ASH FORMING MATTER IN PEAT
-THE ROLE OF IRONMARIA ZEVENHOVEN§, JAAKKO LEHTOVAARA¤, STEFAN STORHOLM†, MIKKO HUPA§
§
Process Chemistry Centre, Åbo Akademi University, Turku, Finland, mzevenho@abo.fi,
+358 2 2154718
¤
Vapo Oy, Jyväskylä, Finland
†
Katterno Group, Jakobstad, Finland
ABSTRACT
In Finland, some 9.6 million ha consist of peat land from which only 0,6% is used for
peat production. With respect to energy consumption peat comes on the 6th place with a
share of some 7.2% thus being placed behind oil, wood, coal, nuclear power and natural
gas. Although heated discussions have and are taking place in Europe on the
sustainability of peat production and its use for heat and power production, peat is
considered as a serious alternative as replacement of imported fossil fuels, especially in
areas were peat production takes place.
Peat may have both positive and negative effects on the combustion process depending
on the combustion technology chosen and the composition of the peat considered.
Therefore thorough analysis of peat composition and ash forming matter is important in
order to understand combustion behaviour.
Many sorts of Finnish peat are found to be very different when compared to peat
originating from other parts of Europe, and they may thus show different behaviour
when fired.
In an attempt to explain fouling and slagging, ash forming matter has been analysed
extensively with different methods. Chemical fractionation, i.e. stepwise leaching with
water, ammonium acetate and hydrochloric acid combined with thermodynamic studies
and SEM/EDX analysis of gently ashed fuels showed to be a useful technique for
prediction of alkali induced fouling.
For explanation and prediction of iron induced slagging, different methods have been
studied in an attempt to distinguish between different forms of iron, such as iron oxides,
organically associated iron and iron silicates, three forms of iron that each may play a
role in slagging in boilers. Sequential leaching using both oxidative and reductive
agents was found useful for determination of these forms of iron. Further research is
needed to explain which form of iron is responsible for initial slag formation in large
scale boilers.
Keywords: Iron, Ash, Slagging, Sequential leaching
INTRODUCTION
In Finland, some 9.6 million ha consist of peat land from which only 0,6% is excavated
for use as fuel. With respect to energy consumption peat comes on the 6th place with a
share of some 7.2% thus being placed behind oil, wood, coal, nuclear power and natural
gas. Although heated discussions have and are taking place in Europe on the
sustainability of peat production and its use for heat and power production, peat is
considered as a serious alternative as replacement of fossil fuels, especially in areas
were peat production takes place.
Peat may have both positive and negative effects on the combustion process dependent
on the combustion technology chosen and the composition of the peat considered.
Therefore thorough analysis of peat composition and ash forming matter is important in
order to understand combustion behaviour.
Many sorts of Finnish peat are found to be very different when compared to peat
originating from other parts of Europe, and they may thus show different behaviour
when fired.
Figure 1 Slag sample taken from the boiler wall. The slag seemed more sintered at the outside (above)
when compared to the side facing the boiler wall, (below) that seemed to contain a large amount of fine
powdered iron oxide.
Iron has shown to be the cause of severe slagging problems in firing West-Finnish peat
in a pf-boiler. Standard fuel analysis i.e determination of the total iron content in the
fuels burned could only partly explain the slagging in the boiler. It is believed that also
the association of iron in the fuels may play a significant role in the slagging process.
Hereto different analytical methods available at Åbo Akademi Universty have been
studied in an attempt to find an easy way of analysing iron in peat.
EXPERIMENTS
Peat samples were analysed focusing on ash forming matter especially iron.
Since some samples contained a high moisture content and the mean particle size was
larger then 5 mm these samples were dried and milled before further analysis was
carried out.
The solubility of iron in aqueous solutions at different pH
The solubility of iron in aqueous solutions at different pH was determined using a
sequential leaching method, i.e. chemical fractionation. Chemical fractionation is a
method based on selective leaching by water, ammonium acetate and hydrochloric acid
and has been used for characterization of all eight fuels. Increasingly aggressive
solvents, i.e. water (H2O), 1M ammonium acetate (NH4Ac) and 1M hydrochloric acid
(HCl) leach samples into a series of four fractions (including the unleached residue) for
analysis. The untreated samples, liquid fractions and the remaining solids were
analyzed [Baxter 1994, Benson and Holm 1995, Skrifvars et al. 1998, Zevenhoven et
al. 2000, 2001, 2001b, 2003, 2005].
All chemical analyses were carried out by an external laboratory. Dry matter was
determined at 105°C. Samples were dissolved in HNO3/H2O2 for determination of As,
Cd, Cu, Co, Hg, Ni, Pb, Sb, and Se. For determination of the other main ash and heavy
metals the samples were molten with LiBO2 and dissolved in HNO3. Analysis took
place either with ICP-AES or ICP-SFMS. Special care was taken to obtain reliable
analyses of Fe. (Analyses of other elements was seen as bonus)
SEM/EDS
In order to allow ash
forming matter to be seen
in SEM, fuels were ashed
in air in a laboratory
furnace at 500, 700 and
900°C respectively. The
ashing time was 15
minutes.
After ashing both untreated
fuel and ashed samples
were mounted on carbon
tape and the samples were
mounted in the SEM.
SEM/EDS analyses were
carried out wit 30, 250 and
5000x magnification. At
30x
magnification
an
overall area analysis was
Figure 2 Chemical fractionation method
performed. At 250x and
5000x point analyses were
performed. At 250x elemental maps were scanned for Al, C, Ca, Cl, Fe, K, Mg, Na, O,
P, Pb, S, Si, Zn. All analyses were calculated on a carbon free basis.
Determination of different forms of iron
Exchangeable + weak
acid soluble fraction
HAc
NH2OH·HCl
Reducible fraction:
Inorganic Fe i.e. oxides
and hydroxides
Analysis
Oxidizable fraction
(e,g,, metals associated
with organic matter and
sulfides)
H2O2
NH4Ac
Residual fraction,
silicates
Figure 3 Sequential leaching for determination of different forms of
iron
The Community Bureau
of Reference (BCR, now
Standards, Measurements
and Testing Program of
the
European
Commission) developed
a sequential extraction
program in 1987 to
harmonize the sequential
schemes
for
the
determination of metals
in soils and sediments.
[Kazi et al.2005, Ure et
al.1993, Wang et al.
2006]. So far, this
procedure
has
been
successfully applied to a
variety of sediments, sewage sludges and soil samples.
The sequential extraction procedure is briefly given in Figure 3. This scheme consists
of three leaching steps by using acetic acid, hydroxyl ammonium chloride and
ammonium acetate separately. Leached by the three steps, the metal species in the
sample are divided into four different fractions: An exchangeable and weak acid soluble
fraction, a reduced fraction, an oxidised fraction and a silicate fraction.
RESULTS AND DISCUSSION
Standard fuel analysis
shows that Peat 16 and 6
were peats with the highest
amount of iron ~5.5% wt
in fuel), whereas Peat 13
represented a peat sort low
in Fe (0.3%wt in fuel)
(Figure 4)
6
5
% ds
4
3
2
1
0
13
1
12
14
2
15
9
4
3
10
5
Figure 4 Content of iron in West-Finnish peat (#16)
7
8
6
16
Figure 5 shows the results
of chemical fractionation
of 22 Finnish peats. It
shows that the major part
of iron in the fuel dissolved
at low pH in HCl. Silicates,
potassium and sodium are
100
remaining mainly insoluble
after
the
leaching
procedure indicating that
they may be present as
alkali silicates.
av H2O
av NH4Ac
av HCl
av rest
90
80
70
%
60
Ca and Mg seem to be
more
organically
associated and are found
mainly ion exchangeable
with NH4Ac.
50
40
30
20
10
0
A substantial part of sulfur
is usually found nonFigure 5 Chemical fractionation results of Finnish peat (#22, 95%
soluble indicating that
confidence limit)
sulphur may be present
covalently bonded in peat.
The chemical fractionation results showed that the procedure can not distinguish
between different forms of iron in peat.
Si
Al
Fe
Ca
Mg
P
Na
K
S
As stated in the introduction combustion behaviour of peat with different iron content
showed different slagging behaviour not always proportional to the total iron content
and thus the form of iron present in the fuel and formed during combustion is believed
to play a significant role
MgO
Al2O3
SiO2
P2O5
SO3
K2O
CaO
Fe2O3
Na2O
Cl
MgO
Al2O3
SiO2
P2O5
SO3
K2O
CaO
Fe2O3
Na2O
Cl
4
5
6
7
4.62
18.25
9.88
3.79
5.03
7.91
18.63
28.65
2.52
0.34
3.55
16.78
19.38
3.91
3.60
7.12
14.02
28.66
1.94
0.39
4.18
17.75
5.65
4.03
4.64
4.37
15.99
41.76
0.95
0.34
4.78
15.72
4.15
4.00
4.94
4.45
19.61
41.04
0.63
0.30
14
15
16
17
3.70
13.87
46.62
3.06
2.09
0.25
10.61
18.69
0.31
0.00
7.25
16.56
20.80
4.19
3.26
0.28
16.74
29.69
0.63
0.13
5.59
18.59
14.49
3.61
1.66
0.23
14.82
40.56
0.36
0.00
6.51
22.09
4.74
4.26
4.06
0.21
16.02
41.25
0.18
0.03
Figure 6 Example of SEM/EDX analyses of peat ashed at 500°C (above) and 900°C (below)
SEM/EDX analyses showed that gentle ashing helped to reveal the inorganic
components without disturbing too much the place they were situated. At ashing
temperatures below 700°C alkali metals remain present in the ash, whereas the alkaline
metals will have volatilised at higher temperatures and will be lost for SEM analysis.
This problem does not appear for iron. Figure 6 shows that iron can be found in
different forms in the fuels, either as discrete “mineral” particles or distributed evenly
over the fuel indicating iron could be present in the organic matrix.
4.5
4
3.5
The oxidation state of iron
in eight peat fuels was
determined with help of the
reducing
agent
of
hydroxyl-amino-hydrochloride and the oxidising
agent peroxide in an acid
environment.
Fe-silicates
Fe2O3, Fe(OH)3
FeS and organic Fe
exchangable Fe
%wt ds
3
2.5
2
1.5
1
0.5
0
Peat 1
Peat 2
Peat 3
Peat 4
Peat 5
Peat 6
Peat 7
Peat 8
Figure 7 Different forms of iron in West-Finnish peat
2.2
Organic Fe/Inorganic Fe
2.0
Iron may be present in
different forms in all eight
peats (#1-8) subjected to
this leaching scheme. The
main part seems to be
either organic reduced iron
(Fe2+) or mineral Fe2O3 or
Fe(OH)3.
In the three peats with high
iron content more than
20% of total iron may be
present as Fe-silicates
giving
possible
slag
properties, which may lead
to formation of low
melting silicates.
1.8
1.6
1.4
1.2
1.0
Peat 1
Peat 2
Peat 3
Peat 4
Peat 5
Peat 6
Peat 7
Peat 8
Figure 8 Ratio of organic//inorganic iron in West-Finnish peat
Fe3+ may be released from
the fuel as oxides and
particles may hit the wall
of the boiler and remain
there.
Fe2+ may be release partly as reduced species, which usually have a lower melting point
than Fe3+ compounds. When still partly reduced when hitting the boiler walls these
particles could from a sticky deposit enabling other particles to stick to the wall as well.
CONCLUSIONS AND FUTURE RESEARCH NEEDED
Iron may be the initiator of slagging phenomena in peat fired boilers. This has
consequences for the way the boiler is operated. For good operation better
understanding of total iron contents and forms of iron present in the fuel was found to
be vital.
During these studies different analytical methods have been used to analyse iron in peat
in an attempt to find a fast and easy way to predict slagging risks in the boiler.
Standard fuel analysis showed total concentration of Fe in fuel and is useful to estimate
the risk of slag formation. This type of analysis is always needed.
SEM showed distribution of Fe in the fuel. Ashing is often needed to reveal ash forming
matter hidden in fuel matrix. Sometimes SEM analyses give valuable extra information.
Sequential leaching with aqueous solutions at different pH have shown in the past to be
very useful to understand deposit formation in heat exchanger areas when caused by
alkali metals. However this method was found not suitable for Fe since no
differentiation could be made between different forms of iron.
Standard fuel analysis i.e determination of the total iron content in the fuels burned
could only partly explain the slagging in the boiler. It is believed that also the
association of iron in the fuels may play a significant role in the slagging process.
Sequential leaching with oxidative and reducing solutions showed that iron may be
present as weakly associated and organically associated (Fe2+), as oxidised minerals
(Fe3+) or as iron silicates. These three different groups all could play a role in the
slagging encountered.
This means that, for determination of slagging risk factors knowledge of total Fe,
organic Fe, inorganic Fe, ratio organic Fe/inorganic Fe and Fe-silicates could be
helpful.
The results as described in this paper show how forms of iron in peat fuels can be
determined. Further studies are needed to study the reaction pathways of different iron
compounds under combustion conditions.
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