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Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part IX. Effects of volatile-char interactions on char–H2O and char–O2 reactivities
Fuel 90 (2011) 1655–1661
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Fuel journal homepage: www.elsevier.com/locate/fuel
Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part IX. Effects of volatile-char interactions on char–H2O and char–O2 reactivities Shu Zhang a,1, Jun-ichiro Hayashi b, Chun-Zhu Li a,c,⇑ a b c
Department of Chemical Engineering, Monash University, P.O. Box 36, VIC 3800, Australia Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-8580, Japan Curtin Centre for Advanced Energy Science and Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia
a r t i c l e
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Article history: Received 4 August 2010 Received in revised form 2 November 2010 Accepted 8 November 2010 Available online 19 November 2010 Keywords: Brown coal Volatile-char interactions Gasification Char reactivity
a b s t r a c t Volatile-char interactions are an important consideration in the design and operation of a gasifier. This study aims to investigate the effects of volatile-char interactions on the in situ char–steam reactivity at 800 °C and the ex-situ char–O2 reactivity at 400 °C. A Victorian brown coal was gasified in 15% steam at 800 °C in a one-stage novel fluidised-bed/fixed-bed quartz reactor, in which the extent of volatile-char interactions could be controlled. The chars after varying extents of volatile-char interactions and/or varying extents of char conversion in steam were also collected for the measurement of their reactivity with air at 400 °C in a thermogravimetric analyser. Our results show that the char–steam gasification reactions were greatly inhibited by the volatile-char interactions. It is believed that the H radicals generated from the thermal cracking/reforming of volatiles slowed the char gasification in three ways: occupying the char reactive sites, causing the char structure to re-arrange/condense and enhancing the release of catalytic species inherently present in the brown coal. The importance of volatile-char interactions to char–steam reactivity was further confirmed by the char–air reactivity. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The need to operate gasification at high temperature and pressure contributes significantly to the limited gasification efficiency and high capital/operating costs. This can be dramatically improved by developing the second generation coal gasification technologies where coal is gasified at lower temperatures. High char reactivity is essential to realising the full potential of a lowtemperature gasification process. Three key factors [1–18] influencing char reactivity are the presence of catalysts in char, the structural features of char itself and the interactions between catalysts and char. Volatiles interact strongly with char in practical gasifiers, particularly in fluidised-bed gasifiers. Volatile-char interactions have been experimentally proved to be a crucial factor during the pyrolysis of brown coal, which favours the release of alkali metallic species, activates the growth of aromatic rings and
⇑ Corresponding author at: Curtin Centre for Advanced Energy Science and Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia. Tel.: +61 8 9266 1131; fax: +61 8 9266 1138. E-mail address: [email protected] (C.-Z. Li). 1 Present address: Curtin Centre for Advanced Energy Science and Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia. 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.11.008
reduces the (ex-situ) char reactivity [3,18–21] Therefore, such problems as the erosion of turbine blades by the volatilised alkali and alkaline earth metallic (AAEM) species and the low char reactivity in a gasifier are closely related with the extent of volatile-char interactions. In the worst case [17], increases in the gasification temperature for a brown coal do not necessarily lead to increases in the char reactivity. Further detailed investigation on the reactions responsible for the observed effects of volatilechar interactions under gasification conditions is warranted. In particular, the effects of volatile-char interactions on the in situ char reactivity during gasification in steam at elevated temperature remain poorly understood. Our recent paper [22] reported that the interactions of volatiles and char could practically terminate the char conversion with the exact char conversion level at which gasification stops depending on gasification temperature. The H radicals originated from the thermal cracking/reforming of volatiles were believed to be responsible for the inhibition of the gasification reactions by occupying the reactive sites of char surface. Although our previous work [22] has demonstrated the importance of volatile-char interactions on char conversion, the exact roles of H radicals require further clarification. Except from occupying reactive sites on char surface, H radicals could also change char structure and AAEM retention/dispersion in char; both would
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alter char reactivity. There is a need to appreciate the individual roles of each factor, viz. the adsorption of H radicals on char surface, the change in the char structure and the catalyst concentration/dispersion, in determining the char reactivity. The ability to control the extent of volatile-char interactions is essential in appreciating the roles of each above-mentioned factor in influencing the char reactivity. Continuing our recent efforts to study the volatile-char interactions [3,10–12,16,17,19–21,23–25] this study aims to examine the effects of volatile-char interactions on the in situ char reactivity during the gasification in steam and on the ex-situ char reactivity in air after the gasification in steam. The extent of volatile-char interactions was controlled by adjusting the volatile-char interaction time and the concentration of volatiles (H radicals) interacting with char independently.
R¼
1 dW W dt
ð1Þ
where, W is the weight (daf basis) of the char at any given time t. After the weight of the char sample became constant, the temperature was further increased at 50 K min1 to 600 °C for an additional 30 min to ensure the complete combustion of any carbonaceous material possibly remaining in the char. The final weight was taken as the weight of ash in the char sample. The AAEM species in char/coal samples were quantified according to the procedure described elsewhere [26] The char/coal samples were ashed and then digested in an acid mixture of HF and HNO3. The residue derived from the evaporation of the acid mixture was re-dissolved in 20 mM methane sulfonic acid (MSA). The AAEM species in the MSA solution were analysed by an ion chromatograph.
2. Experimental
3. Results and discussion
A Loy Yang brown coal with particle sizes between 106 and 150 lm was used in this study. The properties of the coal sample [19] are: C, 70.4; H, 5.4; N, 0.62; S, 0.28; Cl, 0.1 and O, 23.2 (by difference) wt.% (daf) together with an ash yield of 1.1 wt.% (db). Gasification experiments at 800 °C were conducted in a novel fluidised-bed/fixed-bed quartz reactor detailed elsewhere [19]. Briefly, two quartz frits were installed in the reactor that had a diameter of 3.7 cm. The bottom frit acted as a bed (300–355 lm silica sand) supporter and fluidising gas distributor while the top one, in the freeboard, could prevent the char particles from escaping out of the reactor. The water-cooled feeding inlet was located just above the bottom frit from the side. During experiments, 1.8 L/min argon was used for feeding coal (0.8 L/min) and fluidising sand (1.0 L/min). Steam was generated by feeding water directly into the reactor underneath the bottom frit with an HPLC pump. The steam would mix with argon before entering the sand bed. The overall steam concentration was 15 vol.%. Due to the top frit in the freeboard, the char particles elutriated out of the fluidised sand bed due to the poor caking property of the brown coal would accumulate underneath the top frit to form a char bed [19]. The volatiles formed at a later stage must pass through and therefore interact with the char bed formed from the coal fed into the reactor at an earlier stage when the coal particles were continuously fed into the reactor. Therefore, the extent of volatile-char interactions could be changed by one of (or both) two ways: changing the feeding time (i.e. volatile-char interaction time) and/or changing the coal feeding rate (mainly the concentration of volatiles being generated). The experiments in the absence of volatile-char interactions were also carried out for comparison. The reactor was designed in such a way that the gasification of char could be terminated at any time by lifting the whole reactor assembly out of the furnace [19]. Chars were collected after experiments for further analysis. The reactivity of char in air was measured at 400 °C using a Perkin Elmer Pyris 1 thermogravimetric analyser (TGA) following the procedure outlined previously [3]. Briefly, about 4 mg of char sample was placed in a platinum crucible and heated in pure nitrogen (99.999%) atmosphere in the TGA to 105 °C to remove the moisture from the char. The temperature was then increased at a rate of 50 K min1 to 400 °C. After 2 min at 400 °C, the atmosphere was switched from nitrogen to air to commence the reactivity measurement. 400 °C was chosen as the isothermal char–O2 gasification temperature in this study in order to minimise the changes in char structure due to thermal annealing and avoid the possible ignition while ensuring that the gasification reaction could be completed within a reasonable period of time. The specific reactivity (R) of the char was calculated using the equation:
3.1. Gasification in steam in the presence and absence of volatile-char interactions Fig. 1 shows the conversion of coal during the gasification in steam as a function of feeding time (i.e. volatile-char interaction time) when coal particles were fed into the reactor continuously at feeding rates ranging from 15 to 100 mg min1. After somewhat initial rapid gasification within about first 80 min of feeding, the gasification of coal, in the presence of unchanged steam supply, slowed down. This is in broad agreement with our previous observation [22]. Higher feeding rates have resulted in slower coal gasification. In particular, the highest feeding fate of 100 mg min1 used in this study has led to a very limited level of coal conversion even after long feeding time during which the char was in constant contact with 15% steam. For example, doubling the feeding time from 80 to 160 min resulted in little additional coal conversion. In other words, it appears that the char formed in the first 80 min has undergone little further gasification although a gas stream containing 15% steam (i.e. excess steam was supplied) passed through the char continuously. It was not clear from Fig. 1 if the char, for example, after 80 min of feeding at 100 mg min1 was intrinsically deactivated or its
Fig. 1. Coal conversion as a function of feeding time during the gasification in steam at 800 °C with continuous volatile-char interactions at different feeding rates as labelled in the figure.
S. Zhang et al. / Fuel 90 (2011) 1655–1661
Fig. 2. Coal conversion as a function of time. Solid line: the chars were prepared from the gasification in steam in the presence of continuous volatile-char interactions at 100 mg min1 at 800 °C; Dashed line: the chars were prepared from the gasification in steam in the absence of volatiles after a period of feeding (50 min or 160 min).
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radical-laden volatiles through their occupation of reactive sites on char is clear from the data in Fig. 2. On stopping the generation of volatiles (i.e. stopping the feed of coal into the reactor), the char gasification rate increased drastically for both chars produced after feeding coal into the reactor for 50 or 160 min. As was described in detail in our previous study [22], similar to the inhibition of char–H2O gasification by the adsorption of H2, H radicals (and other species) from the reforming of volatiles would adsorb on the char surface during the volatile-char interactions to inhibit the char–H2O reactions. The changes in char structure due to the volatile-char interactions for the set of chars in this study were investigated using FT-Raman spectroscopy [14,15] and the results are reported separately [27]. The FT-Raman spectroscopy showed a drastic decrease in the relative ratio of small (with less than five fused rings or equivalent) and large (with at least six fused rings or equivalent) aromatic ring systems with increasing feeding time in the first 60–80 min for all coal feeding rates used here [27]. Much less changes in the ratio were observed after 100 min of feeding time [27]. These FT-Raman spectroscopic data indicated [27] that the H radicals were able to penetrate deep into the char structure to induce the ring condensation reactions within the char to convert the smaller aromatic ring systems into the bigger ones. It is reasonable to believe that the drastic changes in char structure in the early stages of coal feeding were largely responsible for the gradual slow down of coal gasification rate over the same period of time (e.g. <80 min, Fig. 1). Indeed, the data in Fig. 3 show that the
gasification in steam was inhibited temporarily by the presence of volatiles. Further experiments were carried out to gasify the char with steam but in the absence of volatiles. As is shown in Fig. 2, using a feeding rate of 100 mg min1, two separate feeding times, 50 and 160 min, were selected as the initial points for further comparison. At the end of each feeding time, the feeding of coal into reactor was suddenly stopped (and thus the generation of volatiles was stopped) and the char was then gasified in the absence of volatile-char interactions while steam was supplied throughout the whole experiments. As is shown in Fig. 2, once the coal feeding was stopped, the char which was very difficult to gasify with steam in the presence of volatile-char interactions could be gasified at a relatively high reaction rate. After about 40 min, the char gasification somewhat slowed down, but still much faster than that in the presence of volatile-char interactions. It appears that the selection of the initial feeding time (50 vs. 160 min) had only a small effect on the subsequent gasification rate in steam: the char from short feeding time (50 min) only showed a slightly higher reactivity than that from 100 min feeding time. As the feeding of coal was stopped, for instance, after 50 or 160 min of feeding time (Fig. 2), the production of volatiles and therefore the volatile-derived H radicals would have been terminated immediately. Therefore, the data in Figs. 1 and 2 clearly demonstrate that the volatile-char interactions can drastically reduce the char gasification rate in steam at 800 °C. 3.2. Understanding the effects of volatile-char interactions on the in situ char–H2O reactions at 800°C The data in Fig. 1 show that the volatile-char interactions can greatly impact on the char–H2O reactions at 800 °C. The essence of volatile-char interactions is the interactions between radicals, especially H radicals, with char [12]. The H radicals could inhibit char conversion by one of (or a combination of) three possible ways: occupying reactive sites, changing char structure and/or changing the retention/concentration/dispersion of catalytic species (e.g. Na) in char. The inhibition of char gasification by the
Fig. 3. AAEM concentration in char as a function of feeding time during the gasification in steam at 800 °C with continuous volatile-char interactions at different feeding rates.
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concentrations of main inherent catalysts (Na, Mg and Ca) in char changed little and would not be sufficient to account for the changes in reactivity over the same period. As for the slowdown of char gasification rate from about 60–80 to 100–120 min of feeding time, as the concentrations of the inherent catalysts actually tended to increase, the main factors responsible for the slowdown of char gasification rate would have to be the changes in char structure [27]. The extent of volatile-char interactions must be varied in order to gain a better understanding about the reactions responsible for the effects of volatile-char interactions on char reactivity. The first variable that can be used to change the extent of volatile-char interactions is the length of time over which volatiles interact with char, i.e. the coal feeding time in Fig. 1. Clearly, extending the volatile-char interaction time would tend to enhance the effects of volatile-char interactions, for example, on the changes in char structure. Indeed, the data in Fig. 1 show that increasing volatilechar interaction time tended to slow down the char gasification. In other words, at any feeding rate shown in Fig. 1, coal was gasified more rapidly at the earlier stages (shorter feeding time) than at the later stages. A closer examination of what happens with increasing volatilechar interaction time would bring further insights into the details of volatile-char interactions. At the earlier stage of feeding, the char in the reactor was relatively nascent, which would tend to have higher reactivities [28] than the aged chars. With increasing feeding time, the char accumulated underneath the top frit formed an increasingly thick char bed, enhancing the interactions between
volatiles and char in two ways. Firstly, the increasing thickness of char bed would mean that the H-radical-laden volatiles have a longer distance to travel within the char bed, improving the chance of any reactions between the H-radical-laden volatiles and the char, i.e. improving the ‘‘H utilisation efficiency’’ [25]. Secondly, the extended interactions between volatiles and char would further break down the volatiles to generate additional H radicals, as is supported by the following experimental observation. During the experiments, a bubbler containing a 0.02 M MSA solution was connected to the exit of the reactor (for other purposes). The bubbler was replaced every 20 min with a fresh one while the coal particles were continuously fed into the reactor. The solution in the first two bubblers was quite coloured (yellow) although the second one was much lighter than the first one, due to the tar dissolved into the solution. Surprisingly, starting from the third one (i.e. after 40 min of feeding), the bubbler was always colourless, indicating that little tar was dissolved into the solution. This observation clearly indicates that the increased thickness of char bed at the later stage of feeding has resulted in better reforming/decomposition of volatiles passing through the char bed, which would generate H radicals to interact with the char. Therefore, the increases in the volatile-char interaction time would also help to generate additional radicals and improve the H radical utilisation efficiency to enhance the consequence of volatile-char interactions, for example, causing the aromatic ring systems to condense to make the char structure to become increasingly compact and inert. The second variable used to control the extent of volatile-char interactions in this study was the concentration of volatiles.
Fig. 4. Specific reactivity in air at 400 °C as a function of char conversion. Chars were prepared from the gasification in steam with continuous volatile-char interactions at different feeding rates (labelled in the figure) at 800 °C.
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Increasing coal feeding rate would increase the concentration of volatiles interacting with the char when the supplies of steam and argon carrier gas remained unchanged. The data in Fig. 1 show that the enhanced volatile-char interactions due to the increases in the concentration of volatiles with increasing coal feeding rate had slowed down the char gasification reactions. While some slow char gasification took place at 15 mg min1 feeding rate in the feeding time range of 80 and 160 min, such char gasification was negligible at the coal feeding rate of 100 mg min1. It is clear that increasing volatile concentration would result in the better coverage of the reactive sites on the char surface by H radicals to inhibit the char–H2O reactions. However, in terms of char structure, the FTRaman spectroscopic data indicated [27] that the increases in coal feeding rate (thus volatile concentration) within the range used in this study did not result in further significantly enhanced ring condensation reactions within the char. In other words, even a coal feeding rate of 15 mg min1 has supplied sufficient H radicals for the ring condensation reactions with the char under the present experimental conditions. Calculation based on the data in Fig. 3 indicates that drastic volatilisation (>60%) of Na had taken place even for the shortest volatile-char interaction time used in this study, in abroad agreement with our previous observations [16–21] that the volatilisation of Na from a relatively nascent char, enhanced by the volatile-char interactions, is a relatively rapid process. With increasing volatile-char interaction time, the gasification of char, together with possible re-adsorption/takeup of volatilised Na by char [29] would balance or outweigh the volatilisation of Na to make the Na concentration in char to remain constant or to increase (Fig. 3). In
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agreement with our previous observations [16–21], the volatilisation of divalent Ca and Mg was less affected by the volatile-char interactions. The gasification of char has therefore been the main factor contributing to the increases in their concentrations in char (Fig. 3). The catalytic and non-catalytic gasification of char with steam under the current experimental conditions would take place concurrently. The increases in the catalyst (Na, Mg and Ca) concentrations in char (Fig. 3) are accompanied by the slowdown of coal gasification in the first 80 min (Fig. 1). It is thus suspected that the effects of inherent catalysts were not as significant as the changes in char structure and the occupation of reactive sites by H radicals in influencing the char–H2O reaction rates. 3.3. Effects of volatile-char interactions on the ex-situ char–O2 reactions at 400 °C Chars were collected after the gasification in steam for reactivity measurement in air at 400 °C using TGA. As is shown in Fig. 4, the specific reactivity fell with increasing volatile-char interaction time for the feeding rates from 100 to 30 mg min1. However, no clear trends can be seen after 10 min feeding when the feeding rate was further decreased to a very low level of 15 mg min1. The presence of inherent catalysts is a major factor influencing the reactivity of char in air at 400 °C [3,10–12,16–21]. Na has far more profound catalytic effects than Mg and Ca for the char–O2 reactions at 400 °C. Fig. 5 show the plots of specific reactivity versus Na concentration for the reactivity data shown in Fig. 4. As before [3,10–12,16–21], it is assumed that no Na would be volatilised
Fig. 5. Specific reactivity in air at 400 °C as a function of the Na concentration in char. Chars were prepared from the gasification in steam with continuous volatile-char interactions at different feeding rates at 800 °C. The coal feeding times are indicated.
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Fig. 6. Specific reactivity in air at 400 °C as a function of char conversion. Chars were prepared from the gasification in steam for different holding time (without volatile-char interactions) after 50 min (A) and 160 min (B) feeding, respectively, at 800 °C.
during the char–O2 reactions at 400 °C and the Na concentration at any char conversion level was calculated based on the initial Na concentration in the char at the beginning of char–O2 reaction and the char conversion due to the char–O2 reactions. While increases in Na concentration did result in increases in the char– O2 reaction rate at 400 °C, the relationship between reactivity at 400 °C and the Na concentration in char did depend on the volatile-char interaction time each char had experienced during the gasification in steam at 800 °C. These data mean that, except the Na concentration in char, the structure of char is another critical factor influencing the char–O2 reactivity [3,10–12]. As was noted above, the FT-Raman data [27] of these chars did show drastic changes in char structure with increasing volatile-char interaction time. The structure of char not only determines the ease with which the char structure can be broken down (i.e. gasification) by steam but also the interactions between Na and char, the latter would affect the dispersion of Na in/on char to influence its reactivity. Similarly, the ex-situ char reactivity in air at 400 °C was measured for the chars that have undergone different extents of gasification in steam at 800 °C in the absence of volatile-char interactions (see Fig. 2). The reactivity data in Fig. 6 show that the char–O2 reactivity at 400 °C increases with increasing conversion of char during the char–H2O gasification at 800 °C, especially for the char that had experienced 50 min of volatile-char interaction time. When the char–O2 reactivity is plotted as a function of the Na concentration in char (Fig. 7), it is clear that the Na concentration in char was the dominant factor influencing its char–O2 reactivity: all data fall into the same lines in Figs. 7A and B, respectively. The main reason for the differences in the Na concentration in char prior to char–O2 reaction among the chars in Fig. 7 is the removal of char due to gasification with steam (Fig. 2).
Fig. 7. Specific reactivity in air at 400 °C as a function of the Na concentration in char. Chars were prepared from the gasification in steam for different holding time (without volatile-char interactions) after 50 min(A) and 160 min (B) feeding, respectively, at 800 °C.
It should also be noted from Fig. 7 that the char that had experienced 50 min of volatile-char interaction time (Fig. 7A) has shown a different reactivity–Na-concentration relationship from the char that had experienced 160 min of volatile-char interaction time. This is again because the former char has lower level of aromatic ring condensation than the latter char due to the different extents of volatile-char interactions. It is also now appropriate to re-examine the data in Fig. 5 where the chars formed at a feeding rate of 15 mg min1 show different trends from the chars formed at higher feeding rate. The data in Fig. 1 indicate that the char gasification at a feeding rate of 15 mg min1 in fact continued. At this low coal feeding rate of 15 mg min1, the concentration of volatiles was not sufficient to drastically inhibit the char–H2O gasification while the gasification virtually stopped at a coal feeding rate of 100 mg min1 (Fig. 1). While the gasification time in steam in the presence of volatilechar interactions at 800 °C tended to decrease the subsequent char–O2 reactivity at 400 °C, the char–H2O gasification in the absence of volatile-char interactions tended to increase the subsequent char–O2 reactivity. The data in Figs. 4–7 therefore provide circumstantial evidence that there is a difference in the reaction mechanism between the gasification in the presence of continuous volatile-char interactions and the gasification in the absence of continuous volatilechar interactions. In the presence of volatile-char interactions (Figs. 4 and 5 for the feeding rates of 30–100 mg min1) during the char–H2O gasification at 800 °C, the subsequent ex-situ char– O2 reactivity at 400 °C is determined both by Na concentration and char structure. Once the volatile-char interactions at 800 °C were terminated, subsequent char–H2O gasification at 800 °C would only influence the subsequent char–O2 reactions through the enrichment of Na concentration in char through the removal/ gasification of carbon structure. It appears that the volatile-char interactions have far more important implications in terms of char
S. Zhang et al. / Fuel 90 (2011) 1655–1661
structural features to the ex-situ char–O2 reactivity than the gasification in steam without the volatile-char interactions. 3.4. Conclusions The volatile-char interactions significantly inhibited char conversion during the gasification in steam through three possible mechanisms: the occupation of reactive sites on char surface by H radicals (or similar), the alteration of char structure and the volatilisation of inherent catalysts. The volatilisation of the inherent catalysts is a relatively rapid process. While the occupation of reactive sites by the volatile-derived species (H radicals) can be removed as soon as the generation of volatiles is stopped, the changes in char structure and the volatilisation of inherent catalysts are permanent. The volatile-char interactions in steam at 800 °C also drastically affect the char–O2 reactivity at 400 °C mainly through the changes in char structure and in Na concentration in char. The gasification in steam at 800 °C in the presence and absence of volatile-char interactions showed different influences on char–O2 reactivity at 400 °C. Acknowledgements The authors gratefully acknowledge the support of this study by the Victorian state government under its Energy Technology Innovation Strategy program. Helpful discussions with Dr. F-J. Tian are gratefully acknowledged.
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Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part IX. Effects of volatile-char interactions on char–H2O and char–O2 reactivities Shu Zhang a,1, Jun-ichiro Hayashi b, Chun-Zhu Li a,c,⇑ a b c
Department of Chemical Engineering, Monash University, P.O. Box 36, VIC 3800, Australia Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-8580, Japan Curtin Centre for Advanced Energy Science and Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia
a r t i c l e
i n f o
Article history: Received 4 August 2010 Received in revised form 2 November 2010 Accepted 8 November 2010 Available online 19 November 2010 Keywords: Brown coal Volatile-char interactions Gasification Char reactivity
a b s t r a c t Volatile-char interactions are an important consideration in the design and operation of a gasifier. This study aims to investigate the effects of volatile-char interactions on the in situ char–steam reactivity at 800 °C and the ex-situ char–O2 reactivity at 400 °C. A Victorian brown coal was gasified in 15% steam at 800 °C in a one-stage novel fluidised-bed/fixed-bed quartz reactor, in which the extent of volatile-char interactions could be controlled. The chars after varying extents of volatile-char interactions and/or varying extents of char conversion in steam were also collected for the measurement of their reactivity with air at 400 °C in a thermogravimetric analyser. Our results show that the char–steam gasification reactions were greatly inhibited by the volatile-char interactions. It is believed that the H radicals generated from the thermal cracking/reforming of volatiles slowed the char gasification in three ways: occupying the char reactive sites, causing the char structure to re-arrange/condense and enhancing the release of catalytic species inherently present in the brown coal. The importance of volatile-char interactions to char–steam reactivity was further confirmed by the char–air reactivity. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The need to operate gasification at high temperature and pressure contributes significantly to the limited gasification efficiency and high capital/operating costs. This can be dramatically improved by developing the second generation coal gasification technologies where coal is gasified at lower temperatures. High char reactivity is essential to realising the full potential of a lowtemperature gasification process. Three key factors [1–18] influencing char reactivity are the presence of catalysts in char, the structural features of char itself and the interactions between catalysts and char. Volatiles interact strongly with char in practical gasifiers, particularly in fluidised-bed gasifiers. Volatile-char interactions have been experimentally proved to be a crucial factor during the pyrolysis of brown coal, which favours the release of alkali metallic species, activates the growth of aromatic rings and
⇑ Corresponding author at: Curtin Centre for Advanced Energy Science and Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia. Tel.: +61 8 9266 1131; fax: +61 8 9266 1138. E-mail address: [email protected] (C.-Z. Li). 1 Present address: Curtin Centre for Advanced Energy Science and Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia. 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.11.008
reduces the (ex-situ) char reactivity [3,18–21] Therefore, such problems as the erosion of turbine blades by the volatilised alkali and alkaline earth metallic (AAEM) species and the low char reactivity in a gasifier are closely related with the extent of volatile-char interactions. In the worst case [17], increases in the gasification temperature for a brown coal do not necessarily lead to increases in the char reactivity. Further detailed investigation on the reactions responsible for the observed effects of volatilechar interactions under gasification conditions is warranted. In particular, the effects of volatile-char interactions on the in situ char reactivity during gasification in steam at elevated temperature remain poorly understood. Our recent paper [22] reported that the interactions of volatiles and char could practically terminate the char conversion with the exact char conversion level at which gasification stops depending on gasification temperature. The H radicals originated from the thermal cracking/reforming of volatiles were believed to be responsible for the inhibition of the gasification reactions by occupying the reactive sites of char surface. Although our previous work [22] has demonstrated the importance of volatile-char interactions on char conversion, the exact roles of H radicals require further clarification. Except from occupying reactive sites on char surface, H radicals could also change char structure and AAEM retention/dispersion in char; both would
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alter char reactivity. There is a need to appreciate the individual roles of each factor, viz. the adsorption of H radicals on char surface, the change in the char structure and the catalyst concentration/dispersion, in determining the char reactivity. The ability to control the extent of volatile-char interactions is essential in appreciating the roles of each above-mentioned factor in influencing the char reactivity. Continuing our recent efforts to study the volatile-char interactions [3,10–12,16,17,19–21,23–25] this study aims to examine the effects of volatile-char interactions on the in situ char reactivity during the gasification in steam and on the ex-situ char reactivity in air after the gasification in steam. The extent of volatile-char interactions was controlled by adjusting the volatile-char interaction time and the concentration of volatiles (H radicals) interacting with char independently.
R¼
1 dW W dt
ð1Þ
where, W is the weight (daf basis) of the char at any given time t. After the weight of the char sample became constant, the temperature was further increased at 50 K min1 to 600 °C for an additional 30 min to ensure the complete combustion of any carbonaceous material possibly remaining in the char. The final weight was taken as the weight of ash in the char sample. The AAEM species in char/coal samples were quantified according to the procedure described elsewhere [26] The char/coal samples were ashed and then digested in an acid mixture of HF and HNO3. The residue derived from the evaporation of the acid mixture was re-dissolved in 20 mM methane sulfonic acid (MSA). The AAEM species in the MSA solution were analysed by an ion chromatograph.
2. Experimental
3. Results and discussion
A Loy Yang brown coal with particle sizes between 106 and 150 lm was used in this study. The properties of the coal sample [19] are: C, 70.4; H, 5.4; N, 0.62; S, 0.28; Cl, 0.1 and O, 23.2 (by difference) wt.% (daf) together with an ash yield of 1.1 wt.% (db). Gasification experiments at 800 °C were conducted in a novel fluidised-bed/fixed-bed quartz reactor detailed elsewhere [19]. Briefly, two quartz frits were installed in the reactor that had a diameter of 3.7 cm. The bottom frit acted as a bed (300–355 lm silica sand) supporter and fluidising gas distributor while the top one, in the freeboard, could prevent the char particles from escaping out of the reactor. The water-cooled feeding inlet was located just above the bottom frit from the side. During experiments, 1.8 L/min argon was used for feeding coal (0.8 L/min) and fluidising sand (1.0 L/min). Steam was generated by feeding water directly into the reactor underneath the bottom frit with an HPLC pump. The steam would mix with argon before entering the sand bed. The overall steam concentration was 15 vol.%. Due to the top frit in the freeboard, the char particles elutriated out of the fluidised sand bed due to the poor caking property of the brown coal would accumulate underneath the top frit to form a char bed [19]. The volatiles formed at a later stage must pass through and therefore interact with the char bed formed from the coal fed into the reactor at an earlier stage when the coal particles were continuously fed into the reactor. Therefore, the extent of volatile-char interactions could be changed by one of (or both) two ways: changing the feeding time (i.e. volatile-char interaction time) and/or changing the coal feeding rate (mainly the concentration of volatiles being generated). The experiments in the absence of volatile-char interactions were also carried out for comparison. The reactor was designed in such a way that the gasification of char could be terminated at any time by lifting the whole reactor assembly out of the furnace [19]. Chars were collected after experiments for further analysis. The reactivity of char in air was measured at 400 °C using a Perkin Elmer Pyris 1 thermogravimetric analyser (TGA) following the procedure outlined previously [3]. Briefly, about 4 mg of char sample was placed in a platinum crucible and heated in pure nitrogen (99.999%) atmosphere in the TGA to 105 °C to remove the moisture from the char. The temperature was then increased at a rate of 50 K min1 to 400 °C. After 2 min at 400 °C, the atmosphere was switched from nitrogen to air to commence the reactivity measurement. 400 °C was chosen as the isothermal char–O2 gasification temperature in this study in order to minimise the changes in char structure due to thermal annealing and avoid the possible ignition while ensuring that the gasification reaction could be completed within a reasonable period of time. The specific reactivity (R) of the char was calculated using the equation:
3.1. Gasification in steam in the presence and absence of volatile-char interactions Fig. 1 shows the conversion of coal during the gasification in steam as a function of feeding time (i.e. volatile-char interaction time) when coal particles were fed into the reactor continuously at feeding rates ranging from 15 to 100 mg min1. After somewhat initial rapid gasification within about first 80 min of feeding, the gasification of coal, in the presence of unchanged steam supply, slowed down. This is in broad agreement with our previous observation [22]. Higher feeding rates have resulted in slower coal gasification. In particular, the highest feeding fate of 100 mg min1 used in this study has led to a very limited level of coal conversion even after long feeding time during which the char was in constant contact with 15% steam. For example, doubling the feeding time from 80 to 160 min resulted in little additional coal conversion. In other words, it appears that the char formed in the first 80 min has undergone little further gasification although a gas stream containing 15% steam (i.e. excess steam was supplied) passed through the char continuously. It was not clear from Fig. 1 if the char, for example, after 80 min of feeding at 100 mg min1 was intrinsically deactivated or its
Fig. 1. Coal conversion as a function of feeding time during the gasification in steam at 800 °C with continuous volatile-char interactions at different feeding rates as labelled in the figure.
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Fig. 2. Coal conversion as a function of time. Solid line: the chars were prepared from the gasification in steam in the presence of continuous volatile-char interactions at 100 mg min1 at 800 °C; Dashed line: the chars were prepared from the gasification in steam in the absence of volatiles after a period of feeding (50 min or 160 min).
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radical-laden volatiles through their occupation of reactive sites on char is clear from the data in Fig. 2. On stopping the generation of volatiles (i.e. stopping the feed of coal into the reactor), the char gasification rate increased drastically for both chars produced after feeding coal into the reactor for 50 or 160 min. As was described in detail in our previous study [22], similar to the inhibition of char–H2O gasification by the adsorption of H2, H radicals (and other species) from the reforming of volatiles would adsorb on the char surface during the volatile-char interactions to inhibit the char–H2O reactions. The changes in char structure due to the volatile-char interactions for the set of chars in this study were investigated using FT-Raman spectroscopy [14,15] and the results are reported separately [27]. The FT-Raman spectroscopy showed a drastic decrease in the relative ratio of small (with less than five fused rings or equivalent) and large (with at least six fused rings or equivalent) aromatic ring systems with increasing feeding time in the first 60–80 min for all coal feeding rates used here [27]. Much less changes in the ratio were observed after 100 min of feeding time [27]. These FT-Raman spectroscopic data indicated [27] that the H radicals were able to penetrate deep into the char structure to induce the ring condensation reactions within the char to convert the smaller aromatic ring systems into the bigger ones. It is reasonable to believe that the drastic changes in char structure in the early stages of coal feeding were largely responsible for the gradual slow down of coal gasification rate over the same period of time (e.g. <80 min, Fig. 1). Indeed, the data in Fig. 3 show that the
gasification in steam was inhibited temporarily by the presence of volatiles. Further experiments were carried out to gasify the char with steam but in the absence of volatiles. As is shown in Fig. 2, using a feeding rate of 100 mg min1, two separate feeding times, 50 and 160 min, were selected as the initial points for further comparison. At the end of each feeding time, the feeding of coal into reactor was suddenly stopped (and thus the generation of volatiles was stopped) and the char was then gasified in the absence of volatile-char interactions while steam was supplied throughout the whole experiments. As is shown in Fig. 2, once the coal feeding was stopped, the char which was very difficult to gasify with steam in the presence of volatile-char interactions could be gasified at a relatively high reaction rate. After about 40 min, the char gasification somewhat slowed down, but still much faster than that in the presence of volatile-char interactions. It appears that the selection of the initial feeding time (50 vs. 160 min) had only a small effect on the subsequent gasification rate in steam: the char from short feeding time (50 min) only showed a slightly higher reactivity than that from 100 min feeding time. As the feeding of coal was stopped, for instance, after 50 or 160 min of feeding time (Fig. 2), the production of volatiles and therefore the volatile-derived H radicals would have been terminated immediately. Therefore, the data in Figs. 1 and 2 clearly demonstrate that the volatile-char interactions can drastically reduce the char gasification rate in steam at 800 °C. 3.2. Understanding the effects of volatile-char interactions on the in situ char–H2O reactions at 800°C The data in Fig. 1 show that the volatile-char interactions can greatly impact on the char–H2O reactions at 800 °C. The essence of volatile-char interactions is the interactions between radicals, especially H radicals, with char [12]. The H radicals could inhibit char conversion by one of (or a combination of) three possible ways: occupying reactive sites, changing char structure and/or changing the retention/concentration/dispersion of catalytic species (e.g. Na) in char. The inhibition of char gasification by the
Fig. 3. AAEM concentration in char as a function of feeding time during the gasification in steam at 800 °C with continuous volatile-char interactions at different feeding rates.
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concentrations of main inherent catalysts (Na, Mg and Ca) in char changed little and would not be sufficient to account for the changes in reactivity over the same period. As for the slowdown of char gasification rate from about 60–80 to 100–120 min of feeding time, as the concentrations of the inherent catalysts actually tended to increase, the main factors responsible for the slowdown of char gasification rate would have to be the changes in char structure [27]. The extent of volatile-char interactions must be varied in order to gain a better understanding about the reactions responsible for the effects of volatile-char interactions on char reactivity. The first variable that can be used to change the extent of volatile-char interactions is the length of time over which volatiles interact with char, i.e. the coal feeding time in Fig. 1. Clearly, extending the volatile-char interaction time would tend to enhance the effects of volatile-char interactions, for example, on the changes in char structure. Indeed, the data in Fig. 1 show that increasing volatilechar interaction time tended to slow down the char gasification. In other words, at any feeding rate shown in Fig. 1, coal was gasified more rapidly at the earlier stages (shorter feeding time) than at the later stages. A closer examination of what happens with increasing volatilechar interaction time would bring further insights into the details of volatile-char interactions. At the earlier stage of feeding, the char in the reactor was relatively nascent, which would tend to have higher reactivities [28] than the aged chars. With increasing feeding time, the char accumulated underneath the top frit formed an increasingly thick char bed, enhancing the interactions between
volatiles and char in two ways. Firstly, the increasing thickness of char bed would mean that the H-radical-laden volatiles have a longer distance to travel within the char bed, improving the chance of any reactions between the H-radical-laden volatiles and the char, i.e. improving the ‘‘H utilisation efficiency’’ [25]. Secondly, the extended interactions between volatiles and char would further break down the volatiles to generate additional H radicals, as is supported by the following experimental observation. During the experiments, a bubbler containing a 0.02 M MSA solution was connected to the exit of the reactor (for other purposes). The bubbler was replaced every 20 min with a fresh one while the coal particles were continuously fed into the reactor. The solution in the first two bubblers was quite coloured (yellow) although the second one was much lighter than the first one, due to the tar dissolved into the solution. Surprisingly, starting from the third one (i.e. after 40 min of feeding), the bubbler was always colourless, indicating that little tar was dissolved into the solution. This observation clearly indicates that the increased thickness of char bed at the later stage of feeding has resulted in better reforming/decomposition of volatiles passing through the char bed, which would generate H radicals to interact with the char. Therefore, the increases in the volatile-char interaction time would also help to generate additional radicals and improve the H radical utilisation efficiency to enhance the consequence of volatile-char interactions, for example, causing the aromatic ring systems to condense to make the char structure to become increasingly compact and inert. The second variable used to control the extent of volatile-char interactions in this study was the concentration of volatiles.
Fig. 4. Specific reactivity in air at 400 °C as a function of char conversion. Chars were prepared from the gasification in steam with continuous volatile-char interactions at different feeding rates (labelled in the figure) at 800 °C.
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Increasing coal feeding rate would increase the concentration of volatiles interacting with the char when the supplies of steam and argon carrier gas remained unchanged. The data in Fig. 1 show that the enhanced volatile-char interactions due to the increases in the concentration of volatiles with increasing coal feeding rate had slowed down the char gasification reactions. While some slow char gasification took place at 15 mg min1 feeding rate in the feeding time range of 80 and 160 min, such char gasification was negligible at the coal feeding rate of 100 mg min1. It is clear that increasing volatile concentration would result in the better coverage of the reactive sites on the char surface by H radicals to inhibit the char–H2O reactions. However, in terms of char structure, the FTRaman spectroscopic data indicated [27] that the increases in coal feeding rate (thus volatile concentration) within the range used in this study did not result in further significantly enhanced ring condensation reactions within the char. In other words, even a coal feeding rate of 15 mg min1 has supplied sufficient H radicals for the ring condensation reactions with the char under the present experimental conditions. Calculation based on the data in Fig. 3 indicates that drastic volatilisation (>60%) of Na had taken place even for the shortest volatile-char interaction time used in this study, in abroad agreement with our previous observations [16–21] that the volatilisation of Na from a relatively nascent char, enhanced by the volatile-char interactions, is a relatively rapid process. With increasing volatile-char interaction time, the gasification of char, together with possible re-adsorption/takeup of volatilised Na by char [29] would balance or outweigh the volatilisation of Na to make the Na concentration in char to remain constant or to increase (Fig. 3). In
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agreement with our previous observations [16–21], the volatilisation of divalent Ca and Mg was less affected by the volatile-char interactions. The gasification of char has therefore been the main factor contributing to the increases in their concentrations in char (Fig. 3). The catalytic and non-catalytic gasification of char with steam under the current experimental conditions would take place concurrently. The increases in the catalyst (Na, Mg and Ca) concentrations in char (Fig. 3) are accompanied by the slowdown of coal gasification in the first 80 min (Fig. 1). It is thus suspected that the effects of inherent catalysts were not as significant as the changes in char structure and the occupation of reactive sites by H radicals in influencing the char–H2O reaction rates. 3.3. Effects of volatile-char interactions on the ex-situ char–O2 reactions at 400 °C Chars were collected after the gasification in steam for reactivity measurement in air at 400 °C using TGA. As is shown in Fig. 4, the specific reactivity fell with increasing volatile-char interaction time for the feeding rates from 100 to 30 mg min1. However, no clear trends can be seen after 10 min feeding when the feeding rate was further decreased to a very low level of 15 mg min1. The presence of inherent catalysts is a major factor influencing the reactivity of char in air at 400 °C [3,10–12,16–21]. Na has far more profound catalytic effects than Mg and Ca for the char–O2 reactions at 400 °C. Fig. 5 show the plots of specific reactivity versus Na concentration for the reactivity data shown in Fig. 4. As before [3,10–12,16–21], it is assumed that no Na would be volatilised
Fig. 5. Specific reactivity in air at 400 °C as a function of the Na concentration in char. Chars were prepared from the gasification in steam with continuous volatile-char interactions at different feeding rates at 800 °C. The coal feeding times are indicated.
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Fig. 6. Specific reactivity in air at 400 °C as a function of char conversion. Chars were prepared from the gasification in steam for different holding time (without volatile-char interactions) after 50 min (A) and 160 min (B) feeding, respectively, at 800 °C.
during the char–O2 reactions at 400 °C and the Na concentration at any char conversion level was calculated based on the initial Na concentration in the char at the beginning of char–O2 reaction and the char conversion due to the char–O2 reactions. While increases in Na concentration did result in increases in the char– O2 reaction rate at 400 °C, the relationship between reactivity at 400 °C and the Na concentration in char did depend on the volatile-char interaction time each char had experienced during the gasification in steam at 800 °C. These data mean that, except the Na concentration in char, the structure of char is another critical factor influencing the char–O2 reactivity [3,10–12]. As was noted above, the FT-Raman data [27] of these chars did show drastic changes in char structure with increasing volatile-char interaction time. The structure of char not only determines the ease with which the char structure can be broken down (i.e. gasification) by steam but also the interactions between Na and char, the latter would affect the dispersion of Na in/on char to influence its reactivity. Similarly, the ex-situ char reactivity in air at 400 °C was measured for the chars that have undergone different extents of gasification in steam at 800 °C in the absence of volatile-char interactions (see Fig. 2). The reactivity data in Fig. 6 show that the char–O2 reactivity at 400 °C increases with increasing conversion of char during the char–H2O gasification at 800 °C, especially for the char that had experienced 50 min of volatile-char interaction time. When the char–O2 reactivity is plotted as a function of the Na concentration in char (Fig. 7), it is clear that the Na concentration in char was the dominant factor influencing its char–O2 reactivity: all data fall into the same lines in Figs. 7A and B, respectively. The main reason for the differences in the Na concentration in char prior to char–O2 reaction among the chars in Fig. 7 is the removal of char due to gasification with steam (Fig. 2).
Fig. 7. Specific reactivity in air at 400 °C as a function of the Na concentration in char. Chars were prepared from the gasification in steam for different holding time (without volatile-char interactions) after 50 min(A) and 160 min (B) feeding, respectively, at 800 °C.
It should also be noted from Fig. 7 that the char that had experienced 50 min of volatile-char interaction time (Fig. 7A) has shown a different reactivity–Na-concentration relationship from the char that had experienced 160 min of volatile-char interaction time. This is again because the former char has lower level of aromatic ring condensation than the latter char due to the different extents of volatile-char interactions. It is also now appropriate to re-examine the data in Fig. 5 where the chars formed at a feeding rate of 15 mg min1 show different trends from the chars formed at higher feeding rate. The data in Fig. 1 indicate that the char gasification at a feeding rate of 15 mg min1 in fact continued. At this low coal feeding rate of 15 mg min1, the concentration of volatiles was not sufficient to drastically inhibit the char–H2O gasification while the gasification virtually stopped at a coal feeding rate of 100 mg min1 (Fig. 1). While the gasification time in steam in the presence of volatilechar interactions at 800 °C tended to decrease the subsequent char–O2 reactivity at 400 °C, the char–H2O gasification in the absence of volatile-char interactions tended to increase the subsequent char–O2 reactivity. The data in Figs. 4–7 therefore provide circumstantial evidence that there is a difference in the reaction mechanism between the gasification in the presence of continuous volatile-char interactions and the gasification in the absence of continuous volatilechar interactions. In the presence of volatile-char interactions (Figs. 4 and 5 for the feeding rates of 30–100 mg min1) during the char–H2O gasification at 800 °C, the subsequent ex-situ char– O2 reactivity at 400 °C is determined both by Na concentration and char structure. Once the volatile-char interactions at 800 °C were terminated, subsequent char–H2O gasification at 800 °C would only influence the subsequent char–O2 reactions through the enrichment of Na concentration in char through the removal/ gasification of carbon structure. It appears that the volatile-char interactions have far more important implications in terms of char
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structural features to the ex-situ char–O2 reactivity than the gasification in steam without the volatile-char interactions. 3.4. Conclusions The volatile-char interactions significantly inhibited char conversion during the gasification in steam through three possible mechanisms: the occupation of reactive sites on char surface by H radicals (or similar), the alteration of char structure and the volatilisation of inherent catalysts. The volatilisation of the inherent catalysts is a relatively rapid process. While the occupation of reactive sites by the volatile-derived species (H radicals) can be removed as soon as the generation of volatiles is stopped, the changes in char structure and the volatilisation of inherent catalysts are permanent. The volatile-char interactions in steam at 800 °C also drastically affect the char–O2 reactivity at 400 °C mainly through the changes in char structure and in Na concentration in char. The gasification in steam at 800 °C in the presence and absence of volatile-char interactions showed different influences on char–O2 reactivity at 400 °C. Acknowledgements The authors gratefully acknowledge the support of this study by the Victorian state government under its Energy Technology Innovation Strategy program. Helpful discussions with Dr. F-J. Tian are gratefully acknowledged.
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