Article pubs.acs.org/EF
Interaction of Potassium and Calcium in the Catalytic Gasification of Biosolids and Switchgrass Ross A. Arnold,† Rozita Habibi,† Jan Kopyscinski,‡ and Josephine M. Hill*,† †
Department of Chemical & Petroleum Engineering, University of Calgary, 2500 University Drive Northwest, Calgary, Alberta T2N 1N4, Canada ‡ Department of Chemical Engineering, McGill University, 3610 University Street, Montreal, Quebec H3A 0C5, Canada ABSTRACT: Catalytic gasification is a method of converting biosolids, the solids created during wastewater treatment, into a valuable gaseous stream. One of the challenges with this process is that the components in the ash of the biosolids can interact with the gasification catalyst(s)in particular, calcium and potassium. In this study, the behaviors of different combinations of switchgrass (the source of potassium), biosolids, ash-free carbon black, and mixtures of each feed with added calcium and/or potassium were observed with a thermogravimetric analysis unit. The results were consistent with calcium preferentially reacting with components in the ash, preventing the deactivation of potassium. Any additional calcium available may form bimetallic compounds with the potassium, but this interaction did not increase the rate of reaction. Modeling was performed using the random pore model and extended random pore model, with the appropriate model chosen for each mixture. The extended random pore model was better suited for the data with the highest reaction rates.
1. INTRODUCTION As urbanization continues worldwide, the ability to efficiently treat water becomes more of a concern. While sufficient purification of municipal drinking water is of high importance, the treatment and disposal of wastewater is also a key consideration. Separation and disposal of biosolids, the solids obtained from wastewater treatment, can represent more than 50% of the total cost of wastewater treatment.1 In the past, biosolids have been disposed as a fertilizer, through incineration, through landfilling,2,3 and via anaerobic digestion,4 but there are issues with each of these processes. Notably, spreading biosolids on land as fertilizer has been controversial,5 emissions from incineration are becoming more heavily regulated,6 anaerobic digestion is slow7 and cannot reach complete conversion,8 and landfills pose long-term problems, especially for regions where land is at a premium.9 As such, thermal conversion methods such as gasification have been proposed as an alternative to disposal for biosolids.10 For example, British Columbia-based company Nexterra has developed a commercial-scale gasifier that can process 12 t/day of biosolids and other biomass feedstocks.11 Gasification is a partial oxidation of a carbon-based feed12 to produce syngas,13 a mixture of primarily hydrogen gas and carbon monoxide.14 Gasification of biosolids has been shown to produce syngas of a high quality8 (less than 10% gas-phase impurities). Gasification can take place in the presence of a catalyst, as this lowers the necessary temperature15 and can reduce the tar content16 of the product gas. Alkali metals have been observed to be effective gasification catalysts, with their activity increasing with increasing molecular weight.17 Potassium in particular has been studied due to its availability and high mobility.18−20 Potassium has been added to biomass for gasification through mixing with a precursor (see ref 21 and references therein) or through doping into the biomass matrix.22 Biomass-based sources of potassium are attractive in © 2017 American Chemical Society
terms of reducing costs and increasing the sustainability of gasification.23 Switchgrass, which is a prairie grass native to North America and a commonly used energy crop due to its low-demand and fast-growing nature,24,25 is an effective source of potassium.26,27 Although active for gasification reactions, potassium can be deactivated if the feed contains aluminum and silicon because stable potassium aluminosilicate compounds are formed.28 The composition of biosolids varies but generally comprises ∼50% carbon with various other species, including nitrogen, phosphorus, and calcium. The presence of calcium during potassium-catalyzed gasification reactions has been observed to keep potassium in a catalytically active form.29−32 The mechanism by which calcium is able to promote the catalytic activity of potassium is not well understood, but Wang et al.29,33,34 have proposed two theories: (1) calcium is able to react with kaolinite, a model aluminosilicate compound, suggesting that calcium is able to promote the catalytic activity of potassium by limiting the sites through which potassium can deactivate;33 (2) calcium and potassium can form a bimetallic compound that may possess superior catalytic properties, which would lead to the promoting activity of calcium being unrelated to the presence of mineral matter.29,34 Gasifier design can be optimized according to the interaction mechanism. Thus, in this paper, the interaction between potassium and calcium has been studied by using biosolids and switchgrass, biomass-based sources of calcium and potassium, respectively. Gasification experiments were also performed with ash-free carbon black, to observe the interaction between calcium and potassium in the absence of aluminosilicates. Received: April 5, 2017 Published: April 20, 2017 6240
DOI: 10.1021/acs.energyfuels.7b00972 Energy Fuels 2017, 31, 6240−6247
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Energy & Fuels
rate of 2 deg min−1. Experimental patterns were then compared to patterns of known compounds using the Jade 5.0 database (MDI, Livermore, CA). 2.4. Kinetic Modeling. Kinetic modeling followed the procedure explained previously.42 In this work, only the random pore model (RPM; eq 2) and extended random pore model (eRPM; eq 3) were considered.
2. MATERIALS AND METHODS 2.1. Sample Preparation. Switchgrass (Manitoba, Canada) and class A primary biosolids (provided by Nexterra Systems Corp., Vancouver, British Columbia, Canada) were the studied biomass samples. Proximate, ultimate, and ash analyses of the samples were performed by Loring Laboratories, Calgary, Alberta, Canada. Experiments were also done with an essentially pure carbon: carbon black (CB; Monarch 120, Cabot Corp., Alpharetta, GA). Both the biomass samples were converted to char at 850 °C under N2 flow for 2 h and then sieved to obtain particle sizes below 90 μm. The CB was used as received, with a particle size of 9000 10
0
0.1
0
2
75
0.1
0.1
2
2
7
compared to Ca. The rate of reaction for a 50/50 mixture by mass of SG and BC (curve labeled SG/BC observed) was similar to that for BC, and significantly lower than a weighted average of the rates for the two feeds (dotted line). Thus, there was an unfavorable interaction (inhibition) between the feeds that resulted in a lower than expected gasification rate over the entire range. A similar effect has been observed during CO2 cogasification of SG mixed with a high-ash coal.27 During gasification, the K in SG undergoes an oxygen transfer cycle including a site regeneration step.48 After a carbon in SG is converted, the K can move due to its high mobility either to the next carbon within SG (intraparticle) or to BC (interparticle). On the basis of the observed behavior, a significant amount of K moves to BC, but the K becomes deactivated by the formation of nonreactive potassium aluminosilicate compounds such as KAlSiO4. Adding SG to BC in fact showed an inhibitive effect, likely due to the much higher ash content of BC compared to SG. Since Al is present in much smaller amounts than Si in the biosolid ash, Al is the limiting reactant in the formation of potassium aluminosilicate compounds. For this reason, catalyst/Al ratios are reported instead of catalyst/Si ratios. The K/Al molar ratio increased from 0.004 to 1.0 for BC and BC/SG (Table 2), but still most K was captured by the aluminosilicates in the mixture. When SG ash rather than SG was used in the mixture (BC/ SG ash, Figure 1b), the rate of gasification was higher than that of SG. Increasing the amount of K resulted not only in a higher K/Al ratio, but also in a higher K/C ratio (Table 2). The K/Al mole ratio increased from 0.004 to 1.0 to 2.8 for the BC, BC/
Figure 1. CO2 gasification rate as a function of the char conversion for (a) switchgrass (SG) and biosolids (BC) and their mixture (BC/SG) and (b) BC without and with various amounts of potassium (KOH and SG ash). Symbols represent experimental data points. Solid lines represent the best fit model, and the dotted line refers to the noninteracting BC/SG mixture. (Note that (a) and (b) have different yscales.)
experimental observations (symbols) and the best fit model (solid lines), the theoretical non-interacting char conversion (dotted line) of the mixture is presented. The latter is calculated on the basis of the weighted average of the single feed conversions assuming no interaction between the constituents: X noninteracting mix = βXBC + (1 − β)XSG
time to 50% conversion (min)
(4)
where β is the mass fraction of the BC in the mixture (i.e., β = 0.5 in this study) and XBC and XSG are the conversions of the BC and SG, respectively. 6242
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hydroxide, potassium hydroxide, and/or kaolinite are shown as a function of conversion in Figure 3. CB on its own had a
SG, and BC/SG ash samples, respectively. Thus, for the latter mixture, not all K reacted with the aluminosilicates; the excess K led to an increase in the gasification rate. The K/Al ratio of 11 in SG was much higher than in BC/SG ash (K/Al = 2.8), but the latter exhibited a gasification rate that was greater by a factor of 3. This behavior can be explained by the larger K/C ratio in BC/SG ash compared to SG (K/C = 0.12 vs K/C = 0.02). Previous studies have shown the gasification rate increased with increasing K/C ratio until saturation was reached.49,50 Thus, both the K/Al and K/C ratios are important parameters in understanding and evaluating the gasification behavior. 3.2. Biomass with Catalysts. To verify the components of interest, K and Ca were added to BC and SG, respectively, and the gasification rates determined (Figures 1b and 2). As
Figure 3. Effect of addition of (a) calcium hydroxide and potassium hydroxide and (b) kaolinite on the CO2 gasification rate as a function of the char conversion of carbon black (CB). Symbols represent experimental data points. Solid lines represent the best fit model.
Figure 2. Effect of calcium hydroxide addition on the CO2 gasification rate of SG char as a function of conversion. Symbols represent experimental data points. Solid lines represent the best fit model.
negligible rate of gasification, with a maximum rate of 0.0006 min−1, which is less than the variation observed between triplicate runs of the other mixtures. Ash typically contains compounds that can promote gasification, so a low reaction rate for the ash-free CB was expected. Ca addition (Ca/C = 0.1) increased the maximum rate by an order of magnitude from 0.0006 to 0.007 min−1, while K addition (K/C = 0.1) increased the maximum rate of reaction by 2 orders of magnitude to 0.091 min−1 (Figure 3a). Thus, at the same catalyst/carbon molar ratio (0.1), K was a much more effective catalyst than Ca, consistent with the results of other studies (see ref 51 and references therein). The maximum gasification rates occurred at 30% conversion for the CB/KOH mixture and as the initial rate of reaction for the CB/Ca(OH)2 mixture. Adding Ca to the CB/KOH sample (CB/KOH/Ca(OH)2) resulted again in a promoting effect as stated previously. The effect, however, was only observed at the onset of the gasification. The initial gasification rate of CB/KOH/Ca(OH)2 was higher than that for CB/KOH (i.e., 0.1 min−1 versus 0.078 min−1), but from ∼30% conversion onward, the rates were similar. To obtain additional evidence, kaolinite, a representative aluminosilicate compound, was added to CB and catalyzed CB samples. Adding kaolinite to both CB and CB/Ca(OH)2 did not significantly change the gasification rates of either sample (not shown), confirming that kaolinite had no catalytic ability, and suggesting that Ca might not react with kaolinite to form calcium aluminosilicates. The addition of kaolinite to the CB/ KOH mixture resulted in a significant decrease in the reaction rate at all conversions (Figure 3b). The maximum reaction rate was reduced by ∼40% from 0.087 to 0.053 min−1, consistent with K deactivation through combination with aluminosilicate compounds. In the presence of Ca, the effect of kaolinite was
expected, the addition of K increased the gasification rate of BC (Figure 1b) provided the K/Al ratio was greater than 1, allowing for the presence of excess alkali metal in the system. The addition of KOH to biosolidsK/Al ratio of 1.3 (Table 2)increased the maximum rate of reaction more than 4-fold from 0.007 to 0.030 min−1. The maximum gasification rate of BC/KOH (0.028 min−1) was significantly smaller than that of BC/SG ash (0.052 min−1), but higher than that of SG (0.016 min−1) even though SG had the highest K/Al ratio. Thus, if the K/Al ratio is larger than the stoichiometric requirement for the deactivation reaction (i.e., K/Al > 1), the K/C ratio is the more important parameter (0.02 for SG, 0.04 for BC/KOH, and 0.12 for BC/SG ash). As shown in Figure 2, the addition of Ca(OH)2 to SG increased the reaction rate. The increase was small when the Ca/C ratio was increased from 0.004 (SG without additives) to 0.008. The improvement in the reaction rate was up to 0.0017 min−1, which was more than the maximum variation of 0.0010 min−1 observed between triplicate runs of the same mixture. When the Ca/C ratio was increased further to 0.04, there was a 3-foldfrom 0.015 to 0.045 min−1increase in the maximum rate of reaction and the shape of the curve changed. For SG, alone the maximum rate of reaction occurred at ∼25% conversion, while for SG with a Ca/C ratio of 0.04, the maximum rate of reaction was the initial rate of reaction. Ca had a clear promoting effect on the catalytic behavior of K during gasification. These experiments, however, do not resolve the literature dispute over the nature of the promoting effect of Ca. 3.3. Carbon Black. As mentioned, CB contains essentially no ash (Table 1). The rates of gasification for CB with calcium 6243
DOI: 10.1021/acs.energyfuels.7b00972 Energy Fuels 2017, 31, 6240−6247
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Energy & Fuels greatly reduced (curve labeled “CB/KOH/Ca(OH)2/Kao”), and the maximum rate of reaction was increased again. It appears that Ca was able to mitigate the deactivation effect of aluminosilicates, consistent with results found in previous work.52 Besides the increased rate, the curve shape changed and the maximum gasification rate occurred at lower conversion values. XRD was used to analyze samples before and after gasification. For the CB/Ca(OH)2/kaolinite mixture, the XRD peaks matched those of Ca(OH)2 before gasification and those of CaCO3 after gasification (Figure 4). The presence
KOH-based signals. There were, however, no obvious nonhydrate KOH peaks in the pre-gasification mixture. Similar peaks for kaolinite were seen in both CB/Ca(OH)2/kaolinite and CB/KOH/kaolinite pre-gasification mixtures, with the set of four peaks in the 19.9−22.0° range noticeably absent. The most prevalent species for the CB/KOH/kaolinite mixture after gasification was KAlSiO4 (Figure 5), confirming the suggestion based on the TGA results that K reacted with the added aluminosilicates to lower its catalytic activity. The pregasification XRD pattern for the CB/KOH/Ca(OH)2/kaolinite mixture (Figure 6) was similar to that of CB/Ca(OH)2/
Figure 4. XRD patterns for the post-gasification mixture of carbon black, Ca(OH) 2 , and kaolinite compared to various known compounds.
Figure 6. XRD patterns for post-gasification mixture of carbon black, Ca(OH)2, KOH, and kaolinite compared to various known compounds.
of CaCO3 suggests that the Ca was not reacting to form calcium aluminosilicates, in agreement with literature results in which Ca did not react with kaolinite to form calcium aluminosilicates at temperatures below 850 °C.33,53 Any peaks associated with kaolinite or Ca(OH)2 from the initial mixture were not seen in the post-gasification mixture. The XRD pattern for CB/KOH/kaolinite before gasification had some peaks matching those of KOH·H2O (Figure 5), but the signals were not as well-defined as those of Ca(OH)2 in CB/ Ca(OH)2/kaolinite pre-gasification (Figure 4). KOH combines with water to form a hydrate, which would result in two weaker
kaolinite (Figure 4), in that Ca(OH)2 was the main species present. No peaks matching kaolinite, KOH, or KOH·H2O were seen, so the file patterns for these species were not included in Figure 6. In the post-gasification XRD of CB/ KOH/Ca(OH)2/kaolinite, neither CaCO3 nor KAlSiO4 was present in notable amounts, suggesting an interaction between Ca and K. There were peaks that corresponded to two calcium aluminosilicates: Ca2Al2SiO7 and Ca3Al2(SiO4)3. There was no evidence of the bimetallic compound K2Ca(CO3)2, since no peaks at 2θ = 31.2°, 54.2°, or 56.0° were seen in the XRD profile of the ash. The largest peaks for K2Ca(CO3)2 and Ca2Al2SiO7 both occur near 2θ = 31°, which may explain why both species have been reported in the literature.33,34 No calcium aluminosilicates were observed without K present (Figure 4), which suggests that K assists calcium aluminosilicate formation. The gasification curve changed shape when Ca was added, both when Ca(OH)2 was added to SG char until the Ca/C ratio passed 0.04 (Figure 2) and when Ca was added to the CB/KOH mixture (Figure 3a). In both cases, addition of Ca moved the maximum rate of reaction from 20−30% conversion to the initial rate of reaction, implying a direct interaction between K and Ca. A bimetallic Ca−K compound may form, as observed via XRD by Wang et al.,29,34 but the results in the current study (Figure 3a) demonstrate that this compound does not possess catalytic properties superior to those of K on its own. 3.4. Modeling. The single feeds SG, BC, and CB, as well as the mixtures of BC/SG and CB/Ca(OH)2, showed good agreement when modeled with the random pore model (RPM)
Figure 5. XRD patterns for the post-gasification mixture of carbon black, KOH, and kaolinite compared to various known compounds. 6244
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Table 4. Structural (ψ) and Empirical (c, p) Parameters for the Catalyzed Gasification Mixtures Using the Extended Random Pore Model (eRPM) with a 95% Confidence Intervala
(Figures 1−3). The kinetic parameters of these samples with a 95% confidence interval are shown in Table 3. Table 3. Rate Constants (kj), Structural Parameters (ψ), and R2 Values for Studied Gasification Mixtures That Follow the Random Pore Model (RPM) with a 95% Confidence Interval sample SG BC CB BC/SG SG/Ca(OH)2 (Ca/C = 0.008) SG/Ca(OH)2 (Ca/C = 0.04)
kj (min−1) 1.44 × 10−2 ± 8.06 × 10−5 6.44 × 10−3 ± 8.40 × 10−5 1.84 × 10−4 ± 3.38 × 10−6 5.06 × 10−3 ± 2.04 × 10−5 1.42 × 10−2 ± 1.74 × 10−4 2.67 × 10−2 ± 3.95 × 10−4
ψ
R2
3.36 ± 0.07
0.9975
1.65 ± 0.10
0.9975
49.50 ± 2.15
0.9991
2.24 ± 0.04
0.9998
5.54 ± 0.21
0.9979
2.94 ± 0.16
0.9979
ψ
sample BC/KOH BC/SG ash CB/KOH CB/ Ca(OH)2 CB/KOH/ Ca(OH)2 CB/KOH/ kaolinite CB/ Ca(OH)2/ kaolinite CB/KOH/ Ca(OH)2/ kaolinite
6.88 8.87 14.73 17.73
± ± ± ±
c 0.50 0.70 0.56 0.50
2.6 5.3 333.9 18.2
± ± ± ±
R2
p 0.0 0.1 4.2 0.2
0.41 0.51 0.60 0.027
± ± ± ±
0.03 0.02 0.00 0.004
0.9993 0.9999 0.9999 0.9986
19.40 ± 8.89
378.9 ± 57.6
0.94 ± 0.07
0.9986
11.45 ± 0.94
233.4 ± 6.1
0.66 ± 0.01
0.9997
26.84 ± 1.94
19.7 ± 0.5
0.48 ± 0.01
0.9994
9.05 ± 1.19
359.0 ± 13.9
0.64 ± 0.02
0.9994
a
The intrinsic reaction rate constant kj for the BC/KOH and BC/SG ash samples is assumed to be the same as for the single BC sample. The intrinsic reaction rate constant kj for the catalyzed CB samples is assumed to be the same as for the single CB sample. The deviation of the gasification rate of the catalyzed samples from the single sample is expressed with empirical parameters c and p (see the text).
The reaction rate constants follow the order of the observed gasification rate for the single feedstocks (SG > BC > CB); SG had the highest reaction constant of kSG = 1.4 × 10−2 min−1, whereas the rate constant for BC was about half (kBC = 6.4 × 10−3 min−1) and that for CB was 2 orders of magnitude smaller (kCB = 1.8 × 10−4 min−1). The rate constant for the mixture of BC/SG was slightly smaller than the rate constant for the BC sample, which was in agreement with the experimental data (Figure 1). The structural factors for SG, BC, and BC/SG were of the same order of magnitude, ranging from 2 to 5. Carbon black had a structural factor of ∼50, which is consistent with its low initial surface area with respect to that of the biomass chars. Catalyzed gasification of BC and CB samples could not be fitted with the RPM. To investigate and understand the influence of the catalyst and kaolinite on the gasification, the intrinsic rate constant kj for the pure BC and CB samples was used for the catalyzed mixtures when fitted by the extended random pore model (eRPM). Thus, one out of four parameters was fixed, and the other three (structural factor ψ and the semiempirical parameters c and p) were estimated from the experimental data to describe the deviation from the intrinsic gasification of BC and CB. The results for the eRPM are summarized in Table 4 and illustrated in Figures 1−3 as solid lines. For the catalyzed BC sample, the kinetic parameter c increased with increasing K/C ratio and gasification rate (i.e., for BC/KOH, c = 2.6 with K/C = 0.04 and, for BC/SG ash, c = 5.3 with K/C = 0.12), confirming a previous study42 which correlated the parameter c with the K/C ratio and value of the maximum gasification rate. The structural factor (ψ) increased from 1.6 for BC to 6.7 for BC/KOH and to 8.9 for BC/SG ash, confirming that the catalyst promoted surface area development, resulting in a higher ratio of maximum to initial surface area. The addition of K and Ca catalysts as well as kaolinite influenced the observed gasification rate of carbon black (CB) significantly. In terms of kinetic modeling, the eRPM fit well with the CO2 gasification behavior (Figure 3). The kinetic parameter c showed that adding alkali metal catalyst increased the maximum gasification rate, with a higher value of c for CB/ KOH/Ca(OH)2 (379) than for CB/KOH (334) and for CB/ Ca(OH)2 (18), shown in Table 4. The eRPM fit was not good for the initial 25% conversion of the CB/KOH/Ca(OH)2 sample (Figure 3a). Nevertheless, the larger value for parameter
p (0.94 for CB/KOH/Ca(OH)2 and 0.60 for CB/KOH) indicated that the maximum gasification rate occurred at a lower conversion,42 which is why the eRPM with the inclusion of parameter p is a better representation of the data than the RPM. Adding kaolinite to CB/KOH reduced the observed rate and thus parameter c (233 for CB/KOH/kaolinite and 334 for CB/KOH), while adding kaolinite to CB/Ca(OH)2 had little influence on the gasification rate and parameter c (19.7 for CB/ Ca(OH)2/kaolinite and 18.2 for CB/Ca(OH)2). Furthermore, the deactivation of the K from kaolinite can be reduced by adding Ca(OH)2, as shown by a larger value for parameter c: 359 for CB/KOH/Ca(OH)2/kaolinite compared to 233 for CB/KOH/kaolinite.
4. CONCLUSIONS A series of gasification experiments with mixtures of biomass chars and additives with various carbon, potassium, calcium, and/or kaolinite ratios were performed in a thermogravimetric analyzer to elucidate the interactions between these species. The results of these experiments were consistent with Ca promoting the activity of K by preferentially combining with the aluminosilicate compounds in the ash. A Ca−K compound could also have formed, which resulted in the maximum rate being the initial rate of reaction but did not improve the overall gasification rate. Modeling using the random pore model was a good fit for the mixtures with lower rates of gasification. The extended random pore model was a better fit for catalyzed mixtures of biosolids and of carbon black because the presence of catalysts affects surface area development during gasification.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 403-210-9488. Fax: 403-284-4852. E-mail: jhill@ ucalgary.ca. ORCID
Jan Kopyscinski: 0000-0002-1785-0541 6245
DOI: 10.1021/acs.energyfuels.7b00972 Energy Fuels 2017, 31, 6240−6247
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Energy & Fuels Author Contributions
(10) Seggiani, M.; Vitolo, S.; Puccini, M.; Bellini, A. Cogasification of sewage sludge in an updraft gasifier. Fuel 2012, 93, 486−491. (11) Nexterra Technology. Waste Feedstocks. http://www.nexterra. ca/files/biomass-feedstocks.php (accessed June 14, 2016). (12) Furimsky, E. Gasification of oil sand coke: Review. Fuel Process. Technol. 1998, 56 (3), 263−290. (13) Gikas, P. Electrical energy production from biosolids: a comparative study between anaerobic digestion and ultra-hightemperature gasification. Environ. Technol. 2014, 35 (17−20), 2140−6. (14) Broer, K. M.; Woolcock, P. J.; Johnston, P. A.; Brown, R. C. Steam/oxygen gasification system for the production of clean syngas from switchgrass. Fuel 2015, 140 (0), 282−292. (15) McKee, D. W. Mechanisms of the alkali metal catalysed gasification of carbon. Fuel 1983, 62 (2), 170−175. (16) Sueyasu, T.; Oike, T.; Mori, A.; Kudo, S.; Norinaga, K.; Hayashi, J.-i. Simultaneous Steam Reforming of Tar and Steam Gasification of Char from the Pyrolysis of Potassium-Loaded Woody Biomass. Energy Fuels 2012, 26 (1), 199−208. (17) Wood, B. J.; Sancier, K. M. The Mechanism of the Catalytic Gasification of Coal Char: A Critical Review. Catal. Rev.: Sci. Eng. 1984, 26 (2), 233−279. (18) Karimi, A.; Gray, M. R. Effectiveness and mobility of catalysts for gasification of bitumen coke. Fuel 2011, 90 (1), 120−125. (19) Kopyscinski, J.; Rahman, M.; Gupta, R.; Mims, C. A.; Hill, J. M. K2CO3 catalyzed CO2 gasification of ash-free coal. Interactions of the catalyst with carbon in N2 and CO2 atm. Fuel 2014, 117B, 1181− 1189. (20) Wen, W.-Y. Mechanisms of Alkali Metal Catalysis in the Gasification of Coal, Char, or Graphite. Catal. Rev.: Sci. Eng. 1980, 22 (1), 1−28. (21) Irfan, M. F.; Usman, M. R.; Kusakabe, K. Coal gasification in CO2 atm and its kinetics since 1948: A brief review. Energy 2011, 36 (1), 12−40. (22) Kramb, J.; Gómez-Barea, A.; DeMartini, N.; Romar, H.; Doddapaneni, T. R. K. C.; Konttinen, J. The effects of calcium and potassium on CO2 gasification of birch wood in a fluidized bed. Fuel 2017, 196, 398−407. (23) Brown, R. C.; Liu, Q.; Norton, G. Catalytic effects observed during the co-gasification of coal and switchgrass. Biomass Bioenergy 2000, 18 (6), 499−506. (24) McLaughlin, S.; Bouton, J.; Bransby, D.; Conger, B.; Ocumpaugh, W.; Parrish, D.; Taliaferro, C.; Vogel, K.; Wullschleger, S. Developing switchgrass as a bioenergy crop. Perspectives on New Crops and New Uses; American Society for Horticultural Science: Alexandria, VA, 1999; p 282. (25) Wright, L. Historical Perspective on How and Why Switchgrass Was Selected as a ″Model″ High-Potential Energy Crop; Prepared by Oak Ridge National Laboratory for the Department of Energy; Oak Ridge National Laboratory: Oak Ridge, TN, 2007. (26) Masnadi, M. S.; Grace, J. R.; Bi, X. T.; Lim, C. J.; Ellis, N.; Li, Y. H.; Watkinson, A. P. Single-fuel steam gasification of switchgrass and coal in a bubbling fluidized bed: A comprehensive parametric reference for co-gasification study. Energy 2015, 80 (0), 133−147. (27) Habibi, R.; Kopyscinski, J.; Masnadi, M. S.; Lam, J.; Grace, J. R.; Mims, C. A.; Hill, J. M. Co-gasification of Biomass and Non-biomass Feedstocks: Synergistic and Inhibition Effects of Switchgrass Mixed with Sub-bituminous Coal and Fluid Coke During CO2 Gasification. Energy Fuels 2013, 27 (1), 494−500. (28) Bruno, G.; Buroni, M.; Carvani, L.; Piero, G. D.; Passoni, G. Water-insoluble compounds formed by reaction between potassium and mineral matter in catalytic coal gasification. Fuel 1988, 67 (1), 67− 72. (29) Hu, J.; Liu, L.; Cui, M.; Wang, J. Calcium-promoted catalytic activity of potassium carbonate for gasification of coal char: The synergistic effect unrelated to mineral matter in coal. Fuel 2013, 111, 628−635. (30) Liu, L.; Liu, H.; Cui, M.; Hu, Y.; Wang, J. Calcium-promoted catalytic activity of potassium carbonate for steam gasification of coal char: Transformations of sulfur. Fuel 2013, 112, 687−694.
R.A.A. completed the gasification experiments, mixed the feeds, and prepared the manuscript. R.H. obtained the analysis of the feeds and did some preliminary experiments with biosolids and/or SG. J.K. set up the equipment, consulted on the experimental design, and performed the modeling as well as the analysis of the modeling results. The original project idea came from J.M.H., who obtained all the equipment for the experiments and supervised the other authors. Funding
We recognize funding from the Natural Sciences and Engineering Research Council (NSERC) of Canada through a strategic grant (STPGP 380927). Notes
The authors declare no competing financial interest.
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ABBREVIATIONS BC = biosolid char CB = carbon black eRPM = extended random pore model RPM = random pore model SG = switchgrass char TGA = thermogravimetric analysis XRD = X-ray diffraction c = semiempirical parameter (dimensionless), eRPM (eq 3) kj = reaction rate constant (min−1) m = mass (mg) p = semiempirical parameter (dimensionless), RPM (eq 3) T = temperature (°C) t = time (min) X = char conversion (dimensionless) β = weight fraction (dimensionless) (eq 4) ψ = structural factor (dimensionless), RPM (eqs 2 and 3)
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REFERENCES
(1) Rulkens, W. Sewage sludge as a biomass resource for the production of energy: overview and assessment of the various options. Energy Fuels 2008, 22 (1), 9−15. (2) Yu, M. M.; Masnadi, M. S.; Grace, J. R.; Bi, X. T.; Lim, C. J.; Li, Y. Co-gasification of biosolids with biomass: Thermogravimetric analysis and pilot scale study in a bubbling fluidized bed reactor. Bioresour. Technol. 2015, 175 (0), 51−58. (3) U.S. EPA. Biosolids Generation, Use and Disposal in the United States; EPA530-R-99-009; Washington, DC, 1999. (4) De los Cobos-Vasconcelos, D.; Villalba-Pastrana, M. E.; Noyola, A. Effective pathogen removal by low temperature thermal pretreatment and anaerobic digestion for Class A biosolids production from sewage sludge. J. Water, Sanit. Hyg. Dev. 2015, 5 (1), 56−63. (5) Shober, A. L.; Stehouwer, R. C.; Macneal, K. E. On-farm assessment of biosolids effects on soil and crop tissue quality. J. Environ. Qual. 2003, 32 (5), 1873−1880. (6) Dangtran, K.; Mullen, J. F.; Mayrose, D. T. A comparison of fluid bed and multiple hearth biosolids incineration. Proceedings of the Water Environment Federation 2000, 2000 (1), 368−384. (7) Appels, L.; Lauwers, J.; Degrève, J.; Helsen, L.; Lievens, B.; Willems, K.; Van Impe, J.; Dewil, R. Anaerobic digestion in global bioenergy production: Potential and research challenges. Renewable Sustainable Energy Rev. 2011, 15 (9), 4295−4301. (8) Gikas, P. Ultra high temperature gasification of municipal wastewater primary biosolids in a rotary kiln reactor for the production of synthesis gas. J. Environ. Manage. 2016, 10.1016/j.jenvman.2016.02.043 (9) Girovich, M. J. Biosolids Treatment and Management: Processes for Beneficial Use; CRC Press: Boca Raton, FL, 1996. 6246
DOI: 10.1021/acs.energyfuels.7b00972 Energy Fuels 2017, 31, 6240−6247
Article
Energy & Fuels (31) Perander, M.; DeMartini, N.; Brink, A.; Kramb, J.; Karlström, O.; Hemming, J.; Moilanen, A.; Konttinen, J.; Hupa, M. Catalytic effect of Ca and K on CO2 gasification of spruce wood char. Fuel 2015, 150, 464−472. (32) Ramsurn, H.; Kumar, S.; Gupta, R. B. Enhancement of Biochar Gasification in Alkali Hydrothermal Medium by Passivation of Inorganic Components Using Ca(OH)2. Energy Fuels 2011, 25 (5), 2389−2398. (33) Tang, J.; Guo, R.; Wang, J. Inhibition of interaction between kaolinite and K2CO3 by pretreatment using calcium additive. J. Therm. Anal. Calorim. 2013, 114 (1), 153−160. (34) Jiang, M.-Q.; Zhou, R.; Hu, J.; Wang, F.-C.; Wang, J. Calciumpromoted catalytic activity of potassium carbonate for steam gasification of coal char: Influences of calcium species. Fuel 2012, 99, 64−71. (35) Masnadi, M. S.; Grace, J. R.; Bi, X. T.; Lim, C. J.; Ellis, N. From fossil fuels towards renewables: Inhibitory and catalytic effects on carbon thermochemical conversion during co-gasification of biomass with fossil fuels. Appl. Energy 2015, 140 (0), 196−209. (36) Emami Taba, L.; Irfan, M. F.; Wan Daud, W. A. M.; Chakrabarti, M. H. The effect of temperature on various parameters in coal, biomass and CO-gasification: A review. Renewable Sustainable Energy Rev. 2012, 16 (8), 5584−5596. (37) Franco, C.; Pinto, F.; Gulyurtlu, I.; Cabrita, I. The study of reactions influencing the biomass steam gasification process☆. Fuel 2003, 82 (7), 835−842. (38) Hirano, A.; Hon-Nami, K.; Kunito, S.; Hada, M.; Ogushi, Y. Temperature effect on continuous gasification of microalgal biomass: theoretical yield of methanol production and its energy balance. Catal. Today 1998, 45 (1−4), 399−404. (39) Wu, C.; Williams, P. T. Effects of gasification temperature and catalyst ratio on hydrogen production from catalytic steam pyrolysisgasification of polypropylene. Energy Fuels 2008, 22 (6), 4125−4132. (40) Devi, T. G.; Kannan, M. P. Gasification of Biomass Chars in Air − Effect of Heat Treatment Temperature. Energy Fuels 2000, 14 (1), 127−130. (41) Hanping, C.; Bin, L.; Haiping, Y.; Guolai, Y.; Shihong, Z. Experimental investigation of biomass gasification in a fluidized bed reactor. Energy Fuels 2008, 22 (5), 3493−3498. (42) Kopyscinski, J.; Habibi, R.; Mims, C. A.; Hill, J. M. K2CO3Catalyzed CO2 Gasification of Ash-Free Coal: Kinetic Study. Energy Fuels 2013, 27 (8), 4875−4883. (43) Bhatia, S. K.; Perlmutter, D. D. A random pore model for fluidsolid reactions: I. Isothermal, kinetic control. AIChE J. 1980, 26 (3), 379−386. (44) Zhang, Y.; Ashizawa, M.; Kajitani, S.; Miura, K. Proposal of a semi-empirical kinetic model to reconcile with gasification reactivity profiles of biomass chars. Fuel 2008, 87 (4−5), 475−481. (45) Powers, S. M.; Bruulsema, T. W.; Burt, T. P.; Chan, N. I.; Elser, J. J.; Haygarth, P. M.; Howden, N. J. K.; Jarvie, H. P.; Lyu, Y.; Peterson, H. M.; Sharpley, A. N.; Shen, J.; Worrall, F.; Zhang, F. Longterm accumulation and transport of anthropogenic phosphorus in three river basins. Nat. Geosci. 2016, 9 (5), 353−356. (46) Zhang, Q.; Liu, H.; Li, W.; Xu, J.; Liang, Q. Behavior of Phosphorus during Co-gasification of Sewage Sludge and Coal. Energy Fuels 2012, 26 (5), 2830−2836. (47) Matinde, E.; Sasaki, Y.; Hino, M. Phosphorus Gasification from Sewage Sludge during Carbothermic Reduction. ISIJ Int. 2008, 48 (7), 912−917. (48) Fernandes, R.; Hill, J. M.; Kopyscinski, J. Determination of the Synergism/Antagonism Parameters during Co-gasification of Potassium-Rich Biomass with Non-biomass Feedstock. Energy Fuels 2017, 31, 1842. (49) Mims, C. A.; Krajewski, J. J. Mechanism of methane formation in potassium catalyzed carbon gasification. J. Catal. 1986, 102 (1), 140−150. (50) Kopyscinski, J.; Mims, C. A.; Hill, J. M. Formation of CH4 during K2CO3-Catalyzed Steam Gasification of Ash-Free Coal:
Influence of Catalyst Loading, H2O/H2 Ratio, and Heating Protocol. Energy Fuels 2015, 29 (11), 6970−6977. (51) Nzihou, A.; Stanmore, B.; Sharrock, P. A review of catalysts for the gasification of biomass char, with some reference to coal. Energy 2013, 58, 305−317. (52) Habibi, R. Co-Gasification of Biomass and Non-Biomass Feedstocks. Thesis, University of Calgary, Canada, 2013. (53) Shelby, J. E. Formation and Properties of Calcium Aluminosilicate Glasses. J. Am. Ceram. Soc. 1985, 68 (3), 155−158.
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NOTE ADDED AFTER ASAP PUBLICATION This article published with an incorrect Figure 4 file. The correct figure published June 6, 2017.
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DOI: 10.1021/acs.energyfuels.7b00972 Energy Fuels 2017, 31, 6240−6247
