OIL SHALE AS POWER FUEL

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Nov 8, 2013 (4 years and 4 days ago)

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OIL SHALE AS POWER FUEL
ARVO OTS
Tallinn University of Technology,
Thermal Engineering Department, aots@sti.ttu.ee


1. Introduction
Sold fuels can be divided into two large groups to their formation: humus and sapropel fuels.
The formation sequence (called rank) of humus fuels is as follows: terrestrial plants → peat
→ brown coal (lignite) → coal → anthracite. Humus fuel coalification or ageing is
accompanied by increase in carbon and decrease in the hydrogen and oxygen content.
The formation sequence of sapropel fuels is as follows: prostista and inferior plants →
sapropel → sapropel coal or oil shale. Sapropel fuels ageing carbon/hydrogen ratio does not
change significantly.
Oil shale is sedimentary rock containing organic matter, kerogen.
The significant feature of oil shale organic matter is its low solubility in strong solvents.
Oil shale differs from humus fuels by its high hydrogen and oxygen content of organic matter.
The atomic ratio of hydrogen to carbon (H/C) is about 1.5, which is approximately the same
value as crude oil, but for coals only about 0.3 to 0.4. The high hydrogen to carbon ratio is the
main reason for the high yield of volatile matter and condensable oil during the thermal
decomposition of oil shale organic matter.
Oil shale deposits have been discovered on all continents. The more than 600 oil shale basins
are known. The world oil shale resources is estimated about 11.5 Tt.
The largest oil shale resources belong to the Middle Cambrian, Early and Middle Ordovician,
Later Devonian, Late Jurassic, and Paleogene periods. Mostly the formation oil shale took
place in marine conditions, less often in lakes. Therefore, the geotectonic structure of oil shale
deposits is predominantly of the platform type.
Estonia has significant oil shale resources; they are estimated to be more than 7 Gt. Estonia is
the only country in the world that uses oil shale fired power plants to supply most of its
electricity to domestic customers and can export power to neighboring countries. In addition
to thermal power plants, Estonia also has oil shale thermal processing plants for shale oil
production. However, these processing plants consume less than 20 % of total oil shale
consumption. Power plants and processing factories in Estonia are supplied with oil shale
from two underground mines and two opencast mines.
Estonian oil shale as power fuel undoubtedly belongs in the fuel class with most complicated
organic and mineral matter composition in the world. The composition of oil shale creates
numerous complicated and mutually related problems that can influence power plant
equipments operations and reliability.
The following presents Estonian oil shale as power fuel properties and utilization problems in
power plants.
2. Estonian oil shale geology and reserves
Figure 1 show the map of Estonia, its neighbor countries and Baltic sapropelites area. The
first and oldest formation of sapropelites is the belt of Paleoproterozoic gneisses. The second
formation is Dictyonema shale. The third formation is the major mineral resource of Estonia,
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oil shale. It is present mainly in the Middle Ortovician Kukruse stage, whose beds form Eesti
and Tapa deposits. The original material of oil shale was rich in the flora and fauna of the
Ordovician Sea. Despite a slight overlap of Dictyonema shale and oil shale deposit areas, oil
shale differs significantly from Dictyonema shale in appearance, composition, and heating
value.

Figure 1. Distribution of sapropelites. Oil shale deposits: E – Eesti, T – Tapa, L – Leningrad, V –
Veimarn, B – Tšudovo-Babinskoe.
The total area of the Balti oil shale basin is about 50,000 km
2
. The area of Estonian oil shale
deposits is the largest one within the the Baltic Oil Shale Basin, accounting for two thirds of
industrial source. The oil shale body lies at a depth of between 30 to 60 m, but in the center of
the Pandivere upland, its depth is 100 to 120 meaters.
The Estonian basin is divided into five parts: Central, Eastern, Western, Northwestern, and
Southern. The total area is about 3000 km
2
. The mined area covers a total of 425 km
2
.
Oil shale beds have a stratified structure, with oil shale seams alternating with mineral
interbeds.
Figure 2 shows a cross-section of an Estonian oil shale deposit.
In an oil shale seam, organic matter is tightly bound with sandy-clay minerals and forms a
uniform mixture. Oil shale is a yellow-brownish, relatively soft material. Its compression
strength is 20 to 40 MN/m
2
. The main substances in the interbeds are carbonate minerals –
mainly calcium carbonate, to a lesser extent, dolomite. The interbed substances also contain a
certain amount of terrigenous and organic matter. The compressive strength of the interbed
material is higher than that of oil shale, between 40 to 80 MN/m
2
.
A commercial bed consists of seven indexed oil shale seams, counted from bottom to the top
A, A', B, C, D, E, F
1
and six limestone interbeds, A/A', A'/B, B/C, C/D, D/F, E/F
1
. A majority
of oil shale seams contain lens-like concretions of kerogenous limestone. They are present in
different layers and in various proportions. The thickness of oil shale seams varies from 0.05
to 0.6 m.
3

Figure 2. Cross-section of oil shale deposit. 1 – limestone, 2 – kerogenous limestone, 3 – oil shale.
There are four oil shale seams in the Leningrad oil shale basin, and they are marked with
Roman numbers from top to bottom.
The commercially utilized bed A – F
1
(also called the industrial bed) is 2.7 to 2.9 m thick in
the central part. The bed thickness decreases continuously south down to 2.1 m and west
down to 1.6 m. The total thickness of oil shale seams changes similary. In the central and
eastern parts, the oil shale seams is 2.0 to 2.2 m thick; however, on the southern and western
borders, it is only 1.3 to 1.4 m.
Considering oil shale as fuel, its dry matter consists of three parts: organic – R, sandy-clay
(terrigenous) – T, and carbonate – K. Consequently,
R
d
+ T
d
+ K
d
= 100 %.
The values of R, T, and K differ for different oil shale seams. This can be seen on Figure 3,
where a triple diagram depicting the composition of oil shale seams are presented.
The Tapa oil shale bed lies at a depth of between 60 and 170 m. The total thickness of the bed
is 1.6 to 2.3 m. The total organic matter concentration of the bed is 10 to 25 %, the content of
terrigenous matter is 14 to 20 % and the rest is carbonate matter.
Oil shale resources divided into the following three categories: useable, inventory, and
prospective. The useable and inventory resources can be either active or passive. Active
resources can be used immediately without any limits. Passive resources can become active
after changes in the current technical possibilities, and ecological and economical conditions.
At the moment mining of oil shale is considered economically active if the average energy
rating of the deposit layer is at least 35 GJ/m
2
. A resource is considered passive if the average
energy rating ia between 25 and 35 GJ/m
2
. In addition to economic restriction, there are also
environmental limitations that can convert an oil shale reserve into a passive one.
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Figure 3. Triple diagram of composition of oil shale seams.
The oil shale resource in the Eesti deposit totally is 5 Gt, from witch the active is 1.7 Gt. The
estimated oil shale resource in the Tapa deposit is 2.6 Gt.
Figure 4 shows the dynamic of oil shale production over time and also the operation time of
opencast and underground mines.

Figure 4. Oil shale production and operation time of mines.
The largest consumers of oil shale in Estonia are thermal power plants. Today power plants
consumed about 12 Mt/y. The shale oil production industry consumed the second highest
amount, about 2 Mt/y.
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3. Oil shale properties
In solid fuel as fired, the following three constituents are usually distinguished: organic part –
R, mineral part – M, and moisture – W. The following holds:
R
r
+M
r
+ W
r
= 100 %.
Oil shale mineral matter consists of two components: carbonate and sandy-clay parts. By
separating mineral carbon dioxide content from mineral matter, then
M
r
= T
r
+ K
r
= U
r
+
r
c
)(CO
2
= T
r
+
r
K

+
r
c
)(CO
2
,
where U
r
– mineral part, free from mineral CO
2
,
r
c
)(CO
2
- amount of mineral CO
2
,
r
K

-
carbonate free from mineral CO
2
.
At ashing of oil shale, gaseous carbon dioxide is related due to thermal decomposition of
carbonate minerals, and amount of the ash formed does not correspond to the amount of the
mineral matter. Therefore, the amount of mineral carbon dioxide must be given separately.
3.1. Organic and combustible matter
The elemental composition of the organic matter of oil shale is given in Table 1.
Table 1. Elemental composition of organic matter of oil shale, %.
Element C H O N S Cl
Range 77.11-77.80 9.49-9.82 9.68-10.22 0.30-0.44 1.68-1.95 0.60-0.96
Average 77.45 9.70 10.01 0.33 1.76 0.75
The main characteristics of the organic matter of oil shale are: high hydrogen and oxygen
content, and low nitrogen percent. The mass ratio of C/H is 8, which is similar to the values
found in liquid fuels. Oil shale organic matter contains in average about 1.8 % of organic
sulfur. An important characteristic of the organic matter is a high chlorine content.
One of the combustible component of oil shale is marcasite (pyrite) with general chemical
formula FeS
2
. The sum of organic and pyretic sulfur is called combustible or volatile sulfur:
S
V
= S
o
+ S
p .
Organic matter together with pyretic sulfur is considered combustible matter of oil shale.
Oil shale also contains a small amount of sulfate sulfur whose content does not exceed 10 %.
Figure 5 present the thermal decomposition characteristics of oil shale as the release of
different components during devolatilization.
The first signs of the thermal decomposition of organic matter appear at 150
0
C. Visible
decomposition starts at 200 – 250
0
C. One feature in decomposition process of oil shale
organic matter is thermobitumen formation on temperature between 250 – 425
0
C. It is also
characteristic that the proportion of the condensing phase in volatile matter decreases with
temperature, which is accompanied by increase in the gaseous phase.
Because the devolatilization process of fuel depends on many parameters, the amount of
realized volatile matter can be determined only in fixed (standard) conditions. Due to the low
ratio of C/H, oil shale has very high volatile matter content. Its standard value is 85 – 90 %.
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Figure 5. Material balance of oil shale organic matter as a function of temperature.

The higher heating value (HHV ) of oil shale organic matter is 36.68 MJ/kg and lower heating
value (LHV) is 34.56 MJ/kg. The LHV of volatile matter is approximately 34 MJ/kg.
The properties of oil shale char depend on organic matter decomposition conditions or C/H
ratio in the char. Figure 6 depicts the relationship between the C/H ratio and the carbonizing
temperature. The C/H ratio in oil shale char rises uniformly with an increase in the
carbonizing temperature. In oil shale, the ratio C/H is 8, in the char formed at 900
0
C, the
average C/H ratio is 25. The activation energy of oil shale char oxidation also depends
strongly on the ratio C/H in the char.

Figure 6. Dependence of C/H ratio in oil shale char on carbonizing temperature.
3.2. Mineral matter
Mineral matter is an association of minerals compounds initially present in fuel.
The mineral matter of oil shale can be divided into two large groups: a sandy-clay or
terrigenous part and carbonate part. The sandy-clay part is densely interwind with organic
matter and can be considered as an inherent mineral impurity. Carbonate minerals in oil shale
deposit occur as separate layers or are distributed as limestone concretions in oil shale seams.
The carbonate part can be considered extraneous mineral matter.
Table 2 presents the chemical composition of the sandy-clay and carbonate parts of oil shale.
The main component of the oil shale carbonate part is calcium oxide (48.1 %), followed by
magnesium oxide (6.6 %). Their ratio corresponds to the average oil shale carbonate part
CaO/MgO = 7.3.
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The average ratio of oxides and CO
2
in oil shale carbonate part is MO
c
/CO
2
= 1.217, where
MO
c
and CO
2
denote content of oxides in carbonate minerals and mineral CO
2
content in
mineral matter, respectively.
The main components in the sandy-clay part are SiO
2
, Al
2
O
3
, and K
2
O. The content of
potassium (given as K
2
O) exceeds that of Na
2
O by about 8 to 12 times.
Table 2. Chemical composition of oil shale mineral matter, %.
Sandy-clay part Carbonate part
Component Content Component Content
SiO
2
59.8 CaO 48.1
CaO 0.7 MgO 6.6
Al
2
O
3
16.1 FeO 0.2
Fe
2
O
3
2.8 CO
2
45.1
TiO
2
0.7
MgO 0.4
Na
2
O 0.8
K
2
O 6.3
FeS
2
9.3
SO
3
0.5
H
2
O 2.6
Total 100.0 Total 100.0
Table 3 give the mineralogical composition of oil shale mineral matter.
Table 3. Mineralogical composition of oil shale mineral matter, %.
Group of minerals Mineral Formula Content
Carbonate minerals
Calcite CaCO
3
69.1
Dolomite CaMg(CO
3
)
2
30.6
Siderite FeCO
3
0.3
Total 100.0
Sandy-clay minerals
Quartz SiO
2
23.9
Rutile TiO
2
0.7
Orthoclase K
2
O∙Al
2
O
3
∙6SiO
2
29.0
Albite Na
2
O∙Al
2
O
3
∙6SiO
2
6.0
Anorthite CaO∙Al
2
O
3
∙2SiO
2
1.4
Muskovite [K
1-x
(H
2
O)
x
∙Al
3
Si
3
O
10
(OH)
2
]
2
23.7
Amphipole [NaCa
2
Mg
4
)Fe,Al)Si
8
O
22
(OH)
2
] 2.1
Markasite FeS
2
9.3
Limonite Fe
2
O
3
∙H
2
O 2.9
Gypsum CaSO
4
∙2H
2
O 1.0
Total 100.0
The main minerals in carbonate part are calcite and dolomite.
The main minerals in the oil shale sandy-clay part are quartz, feldspars (mainly as orthoclase),
and micas (mainly as muscovite). Iron present in the oil shale sandy-clay part is bound mainly
to marcasite sulfur, but, to a limited extent, also to siderite.
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Because carbonate minerals are present as independent components of oil shale mineral
matter, and does not bind with organic part, then the content of SiO
2
decreases and CaO
content increases as a function of amount of ash. Figure 7 shows the content of CaO and
SiO
2
in oil shale laboratory ash as a function of the amount of ash.

Figure 7. CaO and SiO
2
content of laboratory ash depending on the amount of ash formed
from dry matter of oil shale.
3.3. Formation of ash
The content of combustible matter is usually determined by using standard amount of ash.
For oil shale, one also has to take into account the content of mineral CO
2
which is realized
during ashing from calcite and dolomite by the following dissociation reactions:
CaCO
3
→ CaO + CO
2
,
CaMg(CO
3
)
2
→ CaO + MgO + 2CO
2
.

Reactions between pyrite (marcasite) and oxygen proceeds according to the following
reaction:
4FeS
2
+ 11O
2
→ 2Fe
2
O
3
+ 8SO
2
.

In principle, SO
2
can also be formed at oxidation of organic sulfur.
Formed CaO react with sulfur dioxide and oxygen:
CaO + SO
2
+½O
2
→ CaSO
4
.

During the formation of laboratory ash, crystal water is released from minerals of the oil shale
sandy-clay part.
Due to mentioned reactions oil shale mineral matter content always lower than sum of ash and
mineral CO
2
. Therefore, in the case of oil shale, establishing the content of combustible
matter by amount of laboratory ash, yields a marked error. The difference between the actual
and apparent content of combustible matter depends on the extent on the mentioned reactions.
3.4. Heating value
The heating value of fuel is primarily determined on the basis of experiments in fixed
conditions. Usually, the heat released during the combustion of a fuel sample is measured in a
calorimetric bomb in standard conditions.
Oil shale as fuel with complicated composition of organic and mineral matter heating value
determined in a calorimetric bomb does not coincide with the actual amount of heat released
during the combustion of fuel in the combustion chamber. This is caused by differences in the
combustion processes in a calorimetric bomb and in the furnace, which are greatly influenced
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by the behavior of fuel mineral matter. Therefore, one should know the phenomena
influencing thermal effects in a calorimetric bomb and in the combustion chamber.
Heat is released mainly during combustion of organic matter, but during the burning of oil
shale, it must be taken into account the thermal effects associated with the decomposition of
carbonate minerals and the behavior of sulfur.
The relationships between thermal effects and the heating value measured in a calorimetric
bomb are presented on Figure 8, as well as the sum of thermal effects Σ∆
Q
. There are: ∆
Q
C

thermal effect due to incomplete combustion of carbon, ∆
Q
c
- thermal effect due to
incomplete decomposition of carbonaceous minerals, ∆
Q
S-D
- thermal effect due to
incomplete oxidation of sulfides,
42
SOH
Q

- thermal effect due to formation of sulfuric acid, -

Q
S-T
- thermal effect due to formation of sulfate.

Figure 8. Thermal effects occurring during combustion of oil shale in a calorimetric bomb
.
When oil shale heating value is low, thermal effect from partial decomposition of carbonate
minerals have a significant influence. Similarly, thermal effects due to sulfation of calcium
oxide in the range of heating value
d
B
Q
= 10 MJ/kg, and thermal effects due to formation of
sulfuric acid over this value are also important. The sum of heat effects Σ∆
Q
decreases with
an increase in heating value.
Heat released during oil shale combustion is dependent on the combustion technology used.
The most important factor is the influence on the behavior of carbonate minerals. The
behavior of carbonate minerals is expressed as the extent of carbonate decomposition
2
CO
k
.
This is the ratio of the amount of CO
2
released from carbonate minerals to the total content of
mineral CO
2
in the initial material.
The average extent of decomposition of carbonate minerals during high-temperature
pulverized firing of oil shale is 0.97. However, in the case of low-temperature fluidized
combustion, the extent of decomposition of carbonates remains in the range of 0.7 to 0.8.
Figure 9 shows the influence of the decomposition extent of carbonates on heat release from
oil shales of various heating values. On the vertical axis, there is
10
(
)
( )
1
r
L
1
r
L
r
L
L
2
CO
2
CO

=
=

=
k
k
Q
QQ
q

and on the horizontal axis, there is the decomposition extent of carbonates.

Figure 9. Influence of decomposition extent of carbonates on heat release during oil shale
combustion.
In the first approximation, the dependence between heat release from oil shale and
decomposition extent of carbonates can be viewed as linear. As oil shale with smaller heating
value contains more carbonate compounds, the influence of the decomposition extent of
carbonates on heat release increases with a decrease in heating value. This reflects, in
principle, the influence of the absolute amount of carbonate minerals on heat release.

Figure 10. Relationship between LHV of oil shale and ash content, amount of mineral carbon
dioxide, and moisture.
Oil shale heating value is the main integral indicator of oil shale quality from the firing
technology point of view. Despite the varying ratio of oil shale components, a strong
relationship exists between oil shale technical characteristics and heating value. Statistical data
offer the possibility of linking the amount of ash, mineral CO
2
content, and moisture with oil
shale heating value. The results are presented on Figure 10. The data include oil shale
analyses from underground and opencast mines. The results show that the amount of ash and
mineral CO
2
content decrease, but moisture content increases with a decrease in oil shale
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heating value. The dependence presented in Figure 10 can also be expressed by the analytical
Formulas.
3.5. Fusion characteristics
The fusion characteristics of ash at three temperatures are as follows: t
A
(or t
1
) - initial
deformation temperature, t
B
(or t
2
). - softening temperature, and t
C
(or t
3
) - fusion temperature.
The fusion temperatures of ash depend on the chemical and mineralogical composition of the
ash. Fusion temperatures are generally considered to depend on the sum of the basic
components, the CaO content of ash, and the ratio of basic components to acidic ones. Ash
fusion temperatures also influenced, to a certain extent, on the composition of the surrounding
gaseous medium. In a reducing and partly reducing atmosphere, fusion temperatures are lower
than in an oxidizing one. The influence of the composition of the surrounding atmosphere is
related primarily to the behavior of iron compounds. In a reducing atmosphere, iron oxides of
higher oxidation numbers are converted to lower oxides and, therefore, they reduce the ash
fusion temperature.
The main components in oil shale ash that influence ash fusibility are CaO, SiO
2
, and Al
2
O
3
.
During the burning of oil shale, ash forms primarily from the carbonate and sandy-clay matter
of a stable chemical composition. This allows one to follow the relationship of the fusion
temperatures of oil shale ash as a function on calcium oxide content in the ash.
Figure 11 presents the relationship of temperatures t
A
, t
B
, and t
C
to the amount of calcium
oxide in oil shale ash. The fusion temperatures are lowest when the CaO content is 25 to 40
%, and they correlate well with the liquidus curve of CaO-SiO
2
-Al
2
O
3
. The absolute values of
fusion temperatures in this region are relatively high and do not differ by more than 70 to
80 K. The value of t
A
is between 1,170 and 1,190 °C.
In the region CaO > 40 %, ash fusion temperatures increase quickly, and temperature t
C

reaches 1,750 to 1,850 °C at a CaO content of 65 to 70 %. The difference between
temperatures t
C
and t
A
is substantial, and can reach 400 to 450 K.

Figure 11. Fusion temperatures of oil shale ash as a function of CaO content.
Oil shale mineral matter is a heterogeneous system containing minerals of carbonate and
sandy-clay parts. Therefore, different ash particles have different fusion temperatures
depending on the proportions of the mentioned components. Consequently, ash as whole
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consists of mixture of particles of different phase state. Integral fusion characteristics of oil
shale ash as a function of temperature are shown on Figure 12. The vertical axis represents the
share of the corresponding phase in ash.
The curves show the proportion of ash components in different states. The first signs of
deformation of oil shale ash appear at about 1,050 °C; the fusion temperature is close to
1,150 °C. At higher temperatures, the amount of the liquid phase in the system increases
rapidly.

Figure 12. Integral fusion characteristics of oil shale ash. Sl – solid state, D – deformed state, Sf
– softened state, L – liquid state.
4. Size distribution characteristics of ground oil shale
The fuel preparation system is an integral part of the essential operations of a power plant.
The polyfractional particles system that is formed during different fuel pretreatment process
(for instance, blasting in mines, crushing, grinding in mills, etc.) is subject to a certain
principles of the distribution law. The grind oil shale polyfractional particles system is
described well by Kolmogorov logarithmic-normal distribution law. According this law the
mass of particles passing through the sieve:
















Φ=−=

m


∆∆
M
ln)(R1)D(
,
where R(∆) – mass of particles remaining on the sieve, ∆ – particle size,
*
M


- particle median
size, m – uniformity factor of polyfractional system.
The Kolmogorov-Fay-Zselew (K-F-Z) diagram is widely used in practice for representation
(visualization) of oil shale particle size distribution. The horizontal axis of the K-F-Z diagram
represents the particle size, and the vertical axis represents the relative amount of the particles
in the system according to Kolmogorov distribution law.
For example, the particle size distribution of oil shale prepared in mills with a gravitational
and an inertial classifier is shown on Figure 13. The median size of pulverized oil shale
13
burned in industrial boilers is between
*
M

= 35 – 60 µm, and sieve residues are R
90
= 20-45%
and R
200
= 10-25%.

Figure 13. Particle size distribution of pulverized oil shale on K-F-Z diagram. 1 – MV-
1050/400/1470 (ventilator mill), 2 – MMT-1500/2510/735 hammer mill (inertial classifier, 3 –
ŠMT-1300/2564/740 hammer mill (gravitational classifier, cylindrical shaft), 4 – ŠMT-
1300/2564/740 hammer mill (inertial classifier), 5 – without blowing ball-tube mill, 6 – ball-tube
mill ŠBM-280/600, 7 – laboratory ball-tube mill.

Figure 14. Content of organic (R), sandy-clay (T), and carbonate (K) matter in different
fractions of oil shale in the K-F-Z diagram.
As was mentioned before, dry matter of oil shale is heterogeneous material consisting of three
components. These components differ in their behavior at grinding and, therefore, the share of
those components in fractions of different particle size differs from that in oil shale before
14
grinding. Fractions of ground oil shale yield different amounts of ash, mineral CO
2
, and
organic compounds. Similarly, various oil shale fractions are characterized by different
heating values, density, and other parameters.
Figure 14 gives the content of

organic (R), sandy-clay (T), and carbonate (K) matter in oil
shale as a function of particle size. The carbonate part predominates in coarser fractions, but
the sandy-clay matter concentrates in finer particles. The distribution of organic matter is
characterized by the line between those representing carbonate and sandy-clay matter.
5. Combustion and mineral matter behavior characteristics
In the classical sense, combustion means chemical reactions between components of fuel
organic matter and oxygen, which results in the formation of gaseous combustion products or
flue gas. In flue gas environment takes place fuel mineral matter conversion processes and ash
formation. The composition of flue gas environment (for instance, oxygen-, carbon dioxide-,
and sulfur dioxide partial pressure) strongly influenced on formed ash physic-chemical
properties, because ash properties determined ash fouling intensity of boiler heat transfer
surfaces, extent of boiler tubs corrosion, thermal resistance of ash deposits on heat transfer
surfaces, etc.
Figure 15

shows the dynamics of the combustion of polyfractional pulverized oil shale at two
different excess air factors. The vertical axis represents the temperature and volumetric
concentration of flue gas components along the axis of the combustion chamber. The
horizontal axis represents time. The figure also expresses the extent of fuel combustion.
An intensive heat release (combustion) and a rapid rise in temperature start after a short
period of 0.02 to 0.04 s. Intensive combustion demonstrated a rapid decrease in oxygen
concentration and an increase in CO
2
concentration. Particles burn with particular intensity
within 0.25 to 0.35 s.

Figure 15. Combustion characteristics of polyfractional pulverized oil shale.
The value of combustion extent 0.8 (zone of combustion of volatile matter) depending upon
the combustion regime, corresponds to the combustion time 0.12 to 0.20 s. The volumetric
heat release rate in the zone of combustion of volatile matter is high and can reach 1.5 to
15
2.5 MW/m
3
. Beginning from the value 0.8, slower char burning dominates, and heat release
diminishes rapidly. The extent of combustion starts to approach asymptotically to one. The
time required for complete combustion of pulverized oil shale is in the range of 1.0 to 1.5 s.
Flue gas in the region of a rapid decrease in oxygen concentration also contains small
amounts of components of incomplete combustion, CO being the most important one.
Carbon dioxide released during oil shale combustion forms both from organic carbon (CO
2C
)
and through the thermal decomposition of carbonate minerals (CO
2CC
). The ratio θ =
CO
2CC
/(CO
2C
+ CO
2CC
) indicates the share of mineral CO
2
in the amount of total CO
2
formed.
Dissociation of carbonate minerals is a dynamic process, and θ increases over time. The
maximum value of θ, is determined by the ratio of mineral to organic carbon content in the
fuel and by the extent of decomposition of carbonate minerals. The time dependence of θ is
influenced by the parameters affecting the thermal decomposition of carbonate minerals. For
oil shale burned at power plants during the complete combustion of organic carbon (η = 1)
and during the complete decomposition of carbonate minerals (
2
CO
k
= 1), the maximum possible
ratio is θ = 0.18-0.22. One can see on Figure 16 that the amount of mineral CO
2
increases the
absolute concentration of CO
2
in flue gas by 2.5 to 3.0 %; the total concentration of CO
2
in
the flue gas is 15 to 16 %.
The phenomena occurring during the combustion of oil shale mineral matter can be divided
into four groups:
1.

Thermal decomposition of the initial minerals into simpler compounds.
2.

Full or partial volatilization and sublimation of minerals. Formation of superfine
particles (aerosols).
3.

Formation of novel minerals through the chemical reactions taking place in mineral
and organic matter during contact within particles and between particles.
4.

Change in the phase state (solid, plastic, liquid, etc.) of ash particles.
One of the most reactive ash components with respect to ash fouling, sulfate-rich deposits
formation, and heat exchange intensity is calcium oxide. The reactivity of calcium oxide
depends on CaO form in ash.
The dynamics of calcium oxide conversion in gaseous atmosphere corresponding on Figure
15 is shown on Figure 16. The vertical axis represents the amounts of the different forms of
calcium oxide in sulfur free ash. CaO
c
- calcium oxide that is bound to carbonate minerals,
CaO
b
- bound calcium oxide, CaO
f
- calcium oxide that stays in free form. Dimensionless
values k
c
= CaO
c
/CaO, k
b
= CaO
b
/CaO, k
f
= CaO
f
/CaO, where:CaO = CaO
c
+ CaO
b
+ CaO
f
.
The horizontal axis represents time.
It can be seen that the decomposition of carbonates starts immediately after pulverized fuel
has entered the combustion chamber. The dissociation of carbonates is slightly slower at
lower values of excess air factor.
The formation of free calcium oxide is immediately followed by the intensive formation of
novel minerals from sandy-clay minerals in reactions with CaO. The primary minerals that
form from free calcium oxide in an oxygen-containing medium are β-2CaO∙SiO
2
, CaO∙Al
2
O
3
,
2CaO∙Fe
2
O
3
, and CaSO
4
. In a reducing atmosphere, calcium sulfide can also form. All these
minerals form when there is an excess of free calcium oxide (k
f
> 0). Therefore, the thermal
decomposition of carbonates does not limit the presence of free CaO in ash or the formation
of novel minerals. It is clear that the rate of formation of minerals depends on temperature. At
higher temperatures, the amount of novel minerals in ash is greater. Temperature also
determines their phase state.
16


Figure 16. Dynamics of calcium oxide conversion.
Oil shale ash is characterized by the ability to react with gaseous sulfur compounds present in
flue gas. This phenomenon is known as ash sulfation.
Sulfation takes place in boiler gas passes through reactions between fly ash components and
sulfur containing gaseous compounds, either in the space of the boiler gas pass or on the heat
exchange surfaces during formation of the ash deposit. In the first case, the concentration of
SO
2
in the flue gas and the SO
2
emissions into the atmosphere diminish. In the second case, a
sulfate ash deposit forms on boiler heat transfer surfaces, and SO
2
concentration in the flue
gas also somewhat decreases.
The main sulfur compound present in flue gas is SO
2
. Flue gas can contain small amounts of
SO
3
. Flue gas can also contain hydrogen sulfide, which is formed during the decomposition of
fuel organic matter. H
2
S is more often present during the incomplete combustion of fuel.
Calcium oxide is the major component in oil shale ash that is subject to sulfation. The
sulfation ability of CaO depends on its form in ash. Calcium oxide is present in free and
bound forms. Free CaO is chemically more active. Free calcium oxide sulfates according to
the formula CaO + SO
2
+ ½O
2
↔ CaSO
4
. Bound calcium oxide is also able to react with SO
2
,
for instance, according to the following reactions:
2CaO∙SiO
2
+ SO
2
+ O
2
→ 2CaSO
4
+ SiO
2
.
Oil shale ash sulfation intensity depends on temperature, partial pressures of SO
2
and oxygen,
and particle sizes. The dynamics of the sulfation of oil shale fly ash at two different
17
temperatures and values of
22
OSO
pp
is shown on Figure 17. Value of m on the vertical axis
denotes mass increas; the horizontal axis denotes time.

Figure 17. Dynamics of sulfation of oil shale ash.
Sulfation of oil shale ash is possible at relatively low partial pressures of SO
2
, as proved by a
significant increase in the ash mass during the sulfation process, even at a low concentration
of SO
2
in the surrounding medium. Figure also shows that at the same temperature (1,000 °C),
but at lower values of

6. Oil shale firing power plants
The chief aim of oil shale mining today is to obtain source material for the production of
electrical energy.
Until between 1920 and 1925, the main fuel resource in Estonia was wood, which provided
more than 90 % of the energy requirements. Imported coal was the second most widely
consumed fuel.
Initially, oil shale was used as a fuel in locomotives. Thereafter, more and more lump oil shale
was burned in industrial boilers.
An important milestone in the history of the oil shale power industry was the switch to oil
shale firing at the Tallinn Power Plant in 1924. The electrical capacity of Tallinn power plant
was 23 MW. This year can be considered the start of the oil shale power industry. Thereafter,
many oil shale-fired power plants were built in north Estonia: Püssi, Kohtla, Kunda, and
Kiviõli.
At that time, a steam boiler with a grate-firing furnace was typical. The key problem with
firing oil shale rich in volatile matter is ensuring soot-free combustion. This issue was
successfully solved.
The next stage was introducing oil shale pulverized firing technology. The first power plants
using oil shale pulverized technology were the Kohtla-Järve Power Plant, which began
operation in 1949, and had designed electrical capacity 48 MW, and Ahtme Power Plant,
which began operation in 1951 and had an electrical capacity of 75.5 MW. Steam boilers at
18
these power plants were slightly modified medium-pressure boilers initially designed for
pulverized firing of coal or brown coal. Operational experience showed that these boilers on
oil shale could be operated only at loads considerably below the design value, because ash
deposits caused fouling of radiant and convection heat transfer surfaces and high-temperature
corrosive-erosive wear of the boiler tubes. These experience indicated for the first time
specific problems with fouling of heat transfer surfaces, high-temperature corrosion and wear
of boiler tubes, cleaning of heating surfaces from ash deposits, and heat transfer. It became
evident that boilers designed for burning coal or brown coal cannot work satisfactorily with
pulverized oil shale due to the very specific and complex composition of oil shale mineral and
organic matter. Systematic application of the results of scientific studies allowed the
operational capacity of medium-pressure boilers to increase, which formed a basis for
designing of original construction of oil shale boilers.
The next stages in development of oil shale power engineering industry started in 1959, when
the first high-pressure pulverized firing boilers specially designed for firing of oil shale were
launched at Balti Power Plant. The designed electrical capacity of Balti Power Plant was 1624
MW.
The first power unit at Eesti Power Plant was inaugurated in 1969. The design capacity of the
power plant was 1610 MW.
At the beginning of 2004, a power unit of electrical capacity of 215 MW with two circulating
fluidized bed combustion boilers was put into operation at Eesti Power Plant. Some months
later, a power unit of the same type was put into operation at Balti Power Plant.
Burned in power plants oil shale has the following proximate characteristics W
r
= 11-13 %, A
r

= 45-57 %,
r
c
)(CO
2
= 16-19 %, and
r
L
Q
= 8.3-8.7 MJ/kg
Profile drawing of Eesti Power plant is shown in Figure 18.
Figure 18. Profile drawing of Eesti Power Plant. 1 – fuel conveyer, 2 – fuel hopper, 3 – fuel
feeder, 4 – mill with classifier, 5 – boiler, 6 – ash cyclones, 7 – electrostatic ash precipitator, 8 –
flue gas fan, 9 – stack, 10 – deaerator, 11 – steam turbine.
7. Oil shale firing technologies
19
7.1. Pulverized firing
Today, pulverized firing is the most widely used combustion technology for solid fuels. This is
true in oil shale fired-power plants by installed power.
Pulverized firing of solid fuels is a high-temperature combustion technology. Maximum flame
temperature in the furnace chamber by burning of pulverized oil shale reach to 1400- 1450
0
C.
Medium-pressure pulverized oil shale boilers in Kohtla-Järve and Ahtme Power Plant have
played an important role in studying the processes occurring during oil shale combustion. The
scientific results obtained have formed the basis for more advanced designs of high-pressure
boilers for Balti and Eesti power plant.
The conceptual solutions of the boilers designed with cooperation of Thermal Engineering
Department of Tallinn University of Technology (STI TTU) and built at the Taganrog Boiler
Factory for Balti and Eesti power plant differ from those typical pulverized firing boilers. The
designers have taken into account the experience gained from the operation and reconstruction
of medium-pressure boilers for pulverized firing of oil shale and also research results.
The main issues which must be taken account by designing of oil shale firing boilers are
following:
1.

Ash fouling mechanism and ash deposits formation dynamics of boiler heat transfer
surfaces.
2.

Arrangement of boiler gas passes and heat exchange surfaces according to ash fouling
mechanism.
3.

Influence of ash fouling mechanism on heat transfer on boiler heat exchange surfaces.
4.

Heat transfer in heat exchange surfaces as transient process due to unlimited growth of
ash deposits.
5.

Boiler heat exchange surface ash deposits cleaning technology.
6.

Influence of boiler heat exchange surface cleaning technology on heat transfer
conditions.
7.

High-temperature corrosion of boiler tube metal under the influence of ash deposits.
8.

Influence of heat exchange surface cleaning technology on corrosive-erosive wear
intensity of boiler tubes.
The longitudinal section of the high-pressure pulverized oil shale firing TP-101 boiler
mounted at Eesti Power Plant is shown on Figure 19.
TP-101 is a balanced-draft, natural-circulation, single-drum boiler with a superheater and
reheater. The steam capacity of the boiler is 320 t/h. The pressure in the drum is 15.3 MPa.
Pressure and temperature of the superheated steam after the boiler are 13.8 MPa and 520 °C.
Steam pressure and temperature at the reheater inlet and outlet are 2.5 MPa/330 °C and
2.2 MPa/525 °C, respectively.
The boiler has four vertical gas passes. The first (rising) gas pass is the furnace chamber. In
the second (downflow) gas pass, there are hanging platen-type superheaters. In the third
(rising) gas pass, there is an economizer, and in the fourth (downflow) gas pass, an air
preheater.
Rear and side walls of the furnace are screened by water tubes. Radiant superheater panels
cover the front wall.
20
In the front wall of the furnace, two rows of eight turbulent burners are placed. Oil shale fed
into the boiler is pulverized in four hammer mills with an inertial classifier. The particle size
distribution of the pulverized oil shale is characterized by the following residues in the sieves:
R
90
= 25-35 % and R
200
= 17-22 %. The median size of pulverized fuel

M

= 45-55 µm.

Figure 19. Boiler TP-101. 1 – burners, 2 – furnace, 3 – outlet screen, 4 – furnace platen
superheater, 5 – intermediate platen superheater, 6 – hanging platens of primary superheater,
7 – hanging platens of reheater, 8 – economizer, 9 – air preheater.
The primary superheater consists of the located in the front wall and on the ceiling of the
furnace chamber, 16 crossflow platens (furnace platens), panels placed on the ceiling of the
upper reversing chamber after the furnace, 32 crossflow platens in the upper reversing
chamber in the horizontal gas pass after outlet screen (intermediate platens), panels in the
ceiling of the downflow gas pass, and 32 long hanging platens in the first row of the
downflow gas pass. Thirty-two long platens of the reheater hang in the second row of the
downflow gas pass.
Ash deposits are cleaned from furnace platens and the platens in reversing chamber by
vibration. In addition to vibrators, long-distance retraceable water blowers are used for
cleaning crossflow platens
In the third (rising) gas pass, there is a economizer. It consists of two sections: lower platens
and upper convection tube bundles. Ash deposit is cleaned from the economizer by vibration.
For additional cleaning of the economizer platens, long-
distance water blowers are also used
periodically.
An air preheater is located in the fourth (downflow) gas pass. It is a three-story heat
exchanger made from smooth tubes.
21
7.2. Fluidized combustion
Fluidized bed combustion of solid fuels, a technology that still in the development stage, is the
newest trend in the thermal power plants. It is very prospective also for firing of oil shale.
A fluidized bed is a floating aerodynamic system of fine inert solid particles in gas flow. If fuel
particles are fed into the fluidized bed, where the temperature is at least equal to the ignition
temperature, continuous combustion without the need for high temperature take place.
Combustion temperature is typically between 800-900
0
C. Consequently, fluidized bed
combustion is low-temperature firing technology.
There are currently three fluidized bed combustion technologies in use: stationary or bubbling
fluidized bed, circulating fluidized bed, and pressurized fluidized bed.
Estonian Power plants has more than two years experience of burning of oil shale in circulating
fluidized boilers.
Figure 20 shows the circulating oil shale boiler designed from Foster Wheeler for burning of oil
shale in Eesti and Balti power plant. The power unit, with an electrical capacity of 215 MW, has
two boilers. The main parameters of the boiler are follows: steam capacity, 342/272 t/h;
primary/reheated steam pressure, 12.7/2.4 MPa; and primary/reheated steam temperature,
535/535
0
C.

Figure 20. Oil shale fired CFB boiler. 1 – raw fuel bunker, 2 – fuel feeder, 3 – grate, 4 – furnace
chamber, 5 – separating chamber, 6 – fluidized bed internal heat exchanger, 7 – separator, 8 –
superheater, 9 – economizer, 10 – air preheater.
The circulating fluidized bed loop consists of the following items making up the solid particles
recirculation system: furnace, gas-solid separator, loop-seal and internal heat exchanger. The
back-pass consists of: superteater, reheater, economizer and air preheater.
The inertial bed material is usually sand. However, if high mineral matter content oil shale is
burned, the bed material forms ash during the fuel combustion process.
The concentration of fuel in a circulating fluidized combustor is very small (usually 0.5 to 2
22
%); heat release is dissipated over the entire combustion chamber and balanced by the
uniform energy absorption of the heat transfer surfaces on the furnace wall. Therefore, flue
gas temperature is not dramatically changed along the furnace height, and there is no need to
add tube bundles into the bed.
Modern circulating fluidized boilers are equipped with an external or internal air fluidized bed
heat exchanger in the back pass after the separator. In this heat exchanger, intensive heat
transfer from the fluidized bed to the heating surface takes place.
When oil shale is burned, sulfur is completely captured by ash, and the concentration of SO
2

in the flue gas is almost negligible. There is no need to add sorbent (limestone) into the
fluidized bed for sulfur capture. The Ca/S molar ratio in oil shale is high, and free lime forms
during thermal decomposition of carbonate minerals.
7.3. Ash fouling
Important issues in design and exploration oil shale firing boiler are: ash fouling and high-
temperature corrosion of heat transfer surfaces, and transient heat exchange.
During the firing of a solid fuel, the flue gas passing through the convective heat transfer
surface always contains ash particles; therefore, the boiler tubes inevitably become covered
with an ash deposit. Ash fouling of the heat exchange surface decreases the heat transfer rate
and also influences boiler tube corrosion.
These processes in oil shale firing boilers related with very complicated composition of oil shale
organic and mineral matter, and also with combustion technology.
Ash fouling of heat transfer surfaces in a boiler is a complex phenomenon; it is a result of
several physical and chemical processes. Composition of the ash deposit on a boiler tube in an
oil shale boiler usually differs significantly from the composition of fly ash passing through heat
transfer surface. This difference is caused by selective deposition of ash particles on the surface,
condensation vapor phase components from flue gas, chemical reactions, sintering, and others
processes within the deposit layer. Therefore, ash deposits of different structure, chemical and
mineralogical composition, strength, thermophysical properties, can be found on heat transfer
surface tubes.
In oil shale pulverized firing boilers, the highest ash fouling rate can be observed in
superheaters and in the economizer located in the high-temperature region of the flue gas.
For oil shale boiler is specify formation of bound ash deposit. A bound deposits forms on the
tubes as result of the transfer of particles onto it and by the condensation of vapors of flue gas
components and through the chemical reactions in deposit. Deposits formed on can vary in
structure and mechanical strength. According to these characteristics, bound deposits can be
divided into two categories: hard deposits of dense structure and high compression strength
and friable deposits of porous structure and low compression strength. The compression
strength of the friable-type and the hard-type deposit is usually 0.1 to 0.5 MN/m
2
and 5 to
20 MN/m
2
, respectively.
Both mechanical and chemical processes influence the formation of the bound deposit. The
growth rate and strength of the bound deposit are influenced primarily by composition of ash
particles, flue gas temperature, composition, velocity, and particle size distribution of the ash.
Photos on Figure 21 provides an overview of the fouled convective heat transfer surfaces of oil
shale fired boilers.
23




Loose deposits on boiler tubes surfaces.

Bounded ash deposits on boiler tubes surfaces.
Figure 21. Ash deposits on heat transfer surface tubes.
24
The typical characteristics shapes of bound ash deposits that form on boiler tube surface are
shown on Figure 22. The deposit shapes mainly depends on flue gas temperature and velocity.
A weakly bound friable deposit forms on the front side of the tube at low gas velocity. The
formation of a hard deposit is associated with a higher gas velocity. The strongly bound
deposit consists of a bottom layer, thin middle crest, and the tops of lower side crests, which
all grow together. The weakly bound friable deposit is wedge-shaped and resembles a loose
deposit; however, it is much more massive.

Figure 22. Shape of a bound deposit formed on a boiler tube. a) – hard deposit on the front side
of the tube, b) – friable deposit on the front side of the tube. 1 – base, 2 – middle crest, 3 – side
crest, 4 – loose crest, 5 – friable deposit on the back side of the tube, 1’- thin hard side layer. I-
IV – stages of formation of the hard deposit.
Figure 23 depicts the growth rate of the ash deposit formed on a boiler tube as a function of
particles velocity and the flue gas temperature. The curves correspond to the temperature
range of flue gas at 560 to 700 °C. The concentration of fly ash in the flue gas was 10 g/m
3
,
the median size of fly ash particles

M

= 35 µm, and the diameter of the largest particle was 250
µm. The ash deposit growth rate depending on velocity has very complicated character. Up to
velocity 10 m/s the weakly bound deposit with maximum growth rate is formed. Between
velocities 8 to 10 m/s, the fouling rate drops quickly, and a hard bound deposit starts to form.
The growth rate of the weakly bound deposit can exceed the growth rate of the hard bound
deposit many times. The latter decreases with an increase in flue gas velocity.
The critical flue gas velocity when hard bound deposits starts to form depends very strongly
on ash particles size distribution characteristics. The line 4 in Figure describes the growth rate
of weakly bound deposits by concentration of fly ash in the flue gas was 12 g/m
3
, the media
size of particles

M

= 5 µm, and the diameter of the largest particle was 25 µm. The rapid
growth of the ash deposit and weak bonds between particles in the layer are caused by a lack
of larger particles with high kinetic energy in the fly ash. The lack of coarser particles and,
therefore, the absence of their selective effect do not allow the formation of a strongly bound
deposit.
Oil shale flue gas also consist condensing compounds of alkali metals and chlorine, mainly
25
potassium sulfate and potassium chloride. Direct condensation or desublimation of alkali
metal sulfates and chlorides on the tube surface is an important step in the formation of the
initial deposit in an oil shale boiler. Alkali metals compounds deposition (condensing) on the
boiler tubes influence on ash fouling rate and deposits chemical composition. The later is
determined corrosive activity of ash deposits.

Figure 23. Influence of velocity and flue gas temperature on the growth rate of the bound
deposit formed on a boiler tube. 1 – average growth rate 2 – maximum growth rate, 3 – strongly
bound deposit, 4 – weakly bound deposit.
The possibility of condensation of alkali metal sulfates and chlorides is determined by the
partial pressure of the condensing compound in flue gas and by the surface temperature. The
condensation possibility of alkali metal sulfates and chlorides present in flue gas is affected
by the behavior of mineral matter during fuel combustion. The type of combustion technology
is important in this respect. It is obvious that volatilization of alkali metals (mainly potassium)
from sandy-clay minerals of oil shale and condensing compounds formation is much more
intensive in high-temperature pulverized firing conditions than in low-temperature fluidized
bed combustion.
The condensing component phase state is determined by dew point, which is the border
between the gaseous and condensed phases. If the temperature is lower than the dew point, the
rate of condensation onto the surface is affected primarily by the partial pressure of the
corresponding compound in the flue gas and by mass transfer conditions.
The dew points of the condensing components in the flue gas of oil shale combustion have
been identified and are shown on Figure 24. The dew points of K
2
SO
4
42
SOK
pd−
t
and potassium
chloride
KCl
pd−
t
are within the range t = 900 to 950 °C and 550 to 650 °C, respectively. The
figure also shows the dew points of sulfuric acid vapor (
42
SOH
pd−
t
= 70 to 80 °C) and water vapor
(
OH
2
pd−
t
= 40 to 50 °C).
The condensation of the vapors of alkali metal sulfates, chlorides, and other compounds
directly onto tube surface is not possible at temperatures of flue gas below their dew points.
26
The thickening of the deposit increases its outside surface temperature, and once it exceeds
the respective dew point, the condensation process stops and the content of alkali metal
compounds in the layer decreases dramatically. It is a one reason while the lower layers of the
ash deposit are enrichment with alkali metal compounds can occur.

Figure 24. Dew points of condensing compounds from oil shale flue gas.
7.4. High-temperature corrosion of boilers tubes
Corrosion of metal is its oxidation under the influence of the surrounding medium. High-
temperature corrosion occurred at temperatures higher than sulfur acid dew point. High-
temperature corrosion typically becomes a significant phenomenon only at metal temperature
above 350 to 400
0
C, though frequently at even higher temperatures. High-temperature
corrosion rate of metal are determined the working resource (lifetime, limited temperature, etc.)
of metal, specially of boiler superheater and reheater tubes. The corrosion rate of metal is
influenced by corrosive compounds present in the ash deposit.
The intensity of high-temperature corrosion of boiler tubes depends mainly on the type of
steel, metal temperature, properties of the ash deposit, and composition of the surrounding
gaseous medium (flue gas). Time is also an important factor. The corrosion intensity of metal
is mainly dependent on the temperature of the metal. The corrosion rate usually shows an
exponential increase with temperature and can be described by Arrhenius’ law.
The most important corrosive constituents formed during oil shale combustion and
accelerating corrosion process are chlorine compounds. The metal corrosion intensity depends
directly on the chlorine content of the ash deposit on the tube surface.
The impact of chlorine on the corrosion of austenitic and pearlitic steel is clearly shown on
comparative diagram on Figure 25, which displays experiments data. Corrosion depth is
shown on the vertical axis, and time is shown on the horizontal axis. The research was
conducted by presence of oil shale and brown coal fly ash. The oil shale ash contained 0.5 %
chlorine and 6 % K
2
O. The brown coal ash did not contain any measurable chlorine, and the
Na
2
O + K
2
O content was 1 %. As a basis for comparison, the corrosion curves in an air
atmosphere without ash are also plotted on the same figure. The corrosion of both samples is
much more extensive in the chlorine-containing ash environment than that in the chlorine-free
ash. The brown coal ash did not enhance corrosion; these results demonstrate essentially the
same behavior as in a pure air atmosphere.

27

Figure 25. A comparison of corrosion depth of two steels in oil shale ash and brown coal ash. a)
– Cr18Ni12Ti austenitic steel, b) – 12Cr1MoV

pearlitic steel.
8. Formation and emission of air pollutants
The main air pollutants formed during the combustion of oil shale are nitrogen oxides, sulfur
dioxide, hydrogen chlorine, and solid particles. The most substantial greenhouse gas emitted
is CO
2
. The concentration of air pollutants in oil shale flue gas depends primarily on the
combustion technology and burning regime, while the emissions of solid particles are
determined by the efficiency of fly ash-capturing devices.
With regard to emissions of air pollutants, the characteristics of oil shale include the low
nitrogen content of organic matter (0.3 %), a large amount of organic and marcasite sulfur
(1.6-1.8 % in as-received fuel), a high Ca/S molar ratio (8-10), , and an abundance of
carbonate minerals (16-19 % mineral CO
2
content as-received fuel).
8.1. Nitrogen oxides emission
During fuel combustion, NO
x
can be formed in the following ways: in a reaction between
nitrogen and oxygen from the air (thermal NO
x
), in a reaction between hydrocarbon radicals
and molecular nitrogen (prompt or fast NO
x
), and from fuel nitrogen (fuel NO
x
).
Most of NO
x
forms from fuel nitrogen during both pulverized firing and fluidized
combustion. The amount of NO
x
in flue gas that forms from fuel nitrogen is proportional to
fuel nitrogen content.
The influence of the nitrogen content of oil shale on the total concentration of nitrogen oxides
(as bound in NO
x
nitrogen) in flue gas is explained on Figure 26,a. In the first approximation,
the NO
x
concentration in flue gas as a function of the nitrogen content of oil shale organic
matter can be treated as linear. By extrapolating the experimental line to the value N = 0, one
can estimate the concentration of nitrogen oxides formed from nitrogen in the air; at 1300
0
C
it is approximately 12 mg/m
3
.
Based on the average nitrogen content of oil shale organic matter (N = 0.33 %), one can determine
that the amount of NO
x
formed from fuel nitrogen is about 40 mg/m
3
, which accounts for 18 % of
28
the total amount of nitrogen in oil shale. The portion of thermal NO
x
is approximately 30 % of the
total amount of nitrogen oxides in flue gas.
The effect of combustion temperature on the formation of NO
x
from fuel nitrogen is weak.
Figure 26,b shows the concentration of NO
x
formed from oil shale nitrogen as a function of
combustion temperature. In the first approximation, this relationship is linear. An increase in
combustion temperature by 100 K increases the amount of bound nitrogen in flue gas by
approximately 4 mg/m
3
.

a) b)
Figure 26. Content of bound nitrogen in flue gas as a function of nitrogen content of organic
matter of oil shale and flue gas temperature. Excess air factor α = 1.25.
The most important parameter that influences the nitrogen oxide content of flue gas is the
oxygen concentration (excess air factor). To determine the influence of excess air factor on
the NO
x
concentration formed from fuel nitrogen in flue gas, experimental results are reduced
to an average nitrogen content of oil shale organic matter of 0.33 % and a temperature of
1,300 °C. Results obtained in this manner are presented on Figure 27.

Figure 27. Concentration of bound nitrogen in flue gas as a function of excess air factor.
Results are reduced to nitrogen content of fuel N = 0.33% and temperature t = 1300 °C.
According to measurements performed by the Department of Thermal Engineering of TUT in
industrial pulverized oil shale boilers, the average concentrations of NO
x
in flue gas leaving
the boiler (flue gas contained 6 % O
2
) depending on the type of boiler is in the range 220 to
29
270 (250) mg/m
3
. Measurements in an industrial oil shale circulating fluidized bed boiler
showed that the NO
x
concentration was in the range of 140 to 160 mg/m
3
.
During oil shale pulverized firing in industrial boilers, the portion of thermal nitrogen oxides
in the total NO
x
is 35 to 40 % (maximum combustion temperature of 1400 to 1500 °C). Since
the concentration of thermal NO
x
is highly dependent on the combustion temperature, the
amount of NO
x
formed from air nitrogen in fluidized bed combustion, where the combustion
temperature remains about 800 to 900 °C, is very small. Therefore, during fluidized bed
combustion, nitrogen oxides form primarily from fuel nitrogen.
During oil shale fluidized bed combustion, the distribution of bound nitrogen between various
individual oxides is different compared to pulverized firing. The N
2
O emissions at pulverized
firing are almost negligible because of the high combustion temperature; however, the
emissions of nitrous oxide from an industrial oil shale circulating fluidized bed combustion
boiler does not exceed 10 mg/m
3
.
8.2. Sulfur dioxide emission
The processes occurring during oil shale combustion in a pulverized firing boiler furnace and
gas passes yield sulfur compounds (mainly as SO
2
) in amounts much less than would be
expected, based on the sulfur content of oil shale. Due to the ability of oil shale ash to bind
sulfur and it is the reason that sulfur mainly is removed from the boiler along with the ash.
The main sulfur-binding component in oil shale is calcium. Therefore, to characterize the
potential of sulfur capture, the molar ratio Ca/S is used.
Because oil shale contains alkali metals, a part of the sulfur is bound with those compounds,
usually in the form of sulfates. However, not all alkali metals present in fuel are converted
into sulfates, only a portion (this depends primarily on the extent of alkali metals volatilized
from mineral matter during combustion).
The emissions of sulfur dioxide and the extent of volatilization of combustible sulfur during
oil shale combustion are influenced by many factors. The most important ones are as follows.
Sulfur content of fuel. The effects of the content of combustible sulfur in fuel dry matter on the
total sulfur amount in slag, in ash from the reversing chamber between the superheater and the
economizer, and in fly ash of oil shale pulverized firing boiler are shown on Figure 28. This
result clearly indicates that the amount of sulfur in fly ash, in ash from the reversing chamber,
and in slag increases with an increase in the sulfur content of oil shale.

Figure 28. Total sulfur content of ash as a function of the content of combustible sulfurin dry
oil shale. 1 – fly ash, 2 - ash from reversing chamber, 3 - slag. Numbers by the curves 1 denote
relative load of boiler.
30
Grinding rate of fuel. The effect of fuel particle size can be indirectly deduced from the
relationship between the total sulfur content of the ash particles and the particle size
distribution. The finest fractions are characterized by the highest sulfur content, whereas
coarser particles contain less sulfur (Figure 29).

Figure 29. Content of sulfur in ash precipitated in gas passes of an oil shale boiler as a function
of median particle size. Relative boiler load: 1 – 0.94, 2 – 0.69.
This conclusion can be also drawn from Figure 28. Finer ash particles have a higher potential
for binding sulfur oxides because of their greater specific surface area, which favors contact
between the sulfur oxides in flue gas and active ash components, mainly free lime. One can
assert that the finer crushing of oil shale will increase the specific surface area of ash particles
containing CaO and, therefore, their ability to capture sulfur oxides from flue gas.
Oxygen concentration in flue gas. As already mentioned, calcium oxide is the main
component of oil shale ash that reacts with SO
2
. The sulfation rate of CaO is proportional to
2
O
p
,where
2
O
p
is oxygen partial pressure in flue gas. The oxygen concentration in flue gas
in practice is expressed by excess air factor. Figure 30 depicts the relationship of the extent of
sulfur capture by ash on excess air factor. The data show that an increase in excess air factor
or the partial pressure of oxygen in flue gas enhances the sulfation of ash particles and the
extent of sulfur capture.

Figure 30. Extent of sulfur capture by ash particles as a function of excess air factor during oil
shale firing.
31
Sulfur dioxide emissions in oil shale pulverized boilers have been measured by the
Department of Thermal Engineering of Tallinn University of Technology. According to
measurements the sulfur dioxide concentration in flue gas leaving the boiler (flue gas
contained 6 % O
2
) was between 1750 to 2700 mg/m
3
, and he extent of sulfur capture by ash,
in the range k
S
= 0.72 to 0.81.
The sulfur dioxide emissions were studied in a pilot scale circulating fluidized bed combustor
and in industrial device. Results show that in both cases the sulfur dioxide concentration in
flue gas is very low <15 mg/m
3
(O
2
concentration in the flue gas was 6 %).
The possibility capture of sulfur dioxide by oil shale ash strongly depends on the behavior of
calcium oxide in combustion process. Important is the form of calcium in the ash.
One parameter that influences the extent of sulfur capture by ash is the molar ratio Ca/S.
In oil shale, the ratio Ca/S is 8 to 10. During the high-temperature pulverized firing of oil
shale and the formation of novel minerals in the boiler, about 70 to 80 % of CaO combines
with the sandy-clay minerals. The calcium oxide bound within novel minerals is chemically
less active toward SO
2
compared to active free CaO. Therefore, it is not always sufficient to
characterize the fuel by the ratio Ca/S. The ratio Ca/S by active (free) CaO is only 1.5 to 2.5,
which is not for sufficient sulfur capture. This is why, during pulverized firing of oil shale, the
extent of SO
2
capture by ash is approximately 0.72 to 0.81.
During fluidized combustion the formation of novel minerals containing calcium oxide is
slow because of the low combustion temperature, and CaO released from carbonate minerals
stays mainly in active free form.
8.3. Carbon dioxide emission
Carbon dioxide belongs to the group of greenhouse gases.
Carbon dioxide is formed during the combustion reactions of organic carbon and from
minerals, which are present in fuels as carbonates. Full conversion of organic carbon to CO
2
is
possible only at complete combustion. The release of carbon dioxide from carbonate minerals
is determined by the behavior of fuel mineral matter during the combustion process.
The combustion technology used to burn the fuel does not significantly affect the amount of
CO
2
formed from organic carbon. However, the combustion technology may strongly
influence the emissions of mineral CO
2
. The concentration of mineral CO
2
formed from
carbonate compounds is determined by the conditions of the thermal decomposition of
minerals and also by the direct combination of gaseous components present in flue gas and
CO
2
-containing minerals.
The total volume of CO
2
released for each mass unit of oil shale depends on the amount of
organic matter, the content of mineral CO
2
, and the decomposition extent of carbonates. The
influence of the extent of carbonate decomposition on the CO
2
concentration in flue gas can
be expressed by the following formula:
1CO
CO
CO
2
CO2
2
CO2
2
)(
)(

=
−=
k
k
V
V

where
1CO
2
co2
)(
=k
V
- the highest possible concentration of CO
2
in flue gas, which corresponds to
complete the decomposition of carbonate minerals (
2
CO
k
= 1), m
3
/kg; and
2
CO2
)(
CO k
V
- CO
2
concentration in flue gas, which corresponds to the arbitrary decomposition extent of
carbonates, m
3
/kg.
32
Based on the relationship between the composition of oil shale and its heating value Figure 31
depicts the relationship between factor
2
CO
ν
慮搠ah攠數瑥et=潦o捡牢潮慴攠de捯mp潳楴楯渠慴⁦潵爠
摩晦敲敮d=汯睥爠桥慴lng⁶慬略s⸠䥦.捡 牢rn慴攠mi湥牡汳n摯ot⁤散ompos攠(
2

k
= 0), the CO
2
emissions from oil shale, whose heating value is 8.5 MJ/kg, are about 22 % lower compared
to conditions when
2
CO
k
= 1.

Figure 31. Changes in carbon dioxide concentration in flue gas depending on extent of
carbonates decomposition at different heating values.