The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
Co-firing of shredder light fraction with waste and
sewage sludge
Marianne Gyllenhammar1, Kent Davidsson2*, Torbjörn Jonsson3
1
Stena Metall AB
Box 4088
S-400 40 Göteborg
Sweden
marianne.gyllenhammar@stenametall.se
2
3
SP Technical Research Institute of Sweden
Box 857
SE-501 15 Borås
Sweden
kent.davidsson@sp.se
tel. +46(0)105165796
fax. +46(0)33131979
Environmental Inorganic Chemistry, Department of Chemical and Biological Engineering, Chalmers
University of Technology
S-412 96
Göteborg
Sweden
torbjorn.jonsson@chalmers.se
* corresponding author
ABSTRACT
Shredder Light Fraction (SLF) is co-fired with normal waste fuel in a 20 MWth bubbling
fluidised bed boiler. The fuels in the three tests were waste, waste + 20% SLF (on energy
basis), and waste + 20% SLF + 4% sewage sludge. Deposit probes at 280 °C were
situated upstream of the convection pass and upstream of the economiser for 24 h.
Collected deposits were weighed and analysed for elements. Probes at 420 °C were also
situated upstream of the convection pass for 24 h in order to study the initial corrosion.
Addition of 20% SLF to ordinary waste increases the deposit growth by a factor of two,
and also increases the corrosion rate. The deposits contain more chlorine, and scanning
electron microscopy analyses of the interface between deposit and metal surface indicate
chlorine-induced corrosion.When sludge is supplied, the deposit growth and the corrosion
rate decreases to the same level as when ordinary waste is fired. The amount of chlorine
in deposits decreases. The results show that during the first 24 hours, co-firing of ≥20%
SLF and ordinary waste is likely to cause corrosion and deposit problems, but that
addition of 4% sewage sludge can mitigate these problems.
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The Swedish and Finnish National
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Keywords: fludized bed, alkali chloride, chlorine, sulphur, sulphation, corrosion
1. INTRODUCTION
Chlorine-rich and alkali-rich fuels, such as short rotation crops and waste, are liable to
form alkali chlorides which cause deposits and corrosion of heat exchanger surfaces in
combustion facilities. Shredder Light Fraction (SLF) is waste from the recovery of metal
scrap. It has an even higher chlorine content, and is therefore usually co-fired in small
fractions with waste.
Co-firing of chlorine-rich fuels with sewage sludge has been shown to counteract the
formation of alkali chlorides, deposits and corrosion.[1][2] Sludge comprises sulphur,
phosphorus, aluminium and a high amount of ash, all of which may have a positive
effect.[3][4] Therefore, it is interesting to study the co-firing of sludge and SLF, and its
effect on deposits and corrosion.
In the present study, SLF is co-fired with sewage sludge in a waste-fired plant. The
formation and composition of deposits are monitored, and corrosion products are
analysed. The aim is to study the effect of sludge on deposits and corrosion.
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The Swedish and Finnish National
Committees of the International Flame
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2. EXPERIMENTAL
2.1 The boiler
The boiler is the 20 MW bubbling fluidized bed (manufactured by Metso Power) of
Lidköping Värmeverk AB. It is schematically shown in Figure 1. During the present
tests, the load was 20 MW and the excess air ratio approximately 1.4. The measured
temperature in the bottom bed was 850-860 °C, and at the top of the furnace it was 830840 °C. The real temperature is 50-70 °C higher at the top. At positions A and B (Figure
1) it was 550-580 and 330-350 °C, respectively.
waste
sewage
sludge
B
2
4
1
A
3
5
Figure 1: Schematic picture of the bubbling fluidised bed boiler at Lidköping Värmeverk AB. 1:
furnace, 2: convection area, 3: economizer, 4: fuel dosing feeder and rotary vane feeder, 5:
flue gas cleaning device with hydrated lime and active carbon. A and B are the measuring
and sampling positions.
2.2 Fuels
Three types of fuels were used: waste, SLF and sewage sludge. Fuel/fuel blend analyses
are given in Table 1. Waste is the ordinary fuel for the boiler. It consists of 50%
household waste and 50% industrial waste. The SLF was produced in the fragmentation
plant at Stena Recycling in Halmstad, Sweden. It results from the handling of metal scrap
from cars, households and industry. It contains combustibles such as wood, plastics,
rubber and textiles, but also metals. Despite the ash content of 35-45%, the higher heating
value is 11-14 MJ/kg dry substance. The particle size in the present case was >15 mm.
Digested and dried sewage sludge was produced in the waste water treatment plant in
Visby, Sweden.
2.3 Experimental procedure
The three combustion tests (Ref, F1 and F2) differ by fuel mixture. Ref is the reference
test in which only waste was fired. In F1, 75% (on energy basis) waste, 21% SLF and 4%
sewage sludge were fired. In F2, 77% waste and 23% SLF were fired. Waste and SLF
were mixed and fed to the boiler in the normal manner (Figure 1). The dried sludge was
fed into the rotary vane feeder. Most sludges available are not dried. To simulate moist
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sludge, water was fed along with the sludge at a flow corresponding to a sludge moisture
of 70%. The three tests were separated in time to ensure that no memory effects would
occur. To approach steady-state conditions, the boiler was run for at least 48 h with the
fuel blend of interest. Thereafter, air-cooled deposit/corrosion probes fitted with steel
rings, were inserted. After 24 h, the probes were taken out and the rings were weighed
and analysed.
Table 1: Chemical analyses of the fuel/fuel blends.
Test:
Ref
F1
F2
waste waste+SLF+sludge waste+SLF
Fuel:
Moisture
%
27
31
29
Ash
%ds
21
27
25
Calorimetric heating value MJ/kg ds
18.8
18.4
18.4
Effective heating value
MJ/kg ds
17.5
17.2
17.2
S
Cl
F
C
H
N
O
Al
As
Ba
Ca
Cd
Co
Cr
Cu
Fe
K
Mg
Mn
Mo
Na
Ni
P
Pb
Sb
Si
Sn
Ti
Tl
V
Zn
ds = dry substance
% ds
% ds
% ds
% ds
% ds
% ds
% ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
mg/kg ds
0.57
0.42
0.01
44
5.8
0.95
27.3
0.36
0.58
0.02
43.2
5.6
1.1
21.8
0.26
0.55
0.03
42.6
5.6
1.2
25
10 600
9
400
29 300
4
7
54
400
5 400
3 600
2 900
200
2
6 000
13
1 000
210
350
39 100
9
2 000
4
10
590
14 600
6
400
32 600
4
10
67
5200
24 800
3 900
3 400
380
7
7 000
38
1 600
230
36
53 300
19
2 200
6
15
1 800
12 300
7
800
22 800
4
9
98
4000
18 100
4 000
3 000
300
8
7 800
43
1 100
350
35
55 300
32
1 600
5
17
2 700
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Research Foundation – IFRF
The deposit rings (position A and B in Figure 1) were of 310 material held at a surface
temperature of 280 °C. Chlorine and sulphur were analysed with ion chromatography,
and the metals with Ion Coupled Plasma-Optical Emission Spectrometry. Corrosion rings
(position A in Figure 1) of the material 16Mo3with a surface temperature of 420 °C were
investigated using gravimetry, Environmental Scanning Electron Microscope (SEM) with
an Energy Dispersive X-ray and X-ray Diffraction.
SO2 and HCl were measured in raw gas upstream of the bag house filter, and emission
control examination of clean gas was done in the stack.
The tests and methods are thoroughly explained elsewhere.[5]
3. RESULTS
3.1 In-flow ratios
The in-flow of elements in the tests are given in Table 2. Cl/(Na+K) is highest in F1,
which should result in the highest concentrations of alkali chlorides. However,
Cl/(2S+3P) is well below one, which means that S and P compete for alkali; especially
since they are in excess of alkali in Ref and F1. In F2, Cl/(2S+3P) and Ca/(S+1.5P) are
higher. Ca readily reacts with S and P, and thereby hinders their reaction with alkali.
Therefore, F2 is expected to result in the formation of more alkali chlorides than Ref and
F1.
Table 2: In-flow and In-flow ratios of elements.
Ref
F1
F2
mol/h
mol/h
mol/h
Na+K
1800
2000
2100
Cl
560
820
740
S
850
560
380
P
170
720
170
Ca
3700
4000
2700
Al
2000
2600
2100
mol/mol
mol/mol
mol/mol
Cl/(Na+K)
0.31
0.42
0.36
Cl/(2S+3P)
0.26
0.25
0.58
(Na+K)/(2S+3P)
0.82
0.60
1.6
Ca/(S+1.5P)
3.4
2.43
4.2
Cl/(K+Na+Pb/2)
0.31
0.41
0.36
Cl/(K+Na+Pb/2+Zn/2)
0.31
0.40
0.34
Al/(Na+K)
1.11
1.34
1.03
3.2 Deposits
Figure 2 shows the amount and composition of deposits at a surface temperature of 280
°C upstream and downstream of the convection pass. Addition of 23% SLF to waste
(compare F2 and Ref) results in twice as much deposits. The fractions of chlorine and
copper are higher. Addition of 4% sludge to waste and SLF (compare F1and F2) results
in half the amount of deposits and a lower fraction of chlorine. The amount and
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composition of the deposits in F1 are similar to those in Ref except for a higher content of
copper. The fraction of alkali is about the same in all the tests, but the Cl/S ratio is much
higher in F2 than in the other two tests.
Deposit growth (mol/m2h)
As expected from the inflow ratios (Table 2), the chlorine content of the deposits is
highest in F2. The increase of the Cl/S ratio is likely to be a result of the higher in-flow
ratios Cl/(2S+3P) and (Na+K)/(2S+3P) in F2 than in the other experiments. A possible
effect of the sludge is therefore that alkali chlorides react with sulphur, and form alkali
sulphates. Other mechanisms should not be neglected. Aluminium silicates, present in the
sludge ash, can bind alkali in relatively inert forms,[6] and it should be noted that
Al/(Na+K) is highest in F1 (Table 2). If these species leave the boiler as ash, it should
have a decreasing effect on alkali and chlorine in the deposit, but it does not fully explain
the shift of the Cl/S ratio in the deposit. Phosphorus should form alkali phosphates and
have an effect on alkali chlorides similar to that of sulphur. The in-flow of phosphorus is
much higher in F1 than in F2, and deposits and fly ash in F1 contain more phosphorus,
why it is likely that some of the phosphorus has formed alkali phosphates.
Ref
0.15
F1
F2
0.1
0.05
0
A
B
A
B
A
B
Zn
Pb
Cu
Na
K
Mg
Ca
Ba
Ti
Mn
Al
Si
P
sulphur
chloride
Figure 2: Deposit growth on the deposit probes at 280 °C upstream (A) and downstream (B) of the
convection pass.
3.3 Corrosion
Figure 3 shows cross-sections of the corrosion rings and the corrosion products formed
on the low-alloyed steel (16Mo3) rings upstream the convection pass. Addition of 23%
SLF to waste (compare F2 and Ref) results in more corrosive deposits due to the higher
concentration of Cl. This causes thicker corrosion products with more Cl at the metal
interface. Addition of 4% sewage sludge to waste and SLF (compare F1 and F2) results
in decreased corrosion rate and corrosion products similar to the those formed in the
reference case. No or very low contents of copper, lead and zinc are detected close to the
corrosion front, why these elements are expected not to be important in the initial (24 h)
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corrosion attack. A more detailed description of the corrosion results is reported
elsewhere.[7]
Figure 3: SEM/Backscattered electron images of polished cross-sections of deposits and corrosion
products formed on 16Mo3 in Ref, F1 and F2 at a surface temperature of 420ºC.[5]
3.4 Ash and content of emitted flue gas
The ash flows were not explicitly measured during the tests, but according to the fuel
analyses, the flow of ash into the boiler increased by 38%, when adding sludge and SLF
(compare F1 and Ref). When SLF was co-combusted with normal fuel the ash content in
the inflowing fuel mix increased 19% (compare F2 and Ref). This higher ash content in
the fuel increases the cost of ash handling.
The emission control examination showed that the emissions were within the limits of the
permit. The dry flue gas cleaning device used hydrated lime and active carbon to control
the emissions of SO2, HCl and dioxins.
4. CONCLUSIONS
Co-firing of ≥20% SLF and ordinary waste increases the total deposition rate, the
deposition of alkali chlorides, and the chlorine-induced corrosion compared with normal
waste during the first 24 h.
Addition of 4% sewage sludge to firing of waste and SLF reduces the total deposition
rate, the content of alkali chlorides in the deposits, and the corrosion rate.
The most important mechanism for the reduction of deposition and corrosion in the
present tests seems to be sulphation of alkali chlorides, but formation of alkali phosphates
also play a part.
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The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
Addition of sewage sludge gives rise to an increased flow of ash.
5. REFERENCES
[1] L.-E. Åmand, B. Leckner, D. Eskilsson, C. Tullin, Deposits on heat transfer tubes
during co-combustion of biofuels and sewage sludge, Fuel 85 (2006) 1313-1322.
[2] K.O. Davidsson, L.-E.Åmand, A.-L. Elled, B. Leckner, Effect of cofiring coal and
biofuel with sewage sludge on alkali problems in a circulating fluidized bed boiler,
Energy & Fuels 21 (2007) 3180–3188.
[3] K.O. Davidsson, L.-E. Åmand, B.-M. Steenari, A.-L. Elled, D. Eskilsson, B. Leckner,
Countermeasures against alkali-related problems during combustion of biomass in a
circulating fluidized bed boiler, Chemical Engineering Science 63 (2008) 5314-5329.
[4] P. Piotrowska, M. Zevenhoven, K. Davidsson, M. Hupa, L.-E. Åmand, V. Barišić, E.
Coda Zabetta, Fate of alkali metals and phosphorus of rapeseed cake in circulating
fluidized bed boiler Part 2: Cocombustion with coal, Energy & Fuels, 24 (2010)
4193–4205.
[5] M. Gyllenhammar, K. Davidsson, T. Jonsson, J. Pettersson, A. Victorén, H.
Andersson, C. Widén, Energiåtervinning av brännbar fraktion från fragmentering av
metallhaltigt avfall, Waste Refinery ISSN nr 164-4706 WR 08, Borås March 2010.
[6] K.O. Davidsson, B.-M. Steenari, D. Eskilsson, Kaolin addition during biomass
combustion in a 35 MW circulating fluidised bed boiler, Energy & Fuels 21 (2007)
1959-1966.
[7] T. Jonsson, J. Pettersson, K. Davidsson, L.-G. Johansson, J.-E. Svensson, Sewage
sludge as additive to reduce the initial fireside corrosion caused by combustion of
shredder residues in a waste-fire BFB boiler, in 9th Liége Conference on Materials for
Advanced Power Engineering, Liége, Belgium 27-29 September 2010.
6. ACKNOWLEDGEMENTS
This work was financed by Waste Refinery and is part of a collaboration project between
Stena Metall AB, Metso Power AB, High Temperature Corrosion Center at Chalmers
University of Technology, SP Technical Research Institute of Sweden and Lidköpings
Värmeverk AB.
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