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The influence of carbonate decomposition on normal spectral radiative emittance in the context of oxy-fuel combustion Jeanette Gorewoda, and Viktor Scherer Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01398 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016

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Figure 1. Spectral emittance of the Rhenish lignite ash, size fraction x < 32 µm. 144x100mm (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. 143x165mm (150 x 150 DPI)


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Figure 3. Particle size distribution for particles sieved with a mesh size of A) 32 µm, B) 125 µm and 160 µm and C) 32 µm for SrCO3 before and after the measurements. 86x214mm (150 x 150 DPI)


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Figure 4. TGA-measurements (x < 32 µm) of MgCO3, CaCO3 and SrCO3. The phase transition is initiated at a temperature of 500 °C for MgCO3, 700 °C for CaCO3 and 900 °C for SrCO3. The temperature history is consistent with the heating procedure in the radiation test rig. 140x89mm (150 x 150 DPI)


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Figure 5. Spectral emittance of SrCO3, size fraction x < 32 µm. The phase transition takes place over a temperature range between 900 °C and 1000 °C (TGA measurement). 145x100mm (150 x 150 DPI)


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Figure 6. Spectral emittance of MgCO3, size fraction x < 32 µm. The phase transition takes place over a temperature range between 500 °C and 800 °C (TGA measurement). 144x100mm (150 x 150 DPI)


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Figure 7. Spectral emittance of CaCO3, size fraction x < 32 µm. The phase transition takes place over a temperature range between 700 °C and 900 °C (TGA measurement). 145x100mm (150 x 150 DPI)


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Figure 8. Spectral emittance of Fe2O3, size fraction x < 32 µm. 145x100mm (150 x 150 DPI)


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Figure 9. Total emittances of CaCO3 and MgCO3, size fraction 125 < x < 160 µm. 145x100mm (150 x 150 DPI)


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Figure 10. Non-Planck weighted averages of the spectral emittances of CaCO3 and MgCO3, size fraction 125 < x < 160 µm. 145x100mm (150 x 150 DPI)


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Figure 11. Total emittances of CaCO3, MgCO3 and SrCO3, size fraction x < 32 µm. 145x100mm (150 x 150 DPI)


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Figure 12. Total emittances of CaCO3, SiO2 and CaCO3 enriched samples based on SiO2, size fraction 125 < x < 160 µm. The vertical line denotes the onset of the phase transition of CaCO3 (TGA-measurement). 145x100mm (150 x 150 DPI)


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Figure 13. Total emittances of CaCO3, SiO2 and CaCO3 enriched samples based on SiO2, size fraction x < 32 µm. The vertical line denotes the onset of the phase transition of CaCO3 (TGA-measurement). 145x100mm (150 x 150 DPI)


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Figure 14. Total emittances of Fe2O3, SiO2 and Fe2O3 enriched samples based on SiO2, size fraction 125 < x < 160 µm. 145x100mm (150 x 150 DPI)


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Figure 15. Total emittances of Fe2O3, SiO2 and Fe2O3 enriched samples based on SiO2, size fraction x < 32 µm. 145x100mm (150 x 150 DPI)


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The influence of carbonate decomposition on normal spectral radiative emittance in the context of oxy-fuel combustion Jeanette Gorewoda1*, Viktor Scherer1

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

1

Department of Energy Plant Technology, Faculty of Mechanical Engineering, Ruhr University

of Bochum, Universitätsstr. 150, 44801 Bochum, Germany

KEYWORDS Emittance, oxyfuel, quartz, carbonate, ash, particle size, chemical composition

ABSTRACT To investigate whether the radiative properties of carbonate rich ash layers in oxyfuel combustion systems might be influenced by the carbonate decomposition to the corresponding oxide, the emittance of Sr, Mg and Ca carbonates is examined. In addition “synthetic coal ashes” were produced from mixtures of CaCO3 and SiO2 as well as Fe2O3 and SiO2. The mixture ratios of the minerals were varied. All samples were prepared from powders with known particle size fractions of x < 32 µm and 125 < x < 160 µm. The powders were investigated for 1 ACS Paragon Plus Environment

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their temperature-dependent normal emittance in a radiation test rig by FT-IR spectroscopy in the temperature range from 500 °C to 1000 °C. The results reveal that the phase transformation from the carbonate to the corresponding oxide has a significant influence on spectral emittance. Whereas the carbonates show characteristic peaks in spectral emittance around 4 µm which stem from the infrared active CO3 group, these peaks vanish after transformation to the oxide. For CaCO3, the most prominent carbonate in typical coal ashes, the emittance of the oxide is significantly lower than for the carbonate. Such a behavior in terms of total and spectral emittance has also been detected, for example, examining a Ca rich Rhenish lignite. Emittance increases with particle size for all samples. An enrichment of SiO2 with Fe2O3 leads to an increase in emittance.

Nomenclature S T x RT Greek ε

detected radiance signal temperature T particle diameter room temperature

ε̅ λ Subscripts BB λ

total emittance wavelength Blackbody spectral dependence

Emittance

1. Introduction Oxy-fuel combustion of solid fuels is one of the options of carbon capture and storage. Major findings of the differences between air and oxy fuel firing are summarized for example in [1– 3]. However, these papers do not address whether oxyfuel firing has an influence on the radiative properties of coal ashes which is the motivation of the current paper.

It is well known that heat transfer in the furnace of coal fired boilers is dominated by radiation. Ash deposits on heat exchanger surfaces influence the heat transfer significantly, because of 2 ACS Paragon Plus Environment

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their low thermal conductivity and their thermal radiation characteristics. Therefore, the reliable design of the combustion system requires the knowledge of the material properties of the ash deposits, particularly their emittance.

Emittance of synthetic and real coal ashes have been examined in several studies during the last years [4–12]. Typically total emittance is decreasing with increasing ash layer temperature [4,5]. Mulcahy [6] and Boow et al. [7] concluded already in 1966 that emittance of ash layers is dependent on chemical composition and physical surface structure. For synthetic ash layers in [7,8] correlations of emittance with mean particle diameter are reported. A decrease in particle diameter leads to a decrease in emittance. This finding was confirmed by Wall et al. [9] based on theoretical investigations (Mie scattering) [10] and by measurements for real ashes by Zygarlicke et al. [11] and Markham et al. [12]. Surface sintering at elevated temperature increases emittance (larger particle sizes) as reported by Mulcahy [6] and Boow et al. [7]. They also reported further increasing emittances for surface fusion. Greffrath [13] reports that fusion - depending on the ash composition - can either increase emittance (formation of glass like layers) or can decrease emittance (formation of reflecting, mirror like surface). Gwosdz [14] reported highly reflective ash deposits in industrial boilers when ash particles become very small. Fe2O3 was identified as a “colouring agent” which increases emittance of ash layers [7]. Goodwin and Mitchner [15] verified the influence of Fe2O3. They measured both components of the complex refractive indices of slags as a function of composition. Their measurement data were used for numerical simulations involving absorption and scattering of radiation by slag particles, e.g. Bhattacharya [16]. Shimogori 2012 [17] proposed a correlation of the spectral emittance of coals ashes based on their F2O3 content. Bhattacharya and Wall [18] report an increasing total emittance with increasing carbon content of the ash and a tendency to a more grey body behaviour (carbon content of the ash was rather high with approx. 50 mass%). Moore et al. [19] presented in-situ measurements of spectral emittance of coal ashes in the outlet of a 3 ACS Paragon Plus Environment

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laboratory scale down-fired entrained flow reactor. They also highlight the influence of iron content and physical ash structure on emittance. Infrared adsorption bands of carbonate minerals have been reported by Huang and Kerr [20] at ambient temperatures. They indicate that especially the infrared active CO3 group dominates the radiative characteristics of carbonates. Details on the spectral emittance of carbonates and their corresponding oxides during phase transition at elevated, boiler relevant, temperatures– the focus of the current paper – are, to the best knowledge of the authors, not available.

The results on whether oxyfuel conditions do influence the ash particle size are contradictory. Fryda et al. [21] did find an increased fly ash particle size (mean particle size around 60 μm) for a hard coal under oxy-fuel conditions but they could not find a difference in particle size for a lignite. In [3] it is reported that sub-micron particles become smaller in size under oxy-fuel conditions but in parallel the number density is decreasing.

The formation and chemical composition of ash deposits depends among other factors on the combustion conditions – oxyfuel or air. Typical ash constituents under air combustion conditions are sulfates, chlorides, carbonates and oxides. Under oxyfuel conditions the composition of the deposits may change. Higher CO2 and SO2 partial pressure due to flue gas recirculation and an altered local combustion temperature can influence the generation of mineral phases in the deposits. The studies of Fryda et al. [21,22] and Sheng and Li [21,22] on the chemical and mineralogical composition of ash deposits indicate no significant differences in the type of mineral phases present in oxy-fuel and air combustion conditions, but an influence on the relative amounts of the minerals phases. Scheffknecht et al. [2] did find an increased amount of sulfates, especially CaSO4 in the ash deposits under oxy-fuel conditions, whereas Kull et al. [23] propose the enhanced presence of carbonates in the ash. Due to the higher CO2 partial pressure under oxyfuel conditions the transformation of carbonates to the oxide typically 4 ACS Paragon Plus Environment

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occurs at higher temperatures, but (Wigley et al. [24]) state that “there is insufficient information in literature to predict the rate of calcite decomposition, even if the local CO2 partial pressure is known”. Konist et al. [25] investigated experimentally the influence of oxy fuel combustion of Ca-rich oil shale fuel on carbonate stability and ash composition. Their results indicated that the elevated CO2 levels delay the decomposition of CaCO3, as expected.

To further motivate our examinations Figure 1 shows the measurement of the spectral emittance of a Ca rich lignite ash as an example (Rhenish lignite, ash content = 5 %, CaO content of ash by mass = 48 %, measured by XRF analysis which only gives elemental composition and, therefore, Ca content is reported as CaO). Measurements have been carried out with the test rig explained in more detail in chapter 2. Obviously emittance is decreasing with sample temperature but also the spectral characteristics are changing which indicate phase transformation of minerals with increasing temperature. The characteristic spectral peaks present below 680 °C (see shaded areas) are vanishing at higher temperatures. In the further proceeding of this paper we will show that a similar behavior, e.g. vanishing characteristic spectral peaks (at approx. same wavelengths) and a decrease of emittance at higher temperature, is depicted by pure CaCO3. Based on these findings and the publications mentioned the question arises whether there are significant differences in emittance of carbonates compared to their corresponding oxides and whether this influences coal ash emittance. Therefore, three different carbonates (Sr, Mg, Ca) are heated up in a radiation test rig. These carbonates forming elements are typical substances occurring in most coal ashes with the following order of content (Ca > Mg > Sr). Phase transition to the oxide occurs at a certain temperature which is different for the three carbonates. Spectral normal emittance has been measured during sample heat-up. Additional measurements have been concentrated on Ca which typically forms the major amount of carbonate in ashes. CaCO3 has been mixed with SiO2 (another major constituent of ashes) in different mass ratios 5 ACS Paragon Plus Environment

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to address the impact of a chemical enrichment with CaCO3 on emittance. These results have been compared with measurements of a mixture of SiO2 with Fe2O3 because, as already mentioned, it is well known that the presence of Fe2O3 in ashes increases emittance. The SiO2/Fe2O3/CaCO3 has been sieved to two particle fractions of different size to address the influence of particle size.

Figure 1. Spectral emittance of the Rhenish lignite ash, size fraction x < 32 µm.

2. Experimental 2.1.

Radiation test rig

A schematic sketch of the radiation test rig is shown in Figure 2. The experimental facility consists of two main parts: a Fourier transform infrared (FT-IR) spectrometer and an electrical heating unit that includes, both, the sample holder system as well as the reference radiator, a black body. The black body, which also acts as the sample mount for the ash sample holder, is placed in an electrical oven with thermal insulation. At the top, thermal radiation is guided 6 ACS Paragon Plus Environment

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through a port (optical access) in the insulation to a FT-IR spectrometer via a gold coated offaxis parabolic mirror. For mineral measurements, samples are placed in the sample holder on top of the black body. The sample (22 mm diameter) temperature is measured by two thermocouples (type K, 0.25 mm diameter) which are installed 1 and 2 mm below the sample surface (in the central axis of the disk shaped sample). The sample surface temperature is then calculated by assuming a linear temperature profile across the sample height (the validity of this assumption has been checked by solving the 3D-heat conduction equation in the sample). For black body (i.e. reference) measurements, the sample holder is removed and the black body is lifted upwards until its opening is in the same position as was the mineral sample before. The reference radiator (black body) is made of a nickel-based alloy. It is laid out as a cylindrical cavity radiator (cavity length 135 mm) with an inner diameter of 50 mm and a conical bottom (emittance > 0.9985, for details see [26]). The black body temperature is determined by thermocouples. The port (optical access) has a diameter of 22 mm to match the sample geometry. To obtain comparable results the sample and black body heat radiation takes the identical optical path. Thermal radiation is detected by a FT-IR (PerkinElmer Frontier MIR/NIR) spectrometer at a resolution of 4 cm-1. The FT-IR is equipped with two deuterated triglycine sulfate DTGS detectors (wavelength range: 0.68 µm to 5 µm and 1.2 µm to 28.6 µm). In order to reduce the signal-to-noise ratio at least 50 scans have been recorded (max. standard deviation 0.35 %). Total statistical error (standard deviation) based on repeated measurement with same sample material at different days does not exceed ±3.2 %. All measurements are performed in air and under the same boundary conditions. Absorption bands of water vapor and CO2 have been extracted from the spectral emittances by interpolating emittances in the relevant wavelength ranges of the absorption bands. Directional measurements are performed perpendicular to the surface of the sample and as a function of the temperature in the range from 500 °C to 1000 °C. Therefore, the sample temperature as well as the black body temperature is increased by steps 7 ACS Paragon Plus Environment

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of 100 °C until a temperature of 1000 °C is reached. Note that 500 °C to 1000 °C are typical ash layer temperatures to be expected on the water walls of boiler furnaces. The heating procedure including hold times at each temperature is depicted in Figure 4. One measurement for a single sample takes 400 minutes.

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.

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

𝜀(𝜆, 𝑇) =

𝑆𝑠𝑎𝑚𝑝𝑙𝑒 (λ, T) 𝑆𝐵𝐵 (λ, T)

(1)

Additionally, the total emittance 𝜀̅(T) has been calculated by weighting the spectral emittance with the radiation intensity of a black body as described by Planck’s law (see Eq. (2)):

𝜆

𝜀̅(T) =

2 ∫𝜆 𝜀𝜆 (𝜆, 𝑇) ∙ 𝑆𝐵𝐵 (𝜆, 𝑇) 𝑑𝜆 1

𝜆2 ∫𝜆1 𝑆𝐵𝐵 (𝜆, 𝑇)

(2)

𝑑𝜆

3. Samples All samples are measured in powder form. In order to get similar samples considering their particle size distribution, each sample was ground to a powder and subsequently sieved into fractions. The size fractions for all samples are 0 to 32 µm, and 125 to 160 µm, respectively. An exception is SrCO3 which is only available for the sieve fraction of 0 to 32 µm.

Heat radiation measurements have been made for the pure substances, and mixtures of the powders. Mixtures of CaCO3 and SiO2 are prepared with a content of 10 %, 30 %, 50 % CaCO3, and mixtures of Fe2O3 and SiO2 with a content of 10 %, 30 %, 50 % Fe2O3 (mass fraction), respectively.

After sieving the pure minerals are inserted into the cylindrical hollow of the sample holder with a diameter of 22 mm and a height of 2 mm (see Figure 2). The sample surface was adjusted to be flush with the surface of the sample holder. 9 ACS Paragon Plus Environment

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The particle size distributions were measured by laser diffraction (Sympatec HELOS). Figure 3 shows the particle size distributions for all samples for the particle fractions 3A: x < 32 µm and 3B: 125 µm < x < 160 µm before and after radiation measurement. The particle size distributions of SrCO3 are depicted in Figure 3C.

The difference between the particle size distribution before and after the radiation measurement is small for most minerals. The particle size distributions of CaCO3 (125 < x < 160 µm) show a weak tendency to a bi-modal size distribution (second peak of small particles at around 30 µm) after heat treatment with an additional maximum for small particles. The reason for the second peak is not completely clear, however, it is known that CaCO3 is a rather dense material whether CaO is rather porous (release of CO2). Especially for larger particles these might lead to a disintegration of a certain number of particles. Note that the MgCO3 size fraction x < 32 µm shows a bimodal distribution before and after thermal treatment. Notably the particle size for SrCO3 is significantly smaller with a mean diameter in the order of 2 µm (larger particles were not commercially available). SrCO3 tends to form sticky agglomerates which explain the shift of the mean particles size to approx. 40 µm after heat treatment.


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Figure 3. Particle size distribution for particles sieved with a mesh size of A) 32 µm, B) 125 µm and 160 µm and C) 32 µm for SrCO3 before and after the measurements.

The carbonate decomposition has been measured in a thermogravimetric analyzer (TGA Leco 601) as depicted in Figure 4. 11 ACS Paragon Plus Environment

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Figure 4. TGA-measurements (x < 32 m) of MgCO3, CaCO3 and SrCO3. The phase transition is initiated at a temperature of 500 °C for MgCO3, 700 °C for CaCO3 and 900 °C for SrCO3. The temperature history is consistent with the heating procedure in the radiation test rig.

The same temperature history has been chosen for the TGA as well as for emittance measurements. MgCO3 is the most unstable mineral and starts to decompose at values around 500 °C and is completely decomposed at 800 °C. CaCO3 decomposition is initiated at around 700 °C, and finalized at 900°C, whereas SrCO3 is the most stable mineral with an onset of reaction at 900 °C and still not finalized at 1000 °C. Note that the equilibrium temperatures of decomposition calculated with FactSage [27] in air are: MgCO3 (360 °C), CaCO3 (775 °C), and SrCO3 (1020 °C), respectively. For a typical dry oxy-fuel atmosphere (30 % O2, 70 % CO2) these temperatures change to MgCO3 (385 °C), CaCO3 (833 °C), and SrCO3 (1120 °C), respectively.


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4. Results and Discussion 4.1.

Carbonates

Figure 5 to 7 present the spectral emittance of the size fraction smaller than 32 µm for the three carbonates (Ca, Mg, Sr) for different temperatures as an example. Only selected temperatures are shown, more data for other temperatures have been measured, but are not presented here. The general trend of the curves is similar and typical for most minerals. Emittance is small at low wavelengths and increases with wavelength to values in the order of 0.8 to 0.9 above 10 microns.

Figure 5. Spectral emittance of SrCO3, size fraction x < 32 µm. The phase transition takes place over a temperature range between 900 °C and 1000 °C (TGA measurement).


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Figure 6. Spectral emittance of MgCO3, size fraction x < 32 µm. The phase transition takes place over a temperature range between 500 °C and 800 °C (TGA measurement).

Figure 7. Spectral emittance of CaCO3, size fraction x < 32 µm. The phase transition takes place over a temperature range between 700 °C and 900 °C (TGA measurement). 14 ACS Paragon Plus Environment

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In all three figures the phase change from the carbonates to the oxides can be clearly observed. The carbonates show typical peaks in the wavelength range of 3-4 µm which vanish when the carbonate transforms to the oxide. These peaks are attributed to the infrared active CO3 group, which corresponds to results of Huang and Kerr [20] which have detected an adsorption band around 3.9 µm for CaCO3 at room temperature. Because the phase transition temperature is different for the three carbonates as mentioned above (see Figure 4), also the change of the emission characteristics occurs at different temperature (TTransition, MgCO3 < TTransition, CaCO3 < TTransition, SrCO3).

For SrCO3 (Figure 5), the spectral emittance increases with phase transformation for wavelengths below 3.5 µm. The increase for the low wavelength range is important for total radiative heat exchange because at the highest measurement temperature (963 °C) the maximum of Planck’s law is at 2.3 µm. Thus, a change in emittance in this wavelength range has comparatively high impact on the total radiative heat exchange or spectrally averaged emittances according to equation 2. Note that phase transition (and hence change in emittance) still goes on for the highest temperature (963 °C) with progressing time (see curves for 90 and 220 minutes after having reached 963 °C). Immediately when having reached 963 °C the carbonate peaks still can be detected but they are completely vanished after 220 minutes.

MgCO3 transforms to the oxide at lower temperatures (Figure 6). When having reached 768 C° phase transformation is already completed (compare also Figure 4). This is reflected in spectral emittance where the carbonate peaks are absent for 768 °C. Note that FactSage predicts start of phase transformation already at 360 °C (initial emittance measurement presented is at 578 °C). Figure 4 shows that the mass loss of MgCO3 is only 6 % at 500 °C and Figure 6 (578 °C line) still reflects the spectral characteristics of the carbonate. Therefore, we assume that the data at


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578 °C are still dominated by MgCO3. Measurements at < 400 °C are not possible with the current set-up because of increasing signal to noise ratio.

The differences in spectral emittances are most striking for CaCO3. A severe reduction of emittance occurs between 3 and 8 µm when the carbonate transforms to the oxide. After transformation again the characteristic peaks around 4 µm have vanished. For the highest temperature of 874 °C phase transformation is completed (see also Figure 4). Note that Huang and Kerr [20] have observed a second absorption band of the CO3 group at around 7 µm which in combination with the adsorption band around 4 m might lead to the strong decrease in the whole range from 3 to 8 µm for CaCO3. Also for SrCO3 a decrease of the emittance at around 7 µm during phase transition could be observed but to a lower extend (also for MgCO3 but there the reduction of emittance is very weak around 7 µm).

As a comparison Figure 8 shows the spectral emittance of Fe2O3 (< 32 µm particle size) for 482 °C and 956 °C. Fe2O3 shows no phase transformation which is reflected by the fact that the characteristics of the spectral emittance remains unchanged. The difference in emittance for the different temperatures for wavelengths above 8 µm is not very important for radiative heat transfer because most of the heat is transferred at lower wavelengths for the temperatures under consideration.


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Figure 9 depicts the spectrally averaged (total) emittances of Mg, Ca for the size fraction 125 < x < 160 µm and Figure 11 for Sr, Mg, Ca for the fraction x < 32 µm. The data for the highest temperature of approx. 1000 °C have been measured after a holding time of 30 minutes at this temperature. The holding times for the other temperatures are according to Figure 4. As a general trend spectrally averaged emittance decrease with temperature as for most minerals [4,5]. This is to a certain part due to the averaging procedure 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 emittances at low wavelengths. Hence, the low emittances of minerals at low wavelengths are shifting the averaged emittances to low values for higher temperatures.

Figure 8. Spectral emittance of Fe2O3, size fraction x < 32 µm. In Figure 9 it becomes clear that the size fraction 125 < x < 160 µm of CaCO3 and MgCO3 show similar values for total emittance (within the limits of measurement error). During phase transition (indicated by an arrow) a distinct reduction of the total emittance of the Ca-mineral can be seen. The CaO has a significantly lower total emittance than the corresponding 17 ACS Paragon Plus Environment

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carbonate. The influence of phase transition (arrow) on total emittance for MgCO3 is not that concise. After transition the total emittances for MgO is higher than for CaO.

Figure 9. Total emittances of CaCO3 and MgCO3, size fraction 125 < x < 160 µm. To give an idea of the influence of Planck-weighting on total emittance Figure 10 depicts the corresponding non-weighted values for the fraction 125 < x < 160 µm of CaCO3 and MgCO3 (compare Figure 9). For low temperatures (< 650 °C) weighted and non-weighted total emittances are rather similar. The higher the temperature the bigger the differences. The nonweighted total emittances are significantly higher than the weighted values. Thus, the selection of a non-weighted total emittance for a boiler CFD-simulation would lead to an overestimation of the heat-take up of the heat transfer surfaces when higher ash layer temperatures above 650 °C are present.


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Figure 10. Non-Planck weighted averages of the spectral emittances of CaCO3 and MgCO3, size fraction 125 < x < 160 µm. Figure 11 presents the results for the small size fraction x < 32 µm. Again the total emittance of MgCO3 and CaCO3 are on the same level before phase transformation. But the magnitude of the total emittance of the x < 32 µm sample (Ca and Mg (before and after calcination)) is significantly lower than for the 125 < x < 160 µm sample. This is expected because a lower particle size results in a lower emittance [11,12]. Again the Ca-mineral shows a drastic reduction in total emittance after phase transformation to CaO, which is much less pronounced for Mg. As before the total emittance of MgO is higher than of CaO. The total emittance for SrCO3 is the lowest for all three carbonates. It has to be mentioned that the particle sizes for Sr are significantly smaller compared to Ca and Mg. Small particles sizes typically go in line with a reduced emittance [11,12]. Therefore, it cannot be stated that the emittance of Sr carbonates is generally lower than those of Ca and Mg, because of the difference in particle size.


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Interestingly the total emittance of SrCO3 increases during phase transformation. This is due to the increase of spectral emittance for wavelengths below 3.5 µm (see Figure 5). As has been already mentioned SrCO3 tends to form larger sticky agglomerates (sintering) at higher temperatures (see the increased particle size in Figure 3C). Typically sintering (larger particles agglomerate) does increase emittance [13].

Figure 11. Total emittances of CaCO3, MgCO3 and SrCO3, size fraction x < 32 µm. As a first summary one can state that during the transformation from the carbonates to the oxides the carbonates are losing their characteristic peaks around 3.5 - 4 µm (and also to a lesser extend around 7 µm) which are caused by the infrared active CO3 group. Ca and Mg minerals show a clear tendency of reduce emittance with reduced particle size. The CaO emittance is lower than the emittance of CaCO3, the most important carbonate group in ash minerals.


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4.2.

CaCO3 and SiO2

The holding times at a certain temperatures for the mixtures are as for the pure carbonates presented before. Figure 12 presents results of spectrally averaged emittance for the CaCO3 enrichment for the size fraction 125 < x < 160 µm and Figure 13 for the fraction x < 32 µm, respectively. When comparing the two figures it becomes evident that for the pure minerals SiO2 as well as for CaCO3 the total emittance is increasing with particle diameter, which is an expected trend according to [7–10]. It is also obvious that the total emittance of CaCO3 is higher than the total emittance of SiO2. As in Figure 9 and 11 the drop in total emittance for both size fractions of CaCO3 occurs when the carbonate is transformed to the oxide (at approx. 750 to 800 °C). After transition the total emittance of SiO2 is higher than for CaO for the size fraction 125 < x < 160 µm and total emittances of SiO2 and CaO are rather similar for x < 32 µm.

Figure 12. Total emittances of CaCO3, SiO2 and CaCO3 enriched samples based on SiO2, size fraction 125 < x < 160 µm. The vertical line denotes the onset of the phase transition of CaCO3 (TGA-measurement).


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For the size fraction 125 < x < 160 µm (Figure 12) an enrichment of SiO2 with CaCO3 leads to an increase in total emittance before the transition to CaO occurs (T < 750 °C). The highest enrichment with 50 % CaCO3 leads to the highest total emittances, the trend for the 30 % and 10 % enrichment is not that clear. Note that the differences lie within the measurement error. When the CaCO3 transition sets in the opposite effects occurs. The SiO2 (higher total emittance than CaO) is now dominating the overall total emittance. An enrichment with up to 50 % CaO is reducing total emittances but the order of magnitude of the total emittance of the mixture remains at a level typical for pure SiO2.

Figure 13. Total emittances of CaCO3, SiO2 and CaCO3 enriched samples based on SiO2, size fraction x < 32 µm. The vertical line denotes the onset of the phase transition of CaCO3 (TGA-measurement). Also for the size fraction x < 32 µm (Figure 13) an enrichment of SiO2 with CaCO3 leads to an increase in total emittance before the transition to CaO (T < 750 to 800 °C). The enrichments of 10 to 50 % CaCO3 scatter in a similar range of total emittance values. Interestingly the mixtures of CaO with SiO2 all show higher total emittance values than the pure substances. The route cause for this behavior is not fully understood, but note that equilibrium calculations with 22 ACS Paragon Plus Environment

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FactSage [27] indicate the formation of Ca-silicates. XRD (PANalytical X’Pert Pro) measurements of the sample composition after the radiation measurement reveal a beginning of the phase transformation with small amounts of CaSiO3 and Ca2SiO4 (in the order of 1 %, note that a sample exposed to a temperature of 1000 °C for five days showed complete transformation to silicates). Also the small particle size accelerates sintering processes which is less pronounced for the larger size fraction of 125 < x < 160 μm. It is believed that the combination of increased sintering propensity and beginning phase transformation to the silicates might be responsible for the emittance effect described.

4.3.

Fe2O3 and SiO2

Figure 14 provides results for spectrally averaged emittance of the Fe2O3 enrichment for the size fraction 125 < x < 160 µm and Figure 15 for the fraction x < 32 µm, respectively.

Figure 14. Total emittances of Fe2O3, SiO2 and Fe2O3 enriched samples based on SiO2, size fraction 125 < x < 160 µm.


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The holding times at a certain temperatures are selected as before. When comparing the two figures it becomes evident that also for pure Fe2O3 (as for SiO2) the total emittance is increasing with particle diameter. It is also obvious that the total emittance of pure Fe2O3 is higher than the total emittance of SiO2, which reflects the findings of the literature indicating the relevance of Fe2O3 for emittance. Accordingly, an enrichment of SiO2 with F2O3 leads to a trend to increase in total emittance. The difference between pure SiO2 and Fe2O3 is more pronounced for the small size fraction with differences of up to 0.15 in total emittance (for approx. 900 °C).

Figure 15. Total emittances of Fe2O3, SiO2 and Fe2O3 enriched samples based on SiO2, size fraction x < 32 µm.

For both size fractions an enrichment of SiO2 with 10 % Fe2O3 has only a minor influence on total emittance (within measurement error). This holds true for the whole range of temperatures. For the size fraction x < 32 µm the trend is very clear, the higher the enrichment with Fe2O3 the higher the total emittance. The same behavior can be observed in principal for the large size fraction, with two exceptions. For 570 °C the total emittance of the 50 % enrichment is slightly 24 ACS Paragon Plus Environment

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higher than for the pure Fe2O3 and for 950 °C the total emittance of the 30 % enrichment is slightly higher than for the 50 % enrichment. But note that the difference of the measurement data between 30 and 50 % enrichment are within the measurement error of the system.

5. Conclusion The effect of carbonate decomposition on spectral and spectrally averaged emittance of Sr, Mg, Ca carbonates is examined. In addition “synthetic coal ashes” were produced from mixtures of CaCO3 and SiO2 as well as Fe2O3 and SiO2. The mixture ratios of the minerals were varied (100 % SiO2, 10 %, 20 % and 50 % enrichment (by mass) with CaCO3/Fe2O3 and 100 % CaCO3/Fe2O3). All samples were prepared from powders with known particle size distribution (size fractions x < 32 µm and 125 - 160 µm). Investigation of the temperature-dependent normal emittance was carried out in a radiation test rig by FT-IR spectroscopy in the temperature range from 500 °C to 1000 °C. The major findings can be summarized as follows: 

For a given chemical composition of a sample the total emittance increases with particle size.



Phase transformation from the carbonate to the corresponding oxide has a significant influence on spectral emittance. Whereas the carbonates show characteristic peaks in spectral emittance around 4 µm which stem from the infrared active CO3 group, these peaks completely vanish after transformation to the oxide.



For CaCO3, the most prominent carbonate in typical coal ashes, the total emittance of the oxide is significantly lower than for the carbonate.



The ash of a Ca rich Rhenish lignite did show similar behavior than CaCO3, e.g. characteristic emittance peaks typical for CaCO3 which vanish at elevated temperatures and a decreasing total emittance with temperature; a strong indication that the Ca is present as CaCO3 in the lignite ash. 25 ACS Paragon Plus Environment

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

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For MgCO3 the total emittance is not significantly influenced by phase transformation (starting around 500 °C) although the characteristic CO3 peaks are vanishing.



SrCO3 shows the highest temperature for phase transformation (around 900 °C). The mean particle size was significantly lower than for the other carbonates (3 µm compared to 20 µm). The total emittance did increase during phase transformation. The sintering tendency of the very fine powder forms larger agglomerates (proven by particle size analysis) which might explain higher total emittances.



The total emittance of SiO2 is lower than the total emittance of CaCO3 and Fe2O3.



An enrichment of SiO2 with CaCO3 leads to a trend to an increase of total emittance.



Interestingly for the size fractions x < 32 µm the mixtures of SiO2 with CaCO3 show a higher total emittance than the pure substances after the phase transformation of the carbonate. It is believed that onset of silicate formation in combination with sintering is the cause for this behavior.



An enrichment of SiO2 with Fe2O3 leads to an increase in emittance, the difference between pure SiO2 and an enrichment with 10 % FeO3 is negligible.



Finally, the measurements reveal the general difficulty of emittance measurements of minerals. The emittance is a result of complex interactions of chemical composition and particle size. These properties can change during heat treatment due to sintering effects but also due to the formation of new mineral phases (which is in addition influenced by the composition of the surrounding gas phase). Therefore, correlation just based on initial particle size distribution and/or on initial chemical elemental composition must be scrutinized critically concerning their range of applicability. The situation gets even more difficult when phase transformation from solid to liquid occurs which might be the case in boilers locally.


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Note that the current emittance measurements have been carried out under air and not in an oxy-fuel atmosphere. One has to keep in mind that carbonate decomposition temperature is a function of CO2 concentration in the gas phase. Carbonate decomposition temperatures will rise with increasing CO2 concentration (predictions of decomposition temperatures can be made with thermochemical equilibrium tools like FactSage [27]). This shift in phase transformation temperature has to be considered when deriving emittance from measurements in air for an oxyfuel firing system. Finally, the question arises what can be learned from synthetic ashes for boiler operation with natural coal ashes. All pure minerals as all real coal ashes are strongly non-grey. The emittance is low at low wavelengths (< 5 m) and increase to values around 0.8 to 0.9 for larger wavelengths. This is of crucial importance for a spectrally resolved simulation of radiative heat transfer in boilers. The more rough the surface structure (i.e. the larger the particle size) of an ash layer the higher the emittance, and in contrary very fine ash powders on a heat transfer surface can effectively reflect thermal radiation, an unwanted effect. The spectral characteristic of carbonates can be found in the coal ash and is not suppressed by other substances (see Figure 1). The emittance of the coal reduces when the carbonate transfers to the oxide. We believe that in the end only the knowledge of the spectral characteristics of the ash minerals (carbonates, sulfates, oxides, etc.) opens up a way for a deeper understanding of the coal ash emittance prediction based on composition. Therefore, a comparison of carbonates with sulfates, the other import mineral class under oxy-conditions will be examined as a next step.

ACKNOWLEDGMENT This work has been funded by the German Science Foundation (DFG) within the Sonderforschungsbereich/Transregio TR 129 “Development of methods and models to describe solid fuel reactions within an oxy-fuel atmosphere“. 27 ACS Paragon Plus Environment

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