NO and N2 Formation Behavior during the High-Temperature O2

High-Temperature O2 Gasification of Coal Char ... examined using a pulse gasification reactor, which minimizes the exothermic heat generation and the ...
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Energy & Fuels 2003, 17, 405-411

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NO and N2 Formation Behavior during the High-Temperature O2 Gasification of Coal Char Hironori Orikasa and Akira Tomita* Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai, 980-8577 Japan Received September 4, 2002. Revised Manuscript Received December 26, 2002

NO- and N2-formation behavior during the high-temperature O2 gasification of coal char was examined using a pulse gasification reactor, which minimizes the exothermic heat generation and the secondary reaction of NO. Coal char heat treated at 1300 °C was gasified with 10% O2 in a temperature range from 1000 to 1300 °C. Although the total nitrogen converted to NO and N2 are independent of gasification temperature, a fraction of NO tends to increase with increasing char conversion and decrease with increasing gasification temperature. These trends can be explained by the secondary N2-formation reaction from NO and char via nitrogen-containing surface species as an intermediate. This result is similar to that observed in the previous study at low temperatures, except that the surface nitrogen species is less stable at high temperature. In another series of experiments, the char prepared at 950 °C was subjected to gasification without pretreatment at 1300 °C. The NO- and N2-formation behavior heavily depends on whether the char was pretreated at 1300 °C. The release of unstable char nitrogen as N2 was observed in addition to the N2 formation from the char-NO reaction when the char was not heat-treated at a severe condition. The relative importance of these two N2-formation routes determines the final gas composition.

Introduction Coal is expected to be one of the main energy resources at least in the first half of the 21st century. NOX suppression from coal combustion system is one of the key issues to minimize its environmental load. Many studies have been conducted to clarify the NOX formation and destruction mechanisms in order to design better coal combustion systems. The reaction mechanism of coal volatile matter in the homogeneous phase has been well understood, but there remain some uncertainties on the heterogeneous reactions, i.e., NOX formation from char nitrogen and NOX reduction over char surface. Nitrogen in char is one of the origins for NOX formation upon combustion, and concurrently char itself plays a role as NOX reductant. Such complexities may be the reason the char-related reactions are hard to be elucidated. In most of studies, the heterogeneous reaction of NOX with char surface has been conducted at relatively low temperatures, i.e., 1000 °C has never been reported to the best of our knowledge. The aim of this study is to understand the NO and N2 formation behavior during O2 gasification of char at above 1000 °C using a laboratory scale fixed-bed reactor. At high temperatures, the exothermicity of O2 gasification is too large to carry out the reaction at a steady state. Therefore, in this study, a small amount of char sample was used to minimize the heat generation and (3) Tomita, A. Fuel Process. Technol. 2001, 71, 53-70. (4) Ashman, P. J.; Haynes, B. S.; Nicolls, P. M.; Nelson, P. F. Proc. Combust. Inst. 2000, 28, 2171-2179. (5) Aihara, T.; Matsuoka, K.; Kyotani, T.; Tomita, A. Proc. Combust. Inst. 2000, 28, 2189-2195. (6) Ashman, P. J.; Haynes, B. S.; Buckley, A. N.; Nelson, P. F. Proc. Combust. Inst. 1998, 27, 3069-3075.

10.1021/ef020194z CCC: $25.00 © 2003 American Chemical Society Published on Web 02/07/2003

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Figure 1. Schematic of pulse gasification apparatus.

a series of a little O2 pulse was fed to the sample bed. By this experimental technique, N2 and NO formation behavior can be examined under the condition with minimal temperature fluctuation and minimal secondary reaction of NO with char. The char was prepared from Blair Athol coal and the gasification was carried out at 1000, 1100, 1200, and 1300 °C. Experimental Section Sample Preparation. Char was prepared using a fixedbed quartz reactor in a He flow. Blair Athol coal with a size of 0.5-1.0 mm was pyrolyzed by heating to 950 °C at a rate of 10 °C/min. The sample was held at 950 °C for 30 min. Resultant char was sieved and those with a size of 0.5-1.0 mm were used throughout this study. The elemental analysis was carried out by a CHN analyzer. The content of C, H, N, and O (by difference) and ash of the parent coal was 75.6, 4.3, 1.7, 9.3, and 8.8 wt % (dry basis), respectively, and that of the char was 81.1, 0.9, 1.5, 3.4, and 13.1 wt % (dry basis), respectively. A CHN analyzer often overestimates the nitrogen content in char due to measurable N2 adsorption on char at room temperature.7 Thus, the amount of N2 adsorbed on the char was estimated by temperature programmed desorption in He. The amount of N2 desorbed between ambient temperature and 600 °C was 0.4 wt % on the char weight basis. By subtracting this amount from the elemental analysis data, the intrinsic nitrogen content was estimated to be 1.1 wt %, and the N/C atomic ratio was 0.012. Apparatus. Schematic of pulse gasification apparatus is shown in Figure 1. About 2.5 mg of char (roughly 10-15 particles) was packed in quartz wool and placed in the vertical Al2O3 tube (inner diameter, 4 mm; length, 500 mm). The thickness of the char bed was close to a monolayer. The tip of an R-type thermocouple was positioned at 5 mm below the sample bed for measuring the bed temperature and controlling the furnace temperature. Char was gasified by a series of O2 pulses. The reactant gas concentration (10% O2 in He) and the (7) Matsuoka, K.; Horii, T.; Chambrion, Ph.; Kyotani, T.; Tomita, A. Carbon 2000, 38, 775-778.

Orikasa and Tomita total gas flow rate (0.5 L/min at 25 °C and 0.1 MPa) were controlled using four mass flow controllers. The purity of gas was >99.99% for O2 and >99.9999% for He. As shown in Figure 1, the gas analysis was made with a gas chromatograph (GC) (Yanaco G-3800) and a quadrupole mass spectrometer (MS) (Anerva AQA-100R). Pure He gas was used as a carrier gas for GC. Another pure He line is connected to a 6-way valve, the reactor, and finally to GC. A loop tube connected to the 6-way valve was used for a pulse generation. Its volume was either 1.5 or 4.3 mL at 25 °C and 0.1 MPa. By rotating this valve, the reactant gas in the loop was introduced into the reactor as a pulse. All of the unreacted O2 and product gases were transferred to GC. In this way, it was possible to analyze a tiny amount of N2. Simultaneously a small amount of the reactor outlet gas was sucked through a silica capillary tube into a MS vacuum chamber for NO analysis. The pressure in the reactor was set at 0.23 MPa in order to meet the pressure drop due to GC column. To adjust the pressure in the loop to the reactor pressure, a backpressure regulator was installed at the end of the reactant gas line. The amount of O2 in a pulse was 14 and 40 µmol when 1.5 and 4.3 mL loop tube, respectively, was used. The contact time of reactant gas with char at each pulse was 0.4 and 1.2 s, respectively, assuming a plug flow. Analysis. A GC with a molecular-sieve 5A column and a thermal conductivity detector was employed for O2, CO, and N2 analysis. Since CO2 strongly adsorbs on column material, the analysis of CO2 by GC was difficult. Therefore it was calculated from the material balance of O atom (eq 4).

1 1 CO2out ) O2in - O2out - COout - H2Oout 2 2

(4)

The analysis of H2O was also quite difficult in the present system, and the corresponding term in eq 4 was neglected in the calculation of CO2. Therefore, the obtained CO2 concentration is always overestimated to some extent. However, judging from the H content determined by the elemental analysis, the overestimation of CO2 content due to this simplification was at most 5%. N2O formation is expected at lower reaction temperature, but it is insignificant at 1000 °C or above due to the thermal decomposition and radical reaction of N2O.2,8 The gas species evolved upon pulse injection could easily be analyzed by GC, but it was rather difficult to detect dilute gases that slowly and continuously evolve between the pulses as well as during the heat-treatment period prior to each experiment. Even though the concentration of these dilute gases was quite low, the total sum was not small because the time periods of these stages were much longer than those of the pulse reactions. The contact time of reactant gas with char sample was around 1.2 s for one pulse, but the interval between each two pulses was 12 min and the heat-treatment time was as long as an order of hour. A quadrupole MS was used for NO analysis. MS signal intensity of m/z ) 30 is mainly due to NO, but there is some contribution from isotopic 12C18O. The concentration of NO was determined by subtracting the latter from the former. When a smaller loop (1.5 mL) was used for a pulse generation, the reliability of NO determination was poor because of an insufficient amount of NO. The formation of other nitrogen containing species, like HCN4,6,9,10 and HNCO,4,11 was reported in the literatures, but (8) Johnsson, J. E.; Åmand, L.; Dam-Johansen, K.; Leckner, B. Energy Fuels 1996, 10, 970-979. (9) Jones, J. M.; Harding, A. W.; Brown, S D.; Thomas, K. M. Carbon 1995, 33, 833-843. (10) Winter, F.; Wartha, C.; Lo¨ffler, G.; Hofbauer, H. Proc. Combust. Inst. 1996, 26, 3325-3334. (11) Nicolls, P. M.; Nelson, P. F. Energy Fuels 2000, 14, 943-944.

O2 Gasification of Coal Char

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Figure 2. Typical char gasification profile. Char heattreatment temperature: 950 °C. Gasification temperature: 1200 °C. most of them would be rapidly converted to NO and/or N2 at higher temperatures and in an oxidizing atmosphere as employed in this study. Procedure. After the char sample prepared at 950 °C was set in the reactor, it was purged with He for 30 min. Then the temperature was raised to 1000 °C at a rate of 10 °C/min. In some cases, the temperature was further raised to 1100, 1200, and 1300 °C at 5 °C/min. The gasification reaction was conducted isothermally in a temperature range between 1000 and 1300 °C. Two sets of reaction were conducted: (1) the char sample was heated to a predetermined reaction temperature, and then the reaction was allowed to start with no holding time; (2) the char was first heat-treated at 1300 °C for 30 min, and then the temperature was lowered to a reaction temperature. In the latter case, the char sample is almost completely annealed, while in the former case the annealing proceeds during heating-up period to the reaction temperature and even in the initial stage of gasification. O2 pulses were injected every 12 min. This interval was determined by considering a relatively long retention time of CO in the GC analysis. It is possible to shorten the retention time by increasing the column temperature, but this is unfavorable in separating O2 and N2 peaks. Thus column temperature was set at 35 °C in this study. The pulse volume was 4.3 mL in most experiments. The pulse volume of 1.5 mL was only employed in some experiments. Repeatability was checked by conducting several experiments under the identical condition. In this paper, all data from several experiments are plotted in the same figure, and thus one can judge the repeatability from the data scattering.

Results Char Gasification. Figure 2 represents one example of char gasification profile, which was obtained at 1200 °C. The amount of unreacted O2 in the initial stage was much lower than O2 injected (40 µmol). However, even at the first pulse the conversion of O2 was not 100%. In

Figure 3. Variation of fN2, fNO, and fN with carbon conversion at different gasification temperatures. Char heat-treatment temperature: 950 °C.

the initial stage, the temperature rapidly increased by about 100 °C upon the pulse injection, and then the temperature decreased to the reaction temperature in a few seconds. This phenomenon is due to the exothermicity of carbon gasification. The amount of all product gases, CO, CO2, N2, and NO, gradually decreased with pulse number, i.e., with char consumption. In a later stage, the extent of temperature increment decreased in accordance with the extent of reaction. After 10 pulses, the char was completely gasified. N2- and NO-Formation Behavior. N2- and NOformation profiles are shown in Figure 3a,b, respectively, as a function of gasification temperature. In these experiments, the char prepared at 950 °C was used, and the maximum heat-treatment temperature was the same as the reaction temperature. It should be noted that all the data from repeated experiments are plotted in this figure. The actual numbers of pulses in one run ranged from 6 to 13, depending on the temperature and initial char feed. The sum of the nitrogen atoms evolved as NO and N2 are shown in Figure 3c. The codes used in these Figures (fN2, fNO, and fN) and carbon conversion are defined as follows.

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fN2 ) fNO )

Orikasa and Tomita

2aN2

(5)

(aCO + aCO2) aNO

(6)

(aCO + aCO2)

fN ) fN2 + fNO )

(2aN2 + aNO)

(7)

(aCO + aCO2)

carbon conversion (% ) ) ∞

n

100

∑ l)1

(aCO + aCO2)/

(aCO + aCO ) ∑ l)1 2

(8)

where aN2, aNO, aCO, and aCO2 are the amount of N2, NO, CO, and CO2 evolved in each pulse, respectively, and f represents instantaneous value at each pulse. It should be noted that at very high carbon conversion (>95%), aN2, aNO, aCO, and aCO2 were quite small, and thus fN2, fNO, and fN values in this region are not reliable enough. At all reaction temperatures fN2 tended to decrease with carbon conversion and converged at a conversion near 100%. At a carbon conversion of around 20%, fN2 for lower reaction temperatures (1000 and 1100 °C) were much larger than those for higher temperatures (1200 and 1300 °C). As for NO formation, fNO were nearly constant until a carbon conversion of around 80% and then increased. In all conversion levels, fNO values were lower for the experiments at higher temperatures. fN showed almost the same tendency as fN2. It should be noted that some of fN values at a low carbon conversion exceeded the N/C atomic ratio of the char prepared at 950 °C (0.012). At 1200 and 1300 °C, fN was almost constant regardless of carbon conversion. FN2, FNO, and FN are defined as follows. F represents cumulative value from the beginning to the end of gasification.

FN2 )

∑aN

2

2

(9)

∑ (aCO + aCO ) 2

FNO )

∑ aNO

(10)

∑ (aCO + aCO ) 2

FN ) FN2 + FNO )

∑ (2aN + aNO) ∑ (aCO + aCO ) 2

(11)

2

FN2, FNO, and FN are shown in Table 1. All of the FN values were less than 0.012, the N/C ratio of the char before the gasification. This means that some amount of nitrogen was lost during the holding time between two pulses and/or before the reaction. Another remarkable feature of Table 1 is that, in this series of experiments (upper half of Table 1), the yields of N2 and NO were much less at higher reaction temperature. An abrupt temperature increase was observed upon pulse injection especially in the initial stage. To suppress this effect, several experiments were carried out

Figure 4. Variation of fN2 with carbon conversion. Gasification temperature: 1000 °C. Pulse volume: 1.5 mL. Table 1. Values for FN2, FNO, and FN at Different Pretreatment and Gasification Temperatures pretreatment temperature (°C)

gasification temperature (°C)

FN2

FNO

FN

1000 1100 1200 1300

1000 1100 1200 1300

0.0060 0.0053 0.0036 0.0028

0.0027 0.0023 0.0013 0.0011

0.0088 0.0076 0.0049 0.0039

1300 1300 1300 1300

1000 1100 1200 1300

0.0020 0.0024 0.0027 0.0028

0.0018 0.0015 0.0013 0.0011

0.0038 0.0039 0.0039 0.0039

using a smaller O2 loop (1.5 mL). As stated in the Experimental section, the reliability of NO concentration was low, and therefore only the meaningful data of fN2 are presented in Figure 4 for the case of gasification at 1000 °C. As the amount of O2 in each pulse is smaller, the carbon conversion at each pulse was lower. The profile of fN2 is much different from that in Figure 3a. Large fN2 values at low carbon conversion, as were observed in Figure 3a, are not seen in Figure 4. FN2 value was 0.0049 that was somewhat smaller than 0.0060 obtained with a pulse volume of 4.3 mL at the same temperature. N2 and NO Formation Using a Char HeatTreated at 1300 °C. In the above experiments, the char was heated in He up to a predetermined temperature and allowed to react with O2 at this temperature. In other words, the char heat-treatment history was different from sample to sample. To examine the true effect of gasification temperature, it is necessary to use a char with the same pretreatment history. Thus a char was heat-treated at 1300 °C, and then it was used for the gasification at 1000, 1100, 1200, and 1300 °C. Figure 5 shows the variation of fN2, fNO, and fN with the carbon conversion for these experiments. fN2 slowly decreased with carbon conversion until a carbon conversion reached around 90%, and then suddenly decreased. As for the effect of gasification temperature, fN2 became larger at higher temperature unlike the results in Figure 3a. fNO slowly increased with carbon conversion, and fNO was smaller at higher gasification temperature. As a whole, fN was almost constant irrespective of carbon conversion and gasification temperature. Table 1 compares the results of two reaction systems. All of the F values are smaller for the experiments with a char heat treated at 1300 °C.

O2 Gasification of Coal Char

Figure 5. Variation of fN2, fNO, and fN with carbon conversion. Char heat-treatment temperature: 1300 °C.

Discussion Use of a Pulse Reactor for High-Temperature Gasification. NO formation behavior at temperatures below 1000 °C has been extensively discussed by many investigators. A research at higher temperature is equally important, because it is closely related to pulverized combustion. However, there have been very few mechanistic studies in this temperature region, partly because of a lack of suitable experimental technique. The extensive heat generation makes the steady state experiment difficult. It was reported that the char particle temperature greatly exceeds the gas temperature by over a few hundred degrees during char combustion, especially at high temperatures, at high O2 concentration and with highly reactive chars.12,13 To minimize this problem, we employed here the pulse gasification technique. A similar approach has been made by Hu et al.14 to investigate the CO2 formation behavior in the high-temperature O2 gasification of coal (12) Young, B. C.; McColler, D. P.; Weber, B. J.; Jones, M. L. Fuel 1998, 67, 40-44. (13) Reichelt, T.; Joutsenoja, T.; Spliethoff, H.; Hein, K. R. G.; Hernberg, R. Proc. Combust. Inst. 1998, 27, 2925-2932. (14) Hu, Y. Q.; Nikzat, H.; Nawata, M.; Kobayashi, N.; Hasatani, M. Fuel 2001, 80, 2111-2116.

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char. They successfully gasified a char with a series of tiny amount of O2 pulse (20% O2, 1.05 mL) to suppress the temperature increase, the numbers of pulses to completely gasify the char sample being as many as 117 at 1200 °C. However, in the present study, we could not employ such conditions, because of the limitation on NO and N2 analysis. The quantities of NO and N2 are much less than CO and CO2. The volume of pulse we mainly used was thus as much as 4.3 mL. With this pulse, we could not avoid a temperature spike of about a hundred degrees at the initial stage of reaction. It can be said that the initial high fN2 and fN value in Figure 3 is a sort of artifact due to this temperature spike. When a 1.5 mL pulse was used instead of one of 4.3 mL, the temperature increase became much less, and as can be seen in Figure 4 the fN2 value in the initial stage was much less than that observed in Figure 3a. The kinetic data analysis was difficult due to this temperature increment, and furthermore this temperature spike influenced the NO/N2 selectivity as will be discussed later. Despite this drawback, the use of pulse reactor made it possible to suppress the consumption of char at each pulse and to monitor the formation of gaseous product under highly reactive conditions. N2 Desorption upon Heat Treatment. The nitrogen balance throughout experiment is as follows. The char prepared at 950 °C contains intrinsic nitrogen of 1.1% as described in the Experimental Section. This char was subjected to gasification at different temperatures, either directly or after further heat treatment at 1300 °C. All the nitrogen in the original char is not necessarily converted during the pulse gasification. Some of them were released on heating in He as well as during the holding period between two pulses. Only the rest of nitrogen participated in the pulse reaction and detected as NO or N2. The product yields presented in Figures 2-5 and in Table 1 correspond to this fraction of nitrogen. This is the reason all the FN values in Table 1 are much less than the N/C ratio of char prepared at 950 °C. It is well-known that coal char contains a certain amount of unstable nitrogen, which is released as N2 and other nitrogen-containing gases when heat-treated in an inert atmosphere. The amount and the form of evolved gases depend on the nature of char and the heat-treatment conditions. Tsubouchi et al.15,16 observed the N2 formation in the temperature range from 600 to 1300 °C in an inert atmosphere. From particular coals, more than half of nitrogen in the sample evolved as N2. Jones et al.9 conducted temperature programmed desorption experiments using a char derived from phenothiazine and observed the N2 evolution at around 1100 °C. Ninomiya et al.17 prepared coal chars at 1000, 1100, and 1200 °C, and gasified with 5% O2 at 800 and 900 °C. They found that the char prepared at lower temperature contained more nitrogen, and during the O2 gasification the portion of nitrogen evolved as NO from a nitrogen-rich char was less than from the chars prepared at higher temperature. They did not quantify (15) Tsubouchi, N.; Ohshima, Y.; Xu, C.; Ohtsuka, Y. Energy Fuels 2001, 15, 158-162. (16) Tsubouchi, N.; Ohtsuka, Y. Fuel 2002, 81, 1423-1431. (17) Ninomiya, Y.; Yokoi, K.; Arai, N.; Hasatani, M. Int. Chem. Eng. 1989, 29, 512-516.

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the N2 evolution, but probably much more N2 might be formed during the gasification. A series of experiments using the char heat treated at 1300 °C (Figure 5) were intended to minimize such N2 evolution from unstable nitrogen. After the heattreatment at such a high temperature, the char is almost completely annealed and free from unstable nitrogen. On the contrary, in the experiments without such heat treatment (Figure 3), the maximum heat treatment temperature was equal to the gasification temperature. The sample for 1000 °C gasification contains more unstable nitrogen than the sample used for the gasification at higher temperatures. This difference reflects the data in Table 1, where the char heat treated at 1300 °C always gave smaller FN values than the char without heat treatment when compared at the same gasification temperature. There are two big differences between the N2-formation behaviors in Figures 3 and 5: (1) extraordinary high fN2 values are seen only in Figure 3 for the initial stage of the gasification at 1000 and 1100 °C and (2) fN2 values become large at higher gasification temperatures in Figure 5, while the opposite occurs in Figure 3. The former result can be explained by the release of unstable nitrogen due to annealing. Since a char prepared at low temperature contains a significant amount of unstable nitrogen, it would release more N2 during gasification than from high-temperature char. The high fN2 value in the beginning of the reaction indicates that the temperature spike resulted in more N2 release. As a total, FN2 value for the gasification at 1000 °C was 0.0060, while that with the heat treatment at 1300 °C was 0.0020 (Table 1). A similar trend was seen for the gasification at 1100 °C. There is no definite explanation for the N2 formation from unstable nitrogen at the present moment. It is, however, likely that the heattreatment may cause the rearrangement of char structure producing more graphitic structure, which activate the exclusion of char nitrogen as N2. The result in Figure 5aslarger fN2 values at higher gasification temperaturescannot be explained by the release of unstable char nitrogen, because the char annealed at 1300 °C is almost free from unstable nitrogen. Therefore this difference simply indicates that the N2 formation via the C-NO reaction is more favorable at higher temperature. NO- and N2-Formation Selectivity. In the studies on the NO formation behavior at temperatures below 1000 °C, it is commonly accepted that the fraction of char-N evolved as NO during the O2-gasification of coal char depends on the amount of char in the bed: (1) The higher the carbon conversion, the higher the fraction to NO,5,6,18,19 and (2) the larger the initial feed of char, the lower the fraction of NO.5,20 This means that the NO/N2 selectivity strongly depends on the extent of the secondary reaction of NO with char (eqs 2 and 3). Jensen et al.21 observed a similar trend in the O2 gasification of coal char. They estimated the fraction of nitrogen that (18) Harding, A. W.; Brown, S. D.; Thomas, K. M. Combust. Flame 1996, 107, 336-350. (19) Tullin, C. J.; Goel, S.; Morihara, A.; Sarofim, A. F.; Bee´r, J. M. Energy Fuels 1993, 7, 796-802. (20) Furusawa, T.; Tsunoda, M.; Sudo, S.; Ishikawa, S.; Kunii, D. Prepr. Am. Chem. Soc., Div. Fuel Chem. 1982, 27, 262-273. (21) Jensen, L. S.; Jannerup, H. E.; Glarborg, P.; Jensen, A.; DamJohansen, K. Proc. Combust. Inst. 2000, 28, 2271-2278.

Orikasa and Tomita

was converted to NO in the absence of secondary reaction in the following manner. They plotted the fraction to NO against char weight in the bed, and evaluated the NO fraction corresponding to “the singleparticle condition” by extrapolating the char weight to zero. This extrapolated NO fraction increased with increasing gasification temperature. They reported that 65% of nitrogen in the char evolved as NO at 850 °C and it was almost 100% at 1150 °C under the single particle condition. On the contrary, in the presence of a larger amount of char as employed in many other studies,5,18,19 which is far from the single particle condition, the NO fraction becomes smaller at higher temperatures due to NO reduction on char surface (eq 3).21 In the present study, we examined the effect of reaction conditions on the NO/N2 selectivity at relatively high-temperature region. HCN and N2O were hardly formed under the present condition, and therefore only NO and N2 formation is taken into consideration in the following discussion. It would be better to discuss the reaction selectivity using the data of Figure 5, because in these experiments the starting materials have experienced the same heat-treatment history. We observed some temperature rise due to high exothermicity in the initial stage, but an extraordinary high fN2 was not observed in contrast to the case of Figure 3. This is because unstable nitrogen was almost absent in these chars due to the high-temperature annealing. With increasing char conversion, the NO/N2 ratio increased, since the chance of C-NO reaction becomes less as the residual char decreases. With increasing gasification temperature, the C-NO reaction takes place extensively and thus the NO/N2 ratio decreased. Such general trend is quite similar to those observed in the low-temperature gasification. It should be noted that fN was almost constant irrespective of the carbon conversion in the case of gasification of heat-treated char (Figure 5c). This is in an apparent contrast with the observation by Ashman.4 In their low-temperature gasification of coal char at 600 °C, the evolution of nitrogen was slower than the oxidation of carbon at low char conversions, and then N/C evolution ratio gradually increased with carbon conversion. In other words, the fN value is initially smaller than the N/C ratio of initial char and becomes larger in a later stage. The nitrogen accumulation on the char surface in the beginning of the C-NO reaction is responsible for this phenomenon.22 At high temperatures, on the contrary, nitrogen accumulation on char surface in the C-NO reaction is not extensive as we observed in the previous study at around 1100 °C.23 The constant fN value at all char conversion levels (Figure 5c) also suggests the absence of nitrogen accumulation in the beginning of the reaction. This does not necessarily deny the aforementioned N2 formation mechanism (eq 3). It is still likely that N2 was formed via the same route even though the lifetime of C(N) species would be very short at high temperature. As for the chars without severe heat treatment, N2 release from unstable nitrogen and N2 formation via eq (22) Chambrion, Ph.; Kyotani, T.; Tomita, A. Energy Fuels 1998, 12, 416-421. (23) Orikasa, H.; Matsuoka, K.; Kyotani, T.; Tomita A. Proc. Combust. Inst. 2002, 29. In press.

O2 Gasification of Coal Char

3 occur simultaneously. If there is some effective means to promote the conversion of unstable nitrogen to N2 gas, the NO emission during the char combustion may be reduced to some extent. For example, keeping char for some time at high temperature under less oxidative atmosphere may result in more N2 release. Promoting the C-NO reaction by raising reaction temperature would also be effective for NO reduction. It is desired that these findings may be used in designing effective NOX reduction systems. Conclusions N2 and NO formation behavior during the char gasification were examined using a pulse reactor in a temperature range from 1000 to 1300 °C. The NO and N2 formation behavior was similar to that observed at a low-temperature region. The NO/N2 ratio was high at a high char conversion and at a low gasification temperature. This trend can be explained as the result of secondary reaction between NO and char. A big difference from the low-temperature reaction is that

Energy & Fuels, Vol. 17, No. 2, 2003 411

there is almost no accumulation of nitrogen during the high-temperature gasification. However, N2 formation mechanism through the C(N)-NO reaction (eq 3) is likely to be most probable even at high temperature. In addition to this mechanism, the release of unstable char nitrogen as N2 was found to be equally important in determining the final gas composition. There is a considerable amount of unstable nitrogen in char prepared at low temperatures (