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Electric power production from coal is on a steep rise in major developing countries, including China, India, Indonesia, South Africa, and Vietnam, albeit declining in developed countries such as the United States. Shortfalls in coal production have been reported in some of these countries, but these issues are being addressed by increasing coal production, as well as by use of imported coal from other countries. The major concerns of Pulverized Coal (PC) or Circulating Fluidized Bed (CFB) combustion type coal-fired power plants are fires and explosions in hazardous areas and global warming and other environmental issues. Fires and explosions have caused a significant number of deaths and injuries to power plant staff. Besides other ignitable materials used in coal-fired power plants, coal dust has been identified as the major source of these fires. For coal to remain as a viable fuel in power production in the power generation industry, protective measures are required during engineering, design, construction and operation of the coal-fired power plants, particularly in electrical areas that are often a source of ignition.
The fire triangle (Figure 1) is a well-known tool that illustrates the three conditions that must be present for a fire or explosion to occur at a particular location: Flammable or combustible material must be present; the material must be mixed with air in the proportions and concentration to form a combustible mixture; and the ignition source must supply enough energy to initiate combustion. A spark or flame is not necessary, as temperature alone can supply the energy to cause ignition of the mixture.
The energies required to ignite various groups of combustible substances have been proven by experimentation. The concepts included in the fire triangle have been specified in codes and standards issued by various world organizations providing guidance in the design of electrical systems, selection of equipment, and construction and operation of power plant facilities.
The National Electrical Code (NEC) in United States and International Electrotechnical Commission (IEC) in other countries define hazardous area locations as those areas, where fire or explosion hazards may exist due to presence of combustible dust or ignitable fibers, flammable gases or vapors, and flammable liquids. Electrical equipment can become a source of ignition in these volatile areas.
Hazardous areas are classified by types, division or zone, and nature/group. There are three types of hazardous conditions, Class IGas and Vapor, Class IIDust, and Class IIIFibers and Flyings. Hazardous area locations are categorized by two methods per NEC:
Coal. The main combustible fuel. In addition, coal dust is a major source of dust explosions in a coal-fired power plant. It can cause primary explosion when the right concentration of finely divided dust, suspended in air, is exposed to a sufficient source of ignition causing combustion. If this primary explosion occurs, additional available dust can disperse and secondary explosions can spread throughout the facility.
Natural gas. As start-up or alternate fuel. Gas can be present in gas compressor station, filter and scrubber station, gas pre-heater areas, gas shut-off valve and filter, gas control valve block, gas relief valve, and gas piping.
Fuel oil. As start-up or alternate fuel. Can be present in oil tanks, fuel unloading and forwarding pump station, fuel oil booster pump and leakage tank, fuel oil control valve block and fuel oil piping to burners. The fuel oil becomes flammable when heated above its flash point.
High temperature. Some heat producing equipment, such as light lamps and lighting fixtures, electric motors and heaters can ignite flammable atmospheres if they exceed the ignition temperature of the hazardous material.
Electrical equipment can cause explosions in coal dust atmospheres when located in coal conveyor galleries, transfer towers, head chute, flop gate bunkers, coal preparation system/crusher house, coal silos, and feed system, totally enclosed portions of coal handling conveyors, coal handling below grade conveyor systems, coal chutes, dust collectors, and other locations where dust can settle, hidden concealed areas, and where other combustible and flammable materials exist.
Ignitions, combustion and explosion within electric equipment and circuit wiring enclosures in hazardous areas are due to hot surfaces and arcs/sparks, or combustible material entering the enclosures. The ideal electrical enclosure should be installed and removed easily, protect the equipment, allow the components housed inside it to be accessed easily, and resist and prevent fire and explosion hazards. NFPA 70, NFPA 79, NEMA 250, UL 50, UL 508A; Canadian Standard Association CSA 22.2, and IEC 60529 provides guidance in the selection of specific type of enclosure to match the environmental protection.
Equipment protection methods include flameproof, encapsulation, oil emersion, dust ignition proof, dust tight, powder filled, non-sparking, non-incendiary and hermetically sealed construction. Many of these methods apply to AC-powered circuits, but few are used for instrument wiring circuit wiring.
When installations are not explosion proof or intrinsically safe, pressurization is often used to maintain the classified area safety. Wiring and enclosures are protected using a positive pressure maintained within the enclosure, junction boxes and conduit as per ANSI/NFPA 496.
Intrinsically safe circuits in which any spark or thermal effect is incapable of causing ignition of a mixture of flammable or combustible material in air, may be used in hazardous classified locations. NEC Article 504 requires that conductors and cables of intrinsically safe circuits shall be physically separated from non-intrinsically safe circuits. Nonincendive circuits, in which any spark or thermal effect produced under intended operating conditions is not capable of igniting the gas-air, vapor-air, or dust-air mixture, may be used for equipment in Class 1, Division 2 and Class II, Division 2 hazardous classified locations.
Class I, Division 1 equipment is used with the assumption that hazardous gases or vapors will be present and eventually seep into the enclosure. Therefore, construction of such equipment must be strong enough to contain an explosion within, and be explosion proof. It must function at a temperature below the ignition temperature of the surrounding atmosphere providing a way for the burning gases to escape from the device as the gases expand during an internal explosion, only after they have been cooled off and their flames quenched. The escape paths could be ground surface or threaded flame path. Further, it is imperative to ensure that all flame paths are protected during handling, shipping, storage, installation and maintenance of explosion proof material and equipment.
Class II, Division 1 equipment is designed and constructed such that it must seal out the combustible dust, operate below the ignition temperature of hazardous substances, and allow for a dust blanket. Therefore, there is no need for heavy explosion-proof construction, or flame paths required for equipment in Class I, Division 1 locations. Class II equipment is called dust-ignition proof.
There is very little difference in the design between Class II and Class III equipment. Class III equipment must minimize entrance of fibers and flyings, prevent escape of sparks, burning material or hot metal particles resulting from failure of equipment, and operate at a temperature that will prevent the ignition of fibers accumulated on the equipment. Class III equipment is generally not used in coal-fired power plants.
There are many devices, fixtures and enclosures that are suitable for Class I, II and III locations. Additionally, a Class I device would have to prevent dust entering the enclosure to be suitable for Class III application.
Explosion proof or dust ignition proofequipment is generally suitable for use in an ambient temperature range of -25C (-13F) to +40C (+104F). Such equipment may not be suitable for use at temperatures lower than -25C (-13F), unless they are identified for lower temperature service.
Equipment that is approved for Class I and Class II or Zone 0 and Zone 1 should be marked with the maximum safe operating temperature. Equipment must be selected so that its maximum surface temperature will be less than the ignition temperature of the coal dust.
Proper installation of electrical equipment in a hazardous location requires use of seals. Special fittings are required to keep hot gases from travelling through the conduit system igniting other areas if an internal explosion occurs in a Class I device.
Coal handling buildings that have dust explosion hazards, structural members should be carefully reviewed for explosion pressures. Also, proper pressure relieving vents and wall panels should be selected, located and provided.
There are a number of ways of protecting electrical equipment so that it does not cause an explosion when used in a surrounding flammable atmosphere, or ignite a layer of dust on the equipment. The two most common ways are explosion-proof equipment and dust-ignition proof equipment
In Coal handling areas, where accumulation of coal dust and its suspension in air are sources of potential hazards, medium voltage switchgear and low voltage switchgear enclosures should be fully gasketed and be provided with filtered and screened air.
Dust ignitionproof equipment should be enclosed in a manner that will exclude dust and constructed so that arcs, sparks, or heat generated inside of the enclosure will not ignite exterior accumulation or atmospheric suspension of a specified dust on or in the vicinity of the enclosure.
Electrical devices including switches, solenoids, panels, should be used with NEMA 9 (dust ignitionproof) enclosures with watertight seals (O-rings), which together with the enclosed equipment in each case, shall be as complete assembly for Class II, Division 1, Group F locations.
NEC Article 500.8 (A) states that the suitability of identified equipment shall be determined by the equipment listing or labeling (for example, UL), evidence of equipment evaluation from a qualified testing laboratory or inspection agency, or evidence acceptable to the authority having jurisdiction.
A design basis document is an engineering document that defines the basis of engineering and design, and the selection of electrical equipment that must meet requirements for each classified area. The design basis document also provides the necessary guidance to inter-discipline team of engineers in the design of their systems and design drawings for classified areas.
Table 2 provides general practical guidelines for classification of electrical areas where combustible and/or flammable materials are located and processes are performed in a coal-fired power plant. The following is a list of NEC rules that are intended to convey an awareness of the complexity of electrical design in hazardous areas:
Sound engineering judgment should be applied and additional areas, not mentioned in Table 2, should be identified. Equipment manufacturers recommendations for area classification of specific equipment should be followed, if they are more stringent than the guidelines provided in Table 2.
When there is more than one leakage source in an area, such as in a manifold having several instruments, valves and flanges, or if there are several pieces of equipment with potential leak sources, the area should be boxed out as an overall 3 dimensional shape covering limits of extreme leakage points.
In addition to the recommended distances from sources, considerations should be given to use easily recognizable boundary limits when defining the horizontal and vertical extent of classified locations. Examples of recognizable boundaries are column lines, walls, ceilings, coordinates, equipment outlines, roads, dikes, etc. Areas identified by recognizable boundaries are helpful for plant installation, operation and maintenance personnel.
To avoid undue expense, precaution should be taken to verify that boxing an overall area or extending a classified area to recognizable boundaries does not include electrical equipment that would otherwise not be included in the hazardous area.
Project documents prepared and used in establishing hazardous areas include process flow diagrams of the systems containing the hazardous materials, piping layouts, equipment general arrangements, and vendor supplied equipment drawings
Two examples may be illustrative. The first shows a typical example of a pulverized coal-fired power plant coal handling system process flow diagram, and the second shows boiler general arrangement with classified hazardous locations.
The design basis document and the associated design drawings that show the extent of horizontal and vertical boundaries of each classified area should be discussed with the owner, the construction contractor and the plant operators for their understanding of the affected areas. It also helps the plant operation personnel to take protective measures that ensure safety of operation and maintenance of plant and better understanding to inspectors and insurance personnel.
Plants can no longer sweep coal dust under the rug and ignore the health and safety hazard it presents, because a single spark can cause a dust explosion that could put a plant out of service, perhaps permanently. Managing dust in a power plant begins with good housekeeping, followed by retrofits using properly designed equipment.
On February 7, 2008, at about 7:15 p.m., a series of huge explosions and fires occurred at the Imperial Sugar refinery northwest of Savannah, Georgia, causing 14 deaths and injuring 38 others, 14 seriously. The facility, which converted raw cane sugar into granulated sugar, had a material-handling system that included the familiar railcar unloader, belt conveyors, bucket elevators, and silo storage. The explosions were fueled by massive accumulations of combustible sugar dust throughout the packaging building.
The initiating explosion occurred in the enclosed belt conveyor below the silos. That explosion lofted the dust accumulated on floors and elevated surfaces throughout the buildings. Secondary explosions and fires heaved thick concrete floors and collapsed brick walls, blocking stairwells and exit routes. The destruction, as shown in the photo above, was complete.
The U.S. Occupational Safety and Health Administration (OSHA) reports that between 1980 and 2008, 422 combustible dust incidents were reported across 64 different industries; electric power generating utilities experienced 28 incidents. The U.S. Chemical Safety and Hazard Investigation Board (CSB) investigates and documents the causes of these industrial dust explosions. Each of the CSB dust explosion reports suggests a common cause: Companies and their employees failed to recognize the implicit danger of airborne and accumulated dust.
The explosion at the Imperial Sugar plant prompted the U.S. House of Representatives to pass a bill requiring OSHA to adopt the National Fire Protection Association (NFPA) recommendations to prevent explosions caused by combustible dust (see Five Requirements for a Dust Explosion), although it never became law.
The dust explosion pentagon (Figure 1) takes its first three elements from the familiar fire triangle and adds two more. If one of these five elements is missing, an explosion cannot occurbut a fire may still occur: combustible dust (fuel), an ignition source (heat), oxygen in air (oxidizer), dispersion of dust particles in sufficient quantity and concentration, and confinement of the dust cloud.
A flame need not be the ignition source. Dry powders can build up static electricity charges when subjected to the friction of transfer, conveyor belt friction on pulleys and idlers, and mixing operations. Adequate precautions should be provided, such as electrical grounding and bonding, or inert atmospheres.
Today, OSHAs instruction No. CPL 03-00-008 is the guiding directive for controlling dust in manufacturing facilities. CPL 03-00-008 and NFPA 654 define the conditions under which plants must immediately remove dust accumulations that are 1/32 inch thick. OSHA standard No. 29 CFR 1910.269(v)(11)(xii) requires the elimination or control of ignition sources when coal-handling operations may produce a combustible atmosphere. NFPA 654, which includes a comprehensive list of dust control, ignition sources, and damage control provisions, is also an invaluable reference.
A good understanding of what these instructions require (and following through on upgrades, as required) will make your next OSHA inspection much less stressful. The sidebar Prepare for Inspections, Not Citations describes what happens during a typical OSHA visit.
The U.S. Occupational Safety and Health Administrations (OSHAs) area offices are required to identify and inspect facilities where employees may be exposed to potential combustible dust hazards and respond if a formal complaint or referral is received. Facilities are selected using a random numeric system or are chosen via a Site-Specific Targeting Plan. OSHAs inspector may elect to do one or more of the following during a visit:
The OSHA inspector may issue citations for housekeeping, general duty, deflagration/explosion hazards, personal protective equipment, electrical, warning sign, egress, and fire protection violations. The collection of air samples is not necessary for a citation to be issued.
OSHA has initiated site visits to obtain facility-specific information on combustible dust recognition, prevention, and protection programs, and to relay notable findings and other facility feedback to OSHA. In one such reported instance, plant representatives expressed concern about the amount of resources needed to comply with OSHA dust standards and commented that this would be particularly difficult for older plants. They also expressed a concern that new standards should not conflict with environmental regulations and that a new dust control system would be required to be within permitted limits.
Ameren has experienced two OSHA combustible dust inspections. In both instances, the inspectors were primarily interested in the silo fill areasthe tripper area directly above the coal silos/bunkers. The inspectors were satisfied with the dust control measures being used by Ameren. The company has added a variety of dust control measures to its plants in a continuing, multi-plant improvement program. This spreads the cost over several years, and the staff gain valuable experience and confidence with the equipment during that time. Dust reduction results can also be compared and verified over time so that the most effective and economical technologies are deployed, as needed.
Coal-fired power plants typically store coal outdoors and normally avoid using enclosed bucket elevators, making the design of a coal-handling system unique and much different than, for example, a grain-handling system at an export terminal.
Most coal-fired plants have crushers, and those in northern climates will often feature enclosed transfer houses and conveyor galleries. Day silos store one or more shifts supply of coal within the power block for operating reliability. Consequently, devastating fires and explosions such as those that occurred at Imperial Sugar and at grain export terminals, particularly during the 1970s, are an ever-present threat at coal-fired plants that ignore basic safety and housekeeping precautions.
At coal-fired plants, high-capacity conveying systems handle thousands of tons of coal per hour. When even a small fraction of this tonnage is released and becomes airborne, an unacceptable danger is present. The danger escalates because the conveyed product is also flammable and may reach explosive concentrations. Airborne dust eventually settles on a variety of surfaces and over time; thick layers accumulate in less-visible or inaccessible areas. These accumulations fuel the most devastating events when a small initial explosion shakes dust-laden equipment, piping, conduit, ducts, and structures, thereby propelling the dormant fuel into a dense, flammable cloud that feeds a rapidly expanding fireball. These secondary explosions typically cause the majority of the damage in a plant.
Consider the bulldozer in Figure 2 that is slowly pushing a blade of coal into a mostly empty, below-grade reclaim hopper. As the coal slides from the left into the hopper, it displaces air within the hopper. As the photo shows, the slow-flowing dry coal creates a cloud of dust. The exiting displaced air floats fine coal particles into a coal dust cloud that drifts upward at the right side of the hopper. Clearly, even relatively slow motions can create a cloud of dust. Now consider the amount of coal dust generated when a fast-moving belt conveyor throws a continuous discharge stream of coal into a transfer chute.
While upgrading housekeeping will reduce visible coal dust, curing the patient is a better practice than only treating the symptoms. In other words, housekeeping must be followed by a program that eliminates coal dust altogether in order for future housekeeping to be effective. A haze of coal dust that obscures vision, and accumulations that block egress, as shown in Figures 3 and 4, are workplace obstructions that are not allowed and major safety hazards that must be eliminated in order to comply with OSHA requirements.
Together, a selection of these dust reduction technologies and improved housekeeping practices are required to meet OSHA requirements for dust control in plants. In addition, OSHA suggests a four-step process for reducing coal dust in plants:
The National Electric Code and National Fire Protection Association 70 classify hazardous locations and provide guidance for electrical equipment design/enclosures as well as whether they should use explosion-proof, dust ignitionproof, dust-tight, purge and pressurized, or intrinsically safe designs. For coal-handling systems, the following area classifications are commonly of interest:
In a practical sense, the first step in eliminating the hazard is to understand dust formation processes and which dust control technologies will be most effective to reduce airborne dust at the source. The available technologies are usually classified as containment, suppression, collection, and flow control. Each individual technology will reduce dust formation at the source, but a holistic approach should be considered for the entire plant. Each technology is discussed in more detail in the following sections.
In the remainder of this article, we explore useful hardware upgrades that will reduce dust production within the typical coal-fired plant that enable operators to comply with the latest OSHA guidance and NFPA design requirements.
Transfer chutes and skirtboards enclose the stream of coal as it spills off the upstream belt and falls or cascades onto the next conveyor. Chute joints can be sealed and dust curtains and/or belt seals can be fitted to chute entrances and skirtboard exits. These improvements help to reduce the flow of air that is carried along with the stream of coal on the belt. Stilling curtains within the skirtboard are another feature that can be added to reduce the turbulent air that is pulled or pumped by the moving belt and coal stream.
The carrying surface of the belt naturally comes in contact with the coal being conveyed. Although multiple belt scrapers are a necessity to clean sticky particles from this surface, some of the smallest, clay-like particles are not easily removed and will adhere to the belt. As these particles dry and the belt flexes over conveyor pulleys and idlers on the return belt strand, they can spill onto the floor or become airborne as they fall from the moving belt or are flung by the rotating rollers. So, although containment can be effective, it is not a complete solution.
Coal-handling systems are typically operated from remote, central control rooms, which monitor and control the system at as a whole. Where a local operator is required, the worker can be stationed in an enclosed cab that is provided with fresh, clean, filtered air. Such is often the case for traveling and mobile equipment like bucketwheel reclaimers and bulldozers. The same is true for stationary locations such as the control panel for a rotary railcar dumper.
As for any power plant system, daily inspections and a planned maintenance program are essential elements in keeping a coal-handling system performing as designed. A walking inspection of the system while it is operating is one of the best methods for detecting emerging problems, such as a bearing thats starting to rattle or equipment that is beginning to overheat.
Small, dry dust particles that become entrained in air currents are the prime cause of airborne dust. When the surface of a coal stockpile becomes too dry, irrigation-style water cannons are used to wet the surface to control windblown dust. As received coal and coal reclaimed from the interior of a stockpile (below what might be a moist outer surface) can become sufficiently dry to create a dust problem as the coal cascades from one conveyor to another. One common solution is to use a dust suppression system that wets small particles that become airborne. This technique suppresses airborne dust by spraying fine droplets of water into the dust cloud. The objective is to increase the weight of these particles by wetting individual particles, a percentage of which will collide into neighboring wetted particles and agglomerate into larger, heavier particles that fall more rapidly.
The effectiveness of a suppression system on airborne dust depends upon dust particle and water droplet characteristics, including their size, solubility, hydrophobic/hydrophilic properties, presence of hygroscopic salts, electric charge, temperature, relative humidity, pressure, and surface tension. The dust particle is much smaller than the water droplet, so air currents can interfere with successful collisions. Smaller droplets are more effective. To create smaller water droplets, the suppressant water is modified by using surfactants to reduce waters surface tension, ultrasonic atomizers to produce a fog of water droplets (1 to 10 micron size), and foam generators to create a blockade of bubbles. A secondary advantage: An increase in effectiveness means less water is required. However, in open outside areas, care must be taken to ensure the water droplets are not so small that they simply blow away with the dust and wind.
Minimizing water usage at the first transfer point can affect the dust problem at the next and subsequent transfer points. Much of the coal can remain relatively dry so that particles that did not agglomerate can become a downstream problem. A common practice for reducing dust formation at the next transfer is to install suppression systems at multiple locations. A different approach developed in the 1980s for the Superior Midwest Energy Terminal increases the moisture content of the coal to reduce dusting, which economically provides dust carryover from the application point to subsequent transfer chutes.
Ventilation of the general area and processing areas is important for coal-handling system dust control. General area ventilationusing outdoor air to control indoor air qualityis a basic requirement for any building, structure, or enclosed work area. Process area ventilation, on the other hand, is used to prevent dust from escaping from chutes, skirtboards, and crushers into the general work area.
For general area ventilation on a coal-handling system, fresh air can be drawn into captive areas of tunnels and enclosures by locating the filter/inlet to the exhaust duct at these furthest, interior locations. An induced draft fan then exhausts air from these areas, and the inlet filter (if the system is equipped with one) helps to reduce the potential for dust accumulations in the ventilation duct. Though the finest particles will remain in suspension and be captured on the filter, a general area ventilation system cannot prevent larger dust particles from settling on floors, equipment, conduit, piping, and other surfaces.
Controlling airborne coal dust that settles on surfaces requires more than dilution air. The containment of process dust is more complex. The American Conference of Governmental Industrial Hygienists publishes Industrial Ventilation, A Manual of Recommended Practice. This manual is a guide to the latest techniques and data for the design, maintenance, and evaluation of industrial exhaust ventilation systems. Sections and examples are devoted to the design of dust capture/intake hoods, ducts, airflow, losses, and the like. Reducing the opportunity for dust to escape into the work area is an important focus of this manual.
Once dust-laden air is extracted from the process, we must safely handle it and meet air emission requirements. A common tool for this task is a fabric dust collector. In some cases, a precleaning cyclone might be advantageous to first capture larger particles. Wet cyclones or scrubbers can also be an effective option. Regardless of the technology used, there are several important criteria to consider:
Once the dust is collected in the hopper of the filters housing, we must do something with it. Just discharging it onto the downstream conveyor at a transfer point can create a secondary dust problem at that location. The dust can also become airborne again at the next downstream transfer station. Dust can be even become more of a nuisance if the coal is being stockpiled and theres a chance it will become windblown over a wide area of the plant. More sophisticated options include these:
Dust collectors are commonly used to vent the tripper and plant silos areas. Coal can oxidize within a silo, especially if there are stagnant, poorly flowing regions within the silo. The risk is that oxidation can accelerate, erupting into a smothering fire usually deep within the coal silo or bunker. Smothering fires generate carbon monoxide (CO), which is also a safety and explosion hazard. Installing a CO monitoring system within the dust-collection system can both warn of the presence of this poisonous gas and provide early warning of an impending fire.
Belt conveyors were once designed for relatively low belt loads and speeds. Ever-increasing belt loads, widths, and speeds became common with the growing size of coal-fired power plants, starting in the 1950s. The impact on coal transfer design was just that, impact. The discharge of coal from the head pulley of a conveyor no longer falls mostly vertically onto shallow-sloped chute surfaces. At high belt speeds, the stream of coal now typically shoots off the conveyor and lands directly into the vertical plate at the front of the head chute.
Violent collisions such as these produce a lot of dust. Some lumps of coal fracture, which creates smaller, lighter dust particles with newly exposed dry surfaces. These are entrained with air and then carried along in the newly formed aerated mixture. The falling aerated stream of coal creates a draft at the entrance to the discharge hood, pulling more air into the chute to continue the process. The slightly higher pressure within the chute allows floating dust to escape through poorly sealed openings and flanges.
In the mid-1980s, new transfer designs using fluid flow principles were introduced that reduced coal dust generation produced by high-capacity conveyor systems. The new approach focuses on transfer chute design to control the flow of coal within the chute. This technique avoids the direct impact of the coal stream on chute surfaces. Instead, the stream is guided. Its velocity and direction are controlled with intersecting, contoured, and form-fitting plate surfaces. The objective is to channel the coal stream so its speed, as it is being loaded onto the downstream belt, closely matches the speed of that belt. This also reduces turbulence and sliding friction at the loading point, which is a primary cause of belt cover wear.
One of the all-too-common problems experienced with coal chutes is plugging. Ameren has installed flow control chutes at several of its coal-fired plants, including the Meramec Plant; Figures 7 and 8 are before and after photos of that plants tripper transfer. This was a particularly challenging transfer system design because of an extremely short skirt board length. This design, common in the 1950s, featured a slow-moving belt fitted with a bend pulley for transitioning from the inclined loading section to the horizontal length of the belt. As units were added, belt speeds increased and the bend pulley was replaced by a convex belt curve. The original transfer chute/skirtboard was replaced with a second-generation design.
Meramec next switched to Powder River Basin coal, which increased the amount of spillage and dust to unacceptable levels. To alleviate the problem, a third-generation design using flow control technology was installed. A hood funnels the flow of coal from the discharging conveyor. The coal is then guided into a concentrated stream by intersecting plates and is directed toward the tail pulley of the tripper conveyor.
This is different from how most chutes are designed, where the primary objective is to direct the flow in the direction of the outbound belt. As seen in Figure 9, there is a lower spoon chute section that does this, but the coal takes an atypical zigzag route. The objective is to gently lower the flowing stream by maintaining plate contact to avoid an uncontrolled freefall where gravitys acceleration expands the falling coal stream and separates it into individual particles or lumps that allows the finer particles to float into a cloud of dust. The lower loading chute has an internal spoon that directs the coal stream onto the tripper belt at a reasonable speed and angle to minimize turbulence, belt wear, and particle size degradation. The spoon is enclosed in a stilling box, which also helps to reduce airborne dust.
As a proof of concept, Ameren conducted an acceptance test over a three day-period. As seen in Figure 10, sample collecting pans were employed. The dust samples collected in these pans confirmed that coal spillage and dust accumulations just outside of the transfer chute were reduced by 98.8%, from an average of 1.810 to 0.022 grams/ton for those samples that were collected
Meramecs new tripper conveyor loading chutes illustrate an issue that is common for many existing power plants. Existing transfers were not originally designed for flow control chutes. The available transfer height and conveyor arrangement was fixed long ago and are definitely not ideally suited for the features employed in flow control technology. Besides the zigzag flow path, the loading point for Meramecs tripper conveyor is moved back toward the tail pulley. This is a compromise for the belt troughing transition distance, which is normally established to avoid overstressing the outer edges of the belt as it transitions from flat to troughed contours as it moves from the tail pulley to the troughing impact idlers. Overstressing a conveyor belt can lead to other problems including poor tracking due to belt cupping and splice failures.
Materials of construction are also important when specifying flow control chutes. Because the coal stream maintains contact with chute surfaces, chute or liner wear will increase dramatically. In one case, chutes have worn through in a matter of weeks instead of years due to errors in selecting and inspecting materials of construction for the particular application.
Vacuum and washdown systems provide the tools necessary to clean up spills and dust accumulations that will inevitably occur. Designs run the gamut from rudimentary to sophisticated. Choices are often determined by clean-up quantities, frequencies, and difficulties. Consider how the selected design reduces the cost of manual labor otherwise needed for clean-up to help financially justify a decision.
Vacuum systems are an effective alternative to shoveling large spills when crushed coal is dry. Truck-mounted systems are often employed when an abnormal event occurs and the services of a contractor is a logical response. At plant locations that are within reasonable distance of a coal-handling system dust collector or scrubber, a permanent central vacuuming system is usually an economical choice. Portable units can be added to plant tools where a central system is not available. Keep in mind, however, that portable vacuum systems are essentially dust collection systems and entail all the hazards associated with contained dust.
With any washdown system, an important consideration is disposal of sludge-laden wash water. Sometimes concrete floors can be resloped to drains using a layer of thinset mortar, but this is not critical. Workers can also hose down dust accumulations, walking them to the collecting basin, and follow up with a wide floor squeegee to help move puddles.
Tunnels will normally slope to a below-grade vault, which is often fitted with a sump pump. It may be necessary to upgrade the design to handle increased water flow, higher solids flow, and larger particles. A slurry pump in an agitated sump should be considered.
For above-grade floors, collection basins fitted with a grating cover should be connected to large-diameter downspouts or nearly vertical drain pipes. The washdown water will be saturated with silt and larger particles that mass into a muddy mess in shallow slopes, where velocities are too low to maintain the sludge in suspension. Naturally, elbows should not be used, but cleanout provisions at any bend should.
Coal is subject to spontaneous heating as it absorbs oxygen from the air, often within the first 90 to 120 days. Freshly mined coal absorbs oxygen more rapidly and spontaneously heats quicker than coal that has been mined and stored. Moisture aids spontaneous heating by assisting oxidation. Water used in fighting a coal pile or silo fire may aggravate the problem if sufficient water does not reach the core of the fire to adequately cool and quench it.
To reduce the potential for stockpile heating and the progression of events to a hazard, coal should not be simply stacked from an elevated discharge into a gravity-formed pile. Coal fines will naturally accumulate in the center of the pile, while the largest particles roll down the surface to the perimeter base, creating voids at the perimeter base of the stockpile. Winds entering the crevices promote oxidation and fan the flames. It is better to stack coal in layers and contour and compact it with mobile equipment, especially for longer-term storage. This also allows rainfall to be more easily shed from the surface, so it can be collected in perimeter ditches.
Finally, hot coal should not be reclaimed from a storage pile. This introduces an ignition source into the coal-handling system. Any smoldering coal should be segregated from the main stockpile and extinguished.
Daniel Mahr, PE ([email protected]) is a project manager with Energy Associates PC. Michael A. Schimmelpfennig, PE ([email protected]) is a consulting engineer for Power Operations Services, Ameren Missouri.
Powder River Basin (PRB) coal has given coal-fired plants an environmentally conscious, inexpensive alternative to traditional bituminous coal since the 1980s. The lower NOx and SO2 in PRB coal reduced power plant emissions, decreasing pollution, and appeasing the 1990 Clean Air Act. At the same time, the low cost and availability made PRB coal not only a viable option, but a fuel of choice. While PRB coal was the predominant emission efficient energy source in 1990, in 2012, it no longer reins as the most environmentally friendly fuel. With the current government crack-down on coal-fired power plants, the existing plants will likely be the last.
Coal-fired power plants are extremely volatile. After all, there is a reason the industry saying is, It isnt if a fire occurs, but when. With the prospect of aging coal-fired power plants and their propensity for combustion, it is essential to understand their fire hazards and insure the fire protection preparedness of coal-fired power plants with extensive fire protection solutions.
Throughout a twenty-five year (1980-2005) study of PRB coal-fired power plants, there were an average of 11 fires or explosions, 29 injuries, and 5 deaths per year. Another study conducted by the United States Department of Labor during the 1996-2009 time period noted 437 workplace coal power-related deaths, averaging 33 deaths per year in the United States. To understand what fire protection is necessary to guard against mishaps, it is crucial to first understand why explosions occur.
For a fire to occur, the fire triangle needs to be present oxygen, fuel, and heat. An explosion happens when two other elements are added to the equation dispersion of dust and confinement of dust, as shown in diagram A. Oxygen and fuel cannot be avoided in a PRB coal-fired power plant, but the heat source can originate from several different sources. A common cause is the conveyor belt. As the coal is being transported from storage to use, the coal-dust begins to fall off the belt and accumulate. Once the dust accumulates to 1/32 of an inch, or about the breadth to leave a footprint, it becomes a fire hazard. NFPA 654 defines combustible dust as, any finely divided solid material that is 420 microns or smaller in diameter and presents a fire or explosion hazard when dispersed and ignited in the air. If a conveyor belt is not in impeccable condition, and one moving part stops, the friction can create a heat source for combustion. Other causes of heat are friction through mixing operation, electrical shortage, tool usage, or storage bin transfer. The fire triangle is difficult to avoid.
Two additional elements are added to the fire triangle to create an explosion. The dispersion of dust happens naturally as the coal is being moved. The sub-bituminous coal is high in oxygen and moisture, making it more susceptible to deteriorate into powder than standard bituminous coal. It easily creates a dust and disperses over pipes, conveyor belts, floors, ceilings, and machinery. The confinement of coal dust happens just as easily. The dust spreads in unseen areas, like coal silos or chutes. A Kansas City coal-fired power plant witnessed this first hand when, on April 4, 2012, an explosion rocked the plant. Dust accumulated in a chute, completely unseen, and caused the fire. Often, it is the hidden dust that causes the devastation, carrying the explosion or causing secondary explosions throughout the plant.
Coal dust is not the only cause of fires in a PRB Coal-Fired Power Plant. Both the turbine and transformer are insulated by oil, making them flammable. There are three different types of oil fires that can take place in or near the turbine or transformer: spray, pool, and three-dimensional. Spray fires happen when highly pressurized oil is released; 50% of the time, this fire happens because of malfunctioning bearings. If there is an unpressurized leak, plants could see a pool fire when the oil catches fire after it has accumulated on the floor or a three-dimensional fire if it catches fire while flowing downhill.
Hydrogen cools generators in coal-fired power plants. Hydrogen is an invisible threat with the capability to catch fire and/or explode. The gas is odorless, colorless, tasteless, and the flames are invisible. It will not be detected without the use of hydrogen sensors. Fire fighting should not commence until after the hydrogen source has been shut off. If hydrogen is still present, it is likely to re-ignite or explode.
The key to reducing the probability of a coal-fired power plant fire or explosion is preparation. Fires generate from several different sources: coal dust, oil, or hydrogen. It is necessary to be knowledgeable about fire ignition in order to avoid it. The main causes of plant fires and explosions are coal dust, equipment error, and human error. Training plant personnel on proper housekeeping and machinery maintenance along with proper fire protection will greatly reduce the chances of a fire or explosion.
Without a stringent housekeeping regimen, even the most advanced fire suppression system will not be able to stop an explosion from happening. A documented housekeeping routine is necessary to reduce the odds of a fire or explosion. According to the Mine Safety and Health Administration, with a robust housekeeping schedule, the fuel source would be eradicated, eliminating secondary explosions. Secondary explosions have the largest death toll of all coal-fired power plant combustions.
Dust collectors alone will not adequately dispose of dust; in fact, 40% of fires and explosions were caused by the dust collectors. An effective option is to wet the dust to weigh it down so it does not float into hidden crevices. Because the dust is microscopic, microscopic water spray must be used. Plants should use a wash down system to keep coal dust at a minimum. Industry surveys have shown that plant personnel who have utilized wash down systems have been happy with the results.
During an outage, it is essential to clear dust completely from bunkers, silos, and conveyor belts. Idle dust can explode. When preparing for the outage, wash down all walls of the bunkers or silos to eliminate the source for explosions.
If dust cannot be completely cleared, another option is to pump carbon dioxide into a sealed bunker or silo. The carbon dioxide would eliminate the possibility of dust combustion by taking away its oxygen.
A bunker or silo should be designed as if a fire is imminent. Access points should be installed on several levels to allow for entrance of fire extinguishment tools. It is important for the water to directly contact the source of the fire in a bunker or silo. Another design choice that will reduce the chances of a fire or explosion is a cone shaped floor or a free flow bottom cone. Many bunkers or silos have a funnel-flow pattern that occurs when the walls inhibit the coal from flowing freely. Most coal will flow down the center, while the remaining coal that has accumulated on the sides will linger stagnantly. Stagnant coal can create a heat source. The key to reduce the likelihood of a bunker or silo fire is in the design.
Several different detectors are needed throughout the facility, depending on the location. Silos, bunkers, and dust collectors are at a high risk for explosions due to the congregation of PRB dust. It is necessary to choose the correct detection device. Carbon monitors, infrared scanning, temperature scanning, or linear heat detectors are adequate options. Linear heat detectors, such as Protectowire, can detect heat along a length of space, instead of a singular spot. This works extremely well along conveyor belts, which are a major fire hazard because they easily create heat through movement or from idler or roller bearing failure.
Sprinkler systems must be installed throughout a plant. The main fire culprits are silos, bunkers, conveyor belts, crusher buildings, dust collectors, coal pulverizers, turbines, generators, and transformers as seen in Diagram B. Hazard location will determine the best system type. Temperature controlled locations are best protected by a wet-pipe system. Non-temperature controlled areas need a dry-pipe system to avoid frozen pipes. Transformers and other areas where quick suppression is important and water damage is not a concern are effectively protected by deluge sprinkler systems.
Three main suppressants dominate coal-fired power plants: water, CO2, foam and/or f500 solutions. It is essential in coal dust-related bunker/silo fires to use a piercing rod or inerting system to smother the fire at its source. In all other areas of a plant, various types of sprinkler systems will effectively suppress fires.
An integral part of finding a solution to fire protection is choosing a company with experience and expertise to implement a comprehensive system. Fire protection providers must have the design capability to plan custom solutions for site obstructions and plant nuances. Each fire susceptible location of a plant must have a fixed sprinkler system that is designed specifically for that area. High value high risk facilities are vastly more complicated than other industries; a fire protection solution provider should be experienced in providing fire protection for plant environments to ensure solutions that are suitable for the specific application. With proper housekeeping schedules, diligence, and fire suppression systems, the safety of people, plant, and production is greatly increased.
Compared with actual situation of the current thermal power plant, this paper studies operation process of coal handling system in thermal power plant. Analyze technical characteristics of coal handling system and operating characteristics of the relevant machinery and equipment. Then, the safe operation of coal handling system and the proper method of the equipment maintenance are summed up.
Coal, by its very nature, is a dusty fuel. That poses a serious risk at coal-fired power plants, because coal dust can be highly explosive. However, actions can be taken to reduce the risk. Implementing strict administrative controls is a good first step, but combining that with other passive and active control measures may be the best way to ensure a plant avoids catastrophe in the long run.
On February 7, 2008, the Imperial Sugar refinery in Port Wentworth, Ga., exploded, ultimately killing 14 people and injuring 38. The resulting investigation determined that the explosion was fueled by massive accumulations of combustible sugar dust throughout the packaging building at the plant.
After that incident, industrial facilities all over the U.S. took notice. Whether sugar, flour, wood, plastics, or coal, uncontrolled industrial dust is a significant explosion risk. Utilities with coal generation facilities, rightly concerned with the safety implications, worked to develop plans to mitigate the combustible dust risk at their coal-fired plants.
The concern was driven in part by the fact that many plants have converted from Appalachian coal to Powder River Basin (PRB) coal for its noteworthy environmental benefits. However, there is a significantly increased risk of dust explosions in these converted plants due to the possibility of spontaneous combustion and the higher explosive severity of PRB coal.
People who take safety seriously dont wait around for regulations to change. They work to mitigate the risk of combustible dust disasters regardless of whether or not they are meeting present regulatory guidelines.
A good first step is to conduct an audit of the current combustible dust risk level at your coal-fired plants. That establishes a baseline for risk and helps management understand what is required to ensure safe operations. To improve working conditions and protect lives of employees, a plan for plant upgrades and a cost estimate for projects is needed.
Acensium was brought in to perform an audit for a major southern utilitys coal plants in 2010. Acensiums leadership had significant experience in PRB coal conversions and meaningful expertise in mitigating coal dust risks, which was a driving factor in the companys selection. The utility assigned a corporate project manager to work with Acensium in planning the project and budgeting the upgrades over a 7-year period.
Together, the team performed onsite evaluations of each plant targeted for upgrades. Data was compiled on all problems identified at each plant, a technical plan to implement solutions was developed, and a budget was created to carry out the plan. The process culminated with a central meeting of all the coal-handling managers from each plant. The meeting lasted approximately two full 8-hour days.
At the clients insistence, improvements were prioritized based on safety impactnot the revenue ranking of each plant. The outcome was $180 million in budgeted safety improvements designed to shrink the risk presented by coal dust in each unique situation. A schedule was developed to implement upgrades across the plants, and the program was launched in the spring of 2011.
The plan to effectively manage coal dust at each facility was comprised of three key components. The approach to dust management for material-handling systems followed a path from administrative controls, to passive engineering controls, and then to active engineering controls.
Administrative Dust Control. One of the initial priorities was to create a culture in the material-handling department in which the danger of combustible dust was understood, and simple, everyday actions were carried out to reduce that risk. Those actions included closing chute inspection hatches (Figure 1), closing doors to material handling buildings, and completing more frequent walkdowns during operation. Typically a large portion of combustible dust can be controlled through culture change and its ensuing positive actions.
Passive Dust Control. The second priority was to implement a foundational change in how the general material stream was handled at each plant. That included replacing end-of-life equipment in the transfer systems in order to capture nominal coal particles before moving on to containing submicron dust. Acensium informed the customer that transfer point technology had advanced significantly since most of the plants had been constructed, and that transfer point features and materials had been developed to allow better material control and prevent spillage, while extending maintenance intervals.
In the mid 1980s, Alan Huth first conceived a hood-and-spoon arrangement for material handling transfers. The concept of the hood was employed to consolidate the material flow as it left the delivery belt with a curved hood arrangement. The hood was constructed with curved or angled panels that closely followed the projected trajectory of the material (Figure 2). By following the trajectory of the material, impact could be reduced, or even eliminated. In addition, the hood had the effect of squeezing the air from the material, leaving a very dense material stream to pass through the chute, with little to no entrained air.
Located just above the receiving belt, at the bottom of the chute, the spoon performs a similar impact-reducing function. As the dense material stream from the hood travels to the receiving belt, the spoon presents a series of curved surfaces that ultimately discharges the material closer to the receiving belt. Some spoon designs claim to discharge the material not only close to the receiving belt, but also at the same velocity. When this is achieved, material flow on the receiving belt appears very settled, absent of the usual turbulence that tends to liberate coal dust on the receiving belt.
Power generation coal-handling operators began to specify stainless steel construction in the late 1990s. The initial surge in use of this material was correlated with excessive corrosion that many facilities experienced after the conversion to low-sulfur, Western coals. While more difficult to work with in the fabrication shop, and even more so in the field, stainless steel offered the advantage of providing a shell that could last as long as the new materials that were being used to line impact and sliding areas inside the chute.
Transfer point designs should seek to minimize impact within the coal chute. Impact is the major cause of liner degradation through direct material fracture (ceramic) or washing of the metal matrix (overlay products). In addition, impact leads to further fracturing of the coal, creating more coal dust with the potential for liberation from the system.
Utilizing computer-aided design tools (Figure 3) to create complex transfer chute geometries, particle velocity can be controlled, allowing a substantial reduction in the volume of airflow that is typically observed in conventional transfer systems. Further, engineering controls that utilize passive dust control help reduce operating and maintenance costs typically associated with active dust control.
Active Dust Control. Active dust control includes suppression and collection of dust particles utilizing chemical surfactants and wet dust collection, respectively. During crushing and silo-fill operations, active collection is often required, because passive engineering controls are not able to contain an acceptable amount of fugitive combustible dust.
Coal dust collection systems, as applied to power generation coal-handling systems, should be referred to as excess air collection systems. Coal dust is a viable fuel for the boiler, if it can be safely conveyed there for combustion. Collecting dust from the conveyance stream only adds additional operational costs. Coal dust should be maintained on the conveyors and loaded safely into the silos.
Coal dust collection systems should be used to remove excess air from transfer systems. This excess air is typically a function of entrained, induced, displaced, or generated air movement. Excess air should be minimized through passive means with the resultant air collected at the proper face velocity to minimize the amount of material removed from the conveyance stream. This design approach utilizes wet dust extraction to protect the environment from fugitive dust release.
All coal dust collection systems serving coal handling shall conform to the latest code for fire protection, and explosion isolation and venting. All ductwork shall conform to adopted codes for transport velocity of combustible dust (Figure 4). Ductwork should have blast gates and bleeder valves arranged appropriately to facilitate balancing the system and to maintain minimum transport velocities. Branch entries should be arranged correctly to prevent dust drop out due to eddy currents.
Washdown systems are another form of active dust control, and when engineered properly, they can deliver acceptable dust control with a lower capital and ongoing cost profile. Washdown systems are typically employed after all passive and active coal dust control measures have been applied. Washdown systems can be as simple as the convenient placement of hose systems up to the design and installation of touchscreen controlled, multizoned, hard-piped systems. Fixed washdown systems should be applied based on risk to the operation and are therefore typically installed in tripper/cascade rooms first (Figure 5). Unfortunately, most plants do not have adequate drainage in these areas to handle new fixed systems and these systems must be designed and installed first.
What really enables a utility, or other manufacturer, to make a concrete improvement in the safety of their plants is employing all of these systems together as part of a holistic, sustainable solution. Particle movement at every point throughout the plant should be considered when developing a dust-mitigation strategy to truly address causes, rather than fixing symptoms and potentially creating new deficiencies down the line.
The highest risk plant in Acensiums recent dust-mitigation project has completely rebuilt its coal-handling system in order to dramatically reduce the explosive dust risk. The utilitys other facilities are implementing upgrades according to schedule. The initial 7-year capital plan has been expanded due to the success of the program to date. Administrative controls recommended by Acensium enabled safety improvements to be delivered in the short-term, while long-term physical improvements continue to be implemented.
Many more utilities are starting similar evaluations to mitigate coal dust risk at their plants. These lead innovators are experiencing safety improvements that are now years ahead of industry contemporaries.
In India, a coal-based thermal power plant generates 93,918.38MW approximately of total energy production. A coal-based thermal power plant converts the chemical energy of the coal into electrical energy. The company is operating coal-based 2300MW thermal power plant which is driven by steam and generates electricity by expanding and raising the steam in the boilers and coupling the turbines to the generators which converts mechanical energy into electrical energy. In this operation, there are many stages and processes that steam and coal have to pass through which involves various unsafe acts and sometimes unsafe conditions while operating and handling equipment or processes that could lead to potential failure of the equipment, interrupting the normal working of the plant that ultimately have a detrimental effect on the production of the plant, probability of accidents, injury to human lives, damage to property and environment becomes quite high. For protecting plant from such condition, a quantitative risk assessment can be very useful as it helps in focusing and prioritizing the risk which are more severe and catastrophic. In this project, a HIRA is prepared for each process, and risk factor has been calculated by assigning frequency of occurrence and severity of the hazards. The finding of the study suggested the possible control measures and corrective actions to reduce or eliminate the risk that can be used by power plant in preventing accidents from occurrence.
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