INCREASING FLEXIBILITY OF COAL POWER PLANT BY CONTROL SYSTEM MODIFICATIONS

Transcription

1 THERMAL SCIENCE: Year 2016, Vol. 20, No. 4, pp INCREASING FLEXIBILITY OF COAL POWER PLANT BY CONTROL SYSTEM MODIFICATIONS by Ate MARUŠIĆ a*, Draže LONČAR a, Jakov BATELIĆ b, ad Valdi FRANKOVIĆ b a Departmet of Eergy, Power Egieerig ad Ecology, Faculty of Mechaical Egieerig ad Naval Architecture, Uiversity of Zagreb, Zagreb, Croatia b HEP Croatia Power Utility, Zagreb, Croatia Itroductio Origial scietific paper DOI: /TSCI M Expadig implemetatio of itermittet reewable eergy sources has already started to chage the role of thermal power plats i eergy systems across Europe. Traditioally base load plats are ow forced to operate as peakig plats. A familiar trasitio i upcomig years is expected i Croatia ad coal power plat operators are preparig accordigly. To evaluate cyclig capabilities ad cotrol system operatio for flexible operatio of selected 210 MW coal plat, series of tests with differet load gradiets were performed ad results were thoroughly aalyzed. Two possible bottleecks are idetified, thermal stress i superheater header, ad achievable rampig rate cosiderig operatioal limitatios of coal feeders, firig system ad evaporator dyamics. Several uexpected readigs were observed, usually caused by malfuctioig sesors ad equipmet, resultig i uexpected oscillatios of superheated steam temperature. Based o superheater geometry ad experimetal data, maximal steam temperature gradiet durig rampig was evaluated. Sice thermal stress was well iside the safety margis, the simulatio model of the whole boiler was used to evaluate achievable rampig o electric side. Key words: coal powered plats, flexible geeratio, thermal stress, simulatio Icreased participatio of itermittet sources i the Croatia power system has forced the system operator to fially cosider flexible geeratio from thermal power plats which are traditioally operated i base load. Due to the high share of hydro power, system eeds for primary ad secodary reserves are covered most of the year. However, low load operatio of hydro power plats is ot profitable durig periods with good hydrological coditios. Therefore some space is left for egagemet of thermal power plat fleet to provide required auxiliary services. Sice most of gas turbies are part of cogeeratio plats ad their operatio profile is defied with heat demad, achievable cotrol reserve of coal power plats has to be estimated. Flexible coal plats are subject of iterest for over two decades, but it gaied sigificat attetio oly i the last decade as, thaks to the high share of reewable resources, coal plats started losig their role as baseload plats [1]. Flexibility improvemets durig this period were based o experiece ad improvemets of the cotrol system. The expoetial growth of reewable resources i the last few years demads eve faster power plat respos- * Correspodig author; ate.marusic@fsb.hr

2 1162 THERMAL SCIENCE: Year 2016, Vol. 20, No. 4, pp es ad better damage mitigatio which ca be achieved oly with optimizatio based o detail dyamic models. Therefore the research iterest i coal plats started to grow rapidly recetly. A ew, complex dyamic model of large coal plats for simulatio ad further ivestigatio of flexibility is described i [2]. Furthermore, advace model for boiler start-up optimizatio takig ito accout thermal stresses is developed recetly [3]. Apart from covetioal coal plats, flexibility of moder coal plats with carbo capture is also addressed [4]. Further, process simulatio model coupled with computatioal fluid dyamics for tagetially hard coal fired 700 C boiler is developed [5]. With detailed simulatio of heatig profiles, calculatios based o existig stadards (i. e. EN [6] ad EN [7]) are iadequate ad other, more detailed approaches should be used. Oe such method for estimatio of optimum fluid temperature durig start-up process, based o fiite elemets method ad takig ito accout two poits at the edge of the collector opeig is described i [8]. The maximum ramp rate ad umber of load cycles i coal plats are geerally limited by thermal stresses, creep ad fatigue of critical thick wall compoets. Furthermore due to specific desig every plat ca have a differet bottleeck. Traditioal base load operatio did ot require steep ramps, hece recommeded rampig rates were kept relatively low, deep withi the safe operatio zoe. However, recet experieces have show that ramp rate improvemets of up to 300% are possible with simple sesor ad cotrol system retrofits. Thermal stresses are caused by temperature chages iside thick wall compoets (e. g. collector shell), therefore the quality of steam temperature cotrol is of essetial value. Cotrol system moderizatio offers key operatioal improvemets that help to icrease operatioal flexibility, decrease costs ad maage emissios. By reducig oscillatios ad excursios through tighter plat cotrol, plats ca improve uit efficiecies ad realize operatioal savigs, while at the same time improvig ramp rates 1-2% [9]. Differet potetial modificatios for flexibility improvemet ad low load operatio ca produce differet results depedig o plat specificatios but also o the eergy market regulatios, eergy system cofiguratio, etc. As preseted i [10] i a case of Norther Irelad, modificatios of oe coal power plat ca have sigificat cosequeces o the whole system, affectig wid curtailmet, carbo emissios ad fiacial savigs. Referece coal power plat The referece plat is a 210 MW plat with pulverized coal-fired oce through boiler, reheatig ad sea-water cooled codeser. The mai boiler desig parameters are preseted i tab. 1. Although the steam geerator was desiged for domestic coal from a earby exploitatio site, sice its commissioig the coal is imported from overseas. Therefore the coal choice depeds oly o market circumstaces ad the compositio of coal ca vary sigificatly. Operatioal measuremets durig burig differet coal types ca be valuable for plat model validatio. Water-steam tract cosists of ecoomizer, evaporator, two superheaters ad two reheaters. After each superheater steam is collected i the collector, because of the collector size ad high pressure, a thick wall is ecessary to esure safe operatio, o the other had, thick walls combied with temperature variatios ca cause sigificat thermal stress. The temperature i the collector is fuctio of umber of variables, most importat beig fuel rate ad feedwater flow. However, oe of these variables is directly cotrolled by steam temperature. While fuel rate is govered by desired overall power, feedwater flow rate is cotrolled by steam pressure. Therefore, steam temperature is cotrolled by two pairs of attemporators

3 THERMAL SCIENCE: Year 2016, Vol. 20, No. 4, pp (desuperheaters). Oe pair is placed after the evaporator ad the secod oe is betwee the first ad the secod superheater. Steam temperature is cotrolled by the secod attemporators pair. The water flow through the first pair of attemporators is cotrolled to keep steam temperature i cotrol rage for the secod attemporator pair. The steam temperature cotrol is realized by cascaded cotrol, where simple model of the process is icluded i the master loop while PI cotroller is placed i subordiated loop. There is a room for loop improvemet by feed-forward sigal based o expected load chages. Table 1. Referece power plat Number of Desig steam Desig steam No. of superheaters reheaters No. of Power Fuel coal feeders temperature pressure 210 MW Hard coal C 150 bar 2 2 Real rampig rates I order to evaluate curret rampig capabilities several tests with differet heat rate gradiets ad differet boudary coditios were coducted. Load chages were betwee 160 MW ad 210 MW which equals 76.2% ad 100%, respectively. Real plat measuremets for +8 MW/mi thermal gradiet are show i fig. 1, together with the electrical power output for the same period. Figure 1. Rampig at 8 MW/mi startig at 8:00 a. m. The steam pressure sigal resolutio was lowered i the acquisitio software by plat operator because of practical reasos durig costat pressure operatio mode. The

4 1164 THERMAL SCIENCE: Year 2016, Vol. 20, No. 4, pp electric power gradiet for the whole rampig period is aroud 1.2 MW/mi, while the electric power gradiet betwee 8:02 ad 8:13 is somewhere aroud 3 MW/mi. This relatively steep ramp is disturbed by sudde steam flow chages caused by the subordiated steam pressure cotrol, as ca be see from lower left diagram i the same figure. Aroud 8:10, two of four turbie valves were suddely opeed because of the high pressure causig the disturbace i the pressure cotrol system. System respoded by closig the valves eve more tha their prior positio. This behavior caused by the pressure cotrol is tha cotiued for about 15 miutes causig the lower steam flow ad the pressure icrease. Mismatch betwee the steam ad the feedwater flow is further icreased by slight thermal power output drop occurrig at the same time. Thermal stress i collector header The drum is the thickest boiler pressure compoet, it is therefore defied as the critical compoet, limitig allowable boiler startup as well as rampig (heatig ad coolig) rates [8]. Thermal stress is ot moitored directly but is calculated from the temperature differece betwee surface temperature ad the temperature i the middle of the collector wall. Durig results aalysis it was observed that oe of the temperature sesors is malfuctioig which ca be see from fig. 2, istead of reactig to steam temperatures before middle sesor, it is reactig almost at the same time. The error is probably caused by the displacemet of the surface sesor, or its bad istallatio, sice the sesor was ot i direct cotact with the wall, its time costat icreased. Hece, oe dimesioal mathematical model of heat trasfer through a pipe wall was created to acquire more realistic heat profile iside a thick collector wall ad ultimately give better approximatio of real thermal stress. Temperature distributio through a circular wall ca be writte as i eq. (1): T t 2 T 1 T 2 c r r r The ratio of ier collector radius ad wall thickess is relatively small, therefore it is reasoable to simplify the problem ad describe it as a flat wall. Flat wall formulatio of the problem i its discretized form ca be writte as: T t Δrc Δr ad boudary coditio o steam side as: T t Δ 1 Δr c r h 2 T T T T 1 1 T T T T where λ is thermal coductivity, ρ desity, c heat capacity, T temperature of discretized segmet, Δr segmet width, r ier radius of segmet ad h heat trasfer coefficiet. Because of the complex geometry exact evaluatio of local h is almost impossible, therefore simple calculatio based o fictive velocity i the collector was used. Calculated h is aroud 1,000 W/(m 2 K) which is i accordace with calculatios from other papers [8, 11]. Because s 1 1 Δr 2 1 (1) (2) (3)

5 THERMAL SCIENCE: Year 2016, Vol. 20, No. 4, pp of the thick heat isulatio o the outer side of the collector, outer boudary coditio was modeled as adiabatic. Recorded steam temperature was used as simulatio iput ad costat outside temperature was used as outside boudary coditio. The results were verified by compariso of simulated ad measured middle wall temperature. This way, better assessmet of itegral wall temperature ad cosequetially thermal stress was possible. Figure 2. Steam temperature ad wall temperatures durig rampig at 8 MW/mi Actual stress at the iside corer of the bore of a collector (critical poit) was calculated based o expressio from EN stadard: dms Lt Et m p t ΔT (4) 2e 1 v ms The first part of the right side expressio stads for pressure caused stress ad rightmost part for thermal stress. α m deotes stress cocetratio factor due to pressure, d ms mea diameter of the collector, e ms mea wall thickess of collector, p pressure i MPa, α t stress cocetratio elasticity, v Poisso s ratio ad ΔT through-the-wall temperature differece defied as: Δ T T T m i (5) where T m is itegral mea wall temperature, ad T i is the temperature at the iside surface of the wall. Actual stress durig 8 MW/mi thermal load is show i fig. 3. Costat pressure operatio mode excludes ay sigificat pressure chages, stress cycles are practically caused oly by temperature chages. The largest stress cycle is approximately 27 N/mm 2, which is sigificatly smaller tha the upper limit of stress rage that does ot cause fatigue damage (190 N/mm 2 as defied i EN [7]). Based o the maximal allowed stress a ivers equatio ca be used to calculate maximal ΔT: dms 190 m 2e ΔT LtEt t 1 v ms Δp (6)

6 1166 THERMAL SCIENCE: Year 2016, Vol. 20, No. 4, pp However, ΔT caot be directly related to steam temperature deviatios sice it is ot oly a fuctio of steam temperature but also of time ad heat trasfer rate at the surface of the collector. Nevertheless durig oe cycle ΔT is always smaller tha the peak to peak steam temperature, ad therefore ca be used as limitig value for peak to peak steam temperature variatio durig load chages. The calculated allowed ΔT ad cosequetially the steam temperature variatio durig operatio with 5 bar pressure oscillatios (largest oe observed durig measuremets) is aroud 32.5 C. Figure 3. Total stress o critical poit I reality the metioed temperature differece ca oly be achieved with high heat trasfer coefficiets (high local velocities) durig steep, uiterrupted loadig gradiets. Achivable rampig rates Although the comprehesive cotrol system modificatio is a geeral practice for coal power plats ramp improvemet, oe has to woder about the achievable rampig rate without sigificat modificatios. As metioed before, much higher (32.5 C) temperature variatios could be allowed without compromisig the boiler life expectacy. Nevertheless durig more tha 30 rampig tests, the temperature variatios of this order were ot possible because of the cotrol system costrais. A sigificat ramp of aroud 5 MW/mi was observed oly oce, caused icidetally by the egagemet of additioal coal feeder. Records of this evet are show o lower right diagram i fig. 4 startig aroud 05:25. Sudde start-up of the additioal coal feeder (show i the upper left diagram i fig. 4) causes thermal load overshoot which is ot compesated by the decrease i other five feeders. As ca be see from the upper right diagram i fig. 4, the temperature overshoot caused by this disturbace was ot larger tha 5 C. These results lead to coclusio that eve without complicated cotrol system modificatios ad with just simple adjustmets of the cotrol system costraits (allowig thermal load overshot durig fast load chages), sigificat rampig rate improvemets ca be accomplished. To cofirm this thesis ad asses maximal achievable rampig rates detailed oe dimesioal boiler model was used. The boiler model was based o the i-house model which is i detail preseted i [12], with small modificatios. The model domai (boiler) is discretized o 150 segmets o the water-steam side (ecoomizer, evaporator, superheaters ad reheaters) ad 25 segmets o the flue gas side. The boiler scheme used for discretizatio is show i fig. 5. The heated

7 THERMAL SCIENCE: Year 2016, Vol. 20, No. 4, pp water-steam pipes are prited i red ad the coectig pipes outside of the firebox are prited i red. Outside cotours of the firebox are prited i gree. Both covectio ad radiatio were take ito accout for calculatio of the heat trasfer o the gas side, ad a simple quasisteady model of combustio was used. Fuel flow ad ambiet air temperature were iput variables, where load is idirectly defied with fuel flow rate. The simulatio was set as opeloop system, that is subordiate local cotrol loops were ot simulated. Figure 4. Maximal achieved ramp rate Figure 5. Boiler compoets scheme used for discretizatio Iitial coditios used i simulatio were steady state coditios durig stable operatio with 80% load, resultig i the power output of 168 MW. Because of the umerical costraits oly 6% load chage was simulated, resultig i the power output icrease of aroud 9.5 MW. Simulatio results are show i fig. 6, where the x axis represets time i miutes. Two differet firig methods were simulated, the ormal ramp, which is similar as actual firig method at the mo-

8 1168 THERMAL SCIENCE: Year 2016, Vol. 20, No. 4, pp met, ad the fuel flow overshoot (clearly visible i upper left diagram), which is traditioally used to produce steeper power gradiets. As ca be see from the lower right diagram, effective power ramps of aroud 4 MW/mi were observed, where the overshot has produced better results durig the rampig ad also caused sigificatly shorter trasiet period (time ecessary for reachig set poit load). Coclusios Figure 6. Simulated boiler resposes Two most importat operatig coditios cocerig flexibility are startup ad rampig. Both of these states are characterized with sudde temperature ad/or pressure chages, hece causig large stress chages i thick wall compoets. Although older thermal power plats were ot desiged for cyclig operatio, most of them are capable of it with oe or small hardware modificatios. Nevertheless extesive cotrol system modificatios are sometimes ecessary to esure high heatig rate gradiets ad safe operatio. To assess thermal stress durig rampig rates, a simple oe dimesioal mathematical model was created ad a sigificat ifluece of heat trasfer rate i the collector was addressed. However, thermal stress caused by rampig is well uder specified limit that causes fatigue eve durig steepest heat gradiets ad despite malfuctioig of the steam temperature cotrol system. Cosiderig small calculated thermal stress durig operatio, the simulatio of maximal load gradiets was carried out. It is show that eve durig maximal firig rate gradiets thermal stress is well iside the safe zoe leadig to the coclusio that, cotrary to some cases foud i literature, the load gradiet i the observed power plat is oly limited

9 THERMAL SCIENCE: Year 2016, Vol. 20, No. 4, pp by its firig mechaism ad thermal iertia. Most simple modificatios such as fuel load overshoot cotrol ca produce sigificat improvemets ad ofte be best solutio for the thermal plat flexibilizatio. Refereces [1] ***, Flexible Geeratio: Backig Up Reewables, Reewables Actio Pla (RESAP), EURELECTRIC, Brussels, 2011 [2] Starkloff, R., et al., Developmet ad Validatio of a Dyamic Simulatio Model for a Large Coal-fired Power Plat, Appl. Ther. Eg., 91 (2015), Dec., pp [3] Taler, J., et al., Optimizatio of the Boiler Start-up Takig Ito Accout Thermal Stresses, Eergy, 92 (2015), 1, pp [4] Lucquiaud, M., et al., Ehaced Operatig Flexibility ad Optimised Off-desig Operatio of Coal Plats with Post-combustio Capture, Eergy Procedia, 63 (2014), Dec., pp [5] Schuhbauer, C., et al., Coupled Simulatio of a Tagetially Hard Coal Fired 700 C Boiler, Fuel, 122 (2014), Apr., pp [6] ***, Brussels: Europea Committee for Stadardizatio, BS EN :2001 Water-Tube Boilers ad Auxiliary Istallatios, Desig ad Calculatio of Pressure Parts, 2001 [7] ***, Brussels: Europea Committee for Stadardizatio, BS EN :2000 Water-Tube Boilers ad Auxiliary Istallatios, I-Service Boiler Life Expectacy, 2000 [8] Dzierwa, P., Taler, J., Optimum Heatig of Pressure Vessels With Holes, J. Press. Vess. Techol., 137 (2014), 1, pp [9] Shakar, R., Feasibility of Wear ad Tear Sesors for Flexible Plat Operatios, EPRI Tech. Update, 2003 [10] Kubik, M. L., et al., Icreasig Thermal Plat Flexibility i a High Reewables Power System, Appl. Eergy, 154 (2015), Sep., pp [11] Kruger, K., et al., Optimizatio of Boiler Start-up Usig a Noliear Boiler Model ad Hard Costraits, Eergy, 29 (2004), 12-15, pp [12] Ločar, D., Applicatio of Fuzzy Logic i Thermal Power Plat Cotrol Systems, Ph. D. thesis, Uiversity of Zagreb, Zagreb, 2001 Paper submitted: March 14, 2016 Paper revised: May 23, 2016 Paper accepted: May 24, 2016