烟气脱硫喷淋塔本体设计与分析

收稿日期:　20030827作者简介:　郭毅(1981),女,河南邓州人,西安交通大学环境工程系在读硕士研究生,主要研读方向为大气环境及污染控制。

烟气脱硫喷淋塔本体设计与分析郭　毅,李荫堂,李　军(西安交通大学,陕西西安　710049)[摘　要]　对湿式烟气脱硫喷淋塔本体尺寸进行了分析,结合已有的工程设计经验,给出了确定喷淋塔本体尺寸的具体步骤,并讨论了设计应考虑的主要问题。

[关键词]　烟气脱硫;吸收塔;喷淋塔;传质单元数[中图分类号]X511 [文献标识码]B [文章编号]10023364(2004)01002903 湿式烟气脱硫中的石灰石法已成为我国火电厂烟气脱硫的首选工艺[1]。

在烟气脱硫系统中,吸收塔是核心装置。

近年来国内外的发展趋势表明,喷淋塔逐渐成为湿式烟气脱硫吸收塔的主流塔型[2]。

目前国内的火电厂烟气脱硫装置主要靠进口。

虽然这些装置大多技术成熟、运行稳定,但是投资和运行费用极高,以至于在我国目前情况下难以推广。

因此,主要装置的自行设计、制造是必要而迫切的。

在喷淋塔内,吸收剂浆液喷雾形成较大的气液接触界面;烟气与液体雾粒一般为逆流,雾粒降落过程中吸收SO 2,落入下部浆池,在池中氧化成石膏后排出。

烟气向上流动,经除雾装置脱除去其携带的雾粒后排出塔外。

根据双膜传质模型推导出的吸收塔高度计算公式,传质单元高度和传质单元数决定吸收区需要的高度;液气比影响操作线与平衡线的相对位置,进而影响设计塔高或脱硫效率。

本文对喷淋塔本体设计进行讨论。

1　喷淋塔高度设计的理论分析根据双膜理论的一系列假设,吸收过程的传质阻力简化为气液界面处被吸收组分(污染物)通过气膜与液膜的扩散阻力,由此控制气液两相的传质速率。

喷淋的作用是造成尽可能大的气液传质界面。

1.1操作线与液气比喷淋塔多为逆流操作,塔内烟气向上流动,吸收液雾滴向下降落。

以污染物在液相中的摩尔分数为x ,污染物在气相中的摩尔分数为y ,则X =液相中污染物的摩尔分数液相中吸收剂的摩尔分数=x1-x Y =气相中污染物的摩尔分数气相中载气组分的摩尔分数=y 1-y由物料衡算可得操作线方程:Y =L mG mX +Y 1-L mG m X 1(1)式中:L m 为吸收剂流量,kmol/(m 2・s );G m 为载气流量,kmol/(m 2・s )。

如图1线AB 所示,操作线斜率L m /G m (此处为无因次)即液气比。

操作线表示塔内任一高度上污染物的平均气相浓度Y 和液相浓度X 。

图1　操作线与平衡线根据操作线方程和相应的相平衡浓度y 3、x 3,计技术交流热力发电・2004(1)µ∼　© 1995-2004 Tsinghua Tongfang Optical Disc Co., Ltd. All rights reserved.算出各自对应的Y3、X3,即得到平衡线OC(图1)。

对于给定的烟气进出口污染物浓度、浆液进口浓度,最小液气比对应的操作线为A C,即理论上吸收液到出口能达到的最高浓度。

液气比增大,操作线斜率增大,操作线上端由C向B移动,浆液出口污染物浓度降低。

实际设计的液气比为最小液气比的1.2～1.5倍[2,3]。

对于给定塔高、烟气进口浓度,液气比增大,可使烟气出口污染物浓度降低,亦即脱硫率升高。

1.2　吸收区高度达到给定的吸收目标需要一定的塔高。

烟气量增大只需相应增大塔的横截面积。

通常烟气中SO2浓度很低。

吸收区高度的计算式理论上表达为:h=H0×N TU(2)式中:H0为传质单元高度;H0=G m/(k y a),k y a为体积吸收系数,其中k y为以污染物气相摩尔差为推动力的总传质系数,a为塔内单位体积中的有效传质面积, m2/m3;实验表明,对于低浓度废气,H0近似为常数[4]。

N TU为传质单元数,近似值N TU=(y1-y2)/Δy m,即气相总的浓度变化除以平均推动力,其中Δy m=(Δy1-Δy2)/ln(Δy1/Δy2);Δy1、Δy2分别为塔底、塔顶的气相总推动力。

N TU是吸收困难程度的度量。

N TU增大,则达到吸收目标所需的塔高随之增大。

根据一些烟气脱硫喷淋塔的经验数据可得N TU =3.6～3.8。

式(2)中分子部分G m(y1-y2)可近似看作需要吸收清除的污染物量,即给定目标;分母部分k y aΔy m 为吸收系数与平均推动力之乘积,可视为吸收能力。

在给定烟气量、烟气进出口浓度后,设计、运行要尽可能使分母所表示的吸收能力大。

吸收能力强,则设计塔高可降低,反之则增高。

或者是给定吸收区高度,吸收能力强,则脱硫率提高。

因此,设计、运行应追求高的体积吸收系数,以及高的平均推动力。

吸收区高度(或给定高度讨论吸收能力)的主要影响因素有:液气比、烟速、浆液p H值等参数,以及吸收塔结构等。

这些参数在技术、经济因素制约下,都存在最佳值。

于是在各种因素综合作用下,塔高也被确定在一个合理范围。

液气比和烟速的影响如下:工程设计中液气比L/G指吸收剂循环量与烟气流量之比(L/m3)。

L/G增大(图1),将使吸收推动力增大,传质单元数减小;气液传质面积增大,使体积吸收系数增大,因而可降低塔高。

如果给定吸收区高度, L/G增大,则脱硫率提高。

但另一方面,L/G增大,液体停留时间有所减少;而且循环泵流量增大,塔内气体流动阻力增大使风机耗能增大,投资和运行费用相应增加。

实际设计和运行应尽可能采用适当小的液气比。

石灰石法喷淋塔中的液气比一般为(15～25) L/m3[2,3]。

有推荐值为16.6L/m3[4]。

实际中L/G 随具体情况而变化,比如添加缓冲剂可大幅度减小液气比[2]。

烟速提高可增大吸收系数。

烟速增大,气液两相界面湍动加强,气膜厚度减薄,传质速率系数提高;烟速增大可减缓液滴下降速度,使体积有效传质面积增加[6],因而可使塔高降低。

但是另一方面,烟速增大,烟气停留时间缩短,要求增加塔高,因此使其对塔高的降低作用削弱。

实际中烟速提高还影响除雾效果。

研究及实践表明,逆流喷淋塔的烟气流速合适的范围是(2.6～3.4)m/s;典型值为3m/s[4]。

设计时需考虑浆液雾化状况、烟气污染物浓度、脱硫率要求等。

2　喷淋塔本体的经验性设计由于传质速率系数影响因素的复杂性,难于理论计算,通常需要从相当规模的实验装置来确定。

因而实际工程设计主要是经验性设计。

2.1　运行工况烟气量由工艺流程确定塔进、出口的运行工况烟气量。

烟气进入塔内,温度迅速降低;放热使一部分水分蒸发,使烟气中水蒸汽饱和。

烟气量温度修正要偏低计算,比如,烟气进口温度120℃、出口50℃,则可按55℃饱和湿烟气计算总流量;另外要计入强制氧化供入的空气量,不过氧化空气量相对份额小,为1%左右。

2.2　喷淋塔内径按平均烟气总流量V和烟速u确定烟气进口以上的塔内径:D i=2V3600πu(3) 由于设计烟速变化不大,所以塔内径随处理的烟气量增大。

若用机组容量表示,且取烟速3m/s,则D i≈0.75M(4)技术交流　νυ　热力发电・2004(1)© 1995-2004 Tsinghua Tongfang Optical Disc Co., Ltd. All rights reserved.式中:M为机组容量,M W。

2.3　吸收区高度工程设计中吸收区高度h一般指烟气进口水平中心线到喷淋层中心线的距离。

h按烟速和停留时间确定。

例如,烟速3m/s,接触反应时间(2.5～5)s,则h=(7.5～15)m。

设计时需考虑喷雾状况、液气比等。

目前实际中h多为(7～10)m。

例如,北京第一热电厂(德国Stienmuller公司)喷淋塔的h约9m。

广东连州电厂(奥地利能源公司)的h不到6m,其脱硫率较低(78%～89%)[8]。

喷嘴布置分2～6层,一般为4层;脱硫率要求低时可减少;低负荷时可停运某一层。

层间距(0.8～2) m[7],离心式喷嘴1.7m[5]。

广东连州电厂喷嘴分2层,层间距1.8m[8]。

实际上,从浆池液面到除雾器,整个高度都在进行吸收反应。

因而实际吸收区高度要比h高出(6～8)m。

2.4　浆池高度浆池容量V l按液气比和浆液停留时间t l确定:V1=L/G×V N×t1(5)式中:L/G为液气比,L/m3;V N为烟气标准状态湿态容积,m3/h;t l=(4～8)min[2]。

选取浆池内径等于或略大于D i,根据V l计算浆池深度。

从液面到烟气进口底边,高度为(0.8～2) m[5]。

2.5　除雾区高度除雾器设计为两段。

每层除雾器上下各设有冲洗喷嘴。

最下层冲洗喷嘴距最上层喷淋层(3～3.5)m;距最上层冲洗喷嘴(3.4～3.5)m[5]。

浆池、吸收区、除雾区,再加上烟气进出口高度,喷淋塔高度约为(27～32)m。

3　塔内气体分布均匀性烟气一般水平方向进入、水平方向引出,而且横截面随容量增大。

另外吸收区路程短,使得吸收区烟气速度分布难以均匀。

为使烟气均匀通过,采取如下措施:(1)采用较大的烟气进口宽高比,进口宽度占到塔径的80%以上;(2)进口向下倾斜6°～10°;(3)可采取顶部出口垂直向上;(4)可采用长方形塔截面;(5)在塔下部设置多孔板、导流板,对烟气整流。

多孔板一方面使烟气分布均匀,另一方面可强化气液接触传质。

在保证同样脱硫率的条件下,L/G可减小20%～30%。

这样,虽然烟气阻力增加,但循环泵费用的降低远大于风机能耗的增加,使系统投资、运行总费用下降。

4　结　论(1)湿法烟气脱硫常规设计的喷淋塔高度变化不大;横截面积随烟气容量增大而增大。

(2)喷淋塔给定烟气、浆液浓度后,设计和运行应尽可能采用小的液气比。

(3)由研究和实践结果可得,一般的烟气脱硫喷淋塔传质单元数为3.6～3.8。

(4)为保证脱硫率,应采取措施使塔内烟气速度分布均匀。

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All rights reserved.THERMAL POWER GENERATIONV ol.33　No.1　Jan.2004Contents &AbstractsPR OSPECT OF DEVE LOPING CFB B OI LERS IN CHINA UN DER MORE STRINGENT RE QUIREMENT OF ENVIR ONMENTA L PR OTECTION XIAO Ping et al(2)…………………………………………………………………………………………………………Along with P aying increasingly attention to environmentel protection in our coutry ,the allow able m ax.concentration of SO 2emmision m ay be reduced to 400mg/m bined with analysis of pollutant emmision in trial combustion test in a 1MW th lage scale combustion test stand for 17coal sorts in engineering projects of 13CFB boiters ,it is consid 2ered that the wide application and development of CFB boilers in m any sectors ,such as combustion of coal sorts with medium and low er sulfer content ,combustion of low grade coal sorts with taking account of desulfuration ,as w ell as for peak load regulating units etc.,can still be obtained even under conditions of more strin gert stand ard of en 2vironmental protection.The development of CFB boiler technology in China h as yet bright future.STU DY AN D RECOMMEN DATIONS ON REPAIR STRATEG Y IN THERMA L POWER PLANTS ZHU H ong bo et al (6)…………Directing against the traditional mode regarding periodical repair in therm al pow er plants ,a fully new repair strategy h as been put forw ard,m aking the equipments as the start 2ing point of studying the said rep air strategy and taking the difference in rep air speciality among equipments as clues ,including four sorts of repair w orks ,namely routine cen 2tralized repair (R CR ),intermittent centralized repair (ICR ),d aily predicted m aintenance and rep air (DPMR ),and accident rep air (AR ).An optim al m athem atic model has been established for R CR,ICR,DPMR and AR.The o perating beh avior of equipments h as been thorou ghly considered in the said model ,and m aking optimization in economic aspects as the target function.NUMERICAL SIMU LATION OF CFB COMBUSTION SYSTEM YONG Yu mei et al (11)……………………………………………B ased on a 75t/h circulating fluidized bed (CFB )boiler desi gned by I nstitute of E ngineering Thermophysics ,a set of m athem atical models ,which ean describe the process of flow,combustion ,and heat transfer for a wide ran ge of p article size in CFB boilers ,h as been established,and the o peration of said CFB boiler being simulated.The computa 2tional results are within a reasonable range ,and obtaining a reasonabe explanation from the mechenisms of CFB boilers.such ,a f irm found ation has been laid for develo ping large scale CFB boilers.MATRIX CA LCU LATION OF CONVERSION λFOR EQUIVA L ENT HEAT RE D UCTION UN DER CONSTANT HEAT QUANTIT YGU O Jiang long et al (15)………………………………………………………………………………………………………………When an equivalent heat reduction under constant heat qu antity is used as partial analysis ,it is necessary to ch ange w ork done obtained into equivalent hea reduction varia 2tion of the live steam throu gh conversion factor λ,and then the relative variation of economic index can be evalu binin 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f ine p article.The concentration of PAH S in flue gas has been measured simultaneously before and after the w et scrubber under four operating conditions.R esults indicate that the w et scrubber h as certain effectiveness on removin g PAH S,especially for MMW (moderate molecular w eight)and H MW (high molecular w eight)PAH S.The w et scrubber under long term operation h as an obvious “memory effect ”for PAH S,and the said memoryeffect h as a negative influence on controlling PAH S emissions.MEASUREMENT AN D ANA LYSIS OF HEAT TRANSFER COEFFICIENT FOR PLATEN HEATING SURFACE IN CFB B OI LERSL ING Xiao cong et al (23)………………………………………………………………………………………………………………The calculation of heat transfer coeff icient for platen heating surface in combuster of circuilating fluidized bed (CFB )boiler h as great influence upon the design and operation of s aid CFB boiler.The measurement of heat transfer coeff icient for platen heating surface in 75t/h and 130t/h CFB boilers has been carried out.The results show th at the heat transfer coeff icient on the gas side of platen heating surface h as direct relationship with its location ,bed temperature ,and solid porticles carried in the flue gas.The said coeff i 2cient increases along with less distance from the grid,and higher bed temperature ,as w ell as with large qu antity of solid particles carried in the flue gas.These results h ave refer 2ence value for design of CFB boilers.APPL ICATION OF NEW PU L VERIZED COA L BURNER WITH THREE CHANNE LS AN D DOUB LE BACKFIOW ON B OI LER N O.3IN HANCHENG POWER PLANT QI H ong mei et al (26)………………………………………………………………………………I n order to solve problems with utility boilers ,such as high consumption of fuel oil in start up course ,and low capability of peak load reyulation in the operation process etc.,a new type pulverized coal burner with three ch annels and double b ackflows h as been develo ped and designed.Practical application shows th at the said new burner has low con 2sumption of fuel oil in the course of start up ,m aking continuous operation reliability of the equipment to be heightened,and the range of load ad aptability to be enlarged,achieving the desired requirements.DESIGN AN D ANA LYSIS OF THE SPRAY T OWER PR OPER FOR F L UE G AS DESU LPHURIZATION GU O Yi et al (29)…………Theoritical analysis of influence factors upon dimensions of the spray tow er proper for w et flue gas desulphuriz ation has been carried bined with existin g experience in project design ,concrete steps for determining dimensions of the spray tow er proper are given ,and m ain factors needing to be considered in desi gn being discussed.RETR OFITTING THE CONTR OL SYSTEM FOR SPEE D SWITCHING OF DOUB L E SPEED MOT ORS USE D FOR DRIVING IN2DUCED D RAFT FANS OF 350MW UNITS PAN Ming hu a et al (32)…………………………………………………………………The experience of retrof itting the high low speed switching facilities for induced draft fans of 350MW units in H u aneng N antong Pow er Plant has been presented.Through start stop speed switching control method by using electric switch carbinet ,the ori ginal isolating switch control is replaced by start stop switching through tw o 6kV high low speed switching carbinets ,realizing the retrof it of switching control system of motors used for driving induced draft fans of tw o boilers.At the same time ,in order to reduce the perturb ation influence of speed switching for incluced draft fans upon the pressure in furnace ,the regulating system of pressure in the furnace h as also been modif ied corre 2spondingly.APPL ICATION OF ANTI WEAR TECHN OLOG Y ON ECON OMIZ OR OF CFB B OI LERS MA Yi fei et al (34)……………………The problems of w ear out and frequent tube burst h ave existed in a long period of time on high and low temperature economizers of circulalting fluidized bed (CFB )boilers ,of Ningbo City Pow er Development Company.For these ,retrof it h as been carried out b y using special shaped anti w ear covers enjoying p atent right.After retrof itting ,tube burst failures have never occurred again on the said economizers ,greatly lenghthening the service life of ecomozers ,obtaining rem arkable economic benef it.OPERATION BEHAVIOR OF START UP SYSTEM FOR ONCE THR OUGH B OI L ERS WITH B Y PASS SYSTEM OF STEAM TUR 2BINE AN D SIMU LATION THERE OF DU O B ao lin et al (36)……………………………………………………………………………The types and w orking principle of start up system for once through boilers and by p ass system for steam turbines have been expounded.The simulation of cold and hot start 2ing process for once throu gh boilers with recircuilating pump h as been carried out b y using the developed computer programme OTBSP.providing b asis for design and operation of start up system for once through boilers.A m athem atic model for com puting combustion rate b ased on pressure raising curve and on opening degree of the high pressure by p ass regulating valve during the starting process of unit h as also been correctly established by using the said programme ,providing b asis for designing the by p ass system of steam turbines and for optimizing the start up curve.STU DY ON PRB LEMS CONCERNING VIBRATION C L IMB AN D INSTABI L IT Y OF BEARING SH OES N O.6AN D N O.7ON IM 2PORTE D T YPE 300MW UNITS SHEN Pei hu a (39)……………………………………………………………………………………cause analysis concerning vibration climb of the bearing shoes N O.6and N O.7of imported type 300MW units in operation has been carried out ,the hasic cause resultin g in im 2p act w ear of geneater ’s sealing liner and great inb alance response of bearing shoes N O.6and N O.7being expounded,pointing out these problems are due to the structure of three support shafting system being used.H ence ,an effective scheme for solvin g and controlling the said vibration climb of beearing shoes N O.6and N O.7has been put for 2w ard.APPR OACH T O T ORSIONA L VIBRATION OF SHAFT SYSTEM FOR HIGH CAPACIT Y TURB O GENERAT ORS SUN H e tai (42)…………………………………………………………………………………………………………………………………………The importance of developing study on torsional vibration for turbo generators has been expounded,the m ain cause leading to occurrence of torsional vibration and kinds of said vibration being analysed,and test method of torsional vibration for sh aft s ystem of turbo generators being emph atically stated.ANALYSIS OF FAU LT IN SPEE D G OVERNING SYSTEM OF FEE D WATER PUMP AN D E L IMINATION MEASURES THERE OF GE jing ye et al (45)………………………………………………………………………………………………………………………© 1995-2004 Tsinghua Tongfang Optical Disc Co., Ltd. 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