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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