The formation of nitric oxide can be subdivided into three mechanisms according to the nitrogen source:

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1 Air Pollution Control 7. Emissions 7.1 Carbon Dioxide Every fossil fuel produces CO 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 emissions. From this it is obvious that natural gas causes the lowest emissions and coal the highest. By way of comparison the CO 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. Nitric Oxide The formation of nitric oxide can be subdivided into three mechanisms according to the nitrogen source: thermal prompt fuel Thermal According to Zeldovich, who first postulated this mechanism in 1946, the thermal is formed after three reactions. First the nitrogen reacts with atomic oxygen in accordance with N + O + N. (7-1) The atomic nitrogen reacts further with O and OH in accordance with N + O + O (7-) N + OH + H. (7-3) In accordance with the three reaction equations the following applies to the formation dx ~ dt = k x~ x~ x~ x~ x~ x~ (7-4) I O N + k II N O + k III N OH and the following to the change of the atomic nitrogen dx ~ dt N = k x~ x~ x~ x~ x~ x~. (7-5) I O N k II N O k III N OH The reactions possess the reaction coefficients (Warnatz)

2 kJ mol m k I = exp (7-6) ( R T) kmol s kJ mol m k II = exp (7-7) ( R T) kmol s 3 10 m k III = (7-8) kmol s 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 formation can be designated as thermal. 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 dx~ N dt = 0 the following simple connection ensues for the formation from the two equations above dx ~ dt = k x~ x~. (7-9) I O N 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 formation. In accordance with the above equations, the N and the O concentration are necessary for the calculation of the formation. The N 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 OH + O H + OH H + O OH + H H O + H. The reaction rates amount to (Warnatz) k H O and k H O and =.0 10 = kJ m exp R T mol kmol s T kJ m exp R T mol kmol s (7-13) (7-14)

3 k OH H = T kJ m exp. (7-15) R T mol kmol s 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~ O = K5(T) (7-16) x~ O x~ O ( x~ / K ) 1/ = (7-17) O 5 with the equilibrium number according to Table -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 x 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. 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 emissions, secondary measures are necessary. 7.. Prompt 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 N molecule and reacts with the latter such that hydrogen cyanide (HCN) and atomic nitrogen are formed. CH + N HCN + N. (7-19) According to the equations (7-) 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 kj/mol so that it is relatively small. Consequently, formation is relatively rapid (prompt ) 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 formation is not yet known. The formation of prompt 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

4 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 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 takes a share of less than 10% of the total x emission (Stapf, Leuckel 1996). Consequently, only little importance can be attributed to this formation mechanism in case of technical combustion processes which take place under conditions of an excess of air. On the other side, the prompt 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 x 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 formation with some 50 ppm lies in the air number range of 0.7 λ In heating boiler installations where the flame temperature may be kept relatively low the thermal formation is small. When determining the total emission the prompt must be considered in this case. In industrial firing installations, however, the prompt can be neglected with respect to the thermal and especially in case of liquid and solid fuels even with respect to the fuel Fuel 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 formation taking place on the basis of this fuel 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 x emissions in these cases. The fuel N content may reach up to % 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 NH 3. 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 NH i. The associated essential reactions are:

5 HCN + O NCH + H (7-0) NCO + H NH + CO (7-1) HCN + O NH + CO (7-) HCN + OH NH + CO. (7-3) On the other hand, inorganic nitrogen compounds are directly converted into NH i radicals. The decisive reaction is as follows: NH 3 + OH NH + H O (7-4) The NH i formation from inorganic compounds takes place more rapidly than the formation from organic compounds. The NH radicals are subsequently decomposed with H and OH in rapid reactions according to NH + H NH + H (7-5) NH + H N + H (7-6) NH + OH NH + H O (7-7) NH + OH N + H O. (7-8) 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 + O + O (7-9) NH + O + OH (7-30) N + OH + H (7-31) NH + OH + H (7-3) NH + O + H (7-33) NH + O + H. (7-34) In case of low-oxygen conditions the formation are suppressed due to the low concentration of oxidizing radicals. Consequently, the NH i radicals are preferentially transformed with nitric oxide into molecular nitrogen. NH + N + H O (7-35) NH + N + OH. (7-36)

6 The issue which of the NH i radicals will be mainly involved in the reactions for the formation of and N is essentially dependent on the thermal and stoichiometric combustion conditions. The overall mechanism of fuel 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 x emissions in the combustion process of coal and liquid fuels (Kremer and Schulz [1984] as well as Kremer, Schulz, et al. [1985]). The fuel 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 x -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 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 formation mechanism by Fenimore Fenimore (197, 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 NH i radicals through the HCN intermediate compound irrespective of its kind of bond. In this case, the NH i radicals will react either with the OH radicals according to the reactions (7-31) and (7-3) so that nitrogen monoxide is formed, or they will be converted with 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 197 x~ x~ gl 1 x~ + x~ = 1 exp x~ gl anf (7-37) for the calculation of the concentration x~ of fuel nitrogen oxide. In this equation x~ anf is the initial concentration of fuel nitrogen oxide which corresponds to the theoretical concentration in case of complete conversion of N contained in the fuel into, whereas x~ is the equilibrium concentration for which the approximation gl E x~ gl = exp(a ) (7-38) T

7 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- 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 in cement furnace plants, and Klöppner et al. (1993 and 1995) described the concentrations of residual oils in swirl combustion chamber systems. Global formation mechanism according to De Soete In contrast to Fenimore, De Soete (1974 and 1981) distinguishes a number of so-called secondary nitrogen compounds (NH 3, HCN and (CN ) 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 + O +... (7-39) NX + N (7-40) He states the following equation for the formation speed: dx ~ dt n = x~ (k x~ k x~ ) (7-41) NX O O and the following equation for the decomposition speed of fuel-nitrogen compounds: dx ~ dt NX n = x~ (k x~ + k x~ ). (7-4) NX O O k o The reaction coefficients and K defined by De Soete for different nitrogen compounds are given in Table 7-. The approximation b ln ~ x o n = 1 exp. (7-43) a

8 is applicable to the exponent n. Consequently, the exponent equals 1 in case of low O concentrations and equals 0 in case of high O 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 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 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 formation mechanism before, the 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 concentration after combustion of methane with an air number of 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.

9 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 formation is virtually suppressed. On the other hand, the fuel 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 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 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 formation. The effect of air gradation is demonstrated in the example in figure 7-6 which shows the concentrations of the N species, HCN and NH 3 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 = 1.5. When the air number of the primary air stage increases, the NH 3 and HCN concentrations decrease at the end of this stage while the concentration increases since more and more oxygen is available. At the end of the secondary stage the NH 3 and HCN concentrations decrease as well as a function of the air number λ p. However, the concentration temporarily decreases, passes a minimum in the range of λ p = and increases. The relatively high 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 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 4 kw. The strong reduction of 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 reduction even in case of fuels without chemically bonded nitrogen (0 % NH 3 ). 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 λ ges

10 to strong soot formation. In case of such fuels the fuel gradation for reduction of x 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 (CH i ). The latter will be formed as a result of the partial oxidation of hydrocarbons contained in the reduction fuel and bring the through the recycling reactions (7-35) and (7-36) + CH i HCN + H i-1 O back into the fuel N mechanism in the form of HCN. The decomposition of HCN through NH i radicals as intermediate compounds into molecular nitrogen is promoted under the reduced combustion conditions. Consequently, the formation in the primary stage of fuel gradation is of subordinate importance for the total x 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 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 Secondary Measures for the Minimization of As secondary measures for the reduction of the emissions selective homogeneous reduction (denoted as SHR or thermal De x ) and the selective catalytic reduction (denoted as SCR) are available. In the selective homogeneous reduction of ammonia (NH 3 ), which is decomposed by OH to NH, is mixed with the combustion gases NH 3 + OH NH + H O (7-44) This NH reacts with the in accordance with NH + N + H O (7-45) NH + N H + OH. (7-46) These three reactions are the most important. In addition a large number of further elementary reactions proceed, in which the N H is finally also converted to N.

11 If the temperature is not sufficiently high, NH 3 thus does not react in the OH in accordance with equation (X). At temperatures which are too high the NH 3 is oxidized. Hence the homogeneous reduction is possible only in a relatively narrow window of temperatures. Figure 7-9 shows the 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, since otherwise this excess leads to formation in the atmosphere. x~ NH 3 x~ < 1, 5 applies as guide value. In the selective catalytic reduction on the surface of the catalyst is converted to N, for which H O, NH 3 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 N, O and H 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 Mechanism If the fuel contains sulfur, such as oil and coal, SO is produced during combustion. It is assumed that each percent by weight of sulfur contained in the fuel results in 500 ppm of SO in the flue gas. In case of an excess of air a part of this SO will oxidize to SO 3 as follows: SO + ½ O SO 3. (7-47) The latter will react with water steam according to the equation SO 3 + H O H SO 4 (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 ( ~ x + ~ H = 0. 1 O xh ). Thus the dew point of pure water steam is SO 4 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 O 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 O excesses whereas only a small rise takes place in case of O excess.

12 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 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: CaCO 3 CaO + CO, (7-49) so that porous CaO particles are formed. These particles absorb the SO according to the following equation: CaO + SO + ½O CaSO 4. (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 SO to the remaining CaO core. The more porous and the smaller the particle is, the more SO may be agglunitated. Both reactions take place only in a definite temperature range. Figure 7-1 gives the necessary explanations and shows the equilibrium curves of both reactions. In case of conventional CO concentrations in the combustion gas of some 10% the temperature must be higher than 750 C so that CO 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 SO may develop in the desired direction only at temperatures below some 1170 C (6% O, 1000 ppm SO ). 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 CaCO 3. Moreover, it must be ensured that the additive cannot be subjected even to short-time temperatures above 100 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 SO doted natural-gas flames. It can be recognized that the SO integration increases as a function of the molar Ca/S ratio. The values of 1 to 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

13 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) Low-temperature desulfurization The so-called dry process uses a lime hydrate meal Ca(OH) 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) CaO + H O (7-51) is shown in Figure 7-1. Proceeding from usual water steam contents in the waste gas between 5 and 0 %, the decomposition temperature of lime hydrate amounts to 375 C or 45 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) + CO CaCO3 + H O. (7-5) The equilibrium line is shown as an example for the three water steam partial pressures of 5,10 and 0 %. In case of CO 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). The reactions and Ca(OH) + SO CaSO3 + H O (7-53) 1 Ca(OH) + SO + O CaSO 4 + H O. (7-54) are in the foreground for desulfurization. Figure 7-1 shows the equilibrium curve of the upper reaction for both water steam concentrations of 10 and 0 %. In case of SO 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 SO 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) will be limited to some 0 to 30 % (Hünlich 1991). This SO 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 Desulfurization rates of 80% can be reached at a relative humidity of 0.8.

14 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 (C H ) that is formed particularly in flames rich in fuel or areas in higher concentration. The aromatized ring structures form then by reaction in the C H with CH or CH under the formation of C 3 H 3, that then can form the first ring by rearrangement. The subsequent rings then form through further attachments of C H. 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 O concentrations, emissions are effectively based on a defined O concentration. If x~ ia is taken as the concentration of an emitted component (, SO, CO etc.) in the flue gas with the O concentration x~, the following shall be applicable to the reference concentration at the A o reference O concentration x~ B : x~ O 0,1 x~ =. (7-51) o B ~ ib x ia 0,1 x~ oa x~ib The reference O 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 (-4).

15 Limit values are frequently defined as the partial density mgi/m 3 G. Proceeding from the volume concentration ρ i (weight i per gas volume), e.g. x~ i the resultant correlation is ρ i = ~ x i ρi, (7-5) where ρ i is the density of the pure component i. Nitric oxide emissions ( x ) are understood as the mixture of nitrogen monoxide () and nitrogen dioxide ( ). As agreed before, the x concentration is specified as the partial density of. In many measuring gauges the will be converted to 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, CO, H O, CH 4,, SO etc., absorb light of special wave length. Fig shows the characteristic wave length of some gases. The decrease of the intensity I 0 of a light beam entering a gas is in accordance with the Beer-law ( a( λ) p s I = IO exp ), (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 I 0 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 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

16 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 content the is converted with ozone to nitrogen dioxide inside an analyzer according the equation + O 3 + O (7-58) The -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 -concentration, if ozone concentration is high. In this measurement procedure the nitrogen dioxide is converted to according the equation + O (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 and. The indicated concentration of X is equivalent to the concentration of. 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, SO, SO 3, HCl, HF, NH 3 are measured analytical. The measurement according to this method are normally made discontinuous and taken to compare with other methods.

17 X O 1,E-03 1,E-04 1,E-05 1,E-06 1,E C 1600 C 1500 C 1400 C 1300 C 100 C ϑ G 1,E-08 0., Excess air number λ Fig. 7-1: Atomic concentration of oxygen in the equilibrium combustion of natural gas Fig. 7-: Thermal -forming in dependence on temperature and time (Beckervordersandforth 1989)

18 No-Recycle +CH i NH 3 Fuel Nitrogen HCN Fuel - Mechanism +O,OH,H +CH i NH NH N +O,OH + +O N Zeldovich- Mechanism Molecular Nitrogen Prompt-- Mechanism +CH i Fig. 7-3: Mechanism of Fuel Formation Fig. 7-4: Reduction of x emission by flue gas recirculation according to Feisst 1991

19 Fuel Primary air st 1 stage λ p <1 nd stage λ S >1 Flue gas Secondary air Fig. 7-5: Principal of air staging 10 4 ppm 10 3 X Ma.%N, T= K x HCN NH Ma.%N, T= K x HCN NH 3 HCN 4 Ma.%N, T= K x HCN NH 3 10 NH 3 HCN λ P NH 3 NH 3 HCN λ P λ P Fig. 7-6:, HCN and NH 3 Emissions at the end of Primary Stage (-----) and Secondary Stage ( ). Total excess air of two stage combustion λ tot = 1. 5

20 5000 Flue gas 83 Vol.% NH 3 3 X in mg/m in NTP dry. (3% O ) Secondary air NH 3 Natural gas Primary air 50 Vol.% NH 3 9 Vol.% NH 3 0 Vol.% NH 3 two stage one stage λ Excess air (one stage) or (two stage) tot λ p Fig. 7-7: x emissions for one and two stage combustion of natural gas with different Ingredients of fuel nitrogen (reference fuel: natural gas and NH 3 ) [Weichert et al. 1995] Fuel st 1 stage nd stage rd 3 stage Flue gas Primary air λ p <1 λ T >1 Secondary fuel Tertiary fuel Fig. 7-8: Principal of fuel staging

21 X End X Initial Temperature in C Fig. 7-9: Temperature window for -reduction by thermal DEX according to Warnatz et al Gas Dew line Range of mixture T in C 00 S turat lin a ion e Liquid 150 HSO+HO X H SO 4 Fig. 7-10: Constitutional diagram of sulphuric acid and steam

22 in C ϑ t P HO = 0, bar 0,18 bar 0,16 bar 0,14 bar 0,1 bar 0,10 bar 0,08 bar ,1 0, X H SO 4 in ppm Fig. 7-11: Temperature of dew point of waste gas for SO SO -,H O-,CO - Partial pressure in bar Technical range Ca(OH) CaO+H O Ca(OH) +CO CaCO +H O 3 %H O HO - CO Partial pressure %H O Temperature in C Technical range SO partial pressure CaO+SO +1/O 1 %O CaSO4 6 CaCO 3+SO +1/O CO +CaSO4 %O 4 (10%CO ) Ca(OH) +SO CaSO +H O CaCo3 CaO+CO 1 6 Fig. 7-1: Equilibrium curves for lime reactions

23 100 % SO - Reduction X X X X ϑ μ ( C) d( m) 900 <15 X1000 < < <40 X SO (t=0) = 1000 ppm Molar ratio Ca/S Fig. 7-13: Influence of the molecular Ca/S relationship and a middle reactor temperature on the desulphuristion with chalk 100 % 80 x 93 C 60 C SO - Reduction x 150 C 100 C 80 C Ca(OH) m /g 35 x x CO = 1 Vol% O = 8 Vol% 3 SO = 00 mg/m.. i N Ca/S = Humidity Fig. 7-14: Influence of humidity on SO reduction

24 1 Emissivity CO CO SO CO Wave length λ in μm 10 Fig. 7-15: Wave length of absorption for some gases Permanent Magnet Heating Wire Glass Tube Moving-Coil Meter Annulus Oxygen Flue Gas Oxygen Flue Gas Balancing Resistor Balue of the Calibrated Gas Zero Point Fig. 7-16: Magnetic susceptibility of oxygen concentration measurement

25 Heating value C-content CO -Emission MJ kg kg C CO kgco kg B kg B kwh MJ Natural gas 39,6 0,59 0,0 0,055 fuel oil EL 4,7 0,86 0,7 0,075 hard coal 9,7 0,77 0,34 0,095 brown coal 8,5 0,8 0,43 0,1 wood 15,0 0,50 0,43 0,1 electrical - - 0,56 0, primary - - 0, 0,06 Table: 7-1: Specific CO -Emissionen of different sources of energy Reaction k 0 [1/s] NH 3 + O ,00 10 NH 3 + N +... CH + O , , , E act [kj/mol] CN + N +... HCN + O +... HCN + N , , Table 7-: Reaction coefficients for forming and reduction of according to De Soete ppm CO 1 ppm x 1 ppm SO x~ i ρ i 1.5 mg CO/m 3 NTP.05 mg /m 3 NTP.93 mg SO /m 3 NTP Tab. 7-3: Factors for conversion of volume concentration into partial density

26 Exemples Unit Relation O in % x CO SO Heating vessel <10 kw Bimsch V 1996 mg/kwh mg/m 3 i. N. 94 Blue Angel mg/kwh (Net Calorific vessel) Blue Angel mg/kwh (Gross calorific vessel) Firing Plants < 100 MW Solid fuels mg/m 3 i. N Heavy oil mg/m 3 i. N Light oil mg/m 3 i. N Natural gas mg/m 3 i. N Combustion engines Diesel > 3 MW mg/m 3 i. N Diesel < 3 MW mg/m 3 i. N Otto mg/m 3 i. N Table 7-4: Examples for limits of emissions