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The NO and N2O formation mechanism during devolatilization and char combustion under fluidized-bed conditions
Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1996/pp. 3325–3334
THE NO AND N2O FORMATION MECHANISM DURING DEVOLATILIZATION AND CHAR COMBUSTION UNDER FLUIDIZED-BED CONDITIONS ¨ FFLER and HERMANN HOFBAUER FRANZ WINTER, CHRISTIAN WARTHA, GERHARD LO Vienna University of Technology Vienna, Austria
In an exclusively electrically heated, laboratory-scale, fluidized-bed combustor (MM-FBC) made of quartz glass, the formation rates of NO, N2O, and HCN are studied by using a high performance FT-IR spectrometer in combination with a long-path, low-volume gas cell. The emission characteristics of single, spherical fuel particles (5–15 mm in diameter) of bituminous coal, subbituminous coal, and beech wood are studied. The bed temperature is varied between 600 and 9008C. The fluidizing velocity is 0.68 m/s and the oxygen partial pressure is varied between 0.05 and 21 kPa. The bed material is silica sand with a mean diameter of 225 lm. Besides changing combustion conditions and fuel type and diameter, an iodine addition technique is applied to study the relative importance of the homogeneous and the heterogeneous chemistry during devolatilization and char combustion. Iodine addition suppresses the radical concentrations to equilibrium levels and has proven as a very effective method for kinetic measurements. With a semitheoretical model, the measurement results are discussed and reaction paths are evaluated. The formation characteristics during devolatilization can be explained mainly by the homogeneous HCN oxidation and NO and N2O formation. NO and N2O formation during char combustion consists of two different paths. NO is mainly heterogeneously formed by char-nitrogen oxidation. But a volatile species, which has been identified as HCN, is simultaneously released in low concentrations. This species is homogeneously oxidized to NCO which further reacts with the heterogeneously produced NO to form N2O.
Introduction To minimize the emission of pollutants, much effort has been put into changing the fuel or the operating conditions of fluidized-bed combustors (FBCs) including bed temperature, bed material, air staging, and excess air. These results can be found in the literature (an overview is given in Ref. 1) but not once do they show controversial trends. For further progress, a deep understanding of the principal formation and destruction mechanism of the pollutants, especially NO and N2O, is essential. NO and N2O emissions are dependent on the fuel and fuel characteristics, the complex homogeneous and heterogeneous formation and destruction paths, temperature and residence times, fluid dynamics, combustor geometry, heat and mass transfer, and so forth [2,3]. Because of these complex interrelations, it is necessary to study these processes separately and to analyze their relative importance. Accompanying laboratory-scale experiments are essential for the correct interpretation of data from pilot plants or industrial boilers. This study focuses on the formation of NO and N2O during devolatilization and char combustion of single fuel particles under FBC conditions. In single fuel particle experiments, no fuel particle–fuel
particle interactions exist and the emissions can be directly related to the single fuel particle. The temperatures of the single fuel particles are measured by implanting thermocouples and do not have to be calculated. Extensive heat and mass transfer studies provide a profound basis on Nusselt and Sherwood number correlations. A flexible, laboratory-scale multimode, fluidized-bed combustor (MM-FBC) is used where combustion conditions are well defined and can be independently varied. The MM-FBC has been optimized to obtain formation rates. In the literature, there exists some work investigating NO and N2O formation of single coal particles under FBC conditions [4–7]. But there is no deep understanding of the fractional conversion of fuel nitrogen to NO and N2O during devolatilization and char combustion [8]. Especially concerning char combustion, different hypotheses are discussed in literature [9] explaining NO and N2O formation. One hypothesis claims that N2O is strictly heterogeneously formed by the direct oxidation of the fuel nitrogen. In other hypotheses, NO is assumed to react with char-nitrogen. N2O can also be formed homogeneously by reaction of a released volatile species (e.g., HCN) with NO. The aim of this work is to get a deeper insight into NO and N2O formation of different fuels during
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FLUIDIZED BEDS TABLE 1 Proximate and ultimate analysis of the fuels. Parentheses indicate values after devolatilization.
Proximate analysis
Heat value (MJ/kg)
Bituminous coal as received
Subbituminous coal as received
Beech wood
(After devol.)
(After devol.)
(After devol.)
32
Density (kg/m3)
1524 (832)
16
as received
17.5
1250 (600)
670 (150)
30
32.2
81.7
Moisture (w-%)
6.0
25.3
5.6
Ash content (w-%)
2.4
12.3
0.3
61.6
30.2
12.4
Ultimate analysis
(After devol.)
Water ash free
Carbon (w-%)
86.8 (85.65)
68.11 (73.1)
49.76 (87.31)
Hydrogen (w-%)
4.27 (0.94)
2.84 (1.05)
5.94 (1.63)
Nitrogen (w-%)
1.48 (1.76)
0.64 (0.9)
0.11 (0.24)
Sulfur (w-%)
0.5 (0.32)
1.52 (0.73)
,0.02 (,0.02)
Oxygen diff. (w-%)
6.95 (11.3)
26.88 (24.2)
44.17 (10.83)
Volatile matter (w-%)
Fixed carbon (C-fix) (w-%)
devolatilization and char combustion under fluidized-bed combustor conditions. The transient NO and N2O emission behavior is characterized and its dependencies on bed temperature and oxygen concentration are shown. To study the relative importance of the radical chemistry during devolatilization and char combustion, an iodine addition technique has been developed and proven as a very useful investigation tool.
Experimental Single, spherical particles are formed from three different solid fuels: a bituminous coal, a subbituminous coal, and beech wood. The fuel particle diameters are 5, 10, and 15 mm. The proximate and the ultimate analyses are given in Table 1. The MM-FBC is made of quartz glass to minimize the catalytic reactivity of the walls and to allow observation inside the combustor. The inner diameter of the main quartz tube is 35 mm and the height is
240 mm (Fig. 1). Ni/CrNi thermocouples are used at different positions to control the combustion temperature. It is exclusively electrically heated by two heating shells. The bed temperature is varied between 600 and 9008C. The bed material is silica sand with a mean diameter of 225 lm. The static bed height is about 40 mm. Air/nitrogen mixtures are used to vary the oxygen partial pressure of the fluidizing gas between 0.05 and 21 kPa. The superficial velocity is 0.68 m/s and controlled by two mass-flow controllers. In the case of iodine addition, solid iodine was placed in a flask which was warmed up in a water bath. Thus, the iodine sublimated and enriched the fluidizing gas which was lead through the flask (Fig. 1). To obtain the instantaneous concentrations of various gaseous species (CO2, CO, CH4, other hydrocarbons, NO, N2O, HCN, and other species) in the flue gas, a high-performance FT-IR spectrometer (BIO-RAD, FTS 60A) was used. The spectrometer
NO AND N2O FORMATION MECHANISM
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Fig. 1. The experimental set-up of the multimode, fluidized-bed combustor (MM-FBC): 1,2, inlet of the fluidizing gas (air, nitrogen); 3, mass-flow controllers; 4, display for the mass flow; 5, preheating zone; 6, heating shells; 7, thermocouple; 8, to the chimney; 9, filter; 10, cooler; 11, pump; 12 and 13, gas cell and FT-IR spectrometer; 14 and 15, fuel inlet; 16, flask for iodine addition in a water bath.
is combined with a long-path, low-volume gas cell (Foxboro LV7, cell volume 4 223 cm3, optical path length 4 7.25 m). The design of the MM-FBC, the operating conditions, the sampling line, the FT-IR recording options, and the gas-cell have been optimized to obtain formation rates (i.e., short residence times, high scan rates of 1.2 scans/s at a high resolution of 0.5 cm11). NH3 has not been detected in significant amounts by the FT-IR, possibly due to adsorption in the sampling line, its low concentration, and time dependence. The single fuel particles were fed into the MMFBC and their formation rates as well as the operating conditions were recorded during devolatilization and char combustion. Additionally, the ignition and the extinction of the flame of the volatile matter, the phenomenological behavior, fuel particle temperatures, and the fluid-dynamical behavior have been investigated. Mass balances of the fuel carbon lead to kinetic data of devolatilization and char combustion. These results have been reported elsewhere [10–12] and were used whenever necessary as a basis to predict the NO and N2O formation rates.
Experimental Results Bituminous Coal Figure 2 shows the typical concentration histories of CO2 and the nitrogen-containing species NO, N2O, and HCN during devolatilization and char combustion of a single bituminous coal particle with an initial diameter of 10 mm at 8008C and an oxygen concentration of 10 kPa in the MM-FBC. During devolatilization, the rapid increase of all species can be clearly seen. The NO concentration in the flue gas reaches its maximum of about 190 ppm 5 10 ppm after the particle has been in the combustor about 60 s (after the first third of the devolatilization period). The HCN concentration also increases rapidly but its maximum is only slightly above 20 ppm 5 2 ppm. These two peaks are followed by the increase of N2O to a maximum of about 20 ppm 5 2 ppm. With decreasing amount of volatile matter in the fuel particle, the formation rates of all species decrease toward their corresponding char combustion levels. Integrating the formation
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FLUIDIZED BEDS
Fig. 2. Concentration histories of CO2, NO, N2O, and HCN during devolatilization and char combustion of a 10mm bituminous coal particle at 8008C and 10 kPa oxygen partial pressure in the fluidizing gas in the MM-FBC.
rates over the devolatilization period leads to the percentage of conversion of the volatile nitrogen, that is, the nitrogen that is released during devolatilization (28 w-% of the fuel nitrogen), to NO (38 w-%), N2O (8 w-%), and HCN (4 w-%) (refer to Table 2). During char combustion, all formation rates are at low levels compared to the formation rates during devolatilization. In the first fifth of char combustion, the concentration levels are about 20 ppm 5 2 ppm NO, 2 ppm 5 0.5 ppm N2O, and 2 ppm 5 0.5 ppm HCN and they are steadily decreasing. But the time scale of char combustion is longer than the time scale of devolatilization (about 10 times) and compensates to a certain extent the low formation rates. Integrating the formation rates over the char combustion period leads to the percentage of conversion of the char nitrogen, that is, the nitrogen that is released during char combustion (72 w-% of the fuel nitrogen), to NO (16 w-%), N2O (1.5 w-%), and HCN (1.4 w-%) (refer to Table 2). The relative importance of devolatilization and char combustion for NO, N2O, and HCN formation can be found by comparing the amount of fuel nitrogen converted to NO, N2O, and HCN for devolatilization (14 w-%) and char combustion (14 w-%) (Table 2). This reveals that devolatilization and char combustion are of nearly equal importance for the formation of these species. Varying combustion conditions, the above-mentioned percentages of conversion (listed in Table 2) will change. Increasing the bed temperature during devolatilization from 600 to 9008C increases the con-
version of volatile nitrogen to NO from 6 to 49 w-% (of the volatile nitrogen), whereas HCN decreases from 20 to 2 w-%. Conversion to N2O stays at low levels between 600 and 7008C (1–2 w-%) but reaches a maximum around 8008C (8 w-%) and slightly declines toward 7 w-% at 9008C. Increasing the oxygen partial pressure in the fluidizing air from 0.05 to 21 kPa leads, in the beginning, to an increase of the conversion to NO from 6 to 38 w-% (around 10 kPa), then it slightly decreases toward 33 w-%. N2O slightly increases from 1 to 9 w-%, whereas HCN slightly decreases from 9 to 2 w-%. For char combustion, a temperature increase from 600 to 9008C increases the conversion of the char nitrogen to NO from 5 to 26 w-% (of the char nitrogen), whereas the conversion to HCN decreases from 7 to 1 w-%. The conversion to N2O stays at low levels; a maximum can be found about 8008C (1.5 w-%). Varying the oxygen partial pressure from 5 to 21 kPa, the conversion to NO decreases from 31 to 16 w-% (at 10 kPa) but, at higher oxygen levels, increases to 24 w-% (at 21 kPa) again. A slight minimum can also be found at 10 kPa in the case of charnitrogen conversion to N2O (1.5 w-%). The conversion to HCN continuously decreases from 4 to 1 w-% (at 21 kPa). An iodine addition technique has been developed and proven to be a very successful tool for studying the relative importance of the radical chemistry during devolatilization and char combustion of a single fuel particle under FBC conditions. Enriching the fluidizing gas with iodine vapor (concentration levels in the MM-FBC are about 1000 ppm) leads to dramatic changes of the species concentrations but does not effect the heterogeneous carbon conversion rates; neither changes of the devolatilization time nor changes of the char burnout have been found. Contrary to the heterogeneous chemistry, the homogeneous is effected: flame ignition is inhibited and the CO levels increase whereas the CO2 levels slightly decrease. This characteristic behavior is based on the inhibiting effect of iodine. It suppresses the radical concentrations to equilibrium levels and slows down radical reactions (e.g., the CO-OH oxidation reaction, compare with Ref. [13]). Figure 3 shows the strong effects of iodine addition during devolatilization. The NO concentration dramatically decreases, N2O completely vanishes, whereas the HCN level doubles. Figure 4 shows the effects of iodine addition on NO, N2O, and HCN during char combustion. The instantaneous conversion of the fuel nitrogen to NO, N2O, or HCN is defined as the instantaneous ratio of the fuel nitrogen measured as NO, N2O, or HCN to the released amount of carbon (as CO2, CO, CH4, and hydrocarbons). Furthermore, this ratio is normalized by division of the nitrogen to carbon ratio of the fuel (N/C, see Table 2). The carbon conversion of the fuel particle is obtained by measuring the
NO AND N2O FORMATION MECHANISM
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TABLE 2 The fuel-nitrogen-to-carbon ratio before and after devolatilization (in parentheses). The increase of this ratio after devolatilization. The splitting of the fuel nitrogen in volatile and char nitrogen. The nitrogen conversion to NO, N2O, and HCN during devolatilization on the basis of volatile nitrogen and fuel nitrogen. The summarized fuel-nitrogen conversion to NO, N2O, and HCN during devolatilization. The same for char combustion.
N/C [1] N/C 1 change (%) Volatile-N (w-%) Char-N (w-%)
Bituminous coal (After devol.)
Subbituminous coal (After devol.)
Beechwood spheres (After devol.)
0.017 (0.021) 24 28 72
0.0094 (0.012) 28 26 74
0.0021 (0.0027) 29 68 32
38 5 3 11 5 1 851 2 5 .5 451 1 5 .5 14 5 2
40 5 5 10 5 2 352 1 5 .5 752 2 5 .5 13 5 3
28 5 3 19 5 3 0.7 5 .2 0.5 5 .2 10 5 2 752 26 5 5
16 5 2 12 5 2 1.5 5 .5 1.0 5 .5 1.4 5 .5 1.0 5 .5 14 5 3
18 5 2 13 5 2 0.5 5 .2 0.3 5 .2 3.5 5 .5 2.5 5 .5 16 5 3
31 5 3 10 5 2 452 1.5 5 1 13 5 2 451 15.5 5 4
Devolatilization NO/vol-N (w-%) NO/fuel-N (w-%) N2O/vol-N (w-%) N2O/fuel-N (w-%) HCN/vol-N (w-%) HCN/fuel-N (w-%) N to (NO, N2O, HCN)/fuel-N (w-%) Char combustion NO/char-N (w-%) NO/fuel-N (w-%) N2O/char-N (w-%) N2O/fuel-N (w-%) HCN/char-N (w-%) HCN/fuel-N (w-%) N to (NO, N2O, HCN)/fuel-N (w-%)
carbon-containing species in the flue gas and by integrating them over time. After devolatilization and the first stages of char combustion, iodine was added to the fluidizing gas. The instantaneous conversion to NO only slightly increases whereas the conversion to N2O drops to zero. The conversion to HCN significantly increases. Comparison to Other Fuels To study the influence of the fuel type on the NO, N2O, and HCN formation characteristics, additional measurements with spherical subbituminous and beech wood particles (10 mm in diameter) were performed at the same operating conditions in the MMFBC. Basically, the concentration histories show qualitatively the same behavior as presented in Fig. 2. As explained in the section on bituminous coal, the conversion of nitrogen in terms of volatile, char, and fuel nitrogen during devolatilization and char combustion are given in Table 2. Although there exist
strong differences in the nitrogen content of the fuels (Table 1), many parallels can be found. The splitting of the fuel nitrogen to volatile nitrogen and char-nitrogen is the same for the two coals (Table 2). The retaining nitrogen to carbon ratio after devolatilization increases (about 27%). The nitrogen conversions during devolatilization and char combustion are very similar for the two coals. The higher conversion of nitrogen to HCN of the subbituminous coal is compensated by the lower conversion to N2O. This is also valid for the beech wood particles during devolatilization. During char combustion, the char-nitrogen conversions to NO, N2O, and HCN of the beech wood are substantially higher than the char-nitrogen conversions of the coals. But the summarized fuel-nitrogen conversion to NO, N2O, and HCN is nearly the same for all three fuels (about 15 w-%) due to the different splitting of the fuel nitrogen of the beech wood. Varying combustion conditions, the listed nitrogen conversions of the fuels will change (compare with the section on bituminous coal). But similar to the concentration histories, they show the same trends.
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Modeling
Fig. 3. Concentration histories of NO, N2O, and HCN during devolatilization of a 10-mm bituminous coal particle at 8008C and 10 kPa oxygen partial pressure in the fluidizing gas in the MM-FBC. The arrows indicate the concentration changes when iodine is added.
The aim of the modeling work is to understand the NO and N2O formation during devolatilization and char combustion presented in the experimental section. The subject of this semitheoretical approach is to use well-known parameters to predict and explain the measured species concentrations. The following measured parameters have been used for input: the combustion conditions of the MM-FBC (bed temperature and temperature profiles, flame characteristics, the input oxygen partial pressure, superficial gas velocity, bed voidage, mean bed particle diameter); the concentration histories of oxygen, CO, CO2, and hydrocarbons in the flue gas; and fuel data (fuel particle diameter, ultimate and proximate analysis before and after devolatilization, particle temperature). For comparison of experimental and modeling results, the bituminous coal has been used. The model calculates the instantaneous concentration profiles of NO, N2O, HCN, NH3, and the nitrogen-containing radicals (NCO, NH2, NH) by considering homogeneous and heterogeneous reactions inside the fluidized bed as well as the homogeneous chemistry in the freeboard (compare with Refs. 2 and 3). In Table 3, the set of the used chemical reactions is given. The measured carbon-conversion rates are taken as the basis for the calculations. The volatile nitrogen is assumed to be released proportional to the carbon release during devolatilization as HCN and NH3. The HCN/NH3 ratio has been varied as an input parameter from 1/1 to 1/0 to study the relative importance of these intermediates on NO and N2O formation. Best agreements with the measurements have been found with a HCN/NH3 ratio of 9/ 1. During char combustion, the char-nitrogen release is taken proportional to the carbon conversion as NO and HCN. Released from the particle’s surface, they are taken as 20 and 4 w-%, respectively. The radical concentrations (OH, HO2, O, H) are estimated by using the CO2 and CO concentration histories (similar to Ref. 14). This enables the decoupling of the nitrogen chemistry from the carbon-oxidation chemistry. Iodine addition decreases the level of the radicals, again estimated by using the CO2 and CO concentrations. The flame of the volatiles is considered in the instantaneous temperature profile of the combustor. A detailed description of the model will be published elsewhere. Discussion
Fig. 4. Instantaneous conversion to NO, N2O, and HCN versus carbon conversion of a 15-mm bituminous coal particle, at 7508C and 10 kPa oxygen partial pressure in the fluidizing gas in the MM-FBC. Iodine is added after about 0.45 carbon conversion during char combustion.
The NO and N2O formation and destruction paths (Fig. 5) have been analyzed and evaluated for the bituminous coal with the model under measurement conditions. The bold arrows indicate the main reaction paths. Typical modeling results are given in Fig. 6 as a concentration versus time plot. It can be
NO AND N2O FORMATION MECHANISM
3331
TABLE 3 The set of chemical reactions used in the model. Arrows in one direction indicate irreversible reactions, arrows in both directions reversible reactions. M indicates a third body Number (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18)
Used reactions NCO ` O → NO ` CO NCO ` OH ↔ NO ` CO ` H NH ` O ↔ NO ` H NCO ` NO ↔ N2O ` CO NH ` NO ↔ N2O ` H NH2 ` NO → N2 ` H2O N2O ` H ↔ N2 ` OH N2O ` OH ↔ N2 ` HO2 N2O ` O ↔ 2 NO N2O ` O ↔ N2 ` O2 N2O ` M ↔ N2 ` O ` M HCN ` O ↔ NCO ` H NH3 ` O ↔ NH2 ` OH NH2 ` O ↔ NH ` OH NO ` CO → 1/2 N2 ` CO2 NO ` 2/3 NH3 → 5/6 N2 ` H2O NH3 ` 3/4 O2 → 1/2 N2 ` 3/2 H2O NH3 → 1/2 N2 ` 3/2 H2
Catalyst
References
Bedmat. Bedmat. Bedmat. Bedmat.
15 15 17 15 18 19 15 15 21 21 22 15 20 23 16 16 16 16
Fig. 5. The reaction paths of fuel nitrogen to NO and N2O. Bold arrows indicate the main reaction paths evaluated from the model for bituminous coal.
seen that iodine addition causes an increase of HCN and a decrease of NO and N2O during devolatilization. During char combustion, the NO level is nearly unaffected, HCN increases and N2O decreases. During devolatilization, the release rates of NO, N2O, and HCN are very high relative to their release
rates during char combustion (Fig. 2), corresponding with the carbon-conversion rates. The conversion of the fuel nitrogen is based on carbon conversion. Basically, the concentration levels of NO are higher because two nitrogen atoms are necessary to form N2O. This implies that more reactions and
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Fig. 6. Modeling results of the concentration histories of NO, HCN, and N2O of a bituminous coal particle (10 mm in diameter, at 8008C, 10 kPa oxygen) during devolatilization and char combustion in the MM-FBC. The arrows indicate the changes when iodine is added. For better presentation, the HCN and N2O concentrations during char combustion are multiplied by 100.
longer residence times are necessary to combine these nitrogen atoms. Due to the longer char combustion period, the lower release rates of NO, N2O, and HCN are compensated to a certain extent, and devolatilization and char combustion are of nearly equal importance for the formation of these species. This is also somehow valid for beech wood. Although the percentage of fuel nitrogen as volatile nitrogen is much higher in comparison to the coals, the conversion rates to NO and N2O are lower (Table 2). The two coals show nearly the same conversion rates for volatile nitrogen and char nitrogen. NO and N2O formation during devolatilization is based on gas-phase chemistry (compare with Ref. 2). This can be clearly seen after iodine addition. The addition of iodine suppresses the radical reactions leading to lower levels of NO and N2O (Fig. 3). HCN is the main precursor of NO and N2O. This can be seen in several figures (Figs. 2–4 and 6). The N2O peak shortly follows the NO and HCN peaks (Figs. 2, 4, and 6). Iodine addition increases HCN and decreases NO and N2O (Figs. 3 and 6), implying that the oxidation reactions of HCN to NO and N2O are inhibited (reactions 12, 1, 2, and 4 in Table 3), leading to higher concentrations of the precursor. The dependencies of bed temperature and oxygen partial pressure in the flue gas (described in the experimental results) reveal similar trends on the formation rates. HCN is decreasing with increasing temperature and oxygen partial pressure, whereas NO and N2O are increasing to a certain extent. The modeling results indicate that HCN is the main pre-
cursor of NO and N2O during devolatilization. The very low levels of NO and N2O cannot be completely explained by model calculations (Fig. 6). Increasing destruction rates due to higher CO concentration levels may be the reason. Modeling results of the bituminous coal indicate that NH3 is of minor importance for NO and N2O formation due to its conversion via reactions 17 and 18 (Table 3). NO and N2O formation during char combustion is based on a different mechanism. Again, this can be clearly seen in the iodine addition experiments (Fig. 4). The effect of iodine addition on the NO level is minor. This indicates that NO formation is based on a heterogeneous mechanism inside the pore structure or at the surface of the fuel because heterogeneous chemistry is nearly independent from radical concentrations. In contrast to NO, HCN steeply increases, whereas N2O steeply decreases. During char combustion, a volatile species which has been identified as HCN is released in low concentrations. This species is homogeneously oxidized to NCO which further reacts with the heterogeneously produced NO to form N2O. Inhibiting the homogeneous oxidation path of HCN (reactions 12, 1, 2, and 4 in Table 3) leads to a steep decrease of N2O and a steep increase of HCN. The NO concentration level is nearly unaffected. It slightly increases because the NO destruction reaction with NCO (reaction 4 in Table 3) to form N2O is inhibited. But the N2O level is much lower than the NO level. Modeling results support this mechanism (Fig. 6). Additional evidence can be found when comparing the two coals during char combustion (Table 2). A decrease of the N2O level is compensated with an increase of HCN and NO. And varying the bed temperature and the oxygen partial pressure (described in the experimental results) leads to a decrease of HCN but in an increase of NO and N2O to a certain extent. Conclusions Devolatilization and char combustion are of nearly equal importance for NO and N2O formation for the two coals. The higher formation rates during devolatilization are compensated by the shorter time scale. In the case of beech wood, devolatilization is more important, although the higher percentage of fuel nitrogen as volatile nitrogen is compensated to a certain extent by lower conversion rates in comparison to the coals. The NO concentration levels are higher than the N2O concentration levels during devolatilization and char combustion due to the formation mechanisms and residence times. Iodine addition has proven to be a very effective method to study the relative importance of homogeneous and heterogeneous chemistry in kinetic experiments.
NO AND N2O FORMATION MECHANISM
Iodine addition experiments confirm that NO and N2O formation during devolatilization is based on a homogeneous mechanism. HCN is the main precursor of NO and N2O (compare with [2]). The formation characteristics can be explained by the homogeneous HCN oxidation and NO and N2O formation reactions (reactions 12, 1, 2, and 4 in Table 3). NO and N2O formation during char combustion is different. NO is primarily heterogeneously formed by char-nitrogen oxidation. But a volatile species, which has been identified as HCN, is simultaneously released in low concentrations. This species is homogeneously oxidized to NCO which further reacts with the heterogeneously produced NO to form N2O. Acknowledgment The authors want to acknowledge Ben Anthony, CANMET, Ottawa who awoke their interest in iodine addition under FBC conditions.
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9. Kramlich, J. C., Cole, J. A., McCarthy, J. M., Lanier, W. S., and McSorley, J. A., Combust. Flame 77:375– 384 (1989). 10. Winter, F., Krobath, P., and Hofbauer, H., Thirteenth International Conference on Fluidized Bed Combustion, ASME, Orlando, FL, 1995, pp. 1477–1487. 11. Winter, F., Wartha, C., and Hofbauer, H., Third International Conference on Combustion Technology for a Clean Environment, Lisbon, Portugal, 1995, Sec. 15.2. 12. Winter, F., Prah, M. E., and Hofbauer, H., “Intra-Particle Temperatures under Various Fluidized-Bed Combustor Conditions: The Effect of Drying, Devolatilization, and Char Combustion”, Combust. Flame, 1995, submitted. 13. Anthony, E. J., Bulewicz, E. M., and Preto, F., Twelfth International Conference on Fluidized-Bed Combustion, ASME, La Jolla, CA, 1993, pp. 41–52. 14. Bulewicz, E. M., Janicka, E., and Kandefer, S., Tenth International Conference on Fluidized-Bed Combustion, ASME, San Francisco, 1989, Vol. 1, pp. 163–168. 15. Miller, J. A. and Bowman, C. T., Prog. Energy Combust. Sci. 15:287 (1989). 16. Johnsson, J. E., Kinetics of Heterogeneous NOx Reactions at FBC Conditions, CHEC Report No. 9003, Technical University of Denmark, Lyngby, 1990. 17. Mertens, J. D., Chang, A. Y., Hanson, R. K., Bowman, C. T., and Masten, D. A., “A Shock Tube Study of the Reactions of NH with NO, O2, and O,” Western States Section/The Combustion Institute Meeting, Livermore, California, 1989, Paper no. WSS/CI-89-96. 18. Glarborg, P., Miller, J. A., and Kee, R. J., Combust. Flame 65:177 (1986). 19. Hanson, R. K. and Saliman, S., in Combustion Chemistry, (W. C. Gardiner Jr., Ed.), Springer-Verlag, New York, 1984, p. 361. 20. Ba¨tz, P., Ehbrecht, J., Hack, W., Rouyeorolles, P., and Wagner, H. G., Twenty-First Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, p. 1107. 21. Davidson, D. F., DiRosa, M. D., Chang, A. Y., and Hanson, R. K., Eighteenth International Symposium on Shock Waves, Springer Verlag, 1991, Vol. 2, p. 813. 22. Glarborg, P., Johnson, J. E., and Dam-Johanson, K., Combust. Flame 99:523–532 (1994). 23. Glarborg, P. and Hadvig, S., Development and Test of a Kinetic Model for Natural Gas Combustion, Report, Nordic Gas Technology Centre, 1991, p. 83.
COMMENTS A. N. Hayhurst, Cambridge University, UK. Your model has CO reducing NO to N2. Did you measure the effect of iodine on the concentration of CO within the bed? Could you give some details of the conditions in the bed; in particular were the bubbles fast and what was the crossflow factor? Also did the burning char particle circulate around the bed (or, on the other hand, float)?
Author’s Reply. One aim of the NO/N2O model was to study different possible effects (especially destruction reactions) which might be of importance for the interpretation of the measurements. Iodine addition increases the CO level which might reduce NO. But the fuel particle is mainly located in the upper part of the fluidized bed (ca 0 to 4 cm below the surface) and
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moves frequently up to the bed’s surface and back again resulting in low gas—bed material contact times decreasing the importance of the heterogeneous destruction reactions. Additionally, the catalytic reactivity of pure silica sand is rather low compared to bed materials containing char and ash. CO was not obtained within the bed but shortly after the particle as described in the paper. The cross-flow factor has not been measured but can be estimated from Kunii and Levenspiel [1] and is in the range of 0.5 to 1 depending on operating conditions.
1. Klein and Rotzoll, 6th N2O Workshop, Turku Finland, 1994. 2. Krammer and Sarofim, Combust. Flame 97:118–124 (1994). 3. Goel, Zhang, and Sarofim, Combust. Flame 104:213– 217 (1996).
REFERENCE
Prof. Mikko Hupa, ABO Akademi University, Finland. How did you take into account the influence of the iodine on the radical levels in your model? How can you exclude the direct influence of the added iodine on the char nitrogen release chemistry?
1. Kunii, D. and Levenspiel, O., Fluidization Engineering, Wiley, New York, 1991. ● Bo Leckner, Chalmers University of Technology, Sweden. It has been shown that N2O is formed heterogeneously during char combustion. In your case the iodine added is supposed to eliminate the gas phase formation of N2O and prevent the conversion of HCN to N2O. This seems to take place, but heterogeneously formed N2O seems to disappear too when iodine is added. Does it mean that there is no heterogeneous formation on N2O or does the iodine influence the surface reaction as well? Adel Sarofim and Shakti Goel, MIT, USA. Until seeing your results, we were reasonably convinced that the heterogeneous mechanism developed at MIT for NO and N2O was the valid one, although there are some experiments (e.g., [1]) that provide reasonable evidence that homogeneous reactions cannot be ruled out. This paper provides interesting data which seem to further support a homogeneous pathway. There seems to be need to examine further the differences between what appears to be conflicting evidence on the relative roles of heterogeneous versus homogeneous reactions. Transient release of nitrogen oxides on oxygen interruption (e.g., [2,3]) are consistent with the role of oxide formation on the surface both for the nitrogen oxides and the CO. Have you examined such transients on cutting off oxygen, and perhaps also iodine? In your presentation you claimed that iodine atoms did not influence the surface reactions based on the lack of change of the carbon oxidation rate on iodine addition. Iodine may compete with NO for the nitrogenous surface complexes and prevent the formation of N2O without affecting the char consumption rate. Examination of the influence of iodine on the transient emissions of oxide complexes on turning off the oxygen might provide useful insights.
REFERENCES
●
Author’s Replies. The concentrations of the OH, H, HO2, and hydrocarbon radicals depend on the carbon oxidation chemistry and are not significantly affected by the nitrogen chemistry and its radicals. The concentrations of the radicals related to carbon oxidation are estimated by the measured concentrations of CO and CO2 similar to Bulewicz et al., 10th FBC Conf., 1989. Iodine addition changes the CO and CO2 concentration levels by suppressing the OH and H radicals. Using the CO and CO2 measurements it allows to us estimate the radicals’ concentration levels also during iodine addition. M. Hupa’s second question and the questions of A. Sarofim and S. Goel and B. Leckner are regarding possible effects of iodine with the surface chemistry especially for N2O formation during char combustion. It has been shown during the presentation and it is shortly listed in the paper that the addition of iodine does not change the carbon conversion rates during devolatilization nor during char combustion. Char combustion is a strictly heterogeneous process but no effects on iodine addition have been found. It is very unlikely that iodine changes only the nitrogen-containing surfaces. A competing reaction of iodine with NO for possible nitrogenous surface complexes and a small contribution of heterogeneously formed N2O cannot be completely excluded with these measurements. But the homogeneous chemistry of iodine is well understood and the transient behavior of NO, N2O and HCN formation during char combustion has been studied by significantly changing the combustion conditions (e.g. temperature, radical concentrations in the gas-phase, etc.) revealing strong evidence that N2O is homogeneously formed. Experiments where the oxygen and the iodine is turned off are interesting and will be considered in our future work but there exist some possible draw-backs of this method, e.g. the time-scale for gas dispersion in the reactor.
THE NO AND N2O FORMATION MECHANISM DURING DEVOLATILIZATION AND CHAR COMBUSTION UNDER FLUIDIZED-BED CONDITIONS ¨ FFLER and HERMANN HOFBAUER FRANZ WINTER, CHRISTIAN WARTHA, GERHARD LO Vienna University of Technology Vienna, Austria
In an exclusively electrically heated, laboratory-scale, fluidized-bed combustor (MM-FBC) made of quartz glass, the formation rates of NO, N2O, and HCN are studied by using a high performance FT-IR spectrometer in combination with a long-path, low-volume gas cell. The emission characteristics of single, spherical fuel particles (5–15 mm in diameter) of bituminous coal, subbituminous coal, and beech wood are studied. The bed temperature is varied between 600 and 9008C. The fluidizing velocity is 0.68 m/s and the oxygen partial pressure is varied between 0.05 and 21 kPa. The bed material is silica sand with a mean diameter of 225 lm. Besides changing combustion conditions and fuel type and diameter, an iodine addition technique is applied to study the relative importance of the homogeneous and the heterogeneous chemistry during devolatilization and char combustion. Iodine addition suppresses the radical concentrations to equilibrium levels and has proven as a very effective method for kinetic measurements. With a semitheoretical model, the measurement results are discussed and reaction paths are evaluated. The formation characteristics during devolatilization can be explained mainly by the homogeneous HCN oxidation and NO and N2O formation. NO and N2O formation during char combustion consists of two different paths. NO is mainly heterogeneously formed by char-nitrogen oxidation. But a volatile species, which has been identified as HCN, is simultaneously released in low concentrations. This species is homogeneously oxidized to NCO which further reacts with the heterogeneously produced NO to form N2O.
Introduction To minimize the emission of pollutants, much effort has been put into changing the fuel or the operating conditions of fluidized-bed combustors (FBCs) including bed temperature, bed material, air staging, and excess air. These results can be found in the literature (an overview is given in Ref. 1) but not once do they show controversial trends. For further progress, a deep understanding of the principal formation and destruction mechanism of the pollutants, especially NO and N2O, is essential. NO and N2O emissions are dependent on the fuel and fuel characteristics, the complex homogeneous and heterogeneous formation and destruction paths, temperature and residence times, fluid dynamics, combustor geometry, heat and mass transfer, and so forth [2,3]. Because of these complex interrelations, it is necessary to study these processes separately and to analyze their relative importance. Accompanying laboratory-scale experiments are essential for the correct interpretation of data from pilot plants or industrial boilers. This study focuses on the formation of NO and N2O during devolatilization and char combustion of single fuel particles under FBC conditions. In single fuel particle experiments, no fuel particle–fuel
particle interactions exist and the emissions can be directly related to the single fuel particle. The temperatures of the single fuel particles are measured by implanting thermocouples and do not have to be calculated. Extensive heat and mass transfer studies provide a profound basis on Nusselt and Sherwood number correlations. A flexible, laboratory-scale multimode, fluidized-bed combustor (MM-FBC) is used where combustion conditions are well defined and can be independently varied. The MM-FBC has been optimized to obtain formation rates. In the literature, there exists some work investigating NO and N2O formation of single coal particles under FBC conditions [4–7]. But there is no deep understanding of the fractional conversion of fuel nitrogen to NO and N2O during devolatilization and char combustion [8]. Especially concerning char combustion, different hypotheses are discussed in literature [9] explaining NO and N2O formation. One hypothesis claims that N2O is strictly heterogeneously formed by the direct oxidation of the fuel nitrogen. In other hypotheses, NO is assumed to react with char-nitrogen. N2O can also be formed homogeneously by reaction of a released volatile species (e.g., HCN) with NO. The aim of this work is to get a deeper insight into NO and N2O formation of different fuels during
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FLUIDIZED BEDS TABLE 1 Proximate and ultimate analysis of the fuels. Parentheses indicate values after devolatilization.
Proximate analysis
Heat value (MJ/kg)
Bituminous coal as received
Subbituminous coal as received
Beech wood
(After devol.)
(After devol.)
(After devol.)
32
Density (kg/m3)
1524 (832)
16
as received
17.5
1250 (600)
670 (150)
30
32.2
81.7
Moisture (w-%)
6.0
25.3
5.6
Ash content (w-%)
2.4
12.3
0.3
61.6
30.2
12.4
Ultimate analysis
(After devol.)
Water ash free
Carbon (w-%)
86.8 (85.65)
68.11 (73.1)
49.76 (87.31)
Hydrogen (w-%)
4.27 (0.94)
2.84 (1.05)
5.94 (1.63)
Nitrogen (w-%)
1.48 (1.76)
0.64 (0.9)
0.11 (0.24)
Sulfur (w-%)
0.5 (0.32)
1.52 (0.73)
,0.02 (,0.02)
Oxygen diff. (w-%)
6.95 (11.3)
26.88 (24.2)
44.17 (10.83)
Volatile matter (w-%)
Fixed carbon (C-fix) (w-%)
devolatilization and char combustion under fluidized-bed combustor conditions. The transient NO and N2O emission behavior is characterized and its dependencies on bed temperature and oxygen concentration are shown. To study the relative importance of the radical chemistry during devolatilization and char combustion, an iodine addition technique has been developed and proven as a very useful investigation tool.
Experimental Single, spherical particles are formed from three different solid fuels: a bituminous coal, a subbituminous coal, and beech wood. The fuel particle diameters are 5, 10, and 15 mm. The proximate and the ultimate analyses are given in Table 1. The MM-FBC is made of quartz glass to minimize the catalytic reactivity of the walls and to allow observation inside the combustor. The inner diameter of the main quartz tube is 35 mm and the height is
240 mm (Fig. 1). Ni/CrNi thermocouples are used at different positions to control the combustion temperature. It is exclusively electrically heated by two heating shells. The bed temperature is varied between 600 and 9008C. The bed material is silica sand with a mean diameter of 225 lm. The static bed height is about 40 mm. Air/nitrogen mixtures are used to vary the oxygen partial pressure of the fluidizing gas between 0.05 and 21 kPa. The superficial velocity is 0.68 m/s and controlled by two mass-flow controllers. In the case of iodine addition, solid iodine was placed in a flask which was warmed up in a water bath. Thus, the iodine sublimated and enriched the fluidizing gas which was lead through the flask (Fig. 1). To obtain the instantaneous concentrations of various gaseous species (CO2, CO, CH4, other hydrocarbons, NO, N2O, HCN, and other species) in the flue gas, a high-performance FT-IR spectrometer (BIO-RAD, FTS 60A) was used. The spectrometer
NO AND N2O FORMATION MECHANISM
3327
Fig. 1. The experimental set-up of the multimode, fluidized-bed combustor (MM-FBC): 1,2, inlet of the fluidizing gas (air, nitrogen); 3, mass-flow controllers; 4, display for the mass flow; 5, preheating zone; 6, heating shells; 7, thermocouple; 8, to the chimney; 9, filter; 10, cooler; 11, pump; 12 and 13, gas cell and FT-IR spectrometer; 14 and 15, fuel inlet; 16, flask for iodine addition in a water bath.
is combined with a long-path, low-volume gas cell (Foxboro LV7, cell volume 4 223 cm3, optical path length 4 7.25 m). The design of the MM-FBC, the operating conditions, the sampling line, the FT-IR recording options, and the gas-cell have been optimized to obtain formation rates (i.e., short residence times, high scan rates of 1.2 scans/s at a high resolution of 0.5 cm11). NH3 has not been detected in significant amounts by the FT-IR, possibly due to adsorption in the sampling line, its low concentration, and time dependence. The single fuel particles were fed into the MMFBC and their formation rates as well as the operating conditions were recorded during devolatilization and char combustion. Additionally, the ignition and the extinction of the flame of the volatile matter, the phenomenological behavior, fuel particle temperatures, and the fluid-dynamical behavior have been investigated. Mass balances of the fuel carbon lead to kinetic data of devolatilization and char combustion. These results have been reported elsewhere [10–12] and were used whenever necessary as a basis to predict the NO and N2O formation rates.
Experimental Results Bituminous Coal Figure 2 shows the typical concentration histories of CO2 and the nitrogen-containing species NO, N2O, and HCN during devolatilization and char combustion of a single bituminous coal particle with an initial diameter of 10 mm at 8008C and an oxygen concentration of 10 kPa in the MM-FBC. During devolatilization, the rapid increase of all species can be clearly seen. The NO concentration in the flue gas reaches its maximum of about 190 ppm 5 10 ppm after the particle has been in the combustor about 60 s (after the first third of the devolatilization period). The HCN concentration also increases rapidly but its maximum is only slightly above 20 ppm 5 2 ppm. These two peaks are followed by the increase of N2O to a maximum of about 20 ppm 5 2 ppm. With decreasing amount of volatile matter in the fuel particle, the formation rates of all species decrease toward their corresponding char combustion levels. Integrating the formation
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Fig. 2. Concentration histories of CO2, NO, N2O, and HCN during devolatilization and char combustion of a 10mm bituminous coal particle at 8008C and 10 kPa oxygen partial pressure in the fluidizing gas in the MM-FBC.
rates over the devolatilization period leads to the percentage of conversion of the volatile nitrogen, that is, the nitrogen that is released during devolatilization (28 w-% of the fuel nitrogen), to NO (38 w-%), N2O (8 w-%), and HCN (4 w-%) (refer to Table 2). During char combustion, all formation rates are at low levels compared to the formation rates during devolatilization. In the first fifth of char combustion, the concentration levels are about 20 ppm 5 2 ppm NO, 2 ppm 5 0.5 ppm N2O, and 2 ppm 5 0.5 ppm HCN and they are steadily decreasing. But the time scale of char combustion is longer than the time scale of devolatilization (about 10 times) and compensates to a certain extent the low formation rates. Integrating the formation rates over the char combustion period leads to the percentage of conversion of the char nitrogen, that is, the nitrogen that is released during char combustion (72 w-% of the fuel nitrogen), to NO (16 w-%), N2O (1.5 w-%), and HCN (1.4 w-%) (refer to Table 2). The relative importance of devolatilization and char combustion for NO, N2O, and HCN formation can be found by comparing the amount of fuel nitrogen converted to NO, N2O, and HCN for devolatilization (14 w-%) and char combustion (14 w-%) (Table 2). This reveals that devolatilization and char combustion are of nearly equal importance for the formation of these species. Varying combustion conditions, the above-mentioned percentages of conversion (listed in Table 2) will change. Increasing the bed temperature during devolatilization from 600 to 9008C increases the con-
version of volatile nitrogen to NO from 6 to 49 w-% (of the volatile nitrogen), whereas HCN decreases from 20 to 2 w-%. Conversion to N2O stays at low levels between 600 and 7008C (1–2 w-%) but reaches a maximum around 8008C (8 w-%) and slightly declines toward 7 w-% at 9008C. Increasing the oxygen partial pressure in the fluidizing air from 0.05 to 21 kPa leads, in the beginning, to an increase of the conversion to NO from 6 to 38 w-% (around 10 kPa), then it slightly decreases toward 33 w-%. N2O slightly increases from 1 to 9 w-%, whereas HCN slightly decreases from 9 to 2 w-%. For char combustion, a temperature increase from 600 to 9008C increases the conversion of the char nitrogen to NO from 5 to 26 w-% (of the char nitrogen), whereas the conversion to HCN decreases from 7 to 1 w-%. The conversion to N2O stays at low levels; a maximum can be found about 8008C (1.5 w-%). Varying the oxygen partial pressure from 5 to 21 kPa, the conversion to NO decreases from 31 to 16 w-% (at 10 kPa) but, at higher oxygen levels, increases to 24 w-% (at 21 kPa) again. A slight minimum can also be found at 10 kPa in the case of charnitrogen conversion to N2O (1.5 w-%). The conversion to HCN continuously decreases from 4 to 1 w-% (at 21 kPa). An iodine addition technique has been developed and proven to be a very successful tool for studying the relative importance of the radical chemistry during devolatilization and char combustion of a single fuel particle under FBC conditions. Enriching the fluidizing gas with iodine vapor (concentration levels in the MM-FBC are about 1000 ppm) leads to dramatic changes of the species concentrations but does not effect the heterogeneous carbon conversion rates; neither changes of the devolatilization time nor changes of the char burnout have been found. Contrary to the heterogeneous chemistry, the homogeneous is effected: flame ignition is inhibited and the CO levels increase whereas the CO2 levels slightly decrease. This characteristic behavior is based on the inhibiting effect of iodine. It suppresses the radical concentrations to equilibrium levels and slows down radical reactions (e.g., the CO-OH oxidation reaction, compare with Ref. [13]). Figure 3 shows the strong effects of iodine addition during devolatilization. The NO concentration dramatically decreases, N2O completely vanishes, whereas the HCN level doubles. Figure 4 shows the effects of iodine addition on NO, N2O, and HCN during char combustion. The instantaneous conversion of the fuel nitrogen to NO, N2O, or HCN is defined as the instantaneous ratio of the fuel nitrogen measured as NO, N2O, or HCN to the released amount of carbon (as CO2, CO, CH4, and hydrocarbons). Furthermore, this ratio is normalized by division of the nitrogen to carbon ratio of the fuel (N/C, see Table 2). The carbon conversion of the fuel particle is obtained by measuring the
NO AND N2O FORMATION MECHANISM
3329
TABLE 2 The fuel-nitrogen-to-carbon ratio before and after devolatilization (in parentheses). The increase of this ratio after devolatilization. The splitting of the fuel nitrogen in volatile and char nitrogen. The nitrogen conversion to NO, N2O, and HCN during devolatilization on the basis of volatile nitrogen and fuel nitrogen. The summarized fuel-nitrogen conversion to NO, N2O, and HCN during devolatilization. The same for char combustion.
N/C [1] N/C 1 change (%) Volatile-N (w-%) Char-N (w-%)
Bituminous coal (After devol.)
Subbituminous coal (After devol.)
Beechwood spheres (After devol.)
0.017 (0.021) 24 28 72
0.0094 (0.012) 28 26 74
0.0021 (0.0027) 29 68 32
38 5 3 11 5 1 851 2 5 .5 451 1 5 .5 14 5 2
40 5 5 10 5 2 352 1 5 .5 752 2 5 .5 13 5 3
28 5 3 19 5 3 0.7 5 .2 0.5 5 .2 10 5 2 752 26 5 5
16 5 2 12 5 2 1.5 5 .5 1.0 5 .5 1.4 5 .5 1.0 5 .5 14 5 3
18 5 2 13 5 2 0.5 5 .2 0.3 5 .2 3.5 5 .5 2.5 5 .5 16 5 3
31 5 3 10 5 2 452 1.5 5 1 13 5 2 451 15.5 5 4
Devolatilization NO/vol-N (w-%) NO/fuel-N (w-%) N2O/vol-N (w-%) N2O/fuel-N (w-%) HCN/vol-N (w-%) HCN/fuel-N (w-%) N to (NO, N2O, HCN)/fuel-N (w-%) Char combustion NO/char-N (w-%) NO/fuel-N (w-%) N2O/char-N (w-%) N2O/fuel-N (w-%) HCN/char-N (w-%) HCN/fuel-N (w-%) N to (NO, N2O, HCN)/fuel-N (w-%)
carbon-containing species in the flue gas and by integrating them over time. After devolatilization and the first stages of char combustion, iodine was added to the fluidizing gas. The instantaneous conversion to NO only slightly increases whereas the conversion to N2O drops to zero. The conversion to HCN significantly increases. Comparison to Other Fuels To study the influence of the fuel type on the NO, N2O, and HCN formation characteristics, additional measurements with spherical subbituminous and beech wood particles (10 mm in diameter) were performed at the same operating conditions in the MMFBC. Basically, the concentration histories show qualitatively the same behavior as presented in Fig. 2. As explained in the section on bituminous coal, the conversion of nitrogen in terms of volatile, char, and fuel nitrogen during devolatilization and char combustion are given in Table 2. Although there exist
strong differences in the nitrogen content of the fuels (Table 1), many parallels can be found. The splitting of the fuel nitrogen to volatile nitrogen and char-nitrogen is the same for the two coals (Table 2). The retaining nitrogen to carbon ratio after devolatilization increases (about 27%). The nitrogen conversions during devolatilization and char combustion are very similar for the two coals. The higher conversion of nitrogen to HCN of the subbituminous coal is compensated by the lower conversion to N2O. This is also valid for the beech wood particles during devolatilization. During char combustion, the char-nitrogen conversions to NO, N2O, and HCN of the beech wood are substantially higher than the char-nitrogen conversions of the coals. But the summarized fuel-nitrogen conversion to NO, N2O, and HCN is nearly the same for all three fuels (about 15 w-%) due to the different splitting of the fuel nitrogen of the beech wood. Varying combustion conditions, the listed nitrogen conversions of the fuels will change (compare with the section on bituminous coal). But similar to the concentration histories, they show the same trends.
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Modeling
Fig. 3. Concentration histories of NO, N2O, and HCN during devolatilization of a 10-mm bituminous coal particle at 8008C and 10 kPa oxygen partial pressure in the fluidizing gas in the MM-FBC. The arrows indicate the concentration changes when iodine is added.
The aim of the modeling work is to understand the NO and N2O formation during devolatilization and char combustion presented in the experimental section. The subject of this semitheoretical approach is to use well-known parameters to predict and explain the measured species concentrations. The following measured parameters have been used for input: the combustion conditions of the MM-FBC (bed temperature and temperature profiles, flame characteristics, the input oxygen partial pressure, superficial gas velocity, bed voidage, mean bed particle diameter); the concentration histories of oxygen, CO, CO2, and hydrocarbons in the flue gas; and fuel data (fuel particle diameter, ultimate and proximate analysis before and after devolatilization, particle temperature). For comparison of experimental and modeling results, the bituminous coal has been used. The model calculates the instantaneous concentration profiles of NO, N2O, HCN, NH3, and the nitrogen-containing radicals (NCO, NH2, NH) by considering homogeneous and heterogeneous reactions inside the fluidized bed as well as the homogeneous chemistry in the freeboard (compare with Refs. 2 and 3). In Table 3, the set of the used chemical reactions is given. The measured carbon-conversion rates are taken as the basis for the calculations. The volatile nitrogen is assumed to be released proportional to the carbon release during devolatilization as HCN and NH3. The HCN/NH3 ratio has been varied as an input parameter from 1/1 to 1/0 to study the relative importance of these intermediates on NO and N2O formation. Best agreements with the measurements have been found with a HCN/NH3 ratio of 9/ 1. During char combustion, the char-nitrogen release is taken proportional to the carbon conversion as NO and HCN. Released from the particle’s surface, they are taken as 20 and 4 w-%, respectively. The radical concentrations (OH, HO2, O, H) are estimated by using the CO2 and CO concentration histories (similar to Ref. 14). This enables the decoupling of the nitrogen chemistry from the carbon-oxidation chemistry. Iodine addition decreases the level of the radicals, again estimated by using the CO2 and CO concentrations. The flame of the volatiles is considered in the instantaneous temperature profile of the combustor. A detailed description of the model will be published elsewhere. Discussion
Fig. 4. Instantaneous conversion to NO, N2O, and HCN versus carbon conversion of a 15-mm bituminous coal particle, at 7508C and 10 kPa oxygen partial pressure in the fluidizing gas in the MM-FBC. Iodine is added after about 0.45 carbon conversion during char combustion.
The NO and N2O formation and destruction paths (Fig. 5) have been analyzed and evaluated for the bituminous coal with the model under measurement conditions. The bold arrows indicate the main reaction paths. Typical modeling results are given in Fig. 6 as a concentration versus time plot. It can be
NO AND N2O FORMATION MECHANISM
3331
TABLE 3 The set of chemical reactions used in the model. Arrows in one direction indicate irreversible reactions, arrows in both directions reversible reactions. M indicates a third body Number (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18)
Used reactions NCO ` O → NO ` CO NCO ` OH ↔ NO ` CO ` H NH ` O ↔ NO ` H NCO ` NO ↔ N2O ` CO NH ` NO ↔ N2O ` H NH2 ` NO → N2 ` H2O N2O ` H ↔ N2 ` OH N2O ` OH ↔ N2 ` HO2 N2O ` O ↔ 2 NO N2O ` O ↔ N2 ` O2 N2O ` M ↔ N2 ` O ` M HCN ` O ↔ NCO ` H NH3 ` O ↔ NH2 ` OH NH2 ` O ↔ NH ` OH NO ` CO → 1/2 N2 ` CO2 NO ` 2/3 NH3 → 5/6 N2 ` H2O NH3 ` 3/4 O2 → 1/2 N2 ` 3/2 H2O NH3 → 1/2 N2 ` 3/2 H2
Catalyst
References
Bedmat. Bedmat. Bedmat. Bedmat.
15 15 17 15 18 19 15 15 21 21 22 15 20 23 16 16 16 16
Fig. 5. The reaction paths of fuel nitrogen to NO and N2O. Bold arrows indicate the main reaction paths evaluated from the model for bituminous coal.
seen that iodine addition causes an increase of HCN and a decrease of NO and N2O during devolatilization. During char combustion, the NO level is nearly unaffected, HCN increases and N2O decreases. During devolatilization, the release rates of NO, N2O, and HCN are very high relative to their release
rates during char combustion (Fig. 2), corresponding with the carbon-conversion rates. The conversion of the fuel nitrogen is based on carbon conversion. Basically, the concentration levels of NO are higher because two nitrogen atoms are necessary to form N2O. This implies that more reactions and
3332
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Fig. 6. Modeling results of the concentration histories of NO, HCN, and N2O of a bituminous coal particle (10 mm in diameter, at 8008C, 10 kPa oxygen) during devolatilization and char combustion in the MM-FBC. The arrows indicate the changes when iodine is added. For better presentation, the HCN and N2O concentrations during char combustion are multiplied by 100.
longer residence times are necessary to combine these nitrogen atoms. Due to the longer char combustion period, the lower release rates of NO, N2O, and HCN are compensated to a certain extent, and devolatilization and char combustion are of nearly equal importance for the formation of these species. This is also somehow valid for beech wood. Although the percentage of fuel nitrogen as volatile nitrogen is much higher in comparison to the coals, the conversion rates to NO and N2O are lower (Table 2). The two coals show nearly the same conversion rates for volatile nitrogen and char nitrogen. NO and N2O formation during devolatilization is based on gas-phase chemistry (compare with Ref. 2). This can be clearly seen after iodine addition. The addition of iodine suppresses the radical reactions leading to lower levels of NO and N2O (Fig. 3). HCN is the main precursor of NO and N2O. This can be seen in several figures (Figs. 2–4 and 6). The N2O peak shortly follows the NO and HCN peaks (Figs. 2, 4, and 6). Iodine addition increases HCN and decreases NO and N2O (Figs. 3 and 6), implying that the oxidation reactions of HCN to NO and N2O are inhibited (reactions 12, 1, 2, and 4 in Table 3), leading to higher concentrations of the precursor. The dependencies of bed temperature and oxygen partial pressure in the flue gas (described in the experimental results) reveal similar trends on the formation rates. HCN is decreasing with increasing temperature and oxygen partial pressure, whereas NO and N2O are increasing to a certain extent. The modeling results indicate that HCN is the main pre-
cursor of NO and N2O during devolatilization. The very low levels of NO and N2O cannot be completely explained by model calculations (Fig. 6). Increasing destruction rates due to higher CO concentration levels may be the reason. Modeling results of the bituminous coal indicate that NH3 is of minor importance for NO and N2O formation due to its conversion via reactions 17 and 18 (Table 3). NO and N2O formation during char combustion is based on a different mechanism. Again, this can be clearly seen in the iodine addition experiments (Fig. 4). The effect of iodine addition on the NO level is minor. This indicates that NO formation is based on a heterogeneous mechanism inside the pore structure or at the surface of the fuel because heterogeneous chemistry is nearly independent from radical concentrations. In contrast to NO, HCN steeply increases, whereas N2O steeply decreases. During char combustion, a volatile species which has been identified as HCN is released in low concentrations. This species is homogeneously oxidized to NCO which further reacts with the heterogeneously produced NO to form N2O. Inhibiting the homogeneous oxidation path of HCN (reactions 12, 1, 2, and 4 in Table 3) leads to a steep decrease of N2O and a steep increase of HCN. The NO concentration level is nearly unaffected. It slightly increases because the NO destruction reaction with NCO (reaction 4 in Table 3) to form N2O is inhibited. But the N2O level is much lower than the NO level. Modeling results support this mechanism (Fig. 6). Additional evidence can be found when comparing the two coals during char combustion (Table 2). A decrease of the N2O level is compensated with an increase of HCN and NO. And varying the bed temperature and the oxygen partial pressure (described in the experimental results) leads to a decrease of HCN but in an increase of NO and N2O to a certain extent. Conclusions Devolatilization and char combustion are of nearly equal importance for NO and N2O formation for the two coals. The higher formation rates during devolatilization are compensated by the shorter time scale. In the case of beech wood, devolatilization is more important, although the higher percentage of fuel nitrogen as volatile nitrogen is compensated to a certain extent by lower conversion rates in comparison to the coals. The NO concentration levels are higher than the N2O concentration levels during devolatilization and char combustion due to the formation mechanisms and residence times. Iodine addition has proven to be a very effective method to study the relative importance of homogeneous and heterogeneous chemistry in kinetic experiments.
NO AND N2O FORMATION MECHANISM
Iodine addition experiments confirm that NO and N2O formation during devolatilization is based on a homogeneous mechanism. HCN is the main precursor of NO and N2O (compare with [2]). The formation characteristics can be explained by the homogeneous HCN oxidation and NO and N2O formation reactions (reactions 12, 1, 2, and 4 in Table 3). NO and N2O formation during char combustion is different. NO is primarily heterogeneously formed by char-nitrogen oxidation. But a volatile species, which has been identified as HCN, is simultaneously released in low concentrations. This species is homogeneously oxidized to NCO which further reacts with the heterogeneously produced NO to form N2O. Acknowledgment The authors want to acknowledge Ben Anthony, CANMET, Ottawa who awoke their interest in iodine addition under FBC conditions.
REFERENCES 1. Winter, F., Ph.D. Thesis, Vienna University of Technology, Vienna, 1995. 2. Kilpinen, P. and Hupa, M., Combust. Flame 85:94–104 (1991). 3. Johnsson, J. E., Fuel 73:1398–1415 (1994). 4. Tullin, C. J., Sarofim, A. F., and Beer, J. M., J. Inst. Energy 66:207–215 (1993). 5. Tullin, C. J., Goel, S., Morihara, A., Sarofim, A. F., and Beer, J. M., Energy Fuels 7(6):796–802 (1993). 6. Klein, M. and Rotzoll, G., Sixth International Workshop on Nitrous Oxide Emissions, A˚bo Akademi, Turku, Finland, 1994, pp. 239–253. 7. Miettinen, H., Paulsson, M., and Stro¨mberg, D., Energy Fuels 9(1):10–19 (1995). 8. De Soete, G. G., “Relative Contribution of Homogeneous and Heterogeneous Reactions to the Emissions of NO and N2O at Fluidized-Bed Conditions,” presented at the Institute of Chemical Engineering, Fuel Technology and Environmental Technology, Vienna University of Technology, 1995.
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9. Kramlich, J. C., Cole, J. A., McCarthy, J. M., Lanier, W. S., and McSorley, J. A., Combust. Flame 77:375– 384 (1989). 10. Winter, F., Krobath, P., and Hofbauer, H., Thirteenth International Conference on Fluidized Bed Combustion, ASME, Orlando, FL, 1995, pp. 1477–1487. 11. Winter, F., Wartha, C., and Hofbauer, H., Third International Conference on Combustion Technology for a Clean Environment, Lisbon, Portugal, 1995, Sec. 15.2. 12. Winter, F., Prah, M. E., and Hofbauer, H., “Intra-Particle Temperatures under Various Fluidized-Bed Combustor Conditions: The Effect of Drying, Devolatilization, and Char Combustion”, Combust. Flame, 1995, submitted. 13. Anthony, E. J., Bulewicz, E. M., and Preto, F., Twelfth International Conference on Fluidized-Bed Combustion, ASME, La Jolla, CA, 1993, pp. 41–52. 14. Bulewicz, E. M., Janicka, E., and Kandefer, S., Tenth International Conference on Fluidized-Bed Combustion, ASME, San Francisco, 1989, Vol. 1, pp. 163–168. 15. Miller, J. A. and Bowman, C. T., Prog. Energy Combust. Sci. 15:287 (1989). 16. Johnsson, J. E., Kinetics of Heterogeneous NOx Reactions at FBC Conditions, CHEC Report No. 9003, Technical University of Denmark, Lyngby, 1990. 17. Mertens, J. D., Chang, A. Y., Hanson, R. K., Bowman, C. T., and Masten, D. A., “A Shock Tube Study of the Reactions of NH with NO, O2, and O,” Western States Section/The Combustion Institute Meeting, Livermore, California, 1989, Paper no. WSS/CI-89-96. 18. Glarborg, P., Miller, J. A., and Kee, R. J., Combust. Flame 65:177 (1986). 19. Hanson, R. K. and Saliman, S., in Combustion Chemistry, (W. C. Gardiner Jr., Ed.), Springer-Verlag, New York, 1984, p. 361. 20. Ba¨tz, P., Ehbrecht, J., Hack, W., Rouyeorolles, P., and Wagner, H. G., Twenty-First Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, p. 1107. 21. Davidson, D. F., DiRosa, M. D., Chang, A. Y., and Hanson, R. K., Eighteenth International Symposium on Shock Waves, Springer Verlag, 1991, Vol. 2, p. 813. 22. Glarborg, P., Johnson, J. E., and Dam-Johanson, K., Combust. Flame 99:523–532 (1994). 23. Glarborg, P. and Hadvig, S., Development and Test of a Kinetic Model for Natural Gas Combustion, Report, Nordic Gas Technology Centre, 1991, p. 83.
COMMENTS A. N. Hayhurst, Cambridge University, UK. Your model has CO reducing NO to N2. Did you measure the effect of iodine on the concentration of CO within the bed? Could you give some details of the conditions in the bed; in particular were the bubbles fast and what was the crossflow factor? Also did the burning char particle circulate around the bed (or, on the other hand, float)?
Author’s Reply. One aim of the NO/N2O model was to study different possible effects (especially destruction reactions) which might be of importance for the interpretation of the measurements. Iodine addition increases the CO level which might reduce NO. But the fuel particle is mainly located in the upper part of the fluidized bed (ca 0 to 4 cm below the surface) and
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moves frequently up to the bed’s surface and back again resulting in low gas—bed material contact times decreasing the importance of the heterogeneous destruction reactions. Additionally, the catalytic reactivity of pure silica sand is rather low compared to bed materials containing char and ash. CO was not obtained within the bed but shortly after the particle as described in the paper. The cross-flow factor has not been measured but can be estimated from Kunii and Levenspiel [1] and is in the range of 0.5 to 1 depending on operating conditions.
1. Klein and Rotzoll, 6th N2O Workshop, Turku Finland, 1994. 2. Krammer and Sarofim, Combust. Flame 97:118–124 (1994). 3. Goel, Zhang, and Sarofim, Combust. Flame 104:213– 217 (1996).
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Prof. Mikko Hupa, ABO Akademi University, Finland. How did you take into account the influence of the iodine on the radical levels in your model? How can you exclude the direct influence of the added iodine on the char nitrogen release chemistry?
1. Kunii, D. and Levenspiel, O., Fluidization Engineering, Wiley, New York, 1991. ● Bo Leckner, Chalmers University of Technology, Sweden. It has been shown that N2O is formed heterogeneously during char combustion. In your case the iodine added is supposed to eliminate the gas phase formation of N2O and prevent the conversion of HCN to N2O. This seems to take place, but heterogeneously formed N2O seems to disappear too when iodine is added. Does it mean that there is no heterogeneous formation on N2O or does the iodine influence the surface reaction as well? Adel Sarofim and Shakti Goel, MIT, USA. Until seeing your results, we were reasonably convinced that the heterogeneous mechanism developed at MIT for NO and N2O was the valid one, although there are some experiments (e.g., [1]) that provide reasonable evidence that homogeneous reactions cannot be ruled out. This paper provides interesting data which seem to further support a homogeneous pathway. There seems to be need to examine further the differences between what appears to be conflicting evidence on the relative roles of heterogeneous versus homogeneous reactions. Transient release of nitrogen oxides on oxygen interruption (e.g., [2,3]) are consistent with the role of oxide formation on the surface both for the nitrogen oxides and the CO. Have you examined such transients on cutting off oxygen, and perhaps also iodine? In your presentation you claimed that iodine atoms did not influence the surface reactions based on the lack of change of the carbon oxidation rate on iodine addition. Iodine may compete with NO for the nitrogenous surface complexes and prevent the formation of N2O without affecting the char consumption rate. Examination of the influence of iodine on the transient emissions of oxide complexes on turning off the oxygen might provide useful insights.
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Author’s Replies. The concentrations of the OH, H, HO2, and hydrocarbon radicals depend on the carbon oxidation chemistry and are not significantly affected by the nitrogen chemistry and its radicals. The concentrations of the radicals related to carbon oxidation are estimated by the measured concentrations of CO and CO2 similar to Bulewicz et al., 10th FBC Conf., 1989. Iodine addition changes the CO and CO2 concentration levels by suppressing the OH and H radicals. Using the CO and CO2 measurements it allows to us estimate the radicals’ concentration levels also during iodine addition. M. Hupa’s second question and the questions of A. Sarofim and S. Goel and B. Leckner are regarding possible effects of iodine with the surface chemistry especially for N2O formation during char combustion. It has been shown during the presentation and it is shortly listed in the paper that the addition of iodine does not change the carbon conversion rates during devolatilization nor during char combustion. Char combustion is a strictly heterogeneous process but no effects on iodine addition have been found. It is very unlikely that iodine changes only the nitrogen-containing surfaces. A competing reaction of iodine with NO for possible nitrogenous surface complexes and a small contribution of heterogeneously formed N2O cannot be completely excluded with these measurements. But the homogeneous chemistry of iodine is well understood and the transient behavior of NO, N2O and HCN formation during char combustion has been studied by significantly changing the combustion conditions (e.g. temperature, radical concentrations in the gas-phase, etc.) revealing strong evidence that N2O is homogeneously formed. Experiments where the oxygen and the iodine is turned off are interesting and will be considered in our future work but there exist some possible draw-backs of this method, e.g. the time-scale for gas dispersion in the reactor.