Char Gasification in the

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Environ. Sci. Technol. 2010, 44, 3719–3723

HCN and NH3 Formation during Coal/Char Gasification in the Presence of NO J I A N - Y I N G L I N , †,‡ S H U Z H A N G , †,§ L I A N Z H A N G , † Z H E N H U A M I N , †,§ H U I L I N G T A Y , †,§ A N D C H U N - Z H U L I * ,†,§ Department of Chemical Engineering, PO Box 36, Monash University, Victoria 3800, Australia, College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, Shanxi, 030024, P. R. China, and Curtin Centre for Advanced Energy Science and Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia

Received January 15, 2010. Revised manuscript received March 31, 2010. Accepted March 31, 2010.

Understanding the conversion of coal-N during gasification is an important part of the development of gasification-based power generation technologies to reduce NOx emissions from coal utilization. This study investigated the conversion of coal-N in the presence of NO during the gasification of three rankordered coals and their chars in steam and low-concentration O2. Our results show that NO can be incorporated into the char structure during gasification. The inherent char-N and the N incorporated into the char from NO-char reactions behave very similarly during gasification. During the gasification in steam, significant amounts of HCN and NH3 can be formed from the incorporatedNstructureinchar,especiallyfortherelatively“aged” chars, mainly due to the availability of abundant H radicals on the char surface during the gasification in steam. During the gasification in 2000 ppm O2, the formation of HCN or NH3 from the N structures in char, including those incorporated into the char from the NO-char reactions, was not a favored route of reaction mainly due to the lack of H on char surface in the presence of O2.

Introduction NOx emissions from power generation using coal and other solid fuels are an important environmental problem. Understanding the mechanism of their formation is an important part of efforts to develop advanced low-emission gasification technologies for power generation or for the production of liquid fuels and chemicals. HCN and NH3 are commonly believed (1–6) to be two important NOx precursors formed during pyrolysis and gasification. Li and co-workers (3–19) found that H radicals play an important role in both HCN and NH3 formation. HCN is mainly generated from the thermal cracking of unstable N-containing compounds in the volatiles while NH3 is formed from more stable N-containing compounds in nascent char (and soot) during coal pyrolysis. HCN and NH3 formation during coal gasification can be more complicated than during pyrolysis. The atmosphere surrounding coal/ * Corresponding author phone: +61 8 9266 1131; fax: +61 8 9266 1138; e-mail: [email protected]. † Monash University. ‡ Taiyuan University of Technology. § Curtin University of Technology. 10.1021/es1001538

 2010 American Chemical Society

Published on Web 04/23/2010

char particles could have drastic effects on HCN or NH3 formation (3, 9, 20). In particular, HCN and NH3 can form through the adsorption and further interactions of H radicals with N-sites during char gasification. However, some investigations (21–23) on gasification in O2 atmospheres suggested that NO is the primary product, and that NO/surface-N and NO/C interaction are responsible for the formation of other products (N2, N2O, HCN and HCNO). At higher temperatures, a significant amount of HCN was observed. The mechanism (22) to account for this observation was proposed as follows: NO + 2C f C(N) + CO or C(O)

(1)

C(N) + 1/2H2or H f C(H,N) f HCN

(2)

C(N) + C(H) f C(H,N) f HCN

(3)

where C(N) is an nitrogen-containing species on the surface. The formation of additional NH3 was not mentioned. On the contrary, an investigation (24) on the formation of NH3 during air-blown gasification concluded that NO does not act as the main precursor for NH3 formation. Clearly, the mechanisms of HCN and NH3 formation are still poorly understood. There is still considerable debate if HCN or NO is the main primary product during gasification in Ocontaining atmospheres. This study compares the formation of HCN and NH3 during coal and char gasification in O2 or H2O in the presence or absence of NO. The main objective was to clarify if NOchar interactions play an important role in the formation of HCN and NH3, in particular, if HCN/NH3 formation from NO-char interactions during gasification would depend on char structure.

Experimental Section Coal Samples. Three rank-ordered coal samples were selected for use in this study: a Loy Yang brown coal (denoted as LY) from Australia, a Shenmu (SM) high-volatile bituminous coal from China, and a Dongshan (DS) low-volatile bituminous coal from China. Their properties are listed in Table 1. Particle sizes for all coal samples were 106-150 µm. Char samples used in this study were prepared in a drop tube reactor (25) in 2000 ppm O2 (balanced with Ar) at 1000 °C with a nominal gas residence time of about 6 s. The procedure to prepare the H-form LY coal (in which all carboxylic groups exist as carboxylic acids) was described elsewhere (26). Briefly, the LY coal was stirred in 0.1 M H2SO4 for at least 16 h under argon. The sample was then washed with deionized water, followed by drying at 99.999%) before entering the fluidized bed of sand. Argon was also the main balance gas for the gasification experiments using 2000 ppm O2 and 8 ppm NO as the gasifying agent. The total gas flow rate was always 1.8 L min-1 measured under ambient conditions. The char yield was determined by weighing the reactor before and after the experiment. While the mass of feedstock (coal or char) was quantified separately by weighing the coal feeder before and after experiment, the ungasified char remained in the reactor and thus was weighed together with the reactor. Quantification of HCN, NH3, and NO. HCN and NH3 in the product gas were collected by bubbling through 0.1 M NaOH solution or 0.02 M CH3SO3H (methanesulfonic acid, MSA) solution respectively (4). Separate experiments were conducted for the collection of HCN and NH3 (4). HCN and NH3 in solution were quantified separately using Dionex 500 ion chromatographs (4). The yields of HCN and NH3 were calculated on the basis of their nitrogen as a percentage of the nitrogen in coal or char fed into the reactor. Separate experiments were conducted to quantify NO in the product gas with a Monitor Laboratories ML9841B NOx analyzer. The product gas passed through two empty traps cooled with ice in order to prevent tar and water from entering the NOx analyzer.

Results and Discussion Formation of HCN and NH3 during Coal Gasification in Steam. Figure 1 shows the cumulative yields of HCN and NH3 during the gasification of three coal samples at 800 °C in 15% H2O with and without externally added NO. The first datum points at ∼10 min in Figure 1 represent the HCN or NH3 yields during the feeding periods while all other HCN or NH3 yields are from the gasification of char in the nonfeeding periods. Within the gasification time in Figure 1, LY and SM coals have completed their gasification: no visible char remained in reactor at the end of the experiments. However, this was not the case for the gasification of DS coal, giving a char yield of around 15% after gasification for more than 720 min. 3720

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FIGURE 1. Cumulative yields of HCN and NH3 based on coal-N from the gasification of three coal samples in 15% H2O and 15%H2O + 8 ppm NO at 800 °C. 0 and 9, LY coal; ∆ and 2, SM coal; O and b, DS coal. Solid symbols: with NO. Hollow symbols: without NO. Around 500 mg (accurately weighed) of coal sample was fed into the reactor in each experiment. The data in Figure 1 show that the gasification of char was a major source of NH3 formation, accounting for about 75, 70, and 50% of the total NH3 observed for the DS, SM, and LY coals, respectively. This is in good agreement with previous studies (6, 9–14). As the formation of NH3 from coal-N requires the presence of a solid phase, little NH3 can be formed from the cracking/reforming of volatiles in the feeding periods (3–6). However, the gasification of char in steam produces significant amounts of H radicals as the intermediates of char-steam reactions for char-N to be gradually hydrogenated into NH3. The data in Figure 1 also show that the HCN yields during the feeding periods were relatively low, particularly for the high-rank DS coal, in broad agreement with previous pyrolysis studies (1, 7). This is because the main reactions contributing to the formation of HCN in the feeding period would be the primary pyrolysis of coal and the thermal cracking of volatiles and char. At 800 °C, these reactions do not contribute significantly to the formation of HCN, particularly with short volatile residence time. Little HCN could be formed from the gasification of char in the feeding periods at 800 °C. However, when the char was held for a long time for gasification with steam, additional HCN was observed (Figure 1). As there were no volatiles generated in the nonfeeding periods, the additional HCN observed all originated from char gasification and accounted for 95, 54, and 19% of the total amounts of HCN for the DS, SM, and LY coals, respectively. The DS coal of the highest rank (see Table 1) gave the highest HCN yield (38.4%) when gasified in 15% H2O, whereas the LY coal of the lowest rank gave the lowest HCN yield (8.3%). The observations here are in broad agreement with our previous ones (11). Figure 1 also shows the effects of NO (8 ppm) addition on the formation of HCN and NH3 during the gasification of the same coals in 15% steam at 800 °C. The yields of HCN and NH3 were still calculated as percentages of coal-N without considering the NO added. While the addition of NO only

FIGURE 2. Cumulative yields of HCN and NH3 from the gasification of LY char in 15% H2O or 15% H2O + 8 ppm NO at 800 °C. Solid symbols: with NO; hollow symbols: without NO. About 300-360 mg of char sample (accurately weighed) was fed into the reactor in each experiment.

FIGURE 3. Cumulative yields of HCN and NH3 from the gasification of DS char in 15% H2O or 15% H2O + 8 ppm NO. Solid symbols: with NO; hollow symbols: without NO. About 290-360 mg of char sample (accurately weighed) was fed into the reactor in each experiment.

had small effects (also see below) on the formation of HCN during the gasification of LY coal, the added NO did result in significant increases in the observed HCN yields for SM and DS coals. The increases in the HCN yield mainly took place in the later stages of char gasification. In other words, extra HCN only formed during the gasification of “aged char”. The gasification of LY coal and DS coal in 15% steam in the presence and absence of 8 ppm NO was also carried out at 900 °C, confirming that the addition of NO had no effects on HCN formation for the gasification of LY coal but resulted in significant additional HCN for the gasification of DS coal. The addition of 8 ppm NO also resulted in increases in the yields of NH3 (Figure 1), particularly for LY and SM coals, again only in the later stages of char gasification. It is intriguing to note that, while the addition of NO caused the biggest increase in the observed HCN yield for the high-rank DS coal, the addition of NO caused the bigger increases in the observed NH3 yields for the lower-rank LY and SM coals. No suitable gas-phase reactions could be used to explain the conversion of NO into HCN or NH3 that would only take place in the later stage of gasification but in the earlier stages of gasification. Taken together, the data in Figure 1 indicate the conversion of externally added NO into HCN and NH3 through reactions such as Reaction 1 mentioned above. In other words, NO was incorporated into the char. The fate of N incorporated into the char from NO was similar to that of char-N during gasification. In particular, the selectivity of the incorporated N to HCN and NH3 was largely determined by the availability of H: abundant H radicals favored the formation of NH3 over the formation of HCN (6, 7, 9, 10, 14). The rapid gasification of LY char meant that there was high concentration of H radicals (as char-H2O reaction intermediates) on the char surface to give the highest NH3 yield and a big increase in NH3 yield when NO was added (Figure 1). The rather slow gasification of DS char only provided a low concentration of H radicals to give the highest HCN yield and the biggest increase in the observed HCN yield when NO was added (Figure 1). Formation of HCN and NH3 during Char Gasification in Steam. The data in Figure 1 appear to indicate that the formation of additional HCN and/or NH3 from externally added NO is only possible with relatively “aged” chars. The LY and DS coals behaved very differently. This difference in the formation of HCN/NH3 from the char-NO-H2O reactions between fresh and aged chars was further confirmed by the gasification of chars prepared under more severe conditions of 1000 °C (than 800 °C) in the presence of 2000 ppm O2. The gasification of the 1000 °C LY char in Figure 2 was much slower (longer time to complete) than the gasification of the 800 °C LY char in Figure 1 under otherwise identical

conditions, whereas the formation of HCN/NH3 was virtually complete within about 150 min in Figure 1 (800 °C char), that for the 1000 °C char (Figure 2) continued after 300 min. Substantial increases in the HCN and NH3 yields from LY char gasification due to the addition of 8 ppm NO were observed (Figure 2). In particular, while the NO addition caused mainly increases in NH3 yield and very little additional HCN for the 800 °C (relatively “nascent”) char in Figure 1, the NO addition caused similar magnitudes of increases in the yields of HCN and NH3 for the 1000 °C (relatively “aged”) char in Figure 2. This is in good agreement with our explanation given above. The more rapid gasification of the 800 °C char must have produced higher concentrations of H radicals on the char surface than the slower gasification of the 1000 °C char, corresponding to a higher NH3 yield (and a lower HCN yield) for the 800 °C char. Even for the gasification of DS char, increasing the severity of char preparation conditions from 800 °C (Figure 1) to 1000 °C (Figure 3) has further slowed the char gasification: the char yield after 720 min has increased from 15 to 38 wt %. There was a notable increase in the yield of HCN due to the addition of NO going from 800 °C (Figure 1) to 1000 °C char (Figure 3). The data confirm that significant HCN and some NH3 can be formed from the char-NO-H2O interactions and that the formation of HCN from the char-NO-H2O interactions is favored by a more aged char. Comparing the data in Figures 2 and 3 with those in Figure 1 also shows that the difference in the effects of NO addition on HCN/NH3 formation between the LY and DS chars decreased with increasing severity (800-1000 °C) of char preparation conditions, which reduced the difference in the structure of char. Another significant difference between the LY coal and DS coal is the presence of high concentrations of finely dispersed alkali and alkaline earth metallic (AAEM) species in the former (12, 26–29), which catalyze the char gasification. To test the effects of AAEM species on the formation of HCN and NH3 from the externally added NO, efforts were thus made to acid-wash the LY coal to remove its finely dispersed AAEM species completely to prepare an H-form LY coal. This H-form LY coal underwent the same treatment at 1000 °C in 2000 ppm O2 and then subject to gasification in 15% H2O. Two observations can be made with the data in Figure 4 for the gasification of H-form LY char. First, the gasification of AAEM-free H-form LY char produced more HCN (>40% of char-N) than that of the AAEM-containing LY char (∼20% of char-N, Figure 2). As the gasification of H-form LY char (Figure 4) did not complete (with a char yield around 16 wt %) within the holding time of 550 min (while that of LY char in Figure 2 was almost complete within 360 min), even more HCN could potentially be formed from the H-form LY char. VOL. 44, NO. 10, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Cumulative yields of HCN and NH3 from the gasification of H-form LY char in 15% H2O or 15% H2O + 8 ppm NO at 800 °C. Solid symbols: with NO; hollow symbols: without NO. About 300-390 mg of char sample (accurately weighed) was fed into the reactor in each experiment.

TABLE 2. Cumulative Yields of HCN and NH3 from the Gasification of Three Coals in 2000 ppm O2 and 2000 ppm O2 + 8 ppm NO at 800°C cumulative yield of HCN, % coal-N

cumulative yield of NH3, % coal-N

LY LY SM DS DS LY LY SM DS DS coal char coal coal char coal char coal coal char O2 12.1 3.6 9.1 4.7 O2 + NO 10.4 4.6 11.6 5.7

1.9 20.3 11.3 18.1 7.9 4.0 17.7 13.3 19.3 9.1

1.9 1.0

Unfortunately, also due to the noncomplete gasification of the H-form LY char, no direct comparison can be made about the NH3 formation from these two chars. There are significant differences in the HCN/NH3 ratio between the two chars (Figures 2 and 4). The rapid gasification of AAEM-containing LY char provided high concentrations of H radicals on the char surface to favor NH3 formation, compared with the slow gasification of AAEM-free H-form LY char. The second observation from the data in Figure 4 is that the addition of NO into the gasification of H-form LY char has caused massive increases in the observed HCN yield (Figure 4), compared with the case of LY char (Figure 2). This is in general agreement with the above-mentioned observation that low-reactivity or “aged” chars (e.g., DS char) have better capacity to form HCN from NO. Formation of HCN and NH3 during Coal and Char Gasification in O2. Table 2 shows the cumulative HCN and NH3 yields from the gasification of three coals and their 1000 °C chars in 2000 ppm O2 and 2000 ppm O2 + 8 ppm NO. Unlike the addition of NO into the coal/char-steam gasification systems, only a little or no additional HCN or NH3 was formed when NO was added into the coal/char-O2 gasification systems. Additional experiments were conducted on the gasification of LY coal in 2000 ppm O2 and 2000 ppm O2 + 8 ppm NO at 700 °C. HCN yields were 8.6% in both cases. Clearly, little H radicals would exist on the char surface during the gasification in 2000 ppm O2 (11). Even if NO can be adsorbed and incorporated into the char structure, little HCN or NH3 can be formed from char-N, either the inherent char-N or the char-N formed from the char-NO reactions, because the formation of HCN and/or NH3 requires hydrogen. The first step for the conversion of NO into HCN or NH3 in our reaction systems is the incorporation of NO into char (e.g., Reaction 1). It was initially suspected that the lack of significant formation of additional HCN/NH3 from the charNO-O2 reactions in Table 2 might be due to the difficulties for NO to be incorporated into the char in the presence of low concentration O2. Further experiments were carried out to investigate the consumption of NO and typical results are 3722

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FIGURE 5. Concentration of NO in reactor exit gases during the gasification of LY char in 2000 ppm O2 (dashed line) and 2000 ppm O2 + 8 ppm NO (solid line) at 800 °C. shown in Figure 5 for LY char. Similar trends were seen with other samples. When 8 ppm additional NO was added into the reaction system, the char-NO-O2 reactions drastically reduced the NO added (Figure 5). As long as relatively nascent char (e.g., in the first 70 min) was present in the reactor, the NO concentration at the exit of reactor was always less than the externally added NO (i.e., 8 ppm input into the reactor). Therefore, NO was incorporated (i.e., consumed) into the char to become char-N in the char-NO-O2 reaction systems. However, only a little or none of the incorporated NO (as char-N) was converted into HCN or NH3, due to the lack of H on the char surface in 2000 ppm O2. The majority of the incorporated NO (as char-N) was converted into N2. The preferential formation of N2 from char-N in O2, especially from the nascent char-N, is in good agreement with our previous observations (10, 11) made under similar conditions. The results in this study clearly demonstrate that NO can be incorporated into the char structure, for example, via Reaction 1. For a nascent char (i.e., at the earlier stages of gasification), the newly incorporated N-containing structures on/in char would be reactive and even mobile to form N2, as is shown in Figure 5 for the gasification in O2. Even during the gasification in steam, the newly incorporated N-containing structures on/in the nascent char would have been sufficiently reactive and/or mobile to form N2 although the NOx analyzer could not be used to quantify NO in the reducing gasification product gas to confirm the extents of NO incorporation into the char. However, for the more “aged” and less reactive chars, the N-containing structures resulting from the incorporation of NO into the char structure would be stablized. Their main fates would be the partial hydrogenation and subsequent decomposition to form HCN or the gradual complete hydrogenation to form NH3; these reactions are similar to the formation of HCN and NH3 (3–6) from the char-N itself.

Acknowledgments This project was partly supported proudly by the International Science Linkages established under the Australian Government’s innovation statement, Backing Australia’s Ability. J.-Y. Lin also acknowledges the scholarship from the China Scholarship Council.

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