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Mar 10, 2013

Grain Dust Control and Suppression Systems – Learn What’s New

Introduction

The methods of dust control used by the Grain Industry include the use of food grade oil and water spray on grain, cyclones, manual housekeeping and baghouses. There has been advancement in the use of long bag technology in the last 10 years. This paper will discuss the advantages and disadvantages of each of these methods of dust control.

Use of Water

While the addition of water can be a very effective method of suppressing grain dust, the worry of some government officials and others that water would be used to increase the weight of grain and thereby the value of the grain was of great concern. As a result, in October of 1994, FGIS issued a final ruling prohibiting the application of water on grain except for malting, milling and similar processing.

Use of Oil

Oil additives have been shown to be nearly as effective as water for the suppression of grain dust. Oil additive systems consist of an application of food grade vegetable or mineral oil to the grain surface which results in the fine dust particles sticking to the grain kernel surface. Oil additive systems require sophisticated equipment to control the proper application rates and to assure adequate coverage. They also require continuous monitoring of grade, quality, and viscosity of the oil being used. In addition, inappropriate or repeated application may cause detrimental biological activity in grain.

FGIS has concurred that oil applications are effective in reducing the amount of emissions in grain handling facilities. While oil applications have resulted in significant dust reductions, oil is slow to develop dust control. Even with extensive missing in the grain flow it can take from 20 to 60 or more seconds for the oil to be effective in controlling the dust. Additionally, oil is not as effective when applied to extremely dusty grain and is not as effective on some grains, the effect of oil also degrades with time and repeated handling.

Another concern with oil additives is the potential for residue build-up through the normal distribution channels. Additionally there is no good method for detecting the level of oil that has already been applied to the grain as it moves from country elevators to various terminals.

Another concern that exists with vegetable oil is that the viscosity changes with cold weather which requires heating of the oil for proper flow during application. Vegetable oil is also subject to oxidation which can lead rancidity. This problem can be reduced by using proper oil handling mineral oil does not have the potential for oxidation and therefore the rancidity problem does not exist. Testing conducted by the US Department of Agriculture to be effective on various grains at from 4 to 6.8 quarts per 1,000 bushels. The FDA allows up to 7 quarts per 1,000 bushels (0.02% by weight) for any grain used for human consumption.

The basic equipment required for an oil suppression system consist of a food grade oil storage tank, a small oil pump, a pressure regulator to control flow and application rate, a pressure gauge, automatic shut-of and limits witches to stop the oil when the grain flow ceases, tubing and a spray nozzle. When the system is operated at temperatures less then 32°F, tank heaters or in-line heaters are necessary to warm the oil. Oil is best applied at 75°F or above.

Weight that would be lost as grain dust can, with oil, be retained with the grain and sold as grain. Additionally, since the dust adheres to the grain thereby reducing the amount of suspended dust particles, the potential for grain dust explosion is decreased. The application of oil also reduces the levels of inhalable and respirable dust. Inhalable dust particles are typically less than 10 microns and can penetrate the respiratory system.

The main disadvantage for oil suppression systems is the effect that some feel oil has on grain quality. Some grain exporters are hesitant to buy grain treated with oil since they are not sure of the acceptance in the world market. Millers and distillers have also expressed concerns about the effect of oil on grain. Mills have stated that oil seems to adversely affect the tempering process in wheat and causes discoloration in flour.

Use of Cyclones

A cyclone is a centrifugal separating device which can retain and discharge a higher percentage of larger particles while allowing many of the smaller particles to discharge from the top outlet of the cyclone. The larger particles are often returned to the grain stream, while the smaller particles are emitted to the atmosphere. While most cyclones are 85 to 95% efficient when handling grain, some of the high efficiency cyclones can achieve 99% efficiency handling grain dust.

The main problem with cyclones is that they all have a visible emission which will not meet new EPA standards.

Some elevators have used cyclones in series with a baghouse. Certain limitations govern the use of this method to lessen the continual recirculation of dust, including:

  • Dust should not be returned directly to the bucket elevator
  • Dust should be returned down stream of the collection points
  • Dust added to conveyors or spouts should be introduces under of inside the grain stream

An additional problem with any cyclone is that it operates efficiently in a narrow airflow range. Once the cyclone is operated above or below its design rate it rapidly becomes less efficient.

Cyclones are never the only control device in new facilities and typically only used for replacement of like devices in some country elevators.

Use of Fabric Filters

The fabric filters (or baghouse) is the most efficient device currently used by the Grain Industry. The typical efficiency on fabric filters is 99.9% on particles 2 microns and larger and there is filter media (bags) available which are efficient on particles as small as 0.2 microns.

The typical fabric filter used for grain filters 1,000 to 50,000 cubic feet of air per minute depending upon the operating conditions at the particular elevator. Fabric filters operate under negative pressure with the aid of a fan to draw air through the filter.

Most fabric use polyester filter bags as the filtration media with air to cloth ratios from 5:1 and 8:1; although sometimes higher air to cloth ratios are used. Filter bags must be cleaned periodically to allow the air to flow through the filter media. This is done by using a high volume of low pressure air produced by a high pressure fan or blower that blows some of the cake off the filter bags; or by using a low volume of high pressure compressed air (80 to 100 psi) to “snap” the cake off the bags. As long as properly operated and maintained all fabric can meet the EPA emission standards. The efficiency of fabric filters can decrease as a result of several various operational problems. A tear or hole worn in a filter bag will result in excess emissions coming from the filter and plugging the inside of other filter bags. Broken bag detectors can alert the operator immediately.

Bag blinding problems have been observed in moist or very humid areas, or when warm moist grain when the filter is located outside in a cooler environment. This problem can be greatly reduced or stopped by some or all of the following: using baghouse heating systems, cleaning the bags by turning off the vacuum fan and continuing to run the bag cleaning system for 5 to 10 minutes, using a filter bag, or by changing the filter bags to a filter media which will not wick or absorb moisture.

Malfunction of the bag cleaning system may cause dust to build up on the bags and could lead to plugging of the system. A simple device such as a photohelic gauge can warn of any problems with bags starting to plug and a magnehelic gauge can give a visual warning.

Sometimes the control panels for pulse-jet filters located outdoors are not properly closed. This can allow moisture to collect inside the control panel and may cause the solenoid calves to short circuit which can result in system failure. This problem can be eliminated by practicing good general operation and maintenance procedures.

What’s New with Dust Control Industry?

The Newest Baghouse Technology

Over the past 10 years the long bag high pressure low volume bag cleaning has been developed and utilized. This technology has proven to clean fabric filter bags up to 27 ft long with bag cleaning comparable to conventional 12 ft long filter bags.

This long bag filter has been utilized by the Grain Industry at grain facilities producing Ethanol where the energy for processing the grain is produced with a coal fired boiler. The typical air requirement for this type of system is 200,000 CFM versus the normal grain elevator requirement of 1,000 to 50,000 CFM.

The need for air pollution technology to handle these higher air flows in user friendly and maintenance friendly way were important design considerations in the development and design of the long bag baghouse. With all bag changing done inside the penthouse and all air headers, diaphragms, solenoids and timer boards are also located inside the penthouse. All maintenance and bag changing is done in an inside weather protected area.

Another advantage to the long bag fabric filter is that this type of baghouse allows for more than twice the filter bag area in the same amount of square feet.

While the Grain Industry rarely needs a baghouse the size and capacity of the long bag filter, this type of baghouse has been successfully used in other industries with airflow requirements of 2,000,000 CFM or more.

Conclusions

The primary technology for controlling grain dust in grain handling facilities is pneumatic dust collection and when combined with good maintenance and general housekeeping is the best method of controlling grain dust. Grain elevators subject to these standards generally use typically use fabric filters. Sometimes in combination with cyclones and/or oil additives, reducing the accumulation of dust and the danger of grain dust requires a total system-wide approach for dust handling.

References

EPA Control Technology Center; July 1994
Oil Suppression of Particulate Matter at Grain Elevators

Office of Technology Assessment; September 1995
Technology and Policy for Suppressing Grain Dust Explosions in Storage Facilities

Feed and Grain; June/July 1991
Controlling Grain Dust Can Be A Challenge

C.B. Parnell, B.J. Lespkan and H.D. Wardlaw, Jr; December 1987
Dust Suppression Results with Mineral Oil Applications for Corn and Milo

Federal Register; October 1994
Final Rule, Prohibition on Adding Water to Grain

National Fire Protection Association; NFPA GIB 1994
Standards for the Preservation of Fires and Explosions in Grain Elevators and Facilities Handling Bulk Raw Agricultural Commodities

*As presented by Robert Frye at GEAPS Exchange 2013 

Speaker Contact Info

Contact info for Robert Frye:
Phone: (913) 384-5511 
Email: bfrye@iac-intl.com

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