Examination exercises - Startsidamzevenho/portfolj/undervisning/FPK... · 2012-10-23 ·...
Transcript of Examination exercises - Startsidamzevenho/portfolj/undervisning/FPK... · 2012-10-23 ·...
Examination exercises Exam: Your grade in the exercises will be used as points in the exam for
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
Exam: 21.3 Enrolment through Min Plan Students from outside need to send me an email adress of a
responsible person that can take care of supervision in your home university
Exam is CLOSED BOOK Allowed: calculator Not allowed: Book, slides, notes, pc, Maol tablebook, GSM
2
Trace elements/-metals/heavy metals
Maria Zevenhoven
3
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-%
6
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– categorised by the fact that they are toxic: Outdated term referring to Pb, Cd, Hg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
8
Heavy metals in fuels
unit : ppmw =mg/kg,
dry
Hg
As
CdCoCrCuMnNiPbSb
Tl V
Amounts of waste in Finland
www.ymparisto.fi
10
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
12
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)
13
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
14
Volatility and stoichiometry
Reducing atmosphere renders elements often more volatile leading to more trace metals in fly ash after cooling down
15
Influence on volatility
CrNi
TlSbPb
Cd
AsHg
16
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
%)
19
Cd in grate fired boiler(wood, bark)
boiler
ESP
bottom ash
fly ash
stack
20
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
23
Emission standards trace elements
24
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
26
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
29
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/
30
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
31
vaporisation
Ash
gas phase oxidation
750-900K
Hg→HgCl2
coal char
Hg
gas phase oxidation
750-900K
Mercury species transformations during pulverised coal combustion
32
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
33
Hg→HgCl2
Hg→Hg+catalytic oxidation
400-600K
Mercury species transformations during pulverised coal combustion
34
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
35
Ash
Hg→Hg+
adsorption
Particulate
HgO, HgCl2, HgSO4,HgS
vapour phase Hg, HgCl2
Mercury species transformations during pulverised coal combustion
Post combustion
36
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
37
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
38
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
39
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