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Fogging systems have been successfully used in the material-handling industry for more than 30 years to control explosive dust at transfer points. Today, fogging systems are an EPA Best Demonstrated Technology for subbituminous coal preparation plants.

The importance of properly managing fugitive dust emissions to ensure employee safety and equipment protection can't be overstated. If you are a plant owner or operator who handles coal, then you are responsible for improving plant housekeeping, providing adequate employee training, and ensuring that the proper safety equipment is available and functional in the case of an emergency. I hear people say that safety is paramount at the plants I visit, yet many plants continue to ignore all the red flags when it comes to aggressively controlling fugitive dust, especially those plants that handle Powder River Basin (PRB) coal. There's no longer any excuse for failing to have the best dust-management systems and practices

A Brief History of Recent Coal Dust Standards

The hazards of combustible coal dust were the subject of an Investigation Report released by the U.S. Chemical Safety and Hazard Investigation Board (CSB) in November 2006. In this landmark report, the CSB reported on its investigation of dust explosions in general industry between 1980 and 2005. The CSB found that 281 combustible dust incidents killed 119 and injured 718 workers and "extensively damaged industrial facilities" in 44 states. The report concluded: "These findings illustrate the seriousness of the combustible dust hazard in U.S. workplaces, yet no comprehensive federal Occupational Safety and Health Administration (OSHA) standard exists to control the risk of dust explosions in general industry."

OSHA responded by initiating a Combustible Dust National Emphasis Program (NEP), defining new standards to eliminate flash fires, explosions, and implosions caused by the ignition of accumulations of fugitive coal dust. (See "A Burning Concern: Combustible Dust," POWER, May 2010.) The first release of the new NEP was on October 18, 2007, but a subsequent catastrophic explosion at a sugar refinery pressed OSHA to take more direct action against fugitive dust. It updated the NEP on March 11, 2008, noting that "OSHA is reissuing the directive (CPL03-00-008) on the Combustible Dust National Emphasis Program to increase its enforcement activities and to focus on specific industry groups that have experienced either frequent combustible dust incidents or combustible dust incidents with catastrophic consequences."

Dry Fogging Is Now a Best Technology

One of the most cost-effective and direct means of controlling fugitive dust is using a dry fog system. The technology isn't new. A 1981 article published in Coal Industry News discussed the use of dry fog for explosive dust control at Westvaco (paper mills) and Dayton Power & Light. In that article, Sonic, the developer of the dry fog dust suppression technology, predicted that "this new technology is going to have a major impact on coal handling in the years ahead."

With recognition long over due, "fogging systems" were recently declared a best demonstrated technology (BDT) for coal-handing equipment used with subbituminous and lignite coals by the U.S. Environmental Protection Agency (EPA) in 40 CFR Part 60 "Standards of Performance for Coal Preparation and Processing Plants; Final Rule," published October 8, 2009. BDT, as defined by the EPA, is the most effective commercially available means of treating specific wastes, in this case, fugitive dust emissions. Public testimony given to the EPA on fogging systems has confirmed that 29-year-old prediction: Dry fog can be the most cost effective dust control system available.

The BDT designation was due in part to the supportive testimony by representatives from PRB coal mines, PRB coal-fired plants, mining associations, Air Utility Group, and various state air regulators. Representatives from PRB coal mines and power plants testified that fogging systems have dramatically reduced capital and operating cost while simultaneously increasing dust control efficiency.

One PRB coal mine presented a detailed cost analysis of competing dust control technologies. Its study concluded that the total ownership cost of dust scrubbers was 50% more and replacement baghouses were 400% more expensive than using atomizing fog for dust control over a 30-year life cycle. Chemical systems are not BDT for subbituminous and lignite coals and therefore were not considered in this analysis.

The PRB mine analysis also estimated that baghouses use 2.5 times more energy than dry foggers.

Fogging Proved Better Than Baghouses

Why did it take regulators so long to recognize the importance of properly managing fugitive dust in general and the benefits of fogging systems in particular? According to David Gilroy of Dust Solutions Inc., "When the demand for PRB exploded in the early 1990s, PRB mines were very busy building new facilities and were required to install baghouse collection systems as BDT. Since this rush to build baghouses, there have been many baghouse fires and some violent explosions caused by the inherent properties of PRB coal. Power plants burning PRB coal were also having the same experiences."

The CSB Investigation Report also found that 18% of all incidents and fatalities involved dust collectors such as baghouses.

The risk of fires, explosions, and noncompliance has prompted nearly every PRB coal mine and more than 100 coal-fired power plants to phase out mechanical dust collection in favor of a fogging system. With dry fog, there is no explosive exhaust stream.

One of the first PRB coal mines to make the switch to dry foggers was Triton Coal (now Black Thunder). The transition was difficult because regulators required the mine to keep the mechanical dust collection systems in service until they could demonstrate that the foggers worked at least as well as the mechanical systems. The foggers proved their value, and, as they say, the rest is history. Wyoming, the home of PRB coal, eventually permitted fogging systems as best available control technology (BACT) for PRB coal conveyor transfer points, giving fog systems a Zero Visible Emission Rating. Today, nearly every PRB mine has a fog system as part of its dust control strategy.

Word of the success of this "new" technology at the PRB coal mines trickled into the power generation industry, especially at plants that were converting to burn PRB coal. PRB coal mines are concerned about handling the coal from the mine to the freight cars. Coal-fired plants are concerned about unloading the coal, but they're even more concerned about storing, handling, and pulverizing a coal with extremely friable outer layers that dry quickly, which makes more fines that increase the potential for baghouse fires and dust explosions. They soon became interested in the potential of fogging.

Fog System Application Guidelines

Like any technology, fog systems must be properly applied to achieve the desired results. With fog systems, a fuel-handling transfer point is a "scrubbing chamber." Gilroy says that each chamber must be designed so that airflow is properly managed, such as by having adequately sized conveyor skirt covers at the conveyor load point. This is an important consideration for foggers because air movement must be slow so that the fog droplets injected into the transfer point will have sufficient retention time to attach to the airborne dust particles and settle them out on the belt. The design must have enough cover height for the air to expand and enough length for agglomeration to occur (Figures 1–4).

PlantSafety_Fig1_med
1. Gravity works. The Dust Solutions Inc. (DSI) dry fogging system agglomerates the airborne dust particles to micron-sized water droplets so that when the particles become heavy enough, they are returned to the product stream by the force of gravity. With conveyor transfer points, there must be sufficient room for the fog to fully develop, particularly in the receiving belt area. In addition, the conveyor skirt boards must be tight, and the inspection and conveyor covers must be in place so air leakage does not affect the efficiency of the system. Courtesy: Dust Solutions Inc.
PlantSafety_Fig2_med
2. Blanket of fog. The system design is based on a unique nozzle design that can produce a very dense fog of 1- to10-micron-size water droplets that literally blanket the dust source and keep the dust particles from becoming airborne in this rail car unloading system. The DSI nozzle requires a maximum of 15 psi water pressure. Courtesy: Dust Solutions Inc.
PlantSafety_Fig3_1_med
3. Droplet size matters. For agglomeration of the dust particles to occur at the dust source point, two conditions must occur: Enough water droplets of the same size as the dust particles must be present, and both dust and water droplets must be constrained to the same volume so that agglomeration can occur. If the water droplet's size is too large, the dust particle will follow the air streamline and miss the water droplet. Source: Dust Solutions Inc.
PlantSafety_Fig4_med
4. Micron-scale water particles. An actual high-speed photograph of droplets generated by a DSI fogger designed to produce water droplets less than 10 microns in diameter. The droplets were captured on a greased microscope slide in a chamber that was at 100% relative humidity to eliminate evaporation of the droplets. The small squares are 2 microns across. Any fugitive dust particles encapsulated by such densely packed fog droplets have little or no chance of escaping. Courtesy: Dust Solutions Inc.

In the past few years, controlled-flow chutes have taken these principles one step further by shaping the material flow and reducing material impact within the chute. Gilroy notes that although some manufacturers claim to have a "dustless" transfer chute design, his customers say that controlled flow transfer points have helped reduce dust generation; however, in many cases, that is not enough, and a fog system must be added to achieve the desired results.

Gilroy pointed out an interesting recent project where dry foggers were added to an existing PRB coal-handling system. Dry Fork Mine (part of Basin Electric Power Cooperative) is a PRB coal mine that installed a passive control chute under a primary crusher to achieve the specified amount of dust control. The staff at Dry Fork Mine has long experience with fog systems, having previously replaced their mechanical dust collectors at the silo fill and conveyor transfer points. When the conveying system was designed for supplying coal from the mine to the Dry Fork Power Station now under construction, Basin Electric specified dry fog combined with passive control chutes for optimum control.

Basin Electric is also installing fogging systems at its new 400-MW Dry Fork Station in Gillette, Wyo. Each transfer point will use both a controlled flow chute and a fog system to achieve the highest degree of dust control efficiency.

—Dr. Robert Peltier, PE is editor-in-chief of POWER

EPA designates Fogging Systems as Best Demonstrated Technology (BDT) for PRB, sub bituminous & lignite coal at Coal Preparation & Processing Plants!

On September 25, 2009 the United States Environmental Protection Agency (EPA) revised emissions control requirements for new coal preparation plants. These revisions were based on a variety of input and comments from those directly involved in the coal industry, including coal mines, power plants, consultants, engineering firms and suppliers of air pollution control systems.

The EPA findings have created new rules that they published in the Federal Registers on October 8, 2009. The new rules designate "Fogging Systems" as a Best Demonstrated Technology (BDT) for coal handing equipment used on sub bituminous and lignite coals.

BDT as identified by the EPA, is the most effective commercially available means of treating specific types of hazardous waste. Public testimony given to the EPA on fogging systems has revealed what we at DSI has been saying for many years, that Dry Fog can be the most cost effective dust control system available, yielding results as good or better than any other control measure.

A Power River Basin (PRB) Coal Mine did a Cost Analysis alt comparing various dust control systems. It concluded that Dry Fog has the Lowest Capital and 30 year Operation and Maintnentce Cost of any BDT system on the market. Lower than Passive Transfer Chutes, Dust Scrubbers Baghouse Collection Systems. In fact Dry Fog is shown to be nearly 4 times less expensive than Baghouse Collection Systems!

Chemical systems are not BDT for sub bituminous and lignite coals and were not even considered in this analysis. However, the requirements to purchase chemicals for the entire life of the systems could put chemical suppression system as the highest cost of operation.

The U.S. Chemical Safety Board investigated combustible dust incidents from 1980 through 2004. It found that 18% of all incidents and fatalities involved dust collectors. This PRB mines analysis estimates that their former baghouses used 2.5 times more energy than Dry Fog. The risk of fires and explosions has prompted nearly every PRB coal mine to phase out collection in favor of Dry Fog and or Passive Control Chutes. This trend has made its way into the facilities of end users (power plants) for the same reasons. With Dry Fog, there is no explosive waste stream and the reduction of dust has been consistently been reported as better than collection systems. Read their full alt Testimony to the EPA .

Affected facilities constructed, reconstructed or modified after May 27, 2009 are required to prepare and operate in accordance with a submitted fugitive coal dust emissions plan. The plan must identify and describe the control measures the owner/operator will use to minimize fugitive coal dust emissions from each open storage pile.

The EPA identified fogging systems and wind barriers as two of the accepted control measures.  DSI DustTamer Wind Fence Systems and Dry Fog Systems can help you comply with these regulations avoiding the use of chemicals. With chemical systems the EPA requires that the user consider in their dust control plan the site-specific impacts associated with the use of chemical dust suppressants (e.g.,water runoff, water quality concerns).

New EPA Coal Dust Control Regs

EPA declares Fogging Systems as Best Demonstrated Technology for PRB Coal On September 25, 2009 the United States Environmental Protection Agency (EPA) revised emissions control requirements for new coal preparation plants. These revisions were based on a variety of input and comments from those directly involved in the coal industry, including coal mines, power plants, consultants, engineering firms and suppliers of air pollution control systems.

The EPA findings have created new rules that they published in the alt Federal Register on October 8, 2009 (see PDF File Page 6). The new rules designate “Fogging Systems” as a Best Demonstrated Technology (BDT) for coal handing equipment used on sub bituminous and lignite coals.

BDT as identified by the EPA, is the most effective commercially available means of treating specific types of hazardous waste. Public testimony given to the EPA on fogging systems has revealed what we at DSI have been saying for many years. Dry Fog can be the most cost effective dust control system available while yielding results as good or better than any other control measure.

A Powder River Basin (PRB) Coal Mine did a alt Cost Analysis comparing various dust control systems. It concluded that Dry Fog has the Lowest 30 year Operation and Maintnence Cost of any BDT Dust Control System on the market! Approximately 25% lower than Passive Transfer Chutes, 35% lower than Wet Dust Scrubbers and 75% lower than a new Bag-house Collection System!

Chemical systems are not BDT for sub bituminous and lignite coals and were not even considered in this analysis. However, the requirements to purchase chemicals for the entire life of the system, typically makes chemical dust suppression systems the system with the highest cost of operation.

The U.S. Chemical Safety Board investigated combustible dust incidents from 1980 through 2004. It found that 18% of all incidents and fatalities involved dust collectors. The PRB mine analysis estimated that their former bag-houses used 2.5 times more energy than Dry Fog. The risk of fires, explosions and non-compliance has prompted nearly every PRB coal mine and over 100 Coal Fired Power Plants to phase out collection in favor of Fogging Systems. With Dry Fog, there is no explosive waste stream. Read the PRB Coal Mines complete Testimony to the EPA.

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