Influence of Load Changes on the Deposit Behavior during

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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Influence of Load Changes on the Deposit Behavior during Combustion of Five Different Hard Coals Matthias Dohrn* and Michael Müller Institute of Energy and Climate Research (IEK-2), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany ABSTRACT: This work focuses on the influence of cyclic temperature profiles on the sintering behavior of hard coal ashes. To extract mechanisms, all ashes are fully characterized concerning chemical and mineralogical composition and thermal behavior including the characterization of the decomposition species and the ash fusion test with two different setups. Also, the characterization should ensure a broad variability of chemical and mineralogical compositions. To determine sintering effects, the sintering strength test was used in combination with mass and density change analysis. All samples were investigated at 12 different temperature profiles, whereas four include different cycling conditions, four use isothermal short-term conditions, and four use isothermal long-term conditions. In addition, the effect of the preparation load for this experiment was examined. Although all ashes contain similar amounts of the common hard coal ash main components like SiO2, Al2O3, and Fe2O3, they still differ a lot in alkaline and alkaline earth elements. Besides, every coal ash contains a prominent mineral, which could characterize the ash. The measurement results from the different temperature profiles indicate many influences of certain minerals and also an alkali threshold of nonreactivity was detected and quantified. To sum up, most cycled samples revealed results similar to those of the isothermal investigated samples at 950 °C. However, the possibility for lower and higher sintering due to cycled temperatures profiles was proofed and responsible minerals and chemical circumstances were identified.

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Due to an increasing need of flexibility concerning coal and load, some parameters like temperature will change cyclically inside the boiler. Viswanathan et al. summarized the main effects for all areas in a power plant caused by cyclic operation. Within that report, the authors mention reduced slagging at low load in general but increased slagging using specific coals.8 Furthermore, Hurley et al. analyzed the effect of cycling on deposit formation of a calcium-rich coal in a full-scale utility boiler. They also concluded a negligible effect of cycling itself.9 Nevertheless, deposit formation of coals with a broad range of compositions with special focus on cyclic operation has not been investigated yet. To experimentally determine the effects of cyclic operation on deposits, five different coals from the world market have been selected. They were chosen to cover a broad range of ash compositions combined with economic relevance. These coals are very well-known and the point of slagging will mainly be avoided. That is why this study focuses on fouling and sintering changes during cyclic operation. The determination of sintering strength is the most promising way to determine differences in ash deposits. Barnhart and Williams10 developed the compressive strength test to precisely detect sintering induced changes in pelletized coal ash. They and also several other authors pointed out some factors for sintering increase like temperature, alkali content,10−13 reducing atmosphere,14 sulfur content, and calcium sulfate.9,10,13,15 In contrase to this, a porosity increase due to the decomposition of carbonates and sulfates results in a decrease in compressive strength.16,17 Furthermore, crystal-

INTRODUCTION Deposit formation during pulverized combustion of hard coal in power plants has been under investigation for several decades. High temperature slagging and low temperature fouling can occur, and this can lead to an unpredicted boiler shutdown. To prevent those cases, many studies have been carried out. In brief, major and minor parameters have been worked out to examine the behavior of a certain coal. Major parameters are gas and particle temperature, ash and flue gas composition, and time, whereas minor parameters include particle size distribution, oxygen level, gas velocity, deposit heat flux, and several more.1 These parameters have been experimentally investigated for several coals in different smallscale and large-scale setups, and on the basis of that data, boilers have been designed. Main deposition mechanisms have been summarized by several authors.2,3 Fouling deposits consist mainly of sulfates and carbonates. Initially, the coals’ calcium content is crucial for the amount of sulfate in the deposits.4 Subsequently, after deposition of those calcium sulfates, sodium hydroxide can diffuse into the deposit and reacts with calcium sulfate to sodium sulfate and in the end to calcium sodium sulfate.5 This sintering leads to hardening of the deposit. Increasing stickiness and reactivity, considering the interaction with low reactive particles like quartz, result in the formation of silicates, which turns the deposit from fouling to strong-bonded slagging. Hence, it has been observed that the initial layer differs from the bulk composition.6,7 Fouling mainly occurs within temperatures below 900 °C, whereas slagging develops at higher temperatures. In between, particle sintering bridges the gap between those two deposit types. An increase in temperature also accelerates the deposit buildup, especially in consideration of hard coals.6 © XXXX American Chemical Society

Received: December 20, 2017 Revised: February 20, 2018

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DOI: 10.1021/acs.energyfuels.7b04020 Energy Fuels XXXX, XXX, XXX−XXX

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Table 1. Ultimate Analysis Including Clorine on Dry, Ash Free Basis with Values for Ash and Water (wt %) and ICP-OES Analysis of All Hard Coal Ashes (450 °C) Normalized and in Oxide Form (wt %)a hard coals

C

H

S

O

N

Cl

Ash

Water

Na2O

K2O

CaO

MgO

Fe2O3

Al2O3

SiO2

TiO2

SO3

P2O5

SKC SKK SKP SKR SKU

76.8 80.8 83.6 73.1 81.0

5.4 4.6 5.2 5.7 5.5

0.9 1.7 0.5 0.4 2.9

15.4 11.3 9.5 18.4 8.9

1.4 1.7 1.1 2.4 1.5

0.0 0.0 0.1 0.1 0.3

6.7 19.2 9.1 18.2 8.4

0.3 2.4 2.0 1.3 2.4

0.9 0.1 0.9 1.2 0.8

1.6 0.6 2.2 3.0 2.3

2.1 7.1 5.9 3.5 3.7

1.0 1.3 3.6 1.9 0.9

10.9 11.5 7.6 4.9 15.4

20.3 22.5 24.0 20.4 19.0

57.5 46.0 45.5 60.2 49.7

0.9 1.3 1.0 0.9 1.0

4.7 8.1 8.6 3.7 7.1

0.2 1.5 0.7 0.5 0.1

a

The sulfur oxide content has been calculated from the ultimate analysis.

Figure 1. Schematic of the experimental setup of the furnace with four tubes.

Figure 2. Temperature profiles inside the furnace with four tubes. B


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Figure 3. XRD analysis of all hard coal ashes.

lization also has to be considered.6,14,18,19 On the one hand, crystallization can for example lead to decreasing sintering strength in case of gehlenite, and on the other hand, anorthite has an increasing effect on sintering strength. Barnhart and Williams also observed a good correlation with density changes before and after sample annealing.

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spectrometry. In all devices the samples were heated in air with 5 K/min from room temperature to 1200 °C. For differential scanning calorimetry a NETZSCH DSC 404 C was used with a platinum sample holder. The thermogravimetric measurement was performed with a NETZSCH STA 449 F3 in a corundum sample holder. Finally, the mass spectrometry data were collected with NETZSCH STA 409 CD with a skimmer coupling system using a quadrupole analyzer. For this measurement a platinum plate was used as the sample holder. Additionally, the ash fusion test was used to determine first melting and a pellet stability range. Those cylindrical pellets were prepared with 5 mm in diameter and 1 kN pressure. They were heated with 5 K/min from room temperature to 1430 °C in air. An additional measurement was performed with 50 °C steps from 800 to 1400 °C and at each step with 60 min dwell time. Hence, kinetically slow shape changes were emphasized. Afterward, ash pellets were prepared with 8 mm in diameter and with a pressure of 8 and 12 kN for 1 min. They were weighted, and the size was measured before and after the experiment to determine weight change and density change. The pellets were placed in corundum boats inside a furnace with four tubes (Figure 1) and were annealed with different numbers of temperature cycles ranging from 600 to 950 °C (Figure 2, top) but with the same integrated temperature. Furthermore, to avoid overestimating effects due to different times at the peak temperature, additional measurements with constant temperature profiles at 600, 700, 850, and 950 °C were performed for 16 and 72 h each (Figure 2, center and bottom). Every experiment used artificial flue gas with 79 vol % N2, 14% CO2, 6% O2, and 1% SO2. Added to this, 4 vol % steam was carried with the aforementioned nitrogen, adjusted by selecting a temperature in the cooling cycle corresponding to the vapor pressure of water. Although sulfur dioxide typically is lower inside a boiler, this experiment used higher concentrations to accelerate conversion processes. For satisfying statistics11 every sample was prepared three times.

EXPERIMENTAL SECTION

Samples from Colombia-Calentur (SKC), South Africa-Kleinkopje (SKK), Poland (SKP), Russia (SKR), and the US (SKU) were collected. They were finely ground and subsequently analyzed with ultimate analysis and parts of the proximate analysis as shown in Table 1. In addition, inductively coupled plasma with optical emission spectroscopy (ICP-OES) was used to determine inorganic species inside the 450 °C ash. Samples (50 mg) of of each ash were mixed with 250 mg of lithium borate, and the solution was annealed at 1000 °C for 30 min in a Pt/Au-crucible. Subsequently, the slag was dissolved with 30 mL of a 5% solution of hydrochloric acid and the solution was diluted to a volume of 50 mL. Then, all samples were again 2-fold diluted 1:10 and analyzed. The results are presented as normalized and as oxides in Table 1. For all experiments fine ground laboratory ashes were prepared at 450, 815, and 1000 °C in air for 72 h to ensure complete conversion of organic matter and of all mineral phases. For X-ray diffraction (XRD) the Bruker device D4 Endeavor was used with Cu Kα anode (40 kV, 40 mA). Mineral composition of the laboratory ashes and the raw coals were identified with Bragg−Brentano geometry at 0.02° steps with 10−15 s per step. To figure out crystallization, recrystallization, and decomposition events of every 450 °C laboratory ash, they were analyzed via differential scanning calorimetry, thermogravimetry, and mass C


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Figure 4. Differential scanning calorimetry, thermogravimetry, and mass spectrometry of all hard coal ashes. The colored bars in the background indicate temperature ranges for the release of H2O (blue), CO2 (black), and SO2 (yellow). Next, the annealed samples were placed into a hydraulic press with a digitalized pressure transducer. This press runs with 0.5 mm/min for 2 min with a base load of 100 N. The resulting load graph was used to determine the peak load for each sample.

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three main oxides of all ashes are SiO2, Al2O3, and Fe2O3 in that order. For this reason, the dominant presence of silicates measured by XRD is appropriate. Especially, quartz, phyllosilicates and hematite were found in the 450 °C ashes. In addition, sulfates and barely carbonates were detected. Inorganic components in coal are ion-exchangeable cations, coordination complexes, and discrete minerals.21 Using XRD, the latter can be identified in raw coals, although it is difficult because of broad scattering peaks resulting from amorphous structures. Accordingly, as shown in Figure 3 only main

RESULTS AND DISCUSSION

Coal Analysis. Using the classification scheme,20 all ashes except SKP are aluminosiliceous ashes, but SKP is a silicoaluminous ash. Typically, hard coal ashes mainly consist of the ternary system SiO2−Al2O3−Fe2O3.1 Consequently, the D


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blue bars). Typically, the kaolinite dehydroxylation combined with the following dehydroxylation of the three-layer clays illite, muscovite, and montmorillonite would have occurred next (Figure 4, second blue bars). Due to the preliminary combustion for 72 h at 450 °C, all kaolinites have already reacted to metakaolinite. For this reason, the amount of threelayer clay inside the ashes could easily be detected by comparing the amount of weight loss between 400 and 500 °C. These peaks are usually undetectable, because of its 10 times lower intensity compared with kaolinite dehydroxylation.28 As expected, the Russian coal ash dropped weight most, followed by the US, the Polish, and the Colombian ash (Figure 4, red lines). Besides, the South African coal ash contained nearly no three-layer clay and that is the reason why it did not lose any weight within this temperature range. All DSC profiles revealed a very small but sharp endothermic peak at 573 °C resulting from the quartz inversion (Figure 4, black lines). Next, mass spectrometer data indicated calcination ranging from 600 to 900 °C (Figure 4, black bars). The amounts were quite low, which is why there were nearly no peaks in the DSC profiles. In addition, the US ash contained a second calcination event. This did not mean that there was another carbonate species inside this ash, but a temperature dependent calcination suppressive mechanism. Moreover, it could be approved via similar decomposition temperatures that preliminary dolomite in the Polish ash has transformed to calcite. With a further increase in temperature, more complex conversions were observed. The desulfurization events were ranging from 800 to 1200 °C (Figure 4, golden bars). The Polish and the Colombian coal ash started to desulfurize at 800 °C. Many authors have mentioned a start of decomposition of anhydrite in combination with metakaolinite, quartz, hematite, or aluminosilicates above 900 °C.9,33 The evaluation of the thermogravimetric and the mass spectrometer results indicated that first decomposition of anhydrite can occur at 800 °C. Besides, at temperatures above 950 °C all clays recrystallized exothermically to new X-ray amorphous phases (Figure 4, black lines). The peak intensity depends on the clay type. Especially, high contents of metakaolinite like in the South African ash resulted in a massive exothermic peak around 980 °C. That is in good agreement with the literature.27,28,31 Small differences in the clay structure and composition were responsible for different DSC profiles in this temperature area. With reference to Grim et al.28,34 combined endothermic and exothermic peaks can occur within one clay type at varying temperatures. That is why DSC profiles cannot sufficiently distinguish different clay minerals. The determination of illite by XRD could still be correct for every coal ash although the profiles differed in shape. The determination of ash melting was conducted with the ash fusion test based on DIN 51730.35 Five phases could be observed for nearly every coal ash (Figure 5). The first phase is stable in shape and was called “unaffected by heat treatment” by Pang et al.,36 but due to several ongoing reactions below this temperature, the term “stable in shape” is more suitable. Next, a long phase of linear shrinkage was followed by a short temperature range with progressive shrinkage. Then, due to the decomposition of sulfates the samples started to grow massively, because the gas inside of closed pores had the endeavor to leave the sample, but it was blocked by silicate structures. Subsequently, with increasing melting, the sample stability collapsed, leading to a degressive line plot shape. The South African coal ash was the most stable coal ash, resulting from low alkali content. The stable in shape phase

minerals like quartz and the two-layer clay (1:1) kaolinite could be detected in every hard coal. In addition to this, three-layer clays (2:1) like illite/muscovite or montmorillonite are also found in every coal. The distinction of those is barely possible, because of their flexible compositions and similar structure. Furthermore, the Colombian and the US coal also revealed the sulfide pyrite, a characteristic mineral responsible for high iron and sulfur contents. Parts of that sulfide seem to have partially oxidized to gypsum in the US coal. Carbonates like calcite could be detected in the Russian and South African coal, whereas the latter also contains siderite. The highest diffraction peaks for carbonates were found in the Polish coal. Many conversion processes cause the resulting mineral composition of the 450 °C laboratory ashes. The formation mechanisms including intermediate steps of hematite and anhydrite out of pyrite are explained in eqs 1−7:22−24 FeS2 + 3O2 → FeSO4 + SO2

(1)

4FeSO4 → 2Fe2O3 + 4SO2 + O2

(2)

4FeS2 + 11O2 → 2Fe2O3 + 8SO2

(3)

FeS2 → FeS + S

(4)

4FeS + 7O2 → 2Fe2O3 + 4SO2

(5)

2CaO + 2SO2 + O2 → 2CaSO4

(6)

CaO + SO3 → CaSO4

(7)

Dolomite is also a good contributor for the formation of anhydrite:25,26 CaMg(CO3)2 → [CaCO3 + MgO] + CO2

(8)

[CaCO3 + MgO] + SO2 + 0.5O2 → [CaSO4 + MgO] + 4CO2

(9)

Moreover, kaolinite did dehydroxylate to metakaolinite which is X-ray amorphous.27 This reaction is normally observed at temperatures in the range 550−600 °C,28 but apparently this reaction starts below 450 °C and its rate is high enough to completely convert the mineral within 72 h. Similarly, the 815 °C ashes no longer contained three-layer clays, because they recrystallized earlier than expected27 to mullite. An explanation might be given by Jasmund et al., who reported an enhanced crystallization of primary mullite, combining illite with kaolinite.29 At 815 °C the reaction between calcium and glassy aluminosilicate resulted in the formation of anorthite.6 The preliminary phases for this formation were calcite and metakaolinite with a probable intermediate crystallization of gehlenite (Ca2Al(AlSi)O7).30 Likewise, clays could have spent the glassy aluminosilicate for that reaction. On top of that, metakaolinite is well-known to recrystallize at 970 °C31 into amorphous silica and Al−Si-spinel.32 Most of the reactions mentioned above can be determined by differential scanning calorimetry (DSC) in combination with thermogravimetric (TG) measurements. On top of that, a mass spectrometer (MS) can distinguish the species, which are responsible for a weight change. Figure 4 contains this information for all hard coal ashes. Those ashes behaved similarly concerning most reactions. Every ash contained some adhesive water, which is responsible for the first DSC, TG, and MS peaks ranging from room temperature to 150 °C and causes a weight loss of 2−2.5 wt % (Figure 4, blue lines and first E


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temperature period. The measurements with an additional dwell time every 50 K followed the results under standard conditions in general. It is obvious that in most cases the height increase was lower, because of longer degassing during previous dwell time periods. Altogether, these measurements allow the detection of critical temperature ranges. In addition, the formation of definite eutectics, which result in melting events was proofed. It seems that the contact between the particles and their interaction is possible from the very beginning. In total, the kinetic measurement partly allows the distinction between melting and decomposition reactions. Constant and Flexible Load Experiments. The differences between cyclic temperature changes and constant temperatures were evaluated by comparing the results of weight change, density change, and sintering strength of all measurements. All measurement results are plotted in Figures 6−8. First, general trends are discussed, after that, minor details are reviewed, and last, differences are extracted. The results of the weight change are plotted in Figure 6. On the one hand, increasing weight occurs due to adsorption and absorption of gaseous species into the sample, and on the other hand, decreasing weight arises from decomposition reactions. Overall, the working pressure for preparing the samples did not have an effect on those exchange reactions. It seems that inward and outward diffusion was high enough for both sample pressures. Concerning the gas phases and the temperature, weight increase can only occur because of carbonation and sulfurization. Thermodynamic equilibrium calculations with FactSage as well as experimental observations have revealed that in the presence of SO2 an exchange reaction replaces all carbonate phases. Hence, samples with high amounts of reaction partners for SO2 will increase more in weight than samples with low amounts. After 16 h annealing at 600 °C, the Colombian ash increased in weight by 10 wt %. It is followed with even intervals to a value of 2.5 wt % weight change by the Polish, Russian, US, and South African ash. The experiment with 72 h annealing time revealed a decrease in weight change for the Russian, the US., and especially the Colombian coal ash, whereas the Polish and

Figure 5. Results from the ash fusion test based on DIN 51730 (black dashed lines) and measurements with additional dwell time every 50 K. Decreasing values correspond with a height decrease. Vertical line parts are height movements during constant temperature phases. Red dashed vertical lines are DIN temperature points.

reached temperatures above 1000 °C, whereas the Polish, the Russian, and the US coal ash only approached values close to 900 °C. The rich in quartz sample from Colombia was in between at 950 °C. The occurrence of a melting phase is possible at temperatures below 900 °C with the combination of aluminosilicates with potassium species.37 On the contrary, the first big melting event corresponded with the start of the progressive melting phase at temperatures around 1200 °C concerning all ashes. Here, a calculation with FactSage (v. 7.1) revealed the eutectics SiO2−Al2O3−CaO and SiO2−Fe2O3− CaO with melting phases starting at 1191 and 1124 °C, respectively. Next, gaseous species were formed because of the decomposition of sulfates. It led to an increase in height. Such a shape increase is based on aluminosilicate structures, which with reference to the network theory38 can form resilient networks to prevent the gas from getting released out of closed pores. That is why ashes like the Colombian ash with higher quartz contents are able to grow massively in height for a long

Figure 6. Weight change during the experiments with standard error of mean. Left side: experiments with 8 kN molding pressure. Right side: experiments with 12 kN molding pressure. All experiments are plotted with standard error of mean (n = 3). F


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Figure 7. Density change during the experiments with standard error of mean.

aluminosilicate. A possible weight increase at 600 °C for the cycled samples did not mainly influence the overall weight change, but at least most of the cycled measurement values are slightly higher than the corresponding values of the constant measurements for 16 h. In conclusion, additional sulfates produced at 600 °C cannot be decomposed at 950 °C, if the same reaction rate for all samples is assumed. Simultaneously, density measurements of all samples were conducted. Here, Figure 7 indicates a deviation between the two different preparation loads. This was because of the difference in pore volumes. High preparation loads result in reduced pore volumes and vice versa. Thus, the amount for a quasi-liquid filling via viscous flow is higher at lower preparation loads. A sharp density increase is often due to massive sintering, occurring at a distinct temperature range. The weight change measurements did not indicate a sharp reaction, but a linear change in weight per temperature. However, the density measurements marked a sintering step between 850 and 950 °C. The height of the density step corresponded well with the amount of alkalis and iron. That is why the US-American coal ash showed the highest density increase close to 50%. In addition, this coal ash is the only one that already increased in density below 850 °C. It is likely to accept the assumption that the reaction of the US-American coal ash occurred slightly below 850 °C and that the reactions of the other ashes have taken place slightly above 850 °C, because of missing standalone reaction partners, which could explain this deviation. In contrast, the South African coal did not sinter at all. The lack of alkalis led to a poor reactivity. This was already stated by Attig et al., who stated a nonreactivity below 0.5 wt % alkalis.12 That value directly depends on the normalization and the amount of evaluated elements. In this case, a slightly higher value of alkalis definitely above 1 wt % can be stated. These results could be verified by comparing the data from the ash fusion test. Here, the South African ash did not shrink below 1000 °C and thus did not sinter. As already stated with the weight change measurements, all samples except for the South African ash seem to react irreversible at 950 °C. For this reason, the cycled samples had similar deviation in accordance with the isothermal measurements at 950 °C. The only prominent effect could be detected for the Russian coal ash which implied higher

South African ashes increased in weight change. Suitable reaction partners for SO2 are alkaline earth metals and alkalis. With reference to the chemical composition, the Colombian ash has the lowest amount of suitable reaction partners, although the experimental results indicate the opposite. Thus, additional molecules had to react, to reach such a high weight increase. XRD verified that in addition to calcium sulfates some iron-alkali-sulfates have formed. Their stability was temporary, because with increasing time their amount was reduced. Here, chemical equilibrium was competing with kinetics. The formation of iron-alkali-sulfates seems to be kinetically favored compared with the stability of hematite, which always decreased as iron-alkali-sulfates increased. With an increase in annealing time the reaction could proceed to the thermodynamically more favored hematite. Those ashes that weight change increased with increasing annealing time, like the ashes from Poland and South Africa, contained the highest amounts of alkaline earth metals. Therefore, the conversion to sulfates was not completed after 16 h. At 700 °C the overall positive amount of weight change of all ashes decreased. This was also related to less formation of alkali-iron-sulfates. The weight change at 850 °C was within the range of ±2%. At this temperature, sulfate formation and carbonate decomposition combined with silicate formation were nearly equal in amount. Furthermore, the South African ash was the only one that still increased in weight with increasing annealing time. This is due to the poor sintering reactivity at that temperature, which was caused by the low alkali content. The measurements at 950 °C revealed a weight loss with reference to decomposition of primary sulfates. That is why the measurement results of the cycled samples are also located in this area. The first decomposition was followed by a process of sintering. This blocked reaction partners of aluminosilicates like calcium, which could not be sulfated in the following low temperature phase. Furthermore, no significant difference or trends between the number of cyclings could be observed. Obviously, no huge differences between measurements at constant and cyclic temperatures occurred concerning weight changes. It seemed that the major effect was affected by the peak temperature time and the reaction of calcium oxide and G


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Figure 8. Compressive strength after the experiments with standard error of mean.

porous sample, which lowered the overall sintering strength. Conversely, the Russian ash increased in sintering strength. Here, low alkaline earth contents and in addition low iron contents together with high sheet silicate and metakaolinite contents led to sample strengthening. Missing iron and alkaline earth elements were responsible for a direct sulfation at 600 °C of alkalines, which originate from the sheet silicates. Subsequently at 950 °C, the alkali sulfates decomposed and the alkalis were liberated and could react with aluminosilicates of former metakaolinite.

density values in every cycled temperature profile in comparison to the isothermal measurements. The density change is minor influenced by the change of temperature. Similar to the weight change results, the values of the cyclic experiments correspond to the constant measurements at 950 °C. Otherwise and in contrast to the weight changes results, the measurement results of the cycled samples were not always slightly lower than those results at 950 °C. This means that different reactions did take place, which leads to the assumption of composition dependent reaction paths. They are further discussed in the results of the sintering strength. The measurements of the sintering strength are plotted in Figure 8. Here, the preparation loads led to an inverse effect in contrast with the density measurements. This was due to particle−particle reactions, which could take place in a sample with higher density more easily. As stated by Hurley et al.9 and other authors,10,13,15 an increase in sintering strength can be on the basis of sulfation. That could be proofed at 600 °C, where higher values in comparison with those measurements at 700 °C could be detected. At 850 °C first sintering could be measured more efficiently than with density measurements. All reactive ashes increased in sintering strength slightly at 850 °C and markedly at 950 °C and also in all cycled experiments. Above all, the Russian coal ash indicated higher values after cycling. This time, the effect could be measured for both preparation loads and supports the before mentioned assumption of an improved reactivity under cycling conditions. All cycled reactive samples passed through sulfation at low temperature and network forming at high temperature. Hence, network forming can be defined as the dominant reaction. The Colombian and US-American ash did not vary a lot from the isothermal measurements, but the Russian and Polish ash did. Therefore, the Russian ash must have a mechanism which is able to improve the reactivity at low temperatures that activates reaction partners for the following high temperature phase. Similarly, the Polish ash needs to have an improved reaction at lower temperatures with the result of a lower strength increase. The latter passed through a massive sulfation at low temperatures, because of its high alkaline earth contents. At higher temperatures these sulfates decomposed and left a highly

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CONCLUSION Five hard coal samples have been completely characterized in chemical and mineralogical composition, ash reactivity, and decomposition behavior. Then, all samples were measured with 12 different temperature profiles to examine the effect of varying temperatures on the sample sintering. The following conclusions could be drawn: • As typical for hard coal ashes, the ash compositions did not differ a lot. The main components are SiO2, Al2O3, and Fe2O3. Differences could be detected for the alkaline, alkaline earth, and iron contents. • All coal ashes revealed some decomposition reactions with different amounts starting from adhering water release to hydroxide release from illites, decarbonation, and at high temperatures desulfation. Moreover, partly strong DSC peaks could be observed close to 980 °C in accordance with the metakaolinite recrystallization to amorphous phases. • The results of the ash fusion test were divided into discrete phases. It started with a stable in shape phase, followed by a linear and progressive shrinkage. Next, induced by silica a swelling occurred, and last, a degressive melting phase could be observed. Samples with high quartz content like the Colombian ash faced a long swelling period and samples without sufficient alkalis did not shrink below 1000 °C. • XRD results indicated unique quantities of minerals for each coal ash. The Colombian ash contained the highest amount of quartz, whereas the Polish ash consists of H


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many carbonates. The South African ash incorporated high amounts of the refractory mineral kaolinite and the Russian ash was rich in sheet silicates. The highest amount of sulfur species was observed in the USAmerican ash. Concerning load flexible operation different general and specific observations could be obtained: 1. In general, the dominant mechanism during cyclic temperature changes is the irreversible network formation at high temperature. That is the reason why all cycled samples showed results close to the isothermal measurements at 950 °C. 2. The preparation load does not affect the weight change during the experiment. Nevertheless, density change decreases with increasing load and sintering strength increases with increasing load. 3. The alkali amount has to pass a threshold of at least 1 wt % in the ash, to sinter significantly below 1000 °C. These observations can be verified by using the ash fusion test. 4. High iron and alkali contents result in high sintering strength. 5. High available alkaline earth contents result in a decrease of sintering strength in cycling experiments due to sulfation at 600 °C and decomposition at 950 °C with residual pore structures. 6. Cyclic temperature profiles can lead to an increase of sintering strength under distinct chemical and mineralogical conditions. To achieve high sintering strength values, lower amounts of alkaline earth elements are necessary, because otherwise a sulfation and beforementioned decomposition and pore buildup would occur. Next, high amounts of alkali rich minerals with low alkali bonding like sheet silicates are needed. They are sulfated at 600 °C in the absence of iron to alkali sulfates and not to alkali iron sulfates. Then, at high temperatures the decomposition and thereafter reaction with aluminosilicates from former metakaolinite can take place. At least the formation of low temperature eutectics due to the alkalis induces a faster reaction rate and thus a higher sintering.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*M. Dohrn. Telephone: +49-2461-61-4504. Fax: +49-2461-613699. E-mail: [email protected]. ORCID

Matthias Dohrn: 0000-0003-0082-4395 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors thank Mirko Ziegner for performing the XRD measurements and the ZEA-3 (Dr. Volker Nischwitz, Dr. Sabine Willbold) for the elemental and element analysis. The work described in this paper has been performed in the framework of the VerSi-EM Project supported by Bundesministerium für Wirtschaft und Energie (FKZ 03ET7062B). I


Article

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