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this course (corresponding to one out of six questions in the exam).

You have to participate in all exercises and carry over calculation and get them approved to be allowed to do the exam

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responsible person that can take care of supervision in your home university

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Trace elements/-metals/heavy metals

Maria Zevenhoven

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This lecture

What are trace elements/trace metals/heavy metals?

Heavy metals in fuels and wastes

Heavy metals in combustion

Emission standards

Emission control

Mercury

4

This lecture

What are trace elements/trace metals/heavy metals?

Heavy metals in fuels and wastes

Heavy metals in combustion

Emission standards

Emission control

Mercury

5

•Trace elements present in a natural material at concentration < 0.1 wt-%

•major elements: > 1 wt-% & •minor elements: 0.1 – 1 wt-%

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Heavy metal or trace element?

EU directives– names heavy metals and sets limits for Cd, Tl, Hg, Sb, As, Pb, Cr, Co, Cu,

Mn, Ni, V

Usually ”heavy metal” used for metals and metalloids with supposed toxic or ecotoxic properties.

BUT not always specified in literature– categorised by density (seldom biologically significant): limits varying between

3.5 and 7 g/cm3

– categorised by atomic weight: definitions vary from Mw>23, >40, 45-65 etc.– categorised by atomic number: Ti-Hf, As-Bi, Na>20 or 20<Na<90 etc.

categorised by chemical properties: very dense alloys, intermetallic compound of Fe, metals reacting with C6H5N etc.

– categorised by the fact that they are toxic: Outdated term referring to Pb, Cd, Hg

7

This lecture What are trace elements/trace

metals/heavy metals?

Heavy metals in fuels and wastes

Heavy metals in combustion

Emission standards

Emission control

Mercury

Kraft recovery– Sources of heavy metals in

kraft recovery– Black liquor– Partitioning of heavy metals

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Heavy metals in fuels

unit : ppmw =mg/kg,

dry

Hg

As

CdCoCrCuMnNiPbSb

Tl V

Amounts of waste in Finland

www.ymparisto.fi

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This lecture

What are trace elements/trace metals/heavy metals?

Heavy metals in fuels and wastes

Heavy metals in combustion

Emission standards

Emission control

Mercury

11

Convectivetransport

Pyrolysis Char burning andfragmentation

Vaporization

Homogeneousnucleation

Coagulation

Heterogeneouscondensation

Fly ash0.1 -1 um

Fly ash1 - 100 um

Mineralinclusions

Excludedminerals

Mineralcoalescence and

fragmentation

Formation of fly ash

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Classification

Eu, Hf, La, Mn, Rb, Sc, Sm, Th, Zr

Ba Be Bi Co Cr Cs Cu Mo Ni Sr Ta Tl U V W

As Cd Ga Ge Pb Sb Sn Te Ti Zn

B Se I

Hg

Br Cl F

Class

III Volatized and emitted fully in the vapor-phase –not enriched on the fly ash

II Enriched in the fly ash and depleted in the bottom ash

I Equally distributed between bottom ash and fly ash

(after Couch, 1995)

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Volatility and temperatures

The higher the temperature in the boiler the bigger the chance that trace metals evaporise!!

Gaseous metals will condense on cooling down

Higher temperatures will increase amounts on small particles

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Volatility and stoichiometry

Reducing atmosphere renders elements often more volatile leading to more trace metals in fly ash after cooling down

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Influence on volatility

CrNi

TlSbPb

Cd

AsHg

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Volatility and pressure

Higher pressure leads to higher boiling points Higher boiling temperatures mean less

evaporation Less evaporation means enrichment in the

bottom ash

Thus: At higher pressure elements are still condensedat higher temperature thus more efficient capture stillpossible at higher T

Exercize

What happens with Mn in a grate firedboiler

What happens with Cd in a grate firedboiler

What would happen– if I add PVC to the waste mixture?– If I raise temperature?– If I raise pressure?

Gratefired boiler

Typical specifications in waste firing– Solid fuels stationary, moving grates,

spreader stokers – Fuel particle size > 1 cm – High furnace temperature (> 1000C) – Ash removed mainly through the grate (80

%)

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Cd in grate fired boiler(wood, bark)

boiler

ESP

bottom ash

fly ash

stack

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Mn in grate fired boiler(wood, bark)

boiler

ESP

bottom ash

fly ash

stack

21

This lecture

What are trace elements/trace metals/heavy metals?

Heavy metals in fuels and wastes

Heavy metals in combustion

Emission standards

Emission control

Mercury

22

100 %

100 ppm

1 %

1 ppm

1 ppb (µg/kg)

1 ng/kg

1 pg/kg -15

CO2 Carbon Dioxide

CO Carbon Monoxide / CxHy Hydrocarbons

PAH Polyaromatic Hydrocarbons

As, Cr, Ni, V, Pb, Cd, Hg,... Heavy Metals

Dioxins, Furans

SO2 Sulfur Dioxide / NOx Nitrgen Oxides-5

-10

0

Flue Gas Emission Components

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Emission standards trace elements

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This lecture

What are trace elements/trace metals/heavy metals?

Heavy metals in fuels and wastes

Heavy metals in combustion

Emission standards

Emission control

Mercury

25

Controlling emissions Most concern: Hg, Se, As, Cd, Pb class III/II Less concern: Cr, Cu, Ni, V, Zn class II/ I Class I and II in bottom ash and (enriched) in fly ashes:

removal depends mainly on dust control system (and its efficiency for 0.1 - 1 m fines)

Class II and III can be (more) effectively removed by the flue gas desulphurisation system (Hg ~ 40%, Se ~70%)

Specific methods based on sorbents can be used, such as activated carbon, clays and aluminium silicates.

For coal combustion / gasification not (yet) widely used, for waste incinerators often used for Hg and As, Cd, Pb

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Summary part 1

Volatility depends on– Class,– Temperature,– Stoichiometry,– Halogens (Clorine!!),– System pressure

Three classes of volatility:– Volatile (mainly in gasphase), – Semi-volatile- (enriched in fly ash)– Non-volatile (enriched in bottom ash)

Control– Trying to get all in solid/molten= condensed phase– Filter/ESP– Washing in excisting equipment/absorbent systems

27

This lecture

What are trace elements/trace metals/heavy metals?

Heavy metals in fuels and wastes

Heavy metals in combustion

Emission standards

Emission control

Mercury

28

An example

HgMost studied volatile element

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Major emissions

To the atmosphere:– waste incineration– non-ferrous metal production– coal combustion– crematoria (due to the use of the metal in dental fillings) – chlorine manufacturing plants using mercury cells

To waste water:– industrial processes using the metal and its compounds– from dental surgeries, hospitals and clinics

Presence in the earth's crust causes releases to the environment from natural sources

http://www.eper.cec.eu.int/eper/

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Hg→HgCl2

coal char

Hg

gas phase oxidation

750-900K

vaporisation

Ash

Hg→Hg+catalytic oxidation

400-600K adsorption

Particulate

HgO, HgCl2, HgSO4,HgS

vapour phase Hg, HgCl2

Mercury species transformations during pulverised coal combustion

Post combustion

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vaporisation

Ash

gas phase oxidation

750-900K

Hg→HgCl2

coal char

Hg

gas phase oxidation

750-900K

Mercury species transformations during pulverised coal combustion

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Hg→HgCl2

coal char

Hg

gas phase oxidation

750-900K

vaporisation

Ash

Mercury species transformations during pulverised coal combustion

•At high temperatures in combustion zone all Hg will be vaporised in elementary form.•As temperatures decrease Hg can be oxidised and form Hg+

and Hg2+

•At low temperatures oxidised Hg thermodynamically most stable when chlorine present•However kinetic limitations cause presence of Hg and Hgp

•CaO may reverse oxidation by capture of Cl from HgCl2

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Hg→HgCl2

Hg→Hg+catalytic oxidation

400-600K

Mercury species transformations during pulverised coal combustion

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Hg→HgCl2

Hg→Hg+catalytic oxidation

400-600K

Mercury species transformations during pulverised coal combustion

•Around 650°C oxidisation via homogeneous reactions with chlorine species

•H2O inhibits, •NO inhibits/promotes oxidation,•O promotes weakly•V2O5 and TiO2 may catalyse oxidation of Hg0 and Hgp•SNCR no effect on oxidation

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Ash

Hg→Hg+

adsorption

Particulate

HgO, HgCl2, HgSO4,HgS

vapour phase Hg, HgCl2

Mercury species transformations during pulverised coal combustion

Post combustion

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Ash

Hg→Hg+

adsorption

Particulate

HgO, HgCl2, HgSO4,HgS

vapour phase Hg, HgCl2

Mercury species transformations during pulverised coal combustion

Post combustion

Around 130°C heterogeneous reactions with solids (flyash, unburned carbon transition metals etc) by adsorption of chlorine and oxidation of Hg on the active sites

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Reducing emissions of Hg0

Chlorine oxidises at low temperatures Carbon in ash captures

– Low NOX, overfire air and fuel reburn increase carbon in ash and thus enhances capture

Smart fuel blending-increases Cl, decreases Ca

SCR and SNCR– V2O5 and TiO2 may catalyse oxidation of Hg0 and Hgp

– SNCR no effect on oxidation

Obs! catalyst poisoning by As may take place

Technology status review-monitoring and control of trace elements 2003, Will Gibb, Will Quick, Mark SalisburyEON UK

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Capturing Hg

Hg0

– difficult to capture, may leave stack– coarse particle fraction in ESP richer in carbon that

adsorbs Hg0

– use of activated carbon sorbent or other sorbents prior to the flue gas treatment system

Hg2+

– may be caught in wet FGD• Sometimes efficiency lower caused by complexation

with sulfite HgS4+ followed by reduction to Hg0

– use of activated carbon sorbent or other sorbents before/in particle control device

Hgp

– may be caught in ESP/Filter or wet FGDTechnology status review-monitoring and control of trace elements 2003, Will Gibb, Will Quick, Mark Salisbury EON UK

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Summary part 2

Three forms of Hg: Hg, Hg+ and Hgp

Hg problem for capture– Should be oxidised

• Increasing Cl in flue gas helps

– Should be adsorbed on carbon or other sorbent systems

40

This lecture

What are trace elements/trace metals/heavy metals?

Heavy metals in fuels and wastes

Heavy metals in combustion

Emission standards

Emission control

Mercury