Mill Inerting

BEST PRACTICES for MILL INERTING & FIRE SUPPRESSION

Effective Date 5-6-2008

PRB COAL USERS’ GROUP (PRBCUG)

Best Practices

MILL INERTING & FIRE SUPPRESION

Revision Number

Effective Date

Pages Affected

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5-6-08

All

Description of Revision Initial Issue

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Table Table of Contents 1.0

INTRODUCTION ………………………………… ……………………………………………………..…….. …………………..…….. 3

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SCOPE………………………….. …………………………………… …………………………………………... ……... 5

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DISCUSSION OF INERTING & FIRE SUPRESSION………………….. 7

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MANUFA CTURER RECOMMENDATIONS RECOMMENDATI ONS …………………..……. …………………..……..…. .…. 16

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INDUSTRY INCIDENTS ………………….……………… ………………….……………………………….. ……………….. 18

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MILL INERTING INERTING BEST PRACTICE ………………………..…..……..…. ………………………..…..……..…. 25

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FIRE SUPRESSION BEST PRACTICE ………………………………… ………………………………….. 28

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DESIGNING DESIGNING A MILL INERTING INERTING SYSTEM ……………..………… ……………..……………..... …..... 29

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MILL MIL L INERTING/FIRE SUPPRESSION DESIGN CHECKL IST………. 31

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DESIGNING A FIRE SUPPRESSION SYSTEM ………………..……… 32

APPENDIX A – NFPA PART 85 REFERENC REFERENCES ES ………………..………….…. ………………..………….…. 33 APPENDIX B – PATENT SOURCE SOURCE INFORMATION………….. INFORMATION…………..………………. ………………. 36

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Table Table of Contents 1.0

INTRODUCTION ………………………………… ……………………………………………………..…….. …………………..…….. 3

2.0

SCOPE………………………….. …………………………………… …………………………………………... ……... 5

3.0

DISCUSSION OF INERTING & FIRE SUPRESSION………………….. 7

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MANUFA CTURER RECOMMENDATIONS RECOMMENDATI ONS …………………..……. …………………..……..…. .…. 16

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INDUSTRY INCIDENTS ………………….……………… ………………….……………………………….. ……………….. 18

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MILL INERTING INERTING BEST PRACTICE ………………………..…..……..…. ………………………..…..……..…. 25

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FIRE SUPRESSION BEST PRACTICE ………………………………… ………………………………….. 28

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DESIGNING DESIGNING A MILL INERTING INERTING SYSTEM ……………..………… ……………..……………..... …..... 29

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MILL MIL L INERTING/FIRE SUPPRESSION DESIGN CHECKL IST………. 31

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DESIGNING A FIRE SUPPRESSION SYSTEM ………………..……… 32

APPENDIX A – NFPA PART 85 REFERENC REFERENCES ES ………………..………….…. ………………..………….…. 33 APPENDIX B – PATENT SOURCE SOURCE INFORMATION………….. INFORMATION…………..………………. ………………. 36

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1.0


INTRODUCTION This paper has been prepared for the Power River Basin Coal Users Group to determine the best practices for coal mill inerting and fire suppression in use by the companies that are burning Powder River Basin (PRB) fuels, or are recommended by equipment manufacturers handling PRB fuels. The purpose of this document is to review the methods of pulverizer inerting systems in use by the power industry as it relates to the use of PRB fuels in the production of power. The paper will identify, summarize, and categorize the best practices of the industry for coal pulverizer inerting and fire suppression. The definition of “best practice” may vary with the type of mill and the demands of the operating units to meet the needs of individual utilities, and there may not be one common solution that can be identified for all cases. Preparation included the review and analysis of responses to questionnaires sent to all PRB Coal Users’ Group members. Following is a brief profile of those responses:

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General Response A total of 121 units replied 77.7% (94 units) have an inerting system installed whether they use it or not 57.4% (70 units) have an inerting system installed and use it 9.8% (12 units) do not inert 32.0% (39 units) provided no information Choice of Inerting Media 66.0% (62 units) of those that do have an inerting system inert with steam 31.9 (30 units) of those that do have have an inerting system inert with water and mist 22.3% (21 units) of those that do have an inerting system inert with CO2 2.1% (2 units) of those that do have an inerting system inert with N 2 9.6% (9 units) of those that do have an inerting system do not use it.

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Type of Mill 56.2% (68 units) of all units had roller mills 25.6 % (31 units) of all units had ball and race mills 8.3% (10 units) of all units had ball tube mills 3.3% (4 units) of all units had attrita mills 7.4% (9 units) of all units did not specify the type of mill

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2.0


SCOPE The purpose of this document is to review the methods of pulverizer inerting systems in use by the power industry as they relate to the utilization of Powder River Basin (PRB) fuels in the production of power. The paper will identify, summarize, and categorize the best practices of the industry for coal pulverizer inerting and fire suppression. The definition of “best practice” may vary with the type of mill and the demands of the operating units to meet the needs of individual utilities, and there may not be one common solution that can be identified for all cases. In this paper the term “coal pulverizer” and the term “coal mill” are used interchangeably with equal meaning. Identification of such systems will vary, since by the evidence presented there are several companies and units that have no inerting or fire suppression system in place, and yet have experienced no mill puffs or explosions. There are also documented explosion incidents by companies that do operate inerting systems on a regular basis on plants designed to fire PRB coal. The methodology employed includes: •

Survey of PRB Coal Users’ Group for inerting practices.

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Review of literature for inerting procedures.

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Review of the requirements of the regulatory bodies for protection.

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Review of GADS information as it relates to pulverizer fires and deratings. Review of PRB Coal Users’ Group survey for effectiveness of the inerting procedures. Review of manufacturers’ recommendations.

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This document also attempts to describe the system utilizations, designs and requirements of the regulatory bodies governing new “inerting” system design and construction, mandated upgrade retrofits, and modifications to reduce the possibility of a mill fire or explosion. The best practice will always be subject to the needs of the individual utility, the unit and the requirements of the system. The application of best practices is always influenced by the individual company, and the unit analysis of risk associated with the installation of the system and the possibility of damage associated with the system. Identification of such systems will vary, since by the evidence presented there are several companies and units that have no inerting or fire suppression system in place, and yet have experienced no mill puffs or explosions. There are also documented explosion incidents by companies that do employ inerting systems on a regular basis on plants designed to fire PRB coal. This paper provides the reader with the tools needed to make informed decisions, when designing or discussing mill inerting systems.

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3.0


DISCUSSION OF INERTING AND FIRE SUPPRESSION 3.1

Mill Inerting Options

Mill inerting is the rapid introduction of an inert media into a coal mill in sufficient amount to reduce the volumetric oxygen content of the coal mill atmosphere to a concentration low enough to inhibit and effectively prevent the rapid combustion of coal dust or carbon monoxide. The level to which the volumetric oxygen content must be reduced to prevent “explosions” varies, according to the literature and manufacturers’ recommendations, from 13% to 15%. There are patents on the design and installation of mill inerting systems. Most of the patents apply specific techniques to specific types of mills and operating conditions. References to several registered patents for various mill types are included in Appendix B of this report. Review of available patent information, and discussion and questionnaire replies from members of the PRB Coal Users’ Group all indicate that mill inerting has taken many forms. Systems described include many combinations of working fluids, working fluid admission locations, sequences of operation, triggering events and mill clearing procedures. The common feature of all inerting systems is the introduction of a working fluid that reduces the oxygen concentration. Some systems follow the initial inerting with a secondary admission used to maintain the inert atmosphere. The primary working fluids in use are CO 2, nitrogen and steam. A flow-through inerting system is designed to admit the working fluid in sufficient quantity to sweep the fines from the mill during the startup and shutdown operations. System controls and instrumentation that determine the total transport media through the mill must be considered in the installation of any flow-through inerting system. The working fluid injection location seems to be an inconsistent matter of choice by the various implementers of the systems. Locations include the primary air inlet ductwork, the un-millable material rejection

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area (pyrites hopper), the fuel inlet piping, the mill outlet area, and the primary size separation area (classifier) inside the mill. Fire suppression systems are distinguished by low flow or rapid dispersion of the working fluid into the mill at low velocity to avoid disturbing mill fines and placing the fines into suspension. Some systems also flood the mill with water to ensure that hot materials are extinguished, and some systems then introduce water to assist moving the material out of the mill via the pyrites hopper discharge. The National Fire Protection Association (NFPA) part 85 Appendix A describes the system, and operational requirements of systems for prevention of explosions and fire suppression in milling systems. At this time it is not a requirement of NFPA part 85 to incorporate these systems into the Burner Management System (BMS). It is however, a recommended practice. From NFPA Annex K.5 Coal Firing General Considerations:

It takes as little as 1.4 kg (3 lb) of pulverized coal in 28.3 m 3 (1000 ft3) of air to form an explosive mixture. Since a large boiler burns 45.4 kg (100 lb) or more of coal per second, the safe burning of pulverized coal necessitates strict adherence to planned operating sequences. (See 6.8.5 for sequences of operation.)

NFPA 85 refers to the following sections in the index for inerting: Multiple Burner Boilers . . . . . . . . . . . .. . . . . . . . 6.6.5.2.5.4(C) 6.7.5.2.5.4(B) Pulverized Fuel Systems . . . . . . . . . . . . . . . . . . 9.4.4 9.4.5.1.1 9.4.6.8 9.5.4.2.1 9.5.4.2.2.2(B) 9.5.4.3.3 9.6.3.4 3.1.1 Steam Inerting Media

Steam is a cost effective method of inerting and fire suppression, and is the inerting and fire suppression media of choice by most users. It works well for both inerting and fire suppression, although 8



it does require a rather significant flow to maintain an inert atmosphere. The majority of the users report using steam as an inerting media with good success. The main problem with steam is that the steam condenses in the mill, which allows the mill atmosphere oxygen content to be restored. The use of steam during mill startup seems to cause few problems as the control of the mill outlet temperature outweighs the amount of moisture added by the inerting system for the PRB fuel. For example, 8,000 pph of inerting steam injected for eight minutes at mill startup results in an additional drying requirement of 1,066 pounds of moisture. During this same time period 36,000 pph of fuel (Assumed for 25% feeder speed on a large hypothetical mill requiring the above referenced steam flow) containing 30% moisture would place 1,439 pounds of moisture in the mill. The sum of both quantities is 2,505 pounds, which is easily within the drying capability of the primary air system. During the same eight minute period at full load this same primary air system has to remove 5,760 pounds of moisture from the fuel. Steam inerting on restart of the mill again provides a method of reducing the oxygen concentration without exceeding the mill’s capacity to vacate the steam and dry the coal as the mill is restarted. Metal surfaces in the mill are at a temperature significantly lower than the steam, causing the steam to condense and allow the volumetric oxygen concentration to increase and begin to approach normal atmospheric conditions. Tests at one facility indicated that the condensation rate is about the same for both hot and cold mills. A few users have actually measured the oxygen concentration in the inerted mill, and the rate of inerting decay due to leakage and condensation One facility actually performed tests to determine the rate at which the inerting occurs, and the recovery rate comparing both steam and CO2. The tests used both a dry and a wet percent O 2 analyzer. The wet analyzer was accurate, as the dry analyzer always read about 20.9% on a dry basis. All tests were conducted on the same

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Foster Wheeler MBF mills at the same plant under the same conditions. Results are interesting: Inerting Steam Flow Rate: Elapsed Time to Inert (<14% O 2 wet): Elapsed Time to 0% O 2: Recovery Time from 14% to 18% O 2:

1,875 pph 30 min Could not get below 12% 2 min

Inerting CO 2 Flow Rate: Elapsed Time to Inert (<14% O 2 wet): Elapsed Time to 0% O 2: Recovery Time from 14% to 18% O 2:

7,250 pph 1.5 min 6 min 1.4 min

Several interesting conclusions can be drawn from the test results: Condensing plays a minor part in the recovery rate, increasing the rate with steam approximately 7% faster than that of CO2. In this case 1,875 pph steam flow is insufficient if the goal is rapid inerting. Also in this case 7,250 pph CO 2 was too much. By the time inerting was accomplished mill internal pressure was 5”wc higher than the primary air pressure indicating that CO 2 was actually leaking into the primary air system via high pressure. Thus, testing can determine an optimal media injection rate. •

•

•

For that reason steam should continue to be added to the mill during either the inerting or fire suppression mode. A lower flow rate of steam could be used to maintain an inert atmosphere for a predetermined period of time, then shut off and the mill monitored via mill temperature and/or CO concentration to verify that the fire is extinguished prior to opening or operating the mill. Inerting nozzles would be installed in the primary air inlet ductwork and the fire suppression nozzles somewhere in the top of the milling area and underneath the classifier section in the mill discharge area. Nozzles and piping in these areas are subject to significant wear and pluggage, and should be oriented away from the primary flow direction, with nozzle inlet piping as short as possible.

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The possibility of suffocation is significantly reduced by the collapse of the steam, which does provide a limited safety factor associated with steam as an inerting and fire protection media. 3.1.2 CO2 Inerting Media

CO2 is an outstanding inerting and fire suppression media. The main problems with CO 2 are 1) That the primary purpose of the CO2 storage tank may be for other equipment, and 2) In a fire suppression mode the system is used infrequently, causing maintenance issues that tend to render the system’s operation less than satisfactory. Care must be taken with the design and location of CO 2 inerting nozzles to prevent liquid CO 2 from being admitted directly onto hot surfaces. This can cause significant damage to mill and duct metal components. According to one manufacturer’s recommendation the required CO 2 inerting flow at startup and shutdown of the same hypothetical mill is about the 23,000 pph for eight minutes. This flow rate results in a total use of approximately 400 pounds of CO 2 per inerting cycle. If the mill fire suppression cycle includes the initial inerting flow described above for eight minutes followed by a 25% flow rate for an additional thirty minutes, then the total single-mill cycle would require 3,275 pounds of CO 2. For a simultaneous 6 mill trip at the above rate, a typical 10 ton storage unit would be drained in this process. The size of a CO 2 system infrastructure that may be employed by the inerting system and possibly the fire suppression system as well as other plant usages (ie, fire protection and generator purging) must be carefully analyzed to assure that adequate supplies are maintained for all emergency situations. See Section 3.1.1 for appropriate CO 2 inerting test results. Confined space entry procedures for an adequate breathable atmosphere must be followed with this media, since it is heavier than air and will settle in the low areas of the mill. Care must also be taken to protect personnel and equipment from the extreme cold of this media. 11



3.1.3 Nitro gen Inerting Media

Nitrogen performs well as an inerting media. The required flow rates are reduced by about a third from that described above for CO2. Nitrogen (N2) is not commonly used in power plants except for boiler caps. Most plants don’t have the infrastructure to support the installation of an inerting system without additional media storage. The cost of this infrastructure is significant and if the material is stored in a liquid form, as with CO 2 care must be taken to protect personnel and equipment from the extreme cold of this media. Confined space entry procedures for adequate breathable atmosphere must be followed with this media. Since it very closely approximates the weight of air, nitrogen is not easily displaced by dispersion. 3.1.4 Flue Gas Recir cul ation Inerting Media

Another inerting philosophy is to take flue gas from the precipitator outlet through a booster fan or into an exhauster mill (sized to handle the flow and temperature). This process uses hot (approximately 300 F) flue gas to dry the coal in a reduced oxygen atmosphere, typically 2-2.5% O 2, which even when combined with mill inleakage will result in an oxygen concentration lower than the normal explosive limits. Flue gas may be a good choice for inerting of milling systems once the unit is in operation at a load high enough to ensure that coal drying and oxygen level can be controlled. Obviously hot flue gas is unavailable at cold startup, and of limited supply after a Master Fuel Trip. Thus, mill inerting and mill fire suppression would both require a backup plan. 3.2

Fire Suppression

Both NFPA and the pulverizer manufacturers offer recommendations concerning the use of a fire suppression system in a coal pulverizer system. The fire suppression system is usually specified for deployment when: •

There is a high mill discharge temperature tripping the mill. 12


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The mill trips for another reason under load. There is evidence of explosive gas in the mill.

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There is evidence of a fire in the mill.

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The design and application of fire suppression varies with the pulverizer manufacturer. Fire suppression systems can be similar to an inerting system where steam or another inert gas is admitted to the mill. The inert media is then maintained until the fire is out, until other dry fire suppression media is subsequently admitted to the system, or until water is admitted to the system. Steam can be admitted into the primary air inlet area of the mill for inerting, and can also be admitted into the main pulverizer, the upper grinding area, and the classifier to assist in fire suppression. Metal surfaces in the mill are at a temperature significantly lower than the steam, causing the steam to condense and form a water blanket to assist in smothering a fire. There are a variety of systems recommended for clearing the mill once the fire is removed. Two of these involve opening the mill and manually removing the coal left in the milling and under-bowl areas after all evidence of a fire in the mill is out. Another technique applies water to the above-bowl and under-bowl areas to flush the coal into the pyrites reject system without opening the mill. All recommendations of both manufacturers and NFPA include a thorough inspection of the milling system internals after an overheating incident to confirm that no damage has occurred to the mill which could allow coal fines to hide out in the milling process. A CO monitoring system as referenced below can indicate the presence of a smoldering fire in the mill well before the fire is detected by high mill outlet temperature. This indication may allow plant operators to respond more quickly to provide the addition of fire suppression media during a fire suppression incident. 3.2.1 CO Monit ori ng

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A limited number of users indicated the inclusion a CO monitoring system for all mills, both in-service and idle, to detect the presence of any smoldering fuel. Inerting would be initiated upon the indication of a presence of CO, and inerting would continue during the associated shutdown of the mill if the CO is not able to be cleared within a reasonable operating time. The mill would thus be inerted upon shutdown and until the presence of CO is gone. Many CO monitors are capable of detecting very small concentrations (<20 ppm), which may not actually indicate the presence of an active “smolder” in a mill. Individual plant operators will develop experience with this monitoring system to allow them to accurately diagnose situations that require corrective action. CO monitoring systems can be time shared with groups of mills and selected or automatically selected to continuously monitor a mill with a high indication of CO. Care should be taken in selecting a single mill monitoring operation to verify that a common problem across all mills or groups of mills is not missed during this operating mode. 3.2.2 Other Systems

A Fike explosion suppression system detects rapid pressure increase inside the mill to trigger the admission of a noncombustible dust to suppress rapid progression of the incipient explosion. Users have experienced incidents with the equipment related to equipment calibration and preventative maintenance. Burst Discs attached to the primary air ductwork may mitigate the impact of an incident by venting the explosive gases to a safe location. Burst discs do not prevent an explosion, but they do provide some equipment and personnel protection. This method of damage control is not the subject of this paper. Additional information concerning the use and installation of burst discs can be found in NFPA 68. 3.3

Inerting vs. Fire Suppression 14



As defined mill inerting is the rapid introduction of an inert media into a coal mill in sufficient volume to reduce the oxygen content of the coal mill atmosphere to a concentration of less than 14% oxygen, thus inhibiting and effectively preventing the rapid combustion of coal dust or carbon monoxide. Mill inerting is applicable to any equipment or location where fuel mixtures are being transported to the furnace. As defined fire suppression is either the reduction of the O 2 concentration of the pulverizer atmosphere or the admission of water mist to suppress the ability of the coal residing in the mill to continue to smolder. Both methods are intended to inhibit the ignition of the pulverized coal material during a return to service or other disturbance of the coal which may be contained in the pulverizer. One pulverizer manufacturer’s recommendations read “ The only time the steam inerting system is not used is in the case of fire in a mill. If fire is suspected, water should be fed into the water deluge connections in the primary air duct”.

Other manufacturer’s recommend a water fogging system be installed in the upper area of the mill, but almost always in conjunction with a water spray at the pulverizer primary air inlet connection. The spray at the primary air inlet keeps the coal that spills from the bowl in that type mill from entering the air inlet duct and presenting an ignition source during mill startup in advance of any mill inerting.

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4.0


MANUFACTURER’S RECOMMENDATIONS Each of the mill manufacturers has produced inerting system recommendations specifically tailored to their own mill. These recommendations should be made available to both the inerting system designer and each plant operator. However, they typically are emblazoned with confidentiality language so as not to be copied and shared with others outside of the organizations. Most are considered by the manufacturer to be confidential information, and thus are not included in this report. There are however, several patents and other public information concerning the design and implementation of these systems contained in appendix B. The manufacturer’s recommendations have several areas in common: •

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Most recommend some inerting media flow at the start and stop cycle of the mill. Time and flow rate varies with the manufacturer and mill configuration. All recommend inerting the mill during a coal interruption and after a mill trip resulting in fuel being contained in the mill. Most involve the monitoring of loss of fuel feed to the mill as one of the conditions that leads to “unexpected” explosive conditions and the need for initiation of inerting procedures. Most recommend the application of the inerting media near the mill inlet. Most recommend a low velocity application of inert media in the areas near the top of the mill for fire suppression. All indicate that a thorough inspection should be made after fire suppression.

Most of the manufacturers recommend running an inerting system cycle when starting a mill. The inerting cycle is typically initiated before the mill is started and runs continuously for some period of time after the mill is started. For mill shutdown the inerting media flow is usually resumed at

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approximately minimum feed rate and maintained until the mill is purged of fuel. The philosophy of the manufacturers has also changed over time. A review of the patent information and general information literature for the newer installations shows a change in the method of application of inerting media. One manufacturer employs two integrated subsystems in more recently furnished mill systems (<8 years old); One for steam inerting and one for fire detection and extinguishing. Inerting is initiated upon trip of a pulverizer containing fuel and high mill temperature. In this case primary air to the mill is closed and mill discharge control devices are opened. Steam is then admitted to the inerting system at a flow rate of approximately 50% of design flow for 15-20 seconds, followed by an additional steam flow of approximately 20% of design flow for 15-20 additional seconds. These two steps are initiated automatically by the control system. If the fuel feed was interrupted, the manufacturer recommends tripping the feeder manually after the steam flow has been established. This manufacturer then recommends a procedure to clear the mill of raw fuel using an inert media. The system checks all mill start permissives, opens the inerting steam valve to a 100% steam flow condition, and restarts the mill. After a period of time the normal air transport control devices are slowly reopened and inerting steam flow slowly reduced to zero, restoring normal 100% air flow control. No other fire suppression or inerting techniques were furnished.

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5.0


INDUSTRY INCIDENTS Several survey respondents reported some data on puffs and explosions, however most simply reported that they had experienced minor, none or some. In 1987, ATSM published the following paper. The source publication is available from ASTM electronically: Industrial Dust Explosions

Cashdollar KL, Hertzberg M Pages: 372 Published: 1987 A brief summary of the abstracts of two papers particularly relevant to this topic are included here, which expresses the perceived solution at the time. Review of Coal Pulverizer Fire and Explo sion Incidents

Zalosh RG Pages: 11 Published: Jan 1987 Download this paper in pdf format for $25 (224K) Download the complete source publication in pdf format for $55 (6.9M) Source: STP958-EB Abst ract

Reports of 26 coal pulverizer fires and explosions are reviewed in order to elucidate causes and preventive measures. Most of the explosions occurred during pulverizer shutdowns or restarts. The most common ignition scenario involved coal dust accumulations in the mill spontaneously igniting and coming in contact with a flammable coal dust-air suspension during a mill shutdown or restart. Detection of spontaneous combustion by pulverizer outlet temperature alarms (and, to a lesser extent, carbon monoxide analyzers) has not been reliable. Improvements are needed in the detection of incipient mill fires and the effective use of inert gas purges to prevent destructive pulverizer explosions. Coal Pulverizer Explosio ns

Carini RC, Hules KR Pages: 15

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Published: Jan 1987 Download this paper in pdf format for $25 (336K) Download the complete source publication in pdf format for $55 (6.9M) Source: STP958-EB Abst ract

In 1981 the Electric Power Research Institute (EPRI) began a research program to study pulverized coal fires and explosions in the U.S. utility industry. Historical trends resulting from an industry-wide survey and experimental results of explosion-related testing are discussed. The survey indicates that explosive events are occurring at a rate of approximately one explosion for each unit every three years resulting in an industry cost of one billion dollars annually. The historical trends show that although coal type has a large influence on explosion frequency, it is not the only significant factor in setting explosion hazard levels. Laboratory experiments were performed in full-scale test rigs that allowed triggering and monitoring of coal system explosions on demand. Ignition events that remained within the coal pipe created only weak pressure rise, while ignition events within the simulated pulverizer volume created explosions exceeding 70 bars (7000 kPa). These experiments showed that explosion characteristics depend on dust concentration, ration of pulverizer volume to coal pipe area, as well as coal type. The source publication is available from ASTM - Publication STP958-EB Industrial Dust Explosions

Cashdollar KL, Hertzberg M Pages: 372 Published: 1987 At this time we are awaiting information from NERC GADS information requested specifically for pulverizer fires and explosions. This data had not been supplied at the time of publication of this paper, however will be made available as a supplement via the PRB coal website. A summary report form 2002-2006 indicating the number of unit years and outages resulting from GADS reported equipment from the bunker outlet to the burner is included. PRB Coal Users’ Grou p Survey Repons es –

Kind of a minor puff.

•

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Yes, caused by; burning bed of caol in the mill Significant incident caused by a mill being removed rapidly from service during startup. Minor puff when operators failed to inert upon starting. Yes several CO 2 leakage of inerting media leaking through burner swing valve. No explosions when putting them in service Puffs Puffs when taking mills out of service. Six incidents since PRB. Since inerting installed only one puff when a problem with the inerting equipment. Several when starting or stopping mills. Running a Silo empty causing explosion back through feeder. None since inerting changed. Many explosions over 25 years, however none since inerting is used when coal feed is lost. Has used steam inerting with mill in service with no detrimental effects. None in last 15 years with no inerting system None Many in the past but none since water inerting installed. None None None None None None. Inerting system has proven to be unreliable due to valves. Yes. Usually in air ducts where dust settles out . Mills have puffed in the past due to hot spot in mill. None Yes at pulverizer ductwork. Yes at burner front and crusher dryer ductwork. Puffs with startup of hot mill. Steam inerting on before startup and before shutdown.

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• • • •

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•

• • • • • • • • • •

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Additi onal Mill Incid ent Report s Furn ished by us ers (comp anies names d eleted).

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•

•


An explosion occurred when workers opened up a mill on a non-operating unit to clean the coal out of the mill. – Probably smoldering coal which ignited when exposed to enough air to support rapid combustion. Two explosions occurred at a single plant separated by 19 years. The reported information stated that there was coal in the “primary air” and the other listed “ductwork” both of which caused “explosions” when the mill was placed in service. The second incident was after inerting was installed, but did not inert the ductwork prior to the point of injection.

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The following reason codes list the equipment associated with the derates in the above table for reference in understanding the equipment deratings and outages reported. Pulverizers, Primary Ai r Fans, and A ssoci ated Ducts

0200 Pulverizer exhauster fan (for indirect firing) 0205 Pulverizer exhauster fan drive 0210 Pulverizer heater (for indirect firing) 0220 Pulverizer system cyclone separator 0230 Pulverizer bag filter 0240 Pulverized coal bin 0250 Pulverizer feeders 0253 Pulverizer feeder motor 0255 Pulverizer feeder coal scales 0256 Seal air system (air to pulverizers) 0260 Primary air fan 0262 Primary air fan lube oil system 0263 Primary air fan drives 0264 Other primary air fan problems 0265 Primary air heater 0266 Primary air heater fouling 0267 Primary air flow instrumentation 0270 Primary air duct and dampers 0280 Pulverizer fires 0290 Pulverizer reduced capacity due to wear 0300 Pulverizer motors and drives 0310 Pulverizer mills 0312 Pulverizer mill classifiers 0313 Pulverizer mill trunnion seals 0314 Pulverizer mill ball charger hopper (ball mills only) 0315 Pulverizer mill coal level controls 0320 Foreign object in Pulverizers mill 0325 Pulverizer skidding 0330 Pulverizer coal leak (pulverizers only) 0331 Pulverizer system coal leaks (other than pulverizers) 0335 Pulverizer lube oil system 0338 Pulverizer control systems (temperature and pressure) 0340 Other pulverizer problems 0341 Pulverizer deluge system 0342 Pulverizer Inert system 0344 Pulverizer inspection 23



0345 Pulverizer overhaul 0346 Pulverizer pyrite removal system 0350 Pulverized fuel and air piping (from pulverizer to wind box) Burners

0358 Oil burner piping and valves 0359 Gas burner piping and valves 0360 Burners 0362 Burner tilts 0361 Burner orifices 0370 Burner instruments and controls (except light-off) 0380 Light-off (igniter) systems (including fuel supply) 0385 Igniters 0390 Burner wind boxes and dampers 0400 Burner wind box fires

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6.0


INERTING SYSTEM BEST PRACTICES Definition of “Best Practices” for the inerting of coal mills grinding PRB fuels is difficult at best due to the variety of operating conditions, mill types and operating procedures incorporated in individual plants. There are plants with no inerting or suppression systems in service that report no mill “puffs”, while plants with similar mills in other locations and similar conditions have reported “puffs”. 6.1

Ball tube mills

Of the reporting units there were a limited number of these mills (12). The operating companies of these mills report CO 2 and steam inerting on startup and shutdowns and trips. Inerting is employed in all cases, and the reports of fires or puffs are in the crusher/dryer inlet sections, which may be a result of fuel trapped in these areas during a shutdown. Inerting is automatic for 90 to 180 seconds on mill trips (operator intervention sometimes shortens), and manual inerting just prior to mill startup. Inerting admission is in the inlet of the crushing and drying section for a period of time determined by the mill size, and inerting during a mill trip until there is no evidence of fire. If the mill is restarted within a short period of time, the mill is inerted prior to the startup for a period of approximately 2-3 minutes with a volume of steam adequate for the mill size to reduce oxygen concentration during initial mill operation. Inerting is repeated on shutdown from the time the mill feed is shut off until the no load mill characteristic is reached. 6.2

Ball and Race mills

Approximately 27% of the units responding to the survey were ball and race-type mills. Of these units there are reports of water, CO 2, steam and nitrogen inerting. The largest percentage of these systems use steam for inerting. Inerting times vary from 3 minutes to 15 minutes. Most report automatic inerting on mill trip, and manual on mill startup. There is limited information on the damage from mill puffs, however more than 60% of the units report a history of mill puffs. Most of the systems were designed in-house or are of unknown origin. The mill

25



operating temperatures varied from 115 F to 160 F, the widest spread of any of the mill operating types when considering only 100% PRB firing units. B&W has a specific inerting and fire suppression procedure for their mills, which specifies time and quantity for the application of the inerting fluid. The procedure is available from B&W (Guideline Specifications for EL Pulverizer Isolation and Inerting). This document does not recommend water as an inerting media, however does state that it can be an effective media for fire suppression. The manufacturer recommends 8 minutes of inerting time after all sources of air are removed from the mill after a trip, and for a period of 2 minutes prior to “de-isolating” the pulverizer. The procedure recommends inerting immediately after tripping of any pulverizer under coal load. Primary air inerting connections are shown near the mill inlet. Additional connections are shown on the coal feeder inlet and in the top of the mill which are for fire suppression and water mist can be used for this application.

6.3

Roller Mills

The bowl arrangement allows for a collapse of the coal after removal of the transport air after a mill trip. This possibility can result in the coal being exposed to the hottest area of the mill, which if allowed to set can begin the smoldering combustion process, which when disturbed can ignite the fuel in the mill. For this reason this type of mill seems to be most susceptible to puffs resulting from coal igniting during the mill starting or restarting processes, in addition to the hot coal that is trapped in a mill after a mill trip for high mill temperature, and hot coal trapped in the mill after a MFT. The manufacturers of this type of mill all recommend an inerting media input at the inlet to the mill near or in the primary air duct which is used during the initial inerting process as a result of a mill trip. This mechanism is designed to keep the collapsing coal mixture from entering the PA duct, which could be transported into the mill on restart. The inerting media location also is used during an automated mill cleanout procedure, which may or may not be automated on the inerting/suppression system installation. Clearing a mill by a washing 26



or “swirling” procedure is recommended by the manufacturers any time a mill experiences a high mill temperature or fire inside the mill. Manufacturers also recommend that a thorough mill inspection be completed to confirm that there is no internal damage or warping that would allow coal dust to be trapped during the normal mill operation. The inerting of this type of mill as described by the manufacturer accomplishes both inerting and fire suppression. The inerting system varies with the manufacturer, but all systems describe an inerting process prior to startup of the coal mill for short period of time and immediately following a mill trip. An additional recommendation from CE (Alstom) requires that inerting media be supplied at a rate equivalent to 100% mill air flow used to clean out a mill into the furnace after a mill trip where there is no evidence of a fire in the mill. Other manufacturer’s have other procedures for clearing the mill after a trip or a fire. All manufacturers state that all evidence of fire should be absent prior to clearing or cleaning the mill under all circumstances. The best practice for this type of coal mill will always involve the injection of an inerting media at or near the primary air inlet to the mill. The inerting media flow rate is dictated in all cases by the flow rate required to drop the oxygen content to below explosive mixture concentrations (12%-14% oxygen) within a specified elapsed time. Admit flow from the inerting media prior to starting the mill, and then for a period of time after the coal feed has attained a coal flow adequate to bring the internal atmosphere to a fuel rich “non-explosive” atmosphere. The timing depends on the coal mill operating characteristics, primarily the time the coal feed takes to establish an adequate flow to the mill. This could be a short as 45 seconds to approximately 120 seconds. The best practice for a normal mill shutdown would be similar. The inerting system should be started at the same time that the feeder is stopped. The inerting system should then continue to operate until the mill has cleared itself of coal and is ready to be cooled down and shut off. This length of time depends on the volume of coal in the mill and the time it takes for the mill to clear of fuel, and should be determined by the plant operating personnel after consulting with the manufacturer’s recommendations and the observed operating characteristics of the plants equipment.

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7.0


FIRE SUPPRESSION SYSTEM BEST PRACTICES Fire Suppression systems typically begin with the application of inerting flow and removal of the coal mill from service upon the evidence of a fire. Manufacturers recommend that upon evidence of a fire the inerting system be put into service and the mill removed from service with the inerting system in operation. Continue to operate the inerting system until the evidence of fire is absent from the mill indications. The manufacturers have differing methods of establishing and verifying the required inerting media flow by time and flow rate. Establish an initial flow for several minutes and then a reduced flow for a period of time to extinguish any smoldering fuel. At the same time or sequentially a fire suppression flow of inert media is admitted into the main pulverizer above bowl and classifier section. This flow must be a low velocity admission to keep any smoldering fuel from being swept up into the atmosphere inside the mill.

Many respondents report using a steam or water misting system, which has admission points at various locations depending on the firing system design. These systems are used on mill trips, feeder loss of coal flow, and at other times when the system is reporting a high mill outlet temperature. Some monitor CO concentrations in the mill and these systems can be activated manually during mill operation.

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8.0


INERTING SYSTEM DESIGN CONSIDERATIONS There are three major topics to consideration when designing a retrofit mill inerting system. 8.1

Choice of inerting media

steam CO2 nitrogen flue gas water

• • • • •

The choice of inerting media should be strongly impacted by media availability during all conditions when inerting may be required. Inerting may be required when the unit is off line, and other operating considerations may restrict the availability of inerting media. The choices of inerting media are usually based upon the immediate availability of process steam because steam is usually the media of choice. The mill manufacturers’ recommendations for media supply quantity, flow rate and duration must be reviewed. Care must be taken to assure that other safety systems using the same media are not potentially impaired by the installation of the new mill inerting equipment. A review of burner management system modifications is also required to check conformance with current NFPA. (This work is outside the scope of this analysis). 8.2

Appli cation requirements • • • • •

flow requirements quantity requirements location of application supplemental or other media requirements additional optional functionality

Application requirement are dictated by the type of system to be installed. There is evidence that many PRB coal users do not have an

29



application of inerting as defined by the mill manufacturer, but still have been successful in limiting the damage of mill explosive events. Even with proper installation of an inerting system as prescribed by the mill manufacturer, a portion of the system and the operating instructions are still based upon the suppression of a mill fire after mill trip instead of inerting only. Neither do the recommendations of the manufacturers’ seem to be an indicator of success of the inerting system design. In most cases the manufacturers recommend several minutes of initial inerting per cycle, and a maintenance flow for an extended period of time following a mill trip until the temperature is no longer high or evidence of fire has dissipated. Several users inject significantly less inerting flow during startup and shutdown than recommended by the manufacturers. Providing an “inert” atmosphere of less than 14% (13% to 15% is stated in the literature) will minimize the possibility of explosion. The primary consideration for injection jet size and location is to make sure the admission of the inerting/fire suppression media is at a low velocity and with enough dispersion to keep from disturbing fines into suspension. 8.3

System Testi ng

Flow rate and volume testing At a minimum system testing should be performed on a representative mill on each system installed. Recommendations for this testing include monitoring system performance for the reduction in oxygen concentrations. It is recommended that this testing be performed on a verified clean mill startup to minimize potential safety issues.

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9.0


MILL INERTING/FIRE SUPRESSION DESIGN CHECKL IST • • • •

•

•

•

•

•

•

•

•

•

What are the mill manufacturer’s recommendations? What are current NFPA requirements? Are there any additional NFPA recommendations? Is there another plant inerting infrastructure in place (ie, CO 2 or N2), or planned to be in place that can adequately supply mill inerting media? If so, has the combined use of the media been adequately analyzed to assure adequate media supply under all normal and emergency conditions? Is process steam available at all times (ie, is this a multiple-unit station)? Is the system to be integrated for automatic operation with manual override? To what degree is the system going to be integrated into the mill control system? If automatic operation, what might be utilized for triggering an intermittent coal feed inerting sequence? If manual for this condition, what operator indications would be utilized for triggering? What integrated system logic will be required for a combined inerting and suppression system installation? Once designed, test the inerting system using a wet O 2 analyzer to determine proper rate of inerting flow (ie, high enough to inert within the allotted time and low enough to not pressurize the mill). Once designed, test the inerting system using a wet O 2 analyzer to determine the rate and frequency of media re-admission to sustain the inert status.

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10.0


FIRE SUPPRESSION SYSTEM DESIGN CONSIDERATIONS

Suppression system design includes many of the same considerations that are associated with the design of inerting systems. Those issues are summarized here and expanded to include additional considerations for the design of the suppression system. The media selection process for a fire suppression system is similar to that of an inerting system and the same design considerations are recommended. One manufacturer states that water is not an inerting media, but it is a cost effective fire suppression agent for coal pulverizers. Additional mill surface cooling takes place with steam and water mist fire suppression which provides a unique advantage to these media. Fire suppression applications are recommended by most manufacturers in the underbowl, above bowl, classification, mill discharge, and air inlet areas on the bowl mills. Testing should be conducted on the fire suppression portion of the system to include both the initial flow rate and the continuing flow rate present in most all situations. Testing should check for adequate media application time to confirm that the fire is out by all the evidence available to the operations personnel. Tests should also be performed to confirm that the inerting and fire suppression media injection velocity and injection location does not disturb pulverized coal dust layout in a manner that will cause the dust to be reintrained into the internal mill atmosphere.

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APPENDIX A - NFPA REFERENCES NFPA 85 2007 revision includes several references to the inerting of coal mills or pulverizers. The section below contains information dealing with the fire suppression, clearing and inspection of coal mills after a fire or a suspected fire in the mill. If inerting is used in mills NFPA 69 and good practice both indicate that warning signs cautioning an asphyxiation hazard shall be posted. 9.5.4.2 Pulverizer Tripping. 9.5.4.2.1 Inerting. 9.5.4.2.1.1 A pulverizer that is tripped under load shall be inerted and maintained under an inert atmosphere until confirmation that no burning or smoldering fuel exists in the pulverizer or the fuel is removed. 9.5.4.2.1.2 Inerting Procedure. (A) The inerting procedure shall be established by the pulverizer equipment manufacturer and the purchaser. (B) The pulverizer equipment manufacturer and purchaser shall consider fuel characteristics, the pulverizer temperature and size, and arrangement of the pulverizer. 9.5.4.2.1.3 Inerting media shall be selected from, but not limited to, the following: (1)

Carbon dioxide

(2)

Steam

(3)

Nitrogen

9.5.4.2.2 Fuel-Clearing Procedures. 9.5.4.2.2.1 For pulverizers that are tripped and inerted while containing a charge of fuel in accordance with 9.5.4.2.1.1, the following procedure shall be used to clear fuel from the pulverizer and sweep the transport lines clean as soon as possible after it has been tripped and there is confirmation that there is no burning or smoldering fuel: (1)

Start up one pulverizer in accordance with the principles and sequences as listed in 9.5.2.1(1) through 9.5.2.1(9).

(2)

Isolate from the furnace all shutdown or tripped pulverizers.

(3)

Continue to operate the pulverizer until empty and in normal condition for shutdown. When the operating pulverizer is empty of fuel, proceed to another tripped and inerted pulverizer and repeat the procedure until all are cleared of fuel.

Exception: An exception to 9.5.4.2.2.1(3) is to restart feeder and return pulverizer to

33



normal operation if furnace conditions allow such operation. 9.5.4.2.2.2 Burning Fuel in Out-of-Service Pulve rizer. (A) In the event that there are indications of burning or smoldering fuel in an out-ofservice pulverizer, the pulverizer shall not be restarted under the normal procedure. (B) Fire-extinguishing procedures shall be followed, or removal of residual fuel shall be accomplished under inert conditions by taking one of the following steps: (1)

Remove fuel through the pyrites removal system. When this procedure is followed, the pulverizer shall be opened and inspected prior to restarting.

(2)

Start the pulverizer with an inert medium, using the starting sequences in 9.5.2.1(1) through 9.5.2.1(9).

(C) Due to the danger of an explosion when they are being opened and cleaned, pulverizers shall not be cleaned manually until they and their contents hav e been cooled to ambient temperature. (D) The procedures of 9.5.4.3 shall be followed. 9.5.4.3 Fires in Pulverized Fuel Systems. 9.5.4.3.1 Indication of a fire in any part of a pulverized fuel system is a serious condition and shall be dealt with promptly. 9.5.4.3.2 Extinguishing media shall be water or inert solids or shall be in accordance with 9.5.4.2.1.3. 9.5.4.3.3 The following procedures for fighting fires shall be used with modifications for specific systems, specific locations of fire, or requirements of the equipment manufacturer: (1)

If sufficient flow capacity of inerting media is provided (at least 50 percent by volume of the minimum primary airflow for the system), inert the pulverizer air fuel flow, shut off the fuel feed, empty the pulverizer of fuel, and shut down and isolate the pulverizer.

(2)

Stop the primary airflow, trip the pulverizer and feeder, isolate the system, inert, and proceed as follows:

(3)

(a)

Do not disturb any accumulation of dust in the pulverizing equipment.

(b)

Do not open any access doors to the pulverizer until the fire is extinguished and all temperatures have returned to ambient.

(c)

After isolation of the pulverizer is verified, follow the procedures as outlined in 9.5.4.3.6 and 9.5.4.3.7.

Extinguish a fire that is detected in an operating low storage pulverizer by shutting off the hot air, increasing the raw fuel feed as much as possible without overloading the pulverizer, and continuing to operate with tempering air.

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BEST PRACTICES for MILL INERTING & FIRE SUPPRESSION (4)


Introduce water into the raw fuel or tempering air stream, or both, and proceed as follows: (a)

The water must be added in such quantities and at such locations as not to cause hang-up or interruption of raw fuel feed or to stir up any deposit of combustible material.

(b)

When all evidence of fire has disappeared, shut off the water, trip the pulverizer, isolate, and inert.

9.5.4.3.4 When fires are detected in other parts of a direct-fired system, such as burner lines, the procedures as outlined in 9.5.4.3.3(1), (2), or (3) shall be followed. 9.5.4.3.5 When fires are detected in storage system components, including but not limited to cyclones, dust collectors, and pulverized fuel bins, the affected components shall be isolated and inerted. 9.5.4.3.6 If fire is detected in an out-of-service pulverizer, it shall be kept out of service and isolated. 9.5.4.3.6.1 All air supply to the pulverizer shall be shut off. 9.5.4.3.6.2 Access doors to a pulverizer shall not be opened until the fire is extinguished by water or other extinguishing media and all temperatures have returned to ambient. 9.5.4.3.7 Pulverizing equipment shall be inspected internally following fires in pulverizing systems. 9.5.4.3.7.1 All coke formations and other accumulations shall be removed to reduce the potential for future fires. 9.5.4.3.7.2 If the pulverizer is wet, it shall be dried. 9.5.4.3.7.3 In no case shall a compressed air jet be used. 9.5.4.3.7.4 All components shall be inspected, and damaged items, including but not limited to, gaskets, seals, lubricants, and liners, shall be replaced.

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APPENDIX B – PATENT AND OTHER INFORMATION The following list of patents and manufactures information is made available for the readers to access. These patents are available at freepatentsonline.com as one of several sources for this information. USP 5,137,218 --Western Farmers Electric – Pulverizer for reducing the possibility of explosions--August 6, 1991 USP 4,244,525 – Cleveland Cliffs Iron Company – Inerting of pulverizing mills for combustible materials– January 13, 1981 USP 4,653,698 – Babcock and Wilcox – Safety system for coal pulverizers – 3/31/1987 USP 5,230,474 – Mitshibishi – Mill inert apparatus for coal pulverizer and method for prevention of explosion– 6/27/93 USP 4,754,932 – Combustion Engineering – Coal pulverizer inerting and fire extinguishing system - 7/5/1983 USP 4,778,113 – Babcock and Wilcox – Apparatus for measuring low level combustibles 2/1/1988 USP 7,261,746 – Aptech Engineering, Inc.- System and Method of Reducing Pulverizer Flammability Hazard and Boiler Nitrous Oxide Output – 8/28/2007 Other References Factory Mutual - Approval Standard for Explosion Suppression Systems- Class Number 5700 – October 1999 Fike Corporation – Safety Technology – Summer 2000 - Volume 12 Issue 1 Babcock and Wilcox Company – Plant Service Bulletin - PSB-16A – Issued 2005 Aptech Engineering Services, Inc. - A Process to Reduce NOX Emission and Reduce the Risk of Explosions in Pulverized Coal Systems. –

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Mill Inerting

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