NO Release and Reactivity of Chars during Combustion - American

Energy & Fuels 1996, 10, 409-416

409

NO Release and Reactivity of Chars during Combustion: The Effect of Devolatilization Temperature and Heating Rate W. X. Wang and K. M. Thomas* Northern Carbon Research Laboratories, Department of Chemistry, Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K.

H. Y. Cai, D. R. Dugwell, and R. Kandiyoti Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, University of London, Prince Consort Road, London SW7 2BY, U.K. Received August 7, 1995X

The study has involved the investigation of the influence of pyrolysis heat treatment temperature (HTT) and heating rate on the reactivity and the release of char-N during temperature-programmed combustion (TPC) of a set of wire mesh reactor chars in a thermogravimetric analyzer-mass spectrometer system. The gas evolution profiles are bimodal and this indicates the presence of species of different reactivity. It was found that increasing pyrolysis temperature and heating rate both produced significant variation in the reactivity of the resultant chars with the former being more influential. This is apparent from the shift of peak positions and the change in the relative intensity of the low- and high-temperature peaks of CO, CO2, and NO evolution profiles. The differences in the CO2 and NO evolution profiles observed between the Gedling entrained flow reactor (EFR) and wire mesh reactor (WMR) chars of similar heat treatment temperatures may be explained by the different extents of pyrolysis experienced by the chars. However, chars produced in the WMR with pyrolysis temperatures up to 1200 °C show little variation in the char-N conversion to NO. This is believed to be due to the highly reactive nature of the chars which give rise to a high extent of reduction of the primary product NO formed during gasification leading to a low NO/char-N ratio. Heat treatment of the chars at lower heating rates and longer soak times to temperatures in the range 1100-1400 °C lead to reduced char reactivity and higher NO/char-N ratios under temperature-programmed combustion conditions. The results are consistent with the reduction of the primary oxidation product NO on the surface and in the pores of the char.

Introduction The current emphasis on global environmental concerns and proposed changes in national energy production strategies suggest that continued extensive use of coal for power generation will depend, at least in part, on the levels of combustion efficiency and pollution abatement that can be achieved. In pulverized fuel combustion, developments related to staged combustion applications, where a fuel-rich primary region combined with secondary and, in some instances, tertiary aeration,1 and low NOx burners have been instrumental in reducing fuel NOx related to the combustion of coal volatiles, and thermal NOx levels. At present, it is considered that 60-95% of the NO formed in low NOx burners is derived directly from the char-N component.2 In circulating fluidized bed combustors, while additional fresh coal and sorbent are fed to the furnace, unreacted char and elutrated fines, both with minimal volatile contents, generally constitute part of the fuel charged to the combustor. Thus the major part of the NOx problem relating to the combustion stage is related to Abstract published in Advance ACS Abstracts, January 15, 1996. (1) Cooke, M. J.; Fird, N. Env. Control Bull. 1990, 3. (2) Phong-Anant, D.; Wibberley, L. J.; Wall, T. F. Combust. Flame 1985, 62, 21. X

0887-0624/96/2510-0409$12.00/0

char derived NOx. Within this context, the post-devolatilization combustion of char particles merits detailed investigation in attempts to improve efficiency and reduce pollutant emission in coal combustion. Carbon/char gasification and combustion have been the subject of many investigations3-8 and the consensus is that the process is controlled mainly by the intrinsic reactivity of the carbon/char, the catalytic effect of the mineral impurities, and the pore structure. One of the main factors in determining the char reactivity is the thermal history of the char. Generally, gasification/ combustion reactivity decreases with the severity of pyrolysis. These effects are generally larger for lower rank coals than for coals of higher rank. Serio et al.9 (3) Essenhigh, R. H. In Chemistry of Coal Utilization, 2nd Supplementary Volume; John Wiley & Sons: New York, 1981; pp 1153. (4) Van Heek, K. H.; Muhlen, H. J. Fuel 1985, 64, 1405. (5) Lahaye, L.; Dentzer, J.; Soulard, P.; Ehrburger, P. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Dordrecht, 1991; pp 143-162. (6) Miura, K.; Hashimoto, K.; Silveston, P. L. Fuel 1989, 68, 1461. (7) Van Heek, K. H. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Dordrecht, 1991; pp 1-34. (8) Morrison, G. F. Understanding Pulverised Coal Combustion; ICTIS/TR34; IEA Coal Research: London, 1986. (9) Serio, M. A.; Solomon, P. R.; Suuberg, E. M. Proc. 1987 Int. Conf. Coal Sci. 1987, 597.

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reported differences of a factor of 1000 in the reactivities of lignite chars, by varying the pyrolysis temperature and heating rate. On the other hand, a difference of a factor of only 2 was observed in the reactivities of bituminous coal chars prepared by pyrolysis between 700 and 900 °C.10 Significant variation in char reactivity at 500 °C was also observed when pyrolysis was carried out in a thermogravimetric analyzer (TGA) system in the temperature range of 600-1000 °C.11 Khan12 compared the reactivities of low- and hightemperature chars and reported much higher reactivities for the 500 °C chars than for chars of higher pyrolysis temperatures. The difference in reactivity between the high- and low-temperature chars was explained by the different hydrogen contents and oxygen chemisorption capacities, and thus the quantity and energetics of the surface active sites. It was proposed that the hydrogen-rich component of the chars were preferentially oxidized during gasification leaving the highly reactive nascent sites. Previous studies13,14 of the variation of coal char reactivity with pyrolysis heat treatment temperature (HTT), heating rate, and residence time using a wire mesh reactor (WMR) and an entrained flow reactor (EFR) showed that factors affecting the reactivity of the chars are in the order

rank g HTT > heating rate > pressure g residence time Pyrolysis carried out in the WMR in the range of 7001300 °C showed that char reactivities decreased by a factor of 40 with increasing HTT whereas chars pyrolysed using heating rates in the range of 2-5000 °C s-1 with a heat treatment temperature of 1000 °C produced a variation in reactivity of a factor of 4. The evolution of coal nitrogen during pyrolysis using various techniques and experimental conditions has been investigated extensively.15-24 Early work15 indicated that the nitrogen released with the volatiles and that which initially remained in the char are the same type of functionality. Pyrolysis studies carried out on coals of different ranks showed higher nitrogen release (10) Van Heek, K. H.; Muhlen, H. J. Fuel Process. Technol. 1987, 15, 113. (11) Jenkins, R. G.; Nandi, S. P.; Walker, P. L., Jr. Fuel 1973, 52, 288. (12) Khan, M. R. Fuel 1987, 66, 1626. (13) Hindmarsh, C. J.; Wang, W. X.; Thomas, K. M.; Cai, H. Y.; Dugwell, D.; Kandiyoti, R. Proc. 1993 Int. Conf. Coal Sci. 1993, 1, 31. (14) Hindmarsh, C. J.; Thomas, K. M.; Wang, W. X.; Cai, H. Y.; Dugwell, D.; Kandiyoti, R. Fuel 1995, 74, 1185. (15) Solomon, P. R.; Colket, M. B. Fuel 1978, 57, 749. (16) Blair, D. W.; Wendt, J. O. L.; Bartok, W. Sixteenth Symposium (International) on Combustion; [Proceedings]; The Combustion Institute: Pittsburgh, 1976; pp 475-489. (17) Baumann, H.; Moller, P. Erdol, Kohle, Erdgar, Petrochem. 1991, 44, 29. (18) Pohl, J. H.; Sarofim, A. F. Sixteenth Symposium (International) on Combustion [Proceedings]; The Combustion Institute: Pittsburgh, 1976; pp 491-501. (19) Freihaut, J. D.; Proscia, W. M.; Seery, D. J. Joint EPRI/EPA Symposium on Stationary Combustion NOx Control; Electric Power Research Institute: Palo Alto, CA, 1987; Vol. 2, pp 36.1-36.37. (20) Nelson, P. F.; Kelly, M. D.; Wornat, M. J. Fuel 1991, 70, 403. (21) Nelson, P. F.; Buckley, A. N.; Kelly, M. D. Twenty-Fourth Symposium (International) on Combustion [Proceedings]; The Combustion Institute: Pittsburgh, 1992; pp 1259-1267. (22) Chen, J. C.; Castagnoli, C.; Niksa, S. Energy Fuels 1992, 6, 264. (23) Chen, J. C.; Niksa, S. Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1992; pp 12691276. (24) Nelson P. F.; Kelly, M. D. Proc. 1993 Int. Conf. Coal Sci. 1993, 2, 140-143.

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from coals of lower rank compared with higher rank coals.19 HCN is generally regarded to be an important nitrogen-containing pyrolysis product during high-temperature pyrolysis. Nelson and others20,21 investigated the pyrolysis of Australian coals and reported that there were significant amounts of NH3 produced at temperatures higher than 700 °C. At even higher temperatures (>800 °C), the yield of ammonia decreased. Chen and others22 found that at pyrolysis temperatures above 1200 °C most of the nitrogen was released as HCN. Nelson and Kelly24 also presented data in which they found that at temperatures above 800 °C, coals of lowvolatile matter content produced less HCN than those with high-volatile matter content. This was attributed to the greater importance of the secondary cracking of tar above 800 °C, but it could be due to the different nature of volatile products. The relatively stable pyrrolic and pyridinic nitrogen species in coals are, in the absence of secondary reactions, released as components of tar molecules. The primary pyrolysis products are transformed by secondary reactions in the gas phase to form intermediates such as HCN, NH3, HOCN, etc. Fluidized bed combustion studies carried out by various investigators25,26 indicated that while char nitrogen contributes a greater proportion of NOx, volatile nitrogen was found to be the main source of N2O. The effects of carbon conversion and bed temperatures were also studied27 in a laboratory scale fluidized bed combustor using small batches of coal samples. It was observed that the conversion of N2O decreases with increasing bed temperatures while the reverse trend was obtained for NO conversion. The instantaneous fuel nitrogen conversion to N2O decreases with increasing carbon burn-off whereas that for NO increases. The results were explained26,27 by a model in which the nitrogen bound in the char was converted to NO by oxidation and subsequently reduced on the char surface and as it diffused out of the pores. Previous investigations on the nitrogen release using model carbons,28,29 macerals, microlithotypes,30,31 whole coals and coal chars31-34 prepared using an entrained flow reactor (EFR) indicated that in both temperatureprogrammed and isothermal combustion at temperatures up to 1000 °C the release of char-N was delayed compared with the carbon and the NO/CO2 increases with burn-off for both model carbons, coals, and coal chars. It was found that fuel-N conversion to NO increases with rank for the raw coals and the EFR chars derived from them. It was also suggested that char nitrogen conversion to NO is to some extent reactivity related with the primary oxidation product NO being reduced on the surface or in the pores of the char.28-30,34 The present study was undertaken in order to examine the relationship between the devolatilization of (25) Pels, R. J.; Wojtowicz, M. A.; Moulijn, J. A. Fuel 1993, 72, 373. (26) Tullin, C. J.; Sarofim, A. F.; Beer, J. M. J. Inst. Energy 1993, 66, 207. (27) Tullin, C. J.; Goel, S.; Morihara, A.; Sarofim, A. F.; Beer, J. M. Energy Fuels 1993, 7, 796. (28) Wang, W. X.; Thomas, K. M. Fuel 1992, 71, 871. (29) Wang, W. X.; Thomas, K. M. Fuel 1993, 72, 293. (30) Hindmarsh, C. J.; Wang, W. X.; Thomas, K. M.; Crelling, C. J. Fuel 1993, 73, 1229. (31) Crelling, C. J.; Thomas, K. M.; Marsh, H. Fuel 1993, 72, 349. (32) Wang, W. X.; Brown, S. D.; Thomas, K. M.; Crelling, C. J. Fuel 1994, 73, 341. (33) Brown, S. D.; Thomas, K. M. Fuel 1993, 72, 359. (34) Wang, W. X.; Brown, S. D.; Hindmarsh, C. J.; Thomas, K. M. Fuel 1994, 73, 1381.

NO Release and Reactivity of Chars during Combustion

Energy & Fuels, Vol. 10, No. 2, 1996 411

Table 1. Pyrolysis Yield and the Characterization Data for the Coals and Chars Used in the Present Study total vol (wt % db)

sample coals GD coal TM coal WMR chars GD700/1/30 GD700/200/30 GD700/1000/30 GD700/5000/30 GD950/1000/30 GD950/5000/30 GD1100/1000/5 GD1200/1000/2 GD1500/1000/2 TM1000/50/2 TM1000/5000/2 EFR char GD-EFR TGA chars GD1100/1.65/1800 GD1300/1.65/1800 GD1400/1.65/1800

tar yield (wt % db)

C (wt % db)

H (wt % db)

N (wt % db)

N/C × 10-2

H/C

74.6 87.9

4.7 3.9

1.6 1.4

1.8 1.4

0.76 0.53

38.4 43.3 45.2 46.2 47.7 49.0 49.7 50.8 53.5 12.3 17.4

15.7 26.0 28.9 28.9 27.4 29.4 28.2 28.0 nd 7.0 9.3

82.0 80.5 81.3 80.7 83.3 83.8 82.9 82.6 84.3 90.5 89.0

2.3 2.2 2.2 2.1 0.9 0.8 0.7 0.4 0.4 0.7 0.5

1.8 1.8 1.7 1.7 1.8 1.6 1.6 1.6 0.9 1.3 1.3

1.9 1.9 1.8 1.8 1.9 1.6 1.7 1.7 0.9 1.2 1.3

0.34 0.33 0.32 0.31 0.13 0.11 0.10 0.06 0.06 0.09 0.07

nd

nd

82.5

1.0

1.6

1.7

0.15

nd nd nd

nd nd nd

91.0 96.5 97.4

0.3

1.9 0.7 0.8

1.8 0.6 0.7

0.04

pulverized coal and the combustion related characteristics of resulting char particles, focusing primarily on the variation in reactivities of the chars with pyrolysis conditions and the overall levels of NO released during temperature-programmed combustion. A wire mesh reactor was used to obtain chars under a wide variety of pyrolysis conditions. Experimental Section Samples Used. The study used two coals of different rank and their characterisation data are presented in Table 1. Both the Gedling and Taff Merthyr coals were obtained from the collection of the European Centre for Coal Specimens. The former is a low-rank high-volatile UK bituminous coal whereas the latter is a low-volatile bituminous coal. The samples were ground and sieved under nitrogen; the 106-150 µm fraction was dried under vacuum at 35 °C for 16 h and stored in sample vials under nitrogen until required. The 38-75 µm fraction of the Gedling coal was used for the preparation of the EFR char because of the difficulty involved forming an entrained flow of the larger size coal particles and also this size fraction is closer to the pulverized fuel (pf) size range. Pyrolysis Studies. The samples were pyrolyzed in an atmospheric pressure wire mesh reactor to various temperatures using different heating rates in a helium atmosphere. A detailed description of the design of the wire mesh reactor has been presented previously.35-37 The present reactor configuration features a flow of helium gas with a velocity of 0.1 m s-1 through the sample holder, in order to minimize secondary reactions of the evolving volatiles. 5-7 mg of the coal samples was heated at a heating rate between 1 and 5000 °C s-1 to temperatures up to 1500 °C with residence times up to 30 s. The tar from the WMR experiments was collected in a liquid nitrogen cooled trap located directly above the heated mesh with He flowing through the mesh and the trap. The trap (with condensed tar) was then placed in an oven at 50 °C for 40 min to allow the light volatiles and moisture, picked up during cooling, to evaporate. The trap was then weighed and the weight of the tar was calculated by difference from the weights of the trap, before and after experiments. The WMR char samples presented in Table 1 are coded as follows: the first two characters represent the coal from which the chars were derived, the first number is the heat treatment temper(35) Gibbins-Matham, J. R.; Kandiyoti, R. Fuel 1989, 68, 895. (36) Gibbins-Matham, J. R.; Kandiyoti, R. Energy Fuels 1988, 2, 505. (37) Cai, H. Y.; Guell, A. J.; Dugwell, D. R.; Kandiyoti, R. Fuel 1993, 72, 321.

ature (°C), the second the heating rate (°C s-1), and the last number is the holding time at peak pyrolysis temperature in seconds. A fraction of the Gedling coal was also heat treated in a Stanton Redcroft TGA system under nitrogen to temperatures of 1100, 1300, and 1400 °C, respectively, with a heating rate of 1.65 °C s-1 and a residence time of 30 min. These pyrolysis studies were carried out to examine possible effects of reactor configuration, heating rate, residence time and, in particular, heat treatment temperature. A fraction of the 38-75 µm size Gedling coal was also pyrolyzed in an entrained flow reactor at 1000 °C under nitrogen with a residence time of ∼1 s. A full description of the apparatus and the experimental conditions employed can be found elsewhere.33,34 Elemental Analysis. The elemental analysis was carried out using a Perkin-Elmer 2400 elemental analyzer. Thermogravimetric Analysis and Mass Spectrometry. Gas evolution profiles of the chars were recorded under temperature-programmed combustion conditions in 20% O2/ Ar mixture (flow rate 50 cm3 min-1). The 1 mm i.d. gas sampling probe situated directly above the sample in a Thermal Sciences thermogravimetric analyzer was interfaced with a VG quadrupole mass spectrometer. The samples were heated from room temperature at a heating rate of 15 °C min-1. Typically up to 5 mg of sample was used to avoid thermal runaway effects due to exothermic and secondary reactions. The following m/z species were monitored throughout the reaction: total pressure (0), 18, 27, 28, 30, and 44. A more detailed description of the equipment and the experimental procedure has been presented elsewhere.28,29

Results and Discussion WMR Pyrolysis Studies. Table 1 shows that the carbon content of the Gedling WMR chars varies in the range of 80.5-84.3% showing only a slight increase with increasing pyrolysis temperature. With increasing pyrolysis temperature, the hydrogen content decreases significantly and when the HTT exceeds 950 °C, little hydrogen is left in the chars. In contrast, char-N content does not show a noticeable decrease until the pyrolysis temperature reaches 1500 °C in the WMR. The results are in contrast to the TGA chars where the carbon content is greater than 90% even at a pyrolysis temperature of 1100 °C. This can be attributed to the much slower heating rate and longer residence time of

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Table 2. Ratios of Various Integrated Gas Concentration and Fuel-N Conversion Data sample coals GD coal TM coal WMR chars GD700/1/30 GD700/200/30 GD700/1000/30 GD700/5000/30 GD950/1000/30 GD950/5000/30 GD1100/1000/5 GD1200/1000/2 GD1500/1000/2 TM1000/50/2 TM1000/5000/2 EFR char GD-EFR TGA chars GD1100/1.65/1800 GD1300/1.65/1800 GD1400/1.65/1800

CO/CO2

NO/N

HCN/N

T50 (°C)

0.35 0.22

0.15 0.29

0.08 0.02

497 555

0.15 0.35 0.25 0.28 0.27 0.15 0.24 0.21 0.24 0.25 0.14

0.12 0.15 0.11 0.14 0.12 0.13 0.11 0.12 0.49 0.25 0.32

0.02 0.07 0.06 0.05 0.04 0.04 0.03 0.04 0.05 0.05 0.05

505 475 452 457 503 498 511 515 615 667 625

0.16

0.12

0.07

540

0.38 0.34 0.23

0.18 0.60 0.50

0.07 0.01 0.02

567 710 720

the chars in the TGA. The high carbon content of the TGA chars is in accordance with the low ash content of the original coal (2 wt % db). The results suggests that like the EFR chars, the WMR chars have not reached equilibrium due to their short residence times. The variation of tar and total volatile matter yields for pyrolysis under helium at atmospheric pressure for the Gedling and Taff Merthyr coals and the analytical data for the WMR chars are presented in Table 1. The char yield was calculated from the mass balance. Significant increases in the yields of tar and total volatile matter for the Gedling coal was observed at 700 °C with increasing heating rate from 1 to 1000 °C s-1. With further increase in heating rate to 5000 °C s-1, the total volatile matter increased only very slightly while the tar yield remained constant. The total volatile matter also increases with increasing pyrolysis temperature for the Gedling coal. For the chars prepared at a heating rate of 1000 °C s-1, volatile matter content increases from 45.2% at 700 °C to 53.5% at 1500 °C but tar yields do not increase above 700 °C. With the exception of the GD700/1/30 char, the tar yields account for more than half of the total volatile matter yields. These results are comparable with previous studies carried out under similar conditions employing the same apparatus.35,37 Effect of Heat Treatment Temperature (HTT). The temperature-programmed combustion behavior of the suite of wire-mesh and TGA chars was investigated using a thermogravimetric analyzer interfaced with a mass spectrometer system (TG-MS). Since the sampling probe was situated directly above the sample surface it allows the detection of reactive intermediate species thus enabling a better understanding of the mechanism of such reactions.38,39 In the present study, the temperature required for the 50% burn-off of the chars (T50) has been employed as an overall measure of reactivity. The use of T50 is justified since it was shown that T50 correlates strongly with isothermal reactivities measured at 500 °C (R500) in 20% O2/Ar where the reaction is in the chemical control region.34 (38) Varey, J. E.; Hindmarsh, C. J.; Thomas, K. M. Fuel 1996, 75, 164. (39) Jones, J. M.; Harding, A. W.; Brown, S. D.; Thomas, K. M. Carbon 1995, 33, 833.

Figure 1. Variation of CO2 evolution profiles with pyrolysis temperature for the Gedling coal and WMR and EFR chars.

Table 2 shows that T50 generally increases with pyrolysis temperature for the Gedling WMR chars. While the variation of char reactivity with pyrolysis temperature is expected, a detailed examination of the CO2 evolution profiles under temperature-programmed combustion conditions reveals interesting features. Figure 1 shows that the CO2 profiles are asymmetric for the WMR chars indicating the presence of more than one peak. In the temperature-programmed combustion profile of the GD700/1000/30 char, the low-temperature CO2 peak is the dominant feature and, with increasing pyrolysis temperature, the higher temperature CO2 peak increases in relative intensity. There are only minor differences in the CO2 evolution profiles for GD950/1000/30, GD1100/1000/5, and the GD1200/ 1000/2 chars possibly due to the longer residence times of the first two chars compared with the GD1200/1000/2 char. In the case of the GD1500/1000/2 the second peak accounts for the major part of the CO2 evolved. The lowtemperature CO2 peak corresponds to the highly reactive species in the char whereas the high-temperature peak to the less reactive sites. Therefore, an increase in pyrolysis temperature suppresses the production of the highly reactive species or sites in the char. This is understandable because increasing the heat treatment temperature has the effects of annealing the carbon structure through the removal of defects such as dislocations and heteroatoms and this is accompanied by a decrease in porosity and surface area. It is evident from Figure 1 that GD700/1000/30 is actually more reactive than the raw coal. A possible explanation for this is that the breakage of the crosslinked network in the raw coal during pyrolysis produces a char with better accessibility for the reactant gas and a higher concentration of active sites. The CO2 evolution profile of the Gedling EFR char is much broader and less asymmetric compared with the WMR chars. The T50 value for GD-EFR is also considerably higher compared with the two WMR chars prepared at a heat treatment temperature of 950 °C. These differences may be explained by the different extent of pyrolysis experienced by the EFR chars due to the smaller particle size used, higher heating rate and



Figure 2. Variation of T50 (°C) values with the H/C ratio for the WMR chars during temperature-programmed combustion.

slightly higher pyrolysis temperature in the entrained flow reactor. The results are similar to studies12 carried out in a TGA system at heat treatment temperatures up to 950 °C and low heating rates where it was observed that increasing peak pyrolysis temperature reduced the reactivity of the resultant chars under both isothermal and temperature-programmed combustion conditions. The reduced char reactivity with increasing pyrolysis temperature was explained by the reduced H/C ratio in the chars and reduced oxygen chemisorption capacity. However, in the present study, the chars were subjected to higher temperature using much faster heating rates. Figure 2 shows a graph of the H/C ratio in the chars against the corresponding T50 values. The results show that overall the two parameters correlate inversely. However, this structural change is accompanied by other structural changes such as total surface area, active surface area, etc. Due to the small sample size (5-7 mg) obtained from the WMR, it was not possible to carry out detailed char characterization measurements. However, related work40 on chars prepared in a fluidized bed reactor at temperatures up to 800 °C and pressures up to 0.9 MPa also gave bimodal temperature-programmed combustion profiles. In this case the reactor was capable of producing sufficient quantities of char to allow for detailed characterization. Proximate analyses of the chars showed that the chars contained significant amounts of volatile matter. The samples which gave bimodal profiles had volatile matter contents in the range 5-20%. The BET, N2 77 K surface areas of the chars varied in the range 3-35 m2 g-1. In a previous study41 of the temperature-programmed combustion of the model carbons, the CO and CO2 gas evolution profiles were bimodal and H2O was mainly associated with the second CO2 peak. It was, therefore, suggested that the first CO2 peak correlated with the edge carbons and the second with the release of hydrogen present in the carbon. In contrast to the model carbons, the temperature-programmed combustion of the WMR chars prepared at 700 °C revealed that H2O was evolved during the whole combustion process. Gas sampling directly above the sample allows the detection of CO before it is converted to CO2 by homogeneous gas phase reactions. The CO release profiles were also (40) La´zro, M. J.; Ibarra, J. V.; Moliner, R.; Gonzalez de Andre´s, A. I.; Thomas, K. M. Fuel, in press. (41) Grant, K. A.; Zhu, Q.; Thomas, K. M. Carbon 1994, 32, 883.

Figure 3. Variation of NO evolution profiles with pyrolysis temperature of the Gedling coal and WMR and EFR chars.

asymmetric being similar to those of the CO2. In some samples the low-temperature CO peak reaches a maximum intensity at a slightly lower temperature than the corresponding CO2 peak. As a result, the CO/CO2 ratio is usually higher at the initial stage of combustion and then decreases until the later stages of gasification where the ratio increases again. This variation in the CO/CO2 ratio with temperature is possibly due to the changes in the type and relative importance of the different active sites on the char surface and the variation of porous structure of the chars with burnoff. Table 2 shows that the integrated CO/CO2 ratios for the set of the WMR chars studied are in the range 0.15-0.35 and there is no clear trend with either peak pyrolysis temperature or heating rate. Figure 3 shows that the high-temperature NO peak accounts for the majority of the total NO released during TPC. In all cases, the high-temperature NO peak is more intense than the corresponding low-temperature NO peak. It is interesting to note that the lowtemperature NO maxima coincides with that of the lowtemperature CO2 peak while the high-temperature NO peak reaches a maximum intensity at a higher temperature than the corresponding CO2 peak. The NO peak of the raw coal is asymmetric and only a small contribution is made by the volatiles which appears as a lowtemperature shoulder. For the GD700/1000/30 char, the low-temperature NO peak is significantly more intense than for the raw coal. With increasing HTT, the intensities of the low-temperature NO peak decreases whereas that of the high-temperature NO peak increases. For the GD1500/1000/2 char, the low-temperature NO peak appears as a small shoulder. The NO evolution profile for the GD-EFR (HTT 1000 °C) is significantly broader than that of the WMR chars heat treated to similar temperatures. The effect of increasing heat treatment temperature appears to be to increase the importance of the less reactive nitrogen species in the char. Effect of Heating Rate. The effects of heating rate on the reactivity of the resultant chars can be observed by the variation in the T50 values of the set of WMR chars pyrolyzed at 700 °C (Table 2). This gradual

414


Figure 4. Variation of CO2 evolution profiles with heating rate for the 700 °C Gedling WMR chars during temperatureprogrammed combustion.

increase in char reactivity with heating rate is further demonstrated in the CO2 evolution profiles (Figure 4) with the peak maxima gradually shifted to lower temperature with increasing heating rates. Furthermore, the CO2 evolution profiles for the set of 700 °C WMR chars are asymmetric and significant variation in the shape of the profiles is evident. The char prepared at a heating rate of 1 °C s-1 is the least reactive of the set and the high-temperature CO2 peak is the dominant component. With increasing heating rate to 200 °C s-1, a sharp shoulder developed on the lower temperature side of the CO2 profile. The lower temperature CO2 peak becomes the dominant feature for the GD700/1000/30 and GD700/5000/30 chars. As reported elsewhere35,42 at 700 °C, tar yields and char reactivity increase with increasing pyrolysis heating rate but reach a plateau at higher heating rates. The NO evolution profiles for the corresponding WMR chars are illustrated in Figure 5. The results show that the char prepared at a heating rate of 1 °C s-1 gives rise to a NO profile having a very small low-temperature NO peak. With increasing heating rate, the NO profiles become more asymmetric and the low-temperature NO peak increases in relative intensity. This variation of NO release profiles is in line with that observed for the CO2 profiles. As observed in Figure 3, the hightemperature NO peaks are also more intense than the low-temperature ones. The low-temperature NO peak reaches a maximum intensity at the same temperature as the low-temperature CO2 peak whereas the hightemperature NO peak reaches a maximum at significantly higher temperature than the corresponding CO2. As observed previously for the EFR chars,33 the NO/ CO2 ratio for all the WMR chars increases with increasing burn-off during temperature-programmed combustion. The increase in the NO/CO2 ratio with increasing char burn-off could be accounted for by the lower reactivity of the nitrogen in the char compared to carbon during combustion and/or by the decreased reduction of the primary product NO formed with the changes in the porous structure. The analytical data for a series (42) Cai, H. Y. PhD Thesis, University of London, 1995.

Wang et al.

Figure 5. Variation of NO profiles with heating rate for the Gedling 700 °C WMR chars during temperature-programmed combustion.

of carbons derived from polynuclear aromatics as a function of burn-off show that nitrogen content in the carbon increased by approximately 25% when burn-off reached 80% in some of the model carbons.43 Pyrolysis of Taff Merthyr coal was also carried out in the WMR to 1000 °C using heating rates 50 and 5000 °C s-1. The T50 values presented in Table 2 show that the Taff Merthyr (TM) chars are significantly less reactive than the Gedling chars pyrolyzed under similar conditions. This is expected from the consideration of the rank of the coals. Coal devolatilization is essentially a complex depolymerization process during which small aromatic molecules are liberated through dissociation and formation of cross-links concurrently. It has been found that it is difficult to correlate pyrolysis tar yield with coal composition over a wide rank range from lignite to semianthracite.44 The enhanced tar and volatile yield with increasing heating rate can be accounted for by the decreasing times of pyrolysis leaving less time for the aromatic molecules liberated in the pyrolysis process to react to form char. This process may also produce chars with high concentration of reactive sites due to the uncapped dissociation sites. The argument may provide an explanation for the observed variation in reactivity and the shape of the CO2 profiles for the set of WMR chars. Overall Char-N Conversion to NO. The temperature-programmed combustion gas evolution profiles were integrated and ratios of various gaseous products calculated. These ratios are used in conjunction with the analytical data to calculate the conversion of char-N to NO. Table 2 shows that the conversion of char-N to NO for the set of the WMR chars pyrolyzed at temperatures up to 1200 °C with various heating rates shows little variation during temperature-programmed combustion. There is no noticeable difference in the overall level of char-N converted to NO between the GD-EFR (43) Spracklin, C. J.; Thomas, K. M.; Marsh, H.; Edwards, I. A. S. Proceedings of the 20th International Conference on Carbon; American Carbon Society; University of California: Santa Barbara, CA, 1991; p 212. (44) Cai, H. Y.; Dugwell, D. R.; Kandiyoti, R. Proc. 1995 Int. Conf. Coal Sci. 1995, 1, 841.


Figure 6. Variation of char-N conversion to NO with T50 (°C) for the different chars during temperature-programmed combustion.

and the Gedling WMR chars with a HTT below 1200 °C. The T50 values which correlate with reactivity also cover a relatively small temperature range (452-515 °C). The corresponding NO/char-N ratio covers a small range of 0.11-0.14. Figure 6 plots the NO/N ratio against T50 for the set of the WMR chars. Data from previous studies on the EFR chars of maceral concentrates,30,34 whole coals,33,34,45 and chars prepared with a slow heating rate45 are also plotted on the graph for comparison. The results show that, overall, there is an inverse correlation between the level of char-N converted to NO and T50 for the different type of chars. This trend may be explained by the increasing reduction of the primary product NO in the pores or on the surface of the char with increasing reactivity. Previous studies30,34 also showed that the char-N conversion to NO is also correlated with the reactivity measured at 500 °C (R500) under isothermal conditions in 20% O2/Ar. Comparing the T50 values of the WMR chars pyrolyzed up to 1200 °C with those of the EFR chars shows that they are very reactive. Therefore, low conversions of char-N to NO were observed. The effect of changes in carbon structure were investigated further by the preparation of chars from Gedling coal with a low heating rate (compared with the WMR) to heat treatment temperatures of 1100, 1300, and 1400 °C and the char-N conversion to NO data during TPC for these samples are also presented in Table 2. The results show that these chars are much less reactive than the WMR chars prepared to similar heat treatment temperatures and the char-N conversion to NO is higher. This may also be related to tar yields. In the TGA system, tar cracking and tar condensation on char surface were believed to be much more severe than the case in WMR, due to the use of a low heating rate and the sample configuration. Indeed, the two points with the highest nitrogen conversion to NO on Figure 6 are for the TGA chars prepared by heat treatment to 1300 °C and 1400 °C. Table 2 and Figure 6 also show that for the WMR chars prepared from the higher rank Taff Merthyr coal, the char-N conversion to NO is significantly higher than for the Gedling coal chars. This again confirms the previously observed correlation between coal rank, reactivity and fuel-N conversion to NO.32-34 Parameters related to reactivity, such as porous structure/surface area, may also be factors in determining the NO/char-N ratio. (45) Gonzalez de Andre´s, A. I.; Thomas, K. M. Fuel 1994, 73, 635. (46) Thomas, K. M.; Grant, K. A.; Tate, K. Fuel 1993, 72, 941.


It is interesting to compare the evolution of HCN (not shown) and NO during temperature-programmed combustion. The results show that HCN is mainly detected in the initial stage of char combustion in accordance with the first CO2 peak whereas NO release is mainly associated with the higher temperature CO2 peak. Table 2 shows that char-N conversion to HCN is low (

NO Release and Reactivity of Chars during Combustion - American

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