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Interaction of Potassium and Calcium in the Catalytic Gasification of Biosolids and Switchgrass Ross Alexander Arnold, Rozita Habibi, Jan Kopyscinski, and Josephine M. Hill Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 20 Apr 2017 Downloaded from http://pubs.acs.org on April 23, 2017
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Interaction of Potassium and Calcium in the
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Catalytic Gasification of Biosolids and Switchgrass
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Ross A. Arnold‡, Rozita Habibi‡, Jan Kopyscinski†, Josephine M. Hill*‡
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‡Department of Chemical & Petroleum Engineering, University of Calgary, 2500 University Dr.
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N.W., Calgary, AB, T2N 1N4, Canada
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†Department of Chemical Engineering, McGill University, 3160 University St., Montreal, QC,
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H3A 0C5, Canada
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ABSTRACT
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Catalytic gasification is a method of converting biosolids, the solids created during wastewater
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treatment, into a valuable gaseous stream. One of the challenges with this process is that the
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components in the ash of the biosolids can interact with the gasification catalyst(s) – in
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particular, calcium and potassium. In this study, the behaviors of different combinations of
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switchgrass (the source of potassium), biosolids, ash-free carbon black, and mixtures of each
15
feed with added calcium and/or potassium were observed with a thermogravimetric analysis unit.
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The results were consistent with calcium preferentially reacting with components in the ash,
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preventing the deactivation of potassium. Any additional calcium available may form bimetallic
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compounds with the potassium, but this interaction did not increase the rate of reaction.
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Modeling was performed using the random pore model and extended random pore model, with
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the appropriate model chosen for each mixture. The extended random pore model was better
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suited for the data with the highest reaction rates.
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1. INTRODUCTION
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As urbanization continues worldwide, the ability to efficiently treat water becomes more of a
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concern. While sufficient purification of municipal drinking water is of high importance, the
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treatment and disposal of wastewater is also a key consideration. Separation and disposal of
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biosolids, the solids obtained from wastewater treatment, can represent more than 50% of the
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total cost of wastewater treatment1. In the past, biosolids have been disposed as a fertilizer,
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through incineration, through landfilling2, 3, and via anaerobic digestion4, but there are issues
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with each of these processes. Notably, spreading biosolids on land as fertilizer has been
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controversial5, emissions from incineration are becoming more heavily regulated6, anaerobic
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digestion is slow7 and cannot reach complete conversion8, and landfills pose long-term problems,
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especially for regions where land is at a premium9. As such, thermal conversion methods such as
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gasification have been proposed as an alternative to disposal for biosolids10. For example, British
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Columbia-based company Nexterra has developed a commercial-scale gasifier that can process
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12 tonnes/day of biosolids and other biomass feedstocks11.
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Gasification is a partial oxidation of a carbon-based feed12 to produce syngas13, a mixture of
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primarily hydrogen gas and carbon monoxide14. Gasification of biosolids has been shown to
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produce syngas of a high quality15 (less than 10% gas-phase impurities). Gasification can take
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place in the presence of a catalyst, as this lowers the necessary temperature16 and can reduce the
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tar content17 of the product gas. Alkali metals have been observed to be effective gasification
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catalysts, with their activity increasing with increasing molecular weight18. Potassium in
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particular has been studied due to its availability and high mobility19-21. Potassium has been
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added to biomass for gasification through mixing with a precursor22 (and references therein) or through
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doping into the biomass matrix23. Biomass-based sources of potassium are attractive in terms of
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reducing costs and increasing the sustainability of gasification24. Switchgrass, which is a prairie
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grass native to North America and a commonly-used energy crop due to its low-demand and
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fast-growing nature25, 26, is an effective source of potassium27, 28. Although active for gasification
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reactions, potassium can be deactivated if the feed contains aluminum and silicon because stable
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potassium aluminosilicate compounds are formed29.
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The composition of biosolids varies but generally comprises ~50% carbon with various other
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species including nitrogen, phosphorus, and calcium. The presence of calcium during potassium-
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catalyzed gasification reactions has been observed to keep potassium in a catalytically active
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form30-33. The mechanism by which calcium is able to promote the catalytic activity of potassium
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is not well understood but Wang et al.30, 34, 35 have proposed two theories: (1) calcium is able to
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react with kaolinite, a model aluminosilicate compound, suggesting that calcium is able to
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promote the catalytic activity of potassium by limiting the sites through which potassium can
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deactivate34; (2) calcium and potassium can form a bimetallic compound that may possess
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superior catalytic properties, which would lead to the promoting activity of calcium being
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unrelated to the presence of mineral matter30, 35. Gasifier design can be optimized according to
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the interaction mechanism.
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Thus, in this paper, the interaction between potassium and calcium has been studied by using
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biosolids and switchgrass, biomass-based sources of calcium and potassium, respectively.
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Gasification experiments were also performed with ash-free carbon black, to observe the
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interaction between calcium and potassium in the absence of aluminosilicates.
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2. MATERIALS AND METHODS 2.1 Sample Preparation. Switchgrass (Manitoba, Canada) and Class A primary
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biosolids (provided by Nexterra Systems Corp., British Columbia, Canada) were the studied
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biomass samples. Proximate, ultimate, and ash analyses of the samples were performed by
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Loring Laboratories, Calgary, Alberta, Canada. Experiments were also done with an essentially
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pure carbon: carbon black (CB, Monarch 120, Cabot Corp., Georgia, USA).
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Both the biomass samples were converted to char at 850 °C under N2 flow for 2 h, then sieved to
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obtain particle sizes below 90 μm. The CB was used as received, with a particle size of 1), the K/C ratio is
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the more important parameter (0.02 for SG, 0.04 for BC/KOH and 0.12 for BC/SG ash).
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As shown in Figure 2, the addition of Ca(OH)2 to SG increased the reaction rate. The increase
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was small when the Ca/C ratio was increased from 0.004 (SG without additives) to 0.008. The
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improvement in reaction rate was up to 0.0017 min-1, which was more than the maximum
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variation of 0.0010 min-1 observed between triplicate runs of the same mixture. When the Ca/C
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ratio was increased further to 0.04, there was a threefold - 0.015 min-1 to 0.045 min-1 - increase in
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the maximum rate of reaction and the shape of the curve changed. For SG alone, the maximum
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rate of reaction occurred at ~25% conversion, while for SG with a Ca/C ratio of 0.04, the
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maximum rate of reaction was the initial rate of reaction. Ca had a clear promoting effect on the
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catalytic behavior of K during gasification. These experiments, however, do not resolve the
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literature dispute over the nature of the promoting effect of Ca.
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3.3 Carbon black. As mentioned, CB contains essentially no ash (Table 1). The rates of
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gasification for CB with calcium hydroxide, potassium hydroxide, and/or kaolinite are shown as
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a function of conversion in Figure 3. CB on its own had a negligible rate of gasification, with a
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maximum rate of 0.0006 min-1, which is less than the variation observed between triplicate runs
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of the other mixtures. Ash typically contains compounds that can promote gasification, so a low
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reaction rate for the ash-free CB was expected. Ca addition (Ca/C = 0.1) increased the maximum
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rate by an order of magnitude from 0.0006 min-1 to 0.007 min-1, while K addition (K/C = 0.1)
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increased the maximum rate of reaction by two orders of magnitude to 0.091 min-1 (Figure 3a).
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Thus, at the same catalyst/carbon molar ratio (0.1), K was a much more effective catalyst than
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Ca, consistent with the results of other studies52 (and references therein). The maximum gasification rates
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occurred at 30% conversion for the CB/KOH mixture and as the initial rate of reaction for the
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CB/Ca(OH)2 mixture. Adding Ca to the CB/KOH sample (CB/KOH/Ca(OH)2) resulted again in
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a promoting effect as stated previously. The effect, however, was only observed at the onset of
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the gasification. The initial gasification rate of CB/KOH/Ca(OH)2 was higher than that for
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CB/KOH (i.e., 0.1 min-1 versus 0.078 min-1) but from ~30% conversion onwards, the rates were
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similar.
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To obtain additional evidence, kaolinite, a representative aluminosilicate compound, was added
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to CB and catalyzed CB samples. Adding kaolinite to both CB and CB/Ca(OH)2 did not
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significantly change the gasification rates of either sample (not shown), confirming that kaolinite
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had no catalytic ability, and suggesting that Ca might not react with kaolinite to form calcium
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aluminosilicates. The addition of kaolinite to the CB/KOH mixture resulted in a significant
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decrease in the reaction rate at all conversions (Figure 3b). The maximum reaction rate was
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reduced by ~40% from 0.087 min-1 to 0.053 min-1, consistent with K deactivation through
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combination with aluminosilicate compounds. In the presence of Ca, the effect of kaolinite was
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greatly reduced (curve labeled CB/KOH/Ca(OH)2/kaolinite), and the maximum rate of reaction
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was increased again. It appears that Ca was able to mitigate the deactivation effect of
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aluminosilicates, consistent with results found in previous work53. Besides the increased rate, the
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curve shape changed and the maximum gasification rate occurred at lower conversion values.
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XRD was used to analyze samples before and after gasification. For the CB/Ca(OH)2/kaolinite
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mixture, the XRD peaks matched that of Ca(OH)2 before gasification, and that of CaCO3 after
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gasification (Figure 4). The presence of CaCO3 suggests that the Ca was not reacting to form
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calcium aluminosilicates, in agreement with literature results in which Ca did not react with
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kaolinite to form calcium aluminosilicates at temperatures below 850 °C34, 54. Any peaks
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associated with kaolinite or Ca(OH)2 from the initial mixture were not seen in the post-
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gasification mixture. The XRD pattern for CB/KOH/kaolinite before gasification had some peaks
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matching KOH∙H2O (Figure 5), but the signal was not as well-defined as Ca(OH)2 in
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CB/Ca(OH)2/kaolinite pre-gasification (Figure 4). KOH combines with water to form a hydrate,
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which would result in two weaker KOH-based signals. There were, however, no obvious non-
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hydrate KOH peaks in the pre-gasification mixture. Similar peaks for kaolinite were seen in both
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CB/Ca(OH)2/kaolinite and CB/KOH/kaolinite pre-gasification mixtures, with the set of four
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peaks in the 19.9° to 22.0° range noticeably absent. The most prevalent species for the
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CB/KOH/kaolinite mixture after gasification was KAlSiO4 (Figure 5), confirming the suggestion
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based on the TGA results that K reacted with the added aluminosilicates to lower its catalytic
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activity. The pre-gasification XRD pattern for the CB/KOH/Ca(OH)2/kaolinite mixture (Figure
17
6) was similar to that of CB/Ca(OH)2/kaolinite (Figure 4), in that Ca(OH)2 was the main species
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present. No peaks matching kaolinite, KOH, or KOH∙H2O were seen, so the file patterns for
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these species were not included in Figure 6. In the post-gasification XRD of
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CB/KOH/Ca(OH)2/kaolinite, neither CaCO3 nor KAlSiO4 were present in notable amounts,
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suggesting an interaction between Ca and K. There were peaks that corresponded to two calcium
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aluminosilicates: Ca2Al2SiO7 and Ca3Al2(SiO4)3. There was not evidence of the bimetallic
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compound K2Ca(CO3)2, since no peaks at 2θ = 31.2°, 54.2°, or 56.0° were not seen in the XRD
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profile of the ash. The largest peaks for K2Ca(CO3)2 and Ca2Al2SiO7 both occur near 2θ = 31°,
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which may explain why both species have been reported in the literature34, 35. No calcium
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aluminosilicates were observed without K present (Figure 4), which suggests that K assists
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calcium aluminosilicate formation.
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The gasification curve changed shape when Ca was added, both when Ca(OH)2 was added to SG
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char until the Ca/C ratio passed 0.04 (Figure 2), and when Ca was added to the CB/KOH mixture
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(Figure 3a). In both cases, addition of Ca moved the maximum rate of reaction from 20-30%
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conversion to the initial rate of reaction, implying a direct interaction between K and Ca. A
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bimetallic Ca-K compound may form, as observed via XRD by Wang et al30, 35, but the results in
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the current study (Figure 3a) demonstrate that this compound does not possess superior catalytic
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properties to K on its own.
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3.4 Modeling. The single feeds SG, BC, and CB, as well as the mixtures of BC/SG and
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CB/Ca(OH)2 showed good agreement when modeled with the random pore model (RPM)
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(Figures 1 to 3). The kinetic parameters of these samples with a 95% confidence interval are
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shown in Table 3.
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The reaction rate constants follow tedhe order of the observed gasification rate for the single
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feedstocks (SG > BC > CB); SG had the highest reaction constant of kSG = 1.4·10-2 min-1,
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whereas the rate constant for BC was ~half (kBC = 6.4·10-3 min-1) and for CB was two orders of
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magnitude smaller (kCB = 1.4·10-4 min-1). The rate constant for the mixture of BC/SG was
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slightly smaller than the rate constant for the BC sample, which was in agreement with the
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experimental data (Figure 1). The structural factors for SG, BC and BC/SG were in the same
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order of magnitude, ranging from 2 to 5. Carbon black had a structural factor of ~50, which is
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consistent with its low initial surface area with respect to the biomass chars.
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Catalyzed gasification of BC and CB samples could not be fitted with the RPM. In order to
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investigate and understand the influence of the catalyst and kaolinite on the gasification, the
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intrinsic rate constant kj for the pure BC and CB samples was used for the catalyzed mixtures
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when fitted by the extended random pore model (eRPM). Thus one out of four parameters was
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fixed, and the other three (structural factor ψ and the semi-empirical parameters c and p) were
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estimated from the experimental data to describe the deviation from the intrinsic gasification of
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BC and CB. The results for the eRPM are summarized in Table 4 and illustrated in Figures 1 to 3
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as solid lines. For the catalyzed BC sample, the kinetic parameter c increased with increasing
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K/C ratio and gasification rate (i.e., BC/KOH: c = 2.6 with K/C = 0.04; BC/SG ash: c = 5.3 with
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K/C = 0.12), confirming a previous study43 which correlated the parameter c with the K/C ratio
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and value of the maximum gasification rate. The structural factor (ψ) increased from 1.6 for BC
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to 6.7 for BC/KOH and to 8.9 for BC/SG ash, confirming that the catalyst promoted surface area
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development, resulting in a higher ratio of maximum to initial surface area.
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The addition of K and Ca catalysts as well as kaolinite influenced the observed gasification rate
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of carbon black CB significantly. In term of kinetic modeling, the eRPM fit well with the CO2
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gasification behavior (Figure 3). The kinetic parameter c showed that adding alkali catalyst
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increased the maximum gasification rate, with a higher value of c for CB/KOH/Ca(OH)2 (379)
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than for CB/KOH (334) and for CB/Ca(OH)2 (18), shown in Table 4. The eRPM fit was not
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good for the initial 25% conversion of the CB/KOH/Ca(OH)2 sample (Figure 3a). Nevertheless,
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the larger value for parameter p (0.94 for CB/KOH/Ca(OH)2, and 0.60 for CB/KOH) indicated
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that the maximum gasification rate occurred at a lower conversion43, which is why the eRPM
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with the inclusion of parameter p is a better representation of the data than the RPM. Adding
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kaolinite to CB/KOH reduced the observed rate and thus parameter c (233 for
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CB/KOH/kaolinite, and 334 for CB/KOH), while adding kaolinite to CB/Ca(OH)2 had little
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influence on the gasification rate and parameter c (19.7 for CB/Ca(OH)2/kaolinite and 18.2 for
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CB/Ca(OH)2). Furthermore, the deactivation of the K from kaolinite can be reduced by adding
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Ca(OH)2, as shown by a larger value for parameter c: 359 for CB/KOH/Ca(OH)2/kaolinite
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compared to 233 for CB/KOH/kaolinite.
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4. CONCLUSIONS
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A series of gasification experiments with mixtures of biomass chars and additives with various
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carbon, potassium, calcium, and/or kaolinite ratios were performed in a thermogravimetric
13
analyzer to elucidate the interactions between these species. The results of these experiments
14
were consistent with Ca promoting the activity of K by preferentially combining with the
15
aluminosilicate compounds in the ash. A Ca-K compound could also have formed, which
16
resulted in the maximum rate being the initial rate of reaction but did not improve the overall
17
gasification rate. Modeling using the random pore model was a good fit for the mixtures with
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lower rates of gasification. The extended random pore model was a better fit for catalyzed
19
mixtures of biosolids and of carbon black because the presence of catalysts affects surface area
20
development during gasification.
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FIGURES
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Figure 1. CO2 gasification rate as a function of char conversion for (a) switchgrass (SG), biosolids (BC) and their mixture (BC/SG); and for (b) BC without and with various amount of potassium (KOH and SG ash). Symbols represent experimental data points; solid lines represent best fitted model and dotted line refers to non-interacting BC/SG mixture. (Note: Figure a) and b) have a different y-scale.)
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2 3 4
Figure 2. Effect of calcium hydroxide addition on CO2 gasification rate of SG char as a function of conversion. Symbols represent experimental data points; solid lines represent best fitted model.
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1 2 3 4
Figure 3. Effect of addition of (a) calcium hydroxide, potassium hydroxide, and (b) kaolinite on the CO2 gasification rate as a function of char conversion of carbon black (CB). Symbols represent experimental data points; solid lines represent best fitted model.
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Figure 4. XRD patterns for the post-gasification mixture of carbon black, Ca(OH)2, and kaolinite compared to various known compounds.
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Figure 5. XRD patterns for the post-gasification mixture of carbon black, KOH, and kaolinite compared to various known compounds.
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Figure 6. XRD patterns for post-gasification mixture of carbon black, Ca(OH)2, KOH, and kaolinite compared to various known compounds.
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TABLES
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Table 1. Proximate, ultimate, and ash analyses of samples as received. Biosolids
Switchgrass
Carbon Black
Moisture
9.9
6.0
-
Ash (dry basis)
26.4
6.3
0.64
Volatile (dry basis)
60.2
76.9
0.20
Fixed carbon (dry basis)
13.4
16.8
99.2
Proximate analysis (wt%)
Ultimate analysis (wt%), ash free Carbon, C
52.8
47.9
99.4
Hydrogen, H
8.8
6.2
-
Nitrogen, N
8.2
0.8
-
Sulfur, S
1.3
0.1
-
Oxygen, O (by difference)
28.9
45.0
Si
8.8
24.6
-
Al
3.3
1.1
-
Ti
1.1
0.01
-
Fe
2.3
0.2
-
Ca
11.2
4.6
-
Mg
7.3
3.9
-
Na
0.2
1.2
-
K
1.1
16.8
-
P
17.3
2.2
-
S
n.a.
1.0
-
Balance
47.5
44.4
-
Ash Analysis (wt%)
3 4
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Table 2. Molar ratios between select elements and gasification times of studied carbon mixtures.
Mixture
Molar ratios K/C
Ca/C
K/Al
Ca/Al
Time to 50% conversion (min)
SG
0.02
0.004
11
2.2
34
BC
0.0001
0.06
0.004
2.0
120
BC/SG
0.01
0.02
1.0
2.0
125
BC/SG ash
0.12
0.09
2.8
2.0
11
SG/Ca(OH)2 Ca/C=0.04
0.02
0.04
11
22
18
SG/Ca(OH)2 Ca/C=0.008
0.02
0.008
11
4.4
30
BC/KOH
0.04
0.06
1.3
2.0
21
CB
0
0
N/A
N/A
1100
CB/KOH
0.1
0
N/A
N/A
6
CB/Ca(OH)2
0
0.1
N/A
N/A
86
CB/KOH/Ca(OH)2
0.1
0.1
N/A
N/A
7
CB/kaolinite
0
0
N/A
N/A
>9000
CB/KOH/kaolinite
0.1
0
2
0
10
CB/Ca(OH)2/kaolinite
0
0.1
0
2
75
CB/KOH/Ca(OH)2/kaolinite
0.1
0.1
2
2
7
4 5
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Table 3. Rate constant (kj), structural parameter (ψ) and R-square value for studied gasification mixtures that follow the random pore model (RPM) with 95% confidence interval.
3 ψ
kj (min-1)
Sample
R-square
SG
1.44×10-2
±
8.06×10-5
3.36
±
0.07
0.9975
BC
6.44×10-3
±
8.40×10-5
1.65
±
0.10
0.9975
CB
1.84×10-4
±
3.38×10-6
49.50
±
2.15
0.9991
BC/SG
5.06×10-3
±
2.04×10-5
2.24
±
0.04
0.9998
SG/Ca(OH)2 [Ca/C = 0.008]
1.42×10-2
±
1.74×10-4
5.54
±
0.21
0.9979
SG/Ca(OH)2 [Ca/C = 0.04]
2.67×10-2
±
3.95×10-4
2.94
±
0.16
0.9979
4 5 6
Table 4. Structural (ψ) and empirical (c, p) parameters for the catalyzed gasification mixtures using the extended random pore model (eRPM) with 95% confidence interval.
7 Sample
ψ
c
p
R-square
BC/KOH
6.88 ± 0.50
2.6 ± 0.0
0.41 ± 0.03
0.9993
BC/SG ash
8.87 ± 0.70
5.3 ± 0.1
0.51 ± 0.02
0.9999
CB/KOH
14.73 ± 0.56
333.9 ± 4.2
0.60 ± 0.00
0.9999
CB/Ca(OH)2
17.73 ± 0.50
18.2 ± 0.2
0.027 ± 0.004
0.9986
CB/KOH/Ca(OH)2
19.40 ± 8.89
378.9 ± 57.6
0.94 ± 0.07
0.9986
CB/KOH/kaolinite
11.45 ± 0.94
233.4 ± 6.1
0.66 ± 0.01
0.9997
CB/Ca(OH)2/kaolinite
26.84 ± 1.94
19.7 ± 0.5
0.48 ± 0.01
0.9994
359.0 ± 13.9
0.64 ± 0.02
0.9994
CB/KOH/Ca(OH)2/kaolinite 9.05 ± 1.19
Note: The intrinsic reaction rate constant kj for the BC/KOH and BC/SG ash samples is assumed to be the same as for single BC sample. The intrinsic reaction rate constant kj for the catalyzed CB samples are 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 text).
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Energy & Fuels
1
AUTHOR INFORMATION
2
Corresponding Author
3
*Phone: (403) 210-9488. Fax: (403) 284-4852. E-mail:
[email protected] 4
Author Contributions
5
The manuscript was written through contributions of all authors. RAA completed the gasification
6
experiments, mixed the feeds, and prepared the manuscript. RH obtained the analysis of the
7
feeds and did some preliminary experiments with Biosolids and/or SG. JK set up the equipment,
8
consulted on experimental design, and performed the modeling as well as the analysis of the
9
modeling results. The original project idea came from JMH who obtained all the equipment for
10
the experiments and supervised the other authors. All authors have reviewed and provided input
11
for the manuscript, and given approval to the final version of the manuscript.
12
Funding Sources
13
Natural Sciences and Engineering Research Council (NSERC) of Canada (STPGP 380927).
14
ACKNOWLEDGEMENTS
15
The authors would like to recognize funding from the Natural Sciences and Engineering
16
Research Council (NSERC) of Canada through a Strategic grant.
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1
ABBREVIATIONS
2
BC
biosolid char
3
CB
carbon black
4
eRPM
extended random pore model
5
RPM
randpm pore model
6
SG
switchgrass char
7
TGA
thermogravimetric analysis
8
XRD
x-ray diffraction
9
c
-
10
kj
min-1 reaction rate constant
11
m
mg
mass
12
p
-
semi-empirical parameter, RPM (eq. 3)
13
T
°C
temperature
14
t
min
time
15
X
-
char conversion
16
β
-
weight fraction (eq. 4)
17
ψ
-
structural factor, RPM (eqs. 2 and 3)
Page 26 of 30
semi-empirical parameter, eRPM (eq. 3)
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