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ADVANCED C&I SYSTEMS. FOR SUPER CRITICAL BOILERS. Presentation By. V.R.Krishnakumar T.R.Rajagopalan P.Rajesh NTPC-SRHQ Operation Services. Present Operational Scenario. Availability Based Tariff implemented
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ADVANCED C&I SYSTEMS FOR SUPER CRITICAL BOILERS
Presentation By V.R.Krishnakumar T.R.Rajagopalan P.Rajesh NTPC-SRHQ Operation Services
Present Operational Scenario • Availability Based Tariff implemented • Continuous Availability and Operational Reliability of Units are very crucial and linked to Tariff • Fast response to load changes demanded from Control centre is assuming greater significance
AREAS IDENTIFIED FOR IMPROVEMENT in SC BOILERS • Temperature measurement • MS Temperature Control using advanced controllers • Intelligent Soot Blowing Systems • Fire side corrosion monitoring
COMMONLY USED TEMP. TRANSDUCERS • Thermocouples • Type J • Type K • Type R • RTDs • PT 100 • Cu 53
RTDs • A resistor that has a very large temp.coefficient installed in a metal protection sheath • The output of the resistor varies proportionately with temp. • Pt is most commonly used as it has a high melting point • It has a well defined resistance vs temp. relationship
THE ISSUES IN STEAM TEMP. MEASUREMENT • Drift in thermocouples over a period of time • RTDs have much better accuracy, but higher cost and poor durability for rugged applications • Performance acceptance test of power plants warrants measurement within a certainty of 0.5ºC at 550 ºC • The temp.gradient over a T/C has a large effect on its performance • 3 ºC under estimation of MS temp.at 538 ºC increases heat rate by about 0.1 per cent
THE ISSUES IN STEAM TEMP. MEASUREMENT contd.. • 6 ºC over estimation of MS temp. can reduce life expectancy of boiler tubes by up to 28% • Non-homogeneous T/Cs will give difft. Outputs at difft. immersion depths • To achieve 0.5ºC accuracy it is necessary to calibrate T/C at various insertion depths and use only those which show same output at difft. insertion lengths • PT100s are capable of meeting the accuracy but their life expectancy in the industrial environment is not known
STUDIES ON TEMP.SENSORS (1) at PG & E and Duke Power (2) at South African energy company Eskom
RECOMMENDATIONS • Develop a database to keep history records of all calibration data • For Performance tests T/Cs may not be used at least for critical m/m like MS, RH, FW, Bled steam etc. • A project should be initiated to scan international trends in temp.m/m • T/Cs should be calibrated in situ to avoid inhomogeneity effects • There are some internationally reputed suppliers for PT100 RTDs suitable for steam applications
STEAM TEMPERATURE CONTROL - ISSUES • Use of conventional PID controllers results in temperature excursions during load fluctuations and takes around 30 minutes to stabilize the temp • This affects the life of critical components in Boiler as well as Turbine • This also affects the heat rate/ economy of generation
Main Steam Temp.Control EPRI study in Mid American’s George Neal Station
RECOMMENDATIONS • Use of Advanced controllers such as Multi Variable Controllers / Fuzzy Logic Controllers in steam temp control applications • Optimization/Tuning of important boiler controls may be taken up periodically • SH and RH steam sensors may be checked for proper response on line. T/Cs exhibiting time constants in excess of 30 seconds may be removed and checked
PREDICTIVE ADAPTIVE CONTROLLER • The dynamic modeling method is used to represent the process and models the process dead time also • Controller adapts to the changes in gain, time constants and time delay(dead time) happening in the process with MW changes • A custom gain scheduling that would automatically load different sets of PID tuning values depending on MW change
BENEFITS COMPARED TO CONVENTIONAL PID • Performance improvement of around 50% i.e. deviations within 6 to 9 ºC from set point compared to 12 to 19 ºC during ramp load changes of 5-10 MW/min
SHIFT IN CONTROL PHILOSOPHY FOR SC BOILERS? • Firing control based on metal /steam temperature measurement rather than MW/Load set point • This may result in better match of load and firing especially during ramp load changes
FIRE SIDE CORROSION MONITORING • Superheaters and Reheaters can fail due to corrosion by molten alkali sulfates • Higher steam temperatures result in increased susceptibility to this corrosion • On-line electrochemical corrosion monitors developed for low temp.applications can be adopted for monitoring of tube corrosions in SC Boilers • Continuous monitoring is attractive as the trend shows variations with load and coal quality
FIRE SIDE CORROSION MONITORING contd.. • Local gas velocity and particulate loading also influence corrosion rates • EPRI study results reveal that the corrosion resistance of SS310 (25%Cr) is superior to that of SS316 (18% Cr)
INTELLIGENT SOOT BLOWING (ISB) • Conventional Soot Blowing based on a time schedule rather than actual fouling conditions in the boiler • ISB Uses a combination of modeling, neural networks, expert systems and heat flux sensors installed inside furnace • Heat flux sensors serve as a tool to deal with spatial firing differences • ISB based on heat flux sensors has a positive impact on tube life and also helps in better control of Steam temp.
