Thermal Power Plant Performance Centered Operational Recommendations

Transcription

1 National Seminar on Thermal Power Plant Performance Management - NSTPPPM 36 Thermal Power Plant Performance Centered Operational Recommendations Dr.K.C. Yadav and Dr. Abid Haleem T I. INTRODUCTION HIS paper briefly states the remedial measures to minimize the loss of performance on account of variations in uncontrollable input parameters. This is achieved by modifying the controllable input/operating parameters and proper maintenance management of the process equipments. Required modifications in controllable input and operating parameters have been recommended for implementation to reduce the avoidable loss component of various processes and equipments in the sections to come. II. MANAGEMENT OF VARIATION IN COAL After exhausting all the efforts of using design specified coal, variation in its parameters has been recommended to be suitably accommodated in the following subsections. Input Coal Quality Management Pit head coal washing and beneficiation should be done. Use of beneficiated coal reduces the cost of transportation and its handling in the respective units. In case of poor coal quality, its blending with better coal and oil support are recommended. Mill Capacity Management Reduction in coal mill capacity on the basis of pulverization characteristics of the coal parameters has been recommended in accordance with mill operating condition curves provided by the manufacturer/supplier. Capacity of the individual mill has been recommended to be further reduced either because of inadequate pulverized coal fineness or because of high current of the mill driving motor. This further reduction in mill capacity beyond the limit can be prevented only by improving the maintenance standards. Combustion Air Management Accurate assessment and correct distribution of combustion air solve many of the steam generator coal combustion problems. Residual oxygen percentage in flue gas represents the excess air, which mislead in the event of air in leakage and mal functioning of the secondary air damper. Carbon dioxide monitoring also suffer from similar problem but of lesser magnitude. Total air flow has been recommended to be modified in accordance with combustion resistance offered by coal impurities, which in turn may provide enough guidance to the forced draught fan operator to vary the Dr.K.C. Yadav, Director, Jindal Institute of Power Technology, OPSTPP, Raigarh, C.G. India. drkcyadav@jindalpower.com Dr. Abid Haleem, Professor & Honorary Director IQAC, Mechanical Engineering Department, Faculty of Engineering & Technology, JamiaMiliaIslamia, Okhla, N. Delhi, India. haleem.abid@gmail.com combustion air quantity to accommodate very wide variation in coal quality. Total air flow through the boiler has been recommended to be restricted sufficiently lower than that of the critical velocity in any part of the steam generators. A reliable, operator friendly, secondary air damper control system with maintenance ease has also been recommended. It has also been recommended to supply supplementary fuel oil or gas to maintain loading conditions nearest possible to the maximum continuous rating, particularly for the units, which are not stable at partial loads. 1. Soot Blowing Management Prescribed frequency of soot blowing with design specified values of steam pressure and temperature keeps the tubes clean to improve the heat transfer. Long retractable soot blowers of many thermal units, do not function satisfactorily and cause lot of soot deposition on platen super heater, re heater, final super heater, low temperature super heater and economizer. Air pre heater soot blowing also should be managed well because its choking results in reduced heat transfer and higher flue gas exhaust temperature. Air pre heater seals are also very important and must be maintained. 2. Flue Gas Exhaust Temperature Management Flue gas exhaust temperature optimization in accordance with flue gas dew point temperature helps a lot in avoiding the poor boiler efficiency. The causes, consequences and remedial measures of the abnormal flue gas exhaust temperature are required to be analyzed in depth including input coal, air and water quality parameters. Flue gas dew point temperature in case of high ash and low sulfur content coals is relatively lower, which has been recommended to be incorporated in the design for a lower flue gas exhaust temperature. Operational efforts of optimally reducing down the flue gas exhaust temperature have been recommended to improve boiler efficiency as there is no possibility of occurrence of flue gas dew point temperature. High ambient temperature increases the average air pre heater metal temperature and allows to further lowering down the flue gas exhaust temperature. Study on effect of flue gas exhaust temperature on boiler efficiency reveals that there will be 1 % improvement in boiler efficiency for every 18 o C to 20 o C drop in flue gas exhaust temperature. Either economizer may be provided with one more standby feed water heating system or flue gas path should be provided with additional fuel firing system before the economizer for better regulation of the flue gas and feed water temperature across the economizer. More heating elements are required to be retrofitted in the air pre heater for further reducing down the flue gas exhaust temperature.

2 National Seminar on Thermal Power Plant Performance Management - NSTPPPM Boiler Drum Level Management Abnormal boiler drum level is caused due to unbalance of heat or mass or both. Energy taken out from the drum as the enthalpy of the steam at the inlet of the super heater has to be balanced by energy entering to the drum as the enthalpy of water at the outlet of economizer and enthalpy of water/stem mixture at the outlet of the evaporator. Heat supplied by the fuel to its combustion products and then to water/steam mixture in the evaporator and then to the boiler drum. Coal quality parameters inclusive of its fineness and temperature play vital role on combustion. Secondary dampers selection and position, burner selection and angle are also equally important. Contrary to the steam generation process, response of its consumption in the turbine and exit from the drum is very quick and hence it has been recommended that to rely upon steam flow regulation to the turbine for quick relief in managing the drum level rather than to relying on coal or feed water regulation. Coordinated flow of coal, feed water and steam has been recommended. It is specifically recommended to change steam flow on first priority and then coal flow and at last feed water flow. Drum level operator should be provided with additional instrument showing coal flow and steam flow to the turbine so that he can maintain better heat and mass balance with matching responses. In addition to proper coal quality management, boiler water/steam management has to be maintained for stable drum level as described in the next section. III. MANAGEMENT OF VARIATION IN WATER Water is an input to the thermal power plant, quality of which influence the efficiency and equipments life quite significantly. Cost of chemical treatment of raw water for the purpose of drinking and bearing cooling is varying with input quality. Cost of de-mineralization also largely depend on raw water quality, which is used as working media to extract heat from the products of combustion and in turn doing work upon the steam turbine. Maximum quantity of input raw water is used for condensation, without any chemical treatment and hence its quantity and quality influences the condensation efficiency and condenser life respectively. Management of different qualities of water in different equipments has been recommended in the following subsections. Condenser Vacuum Management After exhausting all the efforts of cooling water inlet temperature optimization, loss of condenser vacuum on account of high cooling water temperature is determined. This inevitable component of loss in condenser vacuum is deducted from the measured loss to determine the avoidable component of loss of condenser vacuum, which is recommended to be minimized by increasing cooling water flow, keeping tubes clean, minimizing the air ingress, improving the steam quality and effectively utilizing the vacuum creating devices. The non condensable are required to be removed continuously by the effective vacuum creating devices (i.e. steam ejectors or vacuum pumps), mal-operation of which reduce vacuum. The starting ejector can create vacuum up to 540 mmhgcl. It is recommended to sufficiently wait till the capacity of starting ejector is exhausted and stable vacuum is maintained. 10 to 15 minutes after the establishment of stable vacuum by starting ejector, the main ejector should be cut into service followed by immediate withdrawal of starting ejector. Parallel operation of both the ejector shall not only develop the lesser vacuum but also damage the main ejector. Causes and remedial measures of high terminal temperature difference (i.e. poor heat transfer) have been briefly stated for the implementation as follows: Higher gaseous impurities in the steam can be managed by better management of boiler and preboiler system. Air ingress can be avoided by frequent leak detection test and effective steam sealing of low pressure turbine. External tube deposits can gradually increase terminal temperature difference which needs better de mineralized water quality management. Internal tube deposits result in higher terminal temperature difference and higher cooling water differential pressure across the condenser can be effectively minimized by on-line condenser tube cleaning. Initial Filling of De-Mineralized Water in the Thermal Cycle Equipments It is recommended that the de-mineralized make up water pumps should be used to fill condenser hot well up to required level, then condensate extraction pump should be started to divert the extra de-mineralized water to the deaerator until it is half filled. After the establishment of deaerator parameters, boiler feed pump should be started and then feed water should be taken to economizer till water level in the drum is adequate. Boiler fill pumps and emergency lift pumps must not be used for normal start up because these are provided to fill boiler for the purposes other than the start up. Condensate Pre-Heating Management It is recommended to provide control valves on extraction lines to have better control on feed water outlet temperature. Vapour line of every heater should be kept clean to improve the heat transfer. Adequate drip level in the heaters and proper drip cascading improve heater performance. Condensate heating by the auxiliary steam in last low pressure heater has been recommended to be utilized before every startup. Deaeration Management Quantity of the hydrazine injected to the feed water after deaerator to reduce the oxygen less than the minimum displayable value of the instrument, has been recommended to be optimized to reduce non condensable gases in the condenser. Attempt should be made to maximize the physical deaeration by properly maintaining the deaerator parameters and repairing the internals. This leads to minimum chemical deaeration and reduction in formation of non condensable gas in condenser. Auxiliary steam supply to last low pressure heater is beneficial and helps in maintaining the deaerator parameters quickly, which improves physical deaeration.

3 National Seminar on Thermal Power Plant Performance Management - NSTPPPM 38 Feed Water Temperature Management High pressure heaters have been recommended to be essentially put into service above the recommended load, failing which low load operation has been recommended to be preferred to minimize the loss of life of boiler and turbine and overall efficiency. Heat is recommended to be saved at high potential by maintaining adequate drip level in the heaters and proper drip diversion. Practicing exergy analysis for heat exchangers in general, helps in improving the performance, which is also applicable to the regenerative feed heating equipments. Low temperature feed water has been recommended to be heated by introducing an additional heater in between the last high pressure heater and economizer to ensure heat transfer in the boiler under design prescribed differential temperatures and proportions of heat flux. Steam Generation Management Boiler blow downs and phosphate dozing should be optimally utilized. Continuous blow downs has been recommended to be utilized only on the basis of chemical analysis of feed water samples from evaporator and use of emergency blow down should be avoided by better co-ordination of fuel firing to the boiler and steam supply to the turbine. Intermittent blow down must be operated strictly in accordance with authentic recommendations, failing which it may lead to starvation in the evaporator tubes. Steam generation in the evaporator is mainly controlled by fire side parameters and these recommendations have been proposed to be implemented in line with operating manuals. Steam Super Heating / Re Heating Management Pressure dominated steam must not be allowed for expansion in steam turbine. It has been recommended to prefer temperature dominated steam at the outlet of the super heater and re heater, up to the prescribed metallurgical limits. All the efforts of super heater steam temperature and re heater steam temperature control have been recommended to be tried to avoid attemperation. These efforts include boiler feed water temperature management, combustion (fire ball) management, selection of firing elevation, burner tilt etc. As the steam super heating and re heating are controlled by fire side parameter and hence these recommendations have been proposed to be implemented in line with operating instructions. Steam Expansion Management (Turbine Output Management) On line determination of energy and exergy parameters help operation managers to estimate avoidable component of performance loss and in turn to initiate the action to curtail the same. Controllable operating parameters beginning from steam temperature, pressure and purity are recommended to be suitably modified on the basis of axial shift, differential expansion, eccentricity and vibration. Turbine pressure survey should be utilized to determine the internal losses through energy and exergy parameters. Auxiliary Steam Management Significant amount of steam is taken from the main steam line for auxiliary purposes. Temperature and pressure of the main steam are reduced from 540 o C and 155 Kg/cm 2 to 200 o C and 15 Kg/cm 2 by mixing water, which results in large loss of exergy. It will be better to take steam of lower exergy from the different source such as lower temperature header of the super heater, extraction from the turbine, re heater inlet header or any other pressure vessel etc. IV. MANAGEMENT OF VARIATION IN AIR Adverse influence of variation in air parameters have been recommended to be minimized in the subsections to come. Management of Air Cooled Devices If, the performance of various air cooled devices is poor due to reduction in sensible heat addition to the air on account of high ambient temperature, optimally increased air flow can partly reduce the avoidable loss. If, the air cooled device is an indirect contact heat exchanger, high velocity of hot media and recirculation of flow will also help in avoiding the avoidable component. If, the performance of various air cooled devices is poor due to reduction in latent heat addition to the air on account of high humidity, amount of air supply has to be increased to increase the total evaporation up to the most optimized limits and rest of the performance loss has to be treated as inevitable. Necessary control system is recommended to be retrofitted to improve the performance of above referred devices. Management of Wet Cooling Towers Cooling tower range, approach and drift are influenced by the variation of ambient air temperature and relative humidity. Inevitable loss of cooling tower performance is uncontrollable and hence it is recommended to install air flow variation system with induced draught cooling tower fan to partially curtail the loss of condenser vacuum in the situations of high heat and humidity so that the avoidable component of loss of efficiency due to poor condenser vacuum can be set aside. Management of Air Handling Devices Change in air temperature lead to proportional changes in air pressurizing fans, blowers and compressors driving electric motors power consumption. This power consumption is recommended to be accepted as inevitable for high flow, low discharge pressure fans and blowers. Low flow, high discharge pressure compressors have been recommended to be provided with pre cooler and inter cooler to minimize the avoidable loss. Management of Air Conditioning Systems There is absolutely no problem for the men/women and precision instruments if the air conditioning temperature and relative humidity are maintained in economy ranges of 18 o C, 50-60% RH in winter to 28 o C, 50-60% RH in summer rather than being adamant to the air conditioning alignment point of 25 o C, 50% RH or even a lower value than 25 o C in summer. Incumbents may put woolen cloths in the winter as usual and they may also prefer to opt for the internal air circulation in the summer for getting better comfort and save the energy

4 National Seminar on Thermal Power Plant Performance Management - NSTPPPM 39 wastage. This managerial decision of the concerned shall save the energy in general and contribute significantly in reducing the auxiliary power consumption in thermal power stations. V. MANAGEMENT OF VARIATION IN FLUE GAS EROSION Adverse effect of the tube erosion parameters i.e. shape, size grade, impact frequency, impact velocity, free stream velocity of the carrier fluid and surface condition of the tube, depend upon the following boiler operating and input parameters, which has been recommended to be minimized here under: Use of beneficiated coal reduces the frequency of impacting particles and in turn reduces the boiler tube erosion. In case of poor coal quality, coal blending and oil support also reduce the boiler tube erosion. Flue gas volume is proportional to moisture/oxygen/hydrogen/nitrogen in coal and volume of the combustion air. Coal components are uncontrollable and hence an accurate assessment of excess air has been recommended to keep free stream velocity within the erosion limits. Frequent use of soot blowing keeps the tube surface clean which do not allow the cross section area to reduce to a value at which free stream velocity can cross the erosion limits. An improper management of soot blowing itself causes the erosion of the tubes. Baffle plates can be used in high speed zone of boiler to keep the flue gas velocity within the specified ranges. Particle size can be controlled by maintaining pulverizers healthy. Reduced pulverizer capacity operation is essential in case of high ash & moisture content of coal, lower hard groove index and higher particle size (fineness) at its outlet. Furnace Vacuum and differential pressures across the wind box, platen super heater, re heater, final super heater and economizer also influence the impacting particle velocity. Well maintained boiler fans are essential to keep various deferential pressures within the specified ranges. Sufficient clearance must be incorporated at the design stage itself on the basis of erosion severity. Tubes of higher erosion resistance should be used. Boiler should not be allowed to run at higher loads with very poor coal Air ingress through men holes, peep holes/inspection doors and cracks should be minimized. VI. MANAGEMENT OF ENVIRONMENTAL POLLUTION A well maintained electro static precipitator is quite essential to keep the suspended particulate matter well within the ranges as prescribed by the extant pollution law. SO x reduction has become essential for high sulfur coal based stations by making use of fuel desulphurization unit and putting the flue gas desulphurization units at the discharge of the electro static precipitator. Use of low NO x burners has already been proved successful and hence recommended to be retrofitted in the older units. To prevent ozone layer depletion, leakage of the concerned gases has to be stopped. CO 2 is produced in abundance and increases quantity of the green house gases, which can be minimized either by forestation or by putting the decarburization plant before the chimney. Coal and ash transport of many thermal power plants have been observed to be more polluting than the plume effect and hence recommended to be improved. It is recommended to use pipe conveyor belts to transport the coal from coal mines to the plant and ash from plant to the exhausted coal mines. Noise control methods have to be adopted inclusive of optimizing the operation of drains and vents. Safety valve blow off must be prevented by adopting the all ways and means of controlling the steam pressure. VII. MANAGEMENT OF VARIATION IN GRID FREQUENCY Grid frequency is single parameters, which represents the total quality management of the entire power industry inclusive of electricity generation, transmission and distribution. This is incorporated in the turbine governing system, higher values of which reduce the load by initiating the closure of the steam control valves and vice versa. It is recommended to over ride such frequency signal to export power to the grid in accordance with power purchase commitment or otherwise in accordance with any other profit making alternate schedule. High frequency leads to increase the load on all the motors and other electrical appliances, which increase auxiliary power consumption in thermal power plants. An optimized use of auxiliaries helps in reducing this inevitable loss of auxiliary power consumption. It has been recommended for the multi unit auxiliary system to take up the operational requirement by using the minimum numbers of auxiliaries. VIII. CONCLUSION The thermal power plants are required to generate quality electricity at the cost of minimum consumption of input coal. To facilitate the heat liberation from supplied coal, combustion supporting air and initial ignition energy has to be supplied to the furnace as an integral part of the primary cause. Liberated heat is the effect of combustion system, which cause steam generation. Similarly all intermediate effect become the cause for the next process and hence regulation of every process and monitoring its cause and effect in measurable parameters help in improving the performance of associated process. Reasonable guidelines should be provided to concerned operators and supervisors to optimize the performance of most of the thermal power plant processes along with system wise integration of the same. Causal relationships amongst the thermal power plant process parameters also should be established for all the important subsystems separately and integrated for the entire thermal cycle, which provides valuable directions to analyze the process on micro level. These concluding remarks on the recommendations reveal that the total quality management of thermal power generation can be achieved by integrating best practices of all smallest processes and associated equipments.

5 National Seminar on Thermal Power Plant Performance Management - NSTPPPM 40 REFERENCES [1] A. B. Gill (1984) Site Efficiency Engineer, Drakelow Power Station, Power Plant Performance. Butterworths, London Boston Durban Singapore Sydney Toronto Wellington. Butterworth and Co (Publishers) Ltd [2] Bharat Heavy Electricals Limited Operation & Maintenance Instruction Manual, 4X250 MW, OP Jindal Super Thermal Power Plant, Jindal Power Limited, Tamnar, Raigarh (CG) [3] Chandra Shekhar (2003), Optimization of Flue Gas Exhaust Temperature, M.Tech. Thesis submitted in IIT Delhi (India) [4] CEA Reports (cea.nic.in and powermin.co.in) [5] K. C Yadav (2008),Management of Thermal Power Plant Performance Parameter, Ph. D. Thesis, Mechanical Engineering Department, Faculty of Engineering & Technology, JamiaMiliaIslamia, Okhla, New Delhi - 25, India. [6] PERGAMON PRESS, an imprint of Elsevier (1991) set ISBN: , Indian reprint (2008), set ISBN-10: , Modern Power Station Practice by British Electricity International Limited. [7] Tor-Martin and Tveit (2004), A methodology for improving large scales thermal energy systems, Applied Thermal Engineering 24 (2004) [8] V.T. Sathyanathan, K.P. Mohammad (2004), Prediction of unburnt carbon in tangentially fired boiler using Indian coals Fuel, ABOUT THE AUTHOR Dr.K.C. Yadav is a Mechanical Engineering Graduate from GB Pant University, Pantnagar Uttaranchal (1980), Post Graduate of Thermal Engineering from University of Delhi(1982) & PhD in Management of Thermal Power Plant Performance Parameters from Faculty of Engineering & Technology, JamiaMiliaIslamia (A Central University at Delhi ).He had worked in National Power Training Institute (Nagpur, Delhi & Guwahati) for 25 years from 1982to Some of the corporate he has worked in the past 5 years includes Lanco Infra Tech Limited, Jindal Power Ltd, Reliance Power Limited and Adani Power Limited. Dr Yadav has presented many papers in National/ International Seminars, Workshops and Conferences, published papers in an International Journal and coauthored NPTI Books & Simulators manuals. He has also conceptualized, designed and developed education and training curriculum, pertaining to the up gradation of knowledge and skillofthermal Power Plant Personnel Dr. AbidHaleem is Professor of Mechanical Engineering Department, Faculty of Engineering & Technology since January, Presently he is Honorary Director Internal Quality Assurance Cell (IQAC) of JamiaMilliaIslamia (A Central University by an Act of Parliament), New Delhi, India. He has served as Head, Mechanical Engineering and also Headed, MBA (Evening) program in his University. He has also been Honorary Visiting Professor at Department of Management Studies, IIT Delhi. Professor Haleem has more than hundred research papers to his credit, published in good international and national journals. He is Regional Editor Asia Pacific, for Global Journal of Flexible Systems Management, published by Springer. He is on the advisory / research boards of various institutions. He is also involved in various institutional activities (focused institutions & universities) related to policy planning, administration, accreditation, curriculum design, admission, evaluation and examination process etc. He has produced more than fourteen Ph D