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Normal radiative emittance of coal ash sulfates in the context of oxyfuel combustion Jeanette Gorewoda, and Viktor Scherer Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02866 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 13, 2017

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Figure 1. Spectral emittance of the Rhenish lignite ash (Hambach). 150x100mm (150 x 150 DPI)

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Figure 2. A) Test rig for radiation measurements and B) Close-up view of the sample holder sitting on the black body radiator, thermocouple locations are indicated. Reprinted with permission from [24]. Copyright 2016 American Chemical Society. 143x165mm (150 x 150 DPI)


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Figure 3. Particle size distributions for particles sieved with a mesh size of a) 32 µm, b) 125 µm and 160 µm. 104x171mm (150 x 150 DPI)


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Figure 4. MgSO4, particle size fraction x < 32 µm for a) prior and b) after heat treatment. 156x39mm (150 x 150 DPI)


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Figure 5. TGA-measurements of CaSO4 and MgSO4 (x < 32 µm). For MgSO4 the phase transition is initiated during the heating-up process from 900 °C to 1000 °C. The temperature history is identical to the heating process in the radiation test rig, except for the temperature at 500 °C (holding time reduced for TGA measurements by 64 min). 150x100mm (150 x 150 DPI)


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Figure 6. Spectral emittance of CaSO4, size fraction 125 < x < 160 µm. 150x100mm (150 x 150 DPI)


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Figure 7. Spectral emittance of MgSO4, size fraction 125 < x < 160 µm. 150x100mm (150 x 150 DPI)


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Figure 8. Spectral emittance of CaSO4, size fraction x < 32 µm. 150x100mm (150 x 150 DPI)


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Figure 9. Spectral emittance of MgSO4, size fraction x < 32 µm. 150x100mm (150 x 150 DPI)


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Figure 10. Total emittance of CaSO4, particle size fraction 125 < x < 160 µm and x < 32 µm. For 125 < x < 160 µm and x < 32 µm the cooling is started after 953 and 936 minutes, respectively. 150x100mm (150 x 150 DPI)


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Figure 11. Total emittance of MgSO4, particle size fraction 125 < x < 160 µm and x < 32 µm. The phase transition is occurred between the heating and cooling procedure. For 125 < x < 160 µm and x < 32 µm the cooling is started after 6118 at 949 °C and 5850 minutes at 958 °C, respectively. 150x100mm (150 x 150 DPI)


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Normal radiative emittance of coal ash sulfates in the context of oxyfuel combustion Jeanette Gorewoda1*, Viktor Scherer1

Corresponding author: *[email protected], +49-234-3226323

1Department

of Energy Plant Technology, Faculty of Mechanical Engineering, Ruhr

University of Bochum, Universitätsstr. 150, 44801 Bochum, Germany

KEYWORDS Emittance, mid-infrared, near-infrared, oxyfuel, sulfate, ash, particle size, chemical composition

ABSTRACT Oxyfuel ashes are supposed to form more sulfates than ashes from air fired systems. This can be caused by the increased SO2 concentrations due to intensive flue gas recirculation in oxyfuel systems. Therefore, we investigated the spectral emittance characteristics of typical mineral sulfates in coal ashes, namely Mg and Ca sulfates. The samples were prepared in powder form. Two particle size fractions were examined (x < 32 µm and 125 < x < 160 µm). The powders were investigated concerning their temperature-dependent

normal

emittance

in

a

radiation

test

rig.

Spectral 1


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measurements by an FT-IR in the temperature range from 500 °C to 1000 °C were carried out. The results reveal that Ca and Mg sulfate shows characteristic S-O absorption bands in the wavelength region from 3 to 4 m, 4.5 to 6 m and 8 to 9.5 m. MgSO4 transforms to MgO at around 930 °C. The total emittance of the oxide is significantly reduced by  = 0.15 compared to the sulfate. The small size fractions MgSO4 and CaSO4 undergo sintering when being heated which influences emittance. An increase of total emittance up to a value of  = 0.08 is detected for CaSO4. Finally, it is shown that emittance increases with particle size ( in total emittance approx. = 0.1).

Nomenclature S T x RT Greek ε

1.

detected radiance signal Temperature particle diameter room temperature

ε̅ λ Subscripts BB λ

total emittance Wavelength Blackbody spectral dependence

emittance

Introduction

A promising option of carbon capture is oxyfuel combustion. Overviews on the state of research on solid fuel oxyfuel combustion can be found, for example, in [1–4]. However, information is missing in available literature whether oxyfuel firing has an influence on the radiative properties of coal ashes. As heat transfer in the furnace of coal fired boilers is dominated by radiation, it is of utmost importance to know the emittance of the ashes deposited on heat exchanger surfaces, which is the motivation of the current paper. Because of the importance of emittance on boiler furnace heat balance, synthetic and real coal ashes were the subject of several experimental studies [5–13]. Total emittance is decreasing with increasing ash layer temperature [5,6]. Total and spectral 2 ACS Paragon Plus Environment

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emittance of ash layers is dependent on chemical composition and physical surface structure according to Mulcahy et al. [7] and Boow et al. [8]. For synthetic ash layers correlations of emittance with mean particle diameter can be found in [8,9], indicating a decrease in emittance with decreasing particle size. Calculations based on Mie theory [10,11], as well as measurements with natural coal ashes [12,13], confirmed this finding. Also experiences from boiler operation point into that direction. Gwosdz et al. [14] reported reflective ash deposits in industrial boilers, when ash particles become very small. A decrease of furnace exit temperature up to 125 °C was reported. Mulcahy et al. [7] stated that particle sintering leads to an irreversible increase in emittance. For natural ashes they measured an increase of total emittance up to values of  = 0.15. Fe2O3 was identified to increase emittance of ash layers [8,15]. Shimogori at al. [16] presented a correlation for the spectral emittance of coals ashes as a function of Fe2O3 content. Another correlation has been given by Tsuda et al. [17], who included Si-, Al-, Fe-, Ca- and Mg-content in the ash. In-situ measurements of spectral emittance of coal ashes were reported by Moore et al. [18] in an entrained flow reactor. They confirmed the importance of iron content and ash surface structure on emittance. Complex refractive indices of synthetic and natural slags have been measured by Goodwin and Mitchner [15]. Their measurement data have been used by Bhattacharya [19] to calculate single particle emittance based on Mie theory. Liu et al. [20] generated an artificial ash particle cluster consisting of 714 spheres. Using measured absorption coefficients and refractive indices of SiO2, Al2O3, CaO and Fe2O3 they have calculated spectral emittances using a Generalized Multiparticle Mie solution. The general trend of ash layer emittance and dependency on particle size is well represented by the method.

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Combustion conditions, air or oxyfuel, can influence the formation processes and chemical composition of ash deposits due to different gas compositions and temperatures. In particular, higher CO2 and SO2 partial pressure caused by flue gas recirculation may intensify sulfate and carbonate formation. Fryda et al. [21] as well as Sheng and Li [22] reported that the relative amounts of mineral phases differ in oxyfuel and air combustion, but the type of minerals are the same. Kull et al. [23] proposed the enhanced presence of carbonates in the ash. Therefore, Gorewoda et al. [24] measured spectral and total emittance of Ca, Mg and Sr carbonates and mixtures of these carbonates with SiO2. They have reported strong absorption bands at approximately 4 m and 7 m for spectral emittance caused by the infrared active CO3 group. These absorption bands vanished when the carbonates transformed to the corresponding oxide at elevated temperatures. For CaCO3, the most relevant carbonate in coal ashes, spectral and total emittance were decreased significantly after phase transformation. Scheffknecht et al. [2] did find an increased amount of sulfates, especially CaSO 4, in the ash deposits under oxyfuel conditions. The trend of higher sulfur contents in coal ashes under oxyfuel conditions was also reported in [4,25,26]. They stated that the sulfur content in ashes and deposits of oxyfuel firing systems is increased by a factor of 1.5 – 3. Data from NIST [27] report characteristic bands in the infrared spectrum (approximately 3 to 4 m, 4.5 to 6 m and 8 to 9 m) of sulfates at room temperature. Further IR-spectra of sulfates can be found in [28–31]. Lane [26] also has discussed the influence of particle size, temperature and hydration state of the sulfates. Details on the spectral emittance of sulfates at elevated temperatures, typical for boilers, are, to the best knowledge of the authors, not available. 4 ACS Paragon Plus Environment

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As a further motivation of our work, Figure 1 presents the spectral emittance of a SO3rich lignite ash (German Rhenish lignite Hambach) for three different temperatures. The measurements have been carried out in the test rig explained in more detail under “chapter 2: experimental method”. XRF analysis of the ash delivered a content of SO3 of 28.7 %, of 31.9 % CaO, and 17.8 % MgO, respectively (XRF only provides elemental composition, content is usually reported as oxide). An XRD analysis, which allows the qualitative determination of mineral phases, confirmed the presence of CaSO4 and MgSO4. First of all, Figure 1 shows the typical general trend for the emittance of mineral based coal ashes as a function of wavelength. Low emittance at low wavelength and an increase to higher emittance at higher wavelengths. The wavelength ranges, where S-O bands might be expected, are marked by shaded areas. Bands can be observed for 3 to 4 m, and 4.5 to 6 m. No band can be detected at 8 to 9.5 m, although the pure sulfates CaSO4 and MgSO4 show absorption in these wavelength range, as will be shown later in the paper. Obviously, for the highest temperature of 968 °C the spectral bands are vanishing. In particular, MgSO4 shows a phase transformation to the corresponding oxide above 930 °C.

Figure 1. Spectral emittance of the Rhenish lignite ash (Hambach).

Figure 1 is an indication that sulfates might influence ash emittance, however, as in real coal ashes different minerals can contribute to the net emittance and interference between different minerals can lead to complex dependencies, further studies are needed.

Therefore, we decided to simplify the task for the current paper and to concentrate on the pure substances CaSO4 and MgSO4. We are going to focus on four questions: 5 ACS Paragon Plus Environment

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

are there any significant differences in emittance of the two sulfates?



does phase change from the sulfate to the oxide influence the emittance?



how important are particle size, and associated with that, sintering effects at elevated temperatures for emittance?



what can be learned from pure substances for real coal ashes?

To approach these questions, the sulfates (Ca, Mg) were sieved to two particle fractions, were heated up in a radiation test rig, and spectral normal emittance was measured during this procedure.

2.

Experimental method

Figure 2 shows a schematic sketch of the radiation test rig. The facility consists of a Fourier transform infrared (FT-IR) spectrometer and an electrical heating unit. The heating unit includes the sample holder and the reference radiator, a black body. The black body also acts as the sample mount for the ash sample holder. Radiation is guided through a port (optical access, 20 mm diameter) in the insulation of the heating unit to a FT-IR spectrometer via a gold coated off-axis parabolic mirror. For mineral measurements, samples are placed in the sample holder on top of the black body. The sample (22 mm diameter, 2 mm thickness) temperature is determined by two type K Figure 2. A) Test rig for radiation measurements and B) Close-up view of the sample holder sitting on the black body radiator, thermocouple locations are indicated. Reprinted with permission from [24]. Copyright 2016 American Chemical Society. thermocouples (0.25 mm diameter). They are installed 1 and 2 mm below the sample surface (in the central axis of the disk shaped sample). The sample surface temperature is calculated by postulating a linear temperature profile across the sample height (the validity of this assumption has been checked by solving the 3D-heat 6 ACS Paragon Plus Environment

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conduction equation in the sample). For black body reference measurements, the sample holder is removed, and the black body is lifted upwards until its opening is in the same position as the mineral sample before. The reference radiator (black body) is laid out as a cylindrical cavity radiator (cavity diameter 5 mm, cavity length 135 mm) with an emittance > 0.9985 (for details see [32]). The black body temperature is determined by thermocouples. Thermal radiation is detected by a FT-IR (PerkinElmer Frontier MIR/NIR) spectrometer with a resolution of 4 cm-1. The FT-IR is equipped with deuterated triglycine sulfate DTGS detectors (wavelength range: 0.68 µm to 28.6 µm). Per temperature 50 scans are acquired to reduce the signal-to-noise ratio (max. standard deviation 0.35%). The total statistical error (standard deviation), based on repeated measurements with the same sample material at different days, is below ±3.2%. All measurements are performed in air. Directional measurements are performed perpendicular to the surface of the sample. Emittances are measured from 500 °C to 1000 °C in steps of 100 °C. Note that 500 to 1000 °C are reasonable ash layer temperatures on the water walls of dry ash boiler systems. The heating procedure including holding times at each temperature is depicted in Figure 5.

The spectral normal emittance ε(𝜆,T) is determined by comparing the radiation of the sample to the radiation of the black body at the same temperature and wavelength (see Eq. (1)).

𝜀(𝜆, 𝑇) =

𝑆𝑠𝑎𝑚𝑝𝑙𝑒 (𝜆, 𝑇) 𝑆𝐵𝐵 (𝜆, 𝑇)

(1)

The total emittance 𝜀̅(T) is determined by weighting the spectral emittance with Planck’s law (SBB in Eq. (2)):


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𝜆

𝜀̅(𝑇) =

3.

2 ∫𝜆 𝜀(𝜆, 𝑇) ∙ 𝑆𝐵𝐵 (𝜆, 𝑇) d𝜆 1

𝜆2 ∫𝜆1 𝑆𝐵𝐵 (𝜆, 𝑇)

(2)

d𝜆

Samples Description

The two sulfates considered (CaSO4, MgSO4) were prepared by sieving powders into two particle size fractions. Figure 3 shows the particle size distributions of both fractions prior to heating (Figure 3 a: x < 32 µm and Figure 3 b: 125 < x < 160 µm) and for the large size fraction also after heating.

Figure 3. Particle size distributions for particles sieved with a mesh size of a) 32 µm, b) 125 µm and 160 µm.

MgSO4 and CaSO4 with a particle size smaller than 32 µm tend to form agglomerates. MgSO4 shows a bimodal particle size distribution with a second peak at approximately 100 m. This particle size is larger than the mesh size used (32 µm) indicating particle adherence, which is due to the hygroscopic nature of MgSO4. After heat treatment MgSO4 and CaSO4 tends to form sticky agglomerates due to sintering. The general sintering behavior is depicted in Figure 4. In the left picture MgSO4 powder (x < 32 µm) before heat treatment can be seen. In the right picture the sample is shown after heat treatment. The sample forms a solid structure, and resembles the form of the sample holder. This behavior is more pronounced for MgSO4 than for CaSO4. Because of this strong sintering behavior, reliable particle size measurements were not possible. The particle size distributions for the 125 < x < 160 µm size fraction (Figure 3b) are very similar before and after heat treatment, which is due to the fact that larger particles have a weaker propensity to sinter. Some fines around 20 µm are present before heat treatment. MgSO4 seems to produce some further fines after heat treatment. The reason for the production of further fines is not fully understood, however, we assume 8 ACS Paragon Plus Environment

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that the formation of fines is caused by the gas release during transformation of MgSO 4 to MgO. Particularly at larger particle sizes this effect might lead the breakage of certain particles.

Figure 4. MgSO4, particle size fraction x < 32 µm for a) prior and b) after heat treatment.

The sulfate decomposition has been investigated using a thermogravimetric analyzer (TGA Leco 601). Figure 5 represents the results for CaSO4 and MgSO4. As predicted by accompanying equilibrium calculations using FactSage [33], only MgSO4 decomposes to the corresponding oxide in the temperature range under consideration [34]. The initial mass losses of MgSO4 and CaSO4 at low temperatures are due to dehydration. The conversion to the anhydrous form is already completed below 500 °C in accordance with literature [35]. The high temperature decomposition of Mg is represented by eq. 3. (3)

𝑀𝑔𝑆𝑂4 → 𝑀𝑔𝑂 + 𝑆𝑂3 (𝑔)

The decrease of mass for MgSO4, indicating phase transition, is starting at approximately 950 °C, and it is not completed after a holding time of 1050 min at 1000 °C. Thermodynamic equilibrium calculations carried out with FactSage [33] predict MgSO4 decomposition at a temperature of 930 °C. In literature a temperature range of 900 – 1100 °C is reported [35]. Note that we have measured emittance under air. For a typical dry oxyfuel atmosphere (30 % O2, 70 % CO2) the equilibrium temperature increases to 946 °C. Note that decomposition temperature for CaSO4 is at 1453 °C for air as atmosphere.


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Figure 5. TGA-measurements of CaSO4 and MgSO4 (x < 32 µm). For MgSO4 the phase transition is initiated during the heating-up process from 900 °C to 1000 °C. The temperature history is identical to the heating process in the radiation test rig, except for the temperature at 500 °C (holding time reduced for TGA measurements by 64 min).

4.

Results and Discussion

4.1.

Spectral emittance

Figure 6 and 7 depict the normal spectral emittance of Ca and Mg sulfates of the 125 < x < 160 µm size fractions for three different temperatures. The time depicted in the figures represents the holding time at the highest temperature. The black lines represent the emittance measurement during sample heat up and the grey lines during sample cooling after having been heated. The general trend of the spectral emittance is like for real coal ashes (compare Fig. 1), i.e. increasing values from 2 - 8 µm and a plateau for larger wavelengths at approx. 0.85 – 0.95. The spectral characteristics of both sulfates is comparable, with absorption bands at around 3 to 4 m, 4.6 to 6 m and 8 to 9.5 m (see shaded areas). For CaSO4 the absorption bands are more pronounced. For CaSO4 the absorption bands are still present during the cool-down phase because no phase transition to the oxide occurs. This is different for MgSO 4. Spectral characteristics vanish after phase transition (grey lines) and emittance is significantly lower in the spectral range from 3 to 7 m (note that phase transition needs approx. 6118 min for completion based on TGA measurements). It has to be mentioned that the wavelength range below 7 m is of special importance for heat transfer under typical boiler conditions (maximum of Planck’s curve at 2.3 m for 1000 °C). More than 90% of the heat is emitted for a black body below 7 m. Figure 8 and 9 presents the spectral emittance for the smaller size fractions (x < 32 m). In general, the behavior is similar to the larger size fraction, i.e. presence of more pronounced absorption bands 10 ACS Paragon Plus Environment

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for CaSO4 than for MgSO4, and vanishing spectral characteristics for MgSO4 after phase transition. Again phase transition of MgSO4 to MgO reduces emittance. Note that the total level of emittance for the fine fraction is higher for the larger particle sizes, specifically in the wavelength range below 4 m, an effect that corresponds to literature [7,8]. This increase in emittance will be reflected in the values for the total emittance in the following chapter. Figure 6. Spectral emittance of CaSO4, size fraction 125 < x < 160 µm. The arrow indicates the holding time at the highest temperature (968 °C) before cooling. Figure 7. Spectral emittance of MgSO4, size fraction 125 < x < 160 µm. The arrow indicates the holding time at the highest temperature (949 °C) before cooling. Figure 8. Spectral emittance of CaSO4, size fraction x < 32 µm. The arrow indicates the holding time at the highest temperature (968 °C) before cooling. Figure 9. Spectral emittance of MgSO4, size fraction x < 32 µm. The arrow indicates the holding time at the highest temperature (958 °C) before cooling. 4.2.

Total emittance

In engineering applications spectrally averaged total emittance is the basis for the design of industrial scale systems. Figures 10 and 11 shows the total emittance for CaSO4 and MgSO4, respectively. Both size fractions of each sulfate are shown in one diagram. Again the black curves depict the emittance during heating and the grey curves for cooling. Holding times at the highest temperature are as in figure 6 to 9. CaSO4 (Figure 10) shows the typical trend for total emittance for minerals without a phase change, i.e. a decrease of total emittance with temperature. This is mainly due to the averaging procedure (eq. 2), where spectral emittance is weighted by Planck’s law. The shift of the maximum of Planck’s curve for higher temperatures to lower wavelengths leads to an increased weighting of the spectral emittances at low wavelengths. Hence, the low emittances of minerals at low wavelengths (compare Figure 6 to 9) are shifting the averaged total emittances to lower values for higher temperatures. 11 ACS Paragon Plus Environment

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It is obvious that the sample composed of the larger particles shows significantly higher emittances than the fine powder. The difference in emittance between small and large particles is in the order of  = 0.1 (before the onset of sintering, heating curve). This is in accordance with literature [7] and own measurements [8] which demonstrates that larger particles lead to higher emittance. The larger particles of CaSO4 show very similar values of total emittance for heating and cooling. Values for cooling seem to be a little higher than for heating which indicates some sintering effects. But note that the differences are within the range of the measurement error. The effect of sintering, as expected, is much more pronounced for small particles. This is also reflected in the emittance values. The difference in emittance amounts up to  = 0.08 for the lowest temperature. Figure 11 shows total emittance for MgSO4, the sulfate with the phase change to the oxide. This phase change dominates the emittance characteristics of Mg. After phase change (indicated by an arrow) total emittance is drastically reduced. The reduction is in the order of  = 0.15 for both size fractions. The increase of total emittance before phase change (which is much more pronounced for the small particles) is ascribed to sintering effects. As for CaSO4, the difference in emittance between small and large particles is in the order of  = 0.1, at least before the onset of sintering. Before the onset of sintering the emittances of MgSO 4 and CaSO4 are at the same level. MgSO4 seems to be more sensitive to sintering than CaSO4. CaSO4 does not shows sintering effects, reflected in an increase in emittance already at 760 °C. As for CaSO4 also for MgSO4 the total emittance for the larger particles is on a higher level compared to the smaller particles.

Figure 10. Total emittance of CaSO4, particle size fraction 125 < x < 160 µm and x < 32 µm. For 125 < x < 160 µm and x < 32 µm the cooling is started after 953 and 936 minutes, respectively. 12 ACS Paragon Plus Environment

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Figure 11. Total emittance of MgSO4, particle size fraction 125 < x < 160 µm and x < 32 µm. The phase transition (indicated by an arrow) is occurred between the heating and cooling procedure. For 125 < x < 160 µm and x < 32 µm the cooling is started after 6118 at 949 °C and 5850 minutes at 958 °C, respectively.

5.

Conclusion

Literature indicates that the sulfate concentration in oxyfuel ashes might be increased compared to ashes formed in air fired systems. Therefore, the spectral and spectrally averaged emittances of CaSO4 and MgSO4, two typical sulfates occurring in ashes, were examined. The samples were prepared from powders in two particle size fractions x < 32 µm and 125 < x < 160 µm. Measurements of the temperature-dependent normal emittance were carried out by FT-IR spectroscopy. Temperatures were varied from 500 °C to 1000 °C. The four questions raised in the introduction might be answered as follows: Are there significant differences in emittance of the two sulfates? 

Spectral emittance of both sulfates is characterized by the same characteristic absorption S-O bands between 3 to 4 µm, 4.5 to 6 µm and 8 to 9.5 µm, respectively.



The total emittances of CaSO4 and MgSO4 are very similar before the onset of sintering.

Does phase change from the sulfate to the oxide influence the emittance? 

MgSO4 shows a phase transformation to the corresponding oxide at around 930 °C. Phase transformation of the sulfate to the oxide has a significant influence on spectral emittance. Whereas the sulfates show the described characteristic absorption bands, these bands vanish after transformation to the oxide.


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

The total emittance of MgSO4 is significantly higher than for the corresponding oxide. The total emittance differs by a value of  = 0.15.



CaSO4 does not show a phase transformation to the oxide below 1000 °C, and, therefore, spectral characteristics of the S-O-bands are maintained during heat treatment.

How important are particle size, and associated with that, sintering effects at elevated temperatures for emittance? 

Total emittance increases with particle size.



The difference between small size fraction (x < 32 µm) and large size fraction (125 < x < 160 µm) in total emittances for both, of CaSO4 and MgSO4, is in the order of  = 0.1.



Sintering increases emittance. The sintering effect on emittance is weaker for larger particles, as expected. For CaSO4, which shows no phase change to the oxide, an increase of emittance due to sintering up to a value of 0.08 has been detected. The onset of sintering for MgSO 4 occurs earlier, already at approx. 760 °C, compared to CaSO4.

What can be learned from pure substances for real coal ashes? 

The findings obtained for a pure mineral substance like MgSO 4 and CaSO4 cannot directly be transferred to natural coals ashes, because coal ashes contain a multitude of mineral substances which also can form mixed minerals, which complicates the situation even more. Because not only the chemical composition influences emittance but also the change of surface structure with temperature (which depends on the sintering behavior of the individual mineral), a quantitative description of coal ash emittance just based on elemental composition and surface structure is difficult. 14 ACS Paragon Plus Environment

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

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However, we can summarize that natural coal ashes – as all minerals – show a strongly non-grey radiation behavior, i.e. low emittance at low wavelength and high emittance at larger wavelengths. This is important for spectrally resolved simulation of radiative heat transfer. Note that clean, non-oxidized metal water walls of boilers show the opposite spectral characteristics; high emittance at low wavelength and decreasing emittance for higher wavelengths [36].



Also for natural ashes, rough sintered ash layers will show a higher emittance than ash layers of fine particles. This physical effect is rather independent from chemical composition.



Finally, we can state that natural coals ashes show typical spectral band of the S-O group, as has been shown for the ash of a Rhenish lignite (Hambach). An indication that, despite the complex mineral composition of coal ashes, characteristic absorption bands of the S-O group are also present in real coal ashes. Note that carbonate rich natural ashes also show spectral bands which stem from the CO3 group (as proven in our previous publication [24]).

As a next step, we will compare the emittance of mixtures of carbonates and sulfates, including SiO2 as a third mineral. In addition, examinations on FeS2 will be needed, another important mineral in S-rich coal ashes.

ACKNOWLEDGMENT This work has been funded by the German Science Foundation (DFG) within the Sonderforschungsbereich/Transregio TR 129 “Oxyflame-Development of methods and models to describe solid fuel reactions within an oxyfuel-atmosphere“.

REFERENCES 15 ACS Paragon Plus Environment

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