Kinetics and Mechanism of CO2 Gasification of Chars from 11

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Kinetics and mechanism of CO2 gasification of chars from eleven Mongolian lignites Enkhsaruul Byambajav, Yasuyo Hachiyama, Shinji Kudo, Koyo Norinaga, and Jun-ichiro Hayashi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02262 • Publication Date (Web): 07 Jan 2016 Downloaded from http://pubs.acs.org on January 10, 2016

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Kinetics and mechanism of CO2 gasification of chars from eleven Mongolian lignites Enkhsaruul Byambajav,† Yasuyo Hachiyama,§ Shinji Kudo, § Koyo Norinaga, § Jun-ichiro Hayashi§,‡,* †

Department of Chemistry, School of Arts & Sciences, National University of Mongolia University street-1, Baga toiruu, Ulaanbaatar 14201, Mongolia §

‡

Institute for Materials Chemistry and Engineering, Kyushu University Research and Education Center of Carbon Resources, Kyushu University 6-1, Kasuga Koen, Kasuga 816-8580, Japan * Author to whom all correspondence should be addressed. E-mail: [email protected], Tel.: +81 92 583 7796

Abstract: This work investigated kinetics and mechanism of CO2 gasification of chars from eleven types of Mongolian lignites (ash contents; 7–27 wt%-dry) and the corresponding acid-washed (catalyst-free) ones. The catalytic gasification contributed to 83–99% of the initial rate of char conversion to gas. The non-catalytic gasification obeyed first-order kinetics with respect to the fraction of unconverted char, 1–X, with little or no effect of physical property of char such as specific surface area and porosity. The proposed kinetic model, which considered progress in parallel of non-catalytic and catalytic gasification, assumed the presence of three to four different types of catalysts together with a type of precursor, if necessary. The optimized kinetic parameters enabled quantitative description of the measured changes with time of X over its near entire range, X = 0–0.9995. The kinetic analysis revealed that the most and least active catalysts in each char had initial activities differing by two to three orders of magnitude. The initial rate of catalytic gasification, which varied with the lignite type by a factor of 16, was correlated well and linearly with (Ca+Na)/Si or (Ca+Na+Fe)/Si molar ratio. It was suggested that more or less portions of catalytic species had lost their activities prior to the gasification by reaction with Si and Al species. The catalyst also underwent deactivation during the gasification. The rate constant for the deactivation was a function of neither Si nor Si+Al content in the char, but depended on the initial activity of catalyst.

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Nomenclature Ccn

: Concentration of catalyst Cn [–]

Cc1prec : Concentration of precursor of catalyst C1 [–] ! kcn

: Rate constant of the gasification catalyzed by catalyst Cn [min-1 ]

kcn

! mcn [min-1] : Rate of catalytic gasification defined by kcn

! kc1prec : Rate constant for transformation of C1 precursor to C1 [min-1] kloss-n : Rate constant for loss of Cn [min-1] k nc

: Rate constant for non-catalytic gasification [min-1]

mcn

: Amount of catalyst Cn [–]

mc1prec : Amount of precursor of catalyst C1 [–] t X 0

: Time from the commencement of gasification [ min ] : Conversion of char by gasification on the basis of char mass [–] : Time at commencement of gasification (at t = 0)

2


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1. Introduction Mongolia is one of the countries hosting largest coal reserves. Lignite accounts for about 80% of Mongolia’s total estimated coal reserves of 162 billion tons, 1 and it is therefore a most important resource for export and domestic use. Gasification is a most effective use of Mongolian lignites that have particular features; abundance of alkali and alkaline earth (AAEM) metals and iron. These metallic species are inherent precursors of catalysts for gasification, and therefore responsible for fast gasification with steam and CO2. 2–4 There have been a number of studies on the catalysis of AAEM species 5–11 and iron ones, 12–16 and those on the catalytic mechanism. 17 It is also known that such metallic species catalyze the steam or CO2 reforming of pyrolytic volatiles on the surface of char that plays a role of catalyst support. 18–20 Mongolian lignites are much different from those produced in Victoria state of Australia and Indonesia in terms of ash contents, in other words, those of silica/alumina oxides (SiO2, Al2O3, silicates, aluminates and aluminosilicates). These oxides have high reactivity with AAEM and Fe species and therefore propensity to deactivate catalysts and their precursors during the gasification 7,8,21 and probably also the pyrolysis. 22–25 It is thus necessary in kinetic and mechanistic investigation of the gasification of Mongolian lignites to consider catalysis of the inherent metallic species and their deactivation together. It is of no doubt that the gasification of chars from lignites and biomass consists of catalytic and non-catalytic reactions that occur simultaneously, but nonetheless, there have been only a few kinetic models that consider the kinetics distinguishing catalytic reactions from non-catalytic ones. 26–29 It is also recognized that the existing models have difficulty in describing the char behavior over the entire range of its conversion. In practical gasification, unconverted fraction of char, as well as that of tar, is an important factor for the process performance, and its minimization can effectively reduce the frequency of troubles in gasifier downstream and loading onto the processes there. However, there have been no, or if any, very few trials 29,30 to describe kinetic behavior of char from the beginning of gasification to complete or near complete char conversion. Many of the previous kinetic studies evaluated the char reactivity employing measures such as initial rate of gasification or time required for half conversion of char. 29 Such measures are not available in either predicting the kinetics or considering the mechanism at near complete char conversion. For example, higher initial rate of catalytic gasification does not necessarily lead to that in the late stage. 26,27,29 Kinetics of the catalyst deactivation processes should also be considered for predicting their behavior particularly in late and final stages of the char conversion. It is believed that quantitative description of the kinetics from beginning to completion of char conversion is a step necessary for understanding of the mechanism of gasification. Kim et al. 29 investigated steam gasification of chars from the original and Ca-loaded Victorian lignites. They modified previous kinetic models 26–28 that considered simultaneous non-catalytic and catalytic gasification. The modified model quantitatively described the char 3


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conversion over its range up to 0.99 under a variety of operating variables and Ca loading by assuming the presence of two different types of Ca catalysts that had different activities and deactivation kinetics. The report by Kim et al. 29 suggested necessity of assumption of two or even more catalytic components for fully describing the kinetics of gasification of char from Mongolian lignite. The present authors studied on CO2 gasification of chars from the pyrolysis of Mongolian lignites of that contents of metallic species and ash were over wide ranges, and those from the corresponding acid-washed (i.e., catalyst-free) lignites. The kinetic data were analyzed by a parallel catalytic/non-catalytic reaction that assumed presence of three or four types of catalysts with different initial activities and deactivation kinetics, together with that of catalyst precursor. This paper reports that the model with the optimized kinetic parameters describes time dependent change in the char conversion up to 0.999–0.9995, and then discusses factors influencing the initial catalytic activity and kinetics of catalyst deactivation. 2. Experimental 2.1. Lignite samples. As-received eleven lignites were pulverized to sizes smaller than 150 mesh, and dried at 60 °C for 24 h under vacuum. Table 1 shows the ultimate analyses and ash contents of the lignites. The ash content ranges over a range of 6.8–24.2 wt% on a dry basis. The individual lignites will be denoted by the abbreviations shown in the table. Table 1. Elemental composition and ash content. Lignite

ID

Aduun chuluun Baganuur Chandgan tal Khavtgai Khashaat khudag Khuut Shivee-ovoo Tevhsiin govi Tsaidam nuur Tugrug nuur Unset khudag

AC BN CT KG KK KT SO TG TsN TuN UK

C

H

67.6 65.4 76.5 70.8 69.8 70.9 72.0 71.5 73.7 66.2 64.6

5.3 5.6 3.3 4.1 5.6 4.8 5.2 5.2 4.9 5.2 3.6

N

S

1.2 1.5 1.4 1.1 1.5 1.0 1.1 2.2 1.9 1.7 2.0

< 0.2 < 0.2 1.6 0.7 0.3 1.4 1.2 0.9 1.1 1.2 1.3

wt%-daf

ash wt%-dry 7.9 7.3 26.7 10.0 10.3 6.8 13.5 24.2 12.4 11.3 10.0

Table 2 presents the molar abundances of major metallic species. Si, Al and Ca account for 65–89% of the total molar amounts of the listed elements, while that of Na or Fe is more than 10% for some lignites. Si, Al and Fe were quantified by a general sequence of ashing (combustion) and analysis of the ash by X-ray fluorescence spectroscopy (XRF). This sequence was employed for determining the contents of Fe, Si and Al, but not for the AAEM species. This was because of problems such as volatilization of the AAEM species during the ashing and also inaccuracy of their quantification by XRF. Instead, every lignite was treated in aqueous solution of ammonium acetate (concentration; 0.5 mol L-1), 31,32 which extracted cations bound to organic matter, those as water-soluble inorganic salts, and also those as 4


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slightly water-soluble minerals such as CaCO3. 33 The extracted cations were quantified by an inductively coupled plasma atomic emission spectrometry (ICP-AES). The ammonium acetate method is not necessarily appropriate for measuring the total contents of AAEM species, but a best one to quantify AAEM species as catalyst precursors. Previous studies showed that organically bound metal cations (ion-exchanged ones to acidic functional groups of lignite), 2,3,17 inorganic salts such as NaCl 3,11,34 were precursors of highly dispersed Ca catalysts, 15,17,22 while either particulates of oxides, carbonates or other types of minerals were ineffective unless their sizes well below 1 µm. 35 Every lignite was also washed with an aqueous solution of HCl (3 N) at 70 °C for 48 h for removing AAEM species and Fe, and then further with deionized water (resistivity; 18.2 Ω) until no detection of chlorine ion in the washing, 36 and then drying in the same way as the original lignite. Table 2. Contents of metallic species. Lignite ID

Ca

Aduun AC chuluun 0.299 Baganuur BN 0.360 Chandgan CT tal 0.260 Khashaat KK khudag 0.345 Khavtgai KG 0.234 Khuut KT 0.047 Shivee-ovoo SO 0.418 Tevhsiin TG govi 0.321 Tsaidam TsN nuur 0.288 Tugrug TuN nuur0.443 Unset UK khudag 0.195

Mg

Na

0.190 0.064 0.374 0.298 0.139 0.040 0.210 0.321 0.177 0.226 0.128

K Fe mol/kg-organic-matter 0.123 0.002 0.126 0.013 0.003 0.177 0.061 0.006 0.884 0.093 0.005 0.096 0.100 0.012 0.065 0.007 0.001 0.105 0.039 0.003 0.148 0.112 0.014 0.195 0.061 0.004 0.173 0.019 0.003 0.071 0.197 0.003 0.252

Si

Al

0.37 1.34 2.79 0.84 1.05 0.69 0.85 3.03 1.12 0.78 0.54

0.15 0.33 1.42 0.29 0.36 0.34 0.25 1.68 0.44 0.53 0.20

2.2. Pyrolysis and gasification. A portion (ca. 2.8 mg) of the original lignite was subjected to the pyrolysis and subsequent gasification in a thermogravimetric analyzer (TGA; Hitachi Hi-Tech Science Corporation, model STA7200). The mass of lignite sample, in other words, the thickness of lignite/char bed, must be minimized so that the measured rate of gasification was free from the effect of mass transport within the bed, while the gas velocity over the lignite/char bed must be maximized within an acceptable range. The TGA was equipped a particular horizontal digital dual beam system that realized high thermogravimetric sensitivity (0.2 µg) and stability allowing relatively high gas velocity. According to results of preliminary runs, the above-mentioned initial mass of lignite was decided. The total gas flow rate was fixed at 700 ml-stp min-1. In the TGA, the lignite sample was heated in flow of atmospheric N2 (purity > 99.9995 vol%) from ambient temperature to the holding temperature of 900 °C at a rate of 30 °C min-1. The resulting char was further heated for 10 min, and then the N2 flow was switched to that of CO2/N2 mixed gases (50/50 in volume) to start the gasification while the total volumetric flow rate was maintained at 700 ml-stp min-1. This flow rate corresponded to a linear gas velocity 5


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through the horizontal furnace tube of 0.186 m s-1 at 900 °C. The sample mass was monitored until the complete gasification of carbonaceous portion of the char, in other words, until the mass became steady. The acid-washed lignites were subjected to the pyrolysis and subsequent gasification under exactly the same conditions as described above. Reproducibility of the above-described thermogravimetry, which had been confirmed previously for steam gasification, 36 was again confirmed by repeating the runs under the same conditions for every chars. Table 3 shows the char yields from the original and acid-washed lignites. The yields were determined from the mass release from the individual lignites until the end of the pyrolysis (i.e., the end of the holding period of 10 min at 900 °C). The char yields from the original lignites were in a narrow range of 50–57 wt%-daf except BN and TG. The acid washing resulted in more or less decrease in the char yield for all of the lignites. The decrease was in a range of 0.3–2.4 wt%-daf, which was more than the experimental error within ±0.1 wt%-daf. The decrease in the char yield was mainly due to removal of metallic cations that promoted cross-linking and then charring. 37–39 Table 3. Char yield from original and acid-washed lignites. ID AC BN CT KG KK KT SO TG TsN TuN UK

original 53.2 62.1 55.3 50.2 54.3 56.8 55.1 47.4 57.0 53.6 53.4

Char yield, wt%-daf acid-washed Difference 50.9 2.3 61.0 1.1 53.2 2.1 49.3 0.9 51.9 2.4 56.5 0.3 53.8 1.3 46.2 1.2 55.1 1.9 51.4 2.2 52.2 1.2

2.3. Kinetic analysis and modeling The present kinetic model considers simultaneous progress of non-catalytic and catalytic gasification in parallel. In case of steam gasification of chars from lignites and biomass, the non-catalytic gasification follows first-order kinetics with respect to the residual fraction of char, i.e., 1–X. 26–29 The first-order kinetics means that the char undergoes homogeneous reaction. Kudo et al. 40 recently studied steam gasification of seven different types of acid-washed biomass and lignite chars, and reported that first-order rate constant was maintained in the course of gasification while the specific surface area increased from ca. 500 up to even > 2,500 m g-1. Thus, the surface area of char was not a factor influencing the rate of steam gasification. There was, however, no experimental proof of the first-order kinetics for CO2 gasification, and it was therefore examined by employing the acid-washed lignites. The present model also assumes that the catalytic gasification obeys the zeroth-order and first-order kinetics with respect to 1–X and the effective amount of catalyst, respectively, 6


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which was suggested by previous studies 6,26–29,41 The rationality of zeroth-order kinetics for the catalytic gasification was already discussed in previous studies. 26–29 The overall kinetics of char gasification is given by the following rate equation. dX = k nc 1− X + ∑ kcn dt n

(

)

(1)

knc and kcn are the rate constants for the non-catalytic gasification and catalytic one, respectively. kcn denotes the rate constant for the catalytic component Cn (n = 1–4). kcn is ! ). presented as a function of the effective amount of catalyst (mcn) and rate constant ( kcn ! mcn kcn = kcn

(2)

! mcn,0 (at t = 0) kcn,0 = kcn

(3)

The present authors recognize that not the concentration of catalyst (i.e., its amount per mass or volume of char) but its amount retained in each char particle determines the rate of gasification, according to previous reports. 26–29 The activity of a catalyst, Cn, is represented by kcn, which is a direct function of mcn, while k’cn is defined to be common among the components with different n. kcn changes along with the progress of gasification due to change in mcn. It decreases while the catalyst experiences deactivation in the char matrix or on the mineral matter. Volatilization is another process to decrease mcn, 3 but it was not important under the present gasification. This was because the gasification was performed with no gas flow forced to pass through the fixed bed of char particles. 27,28 When a catalyst is dispersed in the char matrix on an atomic scale or in form of nano-sized particles, it undergoes deactivation by coalescence/growth mechanism. Increasing particle size decreases the catalyst activity per mass and then mcn. The other important mechanism of deactivation is the reaction of catalyst with mineral matter such as silica, alumina and aluminosilicates. 22–25 For both mechanisms, the rate of deactivation is a function of the concentration of catalyst in the char, rather than the amount, although it is difficult to determine the order of reaction. The catalyst concentration increases as X increases, because the progress of gasification means the loss of carbonaceous matrix. There is no need to know or determine the absolute value of catalyst concentration as well as that of amount, but it is necessary to express dependency of the concentration on X. The model gives the concentration by a simple equation that describes the gasification-induced enrichment of catalyst.

mcn 1− X Then the kinetics of catalyst deactivation is then given by Ccn =

(4)

dmcn m = −kloss-nCcn = −kloss-n cn dt 1− X

(5)

7


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with an assumption that the rate of deactivation is first-order with respect to the catalyst concentration. Some previous studies reported effectiveness of this assumption, although the concentration and amount of catalyst were not distinguished clearly from each other. 26–29 mcn can also increase by transformation of the corresponding catalyst precursor. Preliminary kinetic analyses showed necessity of assuming the presence of at least a type of catalyst precursor for the chars from AC, KG and KK, details of which will be reported later. The model assumes expediently that a single type of precursor (C1prec) is transformed into the catalyst C1 exclusively. As shown later, C1 was the most active catalyst with regard to kcn,0. Eq.5 is here revised to the following equations.

dmc1 ! = kc1prec Cc1prec − kloss-1Cc1 n = 1 dt dmcn = −kloss-nCcn n ≥ 2 dt The concentration of C1prec is defined in the same way as Ccn. Cc1prec =

mc1prec

1− X The kinetics of the transformation of C1prec is expressed by

(6) (7)

(8)

dmc1prec

" = −kc1prec Cc1prec (9) dt The model defines that the total amount of the catalysts and the catalyst precursor is unity at the beginning of gasification. (10)

∑ mcn,0 + mc1prec,0 = 1 n=1

The rate of catalytic gasification at t = 0 is given by

∑ kcn,0 = ∑ kcn" mcn,0 = ICA-1 n=1

(11)

n=1

This represents the initial catalytic activity. When the catalyst precursor, C1prec, is involved in the char, it is also reasonable to define an initial and potential catalytic activity, including that of the potential catalyst, i.e., C1prec. This type of the initial activity is presented by ! mc1prec,0 + ∑ kcn,0 = ICA-2 kc1

(12)

n=1

The catalytic activities given by Eqns. 11 and 12 are hereafter referred to as ICA-1 and ICA-2, respectively. ICA-1 and ICA-2 are identical to each other when mc1prec,0 = 0. ICA-2 would be a better measure in discussing the relationship between the catalytic activity and the composition of metallic species in the char at the beginning of gasification. 3. Results and Discussion 3.1. Gasification of char from acid-washed lignite. The kinetic analysis confirmed that all of the chars from the acid-washed lignites underwent gasification obeying first-order kinetics with respect to 1–X. As seen in Table 4, the rate constants were determined with high 8


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linearity between logarithmic 1–X and t, and defined as knc’s for the individual chars. Three 1–X vs t profiles for the chars from AC-A, BN-A and TG-A are shown in Figure 1 as examples. The conversions were not measured over the full range of X due to slow gasification. knc depends on the lignite type, but within a narrow range of 4.3Í10-3–6.7Í10-3 min-1 except BN-A and KG-A, of which knc’s are 9.1Í10-3–9.5Í10-3 min-1. The first-order kinetics of gasification with steam has been confirmed for chars from acid-washed lignite and biomass chars that had no or very little amount of catalytic species, 26–29,40 while no report on such kinetics of CO2 gasification. Table 4 demonstrates that not only steam but also CO2 permeate throughout the char matrix causing the gasification with the first order kinetics. In addition, the porous nature of the present chars was not a factor controlling the kinetics of CO2 gasification under the present experimental conditions. Thus, within the range of the present experimental conditions, there was no necessity to employ kinetic models such as random pore models as well as shrinking/grain core models. The application of either of such models could even bring about misunderstanding the mechanism of gasification. The variety of knc with the type of char is attributed to that of chemical structure. Table 4. Rate constant for gasification of char from acid-washed lignite (k nc). Lignite (acid-washed) ID k nc, min -1 Aduun chuluun AC-A 0.00449 Baganuur BN-A 0.00950 Chandgan tol CT-A 0.00426 Khashaat khudag KK-A 0.00468 Khavtgai KG-A 0.00909 Khuut KT-A 0.00590 Shivee ovoo SO-A 0.00670 Tevhsiin govi TG-A 0.00647 Tsaidam nuur TsN-A 0.00582 Tugrug nuur TuN-A 0.00555 Unset khudag UK-A 0.00428 a. Range of X for determination of k nc .

range of X a 0.15 - 0.85 0.05 - 0.90 0.15 - 0.85 0.10 - 0.80 0.30 - 0.90 0.25 - 0.80 0.20 - 0.70 0.05 - 0.90 0.20 - 0.85 0.20 - 0.80 0.10 - 0.80

r2 0.9999 0.9999 0.9995 0.9994 0.9997 0.9991 0.9991 0.9999 0.9995 0.9992 0.9997

0.5

0.2

BN-A

TG-A

AC-A

t, min

Figure 1. Measured time-dependent changes in 1–X (on logarithmic scale) with time for gasification of chars from AC-A, BN-A and TG-A. 9


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3.2. Gasification of chars from original lignite and kinetic analysis Figures 2a and 2b show the time dependent changes in 1–X for all of the chars from the original lignites on decimal and logarithmic scales, respectively. For drawing each calculated curve, Eq.1 was solved numerically by applying a general Runge-Kutta method. The char reactivity depends largely on the lignite type. The gasification of AC char seems to be completed within 6 min, while time longer than 100 min is necessary for completing the gasification of KT char. Figure 2b shows 1–X over the ranges from 1 to 0.001–0.0003. Since the initial char mass was more than 1 mg, the residual mass of char (carbonaceous part) at 1–X = 0.0003 corresponded to a mass of 0.3 µg, which was safely above the mass sensitivity of the TGA, i.e., 0.2 µg. In Figure 2b are seen various shapes of 1–X vs t profiles. This is explained by a variety of changes in the catalytic activity with t or X, in other words, that of kinetics of catalyst generation and deactivation. Figure 3 shows dX/dt as a function of X for the individual chars. The change in dX/dt detected in each graph is attributed mostly to that in the rate of catalytic gasification. The dX/dt contributed by the non-catalytic gasification is greatest at X = 0 (i.e., at the y-intercept) where dX/dt is given by knc. There are, roughly saying, three different types of dX/dt vs X profiles. The dX/dt’s for BN, SO, TG, TsN and TuN chars decrease monotonically with X in manners similar to linear one. Those for CT, KK and UK chars decrease slowly at X < 0.7–0.8 but quickly later. The dX/dt’s in the gasification of AC, KG, KT chars have maxima in the early stages. The dX/dt’s for eight chars (BN, CT, KK, SO, TG, TsN, TuN and UK) appear to reach maxima in very early stages. As seen in Table 5, the time at the maximum dX/dt was within a range of 22–29 s irrespective of dX/dt at t = 0. It was believed that the maxima resulted from the transition of atmosphere around the char in the TGA. In every run of gasification, the flow of N2 was switched to that of the CO2/N2 mixture, but it took a certain period for the atmosphere inside the furnace tube to become steady. It was then concluded that the dX/dt maxima in the gasification of the eight chars were not due to maximized rates of reaction. In optimization of the kinetic parameters for the eight chars, the initial dX/dt (mainly contributed by ICA-1) was optimized so that the calculated 1–X vs t profiles agreed with the measured ones except the above-described initial period. Inflection of 1–X vs t curve was also detected for the chars from KG and AC at X = 0–0.05. This was brought about due to the same reasons as above. Thus, for the ten lignites (except KT), the initial dX/dt was not defined experimentally but estimated by the kinetic analysis, and given by Eq.1.

10


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Table 5. Time at which dX/dt reaches maximum. Lignite ID BN CT KK SO TG TsN TuN UK average

time at (dX/dt) max, s 22 26 29 26 26 25 25 15 24

Table 6 summarizes the optimized kinetic parameters for the catalytic gasification, which were employed to draw the lines in Figures 2a, 2b and 3. It was necessary to assume the presence of the catalyst precursor to transform into C1 (i.e., C1prec) at the beginning of gasification for describing the profiles for AC, KG and KT chars. The model also required four catalytic components (C1–C4) to draw 1–X vs t profiles over the 1–X range from 1 to 0.001 or smaller for seven chars (BN, KG, KT, SO, TG, TsN and TuN). On the other hand, assuming C1–C3 was sufficient for AC, CT, KK and UK chars. The model describes successfully the kinetics of gasification of all the chars over the range of 1–X from 1 to 0.001–0.0005. The measured and calculated profiles for every char are near identical to each other. The present kinetic model contains 10 kinetic parameters at most (kcn,0; n = 1–4, kloss-n; n = 1–4, k’c1prec and mc1prec). It is therefore necessary to validate the individual parameters. The results of the sensitivity analysis for the two types of chars, which are presented in detail in Supporting Information, demonstrated that 1–X vs t profile is sensitive to all of kcn,0 and kloss-n while less sensitive to k’c1prec. Figure 4 compares the measured 1–X in the TsN char gasification with those calculated assuming different number of catalytic components. It is clear that assumption of more catalytic components enables to describe 1–X over a wider range. In fact, in the kinetic analysis, the number of catalytic components was increased one by one on an as-necessary basis, but not beyond necessity. It is seen for every number of catalytic components that the slope of the line becomes steady at certain t. This is due to complete catalyst deactivation and continuation of the non-catalytic gasification with a slope corresponding to knc. Comparing this slope with that of the measured profile, it is understood that the catalytic gasification was important until the end of char conversion. This importance was common among the chars employed. Figure 5 illustrates changes in mcn with t for the gasification of TsN and AC chars. mc1 of AC char changes through a maximum. This is caused by the transformation of C1prec and responsible for the maximum of dX/dt (see Figure 3). It is commonly seen that mcn,0 varies 11


Energy & Fuels

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Page 12 of 33

with n by up to 1–2 or more orders of magnitude. Such variation is associated with that in kloss-n. For the TsN char, kloss-1 is greater than kloss-4 by about three orders of magnitude. The relationship between kcn,0 and kloss-n will be discussed later. The distribution of kcn,0 and kloss-n implies continuous distribution the catalytic activity and kinetics of catalyst deactivation over wide ranges. If so, the present kinetic model represents such continuous distribution in a discrete manner by lumping various types of catalysts into three or four components together with a single type of precursor. Kim et al., 29 investigated steam gasification of chars from Ca-loaded lignite and found two different types of Ca catalysts of that activities and behaviors were clearly different from each other. This result is consistent with the idea of discrete distribution rather than continuous one. Figure 6 summarizes the variations of kcn,0 and kloss-n with n for the eleven chars without distinguishing each char type from the others. Both kinetic parameters range over 3–4 orders of magnitude. There are also variations in the kinetic parameters for every n, which are, in other words, those with the char or lignite type. The distribution of kcn,0, that of kloss-n and their combination thus cause varieties of dX/dt vs X and 1–X vs t profiles, as seen in Figures 2a, 2b and 3. It was believed that every char contained catalyst with distributed initial activity and kinetics of deactivation. The present model successfully described such a property of the catalyst by assuming a least number of catalytic components together with a catalyst precursor. Table 6. Kinetic parameters fro catalytic gasification. ID AC BN CT KG KK KT SO TG TsN TuN UK

Σk c n,0 min-1 4.50 ·10 -1 1.35 ·10 -1 1.27 ·10 -1 1.04 ·10 -1 1.45 ·10 -1 2.80 ·10 -2 1.72 ·10 -1 5.30 ·10 -2 1.45 ·10 -1 2.00 ·10 -1 1.83 ·10 -1

k c1,0 min-1 3.27 ·10 -1 8.25 ·10 -2 6.25 ·10 -2 9.59 ·10 -2 1.05 ·10 -1 2.23 ·10 -2 1.07 ·10 -1 3.83 ·10 -2 9.86 ·10 -2 1.52 ·10 -1 1.18 ·10 -1

k c2,0 min-1 1.15 ·10 -1 4.60 ·10 -2 5.96 ·10 -2 7.70 ·10 -3 3.63 ·10 -2 4.75 ·10 -3 5.99 ·10 -2 1.29 ·10 -2 3.94 ·10 -2 4.34 ·10 -2 5.96 ·10 -2

k c3,0 min-1 8.00 ·10 -3 5.51 ·10 -3 4.95 ·10 -3 8.73 ·10 -4 3.38 ·10 -3 8.46 ·10 -4 5.11 ·10 -3 1.60 ·10 -3 6.38 ·10 -3 4.50 ·10 -3 5.84 ·10 -3

k c4,0 min-1 5.1 ·10 -4 6.5 ·10 -5 1.0 3.7 1.7 6.5 3.2

·10 -4 ·10 -4 ·10 -4 ·10 -4 ·10 -4

12


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Energy & Fuels

Table 6. Continued. ID AC BN CT KG KK KT SO TG TsN TuN UK

ΣCc n,0 k loss- n 1.93 1.15 5.28 4.77 6.26 1.83 1.55 5.18 1.22 1.20 8.46

·10 -1 ·10 -1 ·10 -2 ·10 -2 ·10 -2 ·10 -2 ·10 -1 ·10 -2 ·10 -1 ·10 -1 ·10 -2

k loss-1 2.50 1.68 9.02 5.16 8.20 2.23 2.35 6.90 1.67 1.51 1.21

·10 -1 ·10 -1 ·10 -2 ·10 -2 ·10 -2 ·10 -2 ·10 -1 ·10 -2 ·10 -1 ·10 -1 ·10 -1

k loss-2 min-1 4.23 ·10 -2 3.48 ·10 -2 1.78 ·10 -2 4.60 ·10 -3 1.19 ·10 -2 2.95 ·10 -3 2.55 ·10 -2 7.90 ·10 -3 3.05 ·10 -2 2.40 ·10 -2 2.08 ·10 -2

k loss-3 2.40 3.20 1.20 4.30 7.10 3.60 2.68 7.50 2.97 1.55 1.40

k loss-4 ·10 -3 ·10 -3 ·10 -3 ·10 -4 ·10 -4 ·10 -4 ·10 -3 ·10 -4 ·10 -3 ·10 -3 ·10 -3

2.30 ·10 -4 4.00 ·10 -6 4.70 1.58 2.00 1.60 5.00

·10 -5 ·10 -4 ·10 -5 ·10 -4 ·10 -5

Table 6. Continued. ID AC BN CT KG KK KT SO TG TsN TuN UK

k c1prec min-1 1.88

m c1prec,0 3.55 ·10 -1

0.410

3.20 ·10 -1

0.140

7.00 ·10 -1

m c1,0 3.72 ·10 -1 6.13 ·10 -1 4.92 ·10 -1 5.97 ·10 -1 7.27 ·10 -1 9.74 ·10 -2 6.21 ·10 -1 7.23 ·10 -1 6.80 ·10 -1 7.59 ·10 -1 6.43 ·10 -1

m c2,0 2.55 ·10 -1 3.42 ·10 -1 4.69 ·10 -1 7.37 ·10 -2 2.5 ·10 -1 1.70 ·10 -1 3.48 ·10 -1 2.44 ·10 -1 2.72 ·10 -1 2.17 ·10 -1 3.26 ·10 -1

m c3,0 1.78 ·10 -2 4.10 ·10 -2 3.9 ·10 -2 8.35 ·10 -3 2.33 ·10 -2 2.96 ·10 -2 2.97 ·10 -2 3.01 ·10 -2 4.40 ·10 -2 2.26 ·10 -2 3.19 ·10 -2

m c4,0 3.8 ·10 -3 6.2 ·10 -4 3.5 2.1 3.3 4.5 1.6

·10 -3 ·10 -3 ·10 -3 ·10 -3 ·10 -3

a) ICA-2 and ICD are defined by Eqns. 12 and 14, respectively.

13


Energy & Fuels

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Page 14 of 33

BN

AC

Calculated Measured

t, min

Calculated Measured

t, min

CT

KG

Calculated Measured

t, min

Calculated Measured

t, min

KT

KK

Calculated Measured

t, min

Calculated Measured

t, min

Figure 2a. Measured and calculated changes with time of 1–X on decimal scale.

14


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

SO

TG

Calculated Measured

Calculated Measured

t, min

t, min

TsN

TuN

Calculated Measured

t, min

Calculated Measured

t, min

UK

Calculated Measured

t, min

Figure 2a (continued). Measured and calculated changes with time of (1–X) on decimal scale.

15


Energy & Fuels

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Page 16 of 33

AC

Calculated Measured

BN

Calculated Measured

t, min

t, min

CT

KG

Calculated

Calculated

Measured

Measured

t, min

t, min

KK

KT

Calculated

Calculated

Measured

Measured

t, min

t, min

Figure 2b. Measured and calculated changes with time of (1 – X) shown on logarithmic scale.

16


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

SO

TG

Calculated

Calculated

Measured

Measured

t, min

t, min

TuN

TsN

Calculated

Calculated Measured

Measured

t, min

t, min

UK

Calculated Measured

t, min

Figure 2b (continued). Measured and calculated changes with time of (1 – X) shown on logarithmic scale.

17


Energy & Fuels

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Page 18 of 33

AC

BN

Measured

Measured

Calculated

Calculated

KG

CT

Measured

Measured

Calculated

Calculated

KT

KK

Measured Calculated

Measured Calculated

Figure 3. Measured and calculated changes in dX/dt with X.

18


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

SO

TG

Measured Calculated Measured Calculated

TuN

TsN

Measured

Measured

Calculated

Calculated

UK

Measured Calculated

Figure 3 (continued). Measured and calculated changes in dX/dt with X.

19


Energy & Fuels

Measured 1 component 2 components 3 components 4 components

Figure 4. Measured change in (1 – X) with X for gasification of TsN char and calculated ones assuming one to four catalytic species.

mc1 Amount of catalytic component mcn, -

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 33

TsN

AC

mc1 mc2

mc2

mc3

mc3 mc4

mc1prec

t, min

t, min

Figure 5. Calculated time dependent changes in mcn and mc1prec in the gasification of TsN and AC chars with kinetic parameters listed in Table 6.

20


Page 21 of 33

kc1,0 + k’c1mc1prec,0

kloss-1 kloss-2

kloss-n,, min-1

Kloss-3 kcn,0, min-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

kc2,0

Kloss-4

kc3,0

kc4,0

n

n

Figure 6. Ranges of kinetic parameters for the initial catalytic activity (k’c1(mc1,0+mc1prec,0), kc2,0, kc3,0 and kc4,0) and catalyst deactivation (kloss-1, kloss-2, kloss-3 and kloss-4). 3.3. Relationship between initial catalytic activity and composition of metallic species. In this section is discussed the initial catalytic activity with a focus on the abundance and composition of metallic species in the char. Of these two indicators of ICA-1 and ICA-2, the latter is better to examine the relationship with the composition of metallic species. This is because ICA-2 involves the catalyst precursor that is assumed to transform into C1. Catalytic activities of Na, K, Ca and Fe species are well known, while that of Mg is, if any, much lower than those of the formers. 5,6,42,43 Then, ICA-2 was first of all correlated with the abundance of Ca, Na and Fe. Figure 7 plots ICA-2 against the Ca content, Fe content, the sum of Ca, Na and K contents (Ca+Na+K) or Ca+Na+K+Fe. The abundance of metallic species should normally be evaluated on the basis of mass of char, but that of the original lignite was instead taken due to similarity of char yield among the lignites (see Table 3). It appeared that the Ca content was responsible for ICA-2, while allowing large deviations from the major trend for AC, TG and UK chars. Neither of adding Na+K or Na+K+Fe content improved the correlation. There was no clear relationship between ICA-2 and the Fe content. Though not shown in the figure, ICA-2 was correlated well with neither Na nor K content. Thus, the catalytic roles of Na, K and Fe seemed to be less important than that of Ca, but there was no reason to ignore the catalysis of those metallic species. 21


Energy & Fuels

b AC

ICA-2, min-1

ICA-2, min-1

a AC

UK

UK

TG

Fe content, mol (kg-daf-lignite)-1

Content of Ca, mol (kg-daf-lignite)-1

d ICA-2, min-1

c ICA-2, min-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

AC

AC

CT

TG

TG

Ca+Na+K content, mol (kg-daf-lignite)-1

Ca+Na+K+Fe content, mol (kg-daf-lignite)-1

Figure 7. Plots of ICA-2 against content of metallic species. The trend seen in Figure 7a is here considered. The ICA-2’s of AC and UK are clearly greater than that indicated by the straight line. It was found that the Si and Al were much less abundant in those two chars than in the others (see Table 2). It was also noted that TG had the highest Si and Al contents among the eleven lignites. Such particular features of the three lignites suggested reasonably that Si or Si and Al played roles in deactivation of metallic species before the gasification. It is well known that SiO2, Al2O3 and aluminosilicates react with Ca, K, Na 22–25 and also Fe 44–46 species (metals or oxides). The changes in Gibbs free energy of such reactions support their progress before the gasification. 25,44–46 It was expected that more abundant Si species resulted in more significant deactivation of the catalytic species during the pyrolysis, and therefore the molar ratios of Ca, Na, K and Fe to Si were considered. Figure 8a–d plots ICA-2 against Ca/Si, Na/Si, Fe/Si and (Ca+Na+Fe)/Si ratios. Among these ratios, ICA-2 was correlated best with the (Ca+Na+Fe)/Si ratio. This suggested the contribution of not only Ca but also Na and Fe to the catalytic gasification, and progress of their deactivation during the pyrolysis. As seen in Figure 8d, however, UK, SO, KK and KT had ICA-2’s lower than those expected by the straight line drawn in the figure. A possibility of such insufficient correlation was difference in the catalytic activity (on molar basis) among Ca, Na and Fe species. Then, the relationship between ICA-2 and the abundances of Ca, Na and Fe was further analyzed with the following equation. (13) ICA-2=α !" Ca/Si + β Na/Si + γ Fe/Si #$

(

) (

) (

)

22


Page 23 of 33

The optimization of the coefficients at β = 0.21 and γ = 0.44 improved the correlation factor r2 between ICA-2 and (Ca+Na+Fe)/Si ratio from 0.80 (for β = γ = 1) to 0.88. However, the correlation factor was even greater when it was boldly assumed that β = 1 for the all chars, and γ = 0 for KK, KT, SO and UK chars, but γ = 1 for the other chars. Figure 9a shows the result of this assumption, which improved r2 to 0.985. It was thus speculated that Fe species of some of the chars had lost their catalytic activity until the beginning of gasification, or had inherently no or little activity. More detailed investigation is necessary for understanding the relationship between ICA-2 and the composition of metallic species in the future work. Figure 9b applies the same assumption as in Figure 9a, while ICA-2 is plotted against (Ca+Na+Fe)/(Si+Al) or (Ca+Na)/(Si+Al) ratio. The r2 is slightly lower than that in Figure 9a, but as high as 0.97. The discussion developed in this section strongly suggests the importance of the deactivation of inherent catalyst or its precursor prior to the gasification, and also shows the effectiveness of employing molar ratio of metallic species to Si or Si and Al as a simple but semi-quantitative measure for the extent of pre-deactivation. The present authors recently examined the loss of inherent catalyst during the pyrolysis of a Victorian lignite, which had ash content as low as 1 wt%-dry while retaining Na and Ca with high catalytic activity. Briquettes of the lignite and its mixture with submicron-sized SiO2 particles were prepared and pyrolyzed. Resulting chars were then gasified with CO2 under conditions very similar to those in the present study. It was found that increasing SiO2 content decreased the initial rate of gasification, which was caused mainly by reactions of Na and Ca with SiO2 during the pyrolysis. Details of the above findings will be reported elsewhere.

b

CT KK

SO

ICA-2, min-1

ICA-2, min-1

a

TuN

AC

TuN KG

SO

KK BN TG

SO TsN KK CT KG KT TG

KT

d

AC

TuN BN

UK CT

TsN

UK

ICA-2, min-1

c ICA-2, min-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

SO

UK

KK KT

23


Energy & Fuels

Figure 8. ICA-2 as a function of (a) Ca/Si, (b) Fe/Si, (c) Na/Si or (d) (Ca+Na+Fe)/Si molar ratio.

ICA-2, min-1

(Ca+Na)/Si ratio (Ca+Na+Fe)/Si ratio

SO

UK

KK

KT

a

(Ca+Na)/(Si+Al) ratio (Ca+Na+Fe)/(Si+Al) ratio

ICA-2, min-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

SO

UK

KK

KT

b

Figure 9. Correlation of ICA-2 with composition of metallic species. (a) Plot of ICA-2 against (Ca+Na)/Si ratio (for KK, KT, SO and UK chars) or (Ca+Na+Fe)/Si ratio (for the other chars). (b) Plot of ICA-2 against (Ca+Na)/(Si+Al) ratio (for KK, KT, SO and UK chars) or (Ca+Na+Fe)/ (Si+Al) ratio (for the other chars). 3.4. Factors influencing kinetics of catalyst deactivation. The kinetics of catalyst deactivation, as well as the initial catalytic activity, is crucial to that of gasification. The initial rate of catalyst deactivation (ICD) is here defined by

(

)

ICD = kloss-1 Ccn,0 + Cc1prec,0 + ∑ kloss-nCcn,0

(14)

n=2

taking the presence of C1prec into consideration. Figure 10 shows the relationship of ICD with Si content and ICA-2. The former relationship was examined expecting that more abundant Si caused faster catalyst deactivation by the same mechanism as that during the pyrolysis. However, as seen in Figure 10a, it was difficult to find such a trend. Clearly different from the catalyst deactivation during the pyrolysis, Si did not seem to play an important role in the catalyst deactivation during the gasification. A possible explanation of this result is preferential occurrence of carbonation of catalytic species forming CaCO3, Na2CO3 and K2CO3, while FeCO3 (siderite) was not stable under the present gasification conditions. 47 The progress of carbonation could result in inhibition of silicate formation. Another explanation is that self-deactivation through the growth in size of 24


Page 25 of 33

catalyst particles was faster than their reaction with SiO2, or otherwise, Al2O3 or aluminosilicates. Asami et al. 14,15 and Kim et al. 29 investigated steam gasification of chars from original and Ca-loaded Victorian lignites. The chars had very low contents of Si, and the catalyst deactivation occurred mainly by the self-deactivation. An important finding was that a more active catalyst underwent more rapid deactivation. It was moreover found that the rate constant for the catalyst deactivation was correlated well and linearly with the initial catalytic activity. They proposed to assume two types of Ca catalysts that had clearly different initial activities and kinetics of deactivation. One (Type-1) was Ca species dispersed in the char matrix at an atomic scale, and the other (Type-2) was contributed by nano-sized Ca-based particles. Type-2 catalyst had greater initial activity while undergoing deactivation more rapidly than Type-1 catalyst. Figure 10b shows the relationship between ICD and ICA-2, indicating two types of linear relationships between them for the chars with an exception (TuN). Thus the chars are classified into three groups by ICD normalized by the initial activity; BN, KT, SO, TG and TsN chars with the greater kloss-n/kcn,0 ratio, AC, CT, KG, KK, and UK chars with the smaller ratio, and then TuN char with an intermediuate ratio. The two different kloss-n/kcn,0 ratios seen in Figure 10b could be explained in essence by the distribution of catalytic species with respect to chemial form of catalyst and its size. More detailed analytical investigation is necessary to clarify the effects of such complex distribution on the kloss-n/kcn,0 ratio, which is beyond the scope of the present study. Figure 11 illustrates the relationships between kloss-n and kcn,0 for the individual n. The chars can be classified into three groups for every Cn, while the constituents of the three groups vary with n. Table 7 shows the result of classification. There are a variety of L/M/S combinations of the catalysts in the chars, which is arisen from complexity of the physical/chemical compositions of the catalysts. Chemical interaction among different metallic species and resulting synergy 48–50 in the catalysis may cause more various L/M/S combinations.

a

b AC

ICD, min-1

SO

ICD, min-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

TsN BN

TG KT

KK CT KG

Si content, mol (kg-daf-lignite)-1

TuN

UK

ICA-2, min-1

Figure 10. Plots of initial catalyst deactivation rate against Si content and ICA-2.

25


Energy & Fuels

a

b

kloss-1, min-1

BN

TsN

TuN

S

M

UK TG

CT

KG

TuN

KT

M

CT

KK KG

kc2,0, min-1

c

d L

SO

TsN

AC

S

TuN M TG

BN

kloss-4, min-1

BN

KT

SO

UK TG

kc1,0, min-1

KG

AC

TsN BN

KK

KT

S

L

kloss-2, min-1

AC

Group L SO

kloss-3, min-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

UK

CT

L TsN

SO

S

KG KT

KK

TuN

TG

kc3,0, min-1

kc4,0, min-1

Figure 11. Relationships between kcn,0 and kloss-n. The relationship is classified into three groups by kcn,0/kloss-n ratio. The L, M and S indicate relatively large, medium and small ratios. Table 7. Classification of the rates of deactivation of individual catalytic components according to the kloss-n/kcn,0 slopes drawn in Figure 11. L; larger rartio of kloss-n/kcn,0, S; smaller ratio, M: medium ratio. Lignite AC BN CT KG KK KT SO TG TsN TuN UK

ICD

kloss-1

kloss-2

kloss-3

kloss-4

S L S S S S L L L M S

S L L S S S L L L M M

S L S L L M S L L M S

S L S L L L L L L M S

L S L L S S S -

26


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Energy & Fuels

4. Conclusions The present authors investigated the CO2 gasification of chars from the eleven types of Mongolian lignites, and analyzed the kinetics in detail by the extended parallel reaction model, and have demonstrated the followings. 1) The chars form the acid-washed lignites undergo the CO2 gasification obeying first order kinetics with respect to 1–X and much slowly than those from the original lignites. 2) The parallel reaction model describes the 1–X vs t profiles for all the chars quantitatively over the range of X up to 0.999–0.9995 by assuming three or four types of inherent catalysts that have different initial activities and kinetics of deactivation together with a single type of catalyst precursor. Both the initial activity and the rate of deactivation of the catalytic component range over three to four orders of magnitude. 3) The overall catalytic activity starts to decrease immediately after the beginning of gasification (i.e., at X = 0), or later but until X reaches ca. 0.25. The decrease continues until the end of gasification, but the rate of catalytic gasification is greater than that of non-catalytic gasification even at X = 0.999. 4) More or less portion of the catalytic species is deactivated during the pyrolysis. The initial catalytic activity, ICA-2, is correlated well and linearly with the (Ca+Na+Fe)/Si or (Ca+Na)/Si molar ratio.

Acknowledgment A part of this work was supported by Japan Society for The Promotion of Science (JSPS) for Grant-in-Aid for Scientific Research A (Grant Number 26249120). The authors are also grateful to financial support by Ministry of Education, Culture, Sports, Science and Technology, Japan through ‘Nano-Macro Materials, Devices and Systems Research Alliance’ Project.

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Supporting Information Figure A1 shows the result of sensitivity analysis for the gasification of TsN char. It is seen that changing either of the kinetic parameters only by 5% (from the optimized value) results in clear deviation of the profile from the optimized one. The calculated 1–X is thus sensitive to all the kinetic parameters. Figure A2 illustrates results for the AC char in the same way as in the manner of Figure A1. Again, the 1–X vs t profile is sensitive to all the kinetic parameters, mcn,0 (kcn,0) and kloss-n. On the other hand, as shown in Figure A3, the parameters of mc1prec and k’c1prec do not give so much impact to the calculation as to the other kinetic parameters. Interestingly, change in mc1prec causes slight change in 1–X at not the early but final stage of gasification. k’c1prec seems to the weakest parameter, and the determination of k’c1prec was less accurate than the other parameters.

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IAC-1, +5% IAC-1, - 5%

TsN

mc2,0, +5% mc2,0, - 5%

TsN

mc3,0, - 5%

mc4,0, +5%

mc3,0, +5%

mc4,0, - 5%

TsN

TsN

kloss-2, +5% kloss-2, - 5%

kloss-1, +5% kloss-1, - 5%

TsN

TsN

kloss-3, +5% kloss-3, - 5%

TsN

kloss-4, +5% kloss-4, - 5%

TsN

Figure S1. Result of sensitivity analysis of kinetic parameters for gasification of TsN char. The optimized set of parameters is listed in Table 5. When mc2,0, mc3,0 or mc4,0 was increased or decreased, mc1,0 was decreased or increased so that Σmcn,0 remained unity.

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AC

AC

IAC2, +5% IAC2, - 5%

kc2,0, +5% kc2,0, - 5%

AC

AC

kc3,0, - 5%

kloss-1, +5%

kc3,0, +5%

kloss-1, - 5%

AC

kloss-2, +5% kloss-2, - 5%

AC

kloss-3, +5% kloss-3, - 5%

Figure S2. Result of sensitivity analysis of kinetic parameters for gasification of AC char. The optimized set of parameters is listed in Table 6. When mc2,0 or mc3,0 was increased or decreased, mc1,0 was decreased or increased so that Σmcn,0 remained unity.

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Energy & Fuels

AC

dX/dt, Calculated dX/dt, k’c1pec, +5% dX/dt, k’c1prec, - 5%

AC

dX/dt, Calculated dX/dt, mc1prec,0, +5% dX/dt, mc1prec,0, - 5%

AC

k’c1prec Cc1prec,0, +5% k’c1prec Cc1prec1,0, - 5%

AC

k’c1prec, +5% k’c1prec, - 5%

Figure S3. Result of sensitivity analysis of kinetic parameters for gasification of AC char. The optimized set of parameters is listed in Table 6.

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Kinetics and Mechanism of CO2 Gasification of Chars from 11

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