汽轮机组热耗率分析及实时计算

研究论文

汽轮机组热耗率分析及实时计算

盛德仁, 李　蔚, 陈坚红, 任浩仁

(浙江大学, 浙江杭州　310027)

[摘　要]　归纳分析了影响汽轮机组热耗率的因素, 比较性能实时计算与热力性能试验的异同点, 重点对

影响热耗率实时计算准确性的主要因素及主要参数进行了分析, 为发电厂实施发电成本实时核算体系, 提

供理论分析基础。

[关键词]　热力性能; 热耗率; 实时计算; 汽轮机性能试验

[中图分类号]TK262　　　[文献标识码]A　　　[文章编号]10023364(2003) 05001603

　　能源工业是国民经济的基础产业, 是实现现代化的物质基础。汽轮发电机组是实现电能转换技术的重要设备, 又是消耗一次能源的大户。我国火力发电厂平均供电煤耗率为408g/(kW ・h ) , 比世界发达国家同类指标高出50g/(kW ・h ) 以上, 这表明我国火电机组的节能潜力巨大。汽轮机组的热耗率是衡量火电机组设计、制造、安装、调试、运行管理等环节的综合技术指标。降低机组的热耗率在建立发电厂安全、经济、稳定、可靠的运行系统以及在电力市场化, 竞价上网的成本核算体系中有重要的指导作用。

(2) 机组运行方式。如某些设备因局部故障而采

用高压加热器(高加) 切除运行、过热器减温水喷水(从给水泵出口投入) 、再热器减温水喷水等。

(3) 机组运行参数。运行参数可以分为可控与不

可控2类。可控参数(如主蒸汽温度、压力、真空等) 是否在该工况最优运行参数下运行等。

(4) 一些旁通阀、疏水阀是否存在严重泄漏等。

2　机组性能试验与性能实时计算

汽轮机组的热耗率是通过热力性能试验后计算得

1　额定工况热耗率的影响因素

汽轮机组额定工况热耗率定义:

q =

() P el

(1)

到的。热力性能试验的目的是要知道汽轮机组实际运行性能, 如对刚投产的新机组, 要做性能考核试验, 来检验机组是否达到制造厂家在技术合同上保证的热耗率; 对机组大修前、后也要进行常规热力性能试验, 来检验大修的质量。此时需要消除运行方式、运行参数对热耗率的影响, 采取高精度测试仪表及严格的系统隔离措施, 计算不明泄漏量, 确保性能试验精度。

(2) q t =q n +Δq t

q n =

式中:D 0为主蒸汽流量; h 0为主蒸汽焓值; h fw 为锅炉给水焓值; D rh 为再热蒸汽管道热段流量; h rh 为再热蒸汽热端焓值; D rc 为再热蒸汽管道冷段流量; h rc 为再热蒸汽冷端焓值; P el 为发电机输出功率。

影响额定工况热耗率的因素很多, 归纳为以下几点:

(1) 发电厂主要设备的内在性能, 诸如汽轮机、锅炉等设备状态是否完好, 是否采用高新技术(大容量机组、超临界技术、通流部分全三维设计和高效叶型、高效燃烧等) 。

Δq q n

(3)

式中:q t 为试验工况参数下的机组热耗率; q n 为把试验工况参数修正到额定工况参数后的机组热耗率; Δq t 为由于试验工况参数偏离额定工况参数使得机组热耗

20021021收稿日期:　

盛德仁(1960) , 男, 浙江大学电厂热能动力及自动化研究所, 副教授, 从事发电厂在线性能监测、机组优化运行等方面的教学与研作者简介:　

究工作。

　1　　6

()

研究论文

率增加值。C 为总的试验工况参数修正系数, 即:

Δq Δq Δq C =1+

1+1+・

q n

p 0

q n

t 0

q n

Δp r

Δq q n

t r

Δq q n

t fw

Δq q n

p c

(4)

　　式(4) 表示可控参数(主蒸汽压力p 0、主蒸汽温度t 0、再热蒸汽压损Δp r 、再热蒸汽温度t r 、给水温度t fw 、真空p c ) 偏离额定工况参数引起机组热耗率变化的修正系数。这些修正系数可以用制造厂家提供的参数修正曲线, 或用美国AS ME 标准推荐的参数修正曲线, 或通过汽轮机组详细热力计算, 以及通过热力学简化计算得到。

机组正常运行时, 监测机组实时热耗率的目的是为了及时评价机组运行管理水平。式(2) 可以写成:

Δq ) =C ・(5) q t =q n +Δq t =q n (1+q n

q n

外的节流损失。因而, 大型汽轮机组测量主蒸汽流量

是通过以下间接测量方法得到。

(1) 对单元制机组而言, 在最末高加与锅炉间的给水管道上, 安装节流孔板测量给水流量, 然后换算成主蒸汽流量, 即:

(6) D 0=D fw +D ed -D ab 式中:D fw 为锅炉给水流量; D ed 为过热蒸汽减温水流

量; D ab 为锅炉排污流量(定排和连排之和) 。

常规热力性能试验中的主蒸汽流量计量均采用此方法, 但直接把它应用于热耗率实时计算中, 会产生较大的误差。这是因为:1) 当投过热蒸汽减温水时, 喷水量转换成汽轮机做功的蒸汽量有1个滞后效应, 使式(6) 所得主蒸汽流量产生一个不真实的跃变, 从而, 机组热耗率也产生一个不真实的跃变; 2) 锅炉排污流量的实时监测也较困难。

(2) 通过测量调节级后(监视段) 压力, 利用Flugel 公式计算主蒸汽流量并作适当修正, 则变工况运行时主蒸汽流量为:

D 01=D 0

p 1

(7)

式中:q t 为运行工况参数下机组运行热耗率; q n 为额定工况参数下机组的设计热耗率, C 为总的试验工况参数修正系数。

运行工况下热耗率值是随着机组负荷、设备完好状况及参数变化的。1台机组投产后在一定的运行周期内, 只要机组设备结构不发生改变, 以及未发生机组损伤等情况, 机组的设计性能基本维持不变。若机组运行方式没有改变, 则机组设计热耗率只受到机组运行参数的影响。运行热耗率与该工况下设计热耗率值之差, 就反映了机组运行管理水平。各可控参数值也是随机组负荷变化的, 若机组在额定功率附近运行, 各参数应达设计值; 当机组运行工况严重偏离额定功率时, 各参数值应根据详细热力计算或热力性能试验确定

[1, 2]

式中:D 0为额定(设计) 工况时的主蒸汽流量; p 1为额

定(设计) 工况时的调节级后压力; p 11为变工况运行时的调节级后压力。

假定汽轮机通流部分叶片没有发生腐蚀、结垢、断

-1

裂等情况, 额定(设计) 工况时C =D 0・p l 为定值, 通过热力性能试验对其进行修正。这样式(7) 为:

(8) D 01=C ・p 11　　在实际使用时, 还需用调节级后温度及负荷进行修正。这样, 在正常运行工况范围内(不包括机组的起、停

工况) , 主蒸汽流量可以满足热耗率实时计算的需要。3. 2　再热蒸汽流量

。

3　影响热耗率实时计算准确性因素

在分析了机组热耗率的影响因素后, 重点对影响机组热耗率实时计算准确性的因素进行分析。3. 1　主蒸汽流量

再热蒸汽流量是通过回热系统热平衡计算求得。利用进入各高加的抽汽压力、温度和进、出高加的给水压力、温度与流量, 通过回热系统热平衡计算, 求得高压缸各级抽汽流量。这样, 再热蒸汽管道冷段流量即高压缸排汽流量, 为主蒸汽流量减去高压缸各级抽汽量, 再热蒸汽管道热段流量为再热蒸汽管道冷段流量加上再热器减温水喷水量。3. 3　汽轮机末级排汽焓值

从式(1) 可以看出主蒸汽流量的测量或计算精度, 对热耗率的影响成正比关系, 即1%的流量误差直接影响机组热耗率1%, 因此对主蒸汽流量的计量精度要求很高。由于蒸汽的可压缩性, 如果采用标准节流孔板, 测量其差压值换算成流量, 再进行蒸汽密度修正的方法, 其精度较低。并且, 随着汽轮机组容量增大和初参数提高, 在主蒸汽管道上加装节流孔板, 会增加额

在汽轮机热耗率实时计算中, 实时确定汽轮机末级排汽焓值一直是一个难题, 原因是处于湿蒸汽区时,

()

研究论文

其压力和温度不再是相互独立参数, 且又不具备对汽轮机内蒸汽湿度实用的在线测量手段。末级排汽焓在机组系统热平衡计算、汽轮机低压缸效率等计算中, 又是一个非常重要的参数。汽轮机末级排汽焓的实时计算方法归纳为以下几种。

(1) 能量平衡法　由下式表达:

P el =(D 0h 0+D rh h rh -D rc h rc -

3. 5　热耗率实时计算周期

为了能实时了解汽轮机组的运行情况, 往往希望

热耗率实时计算周期短一些, 但计算周期不能过短, 这是由于整个汽轮机组系统存在较大热惯性和热滞后现象, 且计算周期过短会使机组因负荷变动造成的过渡过程和扰动引起参数波动, 影响实时计算的准确性。计算周期过长, 会失去计算的实时性, 且过长时间的参数平均也会失去计算的真实性。实际运行情况表明, 以(5～8) min 的参数平均值计算1次热耗率实时值较为合理。

∑D j h j -

ηD c h c ) ηm el

(9)

式中:D j 、h j 为各抽汽流量、焓值; D c 、h c 为排汽流量、

η焓值; η发电机效率。m 、el 为机械、如果各抽汽参数都为过热蒸汽, 由式(9) 可以方便

地计算出排汽焓值。但实际上有几个低压抽汽参数已进入湿蒸汽区, 其焓值也不能用抽汽压力和温度来确定, 所以需要迭代计算。此方法的不足之处是其他参数测量误差会累计在排汽焓值上, 并且影响迭代计算的收敛性。

(2) 外推法　根据再热蒸汽入口状态点和在过热区的各抽汽口的状态点连线的变化规律, 外推到湿蒸汽区, 得到湿蒸汽区的各抽汽焓值及排汽焓值。此方法概念简单、计算方便, 但也有明显的不足之处:1) 由于在再热蒸汽入口之后, 处在过热蒸汽区的抽汽口较少, 使得参与拟合的点数较少, 精度较差。2) 过程线外推, 即过程线斜率不变, 也就是级的相对内效率不变。实际情况热力过程处于湿蒸汽区后, 由于湿蒸汽损失及末级余速损失的存在, 级的相对内效率要明显降低, 热力过程线斜率相对要平缓一些。

(3) 外推能量平衡法　先由外推法确定处于湿蒸汽区的抽汽及排汽焓值, 再进行系统能量平衡迭代计算。此方法具有迭代收敛快、累计测量误差小的优点[6], 可很好满足热耗率实时计算的需要。3. 4　减温水喷水调温

4　结　论

(1) 影响汽轮机组热耗率指标的因素主要有机组

的本身性能、运行方式、运行参数及测量数据的不确定度, 因此不仅要求采用新技术、新设备, 更要加强机组

运行的科学管理。

(2) 机组热耗率实时计算与热力性能试验, 从目的到方法有其共同点, 也有很多不同之处。热力性能试验的目的是检验机组的设计、制造质量, 机组热耗率实时计算的目的是更科学地指导机组的运行和管理。

(3) 主蒸汽流量和再热蒸汽流量计算、汽轮机排汽焓确定及计算周期是热耗率实时计算中必须要解决的关键点, 将直接影响实时计算的可信度。

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()

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THERMAL POWER GENERATION

V ol. 32　N o. 5　May 2003Contents &Abstracts

THE ENVIR ON MENTA L VA L UE OF POWER GENERATION B Y USING NATURA L G AS ………………………Zhou H ao et al(2)

The traditional environmental imp act assessment (EIA) h adn ’t convert the environmental benefits o f pow er generation by using natural gas into currency , resulting in gratuitous oc 2cup ation o f their environmental value (EV ). By using the theory o f environmental economics , an assessment o f the EV for pow er generation by using natural gas h as been carried out. The results show th at the EV o f pow er generation with natural gas can reach to R MB 89. 639yu an/(MW ・h) , providing b asis for reference to the EV trading in the pow er m ar 2ket.

STU DY ON POWER GENERATION COST AN D BI DDING PRICES OF SUPP LYING E LECTRICIT Y FR OM THER MA L POWER P LANT ………………………………………………………………………………………………………………………G ao Bo et al(6)

The cost o f a pow er plant include coal consumption , m aintenance and other monetary expenses , they should all be considered in cost analysis. The energy consumption ch aracters o f m ain equipments and entire unites in a pow er plant h ave been analysed , the cost ch aracteristic curves and some m ain influencing factors being studied. The relationship betw een aver 2age and m arginal costs , their role in price bidding , and some questions must p ay attention in w orking out cost ch aracteristic curves being also analysed. The results m ay be useful for reference to carrying out cost ch aracter analysis and practical use in pow er plants.

DEVE LOPMENT AN D APP LICATION OF SERVICE -LIFE MANAGEMENT TECHN OLOG Y FOR IMPORTANT PARTS OF THER 2MA L POWER P LANTS ………………………………………………………………………………………………………Li Yaojun(9)

Three aspects including in the expoitatively developed service -li fe m anagement technology for important p arts in therm al pow er plants are as follow s :service -li fe o f boiler tubes , service -li fe o f boiler p arts , and service -li fe o f steam turbines. On the b asis o f large amount o f experiments and analyses , evalu ation method and calculation model for service -li fe o f important p arts in tberm al pow er plants with the aim o f engineering application h ave been put forw ard , including a series o f correlated qu antitative evalu ation techniques , monitor 2ing h aedw ares , di agnotic so ftw ares , stand ards and guiding regulations; b ased on integrating and blazing new trails , technical platform so ftw are o f a service -li fe m anagement system with initi ative know -how property right regarding important p arts o f therm al pow er plant , such as boiler tubes , m ain boiler p arts , and steam turbines , h as been designed and de 2veloped firstly in our country; a ling -term service -li fe m anagement mode o f pow er generation equipments , combining on -line monitoring with by -p ass line monitoring , h as been established in our country. The implementation o f a complete set o f techniques and monitoring system for service -li fe m anagement h as been success fully realized in tw o pow er plants o f demonstative projects at the first time , h aving extremely obvious effectiveness.

STU DY ON MATHEMATIC MODE L OF PIPING -MAIN SCHEME CIRCU LATING WATER SYSTEM AN D SOL UTION METH OD THEREOF …………………………………………………………………………………………………………Cheng Maohu a et al(13)

The m athem atic model o f du al -piping -m ain scheme circulating w ater system at N anjing Pow er Plant w as established. Through analysing the topology structure o f the complex hy 2draulic piping netw ork system and intruducing virtural flow resistance , the said circulating w ater system w as decomposed into simple hydraulic piping systems , a key problem o f the piping -m ain scheme circulating w ater system in economical operation w as solved , providing an effective method for analysing complex piping -m ain scheme systems.

ANA LYSIS AN D REA L -TIME CA LCU LATION OF SPECIFIC HEAT CONSU MPTION FOR STEAM TURBINE UNIT ……………

……………………………………………………………………………………………………………………Sheng Deren et al(16)

The influencing factors upon speci fic heat consumption o f steam turbine unit h ave been summ arized and analysed , the similarities and di fferences betw een real -time perform ance cal 2culation and therm al perform ance experiments being comp ared , the m ain influencing factors and p arameters upon real -time calculation accuracy o f speci fic heat consumption being emph atically analysed , providing theoretic b asis for implementing the system o f real -time pow er -generating cost cunting in pow er plants.

STU DY ON THE PARTIA L INCREMENT METH OD FOR OPTIMIZING PERFOR MANCE ANA LYSIS OF UTI LIT Y BOI LERS ……

………………………………………………………………………………………………………………………Wu Zhiqun et al(19)

A p arti al increment method o f perform ance di agnosis technology h as been put forw ard. The influence o f boiler operating p arameters ’vari ation due to ch anging the operation mode upon the economic aspect o f said boiler being qu antitatively analysed. Similarity and di fference betw een the conventional method and the p arti al increment method h ave been com 2p ared , the result obtained is as follow s :the calculation accuracy o f p arti al increment method being higher th an th at o f conventional method.

EXPERTMENTA L STU DY ON G AS -SOLI D TWO -PH ASE F LOW WITH STR ONG LY SWIR LING TURBU LENCE ………………

…………………………………………………………………………………………………………………………Su Yaxin et al(22)

T aking CFB as b ackground o f application , a study on strongly swirling turbulence o f gas -solid tw o -ph ase flow in a sep arator with a squ are cross -section h as been carried out by using a three -dimensional laser -b ased p articles dynamic analyser (3D -PA D). The results show:(1) the distribution o f swirling flow field in the said squ are -sh aped sep arator h as a forced swirl in the centre , but a qu asi -free swirl near the side w all , and a p arti al swirl existing in the place o f edge -angle. P articles ’mutu al collision in the place o f edge -angle m akes the qu asi -laminar flow pulsation there to be strengthened , the dynamic energy o f the turbulence and the strength o f p arti al turbulence near the edge -angle to obtain a comp aratively large value , consuming large amount o f energy , being o f benefit to p articles ’sep aration at the edge -angle ; (2) Above the sep arator , the m ax value o f average falling velocity along the vertical direction is near to the right side o f the flow -guiding cone (w hile the inlet is at the leftside). I n the medium and low er -p art o f the sep arator , the m ax falling velocity is situ ated at the right w all surface o f the sep arator. The direction o f velocity in the central p art o f the cross -section is upw ard , and recirculation occurs here.

STU DY ON TECHNICA L RETR OFIT OF FUE L COA L PU L VERIZE D SYSTEM REG AR DING 130t/h BOI LERS IN YI LI THER MA L POWER P LANT N o. 2………………………………………………………………………………………………G uo hu aib ai et al(27)

A fter putting the tittle boilers into operation. troublesome problems , such as insu fficient output cap acity o f the coal pulverized system , low load cap ability o f said boilers , and slag 2ging in the furnace etc. , h ave been existing in a very long period o f time , leading the furnace outlet flue gas temperature to be above 200℃higher th an the desighed value , and hot air temperature to reach 550℃, hence , the units can only be operated under low load condition , and the time o f consecutive operation being short. Throw gh analysis and testing o f the said coal pulverized system , qu ality o f fuel coal , and combustion conditions etc. effective debugging and retro fitting h ave been carried out , m aking the said problems to be satis 2factorily resolved.

KIN DS AN D TECHNICA L FEATURES OF THE STARTING -UP SYSTEM FOR SUPERCRITICA L ONCE -THR OUGH BOI LERS

……………………………………………………………………………………………………………………H uo Dongfang et al(31)

Common kinds and technical features o f the starting -up system for once -through boilers h ave been analysed and comp ared. The starting -up system with start -up drain heat ex 2ch ange and recirculation h as good beh avior under extremely low load operation and for frequent start -up , being suitable for operation under medium load and for tw o -shi ft opera 2tion ; The flashing type h as b ad beh avior under low load operation and for frequent start -up , being suitable for pow er plants with b asic load. There are unique advantages and obvi 2ous deficiencies in practical operauion o f each kind o f starting -up system. H ence , it is necessary to comprehensively consider the factors , such as technical features , capital invest 2ment , and operation mode o f pow er plants etc. for selecting the suitable type o f starting -up system.

IMPACT OF COA LS ’TRIA L COMBUSTION ON CFB BOI LER PR OJECTS ……………………………………W ang Pengli et al(35)

A 1MW circulating fluidized bed (CFB) combustion pilot plant established by the Therm al Pow er R esearch I nstitute (TPRI) and tri al combustion o f coals carried out on the said plant h ave been presented , some properties , such as burning beh avior o f coal sorts , smoke emmision , as w ell as ch aracters o f ash and slag , h aving been measured. B ased on the re 2