Dry Fog Dust Suppression

8/12/2019 Dry Fog Dust Suppression

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Whitepaper

Dust Control

Dry Fog Dust Suppression

Dust Suppression is the application of waterand/or chemicals, either tothe body ofmaterial to prevent fines from being carriedoff into the air,or to the air above the material to return fugitive airborne fines to thematerial bed.A significant advantage of dust suppression is that thematerial does not have to behandled again. The suppressed dust returnsto the main body of conveyed material andthe process without requiringadditionalmaterial handling equipment.There are a number of systemsused for this purpose ranging from garden hose technology, through

water and surfactant sprays, foam and fog generation systems. Thesevarious suppression technologies call for adding different volumes of

moisture tothe material.

Authors:

D. Blyth, Sealpump Engineering Ltd., United KingdomS. Brown, Sealpump Engineering Ltd., United Kingdom


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Introduction

Dust Suppression is the application of waterand/or chemicals, either to the body ofmaterial to prevent fines from being carriedoff into the air, or to the air above the materialto return fugitive airborne fines to the materialbed.

A significant advantage of dust suppressionis that the material does not have to behandled again. The suppressed dust returnsto the main body of conveyed material andthe process without requiring additionalmaterial handling equipment.

There are a number of systems used for thispurpose ranging from garden hosetechnology, through water and surfactantsprays, foam and fog generation systems.These various suppression technologies callfor adding different volumes of moisture tothe material. Fig 1 presents typical amountsof added moisture.

Perhaps the oldest method for controllingfugitive dust is the application of water overthe body of material. By wetting the fines,

either as they lay in the material body or asthey are being picked up into the air, theweight of each dust particle is increased so

they are less likely to become airborne. Themoisture also increases the cohesive force ofthe material body itself, creating larger,heavier groups of particles and making itmore difficult for air movement to carry awaythe dust particles. This can be done byapplying the water through a series ofproperly sized spray nozzles at a point where

the material expands and takes in air, suchas during discharge from the head drum in atransfer chute.

Water can also be applied to create acurtain around a transfer point, so any dustfines that become airborne come into contactwith the water sprays surrounding the openarea around the chute. The water dropletsare expected to make contact with the dustfines, increasing their mass to remove themfrom the air stream.

The most effective sprays come from low-velocity systems. High-velocity sprays canadd energy to the air and the dust particles.This energy is counterproductive to the taskof keeping (or returning) the dust with thematerial body. High velocity air movementcan keep dust particles in suspension.

Water-based suppression systems canbecome more sophisticated as theengineering moves beyond spray nozzletechnology in efforts to improve results. Theeffectiveness of water spray systems isdependent on the velocity of applied water,the size of the nozzles orifice and thelocation of the spray nozzles. Thetechniques to improve plain water-spray dustsuppression include a reduction of dropletsize, an increase in droplet frequency, an

increase of the droplets velocity, or adecrease in the droplets surface tension,making it easier to merge with dust particles.

Dry Fog Dust Suppression

David BlythSealpump Engineering Limited

Redcar, United Kingdomsealpump.com

Plain WaterSpray

0%

Water/SurfactantSpray

Water/SurfactantFoam

Dry Fog

1% 2% 3% 4% 5%

5%

2.5%

0.2%

0.05%

Amount of Moisture Added

Fig 1Dust Suppression

System Types

Stuart BrownSealpump Engineering Limited

Redcar, United Kingdomsealpump.com


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Ultrasonic Dry Fog

UltraFine Fog fugitive dust suppressionworks like a combination of a wet scrubberand a fabric filter. The generated ultra-finefogging blanket acts like a fabric filter in that

a dust particle cannot pass through it withoutcolliding with a droplet. Since the dropletconsists of water, the dust particle doesbecome somewhat wet as in a true floodedscrubber. This phenomenon can be calledagglomeration and solving fugitive dustemission problems with ultra-fine waterdroplet atomisation begins with the theory ofagglomeration. Agglomeration can bedefined as the gathering of mass into alarger mass, or cluster.

Agglomeration probability is greatlyincreased between bodies of similar size.The agglomeration of these bodies producesa large enough mass to cause settling. Forexample, a dust particle of 5 microns willcontinue to follow the air stream around awater droplet of 200 microns, therefore,avoiding collision. With the dust particle anda water droplet of similar size, the air streamis not as great and collision occurs, causing

agglomeration.

Fig 2 shows the aerodynamics of what canhappen when the water droplets are largerthan the dust particle.

Fog suppression is one method to optimisethe application of water to dusty materials.These systems use special ultrasonicnozzles to produce extremely small water

droplets (10 microns or less) in a dispersedmist. These droplets mix and agglomeratewith dust particles of similar size, with the

resulting larger combined particles fallingback to the material body.

Compressed air passes through the nozzlesinner bore through a convergent/divergentsection at high velocities and expands into a

resonator cavity where it is reflected back tocomplement and amplify the primary shockwave. The result is an intensified field ofsonic energy focused between the nozzlebody and the resonator cap. Fig 3

Any liquid capable of being pumped into theshock wave is vigorously sheared into fine

droplets by the acoustic field. Air bypassingthe resonator carries the atomised dropletsdownstream in a soft plume shaped spray.

In Fig 4 the droplets have low mass and lowforward velocity with lowimpingementcharacteristics. Fineatomisation ensuresuniform distribution of theliquid with minimum overspray and waste.

Ultrasonic atomisingnozzles operate at very lowliquid pressures and havelarge orifices. The large

orifices and low pressures virtually eliminateorifice wear and prevent deterioration of thequality of atomisation while greatly extendinguseful nozzle life.

The plume leaving the fog system nozzles isso fine it will not freeze, but the water supply

small droplet

air stream

dust particle follows air

stream

dust particle

impacts droplet

Diagram illustrates the importance of droplet size for particle agglomeration. Airflowaround the large water droplet (left) prevents the dust particle from contacting thedroplet. However, the dust particle easily impacts the small droplet (right) triggeringagglomeration.

large droplet

air stream

Fig 2

Fig 3


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system itself can freeze if drain or heatingelements are not provided.

Atomisation is designed to reduce thesurface tension of the water droplets, while

increasing the number of droplets in a givenarea and eliminating the need for theaddition of surfactants or other additives. Thelow level of water added through the fog/mistsystems - typically at 0.01% to 0.05% byweight of the material - generally will notdegrade the performance of the material.water supply system itself can freeze if drainor heating elements are not provided.

Thermal Penalty for Added Moisture

There is a substantial performance penaltyadded to combustion and other thermalprocesses when the water content of the fuelis increased. In applications like coal-firedpower plants and cement plants, wateradded to the material going into the thermalprocess must be burned off by the process.This can dramatically reduce the processefficiency and increase fuel costs.

It requires 3,064 kilojoules per litre (1,320BTU per pound) to raise water from 21C(70F) to its vaporization temperature of149C (300F). It only takes 9.1 kg or 9.1litres (20 pounds) of water to increase themoisture content of one tonne of material byone percent. As a gallon of water weighsapproximately 4.5 kg (10 pounds), theaddition of less than 2.0 gallons (9.1 litres) of

water to a tonnne of material will raise themoisture content of a tonnne of material by 1percent. Vaporizing this modest amount of

water produces a heat loss of 27,850kilojoules (26,400 BTU). The thermal penalty typically created by thevarious dust suppression methods isdisplayed in Fig 5.

Because a plain water spray requires thehighest volume of moisture for effective dustsuppression, this method extracts thehighest thermal penalty. While the use of asimple water spray for dust suppression maybe a lower cost because the water is readilyavailable and there is less out-of-pocketexpense, the penalty for the addition ofsurplus moisture can be very costly indeed.

For more information contact:-

Airochem Engineering CompanyPlot B-57, M.I.D.C,

KOLHAPUR 416 122India

[email protected]

Fig 4

Plain WaterSpray

0%

Water/SurfactantSpray

Water/SurfactantFoam

Dry Fog

1% 2% 3% 4% 5%

27,852 - 139,260 kilojoules26,000 - 132,000 BTU

Amount of Moisture AddedPercent of Material Weight

Fig 5Dust SuppressionSystem Types

8,356 - 69,630 kilojoules7,920 - 66,000 BTU

1,393 - 5,570 kilojoules1,320 - 5,280 BTU

279 - 1,393 kilojoules264 - 1,320 BTU

Thermal Penalty perTonne of Material

Documents

Dry Fog Dust Suppression