Formation of Nitrogen-Containing Compounds ... - ACS Publications

Energy & Fuels 1997, 11, 1073-1080

1073

Formation of Nitrogen-Containing Compounds during Slow Pyrolysis and Oxidation of Petroleum Coke Edward Furimsky* IMAF Group, 184 Marlborough Avenue, Ottawa, Ontario, Canada K1N 8G4,

Yasuo Ohtsuka† Institute for Chemical Reaction Science, Tohoku University, Sendai 980-77, Japan Received January 17, 1997X

The petroleum coke from a fluid coking process was pyrolyzed in helium and oxidized in 1% and 4% O2 and in air, with the aim to determine N-containing compounds such as HCN, NH3, NO, and N2O. The experiments were performed with and without limestone. NO was the major product during all oxidation runs. N2O was formed only in air. In this case, N2O formation was delayed when compared with that of NO. The addition of limestone decreased formation of HCN and increased that of NH3, whereas NO formation was least affected.

Introduction The evolution of N-containing compounds from carbonaceous solids during pyrolysis, gasification, and combustion has attracted a great deal of attention, although with some delay when compared with Scontaining compounds. Mechanistic and kinetic aspects of the formation of the N-containing compounds, such as HCN, NH3, NO, and N2O, were extensively reviewed.1-3 For obvious reasons, most of the research has been focused on coal with the aim to explain emission formation during combustion. Because coal devolatilization precedes its combustion, an understanding of the emission formation during pyrolysis is the first step in explaining their formation during combustion. In this regard, HCN and NH3 are the primary species of interest. The relative yields of these compounds depend on coal properties and experimental conditions.4-7 Coal rank, porosity, particle size, rate of heating, and residence time are among the important parameters. Ha¨ma¨laïnen et al.8-10 presented results that indicate the effect of the fuel-O/fuel-N ratio on the HCN/NH3 ratio, i.e., the latter decreased with increasing fuel-O/fuel-N ratio. These authors emphasized the importance of phenolic groups during the conversion of * To whom correspondence should be addressed. E-mail: [email protected]. †E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, August 1, 1997. (1) Wojtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel Process. Technol. 1993, 34, 1. (2) De Soete, G. Rev. Inst. Fr. Pet. 1993, 4, 113. (3) Davidson, R. M. Nitrogen in Coal. IEA Coal Research, Report IEAPER/08; IEA Coal Research: London, 1994. (4) Chen, J. C.; Niksa, S. Energy Fuels 1992, 6, 254. (5) Bassilakis, R.; Zhao, Y.; Solomon, P. R.; Serio, M. A. Energy Fuels 1993, 6, 710. (6) Baumann, H.; Mo¨ller, P. Erdoel, Erdgas, Kohle 1991, 44 (1), 29. (7) Serio, M. A.; Zhao, Y.; Wojtowicz, M. A.; Chapernay, S.; Solomon, P. R.; Nelson, P. F. Proceedings of the Fourth International Conference on the Effect of Coal Quality on Power Plants, Charleston, SC, 17-19 August, 1994. (8) Ha¨ma¨laïnen, J. P.; Aho, M. J.; Tummavuori, J. L. Fuel 1994, 12, 1894. (9) Aho, M. J.; Jouni, P.; Ha¨ma¨laïnen, J. P.; Tummavuori, J. P. Combust. Flame 1993, 95, 22. (10) Ha¨ma¨laïnen, J. P.; Aho, M. J. Fuel 1995, 12, 1922.

S0887-0624(97)00009-1 CCC: $14.00

HCN or its precursor to NH3. The involvement of the coal’s hydrogen during the diffusion of HCN and/or its precursors from the pores was also considered. It was reported that the coal’s mineral matter can also influence distribution of HCN, NH3, and N2 during pyrolysis in He.11,12 These effects were even more pronounced when Fe was added to the coal and He was replaced by CO2. In the presence of O2, HCN and NH3 are converted to NO and N2O. It is generally agreed that most of the HCN is converted to N2O, whereas most of the NH3 is converted to NO. Similarly as for HCN and NH3, properties of coal, such as rank, porosity, and fuel-O/ fuel-N ratio as well as the experimental conditions, such as the O2 partial pressure, temperature, rate of heating, and residence time are all important for the relative yields of NO and N2O. The formation of NO and N2O is rapid during combustion using an excess of air. This is usually the case in all commercial combustion systems. Under such conditions, the elucidation of the mechanism of NO and N2O formation is complicated because it is dominated by secondary reactions. Slow oxidation, using a low partial pressure of O2 can provide valuable information on the origin of the precursors to the formation of NO and N2O. Such an approach was used in the present study. Thus, the O2 concentrations used (1-4 vol. %) ensured that most of the O2 was consumed on the exterior of the particles. This minimized effects of diffusion on the formation of N-compounds during the oxidation. Moreover, the coke used for the study has several unique properties, i.e., very low H/C and fuel-O/fuel-N ratios, low volatiles content, and low porosity. Very low porosity ensured that most of the added limestone deposited on the particle exterior. The experimental system used and conditions chosen were suitable for studying the effect of limestone on the coke oxidation. (11) Ohtsuka, Y.; Furimsky, E. Energy Fuels 1995, 9, 141. (12) Goel, S.; Zhang, B.; Sarofim, A. F. Combust. Flame 1996, 104, 213.

© 1997 American Chemical Society

1074 Energy & Fuels, Vol. 11, No. 5, 1997

Furimsky and Ohtsuka

Table 1. Properties of Coke proximate, wt % (db) ash volatiles fixed carbon

4.87 4.71 90.42 ultimate, wt % (db)

carbon hydrogen H/C nitrogen sulfur oxygen CaO

83.12 1.92 0.28 1.69 6.89 1.51 0.35

N2 BET surf. area, m2/g mean particle diameter, µm

6 180

Experimental Section Coke and Limestone. The coke sample was produced during the upgrading of a heavy residue employing a fluid coking process. During withdrawal from the system, the coke was contacted by air and steam while still hot. Some properties of the sample are shown in Table 1. The limestone sample consisted of about 82% CaCO3 and about 5% MgCO3 with SiO2 and Al2O3 accounting for the difference. For coslurrying, the limestone was crushed to -320 mesh. The suspension of this limestone in water was then coslurried with the coke. After filtration, the mixture was vacuum-dried at 150 °C overnight. The total content of CaO determined during the subsequent analysis of the coke was 3.3 wt % compared with 0.35 wt % in the original coke. Procedure. The experiments were performed in an externally heated quartz reactor using a 1 g sample size. Helium was used for the pyrolysis, whereas 1% O2 and 4% O2 (nitrogen balance) mixtures, as well as air, were used for the oxidation experiments. The gas exited from the top and entered the Balston filter before entering the analytical system. The experiments were performed isothermally at 650 °C and in the temperature-programmed mode. In the latter case, the reactor was heated from room temperature to 1000 °C at 20 °C/min. Gas Analysis. HCN, NH3, CO, and CO2 analyses were performed using the on-line Bruel & Kjaer FTIR analyzer Type 301. Analyses were carried out in 2 min intervals. The chemiluminescent NO/NO2 analyzer Model 10 AR, operated in the NOx mode, was used for the analysis of NO. The N2O was analyzed using an FTIR on-line analyzer. The repeatability of the analysis was (5% for NH3 and NO and for HCN at concentrations exceeding 50 ppm. For N2O and HCN below 50 ppm, the repeatability was (10%. The concentrations of the compounds in the gas and a total flow of the gas, normalized to STP conditions, were used for the calculation of the yields of the compounds and their rates of formation. XPS Analysis. The N 1s XPS spectra of coke samples were measured using a Mg KR X-ray source. A long acquisition (several hours) time was required to ensure good resolution. The binding energy was referenced to the Ag 3d peak at 367.9 eV. Pyridinic, pyrrolic, and quaternary nitrogen, and N-oxide were resolved at 398.8, 400.3, 401.3, and 403.0, respectively.

Results Isothermal Pyrolysis and Oxidation. The yields of HCN and NH3 from the isothermal pyrolysis and oxidation of the coke, expressed as the amount of the converted coke’s nitrogen, are shown in Table 2. These results were obtained from the product-time profiles such as the one shown in Figure 1 for the pyrolysis run. In this case, the formation of HCN and NH3 reached the maximum and then approached the detection limit of the analyzer. During pyrolysis, the flow rate and

Figure 1. Formation of HCN and NH3 during isothermal pyrolysis of coke at 650 °C. Table 2. Wt % of Nitrogen in Coke Converted to HCN and NH3 during Isothermal Pyrolysis and Oxidation yield, wt % medium helium

1% O2

4% O2

helium

1% O2

4% O2

NH3

HCN

1 1 1 0.5 0.5 1 1 0.5 0.5 1 1 0.5 0.5

Without Limestone 0.6 0.6 0.3 0.6 0.3 0.6 0.3 0.6 0.3 0.6 0.3 0.6 0.3

1.16 1.07 1.12 1.02 1.07 2.67 0.87 4.12 1.65 3.11 1.80 6.26 2.67

0.68 0.68 0.46 0.62 0.34 1.66 1.05 2.80 1.48 2.80 1.29 5.48 2.65

1 1 1 0.5 0.5 1 1 0.5 0.5 1.0 1.0 0.5 0.5

With Limestone 0.6 0.6 0.3 0.6 0.3 0.6 0.3 0.6 0.3 0.6 0.3 0.6 0.3

2.33 2.43 1.50 1.70 1.50 5.24 2.04 5.63 3.11 4.17 1.75 5.29 4.27

0.15 0.25 0.22 0.28 0.25 1.38 0.68 1.94 1.32 1.94 0.74 3.14 2.52

size, g

flow, L/min

sample size had little effect on the yields of NH3, whereas yields of HCN decreased at the lower flow rate. This indicates an additional conversion of HCN or its precursor to other species at the lower flow rate due to the longer residence time in the reaction zone. The oxidation profiles differed from that shown in Figure 1. Thus, after reaching the maximum, the HCN and NH3 concentrations decreased to a steady-state level in which their formation persisted during the entire oxidation run. During the oxidation, about onethird of the NH3 and about one-half of the HCN was formed during the initial stage, whereas the rest was formed during the steady-state. The former stage coincided with the pyrolysis maxima, suggesting that also during the oxidation, part of the HCN and NH3 formed initially resulted from the pyrolysis of the coke

Nitrogen Compounds of Coke

matrix. In 1% O2, conversion of the nitrogen in coke to NH3 and HCN increased significantly compared with the pyrolysis. The increase in HCN yield was more pronounced relative to that of NH3. The overall conversion of the coke’s nitrogen to HCN and NH3 increased proportionally with increasing gas/coke ratio, i.e., it was highest for the 0.5 g of coke and 0.6 L/min of gas experiment and lowest for the 1 g of coke and 0.3 L/min of gas experiment. However, the NH3 yield increased relative to that of HCN with increasing gas/coke ratio. The increased yields of HCN and NH3 in 1% O2, compared with the pyrolysis, were complemented by the higher carbon conversion of the coke, indicating that the formation of these compounds is associated with the destruction of the coke matrix. The additional yield increase was observed in 4% O2, but not proportionally to the increase in the O2 partial pressure, when compared with 1% O2. This may indicate that part of the HCN and NH3 or their precursors were converted to other N-containing species. However, also for 4% O2, the amount of coke’s nitrogen converted to HCN and NH3 increased proportionally with increasing gas/coke ratio. During pyrolysis, the addition of limestone to coke increased the NH3 yield, whereas that of HCN decreased and approached detection limits of the analyzer. Nevertheless, the nitrogen conversions to HCN + NH3 during the pyrolysis with and without limestone were similar. This indicates that either limestone or CaO and other species could have played a certain role in the overall mechanism of the HCN and NH3 formation. The effect of limestone was further enhanced in the presence of 1% O2, as indicated by the results obtained with and without limestone, namely, the large increase in the NH3 yield and a slight decrease in that of HCN. Also, in 1% O2, the sum of HCN + NH3 increased in the presence of limestone by about 50%, whereas the difference in the overall carbon conversion was very small when compared with that for the experiment in the absence of limestone. The increase in O2 concentration from 1% to 4% had a more pronounced effect on the HCN yield than on the NH3 yield. Similarly, as in 1% O2, in the presence of limestone, the 4% O2 gave yields of HCN that were consistently lower than those in the absence of limestone. Temperature-Programmed Pyrolysis and Oxidation. As the results in Figure 2 show, the limestone had a significant effect on HCN formation during the temperature-programmed pyrolysis and oxidation in 1% O2. These results were used for estimating the data in Table 3. It is evident that the effect of limestone on the NH3 increase is less pronounced than that during the isothermal runs (Table 2). At the same time, the effect of limestone on HCN removal in 1% O2 was much more pronounced during the temperature-programmed runs than that during the isothermal runs. In 4% O2, the effects of limestone on HCN and NH3 yields were less pronounced during the former. Above 700 °C, the HCN concentration approached the detection limits of the analyzer. This could be at least partly attributed to the onset of the limestone decomposition, i.e., generation of additional CaO and CO2. During the isothermal run at 650 °C, these effects would be insignificant.13 (13) Barin, I.; Knacke. O. Thermochemical properties of inorganic substances; Springer-Verlag: Berlin, 1973.

Energy & Fuels, Vol. 11, No. 5, 1997 1075

Figure 2. Formation of HCN during temperature-programmed pyrolysis and oxidation of coke with and without limestone. Table 3. Final Yields from Temperature-Programmed Runs for 1 g Sample yield, wt % limestone

flow, L/min

NH3

HCN

NO

HCN/NH3

1.64 2.20

0.89 0.16

no yes

0.6 0.6

Helium 1.21 1.08 1.41 0.22

no yes no yes

0.6 0.6 0.3 0.3

1% O2 2.38 1.35 3.69 0.71 2.04 0.74 2.77 0.28

4.39 3.92 3.54 3.72

0.57 0.21 0.36 0.10

no yes

0.6 0.6

4% O2 2.00 2.31 1.89 1.45

6.83 7.11

1.16 0.77

no yes

0.6 0.6

Air 3.40 4.15 3.95 3.20

20.5 20.2

1.22 0.81

Also, in this case, the residence time of the coke in the combustion region was more than twice that of the coke during the temperature-programmed runs. It is noted that in 1% O2, the onset of HCN formation was shifted to lower temperatures compared with pyrolysis. This shift was even more pronounced in 4% O2 and coincided with similar shifts in the formation of CO and CO2. This suggests that the release of HCN and NH3 resulted from an O2-aided breakup of N-containing rings, which is enhanced at higher oxidation rates. Formation of NH3 in He and 1% O2 is shown in Figure 3 for the corresponding runs in Figure 2. In He, the maximum NH3 formation occurred at a higher temperature than that of HCN (Figure 2), suggesting that at least a part of the NH3 originated from the latter. The


Figure 3. Formation of NH3 during temperature-programmed pyrolysis and oxidation of coke with and without limestone.

Figure 4. Formation of NO during temperature-programmed pyrolysis and oxidation of coke with and without limestone.

enhanced formation of NH3 in the presence of limestone is evident especially in 1% O2. As the results in Figure 4 show, during pyrolysis, the NO formation increased upon the addition of limestone. Also, NO formation was occurring only in one temperature region. In 1% O2 (Figure 4), two regions of NO formation were observed, i.e., one having a maximum at about 450 °C and the other at about 750 °C. The temperature region of NO formation during pyrolysis overlapped with the higher temperature region observed during the oxidation in 1% O2. The appearance of another region at lower temperatures in 1% O2 coincides with the shift of HCN (Figures 2 and 3) and NH3 formation to lower temperatures in 1% O2 compared with that during pyrolysis. It is believed that most of the N-containing compounds formed in the lower temperature region originated predominantly from the O2aided destruction of the coke matrix. In the higher


Figure 5. Formation of NO and N2O during temperatureprogrammed oxidation of coke in air with and without limestone.

temperature region, both the pyrolysis and O2-aided breakup of the organic matter were the contributors. The results in Table 3 show that NO was the main product under all conditions. During pyrolysis only the effect of limestone on NO formation was evident. In 1% and 4% O2, the differences between the NO yields in the presence and the absence of limestone were not large enough to indicate an effect. Also, limestone had little effect on the total conversion of the coke’s nitrogen, estimated from the yields of HCN, NH3, and NO. However, limestone influenced the distribution of the N-containing compounds, namely, the formation of HCN relative to that of NH3. Increasing the O2 concentration from 1% to 4% increased the yield from about about 8% to about 11%. At the same time, the total carbon conversion in the absence of limestone increased from about 10 wt % to about 41 wt %, respectively. This would indicate a preferential removal of the carbon groups compared with that of the N-containing groups in the coke. However, other N-containing compounds, not measured in the present work, could have been formed also. For example, it is well established that N2 and tars are important products during pyrolysis of chars.14 Special attention was given to the formation of N2O. In He and 1% O2, the amount of N2O was less than the detection limits of the analyzer. In 4% O2 only, a few ppm of N2O appeared between 600 and 700 °C. Air was used in hopes of seeing some evidence for N2O formation. As the results in Figure 5 show, N2O has indeed appeared but at temperatures higher by at least 200 °C compared with the temperature at which NO appears. In air, the NO yield was at least 20 times greater than that of N2O. Moreover, the NO yield about tripled when compared with that in 4% O2. For completeness, HCN and NH3 formation was determined in air as well. The results in Figure 6 and Table 3 indicate trends similar to that observed in 4% O2, i.e., limestone affected (14) Ohtsuka, Y.; Furimsky, E. Energy Fuels 1995, 9, 141.



Figure 6. Formation of HCN and NH3 during temperatureprogrammed oxidation of coke in air with and without limestone. Table 4. Probable Species from Pyrolysis of Carbonaceous Solid species

wt %

carbon hydrogen nitrogen sulfur oxygen

76.4 4.8 1.5 3.0 6.5C4N2

HC CH3CN (CN)2 N2 HCNS C2N CHNO CN CH3NCS NH3 NH2

HCN formation whereas little effect on NH3 formation was observed. Results in Table 3 show that the overall conversion of the coke’s nitrogen, expressed as the sum of HCN + NH3 + NO, about tripled by increasing the O2 concentration from 1% to about 20%. At the same time, the carbon conversion increased from about 10% to almost 100%. It was shown, that most of the unaccounted coke’s nitrogen could be converted to N2.15 Discussion Pyrolysis and Oxidation without Limestone. It is generally accepted that in the case of coals, tar formation (devolatilization) precedes formation of Ncontaining compounds.1-8 The type of species formed during pyrolysis of carbonaceous solids can be identified using the Gibbs energy minimization approach.16 The species identified by this method are shown in Table 4 in decreasing probability of their formation. Dozens of (15) Niksa, S.; Cho, S. Energy Fuels 1996, 10, 463. (16) Furimsky, E.; Boudreau, F.; Zheng, L.; Kovacik, G. Erdoel Kohle 1993, 10, 379.

other N-containing fragments, having a lower probability of formation, were also identified. Because of the low volatiles content in the coke, the role of tar may differ from that observed during the pyrolysis of coals. The appearance of NH3, HCN, and NO above 600 °C during pyrolysis of the coke (Figures 2A-4A), compared with about 400 °C during that of coals,5,14 indicates cracking of the coke’s heterorings. It is believed that if any tar was formed, its structure would significantly differ from that from coal pyrolysis. Thus, in the former case, stable rings and heterorings requiring high temperatures for their cracking would be the predominant species. The results obtained during pyrolysis (Figures 2A and 3A), namely, formation of HCN and NH3 in the same temperature range, indicate that these compounds have a common precursor. Amino groups would be the natural source of NH3. However, such groups are much less stable than heterorings. Therefore, NH3 would be expected to appear at lower temperatures than HCN if such groups would be present. It is believed that in the present case, contribution of the amino groups to NH3 formation was insignificant and cracking of the heterorings was the main contributor to the formation of both NH3 and HCN. It is generally accepted that the cracking of the N-containing heterorings yields HCN and other unstable intermediates that subsequently react to give NH3.1-3 Several authors have suggested that the NH3 is formed from the HCN precursors via reaction with the OH radicals.8-10 It was reported by Aho et al.9 that the HCN/NH3 ratio obtained during pyrolysis of various carbonaceous solids correlated with the fuel-O/fuel-N ratio. By use of the results in Table 3, the HCN/NH3 ratio was about 0.9 for the fuel-O/ fuel-N ratio of about 1. These values are consistent with the trends observed by these authors. The formation of NO during pyrolysis (Figure 4) occurred in the same temperature range as that of HCN and NH3 (Figures 2A and 3A). This suggests that the breakup of heterorings in coke molecules is the origin of NO as well. NO formation requires the presence of oxygen in the coke. Part of the oxygen may include the chemisorbed O2, as well as organic oxygen in the form of furanic rings and phenolic -OH groups. Pyridonic structures were identified in heavy crudes from which the coke was derived.17 Also, some forms of quaternary nitrogen can be associated with oxygen. Such structures may be additional sources of the coke’s oxygen. The quaternary structures are formed on thermal treatment of carbonaceous solids, presumably from pyrrolic and pyridinic structures.18 However, other information indicates an opposite trend.19 It was shown by Pels et al.20 that pyridinic and quaternary nitrogen are predominant forms in chars treated at high temperatures. The results obtained in the present study (Table 5) were obtained from spectra such as those shown in Figure 7. Thus, pyridinic and pyrrolic structures accounted for about 90% of nitrogen in the coke. The content of the quaternary nitrogen was very small, (17) Choi, J. H. K.; Gray, M. R. Fuel Process. Technol. 1991, 28, 77. (18) Stanczyk, K.; Dziembaj, R.; Piwowarska, Z.; Witkowski, S. Carbon 1995, 10, 1383. (19) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuels 1994, 8, 896. (20) Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Carbon 1995, 11, 1641.



Figure 7. N 1s XPS spectra of residues after temperatureprogrammed pyrolysis of coke in He (A) and temperatureprogrammed oxidation in 1% O2 from room temperature to 1000 °C (B). Table 5. Distribution of N-Containing Structures treatment

pyridinic (398.8)

pyrrolic (400.3)

quaternary (401.3)

N-oxides (403.0)

none He-1000 1% O2-650 1% O2-1000 1% O2-1000-Ca

42 29 40 23 32

48 43 44 45 48

3 20 8 21 11

7 9 8 11 9

but in any thermal treatment its content as well as that of N-oxides increased, whereas that of the pyridinic nitrogen decreased. The pyrrolic nitrogen was the least and the pyridinic nitrogen the most affected during the high-temperature treatments. It appears that limestone slowed these reactions. In 1% O2, the formation of NO, NH3, and HCN increased compared with that from pyrolysis. A further increase in the O2 concentration to 4% decreased the NH3 yield, whereas that of NO and HCN increased. If the OH groups and/or radicals were responsible for the conversion of the HCN precursors to NH3,8-10 then in 1% O2, the concentration of such radicals increased. The decreased NH3 yield in 4% O2 compared with 1% O2 would indicate a removal and/or shortening of the lifetime of the OH radicals. It is believed that these effects are significantly dependent on the properties of the carbonaceous solids. For example, rather different observations were made during similar pyrolysis and oxidation experiments of the coke deposited on spent hydroprocessing catalysts.21 In this case, NH3 was the only product during pyrolysis, whereas during oxidation, (21) Furimsky, E.; Nielsen, M.; Jurasek, P. Energy Fuels 1995, 9, 439.

the NH3 yield decreased and HCN became the major product. It was postulated in these studies that part of the NH3 arose from the hydrogenation of HCN or its precursors. It is believed that both the OH radicals and other forms of active hydrogen (e.g., naphthenic and aliphatic hydrogens) could take part. Considering the high H2 pressures employed during hydroprocessing operations, the presence of such active forms of hydrogen in the coke on the catalyst is probable. Part of the NH3 could have been formed from the amino groups. Again, such groups may have been present in the coke on the catalysts. The significantly lower H/C ratio of the coke used in the present study, compared with that of the catalyst’s coke, supports these assumptions. In the presence of O2, the availability of the active hydrogen required for such reactions was significantly diminished. This lead to the increase in the HCN yield relative to the NH3 yield. The suppression of NH3 formation in the presence O2 compared with the pyrolysis was also observed for some coals and chars.15 The absence of N2O during pyrolysis, as well as in 1 and 4% O2, is not surprising if NO is the origin of N2O, as it was postulated in several studies.12,22-24 Thus, after being formed in the pores, NO reacts with a nitrogen radical and/or group still attached to the solid to give N2O while diffusing from the pores. The reduction of NO to N2O, and even to N2, may also be achieved using carbons possessing a high porosity. Thus, it was reported by Rodriguez-Mirasol et al.25 that N2O is more readily reduced on a char surface than NO. It is believed that at least in 1% O2, most of the NO was formed on the external surfaces of the particles because of the very low porosity of the coke. This would apply especially in the temperature region of the first maximum in Figure 4B. Also, most of the NO formed during pyrolysis could have originated on the external surfaces because of the previous exposure of the coke particles to air. In this case, the secondary gas-solid reduction of NO to N2O could not occur. The appearance of N2O in air would indicate the opening of some pores when the coke burn progressed to a higher level. This is supported by the delayed N2O formation when compared with that of NO, as shown in Figure 5. Thus, the rapid buildup of the latter was observed already at about 300 °C whereas that of N2O occurred above 400 °C. Interestingly enough, the rapid buildup of HCN and NH3 in air (Figure 6) coincided with that of NO (Figure 5). This suggests that most of, if not all, the NO originated from the O2-aided breakup of the coke matrix, whereas the N2O arose from the subsequent NO reduction while diffusing from the pores. It was indeed confirmed that the coke’s porosity can be increased by more than a magnitude by its activation.26 Thus, it is well established that gas-phase oxidation of HCN and NH3 to give NO and N2O requires much higher temperatures than 300 and 400 °C, respectively.1-3 Pyrolysis and Oxidation with Limestone. It was indicated earlier that at low O2 concentrations, most of (22) Tullin, C. J.; Sarofim, A. F.; Bee´r, J. M. J. Inst. Energy 1993, 66 (12), 207. (23) Illań-Go´mez, M. J.; Linares-Solano, A.; Salineas-Martıńez de Lecea, C. Energy Fuel 1993, 7, 146. (24) Miettinen, H.; Abul-Milh, M. Energy Fuels 1996, 10, 421. (25) Rodriguez-Mirasol, J.; Ooms, A. C.; Pels, J. R.; Kapteijn, F.; Moulijn, J. A. Combust. Flame 1994, 99, 499. (26) DiPanfilo, R.; Egiebor, N. O. Fuel Process. Technol. 1996, 46, 157.



Table 6. Free Energies of Formation free energy, ∆G (kcal/mol) reaction

900 K

1100 K

H2O + CN ) CO + H2 + 1/2N2 CO2 + CN ) 2CO + 1/2N2

-87.3 -86.0

-92.1 -95.3

the O2 will be consumed on the external surfaces of the coke particles where the effect of limestone on the distribution of N-containing products will be predominant. As the results in Tables 2 and 3 and Figures 2 and 3 show, during pyrolysis the addition of limestone decreased the HCN yields and increased that of NH3, i.e., the HCN/NH3 ratio decreased significantly. HCN could have been removed by reacting with the Ca component of limestone. At the same time, the increased NH3 yield could have resulted from the limestone-aided decomposition of the coke matrix via reaction of the Ca component of limestone with the organic sulfur in the coke. The increased carbon conversion during pyrolysis and gasification upon addition of the Ca species to the coke was indeed experimentally confirmed.27-29 Also, moisture in the coke and/or H2O produced during pyrolysis could facilitate -OH groups via reaction with some limestone components (e.g., CaO). As indicated above, such groups may be at least partly responsible for conversion of HCN or its precursors to NH3.8-10 The carbonate part of the limestone could have participated as well. Thus, it was shown that HCN formation could be suppressed in CO2.14 The free energies of formation in Table 613 suggest that there is a significant driving force for the removal of CN by both CO2 and H2O. In 1% O2, the effect of limestone on the HCN/NH3 ratio was still quite evident. The increase in O2 concentration from 1% to 4% increased the HCN/NH3 ratio, but the effect of limestone on this ratio was less pronounced. In air, these effects were similar as in 4% O2. In general, observations on the effect of limestone are in agreement with the observations made by several authors1,30-32 who proposed the following mechanism involving Ca species:

CaO + 2HCN ) CaCN2 + CO + H2 CaCN2 + H2O + 2H2 + CO2 ) CaO + 2NH3 + 2CO Mechanism of N-Containing Compounds Formation. The N-containing species predicted by thermodynamics (Table 4) suggest that CN and HCN are among the predominant intermediates released from the coke’s matrix during thermal decomposition.16 Subsequently, these species may be converted to NCsCtCsCN, NCsCN, and other compounds. This may involve reactions such as the addition to unsaturated bonds, recombination, and hydrogenation. It is believed that such conversions increase with the resi(27) Furimsky, E.; Plamer, D. Appl. Catal. 1986, 23, 355. (28) Franklin, H. D.; Cosway, R. G.; Peters, W. A.; Howard, J. B. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 39. (29) Figueiredo, J. L.; Moulijn, J. A. Carbon and Coal Gasification, Science and Technology; Martinus Nijhoff: Dordrecht, 1986. (30) Adams, R. C.; Aul, E. F.; Kulkami, S.; McAllister, R. A.; Margerum, S. Report EPA/600/7-86/051; Radian Corp.: Research Triangle Park, NC, 1986. (31) Alexanderson, V.; Sherman, L. M. Cayanamides. In KirkOthmer Encyclopedia of Chemical Technology; Interscience: New York, 1992; Vol. 4, pp 663-675. (32) Leppalahti, J.; Simmell, P.; Kurkela, E. Fuel Process. Technol. 1991, 29, 43.

Figure 8. Tentative mechanism of chemisorption of O2 by coke.

dence time and at least partly account for the decrease in the HCN yield with decreasing flow rates (Tables 2 and 3). The probability for hydrogenation may increase if CN is formed in the particle interior. Thus, if sufficient hydrogen-donating groups are available, the hydrogenation of CN may continue to give HCN and even NH3 while diffusing to the particles exterior. The NH3 can also be formed before HCN if amino groups are present. The hydrogen-donating species may include OH groups and/or radicals, H2O, and paraffinic and naphthenic hydrogens. It was shown that during heating of the coke in air under conditions similar to those used in the present work, the weight slowly increased, then began to decrease above 300 °C before abruptly decreasing at about 400 °C.33 The abrupt decrease in the weight was attributed to the ignition of the coke particles. The initial weight increase was attributed to the chemisorption of O2 by the coke. Based on the hydrocarbon autoxidation mechanism,34 it was postulated that the O2 chemisorption resulted in the formation of peroxy radicals, hydrogen peroxides, and endo-peroxides, as shown in Figure 8.35 Stability of these groups decreases with increasing temperature. At the same time, their oxidizing strength increases with temperature. It is believed that most of the NO was formed during the oxygen transfer from these groups to the coke’s nitrogen, accompanied by the breakup of the fused rings. In this case, nitroxyl radical-like groups, still part of the coke matrix, could be the precursor before NO is eliminated. It is difficult, if not impossible, to postulate N2O formation under similar conditions. Most likely, N2O is a secondary product arising from the NO reduction.24 While the rings were broken, additional sites for O2 chemisorption and/or formation of the oxidizing groups were created. Because of the low porosity of the coke, most of these reactions occurred on the external surface of the particles. HCN and NH3 precursors were also formed during the O2aided breakup of the fused rings, as suggested by their appearance already at 300 °C in air (Figure 6). Thus, thermal cracking of the fused ring at about 300 °C could not be the source. However, the actual temperature of the particles could be higher than the average temperature in the reactor. In this case, thermal cracking in the particle interior could have also contributed to HCN (33) Furimsky, E. Fuel Process. Technol. 1988, 19, 203. (34) Howard, J. A. Adv. Free Radical Chem. 1973, 14, 72. (35) Furimsky, E.; Duguay, D. G.; Houle, J. Fuel 1988, 2, 183.


and NH3 formation. Hydrogen peroxides could be precursors to the formation of OH groups and/or radicals required for NH3 formation.8-10 The hydrocarbons autoxidation mechanism suggests that the aliphatic carbon attached to aromatic rings is one of the most active carbons for the formation of peroxides and peroxy radicals.34 Under O2-limited conditions, e.g., in 1% O2, the active carbons will be oxidized predominantly. The decomposition of peroxides and peroxy radicals is accompanied by the cleavage of C-C bonds. Aromatic rings can be broken to release various fragments if such


reactions occur in their proximity.36 Conditions of the commercial combustion systems represent another extreme of coke oxidation. Thus, an excess of air and high temperatures favor high rates of complete oxidation of all fragments formed during the pyrolysis, as well as that of the solid residue left behind. EF970009V (36) Huntington, T. G.; Mayo, F. R.; Kirshen, N. A. Fuel 1979, 58, 31.

Formation of Nitrogen-Containing Compounds ... - ACS Publications

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