MATHEMATICAL MODELLING AND OPTIMIZATION OF HEAT RATE OF A THERMAL POWER PLANT

Transcription

1 MATHEMATICAL MODELLING AND OPTIMIZATION OF HEAT RATE OF A THERMAL POWER PLANT by VINOD KRISHNA Submitted in fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Mechanical Engineering INDIAN INSTITUTE OF TECHNOLOGY, DELHI July, 1986

2 CERTIFICATE This is to certify that the thesis entitled 'MATHEMATICAL MODELLING AND OPTIMIZATION OF HEAT RATE OF A TuRNAL POWER PLANT' being submitted by Mr. Vinod Krishna to the Indian Institute of Technology, New Delhi for the award of the degree of Doctor of Philosophy in Mechanical Engineering is a record of the candidate's own bonafide research work. Mr. Vinod Krishna has worked under our guidance and supervision and has fulfilled the requirements for the submission of this thesis, which to our knowledge, has reached the requisite standard. The results contained in this work have not been submitted, in part or in full to any other University or Institute for the award of any degree. (Dr. S,K. Gupta) Assistant Professor Mechanical Engg. Department Indian Institute of Technology, Delhi New Delhi "Y o re(6 (Dr. P.B. Sharma) Assistant Professor Mechanical Engineering Department Indian Institute of Technology, New Delhi

3 ACKNOWLEDGEMENT. I feel great pleasure to express my deepest sense of gratitude towards Dr. P.B. Sharma and Dr. S.K. Gupta for their supervision, valuable guidance, kind help and encouragement throughout this work. I am grateful to the Central Electricity Authority and Power Engineers Training Society for sponsoring my candidature for undertaking the work reported in the present thesis. I am also indebted to the Mechanical Engineering Dept. I.I.T., Delhi for providing the necessary facilities for carrying out the present investigation. I wish to acknowledge the support and co-operation of the authorities of Badarpur Thermal Power Station, Indraprastha Thermal Power Station and Singrauli Super Thermal Power Station in carrying out the experimental investigations reported in the present thesis. I am also indebted to Mr. B. Sinha former Director General P.E.T.S. and Mr. Ramlikhani former Chief Engineer, M.P.E.U. for encouragements and useful discussions. I sincerely acknowledge the powerful inspiration and moral support by my extremely cooperative wife, Kiran. Last but not the least the support of Mr. Ashok Vashistha for typing the thesis and Mr. B.B. Arora and N.C. Saraswat in the graphical assistance is gratefully acknowledged. It 7. SC.. 1C;A\JA:A" (VINOD KRISHNA)

4 (1) ABSTRACT. The work reported in the thesis is concerned with modelling and optimization of heat rate of a thermal power plant. Realistic models for the three prime efficiency functions namely; Turbine Heat Rate, Boiler Efficiency and Plant Heat Rate are developed in terms of controllable operating parameters of the plant. Experimental investigations have been carried out on 60MW and 210MW units for validation of the models developed in the present work. These efficiency functions are then optimized with respect to operating parameters of the power cycle and the boiler using a paramety is optimization technique. The work of present investigation is presented under four major heads viz. a steady state model of heat rate, a transient model for turbine heat rate, experimental investigations and heat rate optimization. The steady state model for turbine heat rate developed in the present work incorporates real time effect of auxiliary heat inputs and leak-offs to and from the turbine system. Penalty on heat rate due to deterioration in condenser back pressure has been modelled using a linear model which predicts effect of individual condenser performance factors on heat rate and the results of this model are comparable with heat balance model at near MCR conditions of the plant. The steady

5 state model also incorporates cross effects of boiler excess air on turbine heat rate through two-way coupling. Boiler efficiency and combustion losses have been modelled directly in terms of operating parameters like excess air, particle size and flue gas exit temperature. The effect of these boiler parameters on plant heat rate has been studied using the models for boiler efficiency and turbine heat rate, utilizing a concept of two-way coupling. The present work also reports on the development of a model of turbine heat rate under load transients. In development of this model a lumped reheater time constant approach has been used for comprehending the transient response of various flows.and unit outputs and hence the turbine heat rate. The basic reheater time constant has been determined using an in-situ experiment on a 210MW unit. In the present work a governor test rig was designed,developed and commissioned for determining the governor time constant. The lumped reheater time constant was determined from the knowledge of the governor time constant and the basic time constant of the reheater. The frequence response test of the governor model has further been used for the validation of the results of the transient study. Experiments have been conducted on 60MW and 210MW boiler and turbine units for validation of various formulations developed in the present investigation:

6 These include experiments on 210MW and 60MW boilers for validation of statistical model for unburnt carbon loss developed in the present investigation. Turbine heat run tests were also carried out to compare the results of the present study with those obtained from ASME:PTC-6, acceptance test code. Using the above validated formulations, the turbine heat rate is optimized subject to variations in cycle parameters viz. steam inlet conditions, extraction pressures and temperatures and condenser back pressure. Boiler efficiency is optimized subject to variation in excess air and particle size at various flue gas exit temperatures. Finally an integral objective function of plant heat rate is formulated in terms of operating 'terminal conditions. The plant heat rate is then optimized subject to variations in above turbine cycle and boiler parameters. The results of the present study provide a realistic assessment of both the steady state and transient behaviour of turbine heat rate of a thermal power plant. The results of the optimization study further reveal that the extractions before reheater play a significant role in deciding the optimum values of plant heat rate.

7 CONTENTS Page CERTIFICATE ACKNOWLEDGEMENTS ABSTRACT NOMENCLATURE (1) (iv) CHAPTER -1 INTRODUCTION Introduction Aims and Objectives 1.3 Scope Steady state models Transient models Optimization Experimental investigations Importance of the problem and main thrust 1.5 Order of presentation 11 8 CHAPTER-2 LITERATURE SURVEY 2.1 Review of relevant literature and problem formulation The performance measures under steady state condition The plant performance under load transients Optimization 2.2 Need for present work 2.3 Methodology of present work

8 Page The modelling techniques The optimization techniques Experimental investigations CHAPTER-3 MODELLING OF HEAT RATE UNDER STEADY-STATE CONDITION 3.1 Introduction 3.2 The system 3.3 A model for turbine heat rate under steady state condition Formulations for steam flows J, Main, reheat and extraction steam flows.2 Auxiliary steam flows Steam property formulations Effect of terminal conditions on turbine heat rate Effect of load, auxiliary steam consumption and main steam properties Effect of final feed temperature Effect of reheat spray on turbine heat rate 3.5 Effect of condenser performance on turbine heat rate Condenser performance parameters Condenser back pressure corrections factor on turbine heat rate-a nonlinear model,1 Evaluation of condenser correction factor 45 49

9 3.5.3 A reduced order model for determination of effect of condenser performance on heat rate Effect of individual condenser performance parameters on it's back pressures Evaluation of condenser correction factor Plant heat rate as affected by combustion parameters Two way coupling between boiler and turbine A model for two way coupling.1. Effect of excess air and particle size on boiler efficiency.2 Effect of excess air on THR A statistical model for combustion losses and formulation of boiler efficiency COmbustion and residence time of a pulverized fuel particle A probability density function 70 for unburnt carbon loss.3 An empirical model for unburnt carbon loss.4 Boiler efficiency formulations and unaccountable losses An integrated model for plant heat rate as affected by boiler parameters-excess air and particle size.1 Plant heat rate as affected by flue gas outlet tempera-' ture

10 3.7 Results and discussions 80 CHAPTER-4 TURBINE HEAT RATE UNDER LOAD-TRANSIENTS 4.1 Introduction 4.2 The system and it's transfer function representation 4.3 Modelling of system parameters for sudden load changes 4.4 Transients in turbine heat rate neglecting variations in feed enthalpya case study 4.5 Transient variation in turbine heat rate considering variation in feed enthalpy Determination of reheater time constant Governor test rig for determination of governor time constant Design aspects of governor test rig Steady state load test Load transient test.4 Estimation of governor time constant Determination of basic time constant of reheater and lumped reheater time constant Results and discussions 122 CHAPTER-5 EXPERIMENTAL INVESTIGATIONS 5.1 Field measurements on 210MW/60MW boilers Coal sampling and particle size analysis

11 5.1.2 Determination of unburnt carbon loss Experiments to establish interrelation between boiler and turbine side parameters due to two-way coupling,l Determination of values of reheat spray flow rates corresponding to different excess air values and burner tilt at various loads 2 To determine constants in the relationship between final feed temperature and reheat spray Boiler losses-a comparative study of the proposed efficiency model with boiler test code Turbine heat run-a comparative study with ASME: PTC-6 code 5.3 Results and discussions Boiler performance tests Turbine heat run tests CHAPTER-6 OPTIMIZATION OF HEAT RATE 6.1 Introduction 6.2 The objective function and various constraints The objective function for turbine heat rate The variables and constraints The objective function, variables and constraints for boiler 166 efficiency Objective function for plena heat rate 168

12 6.3 Optimization of turbine heat rate subject to variation in extraction parameters Optimization of 'THR' at constant condenser back pressure condition Optimization of 'THR' accounting for variations in condenser back pressure Optimization of plant heat rate Optimizing boiler efficiency Integral optimization of turbine heat rate and boiler efficiency An analytical approach of turbine heat rate optimization Optimizing number of feed heaters. 6.6 Results and discussions CHAPTER-7 DISCUSSION OF RESULTS 7.1 Heat rate under steady state condition Turbine heat rate under load transients 7.3 Experimental investigations Field tests on 210MW and 60MW boilers Turbine, heat run tests 7.4 Heat rate optimization Optimization of THR subject to variation in extraction parameters Optimization of THR subject to variation in extraction parameters and condenser performance 208

13 7.4.3 Boiler efficiency optimization Optimization of plant heat rate 210 CHAPTER-8 MAIN CONCLUSIONS REFERENCES APPENDICES APPENDIX-A Flow measurements using'annubar'flow sensors and determination of flow constants in various formulations of mass flow rates A-1 Main steam, reheat steam and extraction steam flow rates and flow constants 232 A-2 Leak-off flows and flow constants 236 A-3 Mass balance of HP & LP turbines A-4 Mass flows as functions of pressures APPENDIX-B Some important design and operating parameters of boiler turbine and auxiliaries for a 210 MW unit B-1 Boiler design and operating parameters B-2 Fuel,furnace and combustion data B-3 Turbine cycle: design and operating parameters APPENDIX-C APP ENDI X-D Salient features of a 210MW power plant simulator Power cycle heat balance diagrams and control volume approach D-1 Power cycle heat balance diagram 0-2 Control volume approach for boiler testing

14 APPENDIX-E APPENDIX-F Algorithms and flow diagrams of computer programs 271 Notes on some formulations F-1 Leak-off flow formulations-value of F-2 Determination of effect of individual factorsinfluencing back pressure 278