NH3 Formation and Destruction during the Gasification of Coal in

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Environ. Sci. Technol. 2007, 41, 5505-5509

NH3 Formation and Destruction during the Gasification of Coal in Oxygen and Steam LACHLAN J. MCKENZIE, FU-JUN TIAN, AND CHUN-ZHU LI* Department of Chemical Engineering, P.O. Box 36, Monash University, VIC 3800, Australia

This study was conducted to investigate the formation and destruction of NH3 during the gasification of coal in atmospheres containing O2 and steam. A Victorian brown coal was gasified in a novel bench-scale fluidized-bed/ fixed-bed reactor at 800 °C in atmospheres containing 2000 ppm O2, 15% H2O, or 2000 ppm O2 + 15% H2O. A NH3 standard gas was also used to study the destruction of NH3 in the gas phase and through gas-solid interactions. Sand, char, and coal ash were all found to enhance the destruction of NH3. An atmosphere containing O2 alone does not favor the conversion of char-N into NH3 but favors the destruction of NH3 through various mechanisms. The introduction of H2O into the gasification system greatly favors the conversion of char-N into NH3 and inhibits the destruction of NH3. The formation and destruction of NH3 in an atmosphere containing 15% H2O was similar to that in an atmosphere containing 15% H2O and 2000 ppm O2, indicating the dominant effects of steam in the formation and destruction of NH3 in a gasifier.

Introduction Drastic reductions in CO2 emission must be achieved soon if the large reserves of cheap coal are to remain as an energy source in the carbon-constrained future. The gasificationbased power generation technologies have great potential to reduce the emissions of CO2 from a coal-fired power plant (1, 2). However, in addition to reduced CO2 emissions, a true near-zero-emission power generation plant must also have low emissions of other pollutants such as NOx, SOx, and particulates. The gasification-based technologies do not necessarily guarantee a lower emission of NOx than the existing pulverized-fuel (pf) combustion technologies. This is because the conversion of coal-N during gasification is exceedingly complex (3). Coal-N can be converted into several species such as NH3, HCN, NOx/N2O, N2, and tar-N during gasification (3). NH3 appears to be the most important N-containing species from a commercial/demonstration gasifier (4). When the fuel gas is burnt in a gas turbine, the NOx precursors (especially NH3 and HCN) may be converted into NOx. There are broadly two approaches (3) with which the NOx emission from a gasification-based power plant can be minimized. The first approach is to rely on the cleaning of gasification product gas prior to combustion to remove the NOx precursors. The second approach is to minimize the formation of NOx and NOx precursors during gasification, * Corresponding author fax: [email protected]. 10.1021/es0705691 CCC: $37.00 Published on Web 06/30/2007

+61 3 9905 9623; e-mail:

 2007 American Chemical Society

e.g., through optimizing reaction conditions. Minimizing the formation of NOx and NOx precursors during gasification will also reduce the load on the gas cleaning system when both approaches are used simultaneously. Compared with the formation of NOx during combustion, our current knowledge on the coal-N chemistry during gasification is still very limited and lags behind the requirement of technology development. Nitrogen exists mainly in coal as heteroaromatic ring structures (3), which must be hydrogenated in order to form NH3 observed in the gasification product gas. Recent studies by our group have clarified the main mechanism of NH3 formation during pyrolysis (3, 5-10). The primary step in NH3 formation is the initiation of the N-containing heteroaromatic ring structures in coal/char via thermal cracking and interaction with radicals. NH3 then forms via the hydrogenation of the activated N-containing ring structures in the char; thus the presence of a significant amount of donatable hydrogen on the pyrolyzing char surface is critical for NH3 formation. During gasification in steam, the steam acts as an external source of H, and NH3 formation is greatly increased over that from pyrolysis (10-12). The reaction conditions in a commercial gasifier (especially a fluidized-bed gasifier) are very complicated. Both oxygen and steam will be present simultaneously and many factors influence the formation and destruction of NOx precursors (particularly NH3). While there has been research conducted into NOx and NOx precursor formation during gasification with oxygen (e.g., 3, 10, 13-18) and combustion (e.g., 19-22), there is very little information (23-25) specifically concerned with N release from coal during gasification with both steam and oxygen. For example, little is known about competitive reactions of the O2-derived and H2Oderived radicals with the char-N. Furthermore, NH3 in the product gas will be in constant direct contact with char and/ or ash, especially in a fluidized-bed gasifier. It is unclear if and how interactions of NH3 with char or ash could lead to its destruction. The purpose of this study was to clarify the main pathway of NH3 formation from coal-N during the gasification of Loy Yang coal with oxygen and steam at 800 °C. A fluidizedbed/fixed-bed reactor was employed to separately examine the destruction of NH3 in the gas phase and during charNH3 interactions. Our investigations show that even in the presence of oxygen, the dominant factor affecting char-N selectivity toward NH3 formation is the presence of H. Our results have significant implications to the destruction of NH3 in a gasifier.

Materials and Methods Coal Sample. Loy Yang brown coal from the Latrobe Valley in Victoria, Australia, was used in all experiments. This vast reserve of brown coal is currently being considered for future power generation using gasification-based technologies. The coal sample prior to gasification had a particle size range of 106-150 µm. It featured a low ash yield of 1 wt % (dry basis). Its elemental analyses were as follows: C, 68.5; H, 4.8; N, 0.55; S, 0.32 wt % on a dry and ash-free basis. Gasification. A one-stage fluidized-bed/fixed-bed reactor (26) was used throughout this study, as is shown schematically in Figure 1. The reactor consisted of two sintered quartz frits within an externally heated, cylindrical, quartz tubular reactor (about 130 mm in length). Coal in a feeder was entrained in a gas stream and fed into the reactor via a water-cooled injection probe. About 500 mg (accurately weighed) of coal was used in each experiment. Once inside the reactor, the VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Destruction of NH3 Standard at 800 °C in Different Atmospheres without Coal Being Fed into the Reactor

FIGURE 1. Schematic diagram of one-stage fluidized-bed/fixedbed reactor (not to scale) (26). Reprinted with permission from Elsevier. 2002. coal particles were rapidly (>103-4 K s-1) heated up (as in a fluidized bed) to release volatiles. While the volatiles passed through the top frit and exited the reactor, the corresponding char was retained underneath the top frit to form a fixed bed. Char was held in the reactor for an extended period for in situ gasification. All experiments were conducted at 800 °C with a nominal coal-feeding rate of 60 mg min-1. In some sets of experiments, the coal was injected directly into a hot bed of fluidized zircon sand (particle size range 90-125 µm). In other sets of experiments (those employing an NH3 standard gas), it was necessary to remove the sand bed and operate the reactor as a kind of drop-tube/fixed-bed reactor with heating rate no less than 103-4 K s-1. Three different gasification atmospheres were selected for study: 15% H2O, 2000 ppm O2, and 2000 ppm O2 + 15% H2O (all on volumetric basis). In all cases, the balance gas was ultrahigh purity (99.999%) argon. During experiments that required steam, an HPLC pump was used to feed the deionized water at a constant flow rate into the reactor (just below the bottom frit) to produce steam. The steam was well mixed with the incoming “fluidizing gas” (Figure 1) to become a part of the fluidizing gas before passing into the main body of the reactor. The steam flow rate was thus controlled by the flow rate of water delivered by the HPLC pump. A concentration of 2000 ppm O2 was selected as it provided a gasification rate of magnitude similar to that in 15% H2O (based on the weight loss as a function of gasification time), thus any changes in coal-N release caused by the use of a different atmosphere would not be obscured by markedly different reaction rates. The 2000 ppm O2 would also be a condition present in some parts of a gasifier. Each gasification experiment is divided into 2 periods. The “feeding” period (about 8 min) refers to the time when coal particles were continuously fed into the reactor. At 800 °C under all conditions in this study, the char gasification rate was relatively slow and a significant amount of char remained ungasified at the end of the “feeding” period. The remaining char was further gasified in the “not feeding” periods during which only gasifying agents (O2, H2O, or O2/H2O) were fed into the reactor without additional coal particles being fed into the reactor. Quantification of HCN and NH3. Gas exiting the reactor system was passed through a solution of NaOH or 0.05 M CH3SO3H (methanesulfonic acid, MSA) for the collection of HCN and NH3, respectively. It was necessary to use a near saturated concentration of NaOH for collection of HCN during and immediately after the coal feeding period (to 5506

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atmosphere

without sand bed

with sand bed

2000 ppm O2 + 15% H2O 15% H2O 2000 ppm O2

4% 6% 19%

23% 18% 83%

ensure the solution was not neutralized by the relatively high concentration of CO2). NaOH (0.1 M) was used for all other periods. Separate experiments were conducted for the collection of HCN and NH3 due to the non-negligible solubility of HCN in MSA solution and NH3 in NaOH solution (5). HCN (as CN-) and NH3 (as NH4+) were quantified separately using Dionex 500 ion chromatographs. Quantification of NO. Separate experiments were conducted for the quantification of NO. The exit gas from the reactor was passed through two empty ice-cooled absorption bubblers designed to act as a trap to condense tar and excess water out of the exit gas stream. The cold gas then passed into a Monitor Labs ML9841B chemiluminescent NOx analyzer.

Results and Discussion As will be detailed later, very different NH3 yields were obtained when the same Loy Yang brown coal was gasified in atmospheres containing O2 or O2+H2O. These differences may originate from the differences in the primary formation of NH3 during the gasification of coal-N in different atmospheres, or from the differences in the interactions of char (or ash) with NH3 or the gas-phase destruction of NH3 in different atmospheres. This necessitated an investigation of the destruction of NH3 in the gas phase as well as interactions of NH3 with char or ash using an NH3 gas standard. Effects of Gas Atmosphere on Gas-Phase NH3 Destruction. When a 17 ppmv NH3 standard gas in argon was fed into the reactor via the coal injection probe (Figure 1) but without coal being fed into the reactor (blank experiments), the destruction of NH3 was found to change with gas atmosphere (O2, H2O, or O2/H2O), as shown in Table 1. NH3 (17 ppmv) represents a concentration of magnitude similar to that formed during the initial not-feeding period gasification of Loy Yang coal in steam-containing atmospheres in our reactor system. The destruction of NH3 in Table 1 was calculated by quantifying the amount of NH3 in the standard gas entering the reactor and the amount of NH3 exiting the reactor over a given period of time. The average gas residence time was around 2 s. In agreement with our previous study (27), the presence of a sand bed led to much higher NH3 destruction than in the absence of a sand bed (empty reactor) for all conditions tested (Table 1), indicating that the sand bed with its large surface area catalyzed the destruction of NH3. In the absence of sand in the reactor, the destruction of NH3 was strongly affected by the composition of atmosphere. In an atmosphere of 2000 ppm O2, the destruction of NH3 was 19%, which is greater than the 4-6% destruction seen with atmospheres containing 15% H2O. H2O was able to greatly inhibit the destruction of NH3 even in the presence of O2: NH3 destruction was practically the same (within experimental errors) in 15% H2O and in 2000 ppm O2 + 15% H2O. It was initially suspected that H2O inhibited the consumption of NH3 due to the shift of the following equilibrium toward the left with the introduction of H2O:

4NH3 + 3O2 f 2N2 + 6H2O

(1)

However, our calculation indicates that the reaction system was far from being at equilibrium and thus was controlled

TABLE 2. Average Destruction of NH3 in the Standard Gas Fed into the Reactor by Char or Ash NH3 destruction in atmosphere containing time interval since the end of feeding period, min

2000 ppm O2 + 15% H2O

15% H2O

2000 ppm O2

0-20 20-60 60-100 100-140 140-180 180-220 220-360 blanka

42% 39% 1% 0% 1% 1% 5% 4%

27% 25% 10% 9% 11% 8% 6% 6%

87% 89% 98% 76% 37% 38% 38% 19%

a “Blank” refers to the destruction of NH standard in the absence 3 of char and sand in the reactor.

by the reaction kinetics. It is thus believed that the inhibition of NH3 destruction by the presence of H2O is due to the preferential adsorption of H2O on the sites on the reactor wall that would otherwise catalyze the destruction/oxidation of NH3. Char-NH3 Interactions. Char-NH3 interactions were examined by running two different types of experiments. In the first type of experiments, coal was fed into the fluidized sand bed in the reactor for continuous gasification at 800 °C. It was confirmed that the observed NH3 yield remained unchanged with or without sand in the reactor. This is due to the non-melting nature of Loy Yang coal. The majority of the char would not have agglomerated with the sand but would have been elutriated out of the sand bed to stay underneath the top frit (Figure 1) almost immediately after coal-feeding (26). Any NH3 that is formed from char-N would have very limited, if any, contact with the sand bed. In the second type of experiments, coal was fed into the empty reactor (without sand) for gasification. A stream of NH3 standard gas was also continuously flowed through the reaction system throughout the experiment, acting as a “tracer” to investigate the destruction of NH3 under gasification conditions. NH3 destruction by the char-NH3 interactions was calculated using the following formula:

%NH3 destruction )

(

1-

NH3(std gas + coal) NH3(coal) + NH3(std)

)

× 100% (2)

where “NH3(std gas + coal)” refers to the total NH3 exiting reactor during a gasification experiment where a stream of NH3 standard gas was continuously fed into the reactor, “NH3(coal)” refers to the NH3 formed from coal-N as determined in the first type of experiment, and “NH3(std)” refers to the NH3 fed into reactor with the standard gas. Table 2 shows the effects of gas atmosphere on NH3 destruction by char. Because NH3 formation was quantified in reaction-time-resolved fractions during the course of gasification, it was possible to calculate the average destruction for different periods of the reaction. Compared with the data from blank experiments (no sand or char in the reactor) as shown in the last row of Table 2, char enhanced the destruction of NH3 in all atmospheres especially during the first 60 min, when there were significant amounts of ungasified char remaining in the reactor. However, the extent of NH3 destruction depended on the reaction atmosphere. In 15% H2O, NH3 destruction was decreased from 27% to 8% over a period of 180 min, most likely due to decreases in the amount of char in the reactor. NH3 destruction in an atmosphere containing 2000 ppm O2 + 15% H2O was higher, decreasing from 42% to 1% in 60 min. The more rapid

FIGURE 2. NO concentration in reactor exit gases during the gasification of Loy Yang coal at 800 °C in 2000 ppm O2 with and without NH3 standard gas fed into the reactor. Also shown is the destruction of NH3 in the standard gas. decrease in NH3 destruction in an atmosphere containing 2000 ppm O2 + 15% H2O than that in 15% H2O was due to the more rapid removal of char in the former case than in the latter case. However, NH3 destruction in an atmosphere containing 2000 ppm O2 was the highest, increasing from 87 to 98% in 100 min. Because the amount of char in the reactor decreased continuously as it was gasified (e.g., only about 15-20% of coal fed into the reactor remained as char after 60 min), these data indicate that the ability of char to enhance the NH3 destruction in O2 increased drastically with gasification, at least for the first 100 min of char gasification (Table 2). Further experiments were conducted in an effort to determine the products of NH3 destruction in 2000 ppm O2 in the presence of char. NO exiting the reactor system was quantified both with and without an external NH3 standard gas flowing through the reactor during gasification, as shown in Figure 2. It is interesting to note that significant amounts of NO did not form until 40 min after the completion of coal feeding. In contrast, during the first 40 min after coal feeding, over 85% of NH3 in the standard gas was destroyed. It appears that the major product of NH3 oxidation enhanced by the relatively fresh char was N2. At the later stages (>40 min in Figure 2), NO became an important product of char-O2NH3 interactions. At gasification times longer than 130 min (Figure 2), the majority of char was gasified. Our calculations show that in an empty reactor at 800 °C, 4% of the NH3 standard is converted to NO, but in a reactor containing only ash (i.e., gasification times >130 min) conversion to NO is 9%. Thus, the NO observed after 130 min (∼1.7 ppm) was from both the homogeneous oxidation and the catalytic (via ash) heterogeneous oxidation of NH3. The data in Table 2 and Figure 2 may be understood by considering the possible mechanisms of char-NH3-O2-H2O interactions. The consumption of NH3 would require the adsorption of an NH3 molecule onto an active C site. In the presence of O2 and absence of H2O, the char surface is deficient of H, favoring the dissociation of NH3; e.g., NH3 ) NH2 + H. Furthermore, O2-derived species present on the char surface would act as scavengers of H to form water. With gasification in O2, the char surface becomes increasingly deficient of H, explaining the increased effectiveness of char in enhancing NH3 consumption with gasification time (Table 2). However, during gasification in steam-containing atmospheres, the dissociation of H2O on char surface means that the char surface is likely to be rich in H (adsorbed radicals). Furthermore, as the char structure is broken down during gasification, the free C sites, even if generated by reaction with O2 in the case of O2+H2O gasification, are likely to be VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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covered or even saturated by the H radicals. The high H concentration on the char surface does not favor the dissociation of NH3 on the surface, e.g., the reaction NH3 ) NH2 + H is shifted toward the left. Therefore, the concentration of H on the char surface, i.e., char surface structure, plays an important role in the consumption of NH3, explaining the role of H2O in preventing NH3 from being consumed at the char surface (see Table 2). It follows from the above description that the more H-deficient char from gasification with O2 would result in decreases in the mobility of N-containing species on char surface, e.g., due to the increased strength of C-N bonding. This would mean that the adsorbed N-containing species (e.g., -C(N)) would tend to be gradually oxidized to NO. Indeed, significant amounts of NO were only observed with “aged” char, e.g., after gasification for 40 min in O2 (Figure 2). On the contrary, the relatively fresh char (less H deficient, e.g., less than 40 min of gasification) would allow greater mobility of the N-containing species on the char surface. This would promote movement and combination of those N-containing species, favoring the formation of N2. The above data indicate that, by properly adjusting the reaction conditions, the char-NH3-O2 interactions could potentially be used to destroy the NH3 formed during coal gasification. This is clearly worth further exploration in the design of industrial-scale gasifiers. Ash-NH3 Interactions. The possible destruction of NH3 by ash-NH3 interactions was also investigated. Complete char gasification had occurred by 180 min from coal feeding in all atmospheres used in this study. Thus, NH3 destruction (Table 2) after this time represents that due to Loy Yang coal ash combined with that due to any gas-phase decomposition. As is shown in Table 2, when H2O was present in the atmosphere, NH3 destruction in the presence of ash was practically the same as that due to gas-phase reaction in an empty reactor (blank decomposition). However, in atmospheres containing 2000 ppm O2 only, NH3 destruction (38%) in the presence of ash was significantly higher than that in the blank experiment (19%) (Table 2). This could be due to the properties of ash formed in 2000 ppm O2 being different from those formed in 15% H2O and 15% H2O + 2000 ppm O2, or due to the steam affecting the ability of ash to enhance the destruction of NH3. An experiment was conducted to clarify the cause of the enhanced NH3 decomposition in 2000 ppm O2. Loy Yang coal was gasified to completion (i.e., no char remaining) in 2000 ppm O2 and then 15% H2O was introduced into the reaction atmosphere. NH3 standard gas was flowed into the reactor for the entire duration of the experiment. The results indicate that the NH3 destruction decreased drastically from 33% to 6-9% immediately upon the introduction of H2O. This suggests that the absence of steam, not differences in ash properties, was the determining factor in the ability of the ash to enhance NH3 destruction in 2000 ppm O2. As before, the preferential adsorption of H2O on ash surface is believed to be the main reason for the decreases in the ability of ash to enhance the NH3 destruction. Clearly, this should be considered in the design of commercial gasifiers. Formation of NH3 During Gasification. Figure 3 shows the cumulative NH3 formation with time during the gasification of Loy Yang coal in various atmospheres at 800 °C. The first datum points on the plot represent the coal-feeding periods during which coal was being continuously fed into the reactor. All datum points afterward represent the notfeeding periods during which no coal was fed into the reactor, but remaining char was held under the top frit for gasification (Figure 1). The data in Figure 3 clearly indicate that the formation of NH3 was strongly dependent on the composition of atmosphere surrounding the coal/char particles. The NH3 5508

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FIGURE 3. Cumulative conversion of coal-N to NH3 with time during the gasification of Loy Yang coal in different atmospheres at 800 °C. yield during the gasification in 2000 ppm O2 was much lower than those during gasification in 15% H2O or 15% H2O + 2000 ppm O2. While some differences were observed during the feeding periods (including the NH3 formation from pyrolysis and volatile-char interactions), the main difference occurred during the not-feeding periods during which the gasification of char and, to minimal extent, soot was the only source of NH3. Very little NH3 (∼ 2% of coal-N) was formed during the gasification of char in O2. Once H2O was present, the effects of O2 on NH3 formation were minimal and high yields of NH3 were observed. The gasification rate in 15% H2O + 2000 ppm O2 was higher than that in 15% H2O, explaining why the NH3 yield in the former case reached ultimate yield than in the latter case. HCN formation during gasification in the three different atmospheres was also quantified. The final HCN yields were between 6 and 8% coal-N, independent of gasification atmosphere. The extremely small (if any) difference among HCN yields from each of the three different atmospheres confirms that the hydrolysis of HCN to form NH3 either at the char surface or in the gas phase cannot account for the changes in NH3 yields with gas atmosphere surrounding the coal/char particles. In fact, our previous studies (5-9) have indicated that HCN mainly forms from the thermal cracking of volatiles. Changing gasification atmosphere would have minimal effects on the release and subsequent cracking of volatiles. There are two possible factors contributing to the effects of H2O introduction on the observed NH3 yield: decreased homogeneous/heterogeneous destruction of NH3 or, a change in selectivity (i.e., a change in major reaction pathways) of coal-N/char-N toward NH3. If a change in the gas-phase destruction of NH3 with atmosphere (with or without H2O) were the only cause of the differences in the final NH3 yields formed in different atmospheres, our calculation indicates that the destruction of NH3 in 2000 ppm O2 would have to exceed 70%. Clearly this is not the case: the experimentally determined gas-phase NH3 destruction was less than 20% (Table 1). In fact, the residence time (, 1 s) of NH3 formed from char-N (underneath the top frit in Figure 1) was much shorter than that (∼ 2 s) of NH3 standard gas fed into the reactor via the coal injection probe (Figure 1). The actual gas-phase destruction of NH3 originating from char-N would be much smaller than 20%. Therefore, the difference in gas-phase NH3 destruction among different atmospheres is negligible or, at most, only partially responsible for the differences in the final NH3 yields observed from the gasification of coal in these atmospheres.

Heterogeneous destruction via interaction with char or ash might appear to be responsible for the differences in the final NH3 yields among various atmospheres, especially considering that char is extremely effective at decomposing NH3 in an atmosphere containing 2000 ppm O2 (Table 2). However, we believe that the changes in char-N reaction routes with atmosphere are more likely to be the main cause. The presence of H radicals on the char is critical for the formation of NH3 from char-N (7, 8, 10, 11). The formation of NH3 from char-N is a stepwise process, involving intermediates such as adsorbed -C(NH2) or -C(NH):

(8)

(9) (10)

(11) -C(H)

-C(H)

-C(H)

Char-N 798 -C(NH) 798 -C(NH2) 798 NH3

(3)

where -C(H) represents the H (radicals) adsorbed on char surface that have high mobility. In other words, the same intermediates are involved in the formation of NH3 (forward reaction 3) and in the destruction of NH3 (backward reaction 3) on the same char surface. As was described previously, the presence of 2000 ppm O2 would have made the char surface H deficient, shifting reaction 3 toward the left for the destruction of NH3. During the gasification in 2000 ppm O2, the lack of H on the char surface would not favor the formation of NH3. Therefore, by studying the destruction of NH3 in the char-NH3-O2 interactions, we have provided logical reasoning to conclude that NH3 formation itself during the gasification of char-N in 2000 ppm O2 was very limited. The subsequent destruction of formed NH3 could only play a secondary role in further reducing the NH3 yield from the gasification of char-N in O2. In the atmosphere containing 15% H2O, the char surface always had abundant H radicals. This shifted reaction 3 toward the right for the formation of NH3. In 15% H2O + 2000 ppm O2, O2 mainly played the role of carbon removal. As soon as a free C site was formed, it was likely to be covered by H and thus still favored the formation of NH3. In summary, the presence of H2O generally favors the formation of NH3 (Figure 3) and inhibits the destruction of NH3 through NH3-char/ash (Table 2) or NH3-sand (Table 1) interactions. The ability of O2 to destroy NH3 in a gasifier can be greatly compromised by H2O.

(12)

Acknowledgments

(22)

We gratefully acknowledge the financial and other support received for this research from the Cooperative Research Centre (CRC) for Clean Power from Lignite, which is established and supported under the Australian Government’s Cooperative Research Centres program.

(23)

Literature Cited (1) Li, C.-Z. Editorial: Special Issue - Gasification: a Route to Clean Energy. Process Saf. Environ. Prot. 2006, 84, 407-408. (2) Li, C.-Z. Chapter 1: Introduction. In Advances in the Science of Victorian Brown Coal; Li, C.-Z., Ed.; Elsevier: Oxford, 2004. (3) Li, C.-Z. Chapter 6: Conversion of Coal-N and Coal-S during Pyrolysis, Gasification and Combustion. In Advances in the Science of Victorian Brown Coal; Li, C.-Z., Ed.; Elsevier: Oxford, 2004. (4) Leppalahti, J.; Koljonen, T. Nitrogen evolution from coal, peat and wood during gasification: Literature review. Fuel Process. Technol. 1995, 43, 1-45. (5) Tan, L. L.; Li, C.-Z. Formation of NOx and SOx precursors during the pyrolysis of coal and biomass. Part I. Effects of reactor configuration on the determined yields of HCN and NH3 during pyrolysis. Fuel 2000, 79, 1883-1889. (6) Tan, L. L.; Li, C.-Z. Formation of NOx and SOx precursors during the pyrolysis of coal and biomass. Part II. Effects of experimental conditions on the yields of NOx and SOx precursors from the pyrolysis of a Victorian brown coal. Fuel 2000, 79, 1891-1897. (7) Li, C.-Z.; Tan, L. L. Formation of NOx and SOx precursors during the pyrolysis of coal and biomass. Part III. Further discussion

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Received for review March 6, 2007. Revised manuscript received May 17, 2007. Accepted May 25, 2007. ES0705691 VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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