Article pubs.acs.org/EF
TGA and Drop Tube Furnace Investigation of Alkali and Alkaline Earth Metal Compounds as Coal Combustion Additives Katherine Le Manquais,†,‡,* Colin Snape,† Jim Barker,‡ and Ian McRobbie‡ †
Department of Chemical and Environmental Engineering, Faculty of Engineering, University of Nottingham, NG7 2RD, U.K. Innospec Limited, Oil Sites Road, Ellesmere Port, Cheshire, CH65 4EY, U.K.
‡
ABSTRACT: Even though alkali and alkaline earth metal compounds are well-known catalysts for the combustion of coal, there has been no significant investigation into the importance of the anion across a broad selection of salts. The results described here compare the burnout of bituminous coal samples containing 21 Group I and II compounds on a thermogravimetric analyzer, thereby furnishing a wide-ranging systematic evaluation of anion effects for the first time. A variety of acetates, bicarbonates, carbonates, chlorides, hydroxides, nitrates, and sulfates were studied. Testing was also extended to a drop tube furnace (DTF), so that individual combustion additives could be assessed under conditions more similar to those found in a pulverized fuel (PF) boiler. All of the catalysts were subsequently found to increase the rate of TGA char combustion, but establishing definitive carbon burnout improvements proved to be more difficult on the DTF. This was probably due to a combination of the experimental variability associated with this setup, poor additive-coal contact, and the intrinsic volatility of the tested salts, particularly sodium carbonate’s removal at high temperatures and the loss of calcium nitrate in an oxidizing environment. Despite these uncertainties, the attained DTF reactivity ranking was quite similar to that of thermogravimetric analysis (TGA). The alkali chlorides were identified as the most active additives, with their higher melting points possibly enhancing both their retention and the catalyst−coal contact achieved during devolatilization. However, the burnout improvements associated with the other Group I salts appeared to be limited by the propensity of the cations to interact with the coal matrix, while the activities of the less effective Group II compounds seemed to be restricted by their higher ionization energies and different bonding. reactivity after a relatively short heat treatment time,5 a combination of diffusion and chemical control now seems to be much more likely.6 This makes the rate of combustion dependent on three interacting factors: the extent diffusion limits the reaction, the internal surface area of the char particles, and the internal surface’s innate reactivity.7 Thermogravimetric analysis (TGA) provides a convenient, rapid, and quantitative means of evaluating a coal’s combustion rate by measuring the mass changes that occur when burning a small amount of it. These instruments easily lend themselves to assessing a large number of coal samples as they are relatively compact, cheap, and fast to run. Their current prevalence has produced a new generation of research into suitable combustion additives, most of which has concentrated on readily available inorganic salts. For example, several studies have already focused on certain alkali and alkaline earth metal compounds 8−12 that are also known to enhance the combustion of graphite13 and the suppression of soot.14−16 These include barium carbonate, cesium chloride, calcium nitrate, and various other alkali carbonates, hydroxides, and nitrates.8−12,17 However, it currently remains difficult to clarify the underlying chemistry behind these catalysts’ performances since, even though many studies have been conducted, very few of them have thoroughly investigated a comprehensive number of compounds in a systematic manner. Researchers have instead chosen to explore catalytic mixtures (such as combinations of
1. INTRODUCTION Nearly complete carbon burnout has become inherently more difficult to achieve during coal combustion because of the introduction of low NOx burners and increasing amounts of unreactive inertinite in some internationally traded coals. This has provided the opportunity to develop coal additives for decreasing ignition temperatures and/or increasing the rate of combustion, both during iron-making and in pulverized fuel (PF) boilers. Even a 0.1% improvement in carbon burnout would save a typical 2000 MW power station up to 7000 t year−1 of coal, presumably without producing any additional carbon dioxide emissions. These catalysts would also lessen the required boiler residence time or allow combustion to take place at lower temperatures, leading to smaller emissions of NOx and SO3.1 Furthermore, such additives would reduce the carbon content of the produced pulverized fuel ash (PFA), thereby increasing boiler efficiency and obviating the need for postcombustion ash beneficiation. This treatment reduces the ash’s carbon content to below 6% (ASTM C618-08a) making it suitable for use in side-products, such as concrete. It also avoids a power station or industrial furnace needing to store or dispose of the ash, either by lagooning or as landfill waste.2 A variety of inorganic additives have previously been used to improve the efficiency of coal-fired systems,3 but their application was normally based on operational trial and error rather than on a detailed understanding of the process involved. This was often because of the assumption that diffusion was the rate limiting factor at the high temperatures experienced in a PF boiler, not intrinsic char reactivity.4 However, given that thermal annealing is known to create char with a low intrinsic © 2012 American Chemical Society
Received: July 5, 2011 Revised: February 6, 2012 Published: February 10, 2012 1531
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Table 1. Proximate, Ultimate, and Maceral Analyses of 53−75 and 106−150 μm ATC Coal
proximate manalysis
ultimate analysis
maceral analysis
moisture (wt %) ash (wt %) fixed carbon (wt %, daf) volatiles (wt %, daf) C (% daf) H (% daf) N (% daf) S (% daf) O (% daf) vitrinite (% daf) liptinite (% daf) inertinite (% daf)
metal and semimetal oxides17) or a narrow selection of additives (e.g., just the compounds barium carbonate, iron(III) oxide, and manganese oxide10). This is probably because cost, availability, or the assessment of a specific patented formulation has been the main motivation behind these investigations. The need for a more comprehensive survey has consequently been identified.8 Additionally, the effectiveness of these catalysts under realistic PF boiler conditions is still largely unknown because only two drop tube furnace (DTF) studies have hitherto reported burnout results from chars created under high temperatures, fast heating rates, and short residence times. The first of these investigations looked into the combustion acceleration associated with significant quantities of potassium carbonate,18 while the other compared chars catalyzed by different Group I and II salts with those produced by TGA.8 Similar investigations have previously contrasted the DTF firing of uncatalysed coal chars with analogous TGA data, but different reactivity trends have often been reported with the results from TGA arguably being closer to those observed in a full-scale boiler,19−22 even though DTFs are more expensive and time-consuming to run. Conducting burnout studies on a DTF nevertheless appears to be an industry requirement for further examining potential additives, as confirmation of their behavior under a more realistic environment is clearly needed before trials in a pilot plant or boiler can be undertaken. Testing is required to accurately gauge the carbon-in-ash content reduction that could be achieved by this type of additive, in addition to assessing the impact a boiler’s operating parameters might have on it. The purpose of this study was to systematically rank seven common anions and the alkali and alkaline earth metal cations according to their catalytic effect on coal combustion. TGA was employed to test more than 20 compounds, and the results were then assembled to form a wide-ranging activity grading for the first time. This ranking was used to select five catalysts for DTF burnout trials, and the ability of TGA to reproduce the behavior of these additives under high temperatures and fast heating rates was explored. A further significant objective was the investigation of mole-based additive weightings, where an equal number of catalyst particles could be directly compared, since previous reactivity rankings seem to have been influenced by the salts’ molecular masses.8 As well as allowing the most effective combustion additives to be identified, these results provide an interesting insight into the catalytic roles that these compounds might play as part of a coal’s inherent mineral matter.
standard
53−75 μm
106−150 μm
ISO 11722:1999 BS 1016-104:1991 by difference ISO 562:2010 ISO 17247:2005 ISO 17247:2005 ISO 17247:2005 ISO 19579:2006 by difference ISO 7404-3:2009 ISO 7404-3:2009 ISO 7404-3:2009
4.5 13.4 68.9 31.1 85.2 5.1 2.1 0.6 7.0 45.6 3.6 50.8
3.2 11.1 66.9 33.1 84.2 5.3 2.2 0.6 7.7 46.8 2.8 50.4
2. EXPERIMENTAL PROCEDURE 2.1. Sample Preparation. The bituminous coal used in this study was from the Arthur Taylor Colliery (ATC) in South Africa. Its proximate, ultimate, and maceral analyses are set out in Table 1, and its
Table 2. Ash and Trace Element Composition of 53−75 μm ATC Coala
a
elemental oxide
composition (%, by mass)
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 MnO SO3 P2O5 trace element
43.7 30.3 5.4 8.9 1.7 0.3 0.5 1.8 acetate > nitrate. Here, the compounds with the lowest molecular masses were probably the most successful simply because their coal samples contained the highest number of catalyst atoms. Sodium sulfate (Na2SO4) and carbonate (Na2CO3) might also have benefited from possessing double the amount of cation in their formulas, although alkali carbonates have been found to be similarly active during gasification where their reactivity has been attributed to their ability to disperse uniformly.28 For 0.00005 mol g−1 of the sodium compounds, a very different reactivity grading was exhibited in Figure 2: chloride > sulfate > acetate > bicarbonate > carbonate > nitrate. In fact, sodium chloride appeared to be so successful that the burnout time achieved by it almost matched that from the 1% loading,
Figure 1. Schematic of the DTF setup.24 the inlet probe and correcting the total inlet gas flow to ∼12 L min−1, a 400 ms residence time was employed. The DTF was then heated to 1300 °C using the three heaters. Coal was introduced at a rate of 3 to 5 g hr−1 using a screw feeder, but because some of the additives were hygroscopic, they made the coal sticky and difficult to feed at a consistent rate. An atmosphere of 1% oxygen in nitrogen (mol mol−1) was maintained during devolatilization, rather than true pyrolysis conditions, to allow for the combustion of any tars formed. Then, 1 g portions of the produced DTF chars were refired at 1300 °C in 10% oxygen in nitrogen (mol mol−1) for 600 ms, in order to achieve the highest attainable carbon burnouts. This equated to a probe separation of 65 cm. The collected chars and ashes were weighed and their carbon contents were assessed by elemental analysis (EA) using a Flash EA 1112 CHNS-O analyzer. These values were used to conduct a mass 1533
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Figure 2. Time until 90% carbon conversion from the TGA burnout of TGA chars containing a selection of sodium salts.
Table 3. TGA Burnout Ranking of All the Alkali and Alkaline Earth Metal Compounds Tested on a Basis of 0.00005 mol g−1 of Additive
no catalyst with lithium nitrate with magnesium nitrate with strontium nitrate with sodium nitrate with potassium nitrate with calcium nitrate with strontium chloride with cesium nitrate with rubidium nitrate with lithium chloride with calcium chloride with sodium carbonate with barium chloride with magnesium chloride with cesium chloride with rubidium chloride with sodium bicarbonate with potassium chloride with sodium acetate with sodium sulfate with sodium chloride
formula
apparent 1st order rate constant (min−1)
apparent 1st order rate constant (g min−1 mol[cation]−1)
burnout time to 10% carbon remaining (min)
LiNO3 Mg(NO3)2·6H2O
0.049 0.049 0.051
982 1012
56.1 56.6 55.0
Sr(NO3)2
0.051
1013
55.0
NaNO3 KNO3
0.051 0.051
1014 1015
54.9 55.4
Ca(NO3)2.4H2O SrCl2
0.052 0.052
1033 1049
53.7 47.5
CsNO3 RbNO3 LiCl CaCl2·2H2O Na2CO3
0.053 0.054 0.055 0.055 0.055
1056 1087 1091 1096 553
51.2 49.9 49.3 48.4 47.3
BaCl2·2H2O MgCl2·6H2O
0.056 0.056
1115 1115
47.8 43.0
CsCl RbCl
0.058 0.060
1152 1197
45.3 41.0
NaHCO3
0.061
1212
45.7
KCl
0.061
1222
40.4
NaCH3CO2 Na2SO4 NaCl
0.061 0.066 0.067
1223 657 1332
45.6 40.2 36.6
despite this sample containing less than a third of the number of catalyst particles. This is unlike gasification where the salts of weak acids, or those that decompose to form weak acids, have normally given the best results. Strong acids are consequently believed to inhibit the formation of the alkali−carbon complexes that become gasification active sites.29 However, under the combustion conditions experienced in the TGA, salts of similar strength weak acids (sodium acetate, bicarbonate and carbonate) showed comparable levels of activity but were
outperformed by sodium chloride, the salt of a stronger acid. As has already been described in a parallel cycle for halide catalyzed soot prevention, this could be due to sodium chloride’s physical properties.30 In this scheme, the dissociation of oxygen occurs on the anion and is followed by a shuttle mechanism that transfers it to the surface of the soot particle. Mobility of the catalyst particles is therefore a major parameter in determining the amount of soot-oxygen contact, and it depends on temperature and the catalyst’s melting point. 1534
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Figure 3. 1st order rate constants from the TGA burnout of TGA chars containing different Group I nitrates.
Figure 4. 1st order rate constants from the TGA burnout of TGA chars containing different Group I chlorides.
apparent first order rate constant of 0.049 min−1. Two different activity trends were discovered in Figure 3, one for each of the additive treat rates, reinforcing that a compound’s cation, its anion, and the total number of catalyst particles added to the coal could all play a role in determining its observed reactivity. Thus, while the results from the 0.00005 mol g−1 basis appeared to be fairly stable across all of the different alkalis, those from the 1% mass loading seemed to track the cations’ positions in the periodic table, presenting a rough activity ranking of cesium > rubidium > potassium > sodium > lithium > no catalyst. This implied that once these salts decomposed to their corresponding oxides upon heating, a relationship might have existed between the cation’s atomic number and the catalyzed coal’s burnout rate constant. A corresponding trend has previously been reported during gasification.29 The catalytic activities of the alkali nitrates can accordingly be rationalized from their inclination to react with the coal’s carbon matrix, either before or during burnout. Explicitly, even though all of the alkalis demonstrate some initial mobility, lithium and sodium are known to possess the highest inclination to form bulk carbonate, which cannot participate in exchange reactions. Hence, while cesium, rubidium, and potassium can migrate into and across the char, these smaller elements tend to agglomerate on the particles’ surfaces.
Halide anions are thought to possess improved migration capabilities from the in situ formation of intimate contact via wetting or gas phase transport.30 Given that the loose sootcatalyst contact therein portrayed is similar to the physical mixture dispersion being employed here, a corresponding mechanism is feasible and would help to explain the good performance of sodium chloride. Potassium acetate has also been shown to wet the surface of carbon composites during their catalytic oxidization,31 but there was no obvious indication that sodium acetate’s burnout time benefited from a similar phenomenon in Figure 2even though this run did simultaneously produce the third highest combustion rate constant in Table 3. Since the tested chlorides appeared to possess characteristics that might make them exceptional coal combustion additives, it was decided that further halide compounds should be investigated. Some alkali and alkaline earth metal nitrates were also run as a comparison, because this anion produced the smallest combustion enhancements in Figure 2 and was therefore considered to be least likely to benefit from the intimate contact described above. Starting with data from the Group I nitrates, Figure 3 depicts the relative combustion rate constants of these samples per mol of catalyst cation applied. The uncatalysed coal, which is not shown, produced an 1535
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Figure 5. Association between the Group I chlorides’ melting points and their TGA char TGA burnout rate constants on a 0.00005 mol g−1 of compound basis.
Evidence of this has been provided by X-ray diffraction,32 gas change step-response experiments with labeled molecules,33 and controlled-atmosphere electron microscopy.34 Furthermore, as the Group I cations' propensity to disperse decreases, so does their ability to form complexes, the active sites of both gasification and combustion. Since hydrolysis experiments have indicated that all alkali metal complexes are probably equivalent,35 the creation of fewer of them will severely impede a combustion additive’s performance. Figure 4 presents the same information for the Group I chlorides where, as anticipated, they mostly exhibit larger burnout improvements than their equivalent nitrates, although the relationship between the additives’ activities and their properties has changed somewhat. Specifically, the 0.00005 mol g−1 results portray an evident but not statistically significant activity ranking of sodium > potassium > rubidium > cesium > lithium, which is very different from that from the nitrates. It can be explained, however, by considering the alkali chlorides’ melting points, since prior to burnout they would all have remained liquids at the high temperatures applied during TGA devolatilization. Because of this, volatilization losses are likely to have been reduced, and catalyst−coal contact is likely to have been improved. The additive particles might also have been better placed to affect the coal’s devolatilization rate, thereby intrinsically altering the created char structure. All of these factors would have led to more reactive chars and, as shown in Figure 5, a roughly linear association between the additives’ melting points and their ensuing combustion rate constants. Next, it seemed important to compare all of the tested alkali salts on a given basis, thereby providing an overall, systematic, and wide-ranging TGA ranking that covered both cation and anion effects. This is set out in Table 3, where a treat rate of 0.00005 mol g−1 is used because most of the existing published data has been on a % mass basis. Results from some alkaline earth compounds are also included, since similarities between the Group I and II elements make a comparable catalytic mechanism highly likely.36 These alkaline earth salts generally demonstrated lower activities than their equivalent alkali compounds, with 6 out of the 7 result pairings from Table 3 indicating that the Group I salt was more effective. This was most obvious with the chlorides, where all the Group I salts performed better than Group II salts from the same row of the periodic table. Similar behavior has been documented in the
literature, where it has been attributed to the metal ions’ first ionization energies and their different valences, which implies that there could be different types of bonding to the coal’s macromolecular structure.12 Hence, although the activity of a Group II site is probably comparable to that of a Group I site, it might only form at the additive−carbon phase boundary.36 This suggests that the main problem could be one of dispersion, retention, or the resulting carbon to additive ratio, although more restrictive bonding might also make the alkaline earth metal compounds poorer at dissociating oxygen.33 For the nitrates tested, the observed trend was less defined than that described above, with calcium nitrate seeming to outperform potassium nitrate by 0.001 min−1. However, this and all the other differences between the Group I and II compounds were within the expected error of ±33%,8 meaning that none of them can be considered to be statistically significant in isolation. The catalytic performances of the Group II cations were also expected to follow their atomic numbers in Table 3; chiefly because magnesium, similar to lithium and sodium, has formerly demonstrated little inclination to create carbon complexes.35 They instead demonstrated an activity trend more reminiscent of the calcium > barium ∼ strontium > magnesium ranking that has occasionally been suggested for gasification.37 Likewise, the relative rate constants of the Group II chlorides did not appear to correlate with the additives’ melting points, although once again none of the witnessed rate differences were outside of the minimum expected coefficient of variation (±33%). This may have been due to the presence of crystallization water in some of the compounds, which has previously been shown to influence a catalyst’s effectiveness.12 However, it remained difficult to comprehensively identify an underlying reason for these trends. 3.2. Additive Behavior on the DTF. Before the additives’ performances could be determined in the DTF, the combustion characteristics of the underlying samples needed to be established by assessing the repeatability and reproducibility of the entire DTF firing procedure. Table 4 provides this information as averages and standard deviations in the carbon contents of the uncatalyzed DTF ash residues and the calculated carbon burnouts. In each particle size range, these values were derived from the combustion of five ostensibly identical chars. Before the introduction of an additive, the 1536
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ultimately made the data difficult to interpret. For example, two of the additives, potassium nitrate and sodium carbonate, demonstrated slightly diminished carbon burnouts, but neither of these results deviated by more than the expected result standard deviation in Table 4 (±5%). Hence, these reductions could just be a reflection of the coal’s or the experimental setup’s inherent variability. In contrast, the 106−150 μm particle size range in Figure 6 appeared to be sufficiently removed from the extremes of burnout to allow the additives to influence the extent of carbon conversion. However, none of the observed burnout improvements were larger than the results’ predicted standard deviation (±13%), questioning whether these additives would provide any benefit in a full-scale boiler and limiting the strength of any conclusions that can be drawn. This might be because there was some separation of the physically mixed coal and additive particles as they were entrained in the combustion gases and carried through the DTF under a laminar flow regime. Compared to the TGA experiments, this would have reduced the proximity of the catalyst particles to the carbon surface and, accordingly, the achieved combustion acceleration. Additional testing, such as conducting multiple DTF firings with the same additive or performing pilot or large-scale boiler trials, is required to investigate these results further. It is also possible that another experimental technique, like a wire mesh reactor,38 would be capable of providing more reliable results while still maintaining a fairly realistic combustion environment. Nevertheless, the attained additive activity ranking from Figure 6 (sodium nitrate ∼ cesium chloride > sodium carbonate > no catalyst > potassium nitrate ∼ calcium nitrate) was quite similar to that from TGA, albeit with two noteworthy differences. First, there was an improvement in the relative performance of sodium nitrate. This additive might have outperformed calcium and potassium nitrate purely because of the number of catalyst particles supplied at this 1% loading, but it was not established why it was now more effective than cesium chloride. These results might therefore have been subject to the large experimental errors discussed above. Second, there also seemed to be a substantial reduction in the relative reactivity of sodium carbonate. Alkali carbonates are known to be exceptionally mobile during reactions,39 and while this capability makes them very active TGA combustion additives, it could also facilitate their removal in the DTF.
Table 4. Uncatalyzed DTF Burnout Variability, Expressed as Means and Standard Deviations in the Carbon Contents of the Created DTF Ash Residues and the Calculated Carbon Burnouts mean
standard deviation
particle size fraction (μm)
EA carbon content of DTF ash (%)
carbon burnout (%)
EA carbon content of DTF ash (%)
carbon burnout (%)
53−75 106−150
37.7 56.8
86.6 66.1
±6.2 ±10.3
±4.9 ±12.8
standard deviation associated with the entire DTF process was thus estimated at between ±5% and ±13%. Repeatability and reproducibility were therefore poorer than with the TGA methodology, which demonstrated a coefficient of variation of ±1−3% without a catalyst but an error of ±33−50% with one.8,26 Previous work has indicated that this unpredictably mainly originates from fluctuating process variables during DTF devolatilization. Likely culprits include the gases’ flow rates and pressures, the recycle flow rate as applied by a vacuum pump, the coal feed rate, and any automatic adjustments to the furnace’s temperature.26 Concentrating on the effects of the additives, Figure 6 shows the DTF carbon burnouts achieved by chars containing the selected Group I and II salts at a 1 part catalyst to 99 parts coal ratio. Results from both particle size ranges (53−75 μm and 106−150 μm) are presented. For the smaller coal fraction, combustion produced reasonably consistent levels of carbon burnout across the unmodified and catalyzed samples, either implying that none of the catalysts were very effective or that the reaction was entirely mass transfer limited. Perhaps this was to be expected under the high temperatures and fast heating rates experienced in the DTF, where small coal particles combust more quickly simply because they possess a larger surface area to mass ratio.26 The particles might therefore have burned at the highest achievable rate, both with or without a catalyst, and this value (∼87%) could represent the maximum carbon burnout that was possible under these DTF conditions. However, it was difficult to conclusively prove whether the reaction was occurring at the fastest possible rate and, likewise, if this would be the case in a PF boiler where there would be increased char deactivation from longer residence times. This
Figure 6. DTF carbon burnouts achieved by chars containing the selected Group I and II additives on a 1 part catalyst to 99 parts coal mass loading. 1537
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For instance, most of the inherent sodium in coal has been shown to volatilize during industrial pyrolysis at 900 °C, with less than 10% being retained in the succeeding char.40 Consequently, under the fast heating rates and 1300 °C of DTF devolatilization, a substantial loss of sodium carbonate probably occurred, so that very little additive remained to catalyze the ensuing refiring. Additional evidence of this is also depicted in Table 5, where the TGA burnout results from these
To summarize, as the inherent errors associated with the DTF methodology were very large, it was difficult to completely corroborate any of the above theories. Even the most effective DTF additivessodium nitrate and cesium chloridedisplayed carbon burnout enhancements of only 12%, slightly lower than the expected standard deviation for the 106−150 μm particle size range (±13%). TGA screening therefore remains essential, chiefly to provide a clearer insight into the relative effects of the additives. Yet, even with this TGA data and some perceptible if not statistically significant DTF burnout improvements, it was still impossible to predict the impact of these Group I and II additives on PF combustion. Additionally, in order to comprehend the underlying chemistry behind these results, more detailed information is required on the additives’ catalytic mechanisms. This subject will be addressed in an upcoming publication by carrying out a detailed investigation into the behavior of the combustion additive rubidium chloride. Variations in parent coal properties, such as inorganic ash composition, are also known to have a significant but little understood effect on catalyst performance41 and rubidium chloride’s susceptibility to interactions with ATC’s inherent mineral matter will also be investigated.
Table 5. 1st Order Rate Constants from the TGA Burnout of 106−150 μm DTF Chars Containing the Selected Group I and Group II Additives on a 1 Part Catalyst to 99 Parts Coal Loading apparent 1st order rate constant (min−1)
apparent 1st order rate constant (g min−1 mol[metal]−1)
KNO3
0.059 0.064
643
Na2CO3
0.066
364
NaNO3
0.091
761
Ca(NO3)2·4H2O
0.094
1563
CsCl
0.098
1625
formula no catalyst with potassium nitrate with sodium carbonate with sodium nitrate with calcium nitrate with cesium chloride
4. CONCLUSION A variety of Group I and II acetates, bicarbonates, carbonates, chlorides, hydroxides, nitrates, and sulfates were confirmed as effective TGA coal combustion catalysts. Of the chosen sodium salts, the chloride and carbonate demonstrated the highest levels of activity at a 1 part catalyst to 99 parts coal ratio, while the chloride was also extremely reactive on the 0.00005 mol g−1 basis. Additionally, with 1 part catalyst to 99 parts coal, the burnout activities of the Group I nitrates were found to track their cations’ atomic numbers, a trend that is supported by the inclination of these cations’ to interact with the coal’s carbon matrix. However, this reactivity pattern changed for the Group I chlorides, with data from the 0.00005 mol g−1 treat rate demonstrating a linear relationship between the chlorides’ melting points and their combustion rate constants. Several alkaline earth salts were also tested, and these typically displayed smaller TGA burnout improvements than alkali compounds from the same row of the periodic table. Subsequently, some of the additives were also investigated using a DTF. However, as the standard deviation of the results was estimated at between ±5% and ±13%, it was difficult to draw any firm conclusions from them. For the 106−150 μm particle size range, the five chosen additives generated an approximate, but not statistically significant, activity grading of sodium nitrate ∼ cesium chloride > sodium carbonate > potassium nitrate ∼ calcium nitrate. This ranking was similar to that obtained from TGA, although there were two major differences: a comparative improvement in the effectiveness of sodium nitrate and a worsening in the performance of sodium carbonate. In addition, calcium nitrate’s relative reactivity seemed to improve during the TGA burnout of these DTF chars.
chars indicate that sodium carbonate again functioned poorly compared to its previous performance in Figure 2. The loss of Group I and II additives from volatilization, and the subsequent decrease in the observed catalytic combustion effect, is a topic that will be investigated further in a future publication. One other change shown in Table 5 was the increased effectiveness of calcium nitrate, which was more active than would otherwise have been anticipated from its apparent first order TGA rate constant in Table 3 or its DTF burnout improvement in Figure 6. This could have just been an anomaly from inconsistent additive dispersion in the DTF char, especially when combined with the small mass of coal tested per TGA run, but it might also have been an indication of the Group I and II salts behaving differently. Indeed, alkaline earth metal compounds have been reported to have much higher retentions during industrial pyrolysis because their divalent bonding makes them harder to volatilize. For example, at 900 °C, as much as 60 to 70% of a coal’s original calcium can remain in its char.40 This could allow an alkaline earth metal salt to stay in contact with the coal under an intense devolatilization environment and, potentially, to act as a catalyst for longer. However, upon the introduction of oxygen, Group II salts are inclined to form carbonates, which lose contact with the char, leading to the release of the metal cation.40 This could explain calcium nitrate’s comparatively good reactivity in Table 5, where the chars were subjected to very low levels of oxygen in the harsh surroundings of DTF devolatilization and higher oxygen concentrations only in the comparatively mild environment of the TGA. Group I catalysts still produced the best results during combined TGA pyrolysis and burnout (Table 3), where additive volatility seemed to be less of a concern because of lower temperatures, and during DTF burnout where a high temperature oxidizing environment was encountered (Figure 6).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 1538
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(36) Floess, J. K.; Longwell, J. P.; Sarofim, A. F. Energy Fuels 1988, 2, 756−764. (37) Radovic, L. R.; Walker, P. L.; Jenkins, R. G. J. Catal. 1983, 82 (2), 382−394. (38) Thompson, D.; Argent, B. B. Fuel 1999, 78, 1679−1689. (39) Wu, L.; Qiao, Y.; Gui, B.; Wang, C.; Xu, J.; Yao, H.; Xu, M. Energy Fuels 2012, 26, 112−117. (40) Quyn, D. M.; Hayashi, J. I.; Li, C. Z. Fuel Process. Technol. 2005, 86 (12−13), 1241−1251. (41) Le Manquais, K.; Snape, C.; McRobbie, I.; Barker, J. Combustion Enhancing Additives for Coal Firing; ICCS&T : Nottingham, 2007, p IP21.
ACKNOWLEDGMENTS The authors are grateful to Innospec Inc. for their financial support of this PhD studentship.
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