Air Pollution Control
7.
Emissions
7.1
Carbon Dioxide
Every fossil fuel produces CO2 emissions according to its carbon content. Carbon dioxide is
admittedly not poisonous, yet is blamed for a warming of the earth’s atmosphere. Table 7.1
shows guide values for the specific CO2 emissions. From this it is obvious that natural gas
causes the lowest emissions and coal the highest. By way of comparison the CO2 emissions
of the German electricity power plants (mean value for all used fuels and nuclear power) are
specified in relation to the electric energy on one hand and in relation to the primary energy
on the other hand, an average power plant is assumed to have an efficiency of 38%.
7.2
Nitric Oxide
The formation of nitric oxide can be subdivided into three mechanisms according to the
nitrogen source:
•
•
•
thermal NO
prompt NO
fuel NO.
7.2.1 Thermal NO
According to Zeldovich, who first postulated this mechanism in 1946, the thermal NO is
formed after three reactions. First the nitrogen reacts with atomic oxygen in accordance with
N 2 + O → NO + N .
(7-1)
The atomic nitrogen reacts further with O2 and OH in accordance with
N + O 2 → NO + O
(7-2)
N + OH → NO + H .
(7-3)
In accordance with the three reaction equations the following applies to the NO formation
d~
x NO
= kI ⋅ ~
xO ⋅ ~
x N 2 + k II ⋅ ~
xN ⋅~
x O2 + k III ⋅ ~
xN ⋅~
x OH
dt
(7-4)
and the following to the change of the atomic nitrogen
d~
xN
= kI ⋅ ~
xO ⋅ ~
x N 2 − k II ⋅ ~
xN ⋅ ~
x O 2 − k III ⋅ ~
xN ⋅~
x OH .
dt
The reactions possess the reaction coefficients (Warnatz)
(7-5)
⎡ − 318kJ ⋅ mol −1 ⎤ ⎡ m 3 ⎤
k I = 1.8 ⋅ 1011 ⋅ exp ⎢
(R ⋅ T ) ⎥⎦ ⎢⎣ kmol ⋅ s ⎥⎦
⎣
(7-6)
⎡ − 27kJ ⋅ mol −1 ⎤ ⎡ m 3 ⎤
k II = 9.0 ⋅ 10 6 ⋅ exp ⎢
(R ⋅ T ) ⎥⎦ ⎢⎣ kmol ⋅ s ⎥⎦
⎣
(7-7)
⎡ m3 ⎤
k III = 2.8 ⋅ 1010 ⎢
⎥.
⎣ kmol ⋅ s ⎦
(7-8)
The first reaction is rate-limiting. In view of its high activation energy this reaction first
proceeds rapidly enough at high temperatures so that this NO formation can be designated as
thermal NO.
In view of the rapid further reaction of the nitrogen atoms in accordance with the equations
(II) and (III) their concentration can be regarded as quasi-stationary. Using d~
x N dt = 0 the
following simple connection ensues for the NO formation from the two equations above
d~
x NO
= 2⋅ kI ⋅ ~
xO ⋅ ~
x N2 .
dt
(7-9)
A smaller reaction coefficient and with it a low temperature or a low nitrogen concentration,
e.g. by applying pure oxygen instead of air, are consequently necessary for a low thermal NO
formation.
In accordance with the above equations, the N2 and the O concentration are necessary for the
calculation of the NO formation. The N2 concentration is known from the composition of the
combustion gas. The O concentration is higher in the flame front than in the surrounding gas
so that the application of the thermodynamic equilibrium leads to incorrect values.
Essentially O is formed through the three equations
H + O 2 → OH + O
H + OH → H 2 + O
OH + H 2 → H 2 O + H .
The reaction rates amount to (Warnatz)
70.3kJ ⎤ ⎡ m 3 ⎤
⎡
k H −O 2 = 2.0 ⋅ 1011 ⋅ exp ⎢ −
⎢
⎥
⎣ R ⋅ T ⋅ mol ⎥⎦ ⎣ kmol ⋅ s ⎦
(7-13)
and
26.3kJ ⎤ ⎡ m 3 ⎤
⎡
k H 2 −O = 5.1 ⋅ 101 ⋅ T 2.67 ⋅ exp ⎢ −
⎢
⎥
⎣ R ⋅ T ⋅ mol ⎥⎦ ⎣ kmol ⋅ s ⎦
and
(7-14)
k OH − H 2 = 1.0 ⋅ 10 ⋅ T
5
1.6
3
⎡ 13.8kJ ⎤ ⎡ m ⎤
⋅ exp ⎢−
⎢
⎥.
⎣ R ⋅ T ⋅ mol ⎥⎦ ⎣ kmol ⋅ s ⎦
(7-15)
An equilibrium between the three equations above can be assumed with good approximation
at temperatures above 1300 °C. The following then results for the O concentration
~
x O2
= K 5 (T )
~
x2
(7-16)
O
(
~
xO = ~
x O2 / K 5
)
1/ 2
(7-17)
with the equilibrium number according to Table 2-6. Figure 3-5 shows the concentration for
an example in dependence on the excess air number. This number has only a weak influence
in the range λ > 1.1. But in the range λ < 1 the O-concentration decreases rapidly with the
excess air number. Below λ < 0.7 the concentration becomes very low. Therefore often a
two step combustion is propagated for low NOx emission: in the first step with λ = 0.7 to
λ = 0.9 and then after heat transfer the second step with an overall excess air number greater
than one.
Essentially low temperatures are thus necessary for the minimizing of the thermal NO. Such
low combustion temperatures are achieved through a high heat transfer during the
combustion and a corresponding combustion chamber construction or through flue gas
recirculation, as a result of which the heat capacity of the gas increases and thus the
temperature is reduced. If such primary measures are not possible or do not lead to
sufficiently low NO emissions, secondary measures are necessary.
7.2.2 Prompt NO
As shown in section 4.1 with figure 4-1 for the reaction mechanism of methane, a CH radial
is formed intermediately during the conversion. This radical causes the relatively rapid
splitting of the triple bond of the N2 molecule and reacts with the latter such that hydrogen
cyanide (HCN) and atomic nitrogen are formed.
CH + N 2 → HCN + N .
(7-19)
According to the equations (7-2) and (7-3) this atomic nitrogen reacts further so that nitric
oxide is formed. This mechanism had been initially described by Fenimore (1979). The
activation energy of the above reaction amounts to some 75 – 90 kJ/mol so that it is relatively
small. Consequently, NO formation is relatively rapid (prompt NO) and starts even at lower
temperatures of about 700 °C.
The formation and the decomposition of the CH radical are complex and not yet sufficiently
defined. Therefore, a universally applicable reaction approach for the NO formation is not
yet known.
The formation of prompt NO is restricted to the region of the flame front since the required
hydrocarbon radicals occur only there. The low concentration of these radicals and their
competitive reactions affecting the fuel decomposition are responsible for the fact that the
absolute volume of nitrogen monoxide formed on the basis of the prompt NO mechanism is
relatively small in most cases.
Comprehensive theoretical and experimental investigations into the combustion of natural
gas with air on laminar pre-mixing flames and turbulent pre-mixing and diffusion flames
came to the result that in case of stoichiometric and hypo-stoichiometric air/fuel ratios ( λ ≥
1) the prompt NO takes a share of less than 10% of the total NOx emission (Stapf, Leuckel
1996). Consequently, only little importance can be attributed to this NO formation
mechanism in case of technical combustion processes which take place under conditions of
an excess of air. On the other side, the prompt NO formation under fuel-rich conditions, e.g.
in case of a two-stage combustion process in the primary stage, may have an essential
influence on the amount of the total NOx emission (e.g. Tomeczek and Gradon 1997 as well
as Glassman 1996). This assessment is confirmed by measuring results derived from premixed methane-air flames where the maximum of prompt NO formation with some 50 ppm
lies in the air number range of 0.7 ≤ λ ≤ 0.8 .
In heating boiler installations where the flame temperature may be kept relatively low the
thermal NO formation is small. When determining the total NO emission the prompt NO
must be considered in this case. In industrial firing installations, however, the prompt NO can
be neglected with respect to the thermal NO and especially in case of liquid and solid fuels
even with respect to the fuel NO.
7.2.3
Fuel NO
In case the fuel contains chemically bonded nitrogen (fuel nitrogen) in organic (e.g. amines,
amides, nitrides, pyritine) or inorganic nitrogen compounds (e.g. ammonia, HCN), nitric
oxide will be generated during the combustion process on the basis of another mechanism.
The NO formation taking place on the basis of this fuel NO mechanism is of specific
importance both for the combustion of fossil fuels (coal, natural oils etc.) and for the thermal
disposal of gaseous, liquid and solid nitrogen-containing process residues (flues) since the
fuel nitrogen may present the main source of NOx emissions in these cases. The fuel N
content may reach up to 2% by weight in case of coals and residual oils but residual matter
and flues from chemical industry may contain more than 50% by weight of chemically
bonded nitrogen, e.g. in the form of NH3.
If the fuel nitrogen is contained in organic compounds, HCN is initially produced as
intermediate compound in a number of very rapid decomposition reactions under separation
of hydrogen atoms. This intermediate compound are converted through the radicals to CO
and NHi. The associated essential reactions are:
HCN + O
→ NCH + H
(7-20)
NCO + H
→ NH
+ CO
(7-21)
HCN + O
→ NH
+ CO
(7-22)
HCN + OH → NH2 + CO .
(7-23)
On the other hand, inorganic nitrogen compounds are directly converted into NHi radicals.
The decisive reaction is as follows:
NH3 + OH
→NH2 + H2O
(7-24)
The NHi formation from inorganic compounds takes place more rapidly than the formation
from organic compounds.
The NH2 radicals are subsequently decomposed with H and OH in rapid reactions according
to
NH2 + H
→ NH
+ H2
(7-25)
NH + H
→ N
+ H2
(7-26)
NH2 + OH → NH
+ H2O
(7-27)
NH + OH → N
+ H2O .
(7-28)
The further reaction sequence depends on the fact if oxidizing or reducing conditions are
existing. In case of oxidizing conditions, the nitric oxide is produced on the basis of the
following reaction equations:
+ O2
→ NO
+O
(7-29)
NH + O2
→ NO
+ OH
(7-30)
+H
(7-31)
+ H2
(7-32)
N
N
+ OH → NO
NH + OH → NO
NH + O
→ NO
+H
(7-33)
NH2 + O
→ NO
+ H2 .
(7-34)
In case of low-oxygen conditions the NO formation are suppressed due to the low
concentration of oxidizing radicals. Consequently, the NHi radicals are preferentially
transformed with nitric oxide into molecular nitrogen.
NH2 + NO → N2
+ H2O
(7-35)
NH + NO → N2
+ OH .
(7-36)
The issue which of the NHi radicals will be mainly involved in the reactions for the formation
of NO and N2 is essentially dependent on the thermal and stoichiometric combustion
conditions.
The overall mechanism of fuel NO formation is shown in figure 7-1. A detailed description
of this mechanism may be found in literature, e.g. in the publications of Jahnson et al. (1988
and 1989), Jahnson (1991), Kolb (1990) and Sybon (1994). Moreover, catalytic, noncatalytic, heterogeneous gas-solid reactions with ash, coal, coke or other solid particles may
be of importance for the NOx emissions in the combustion process of coal and liquid fuels
(Kremer and Schulz [1984] as well as Kremer, Schulz, et al. [1985]).
The fuel NO formation is coupled to the fuel oxidation through the radicals. Until now the
complex overall reaction mechanism makes it impossible to get exact calculations of the
nitric oxide emission for technical combustion systems. Nonetheless, a simplified
quantitative description of NOx-formation resulting from combustion of nitrogen-containing
fuels is possible, as Fenimore and De Soete documented in their investigations. Both used
laminar pre-mixing flames for their experiments since in this flame type the kinetics of fuel
NO formation and reduction may be evaluated almost independently of the mixing process
between fuel and the oxidizing agent. This aspect will be discussed in detail below.
Global NO formation mechanism by Fenimore
Fenimore (1972, 1976, 1979, 1980) used his combustion tests with different nitrogen
compounds for the establishment of a global reaction mechanism in which the fuel nitrogen
gets completely converted into a species of the NHi radicals through the HCN intermediate
compound irrespective of its kind of bond. In this case, the NHi radicals will react either with
the OH radicals according to the reactions (7-31) and (7-32) so that nitrogen monoxide is
formed, or they will be converted with NO into molecular nitrogen according to the reactions
(7-35) and (7-36). This fact has been the basis for the following relation developed by
Fenimore in 1972
~
⎡ 1~
x NO
x NO + ~
x NOanf ⎤
1
exp
=
−
−
⎢
⎥
~
~
x NOgl
x NOgl
⎥⎦
⎣⎢ 2
(7-37)
for the calculation of the concentration ~
x NO of fuel nitrogen oxide. In this equation ~
x NOanf is
the initial concentration of fuel nitrogen oxide which corresponds to the theoretical NO
concentration in case of complete conversion of N contained in the fuel into NO, whereas
~
x NOgl is the NO equilibrium concentration for which the approximation
E
~
x NOgl = exp( A − )
T
(7-38)
is applicable. The variables A and E are dependent on the air number and must be determined
by means of experiments. Thus this global mechanism makes it possible to describe the fuelNO formation according to Fenimore independent of the reaction kinetics and, consequently,
independent of the residence time in the combustion processes.
For example, Scheuer (1987) and Gardeik (1985) used this Fenimore mechanism to describe
the formation and decomposition of NO in cement furnace plants, and Klöppner et al. (1993
and 1995) described the NO concentrations of residual oils in swirl combustion chamber
systems.
Global NO formation mechanism according to De Soete
In contrast to Fenimore, De Soete (1974 and 1981) distinguishes a number of so-called
secondary nitrogen compounds (NH3, HCN and (CN2) formed by pyrolysis reactions from
primary nitrogen compounds introduced together with the fuel. De Soete used the results of
combustion tests to determine for the a.m. secondary nitrogen compounds the reaction speeds
required for the formation either of nitrogen monoxide or molecular nitrogen on the basis of
the following global reaction mechanism:
NX + O2 → NO
+ ...
(7-39)
NX + NO → N2
+ ... .
(7-40)
He states the following equation for the NO formation speed:
d~
x NO ~
= x NX ⋅ (k O 2 ⋅ ~
x On 2 − k NO ⋅ ~
x NO )
dt
(7-41)
and the following equation for the decomposition speed of fuel-nitrogen compounds:
d~
x NX
= −~
x NX ⋅ (k O 2 ⋅ ~
x On 2 + k NO ⋅ ~
x NO ) .
dt
(7-42)
The reaction coefficients k o2 and KNO defined by De Soete for different nitrogen compounds
are given in Table 7-2. The approximation
⎡ ⎛ ln ~
x o2
n = 1 − exp ⎢− ⎜⎜
⎢⎣ ⎝ a
⎞
⎟⎟
⎠
b
⎤
⎥.
⎥⎦
(7-43)
is applicable to the exponent n. Consequently, the exponent equals 1 in case of low O2
concentrations and equals 0 in case of high O2 concentrations. The constants a and b shall be
determined experimentally for each single application case.
Among others, the mechanism of De Soete made it possible to describe the NO formation
even in a cement kiln plant and in a swirl combustion chamber system of Jeschar, Jennes et
al. (1996 and 1999) and Malek, Scholz et al. (1993). Adaptation parameters to be determined
experimentally are required for each application case, i.e. both for the global mechanism of
De Soete and the global mechanism of Fenimore.
7.2.4
Primary Measures for the Reduction of Nitric Oxides
Measures aiming at the reduction of pollutant emissions from combustion processes are
generally subdivided into so-called primary and secondary measures. Primary measures are
intended to restrict the formation of pollutants from combustion by means of suitable process
modification. On the other hand, secondary measures are intended to reduce the pollutants
formed during the combustion process in the flue gas flow after combustion.
As shown in the description of the NO formation mechanism before, the NO formation may
be reduced by the following three conditions:
-
Low combustion temperatures
Short retention time at high temperatures
Low oxygen concentration.
Figure 7-3 is an example of the influence of the first two conditions on the NO concentration
after combustion of methane with an air number of 1.05. The strong dependency on
temperature mainly above 1600 °C and on the retention time can be recognized.
These three conditions above may be reached by taking suitable primary measures with
regard to fuel engineering like
-
Flue gas recirculation
Air gradation
Fuel gradation.
These measures will be the more effective the more the combustion process can be decoupled
from the use of energy.
Flue gas reecirculation
In principle, two variants of flue gas recirculation may be distinguished: External and internal
recirculation. In case of the external flue gas recirculation a defined "cold" flue gas volume
from an external source is supplied to the firing installation. On the other side, the internal
flue gas recirculation causes the recirculation through defined flow control in the combustion
system such that flue gas from the fire chamber environment gets suck into the flame area.
Such a mode of flow control can be reached by modification of the burner geometry, e.g.
through the injector effect or through swirling the combustion air.
Recycling of flue gases reduces the oxygen concentration in the supplied combustion air. The
mixing of fuel and combustion air is retarded, the combustion temperature is lowered through
the additional flue gas ballast and the retention time is reduced in high temperature ranges.
Thus the thermal NO formation is virtually suppressed. On the other hand, the fuel NO
formation is less strongly influenced by lowering the combustion temperature as a result of
flue gas recirculation. This principle is shown in figure 7-4 where the relative nitric oxide
formation for different fuels is compared as a function of flue gas recirculation (Feist 1991).
The NO formation is only insignificantly reduced by flue gas recirculation mainly in case of
fuels with a high share of chemically bonded nitrogen, such as heavy fuel oil and coal.
Air gradation
The principle of air gradation is shown in figure 7-5. The combustion air is split into two
separate air flows in order to avoid peak temperatures in the flame zone and to reduce the
oxygen partial pressure. In the first combustion stage (primary stage) the fuel is converted
sub-stoichiometrically. Under these conditions of air lack the NO formation is suppressed to
a large extent and the N compounds introduced together with the fuel is decomposed at the
same time under sufficiently high temperatures so that molecular nitrogen is formed. The
second combustion stage (secondary or burn-out stage) is operated hypo-stoichiometrically
so that the required burn-out effect can be ensured as high as possible. If applicable, heat
must be discharged at the end of the of the primary stage so that the end temperature reached
in the air-rich secondary stage remains below the limit of the thermal NO formation.
The effect of air gradation is demonstrated in the example in figure 7-6 which shows the
concentrations of the N species NO, HCN and NH3 measured by Takagi et al. (1979) at the
end of the primary stage and the secondary stage as a function of the air number λ p of the
primary stage. The total air number is always λ ges = 1.25. When the air number of the
primary air stage increases, the NH3 and HCN concentrations decrease at the end of this stage
while the NO concentration increases since more and more oxygen is available. At the end of
the secondary stage the NH3 and HCN concentrations decrease as well as a function of the
air number λ p . However, the NO concentration temporarily decreases, passes a minimum in
the range of λ p = 0.7 − 0.8 and increases. The relatively high NO concentration at low air
numbers of the primary stage can be attributed to the fact that very oxygen-rich conditions
are prevailing in the secondary stage. The oxygen supply in the secondary stage decreases as
a function of the increasing air number λ p .
Figure 7-7 shows the NO concentrations at the end of the burn-out stage in case of air
gradation where the share of fuel nitrogen had been changed by use of different natural
gas/ammonia mixtures. The tests have been made by Weichert et al. (1995) with a grade flare
having a thermal output of 24 kW. The strong reduction of NO emission through air
gradation can be recognized again. Consequently, the optimum air number of the primary
stage is dependent on the share of fuel nitrogen and will rise together with the latter. The
figure furthermore demonstrates that the air gradation may contribute to a considerable NO
reduction even in case of fuels without chemically bonded nitrogen (0 % NH3).
Fuel gradation
Problematic fuels like coal, heavy fuel oils or liquid process residues will hardly show any
flame-stabilizing properties in the combustion under reduced conditions but have a tendency
to strong soot formation. In case of such fuels the fuel gradation for reduction of NOx
emissions offers advantages in comparison with the air gradation since the primary stage can
be operated neary stoichiometrically.
As shown in the diagram in figure 7-8, the fuel gradation is a three-stage combustion process.
The effect of the fuel gradation is that nitrogen monoxide formed already is transformed into
molecular nitrogen again. Different reaction conditions are necessary for each of the single
combustion stages. In the first combustion stage (primary stage) the primary fuel is converted
neary stoichiometrically, i.e. with a low excess of air. The second combustion stage
(secondary or reaction stage) is operated sub-stoichiometrically through the addition of
secondary fuel. Suitable fuels for this purpose are the primary fuel as well as other fuels like
natural gas. Under fuel-rich conditions the nitric oxides formed in the primary stage are
largely converted into molecular nitrogen under the essential participation of fuel radicals
(CHi). The latter will be formed as a result of the partial oxidation of hydrocarbons contained
in the reduction fuel and bring the NO through the recycling reactions (7-35) and (7-36) NO
+ CHi → HCN + Hi-1O back into the fuel N mechanism in the form of HCN. The
decomposition of HCN through NHi radicals as intermediate compounds into molecular
nitrogen is promoted under the reduced combustion conditions. Consequently, the NO
formation in the primary stage of fuel gradation is of subordinate importance for the total
NOx emission. The third stage (tertiary stage) is the post-combustion or burn-out stage. Burnout air is added so that this process as a whole is operated hypo-stoichiometrically in order to
ensure a burn-out effect as high as possible. Since this stage involves much lower
temperatures due to the air addition and as a result of the heat loss in both preceding
combustion stages, the renewed thermal NO formation is suppressed to a large extent.
The fuel gradation method is employed in practice only in very rare cases since the efforts
are very high. For further information see such publications like Chen et al. (1986),
Mechenbier (1989), Kolb (1990) and Sybon (1994) in literature.
7.2.5 Secondary Measures for the Minimization of NO
As secondary measures for the reduction of the NO emissions selective homogeneous
reduction (denoted as SHR or thermal DeNOx) and the selective catalytic reduction (denoted
as SCR) are available.
In the selective homogeneous reduction of ammonia (NH3), which is decomposed by OH to
NH2, is mixed with the combustion gases
NH 3 + OH → NH 2 + H 2 O
(7-44)
This NH2 reacts with the NO in accordance with
NH 2 + NO → N 2 + H 2 O
(7-45)
NH 2 + NO → N 2 H + OH .
(7-46)
These three reactions are the most important. In addition a large number of further
elementary reactions proceed, in which the N2H is finally also converted to N2.
If the temperature is not sufficiently high, NH3 thus does not react in the OH in accordance
with equation (X). At temperatures which are too high the NH3 is oxidized. Hence the
homogeneous reduction is possible only in a relatively narrow window of temperatures.
Figure 7-9 shows the NO reduction as a function of the temperature for an example. From
here it is obvious that the window of temperatures is approximately in the range of 900 °C to
1000 °C. Beyond this, the excess ammonia may not be too high compared to NO, since
x NH3 ~
x NO < 1,5 applies as
otherwise this excess leads to NO formation in the atmosphere. ~
guide value.
In the selective catalytic reduction NO on the surface of the catalyst is converted to N2, for
which H2O, NH3 is converted as well. The exact reaction mechanism is not known. In
catalytic reactions the reaction partner is first adsorbed on the surface. At the same time
molecules such as N2, O2 and H2 are dissociated. The atoms (N, H, O, etc.) can move
relatively easily. The adsorbed species springs onto a neighboring surface position. In
addition a low adsorption of energy must exist between the species and the surface material.
The reaction rate depends on the quantity of the occupied surface positions. The molecules
formed must further desorb from the surface. A species, which is too strongly adsorbed on
the surface and cannot desorb, blocks the surface positions. These poison the catalyst. Known
catalyst poisons are sulfur and lead.
7.3
Sulfur dioxide
7.3.1
Mechanism
If the fuel contains sulfur, such as oil and coal, SO2 is produced during combustion. It is
assumed that each percent by weight of sulfur contained in the fuel results in 500 ppm of SO2
in the flue gas. In case of an excess of air a part of this SO2 will oxidize to SO3 as follows:
SO2 + ½ O2 → SO3 .
(7-47)
The latter will react with water steam according to the equation
SO3 + H2O → H2SO4
(7-48)
so that sulfuric acid is produced. The acid condenses on walls provided the wall temperature
is below the dew-point temperature. In figure 7-10 the boiling and dew curves of a sulfuric
acid - water mixture is shown. In the gas phase the sum of the concentrations of both
x H 2O + ~
x H 2 SO4 = 0.1 ). Thus the dew point of pure water steam is
components is always 0.1 ( ~
45 °C which corresponds approximately to the combustion gas of an oil firing installation. It
should be noted that even small concentrations of sulfuric acid in the gas phase are in
equilibrium with high concentrations in the liquid. Consequently, the condensed-out sulfuric
acid has a high concentration and a strong corrosive effect.
Figure 7-11 shows the dew point of sulfuric acid as a function of the O2 concentration in the
combustion gas for two different sulfur contents in the fuel. It is obvious that the acid dewpoint temperature rises drastically especially in case of very low O2 excesses whereas only a
small rise takes place in case of O2 excess.
The following desulfurization measures are applicable:
- High-temperature desulfurization where limestone meal is blown in at temperatures of
about 1000°C;
- Dry process where lime hydrate meal is blown in at low temperatures;
- Semi-dry process where a suspension of lime hydrate meal is blown in; and
- Wet process where the combustion gas is blown through a layer of lime milk.
7.3.2. High-Temperature Desulfurization
In the high-temperature desulfurization process lime meal is blown into the high-temperature
zone of the combustion chamber. Decarbonization takes place according to the following
equation:
CaCO3 → CaO + CO2 ,
(7-49)
so that porous CaO particles are formed. These particles absorb the SO2 according to the
following equation:
CaO + SO2 + ½O2 → CaSO4.
(7-50)
A tight sulfate shell is formed around the particle which, in the course of the progressing
conversion time, will obstruct the diffusion of the SO2 to the remaining CaO core. The more
porous and the smaller the particle is, the more SO2 may be agglunitated.
Both reactions take place only in a definite temperature range. Figure 7-12 gives the
necessary explanations and shows the equilibrium curves of both reactions. In case of
conventional CO2 concentrations in the combustion gas of some 10% the temperature must
be higher than 750°C so that CO2 can be split off. According to Kainer et al. (1986) the
temperatures must be above 900°C so that the decomposition reaction may take place with a
sufficient rate. In case of temperatures above some 1150°C the CaO particle will sinter such
that the internal surface and, consequently, the reactivity will decrease. The particle will be
"burnt dead" in this case. The sulfate reaction of CaO with SO2 may develop in the desired
direction only at temperatures below some 1170°C (6% O2, 1000 ppm SO2). The calcium
sulfate formed already would decompose again at higher temperatures. These states of
equilibrium show a temperature range of some 900°C to 1150°C which must be maintained
for the high-temperature desulfurization with CaCO3. Moreover, it must be ensured that the
additive cannot be subjected even to short-time temperatures above 1200°C so that the
deactivation of additive can be avoided.
Figure 7-13 uses the example of Schopf et al. (1985) to show the obtained desulfurization
rates. Tests had been made in a swirl combustion chamber with SO2 doted natural-gas flames.
It can be recognized that the SO2 integration increases as a function of the molar Ca/S ratio.
The values of 1 to 2 correspond to desulfurization rates of some 80%. The optimum
combustion chamber temperature amounts to 1000°C. The desulfurization effect is
significantly lower even at a temperature deviation of ± 100 K, and about double of
limestone quality would be required already for the same desulfurization rate.
Figure 7-14 shows the desulfurization rate for both grain sizes < 15 µm and < 40 µm in
combination with the optimum combustion chamber temperature of 1000°C each. It can be
recognized that very fine grain sizes (smaller than 15 µm if possible) are required especially
for higher desulfurization rates.
Apart from the grain size the kind of limestone has an effect on desulfurization. Examples
may be found in the literature on research work of Mehlmann et al. (1987).
7.3.3. Low-temperature desulfurization
The so-called dry process uses a lime hydrate meal Ca(OH)2 for desulfurization in the lowtemperature range whereas a suspension of lime hydrate meal and water is blown-in in the
co-called semi-dry process. The dehydration equilibrium
Ca(OH ) 2 → CaO + H 2 O
(7-51)
is shown in Figure 7-12. Proceeding from usual water steam contents in the waste gas
between 5 and 20 %, the decomposition temperature of lime hydrate amounts to 375 °C or
425 °C. Since the low-temperature desulfurization takes place at temperatures below 300 °C,
no direct meal dehydration takes place. However, re-carbonatization is possible in
compliance with the following equation:
Ca (OH ) 2 + CO 2 → CaCO 3 + H 2 O .
(7-52)
The equilibrium line is shown as an example for the three water steam partial pressures of
5,10 and 20 %. In case of CO2 partial pressures above the equilibrium values the reaction will
take place in the direction stated above. The group of curves is limited by the equilibrium
line of limestone decomposition. The re-carbonatization according to the above reaction is
known from the hardening of mortar consisting of Ca(OH)2.
The reactions
Ca (OH ) 2 + SO 2 → CaSO 3 + H 2 O
(7-53)
1
Ca (OH ) 2 + SO 2 + O 2 → CaSO 4 + H 2 O .
2
(7-54)
and
are in the foreground for desulfurization. Figure 7-12 shows the equilibrium curve of the
upper reaction for both water steam concentrations of 10 and 20 %. In case of SO2 partial
pressures above the equilibrium values the reactions will again take place in the indicated
direction. The equilibrium values of the second reaction are so small that they are no longer
included in the figure. Consequently, desulfurization with lime hydrate is possible up to very
low SO2 concentrations. In this case the desulfurization effect will be determined by the
reaction kinetics.
In case of dry waste gases the desulfurization by blowing-in Ca(OH)2 will be limited to some
20 to 30 % (Hünlich 1991). This SO2 integration is largely dependent on the relative
humidity of the gas and rises as a function of the latter as it is shown in the example in Figure
7-15 . Desulfurization rates of 80% can be reached at a relative humidity of 0.8.
7.4
Hydrocarbon and Soot
Unburned hydrocarbon, polycyclic aromatized hydrocarbon and soot are differentiated in the
hydrocarbons. Admittedly the formation of these harmful substance has been experimentally
investigated many times, however an adequate theoretical understanding does not exist yet.
Unburned Hydrocarbons
Unburned hydrocarbons form through local extinguishing of the flame either on cold walls or
through elongation. If the extinguishing range, which possesses the dimension of the flame
front density, is fallen below on the cold walls then the heat flow is so high that the reaction
freezes on one hand and the radicals are destroyed on the wall by surface reactions on the
other hand. In industrial kilns the wall temperatures are, as a rule, so high that this effect does
not occur. If the flame fronts are greatly elongated, which for example can be caused by a
strong turbulence, then local extinguishing of the flame occurs, as was explained in this
section. If no new ignition takes place then the fuel leaves the reaction with incomplete burn
up.
Polycyclic Aromatized Hydrocarbons
Polycyclic aromatized hydrocarbons are formed from small hydrocarbon structural elements.
The most important precursor is the acetylene (C2H2) that is formed particularly in flames
rich in fuel or areas in higher concentration. The aromatized ring structures form then by
reaction in the C2H2 with CH or CH2 under the formation of C3H3, that then can form the first
ring by rearrangement. The subsequent rings then form through further attachments of C2H2.
Soot
Soot is formed by a further growth of the polycyclic aromatized hydrocarbons. Only the very
fine and invisible soot particles are going into the lungs and thus carcinogenic. The structure
of the soot can be characterized only with difficulty. The molar C/H ratio is one.
7.5
Emission Data
The level of emission concentration in flue gas is dependent on the air number. The higher
the air number, the lower the concentration. Consequently, emissions from different plants
must be based on defined air numbers so that emissions can be compared with each other.
Since the air number is normally determined on the basis of the measured O2 concentrations,
x iA is taken as the
emissions are effectively based on a defined O2 concentration. If ~
concentration of an emitted component (NO, SO2, CO etc.) in the flue gas with the O2
x o 2A , the following shall be applicable to the reference concentration ~
concentration ~
x iB at the
x :
reference O concentration ~
2
O2B
x o2B ~
0,21 − ~
~
⋅ x iA .
x iB =
0,21 − ~
x o2A
(7-51)
The reference O2 concentration is assumed with 3% or 6% in most cases. This corresponds to
air numbers of λ = 1.15 or 1.4 according to equation (2-4).
Limit values are frequently defined as the partial density ρ∗i (weight i per gas volume), e.g.
x i the resultant correlation is
mgi/m3G. Proceeding from the volume concentration ~
ρ ∗i = ~
x i ⋅ ρi ,
(7-52)
where ρ i is the density of the pure component i.
Nitric oxide emissions (NOx) are understood as the mixture of nitrogen monoxide (NO) and
nitrogen dioxide (NO2). As agreed before, the NOx concentration is specified as the partial
density of NO2. In many measuring gauges the NO will be converted to NO2 prior to
measurement. Table 7-3 shows the factors for conversion of volume concentration into
partial density for the most essential emissions.
In comparable plants, such as heat generation installations (heating boilers) the emissions are
considered as well with reference to the available heat (in kWh). Thus plants are independent
of the reference oxygen content. Table 7-4 shows a few emission limit values as examples
7.6
Concentration measuring methods
Different properties of the gas components are used to measure their concentration. The main
measuring methods are described below:
Light absorption method
Gases with free charge carriers, as CO, CO2, H2O, CH4, NO2, SO2 etc., absorb light of special
wave length. Fig. 7-15 shows the characteristic wave length of some gases. The decrease of
the intensity I0 of a light beam entering a gas is in accordance with the Beer-law
I = I O ⋅ exp(− a (λ ) ⋅ p ⋅ s ) ,
(7-57)
with p as partial pressure and there the concentration of the gas, s as the absorption length
and a as a coefficient depending on the kind of gas. The intensities I and I0 are measured. The
concentration of the gas is than calculated using Eq. (7-57) with the known length of the
probe tube. The gas is dried before measured, because the absorption wave length of water
steam overlaps the absorption wave length of many other gases. Therefor it is differed
between the concentration in the dry gas and the wet gas. For drying the gas is mostly cooled
down, so that the water steam condenses out.
Determination of oxygen concentration
Oxygen does not emit radiation. Thus the concentration has to be measured with other
properties of the gas. Mostly the magnetic susceptibility is used. This measuring method is
explained using Fig. 7-16.
The torsion balance (A) has a weight of only a few milligram. It consists of a torsion band
(C) with a mirror (D), a barbell with two nitrogen cooled glass balloons (E) and a wire
winding (H), encasing the last one. The balance hangs in a asymmetric magnetic field,
generated by the wedge shaped pole shoe (N) and (S). When oxygen containing gas flows
inside the cell, the oxygen aspires to move to the level with highest magnetic flux and
thereby tries to push apart the two diamagnetic glass balloons (E). The torsional moment
acting on the torsion balance is compensated by an artificial contra moment. This contra
moment can simply be measured. It is equal to the torsional moment and directly
proportional to the oxygen content of the cell gas.
Chemiluminescence
To determine the NO content the NO is converted with ozone to nitrogen dioxide inside an
analyzer according the equation
NO + O3 → NO2 + O2
(7-58)
The NO2-molecule is thereby shifted to an activated state and emits short wave radiation
(chemiluminescence). This radiation is amplified by a photo multiplier and than measured.
The radiation intensity is only depending on the NO-concentration, if ozone concentration is
high. In this measurement procedure the nitrogen dioxide is converted to NO according the
equation
2 NO2 → 2 NO + O2
(7-59)
Therefor the gas is heated up to high temperatures. The radiation intensity of the ozone
reaction is therewith determined with the summation of the concentrations of NO and NO2.
The indicated concentration of NOX is equivalent to the concentration of NO2.
Wet chemical methods
In wet chemical methods the gas is conveyed through several series connected fritted wash
bottles. Inside the bottles are different solutions, e.g. caustic soda and hydrogen peroxide in
determining the concentration of fluoride. The high of the concentration is measured with
special ion sensitive electrodes.
With the wet chemical methods different gas components like NO, SO2, SO3, HCl, HF, NH3
are measured analytical. The measurement according to this method are normally made
discontinuous and taken to compare with other methods.
1,E-03
1800 °C
1,E-04
1600 °C
1500 °C
1400 °C
1300 °C
1,E-05
XO
1200 °C
1,E-06
ϑG
1,E-07
1,E-08
0.,7
0.8
0.9
1
1.1
1.2
1.3
1.4
Excess air number λ
Fig. 7-1:
Atomic concentration of oxygen in the equilibrium combustion of natural gas
Fig. 7-2:
Thermal NO-forming in dependence on temperature and time
(Beckervordersandforth 1989)
No-Recycle
+CHi
NH3
Fuel -NO
Mechanism
Fuel
Nitrogen
NH2
+O,OH
+O,OH,H
HCN
NH
+NO
+CHi
N
Prompt-NOMechanism
NO
ZeldovichMechanism
N2
+O
Molecular
Nitrogen
+CHi
Fig. 7-3:
Mechanism of Fuel – NO – Formation
Fig. 7-4:
Reduction of NOx – emission by flue gas recirculation according to Feisst 1991
Fuel
nd
st
2 stage
1 stage
λS>1
λp<1
Primary air
Flue gas
Secondary air
Fig. 7-5:
Principal of air staging
4
10
1.35 Ma.%N, T=1050-1200 K
4 Ma.%N, T=1050-1200 K
1.35 Ma.%N, T=800-900K
ppm
NO
3
10
NO
NO
NOx
NO
NO x
NO
2
X 10
NOx
NO
HCN
NH3
HCN
HCN
HCN
NH3
NH3
NH3
10
NH3
NH3
HCN
HCN
1
1.25
Fig. 7-6:
1
0.8
λP
0.7
0.6
1.25
1
0.8
λP
0.7
0.6
1.25
1
0.8
λP
0.7
NO, HCN and NH3 Emissions at the end of Primary Stage (-----) and Secondary
Stage (
). Total excess air of two stage combustion λ tot = 1.25
0.6
Flue gas
83 Vol.% NH3
Secondary air
NH3
50 Vol.% NH3
1000
Natural gas
Primary air
500
9 Vol.% NH3
3
XNO in mg/m in NTP dry. (3% O2)
5000
0 Vol.% NH3
100
two stage
50
0.6
0.7
0.8
0.9
one stage
1.1
1
1.2
1.3
Excess air λtot (one stage) or λp (two stage)
Fig. 7-7:
NOx emissions for one and two stage combustion of natural gas with different
Ingredients of fuel nitrogen (reference fuel: natural gas and NH3)
[Weichert et al. 1995]
Fuel
st
1 stage
Primary air
nd
2
stage
λp <1
λT >1
Secondary
fuel
Fig. 7-8:
Principal of fuel staging
rd
3 stage
Tertiary
fuel
Flue gas
1.0
0.8
0.6
XNO End
XNO Initial
0.4
0.2
0
600
700
800
1000
900
1100
1200
Temperature in °C
Fig. 7-9:
Temperature window for NO-reduction by thermal DENOX according to
Warnatz et al. 1997
350
Gas
De
w
lin
300
e
Range of
mixture
r at
ion
lin
e
T in °C
250
Sa
tu
200
Liquid
150
H2SO4+H2O
100
0
0.2
0.4
0.6
XH
2SO 4
Fig. 7-10: Constitutional diagram of sulphuric acid and steam
0.8
1.0
160
150
PH O = 0,2 bar
2
0,18 bar
0,16 bar
0,14 bar
0,12 bar
0,10 bar
0,08 bar
ϑt in °C
140
130
120
110
100
0,1
0,5
1
X H2SO4
5 10
in ppm
50
100
Fig. 7-11: Temperature of dew point of waste gas for SO3
0
10
SO2-,H2O-,CO2- Partial pressure in bar
10
-1
H2O - CO2
Technical
range
CaCo3QCaO+CO2
-2
10 Ca(OH)2Q
CaO+H2O
10
10
Ca(OH)2+CO2Q
CaCO3+H2O
%H2O
20
10
5
-5
10
-6
10
10
10
Technical range
SO2 partial pressure
-3
-4
-7
-8
Partial pressure
CaO+SO2+1/2O2
QCaSO4
%O2
6
CaCO3+SO2+1/2O2Q
CO2+CaSO4
Ca(OH)2+SO2Q
%O2
CaSO4+H2O
(10%CO2)
%H2O
20
10
-9
1
6
10
100
300
1
500
700
900
Temperature in °C
Fig. 7-12: Equilibrium curves for lime reactions
1100
1300
100
%
X
80
900
SO2- Reduction
1100
1000
1000
60
X
ϑ (°C)
d(μm)
900 <15
X1000 <15
1100 <15
1000 <40
X
40
X
20
XSO2 (t=0) = 1000 ppm
0
0
1
3
2
Molar ratio Ca/S
4
Fig. 7-13: Influence of the molecular Ca/S relationship and a middle reactor temperature on
the desulphuristion with chalk
100
%
SO2- Reduction
60°C
x
93°C
80
60
2
80°C
x
40
100°C
20
0
CO2 = 12 Vol%
O2 = 8 Vol%
150°C
0
Ca(OH)2 m /g
35
35
x
x
18
18
0.2
3
SO2 = 2200 mg/m . i.N
Ca/S = 3
0.4
0.6
Humidity
Fig. 7-14: Influence of humidity on SO2 reduction
0.8
1.0
1
CO2
CO2
SO2
Emissivity
0.75
0.5
N2O
0.25
CO
NO
NO2
0
1
3
2
4
6
5
7
8
9
Wave length λ in μm
Fig. 7-15: Wave length of absorption for some gases
Heating Wire
Moving-Coil
Meter
Glass Tube
Permanent
Magnet
Annulus
Oxygen
Flue Gas
Balancing Resistor
Oxygen
Flue Gas
Balue of the Calibrated Gas
Zero Point
Fig. 7-16: Magnetic susceptibility of oxygen concentration measurement
10
Natural gas
fuel oil EL
hard coal
brown coal
wood
electrical
primary
Heating value
⎡ MJ ⎤
⎢ kg ⎥
⎣ B⎦
C-content
⎡ kg C ⎤
⎢ kg ⎥
⎣ B⎦
39,6
42,7
29,7
8,5
15,0
-
0,59
0,86
0,77
0,28
0,50
-
CO2-Emission
kg
⎡ CO 2 ⎤ ⎡ kg CO 2 ⎤
⎢ kWh ⎥ ⎢ MJ ⎥
⎣
⎦ ⎣
⎦
0,20
0,055
0,27
0,075
0,34
0,095
0,43
0,12
0,43
0,12
0,56
0,22
0,22
0,06
Table: 7-1: Specific CO2-Emissionen of different sources of energy
Reaction
Eact
[kJ/mol]
134
CH2 + O2 → NO + ...
k0
[1/s]
4,00 ⋅10 6
1,80 ⋅108
3,00 ⋅108
1,10 ⋅1010
CN2 + NO → N2 + ...
1,00 ⋅1010
134
HCN + O2 → NO + ...
3,00 ⋅1012
NH3 + O2 → NO + ...
NH3 + NO → N2 + ...
HCN + NO →N2 + ...
113
168
287
252
Table 7-2: Reaction coefficients for forming and reduction of NO according
to De Soete 1981
~
xi
1 ppm CO
ρ∗i
1.25 mg CO/m3 NTP
1 ppm NOx
2.05 mg NO2/m3 NTP
1 ppm SO2
2.93 mg SO2/m3 NTP
Tab. 7-3: Factors for conversion of volume concentration into partial density
Exemples
Unit
Relation
O2 in %
NOx
CO
SO2
Heating vessel <120 kW
Bimsch V 1996
Blue Angel
mg/kWh
80
mg/m3i. N.
94
mg/kWh
70
60
mg/kWh
60
50
100
(Net Calorific vessel)
Blue Angel
(Gross calorific vessel)
Firing Plants < 100 MW
Solid fuels
mg/m3 i. N.
7
500
250
2000
Heavy oil
mg/m3 i. N.
3
450
170
1700
Light oil
3
mg/m i. N.
3
250
170
1700
Natural gas
mg/m3 i. N.
3
200
100
35
Combustion engines
Diesel > 3 MW
mg/m3 i. N.
2000
650
-
Diesel < 3 MW
3
mg/m i. N.
4000
650
-
Otto
mg/m3 i. N.
500
650
-
Table 7-4: Examples for limits of emissions