Nitrogen oxide formation from australian coals

C O M B U S T I O N A N D F L A M E 62: 21-30 (1985)

21

Nitrogen Oxide Formation from Australian Coals D. PHONG-ANANT, L. J. WIBBERLEY, and T. F. WALL Department of Chemical Engineering, University of Newcastle, Australia 2308

The evolution of fuel nitrogen during devolatilization and the formation of NOx during combustion were studied for two Australian coals in crucible, thermobalance, and rapid heating (drop-tube furnace) experiments. The evolution of coal nitrogen during devolatilization was dependent on both temperature and mode of heating. Under near stoichiometric combustion, 20-30% of coal nitrogen was converted to NOx. Conversion increased markedly with increased fuel-lean conditions. The NOx formed from volatiles was proportional to the fraction of coal nitrogen evolved as HCN and NH3. The combustion of char at various temperatures and stoichiometries showed that the conversion of char nitrogen to NOx depended primarily on char burnout. The contribution of char nitrogen to NOx formation was greater than that of volatile nitrogen under fuel-rich conditions.

INTRODUCTION During the combustion of pulverized coal (p.f) nitrogen oxides are formed as a result of the oxidation of nitrogen compounds in coal (fuel NO) and the fixation of atmospheric nitrogen (N2) at high temperatures (thermal NO). As these oxides are predominantly nitric oxide (NO) with a smaller fraction, usually less than 5 %, appearing as nitrogen dioxide (NO2) [1], the sum equivalence of nitrogen oxides (NOx) is termed "the total N O . " The formation of thermal NO is dependent on both combustion temperature and stoichiometry but is usually insignificant at temperatures below 1800K. Its formation is well described by the Zeldovich or modified Zeldovich mechanisms. Another source of thermal NO is by a separate mechanism involving the reaction of hydrocarbon fragments and molecular nitrogen in the flame. However, the formation of this socalled "prompt NO" [2, 3] has a weak temperature dependence, a short lifetime of several Copyright © 1985 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017

microseconds, and is only significant in very fuel-rich flames. For coals containing around 1% nitrogen, prompt NO has been estimated to be less than 5 % of total NO [4] and therefore is considered negligable in comparison with fuel NO. In contrast to thermal NO, fuel NO is that formed from the gas phase oxidation of devolatilized coal nitrogen and from the heterogeneous oxidation of char nitrogen in the tail of the flame. Although only 10-50% of coal nitrogen is normally converted to NO [8], this corresponds to a flue gas NO concentration of 200-800 ppm in comparison with less than 50 ppm for thermal NO. Fuel nitrogen is therefore the primary source of total NO emissions [5-7]. Previous research with overseas coals has shown that the formation of fuel NO depends mainly on the conditions of combustion (temperature and local oxygen potential) and to a varying extent on the nitrogen content of coal. Combustion conditions are important as they

0010-2180/85/$03.30

22 affect the partitioning of nitrogen between volatiles and char [7-9] and the conversion efficiency of volatile and char nitrogens to NO. However, there is considerable disagreement in the literature as to the relative importance of these factors. Pershing and Wendt [7] reported that fuel NO was insensitive to flame temperature at normal combustion temperatures. Conversely, P0hl and Sarofim [10] and Blair et al. [11] showed that the fraction of coal nitrogen released in the volatiles increased with increasing pyrolysis temperature and particle heating rates, which implies a temperature dependence for the formation of NO. However, Pershing and Wendt [12] considered that only large variations in pyrolysis temperature may give changes in fuel NO, through changes in the partioning of nitrogen between volatiles and char. Both researchers have shown that 60-80% of coal nitrogen was evolved with volatiles and have concluded that volatile rather than char NO is the major contributor to NOx emissions. As the result of these earlier investigations, more emphasis has been given to the study of the evolution of volatile nitrogen and subsequent NO formation in the gas phase. Regarding the effect of coal itself, some studies [7, 13] claim that coal type and coal composition has little or no effect on the conversion efficiency to NO. However, other workers [8, 14] have shown that the evolution of the volatile nitrogen species NH3 and HCN increased with increasing fuel nitrogen and was dependent upon coal rank. Cyanide and amine radicals are thought to be the major intermediates for NO formation [9, 15, 16]. In general, HCN formation was greater than NH3 with bituminous coals while NH3 was predominant with the lower rank coals. Very little of either were formed from anthracites. The characteristics of HCN and NH3 formation have been used in NOx control strategies using staged combustion, where emissions have been correlated with the formation of these species during devolatilization. Fuel NO is also dependent on the proportion of nitrogen remaining in char [14].

D. PHONG-ANANT ET AL. The aim of the present study was to investigate further the fate of fuel nitrogen, during both devolatilization and combustion of two widely different Australian pulverized coals. EXPERIMENTAL PROCEDURE

Experiments were carried out for a wide range of heating conditions and included crucible and thermobalance pyrolysis experiments to study the evolution of volatiles and fuel nitrogen at low heating rates and experiments in a drop-tube furnace to study pyrolysis and NOx formation under simulated p.f firing conditions. Coals U s e d

The coals used in the experiments were a subbituminous coal from Liddell in New South Wales and a soft brown coal from Morwell in Victoria. All samples, except for those used in the crucible experiments, were sieved to + 6390 #m and then air classified to remove the remaining undersize. Coals for the crucible experiments were seived to - 2 0 0 #m. Table 1 gives the ultimate and proximate analyses of the two coals used in the experiments.

TABLE 1

Compositions of Liddell and Morwell Coals Used in the Experiments (63-90/zm) Liddell

Morwell

Proximateanalysis % Ash (d.b.) % Moisture (air dried) % Volatile (d.b.)

25.6 0.4 25.3

2.4 10.6 51.9

Ultimate analysis (daf) % Sulphur % Chlorine % Carbon % Hydrogen % Nitrqgen % Oxygen

0.55 0.05 80.94 5.03 1.81 11.62

0.57 0.07 69.21 4.80 0.59 24.76

NOx FORMATION FROM COAL COMBUSTION

Pyrolysis Experiments The crucible experiments were conducted in a laboratory muffle furnace to determine volatiles and nitrogen evolution at low temperatures and low heating rates. About 5 gm of pulverized coal was placed in covered standard (volatile) silica crucibles which were then heated to 500-1500K until obtaining negligable weight loss. Nitrogen in all char and coal samples were analyzed using the Kjeldahl method (AS1038 Part 6-1971). Further crucible pyrolysis experiments were made using a thermobalance. These experiments allowed continuous measurement of both weight loss and gas composition during heating. In these experiments a 25 mm diameter platinum crucible was suspended from strain gauge in a 35 mm diameter silica tube which was heated in an electric vertical tube furnace. A 200 mg sample of oven dried and vacuum degased coal was placed in the crucible and heated in an argon flow of 100 ml min- 1 at 0.17K s- 1 (10K min- I) to 1443K. This temperature was maintained for a further 60 min. HCN, NH3, and the other pyrolysis gases (H2, CO, CH4, and N2) were measured using specific-ion electrodes, spectrophotometric analysis, and gas chromatography. Tar nitrogen was taken as the difference between the coal nitrogen and that in the char and the other pyrolysis gases. In the drop-tube experiments, pulverized coal and argon were injected into the hot zone of a vertical tube furnace via a hot water-cooled probe (360K). The isothermal hot zone consisted of an electrically heated alumina muffle tube 45 mm in diameter and 100 mm long. The char and product gases were collected by a similar water-cooled probe inserted from below. The char was retained on a filter located in the heated (360K) gas sampling line. The hot zone (gas) temperature was varied from 800 to 1873K. In these experiments, the estimated particle heating rates were 104-105K s- 1 and the residence of the particles in the hot zone was 1-2 s. Gas and char analysis employed the same techniques as was used in the thermobalance experiments.

23

Combustion Experiments (a) Coal Combustion The experiments were conducted under stoichiometric conditions at furnace temperatures ranging from 800 to 1900K. The coal feed rate was about 5 g h - l with a gas flow rate of 600 ml min- ~. This gave a residence time of 1-2 s. Both air and a nitrogen-free Ar/O2 mixture were used, to distinguish between thermal and fuel NO. Experiments were also performed for a range of combustion stoichiometries at 1773K. In these experiments the coal feed rate was varied between 2 and 10 g h -1, which gave fuel equivalence ratios, $ (the ratio of the fuel used to that required stoichiometric combustion), of 0.4-2.

(b) Char Combustion Chars were combusted in similar experiments to those of the pulverized coal. The char was obtained by pyrolyzing the pulverized coal in the drop-tube at 1273, 1473, 1673, and 1873K.

RESULTS AND DISCUSSION

Pyrolysis and Evolution of Coal Nitrogen (a) Crucible Experiments Figure 1 shows that at low heating rates, the proportion of nitrogen evolved at a given temperature varies greatly for different coals, and there is no clear effect of rank. However, all coals show a similar temperature dependence for nitrogen evolution after the early stages. The main difference between the Australian and overseas coals is that the Australian coals appear to release nitrogen at an earlier stage of devolatilization.

(b) Thermobalance Experiments Figure 2 shows differential thermograms and pyrolysis gas compositions for the Liddell and

24

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Fig. 1. Summary of coal pyrolysis and nitrogen evolution for a wide range of coals and at slow heating rates: • , Liddell subbit (results from present investigation); ©, Morwell brown (results from present investigation); x , Saxony bit, [17]; II, Pittsburgh #8 bit [10]; ~ , Montana #A lig. [10].

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Morwell coals at low heating rates. The results are summarised as follows: The first weight loss peak (more clearly defined for Liddell coal) at 400-500K is probably due to the release of hydroxyl functional groups [18]. For both coals, the main weight loss peak near 800K is due to the evolution of tars, and also coincides with formation of HCN and NH3. The majority of other pyrolysis gases (CO, CH4, H2, and N2) appeared at higher temperatures, after tars evolution (Fig. 3), with the formation of methane obaining maximum rate near 900K, followed by maxima for H2 and CO above 120OK. Carbon monoxide was formed up to 1443K, the highest temperature obtained in these experiments. Nitrogen (N2) was not formed until l l00K. In contrast to the rapid evolution of the tars, HCN, NH3, and CH4, the formation of CO, H2, and N2 proceeded at a lower, but more uniform rate. Although NH3 was formed over the same temperature range for both coals, considerably more NH3 was formed from the Morwell coal. This occurred over two narrow temperature regimes at around 700 and 1000K. With the

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NOx FORMATION FROM COAL COMBUSTION Liddell coal, the first NH3 peak is almost nonexistent. The first stage of NH3 formation was probably related to the release of amine groups (which are more predominant in the low rank Morwell coal), while the second regime of NH3 formation probably results from the disruption of larger aliphatic groups, as this temperature also coincides with the onset of H2 formation (Fig. 3). The overall yield of the HCN and NH3 was only 4 and 12 % of the total coal nitrogen for the Liddell coal, while the Morwell coal gave 6% of the coal nitrogen as HCN and almost 33% as NH3. Both coals gave a similar yield of N2, reaching 15% at 1400K. The higher yield of NH3 from the Morwell coal reflects the higher proportion of amine and aliphatic groups in this fuel. The partitioning of coal nitrogen between the volatiles and char is shown in Fig. 4. These results show that a significantly greater proportion of the coal nitrogen was evolved from the low-rank Morwell coal, with only 11% of coal nitrogen remaining in the char formed at 1443K: 22 % was retained in the Liddell char at the same temperature. However, in the initial stages of devolatilization (up to 900K), the concentration of nitrogen was essentially constant in both chars (char nitrogen is proportional to the weight of char), thereafter decreasing as the more stable ring structures of the char were disrupted. Similar devolatilization behavior has been found for many American coals [11, 13].

(e) Drop-Tube Experiments The results of both the drop-tube experiments and previous heated grid and the heated carbonribbon experiments [11] are shown in Fig. 5. The relative rates of nitrogen and volatiles evolution differ significantly with the heating rate, with more nitrogen being evolved with the volatiles (mainly tars) at the higher heating rates, although the total volatiles yield was lower. At the higher heating rates of these experiments, slightly less coal nitrogen was evolved than at similar temperatures in the crucible experiments. This is most likely due to the shorter residence times which give incom-

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26 100

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Fig. 5. The evolution of nitrogen volatiles versus total weight loss for all coals and heating conditions--drop tube (results from present investigation): ®, Liddel subbit. A, Morwell brown; heated grid: A, Ohio #2; Carbon ribbon: V, Arkwright bit; > , Illinois #6 bit; (>, Wyodak subbit. (Crucible: O, ©).

plete devolatilization. However, with both types of experiment, there is no definite effect of either rank or volatile content on the characteristics of (total) nitrogen evolution. For both coals, nitrogen evolution commenced at similar temperatures to the crucible experiments (400-500K), but showed a smaller temperature dependence. These differences in devolatilization behavior show the effect of higher heating rates and more dispersed coal particles during pyrolysis. These conditions minimize secondary reactions of the volatiles (e.g., cracking of volatiles to yield pyrolytic carbon and lighter hydrocarbons), and therefore increase the volatile nitrogens yield (Fig. 5). In addition, under high heating rates the coal disrupts to form larger tar molecules of similar composition to that of the parent coal which also contributes to a higher volatiles yield [13, 19] and hence increased tar nitrogen. Although the increase in volatile formation (at low temperatures) was masked by the short residence times in these experiments (only 1-2 s), the higher

concentration of nitrogen in the volatiles gave a higher overall rate of nitrogen evolution. The additional volatile nitrogen was probably in the form of low temperature tar nitrogen. However, with the exception of the increased NH3 from the Morwell coal, this did not result in the formation of additional NH3 and HCN until above 1000K. Again this was probably due to the short residence time in the hot zone. For the Liddell coal, the maximum conversion of coal nitrogen to NH3 and HCN occurred at 1300 and 1700K, respectively. The yield of NH3 was similar to that observed in the thermobalance experiments, about 12%. However, the yield of HCN under rapid heating conditions was four times greater, 16% in contrast to only 4% in the thermobalance experiments. The overall increase in HCN formation is probably due to a combination of reduced secondary reactions and increased thermal decomposition of char nitrogens between 1000 and 1700K (the high temperature char in the drop-tube experiments contains more nitrogen). At higher temperatures, HCN is unstable and N2 predominates. NH3 decreases above 1300K. For the Morwell coal, the proportion of volatile nitrogens converted to NH3 and HCN was only half of that formed from Liddell coal (Fig. 6). Although more nitrogen was released with the volatiles below 1250K under the rapid heating conditions (Fig. 5), most of the nitrogen was released after the bulk of the volatiles. Above 1200K, the proportion of nitrogen released with the volatiles was similar for both heating conditions, and consisted mainly of N2. The NH3 released between 1073 and 1273K represented only 6% of the total coal nitrogen, and decreased at higher temperatures. This is significantly less than observed in the thermobalance experiments where a long residence time, low heating rate, and a packed coal bed favor the formation of NH3--possibly via secondary reactions including hydrogenation of coal nitrogen. However, with rapid heating, short residence times, and well-dispersed coal particles, secondary reactions are minimized and N2 is formed rather than NH3. As for the Liddell coal, HCN was not formed until above

NOx FORMATION

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Fig. 7. The effect of temperature on the formation of NO during simulated p.f combustion in the drop tube furnace. Results are shown for combustion in both air and an oxygen/argon mixture and include results for thermal NO\. Details of burnout and residual oxygen concentrations are also shown: , Liddell; - - - , Morwell.

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In these experiments, coal and char were burned under simulated p.f combustion conditions at temperatures of 800-1900K, for a range of stoichiometries in both air and an oxygen-argon mixture. Note, as the concentration of NOz was less then 5 ppm for most of the experiments, total NOx is expressed as NO. The results are shown in Figs. 7 and 8 and are summarized in terms of four temperature regimes: 1. Pre-ignition: < 8 5 0 K for Morwell coal and < 1050K for Liddell coal. Only partial oxidation of devolatilization products, NO < 25 ppm. 2. Volatiles combustion: 8 5 0 - 1 4 0 0 K for coal and

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28

D. PHONG-ANANT ET AL.

coal. The step increase in NO formation coincided with volatiles ignition [20]. 3. Char combustion: 1300-1800K, NO formation obtains a maximum value. A slight reduction in NO occurred above 1700K for the Liddell coal and was probably due to reduction of NO by char and combustion under more intense reducing conditions (diffusion control of combustion). 4. Thermal NO: >1800K. Thermal NO increased rapidly above 1800K and was most apparent for the low nitrogen Morwell coal. Figure 7 shows that combustion of Morwell coal in air produced significantly more NO than with the O2/argon mix. As thermal NO (blank run, air only) was negligible below 1800K (at 1800K thermal NO = 50 ppm), the increase in NO formation in the presence of N2 (air) cannot be attributed to thermal NO alone and must represent some form of "prompt N O . " Prompt NO is thought to result from the reaction of hydrocarbon fragments and molecular nitrogen to form N, NH, CN, HCN, or NH3, which are then oxidized to NO or dissociate to N2, depending on oxygen potential [4]. The main difference between thermal and prompt NO is that the formation of prompt NO is only weakly temperature dependent [1-4] and is normally only associated with fuel-rich oil or gas flames [6-8]. However, prompt NO has also been reported under fuel-lean conditions (~b = 0.6-0.8 [1, 7, 9]) with p.f combustion, where locally reducing conditions persist in the flame and in the boundary layer of the devolatilizing/ burning coal regardless of the overall stoichiometry. The relative importance of fuel and atmospheric nitrogen in NOx formation is therefore dependent on volatile content, volatiles composition, and also burner aerodynamics--the effects of which warrant further investigation.

represented only 20-25% of the total fuel nitrogen. In contrast to previous studies where the conversion of fuel nitrogen to NO decreased with increasing nitrogen content [7, 8], the low nitrogen Morwell coal formed proportionally less NO than the Liddell coal. This may be attributed to the high volatile content of this coal, which increases the homogeneous reduction of volatile nitrogen to N2 during the early stages of combustion. Figure 9 shows that the conversion of coal nitrogen to NO was strongly dependent on the stoichiometry of combustion, with NO increasing markedly under fuel-lean conditions (~ < 1). Note that complete burnout was not achieved until ~b ~- 0.5 due to the laminar flow conditions in the reactor. (c) Conversion of Char Nitrogen to NO Figure 10 shows the proportion of char nitrogen converted to NO as a function of char burnout, at 1273, 1473, 1673, and 1873K. For the Liddell coal, the formation char NO increased in proportion to char burnout, up to a maximum conversion of 38 % at 65 % burnout. At this stage of combustion, the char contained only traces of nitrogen. Combustion temperature had no effect on the conversion efficiency. However, with the Morwell coal, the conversion efficiency ini

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(b). Conversion of Coal Nitrogen to NO Figure 8 shows that at p.f combustion temperatures (1500-1800K) the conversion of fuel nitrogen to NO was almost independent of temperature under stoichiometric conditions and

Fig. 9. The effect of combustion stoichiometry(fuel equivalence ratio) on both burnout and the conversion of coal nitrogen to NO" 0 , I , Liddell at 1773K, ©, [], Morwell at 1473K; ---, Pittsburgh #8 bit (after Pohl and Sarofim [10]).

NOx FORMATION FROM COAL COMBUSTION 70

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(d) Relative Contributions of Volatile and Char NO Under fuel-rich combustion (~ > 1), char nitrogen forms 60-90 % of total fuel NO. However, under excess air conditions (~ < 1) the contribution from the volatile nitrogen increases rapidly and exceeds that of the char at q~ = 0 . 6 0.8. Studies with overseas coals have found that up to 60-80% of coal NO is formed from the volatiles. This corresponds to complete conversion of the nitrogen volatiles. This was not observed with the coals used in the present study. Figure 11 shows that under normal p . f combustion conditions [¢ = 0.7, T = 1673K (MorweU), 1773K (Liddell)] volatiles nitrogen formed only 57-61% of the total NO, which corresponds more closely to the nitrogen evolved as NH3 and HCN during pyrolysis (with

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30 some volatiles NO f r o m the M o r w e l l coal being attributed to p r o m p t NO). This accounts for less than h a l f o f the volatiles nitrogen.

CONCLUSIONS The results for NOx f o r m a t i o n f r o m the two A u s t r a l i a n coals c o n f i r m the conclusions o f previous studies [14], indicating that the geographic origin o f coal has a m i n i m a l effect c o m p a r e d with that o f heating rate and c o m b u s tion s t o i c h i o m e t r y . The f o r m a t i o n o f volatile nitrogens for these coals was significantly i n c r e a s e d at the higher heating rates o f the d r o p - t u b e e x p e r i m e n t s and contained a g r e a t e r p r o p o r t i o n o f H C N than NH3. The results c o n f i r m that the c o n v e r s i o n o f volatile nitrogen to NOx d o m i n a t e s that from char nitrogen only u n d e r fuel-lean conditions. The c o m b u s t i o n o f the low rank V i c t o r i a n b r o w n coal in air showed a significantly higher NOx f o r m a t i o n than that in a n i t r o g e n - f r e e oxidant. This unique o b s e r v a t i o n has yet to be explained. C h a r c o m b u s t i o n e x p e r i m e n t s show that for the L i d d e l l coal, c o n v e r s i o n o f c h a r nitrogen to NO is p r i m a r i l y d e p e n d e n t on char burnout, with v e r y little increase in c o n v e r s i o n o v e r the final 35% o f char burnout. H o w e v e r , with the M o r w e l l coal, the f o r m a t i o n o f char N O was increased with t e m p e r a t u r e and was p r o b a b l y due to the h i g h e r c o n c e n t r a t i o n o f residual volatile m a t t e r in the M o r w e l l chars which caused its reduction to N2.

D P ~ O N G - A N A N T ET AL. REFERENCES

1. Sarofim, A. F., and Flagon, R. C., Prog. Energy Combust. Sci. 2:1 (1976). 2. Fenimore, C. P., Thirteenth Symposium (International) on Combustion, The Combustion Institute, 1971, p. 373. 3. Hayhurst, A. N., and McLean, H. G., Nature 251:303 (1974). 4. Hayhurst, A. N., and Vince, I. M., Prog. Energy Combust. Sci. 6:35 (1980). 5. Pershing, D. W., Martin, G. B., and Berkau, E. E., AIChE Symposium Series No. 148, 71, 1975, p. 19. 6. Pereira, F. J., Beer, J. M., Gibbs, B. M., and Hedley, A. B., Fifteenth Symposium (lnternationaO on Combustion, The Combustion Institute, 1975, p. 1149.

7.

8.

9. 10.

11.

12. 13. 14. 15. 16. 17. 18.

The authors gratefully acknowledge the support from the Australian National Energy Research, Development and Demonstration C o u n c i l , t h e I n s t i t u t e o f C o a l R e s e a r c h at t h e University of Newcastle, and the assistance of the laboratory and workshop staff of the D e p a r t m e n t o f C h e m i c a l E n g i n e e r i n g , University of Newcastle,

19. 20.

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Received 9 May 1984; revised 1 April 1985