Ash Forming Matter in Peat
- The Role of Iron
Maria Zevenhoven,
Jaakko Lehtovaara, Stefan Storholm,
Mikko Hupa
Peat in
Europe
One-third of the peat and
peat-topped soils in
Europe found in Finland,
More than a quarter found
in Sweden.
The remainder found in
Poland, the UK, Norway,
Germany, Ireland, Estonia,
Latvia, The Netherlands
and France.
Small areas of peat and
peat-topped soils also in
Lithuania, Hungary,
Denmark and the Czech
Republic.
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The distribution of peatland in Europe
L. Montanarella1, R.J.A. Jones1,3 and
R. Hiederer2
Mires and Peat, Volume 1 (2006),
Article 01, http://www.mires-andpeat.net,
2
ISSN 1819-754X
Peat in Finland
Peat energy today
7 % of total energy in
Finland.
The highest values
of humification
degree are located in
Finnish Lake area, in
Central Finland.
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Composition of peat
60
Data taken from ÅA database, min and max
values indicated,
#16 finnish peat, irish peat #1
50
% d.s.
40
irish
finnish
30
20
10
0
Ash
C
H
N
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O
4
Slagging caused by peat ash
Iron in Peat
Wall tube
Iron initiator of furnace wall
slagging
Start by an iron oxide layer
on wall tube surfaces.
When this layer has grown
thick enough, also the bulk fly
ash starts sticking
2 cm
Flue gas
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Slagg composition
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Wall tube
Iron initiates
slagging
Initial iron oxide
layer on wall
tube surfaces.
Flue gas
On this layer
silicate fly ash
starts sticking
Further
transformation to
glassy slag
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Analysis of Fe in Finnish peat
Standard fuel analysis
SEM/EDX (samples 1-8, peat and peat ash)
XRD (1,3,6,8, peat ash)
TOF-SIMS (1,3,6,8, peat)
ESCA (1,3,6,8, peat)
Sequential leaching in aqueous solutions: water,
ammonium acetate and hydrochloric acid (peat
1-8)
Sequential leaching with oxidative and reducing
agents (peat 9-15)
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Fe in Finnish peat
60000
50000
mg/kg ds
40000
30000
20000
10000
0
12
1
11
13
2
14
9
4
3
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10
5
7
8
6
15
10
Selective leaching in aqueous solutions
Total ash
- all major ash elements
H2 O
NH4 Ac
Water leachable
- alkali sulfates
Buffer solution leachable
- organically associated
/carbonates/chlorides
HCl
Acid leachable
- carbonates/ sulfates
Non-soluble rest
- silicates
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Finnish peat (sample 16)
70000
Rest fraction, analysed
Leached in HCl
Leached in Acetate
Leached in H2O
Untreated Fuel
60000
mg/kg
50000
40000
30000
20000
10000
0
Si
Al
Fe
Ti
Mn
Ca
Mg
P
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Na
K
S
Cl
12
Peat chemical fractionation
100
av H2O
av NH4Ac
av HCl
av rest
90
80
70
%
60
50
40
30
20
10
0
Si
Al
Fe
Ca
Mg
P
Na
K
S
Figure 5 Chemical fractionation results of Finnish peat (#22, 95% confidence limit)
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Summary
Analytical methods
Standard fuel analyses
Shows total concentration of Fe in fuel
Always needed
SEM (fuel, ash)
Shows distribution of Fe in fuel - Sensitive for Si particles
Ashing often needed to reveal ash forming matter hidden in fuel matrix
Gives sometimes valuable extra information, semi-quantitative method
XRD (ash)
Showed possible presence of reduced iron: FeO, Fe
Showed non crystalline Fe, probably organically associated
Concentration of Fe too low, fuel needs to be ashed.
ESCA (fuel)
Could not distinguish between Fe2+ and Fe3+
During sample preparation surface oxidises, destroying information on
reduced species
TOF-SIMS (fuel)
Similar to SEM
Shows association of Fe , but Fe seems not be on the surface
TOF-SIMS is a surface analytical tool only
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Leaching test to better
distiguish between different
forms of iron
Determination of forms of Fe
all major ash elements
Exchangeable + weak
acid soluble fraction
HAc
NH2OH·HCl
Analysis
Reducible fraction:
Inorganic Fe i.e. oxides
and hydroxides
Oxidizable fraction
(e,g,, metals associated
with organic matter and
sulfides)
H2O2
NH4Ac
Residual fraction,
silicates
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Distribution of Fe
45000
40000
35000
Fe-silicates
Fe2O3, Fe(OH)3
FeS and organic Fe
exchangable Fe
mg/kg
30000
25000
20000
15000
10000
5000
0
Peat 1
Peat 2
Peat 3
Peat 4
Peat 5
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Peat 6
Peat 7
Peat 8
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Organic/inorganic ratio for Fe
2.2
Organic Fe/Inorganic Fe
2.0
1.8
1.6
1.4
1.2
1.0
Peat 1
Peat 2
Peat 3
Peat 4
Peat 5
Peat 6
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Peat 7
Peat 8
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Summary
Chemical fractionation by Water+NH4AC+HCl
Very good for alkali induced fouling
Not suitable for Fe, no differentiation between organic and inorganic Fe
Fractionation by Redox solutions:
Three Fe groups to consider:
Organic= Fe2+ + exchangeable
Fe3+ = oxides/hydroxides
Fe-Silicates
Important
Total amount of organic Fe and inorganic Fe
Ratio of organic Fe/inorganic Fe
....
...
Fe(OH)x
Coarser particles of silicates
combustion
Fe silicate
Fines from organic Fe
Iron oxide
particles
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