The Swedish and Finnish National
Committees of the International Flame
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Influence of O2 during Sulphation of KCl in a Biomass
Fired CFB Boiler
Håkan Kassman1, 2, *, Johannes Öhlin1, Jessica Bohwalli1, Lars-Erik Åmand1
1
Chalmers University of Technology
Department of Energy and Environment, Division of Energy Technology
Gothenburg
Sweden
hakanka@chalmers.se; jooh@chalmers.se; jessica.bohwalli@chalmers.se; lars-erik.amand@chalmers.se
2
Vattenfall Power Consultant AB
P.O. Box 1046
Nyköping
Sweden
hakan.kassman@vattenfall.com
* corresponding author
ABSTRACT
Sulphur/sulphate containing additives, such as elemental sulphur (S) and ammonium
sulphate ((NH4)2SO4), can be used for sulphation of the alkali chlorides (mainly KCl)
during biomass combustion. A more efficient sulphation of KCl is achieved for
ammonium sulphate compared to sulphur. The presence of gaseous SO3 is thus of greater
importance than that of SO2. The concentration of O2 and the presence of combustibles
could also have an impact on the sulphation efficiency when injecting ammonium
sulphate. This paper is based on results obtained during co-combustion of wood chips and
straw pellets in a 12 MW circulating fluidised bed (CFB) boiler. Ammonium sulphate
was injected at three different positions in the boiler and they were in the top of the
combustion chamber, in the cyclone inlet, and in the cyclone. The sulphation of KCl was
investigated at three air excess ratios (λ= 1.1, 1.2 and 1.4). Several measurement tools
including, IACM (on-line measurements of gaseous alkali chlorides), deposit probes
(chemical composition in deposits collected), and gas analysis were applied.
Keywords: Sulphation, KCl, In situ alkali chloride monitor (IACM), ammonium
sulphate, combustion of biomass,
1. INTRODUCTION
Biomass generally contains relatively high amounts of alkali (mainly potassium, K) and
in some biomass fuels, such as straw, the chlorine (Cl) content is also rather high. The
content of sulphur (S) is normally relatively low in biomass fuels. High levels of alkali
chlorides in the flue gas can cause enhanced deposit formation and high content of KCl in
deposits may cause accelerated superheater corrosion during biomass combustion.
Deposit formation and superheater corrosion can be reduced by co-combustion or by the
use of additives. Coal, peat and sludge are among the fuels, which can be used for co-
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combustion with biomass. Elemental sulphur or other sulphur/sulphate containing
additives can be used for the sulphation of alkali chlorides. Here the additive reacts with
KCl and converts it to a less corrosive alkali sulphate.
Both homogeneous (gas phase) and heterogeneous (liquid or solid phase) mechanisms
have been proposed for formation of alkali sulphates from alkali chlorides found in
deposits or in ash particles [1]. Presented below are the overall sulphation reaction, R1, as
well as reactions R2 and R3, which are of particular interest for sulphation of gaseous
KCl. The sulphation rate in the gas phase is limited by the presence of sulphur trioxide
(SO3), and the oxidation of SO2 to SO3 (R3) is the rate-limiting step for this
homogeneous mechanism [2, 3]. Meanwhile, the reactions between KCl and SO3 in gas
phase have only been investigated to a certain extent. Calculations concerning the
reactions of SO3 with the O/H radical pool were presented in [4]. These calculations
suggested that the oxidation of SO2 to SO3 involved recombination of SO2 with O and
OH radicals, and that the SO3 concentration could be limited by H radicals (R4 – R6).
(R1)
(R2)
(R3)
(R4)
(R5)
(R6)
2KCl + SO2 + H2O + ½ O2 → K2SO4 + 2HCl
2KCl + SO3 + H2O → K2SO4 + 2HCl
SO2 + ½ O2 ↔ SO3
SO2 + O2 ↔ SO3 + O
SO2 + OH ↔ SO3 + HO2
SO3 + H ↔ SO2 + OH
Two sulphur containing additives were evaluated in [5] for sulphation of gaseous KCl:
elemental sulphur (S) and ammonium sulphate ((NH4)2SO4). Ammonium sulphate
lowered the amount of gaseous KCl and also reduced the chlorine content in the deposits
significantly better than sulphur. Thus the presence of gaseous SO3 was of greater
importance than that of SO2 for sulphation of gaseous KCl. These results by Kassman et
al [5] support that sulphation of gaseous KCl takes place according to reaction R2.
The scope of this paper is improved knowledge concerning the influence of O2 and the
impact from combustibles during sulphation of gaseous KCl. The influence of O2 was
investigated at three air excess ratios (λ= 1.1, 1.2 and 1.4) during injection of ammonium
sulphate (AS) in a full-scale CFB boiler, which is mainly used for research purposes. The
impact of combustibles was investigated by injecting AS at three different positions in the
boiler.
2.
Experimental
2.1
Research boiler and operating conditions
The experiments were performed in the 12 MW CFB boiler at Chalmers University of
Technology (CTH) shown in Figure 1. This research boiler offers the possibility to
perform measurement campaigns in a full scale boiler, while maintaining control over
important operation parameters such as load, air supply and composition of the fuel mix.
The boiler has been described earlier in several publications including [5-10]. The
combustion chamber has a square cross-section of about 2.25 m2 and a height of 13.6 m.
Fuel is fed from a fuel chute (located at the front of the boiler) to the lower part of the
combustion chamber.The bed material is recirculated through the cyclone back to the
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combustion chamber, whereas the combustion gases enter the convection pass where the
gases are cooled down to 150 C before cleaning in a secondary cyclone and a bag house
filter.
14
13
20
7
O
O
19
18
O
17
12
O
10
8
O
O
11
23
22
21
1
6
5
9
2
o
o
3
4
16
15
Figure 1: The 12 MW CFB boiler. 1. furnace; 2. fuel chute; 3. air plenum; 4. primary air; 5. secondary air;
6. fuel feed and sand; 7. cyclone outlet; 8. primary cyclone; 9. particle seal; 10. secondary cyclone; 11: bag
house filter; 12. flue gas fan; 13. IACM (In-situ Alkali Chloride Monitor); 15. bed material; 16. ammonium
sulphate (AS); 17-19. injection of AS; 17. top of the combustion chamber; 18 cyclone inlet; 19. in the
cyclone; 20-23. measurement positions; 20. before the convection pass; 21. in the convection pass, 22. after
the convection pass; 23. before the stack.
Table 1. Operating parameters for Ref during each air excess ratio
Parameter
Ref (λ = 1.1)
Ref (λ = 1.2)
Load (MW)
6.5
6.1
Bed temperature (°C)
836
839
Temperature, top of furnace (°C)
882
875
Temperature, cyclone outlet (°C)
883
841
Temperature, after bag filter (°C)
165
162
Pressure drop in furnace (kPa)
6.2
6.3
Excess air ratio
1.12
1.22
Total air flow to combustor (kg/s)
2.71
2.71
Primary air/total air flow (%)
53.9
53.4
-3-
Ref (λ = 1.4)
5.4
868
876
818
162
6.2
1.36
2.71
54.4
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Measurements were carried out during three air excess ratios (λ= 1.1, 1.2 and 1.4) and
selected operating conditions for each air excess ratio are presented in Table 1. Silica
sand (dp=0.3 mm) was used as bed material. The fuel properties are presented in Table 2.
The base fuel was wood chips and straw pellets (made from wheat straw and
manufactured in Köge, Denmark) were used as additional fuel to increase the level of
gaseous KCl with a ratio of about 20% of the energy input to the boiler.
Table 2. Fuel properties
Straw pellets
Proximate analysis
Water (wt-%, raw)
Ash (wt -%, dry)
Combustibles (wt -%, dry)
Volatiles (wt -%, daf)
Ultimate analysis (wt-%, daf)
C
H
O
S
N
Cl
Ash analysis (g/kg dry ash)
K
Na
Al
Si
Fe
Ca
Mg
P
Ti
Ba
Lower heating value (MJ/kg)
H, daf
H, raw
daf = dry and ash free, raw = as received
Wood chips
6.3
5.1
94.9
80.1
40.5
0.9
99.1
81.7
49.4
6.2
43.5
0.10
0.58
0.29
50.0
6.0
43.7
0.01
0.15
0.01
139
3.9
1.9
250
1.8
65
14
11
0.1
0.7
128
6.6
5.4
31.6
4.9
234
28
15
0.3
1.7
18.4
16.2
18.7
10.1
2.2
Experimental procedure
Ammonium sulphate (AS) was injected into the boiler according to two different
experimental procedures. The first one was a so-called transient test which was carried
out for each excess air ratio according to Table 3. The purpose with the transient test was
to investigate if the presence of combustibles (i.e radicals) could have an impact on the
sulphation efficiency when injecting AS. Increasing amounts of ammonium sulphate
were injected in a sequence. Three different positions in the boiler were selected. They
were injection in the top of the combustion chamber (17), in the cyclone inlet (18), and in
the cyclone (19). A typical sequence consisted of a Ref, AS1, AS2; AS3, AS4, AS5, AS6
and each of them was performed during 25 minutes. Table 3 shows which test cases were
included for excess air ratio λ= 1.2.
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The purpose with second experimental procedure was to investigate the influence of O2
during sulphation of gaseous KCl. Ammonium sulphate was injected into the cyclone at
three different air excess ratios (λ= 1.1, 1.2 and 1.4). Each measurement in the Test plan
(Table 4) was carried out during three hours and several measurement tools including,
IACM, deposit probe measurements, and conventional gas analysis were applied. The
deposit measurements were carried for a reference case and two different flows of
ammonium sulphate (low flow = ASL, high flow = ASH). The selected flows of AS were
7.7 l/h (ASL) and 15.5 l/h (ASH).
Ammonium sulphate ((NH4)2SO4) is a part of the ChlorOut concept. It consists of IACM
[11], an instrument for on-line measurements of gaseous alkali chlorides, and a sulphatecontaining additive that converts alkali chlorides to less corrosive alkali sulphates [12].
The additive is often (NH4)2SO4, and, therefore, a significant NOx reduction is also
achieved parallel to the sulphation of alkali chlorides [7].
Table 3. Injection points and flow of AS during the transient test (excess air ratio λ= 1.2)
Test case and flow /
Top of combustion Cyclone inlet
In the cyclone
Injection point
chamber (17)
(18)
(19)
Ref, Flow of AS = 0 l/h
x
x
x
AS1, Flow of AS = 5 l/h
-a
x
x
AS2, Flow of AS = 7.5 l/h
-a
x
x
AS3, Flow of AS = 10 l/h
x
x
x
AS4, Flow of AS = 15 l/h
x
x
x
AS5, Flow of AS = 20 l/h
x
x
x
AS6, Flow of AS = 30 l/h
x
x
-a
a = No test was included in the sequence at this flow.
Table 4. Test plan – influence of O2 and deposit measurements.
Parameter/Test case
Reference case
Ammonium sulphate low
(Ref)
(ASL) 7.7 l/h
Excess air ratio λ= 1.1 Ref-1.1
ASL-1.1
Excess air ratio λ= 1.2 Ref-1.2
ASL-1.2
Excess air ratio λ= 1.4 Ref-1.4
ASL-1.4
Ammonium sulphate high
(ASH) 15.5 l/h
ASH-1.1
ASH-1.2
ASH-1.4
2.3
Measurement equipment
A so-called IACM (In-situ Alkali Chloride Monitor) located at (13) in Figure 1 measured
the alkali chlorides in the gas phase [11, 13]. IACM measures the sum of the KCl (g) and
NaCl (g) concentrations on-line but is unable to distinguish between these two species.
The result is expressed as KCl, which is the dominating gaseous alkali specie at
temperatures prevailing in a CFB boiler during biomass combustion. IACM also
measures SO2 simultaneously. A schematic view of IACM is shown in Figure 2. Light
from a xenon lamp is sent across the furnace or flue gas channel (measurement path). The
light, which arrives at the receiver, is analysed by a spectrometer. The measuring
principle is based on measurement of molecular absorption at characteristic wavelengths
in the Ultra Violet (UV) - visible region (VIS). The evaluation is made by means of
Differential Optical Absorption Spectroscopy (DOAS). The detection limit at a
measuring length of 5 metres is 1 ppm for KCl and NaCl and 4 ppm for SO2 respectively.
IACM has been used in the present boiler in several previous projects related to alkali
chloride issues [5-10].
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Measurement
path
Sender
UV-light
Receiver
Spectrometer
Fan
Hot
flue
gas
Fan
Computer
Figure 2: Schematic view of an IACM installation
Flue gas was also extracted through a heated probe and heated sampling lines to a FTIR
(Fourier Transform Infra-Red) analyser for the determination of HCl, SO2, N2O, NO,
NO2 and NH3 on hot wet flue gases and further to on-line IR-VIS instruments measuring
CO, SO2 and N2O and a paramagnetic analyser for O2 on cold dry gases. A
chemiluminescence analyser was used (in connection to the cold system) for the
measurement of NO. Gas concentrations were measured in locations before the
convection path (20), after the convection path (22) and before the stack (23).
The deposit measurements were carried out in (20) using a temperature controlled deposit
probe. Deposits were collected at a ring temperature of 500°C after 3 hours’ exposure on
steel rings made of Sanicro 28, a high-alloyed Fe based steel. The deposits were analysed
by wet chemistry (ICP-OES and IC). The chemical analysis by ICP-OES and IC was
made on all the collected deposits on the ring.
3.
Results
3.1
The transient test – injection at different positions
Increasing amounts of ammonium sulphate (AS) were injected in a sequence during the
transient test. AS were injected in three different positions at excess air ratio λ= 1.2. The
different positions were injection in the top of the combustion chamber (17), in the
cyclone inlet (18), and in the cyclone (19). A typical sequence consisted of a Ref, AS1,
AS2; AS3, AS4, AS5, AS6 and additional information concerning the transient test can
be found in Table 3. The level of gaseous KCl in Figure 3 was approximately 45 ppm
during each of the reference cases (Ref) without injection of ammonium sulphate. The
level of gaseous KCl was reduced to ~ 20 ppm during test case AS1 (5l/h) when injecting
AS to the cyclone. It required a significantly greater flow of AS (15l/h) to obtain a similar
reduction in the cyclone inlet. The level of gaseous KCl was only lowered to
approximately 30 ppm during the highest flow of AS (30l/h) when injecting in the top of
the combustion chamber.
The transient test revealed that the position had a great impact on the sulphation
efficiency for gaseous KCl. The injection point in the top of the combustion chamber is
characterised by a higher concentration of combustibles. This indicates that the
concentration of SO3 was limited by the presence of H radicals [4]. Consequently, SO3
was partly consumed according to R6, instead of sulphation of KCl according to R2.
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50
Inlet Cyclone
Cyclone
40
KCl-IACM (ppm)
Top Comb Chamber
30
20
10
0
Ref
AS-1
AS-2
AS-3
AS-4
AS-5
AS-6
Figure 3: Concentration of KCl during the transient test. AS were injected at excess air ratio λ= 1.2. The
positions were injection in the top of the combustion chamber (17), in the cyclone inlet (18), and in the
cyclone (19).
3.2
Influence of O2 – Injection during different air excess ratios
Ammonium sulphate was injected into the cyclone (19) at different air excess ratios in
order to investigate the influence of O2 during sulphation of KCl. The air excess ratios
were λ = 1.1, 1.2 and 1.4. The operating parameters for each air excess ratio are presented
in Table 1 and the different test cases are presented more in detail in Table 4. The test
plan included a reference case (Ref) and two different flows of ammonium sulphate (ASL
and ASH) for each air excess ratio. ASL and ASH corresponded to an injection of 7.7 and
15.5 l/h of ammonium sulphate respectively.
60
λ =1.1
λ =1.2
50
λ =1.4
KCl (ppm)
40
30
20
10
0
Ref
ASL
ASH
Figure 4: Concentration of KCl during injection of ammonium sulphate in the cyclone at three different air
excess air ratios (λ = 1.1, 1.2, 1.4).
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The level of gaseous KCl in Figure 4 was approximately 50 ppm during each of the
reference cases (Ref) without injection of ammonium sulphate. KCl was reduced to less
than 20 ppm at a λ of 1.4 and somewhat above 20 ppm at a λ of 1.2 during test case ASL.
The reduction of KCl was less efficient during the lowest air excess ratio (λ = 1.1). It
required twice the flow of AS (ASH) to obtain a reduction similar to ASL for λ = 1.2.
100
λ =1.1
λ =1.2
SO2 (ppm)
80
λ =1.4
60
40
20
0
Ref
ASL
ASH
Figure 5: Concentration of SO2 during injection of ammonium sulphate in the cyclone at three different
excess air ratios (λ = 1.1, 1.2, 1.4). SO2 was measured after the convective pass at (22).
150
λ =1.1
λ =1.2
120
CO (ppm)
λ =1.4
90
60
30
0
Ref
ASL
ASH
Figure 6: Concentration of CO during injection of ammonium sulphate in the cyclone at three different
excess air ratios (λ = 1.1, 1.2, 1.4). CO was measured before the stack at (23).
The concentration of O2 had an impact on the sulphation efficiency when injecting
ammonium sulphate in the cyclone. This could possibly be explained by the formation of
SO2 from SO3 according to R3 during lower air excess ratios or the presence of
combustibles. Figure 5 shows the concentration of SO2 measured after the convective
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pass at (22). The results for SO2 were somewhat contradictory since the highest
concentrations were found during the highest and lowest air excess ratios. This effect
could possibly be a sampling artefact and it needs to be further investigated.
Figure 6 shows the concentration of CO in the stack (23). Similar trends were observed
during the air excess ratios 1.2 and 1.4. Injection of ammonium sulphate resulted in a
minor increase of CO at very low levels and this effect has previously been discussed in
for instance [7]. The OH radicals needed for the final step of the CO oxidation are instead
consumed for the formation of NH2 radicals from NH3. These results are only relevant at
low levels of CO. The trend during the lowest air excess ratio was the opposite. CO was
above 100 ppm during Ref indicating the presence of combustibles and it decreased
during injection of AS. The lowest level of CO was actually achieved during the lowest
air excess ratio at ASH. This effect has previously been described in [14], in which
ammonium sulphate was used to both lower NO and CO during combustion of biomass
in a boiler with high emissions of CO.
Figure 7 shows the concentration of NO during injection of ammonium sulphate in the
cyclone at the different excess air ratios. The formation of NO was strongly favoured by
an increasing air excess ratio. The reduction of NO is, however, significantly better at
higher air excess ratios. This results in a similar final NO concentration at ASH although
the concentration for λ = 1.4 was almost twice the one for λ = 1.1 during Ref. Further
aspects on ammonium sulphate as an additive for NO reduction in comparison with
ammonia and urea are treated in [7].
150
λ =1.1
λ =1.2
120
NO (ppm)
λ =1.4
90
60
30
0
Ref
ASL
ASH
Figure 7: Concentration of NO during injection of ammonium sulphate in the cyclone at three different
excess air ratios (λ = 1.1, 1.2, 1.4). NO was measured before the stack at (23).
3.3
Concentration in the deposits during different air excess ratios
Deposit measurements were also carried out for the test cases at different air excess ratios
treated in 3.2. Consequently, the test plan consisted of a reference case (Ref) and two
different flows of ammonium sulphate (ASL and ASH) for each air excess ratio. Figure 8
shows the composition in deposits from the whole ring analysed by ICP-OES and IC. In
general, the deposit growth rate was greater during lower excess air ratios and injection of
ammonium sulphate lowered the growth rate somewhat. The main elements in the
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deposits were sulphur (S), chlorine (Cl), potassium (K), calcium (Ca) and silicon (Si). Ca
and Si are not further discussed since they were not affected due to injection of
ammonium sulphate or due to different excess air ratios. Significantly more chlorine and
less sulphur were found in the deposits in the reference cases at all air ratios.
3000
Si
2500
Deposit (mg/h*m2)
P
Mn
2000
Mg
Al
1500
Ca
Na
1000
K
Cl
500
S
0
Ref-1.1 ASL-1.1 ASH-1.1 Ref-1.2 ASL-1.2 ASH-1.2 Ref-1.4 ASL-1.4 ASH-1.4
Figure 8: Composition of deposits given as elements during injection of ammonium sulphate in the
cyclone at three different excess air ratios (λ = 1.1, 1.2, 1.4).
25
λ = 1.1
λ = 1.2
Cl (mol %)
20
λ = 1.4
15
10
5
0
Ref
ASL
ASH
Figure 9: Mole % of chlorine (Cl) in the deposits during injection of ammonium sulphate in the cyclone at
three different excess air ratios (λ = 1.1, 1.2, 1.4).
The mole-% of chlorine during Ref, ASL and ASH are shown in Figure 9. The highest
mole-% of chlorine was found in the deposits without injection of ammonium sulphate.
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Significant amounts of chlorine were also found during the lowest flow of ammonium
sulphate at air excess ratio λ= 1.1. Nevertheless, only low amounts of chlorine remained
during the highest flow at λ= 1.1. There was no chlorine detected in the deposits even at
the lowest flow of ammonium sulphate (ASL) during the highest excess air ratio (λ =
1.4).
4.
Conclusions
The selected position in the boiler had an impact on the sulphation efficiency when
injecting ammonium sulphate at air excess ratio λ= 1.2. Less amounts of gaseous KCl
were reduced in the top of the combustion chamber due to the presence of combustibles
(i.e. H-radicals). The air excess ratios had an impact on the sulphation efficiency when
injecting ammonium sulphate in the cyclone. Less gaseous KCl were reduced during air
excess ratio λ= 1.1 compared to excess ratios λ= 1.2 and 1.4, respectively. The highest
mole-% of chlorine was found in the deposits without injection of ammonium sulphate.
Significant amounts of chlorine were also found during the lowest flow of ammonium
sulphate at air excess ratio λ= 1.1. Nevertheless, only low amounts of chlorine remained
during the highest flow at λ= 1.1.
The reduction of gaseous KCl is, consequently, less efficient in the presence of
combustibles and during low air excess ratios when injection ammonium sulphate.
5.
Acknowledgements
The main financial support for this work was provided by the Swedish Energy
Administration. The additional support from Vattenfall´s Thermal Technology
Programme is also greatly appreciated. Further the authors acknowledge Akademiska
Hus AB for maintaining and operating the boiler.
6.
References
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emissions, Proceedings of the Combustion Institute 31 (2007) 77-98.
[2] S. Jimenez, J. Ballester, Formation of alkali sulphate aerosols in biomass combustion,
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[3] P. Glarborg, P. Marshall, Mechanism and modeling of the formation of gaseous alkali
sulfates, Combust. Flame 141 (2005) 22-39.
[4] L. Hindiyarti, P. Glarborg, P. Marshall, Reactions of SO3 with the O/H radical pool
under combustion conditions, J. Phys. Chem. A 111 (2007) 3984-3991.
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chlorides and deposits during Co-combustion of coal and biomass, 19th International
Conference on Fluidized Bed Combustion, Vienna, Austria, 2006.
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[7] H. Kassman, M. Holmgren, E. Edvardsson, L. E. Åmand, J. Öhlin, Nitrogen
containing additives for simultaneous reduction of KCl and NOx during biomass
combustion in a CFB boiler, 9th International Conference on Circulating Fluidized Beds,
Hamburg, Germany, 2008.
[8] K. O. Davidsson, L. E. Amand, B. M. Steenari, A. L. Elled, D. Eskilsson, B. Leckner,
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circulating fluidized bed boiler, Chem. Eng. Sci. 63 (2008) 5314-5329.
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when co-fired with high potassium fuels, Biomass Bioenergy 34 (2010) 1546-1554.
[10] H. Kassman, J. Bohwalli, J. Pettersson, L.-E. Åmand, Ammonium Sulphate and CoCombustion with Peat - Two Strategies to Reduce Gaseous KCl and Chlorine in Deposits
during Biomass Combustion, Impact of Fuel Quality, Saariselkä, Finland, 2010.
[11] European Patent EP 1221036 (2006).
[12] European Patent EP 1354167 (2006).
[13] C. Forsberg, M. Broström, R. Backman, E. Edvardsson, S. Badiei, M. Berg, H.
Kassman, Principle, calibration, and application of the in situ alkali chloride monitor,
Rev. Sci. Instrum. 80 (2009).
[14] H. Kassman, C. Andersson (Forsberg), J. Carlsson, U. Björklund, B. Strömberg,
Decreased emissions of CO and NOx by injection of ammonium sulphate into the
combustion chamber, Värmeforsk (Ed.) Värmeforsk report No 908 (Summary in
English), 2005.
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