Flexible Operation - Challenges for Thermal Power Plants

K B Batra

Technical Services, Noida

Total Installed Capacity of India (309244MW)As on 30.11.2016( Source: CEA and MNRE)

Coal61%

Gas8%

Oil0%

Hydro(Renewable)14%

Nuclear2%

RES**(MNRE)15%

Coal Gas Oil Hydro(Renewable) Nuclear RES**(MNRE)

2 16th Dec 2016

BHEL’s Contribution In Indian Power Sector

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

200000

Thermal Nuclear CCPP Diesel Hydro RE

All India 187802.88 5780 25282.13 918.89 43133.43 46326.82

BHEL 113264 3340 7560.13 199.42 20149.41 175

187802.88

5780

25282.13

918.89

43133.43 46326.82

MW

CA

PA

CIT

Y

TYPE OF POWER PLANTS

All India BHEL

16th Dec 20163

Indian Renewable Energy Sector (46326.82 MW)Source: MNRE

Wind Power 62%

Small Hydro11%

Biomass/Cogneneration9%

Solar Power18%

Waste to Power0%

Wind Power Small Hydro Biomass/Cogneneration Solar Power Waste to Power

4 16th Dec 2016

Installed RE Capacity Vs. Revised RE TargetsA Long Way To Go…..

8727.64 MW

28279.4 MW

4882.33 MW 4323.37 MW

100000 MW

60000 MW

10000 MW5000 MW

0

20000

40000

60000

80000

100000

120000

Solar Power Wind Power Biomass Small Hydro

Installed Capacity (October 2016) Revised Targets(Till 2022)

5 16th Dec 2016

Share of RE in Future Energy Mix Source: MNRE

6 16th Dec 2016

7

Renewable Generation - Challenges

Intermittent and variable

Season and Weather dependent

Location and time of day dependent

Does not match the load demand curve

Wind generation is unpredictable

Solar generation is predictable but non controllable

16th Dec 2016

8

Integration of Renewable Energy in Grid

Balancing by conventional energy sources (large part of which isthermal) is required

Greater the penetration of RE in Grid greater is the requirementof balancing

16th Dec 2016

Expected All India Duck curve with 20GW Solar Power in Grid

9 16th Dec 2016

10

Expectation from Thermal plants

Backing down and cyclic loading

Frequent start/stops may be required

Higher ramping rates during loading and unloading

But base load conventional plants are not designed for such cyclic loading.

16th Dec 2016

11

Start-up of Steam turbines (BHEL make)

Start type Outage hours Mean HP Rotor

temperature

(deg C)

Start-up time

(Rolling to full

load in min.

approx)

Cold Start 190 hr 150 deg C 255

Warm Start 48 hr 380 deg C 155

Hot Start 8 hr 500 deg C 55

Normal Mode : 2000-2200 starts

Slow Mode : 8000 starts

Fast Mode : 800 starts

16th Dec 2016

Effect of Load Cycling on Power Plant Components

12

Creep – Slow and continuous deformation of materials due to high temperature exposure even at constant load

Thermal Fatigue – Failure of metal when subjected to repeated or fluctuating stresses due to thermal cycling of components

Components affected – HP/IP rotors, Blades, Casings, Valves, Header, Y-Piece, T-piece, MS/HRH Pipelines

16th Dec 2016

13

Life Expenditure of Components

Operating Steam

Pressure

Life Time Consumption

Creep Damage

Stress

Fatigue Damage

Creep Rupture Strength

Mechanical Stress Thermal StressType of

Material

Operating

Stress

Operating Steam

temperature

Steam Pressure

inside a thick –

walled

component

Physical properties of a

material

Geometrical

Dimensions of a thick

walled components

Temperature

Difference inside a

thick –walled

component

16th Dec 2016

14

Life Expenditure Computation

The consumed life of a component is the sum of the life consumed by Creep & Low Cycle Fatigue

MINER SUM MC IS INDICATOR OF THE LIFE EXPENDED DUE TO CREEP

&

MINER SUM MF IS INDICATOR OF THE LIFE EXPENDED DUE TO LOW CYCLE FATIGUE

16th Dec 2016

15

FOR STATIONARY COMPONENTS :

M = MC + MF = 1 WARNING POINT

FOR ROTATING COMPONENTS :

M = M C + MF = 0.5 WARNING POINT

Approaching the Warning Point of Effective Miner Sumindicates that the life of the component has reached its limit.

16th Dec 2016

Life Expenditure Computation

16

Impact of Cycling on Equipment and Operation

Critical components are subjected to thermal stresses which are cyclic in nature

Higher fatigue rates leading to shorter life of components

Advanced ageing of Generator insulation system due to increased thermal stresses

Efficiency degradation at part loads

More wear and tear of components

Damage to equipment if not replaced/attended in time

Shorter inspection periods

Increased fuel cost due to frequent start-ups

Increased O&M cost

16th Dec 2016

17

Other Operational Risks

Ventilation in HP and LP Turbine at lower loads

Droplet erosion of LP blades

Excitation of LP blades due to ventilation

Frequent start/stop of major auxiliaries (PA/FD/ID fans, BFP) reduces their reliability

Increased risk for pre-fatigued components

16th Dec 2016

Age of Thermal Power Plants In India(in Years)

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

0-5 years 6-10 years 10-15 years 15-20 years 20-25 years > 25years

43357 MW

22610 MW

8359 MW 7780 MW5630 MW

> 25years, 29549 MW

MW

CA

PA

CIT

Y

AGE GROUP

19

Assumed Load Demand Curve on Thermal Machines

16th Dec 2016

0

20

40

60

80

100

120

55%

80%

100%

80%

2%/min 3%/min

20

Impact Assessment of Load Cycling

Impact of cyclic operation on BHEL supplied equipment withassumed load curve has been investigated.

Lower load is limited to 55% of rated and a ramp down rate of2%/min and ramp up rate of 3%/ min. is considered.

It is assumed that main steam and HRH temperatures are keptconstant and Unit is operated in sliding pressure mode.

16th Dec 2016

Cyclic Operation - Findings

Preliminary studies indicate that load backing from 100%-55%load at a ramp rate of 2%-3% per minute will not have significantimpact on life consumption of Turbine, Boiler, Generator & ESP.

However this mode of operation will have additional cost interms of lower efficiency at part loads.

16th Dec 201620

Backing down below 55% load and/or increase in ramp rates willhave effect on the fatigue life of the equipment.

Backing down below 55% load will also have other negativeimpacts on the equipment as discussed earlier and need furtherinvestigation in detail.

22

Mitigating the Effect of Cycling

Additional Condition monitoring systems/ Sensors

Improved design of Boiler and Turbine to allow faster ramping and increased number of cycles

Adaptation of Control System

Low cycling regime for older plants (may require RLA)

Replacement of fatigued/ worn-out components

Shorter inspection period

16th Dec 2016

23

Condition Monitoring for Flexible operation

Complete operation data is available

Continuous online consumption of life expenditure

Detection of highly stressed parts for inspection

Scheduling of RLA

Exploring the margins available for optimization of operating modes

Online monitoring of Generator components as early warning system

16th Dec 2016

24

Condition Monitoring Systems

Turbine Stress Controller (TSC)

Boiler Stress Monitoring System (BOSMON)

Blade Vibration Monitoring System (BVMS)

Stator End Winding Vibration Monitoring

Rotor Flux Monitoring

Partial Discharge Monitoring

Additional sensors for health monitoring

16th Dec 2016

25

Renewables integration - Overall impact

Thus increased penetration of renewables will lead to

Increased cost due to cycling resulting in higher tariff from conventional sources

Reduced equipment life and thus earlier replacement of plants

26

Renewables integration - Overall impact

Thus increased penetration of renewables will lead to

Increased cost due to cycling resulting in higher tariff from conventional sources

Reduced equipment life and thus earlier replacement of plants

Increased CO emissions, partly offsetting the gains from renewables

16th Dec 2016