Investigation of Vaporization of Alkali Metals from Solidified Gasifier

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Investigation of vaporization of alkali metals from solidified gasifier slags by Knudsen effusion mass spectrometry Laszlo Bencze, Ma#gorzata Ry#-Matejczuk, Elena Yazhenskikh, Mirko Ziegner, and Michael Müller Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01620 • Publication Date (Web): 29 Dec 2015 Downloaded from http://pubs.acs.org on January 3, 2016

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Investigation of vaporization of alkali metals from solidified gasifier slags by Knudsen effusion mass spectrometry L. Bencze*a, M. Ryś-Matejczuk, E. Yazhenskikh, M. Ziegner and M. Müller Forschungszentrum Jülich GmbH, Institute for Energy and Climate Research (IEK-2), D52425 Jülich, Germany *corresponding author, aOn leave from Eötvös Loránd University, H-1117 Budapest, Pázmány Péter sétány 1/A, Hungary, E-MAIL: [email protected]

KEYWORDS: Knudsen effusion, mass spectrometry, slags, vaporization, coal gasification

ABSTRACT: One problem in coal gasification is the release of alkali species which can cause fouling and corrosion. Depending on their composition, slags have a high potential for alkali retention. Therefore, the vapor pressures of alkali species (Na, K) over four real solidified gasifier slag samples were determined in over the temperature range 1134 to 1591 K by Knudsen effusion mass spectrometry (KEMS). In addition, the residues from the KEMS investigations were studied by X-ray diffraction (XRD) and scanning electron microscope / energy dispersive X-ray (SEM/EDX) to determine the phase composition of the samples and map the alkali distribution. A strong correlation between the composition of the slags in terms of amount of acidic oxides (SiO2, Al2O3) and basic oxides (CaO, FeO) and the volatility was determined. The volatility results showed that higher acidic oxide content leads to higher alkali retention capability. The XRD and SEM/EDX analysis of the KEMS residues showed that an anorthite type phase contains more Na than all the other phases in the equilibrium.

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1. Introduction Coal is one of the most important energy resources worldwide. Due to its broad availability and low costs, coal is the major source for heat and power production in many countries [1]. However, coal resources are limited and coal utilization can cause environmental problems. Therefore, environmentally benign, high efficiency coal conversion technologies are needed. High-temperature gasification has been proven to be efficient in energy conversion and minimizing harmful emissions when using high-ash fuels [2-4]. One promising technology is the integrated gasification combined cycle (IGCC) using entrained-flow gasification technology. To account for the ash content of coals, entrained-flow gasifiers may be equipped with a cooling screen. By maintaining the process temperature optimized to the slag melt properties of the specific coal, a firmly adhering slag coat is applied to this refractory screen. The draining slag melt is finally discharged as a solid material, after passing through a water quench. However, part of the slag may also be entrained with the gas. This entrained slag, in combination with the release of volatile inorganics such as alkali species, which are also entrained with the syngas, can cause fouling and corrosion in downstream processes. By way of example, such problems may arise in a syngas cooler. The influence of process parameters and coal composition on the release of these species from high temperature gasification was recently investigated [5, 6]. The release of alkali species increased with increasing temperature and decreasing pressure, and a significant dependence on the overall ash composition was observed. For instance, it was shown that chlorine promotes the release of alkali metals, and alumino-silicates can capture alkalis. In entrained flow slagging gasification, gaseous alkali species are in contact with the draining slag which also has a high potential for alkali retention [7, 8]. Alkalis absorbed by the draining slag are removed together with the slag from the gasifier and thus cannot contribute to fouling in downstream processes. According to thermodynamics, the equilibrium vapor pressures of the alkali metal species depend on the temperature and composition of the coal slag, which can be regarded as a silicate melt or glass. Furthermore, it is known that the properties of silicate melts depend on their structure [9] and that the polymerization of slag has a strong influence on its alkali oxide activity [7]. Therefore, the alkali-retention potential of the coal slag depends on the process ACS Paragon Plus Environment

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parameters of the gasifier and also the type of coal. To reliably adjust the slag composition for optimum alkali retention, e.g. by use of additives or coal blending, a better understanding of the relationship between slag composition, slag structure and resulting alkali retention potential is needed. This in turn requires additional knowledge of thermodynamic equilibria in defined synthetic systems, and further information on the occurrence of relevant alkali containing phases in real gasifier slags. In this paper the vapor pressures of alkali species (Na(g), K(g)) over four solidified slag samples from a pilot gasifier were determined in the temperature range 1134 to 1591 K by Knudsen effusion mass spectrometry (KEMS). The results were correlated with the composition and structure of the slag to identify ash compositions with the potential to result in slags capable of alkali retention. 2. Experimental Section 2.1 Samples Four slag samples from gasification of hard coals at temperatures above the ash melting temperature in a pilot scale entrained flow gasification facility in Freiberg, Germany, were used in this investigation. The chemical analysis, which was performed by the Central Division of Chemical Analysis of Forschungszentrum Jülich using inductively coupled plasma optical emission spectroscopy (ICP-OES), is given in Table 1. The hemispherical and fluid temperatures determined by hot stage microscopy are also given in Table 1. These two temperatures give an indication of the melting range of the slags. Samples of the same bulk material were also used in previous surface tension investigations [10]. For confidentially reasons, the authors are unable to provide the corresponding gasifier conditions and the origin, rank, and composition of the coals used by the industrial project partner from whom the samples were sourced. 2.2 Experimental techniques The KEMS apparatus. The KEMS experiments were carried out using a Finnigan MAT 271 (Finnigan MAT, Bremen, Germany) 90o magnetic sector-field mass spectrometer. The Knudsen cell, with an orifice diameter of 0.3 mm, was made of iridium to prevent any chemical reaction and mixing between ACS Paragon Plus Environment

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the cell and the slag sample. The cell was surrounded by a tantalum radiation shield. The scheme of a Knudsen cell - mass spectrometer system is presented in Fig. 1 [11]. The applied ionizing electron energy and the total emission current were 70 eV and 0.7 mA, respectively. The cell was heated by radiation and electron bombardment from a hot tungsten filament. The temperature was controlled using a W/WRe thermocouple and measured using a digital pyrometer (Dr. Georg Maurer GmbH, Kohlberg, Germany). The temperature was calibrated using the melting points of five pure metals, i.e. Ag, Au, Cu, Ni and Pt, by plotting the displayed melting points against the true literature melting points. As a first approximation, a linear function was fitted by regression. Further deviation from the true temperatures between the melting points was attributed to errors in the built-in industrial calibration of the digital pyrometer. This deviation was determined using a novel, in-house temperature calibration method, in which the above-mentioned pure metals were repetitively measured by KEMS in order to filter the statistical error in the ion intensity data. The ln(I x T) was plotted against 1/T, where I and T denote the ion intensity and temperature, respectively and a straight line was fitted on the basis of the ClausiusClapeyron relationship. Nevertheless, reproducible ‘wavy’ deviations from the straight line were observed, which cannot be attributed to the statistical error in the intensities. These deviations therefore represent errors in the built-in calibration of the digital pyrometer. These deviations are unevenly distributed and the maximum occurs at the borders of the neighboring sensitivity ranges. Thus, a temperature correction was determined as a function of the displayed temperature. By repeating the measurements on pure metals several times, the error of the built-in temperature measurement of the pyrometer was obtained by decreasing the statistical error in the ion intensities. The ion intensities were measured in counts using a secondary electron multiplier with an appropriate supply voltage to assure gain factors independent of the masses of the ions. A movable shutter between the cell and the ion source served to distinguish the sample ions from background ions. Approximately 100 mg of the glassy slag, powdered in an agate mortar, was used for the KEMS experiments. The powdered sample was loaded into the cell, gently pressed using a metal piston and weighed with an ACS Paragon Plus Environment

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accuracy of 0.1 mg. After reaching the appropriate vacuum, the temperature was increased until detectable and shutterable Na+ and K+ ions appeared in the mass spectrum. The mass loss of the samples due to the KEMS experiments was also checked by weighing the samples after the experiments. X-ray diffractometer. The phase composition of the KEMS residues was checked by X-ray diffraction (D4 ENDEAVOR Diffractometer, Bruker AXS GmbH, Karlsruhe Germany, parafocussing BraggBrentano geometry). The diffractometer is equipped with a Cu LFF tube operated at 40 kV and 40 mA, a curved graphite secondary monochromator and a NaI scintillation counter. The beam path is defined by motorized primary and secondary slit systems; 2.5° soller slits and a 0.2 mm receiving slit. The phase identification was carried out using the ICDD PDF-2 database (Release 2004) and the XRD analysis software HighScorePlus (PANalytical B.V., Almelo, The Netherlands). Quantitative analysis was performed by the Rietveld method using the TOPAS profile fitting software (Bruker AXS GmbH, Karlsruhe Germany). Crystal structures were obtained from the Inorganic Crystal Structure Database (ICSD, FIZ Karlsruhe, Germany). SEM/EDX. The internal phase composition was determined by scanning electron microscopy (SEM, LEO 440, Zeiss, Oberkochen, UK) coupled with energy-dispersive X-ray analysis (INCA Energy 300, Oxford Instruments Analytical, UK). The electron microscope operated with an accelerating voltage of 20 kV and a sonda-current of approximately 1 nA, and the samples were coated with a 2-3-nm-thick Ptlayer by sputtering to obtain the instrument necessary sensitivity and sample electrical conductivity.

3. Results and discussion 3.1 Determination of partial pressures by KEMS The equilibrium partial pressures of the vapor species in equilibrium with the condensed sample were calculated from the measured ion intensities using the following well-known equation: pj =

KT

σj

I ij

(1)

∑η γ i

i i


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where K is the sensitivity constant of the instrument, T is the temperature, pj and σj are the vapor pressure and ionization cross section of species j, respectively. Iij is the ion intensity of ion i originating from species j, and ηi and γi are the isotopic abundance and the multiplier’s gain factor of ion i, respectively. For mono-isotopic ions, ηi = 1 and for Faraday cup measurements, γi = 1. When measuring ion currents via a multiplier, the gain factors of the ions (γi) are much higher than 1 and depend on the masses of the ions. When measuring ion counts via a multiplier in the plateau region of the characteristics of the multiplier, the gain factors of the ions are also higher than 1 but independent of the ions’ masses similar to the case of the Faraday cup. Since all ions were measured using the counting mode of the multiplier, the gain factors were not determined, as they were implicitly the same for all ions. Nevertheless, the mass independence of the detection of ion intensity was confirmed for the local multiplier. The sensitivity constant (K) of the device was determined using pure Ni. The vapor pressure of Ni(g) over pure Ni(s) was taken from the IVTANTHERMO database [12]. The ionization cross sections of the species Ni(g), Na(g) and K(g) at the applied 70 eV were calculated from the Lotz’ s formula[13]. To avoid any reaction and alloying between the iridium Knudsen cell and the calibrating Ni, the cell was completely lined with alumina during the calibration experiments. For slag experiments, the alumina liner was removed from the cell and the cell was heated to very high temperature to remove Ni residue. After slag experiments, the cell was cleaned with hydrofluoric acid, dried, and heated in the mass spectrometer to very high temperatures to remove any residue of the former sample. During recording the temperature and the ion intensities, the temperature was changed by ± 10 or ± 20 °C by the data-acquisition program of the device to check the thermodynamic equilibrium. The program followed first an increasing, then subsequently decreasing temperature profile. It is a known phenomenon that rapidly cooled glassy melts are frozen as a solid glass rather than an equilibrium crystalline phase. Therefore, heating of the samples above their melting points was avoided. Only in case of S1-3 was the highest temperature of the third measurement run above the sample melting point, since the volatility of


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this sample is the lowest of the four studied samples. Therefore it was possible to heat this sample to somewhat higher temperature without any compositional change caused by distillation.

3.2 Thermodynamics and Knudsen effusion relationships The vaporization of alkali oxides (Alk2O, Alk=Na, K)) from the slag, if only one alkali oxide (Na2O or K2O) is present, is the same as that of the pure compounds, i.e. the following equilibria hold: Alk2O(slag) ⇔ Alk2O(g)

(2)

Alk2O(g) ⇔ 2 Alk(g) + ½ O2(g)

(3)

The corresponding equilibrium constants for Reactions 2 and 3 are as follows:

( p Alk O(g) / p o ) K ( 2) = a

(4)

2

a Alk 2 O(slag) 2

⎛ p Alk(g) ⎞ ⎛ p O 2 (g) ⎜ ⎟ ⎜ ⎜ p o ⎟ ⎜ p o ⎝ ⎠ ⎝ K a (3) = ⎛ p Alk2O(g) ⎞ ⎜ ⎟ ⎜ p o ⎟ ⎝ ⎠

⎞ ⎟ ⎟ ⎠

1/ 2

=

(p

) (p 2

Alk(g)

)

1/ 2

O 2 (g)

p Alk2O(g)

o −3 / 2

(p )

−3 / 2

= K p (3) ( p o )

(5)

where po denotes the standard pressure (1 bar = 105 Pa), and Ka and Kp denote the equilibrium constants expressed in activity and pressure. The fugacity coefficients are considered to be 1 for all gaseous species since the species studied by KEMS are quasi-ideal gases due to their low pressure and high temperature. Therefore, the fugacity coefficients do not appear in Eqs. 4 and 5. The equilibrium constants (Ka(2), Ka(3) and Kp(3)) should be independent of composition, and are therefore the same for the pure component oxides. The vapor pressure of the oxide species (Alk2O(g)) is very low relative to the other gaseous species so that the alkali oxides evaporate with a high degree of decomposition. Regarding the congruency condition for Reaction 3, in a Knudsen cell, the vapor pressure of O2 can be expressed as (see also [14]):

pO 2 (g) =

pAlk(g)

M O2

4

M Alk

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if only one type of alkali oxide (Na2O or K2O) is in the slag. M denotes the molar mass of the corresponding gaseous species. Substituting Eq. 6 into Eq. 5: 1/ 4

pAlk 2 O(g)

5/2 pAlk(g) ⎛ M O 2 ⎞ ⎜ ⎟ = 2 K p (3) ⎜⎝ M Alk ⎟⎠

(7)

The activity of the component oxides can be obtained directly from Eq. 7 by applying it to both the pure component and the mixture, since the omitted parameters (Kp, molar masses) do not depend on composition:

aAlk 2 O =

pAlk 2 O(g) ∗ pAlk 2 O(g)

⎛ pAlk(g) ⎞ ⎟ = ⎜ ∗ ⎜ pAlk(g) ⎟ ⎝ ⎠

5/ 2

(8)

The asterisk * denotes the pure substance. This means that the measurable vapor pressure of Alk(g) over pure Alk2O(s,l) and the mixture (i.e. slag) is also theoretically suitable to determine the activity of Alk2O. Alk(g) is even more suitable in a practical sense, as the vapor pressure of Alk2O(g) is often below the sensitivity limit (~10-6 Pa) of KEMS. In the case of simultaneous vaporization of two alkali oxides, i.e. Na2O and K2O, the equation expressing the congruency criteria of the mixed vaporization is:

pO 2 (g) =

M O 2 ⎛ p Na(g) pK(g) ⎜ + 4 ⎜⎝ M Na MK

⎞ ⎟ ⎟ ⎠

(9)

This means that, for Na2O, for example, the vapor pressure of Na2O(g) over the slag is:

p Na 2 O(g)

1/ 4

2 pNa(g)

⎛ M O 2 ⎞ ⎜ ⎟ = 2 K p (3, Alk = Na ) ⎜⎝ M Na M K ⎟⎠

(

M K p Na(g) + M Na pK(g)

)1 / 2 (10)

Whereas that over pure Na2O(s) is:

p*Na 2 O(g)

(p*Na(g) )5 / 2

⎛ M O 2 ⎜ = 2 K p (3, Alk = Na ) ⎜⎝ M Na

1/ 4

⎞ ⎟ ⎟ ⎠

(11)

Thus the activity of Na2O, combining Eqs. 10 and 11, is: ACS Paragon Plus Environment

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a Na 2 O =

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p Na 2 O(g) p ∗Na 2 O(g)

⎛ p Na(g) ⎞ ⎟ = ⎜ ∗ ⎜ p Na(g) ⎟ ⎝ ⎠

2

1/ 2

⎛ M K p Na(g) + M Na pK(g) ⎞ ⎜ ⎟ ∗ ⎜ ⎟ M K p Na(g) ⎝ ⎠

(12)

A boundary case of Eq. 12 is Eq. 8 when the vapor pressure of K(g) is zero. An analogous, symmetric formula holds for K2O. Therefore, in the case of simultaneous vaporization of both oxides, the activities of Na2O and K2O cannot be determined exclusively from the partial pressures of their own gaseous alkali atomic species due to the common gas phase decomposition product, O2(g).

3.3 Evaluation of the thermodynamic data from the KEMS experiments During the first heating of the slag samples, a slight but negligible evolution of non-condensable gases was observed. These non-condensable gases disappeared at high temperatures where only alkali atomic species (Na(g), K(g)) and O2(g) were present in the equilibrium vapor. At high temperature, gas eruptions were infrequently observed (about once an hour), causing a slight, temporary worsening of vacuum, and enhanced ion intensity of the not shutter-able m/e = 39 ion. At mass number 39, at the applied high resolution of the mass spectrometer, two resolved ions could be detected: the shutter-able K+ ion originating from K(g) and a 0.06 a.m.u. heavier (likely C3H3+) ion. The latter is thought to originate from organic non-condensable gaseous molecules (e.g. C6H6(g), etc.) that may be present in gas-inclusions of the slag sample. The measured mass difference between the two peaks corresponds to the literature mass difference between K+ and C3H3+. Due to the high volatility of this type of precursor organic compound, evolved molecules do not condense on the surface of the shutter at elevated temperatures. Continuous, very low background C3H3+ intensity was observed throughout the measurement. In the case of an acute eruption, the intensity of C3H3+ increased until evolution of the precursor ceased, the additional gas was removed from the system. The intensities of Na+ and K+ were unchanged during these rare eruptions, since the vapor pressure of alkali species in the gas inclusions and above the solid sample were probably the same, proving the existence of the thermodynamic phase equilibrium. In the case of sample S1-3, the frequency of eruptions was noticeably higher than that of


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the other samples. Na+ and K+ could not be detected below 900 °C. The intensity of O2+ was not measured for two reasons; O2(g) in the molecular beam does not condense on the shutter surface (its critical temperature is far below room temperature) and a significant part of O2+ signal originates from background O2(g). This species is partly shutter-able due to the reflections of O2(g) molecules on the shutter surface but no total shutter-ability can be reached. However, the partial pressure of O2(g) can be calculated from the obtained partial pressures of Na(g) and K(g) (see below) due to the congruency criteria of the vaporization of both oxides since no other oxide components of the slag evaporated in the temperature range of the experiments. The results of each individual measurement can be found in ESI (Table S-I). Table S-I presents the measured vapor pressures of the alkali atomic species and the alkali oxide activities for all individual measurements. The activities were obtained using Eq. 12 instead of Eq. 8. Nevertheless, using Eq. 8 only results in a very small change in the value of Na2O activity and a maximum change of factor 5 (half an order of magnitude) in the value of K2O activity. The extent of this change depends on the order of magnitude of the activity. The shift in the activity can be estimated from Eq. 12. Since p(K(g)) is usually much lower than p(Na(g)), applying Eq. 12 results in a small change in the activity of Na2O obtained by Eq. 8. Application of the same relationship to the vapor pressures results in a larger change in the value of the activity of K2O. The reference states for the determined activities are pure solid Na2O(s) and K2O(s). The vapor pressures (p*) of Na(g) over pure solid Na2O(s) and K(g) over pure K2O(s) were taken from Ref. [12]. The vapour pressures (p*) of alkali species Na(g) and K(g) over pure Na2O(s) from 1000 to 1273 K and over pure K2O(s) from 800 to 1000 K depending on temperature (T) according to Ref. [12] are: ln(p*(Na(g))/Pa) = 26.30995−29160.87417/(T/K); and ln(p*(K(g))/Pa) = 25.56165−24791.26680/(T/K), respectively. Since three of the slag samples (S-1, S-2 and S-4) remained solid during their investigations from 1134 to 1476 K, it was logic to choose solid pure alkali oxides as reference states for the activities. However, it should be noted that the melting points of pure Na2O(s) and pure K2O(s) are 10 ACS Paragon Plus Environment

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1405 and 1013 K, respectively; therefore the solid reference states were extrapolated as metastable reference states above these temperatures. S1-3 melted near the upper limit of its temperature range of investigation (1294 to 1591 K). Table S-I shows that the activities of Na2O and K2O are very low, indicating strong interaction between these alkali oxides and the other components of the samples. Such low activity indicates the formation of solid compounds between the alkali oxides and the other components. As also shown in Table S-I, each sample was measured three or four times (i.e. for 3 days) subsequently. After the investigation of each representative sub-sample, the material was removed, the cell was cleaned using HF, a fresh sub-sample was loaded into the cell and the measurements were repeated. Three or four sub-samples were investigated for each of the four bulk samples in order to obtain reliable mean values of vapor pressures and activities. The first run shows slightly higher vapor pressure data than the next (repeated) runs for the measurement of all samples and the data of the repeat runs are almost congruent, proving that no further compositional or structural change within the sample and the achievement of thermodynamic equilibrium. It is assumed that the direction to the thermodynamic equilibrium accompanied by structural change must have been jointly responsible for the change of the vapor pressure during the first run of the KEMS experiments, rather than a compositional change (e.g. evaporation of Na2O and K2O), since the initial structure of the ~90% glassy slag became crystalline for all the four samples. It is assumed, therefore, that the crystallization takes place completely during the first run. Fig 2 shows the X-ray diffraction pattern (XRD) of S1-1 before and after the KEMS experiments (at least three examples) where evidence of the crystallization can be seen. After the KEMS experiments, the concentrations of Na and K in the residues were slightly lower than those in the raw slag samples, as shown in Table 2. During the KEMS experiments, only Na(g), K(g) and O2(g) species could be detected. Therefore, the remainder of the observed mass loss may arise from the non-condensable gases which are evolved from the slags at lower temperatures.


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In spite of the decrease of the concentrations of Na and K in the solid phase due to evaporation during the KEMS experiments, the vapor pressures of Na(g) and K(g) were constant after the first KEMS run. Simultaneously, the sample residue crystallized, therefore a thermodynamic equilibrium must have occurred. As an example, the temperature dependence of the vapor pressure between runs for S1-3 is presented in Fig. 3. It can be seen that the vapor pressures in the second and third runs practically coincide, meaning that no further vapor pressure change occurs with time. The same phenomenon was observed during the measurements of the other sub-samples and samples. Nevertheless, upon reaching the final and also highest temperature of Run 3 for S1-3, the sample was definitely molten since the sample residue after Run 3 appeared visually to be a molten transparent glass. Therefore, S1-3 can be considered a crystalline solid during Run 2 only and at those temperatures of Run 3 that are still below the melting point of this sample. All repeat runs were performed by heating, and then the sample was cooled down to the initial, lowest temperature before the next run was performed. The other three slag samples did not melt during the KEMS measurements within the applied temperature range. The melting point of S1-3 is the second highest (around 1300 oC) of the four samples, according to former investigations [10]. Though the structure of the frozen S1-3 sample after melting (in Run 3) is completely different (an amorphous solid glass) from that of the crystalline solid sample before melting, only a slight decrease in the vapor pressures of alkalis due to melting and undercooling was observed. Actually, a slight increase in the vapor pressure was expected, rather than a decrease, since the solidified glass is a thermodynamically metastable state and it can be considered to be an undercooled liquid. In this case, by extrapolation of the liquid part of the Clausius-Clapeyron plots to the temperatures that previously belonged to the solid crystalline phase, a slight increase in the vapor pressures should have been observed. In contrast, the opposite behavior occurred, i. e. a slight decrease in the vapor pressure after melting. Nevertheless, this phenomenon occurred only for sample S1-3, which was completely molten at the upper temperatures of the last run. The relationships can be seen in Fig. 4.


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Figs. 5 and 6 present only the selected, equilibrium (final) Clausius-Clapeyron plots for Na(g) and K(g) over each measured sub-sample for all samples in the measured temperature range without showing any measured (T, p)-data. The corresponding plots of sub-sample 1 for S1-1, -2 and -4 were either deviating due to the overly high applied temperatures causing distillation of the sample or they were measured over a too small temperature range. Therefore, an additional sub-sample (4) was measured for these samples in order to obtain more reliable mean data. The deviating plots for sub-sample 1 of S1-1, -2 and -4 are not shown in Fig. 5 and 6. These summarizing figures assist with further selection. The deviations between the different sub-samples may be due to measurement uncertainties originating from pressure calibration error, possible residual sample inhomogeneity, etc. Nevertheless, in order to avoid sample inhomogeneity, the bulk samples were homogenized in an agate mortar before separating any subsample loading it into the Knudsen cell, as was described in the Experimental chapter above. It can also be seen that the uncertainty of the vapour pressure of K(g) is somewhat higher than that for Na(g) since the concentration of K2O in the samples was much lower than that of Na2O. A further averaging (or selection) procedure on the measurements of the individual sub-samples is shown in Table S-II and the selected Clausius-Clapeyron plots are shown in Figs. 7 and 8. It can be seen in Figs. 7 and 8 that the volatility of S1-3, regarding both Na(g) and K(g), is the lowest of all samples. This means that the alkali retention potential of this sample is the largest. This effect cannot be explained only by comparing the alkali concentration of this sample to that of the others since the Nacontent of S1-3 is the second highest whereas the K-content is the third lowest of all four samples. Regarding to the Na-content of S1-3, S1-1 is the most comparable, and for the K-content, it is S1-2. Nevertheless, there is a large difference between the volatilities of S1-3 and the other two samples (S11, S1-2). The reason can be found in the different concentration of the other components which also have an effect on the activities of alkali oxide components. By relating the activities of Na2O and K2O to the compositions of the samples (see Table 1), it is clear that the components Al2O3, SiO2, CaO, MgO and Fe2O3 have a significant effect on the activities. SiO2 has the largest effect. It is an acidic oxide, acts as a strong network former within the slag, and forms silicates with the alkali oxides during ACS Paragon Plus Environment

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crystallization. The effect of SiO2 is followed by Al2O3, which forms aluminates or appears together with silica as aluminosilicates during crystallization. In the slag, aluminium is an intermediate. It can substitute silicon in silicate melts in 4-fold coordination by charge compensation with a cation, e.g. Na or K, in the vicinity [15, 16]. Comparing the alumina and silica content of S1-3, S1-1 and S1-2 it is evident that the activities of alkali oxides should be the lowest in S1-3 since the sum of the alumina and silica concentrations is the highest in S1-3. It is also not surprising that the alkali oxide activities are the highest in S1-4 since the mole fractions of Na2O and K2O are the largest and the mole fractions of Al2O3 and SiO2 are the second lowest of all four samples. The sum of the alumina and silica concentrations is very similar in S1-1 and S1-4 but the mole fractions of Na2O and K2O are much lower in S1-1. Therefore, lower Na2O and K2O activities are expected in S1-1. The components CaO, MgO and Fe2O3, which latter should be present as FeO in the slag under gasification conditions, have a weaker influence on the activities of the alkali oxides. All of them are basic oxides and act as network modifiers which should increase the alkali activity. Since S1-3 contains the lowest amount of these network modifiers, together with relatively high SiO2 and Al2O3 content, it shows by far the lowest alkali activity.

3.4 Investigation of the KEMS residues of the slag samples by SEM/EDX Further information on the structure and the precise internal composition of the equilibrium solid phases can be obtained from scanning electron microscope/energy dispersive X-ray (SEM/EDX) investigations. Though XRD detects the crystal structure of all individual phases present in a solid sample (see e.g. Fig. 2 above) if the relative amount of a phase is above ~3%, the precise atomic composition of all individual phases can be obtained only from SEM/EDX experiments. Fig 9 presents the SEM picture of a cross section of S1-1 after the KEMS investigation. The KEMS residue of S1-3 was analysed by neither XRD nor SEM/EDX since this sample was melted during its third KEMS run as the highest temperature used in the investigation was above the S1-3 melting point. By rapid cooling of such a molten glass, an amorphous, solid glass is formed. The temperature was elevated above the melting point since the activities in this sample are the lowest of all 14 ACS Paragon Plus Environment

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and the intention was to observe the vaporization properties of this sample, even as an undercooled glass. Table 3 summarizes the experimental results of the XRD and the SEM/EDX investigations, as well as the calculated data obtained by thermodynamic equilibrium calculations using the software package FactSage supplemented with the thermodynamic database of Forschungszentrum Jülich and GTT Technologies Germany [17]. Table 3 shows that the agreement between XRD and SEM/EDX data is very good. Furthermore, the agreement between the experimental (XRD, SEM/EDX) and the calculated data (FactSage, FZ Jülich/GTT database, Ref. [17]) is also good. The temperatures of the calculated data were selected so that no liquid phases were present in equilibrium and to reflect low enough equilibrium temperatures since the experimental samples were rapidly cooled by simply switching off the power supply of the furnace after the KEMS measurements. This cooling rate is somewhat lower than that obtained by standard quenching. Therefore, it is not really known which temperature corresponds to the frozen state of the sample. However, it must be a temperature at which no liquid phases are present and it must be lower than the lowest temperature applied during the KEMS measurements. These facts determined the selection of temperatures at which the equilibrium calculations were performed. Surprisingly good agreement was obtained between the XRD and calculated data for sample S1-2. Table 3 shows that there is some deviation from the theoretical chemical formulae of the phases due to contamination with additional elements. The cleanest phase is SiO2. It can be observed that Fe replaces Mg, K replaces Na and Ti replaces Si. It can be seen from Table 3 that in case of S1-1, there is only one equilibrium phase that contains alkalis: the multi-component (“contaminated”) anorthite phase. The diopside and wollastonite phases in S1-1 are completely free of alkalis. Therefore, during the evaporation (distillation) of alkalis from S1-1, the activity of alkali oxides should decrease immediately by decreasing the overall alkali concentration of the sample and also, therefore, by decreasing the alkali concentration of the anorthite phase. However, after the second KEMS run on S1-1, no further decrease of ion intensities was observed.


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In the case of S1-4, three phases contain alkalis. Beside the multi-component anorthite phase, the “contaminated” diopside and the “contaminated” wollastonite also contain alkalis, and the latter two contain Na in much lower concentrations. Due to the fact that three S1-4 phases contain alkalis, the effect of the evaporation (distillation) of alkalis on the activities is different. Instead of an activity decrease, the activity remains constant until the contaminated anorthite phase, being the main source of alkalis, disappears from the system. Therefore, the activity can be maintained for a long time during distillation even if the overall alkali content of the sample continuously decreases. Since the alkalis are present in the anorthite phase of all three samples, and this phase is the main source of alkalis, there must be a correlation between the alkali concentration of the anorthite phase and the activities. Indeed, the increasing concentration of Na in the anorthite phase in the order S1-2 → S1-1 → S1-4 corresponds to an increasing activity of Na2O in the same order. The relationships for K are more complicated since the diopside phase contains more K in S1-2 than the anorthite phase in the same sample, but the concentration of K in the anorthite phases of S1-1, S1-2 and S1-4 are about the same. Therefore, no correlation can be obtained between the internal K-concentrations of the anorthite phases and the K2O-activities of the three samples. In S1-4, the contaminated wollastonite phase includes a higher concentration of K than the other two phases (anorthite and diopside). The complicated effects cause that the activity of K2O increases in the order S1-1 → S1-2 → S1-4, although the activity of K2O does not differ significantly between S1-1 and S1-2.

3.5 Practical implications The present results confirm earlier findings on boiler slags [7, 8], where it was shown that acid slags containing relatively high amounts of network formers, such as SiO2 and Al2O3, and relatively low amounts of network modifiers, such as CaO and FeO, exhibit high alkali retention capability. Therefore, as far as alkali emissions from the gasifier are concerned, gasification of coals with acid ashes or slags is the most preferable. This can be achieved by using a coal with appropriate ash composition, blending coals or the use of suitable additives. On the one hand, the release of alkali species during gasification is 16 ACS Paragon Plus Environment

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already relatively low [5]. Furthermore, the slag draining down the gasifier walls has a high potential to retain the already captured alkalis and to adsorb further alkalis from the gas phase, which will then no longer contribute to fouling and corrosion processes. However, it should be noted that the viscosity of a slag increases with increasing acidity, which might be a limiting factor since effective removal of slag from the gasifier is also a crucial issue. Although the equilibrium between gas phase and slag cannot be assumed for an entrained flow gasifier with relatively short gas residence times and thus relatively short gas-slag contact times, the retention potential of the slag determines the lowest alkali concentration that can be achieved.

4. Conclusions The vaporization of four solidified slag samples from a pilot gasifier was determined by KEMS. Na(g), K(g)) and O2(g) were the main observed species in the equilibrium vapor. The predominantly amorphous original slag samples became crystalline during the KEMS investigations. Post analysis of the samples by XRD and SEM/EDX showed that an anorthite type phase contains more Na than all the other phases in equilibrium with this phase. It is therefore predominantly this anorthite type phase which determines the measured vapour pressures. The increasing concentration of Na in the anorthite phases of the crystalline solid samples in the order S1-2 → S1-1 → S1-4 results in an increasing activity of Na2O in the same order. S1-4, which has the lowest amount of the acidic oxides SiO2 and Al2O3, a relatively high amount of the basic oxides CaO and FeO, and the highest amount of alkali oxides, was the most volatile sample with regards to both Na(g) and K(g). In contrast, S1-3, having the highest amount of SiO2 and Al2O3 and a relatively low amount of CaO, FeO and alkali oxides, showed the lowest Na(g) and K(g) volatility. Thus, the volatility results clearly show that higher amounts of acidic SiO2 and Al2O3 lead to higher alkali retention capability of the slag. Therefore, if high alkali retention is required, a suitable slag can be achieved by using a coal with appropriate ash composition, blending coals or the use of suitable additives. ACS Paragon Plus Environment

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Acknowledgments The work described in this paper has been performed in the framework of the HotVeGas project financially supported by Bundesministerium für Wirtschaft und Energie (FKZ 0327773F). We gratefully acknowledge the SEM/EDX investigations performed by Dr. E. Wessel. The assistance of the ZCH team of Forschungszentrum Jülich concerning the elemental ICP-OES analysis of the samples is also kindly acknowledged.

Nomenclature: K: sensitivity constant of the mass spectrometer T thermodynamic temperature in K pj equilibrium vapor pressure of species j

σj ionization cross section of species j Iij ion intensity of ion i originating from species j

ηi isotopic abundance of ion i γi multiplier’s gain factor for ion i Mj: molar mass of gaseous species j aA: thermodynamic activity of component A Ka: thermodynamic equilibrium constant expressed in activity Kp: thermodynamic equilibrium constant expressed in pressure

References [1]

IEA, International Energy Agency, World Energy Outlook, 2010.

[2]

Muller, M. Integration of hot gas leaning at temperatures above the ash melting point in IGCC.

Fuel 2013, 108, 37−41. ACS Paragon Plus Environment

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[3]

Energy & Fuels

Valenti, M. Coal gasification: An alternative energy source is coming of age. Mech. Eng. 1993,

114 (1), 39−43. [4]

Higman, C.; van der Burgt, M. Gasification; Elsevier: Amsterdam, Netherlands, 2009.

[5]

Bläsing, M.; Müller, M. Release of alkali metal, sulphur, and chlorine species from high

temperature gasification of high- and low-rank coals, Fuel Process Technol. 2013, 106, 289–294. [6]

Bläsing M.; Melchior, T.; Müller, M. Influence of the Temperature on the Release of Inorganic

Species during High-Temperature Gasification of Hard Coal, Energ Fuel 2010, 24, 4153-4160. [7]

Müller, M.; Willenborg, W.; Hilpert, K.; Singheiser, L. Structural dependence of alkali oxide

activity in coal ash slags. VII. International Conference on Molten Slags Fluxes and Salts, The South African Institute of Mining and Metallurgy, 2004, pp. 615-618. [8]

Willenborg, W.; Müller, M.; Hilpert, K. Alkali Removal at About 1400 °C for the Pressurized

Pulverized Coal Combustion Combined Cycle. 1. Thermodynamics and Concept, Energy & Fuels, 2006, 20, pp. 2593-2598. [9]

Mysen, B. O. Structure and Properties of Silicate Melts, Elsevier, Amsterdam, 1988.

[10]

Melchior, T.; Pütz, G.; Müller, M. Surface Tension Measurements of Coal Ash Slags under

Reducing Conditions at Atmospheric Pressure, Energy & Fuel , 2009, 23, 4540–4546. [11]

Hilpert, K. Fresenius J Anal Chem, 2001, 370, 471-478.

[12]

IVTANTHERMO, Glushko Thermocenter of RAS, Database on Thermodynamic Properties of

Individual Substances and Thermodynamic modelling Software 1992-2000. [13]

Drowart, J.; Chatillon, C.; Hastie, J.; Bonnell, D. Pure and Appl. Chem. High Temperature Mass

Spectrometry: Accuracy of the Method and influence of the Ionisation Cross Sections. IUPACTechnical Report 2005, 77(4), 683. [14]

Stolyarova, V. L.; Semenov, G. A. Mass Spectrometric Study of the Vaporization of Oxide

Systems, John Wiley & Sons; 1st edition (1994).


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[15]

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Mysen, B. O. Structural behavior of Al3+ in silicate melts: In situ, high-temperature

measurements as a function of bulk chemical composition. Geochimica et Cosmochimica Acta, 1995, 59, 455-474. [16]

Neuville, D. R.; Mysen, B. O. Role of aluminium in the silicate network: In situ high

temperature study of glasses and melts on the join SiO2-NaAlO2. Geochimica et Cosmochimica Acta, 1996, 60, 1727-1737. [17]

Hack, K.; Jantzen, T.; Müller, M.; Yazhenskikh, E.; Wu, G. A novel thermodynamic database

for slag systems and refractory materials, in Proceedings of the 5th International Congress on the Science and Technology of Steelmaking, ICS 2012, Dresden, Germany, 2012.


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Table 1. Composition of the slag samples in mole percent determined by ICP-OES, and their hemispherical temperature Th and fluid temperature Tf in °C determined by hot stage microscopy. Sample

Al2O3 BaO

CaO

Fe2O3 K2O

MgO Na2O SiO2

TiO2

Th

Tf

S1-1

1.999

S1-2

16.891 0.382 16.135 2.294 0.771 4.223 1.842 56.984 0.480 1299 1460

S1-3

19.197 0.068 4.362

6.151 0.883 1.983 1.471 65.059 0.827 1267 1302

S1-4

8.238

3.030 1.049 7.712 5.846 55.152 0.462 1167 1234

0.071 23.554 3.381 0.180 9.660 1.506 59.366 0.282 1238 1379

0.161 18.35

Table 2. Partial and total mass losses of the components of the slag samples in mg, obtained from the total mass loss and from the ICP-OES analyses of the samples before and after the investigations by KEMS Sample

initial mass K2O mass loss Na2O mass loss total mass loss

S1-1

104.0

0,1

1,1

4,9

S1-2

105.7

0.5

1.1

7.8


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Table 3. Results of the XRD and SEM/EDX analyses, as well as the results of thermodynamic equilibrium calculations Phases

Chemical formula (SEM/EDX)

Phases (XRD)

(SEM/EDX)

General

Phases obtained by

chemical

thermodynamic

formula (XRD)

calculation in this work using database [17]

KEMS residue of S1-1 (cooled down in the KEMS apparatus spontaneously) contam.

Na0.16K0.05Ca0.38Mg0.01Fe0.21Al0.55Si2.93Ti0.04O8

not detected

T=1173 K not detected

separate feldspars

anorthite

(almost pure albite

(plagioclase)

(~7%) and pure anorthite (~0,2%) phases)

quartz

SiO2

quartz and

SiO2 (~38%)

β-quartz (~44%)

CaMgSi2O6

pyroxene (~17%,

(~32%)

almost pure

cristobalite contam.

Ca0.88Mg0.71Fe0.19Al0.02Si1.94Ti0.01O6

diopside

diopside (pyroxene) contam. iron

diopside) Fe0.984Si0.003Ca0.008P0.005 (very small amount)

not detected

not detected

-

Ca0.81Mg0.04Fe0.05Si1.02O3

wollastonite

CaSiO3 (~30%)

wollastonite (~27%)

not detected

not detected

not detected

Fe2O3 (~5%)

(Fe) contam. wollastonite not detected


Na0.10K0.03Ca0.61Mg0.03Al1.57Si2.35Ti0.01O8

anorthite

anorthite

T=1173 K CaAl2Si2O8

separate feldspars

(~84%)

(anorthite (~48%) and albit-ortoclase (20))

quartz

SiO2

quartz

SiO2(~4%)

cristobalite (~19%)

contam. Fe3P

Fe0.758Ca 0.003P0.239 (very small amount)

not detected

not detected

not detected

diopside???


diopside

CaMgSi2O6

pyroxene (~10%,

(~12%)

almost pure diopside)



anorthite

anorthite (plagioclase)

T=1134 K CaAl2Si2O8

separate feldspars

(~34%)

(albite-ortoclase (~27%) and pure anorthite (~5%) phases


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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

contam.

Energy & Fuels Na0.05K0.01Ca0.83Mg0.54Fe0.28Al0.18Si2.00Ti0.03O6

diopside

diopside

CaMgSi2O6

pyroxene (~21%,

(~49%)

almost pure

(pyroxene)

diopside)

quartz

SiO2

quartz

SiO2 (~3%)

-

contam.

Na0.04K0.04Ca0.65Mg0.04Fe0.08Al0.09Si1.09O3

wollastonite

CaSiO3 (~14%)

wollastonite (~28%)

not detected

not detected

not detected

not detected

Fe2O3 (~7%)

not detected

not detected

not detected

not detected

nepheline (KAlSiO4,

wollastonite

~12%)-


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Figure 1. Scheme of a Knudsen effusion mass spectrometer [11]


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Figure 2. XRD pattern of S1-1 before (top) and after (bottom) the KEMS experiments.


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Run 1 Run 2 Run 3

-1

pNa(g)/Pa

10

-2

10

-3

10

1600

1550

1500

1450

1400

1350

T/K (reciprocal scale)

Figure 3. Clausius-Clapeyron plots for Na(g) over sample S1-3 (Sub-sample 3, Runs 1 to 3)

Run 3: heating and melting Run 4: cooling

-1

10

pNa(g)/Pa

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

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-2

10

-3

10

1600

1550

1500

1450

1400

1350

T/K (reciprocal scale)

Figure 4. Clausius-Clapeyron plots for Na(g) over sample S1-3 (Sub-sample 3, Run 3: heating and melting, Run 4: cooling)


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0

10

pNa(g) / Pa

2 -1

10

2

S1-4

3 2 3 4

-2

10

4

-3

10

1562

1491

1420

3 2

3

S1-3

4

S1-1

S1-2

1

1349

1278

1207

1136

T/K

Figure 5. Clausius-Clapeyron plots for Na(g) over each measured sub-sample of all four samples. The numbers beside the lines denote the studied sub-sample.

4 0

10

32

S1-2

-1

10

pK(g) / Pa

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

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-2

10

S1-4 3

-3

2

10

4 2

2

1

-4

10

3

S1-3

3 S1-1 4

1600 1550 1500 1450 1400

1350

1300

1250

1200

1150

T/K

Figure 6. Clausius-Clapeyron plots for K(g) over each measured sub-sample of all four samples. The numbers beside the lines denote the studied sub-sample.


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Energy & Fuels

0

10

-1

pNa(g) / Pa

10

S1-4 -2

10

S1-1 -3

10

S1-2

S1-3 1600 1550 1500 1450 1400 1350

1300

1250

1200

1150

T/K

Figure 7. Selected Clausius-Clapeyron plots of Na(g) for all four samples.

0

10

-1

10

pK(g) / Pa

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

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-2

10

S1-4 -3

10

S1-2 -4

10

S1-3 1600 1550 1500 1450 1400

1350

1300

S1-1 1250

1200

1150

T/K

Figure 8. Selected Clausius-Clapeyron plots of K(g) for all four samples.


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Figure 9. SEM image of S1-1 after the KEMS investigation. The numbers indicate the places of the EDX analyses.


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Investigation of Vaporization of Alkali Metals from Solidified Gasifier

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