particulates #1

HELSINKI UNIVERSITY OF TECHNOLOGY ENE-47.153

PARTICULATES #1PARTICULATES #1•• IntroductionIntroduction

•• AshAsh--forming elements in fuelsforming elements in fuels

•• Particulate emission standardsParticulate emission standards

•• Options for particulate control emissionsOptions for particulate control emissions

•• Gravity settlersGravity settlers

•• Gas cyclonesGas cyclones

•• Electrostatic precipitatorsElectrostatic precipitators

see: see: www.hut.fi/~rzevenho/gasbookwww.hut.fi/~rzevenho/gasbook


Particulate (emissions) control : why ?Particulate (emissions) control : why ?

•• Regulations considering environmental / health hazardRegulations considering environmental / health hazard

•• Protection of gas turbines / expansion turbinesProtection of gas turbines / expansion turbines

•• Protection / avoid problems with other gas cleanProtection / avoid problems with other gas clean--up up equipmentequipment

•• The particulate solid or the gas may be a valuable productThe particulate solid or the gas may be a valuable product

•• Dust explosion risks.Dust explosion risks.


Ashes and solid residues during typical Ashes and solid residues during typical pulverised coal combustionpulverised coal combustion


Typical size distribution for fly ash and Typical size distribution for fly ash and bottom ash from pulverised coal combustionbottom ash from pulverised coal combustion


AshAsh--forming elements and ash formation #1forming elements and ash formation #1

CoalificationCoalification Mineral impurities during combustionMineral impurities during combustionor gasificationor gasification



Pulverised coal combustion Fluidised bed coal combusPulverised coal combustion Fluidised bed coal combustiontion



Biomass fuelsBiomass fuels


Ash content of fuels (dry %Ash content of fuels (dry %--wt)wt)

Problem: “Ash content of fuel” Problem: “Ash content of fuel” ⇐⇐ Ash forming elements Ash forming elements ⇒⇒ Fly ash, bottom ashFly ash, bottom ash

Fossil fuels Biomasses & waste derived fuels

Coal, lignite 5 - 40 Wood 0.1 - 0.5Bark 2 - 8

Oil < 0.1 Straw 4 - 8Natural gas -

Light fuel oil < 0.01 Sewage sludge 15 - 20Heavy fuel oil ~0.04 Car tyre scrap 5 - 8

Munical solid waste (MSW) 5 - 25Refuse derived fuel (RDF) 10 - 25

Peat 4 - 10 Packaging derived fuel (PDF) 5 - 15Auto shredder residue (ASR) ~25

Petroleum coke, “petcoke” ~1 Leather waste ~5Estonian oil shale ~40

Orimulsion™ ~1.5 Black liquor solids 30 - 40


Ash production from western US coal Ash production from western US coal combustion in a 500 combustion in a 500 MWMWelecelec

pulverised coal power plantpulverised coal power plant

Bituminous Wyoming PowderRiver Basin

Montana PowderRiver Basin

Coal ash content, %-wt 9.5 4.8 3.7Bottom ash, ton/year 24560 17280 8600

Fly ash, ton/year 98260 69100 34390Total ash, ton/year 122820 86380 42990


DustDustemission emission standards standards

for EU for EU Large Large

combustion combustion plants (LCPs)plants (LCPs)

(directive (directive 2001/80/EC)2001/80/EC)


Dust Dust emission standards for waste (coemission standards for waste (co--) firing ) firing and cement plants for EU and cement plants for EU (directive 2000/76/EC)(directive 2000/76/EC)

CCcoco--firingfiring = (= (VVwastewaste . . CCwastewaste + + VVprocessprocess.C.Cprocessprocess)/( )/( VVwastewaste + + VVprocessprocess), V = exhaust volume ), V = exhaust volume


Removal of particulates from (flue) gasesRemoval of particulates from (flue) gases

1. Methods based 1. Methods based on external forceson external forces

Decreasing Decreasing particle particle

sizesize

Gravity settlersGravity settlers

Cyclones Cyclones & centrifuges& centrifuges

Electrostatic Electrostatic precipitatorsprecipitators

2. Methods based 2. Methods based on barrierson barriers

Bag filtersBag filtersCeramic barrier filtersCeramic barrier filters

Granular bed filtersGranular bed filters

Wet scrubbersWet scrubbers


Parameters determining particulate controlParameters determining particulate control

ProcessProcess ParticleParticle

TemperatureTemperature Size distributionSize distributionPressurePressure ShapeShapeGas flowGas flow Surface propertiesSurface propertiesConcentrationConcentration Chemical composition :Chemical composition :

-- carbon contentcarbon content-- alkali contentalkali content-- tar contenttar content-- sulphur contentsulphur contentMelting point, softening pointMelting point, softening pointChemical stabilityChemical stability


Particulate removal efficiencies needed Particulate removal efficiencies needed for various coal firing methodsfor various coal firing methods


Efficiencies of several particulate control devicesEfficiencies of several particulate control devices


An inertial separator: a settling chamberAn inertial separator: a settling chamber


A gas cyclone A gas cyclone more than a useful premore than a useful pre--separator ?separator ?

Advantages

Simple, cheap and compactLarge capacity

DisadvantagesLarge pressure drop

Low efficiency

“Catch” removal problems

No removal below ~5 µm

Problems above ~ 400 °C


Cyclones:Cyclones:processes processes

determining determining separationseparation


A “standard” cyclone (A “standard” cyclone (LappleLapple))

Lb

Lc

De

Dd

W

H

D

SHigh Conventional High

efficiency throughputHeight of inlet

H/D 0.5 ~0.44 0.5 0.75 ~ 0.8Width of inlet

W/D 0.2 ~ 0.21 0.25 0.375 ~ 0.35Diameter of gas exit

De/D 0.4 ~0.5 0.5 0.75Length of vortex finder

S/D 0.5 0.625 ~ 0.6 0.875 ~0.85Length of body

Lb/D 1.5 ~1.4 2.0 ~1.75 1.5 ~1.7Length of cone

Lc/D 2.5 2 2.5 ~2.0Diameter of dust outlet

Dd/D 0.375 ~ 0.4 0.25 ~ 0.4 0.375 ~ 0.4


Removal efficiency for Removal efficiency for LappleLapple cyclonecyclone

Number of gas turns Number of gas turns ((i.e.i.e. revolutions) before enteringrevolutions) before entering

the vortex finder:the vortex finder:

Grade efficiency:Grade efficiency:

““Cut size”:Cut size”:

Typical material properties:Typical material properties:

dynamic gas viscosity :ηgas ≈ 1.8×10-5×(T/293)2/3 Pa.sdensities : ρsolid = typically 500 … 3000 kg/m³ρgas = 1.2 kg/m³ at 20°C, 1 bar (air)

NL L

Hb

c

=+

2

Eff ddd

p

p

( ) =

+⎛

⎝⎜

⎞

⎠⎟

1

1 50

2

dW

NVgas

in solid gas50

92

=−

ηπ ρ ρ ( )


Forces on particles in cyclonesForces on particles in cyclones

Centrifugal forceCentrifugal forcemmpp’ ’ ωω²² r = r = mmpp’ ’ vvtt²² / r/ r

r

Drag force (Stokes)Drag force (Stokes)3 3 ππ vvrr ddpp ηηFF

into cyclone withinto cyclone withvelocity vvelocity vi i , , inlet area Ainlet area A

Force balance gives equilibriumForce balance gives equilibriumradial position: radial position:

1.1. mmpp’ ’ vvtt²² / r = / r = 3 3 ππ vvrr ddpp ηηFF

2.2. vvrr ≈≈ vvii A/ (2A/ (2ππr h)r h)(h=length of cylindrical section)(h=length of cylindrical section)

3. 3. vvttrrnn = = vviiRRnn, n ~ 0.5....0.55, n ~ 0.5....0.55givesgives

(r/R)(r/R)nn = = ππhhρρssvviidd22pp/(9 A/(9 AηηFF))

for capture: large r/R neededfor capture: large r/R needed

R


Advanced (?) gas cyclone designsAdvanced (?) gas cyclone designs

Cyclone withCyclone withvortex collectorvortex collector

pocketspockets←←

Aerodyne rotaryAerodyne rotaryflow cycloneflow cyclone

→→


Typical layTypical lay--out out of a wireof a wire--andand--

plate ESPplate ESP


Electrostatic Electrostatic precipitators(ESPsprecipitators(ESPs))Where (conventional Where (conventional pulverisedpulverised coal combustion):coal combustion):

Before (wet) scrubber for SOBefore (wet) scrubber for SO22 controlcontrolBefore (hot side) or after (cold side) air preheatBefore (hot side) or after (cold side) air preheat

Usually before SCR for Usually before SCR for DeNOxDeNOx (“hot side, low dust”)(“hot side, low dust”)Before (hot side) or after (cold side) air preheatBefore (hot side) or after (cold side) air preheat

Alternative:Alternative:baghousebaghouse filter, because 1) higher efficiency and filter, because 1) higher efficiency and

2) less effect of particle electric properties 2) less effect of particle electric properties 4 process steps:4 process steps:

1. Particle charging1. Particle charging2. Particle movement relative to gas flow2. Particle movement relative to gas flow

3. Particle collection at deposition surface3. Particle collection at deposition surface4. Particle removal from deposition surface (often discontinuous4. Particle removal from deposition surface (often discontinuous))


ESP : basic principle, efficiencyESP : basic principle, efficiency


Tubular ESP :Tubular ESP :basic design basic design

featuresfeatures


Electric field in ESP, configuration factorElectric field in ESP, configuration factorElectric field, E (V/m), Electric field, E (V/m),

and electric potential, and electric potential, φφ (V):(V):

EE = = -- ∇∇ φφ

Electric field as function of Electric field as function of distance x from wire :distance x from wire :

E(x) = E(x) = ∆∆U /x FU /x F

∆∆U = voltage differenceU = voltage differenceF= F= ““configirationconfigiration factor of factor of

the electrode systemthe electrode system””wirewire--inin--tube : F= tube : F= lnln (R/r)(R/r)

equipotential lines field intensity

electric field lines

equipotential lines field intensity

electric field lines


ESP configuration factor ESP configuration factor

a. Wirea. Wire--in tubein tubeb. Wireb. Wire--plateplate

c. Multiple wire c. Multiple wire -- plateplate

δδ=d/r , =d/r , i.e.i.e.relative electrode spacingrelative electrode spacing


An ESP An ESP at Kotka Finland at Kotka Finland


ESP : particle charging #1, ESP : particle charging #1, using corona dischargeusing corona discharge ((uniuni--polar, one direction)polar, one direction)

Diffusion chargingDiffusion charging

Small particles ( < 1 µm)Small particles ( < 1 µm)

charge charge qqmaxmax ~ 10~ 1088 e e ddpp Note: charge e = 1.6 ×10Note: charge e = 1.6 ×10--1919 CC

Field chargingField charging ((PauthenierPauthenier (1932)(1932)

Larger particles ( > 1 µm) Larger particles ( > 1 µm)

relative dielectric constant, relative dielectric constant, εεrr

dielectric constant of vacuum, dielectric constant of vacuum, εε00 = 8.854×10= 8.854×10--1212 C/(V m)C/(V m)

charge charge qqmaxmax ~ 12 ~ 12 ππ EE1 1 ddpp² ² εε 0 0 εε rr / (/ (εε rr + 2)+ 2)

EE--field Efield E11 in charging zone ~ 3×10in charging zone ~ 3×1066 V/mV/m


Classical particle charging theory calculationsClassical particle charging theory calculations


Alternative methods for charging : Alternative methods for charging : 1) 1) UniUni--polar (+ or polar (+ or --) : bi) : bi--polar coronapolar corona

2) 2) UniUni--directional directional ↔↔ bibi--directional field chargingdirectional field charging3) Pulsed corona techniques3) Pulsed corona techniques

4) Impact (contact, 4) Impact (contact, tribotribo,...) charging,...) charging

Electrical mobility, Electrical mobility, vvee, of charged particle, in E, of charged particle, in E--field Efield E22 ~ 10~ 1044 V/m:V/m:Coulomb force = Stokes' drag force Coulomb force = Stokes' drag force

qqppEE22 ≈≈ 33ππ vvee ηη gasgas ddpp, with , with ηηgasgas = dynamic gas viscosity (Pa.s)= dynamic gas viscosity (Pa.s)Result 1: diffusion charging : Result 1: diffusion charging : vvee ≈≈ 101088 e Ee E22 / ( 3 / ( 3 ππ ηηgasgas ) ~ 0.01 m/s) ~ 0.01 m/s

Result 2: field charging : Result 2: field charging : vvee ≈≈ EE11EE2 2 εε 00 ddpp/ (/ (ηη gasgas((εεrr + 2)) ~ 0.1 + 2)) ~ 0.1 -- 1 m/s1 m/s

ESP : particle charging #2, ESP : particle charging #2, particle drift velocityparticle drift velocity


ESP efficiency : Deutsch equationESP efficiency : Deutsch equationSetSet--up: vertical gas flow, velocity up: vertical gas flow, velocity uugasgas, plate , plate

height H, spacing Dheight H, spacing DMass balance for particle concentration, c:Mass balance for particle concentration, c:

uugaga D L ( D L ( ccxx -- ccx+x+∆∆xx ) = ) = vvee ½( ½( ccxx + + ccxx+ + ∆∆ xx ) ) ∆∆x Lx L= mass removed= mass removed

vvee = charged particle electrical mobility= charged particle electrical mobility

⇒⇒ uugasgasDD dc/dc/dxdx = = -- vvee ccIntegrate, c = Integrate, c = ccinin at x = 0, to position x :at x = 0, to position x :

c(x) = c(x) = ccinin exp ( exp ( -- vvee x / (x / (uugasgas D))D))ccoutout = = ccinin exp ( exp ( -- vvee A / A / QQgasgas) @ x = H) @ x = H

for gas flow Q (m³/s) and plate area A = 2 LH for gas flow Q (m³/s) and plate area A = 2 LH (2 sides !!!!!)(2 sides !!!!!)

x

cx

cx+ ∆x

ugas

D

∆x ve

L

H

Efficiency Efficiency ηηESPESP = 1 = 1 -- exp ( exp ( -- vvee A / A / QQgasgas ) Deutsch Equation) Deutsch Equation

Corrected (Corrected (MattsMatts--ÖhnfeldtÖhnfeldt) : ) : ηη ESPESP = 1 = 1 -- exp exp -- ((vvee A / A / QQgasgas ))kk k = 0.4...0.6k = 0.4...0.6


ESP and fly ash ESP and fly ash resistivityresistivity

Fly ash Fly ash sulphursulphur, temperature, temperature MoistureMoisture

(300 °F ~ 150 °C, 200 °F ~ 95 °C, 450 °F ~ 220 °C)(300 °F ~ 150 °C, 200 °F ~ 95 °C, 450 °F ~ 220 °C)


Particle Particle resistivityresistivity and electric drift velocityand electric drift velocity


““RebouncingRebouncing” of ” of particles with (too) highparticles with (too) high

conductivityconductivity


Difficult conditions for Difficult conditions for ESP (ESP (→→) )

and options for and options for improvement (improvement (↓↓))


ESP performance in relation to fuelESP performance in relation to fuel--sulphur: sulphur: the effect of fuel switchingthe effect of fuel switching

(PRB is coal from Powder River Basin, Western USA)


ESP : flue gas conditioning ESP : flue gas conditioning (EPRICON)(EPRICON)


Typical coldTypical cold--side ESP for coal fly ash:side ESP for coal fly ash:design datadesign data

Temperature 120 - 200°C Power / collector areaGas flow velocity 1 - 3 m/s ash resistivity 104-107 ohm.cm ~ 43 W/m2

Gas flow / collector area 15 - 125 s/m ash resistivity 107 - 108 ohm.cm ~ 32 W/m2

Plate-to-plate distance 0.15 - 0.4 m ash resistivity 109-1010 ohm.cm ~ 27 W/m2

Electric drift velocity 0.02 - 2 m/s ash resistivity ~1011 ohm.cm ~ 22 W/m2

Corona current / collector area 50 - 750µA/m2 ash resistivity ~1012 ohm.cm ~ 16 W/m2

Corona current / gas flow 0.05 - 0.3 J/m3 ash resistivity ~1013 ohm.cm ~ 11 W/m2

particulates #1

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