Impacts of Burning High Impacts of Burning High Sulphur Fuels in Lime KilnsSulphur Fuels in Lime Kilns

Sabrina Sabrina Francey and HonghiFrancey and Honghi TranTran

2009 TAPPI EPE ConferenceOctober 14, 2009

Memphis, TN

2

Introduction

• Lime Kiln Fuel: most burn fuel oil and/or natural gas

• Due to high energy costs in recent years, growing interest in burning alternative fuels:– Petroleum coke (petcoke)– Wood residue-based biofuels (directly fired,

gasified, pyrolysed) – Precipitated lignin

3

Introduction

• Fuel composition, heating value and adiabatic flame temperature affect lime kiln operations

• Fuel sulphur content important because influences – Lime availability– Ring formation– TRS and SO2 emissions

4

Fuel Sulphur Content

25.41.0-3.0Precip. Lignin

18-210-0.1Bark

18-210-0.03Wood

34.26.5Petcoke

48.20Natural Gas

40.02.1Fuel Oil (#6)

LHV (MJ/kg, dry)

S Content(wt%S)

Fuel

5

Introduction

• In addition to fuels, waste streams burned in lime kilns also contain sulphur: – Non-condensible gases (NCGs)– Stripper off-gases (SOGs) – Tall oil

6

Motivation

• Burning sulphur-containing fuels and waste streams in lime kilns is not new

• The fate of fuel sulphur and its impact on lime availability, ring formation and emissions is not well understood

7

Major Objectives

Examine Fate of fuel sulphur

Impacts of burning high sulphur fuels on lime kiln and chemical recovery operations

8

Fate of S in Lime Kilns

• Fuel S oxidized in flame to SO2

• Sulphur Absorption by Lime– CaSO4 Formation

SO2 + CaO + ½ O2 → CaSO4

– Na2SO4 Formation

9

Fate of S in Lime Kilns

• Fuel S oxidized in flame to SO2• Sulphur Absorption by Lime

– CaSO4 Formation

SO2 + CaO + ½ O2 → CaSO4

– Na2SO4 Formation

Na2CO3 + SO2(g) + ½O2(g) → Na2SO4(s) + CO2(g)

2NaOH(s,l) + SO2(g) + ½O2(g) → Na2SO4(s) + H2O(g)

10

Fate of S in Lime Kilns

• Not all SO2 produced can react with lime– Poor gas-solid contact– Low specific surface area of lime nodules

• Portion of SO2 may pass through kiln and contribute to increased SO2 emissions

11

Fate of S in Chemical Recovery Cycle

• Slaking/Causticizing: – Some CaSO4 removed from cycle with grits– Remainder CaSO4 released to liquor and

reacts:CaSO4 + Na2CO3(aq) → Na2SO4(aq) + CaCO3(s)

12

Fate of S in Chemical Recovery Cycle

• Na2SO4 is water-soluble– Exits causticizer with white liquor– Becomes part of sulphate deadload

• Na2SO4 is reduced to Na2S in Recovery Boiler ⇒ increase in sulphidity

13

Kraft Chemical Recovery Cycle

RecoveryBoiler

Digester/Washing

EvaporatorsCausticizing

Lime Kiln

14

Fate of Fuel S

RecoveryBoiler

Digester/Washing

Evaporators

CaSO4Na2SO4

SO2Fuel S Na2SO4

(deadload)

Na2S

Causticizing

Lime Kiln

15

Lime Kiln S Balance

• Model Lime Kiln S Balance developed

0% Petcoke

80% Petcoke

40% Petcoke

100% Petcoke

60% Petcoke

20% Petcoke

Cases

Covers S input range of3.77-12.75 kg S/t CaO

16

S Absorption Efficiency Correlation

• In model, S absorption by lime governed by S absorption efficiency correlation– Derived from data collected at 2001 mill study*

at Brazilian kraft mill• S absorption efficiency represents ability of

lime to absorb S in kiln

* “Effects of CNCG Burning on Lime Composition and SO2 Emissions from a Lime Kiln”, Tran, H.N., et al.

nKiltoInputSTotalLimewithnKilLeavingSEfficiencyAbsorptionS =

17

S Absorption Efficiency Correlation

* “Effects of CNCG Burning on Lime Composition and SO2 Emissions from a Lime Kiln”, Tran, H.N., et al.

0

200

400

600

800

1000

0 5 10 15 20Total S Input (kg S/t CaO)

SO2

Emis

sion

s (p

pm)

80

85

90

95

100

S A

bsor

ptio

n Ef

ficie

ncy

(%)

• S Absorption Efficiency decreases as S input to kiln increases

⇒ Increased SO2emissions

18

Impacts on Lime Kiln Operations

• Three main areas:– Lime Availability and Make-Up Requirements– Air Emissions– Ring Formation

19

Lime Availability and Make-Up Lime Requirements

• Fuel Composition impacts lime availability, which dictates make-up lime requirements

• High Sulphur Content Fuels– Absorption of S via sulphation ⇒ ↓ availability– Petcoke, Lignin

20

Effect of Petcoke Substitution on Lime Availability

88

90

92

94

0 20 40 60 80 100Petcoke Substitution (%TL)

Lim

e A

vaila

bilit

y (%

)

No S AbsorptionCADSIMComplete S AbsorptionModel

21

Makeup Lime Requirements

• CaSO4 reaction in slaker and causticizer:

CaSO4 + Na2CO3(aq) ↔ Na2SO4(aq) + CaCO3(s)

Note: Ca in CaSO4 ends up as CaCO3 (returned to kiln)

S Input of 1 kg/t CaO 0.34% Lime Availability⇒

22

Simple Sulphur Balance

1 mol CaO Basis, 0% petcoke substitution

1 mol CaO 1 mol CaCO3

Na2CO3 NaOH

LIME KILN

Causticizing

23

Simple Sulphur Balance

0.024 mol CaSO41 mol CaO

1.024 mol CaCO3

0.024 mol Na2SO4

LIME KILN

Causticizing

Fuel (100% Petcoke)0.024 mol S

1 mol CaO Basis, 100% petcoke substitution (6.5 wt%S), 100% S absorption in kiln, 100% SO4 to CO3 conversion in liquor

Na2CO3

NaOH

24

Makeup Lime Requirements

• Initial Transient Period– Makeup lime increase to compensate for Ca

converted to CaSO4

• Makeup will level off– Ca in CaSO4 returned to kiln as CaCO3 from

causticizing• Higher mud load to kiln

– More “non-available” Ca carried around in form of CaSO4

25

Air Emissions

• SO2 emissions– May increase with use of high S fuels (e.g.

petcoke, lignin)– Depends on:

• Total S input to kiln• Flue gas scrubber or ESP

26

Flue Gas SO2 Concentration vs. Petcoke Substitution

0

100

200

300

400

500

0 20 40 60 80 100

Petcoke Substitution (%TL)

SO2 C

once

ntra

tion

(ppm

)

• For Substitution < 40% – insignificant increase

SO2– Kilns with scrubbers:

SO2 emissions non-issue

• Kilns that burn >40% petcoke, burn CNCG and SOG with petcoke: – high SO2 emissions

inevitable (esp. for kilns without scrubbers)

SO2 Emissions

27

Fuel Substitution vs. SO2 Concentration

0

200

400

600

800

1000

0 20 40 60 80 100Substitution of Fuel Oil with Alternative Fuel (%TL)

SO2 C

once

ntra

tion

(ppm

)

0 wt%S2 wt%S

4 wt%S

6 wt%S

8 wt%S

10 wt%S

28

Ring Formation

• Formation: due to presence of “sticky”molten Na compounds in lime

• Growth: driven by recarbonation reactions• High S Fuel Burning: may aggravate ringing

since sulphation of CaO can harden ring deposits

29

Impact on Chemical Recovery Operations

• S absorption by lime leads to increased sulphidity

• Two Options:1. Operate at higher sulphidity2. Maintain same sulphidity and TTA

• Purge lime dust (CaSO4 enriched)• Purge RB precipitator dust (Na2SO4 enriched)• Na makeup equivalent to Na loss with purge

30

Sulphur Purge Points

RecoveryBoiler

Digester/Washing

Evaporators

SO2Fuel S Na2SO4

(deadload)

Na2S

Causticizing

Lime Kiln

Grits & Dregs

Precipitator Dust (Na2SO4), Stack (SO2)

Lime Dust (CaSO4)

CaSO4Na2SO4

*Also: Other Losses/Spills

31

How high will sulphidity go?

• Difficult to estimate sulphidity and Na makeup requirement

• Depends on factors incl.– S absorption efficiency of kiln– Kinetics of reaction between CaSO4 and

Na2CO3– Amount of grits, dregs, lime dust and RB

precipitator dust disposal– Amount of S loss thru SO2 and particulate

emissions from LK and RB stacks

32

• Impacts of High Sulphur Fuel Use may include:– Lower lime availability and increased make-up

lime requirements– Increased SO2 emissions exiting kiln– Ring hardening– Increased liquor sulphidity

• Effects are manageable, but must be taken into consideration

Summary