Patent Publication Number: US-9421551-B2

Title: Pulverizer mill protection system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation in part of International Application No. PCT/US2013/039107, filed May 1, 2013, which claims priority to U.S. Provisional Patent Application No. 61/640,853, filed May 1, 2012. All of said applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to a protection system for pulverizer mills typically used in coal fueled power plants, and other industrial coal burner facilities that may incorporate boilers, kilns or process heaters. An embodiment of the invention inhibits and suppresses fires, explosions and/or puffs in pulverizer mills, and provides control over high mill outlet temperature excursions. An embodiment of the invention can also be utilized on other solid fuel systems that incorporate pulverizer mills and/or pneumatic conveying systems for pulverized or granulated solid fuels. Various embodiments of the invention can include many advanced features and performance parameters providing benefits over existing steam, water, water fog or carbon dioxide inerting systems. 
     Fires and explosions in coal pulverizer mills can cause tremendous financial and operational burdens on coal fired power plants, as well as other coal fired boilers and industrial processes, especially those burning highly volatile coals such as Powder River Basin (PRB) coal. Along with posing a risk to worker safety, these events lead to financial losses incurred in repairs, lost power generation and litigation. Apart from regular mill maintenance, current techniques in dealing with fires and explosions rely on reaction to an event rather than prevention. Mill inerting, fire suppression, explosion venting and explosion suppression are all offline techniques, in that the mill must either be taken out of service for these techniques to be effective or the mill must be taken out of service following an explosion. Mill fires and explosions have many possible causes ranging from operator error to coal feed interruptions. 
     There are a variety of issues that can lead to mill fires or explosions. These may be maintenance related, caused by equipment failure or improperly following operational guidelines. However, many mill fires and explosions are caused by “hot restarts,” a standard operating procedure which is generally accepted in the industry. A hot restart refers to starting a mill immediately after a trip. Trips often occur while the pulverizer is loaded with coal. With vertical spindle style mills, a loss of airflow during such a trip means that coal that previously was suspended above the grinding bowl or table falls down to the hot underbowl area where temperatures often exceed 650° F. In this high temperature region of the mill, coal quickly dries and, especially in the case of PRB coal and similar highly volatile coals, spontaneously ignites and begins to smolder. When airflow is reintroduced, coal that was previously in mounds with little access to oxygen become suddenly suspended. Once suspended, more surface area is exposed to oxygen, resulting in the often catastrophic combination of high air-to-fuel ratio, high temperatures and an ignition source that could result in an explosion. 
     Often mill fires start after a mill has stopped either due to a trip or as part of a controlled shutdown. Temperatures in the mill rise for a period after the mill is taken off when the heat stored in the thick metal mill housing migrates into the vessel. The resulting rise in temperature causes any coal remaining in the mill to dry and ignite. Left undetected, such fires can grow into major issues when primary airflow is reintroduced. Due to this threat, control room operators are prone to error. Operators are often tasked with watching indicated mill outlet temperature for hours after a mill is taken offline and introducing cold airflow if temperatures rise above normal operating levels. 
     While manual startup and shutdown is often preferred over automatic routines for a variety of reasons, a small oversight on the part of the operator may lead to catastrophic events. For instance, if a feeder is started late during the startup process, temperatures may spike because an absence of coal flow means an absence of the moisture content of the coal. If coal is introduced too late into the startup procedure for temperatures to be kept below blast gate trip temperatures, again the hazardous combination of high temperatures and dry coal is likely. During shutdown, if an operator fails to stop hot airflow when fuel feed is stopped, air-to-fuel ratio and temperature will go high, increasing the potential of an explosion or fire. 
     Mill fires have been known to erupt because a mill, still loaded with coal, which has been isolated from air supply, is opened for inspection. These fires most often occur when a mill has not fully cooled to ambient temperatures. Opening the mill stirs up previously settled coal dust and introduces O 2  into a mill that may or may not have previously contained an inert atmosphere. This scenario may lead to injury or death. 
     Improperly maintained or otherwise malfunctioning equipment is another major cause to mill fire causation. Coal feed interruptions, resulting from mechanical issues or plugged coal feed pipes, often result in high temperatures and air-to-fuel ratios. Improper airflow or temperature indications also have the potential of causing issues. For instance, an indicated temperature that is much lower than actual mill outlet temperature can lead to driving the mill temperature dangerously high. Improper airflow indication has the potential to lead to coal spillage into the underbowl because of insufficient airflow. Stuck or otherwise compromised hot or cold air dampers also have the potential of causing high temperatures, insufficient velocities or high air-to-fuel ratios, while worn or eroded pulverizer components may allow for coal to settle or spill over into the underbowl. 
     There are inerting systems and explosion suppression or venting systems known in the art. Inerting systems are designed to limit the amount of oxygen in the mill by injecting a noncombustible, nonreactive gas into the vessel. Gases used for this purpose are typically steam, nitrogen, carbon dioxide or flue gases. While inerting is effective at extinguishing smoldering or burning materials inside the mill, this method only works when a mill is isolated. This means that the mill operations, coal and airflow must cease and that all inlet and outlet gates are closed. Under these mandatory conditions the pulverizer is inerted and a based characterization test performed during commissioning. It is perceived that all other parameters that affected this characterization test remain constant after commissioning. However, if a damper leak is detected or an improper measurement of media flow rate, this may lead to oxygen levels that exceed the recommended fifteen percent. Without reliable and continuous O 2  measurement, such issues may go undetected. Steam inerting can be effective in displacing oxygen in the pulverizer, but it must be insured that the steam does not condense due to falling below saturation temperature and pressure. Generally, steam cannot be relied upon to extinguish a fire. 
     Explosion suppression and venting solutions react to dust explosions by either rapidly combining the combustible coal dust with a noncombustible dust (phlegmatization) and by venting the explosion pressure with explosion doors, respectively. These methods are purely reactive and result in pulverizer downtime and unit derates. In the case of explosion suppression systems, the mill must be isolated and cleaned out and the suppressant canisters reloaded. In the case of explosion venting, explosion doors must be replaced. In either case, further downtime is incurred since the mill must be thoroughly inspected after an explosion. 
     Existing inerting systems concentrate on inhibiting mill fires during start up and shut down. Existing steam, water and carbon dioxide systems typically provide minimal or no fire suppression capability inside the pulverizer, while the mill is in service. 
     The objective of more rapid cooling of pulverizers is to shorten the time required for maintenance outages and inspections by permitting quicker entry into the pulverizer internals after it has been removed from service. If a fire occurs, the fire can be extinguished quickly preventing damage, costly repairs and extended downtime that is typical following a mill fire incident. 
     Temperatures within coal pulverizer mill internals vary greatly and can reach 700 degrees Fahrenheit during normal operation, especially while firing high moisture sub-bituminous coals. During normal and continuous operation, the highest temperatures are constrained or isolated to areas of the mills where there is usually no coal, dust or combustible material. Certain conditions such as interruptions in raw coal feed or other mechanical and operational anomalies can allow the high temperatures inside the mill to migrate to other areas of the pulverizer mill where pulverized and granulated combustion material (coal or other solid fuel) exist. This usually manifests itself as a temperature excursion where mill outlet or discharge temperature is abnormally high. There is a high risk of fires or puff evolving while mill outlet temperature is abnormally high. 
     Coal pulverizer mill fires and explosions present a major safety and financial concern for owners and operators of coal fired boilers and utilities. Such incidents can damage or completely destroy the mill and ancillary equipment. Workers in the vicinity of the mill may be injured or killed by thermal injury, hot gases and/or flying debris. Another concern is combustible dusts on and around ancillary equipment in the area that can result in secondary explosions or fires. 
     A commonly relied upon method of suppressing a fire inside a coal pulverizer/mill is increasing coal feed to flood the mill with fuel to decrease temperatures in the mill and smother the fire by inducing a fuel rich environment. Flooding the mill that is a closed system can reduce the air/oxygen available to support combustion inside the mill as well as maintain air to fuel ratios below the level necessary to support a pulverizer puff or explosion. To address internal mill fires, most fire suppression systems known in the art douse the mill externally with water, and are ineffective at suppressing fires inside the classifier and grinding/pulverization zones of coal mill/pulverizers. Other strategies to control, suppress or mitigate damage caused by a mill fire can include closing air inlet dampers (primary air dampers, hot air blast gates or other guillotine type isolation dampers) and fuel or burner lines or conduits using burner shut-off valves or isolation gates, in order to remove sources of oxygen (bottling), and filling the mill with steam or water fog (inerting). These methods typically require several hours to completely suppress a fire and most often do not suppress the fire quickly enough to prevent substantial damage to the mill or pulverizer system. Heat and combustibles, such as gases and coal dust remaining in the mill after suppression, present the risk of re-ignition. This system will address neutralizing the combustible material inside a mill while it is out of service as well as enhancing cooling limiting risk associated with re-ignition. The high temperatures inside the mill after suppression mean that a long cooling period is required before maintenance crews may enter the mill to assess and repair damage. Similarly, in non-emergency maintenance and inspection situations, the mill must be cooled from operating temperatures (it is also typical that the process of removing a pulverizer from service sometimes allow the mill to heated above normal operating temperatures), adding several hours to the process of maintaining and inspecting the mill. 
     In addition, the numerous components typically contained in a coal pulverizer mill create a number of enclosed and isolated spaces within the coal pulverizer mill, in which fires can ignite. Accumulations and settling of fine coal particles inside the mill/pulverizer components can spontaneously ignite, particularly in mills with highly reactive sub-bituminous coals. In addition to spontaneously igniting, accumulations of coal pulverized to small particles can be easily ignited during the start-up process where these particles are agitated and exposed to a high air to fuel ratio environment as well as the possibility of high temperature excursions. Raw coal supply interruptions due to imprecise feeder control and stoppages above and below the feeder are another common source of fires and puffs. Interruptions in raw coal feed can be caused by environmental conditions such as frozen coal, wet coal from precipitation and mechanical anomalies such as broken feeder belts, seized bearings and other causes. Also, accumulations of raw coal that has spilled over into the under bowl section are exposed to temperatures of 500° F. to 750° F. while firing sub-bituminous coal, and are another common cause of mill fires. As such, there is a need for a fire suppression system that can effectively suppress and extinguish fires and explosions in all internal areas of the pulverizer mill. 
     SUMMARY OF EMBODIMENTS OF THE INVENTION 
     One object of the present invention is to provide a system that prevents or inhibits pulverizer mill explosions, fires and/or puffs during mill start-up and shut down. Another object of the invention is to prevent or inhibit pulverizer mill fires by controlling high mill outlet temperature excursions. Another object of the present invention is to provide a capable and effective fire suppression system to address fires in the mill internals. 
     Yet another object of the invention is to provide a system that aids in controlling combustible dusts, vaporous gases, and accumulations of smoldering coal that are sometime common with highly reactive coals such as Powder River Basin coal and other sub-bituminous coals. Another object of the invention is to control combustible dusts and gases utilizing a solution with micelle encapsulation properties. Yet another object of the present invention is to provide for rapid and more uniform cooling of coal pulverizing mills for inspection and maintenance purposes. 
     Yet another object of the invention is to prevent or inhibit mill fires, explosions and-or puffs due to coal feed interruptions (wet coal, feeder problems, etc.). 
     Yet another object of the invention is to neutralize the hazards associated with residual coal dusts inside the mill/pulverizer after a pulverizer is taken out of service that can reignite either during the mill start-up process. 
     Yet another object of the invention is to provide an internal fire suppression extinguishing system capable of extinguishing fires in seconds, preventing damage to the mill, piping, external wiring, instrumentation and other ancillary equipment. This system is intended for internal fire suppression but can incorporate an external fire suppression using shared components. 
     Yet another object of the present invention is to provide external fire suppression that helps control and manage combustible dust on the mill exterior and improves housekeeping in the mill bay areas. 
     Yet another object of the present invention is to provides an effective tool to manage mill outlet temperature excursions before they evolve into fires, and avert derates from tripped mills, forced outages and mill damage. 
     Yet another object of the present invention is to provide rapid cooling of the mill internals to reduce mill downtime for emergency repairs, preventive maintenance, inspections or mechanical adjustments. 
     Yet another object of the present invention is to provide vapor encapsulation to eradicate combustible gases such as methane that can cause coal dust to ignite more easily and increase explosion force. 
     Yet another object of the present invention is to provide a fire suppression system that can be operated while the mill is in service to prevent mill fires (due to high mill outlet temperature excursions) from spreading to the burner lines. 
     Yet another object of the present invention is to provide a fire suppression system requiring less water than prior suppression systems, reducing thermal stresses and cracking of grinding elements, grinding and bull rings, rotating throats, mill side liners, and other internal components. 
     Yet another object of the present invention is to provide a suppression system that can function as mill internal wash down, reducing the chance of residual coal dust in the mill interior when removed from service. 
     Yet another object of the present invention is to provide a suppression system that can be integrated into a total fuel burning system protection system that incorporates the bunker/silos, trippers, feeders, mills and burner lines. 
     These and other objects of the invention may be achieved in various embodiments of the invention described below. One embodiment of the invention comprises a fire suppression, cooling and mill inerting system that inhibits coal pulverizer mill fires, explosions and/or puffs, as well as control vaporous combustible gases emitted from the coal inside an idle mill. The system injects a fire suppression solution as a mist through multiple nozzles located at various points in the coal mill/pulverizer. The fire suppression solution can be comprised of water and a fire suppression agent. Preferably, the fire suppression agent is an agent that provides for micelle encapsulation such as the water additive suppression agent currently sold by Hazard Control Technologies, Inc. under the name F-500 MULTI-PURPOSE ENCAPSULATOR AGENT (hereinafter “F-500”). Injection points are located at the primary air duct, classifier and grinding zone of the mill. Additional nozzles may be placed around the exterior of the mill for the purpose of extinguishing external fires and managing combustible dust on the mill exterior. The system can be operated in a stand-alone configuration or as part of a total fuel burning system protection scheme that incorporates the bunker/silos, trippers, feeders, mills and burner lines. The system can be used while the mill is in service to prevent mill fires from spreading to the burner lines and may be utilized during start-up/shut-down when the risk of mill explosion and puff are particularly high. The system can provide protection while the mill is in service in addition to during start-up and shut down (i.e. starting and stopping the pulverizer). 
     It is believed that fires can be suppressed in a fraction of the time when compared to traditional methods utilizing steam or water fog. This quick action reduces the chance of damage to the mill, piping, external wiring, instrumentation and other ancillary equipment. Less water is required compared to traditional methods, reducing thermal stresses and cracking of grinding elements, grinding/bull rings, rotating throats, mill side liners and other internal components. The system provides vapor encapsulation to eradicate combustible gases, such as methane, that can cause coal dust to ignite more easily and increase explosion force. This, along with the speed at which the solution cools mill internals, reduces the risk of reignition. 
     An embodiment of the invention comprises a system utilizing the F-500 suppression solution or similar agent that provides for micelle encapsulation or greater thermal capacity for cooling. The system can benefit routine maintenance operations. The rapid cooling provided by the system shortens downtime required for emergency repairs, preventive maintenance, inspections or mechanical adjustments. Since the F-500 suppression solution is a non-corrosive, biodegradable and non-toxic agent, the system is viable for use in non-emergency, routine maintenance situations as no special cleaning equipment is required after its use. Maintenance crews can enter the confined space without risk of injury due to trapped steam, heat or hazardous fumes. In instances where a mill is to be removed from service, the system can be used for a mill internal wash down to reduce residual coal dust in the mill interior. External fire suppression nozzles may also be used to help control combustible dust on the mill exterior and improve housekeeping in the mill bay areas. 
     An embodiment of the invention comprises a system having a first set of injection points for introducing a fire suppression solution located in a circular array around the raw coal feed inlet on the upper housing of the mill above the classifier cone, and a second set of injection points located in a circular array on the outer edge of the upper housing outside the classifier region. The system can also include a third set of injection points located at the pulverizer primary air inlet; also referred to as under bowl, under table, primary air windbox, wind belt as well as other terms referring to the inlet ducting where hot air for drying and transporting the coal first enters the mill. The system uses the injection arrays and the properties of the fire suppression solution injected through the arrays to manage temperature excursions and reduce peak temperatures during operational and mechanical anomalies that cause high pulverizer mill discharge temperature excursions. The system provides effective fire suppression as well as aids in controlling the environment inside the mill to prevent fires from occurring in the first place. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional schematic view showing a pulverizer mill protection system according to a preferred embodiment of the invention; 
         FIG. 2  is a top schematic view showing the classifier and grinding zone injection locations on the upper housing of the system of  FIG. 1 ; 
         FIG. 3  is a perspective view of the pulverizer of the system of  FIG. 1 ; 
         FIG. 4  is a cross sectional view of the system of  FIG. 1  taken along lines  4 - 4  of  FIG. 3 ; 
         FIG. 5  is a cross sectional perspective view of the system of  FIG. 1 ; 
         FIG. 6  is a top plan view of the system of  FIG. 1 ; 
         FIG. 7  is another top plan view of the system of  FIG. 1 ; 
         FIG. 8  is a partial perspective view of the system of  FIG. 1 ; 
         FIG. 9  is another partial perspective view of the system of  FIG. 1 ; 
         FIG. 10  is a perspective view of a primary air duct nozzle assembly according to a preferred embodiment of the invention; 
         FIGS. 11-14  are various perspective views of a classifier zone nozzle assembly according to a preferred embodiment of the invention; 
         FIGS. 15-18  are various perspective views of a grinding zone nozzle assembly according to a preferred embodiment of the invention; 
         FIG. 19  is a partial cross sectional view of the system of  FIG. 1 ; 
         FIG. 20  is another partial cross sectional view of the system of  FIG. 1 ; 
         FIG. 21  is a schematic view of a pulverizer mill protection system according to a preferred embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION AND BEST MODE 
     A pulverizer mill protection system according to a preferred embodiment of the invention is illustrated in  FIGS. 1-21 , and shown generally at reference numeral  10  in  FIG. 21 . The system  10  generally comprises five subsystems: concentrate and/or solution storage tank(s)  100 , a flow control cabinet  110 , an equipment control/pumping enclosure  120 , an air distribution system, and injection piping and nozzles installed in a pulverizer mill  12 , described in detail below. 
     The equipment control/pumping enclosure  120  can accommodate all mill types/models, and houses multiple water pumps and multiple chemical pumps. The enclosure  120  includes isolation and/or bypass valves for water pump isolation/bypass, and chemical metering pump bypass (to allow clean water to be used for housekeeping). The equipment control/pumping enclosure  120  houses a programmable logic controller, and control equipment. The enclosure can be heated and ventilated, and can accommodate any voltage configuration. 
     The chemical storage tank(s)  100  can be standard 330 gallon size. The storage tank(s)  100  can be connected via a common header to one or more control/pumping skids  110 . The number of tanks  100  and skids  110  can vary depending on the size and number of coal pulverizer mills  12  on site and the need for optional external fire suppression. 
     A flow control cabinet  110  is assigned to each mill  12 . The cabinet includes electronically actuated solenoid valves, and individually controlled, multi point outlet zones within each cabinet  110 . Preferably, the cabinet  110  includes two outlets for two sets of classifier zone nozzles  22 , two zones for two sets of grinding zone nozzles  32 , and one zone for the primary air inlet zone nozzles  42 . 
     The system  10  includes a fire suppression solution. Preferably, the fire suppression solution is provided by mixing a fire suppression agent such as F-500 directly into a flowing water stream at a concentration of about one percent. The F-500 concentrate can be held in an IBC chemical storage tote and fed into the plant service water stream by way of a rotary water dosimeter or chemical feed pump. 
     The suppression solution can be delivered to the mill  12  by opening an electronically or pneumatically actuated valve located at the header outlet. The valve may be opened either by pushbutton at the control/pumping skid when in “Hand” mode or remotely by way of remote I/O switching from the control room when set to “Auto” mode. A VFD controlled booster pump ensures proper pressures required for delivery of the solution to injection nozzles in the pulverizer mill  12 . The remote I/O provides the possibility of manually triggering the system from the control room or being part of an automated system triggered either by high temperature measures at the mill exit or a number of currently available methods of detecting mill fires. 
     The solution is delivered to the mill  12  in steel piping and flexible stainless steel hose  14  to one of a plurality of injection nozzles positioned on the pulverizer mill  12  and jettisoned into one of three regions of the mill  12 : the classifier zone  20 , grinding zone  30  and pulverizer inlet/under bowl zone  40 . As shown in  FIGS. 1 and 4 , the classifier zone  20  refers to the area within the classifier cone  21 , also known as an oversize return or tailing discharge. The grinding zone  30  refers to the area within the mill  12  outside of the classifier cone  21  and above the bull ring  34 , as shown in  FIGS. 1 and 4 . The grinding zone  30  includes the area in which the grinding elements  50  are positioned. The grinding elements  50  can be comprised of a plurality of journal grinder and spring pressure assemblies  50 , as shown in  FIGS. 4 and 5 . The system  10  can be utilized with mills having alternative grinding elements, such as ball and race or roll and race grinding assemblies. The pulverizer inlet/under bowl zone  40  refers to the area below the bull ring  34  and above the bull gear  36 , as shown in  FIGS. 1 and 4 . 
     The system  10  includes three groups of injection nozzle assemblies  22 ,  32 ,  42  positioned to introduce the fire suppression solution into the three regions  20 ,  30 ,  40  of the mill  12 . The first group is comprised of classifier injection nozzle assemblies  22  positioned in a first circular array around the raw coal feed inlet  15  and outlets  18  through the upper housing  16  of the mill  12  above the classifier cone  20 , as shown in  FIGS. 2, 6 and 7 . The second group of nozzle assemblies is comprised of grinding zone injection nozzle assemblies  32  positioned in a second circular array proximate the outer edge of the upper housing  16 , outside the classifier zone  20 , as shown in  FIG. 2 . The third group of nozzles is comprised of pulverizer inlet/under bowl injection nozzle assemblies  42  positioned in the primary air duct  44 , a short distance upstream from the pulverizer  12 , as shown in  FIG. 4 . A distribution valve enclosure  46  provides automated valves that control the flow of water and encapsulator agent flow to the various nozzles  22 ,  32 ,  42 . 
     The stainless steel braided hoses  14  can be connected to the nozzles  22 ,  32 ,  42  by way of a quick release coupling. This allows the maintenance crews to move the hoses out of their way thus reducing tripping hazards when servicing the top of the mill. 
     Upon triggering by either manual activation or by automated detection system, the nozzle assemblies  22 ,  32 ,  42  disperse a fine mist of the fire suppression solution into the mill  12 . The classifier injection nozzle assemblies  22  deliver fire suppression solution into the classifier zone  20 . The grinding zone injection nozzles  32  deliver fire suppression solution “S” into the grinding zone  30 , as shown in  FIG. 20 . The pulverizer inlet/under bowl injection nozzle assemblies  42  deliver suppression solution into the pulverizer inlet/under bowl zone  40 . As such, the suppression solution can be effectively delivered to all areas of the mill  12 . 
     The pulverizer mill protection system  10  provides an effective tool to manage mill outlet temperature excursions before they evolve into fires and averts de-rates due to tripped mills, forced outages and mill damage. Because the system  10  operates continuously while the mill  12  is in service, fires are suppressed in a fraction of the time when compared to traditional methods utilizing steam or water fog. The rapid fire suppression/extinguishing means fires are eliminated in seconds; preventing damage to the mill, piping, external wiring, instrumentation and other ancillary equipment. This is achieved by injecting a water and F-500 solution as a fine mist through numerous nozzles strategically placed in the pulverizer. The system also prevents or inhibits mill fires, explosions and puffs due to coal feed interruptions or during mill start-up and shutdown. In addition, the system prevents mill fires from spreading to the burner lines. Less water is required and cooling is more uniform compared to traditional steam and water fog systems, reducing thermal stresses/cracking of grinding elements, grinding and bull rings, rotating throats, mill side liners and other internal components. The same F-500 system can be used inside and outside the mill  12 . The F-500 EA MPS can be integrated to protect the entire fuel burning system, including the bunker/silos, trippers, feeders, mills and burner lines. 
     The pulverizer mill protection system  10  can be useful for routine maintenance operations. The rapid cooling of mill internals reduces mill downtime for emergency repairs, preventive maintenance, inspections or mechanical adjustments—possibly shortening mill outages from twenty-four hours to a few hours or less. Since the F-500 is a non-corrosive, biodegradable and nontoxic agent, the system  10  is viable for use in non-emergency, routine maintenance situations as no special cleaning equipment is required after its use. Maintenance crews may enter the confined space without risk of injury due to trapped steam, heat or hazardous fumes. In instances where a mill is to be removed from service, the system may be used for a mill internal wash down to reduce residual coal dust in the mill interior. External fire suppression nozzles may also be used to help control combustible dust on the mill exterior and improve housekeeping in the mill bay areas. 
     The system  10  includes a solution delivery system for delivering the fire suppression solution to the nozzles  22 ,  32 ,  42 . As shown in  FIG. 6 , the solution delivery system can be comprised of manifolds  23 ,  33 . Classifier zone manifolds  23  are connected to the classifier zone nozzle assemblies  23  by solution delivery piping  70 , shown in  FIG. 6 . Ring manifolds  33  are connected to the grinding zone nozzle assemblies  33  by piping  70 , shown in  FIG. 6 . 
     A programmable logic controller (PLC) and a thermocouple  60  attached to the mill  12 , as shown in  FIG. 19 , can be operatively connected to the solution delivery system such that the fire suppression solution can be delivered at varying intervals and at varying flow ranges to the nozzle assemblies  22 ,  32 ,  42  depending on the temperature in the mill  12 . In addition, the flow of suppression solution can be modulated. 
     The system  10  can control high mill outlet temperature excursions by regulating the outlet temperature. This can be particularly important when hot and cold air dampers fail to control the mill outlet temperature satisfactorily and/or when high mill outlet temperatures are caused by circumstances outside of an operator&#39;s control, such as interruptions in raw coal feed. When the thermocouple  60  detects high temperatures from the coal outlets  18 , the solution delivery system is triggered to deliver suppression solution to the nozzles  22 ,  32 ,  42 . The nozzles  22 ,  32 ,  42  disperse suppression solution “S” in the mill  12 , thereby lowering the temperature of the fuel/air mixture exiting the outlets  18 . 
     Carbon Monoxide (CO) monitoring equipment can be installed in the mill outlets  18 . Not all fires and puffs are preceded by a measureable CO spike, and not all fires and puffs are preceded by a measurable temperature excursion. By having both CO and temperature monitoring equipment, the likelihood of the onset of a fire going undetected is greatly reduced. 
     The system  10  incorporates mill outlet temperature management and continuous encapsulation of combustibles. The system  10  operates continuously while the mill  12  is in service, and pro-actively manages temperatures in the mill  12  to reduce the chance of a complete shut down due to a major event. 
     The system  10  includes a seal air distribution subsystem for delivering atmospheric air at high pressure to the nozzles  22 ,  32 ,  42 . As shown in  FIG. 7 , the seal air distribution system can be comprised of classifier zone seal air manifolds  25  connected to the classifier zone nozzle assemblies  22  by piping  80 , and grinding zone manifolds  35  are connected to the grinding zone nozzle assemblies  32  by piping  80 . An underbowl/primary air seal manifold  45  is connected to the underbowl/primary air duct nozzle assemblies  42 . 
     The seal air distribution system draws in ambient air, and delivers it at high pressure to the nozzle assemblies  22 ,  32 ,  42 . The pressurized air keeps the nozzle assemblies  22 ,  32 ,  42  clean, and the nozzle assemblies  22 ,  32 ,  42  disperse the air to help prevent contamination of the interior of the mill  12 . 
     As shown in  FIGS. 8 and 9 , three primary air zone nozzle assemblies  42  are mounted in the primary air duct  44 . As shown in  FIG. 10 , each primary air zone assembly  42  comprises a pair of NPT adapters  42   a  joined to a pair of air/solution check valves  42   c  by NPT fittings  42   b  that are connected to a flange mount cap  42   d . The flange mount cap  42   d  is mounted to the outside of the primary air duct  44 , and a nozzle wand  42   e  extends from the opposite side of the mount cap  42   d . Three fine mist, low flow nozzles  42   f  are connected to the wand  42   e . The nozzles  42   f  can be connected to the wand by NPT half couplings, or other suitable connection means. One of the adapters  42   a  can be connected to the seal air distribution system, and the other adapter  42   a  can be connected to the solution delivery system. A flow switch is connected to the solution inlet, and detects when a nozzle  42   f  is eroded. Another flow switch is connected to the air inlet, and detects when a nozzle  42   f  is plugged up. 
       FIGS. 11-14  illustrate a preferred construction of the classifier zone nozzle assemblies  22 . As shown in  FIGS. 11-14 , each classifier zone nozzle assembly  22  can be comprised of a pair of NPT to tube fittings  22   d ,  22   e . One tube fitting  22   d  connects to the solution delivery system, and the other fitting  22   e  connects to the seal air distribution system. The solution inlet fitting  22   d  is connected to a solution side check valve  22   g , and the seal air inlet fitting  22   e  is connected to an air side check valve  22   h , which is connected to a Ten NPT adapter  22   c , which connects to a NPT threaded tee fitting  22   b . The tee fitting  22   b  connects the solution inlet  22   d  and the seal air inlet  22   e  to a pipe flange mounting cap  22   a . A high flow, fine mist nozzle  22   f  positioned on the opposite side of the mounting cap  22   a  can be connected to the tee fitting  22   b  by an NPT threaded elbow. The mounting cap  22   a  is mounted on the mill  12 , with the solution and seal air inlets  22   d ,  22   e  positioned exterior to the mill  12 , and the nozzle  22   f  extending into the interior of the mill  12 , as shown in  FIGS. 19 and 20 . 
       FIGS. 15-18  illustrate a preferred construction of the classifier zone nozzle assemblies  32 . As shown in  FIGS. 15-18 , each classifier zone nozzle assembly  32  can be comprised of a pair of NPT to tube fittings  32   d ,  22   e . One tube fitting  32   d  connects to the solution delivery system, and the other fitting  32   e  connects to the seal air distribution system. The solution inlet fitting  32   d  is connected to a solution side check valve  32   g , and the seal air inlet fitting  32   e  is connected to an air side check valve  32   h , which are connected to a NPT tee fitting  32   b . The tee fitting  32   b  connects the solution inlet  32   d  and the seal air inlet  32   e  to a pipe flange mounting cap  32   a . A machined cylindrical nozzle body  32   i  positioned on the opposite side of the mounting cap  32  can be connected to the tee fitting  32   b  by an elbow fitting. A low flow, medium droplet, full cone nozzle  32   f  is connected to the nozzle body  32   i . The mounting cap  32   a  is mounted on the mill  12 , with the solution and seal air inlets  32   d ,  32   e  positioned exterior to the mill  12 , and the nozzle  32   f  extending into the interior of the mill  12 , as shown in  FIGS. 19 and 20 . 
     The seal air distribution system can provide a continuous stream of air to the each of the nozzle assemblies  22 ,  32 ,  42  to prevent plugging of the nozzles  22 ,  32 ,  42 . When solution is supplied to the mill  12 , a single air valve (solenoid type) is closed halting air supply to all nozzles  22 ,  32 ,  42 . Check valves  22   h ,  32   h ,  42   c  on the air side of the nozzle assemblies  22 ,  32 ,  42  keeps solution from running up into the air lines. When the air supply is shut off, the flow switch on the air supply side indicates when a nozzle assembly  22 , is plugged. Because there is a flow switch per nozzle assembly  22 ,  32 ,  42  the exact nozzle that is plugged may be indicated. 
     Higher than normal pressure in the solution delivery lines typically indicates a partially plugged nozzle. The system  10  may continue to operate, but the spray effectiveness will be compromised. Pressure is measured per spray zone. As such, the particular nozzle that is partially plugged is not known, however, the spray zone that the particular nozzle resides in is indicated. However, if seal airflow to a particular nozzle drops below the seal air flow switch threshold, the exact nozzle that is partially or completely plugged will be indicated. Low pressure in the solution delivery lines typically indicates a worn nozzle. As the nozzle wears, due to the abrasion of swirling coal dust, the orifice diameter expands. An expanded orifice diameter equates to higher than normal flow at a given pressure. Once flow goes above the solution flow switch threshold, the eroded nozzle will be indicated. Nozzle assemblies  22 ,  32 ,  42  can be easily replaced by disconnecting air and water lines and removing the nozzle assembly  22 ,  32 , 42 . 
     In the event of temperature or CO excursions, a few short bursts of suppression solution are sprayed into the classifier zone  20 , and under bowl zone  40  occurs well below the blast gate temperature setpoint. If the temperature or CO continue to rise, longer bursts of solution are sprayed into the classifier zone  20 , grinding zone  30 , and underbowl zone  40 . At a temperature just below the blast gate set point, all spray zone nozzle assemblies  22 ,  32 ,  42  spray continuously. After the blast gate closes, solution is continuously sprayed in all zones  20 ,  30 ,  40  in order to completely cool the interior of the mill  12 , and encapsulate combustibles. 
     In the event of a coal feed interruption, the feeder trips or other indication of interrupted coal feed into the mill  12  triggers intermittent bursts of solution that keep the temperature of the mill  12  under control until the blockage is cleared. Suppression solution is sprayed into the grinding zone  30  and under bowl zone  40 . 
     At start-up of the mill  12 , the system  10  sprays solution into the classifier zone  20 , grinding zone  30  and underbowl zone  40  in frequent bursts that taper off as temperatures in the mill  12  stabilize. During shutdown, the system  10  sprays suppression solution into the classifier zone  20 , grinding zone  30  and underbowl zone  40  initially in short, infrequent bursts that gradually increase in frequency. The shutdown cycle ends with a deluge of continuous solution flow from all nozzles in order to encapsulate combustibles in the mill internals. If the mill is taken offline for a long period of time, solution can be continuously sprayed to completely cool the mill  12  and encapsulate combustibles for maintenance purposes. 
     The system  10  can be set into a manual hand mode, in which individual spray zones  20 ,  30 ,  40  or entire mills can be sprayed with suppression solution at the direction of the operator. Reasons for manual operation can include observation of burning coal in the pyrite reject area, cooling the mill  12  prior to entering the confined space, encapsulating combustibles (effectively inerting) in the confined space of the mill  12  prior to maintenance, internal wash down either with solution or clean water by opening the solution bypass valve. 
     While the system  10  is described above and shown in the drawings as being used in a Bowl type pulverizer mill, the system  10  is not so limited. The system  10  can be incorporated into varying pulverizer designs, such as Attrita, Ball Tube, CE Deep Bowl, CE Shallow Bowl, EL, Dooson Babcock E-Type, MBF, Ball and Race, and MPS pulverizers. 
     The mill protection system  10  is capable of not only responding to existing fires, but can also address many of the issues that lead to mill fires and explosions. The system  10  addresses these issues with advanced sensing equipment, carefully placed spray nozzles of various designs, utilization of an effective encapsulation/wetting agent and high speed reaction to early indications of mill issues. The system  10  can spray at varying densities to address issues ranging from high outlet temperatures to high levels of carbon monoxide (CO). The system  10  also prevents possible issues by removing combustibles in the mill  12  after a shutdown or mill trip and rapidly cooling the mill  12 , thereby eliminating issues related to hot restarts and allowing personnel to enter the mill for inspection after an event or for regular maintenance. At its lowest flow rate, the system  10  fills the vessel with a fine mist, thereby assisting in cooling and encapsulating combustible dusts without agitation. At higher flow rates, the system  10  can fully control mill temperatures without the assistance of coal flow or damper position changes. At the maximum flow rate, the system  10  can fully deluge the mill  12 , flooding the bowl and underbowl and washing coal out of the pulverizer. The coal dust can be carried away by the pyrites removal system. The system  10  can have a maximum spray density of greater than 2.5 gpm/ft 2  so that a fire can be quickly suppressed. The system  10  can be equipped to address fires in ancillary equipment via external spray headers. 
     The system  10  can include an independent, highly sensitive thermocouple at the mill outlet for obtaining an accurate mill temperature. When the mill  12  is at a steady state, this temperature is compared to existing temperature measurement elements also located at the mill outlet in order to verify that the temperature the system is responding to is accurate. The rapid response of this element means that the system  10  responds to increases in temperature quickly, often before plant control systems detect a change. This additional element also provides supplementary data for troubleshooting mill operating parameters. In the event of a temperature spike, the system  10  can initiate spraying in 250 milliseconds or less. In addition to responding to temperature excursions, the system  10  can also utilizes a fulltime, continuous carbon monoxide (CO) monitoring subsystem, which unlike CO monitors known in the art, has one sensor and gas extraction system per mill. Known CO monitors extract gas samples from all mills to a single sensor, one at a time. For larger units with a high mill count and long sample line runs, this can mean that an increase in CO level may not be indicated for several minutes after the initiation of combustion. 
     The mill protection system  10  can have various modes of operation that initiate based on the current operating mode of the attached pulverizer. If the mill  12  is in startup mode or running at a steady state, the system  10  is in temperature excursion mode. In this mode the system  10  sprays at varying densities based on indicated temperature and rate of temperature rise. At a temperature just above the normal mill operating temperature setpoint, the system  10  sprays at a low flow rate in the underbowl/primary air inlet zone (PAZ)  40 . As temperature increases, the system sprays at higher flow rates in the underbowl and upper mill zones, the grinding zone (GZ) and in the classifier cone (CZ). In this mode of operation, the system  10  is capable of mitigating temperature excursions whether due to a reduction or loss of coal feed or a stuck or slowly-responding hot or cold air damper. At its highest flow rate, the system  10  is capable of delivering enough moisture to the vessel to maintain outlet temperatures at or below the normal operating temperature of the pulverizer. If coal flow completely stops, the mill temperature is maintained at safe levels allowing additional time to restore flow. If dampers stick due to a mechanical or electrical malfunction, the system  10  can maintain safe temperatures without tripping the mill  12 . Again, in this situation, this provides additional time to either correct the damper issue or bring the mill  12  offline safely. Because the system  10  can maintain mill temperature independently, operator errors do not generate a potential for catastrophic events. Sudden spikes in temperature due to starting coal feed late or introducing hot airflow early are mitigated by the system  10 . 
     When the mill  12  is being shutdown, the system  10  begins to introduce a fine mist spray in the PAZ  40  when the feeder is stopped. This assists in cooling the mill  12  while the remaining coal is swept out with tempering air. When the hot air damper and/or gate are completely shut and low mill amps indicate that the grinding zone  30  has been completely swept out, the system  10  deluges the mill  12 , completely cooling the mill  12  and washing out combustibles, which are removed via the pyrites removal system. The outlet temperature is checked at the end of this deluge. If the temperature is above setpoint, the system  10  continues to spray until a safe temperature is attained. With the mill  12  cooled to a safe temperature and completely washed out, the mill  12  is safe on the next start. Also, there is no coal left to be heated to ignition and no heat in the mill  12  to do so. For a period of two hours after the mill  12  is stopped, the system  10  monitors mill outlet temperature. If this temperature rises above a predetermined setpoint, a short spray sequence drops the temperature back down. No operator intervention is required. 
     In the event of a mill trip, the system immediately introduces spray. Similar to the shutdown sequence, the mill  12  is rapidly cooled and combustibles are washed out through the pyrites system. This method of clearing the mill  12  of coal greatly reduces the risks associated with hot restarts. The sequence completes in two minutes and it is recommended that the operator run the mill with tempering air for one minute to dry out the mill prior to starting. In the event that the mill  12  is not immediately restarted, the system  10  monitors mill outlet temperatures and responds with a spray sequence if temperatures rise above the setpoint temperature. 
     Several manual modes of operation are possible with the system  10 . Spray may be initiated in any spray zone  20 ,  30 ,  40  either from the control room or using hand/off/auto (HOA) switches at the main control panel. Spray media can be either pure water or a solution of water and encapsulation/wetting agent. With manual operation, the system  10  can be used to encapsulate combustibles and assist in cooling by activating the fine mist nozzles in the pulverizer inlet/under bowl zone  40  and the classifier zone  20 . This method of manual operation can be useful as an extra safety precaution prior to entering the mill  12  for inspection. If a fire is discovered in the mill  12 , either by direct observation of burning material or as indicated by elevated CO levels, spray may be manually activated by initially activating the fine mist spray nozzles in the PAZ  40  and CZ  20  and then stepping up spray intensity by adding a second, high flow zone in the PAZ  40 . A full deluge may also be performed by initiating all spray zones: two sets of nozzles in the PAZ  40 , two sets in the GZ  30  and the set of nozzles located in the classifier cone (CZ)  20 . Beyond directly activating spray zones, any of the automatic modes of operation can be manually forced. This can be useful for washing out the mill  12  by forcing the loaded mill trip sequence. 
     A pulverizer mill protection system and method of using same are described above. Various changes can be made to the invention without departing from its scope. The above description of the preferred embodiments and best mode of the invention are provided for the purpose of illustration only and not limitation—the invention being defined by the claims and equivalents thereof.