Air Pollution Control 7. Emissions 7.1 Carbon Dioxide · Air Pollution Control 7. Emissions ......

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Air Pollution Control 7. Emissions 7.1 Carbon Dioxide Every fossil fuel produces CO 2 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 CO 2 emissions. From this it is obvious that natural gas causes the lowest emissions and coal the highest. By way of comparison the CO 2 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 NO O N 2 + + . (7-1) The atomic nitrogen reacts further with O 2 and OH in accordance with O NO O N 2 + + (7-2) H NO OH N + + . (7-3) In accordance with the three reaction equations the following applies to the NO formation OH N III O N II N O I NO x ~ x ~ k x ~ x ~ k x ~ x ~ k dt x ~ d 2 2 + + = (7-4) and the following to the change of the atomic nitrogen OH N III O N II N O I N x ~ x ~ k x ~ x ~ k x ~ x ~ k dt x ~ d 2 2 = . (7-5) The reactions possess the reaction coefficients (Warnatz)

Transcript of Air Pollution Control 7. Emissions 7.1 Carbon Dioxide · Air Pollution Control 7. Emissions ......

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

NNOON2 +→+ . (7-1) The atomic nitrogen reacts further with O2 and OH in accordance with

ONOON 2 +→+ (7-2)

HNOOHN +→+ . (7-3) In accordance with the three reaction equations the following applies to the NO formation

OHNIIIONIINOINO x~x~kx~x~kx~x~k

dtx~d

22⋅⋅+⋅⋅+⋅⋅= (7-4)

and the following to the change of the atomic nitrogen

OHNIIIONIINOIN x~x~kx~x~kx~x~k

dtx~d

22⋅⋅−⋅⋅−⋅⋅= . (7-5)

The reactions possess the reaction coefficients (Warnatz)

( ) ⎥⎦

⎤⎢⎣

⎡⋅⎥

⎤⎢⎣

⎡⋅⋅−

⋅⋅=−

skmolm

TRmol318kJexp101.8k

3111

I (7-6)

( ) ⎥⎦

⎤⎢⎣

⎡⋅⎥

⎤⎢⎣

⎡⋅⋅−

⋅⋅=−

skmolm

TRmol27kJexp109.0k

316

II (7-7)

⎥⎦

⎤⎢⎣

⎡⋅

⋅=skmol

m102.8k3

10III . (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 0dtx~d N = the

following simple connection ensues for the NO formation from the two equations above

2NOINO x~x~k2

dtx~d

⋅⋅⋅= . (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

OOHOH 2 +→+

OHOHH 2 +→+

HOHHOH 22 +→+ . The reaction rates amount to (Warnatz)

⎥⎦

⎤⎢⎣

⎡⋅⎥⎦

⎤⎢⎣⎡

⋅⋅−⋅⋅=− skmol

mmolTR

70.3kJexp102.0k3

11OH 2

(7-13)

and

⎥⎦

⎤⎢⎣

⎡⋅⎥⎦

⎤⎢⎣⎡

⋅⋅−⋅⋅⋅=− skmol

mmolTR

26.3kJexpT105.1k3

2.671OH2

(7-14)

and

⎥⎦

⎤⎢⎣

⎡⋅⎥⎦

⎤⎢⎣⎡

⋅⋅−⋅⋅⋅=− skmol

mmolTR

13.8kJexpT101.0k3

1.65HOH 2

. (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

)T(Kx~x~

52O

O2 = (7-16)

( ) 2/15OO K/x~x~

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. . (7-19) NHCNNCH 2 +→+ 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 NO

≥x 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 pre-mixed methane-air flames where the maximum of prompt NO formation with some 50 ppm lies in the air number range of 0.7 8.0≤≤ λ . 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:

N + O2 → NO + O (7-29)

NH + O2 → NO + OH (7-30)

N + OH → NO + H (7-31)

NH + OH → NO + H2 (7-32)

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, non-catalytic, 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

⎥⎥⎦

⎢⎢⎣

⎡ +−−=

NOgl

NOanfNO

NOgl

NO

x~x~x~

21exp1

x~x~ (7-37)

for the calculation of the concentration of fuel nitrogen oxide. In this equation 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

NOx~ NOanfx~

NOglx~ is the NO equilibrium concentration for which the approximation

)TEAexp(x~ NOgl −= (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 fuel-NO 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:

)x~kx~k(x~dtx~d

NONOnOONX

NO22

⋅−⋅⋅= (7-41)

and the following equation for the decomposition speed of fuel-nitrogen compounds:

)x~kx~k(x~dtx~d

NONOnOONX

NX22

⋅+⋅⋅−= . (7-42)

The reaction coefficients and K2ok NO defined by De Soete for different nitrogen compounds

are given in Table 7-2. The approximation

⎥⎥⎦

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−−=

bo

ax~ln

exp1n 2 . (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 of the

primary stage. The total air number is always pλ

gesλ = 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 . However, the NO concentration temporarily decreases, passes a minimum in

the range of pλ

8.07.0 −=pλ 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 pλ .

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. Burn-out 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

OHNHOHNH 223 +→+ (7-44)

This NH2 reacts with the NO in accordance with

OHNNONH 222 +→+ (7-45)

OHHNNONH 22 +→+ . (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 otherwise this excess leads to NO formation in the atmosphere. 5,1x~x~ NONH3

< applies as

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 components is always 0.1 ( 1.0~~

422=+ SOHOH xx ). Thus the dew point of pure water steam is

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 dew-point 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 low-temperature range whereas a suspension of lime hydrate meal and water is blown-in in the co-called semi-dry process. The dehydration equilibrium (7-51) OHCaO)OH(Ca 22 +→ 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: OHCaCOCO)OH(Ca 2322 +→+ . (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 OHCaSOSO)OH(Ca 2322 +→+ (7-53)

and

OHCaSOO21SO)OH(Ca 24222 +→++ . (7-54)

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 HydrocarbonsUnburned 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 HydrocarbonsPolycyclic 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, emissions are effectively based on a defined O2 concentration. If iAx~ is taken as the concentration of an emitted component (NO, SO2, CO etc.) in the flue gas with the O2

concentration Ao2x~ , the following shall be applicable to the reference concentration at the

reference OiBx~

2 concentration BO2x~ :

iAAo

BoiB x~

x~21,0x~21,0

x~2

2 ⋅−

−= . (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 (weight i per gas volume), e.g.

mg

∗ρi

i/m3G. Proceeding from the volume concentration ix~ the resultant correlation is

, (7-52) iii x~ ρ⋅=ρ∗

where is the density of the pure component i. iρ 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

( )( spaexpII O ⋅⋅λ−⋅= ) , (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

1,E-04

1,E-05

1,E-06

1,E-07

1,E-080.,7 0.8 0.9 1 1.1 1.2 1.3 1.4

Excess air number λ

1800 C°1600 C°1500 C°1400 C°1300 C°1200 C°

ϑG

XO

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)

Fuel Nitrogen

No-Recycle

+CHi

NH3

NH2

NH

+O

+NO

+O,OH

N

N2

NO

HCN+O,OH,H

+CHi

+CHiPrompt-NO-Mechanism

Fuel -NOMechanism

Zeldovich-Mechanism

MolecularNitrogen

Fig. 7-3: Mechanism of Fuel – NO – Formation

Fig. 7-4: Reduction of NOx – emission by flue gas recirculation according to Feisst 1991

Fuel

Primary air

1 stagest

λp<1

2 stagend

λS>1

Secondary air

Flue gas

Fig. 7-5: Principal of air staging

1.35 Ma.%N, T=1050-1200 K104

ppm

103

102

10

1

NONOxNO

HCN

NH3

NH3

HCN

1.25 1 0.8 0.7 0.6λP

1.35 Ma.%N, T=800-900K

NO

NOxNO

HCNNH3

NH3

4 Ma.%N, T=1050-1200 K

NONOxNO

HCNNH3

HCN

NH3HCN

1.25 1 0.8 0.7 0.6λP

1.25 1 0.8 0.7 0.6λP

X

Fig. 7-6: NO, HCN and NH3 Emissions at the end of Primary Stage (-----) and Secondary Stage ( ). Total excess air of two stage combustion 25.1tot =λ

5000

1000

500

100

500.6 0.7 0.8 0.9 1 1.1 1.2 1.3

Excess air (one stage) or (two stage)λ λtot p

50 Vol.% NH3

9 Vol.% NH3

0 Vol.% NH3

two stage one stage

83 Vol.% NH3X

in m

g/m

in N

TP d

ry. (

3% O

)N

O2

3

NH3

Secondary air

Flue gas

Natural gasPrimary air

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

Primary air

1 stagest

λp <1

2 stagend

Secondaryfuel

Flue gas

Tertiary fuel

3 stagerd

λT >1

Fig. 7-8: Principal of fuel staging

1.0

0.8

0.6

0.4

0.2

0600 700 800 900 1000 1100 1200

Temperature in °C

XNO Initial

XNO End

Fig. 7-9: Temperature window for NO-reduction by thermal DENOX according to Warnatz et al. 1997

350

300

250

200

150

1000.2 0.4 0.6 0.8 1.00

T in

C °

XH SO2 4

Gas

Dew

lin

e

Liquid

H SO +H O2 4 2

Range ofmixture

Stu

rat

aio

n e

lin

Fig. 7-10: Constitutional diagram of sulphuric acid and steam

160

150

140

130

120

110

1000,1 0,5 1 5 10 50 100

ϑt

°in

C

in ppmXH SO2 4

0,2 bar0,18 bar0,16 bar0,14 bar0,12 bar0,10 bar0,08 bar

P =H O2

Fig. 7-11: Temperature of dew point of waste gas for SO3

100

10-1

10-2

10-3

10-4

10-5

10-6

10-7

10-8

10-9

Technicalrange

SO-,H

O-,C

O- P

artia

l pre

ssur

e in

bar

22

2

Temperature in °C

100 300 500 700 900 1100 1300

H O - CO2 2 Partial pressure

Technical rangeSO partial pressure2

20105

2010

%H O2

16%O2

16

Ca(OH)CaO+H O

2

2

Ca(OH) +COCaCO +H O

2 2

3 2

CaO+SO +1/2OCaSO

2 2

4

Ca(OH) +SOCaSO +H O

2 2

4 2

CaCO +SO +1/2OCO +CaSO

3 2 2

2 4

%O(10%CO )

2

2

CaCo3 CaO+CO2

%H O2

Fig. 7-12: Equilibrium curves for lime reactions

X

X

X

X

100

80

60

40

20

0

%

0 1 2 3 4

10001100

900 1000SO

Red

uctio

n2-

Molar ratio Ca/S

ϑ μ ( C) d( m) 900 <15X1000 <15 1100 <15 1000 <40

°

XSO2 (t=0) = 1000 ppm

Fig. 7-13: Influence of the molecular Ca/S relationship and a middle reactor temperature on the desulphuristion with chalk

x

x

100

80

60

40

20

00 0.2 0.4 0.6 0.8 1.0

Humidity

SO- R

educ

tion

2

93 C° 60°C

100°C

150°C

35351818

Ca(OH) m /g22

x x80°C

%

CO = 12 Vol%O = 8 Vol%

SO = 2200 mg/m i NCa/S = 3

2

2

2

3. .

Fig. 7-14: Influence of humidity on SO2 reduction

1054

Wave length in mλ μ

0.25

0.5

0.75

1

0

Emis

sivi

ty

1 2 3 6 7 8 9

CO2 CO2 SO2

N O2CO

NO

NO2

Fig. 7-15: Wave length of absorption for some gases

Heating Wire

Glass Tube

Moving-Coil Meter

Permanent Magnet

Annulus

Oxygen

Flue Gas

Balue of the Calibrated Gas

Zero Point

Balancing Resistor Oxygen

Flue Gas

Fig. 7-16: Magnetic susceptibility of oxygen concentration measurement

Heating value C-content CO2-Emission

⎥⎦

⎤⎢⎣

BkgMJ

⎥⎦

⎤⎢⎣

B

C

kgkg

⎥⎦

⎤⎢⎣

⎡kWhkg

2CO ⎥⎦

⎤⎢⎣

⎡MJ

kg2CO

Natural gas 39,6 0,59 0,20 0,055 fuel oil EL 42,7 0,86 0,27 0,075 hard coal 29,7 0,77 0,34 0,095

brown coal 8,5 0,28 0,43 0,12 wood 15,0 0,50 0,43 0,12

electrical - - 0,56 0,22 primary - - 0,22 0,06

Table: 7-1: Specific CO2-Emissionen of different sources of energy

Reaction k0

[1/s] Eact

[kJ/mol] NH3 + O2 → NO + ... NH3 + NO → N2 + ... CH2 + O2 → NO + ... CN2 + NO → N2 + ... HCN + O2 → NO + ... HCN + NO →N2 + ...

61000,4 ⋅ 81080,1 ⋅ 81000,3 ⋅

101010,1 ⋅

101000,1 ⋅

121000,3 ⋅

134

113

168

134

287

252

Table 7-2: Reaction coefficients for forming and reduction of NO according

to De Soete 1981

ix~ ∗ρi 1 ppm CO

1 ppm NOx

1 ppm SO2

1.25 mg CO/m3 NTP

2.05 mg NO2/m3 NTP

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

(Net Calorific vessel)

Blue Angel

(Gross calorific vessel)

mg/kWh

mg/m3i. N.

mg/kWh

mg/kWh

80

94

70

60

100

60

50

Firing Plants < 100 MW

Solid fuels

Heavy oil

Light oil

Natural gas

mg/m3 i. N.

mg/m3 i. N.

mg/m3 i. N.

mg/m3 i. N.

7

3

3

3

500

450

250

200

250

170

170

100

2000

1700

1700

35

Combustion engines

Diesel > 3 MW

Diesel < 3 MW

Otto

mg/m3 i. N.

mg/m3 i. N.

mg/m3 i. N.

2000

4000

500

650

650

650

-

-

-

Table 7-4: Examples for limits of emissions