air pollution control techniques for aerosol- and dust ... · bag house filters, filter media...

Department of Mechanical Process Engineering

Vienna University of Technology Institute of Chemical Engineering

Air Pollution Control Techniques for Aerosol- and Dust emissions

Wilhelm HoeflingerVienna University of Technology, Institute of Chemical Engineering,

Vienna, AUSTRIA

Presentation at the Tempus Retraining Meeting, Vienna, Austria, 15-19 Nov. 2010



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Content• Dust, Aerosol:

Definitions,

European concentration regulations,

Dust measurement techniques

• Dust SeparatorsDifferent kinds of dust separators

Bag house filters,

Filter media

Standard test facilities for testing different filter media

Electrostatic enhancement of bag house filtration, hybrid filters



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Definitions• Technically, dust or an aerosol are suspensions of fine

particles in a gas.

• Dust: particles in a gas below appr. 100 micrometer

• Aerosol: particles in a gas below 10 micrometer– Solid aerosol, liquid aerosol

– Smoke, haze: aerosol with high concentration

http://en.wikipedia.org/wiki/Suspension_%28chemistry%29

http://en.wikipedia.org/wiki/Gas



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Definitions

Emission - Immission

Immission (mg/m 3)

Emission (mg/m3)



Characterisation of Emissions and Immissions

• TSP: Total suspended particles [mg/m³]

• PM10: Particulate matter smaller than 10 µm [mg/m³]

• PM2,5: Particulate matter smaller than 2,5 µm [mg/m³]

• PM1: Particulate matter smaller than 1 µm [mg/m³] (USA)

Particle size: aerodynamic diameter



Prof. Höflinger; Staubabscheiden 6

Aerodynamic diameter dae

dae = ρ1/2 . dreal

( ) 2

181: dgvStokes PTS ⋅⋅⋅⋅

= ρη

Aerodynamic diameter is the diameter of a sphere of unit density (1g/cc) that has the same gravitational settling velocity as the particle in question.

Settling velocity:



European limit values for fine particle fractions (Environment regulations)

Immission: EU- Council Directive 1999/30 EC, PM10

EU- Council Directive 2008/50EC, PM2.5

Emission: no European limit values

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24 hour mean limit

PM10

Step 1: since 1.1.200550µg/m3

35 exceedences possible per year

Step 2: since 1.1.201050µg/m3

7 exceedences possible per year

Annual mean limit

PM10

Step 1: since1.1.200540µg/m3

Step 2: since1.1.201020µg/m3

EU- Council Directive 99/30 EC, PM10Immission law

Up till now many EU- countries could not reach these limiting values for PM10 extension of the fulfilling deadline: June 2011

→ calls for more intensive separation actions for particle emissions in the future



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EU- Council Directive 2008/50EC, PM2.5Immission law.

Annual mean target

PM2,5

Since 1.1. 201025µg/m3

Annual mean limit

PM2,5

Step 1 since 1.1. 201525µg/m3

Step 2 since 1.1.202020µg/m3

Further more a stricter regulation:



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Staubdeposition im menschlichen Atmungstrakt

A-Dust alveolic Dust 50% smaller than 4 µm

Thoracic-

Dust

Dust 50% smaller than 10 µm

E-Dust inhalable 50% smaller than 100 µm

Working place dust regulations Respirable particle size

EN 481, ISO 778 Workplace (indoor)Inhalable dust fraction: E-dust < 100µmThoracic dust fraction: < 10µmAlveolic dust fraction: A-dust < 4µm



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Particle size range of different kinds of dust



Particle size distribution of atmospheric dust

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Generation and sources of dust1) Condensation processes (from gases and

vapours)

2) Dispersion processes (from solid masses, redispersion of already separated dust)

3) Combined processes of 1) and 2)

4) Augmentation of micro organism

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Concerning the sources, particle emission can be divided up into natural and anthropogenic aerosols

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Anthropogenic Aerosolse.g:Combustion processes coal (flyash)Cement productionPetrol and waste combustionIron and steel industryagriculture

Natural aerosolse.g:Soil dustSea spraySmoke from wildfiresBiological particlesVulcano`s



Dust sampling and measuring methods dust concentration TSP, PM10, PM2,5 (mg/m3)

particle size distribution:

mass (volume) size related Q3, q3 or number sized related Q0, q0

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Immision samplingSuck off from ambient atmosphere Emission sampling

Isokinetic suck-off from a flow channel

Flow situation for different suck-off velocities



Dust collection devices for emission measurement

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Sampling probe (VDI 2066)



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Often used measurement techniquesHigh Volume sampler:(PM10, PM2.5 PM1) with discontinuous filter sampling, particle mass

1 stage impactor with beta radiation:(PM10, PM2.5 PM1) with continuous filter sampling, particle mass

Cascade impactor:particle size distribution 0.1 to 20µm, discontinuous, particle mass,

Scattered light sensor:particle size distribution 0.25 to 40µm, continuous particle number

Scanning mobility sizer (SMPS): particle size distribution 0.02 to 1µm, continuous, particle number



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High Volume sampler:PM10, PM2.5 PM1 discontinuous measurement

24 hour measuring device

Impactor part

Filter part



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1 stage impactor with beta radiation:PM10, PM2.5, continuous measurement

PM10 or PM2.5 impactor

Moving filterband Beta radiation



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Cascade impactor: mass sized particle size distribution between 0,1 and 20 µm, discontinuous measurement

Aerosol flow In

Clean air out



ELPICascade impactor with electrical particle charging0,3 - 10 µm

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Continuous measurementParticle number size distribution



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Scattered light sensor: number sized particle size distribution 0.25 – 40 µm, continuous measurement

•



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Scanning mobility particle sizer SMPS: number sized particle size distribution 0.02 – 1 µm, continuous measuring

DMA (Differential mobility analyser)

CPC (Condensation particle counter)



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5 different types of dust separatorsSettling chamber cyclone

Bag house filter

Electrostatic precipitator

Wet scrubber



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Characterisation of dust separators- Fractional separation efficiency T(x)

E: Total separation efficiency

qf: Particle size distribution clean gas

qe: Particle size distribution raw gas

( ))(

)(11)(

xqxqE

xTe

f⋅−−=⇒

- Pressure drop, energy consumption

→ Filtering separator: best separation efficiency

High pressure drop

Goal of further investigations



Cyclone

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Tangential cyclone

Axial cyclone

Low costLowest separated particle size: 5 – 10 µmGas volume flow influences separation efficiency



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Improvement of the cyclone separation efficiency

Multi cyclone Rotary flow dust collector

Pocket cyclone

Cooled wall cyclone

Application as pre-separator and for hot gas cleaning



Multi cyclone mostly for pre-separation (coarse particles), downstream bag house filter for fine particle separation

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Electrostatic separator

Plate type

Tube type

Wet electr. Separator (sticky particles)

Electric dust resistance

→ problem for separation efficiency



„Deutsch“- Equation: relates separation efficiency to the apparatus parameters v, s and L

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2 s

L

B

gePlattenlän :LdeagselektroNiederschl - rodeSprühelekt Abstand:

1100

s

e vsLw

G

e

⎟⎟⎠

⎞⎜⎜⎝

⎛−⋅= ⋅

⋅

η

v

v: gas velocitywe: migration velocity



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Advantages:Low pressure dropLow maintenanceGood separation efficiency for constant raw gas conditions

Disadvantages:High invest costLarge local wantschanging of the particle resistivity can lower separation efficiency (back corona)

Electrostatic separator



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Wet scrubber

Venturi scrubber

Vortex scrubber

Centrifugal scrubber

Nozzle scrubber

Separation efficiencyPollution is shifted into the liquid

Spray tower



Addvantages and disadvantages of wet scrubbers

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Droplet separation efficiency

- High relative velocities required between droplet and dust particle→ high energy consumption

- Particle pollution is shifted from the air into the water (water treatment necessary)

-Together with dust removal sorption of gaseous pollutants (SO2, NOx, CO) possible

- separation of sticky and viscous particles possible

Energy consumption



Wet scrubbing plant

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Filtering separator, 2 kinds: Depth filter: low raw gas dust concentrations (mg/m3)Cleanable filter: high raw gas dust concentrations(g/m3)

Depth filter Cleanable filter

Excellent separation efficiency



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Operation behaviour of cleanable filters

Clean gas concentration

Pressure drop

Time

Important part of the cleanable filter:

FILTER MEDIUM



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Filter medium for cleanable filters: mostly Needle feltsdifferent materials, surface treated (calandered, singed, laminated) to prevent the particle penetration into the depth and to reduce the residual pressure drop



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Surface treated area raw gas side

Clean gas side

hp50,

0

hpi

hp

3

hp

2

hp

1

At

otsurface area of

all sample

s

A1A2A3

Ai

O1

O2O3

Oi

hp

i

PF-layer

.

.

.

.

.

.

.

.

.

Microscopical (transmitting light) and image analysisEvaluation of the porous situation at the surface treated raw gas side and method to optimise the surface treatment of the filter medium



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Image analysis: Conversion of a coloured image into a binary black/white image and elliptic pore approximation

Threshold 130

Threshold: 160to high

Threshold: 115

to low

Elliptic pore approximation

Ap,totOp,totE0dh



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sharp

Determination of the pore depth distribution by an reflecting light microscope

1,0

0,5

0,0hp50,0 hpmax hphp=h2-h1

Q0(hp)

h1

h2

sharp

Pore depth distribution



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Pore depth distribution together with surface porosity E0→

model pore

Q0(hp)0,0 0,5 1,0

hp50,0

hpmax

hp

0,500 * phEH =

H: measure for the dust holding capacity

0

200

400

600

800

1000

1200

1400

1600

-0,5 -0,4 -0,3 -0,2 -0,1 0 0,1 0,2 0,3 0,4 0,5

surface porosity E0 [-]

dept

h of

por

e (h

P) [µ

m]

870

FM7

FM6FM5

FM3

FM1FM2

FM4



Different fiber materials for filter media

42The cost increase with max. operating temperature



Hot gas cleaning 500 – 1200 °C

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- heat recovering, dedusting upstream a heat exchanger- avoid corrosion and sticky problems- dry additive reactions are better at high temperatures, protect Nox catalysator- protect gas turbines from dusty gas

Silicium carbid filter candle



Large sized bag house filterSeveral thousend square meter of filter area

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filter medium must be carefully chosen



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Choice of the filter medium Standard lab test equipment for characterisation

of the operation behaviour of cleanable filter media

Test parameter:

-Residual pressure drop

-Average clean gas concentration

-Dust load of the filter medium



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Different national standard test regulations

Dust feeder

Blow tube

Pressure tankFilter sampleRaw gas channel

Absolute filter

Dust loaded carrier gas

Dust feeder

Blow tube

Pressure tankFilter sampleRaw gas channel

Absolute filter


Absolute filter

Photometer

Raw gas channel

Dust feeder

Back-up filter

Discharge tube

Vakuum pump

Filter sample

Cleaning system

Dust

Absolute filter

Photometer

Raw gas channel

Dust feeder

Back-up filter

Discharge tube

Vakuum pump

Filter sample

Cleaning system

Dust

USA ASTM German VDI3926 Typ I

German VDI 3926 Typ2



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Different national standard test regulations

Dust container

Air inlet

Control valveVakuum

pump

Clean gas ductPressure transducerAbsolut filter

Mass-flowcontroller

Raw gas channelFilter sample

Photometric concentrationmonitor


Pressure tank

Baseplate

Inspection glass

Dust container

Air inlet

Control valveVakuum

pump

Clean gas ductPressure transducerAbsolut filter

Mass-flowcontroller

Raw gas channelFilter sample

Photometric concentrationmonitor


Pressure tank

Baseplate

Inspection glass

German VDI3926 Typ III

JIS Z 8909-1 Japan



Aging of the filter medium

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30 initial cycles pressure controlled

Aging 10.000 cycles, time controlled cycle time: 5 seconds

30 pressure controlled cycles used for filter media comparison

Measurement of these parameters not at the beginning, but after a so called aged period.

should bring the filter medium in a short time into a state, which is comparable to a long industrial operation time



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Development of an international ISO standard (Draft)Test procedure

Round Robin test which compares different standards shows largedifferences

One of the problems: aging behaviour unclear

Aging: key issueFilter media are usually several years in operation and comparing filter tests should focus also on the filtration behavior along operation time.That means the filter media should be aged in a short time which is comparable to a situation after a long operation time. Tests with very short cycle times (5 – 100 seconds, many cycles up to 10.000)



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Combination of electrostatic charging or an electrostatic separator with a bag house filter

- Electrostatic enhanced filtration

- Hybrid filter: Electrostatic filter (ESP) with downstream bag house filter

- Electrostatic particle agglomeration upstream of a bag house filter

Reason: due to the more stricter air quality regulations for particulates, electrostatic filters can not fulfill these requirements any more

Baghouse filter can fulfill it, but disadvantagous is high pressure drop and premature clogging

Combination can fulfill high separation efficiency also with low pressure drop



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Electrostatic enhancement of cleanable dust filter

Longer cycle times

Lower pressure drop

Lower particle penetration

Riebl et al: TU Cottbus



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Electrostatic enhancementCharged dust particles produce lower dust cake resistance

ESFF/MAX9 Conceptual Design



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Hybrid Filter Combination of ESP and bag house filter

Large dust masses ar separatedin the ESP (90%) which works not very efficiently but cheaply

Remaining dust masses (low conc.) are separated down stream in the bag house filter

-longer cycle and operation times

-Lower pressure drop and pressurised air consumption

-Overall: cost-efficient solution especially by retrofitting an ESP



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Hybrid filter designRetrofit already existing E-filter with a down stream bag filter

Redesign a hybrid filter in one housing



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Hybrid filter

Pre collection in the precipitator section of the filter reduces the dust load and wear on the filter bags.

Less dust on the filter bags results in lower pressure drop, fewer cleaning cycles, and significant compressed air savings.

Reduced pressure loss compared to a traditional fabric filter solution.

Reduced energy consumption compared to a traditional fabric filter solution.

Constant low emissions in spite of varying operational conditions.

Use of existing ESP structure and footprint makes the Hybrid solution cost effective.



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Electrostatic agglomeration upstream the bag house filter

Indigo Agglomerator

Australia



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Thank you very much for your attention

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