REACTIONS OF POTASSIUM CHLORIDE WITH CHROMIUM AS
A FIRST STEP TOWARDS HIGH TEMPERATURE CORROSION
IN BIOMASS COMBUSTION
LEHMUSTO, J., YRJAS, P. AND HUPA, M.
Åbo Akademi University, Laboratory of Inorganic Chemistry
Piispankatu 8, FIN-20500 Turku, Finland
E-mail: juho.lehmusto@abo.fi and telephone: +358 2 215 4038
ABSTRACT
Reactions between potassium chloride (KCl) and chromium oxide (Cr2O3) as well as
pure chromium (Cr) have been studied as model reactions for high temperature
corrosion. The compounds have been used as powders because of their big area-to-mass
ratio, which results in clearer signals in thermogravimetric measurements. The changes
in mass and heat of reaction were measured by means of differential thermal analysis
and thermal gravimetry (DTA/TG). The runs were performed mainly in synthetic air,
but also nitrogen was used. After the DTA/TG runs the samples were analyzed with a
scanning electron microscope (SEM).
Chromium oxide seems to be inert in the used conditions, but pure chromium reacted
with potassium chloride forming chromium oxide as product. In some runs, a yellow
intermediate, probably potassium chromate (K2CrO4) was detected. The reaction needs
both oxygen and potassium chloride to proceed, but already a tiny amount of potassium
chloride is enough to maintain the oxidation of chromium.
Keywords: High temperature corrosion, pure chromium, biomass combustion.
INTRODUCTION
Recovery of energy from biomass and various waste–derived fuels by combustion has
become important due to reduction of detrimental CO2 emissions. Biomass does,
however, release significant amounts of chlorine and alkali metals, as e.g. HCl(g),
KCl(g), KOH(g) and NaCl(g), into the gas phase during combustion. These compounds
may cause deposits, which interfere with operation and eventually may lead to corrosion
and/or blockage of the gas path. Due to severe corrosion caused by these compounds,
steam temperatures in biomass combustion are kept at significantly lower levels and,
consequently, the power production efficiency is lower than in boilers fired with
conventional fuels such as coal. In addition, chlorine bound to the chlorides mentioned
above may produce unacceptably high emissions of HCl and dioxins. HCl can, as well
as released alkali species, form aerosols and condense during cooling in the flue gas.
To prevent and diminish the problems mentioned above, better and more detailed
knowledge of the reactions of potassium chloride during combustion is needed. It has
been suggested that the reaction between solid potassium chloride and chromium oxide
is the one responsible for starting the complex series of corrosion reactions (Eq. 1)
(Pettersson, 2005). If the conditions are dry enough, the reaction occurs with oxygen
(Eq. 2) (Li, 2004). Chromium oxide forms a protective layer on the surface of stainless
steels used for furnace tubes in power plants, but the reaction with potassium chloride
may result in breaking of this layer followed by rapid oxidation of the steel. Reactions
of solid potassium chloride with chromium usually start to have significance at
temperatures between 400 and 600 C, even though the melting point of potassium
chloride is 772 C.
2KCl(s) + Cr2O3(s) + H2O(g)
4KCl(s) + Cr2O3(s) + 5/2O2(g)
K2CrO4(s) + 2HCl(g)
(1)
2K2CrO4(s) + Cl2(g)
(2)
It has been suggested that once the protective oxide layer is destroyed, the corrosion
reaction proceeds with the reaction between metallic chromium and potassium chloride
(Spiegel, 1999). However, the detailed reaction mechanism is still ambiguous; one
proposition includes the so called “chlorine cycle”, where the chloride reacts with
chromium without being consumed (Eqs. 3–6) (Shinata, 1987). Eq. 6 is the net reaction
of the three previous reactions. Another proposition for the reaction mechanism
contains the idea of chloride ions rather than molecular chlorine (Pettersson, 2005).
4Cr + 12 KCl
4CrCl3 + 12K
(3)
4CrCl3 + 3O2
2Cr2O3 + 6Cl2
(4)
6Cl2 + 12K
12KCl
(5)
4Cr + 3O2
2Cr2O3
(6)
To get more detailed information about the reactions and their dependencies on
conditions, such as temperature; the amount of potassium chloride and the composition
of the gas atmosphere, studies with pure substances are needed. In this study
experimental work with model substances has been carried out to shed more light on the
reaction mechanisms relevant to corrosion problems during combustion of biomass and
waste–derived fuels. Later also authentic superheater tube materials and their reactions
with potassium chloride will be studied.
EXPERIMENTAL
Pure Cr, Cr2O3 and KCl were used in the reaction mixtures for powder samples as well
as for Cr-granule samples. KCl was ground with a ball mill prior to the mixing, however
the powders were not sieved to any particular particle size. The runs were performed
using differential thermal analysis and thermal gravimetry (DTA/TG). The sample was
heated at a controlled heating rate to the wanted temperature and the changes in mass
and heat of reaction were observed as a function temperature and/or time. During most
of the runs synthetic air was used, but also pure nitrogen as well as nitrogen mixed with
carbon monoxide was used. After the DTA-TG runs the samples were studied and
analyzed with a scanning electron microscope equipped with an energy dispersive x-ray
analyzer (SEM/EDXA).
RESULTS
The DTA/TG runs were started with a mixture of pure chromium oxide and potassium
chloride, but no changes in mass and no endothermic or exothermic peaks were
observed under the test conditions. The smooth peak, which can be seen in every
DTA/TG-curve at around 200 C, is not a real reaction peak, but just a baseline drift of
the equipment. After this the chromium oxide was replaced with metallic chromium
powder. In both cases the samples were heated up to 700°C in synthetic air with a
heating rate of 2°Cmin-1. Potassium chloride started to react with chromium
approximately at 550 C and the mass of the sample started to increase, first slower and
then rapider, which indicates the formation of chromium oxide (Fig. 1).
KCl(s) + Cr 2O3(s) in synthetic air
-1
o
120
2,0
o
c
2 Cmin to 700 C
Temperature difference / C
Relative mass / %
130
1,5
DTA
110
1,0
TG
100
0,5
90
Exo
0,0
Endo
80
-0,5
100
200
300
400
500
600
o
Relative mass / %
130
KCl(s) + Cr(s) in synthetic air
c
-1
o
2 Cmin to 700 C
2,0
120
1,5
110
DTA
1,0
TG
100
0,5
90
o
a)
Temperature difference / C
Temperature / C
0,0
Exo
80
Endo
100
200
300
400
500
-0,5
600
o
b)
Temperature / C
Figure 1. DTA/TG-curves for KCl/Cr2O3 (a) and KCl/Cr (b) heated up to 700°C in synthetic air. The
smooth peak at around 200 C results from baseline drift.
The importance of oxygen for the reaction was tested by replacing the synthetic air with
nitrogen gas (N2). In both cases the heating program was the same; 2°Cmin-1 up to
700°C (Fig. 2).
KCl(s) + Cr(s) in N2
-1
o
2,0
120
o
c
2 Cmin to 700 C
Temperature difference / C
Relative mass / %
130
1,5
DTA
110
1,0
100
0,5
TG
90
Exo
0,0
Endo
80
-0,5
100
200
300
400
500
600
o
Temperature / C
Relative mass / %
KCl(s) + Cr(s) in synthetic air
c
-1
o
2 Cmin to 700 C
2,0
120
1,5
110
DTA
1,0
TG
100
0,5
90
o
130
Temperature difference / C
a)
0,0
Exo
80
Endo
100
200
300
400
500
-0,5
600
o
b)
Temperature / C
Figure 2. DTA/TG-curves for KCl/Cr heated up to 700°C in N2 (a) and in synthetic air (b). The smooth
peak at around 200 C results from baseline drift.
The product of the reaction was identified to be chromium oxide, Cr2O3. The
identification was done by comparing the elemental spot analyses of the product and
pure chromium oxide. The SEM images revealed that the structure of the formed oxide
was highly porous, consisting of small oxide particles, which are very loosely attached
to each other (Fig. 3). In some samples a yellow color was noted and it was indirectly
identified as potassium chromate, K2CrO4 (Fig. 4). The identification was done with
both elemental line analysis and elemental mapping: in some areas potassium,
chromium and oxygen were found, but no chlorine.
a)
b)
c)
Figure 3. SEM images from an untreated Cr-particle (a), a reacted Cr-particle (b) and formed Cr 2O3 (c).
a)
Cr
Cl
O
K
b)
Figure 4. An optical microscope image of K2CrO4 (a) and an elemental analysis from a KCl/Cr sample
heated to 700°C (b). The clear crystal in (a) is unreacted KCl.
The role of potassium chloride was investigated by using Cr-granules in two runs, the
other one without KCl and the other one with KCl. In the sample with KCl the amount
of potassium chloride was 0,6 mass-% of the total sample. In both runs the samples
were first heated 10°Cmin-1 to 400°C, then 2°Cmin-1 up to 700°C, where they were held
for 240 minutes (Figs. 5 and 6).
a)
b)
c)
Figure 5. Optical microscope images of Cr-granule with one KCl crystal (circled) before the DTA/TG
run (a), Cr-granule without KCl (b) and Cr-granule with KCl after the DTA/TG run (c).
Cr-granula in synthetic air
o
-1
o
o
-1
o
o
10 Cmin to 400 C, 2 Cmin to 700 C, 240 min @ 700 C
o
700 C
Relative mass / %
o
Temperature difference / C
130
2
125
T
120
1
115
DTA
0
110
Exo
105
Endo
-1
TG
100
40
80
120
160
200
240
280
320
360
400
Time / min
a)
Cr-granula + KCl-crystal (0,6 mass-%) in synthetic air
o
-1
o
o
-1
o
o
10 Cmin to 400 C, 2 Cmin to 700 C, 240 min @ 700 C
o
700 C
Relative mass / %
o
Temperature difference / C
130
2
125
T
120
1
115
DTA
0
110
105
-1
Exo
TG
100
Endo
40
80
120
160
200
240
280
320
360
400
Time / min
b)
Figure 6. DTA/TG-curves for Cr-granule without (a) and with (b) potassium chloride. The peaks before
60 minutes result from baseline drift.
CONCLUSIONS
In this work the reaction mechanisms between both pure chromium oxide and
potassium chloride as well as between pure chromium and potassium chloride were
studied. Pure compounds were used as model substances for the high temperature
corrosion reactions, which occur on the surfaces of the superheater tubes in combustion
power plants. The runs were performed using differential thermal analysis and thermal
gravimetry (DTA/TG). During most of the runs synthetic air was used, but also pure
nitrogen as well as nitrogen mixed with carbon monoxide was used. After the DTA-TG
runs the samples were studied and analyzed with a scanning electron microscope
equipped with an energy dispersive x-ray analyzer (SEM/EDXA).
Even though the studies on the detailed reaction mechanism of this corrosion reaction
have just started, already some conclusions can be drawn from the results presented
earlier in this paper.
Chromium oxide seems to be inert in the used conditions.
The reaction needs both potassium chloride and oxygen to proceed.
Although potassium chloride is essential, already a tiny amount is sufficient.
The reaction proceeds at least via one solid intermediate, potassium chromate.
The product is highly porous chromium oxide, consisting of small particles.
REFERENCES
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KCl. Materials Science Forum Vols. 461-464, pp. 1047–1054.
PETTERSSON, J., ASTEMAN, H., SVENSSON, J.-E., JOHANSSON, L.G. (2005). KCl Induced
Corrosion of a 304-type Austenitic Stainless Steel at 600°C; The Role of Potassium. Oxidation of Metals
Vol. 64, pp. 23–41.
SHINATA, Y. (1987). Accelerated Oxidation Rate of Chromium Induced by Sodium Chloride, Oxidation
of Metals Vol. 27, pp. 315–332.
SPIEGEL, M. (1999). Salt melt induced corrosion of metallic materials in waste incineration plants,
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