Air Pollution Control

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Air Pollution Control Part BFiltration
Basic Principles

• Yaacov Mamane
• Visiting Scientist, CNR
• Rome, Italy

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Baghouse Dust Collectors
Fabric dust collectors are commonly known as
baghouses and for some applications are one of
the most efficient and cost effective dust
collector models. In baghouse collectors, the
dust filled air stream passes through fabric bags
that filter the dust particles. Bags are made of
different material such as woven or felted
cotton, synthetic, or glass-fiber.
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Design and Installation of a Baghouse System.
                                                  
                                                  
                                                  
                                                  
            
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TYPES OF FILTERS
Porous membrane filter
Fiber filter
Granular bed filter
Capillary pore membrane filter
Packing density/solidity
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FILTRATION
Fiber filter
Packing density/solidity
For fiber filter, a lt 0.1 For woven filter, a
0.3
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Fabric Filters
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Single Fiber Efficiency

• Definition the fraction of particles approaching
  a fiber in the region defined by the projected
  area of the fiber that are ultimately collected
  on the fiber

Cross section of fiber
projected area stream
u0
Air flow
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• Total length of fiber in a unit volume
• Number of particle collected when a unit volume
  of aerosol passes through an element of unit
  cross section and thickness dh
• Filter Efficiency

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Filtration Mechanisms

• Diffusion (Brownian displacement)
• Impaction (gravitational settling)
• Interception
• Electrostatic Forces

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• Diffusion (Lee Liu, 1982)

Lee, K. W. and Liu, B. Y. H., Aerosol Sci.
Technol., 147-61, 1982
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Filtration Mechanisms Brownian Displacement
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• Impaction (Yeh Liu, 1974)

Yeh. H. C. and Liu, B. Y. H., J. Aerosol Sci.,
5191-217, 1974
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Filtration Mechanisms Gravitational Settling
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Filtration Mechanisms Gravitational Settling
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Filtration Mechanisms Gravitational Settling
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• Interception (Krish Stechkina, 1978)

Krish, A. A. Stechkina, I. B., The theory of
Aerosol Filtration with Fibrous Filters, in
Fundamentals of Aerosol Science, Ed. Shaw, D. T.,
Wiley, 1978.
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Filtration Mechanisms Interception
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• Enhanced collection of diffusing particles due to
  interception
• Gravitational Settling
• Total Efficiency

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Filtration Mechanisms Electrostatic Attraction
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• Single fiber efficiency of different mechanisms
• (h 1mm, a 0.05, df 2mm U010 cm/s)

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Total Filter Efficiency
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• Total collection efficiency as a function of dp
  for 2 face velocities (h 1 mm, a 0.05, df 2
  mm)

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• Total collection efficiency as a function of face
  velocity for 4 particle sizes (h 1 mm, a
  0.05, df 2 mm)

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Filtration characteristics of a fibrous filter h
1 mm, ? 0.05 and df 2 mm
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At minimum efficiency

• Pressure Drop

Q Is increase in pressure drop always bad?
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Types of Baghouse Filters
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Shaker Baghouse
Shaker Cleaning Parameters
Theodore Buonicore, Air Pollution Control
Equipment, CRC Press, 1988.
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Reverse-Air
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Reverse-Flow Cleaning Parameters
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Pulse-Jet
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Air/Cloth Ratio
Filtration velocity
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Pressure Drop
K2
Areal Dust Density
Filter drag
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Pressure Drop(Filter Drag Model)
Time (min) DP, Pa
0 150
5 380
10 505
20 610
30 690
60 990
K1 K2 to be determined empirically DPf
fabric pressure drop in.W.C. DPp particle
layer pressure drop in.W.C. DPs structure
pressure drop, in.W.C. W area dust density,
kg/m2 S filter drag V velocity, ft/min

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Parallel Flow Operation
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Number of Compartments
Flow rate
Filtering velocity
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Filter drag
Areal dust density
Pressure drop
Actual filtering velocity
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Theodore Buonicore, Air Pollution Control
Equipment, CRC Press, 1988.
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Basic Steps in Fabric Filters Control System

• Initial capture
• Removing the particles from gas stream and
  collecting them in dust cakes
• Gravity settling during cleaning
• Discharging accumulated particles to the hoppers
  by cleaning system that
• create disturbances in order to break off layers
  of the accumulated particles
• Solids removal from hoppers

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Types of Filtering Systems

• Housing design
• Closed pressure
• Closed suction
• Filter shape and arrangements
• Cylindrical filters
• Panel type filters
• Cleaning method
• Mechanical shake (inertia)
• Reverse flow (Drag forces)
• Pulse jet (Inertia / drag forces)

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Housing Design
Closed suction
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Filter Shape and Arrangements
Panel type filter
Cylindrical filter
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Fabric Characteristics and Selection

• Fabric type
• Fiber
• Yarn type
• Weave
• Finish

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FILTRATION
Fiber filter
Packing density/solidity
For fiber filter, a lt 0.1 For woven filter, a
0.3
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Yarn Type
Filament Show better release characteristics for
certain dust and fumes, especially with less
vigorous cleaning methods Staple Produces a
fabric of greater thickness and weight with high
permeability to air flow. Certain fumes or dusts
undergoing a change of state may condensate on
fiber ends and become harder to remove from the
fabric
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Weave
Plain Simplest and least expensive method of
fabric construction. It has a high thread count,
is firm, and wears well Twill Fabric with greater
porosity, pliability, and resilience. Therefore
they are commonly used where strong construction
is essential Satin The yarns are compacted which
produces fabric body and lower porosity, and they
are often used in baghouses operating at ambient
temperatures
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Finish Fabrics Treatment

• Finishes are often applied to fabrics to lengthen
  fabric life
• Cotton and wool can be treated to provide
  waterproofing,
• mothproofing, mildew proofing, and fireproofing
• Synthetic fabrics can be heat-set to minimize
  internal
• stresses and enhance dimensional stability
• Glass fabrics are lubricated with silicon or
  graphite to
• reduce the internal abrasion from brittle yarns

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Operating Principles - Air/Cloth Ratio
Typical Air-to-Cloth Ranges
Air-to-Cloth Ratio Baghouse Cleaning Method
0.01 0.03 (m3/sec)/m2 Shacking
0.005 0.015 (m3/sec)/m2 Reverse air
0.025 0.075 (m3/sec)/m2 Pulse jet
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Air-to-Cloth ratio examples
Maximum filtering velocity, m/sec Dusts
0.0075 Activated charcoal, Carbon black, Detergents, Metal fumes
0.01 Aluminum oxide, Carbon, Fertilizer, Graphite, Iron ore, Lime, Paint pigments, Fly ash, Dyes
0.01125 Aluminum, Clay, Coke, Charcoal, Cocoa, Lead oxide, Mica, Soap, Sugar, Talc
0.0125 Bauxite, Ceramics, Chrome ore, Feldspar, Flour, Flint, Glass, Gypsum, Plastics, Cement
0.01375 Asbestos, Limestone, Quartz, Silica
0.015 0.01625 Cork, Feeds and Grain, Marble, Oyster shell, Salt
0.0175 Leather, Paper, Tobacco, Wood
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Operating Characteristics of Fabric Filters
99 percent Collection efficiency
Greater than 0.5 microns Particle size collection
1.25 15 cm, water gauge Pressure drop
288 ºC (peak), 260ºC (continuous) Maximum operating temperatures
0 370 gm/m3 Dust concentration handled in particulate collection
Unlimited Gas volume
Fraction of a m2 to several hundred m2 Cloth area
0.005 0.075 m/sec Filtering velocities
18 months to 2 year Average bag life
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Fabric Filter Design Criteria
Physical and chemical properties of the
dust Predicting the gas flow rate Fabric
construction Proper air-to-cloth (A/C) ratio Bag
cleaning methods The ratio of filtering time to
cleaning time Cleaning and filtering stress Bag
spacing The compartment design Space and cost
requirements
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Problem Simple cloth area calculation Calculate
the number of bags required for an 8-compartment
pulse-jet baghouse with the following process
information and bag dimensions Q, process gas
exhaust rate 45 (m3/sec) A/C, gross air-to-cloth
ratio 0.02 (m3/sec)/m2 Bag dimensions bag
diameter 0.15 m bag height 3.6 m
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Example Solution

1. Calculate the total gross cloth area

or
Ac cloth area, m2 Q process exhaust rate,
m3/sec vf filtration velocity, m/sec
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Example Solution
2. Determine the amount of fabric required per bag
Ab area of bag, m2 ? 3.14 Given d 0.15 m, bag
diameter h 3.6 m, bag height Ab 3.14 x 0.15 m
x 3.6 m 1.70 m2 required per bag
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Example Solution
3. Calculate the number of bags required in the
baghouse
Number of bags
From step 1 Ac 2,250 m2 From step 2 Ab 1.70
m2
or 1,328 bags
Number of bags
So there will be an even number of bags in each
of the 8 compartment, round the value 1326.96 up
to the next highest multiple of 8 (i.e. 1,328).
Thus, there will be 166 bags (1,328/8) in each
compartment
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Example Solution
4. Calculate the net air-to-cloth ratio
The net air-to-cloth ratio is the A/C ratio when
one compartment is taken off-line for bag
cleaning or maintenance
Given Q 45 mt3/sec, process exhaust gas
rate The total number of compartments is 8 From
step 1 Ac 2,250 m2, total cloth area
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Example Solution
5. Calculate the net, net air-to-cloth
ratio (when two compartment are off-line)
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Common Industrial Application
Sources Industry
Electric arc furnaces, Sintering plants, Boilers Steel
Cupolas Foundries
Lead furnaces, Copper smelting furnaces, Zinc furnaces Nonferrous metals
Cleaning operation, Grinding mills, Screening operations, Air classifiers, Dryers, Kilns, Calciners Grain handling
Raw mills, Kilns, Finish mills Cement
Drum mixers Asphalt concrete
Melting furnaces Glass
Dryers, Grinding mills Chemical
Coal-fired boilers Power plants
Incinerators Waste disposal
Cooling of the gas stream or use of refractory
filter bags may be required
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Cost (for a given unit of pollutant)
Operation cost 1982 US / ton removed Capital cost US 1982 Efficiency
3.14 49,000 99.9 Reverse air baghouse
1.68 10,500 87.0 High efficiency cyclone
2.84 96,500 98.3 EPS
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Advantages of Fabric Filters

• Very high collection efficiencies (99 percent)
• Reduced sensitivity to particle size distribution
• Flammable dust may be collected
• Use of special fibers enables submicron removal
  of smoke and fumes
• Collection efficiency not affected by sulfur
  content of the combustion fuel
• Wide range of configurations, sizes, and inlet
  and outlet locations
• No high voltage requirements

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Disadvantages of Fabric Filters

• Shortening fabric life (in the presence of high
  acid or alkaline atmospheres)
• Maintenance problematic and expensive.
• Maximum operating temperature is limited to 288
  ºC. Fabric bags tend to burn or melt readily at
  temperature extremes.
• Impossibility to collect hygroscopic materials
  (plugging, loss of cleaning
• efficiency).
• Certain dust may require special fabric
  treatments to aid in reducing
• leakage or to assist in cake removal.
• High concentrations of dust present an explosion
  hazard.

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Baghouse vs ESP(experience of Elkem Materials)
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References

• TM 5-815-1/AFR 19-6
• http//www.hnd.usace.army.mil/techinfo/UFC/UFC3-43
  0-03/TM58151/chap9.pdf
• 2. US Environmental Protection Agency, Bag
  cleaning
• http//yosemite.epa.gov/oaqps/eogtrain.nsf/Display
  View/SI_412A_2?OpenDocument
• 3. Ron Zevenhoven, Particulates
• http//www.hut.fi/rzevenho/partic_2.PDF
• 4. Air Pollution Control, Particulate matter
• http//www.continuingeducation.com/engineers/airpo
  llutionpm/baghouses.html
• 5. Concepts in Environmental Engineering (APTI
  Course)
• http//www.epin.ncsu.edu/apti/ol_2000/home/homefra
  m.htm
• 6. US Environmental Protection Agency, Fabric
  filter design review
• http//yosemite.epa.gov/oaqps/EOGtrain.nsf/fabbfcf
  e2fc93dac85256afe00483cc4/
• 7005aec7427d543e85256b6d004f9376/FILE/si412a