NTPC RIHAND Pulverised Fuel Plant Operation & Maintenance PDF

,uVhihlh The Safe Operation and Maintenance of Pulverised Fuel fjgUn NTPC Plant RIHAND LOCATION MANAGEMENT INSTRUCTIONS The Safe Operation and Maintenance of Pulverised Fuel Plant...

,uVhihlh The Safe Operation and Maintenance of Pulverised Fuel fjgUn NTPC Plant RIHAND LOCATION MANAGEMENT INSTRUCTIONS The Safe Operation and Maintenance of Pulverised Fuel Plant RIHAND-LMI-OD-OPS-SYST-009 Rev 04, Aug 2022 {Document Reviewed on 02.12.2024} Approved for Implementation by: HOP (Rihand) RIHAND-LMI-OD-OPS-SYST-009 Rev.04 1 | 21 ,uVhihlh The Safe Operation and Maintenance of Pulverised Fuel fjgUn NTPC Plant RIHAND INDEX Page S.No. Topic No. 1.1 Introduction 3 1.2 Superseded Documents 3 1.3 Reference Documents 3 1.4 Records of Revision 3 1.5 Scope 3 2.0 Discussion and Background Information 4 2.1 The development of an explosive mixture 6 2.2 Sources of ignition 6 2.3 Instructions and training of P.F. plant operations and maintenance staff 8 2.4 Principles involved in the safe operation of P.F plant 9 2.5 Principles involved in the safe maintenance of P.F plant 9 2.6 Operational procedures 10 2.6.1 Fires in milling systems 10 2.6.2 Actions to be taken in a Mill System containing a fire 11 2.6.3 Putting a Mill System into Service 12 2.6.4 Taking a Mill System out of Service 12 2.6.5 Coal Flow Failure 13 2.6.6 Furnace Precautions 13 2.6.7 Burner Management System (BMS) / FSSS 14 2.6.8 Bunkers 14 2.6.9 Boiler ash and dust hoppers 15 3.1 Responsibility 16 3.2 Review 18 3.3 Distribution List 18 3.4 LMI Checklist - Annexure-I 18 Annexure-I 19 RIHAND-LMI-OD-OPS-SYST-009 Rev.04 2 | 21 ,uVhihlh The Safe Operation and Maintenance of Pulverised Fuel fjgUn NTPC Plant RIHAND 1.1 INTRODUCTION The ignition of finely divided combustible material in suspension in air can lead to a significant explosion and emission of flame. All such dusts, whether hot or cold, will be considered dangerous. This combustible material can be present throughout the boiler, from the milling plant to the precipitator hopper. It is essential therefore, that personnel. i) Are made aware of the risk of explosions and fires emanating from pulverized fuel (P.F) and hot fly ash. ii) Are provided with the necessary understanding as to now they can arise. iii) Are trained in the best manner in avoiding them. This LMI provides information & defines acceptable standards of practice to establish safe PF operation so that the personnel are made aware of the phenomenon of explosions, fires and consequence of PF fire. It will also provide detail requirement how to prevent such fire and to deal with any accidental fire. 1.2 SUPERSEDED DOCUMENTS RIHAND-LMI-OD-OPS-SYST-009 Rev.03 Nov 2021 1.3 REFERENCE DOCUMENTS COS-ISO-00-OD/OPS/SYST/009Rev. No.: 0 Aug. 2022 1.4 RECORDS OF REVISION  LMI format changed as per RIHAND-LMI-OD-GEN-002 Rev 04, July’2021 Clause 2.2.1.4 revised, Mill outlet temperature for Imported coal & Pellet firing provided. Clause 2.6.6 elaborated. 1.5 SCOPE This LMI describes the operation and maintenance practices necessary to give adequate awareness and understanding in order to minimize the possibility of explosions and fires occurring in and adjacent to P.F. plant. This LMI will be applicable to the pulverized fuel (PF) plant of 6 X 500 MW units of NTPC Rihand, each unit having its own system of PF mill, Raw Coal (R.C.) feeders & associated P.F. piping & auxiliaries with following technical specifications: STAGE#1 A: PF Mills  No. of mills per unit- 8  Manufacturer: M/S GE Power India Ltd. RIHAND-LMI-OD-OPS-SYST-009 Rev.04 3 | 21 ,uVhihlh The Safe Operation and Maintenance of Pulverised Fuel fjgUn NTPC Plant RIHAND  Type -HP1003 Mill  Coal input per mill 57 T/Hr. (Maximum) B: RC Feeders  No. of feeders per unit- 8.  Manufacturer: M/S Schenck process solution India Pvt Ltd.  Normal capacity- -74 T/Hour STAGE#2 A: PF MILL  Total 10 no. of mill.  Type: XRP-1003 Bowl Mills, Conical Grinding rolls 3 no. per mill  Design capacity of Mill: 60 MT/hr. for pulverized fuel fineness 70% through 200 mess with a raw coal of grind ability index 55 HGI & and total moisture of 14%.  Manufacturer: BHEL B: RC Feeders  Type: 36 Gravimetric Feeder  Minimum - 16.4 T/Hr, Maximum - 82 T/Hr.  Manufacturer: BHEL STAGE#3 A: PF MILL  Total 9 no. of mill.  Type: XRP-1003 Bowl Mills with Planetary Gearbox,  Design capacity of Mill: 68 T/hr. maximum with 55 HGI & total moisture of 12% in raw coal.  Manufacturer: BHEL B: RC Feeders  Type: 36 Belt type gravimetric feeder  Minimum - 16 T/Hr., Maximum - 80 T/Hr.  Manufacturer: BHEL 2.0 DISCUSSION AND BACKGROUND INFORMATION 2.0.1 The Possible sites of Pulverized Fuel Explosions This LMI mainly considers explosions initiated in suspensions of P.F. in air, although brief reference is made to coal; gas explosions. These suspensions are an essential part of the processes inside P.F. mill systems and boiler furnaces. However they can also arise as a result of the following: RIHAND-LMI-OD-OPS-SYST-009 Rev.04 4 | 21 ,uVhihlh The Safe Operation and Maintenance of Pulverised Fuel fjgUn NTPC Plant RIHAND I. Leakage of P.F. from the plant II. Careless cleaning up of deposits of P.F outside the plant III. The disturbance of deposits of P.F outside the plant, by other circumstances, including P.F explosions. IV. The release to atmosphere of hot P.F fly ash containing a high concentration of unburnt carbon, such as can occur at precipitator hoppers. Although no specific reference is made to explosions resulting from ignition of oil, it is nevertheless an important area of potential risk which could result in damage being sustained to the P.F Plant and would therefore be considered when formulating operational procedures. 2.0.2 Factors that Cause an Explosion Two factors brought together can cause either a P.F. or coal gas explosion, namely. i) A concentration of combustibles in suspension in air, falling within the explosive concentration limits. ii) A source of ignition of adequate energy. 2.0.3 Plant which may be affected by P.F Explosions 2.0.3.1. In discussing the development of explosive mixtures, it is necessary to give consideration to all those plant items that can affect, or are affected by, an explosion. 2.0.3.2. Pulverized Fuel Plant is defined as that bounded by, and including: I. The top of the coal bunker II. The P.F. burners fed by the mill III. In the case of mills which each have an associated primary air fan; the discharge of the primary air fan. IV. For all other mills, the points at which the hot air and tempering air ducts leave their respective main supply ducts. 2.0.3.3. The Mill system is that part of the P.F plant bound by the bunker outlet(s) and (ii) (iii) or (ii) and (iv) above. 2.0.3.4. In addition, the plant areas encompassing the furnace, the gas passes and precipitator sections also need to be considered. 2.0.3.5. Whilst this Directive deals primarily with mills and P.F pipe work system, it is important to remember that conditions in the coal bunker and the primary air ducting can influence the development of an explosion in the mill and, conversely, the bunker and the primary air ducting may be affected as a result of a mill explosion. RIHAND-LMI-OD-OPS-SYST-009 Rev.04 5 | 21 ,uVhihlh The Safe Operation and Maintenance of Pulverised Fuel fjgUn NTPC Plant RIHAND 2.1. THE DEVELOPMENT OF AN EXPLOSIVE MIXTURE 2.1.1. In a Milling system 2.1.1.1 At mill startup and shutdown, the development of an explosive mixture of P.F and air is an unavoidable event. In mill bodies and classifiers, the air/fuel ratio at start-up is dependent on the shutdown condition and on any subsequent work carried out which may have affected the quantity of coal in the system. In P.F pipe work systems and mills, particularly the vertical spindle type, the air/fuel ratio must change from very weak to normal, as the mill is brought into service. The air/fuel ratios normally found in milling systems lie between 1.5/1 and 3.5/1. However, experience within the industry shows that explosions cannot be sustained in mixtures where the air to fuel ratio is weaker than 5/1. 2.1.1.2 An explosive mixture can also result during operation. Caused by: Loss of coal feed. Air ingress on suction mills. Faulty control or indication systems. Weak mixtures arising from manual control of air and/or fuel supply. 2.1.1.3. It is also possible for accumulations of static coal to ignite, producing coal gas which, in the presence of air and an ignition source, may explode. This is a rare event and most likely to happen in the coal bunker outlet/feeder area. It requires a larger accumulation of coal than normally occurring inside the mill. 2.1.2. In the Boiler Furnace The accumulation of an explosive mixture in the furnace is an abnormal event. It follows the failure to ignite an accumulating cloud of fuel, or loss of ignition during operation. There is time for an explosion to be avoided if the operator is made aware of the situation and takes prompt and effectively planned action. Complete loss of ignition is a rare event that is usually preceded by a period of deteriorating combustion conditions. Partial loss of ignition can also occur, with potentially damaging consequences when re-ignition occurs. Unstable combustion conditions are frequently associated with fuel quality changes 2.2. SOURCES OF IGNITION 2.2.1. Fires due to Spontaneous Combustion in a Mill System 2.2.1.1. Pulverized fuel in suspension cannot be ignited by the primary air. Any credible primary air temperature is inadequate to do so. However, Mechanical considerations limit the mill outlet temperature within the P.F. Pipe work system. RIHAND-LMI-OD-OPS-SYST-009 Rev.04 6 | 21 ,uVhihlh The Safe Operation and Maintenance of Pulverised Fuel fjgUn NTPC Plant RIHAND 2.2.1.2 If any uncooled deposits of coal are allowed to stand in a mill system, they are likely to catch fire eventually, even when the air is at room temperature. This is due to spontaneous combustion which has a greater probability as the fuel and surrounding air temperature increases. Classifier rejects returned to mill inlets may be particularly susceptible. Any coal, or other combustible material, lodging in this area presents a fire risk. Similar consideration applies to dry coal deposits in feeder bodies, particularly on pressurized mill systems. 2.2.1.3. Deposition in large pipework systems is commonplace. Deposits are generally cleared it the mill is purged but are constantly being reformed and replaced as the air to fuel ratios change during normal operation. Whilst deposition would be minimized, a safety hazard only exists in the event of a blockage or following a mill trip which leaves a deposit. This deposit will be prone to spontaneous combustion. 2.2.1.4. A minimum transport velocity of 20 m/s will minimize deposition of P.F. in a typical pipework arrangement. However, the minimum figure for a particular system may be different from this, depending on the aerodynamic characteristics of the pipe work and the air/fuel ratio in use. On systems with long (>10m) sections of horizontal pipework, deposition would be expected. In situations where depositions are known to occur, particular attention must be paid to mill and P.F pipework purging procedures. In the vertical spindle mill the blockage of mill rejects passage will cause accumulation of rejects in the chamber and the friction between pyrites scraper and accumulated rejects, in the presence of primary air, presents a fire risk. More clearance than specified between Scraper and mill bottom plate also will lead to accumulation of rejects in scraper chamber which is prone to fire risk. Trimming of mills will be done regularly, ensuring specified clearance to minimize coal rejects. Any passing of pyrites hopper valves or leakages at pyrites hopper flanges will be prone to spontaneous combustion in rejects hopper. Excessive temperature of pulverized fuel in the mill will be avoided, and would not be allowed to exceed a temperature of 95-degree C. However, this Limiting value of Mill outlet temperature to be further reduced in case of imported Coal as per SOP on imported Coal as below: Sl No Specific Coal consumption (ton/MWh) Mill outlet temp (Deg C) 1 0.5 60 2 0.55 63 3 0.6 65 4 0.65 70 5 0.7 80-85 RIHAND-LMI-OD-OPS-SYST-009 Rev.04 7 | 21 ,uVhihlh The Safe Operation and Maintenance of Pulverised Fuel fjgUn NTPC Plant RIHAND In case of Pellet firing Mill inlet temperature set point would be controlled in such a way that operator cannot give more than 180 C set point & Alarm for mill outlet temperature >70 O C to be provided. 2.2.2 Ignition from the Furnace 2.2.2.1. Furnace over pressurizations can result in ignition sources being introduced into milling systems in the case of correctly operating mills, the temporary reduction of P.F transport velocity will be sufficient to enable significant reverse flow to occur but, with standing, un isolated mill systems reverse flow can occur in the absence, or non-operation of, the non- return valves. 2.2.2.2. The most likely causes of ignition from the furnace into a running or incoming mill and P.F. pipework system are: I. A pressure disturbance in the furnace that reverses the forward flow of air/fuel, so that a flame may be propagated, or carried from the furnace into the P.F pipework. II. Failure of the primary air supply to the mill, with the mill open to the furnace and continuing to pulverize coal. III. Failure to ensure that the hot and cold air supplies to running mills are not unduly disturbed by incoming mills. IV. Failure to ensure, when bringing additional suction mills into service, that the exhausters are running before the mill outlet dampers are opened. V. Operating at too low a load. 2. It would be noted that correctly operating non-return valves in the P.F lines from a running mill give total protection against furnace ignition sources only when the forward flow of air/fuel is halted or reversed and not while it is merely slowed down. 2.2.3. Ignition in the Boiler furnace other than the Normal Flame Other sources of ignition in the furnace are hot refractory, ash and slag deposits. 2.2.4. Ignition outside the Boiler Such sources. e.g. from welding and burning operations, are dangerous if P.F. is escaping from the plant, or if cleaning operations involve the risk of creating clouds of dust containing significant quantities of P.F. in their vicinity. These will be eliminated, as far as is reasonably practicable, while the risk exists. There is also a danger that a source of ignition may emerge from the boiler. 2.3. INSTRUCTIONS AND TRAINING OF P.F. PLANT OPERATIONS AND MAINTENANCE STAFF. Fires and explosions can be avoided by maintaining high standards of operation and maintenance. This objective requires adequate instruction, training and supervision. At each site a glossary of standard terms related to the plant will be compiled and included in the RIHAND-LMI-OD-OPS-SYST-009 Rev.04 8 | 21 ,uVhihlh The Safe Operation and Maintenance of Pulverised Fuel fjgUn NTPC Plant RIHAND station P.F Operating Instructions. Each person concerned with the operation or maintenance of P.F. plant will be trained to a competent level and the continuing competence of operations staff will be reviewed annually. Personnel who might be promoted temporarily to a post where they will be directly concerned with the operation of P.F. Plant will also be instructed and trained accordingly. Records of training will be maintained. 2.4. PRINCIPLES INVOLVED IN THE SAFE OPERATION OF P.F PLANT 2.4.1. The basis of safe firing of P.F rests upon the availability and reliability of oil burners. Without oil burners, high speed would not be possible, and it is therefore essential that oil installation is fully understood by all staff operating P.F. Pl ant. 2.4.2. Milling systems will not be put into service if a fire is suspected in any part including the coal feeder or bunker. 2.4.3. P.F. fuel oil and gas will not be allowed to accumulate as a consequence of leakage. 2.4.4. The velocities in the P.F. pipes on a running mill will be sufficient to minimize deposition of fuel and be above 15 m/s or above or as specified by manufacturers. 2.4.5. The minimum number of personnel will be allowed in the vicinity of P.F. plant and only then under controlled conditions when unstable operating conditions are suspected. 2.4.6. Operations will be instructed on how partial or complete loss of ignition, mill fires, and coal flow failure are indicated and how to proceed in such circumstances. 2.4.7. A clear line of command and communication will be established to ensure that any abnormalities are urgently brought to the attention of the appropriate staff. 2.4.8. All P.F. stations must have an evacuation procedure, adequate for the risk category of the particular plant. The evacuation Zone and procedure will be detailed in the Location P.F. Operating Instructions. 2.4.9. Following any indication of over pressurization in a P.F. system, including the operation of pressure relief vents, the milling system will be treated as containing an ignition source. 2.4.10. On a standing mill, keep the mill body temperature as low as is practicable. 2.4.11. On a running mill, the maximum working mill outlet temperature permitted in normal operation will depend on the nature of the coal and the design of the milling system. Establish an acceptable maximum mill outlet temperature for each individual installation. The alarm temperature will be set 5-degree D higher than this maximum temperature. 2.5. PRINCIPLES INVOLVED IN THE SAFE MAINTENANCE OF P.F PLANT 2.5.1.1. Maintain the design features, e.g. mill fire detectors, that protect against fires and explosions. Management will define these features, specify the inspection frequencies, record inspection results and any remedial action taken and the action to be taken in RIHAND-LMI-OD-OPS-SYST-009 Rev.04 9 | 21 ,uVhihlh The Safe Operation and Maintenance of Pulverised Fuel fjgUn NTPC Plant RIHAND the event of defects arising. All practicable steps will be taken to eliminate sites in the mill system where P.F and raw coal may lodge. All leaks of P.F fuel oil and gas will be dealt with promptly. 2.5.1.4. Air leaks on suction system may render the air/fuel mixture explosive and must be rectified. 2.6. OPERATIONAL PROCEDURES 2.6.1. Fires in milling systems 2.6.1.1. Do not Start or shut down a mill without checking for evidence of a fire in the system. There is a need to clarify the different indications given when there is a fire in a mill and when there has been a loss of coal to the mill. In general, a fire in a mill system is indicated by: I. Rapidly rising and uncontrollable mill outlet temperature with no change in power consumption. II. Severe pressure fluctuations III. Lifting of relief vents on the mill or feeder (if fitted). IV. Visible signs of overheating, smoke emission or sparks. V. Increased response from any mill fire detection system. A loss of coal feed, however, is indicated by: I. Increase in mill outlet temperature system. 2.6.1.2. Care will be taken not to confuse the high mill outlet temperature caused by a fire with that caused by loss of coal feed. If an operator receives indication from the pulverized fuel plant instrumentation, especially during a transient situation which either:  Appears to be misleading or inconsistent  Which may lead to an emergency situation He will: I. Evacuate restricted areas to minimum staffing levels II. If the emergency has not been pre-determined, consider, together with his supervisor, the appropriate course of action. Within the Location P.F operating Instructions, an appropriate course of action will be defined for pre-determined emergency situations. 2.6.1.3. If a fire is suspected in a mill system: RIHAND-LMI-OD-OPS-SYST-009 Rev.04 10 | 21 ,uVhihlh The Safe Operation and Maintenance of Pulverised Fuel fjgUn NTPC Plant RIHAND I. Evacuate restricted areas to minimum staffing levels. II. Do not open up any part of the mill system to the atmosphere, e.g. by opening up coal feeder doors. III. If the mill is shut down when the fire is detected, check that all isolation features are tightly closed. IV. If the mill is off-loaded DO NOT attempt to put it on-load. V. If the mill is on-load DO NOT attempt to take it off-load until a fire is confirmed. 2.6.2. Actions to be taken in a Mill System containing a fire 2.6.2.1. When a fire is detected in the mill body or classifier and does not appear to be in mechanical danger, increase coal feed to the maximum practicable with the coal feeder on manual control. Increase cold air flow to the maximum practicable and evacuate the personnel in the vicinity of the affected mill system. Run like this for a period specified in the P.F Operating Instructions, and then reduce coal feed to normal. 2.6.2.2. If the fire persists, or the mill is in mechanical danger, or the coal feed is lost, withdraw personnel from the area and prepare to shutdown the mill system. In shutting down a mill system that contains a fire, the primary objective is to minimize the risk of an explosion occurring, whether from flash back or from ignition of the air/fuel mixture by the fire as the air/fuel ratio traverses the explosive range. The following methods are for dealing with confirmed fires in the mill systems. A. Stop the air flow through the system by closing the isolating damper/valves. B. Trip the primary air fan or exhauster, mill coal feeder, seal air fan and classifier. C. After the fire has been positively extinguished and the mill has cooled off, purge the mill system thoroughly. If advice on the length of the necessary purge time is required, the operation services, Boiler Group will assist. D. In cases where firefighting equipment is installed, the P.F Operating instructions will give guidance on the circumstances and methods of use. E. Water would not be admitted to mill having fire to avoid cracking of mill internals. F. To quench the fire, admit inert gas or steam to the mill. G. Mill inerting system lines to be kept available. H. All Mills to be equipped with remote controlled motorized valves. RIHAND-LMI-OD-OPS-SYST-009 Rev.04 11 | 21 ,uVhihlh The Safe Operation and Maintenance of Pulverised Fuel fjgUn NTPC Plant RIHAND 2.6.3. Putting a Mill System into Service 2.6.3.1. Do not start up a mill system if there is evidence of fire in that system. 2.6.3.2. On medium and high-speed mills, it is necessary to establish the air flow before coal is admitted and, therefore, for these types of mill, the mill outlet isolating valves have to be opened first in the start-up sequence. At this point, carry out a check on the mill fire detection instrumentation, where this equipment is fitted, to ensure that there is no indication of a fire in the mill. 2.6.3.3. When the mills are in service, maintain air/fuel mixture velocities in the P.F lines above; the minimum transport velocity for the installation, determined as follows: I. Where no estimate has been made of the velocity to minimize deposition, the minimum transport velocity will be taken as 20 m/s. II. Where an estimate of the minimum velocity to minimize deposition has been made, and is greater than 15 m/s, then that estimate will be regarded as the minimum transport velocity. III. Where an estimate of the minimum velocity to minimize deposition has been made, and give the figure below 15 m/s the minimum transport velocity will be deemed to be 15 m/s. 2.6.4. Taking a Mill System out of Service 2.6.4.1. Do not shutdown a mill system if there evidence of fire in the system. 2.6.4.2. As far as practicable, when taking a mill system off-load under normal circumstances, maintain the minimum transport velocity and: I. Firstly, stop the coal feed and increase the proportion of cold air in the primary airflow, if necessary, to control the rise in mill outlet temperature. On medium and high-speed mills, run the mill until the remaining coal is at or above the minimum level. II. In the case of medium and high-speed mills increase the airflow through the mill as the mill empties, adjusting the mill air inlet temperature to control the outlet temperature within the specified limit. When the mill is judged to be empty, stop the mill motor III. Mills would be cooled to as low a temperature as practicable both immediately before and during shutdown. This is particularly relevant in the case of tube ball mills which contain an appreciable amount of fine coal. With such mills, the mill outlet temperature at shutdown would be as near 50 degree C as is practicable. Medium and high-speed mills contain much coarser coal when shut down and some relaxation of the recommended mill outlet temperature at shut down is possible. A target temperature of