnout degree of coal particles and high carbon content in thefly ash[18].In2009,Li et al.retrofitted the combustion system with many items.Fig.1b is a schematic diagram of the retrofitted combustion system.A fuel louver concentrator replaced the original cyclone as it has much lower resistance than the cyclone.The fuel/airflowfirst passes through the louver concentrator and separates into fuel-rich flow and fuel-leanflow.The fuel-richflow further divides into two and is injected into the furnace through the original primary air ducts facing the furnace center while the fuel-leanflow as vent air is injected into the furnace near the wall.The vent air ducts are con-trolled by valves.The horizontal secondary air is modified by the F-tier secondary air with an angle of declination of20°.In this retrofitted boiler,measurements were conducted at loads of660,550and330MW e with a constant opening of the vent air valves.3.Data acquisition methods and experimental conditionsThe formal in situ experiments were carried out in the2026tph down-fired pulverized-coal boiler to investigate certain aspects of the combustion process and NO x formation in the furnace.During the experiments,soot blowing and sewer bleeding were not per-mitted.The coal used in the experiments was a mixture of anthra-cite and lean coal.Sample characteristics of coals before and after retrofit are presented in Table1.The following parameters were measured:(1)The gas temper-ature of the furnace was measured with a leucoscope through observing doors in the front,rear and side walls.The layout of the observing doors is shown in Fig.2.The measurement error is 50°C.(2)The gas temperature of the furnace was measured with a nickel chromium–nickel silicon thermocouple through observing doors1,2and3.As shown in Fig.2,observing door1is in the air-flow zone of the tier D and E slots,observing door2is in the airflow zone of the tier F slots,and observing door3is above the arches. The end of the bare thermocouple was exposed in the furnace,so the temperature measured should be higher than the local gas temperature because of highflame radiation.However,because of radiation between the bare thermocouple and the water-cooled wall,and the close proximity of the two,the temperature mea-sured should be lower than the local gas temperature.The calcula-tions have indicated that in the region of highest temperature the ‘‘true’’temperature do not exceed the measured one by more than 8%[3,19].The thermocouples used were thoroughly checked be-fore leaving the factory,giving high confidence in the temperature measurement results.To minimize errors due to ash deposition,Table1Characteristics of the coal used in the experiments before and after retrofit.Quantity Before retrofit After retrofitProximate analysis,wt.%(as received)Volatile10.729.29Ash30.8431.51Moisture0.54 2.48Fixed carbon57.9056.72Net heating value(kJ/kg)23,53121,250Ultimate analysis,wt.%(as received)Carbon59.7059.48Hydrogen 2.95 2.52Oxygen 3.81 3.53Nitrogen0.820.83Sulfur 1.340.79Z.Li et al./Applied Energy88(2011)2400–24062401the thermocouples were frequently retracted from the furnace and, where necessary,any deposits were carefully removed.Moreover, the probes were replaced if there was any thermal distortion ob-served.(3)The primary air/flow temperature distribution was measured with the same thermocouple inserted parallel to the axis of the fuel-rich pipe,as shown in Fig.1.(4)Gas compositions were sampled using a water-cooled stainless steel probe2.5m in length for analysis of the local mean O2,CO and NO x concentrations.As shown in Fig.3,the probe comprised a centrally located10mm (inner diameter)tube,through which quenched samples were evacuated,surrounded by a tube for probe cooling.The probe was cleaned frequently by blowing high-pressure air through it to maintain a constant suction rate.The water-cooled probe was inserted into the furnace through observing doors1–3(see Fig.2).The gases withdrawn were analyzed online using a Testo 350M system.Theflue gas after the air heater was also analyzed online.Calibrations with standard mixtures including zero concen-trations were performed before each measurement session.The measurement error for O2and CO2concentrations was1%,while that for CO and NO x concentrations was50ppm.(5)Unburnt car-bon infly ash was determined by collectingfly ash using a particle-sampling device with constant suction speed.Values of the main operating parameters for the three operating loads are listed in Table2.4.Results and discussionFig.4shows the gas temperature distribution of the fuel-burn-ing zone along the cyclone axes of the burners;zero points were set at the tips of the burner nozzles in the furnace.With decreasing load,the gas temperature decreases and the ignition position of the primary coal/airflow becomes farther along the axis of the fuel-rich pipe in the burner region under the arches,especially as the load decreases from550to330MW e.For the rated load,the gas temperature rises rapidly as the measurement point extends to lower positions,exceeding1000°C at a position400mm from the end of the fuel-rich nozzle and exceeding1200°C at 1400mm.For the load of550MW e,the gas temperature is a little lower than that for the rated load in the burner region,exceeding 1000°C at a position800mm from the fuel-rich nozzle.For the half load,the gas temperature rises slowly and the temperature gradi-ent is lower than in the other two cases,barely reaching1000°C at 2400mm.The mass ratio of coal/air decreases as the load decreases,indi-cating that the concentration of pulverized-coal falls in the primary air and the momentum of coal/airflow decreases,resulting in aTable2Boiler operating conditions and measurement results.Quantity Before retrofit After retrofit660MW e660MW e550MW e330MW eTotalflux of the primary air(kg/s)121127110.574Temperature of the primaryair(°C)130124125114Totalflux of the secondaryair(kg/s)657620511396Temperature of thesecondary air(°C)398405397362 Coal feeding rate(ton/h)268.6282.7244.6147.7 O2at the furnace exit(dryvolume%)3.35 2.58 2.02 5.31O2influe gases(dry volume%)4.76 3.99 3.047.27NO x influe gas(mg/m3at6%O2dry)1181244821631610Carbon infly ash(%)9.55 2.43 6.02 3.24 Exhaust gas temperature(°C)136129.8119.3114.3Thermal efficiency of the boiler(%)84.5493.6692.5992.862402Z.Li et al./Applied Energy88(2011)2400–2406shallower penetration depth and shorter residence time of the pri-mary coal/airflow in the lower furnace.For these reasons,the gas temperature measured along the axis direction of the fuel-rich pipe decreases and the ignition of the primary coal/airflow in the bur-ner region under the arches is farther along the axis.The mass ratio of coal/air is defined to indicate the coal concen-tration in the primary air.As the load falls from660to330MW e, the mass ratio of coal/air drops from0.618to0.554,which indi-cates an obvious decrease in the coal concentration;thus,the heat and time needed for coal ignition increase.This is one of the rea-sons for the low gas temperature in the burner region and far igni-tion position of the coal particles at half load.For the load of 550MW e,the mass ratio of coal/air is0.615,which is little different to that for the rated load.Furthermore,the quantity of primary air and coal feed rate both decrease as the load decreases,as does the momentum of coal/airflow,leading to shallow penetration of the pulverized-coal in the lower furnace.The short residence times then lower the overall temperature of the burner zone to the ex-tent that the fuel-richflow cannot obtain sufficient heat from the recirculating up-flowing gases(see Fig.1).Fig.5presents furnace gas temperature variations as measured using the thermocouple inserted through observing doors1–3.On the whole,the gas temperature measured through observing doors 1–3decreases with decreasing load.In the case of the temperature distribution measured through observing door1,the gas tempera-turefirst increases and then decreases because some of the high-temperature gas recirculates into the near-wall zone.As the measurement points move deeper into the furnace,the measured temperature begins to fall as the thermocouple enters the fuel-rich flow.At the rated load,the gas temperature is steadily around 1000°C at points further than1400mm,while for the load of 550MW e,the measured temperature is a little lower.At the same position but for a load of330MW e,the temperature falls below 700°C gradually,indicating that the fuel-richflow has not ignited. This temperature sequence is the same as that for the temperature gradients of the fuel-richflow mentioned above,indicating that ignition conditions worsen as the load decreases.In the cases of measuring the temperature through observing doors2and3,once the temperature reached1250°C,no further measurements were taken at deeper positions for the simple rea-son of protecting the thermocouple from burnout.From the rela-tively limited temperature distribution,Fig.5shows that in the zone near observing door2,the temperature rises rapidly at loads of660and550MW e,and at400mm,temperatures already exceed 1250°C.This indicates that coal burns more intensely in the lower furnace in both cases.At the load of330MW e,the temperature rises to nearly1200°C at the position1800mm from the side wall, increasing more slowly than in the other two cases.As illustrated above for the gas temperature in the burner region,the momentum of coal/airflow at half load is low,which results in a shallow pen-etration depth and short residence time of the primary coal/air flow and low furnace temperature in the lower furnace.Further-more,the F-tier air accounts for a large proportion of the secondary air entering the furnace under the arches;therefore,much of the air does not take part in combustion immediately,which decreases the temperature in the region near observing door2.In the zones near observing door3above the arches,the temperature increases to more than1100°C at a load of330MW e,which is just a little lower than the temperature near observing door2.This can be ex-plained by the delay in the fuel-richflow ignition causing much of the fuel to burn in the upper furnace.Fig.6presents furnace temperature variations measured through observing doors for the three loads.The furnace tempera-tures shown in Fig.6are averages of values measured through observing doors at the same level.In the three cases,gas tempera-ture peaks are all in the lower furnace,and the distances from the peak positions to the exit of the furnace are relatively large.Resi-dence times for coal burning in the higher-temperature zone are thus longer,which favors fuel burnout.The difference between temperatures in the burning region and the lower position of the burnout region decreases as the load decreases,and is just35°C at a load of330MW e.This is because the momentum of coal/air flow decreases,which results in a shallower penetration depth and shorter residence time of the primary coal/airflow in the lower furnace;the ignition position becomes farther from the fuel-rich nozzle,and more pulverized-coal combusts completely in the upper furnace.Moreover,the temperature in the furnace decreases with the decreasing load.The reason for this is that the total heatZ.Li et al./Applied Energy88(2011)2400–24062403provided to the furnace also falls with decreasing load,decreasing the furnace temperature.Fig.7shows the variations in gas species concentrations in the zones near observing doors1–3.Gas species concentration mea-surements begin400mm from the side wall to avoid the effect of an air leak.In all three cases,the O2concentration sequence is C(door1)>C(door2)>C(door3),which describes well theflow of coal as illustrated in Fig.1b.Fuel-richflowsfirst inject down-ward into the furnace and then reverse their direction upwards in the down-fired boiler.The coal/airflows pass sequentially through the observation doors1,2,and3and O2is consumed con-tinuously along the path of theflame.Thus,in each case,the O2 concentration in the zone near observation door1was the highest, that in the zone near observation door2was intermediate and that near observation door3was the lowest.In the airflowflow zone of the D and E tiers and in the zone near the wall,a great quantity of hot gas lowers the measured O2con-centration.When the measurement points are deeper and closer to the fuel-richflow,the measured value increases.O2concentra-tions near observing door1increase with decreasing load,espe-cially from550to330MW e.For loads of660and550MW e,at the position1800mm from the wall,O2concentrations are nearly 14%,suggesting that the fuel-richflow is beginning to react and a certain amount of O2is being consumed.For the load of 330MW e,however,O2concentrations are higher than18%,indi-cating that the fuel-richflow has not ignited,which agrees with the conclusion drawn from the temperature analysis.In the near-wall zone of observing doors2and3,O2concentrations at half load are higher than in the other two cases,even as high as8%in the near-wall zone of observing door3.At all three loads,CO concentrations strictly rule contrary to O2 concentrations.In the zones near observing doors1–3,the higher the O2concentration,the lower the CO concentration.In the air-flowflow zone of the D and E tiers,CO concentrations decrease with decreasing load because of the delayed ignition of the fuel/ airflow.In the near-wall zone of observing doors2and3,CO con-centrations in the case of the half load are lower than in the other two cases because of the highly oxidizing atmosphere in the furnace.2404Z.Li et al./Applied Energy88(2011)2400–2406In the zone near port1at loads of660and550MW e,volatiles are released and form a certain amount of NO x.However,biased combustion restricts the formation of NO x,and hence,NO x concen-tration here is only645mg/m3and630mg/m3respectively at a position1800mm from the wall.However,for the load of 330MW e,the NO x concentration is as high as3706mg/m3at the same position,because the high O2concentrations form the strong oxidative atmosphere near the observing door1.Moreover,the fuel/airflows with weak rigidity mix with the fuel gas quickly in the furnace;these are the reason for the high NO x concentration. Compared with those measured through observing door1,NO x concentrations for660and550MW e operation increase rapidly in the airflow zone of observing door2because fuel–NO forms con-stantly with further combustion of the pulverized-coal and ther-mal NO increasingly forms under the high-temperature condition in the lower furnace.The NO x concentration at a load of 550MW e is lower than that for the rated load because of the re-duced fuel–NO formation with a lower coal feed rate and the low-er-furnace temperature in this zone.In the case of330MW e,NO x concentrations decrease sharply,because the reducing atmosphere become higher with a great amount of O2consuming.In the zone near observing door3,the NO x concentration decreases with decreasing load.Table2lists results obtained for theflue gas after the air hea-ter.It is obvious that as the load decreases,the exhaust gas tem-perature decreases from129.8°C to114.3°C,while NO x emissions decrease from2448to1610mg/m3.Overall,the NO x content in theflue gas at the three loads was at high levels, exceeding the proposed Chinese emission standard for anthracite (1100mg/Nm3).To decrease these emissions,further technolo-gies,such as over-fire air[15,20],selective catalytic reduction and selective non-catalytic reduction[21,22]should be taken into account.All three loads result in low carbon content in thefly ash and great boiler thermal efficiency higher than92%.The O2 concentrations at the furnace exit when operating at660and 550MW e were2.58%and2.02%,respectively,with the small dif-ference between the two caused by thefluctuation of air supply during operation.The carbon in thefly ash at a load of550MW e was6.02%,which is higher than that at660MW e because the lower-furnace temperature was lower by as much as50°C than the temperature at the higher operating load.To enhance heat absorption in the reheater and to ensure sufficient temperature of the reheat steam,the method of increasing the excess air coef-ficient is commonly adopted when operated at330MW e.Thus, the O2concentration at the furnace exit at330MW e was5.31%, much higher than that at660or550MW e.Despite the high O2 concentration,the quantity of air supplied,including both pri-mary and secondary air,was low for the330MW e case,being only63%of the quantity supplied at660MW e and76%of the quantity supplied at660MW e,as shown in Table2.Therefore, the mean velocity of the fuel gas was lower and the residence time of pulverized-coal was longer in the furnace.These factors explain the high degree of burnout of pulverized-coal and low carbon content in thefly ash.Table2also shows the main operating parameters and mea-surement results at a load of660MW e before the combustion sys-tem retrofit.After retrofit,the exhaust gas temperature decreased from136to129.8°C,the carbon content in thefly ash decreased from9.55%to 2.43%and the boiler efficiency increased from 84.54%to93.66%.However,the NO x content in theflue gas (O2=6%)was higher than before the retrofit,because premature ignition of the pulverized-coal prolonged the residence time of coal particles in the lower furnace with high temperature.Furthermore, the O2concentration in theflue gas decreased from4.76%to3.99%, which increased the NO x concentration after conversion of O2at 6%.5.ConclusionIndustrial experiments were performed in a retrofitted 660MW e full-scale down-fired boiler.Measurements of ignition of the primary air/fuel mixtureflow,gas temperature distribution of the furnace and the gas components in the furnace were con-ducted at loads of660,550and330MW e.The results were as follows.With decreasing load,the gas temperature decreases and the ignition position of the primary coal/airflow becomes farther along the axis of the fuel-rich pipe in the burner region under the arches, especially as the load decreases from550to330MW e.With decreasing load,the gas temperature measured through observing doors1–3decreases,the ignition conditions worsen. Moreover,the combustion intensity of pulverized-coal reduces in the lower furnace.The furnace temperature also decreases with decreasing load,as does the difference between temperatures in the burning region and in the lower position of the burnout region.O2concentrations near observing door1increase with decreas-ing load,while O2concentrations at half load are higher than in the other two cases in the near-wall zone of observing doors2and3. CO concentrations strictly rule contrary to O2concentrations.The higher the O2concentration,the lower the CO concentration near observing doors1–3.NO x concentrations(O2=6%)near observing door1were low at loads of660and550MW e than at the load of 330MW pared with those measured through observing door 1,NO x concentrations for660and550MW e operation increase rapidly in the airflow zone of observing door2while NO x concen-trations at the load of330MW e decrease.In the zone near observ-ing door3,the NO x concentration decreases with decreasing load.With decreasing load,the exhaust gas temperature decreases from129.8°C to114.3°C,while NO x emissions decrease from 2448to1610mg/m3.All three loads result in low carbon content in thefly ash and great boiler thermal efficiency higher than92%.Compared with the case of660MW e before retrofit,the exhaust gas temperature decreased from136to129.8°C,the carbon con-tent in thefly ash decreased from9.55%to2.43%and the boiler efficiency increased from84.54%to93.66%.AcknowledgmentThis work was sponsored by the Hi-Tech Research and Development Program of China(863Program)(Contract No. 2006AA05Z321).References[1]Li ZQ,Jing JP,Liu GK,Chen ZC,Liu CL.Measurement of gas species,temper-atures,char burnout,and wall heatfluxes in a200-MWe lignite-fired boiler at different loads.Appl Energy2010;87:1217–30.[2]Li ZQ,Jing JP,Chen ZC,Ren F,Xu B,Wei HD,et bustion characteristicsand NO x emissions of enhanced ignition-dual register and central-fuel-rich swirl burners in a300-MWe wall-fired pulverized coal utility bust Sci Technol2008;180:1370–94.[3]Costa M,Azevedo JLT,Carvalho bustion characteristics of a front-wall-fired pulverized coal300MW e utility bust Sci Technol 1997;129:277–93.[4]Costa M,Silva P,Azevedo JLT.Measurements of gas species,temperature,andchar burnout in a low-NO x pulverized coal-fired utility bust Sci Technol2003;175:271–89.[5]Costa M,Azevedo JLT.Experimental characterization of an industrialpulverized coal-fired furnace under deep staging bust Sci Technol2007;179:1923–35.[6]Kouprianova VI,Tanetsakunvatanab V.Optimization of excess air for theimprovement of environmental performance of a150MW boilerfired with Thai lignite.Appl Energy2003;74:445–53.[7]Li S,Xu TM,Hui SE,Wei XL.NO x emission and thermal efficiency of a300MW eutility boiler retrofitted by air staging.Appl Energy2009;86:1797–803.Z.Li et al./Applied Energy88(2011)2400–24062405[8]Toshikazu T,Hirofumi O,Dernjatinc P,Savolainenc K.Reducing the minimumload and NO x emissions for lignite-fired boiler by applying a stable-flame concept.Appl Energy2003;74:415–24.[9]Li ZQ,Yang LB,Qiu PH,Sun R,Chen LZ,Sun SZ.Experimental study of thecombustion efficiency and formation of NO x in industrial pulverized coal combustion.Int J Energy Res2004;28:511–20.[10]Li ZQ,Ren F,Zhang J,Zhang XH,Chen ZC,Chen LZ.Influence of vent air valveopening on combustion characteristics of a down-fired pulverized-coal 300MW e utility boiler.Fuel2007;86:245–62.[11]Ren F,Li ZQ,Jing JP,Zhang XH,Chen ZC,Zhang JW.Influence of the adjustablevane position on theflow and combustion characteristics of a down-fired pulverized-coal300MW e utility boiler.Fuel Process Technol2008;89: 1297–305.[12]Li ZQ,Ren F,Chen ZC,Wang JJ,Chen Z,Zhang JW.Influence of oil atomized aironflow and combustion characteristics in a300MW e down-fired –Pac J Chem Eng2010;5:488–96.[13]Li ZQ,Ren F,Chen ZC,Chen Z,Wang JJ.Influence of declivitous secondary air oncombustion characteristics of a down-fired300-MWe utility boiler.Fuel 2010;89:410–6.[14]Li ZQ,Ren F,Chen ZC,Liu GK,Xu ZX.Improved NO x emissions and combustioncharacteristics for a retrofitted down-fired300-MWe utility boiler.Environ Sci Technol2010;44:3926–31.[15]Garcia-Mallol JA,Steitz T,Chu CY,Jiang PZ.Ultra-low NO x advanced FW archfiring:central power station applications.In:2nd US China NO x and SO2 control workshop,Dalian;2005.[16]Zhang J.Experimental study and numerical simulation on separation char-acteristics of cyclone separator of downfired boiler.Harbin:School of Energy Science and Engineering,Harbin Institute of Technology;2006[in Chinese]. 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DESY02-096ISSN0418-9833 July2002Search for Excited Electrons at HERAH1CollaborationAbstractA search for excited electron(e∗)production is described in which the electroweak decayse∗→eγ,e∗→eZ and e∗→νW are considered.The data used correspond to anintegrated luminosity of120pb−1taken in e±p collisions from1994to2000with the H1detector at HERA at centre-of-mass energies of300and318GeV.No evidence for a signalis found.Mass dependent exclusion limits are derived for the ratio of the couplings to thecompositeness scale,f/Λ.These limits extend the excluded region to higher masses thanhas been possible in previous direct searches for excited electrons.To be submitted toPhysics Letters BC.Adloff33,V.Andreev24,B.Andrieu27,T.Anthonis4,A.Astvatsatourov35,A.Babaev23,J.B¨a hr35,P.Baranov24,E.Barrelet28,W.Bartel10,S.Baumgartner36,J.Becker37,M.Beckingham21,A.Beglarian34,O.Behnke13,A.Belousov24,Ch.Berger1,T.Berndt14,ot26,J.B¨o hme10,V.Boudry27,W.Braunschweig1,V.Brisson26,H.-B.Br¨o ker2,D.P.Brown10,D.Bruncko16,F.W.B¨u sser11,A.Bunyatyan12,34,A.Burrage18,G.Buschhorn25, L.Bystritskaya23,A.J.Campbell10,S.Caron1,F.Cassol-Brunner22,D.Clarke5,C.Collard4, J.G.Contreras7,41,Y.R.Coppens3,J.A.Coughlan5,M.-C.Cousinou22,B.E.Cox21,G.Cozzika9,J.Cvach29,J.B.Dainton18,W.D.Dau15,K.Daum33,39,M.Davidsson20,B.Delcourt26,N.Delerue22,R.Demirchyan34,A.De Roeck10,43,E.A.De Wolf4,C.Diaconu22,J.Dingfelder13,P.Dixon19,V.Dodonov12,J.D.Dowell3,A.Droutskoi23,A.Dubak25,C.Duprel2,G.Eckerlin10,D.Eckstein35,V.Efremenko23,S.Egli32,R.Eichler32, F.Eisele13,E.Eisenhandler19,M.Ellerbrock13,E.Elsen10,M.Erdmann10,40,e,W.Erdmann36, P.J.W.Faulkner3,L.Favart4,A.Fedotov23,R.Felst10,J.Ferencei10,S.Ferron27,M.Fleischer10,P.Fleischmann10,Y.H.Fleming3,G.Fl¨u gge2,A.Fomenko24,I.Foresti37,J.Form´a nek30,G.Franke10,G.Frising1,E.Gabathuler18,K.Gabathuler32,J.Garvey3,J.Gassner32,J.Gayler10,R.Gerhards10,C.Gerlich13,S.Ghazaryan4,34,L.Goerlich6,N.Gogitidze24,C.Grab36,V.Grabski34,H.Gr¨a ssler2,T.Greenshaw18,G.Grindhammer25, T.Hadig13,D.Haidt10,L.Hajduk6,J.Haller13,B.Heinemann18,G.Heinzelmann11,R.C.W.Henderson17,S.Hengstmann37,H.Henschel35,R.Heremans4,G.Herrera7,44,I.Herynek29,M.Hildebrandt37,M.Hilgers36,K.H.Hiller35,J.Hladk´y29,P.H¨o ting2,D.Hoffmann22,R.Horisberger32,A.Hovhannisyan34,S.Hurling10,M.Ibbotson21,C¸.˙Is¸sever7, M.Jacquet26,M.Jaffre26,L.Janauschek25,X.Janssen4,V.Jemanov11,L.J¨o nsson20,C.Johnson3,D.P.Johnson4,M.A.S.Jones18,H.Jung20,10,D.Kant19,M.Kapichine8,M.Karlsson20,O.Karschnick11,J.Katzy10,F.Keil14,N.Keller37,J.Kennedy18,I.R.Kenyon3, C.Kiesling25,P.Kjellberg20,M.Klein35,C.Kleinwort10,T.Kluge1,G.Knies10,B.Koblitz25, S.D.Kolya21,V.Korbel10,P.Kostka35,S.K.Kotelnikov24,R.Koutouev12,A.Koutov8,J.Kroseberg37,K.Kr¨u ger10,T.Kuhr11,mb3,ndon19,nge35,ˇs toviˇc ka35,30,ycock18,E.Lebailly26,A.Lebedev24,B.Leißner1,R.Lemrani10,V.Lendermann10,S.Levonian10,B.List36,E.Lobodzinska10,6,B.Lobodzinski6,10,A.Loginov23,N.Loktionova24,V.Lubimov23,S.L¨u ders37,D.L¨u ke7,10,L.Lytkin12,N.Malden21,E.Malinovski24,S.Mangano36,R.Maraˇc ek25,P.Marage4,J.Marks13,R.Marshall21,H.-U.Martyn1,J.Martyniak6,S.J.Maxfield18,D.Meer36,A.Mehta18,K.Meier14,A.B.Meyer11,H.Meyer33,J.Meyer10,S.Michine24,S.Mikocki6,stead18, S.Mohrdieck11,M.N.Mondragon7,F.Moreau27,A.Morozov8,J.V.Morris5,K.M¨u ller37, P.Mur´ın16,42,V.Nagovizin23,B.Naroska11,J.Naumann7,Th.Naumann35,P.R.Newman3, F.Niebergall11,C.Niebuhr10,O.Nix14,G.Nowak6,M.Nozicka30,B.Olivier10,J.E.Olsson10, D.Ozerov23,V.Panassik8,C.Pascaud26,G.D.Patel18,M.Peez22,E.Perez9,A.Petrukhin35, J.P.Phillips18,D.Pitzl10,R.P¨o schl26,I.Potachnikova12,B.Povh12,J.Rauschenberger11,P.Reimer29,B.Reisert25,C.Risler25,E.Rizvi3,P.Robmann37,R.Roosen4,A.Rostovtsev23, S.Rusakov24,K.Rybicki6,D.P.C.Sankey5,S.Sch¨a tzel13,J.Scheins10,F.-P.Schilling10,P.Schleper10,D.Schmidt33,D.Schmidt10,S.Schmidt25,S.Schmitt10,M.Schneider22,L.Schoeffel9,A.Sch¨o ning36,T.Sch¨o rner25,V.Schr¨o der10,H.-C.Schultz-Coulon7,C.Schwanenberger10,K.Sedl´a k29,F.Sefkow37,V.Shekelyan25,I.Sheviakov24,1L.N.Shtarkov24,Y.Sirois27,T.Sloan17,P.Smirnov24,Y.Soloviev24,D.South21,V.Spaskov8, A.Specka27,H.Spitzer11,R.Stamen7,B.Stella31,J.Stiewe14,I.Strauch10,U.Straumann37, S.Tchetchelnitski23,G.Thompson19,P.D.Thompson3,F.Tomasz14,D.Traynor19,P.Tru¨o l37, G.Tsipolitis10,38,I.Tsurin35,J.Turnau6,J.E.Turney19,E.Tzamariudaki25,A.Uraev23,M.Urban37,ik24,S.Valk´a r30,A.Valk´a rov´a30,C.Vall´e e22,P.Van Mechelen4,A.Vargas Trevino7,S.Vassiliev8,Y.Vazdik24,C.Veelken18,A.Vest1,A.Vichnevski8,K.Wacker7,J.Wagner10,R.Wallny37,B.Waugh21,G.Weber11,D.Wegener7,C.Werner13,N.Werner37, M.Wessels1,G.White17,S.Wiesand33,T.Wilksen10,M.Winde35,G.-G.Winter10,Ch.Wissing7,M.Wobisch10,E.-E.Woehrling3,E.W¨u nsch10,A.C.Wyatt21,J.ˇZ´aˇc ek30,J.Z´a leˇs´a k30,Z.Zhang26,A.Zhokin23,F.Zomer26,and M.zur Nedden251I.Physikalisches Institut der RWTH,Aachen,Germany a2III.Physikalisches Institut der RWTH,Aachen,Germany a3School of Physics and Space Research,University of Birmingham,Birmingham,UK b4Inter-University Institute for High Energies ULB-VUB,Brussels;Universiteit Antwerpen (UIA),Antwerpen;Belgium c5Rutherford Appleton Laboratory,Chilton,Didcot,UK b6Institute for Nuclear Physics,Cracow,Poland d7Institut f¨u r Physik,Universit¨a t Dortmund,Dortmund,Germany a8Joint Institute for Nuclear Research,Dubna,Russia9CEA,DSM/DAPNIA,CE-Saclay,Gif-sur-Yvette,France10DESY,Hamburg,Germany11Institut f¨u r Experimentalphysik,Universit¨a t Hamburg,Hamburg,Germany a12Max-Planck-Institut f¨u r Kernphysik,Heidelberg,Germany13Physikalisches Institut,Universit¨a t Heidelberg,Heidelberg,Germany a14Kirchhoff-Institut f¨u r Physik,Universit¨a t Heidelberg,Heidelberg,Germany a15Institut f¨u r experimentelle und Angewandte Physik,Universit¨a t Kiel,Kiel,Germany16Institute of Experimental Physics,Slovak Academy of Sciences,Koˇs ice,Slovak Republic e,f 17School of Physics and Chemistry,University of Lancaster,Lancaster,UK b18Department of Physics,University of Liverpool,Liverpool,UK b19Queen Mary and Westfield College,London,UK b20Physics Department,University of Lund,Lund,Sweden g21Physics Department,University of Manchester,Manchester,UK b22CPPM,CNRS/IN2P3-Univ Mediterranee,Marseille-France23Institute for Theoretical and Experimental Physics,Moscow,Russia l24Lebedev Physical Institute,Moscow,Russia e25Max-Planck-Institut f¨u r Physik,M¨u nchen,Germany26LAL,Universit´e de Paris-Sud,IN2P3-CNRS,Orsay,France27LPNHE,Ecole Polytechnique,IN2P3-CNRS,Palaiseau,France28LPNHE,Universit´e s Paris VI and VII,IN2P3-CNRS,Paris,France29Institute of Physics,Academy of Sciences of the Czech Republic,Praha,Czech Republic e,i 30Faculty of Mathematics and Physics,Charles University,Praha,Czech Republic e,i31Dipartimento di Fisica Universit`a di Roma Tre and INFN Roma3,Roma,Italy232Paul Scherrer Institut,Villigen,Switzerland33Fachbereich Physik,Bergische Universit¨a t Gesamthochschule Wuppertal,Wuppertal, Germany34Yerevan Physics Institute,Yerevan,Armenia35DESY,Zeuthen,Germany36Institut f¨u r Teilchenphysik,ETH,Z¨u rich,Switzerland j37Physik-Institut der Universit¨a t Z¨u rich,Z¨u rich,Switzerland j38Also at Physics Department,National Technical University,Zografou Campus,GR-15773 Athens,Greece39Also at Rechenzentrum,Bergische Universit¨a t Gesamthochschule Wuppertal,Germany40Also at Institut f¨u r Experimentelle Kernphysik,Universit¨a t Karlsruhe,Karlsruhe,Germany 41Also at Dept.Fis.Ap.CINVESTAV,M´e rida,Yucat´a n,M´e xico k42Also at University of P.J.ˇSaf´a rik,Koˇs ice,Slovak Republic43Also at CERN,Geneva,Switzerland44Also at Dept.Fis.CINVESTAV,M´e xico City,M´e xico ka Supported by the Bundesministerium f¨u r Bildung und Forschung,FRG,under contract numbers05H11GUA/1,05H11PAA/1,05H11PAB/9,05H11PEA/6,05H11VHA/7and 05H11VHB/5b Supported by the UK Particle Physics and Astronomy Research Council,and formerly by the UK Science and Engineering Research Councilc Supported by FNRS-FWO-Vlaanderen,IISN-IIKW and IWTd Partially Supported by the Polish State Committee for Scientific Research,grant no.2P0310318and SPUB/DESY/P03/DZ-1/99and by the German Bundesministerium f¨u r Bildung und Forschunge Supported by the Deutsche Forschungsgemeinschaftf Supported by VEGA SR grant no.2/1169/2001g Supported by the Swedish Natural Science Research Councili Supported by the Ministry of Education of the Czech Republic under the projectsINGO-LA116/2000and LN00A006,by GAUK grant no173/2000j Supported by the Swiss National Science Foundationk Supported by CONACyTl Partially Supported by Russian Foundation for Basic Research,grant no.00-15-965843Among the unexplained features of the Standard Model(SM)of particle physics is the exis-tence of three distinct generations of fermions and the hierarchy of their masses.One possible explanation for this is fermion substructure,with the constituents of the known fermions being strongly bound together by a new,as yet undiscovered force[1,2].A natural consequence of these models would be the existence of excited states of the known leptons and quarks.Assum-ing a compositeness scale in the TeV region,one would naively expect that the excited fermions have masses in the same energy region.However,the dynamics of the constituent level are un-known,so the lowest excited states could have masses of the order of only a few hundred GeV. Electron1-proton interactions at very high energies provide an excellent environment in which to search for excited fermions of thefirst generation.These excited electrons(e∗)could be singly produced through t-channelγand Z boson exchange.Their production cross-section and par-tial decay widths have been calculated using an effective Lagrangian[3,4]which depends on a compositeness mass scaleΛand on weight factors f and f′describing the relative coupling strengths of the excited lepton to the SU(2)L and U(1)Y gauge bosons,respectively.In this model the excited electron can decay to an electron or a neutrino via the radiation of a gauge boson(γ,W,Z)with branching ratios determined by the e∗mass and the coupling parameters f and f′.In most analyses[5–7]the assumption is made that these coupling parameters are of comparable strength and only the relationships f=+f′or f=−f′are considered.If a relationship between f and f′is assumed,the production cross-section and partial decay widths depend on two parameters only,namely the e∗mass and the ratio f/Λ.In this paper excited electrons are searched for in three samples of data taken by the H1 experiment from1994to2000with a total integrated luminosity of120pb−1.Thefirst sample consists of e+p data accumulated from1994to1997at positron and proton beam energies of27.5GeV and820GeV respectively,and corresponds to an integrated luminosity of37pb−1.A search for excited electrons using this sample of data has been previously published[8].The strategy for the selection of events has been modified from the procedures described in[8]to optimize the sensitivity to higher e∗masses.The two other samples were taken from1998 to2000with an electron or positron beam energy of27.5GeV and a proton beam energy of 920GeV.The integrated luminosities of the e−p and e+p samples are15pb−1and68pb−1, pared to previous H1results[8]the analysis presented here benefits from an increase in luminosity by a factor of more than three and an increase of the centre-of-mass energy from300GeV to318GeV.We search for all electroweak decays e∗→eγ,e∗→eZ and e∗→νW,considering the subsequent Z and W hadronic decay modes only.This leads tofinal states containing an electron and a photon,an electron and jets or jets with missing transverse energy induced by the neutrinos escaping from the detector.The Standard Model backgrounds which could mimic such signatures are neutral current Deep Inelastic Scattering(NC DIS),charged current Deep Inelastic Scattering(CC DIS),QED Compton scattering(or Wide Angle Bremsstrahlung W AB),photoproduction processes(γp)and lepton pair production via the two photon fusion process(γγ).The determination of the contribution of NC DIS processes is performed using two MonteCarlo samples which employ different models of QCD radiation.Thefirst was produced withthe DJANGO[9]event generator which includes QEDfirst order radiative corrections based onHERACLES[10].QCD radiation is implemented using ARIADNE[11]based on the ColourDipole Model[12].This sample,with an integrated luminosity of more than10times the exper-imental luminosity,is chosen to estimate the NC DIS contribution in the e∗→eγanalysis.The second sample was generated with the program RAPGAP[13],in which QEDfirst order radia-tive corrections are implemented as described above.RAPGAP includes the leading order QCDmatrix element and higher order radiative corrections are modelled by leading-log parton show-ers.This sample is used to determine potential NC DIS background in the e∗→νW֒→q¯q and e∗→eZ֒→q¯q searches,as RAPGAP describes better this particular phase space domain[8]. For both samples the parton densities in the proton are taken from the MRST[14]parametriza-tion which includes constraints from DIS measurements at HERA up to squared momentum transfers Q2=5000GeV2[15–18].Hadronisation is performed in the Lund string fragmenta-tion scheme using JETSET[19].The modelling of the CC DIS process is performed using the DJANGO program with MRST structure functions.While inelastic W AB is treated using the DJANGO generator,elastic and quasi-elastic W AB is simulated with the W ABGEN[20]event generator.Direct and resolvedγp processes including prompt photon production are generated with the PYTHIA[21]event generator.Finally theγγprocess is produced using the LPAIR generator[22].For the calculation of the e∗production cross-section and to determine the efficiencies,events have been generated with the COMPOS[23]generator based on the cross-section for-mulae given in reference[3]and the partial decay widths stated in reference[4].Initial stateradiation of a photon from the incoming electron is included.This generator uses the narrowwidth approximation(NW A)for the calculation of the production cross section and takes intoaccount the natural width for the e∗decay.For all values of f/Λrelevant to this analysis thisassumption is valid even at high e∗masses where the natural width of the e∗is of the order ofthe experimental resolution.To give an example,for M e∗=250GeV this resolution is equal to7GeV,10GeV,and12GeV for the eγ,eZ andνW decay modes,respectively.All Monte Carlo samples are subjected to a detailed simulation of the response of the H1detector.The detector components of the H1experiment[24]most relevant for this analysis arebriefly described in the following.Surrounding the interaction region is a system of drift andproportional chambers which covers the polar angle2range7◦<θ<176◦.The tracking sys-tem is surrounded by afinely segmented liquid argon(LAr)calorimeter covering the polar angle√range4◦<θ<154◦[25]with energy resolutions ofσE/E≃12%/E⊕2%for hadrons,obtained in test beam measurements[26,27].The tracking system and calorimeters are surrounded by a superconducting solenoid and an iron yoke instrumented with streamer tubes.Backgrounds not related to e+p or e−p collisions are rejected by requiring that a primary interaction vertex be reconstructed within±35cm of the nominal vertex position,by usingfilters based on the event topology and by requiring an event time which is consistent with the interaction time.Electromagnetic clusters are required to havemore than95%of their energy in the electromagnetic part of the calorimeter and to be isolated from other particles[28].They are further differentiated into electron and photon candidates using the tracking chambers.Jets with a minimum transverse momentum of5GeV are recon-structed from the hadronicfinal state using a cone algorithm adapted from the LUCELL schemein the JETSET package[19].Missing transverse energy(E misst )is reconstructed using the vec-tor sum of energy depositions in the calorimeter cells.The analysis presented in this paper is described extensively in[29].The e∗→eγchannel is characterized by two electromagnetic clusters in thefinal state. The main sources of background are the W AB process,NC DIS with photon radiation or a high energyπ0in a jet and the production of electron pairs viaγγfusion.Candidate events are selected with two electromagnetic clusters in the LAr calorimeter of transverse energy greater than20GeV and15GeV,respectively,and with a polar angle between0.1and2.2radians. The sum of the energies of the two clusters has to be greater than100GeV.If this sum is below 200GeV,the background is further suppressed by rejecting events with a total transverse energy of the two electromagnetic clusters lower than75GeV or with more than two tracks spatially associated to one of the clusters.The numbers of events passing the analysis cuts for the SM background processes and for the data in each of the three samples are given in Table1.About half of the background originates from NC DIS events with most of the remainder being due to W AB events.The efficiency for selecting the signal varies from85%for an e∗mass of150GeV to72%for an e∗mass of250GeV.As in all other channels the efficiencies are derived using samples of1000e∗events generated at different e∗masses.The various sources of systematic error are discussed later.Distributions of the invariant mass of the candidate electron-photon pairs of the three data samples together and for the SM expectation are shown in Fig.1a.Sample e−p920GeVChannel SM background SM background SM background e∗→eγ7.2±1.0±0.1 4.0±0.7±0.215.6±1.7±0.4 e∗→eZ֒→q¯q7.1±2.1±2.8 5.6±0.4±1.225.3±1.9±5.5 e∗→νW֒→q¯q 2.4±0.2±0.7 3.9±0.2±0.7 6.1±0.4±1.5 Table1:Number of candidate events observed in the three decay channels with the correspond-ing SM expectation and the uncertainties on the expectation(statistical and systematic error).The e∗→eZ֒→q¯q channel is characterized by an electromagnetic cluster with an associated track and two high transverse energy jets.The analysis for this channel uses a sample of events with at least two jets with a transverse energy above17GeV and16GeV,respectively,and an electron candidate with a transverse energy E e t>20GeV.These two jets and the electron must have polar angles smaller than2.2radians.Furthermore,to avoid possible double counting of events from the e∗→eγchannel,events with two electromagnetic clusters with transverse energies above10GeV and a total energy of the two clusters greater than100GeV are removed. The main SM contribution is NC DIS as photoproduction events do not yield a significant rate of electron candidates with large E e t.For20GeV<E e t<65GeV,a cut is made on the602468101214161820invariant mass (GeV)e v e n t s0510152025invariant mass (GeV)e v e n t s 012345678910invariant mass (GeV)e v e n t s Figure 1:Invariant mass spectra of candidate (a)eγpairs for the e ∗→eγanalysis,(b)elec-tron and two jets for the e ∗→eZ analysis and (c)neutrino and two jets for the e ∗→νW anal-ysis.Solid points correspond to the data and the histogram to the total expectation from different SM processes.electron polar angle.This ranges from θe <1.35for E e t =20GeV to θe <2.2radians for E e t =65GeV and depends linearly on E e t .The dijet invariant mass has to be in the range −15<M jj −M Z <7GeV .If there are more than two jets,the pair with invariant mass closest to the nominal Z boson mass is chosen as the Z candidate.The two jets chosen are ordered such that E jet 1t >E jet 2t .In many SM events the direction of jet 2is close to the proton direction.To ensure that this jet is well measured,an additional cut on its polar angle,θjet 2>0.2radians,7is applied if its transverse momentum is lower than30GeV.For an electron transverse energy 65GeV<E e t<85GeV two jets with an invariant mass M jj>M Z−30GeV are required. At very high transverse energy,E e t>85GeV,the contribution from NC DIS is very low and no further cuts on M jj are needed.The number of events which remain in the data after these cuts are summarized in Table1and compared with the expected SM contribution(mostly NC DIS events).The efficiency for selecting the signal varies from44%for an e∗mass of150GeV to 62%for an e∗mass of250GeV.Distributions of the invariant mass of the electron and the two jets are shown in Fig.1b for data and for the SM expectation.The e∗→νW֒→q¯q channel is characterized by two jets and missing transverse energy E miss t. The main background originates from CC DIS events with a moderate contribution from photo-production,whereas the NC DIS contribution is suppressed for large E misst .The analysis startsfrom a sample of events with at least two jets with transverse energies above17GeV and16GeV,missing transverse energy E misst >20GeV and no isolated electromagnetic cluster withtransverse energy above10GeV.The jets must have a polar angle below2.2radians.Jets in which the most energetic track enters the boundary region between two calorimeter modules and central jets(θ>0.5radians)are required to contain more than two tracks.This cut re-moves NC DIS events in which the scattered electron is misidentified as a jet.Only events with S=V apevents in a mass interval that varies with the width of the expected signal.At M e ∗=150GeV ,a width of the mass interval of 30GeV is chosen for the e ∗→eγdecay mode and 60GeV is chosen for the decay channels with two jets.Systematic uncertainties are taken into account as in [8].The limits on the product of the e ∗production cross-section and the decay branching ratio are shown in Fig.2.As stated in the introduction,most experiments give f/Λlimits under the assumptions f =+f ′and f =−f ′.The H1limits for each decay channel and after combi-nation of all decay channels are given as a function of the e ∗mass in Fig.3a,for the assumption f =+f ′.With this hypothesis the main contribution comes from the e ∗→eγchannel.The values of the limits for f/Λvary between 5×10−4and 10−2GeV −1for an e ∗mass ranging from 130GeV to 275GeV .These results improve significantly the previously published H1limits for e +p [8]collisions.The LEP experiments [5,6]and the ZEUS collaboration [7]have also reported on excited electron searches.Their limits are shown in Fig.3b.The LEP 2experiments have shown results in two mass domains.In direct searches for excited electrons limits up to a mass of about 200GeV are given.Above 200GeV their results are derived from indirect searches only.The H1limit extends the excluded region to higher masses than reached in previous direct searches.Figure 2:Upper limits at 95%confi-dence level on the product of the pro-duction cross-section σand the decaybranching ratio BR for excited elec-trons e ∗in the various electroweakdecay channels,e ∗→eγ(dashed line),e ∗→eZ ֒→q ¯q (dotted-dashedline)and e ∗→νW ֒→q ¯q (dotted line)as a function of the excited electronmass.The signal efficiencies used tocompute these limits have been deter-mined with events generated under theassumption f =+f ′.1010110e * mass (GeV)σ*B R (p b )More generally,limits on f/Λas a function of f ′/f are shown in Fig.4for three e ∗masses (150,200and 250GeV ).It is worth noting that excited electrons have vanishing electromag-netic coupling for f =−f ′.In this case the e ∗is produced through pure Z boson exchange.As a consequence the production cross-section for excited electrons at HERA is much smaller in the f =−f ′case than in the f =+f ′case.For e ∗masses between 150and 250GeV the ratio910101010e* mass (GeV)f /Λ (G e V -1)10101010e* mass (GeV)f /Λ (G e V -1)Figure 3:Exclusion limits on the coupling f/Λat 95%confidence level as a function of the mass of excited electrons with the assumption f =+f ′.(a)Limit for each decay channele ∗→eγ(dashed line),e ∗→eZ ֒→q ¯q (thick dotted-dashed line),e ∗→νW ֒→q ¯q (dotted line)and for the combination of the three channels (full line).It must be noted that a part of the data included in the present result were used to obtain the previous H1limit [8].(b)Comparison of this analysis with ZEUS results [7](dashed line)and LEP 2results on direct searches [5](dotted-dashed line)and on indirect searches [6](dotted line).of the cross-sections for f =+f ′and f =−f ′varies between 170and 900.For high e ∗masses and some values of the couplings,no limits are given because the natural width of the e ∗would become extremely large.In summary,a search for excited electron production was performed using all the e +p and e −p data accumulated by H1between 1994and 2000.No indication of a signal was found.New limits have been established as a function of the couplings and the excited electron mass for the conventional relationship between the couplings f =+f ′.The dependence of the f/Λlimit on the ratio f ′/f has been shown for the first time at HERA.The data presented here restrict excited electrons to higher mass values than has been possible previously in direct searches.We are grateful to the HERA machine group whose outstanding efforts have made and continue to make this experiment possible.We thank the engineers and technicians for their work in constructing and now maintaining the H1detector,our funding agencies for financial support,the DESY technical staff for continual assistance and the DESY directorate for the hospitality which they extend to the non-DESY members of the collaboration.10Figure 4:Exclusion lim-its on the coupling f/Λat 95%confidence level as a function of the ratio f ′/f 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