RECOVERY BOILER GAS EMISSIONS AND EMISSION CONTROL

Mikko Hupa

Åbo Akademi UniversityTurku, Finland

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 Nitrogen Oxides-5

-10

0

FLUE GAS EMISSION COMPONENTS

Co

nce

ntr

atio

n

RECOVERY BOILEREMISSIONS - OUTLINE

• Particulates• Sulfurous Gases• Nitrogen Oxides• Conclusion

CARRY OVER PARTICLES

___

20 μm

CARRY OVER PARTICLE

_

2 μm

TYPICAL PARTICULATEEMISSIONS

50 – 100 mg/m3n (8 % O2, dry flue gas)

0.022 – 0.044 gr/DSCF (8 % O2, dry gas)

PARTICULATE EMISSIONS -CONCLUSIONS

• Carry-over affects fouling – not emissions

• Particulate emissions mostly fume• High furnace temperatures

increase fume production• Particulate emissions controlled

by dust removal systems (ESP)

RECOVERY BOILEREMISSIONS - OUTLINE

• Particulates• Sulfurous Gases• Nitrogen Oxides• Conclusion

SULFUROUS GASES

• Sulfur Dioxide, SO2

• Reduced Sulfur Compounds, TRS

°C

200

400

600

800

1000

LOWER FURNACE

TERT. AIR

SUPER HEATER

BOILER BANK

ECONO- MIZER

H2S(g) SO2(g) Na2SO4(s) Na2CO3(s)

Na2CO3 (l,s)

O2 (g)

O2 (g)

CO2 (g)

H2S (g)

SO2 (g)

2NaOH(g)

Na2SO4 (l,s)

REACTIONS OF S AND Na IN FLUE GASES(hot bed, low sulphidity)

°C

200

400

600

800

1000

LOWER FURNACE

TERT. AIR

SUPER HEATER

BOILER BANK

ECONO- MIZER

H2S(g) SO2(g) Na2SO4(s) Na2CO3(s)

O2 (g)

O2 (g)

CO2 (g)

NaHSO4(s)

O2 (g)

H2O (g)

SO3(g)

H2S (g)

SO2 (g)

2NaOH(g)

Na2SO4 (l,s)

Na2CO3 (l,s)

2NaHSO4 (l,s)

SO3 (g)

REACTIONS OF S AND Na IN FLUE GASES(S/Na2 = 1.5, cool bed, high sulphidity)

H 2S(g) SO 2(g) Na2SO4 (s) Na2CO3 (s)

O 2 (g)

O2 (g)

CO2 (g)

NaHSO4(s)

O 2 (g)

H2O (g)

SO 3(g)

H 2S (g)

SO2 (g)

2NaOH(g)

Na2SO 4 ( l,s)

Na2 CO3 (l,s)

2NaHSO4 ( l,s)

SO3 (g)

S/Na2 = 1.5, cool bed, high sulphidity

S/Na2 = 0.8, hot bed, low sulphidity

°C

200

400

600

800

1000

LOWER FURNAC E

TERT. AIR

SUPER HEATER

BOILER BANK

ECONO- M IZER

H2 S(g) SO2 (g) Na2SO 4(s) Na2CO3(s)

Na2CO 3 (l,s)

O 2 (g)

O2 (g)

CO2 (g)

H2S (g)

SO2 (g)

2NaOH(g)

Na2SO 4 (l,s)

REACTIONS OF S AND Na INFLUE GASES

SULFUROUS GASES

• Sulfur Dioxide, SO2

• Reduced Sulfur Compounds, TRS

TRS: THRESHOLD OF ODOR

Hydrogen sulfide, H2S 0.4 to 5 ppbMethyl mercaptan, CH3SH 2 to 3 ppbDimethyl sulfide, (CH3)2S 2 ppb

Compare with:

Sulfur dioxide, SO2 1 to 5 ppm

Hydrogen sulfide, H2S 0.4 to 5 ppbMethyl mercaptan, CH3SH 2 to 3 ppbDimethyl sulfide, (CH3)2S 2 ppb

Compare with:

Sulfur dioxide, SO2 1 to 5 ppm

TYPICAL TRS EMISSIONS

Direct Contact Evaporator 20 ppm

”Low Odor” DCE 5 ppm

No DCE < 1 ppm

TRS FORMATION IN DIRECT CONTACT EVAPORATORS

Flue gas CO2 releases the sulfur from black liquor:

Na2S(bl) + CO2(g) + H2O(g)

H2S(g) + Na2CO3(bl)

TRS: LOW ODOR SYSTEMS

O2

• Black liquor sulfide is oxidized tothiosulfate:

Na2S Na2S2O3

• No release of H2S in the DCE ofpreoxidized liquor

TRS EMISSIONS FROM THE RECOVERY FURNACE

• Minor TRS emissions also from incomplete oxidation of the lower furnace gases

• Can be controlled by improved mixing of the tertiary air

RECOVERY BOILEREMISSIONS - OUTLINE

• Particulates• Sulfurous Gases• Nitrogen Oxides• Conclusion

NITROGEN OXIDES, NOx

nitrogen(mon)oxide,nitric oxide

nitrogendioxide

nitrousoxide

NOx = NO + NO2 (+ N2O)

EMISSION UNITS(Example NOx)

100 ppm (3 % O2)

= 72 ppm (8 % O2)= 83 ppm (8 % O2, dry gas)

= 237 mg NO2/m3 (3 % O2, dry)= 71 mg NO2/MJ (LHV)

= 0.104 gr/DSCF (3 % O2, dry)= 0.00116 gr/BTU (LHV)

100 ppm (3 % O2)

= 72 ppm (8 % O2)= 83 ppm (8 % O2, dry gas)

= 237 mg NO2/m3 (3 % O2, dry)= 71 mg NO2/MJ (LHV)

= 0.104 gr/DSCF (3 % O2, dry)= 0.00116 gr/BTU (LHV)

NOx EMISSION FACTORS

Oil, uncontrolledlow-NOx

Coal, uncontrolledlow-NOxSCR

Wood, grate & FBC

BLRB

4003002001000

0 0.002 0.004 0.006

mg NO2/MJ

gr NO2/BTU

ORIGINS OF NOx

• Thermal NOOxidation of N2 in combustion air Only at temperatures > 1400 C/ 2500 F

• Fuel NOOxidation of fuel bound nitrogenDoes not require high temperatures

FUEL NITROGEN CONTENTS

Fuel % in N in DS___________________________________________________________________________________________________________________________________

Coals 1 – 2 Oil 0.3 – 1Bark 0.2 – 0.4 Wood 0.1 – 0.2Black Liquor 0.05 – 0.15

BLACK LIQUOR NITROGEN REACTION ROUTES

NBL

NH3

Ngreen liquor

NO

Nvol

N2

NO

NON2

Nchar

Nsmelt

DEVOLATILIZATION BURN-OUT

LIQUOR NITROGEN CONTENTS (Wt %)

0

0.10

0.20

0.75

Soft

Woo

d/K

raft:

Sc-1

Sc-4

Sc-5

Sc-7

Sc-8

US-1

2US

-13

US-

20H

ard

Woo

d/Kra

ft:Sc

-2Sc

-10

US-1

9U

S-11

Baga

sse/S

oda:

US-14

Stra

w/S

oda:

Asia

-15

HW

/NSS

C: Sc-

23

NITROGEN DISTRIBUTION BETWEEN FLUE GAS (NO) AND SMELT (NaOCN)

(Boiler tests by Andritz, 2006)

NITROGEN CHEMISTRY IN KRAFT RECOVERY

Digester01-201-13

Woodorg-N

DigesterEvaporators Recovery Boiler

3

Pulp

BLorg-N

WeakWashGreen

LiquorOCN -

White Liquor NH3

Causticizing

BL

NH3, org-N

NH3NO

NH3

NaOCN

NITROGEN IN RECOVERY CYCLE

White liquor

Wood chips

Weakblackliquor

BlackLiquor

Smelt Greenliquor

Wash solutions Lime

mud

Lime mudNH3

White liquor

NH3

Secondary condensate

NH3

N2

NOx

NH3

EVAPORA-TION

RECOVERY BOILER

SMELT DISSOLVER CAUSTICIZINGDIGESTER

(+WASHING)

Water

Greenliquor

NH3

White liquor

White liquor

NH3

Lime mud

NH3

RECOVERY BOILER NOx -CONCLUSIONS (I)

• NOx emissions 50-100 ppm (8 % O2)• Low compared with other combustion

systems• Mostly fuel NO

RECOVERY BOILER NOx -CONCLUSIONS (II)

• 20 – 30 % conversion of NBL into NO• NBL content varies 0.05 – 0.15 %• Soft Wood < Hard Wood < Straw

RECOVERY BOILER NOx -CONCLUSIONS (III)

• NO can be controlled by operation:- air staging systems- avoiding droplet burn-out in flight

• NO removal techniques are being tested:- ammonia addition (SCR, SNCR)

RECOVERY BOILER NOx -CONCLUSIONS (IV)

• Part of the BL nitrogen stays in the smelt in the form of sodium cyanite (NaOCN)

• Smelt cyanite form ammonia in green liquor

• GL ammonia may cause additional emissions

RECOVERY BOILEREMISSIONS - OUTLINE

• Particulates• Sulfurous Gases• Nitrogen Oxides• Conclusion

BL RECOVERY BOILER EMISSIONS

1970 1980 1990 2000+____________________________________________________________________________________________________________________________________________________________________

SO2 (ppm) 1500 500 100 ~ 0NOx (ppm) 70 70 80 50TRS (ppm) 300 200 5 ~ 0Particulate (mg/m3) 500 300 100 30____________________________________________________________________________________________________________

FLUE GAS EMISSION COMPONENTS100 %

100 ppm

1 %

1 ppm

1 ppb(µg/kg)

1 ng/kg

1 pg/kg -15

CO 2 Carbon Dioxide

CO Carbon Monoxide / C xH y Hydrocarbons

PAH Polyaromatic Hydrocarbons

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

Dioxins, Furans

SO 2 Sulfur Dioxide / NO x Nitrogen Oxides-5

-10

0