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Formation of NOx precursors during the pyrolysis of coal and biomass. Part IX. Effects of coal ash and externally loaded-Na on fuel-N conversion during the reforming of coal and biomass in steam
Fuel 85 (2006) 1411–1417 www.fuelfirst.com
Formation of NOx precursors during the pyrolysis of coal and biomass. Part IX. Effects of coal ash and externally loaded-Na on fuel-N conversion during the reforming of coal and biomass in steam Fu-Jun Tian a, Jiang-long Yu a,1, Lachlan J. McKenzie a, Jun-ichiro Hayashi b, Chun-Zhu Li a,* b
a Department of Chemical Engineering, P.O. Box 36, Monash University, Monash, VIC 3800, Australia Centre for Advanced Research of Energy Conversion Materials, Hokkaido University, N13-W8, Kita-ku, Sapporo 060-8628, Japan
Received 27 July 2005; received in revised form 11 January 2006; accepted 11 January 2006 Available online 9 February 2006
Abstract Ash interacts strongly with char and volatiles in a gasifier, especially in a fluidised-bed gasifier. This study aims to investigate the effects of ash or ash-forming species on the conversion of fuel-N during gasification. A Victorian (Loy Yang) brown coal and a sugar cane trash were gasified in two novel fluidised-bed/fixed-bed reactors where the interactions of ash with char and/or volatiles could be selectively investigated. Our results show that the interaction of ash with char and/or volatiles could lead to increases in the yield of NH3 and decreases in the yield of HCN although the increases were not always matched exactly by the decreases. Loading NaCl or Na2CO3 into the brown coal was also found to affect the formation of HCN and NH3 during gasification. In addition to the possible catalytic hydrolysis of HCN into NH3 particularly at high temperatures, two other causes were identified for the changes in the HCN and NH3 yields. It is believed that some ash species could migrate into the char matrix to affect the local availability of H radicals or to catalyse the formation of NH3 selectively. The interactions of ash (or Na loaded into the coal) with volatiles could enhance the formation of soot-N, which would be gasified favourably to form NH3. q 2006 Elsevier Ltd. All rights reserved. Keywords: NH3; HCN; Ash
1. Introduction Conversion of fuel-N has long been the subject of intensive research in order to reduce the emissions of NOx/N2O from the power generation through the combustion/gasification of biomass and coal. Numerous studies [1–14] have identified HCN and NH3 as two major precursors of NOx/N2O during the gasification/combustion of biomass and coal. HCN is formed mainly from the thermal decomposition of the thermally less stable fuel-N structures (e.g. volatile-N) [5,6,8–10,12,13]. NH3 is mainly produced from the primary pyrolysis of coal (or biomass) and/or the hydrogenation of char-N [1,6,8,9,11]. During gasification, the formation of NH3 is favoured by the introduction of steam as a potential source of hydrogen to
* Corresponding author. Tel.: C61 3 9905 9623; fax: C61 3 9905 5686. E-mail address: [email protected] (C.-Z. Li). 1 Present address: Faculty of Energy and Power Engineering, Shenyang Institute of Aeronautical Engineering, 52 Huanghe Bei Ave, Shenyang 110034, Liaoning Province 110034, People’s Republic of China.
0016-2361/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2006.01.008
increase the availability of H radicals during gasification [1,11,12]. During the gasification of a brown coal at 500 8C in 4% O2, which might generate and consume H and other radicals, HCN can become very important: HCN yield is as high as 40% of total coal-N [1]. Hence, it is believed that the availability of H radicals would be a decisive factor for the conversion of fuel-N (especially char-N) into HCN and NH3. NH3 is always found as an important N-containing species from fuel-N in the product gas from an industrial gasifier: NH3 is normally at a much higher concentration than HCN [14,15]. However, the lab-scale pyrolysis and gasification of biomass and coal often indicates that HCN is as an important NOx precursor as NH3 [1,6,8,9,12]. It is clear that, in addition to the direct formation of HCN and NH3 from fuel-N, interconversion routes for HCN to NH3 (or their precursors) must exist and play an important role in the formation of NH3 in a large reactor. The effects on fuel-N conversion of inherent mineral matter in coal (e.g. through demineralising coal [16,17]) and externally loaded catalysts (alkali and alkaline earth metals (AAEM) [16,18] and iron [17,19,20]) have attracted much attention in the literature. The presence of abundant AAEM
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species, often well dispersed at the atomic scale, is an important feature of Victorian brown coal and biomass. During pyrolysis and gasification (in CO2), the catalysts (AAEM and iron species), either inherently present or externally loaded into coal, enhanced the formation of N2 and suppressed the formation of HCN [16–20]. The effects of these catalysts on the NH3 yield are still controversial [17–20]. Upon or during combustion and gasification especially in an industrial system, these inorganic species will be converted into ash and remain in the gasifier/combustor for extended time. The contact between the inorganic species and the fuel (including volatiles) is a particularly important aspect of consideration for a fluidisedbed reactor. However, little is known about if and how ashforming species would affect the distribution of fuel-N during the gasification of biomass and coal in steam. The purpose of this study is to investigate the catalytic effects of ash on the conversion of fuel-N during the gasification of a Victorian brown coal and a cane trash sample in steam using two novel fluidised-bed/fixed-bed reactors. The gasification of Na2CO3-loaded coal and NaCl-loaded coal was also studied in order to better understand the effects of Na as a major catalytic species present in the coal ash on the conversion of fuel-N. 2. Experimental 2.1. Samples A brown coal (Loy Yang, LY, Victoria, Australia) sample and a cane trash (Queensland, Australia) sample were used in this study. The properties of the LY coal and the cane trash are given in Table 1. To clarify the catalytic effects of Na on the conversion of coal-N, high purity grade Na2CO3 (O99.8%, APS Finechem) and NaCl (O99.9%, APS Finechem) were used to prepare Na2CO3-loaded and NaCl-loaded LY coal samples respectively. In both cases, the Na loading level was 0.34 wt% (dry basis). Briefly, following the procedure described elsewhere [21], an accurately weighed amount of Na2CO3 (or NaCl) was dissolved in distilled/deionised water. An accurately weighed amount of coal was added into the solution and the slurry was then gently stirred for 15 min. The slurry was then stirred every 20 min for the following 5 h before it was partially covered with a sheet of Parafilmw plastic film and placed in an oven at 40 8C in air to evaporate the water. NaCl thus loaded into the brown coal is expected to exist in the coal in a manner similar to the NaCl inherently present in the brown coal [22]. However, a significant proportion of Na2CO3 loaded into the
coal is expected to have undergone the ion exchange process and to exist in the form of carboxylates [21,23]. 2.2. Gasification in steam Two reactors were used in this study. The first one was a one-stage fluidised-bed/fixed-bed reactor as described elsewhere [12,13]. The reactor was equipped with two quartz frits. The bottom frit acted as the gas distributor for the fluidised bed of zircon sand and the top frit was placed in the freeboard to prevent the char particles from being elutriated out of the reactor. The reactor was heated with an external furnace and coal particles entrained in an inert gas were fed via a watercooled feeding probe directly into the hot fluidised bed of sand [1,12,13]. Particle heating rates are expected to be no lower than 103–104 K sK1 [1,12,13]. With this reactor configuration, a thin char bed was formed underneath the top frit in the freeboard where volatiles would interact with the char when passing through the char bed and the frit. Therefore, this reactor had combined features of a fluidised-bed reactor (e.g. fast particle heating rates) and of a fixed-bed reactor (e.g. a thin char bed underneath the frit). The presence of the top frit in the freeboard allowed the in situ gasification of char without the need of a particle recirculation facility for this bench-scale study. During the gasification experiments, while argon was used as the feeding and fluidising gas, steam as the gasifying agent was generated inside the reactor by feeding deionised water at a constant flow rate into the reactor (just beneath the gas distribution frit) with an HPLC pump. The steam formed would have been well mixed with the argon fluidising gas before entering the fluidised bed. In all cases, the overall steam concentration for gasification was fixed at 15 vol% of the total gas flow rate. To examine the catalytic effects of ash on NH3 and HCN yields during the gasification of LY coal, a given amount of LY coal was firstly fed into the reactor at 600 8C under pyrolysis conditions. At the same temperature, a stream of air was subsequently directed into the reactor to burn all the char, leaving only the ash (termed as ‘pre-existing ash’ thereafter) in the reactor. In the presence of the ash, the subsequent gasification of LY coal with steam was then carried out by feeding fresh LY coal into the reactor heated at the required temperature. The second reactor was a two-stage fluidised-bed/fixed-bed reactor schematically shown in Fig. 1: a fluidised-bed reactor as the bottom stage in direct tandem with a fixed-bed reactor as the top stage with the two stages being physically separated with a quartz frit [24,25]. The pre-existing ash in this two-stage
Table 1 Properties of LY brown coal and cane trash samples used in this study Sample
LY coal Cane trash a
By difference.
Particle size (mm)
Proximate analysis, wt% (db)
Ultimate analysis, wt% (daf)
Ash
Volatile matter
C
H
N
S
Oa
106–150 125–212
1.0 7.6
51.5 NA
68.5 49.5
4.8 6.1
0.55 0.31
0.32 0.08
25.8 44.0
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3. Results and discussion 3.1. Increases in the yield of NH3 due to the interactions of pre-existing ash with char and volatiles
Fig. 1. A schematic diagram of the two-stage fluidised-bed/fix-bed reactor (modified from Ref. [24]).
reactor was prepared by heating 2.5 g of raw LY coal held in the top stage at 10 K minK1 from room temperature to 600 8C under pyrolysis conditions and then burning the char with air at 600 8C. The bottom stage was identical to the one-stage fluidised-bed/fixed-bed reactor described above and high particle heating rates were again achieved by injecting coal particles directly into the hot fluidised sand bed. The products, including HCN, NH3 and their precursors, from the in situ pyrolysis and gasification of coal or biomass in the bottom stage (fluidised bed) of the reactor would thus enter the top stage (fixed bed) to interact with the pre-existing ash. The difference between the results from the experiments in the presence and absence of the pre-existing ash allows for the quantification of the effects of ash–volatile interactions on the observed HCN and NH3 yields. Different from the one-stage reactor where the pre-existing ash interacted with the gasifying coal/biomass particles, the configuration of the two-stage reactor allowed for the physical separation of the ash particles in the top stage from the gasifying char particles in the bottom stage.
Fig. 2 shows the impacts of pre-existing ash on the NH3 yield during the gasification of LY coal in steam in the onestage fluidised-bed/fixed-bed reactor at 700 8C. The amount of pre-existing ash in the reactor prior to gasification is expressed by the weight of raw LY coal fed into the reactor to produce the ash prior to the actual gasification experiment (see Section 2.2 for more details). The data in Fig. 2 show that the NH3 yield increased significantly with increasing amount of pre-existing ash in the reactor prior to the gasification experiment, increasing from about 40% in the absence of pre-existing ash to about 58% in the presence of the ash from the combustion of 2.5 g of LY coal previously fed. Under all conditions shown in Fig. 2, the gasification experiment lasted for about 4 h. However, some ungasified char particles could still be observed in the reactor at the end of each experiment. This leads to an uncertainty whether the ash only increased NH3 formation rate within the 4-h gasification period or also simultaneously changed the overall selectivity of coal-N/char-N to NH3. Both effects could have resulted in the changes in the observed NH3 yield as shown in Fig. 2 when the gasification of char was not complete. This ambiguity was resolved by carrying out further experiments at a higher temperature of 800 8C at which the complete gasification of the LY coal could be achieved with a long period of holding time (over 10 h). Fig. 3 shows the accumulated yields of NH3 during the gasification of raw LY coal in the presence and absence of pre-existing ash. The first points in Fig. 3 (and other figures with ‘reaction time’ on horizontal axis) always refer to the ‘feeding’ periods when coal (or cane trash) particles were continuously fed into the reactor. The other points refer to the ‘not-feeding’ periods after the feeding of coal or cane trash had
2.3. Quantification of NH3 and HCN The product gas from the reactor passed through bubblers containing an absorption solution. NH3 in the product gas was absorbed in a 0.02 M CH3SO3H solution and HCN was absorbed in a 0.1 M NaOH solution in separate experiments. HCN (or NH3) was collected into reaction-time-resolved fractions by rapidly changing the absorption bubblers during an experiment. NH3 and HCN were quantified using a Dionex 500 ion chromatograph with two separate analytical methods following the procedures developed previously [8,9].
Fig. 2. Effects of pre-existing ash on NH3 yield during the gasification of LY brown coal in steam in the one-stage fluidised-bed/fixed-bed reactor at 700 8C. The amount of pre-existing ash in the reactor is measured by the amount of coal fed into the reactor (shown in x-axis as ‘weight of fed coal’) to produce coal ash prior to gasification.
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NaCl [28]. Fig. 4 shows the accumulated NH3 yields with gasification time during the gasification of Na-loaded LY coal samples at 700 and 800 8C in steam. The loading of Na2CO3 into LY coal has clearly increased the yield of NH3 during the gasification at 700 8C as well as at 800 8C. While loading NaCl into coal did not lead to significant increases in the NH3 yield during the gasification at 700 8C, it significantly increased the NH3 yield during the gasification at 800 8C. The less pronounced catalytic effects of Na loaded into coal as NaCl during the gasification at 700 8C are probably due to the higher retention of Cl as NaCl in the char at 700 8C, which has little catalytic activity [23]. It is clear from Fig. 4 that Na in the brown coal, both as Na carboxylates and as NaCl, could increase its NH3 yield during gasification in steam. In other words, Na is likely to be an important species in the ash that has caused changes in the observed NH3 yield (Figs. 2 and 3). The corresponding HCN yields from the gasification of the Na2CO3-loaded LY coal in steam are shown in Fig. 5. The loading of Na2CO3 into the brown coal reduced its HCN yield by about 3% at 700 8C (Fig. 5). The reduction in HCN yield due to the loaded Na2CO 3 increased to about 10% when temperature was increased to 900 8C. It is clear that the Na2CO3 loaded into the coal has caused increases in the yield of NH3 and simultaneous decreases in the yield of HCN. The data in Figs. 2–5 show that the presence of pre-existing ash in the bed or the loading of Na into the coal can lead to significant changes in the selectivity of fuel-N to HCN and NH3. The understanding of these data may require the Fig. 3. Accumulated NH3 yield as a function of reaction time during the gasification of LY brown coal in steam in the one-stage fluidised-bed/fixed-bed reactor. (a) 700 8C; (b) 800 8C.
stopped. The results in Fig. 3 indicate that the final NH3 yields at 800 8C both in the presence and absence of pre-existing ash were very similar to their counterparts at 700 8C, respectively. These results confirm that the species in the pre-existing ash in the reactor prior to gasification could increase the total NH3 yield from the gasification of LY coal by increasing the selectivity of coal-N/char-N to NH3. 3.2. Changes in NH3 and HCN yields due to Na externally loaded into coal The main ash-forming species in the Loy Yang brown coal are AAEM species such as Na, Mg and Ca. The Loy Yang brown coal used in this study contained 0.1% Na, 0.09% Ca, 0.08% Mg, 0.01% Al and 0.14% Fe (with negligible amount of FeS2). These AAEM species are catalysts for the gasification of coal in steam [23,26,27]. Among the AAEM species, Na is a particularly good catalyst and also easy to volatilise during pyrolysis and gasification, which means that Na is likely to be widely distributed in a fluidised-bed gasifier. Based on this consideration, attempts were made using the Na2CO3-loaded LY coal and NaCl-loaded LY coal to understand the catalytic effects of Na on coal-N conversion because Na in coal exists in coal either in the form of ion-exchangeable Na carboxylates or
Fig. 4. Accumulated NH3 yield as a function of reaction time during the gasification of LY raw coal and Na-loaded coal in steam in the one-stage fluidised-bed/fixed-bed reactor. (a) 700 8C; (b) 800 8C.
F.-J. Tian et al. / Fuel 85 (2006) 1411–1417
Fig. 5. Accumulated HCN yield as a function of reaction time during the gasification of LY raw coal in steam in the one-stage fluidised-bed/fixed-bed reactor at 700 and 900 8C.
consideration of the specific configuration of the one-stage fluidised-bed/fixed-bed reactor used to obtain the data. With this reactor, a significant portion of char particles would have been suspended in the freeboard zone of the fluidised bed and/ or held just underneath the top frit of this reactor to form a thin char bed. The fine ash particles would have been in direct contact with the coal/char particles held underneath the frit. While transportation of AAEM species from ash particles to char particles may be hard if only through solid–solid contact, volatilisation/evaporation of the AAEM species (particularly Na) could assist the migration of AAEM species from ash onto char particles in the present study and thus exert catalytic effects on the gasification. Indeed, Takarada and co-workers [29] observed the transportation/migration of potassium catalyst from a brown coal char to a bituminous coal char during the gasification of two physically-blended coals in steam at temperature from 600 to 700 8C. This provides a plausible explanation why the pre-existing ash in the bed (Figs. 2 and 5) could show similar effects to the Na loaded into the coal matrix.
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bed held underneath the frit and the soot-N is subsequently converted into NH3. As the configuration of the one-stage fluidised-bed/fixed-bed reactor strongly encourages the interactions of char (AAEM species and/or pre-existing ash) and volatiles, the results in Figs. 2–5 obtained using this reactor do not allow for a clear understanding of the mechanisms by which ash/Na affects the pathways of fuel-N during pyrolysis and gasification. In other words, a clear understanding of the relative importance of these three possible causes for the changes in the selectivity of fuel-N to HCN and NH3 requires the separation of the reactions in the solid phase from those involving gaseous species. In this study, this was achieved by using a two-stage fluidised-bed/fixed-bed reactor. As was detailed in Section 2.2, this reactor configuration allows for the investigation of the interactions between the pre-existing ash and the nascent volatiles (gaseous products) in isolation from the corresponding char particles. Fig. 6 presents the accumulated HCN and NH3 yields as functions of gasification time during the gasification of LY brown coal in steam with or without pre-existing ash in the top stage of the two-stage reactor. Both at 700 and 800 8C, the ash has caused the NH3 yield to increase and the HCN yield to decrease, in broad agreement with the results obtained using the one-stage fluidised-bed/fixed-bed reactor (Figs. 4 and 5). The data in Fig. 6 also indicate that, while the increase in the NH3 yield appears to be balanced by the decrease in the HCN yield at 800 8C (Fig. 6b), the change in the yield of NH3 seems
3.3. Further understanding the changes in the selectivity of HCN and NH3 There are three possible causes for the changes in the selectivity of fuel-N to HCN and NH3 observed in Figs. 2–5. Firstly, the ash or Na in coal/char may catalyse the hydrolysis of HCN [30,31] to form NH3. Secondly, the ash or Na in coal/ char may catalyse the gasification of coal in steam, enhancing the concentration of H radicals on the coal/char surface to favour the formation of NH3 because the formation of NH3 is largely dependent on the availability of H radicals [6,8–12]. Alternatively, the species in the ash or the Na loaded into the coal may have selectively catalysed the formation of NH3. In either case, the catalytic effect mainly originates from the reaction in/on the solid char. Thirdly, the ash or Na in coal/char may catalyse the formation of soot from the precursors of HCN in the volatiles as the volatiles passed through the thin char/ash
Fig. 6. Accumulated NH3 and HCN yields as functions of reaction time during the gasification of LY brown coal in steam in the two-stage fluidised-bed/fixedbed reactor. (a) 700 8C; (b) 800 8C.
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to be bigger than that of HCN at 700 8C (Fig. 6a). These data indicate that, while no evidence is available to ascertain the importance of catalysed HCN hydrolysis for the formation of NH3, the catalysed hydrolysis of HCN is not the only cause for the changes in the selectivity of fuel-N to NH3 and HCN, certainly at 700 8C (Fig. 6a). In fact, based on the study by Scha¨fer and Bonn [30,31], we estimate that the short residence time of HCN and steam in reaction zone in our reactor system at 800 8C or lower does not seem to allow the homogeneous or heterogeneous hydrolysis of HCN to take place to very significant extents. However, the hydrolysis of HCN at 900 8C may be very significant. In fact, at low temperatures (e.g. 700 8C), the volatiles may not have been reformed by steam completely and would then form soot on the catalytic species on ash or char to deactivate the catalysts for the hydrolysis of HCN. The data in Figs. 2 and 6a also indicate that placing the same amount of pre-existing ash (from the combustion of 2.5 g of raw LY coal) in the fluidised bed has caused a much larger NH3 yield increase (up to 18% of coal-N, Fig. 1) than placing the ash in the top stage (around 10%, Fig. 6a) at 700 8C. Clearly, the interactions between the ash and gasifying char, most likely due to the migration of AAEM species from ash into the char e.g. to enhance the H availability as mentioned above, have caused additional NH3 formation. Further examination of the data in Fig. 6 revealed that the increases in the NH3 yield and the decreases in the HCN yield took place mainly during the ‘feeding’ periods (the first datum points), indicating a possible link with the fate of volatile-N. It is believed that the interactions between the un-reformed volatiles (more abundant at 700 8C) and ash condensed volatile-N into soot-N (or its precursors) and that the soot-N was then hydrogenated to favourably form NH3 in the presence of steam. This agrees with our previous data [1,11,12] indicating that the gasification of char-N/soot-N would produce NH3 more favourably than HCN. In the absence of ash in the top stage of reactor during gasification of LY coal in steam at 700 or 800 8C, most of un-reformed volatile-N would have exited the reactor as tar-N. In other words, a part of the increased NH3 yield in Fig. 6a is at expense of volatile-N (tarN). As volatile-N is an important source of HCN, the formation of soot-N would necessarily reduce the yield of HCN. The formation and subsequent gasification of soot-N to form NH3 was further investigated with the gasification of cane trash in steam, during which massive amount of volatiles (up to 80 wt% of cane trash) would be produced. The results for the cane trash sample under identical conditions as those in Fig. 6 for LY coal are shown in Fig. 7, in which the pre-existing ash was also prepared from 2.5 g of LY coal, not cane trash, for easy comparison of the data in Figs. 6 and 7. As expected, the presence of the pre-existing ash in the top stage has resulted in much bigger increases in the yield of NH3 during the gasification of cane trash (Fig. 7) than during the gasification of LY brown coal (Fig. 6). As was in the case of LY coal (Fig. 6), the increases in the yield of NH3 were much bigger than the corresponding decreases in the yield of HCN for the gasification of cane trash. Again, the main changes in the yields
Fig. 7. Accumulated NH3 and HCN yields as functions of reaction time during the gasification of cane trash in steam in the two-stage fluidised-bed/fixed-bed reactor. (a) 700 8C; (b) 800 8C.
of HCN and NH3 took place during the ‘feeding’ periods. Therefore, the data on cane trash (Fig. 7) appear to provide stronger evidence for the importance of the formation and subsequent gasification of soot-N for the formation of NH3 at the expenses of HCN. 4. Conclusions Impacts of ash on the formation of NH3 and HCN during the gasification of Victorian (Loy Yang) brown coal and sugar cane trash in 15% steam were investigated using two novel fluidised-bed/fixed-bed reactors. Loading NaCl or Na2CO3 into the brown coal was also found to affect the formation of HCN and NH3 during the gasification of the brown coal. The interaction of ash with char and/or volatiles (or Na loaded into the coal) tended to result in increases in the yield of NH3 and decreases in the yield of HCN although the increases were not always matched exactly by the decreases. In addition to the possible catalytic hydrolysis of HCN into NH3 particularly at high temperatures, two other routes were revealed through which the selectivity of fuel-N to HCN and NH3 could be altered by ash or Na loaded into the coal. The first route is due to the catalytic effects of ash or loaded Na on the gasification reactions in/on the solid char, possibly by enhancing the availability of H radicals for the hydrogenation of char-N or by catalysing the hydrogenation of char-N itself for the formation
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of NH3. It is believed that some species (e.g. Na) of the ash in the fluidised bed have migrated into the coal/char matrix. The second route is the formation of soot-N from volatile-N and the subsequent gasification of soot-N by steam to form NH3. Acknowledgements The authors gratefully acknowledge the financial support of this study by the New Energy and Industrial Technology Development Organisation (NEDO) in Japan. F-J Tian also gratefully acknowledges the MIPRS and MGS scholarships from Monash University. References [1] Chang L, Xie Z, Xie K-C, Pratt KC, Hayashi J-i, Chiba T, et al. Fuel 2003; 82:1159. [2] Hamalainen JP, Aho MJ. Fuel 1995;74:1922. [3] Hamalainen JP, Aho MJ. Fuel 1996;75:1377. [4] Leppalahti J. Fuel 1995;74:1363. [5] Ledesma EB, Li C-Z, Nelson PF, Mackie JC. Energy Fuels 1998;12:536. [6] Li C-Z, Tan LL. Fuel 2000;79:1899. [7] Nelson PF, Kelly MD, Wornat MJ. Fuel 1991;70:403. [8] Tan LL, Li C-Z. Fuel 2000;79:1883. [9] Tan LL, Li C-Z. Fuel 2000;79:1891. [10] Tian F-J, Li BQ, Chen Y, Li C-Z. Fuel 2002;81:2203.
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Formation of NOx precursors during the pyrolysis of coal and biomass. Part IX. Effects of coal ash and externally loaded-Na on fuel-N conversion during the reforming of coal and biomass in steam Fu-Jun Tian a, Jiang-long Yu a,1, Lachlan J. McKenzie a, Jun-ichiro Hayashi b, Chun-Zhu Li a,* b
a Department of Chemical Engineering, P.O. Box 36, Monash University, Monash, VIC 3800, Australia Centre for Advanced Research of Energy Conversion Materials, Hokkaido University, N13-W8, Kita-ku, Sapporo 060-8628, Japan
Received 27 July 2005; received in revised form 11 January 2006; accepted 11 January 2006 Available online 9 February 2006
Abstract Ash interacts strongly with char and volatiles in a gasifier, especially in a fluidised-bed gasifier. This study aims to investigate the effects of ash or ash-forming species on the conversion of fuel-N during gasification. A Victorian (Loy Yang) brown coal and a sugar cane trash were gasified in two novel fluidised-bed/fixed-bed reactors where the interactions of ash with char and/or volatiles could be selectively investigated. Our results show that the interaction of ash with char and/or volatiles could lead to increases in the yield of NH3 and decreases in the yield of HCN although the increases were not always matched exactly by the decreases. Loading NaCl or Na2CO3 into the brown coal was also found to affect the formation of HCN and NH3 during gasification. In addition to the possible catalytic hydrolysis of HCN into NH3 particularly at high temperatures, two other causes were identified for the changes in the HCN and NH3 yields. It is believed that some ash species could migrate into the char matrix to affect the local availability of H radicals or to catalyse the formation of NH3 selectively. The interactions of ash (or Na loaded into the coal) with volatiles could enhance the formation of soot-N, which would be gasified favourably to form NH3. q 2006 Elsevier Ltd. All rights reserved. Keywords: NH3; HCN; Ash
1. Introduction Conversion of fuel-N has long been the subject of intensive research in order to reduce the emissions of NOx/N2O from the power generation through the combustion/gasification of biomass and coal. Numerous studies [1–14] have identified HCN and NH3 as two major precursors of NOx/N2O during the gasification/combustion of biomass and coal. HCN is formed mainly from the thermal decomposition of the thermally less stable fuel-N structures (e.g. volatile-N) [5,6,8–10,12,13]. NH3 is mainly produced from the primary pyrolysis of coal (or biomass) and/or the hydrogenation of char-N [1,6,8,9,11]. During gasification, the formation of NH3 is favoured by the introduction of steam as a potential source of hydrogen to
* Corresponding author. Tel.: C61 3 9905 9623; fax: C61 3 9905 5686. E-mail address: [email protected] (C.-Z. Li). 1 Present address: Faculty of Energy and Power Engineering, Shenyang Institute of Aeronautical Engineering, 52 Huanghe Bei Ave, Shenyang 110034, Liaoning Province 110034, People’s Republic of China.
0016-2361/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2006.01.008
increase the availability of H radicals during gasification [1,11,12]. During the gasification of a brown coal at 500 8C in 4% O2, which might generate and consume H and other radicals, HCN can become very important: HCN yield is as high as 40% of total coal-N [1]. Hence, it is believed that the availability of H radicals would be a decisive factor for the conversion of fuel-N (especially char-N) into HCN and NH3. NH3 is always found as an important N-containing species from fuel-N in the product gas from an industrial gasifier: NH3 is normally at a much higher concentration than HCN [14,15]. However, the lab-scale pyrolysis and gasification of biomass and coal often indicates that HCN is as an important NOx precursor as NH3 [1,6,8,9,12]. It is clear that, in addition to the direct formation of HCN and NH3 from fuel-N, interconversion routes for HCN to NH3 (or their precursors) must exist and play an important role in the formation of NH3 in a large reactor. The effects on fuel-N conversion of inherent mineral matter in coal (e.g. through demineralising coal [16,17]) and externally loaded catalysts (alkali and alkaline earth metals (AAEM) [16,18] and iron [17,19,20]) have attracted much attention in the literature. The presence of abundant AAEM
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species, often well dispersed at the atomic scale, is an important feature of Victorian brown coal and biomass. During pyrolysis and gasification (in CO2), the catalysts (AAEM and iron species), either inherently present or externally loaded into coal, enhanced the formation of N2 and suppressed the formation of HCN [16–20]. The effects of these catalysts on the NH3 yield are still controversial [17–20]. Upon or during combustion and gasification especially in an industrial system, these inorganic species will be converted into ash and remain in the gasifier/combustor for extended time. The contact between the inorganic species and the fuel (including volatiles) is a particularly important aspect of consideration for a fluidisedbed reactor. However, little is known about if and how ashforming species would affect the distribution of fuel-N during the gasification of biomass and coal in steam. The purpose of this study is to investigate the catalytic effects of ash on the conversion of fuel-N during the gasification of a Victorian brown coal and a cane trash sample in steam using two novel fluidised-bed/fixed-bed reactors. The gasification of Na2CO3-loaded coal and NaCl-loaded coal was also studied in order to better understand the effects of Na as a major catalytic species present in the coal ash on the conversion of fuel-N. 2. Experimental 2.1. Samples A brown coal (Loy Yang, LY, Victoria, Australia) sample and a cane trash (Queensland, Australia) sample were used in this study. The properties of the LY coal and the cane trash are given in Table 1. To clarify the catalytic effects of Na on the conversion of coal-N, high purity grade Na2CO3 (O99.8%, APS Finechem) and NaCl (O99.9%, APS Finechem) were used to prepare Na2CO3-loaded and NaCl-loaded LY coal samples respectively. In both cases, the Na loading level was 0.34 wt% (dry basis). Briefly, following the procedure described elsewhere [21], an accurately weighed amount of Na2CO3 (or NaCl) was dissolved in distilled/deionised water. An accurately weighed amount of coal was added into the solution and the slurry was then gently stirred for 15 min. The slurry was then stirred every 20 min for the following 5 h before it was partially covered with a sheet of Parafilmw plastic film and placed in an oven at 40 8C in air to evaporate the water. NaCl thus loaded into the brown coal is expected to exist in the coal in a manner similar to the NaCl inherently present in the brown coal [22]. However, a significant proportion of Na2CO3 loaded into the
coal is expected to have undergone the ion exchange process and to exist in the form of carboxylates [21,23]. 2.2. Gasification in steam Two reactors were used in this study. The first one was a one-stage fluidised-bed/fixed-bed reactor as described elsewhere [12,13]. The reactor was equipped with two quartz frits. The bottom frit acted as the gas distributor for the fluidised bed of zircon sand and the top frit was placed in the freeboard to prevent the char particles from being elutriated out of the reactor. The reactor was heated with an external furnace and coal particles entrained in an inert gas were fed via a watercooled feeding probe directly into the hot fluidised bed of sand [1,12,13]. Particle heating rates are expected to be no lower than 103–104 K sK1 [1,12,13]. With this reactor configuration, a thin char bed was formed underneath the top frit in the freeboard where volatiles would interact with the char when passing through the char bed and the frit. Therefore, this reactor had combined features of a fluidised-bed reactor (e.g. fast particle heating rates) and of a fixed-bed reactor (e.g. a thin char bed underneath the frit). The presence of the top frit in the freeboard allowed the in situ gasification of char without the need of a particle recirculation facility for this bench-scale study. During the gasification experiments, while argon was used as the feeding and fluidising gas, steam as the gasifying agent was generated inside the reactor by feeding deionised water at a constant flow rate into the reactor (just beneath the gas distribution frit) with an HPLC pump. The steam formed would have been well mixed with the argon fluidising gas before entering the fluidised bed. In all cases, the overall steam concentration for gasification was fixed at 15 vol% of the total gas flow rate. To examine the catalytic effects of ash on NH3 and HCN yields during the gasification of LY coal, a given amount of LY coal was firstly fed into the reactor at 600 8C under pyrolysis conditions. At the same temperature, a stream of air was subsequently directed into the reactor to burn all the char, leaving only the ash (termed as ‘pre-existing ash’ thereafter) in the reactor. In the presence of the ash, the subsequent gasification of LY coal with steam was then carried out by feeding fresh LY coal into the reactor heated at the required temperature. The second reactor was a two-stage fluidised-bed/fixed-bed reactor schematically shown in Fig. 1: a fluidised-bed reactor as the bottom stage in direct tandem with a fixed-bed reactor as the top stage with the two stages being physically separated with a quartz frit [24,25]. The pre-existing ash in this two-stage
Table 1 Properties of LY brown coal and cane trash samples used in this study Sample
LY coal Cane trash a
By difference.
Particle size (mm)
Proximate analysis, wt% (db)
Ultimate analysis, wt% (daf)
Ash
Volatile matter
C
H
N
S
Oa
106–150 125–212
1.0 7.6
51.5 NA
68.5 49.5
4.8 6.1
0.55 0.31
0.32 0.08
25.8 44.0
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3. Results and discussion 3.1. Increases in the yield of NH3 due to the interactions of pre-existing ash with char and volatiles
Fig. 1. A schematic diagram of the two-stage fluidised-bed/fix-bed reactor (modified from Ref. [24]).
reactor was prepared by heating 2.5 g of raw LY coal held in the top stage at 10 K minK1 from room temperature to 600 8C under pyrolysis conditions and then burning the char with air at 600 8C. The bottom stage was identical to the one-stage fluidised-bed/fixed-bed reactor described above and high particle heating rates were again achieved by injecting coal particles directly into the hot fluidised sand bed. The products, including HCN, NH3 and their precursors, from the in situ pyrolysis and gasification of coal or biomass in the bottom stage (fluidised bed) of the reactor would thus enter the top stage (fixed bed) to interact with the pre-existing ash. The difference between the results from the experiments in the presence and absence of the pre-existing ash allows for the quantification of the effects of ash–volatile interactions on the observed HCN and NH3 yields. Different from the one-stage reactor where the pre-existing ash interacted with the gasifying coal/biomass particles, the configuration of the two-stage reactor allowed for the physical separation of the ash particles in the top stage from the gasifying char particles in the bottom stage.
Fig. 2 shows the impacts of pre-existing ash on the NH3 yield during the gasification of LY coal in steam in the onestage fluidised-bed/fixed-bed reactor at 700 8C. The amount of pre-existing ash in the reactor prior to gasification is expressed by the weight of raw LY coal fed into the reactor to produce the ash prior to the actual gasification experiment (see Section 2.2 for more details). The data in Fig. 2 show that the NH3 yield increased significantly with increasing amount of pre-existing ash in the reactor prior to the gasification experiment, increasing from about 40% in the absence of pre-existing ash to about 58% in the presence of the ash from the combustion of 2.5 g of LY coal previously fed. Under all conditions shown in Fig. 2, the gasification experiment lasted for about 4 h. However, some ungasified char particles could still be observed in the reactor at the end of each experiment. This leads to an uncertainty whether the ash only increased NH3 formation rate within the 4-h gasification period or also simultaneously changed the overall selectivity of coal-N/char-N to NH3. Both effects could have resulted in the changes in the observed NH3 yield as shown in Fig. 2 when the gasification of char was not complete. This ambiguity was resolved by carrying out further experiments at a higher temperature of 800 8C at which the complete gasification of the LY coal could be achieved with a long period of holding time (over 10 h). Fig. 3 shows the accumulated yields of NH3 during the gasification of raw LY coal in the presence and absence of pre-existing ash. The first points in Fig. 3 (and other figures with ‘reaction time’ on horizontal axis) always refer to the ‘feeding’ periods when coal (or cane trash) particles were continuously fed into the reactor. The other points refer to the ‘not-feeding’ periods after the feeding of coal or cane trash had
2.3. Quantification of NH3 and HCN The product gas from the reactor passed through bubblers containing an absorption solution. NH3 in the product gas was absorbed in a 0.02 M CH3SO3H solution and HCN was absorbed in a 0.1 M NaOH solution in separate experiments. HCN (or NH3) was collected into reaction-time-resolved fractions by rapidly changing the absorption bubblers during an experiment. NH3 and HCN were quantified using a Dionex 500 ion chromatograph with two separate analytical methods following the procedures developed previously [8,9].
Fig. 2. Effects of pre-existing ash on NH3 yield during the gasification of LY brown coal in steam in the one-stage fluidised-bed/fixed-bed reactor at 700 8C. The amount of pre-existing ash in the reactor is measured by the amount of coal fed into the reactor (shown in x-axis as ‘weight of fed coal’) to produce coal ash prior to gasification.
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NaCl [28]. Fig. 4 shows the accumulated NH3 yields with gasification time during the gasification of Na-loaded LY coal samples at 700 and 800 8C in steam. The loading of Na2CO3 into LY coal has clearly increased the yield of NH3 during the gasification at 700 8C as well as at 800 8C. While loading NaCl into coal did not lead to significant increases in the NH3 yield during the gasification at 700 8C, it significantly increased the NH3 yield during the gasification at 800 8C. The less pronounced catalytic effects of Na loaded into coal as NaCl during the gasification at 700 8C are probably due to the higher retention of Cl as NaCl in the char at 700 8C, which has little catalytic activity [23]. It is clear from Fig. 4 that Na in the brown coal, both as Na carboxylates and as NaCl, could increase its NH3 yield during gasification in steam. In other words, Na is likely to be an important species in the ash that has caused changes in the observed NH3 yield (Figs. 2 and 3). The corresponding HCN yields from the gasification of the Na2CO3-loaded LY coal in steam are shown in Fig. 5. The loading of Na2CO3 into the brown coal reduced its HCN yield by about 3% at 700 8C (Fig. 5). The reduction in HCN yield due to the loaded Na2CO 3 increased to about 10% when temperature was increased to 900 8C. It is clear that the Na2CO3 loaded into the coal has caused increases in the yield of NH3 and simultaneous decreases in the yield of HCN. The data in Figs. 2–5 show that the presence of pre-existing ash in the bed or the loading of Na into the coal can lead to significant changes in the selectivity of fuel-N to HCN and NH3. The understanding of these data may require the Fig. 3. Accumulated NH3 yield as a function of reaction time during the gasification of LY brown coal in steam in the one-stage fluidised-bed/fixed-bed reactor. (a) 700 8C; (b) 800 8C.
stopped. The results in Fig. 3 indicate that the final NH3 yields at 800 8C both in the presence and absence of pre-existing ash were very similar to their counterparts at 700 8C, respectively. These results confirm that the species in the pre-existing ash in the reactor prior to gasification could increase the total NH3 yield from the gasification of LY coal by increasing the selectivity of coal-N/char-N to NH3. 3.2. Changes in NH3 and HCN yields due to Na externally loaded into coal The main ash-forming species in the Loy Yang brown coal are AAEM species such as Na, Mg and Ca. The Loy Yang brown coal used in this study contained 0.1% Na, 0.09% Ca, 0.08% Mg, 0.01% Al and 0.14% Fe (with negligible amount of FeS2). These AAEM species are catalysts for the gasification of coal in steam [23,26,27]. Among the AAEM species, Na is a particularly good catalyst and also easy to volatilise during pyrolysis and gasification, which means that Na is likely to be widely distributed in a fluidised-bed gasifier. Based on this consideration, attempts were made using the Na2CO3-loaded LY coal and NaCl-loaded LY coal to understand the catalytic effects of Na on coal-N conversion because Na in coal exists in coal either in the form of ion-exchangeable Na carboxylates or
Fig. 4. Accumulated NH3 yield as a function of reaction time during the gasification of LY raw coal and Na-loaded coal in steam in the one-stage fluidised-bed/fixed-bed reactor. (a) 700 8C; (b) 800 8C.
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Fig. 5. Accumulated HCN yield as a function of reaction time during the gasification of LY raw coal in steam in the one-stage fluidised-bed/fixed-bed reactor at 700 and 900 8C.
consideration of the specific configuration of the one-stage fluidised-bed/fixed-bed reactor used to obtain the data. With this reactor, a significant portion of char particles would have been suspended in the freeboard zone of the fluidised bed and/ or held just underneath the top frit of this reactor to form a thin char bed. The fine ash particles would have been in direct contact with the coal/char particles held underneath the frit. While transportation of AAEM species from ash particles to char particles may be hard if only through solid–solid contact, volatilisation/evaporation of the AAEM species (particularly Na) could assist the migration of AAEM species from ash onto char particles in the present study and thus exert catalytic effects on the gasification. Indeed, Takarada and co-workers [29] observed the transportation/migration of potassium catalyst from a brown coal char to a bituminous coal char during the gasification of two physically-blended coals in steam at temperature from 600 to 700 8C. This provides a plausible explanation why the pre-existing ash in the bed (Figs. 2 and 5) could show similar effects to the Na loaded into the coal matrix.
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bed held underneath the frit and the soot-N is subsequently converted into NH3. As the configuration of the one-stage fluidised-bed/fixed-bed reactor strongly encourages the interactions of char (AAEM species and/or pre-existing ash) and volatiles, the results in Figs. 2–5 obtained using this reactor do not allow for a clear understanding of the mechanisms by which ash/Na affects the pathways of fuel-N during pyrolysis and gasification. In other words, a clear understanding of the relative importance of these three possible causes for the changes in the selectivity of fuel-N to HCN and NH3 requires the separation of the reactions in the solid phase from those involving gaseous species. In this study, this was achieved by using a two-stage fluidised-bed/fixed-bed reactor. As was detailed in Section 2.2, this reactor configuration allows for the investigation of the interactions between the pre-existing ash and the nascent volatiles (gaseous products) in isolation from the corresponding char particles. Fig. 6 presents the accumulated HCN and NH3 yields as functions of gasification time during the gasification of LY brown coal in steam with or without pre-existing ash in the top stage of the two-stage reactor. Both at 700 and 800 8C, the ash has caused the NH3 yield to increase and the HCN yield to decrease, in broad agreement with the results obtained using the one-stage fluidised-bed/fixed-bed reactor (Figs. 4 and 5). The data in Fig. 6 also indicate that, while the increase in the NH3 yield appears to be balanced by the decrease in the HCN yield at 800 8C (Fig. 6b), the change in the yield of NH3 seems
3.3. Further understanding the changes in the selectivity of HCN and NH3 There are three possible causes for the changes in the selectivity of fuel-N to HCN and NH3 observed in Figs. 2–5. Firstly, the ash or Na in coal/char may catalyse the hydrolysis of HCN [30,31] to form NH3. Secondly, the ash or Na in coal/ char may catalyse the gasification of coal in steam, enhancing the concentration of H radicals on the coal/char surface to favour the formation of NH3 because the formation of NH3 is largely dependent on the availability of H radicals [6,8–12]. Alternatively, the species in the ash or the Na loaded into the coal may have selectively catalysed the formation of NH3. In either case, the catalytic effect mainly originates from the reaction in/on the solid char. Thirdly, the ash or Na in coal/char may catalyse the formation of soot from the precursors of HCN in the volatiles as the volatiles passed through the thin char/ash
Fig. 6. Accumulated NH3 and HCN yields as functions of reaction time during the gasification of LY brown coal in steam in the two-stage fluidised-bed/fixedbed reactor. (a) 700 8C; (b) 800 8C.
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to be bigger than that of HCN at 700 8C (Fig. 6a). These data indicate that, while no evidence is available to ascertain the importance of catalysed HCN hydrolysis for the formation of NH3, the catalysed hydrolysis of HCN is not the only cause for the changes in the selectivity of fuel-N to NH3 and HCN, certainly at 700 8C (Fig. 6a). In fact, based on the study by Scha¨fer and Bonn [30,31], we estimate that the short residence time of HCN and steam in reaction zone in our reactor system at 800 8C or lower does not seem to allow the homogeneous or heterogeneous hydrolysis of HCN to take place to very significant extents. However, the hydrolysis of HCN at 900 8C may be very significant. In fact, at low temperatures (e.g. 700 8C), the volatiles may not have been reformed by steam completely and would then form soot on the catalytic species on ash or char to deactivate the catalysts for the hydrolysis of HCN. The data in Figs. 2 and 6a also indicate that placing the same amount of pre-existing ash (from the combustion of 2.5 g of raw LY coal) in the fluidised bed has caused a much larger NH3 yield increase (up to 18% of coal-N, Fig. 1) than placing the ash in the top stage (around 10%, Fig. 6a) at 700 8C. Clearly, the interactions between the ash and gasifying char, most likely due to the migration of AAEM species from ash into the char e.g. to enhance the H availability as mentioned above, have caused additional NH3 formation. Further examination of the data in Fig. 6 revealed that the increases in the NH3 yield and the decreases in the HCN yield took place mainly during the ‘feeding’ periods (the first datum points), indicating a possible link with the fate of volatile-N. It is believed that the interactions between the un-reformed volatiles (more abundant at 700 8C) and ash condensed volatile-N into soot-N (or its precursors) and that the soot-N was then hydrogenated to favourably form NH3 in the presence of steam. This agrees with our previous data [1,11,12] indicating that the gasification of char-N/soot-N would produce NH3 more favourably than HCN. In the absence of ash in the top stage of reactor during gasification of LY coal in steam at 700 or 800 8C, most of un-reformed volatile-N would have exited the reactor as tar-N. In other words, a part of the increased NH3 yield in Fig. 6a is at expense of volatile-N (tarN). As volatile-N is an important source of HCN, the formation of soot-N would necessarily reduce the yield of HCN. The formation and subsequent gasification of soot-N to form NH3 was further investigated with the gasification of cane trash in steam, during which massive amount of volatiles (up to 80 wt% of cane trash) would be produced. The results for the cane trash sample under identical conditions as those in Fig. 6 for LY coal are shown in Fig. 7, in which the pre-existing ash was also prepared from 2.5 g of LY coal, not cane trash, for easy comparison of the data in Figs. 6 and 7. As expected, the presence of the pre-existing ash in the top stage has resulted in much bigger increases in the yield of NH3 during the gasification of cane trash (Fig. 7) than during the gasification of LY brown coal (Fig. 6). As was in the case of LY coal (Fig. 6), the increases in the yield of NH3 were much bigger than the corresponding decreases in the yield of HCN for the gasification of cane trash. Again, the main changes in the yields
Fig. 7. Accumulated NH3 and HCN yields as functions of reaction time during the gasification of cane trash in steam in the two-stage fluidised-bed/fixed-bed reactor. (a) 700 8C; (b) 800 8C.
of HCN and NH3 took place during the ‘feeding’ periods. Therefore, the data on cane trash (Fig. 7) appear to provide stronger evidence for the importance of the formation and subsequent gasification of soot-N for the formation of NH3 at the expenses of HCN. 4. Conclusions Impacts of ash on the formation of NH3 and HCN during the gasification of Victorian (Loy Yang) brown coal and sugar cane trash in 15% steam were investigated using two novel fluidised-bed/fixed-bed reactors. Loading NaCl or Na2CO3 into the brown coal was also found to affect the formation of HCN and NH3 during the gasification of the brown coal. The interaction of ash with char and/or volatiles (or Na loaded into the coal) tended to result in increases in the yield of NH3 and decreases in the yield of HCN although the increases were not always matched exactly by the decreases. In addition to the possible catalytic hydrolysis of HCN into NH3 particularly at high temperatures, two other routes were revealed through which the selectivity of fuel-N to HCN and NH3 could be altered by ash or Na loaded into the coal. The first route is due to the catalytic effects of ash or loaded Na on the gasification reactions in/on the solid char, possibly by enhancing the availability of H radicals for the hydrogenation of char-N or by catalysing the hydrogenation of char-N itself for the formation
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of NH3. It is believed that some species (e.g. Na) of the ash in the fluidised bed have migrated into the coal/char matrix. The second route is the formation of soot-N from volatile-N and the subsequent gasification of soot-N by steam to form NH3. Acknowledgements The authors gratefully acknowledge the financial support of this study by the New Energy and Industrial Technology Development Organisation (NEDO) in Japan. F-J Tian also gratefully acknowledges the MIPRS and MGS scholarships from Monash University. References [1] Chang L, Xie Z, Xie K-C, Pratt KC, Hayashi J-i, Chiba T, et al. Fuel 2003; 82:1159. [2] Hamalainen JP, Aho MJ. Fuel 1995;74:1922. [3] Hamalainen JP, Aho MJ. Fuel 1996;75:1377. [4] Leppalahti J. Fuel 1995;74:1363. [5] Ledesma EB, Li C-Z, Nelson PF, Mackie JC. Energy Fuels 1998;12:536. [6] Li C-Z, Tan LL. Fuel 2000;79:1899. [7] Nelson PF, Kelly MD, Wornat MJ. Fuel 1991;70:403. [8] Tan LL, Li C-Z. Fuel 2000;79:1883. [9] Tan LL, Li C-Z. Fuel 2000;79:1891. [10] Tian F-J, Li BQ, Chen Y, Li C-Z. Fuel 2002;81:2203.
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