Topic 4: Particulate Controls – Baghouse PDF

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Topic 4: Particulate Controls – Baghouse 2  Oldest method  Fabric collectors use filtration to separate dust particulates from dusty gases.  They are one of the most efficient and cost effective , able to achieve more than 99% efficiency for very fine particulates.  Baghouse can be differentiate...

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Topic 4: Particulate Controls – Baghouse

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Oldest method Fabric collectors use filtration to separate dust particulates from dusty gases.

They are one of the most efficient and cost effective , able to achieve more than 99% efficiency for very fine particulates. Baghouse can be differentiate base on 1. Type of fabric 2. Cleaning mechanism(s) 3. Equipment geometry 4. Mode of operation 3

(a) bottom feed; (b) top feed; (c) exterior filtration.

The process can be continuous or intermittent

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The number of bags in a baghouse may vary from less than a dozen to several thousand.  Baghouse with large number of bags generally will be separated into compartment for cleaning  The bags can be of woven or felted cotton, synthetic, or glass-fiber material in either a tube or envelope shape.  The fabric filter (bags) is capable of providing high collection efficiencies for particles as small as 0.1 µm and will remove a substantial quantity of those particles as small as 0.01µm.

Tube shape

Envelope shape 5

•Particles deposit on surface of fabric •Woven fabric has low collection efficiency •A dust cake would eventually forms; this, in turn, acts predominantly as a sieving mechanism.

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Provide good to excellent chemical resistance to mineral acids, organic acids, alkalis, and organic solvents Dust cake is minimal or almost nonexistent and the primary filtering mechanisms are a combination of inertial impaction, direct interception, diffusion by electrostatic force or Brownian movement 7

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Inertial impaction A particle is so large that it is unable to quickly adjust to the abrupt changes in streamline direction near a filter fiber. The particle, due to its inertia, will continue along its original path and hit the filter fiber. Predominant when high gas velocities and dense fiber packing of the filter media is present. 8

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Direct Interception – Interception occurs when a particle which is following a gas streamline comes within one particle radius of a filter fiber. The particle touches the fiber and is captured, thus being removed from the gas flow.  Electrostatic forces – The presence of an electrostatic charge on the particles and the filter can increase dust capture. 9

Brownian movement – Gas

moves in random zigzagging path known as Brownian motion. As the gas molecules collide with the dust particles ≤ . µm , they also start to move in similar way. The smaller a particle is and the slower the flow, the more time it will have to zigzag around, hitting and sticking to a filter fiber.

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When the dust layer thickness reaches a level where flow through the system is hindered, bag cleaning is initiated. Cleaning can be done while the baghouse is still online (filtering) or in isolation (offline).

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Gas-to-cloth ratio, G/C Filter medium Type of fabric Filtering Method Material of construction Temperature limitation Volume Dew point State or federal regulation Must be build as partner for process and not as restrictor

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G/C describe how much a dirty gas passes through a given surface area of filter in a given time. It is also known as filteration velocity,

Vf

Vf = Q = Ac =  

filtration velocity, ft/min (cm/sec) volumetric air flow rate, ft3/min (cm3/sec) area of cloth filter, ft2 (cm2)

A high GC means a large volume of air passes through the fabric . A low GC means a small volume of air passes through the fabric 13

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Estimating G/C too high will results in excessive pressure drop, reduced collection efficiency, bags becoming caked solidly with dust and more frequent cleaning that leads to reduced fabric life. Estimating G/C too low would increase the size and cost of the baghouse. G/C must be compatible with the fabric selection and cleaning type.

Selection of fabric composition depends on gas and dust characteristic Selection either woven or felt fabric largely depends on type of cleaning

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Over time, the depth of the dust layer increases, the layer of the dust layer Dp can be calculated from

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The fabric (filter media) use would depends on:  Temperature  Corrosiveness  Hydrolysis  Dimensional stability  Cost Some of fabric use Cotton— low temperature capability; good flex abrasion resistance; still used in some low temperature applications.

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Polypropylene— very sleek fabric; demonstrates good cake resistance to blinding. Does not readily absorb moisture and good selection for a low temperature, high moisture condition, production.  Polyester— very sturdy material; good resistance to acids and alkalies, slightly higher temperature capability than polypropylene. Costs about the same as polypropylene. Used in most routine low temperature applications including quarry, woodworking, and sand handling operations.  Fiberglass— normally used in high temperature applications. Improvements in finishes and techniques of fabrication, installation, and operation have paid dividends in extended bag life. Fiberglass is the primary fabric used in the boiler market. Cost is between polyester and Nomex. 18

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Nomex— extremely sturdy material with respect to flex abrasion. Superior to glass in resistance to fluorides and abrasion. Good temperature capability. Poor acid resistance; not used in gas streams containing SO2 and SO3. Costs about 2.5 times that of polypropylene and polyester. Used in asphalt, steel, carbon black, and cement industries.  Teflon— generally chemically inert and for that reason useful in severe environments. Very expensive, but the cost is usually justified because of superior bag life. Used in the carbon black industry, lead smelting, coal-fired boilers, and various unusual applications. Costs about 10 times that of polyester for the same size and weight bag.

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Pressure drop (∆p) is the resistance to air flow across the baghouse.

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The higher the pressure drop, the higher the resistance to air flow.

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The pressure drop is determined by measuring the difference in total pressure at two points, usually the inlet and the outlet. The total system pressure drop can be related to the size of the fan that would push or pull the exhaust gas through the baghouse.

Pressure drop is usually expressed in mmHg or inH20. The simplest equation used to predict pressure drop across a filter is derived from Darcy's law governing the flow of fluids through porous materials

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K 2ci t

Filter Pressure Drop, (∆p) = k1Vf k1 = fabric resistance, inH2O/(ft/min) [cmH2O/(cm/sec)] Vf = filtration velocity, ft/min (cm/sec) k1 is the fabric resistance (also called drag) and is a function of exhaust gas viscosity and filter characteristic such as thickness and porosity. Porosity describes the amount of void volume in the filter

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K 2ci t

Once there is a dust cake build up, the pressure drop

K2 = Ci = Vf= T =

resistance of the cake, in. H2O/(lb/ft2-ft/min)[cm H2O/(g/cm2-cm/sec)] dust concentration loading, lb/ft3(g/cm3) filtration velocity, ft/min (cm/sec) filtration time, min (sec)

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The term k2 is the dust-fabric filter resistance coefficient and is determined experimentally. k2 depends on gas viscosity, particle density and dust porosity. The dust porosity is the amount of void volume in the dust cake. The porosity is related to the permeability.  k2 is also dependent on the size of the particles in the gas stream. If the particles are very small (< 2µm) k2 is high.  Filtration velocity also has an effect on k2.  If k2 is high, then the pressure drop will tend to increase and the bags will have to be cleaned more frequently. 22

K 2ci t

The total pressure drop, ∆pt equals the pressure drop across the filter plus the pressure drop across the cake and structural pressure drop is given as: ∆pt = ∆p (Filter) + ∆pc (Dust cake) + ∆ps * ∆ps is structural pressure drop

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There is no one formula that can determine the collection efficiency of a baghouse. Some theoretical formulas have been suggested, but these formulas contain numerous (3 to 4) experimentally determined coefficients. -(jL +ft) j f

E = 1- e

= Constant base on fabric, ft-1 = Constant based on cake, s-1

T

= Time of operation to develop the cake thickness, s

L

= Fabric thickness, ft

E

= Collection efficiency (dimensionless)

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Vendors normally design and size the units strictly on experience Vendors may collect empirical data available for the source in question or from a similar industry which is using a baghouse and conservatively apply it. Or run a pilot unit on a slip stream and vary the G/C. The pilot plant should run for a few months in order to obtain data representative of a long-term operation. A well designed, well maintained fabric filter that is operated properly generally collects particles from 0.5 to 100 µm at efficiencies of greater than 99% The remaining design involves optimizing filtering velocity to balance capital costs vs operating costs

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Very high collection efficiency with even the finest particles. These units usually have the capability of achieving efficiencies of 99% almost automatically— provided they are properly constructed and maintained in satisfactory operating condition.

Simple to operate

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Consist of many moving parts and require frequent maintenance

May not be used readily in high temperatures unless special fabrics are used The number of variables necessary to design a fabric filter is very large. A qualitative description of the filtration process is possible, although quantitatively the theories are far less successful. 27

A major malfunction will result in one or more of the following identifying symptoms: 1. Abnormally high pressure drop across the collector 2. Visible emission of dust in the exhaust stack 3. Inadequate face velocity or “puffing” at pickup points and hoods 4. Lower-than-normal dust discharge 5. Loud or unusual noises 6. Severe corrosion of material

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Typical causes of bag failure are : 1. High G/C abrasion 2. Metal-to-cloth abrasion 3. Chemical attack 4. Bag-to-bag abrasion 5. Inlet velocity abrasion (on inside-out cleaning) 6. Accidents 7. Upset conditions (e.g., temperature) 8. Thread mismatched 9. Cuff mismatched 10. Improper installation

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