Laboratory Studies on Devolatilization and Char Oxidation under

Fuel nitrogen release and its conversion to nitrogen oxides in pressurized fluidized bed combustion have been recently studied in a laboratory-scale b...
14 downloads 0 Views 129KB Size
Download PDF
Energy & Fuels 1996, 10, 357-363

357

Laboratory Studies on Devolatilization and Char Oxidation under PFBC Conditions. 2. Fuel Nitrogen Conversion to Nitrogen Oxides Yong Lu Department of Energy Engineering, Helsinki University of Technology, Espoo, FIN-02150, Finland Received June 19, 1995X

Fuel nitrogen release and its conversion to nitrogen oxides in pressurized fluidized bed combustion have been recently studied in a laboratory-scale batch reactor, which can be controlled at well-defined operating conditions. The conversion to nitrogen oxides during devolatilization and char oxidation was investigated under the base operating conditions (Tb ) 850 °C, p ) 11 bar, O2 ) 3% v/v, Vf ) 0.24 m/s) with 10 different fuels, from high-volatile peat to relatively low-volatile coke. The effects of other parameters, such as bed temperature, oxygen concentration in the fluidizing gas, pressure, fluidizing velocity, and particle size, on the fuel-N behavior were compared in detail with three of the fuels: lignite, Illinois No. 6 coal, and Kiveton Park coal. A special focus of this study was the influence of pressure which was divided into two parts: oxygen partial pressure and total pressure. Results showed that fuel nitrogen conversion to NOx and N2O was dependent on fuel type and operating parameters, relating strongly to the combustion behavior of fuel batch. Concerning the conversion to NOx, the influence of oxygen partial pressure by changing oxygen concentration in the inlet gas was more marked than that by changing pressure, and total pressure showed a different trend to that when the oxygen partial pressure was changed. Both homogeneous and heterogeneous reactions contributed to the formation of NOx and N2O. The observations indicated that in the batch reactor the reduction mechanism also played a significant role in the emissions of nitrogen oxides under the examined conditions.

Introduction Pressurized fluidized bed combustion (PFBC) has low NOx emissions because of the relatively low combustion temperature and inherent dynamic characteristics, such as higher bed height and lower fluidizing velocity, which enhance the NOx reduction with a longer residence time in the dense bed compared to the residence time in an atmospheric fluidized bed.1 Potentially, a further NO decrease to comply with stricter environmental requirements can be achieved by modifying operations such as air-staged combustion and ammonia injection. NOx and N2O emissions from the demonstration a PFBC power plant have been reported at a relatively lower level than that from atmospheric fluidized bed combustion (AFBC).2 Similar behavior of NOx and N2O under pressurized conditions has also been recognized from the study by the author and his colleagues at the Otaniemi PFBC test rig3 and other studies of PFBC pilot reactors, particularly with respect to decreasing NOx emissions.4 However, the main reason for these observations is still not clear since the influences on other parameters are simultaneously occurring in changing pressure, for instance the profiles of temperature and oxygen concentration in the combustor.3 In addition to optimizing Abstract published in Advance ACS Abstracts, February 1, 1996. (1) McDonald, D.; Anderson, J. 12th Int. Conf. Fluidized Bed Combust. San Diego, CA 1993, 555-564. (2) Dahl, A. 12th Int. Conf. Fluidized Bed Combust. San Diego, CA 1993, 931-940. (3) Lu, Y.; Jahkola, A.; Hippinen, I.; Jalovaara, J. Fuel 1992, 71 No. 6, 693-699. (4) Takeshita, M. IEA Coal Res. 1994, IEACR/75, 90. X

0887-0624/96/2510-0357$12.00/0

the operating conditions for minimizing the emissions of NOx, N2O, SO2, and CO, a comprehensive understanding of the mechanisms of the formation and reduction of nitrogen oxides under PFBC conditions is required. The kinetics of formation and reduction of nitrogen oxides in FBC are complex. Despite the considerable experimental efforts devoted to oxygen partial pressure effects on nitrogen chemistry, which is usually made in studying the effect of pressure, only a few studies on pressure have taken into account the influences of the total pressure and oxygen partial pressure separately. Aho et al.5 found that oxygen partial pressure and total pressure showed opposite influences on fuel-N conversion to NOx; i.e., NOx emissions increased with oxygen partial pressure but decreased with total pressure. The overall effect of increasing pressure reduced NOx at the temperatures encountered typically in FBC. Concerning fuel-N conversion to N2O, however, oxygen partial pressure and total pressure showed a minor influence. Recently, the effect of CO2 partial pressure has also been studied in the same reactor, and no significant influence on the conversion of fuel-N to NOx and N2O was found.6 However, the conditions in their pressurized entrained flow reactor are not similar to those in a fluidized bed particularly when considering the particle size and particle temperature, which have been proved to be very significant factors in the combustion kinetics and nitrogen chemistry. (5) Aho, M.; Paakkinen, K.; Rantanen, J.; Pirkonen, P. Presented at The Nordic Seminar on Combustion and Gasification Reactivities of Solid Fuels, Mar 16, 1993, Jyva¨skyla¨, Finland. (6) Aho et al. LIEKKI-2 Combustion Research Program: Report L951, 1995, Naantali, Finland.

© 1996 American Chemical Society

358

Energy & Fuels, Vol. 10, No. 2, 1996

In studies on nitrogen chemistry involving a gasphase kinetic modeling, Hupa et al.7 have recently observed that the conversion to NO from HCN, modeled as fuel-N, decreased and the conversion to NO2 from HCN increased when pressure was increased. On the other hand, the effect of pressure on N2O was complex, and the conversion of HCN to N2O could be either increase or decrease at higher pressure. On the basis of their kinetic modeling8 and laboratory experiments by Hulgaard,9 it was concluded that gas-phase chemistry is very likely to play a significant role in N2O formation in FBC. However, the studies from other investigators reported that char-N is a greater contributor to N2O emissions compared to volatile-N,10 and the main source of N2O is the heterogeneous reduction of NO with char-N over the surface of particles.11,12 Wallman et al.12 have also studied the effects of pressure on the formation of NO and N2O in a bench-scale pressurized fluidized bed reactor with pressure range of 2-20 bar. Their finding, that increasing pressure decreased NO emissions and increased N2O emissions slightly, was attributed to the effect of fuel concentration in the bed since bed fuel concentration was directly proportional to the operating pressure in their tests. Research on PFBC behavior has been carried out at the Helsinki University of Technology since the mid1980’s. The work, carried out at the Otaniemi PFBC test rig, has concentrated on the combustion and emission behavior using various solid fuels. In addition to a detailed study of fundamental phenomena, a laboratory-scale PFBC batch reactor was implemented to determine the combustion characteristics under welldefined operating conditions, which are usually limited at the test rig in continuous operation. This study, which belongs to LIEKKI-2 (Finnish national research program) project and aims at the comparison of fuel reactivity and nitrogen oxide formation under pressurized FBC conditions, has been divided in two parts. The first part, regarding volatile release and char reactivity during devolatilization and char oxidation, can be found elsewhere.13 This paper is the second part, focusing on the fuel-N conversion to nitrogen oxides, such as NO, NO2, and N2O. The influences of pressure in this study involve both oxygen partial pressure and total pressure. Oxygen partial pressure was set with changing either O2 concentration in the fluidizing gas or system pressure, and total pressure with changing O2 concentration and system pressure simultaneously in order to keep oxygen partial pressure constant. The effects of other parameters, such as bed temperature, fluidizing velocity, and particle size, on fuel-N conversion to nitrogen oxides are also investigated in detail. The results presented assist in understanding the behavior of fuel nitrogen conversion and the development of kinetic modeling. (7) Hupa et al. LIEKKI-2 Combustion Research Program: Report L94-1, 1995, Naantali, Finland, 1994, 189-203. (8) Kilpinen, P. Ph.D. Thesis, Åbo Akademi University, Åbo, Finland, 1992. (9) Hulgaard, T. Ph.D. thesis, Technical University of Denmark, Lyngby, Denmark, 1991. (10) De Soete, G. G. Presented at the 5th Int. Workshop on Nitrous Oxide Emissions, Jul 3-4, 1992, Tsukuba, Japan. (11) Tullin, C.; Sarofin, A.; Beer, J. 12th Int. Conf. on Fluidized Bed Combust. San Diego, CA, 1993, 599-609. (12) Wallman, P.; Ivarsson, E.; Carlsson, R. 11th Int. Conf. Fluidized Bed Combust. Montreal, Canada, 1991, 1021-1025. (13) Lu, Y. Energy Fuels 1996, 10, 348-356.

Lu

Figure 1. Typical combustion profiles of NOx and O2 concentration in the PFBC batch reactor.13

Experimental Section In burning a fuel batch the process occurs in two consecutive stages: devolatilization followed by char oxidation. Figure 1 depicts the release of fuel-N in NOx as a function of burning time. During the first stage a proportion of the bound nitrogen is released into the gas phase as volatile-N. The rest of the coal nitrogen remains as char-N and is released at a rate similar to that of char burning. Subsequently, both volatile-N and char-N are converted to NO, NO2, N2O, and N2 in a variety of homogeneous and heterogeneous reactions. Therefore, the behavior of volatile-N and char-N, and their conversion to nitrogen oxides, can be studied separately by means of the batch reactor. The details of test facility, fuel selection, test settings, operating process, and gas measurements have been presented in part 1 of this study.13 NOx was measured with a conventional analyzer and the data graphically reconstructed in situ as combustion profiles on the monitor. All data were saved in 2 s using a data logging system. N2O, NO, NO2, and other nitrogen species, such as NH3 and HCN, were analyzed with a Fourier transform infrared (FTIR) spectrometer, which is supported with a KBr beamsplitter, MCT detector, and 0.5 L gas cell having an optical path length of 6 m. On the basis of the selected FTIR spectrometer parameter, including four mirror scans with moving velocity of 1.3 cm/s and resolution of 2 cm-1, a spectrum sampling time is about 9 s. Because of low concentrations of N2O and NO2 in the off-gas, particularly during char oxidation, their strongest absorbance bands in the mid-infrared range were used for qualitative analysis. The overlapping bands of CO2 and CO for analyzing N2O and H2O for NO2 can be subtracted totally using reference spectra in various concentrations. The detection limit of N2O and NO2 analysis is less than 0.5 ppm by means of this method. In order to estimate the accuracy of the FTIR, the profiles of CO and CH4 concentrations from the FTIR analyzer were compared with those obtained using a continuous CO and CH4 analyzer (URAS 10E). The concentration profiles from both analyzers are in a good agreement as seen in Figure 2. The conversion of fuel-N to nitrogen oxides was calculated based on the nitrogen species in off-gas and its content in the fuel batch, respectively. The concentration of molecular nitrogen (N2) in the off-gas was not measured, and the conversion of volatile-N and char-N to NOx was calculated based on the assumption that the nitrogen:carbon ratio (N/C) of the chars is similar to those of the parent coals at FBC conditions.14,15 Laughlin et al.14 concluded, from their studies (14) Lauglin, K.; Gavin, D.; Reed, G. Fuel 1994, 73, 1027-1033. (15) Hayhurst, A. N.; Lawrence, A. D. Combust. Flame 1995, 100, 591-604.

Devolatilization and Char Oxidation. 2

Energy & Fuels, Vol. 10, No. 2, 1996 359

Figure 2. Comparison of CO and CH4 concentrations between a conventional and an FTIR analyzer. Table 1. Main Results of NOx Conversion and Operating Conditionsa moisture volatile ash N f Nv Nc (wt %) (wt % db) (wt % db) (%) (%) (%)

fuel peat B peat A lignite Iowa coal Dawmill coal Illinois No.6 coal Kiveton Park coal Harworth coal Polish coal coke

14.3 17.9 11.5 17.8 3.5 3.8 4.3 1.8 2.6 0.2

72.5 69.6 53.2 43.2 34.3 33.9 30.1 29 28.4 1.7

2.5 3.6 4.6 7.5 9.8 10.6 21.3 21.8 17.4 11.1

8 5 21 16 20 18 20 22 24 35

6 4 15 8 12 13 11 11 17

13 6 25 21 23 19 23 27 26 35

a N , N , and N ) conversion to NO of fuel, volatile and char f v c x nitrogen. Tb ) 850 °C, p ) 11 bar, O2 ) 3% v/v, and Vf ) 0.24 m/s, Gf ) 5.28 mol/min.

on coal and char nitrogen chemistry in PFBC, that the influence of operating conditions on the proportion of nitrogen remaining in the char was negligible in the ranges of operating parameters studied. Fuel nitrogen conversion for two pathways to NOx and N2O is expressed as

Nf f NOx conversion )

∫0tw[NOx] dt mcxN/MN

,%

∫0tw[N2O] dt

2 Nf f N2O conversion )

mcxN/MN

,%

where w is gas flow rate in mol/s, [NOx] and [N2O] are the concentrations in the off-gas, mc is the weight of the fuel batch in g, xN is the weight fraction of nitrogen in the fuel in wt %, and MN is the atomic weight of nitrogen in g/mol.

Results and Discussion Fuel Nitrogen Conversion to Nitrogen Oxides. Fuel-N conversion to nitrogen oxides for all selected fuels was investigated under the base testing conditions.13 Table 1 summarizes the total NOx conversion from fuel-N, and conversion from volatile-N and char-N separately, also the proximate analysis of the fuels are listed in the table. The results show that total conversion of fuel-N to NOx covers a wide range of 5-35%, the lowest one was achieved from burning peat A, the middle ones from coals, and the highest one from coke.

It indicates that fuel-N conversion to NOx is roughly dependent on the fuel rank, i.e., the higher the fuel rank, the higher the conversion of fuel-N to NOx. The exceptions, e.g., peat B, which contains more volatiles, shows higher NOx conversion than peat A, and lignite gives higher NOx compared with Iowa and Illinois No. 6 coals, suggest that fuel-N conversion could also be affected by other fuel properties, such as fuel nitrogen bound form, nitrogen content, and the propositions of fuel ash. The same trend between the NOx conversion and fuel rank was also noted at the PFBC test rig for the same fuels,3 and combustion of gasification residues has also revealed a higher NOx conversion than those did for other fuels at the test rig.16 Compared to burning coke, the processes of devolatilization and char burnout of peat are performed at much lower O2 concentration because of the oxygen depletion (seen Figure 4 in part 1) and led to both lower volatile-N and char-N conversions to NOx. The low oxygen conditions may affect the NO formation in two ways: (1) a reducing atmosphere promotes fuel-N conversion to N2 as air staged combustion does for reducing NO formation;3 and (2) providing more NO reducing gases, such as carbon monoxide and hydrocarbons, that enhance the destruction of formed NO. On the other hand, the higher combustion rate of peat leads to higher particle temperatures17 and also contributes to lower NOx conversion because of the acceleration of NO destruction on the char surface. Another finding was that volatile-N exhibited lower conversion to NOx than did char-N whatever the type of fuel (Table 1). This result could mainly be attributed to O2 depletion during devolatilization and the presence of more NO reducing agents, such as char surface and hydrocarbon compounds. Formation of nitrous oxide seemed to be very dependent on fuel rank under the base testing conditions. Nitrous oxide was not observed in burning high-volatile fuels, such as peat, lignite, and Iowa coal. N2O was obtained in a few ppm level for all bituminous coals and the highest N2O concentration was obtained with Kiveton Park coal, a medium-volatile bituminous coal from UK.13 The release profile of N2O with burning time was similar to the profiles of other components in the offgas. The peak of N2O was observed during devolatilization and its decline in char oxidation. However, the ending time for N2O conversion was earlier than the times for the NOx and CO2 conversion, which suggested that N2O formation from char-N is related to carbon conversion and NO concentration; i.e., the reaction of NO with char-N is the principal path for N2O formation in the presence of oxygen.11,18 Nitrous oxide concentration was in many cases lower than the detection limit (0.5 ppm) and this level was too low to determine the conversion of fuel-N to N2O. This phenomenon was mainly attributed to the high temperature in the freeboard area (∼900 °C) at the base testing conditions, seen Figure 3 in part 1. Unlike the fuel pyrolysis in the same reactor, nitrogen intermediates, such as HCN and NH3, were not detected (16) Kudjoi, A.; Hippinen, I.; Lu, Y.; Jahkola, A. 13th Int. Conf. Fluidized Bed Combust., Orlando, FL, 1995, 117-123. (17) Ross, I. B.; Patel, M. S.; Davidson, J. F. Trans. Inst. Chem. Eng. 1981, 59, 83-88. (18) Kunii, D.; Wu, K.; Furusawa, T. Chem. Eng. Sci. 1980, 35, 170177.

360

Energy & Fuels, Vol. 10, No. 2, 1996

Figure 3. Profiles of conversion of fuel-N to NOx as a function of fractional carbon conversion at different O2 concentrations in the fluidizing gas.

Figure 4. Profiles of conversion of fuel-N to NOx as a function of fractional carbon conversion at different system pressures

from devolatilization. It suggests that although the amount of fuel-carbon and fuel-nitrogen release can be the same during the two processes,15 the products from pyrolysis and devolatilization could apparently be different. This phenomenon also implies that the secondary reactions of nitrogen intermediates can be completed in the reactor under the examined conditions. Effect of Pressure. Oxygen Partial Pressure. Oxygen partial pressure for batch combustion can be changed by increasing either O2 concentration in the fluidizing gas or system pressure. Figure 3 shows the profiles of fuel-N conversion to NOx as a function of the fractional carbon conversion with O2 concentrations of 10, 6, 3, and 1% v/v and Figure 4 with system pressures of 5, 8, and 11 bar absolute. The main results of nitrogen conversion at different oxygen partial pressures and total pressures are summarized in Table 2. It was surprising to note that the conversion of fuel-N to NO obviously decreased with increasing O2 concentration for all tested coals. This observation is in contrast to the results from the test rig3 and those from other investigators at the reactors in various scales. It can be seen in Table 2 that when O2 increases from 1 to 10% v/v the conversion decreases from 23 to 13% for

Lu

lignite,20 to 13% for Illinois No. 6, and 20 to 15% for Kiveton Park coal. It can also be noted that only the conversion of volatile-N to NOx shows a slight increase with increasing O2 concentration. Therefore, such a relationship between NOx formation and O2 concentration is mainly attributed to the conversion of bound nitrogen to NOx during char oxidation rather than during devolatilization. This phenomenon may attributed to three possible reasons: (1) the burning rate increases markedly with increasing O2 concentration and thus the burning temperature of char particles, resulting in the accelerating the NO reduction over particle surface; (2) higher O2 concentration increases the volatile yield thus decreasing the char loading, which is the main source of NO formation as found in this work and elsewhere;12 (3) the reaction of NO with char-N to form N2O is enhanced with increasing O2 concentration. A higher N2O concentration in off-gas was observed in this study especially at 10% v/v of O2. A similar phenomenon was also recognized by Tullin et al.11 and Kunii et al.18 Also, Tullin et al.11 reported that the addition of NO in 200 ppm increased N2O conversion from 0.15 to 0.23 in a mixture of 12% v/v O2 in helium, and de Soete10 reported that no N2O was formed from the reaction of NO with char nitrogen in the absence of oxygen. Tullin et al. further pointed out another explanation; i.e., CO in the char pores increased with O2 concentration in a diffusion controlled regime and caused an enhancement of the NO reduction reactions.11 This assumption was unclear in this case, and the dependence of NOx conversion on CO concentration was not remarkable. In contrast to char-N, volatile-N conversion to NO increased slightly with increasing O2 concentration. The effect of O2 concentration during devolatilization was more pronounced at low O2 concentration due to O2 depletion. When O2 concentration went down to 1% v/v the devolatilization process was under more reducing conditions and apparently lower conversion to NOx was obtained compared with those at other O2 concentrations. It could be due to lower concentrations of NO reducing agents (CO and hydrocarbons) at higher O2 concentration.13 A higher emission of nitrous oxide was observed with increasing O2 concentration in burning bituminous coals, particularly during devolatilization. This increase of N2O emission was more apparent when O2 increased from 3 to 10% v/v. The dependence of N2O on oxygen input could be weakened due to the high reactor temperature in this study. The level of N2O ppm was relatively low during char oxidation in the all cases of changing O2 concentration, which makes difficult to conclude the effect of O2 concentration on the conversion of char nitrogen to nitrous oxide. No marked increase of N2O was found in burning lignite. A clear difference of fuel-N conversion to nitrogen oxides was noted in changing system pressure compared to O2 concentration. Within the system pressure range investigated the conversion of fuel-N to NOx was obtained in the range of 17-21% and the minimum conversion was always obtained at pressure of 8 bar for all tested coals, as shown in Table 2. This phenomenon can mainly attributed to the conversion to NOx from char-N rather than from volatile-N; i.e., char-N to NOx decreases from 5 to 8 bar and then increases from 8 to

Devolatilization and Char Oxidation. 2

Energy & Fuels, Vol. 10, No. 2, 1996 361

Table 2. Main Results of Pressure Effect on Nitrogen Conversion to NOxa lignite

fuel O2, % v/v Nf, % Nv, % Nc, % p, bar Nf, % Nv, % Nc, % P (O2, % v/v) Nf, % Nv, % Nc, % a

1 23 13 27 5 19 11 23 5 (6.6) 17 12 20

3 21 15 25 8 18 11 22 8 (4.1) 20 13 24

Illinois No. 6 coal 6 19 16 21 11 21 15 25 11 (3.0) 21 15 25

10 13 15 13

1 20 12 22 5 21 12 23 5 (6.6) 18 12 20

3 18 13 19 8 17 12 19 8 (4.1) 18 12 20

6 16 14 17 11 18 13 19 11 (3.0) 18 13 19

Kiveton Park coal 10 13 14 13

1 20 10 24 5 21 11 25 5 (6.6) 19 11 22

3 20 11 23 8 17 10 19 8 (4.1) 19 11 21

6 15 11 17 11 20 11 23 11 (3.0) 20 11 23

10 15 13 16

Nf, Nv, and Nc ) conversion to NOx of fuel, volatiles, and char nitrogen.

Figure 5. Profiles of conversion of fuel-N to NOx as a function of fractional carbon conversion at different total pressures.

11 bar, and no marked change of volatile-N to NOx in changing system pressure. One explanation for the trend is that the conversion to NO decreases and conversion to NO2 increases with increasing system pressure. At higher system pressure char burns faster owing to the rise of oxygen partial pressure and this in turn increases the burning temperature, resulting in more NO reduction. On the other hand, higher pressure increases NO2 formation from NO oxidation in two ways: (1) owing to higher oxygen partial pressure for the oxidation of NO, and (2) the direct oxidation of char-N to NO2 due to the longer residence time of formed NO in the char particles. An increase of NO2 concentration with increasing pressure was observed by means of the FTIR analyzer. These findings also showed a good agreement with the calculations from the modeling, which indicated that the conversion of HCN (as fuel-N) to NO decreased and to NO2 increased with increasing pressure.7 Also, it was noted that the influences of pressure on both volatile-N and char-N conversion to NOx were less pronounced than those in changing O2 concentration. This finding differs from its effect on combustion characteristics; i.e., pressure showed a stronger effect on devolatilization than did O2 concentration, but a weaker effect on char burnout.13 As discussed in part 1,13 higher system pressure increases oxygen partial pressure, accelerating the oxidation reactions as O2 concentration does, but unlike O2 concentration, pressure has virtually no influence on the driving force for

oxygen transfer to particles and diffusion in particle pores because the higher oxygen partial pressure is offset by a proportional lowering of the oxygen diffusion and thereby the weakened effect of pressure on fuel-N to nitrogen oxides. Meanwhile, pressure also increases simultaneously the partial pressures of other gaseous products, which could affect the formation and reduction of nitrogen oxides in direct and indirect ways. Unlike the influence of O2 concentration, the emissions of nitrous oxide were slightly decreased at higher system pressure. However, the quantitative difference of bound nitrogen conversion to nitrous oxide in changing pressure was not clear due to the relatively low level of N2O ppm. Total Pressure. Effect of total pressure was studied at a constant oxygen partial pressure (pO2) and different system pressure. In this work pO2 was fixed at 0.33 bar, therefore, when the system pressure was changed from 5 to 8, to 11 bar absolute, the O2 concentration in fluidizing gas corresponded from 6.6 to 4.13 to 3% v/v. The main results are also listed in Table 2. It was interesting to note that the effect of pressure on nitrogen oxide formation at fixed oxygen partial pressure differs from that under variable oxygen partial pressure. No remarkable change of NOx conversion with total pressure, including NO formation from both volatile-N and char-N, was observed for Kiveton Park coal and Illinois No. 6 coal (Figure 5). Meanwhile, the increase of NO2 was not significant with increasing total pressure compared to the case in changing pressure only. One explanation can be the balance of NO2 formation by increasing pressure and decreasing O2 concentration simultaneously; i.e., oxygen partial pressure is the major factor on NO2 formation. Also, no clear influence of pressure on N2O was observed in changing total pressure. These suggested that oxygen partial pressure predominates for fuel-N conversion to nitrogen oxides, at least in burning bituminous coals. In burning lignite, a significant increase of NOx conversion with increasing total pressure was obtained, which is similar with decreasing O2 concentration but different with increasing pressure. These results indicate that O2 concentration plays more pronounced role in NOx conversion than system pressure for high volatile fuel under the examined conditions. Effect of Other Parameters. Effects of other parameters, such as bed temperature, fluidizing velocity, and particle size, on the conversion of fuel-N to nitrogen oxides were also investigated. Figure 6 presents the fuel-N conversion to NOx as a function of carbon conversion at a bed temperature of 650, 750, 850

362

Energy & Fuels, Vol. 10, No. 2, 1996

Figure 6. Profiles of conversion of fuel-N to NOx as a function of fractional carbon conversion at different bed temperatures.

and 900 °C. A significantly lower NOx conversion was obtained at higher temperatures. In addition, decreases in both volatile-N and char-N conversion to NOx were observed for lignite with increasing temperature, but mainly in char-N conversion to NOx was notable for the bituminous coals, Illinois No. 6 and Kiveton Park coal. This phenomenon might be attributed to the higher burning rate at higher bed temperature, which leads to more consumption of oxygen and deeper O2 depletion. Meanwhile, higher temperature enhances NO reduction over particle surface, including both bed material and residual char. Also, it can be seen that there is a greater contribution of temperature to the conversion of bound nitrogen to NOx during char oxidation than during volatile release. The typical trend between N2O and temperature, i.e., N2O formation decreases with increasing bed temperature, was achieved in the trials for the tested fuels. Figure 7 shows N2O concentration as a function of burning time at bed temperature of 650, 750, 850, and 900 °C for both Illinois No. 6 coal and lignite. Obviously, temperature is a very critical factor on N2O chemistry, and there is almost no N2O emission when bed temperature was kept above 900 °C. These results agree well

Lu

with the observations from the PFBC test rig.3 Nitrous oxide decomposition, including both thermal decomposition and heterogeneous decomposition on the particle surface, are the major contributions to relatively low N2O at higher temperatures. The conversions to N2O for bituminous coals are about 9-10, 6-7, 1-2, and 0-1% corresponding to the bed temperatures of 650, 750, 850, and 900 °C. A high N2O conversion of 12% was also noted for lignite at the low temperature of 650 °C, but its decline with increasing bed temperature was more pronounced than those for the other coals. This observation also implies one of the explanations for the dependence of N2O emission on fuel rank, i.e., compared with higher ranked fuels lower one burns faster and leads to deeper O2 depletion and higher particle temperature of particle, hence less N2O formation and more N2O reduction. The changes of combustion and fuel nitrogen behavior at low temperature, such as the release form of bound nitrogen and gaseous product,13 may also contribute the trend between nitrous oxide and temperature. A significant increase in fuel reactivity was obtained with increasing gas velocity for all tested coals because of the improvement of both mass transfer and heat transfer,13 including the transfer between particles and between particles and bubbles based on the two-phase theory in fluidizing bed. However, its effect on fuel-N conversion to nitrogen oxides was minor. The trends between NOx conversion and fluidizing velocity were irregular for different coals. At a velocity of 0.24 m/s the lowest conversion to NOx was obtained for lignite (Figure 8), but the highest conversion was for Kiveton Park coal and Illinois No. 6 coal. For all fuels a clear decrease of volatile-N conversion to NOx was observed at a velocity of 0.14 m/s compared with those at higher velocities, and a slightly deeper O2 depletion at lower velocity is a likely reason. Figure 9 shows the fuel-N conversion to NOx as a function of carbon conversion at various fuel batch particle sizes from 0.55 to 2.19 mm for lignite. The conversion increased with bigger particle size, which is attributed to both increases of volatile-N and char-N conversion with increasing particle size. A more pronounced trend between conversion and particle size was obtained for Kiveton Park coal compared with

Figure 7. N2O concentration as a function of burning time with (a) Illinois No. 6 coal and (b) lignite at a bed temperature of 650, 750, 850, and 900 °C.

Devolatilization and Char Oxidation. 2

Figure 8. Profiles of conversion of fuel-N to NOx as a function of fractional carbon conversion at different gas velocities.

other fuels. Compared with bigger particles, smaller particles have a relatively higher reactivity,13 leading to the deeper O2 depletion and higher burning temperature15 and then the enhancement of NO reduction over the particle surface. Also, the changes in ash content and combustion behavior for different batch sizes might also contribute to the lower formation of NOx for smaller particle sizes compared with the larger sizes. Further work is currently being undertaken to ascertain the influence of the pressure and other parameters, such as bed temperature and batch size, on the formation and reduction of nitrogen oxides in PFB combustion of various chars, including gasification residues. Conclusions Concerning fuel nitrogen conversion to nitrogen oxides, the following conclusions can be made under the examined pressurized fluidized bed combustion conditions in the laboratory batch reactor: Fuel nitrogen conversion to nitrogen oxides can cover a wide range (5-35%) depending on the fuel type and operating conditions. The behavior of O2 depletion in the process of burning batch fuel makes a great contribution to combustion behavior and fuel-N conversion. Under the base testing conditions the conversion of fuel-N to nitrogen oxides depends on fuel rank, i.e., the lower the fuel rank, the lower the conversion to both NOx and N2O. Volatile-N showed lower conversion to NOx than did char-N regardless of the fuel type. The influence of operating parameters on the conversion of volatile-N to nitrogen oxides differed from did char-N. Oxygen partial pressure affected fuel-N conversion to nitrogen oxides significantly. The influence of O2 con-

Energy & Fuels, Vol. 10, No. 2, 1996 363

Figure 9. Profiles of conversion of fuel-N to NOx as a function of fractional carbon conversion at different particle sizes.

centration on conversion to NOx was more pronounced than did pressure in their ranges examined. Increasing system pressure caused a decrease in the conversion to NO, but an increase to NO2, leading to a nadir of conversion to NOx at 8 bar. The influence of total pressure on the conversion to NOx depends on fuel type. There is no remarkable effect for bituminous coals in changing total pressure, but a significant effect for lignite. The conversion of the bound nitrogen to nitrous oxide is complicated in changing oxygen partial pressure. Higher N2O emissions were observed at higher O2 concentration in the inlet gas, but at lower system pressure. Total pressure showed no influence on the fuel-N conversion to nitrous oxide. Increasing bed temperature and decreasing particle size yielded a lower conversion of fuel-N to NOx because combustion rate was higher, so as, in turn, was deeper O2 depletion. The effect of fluidizing velocity on fuel-N conversion to nitrogen oxides was minor. Bed temperature is critical for N2O formation and survival; i.e., N2O decreased extremely with increasing bed temperature. Lower N2O concentration was due to the high freeboard temperature in all cases examined. Therefore, further work is needed in order to conclude the behavior of volatile-N and char-N in forming nitrous oxide. Acknowledgment. The research work presented in this paper is a part of the LIEKKI-2 national combustion research program. The work was financed by the Ministry of Industry and Trade of Finland, A. Ahlstrom Oy, Imatran Voima Oy, and Helsinki University of Technology. The author gratefully acknowledges the personnel of the author’s laboratory. EF950116U