Thermodynamic Prediction of the Effect of Excess Air Ratios and

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Thermodynamic prediction of the effect of excess air ratios and combustion temperatures on the equilibrium phases formed during ash fusion from coal and cedar nut shells Wojciech Jerzak Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01563 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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Thermodynamic Prediction of the Effect of Excess Air Ratios and Combustion Temperatures on the Equilibrium Phases formed during Ash Fusion from Coal and Cedar Nut Shells Wojciech Jerzak* AGH University of Science and Technology, Department of Heat Engineering and Environment Protection, 30 Mickiewicza Av, 30-059 Krakow, Poland. Corresponding Author * E-mail: [email protected]

Abstract An analysis of equilibrium phase formation in the ashes from Polish coal and cedar nut shells (CNS) was performed for different combustion conditions in a bubbling fluidized bed boiler. Equilibrium calculations were performed using Factsage 6.3 for five excess air ratio values (λ): 0.7; 0.9; 1.0; 1.2; 1.6, at a temperature range of 650 ≤ T ≤ 1050 °C. The masses and compositions of ashes synergistically depend on λ and T. An increased combustion temperature results in a

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decrease in the mass of: (i) ashes formed from coal and CNS, (ii) liquid slag (in the case of CNS) – resulting from a lower content of K2CO3(l) (the lower the λ, the greater the decrease in K2CO3(l)), and the migration of potassium into the exhaust gas in the form of compounds: KCl(g); KOH(g) and K2SO4(g). Ashes from both coal and CNS contain compounds present: (i) throughout the whole temperature range, regardless of λ value, e.g. KAlSi2O6(s), Mg2Al4Si5O18(s) – in the case of coal; e.g. Mg2SiO4(s), MgO(s) – in the case of CNS, (ii) only in some temperature ranges, e.g. Al2SiO5(s) (T < 780 °C), Al6Si2O13(s) (T > 780 °C) – in the case of coal; e.g. Ca5(PO4)3(OH)(s) (T < 900 °C), Ca3(PO4)2(s) (T < 900 °C) – in the case of CNS, (iii) depending on λ, e.g. FeS(s), Fe2Al4Si5O18(s)(λ ≤ 1.0), Fe2TiO5(s), CaSO4(s)(λ > 1.0) – in the case of coal; e.g. FeO(s) (λ < 1.0), Mg2Fe2O4(s) (λ ≥ 1.0) – in the case of CNS.

Keywords: Thermodynamic modeling, Combustion, Coal, Ash Fusion, Excess Air Ratio, Bubbling Fluidized Bed

1. INTRODUCTION The decline in fossil fuel reserves and the growing awareness of greenhouse gas emissions have made the use of biomass for heat and power production very popular in recent years. However, the melting temperature of ash formed during the combustion of biomass with a high content of alkali metals is significantly lower than that of coal.1,2 The formation of a liquid phase consisting of low melting silicates and alkali metal sulfates is one of the factors initiating an agglomeration in the fluidized bed and ash deposition on the heat exchange surfaces (slag problem). Effective methods preventing this agglomeration in the fluidized bed include: the


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mixing of problematic biomass fuels with coal and the application of various types of mineral additives that bind alkali metals.3–6 However, the co-combustion of coal and biomass may increase the tendency for ash slagging.7,8 The mineral composition of deposit ashes resulting from combustion of coal and biomass depends on: the value of the excess combustion air ratio (λ), 9-14 combustion temperature (T), 15-20 and the time spent by fuel particles in furnace zones at different temperatures (τ) .21-23 These factors (λ, T, τ) synergistically affect the changes in the composition of resulting ashes. The small number of published experimental results compiled in Table 1 and the large variation in fuels make it very difficult to determine any trend in the changes in ashes composition related to the values λ and T. An increased content of Al6SiO13(s) and Al2Si2O7(s) mullites when λ changed from 0.7 to 1.0 has been noted based on an examination of volatile ash deposited from “Zhundong coal” combustion.10 An effect of λ on the growth and/or decrease in some elements in volatile ashes from biomass is not evident, despite the fact that the combustion of switchgrass and hardwood has been performed in the same positions.12 One of the reasons for this situation is the temperature in the combustion chamber. In turn, Vamvuka et al.14 reported significant changes in the content of potassium, silicon, phosphorus, magnesium and calcium oxides present in volatile ashes from combustion with λ = 1.4 and λ = 1.7 residues from orange plantations. The consequence of a coal combustion temperature increase from about 575 to 800 °C is a decrease in sodium content in the volatile ash.15,16 The results of the experimental studies carried out by Xiang et al.15 and presented in Table 1 were obtained using two research methods (wet chemical analysis and X-ray fluorescence). An increase in wood biomass combustion temperature from 600 °C to 1300 °C enables the elimination of unburned carbon from the ash in the form of CaCO3(s), and K2Ca(CO3)2(s), and a reduction of potassium content in ash of up to


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63 % depending on the type of wood.19 The decrease in potassium content in the ash is related to the process of its evaporation, which starts at 800 – 900 °C.19 An increase in the temperature of corn straw combustion from 550 to 900 °C results in a marked decrease in SiO2(s) content (from 35 % to ca. 7 %) in the ash.18 The following tendency has been observed by evaluating the effect of coal residence time in the combustion chamber on the composition of ash from different types of coals: the longer the residence time, the more Al and Si, and less Fe in the ash.21–23 Table 1. Effect of Increasing: the Air Excess Ratio, Temperature, and Residence Time on Changes in Fly Ash Compositions Changes in ash composition Parameter

Fuel

Ref. Decrease

λ low → high 0.7 → 1.0 1.3 → 1.35 1.1 → 1.3 1.1 → 1.3 1.2 → 2.3 1.2 → 2.3 1.4 → 1.7 T (oC) 550 → 800 575 → 815 400 → 1200 550 → 800 550 → 900 600 → 1300 700 → 1000 τ (min) 10 → 2 50 → 20 65 → 180

Increase

coal coal coal switchgrass hardwood herbage-grass miscanthus residues

Al, Fe, K, Mg, Na, Ti Si Fe, Na, Si Al Na, K K Si Al, Ca, Fe, Mg, Na Ca, Fe, Mg, P, Si K Al, Si, Na Ca, Fe, K, Mg Ca, P, Mg, Si K

9 10 11 12 12 13 13 14

coal coal cotton stalks pine corn straw wood corn straw

Fe, Mg, Na, P, Si Na K Fe, Al Si K, Na

Ca Al, Ca, Si

15 16 17 15 18 19 20

coal coal coal

Ca, Fe, K Fe Ca, Fe, K, Mg

Al, Mg, Na, Si Al, Si Al, Si

21 22 23

Fe, Al Ca, P


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The combustion temperature determines not only the ash mineral content but also its mass. 19,24,25 Heating ash from wood biomass from 600 °C to 1300 °C, the weight loss can range between 22.9 – 47.8 % depending on the type of wood.19 In the case of narrower ranges of combustion temperatures (e.g., 400 → 815 °C and 600 → 900 °C), the decrease in ash content was 7.67 → 4.98 % ash (cotton stalk)24 and 4.8% → 3.75% ash (Eucalyptus globulus bark).25 Heating the ashes from coal, lucerne, wheat stalks and wood allows the determination of the intensity of its melting.3,26,27 The beginning of ash sample melting can be related to the Initial Deformation Temperature (IDT). The temperature of the ash deformation depends on the composition of the fuel and combustion temperature. When the combustion temperature T ≥ 600 °C, then the IDT for coal is in the range of 1145 – 1619 °C,2,5,26 while for the biomass it is 631 – 1270 °C.2,7,18,24 In the case of wheat stem combustion in the temperature range of 700 – 860 °C, the liquid phase share is lower than 5 wt% of the total ash. Above 860 °C, there is a rapid increase in the liquid phase and at 900 °C it accounts for as much as 30 wt% of the total ash.27 Rizvi et al.28 proved that it is possible to lower the mass of liquid slag with an increasing temperature. The drop in mass of liquid slag with a temperature increase was noted for: peanut shells (700 → 1300 °C), sunflower (900 → 1100 °C), and miscanthus (1000 → 1500 °C). 2. MODELING STRATEGY 2.1. Fluidized Bed Combustion Conditions The differentiated values of the ratio of excess air and temperature assumed for the calculations reflect the actual combustion conditions in the Bubbling Fluidized Bed (BFB) presented in the literature.29-36A partial combustion of coal and cedar nutshells in fluidized bed was proposed, which is consistent with the proposal of Moradian et al.29, who assumed that the combustion in primary bed relates to 60 wt% of fuel, while the remaining 40 wt% is burned over


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the fluidized bed. To justify the reasons for adopting such different combustion conditions, the BFB was divided into three zones with varying ratios of excess air, as shown in Figure 1.

Fig. 1. Bubbling fluidized bed combustion conditions assumed for calculations Additionally, a temperature range was assigned to each zone. Zone 1 (sub-stoichiometric) is located in a fluidized bed. In Fig. 1 the chute of the bed material is marked as a factor causing lowering of the temperature at the wall of the FBC boiler. Bed material is not included in the calculation. However, bed material can react with alkali from ashes and create new low melting compounds that are directly responsible for bed material agglomerations.2,4 Fuel with a temperature near the ambient temperature is introduced by the wall, due to the use of water cooling. As a result, the fuel and bed material discharge area is characterized by the lowest fluid bed temperature and the largest combustion air deficit (λ = 0.7). The lowest bed temperature in BFB recommended in the literature is 650 °C.30,31 The following fluidized bed conditions were assumed for the calculations: 650 ≤ T ≤ 900°C, 0.7 ≤ λ ≤ 0.9.35 The unburned fuel particles from zone 1 are lifted over the bed and passed to zone 2 (stoichiometric) and


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zone 3 (over-stoichiometric). Secondary combustion air is injected in zone 3; thus, the highest excess of combustion air is observed by the wall (λ = 1.6). Fluidizing air typically accounts for 50 – 70% of the total combustion air (primary air ratio λ = 0.6 – 0.8).31–34 The rest of the combustion air is supplied above the fluidized bed as secondary air (sometimes divided into secondary and tertiary). If there is a distribution of combustion air above the fluidized bed to secondary (SA) and tertiary (TA), their percentages of total combustion air are similar, e.g. SA/TA = 25/2532 or SA/TA = 17/16.34 Temperatures exceeding as high as 1000 °C can be noted in BFB; therefore, the maximum temperature for calculations was assumed to be 1050 °C.32,36 The purpose of BFB division into three zones is to show the differences in the compositions of ash formed in each of the zones. 2.2. Fuels Polish coal and cedar nut shells (hereinafter referred to as CNS) were used in this study. The annual harvest of cedar nuts from Siberia is estimated to be 10 – 12 million tonnes, while the shell accounts for 51 – 59 wt% of the peanut.37 CNS is the waste produced in cedar oil pressing plants. CNS is a fuel recommended by manufacturers of retort boilers and burners (single or modular) with a heat output of 34 – 300 kWh.38,39 Table 2 compares the masses of the elements contained in 1 kg of dry fuel (coal and CNS) calculated based on ultimate and proximate analyses of fuels, and elementary analyses of ashes, which can be found in previous studies.40,41 Coal contains more sulfur and chlorine than CNS. The confrontation of the constituent ash components of coal: Si > Al > Fe > K, and of CNS: K > Mg > Si > Ca, gives an image of their evident differentiation.


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According to Table 3, 19.66 wt% of ash is generated from 1 kg of dry coal, while only 0.85 % from CNS. In addition, CNS contains about 2.5-fold more moisture and volatile components compared to coal. Table 2. Elemental Composition of 1 kg dry Coal and CNS Used in Calculations Sample

Coal

CNS

C

646.91

540.02

H

41.99

62.69

N

8.89

2.81

S

8.79

0.3

Cl

1.10

0.06

Oa

190.73

388.71

Si

45.29

0.73

Al

29.45

0.19

Fe

10.19

0.19

K

7.07

2.35

Ca

4.03

0.47

Ti

1.96

0.005

Mg

1.6

1.05

Mn

1.25

0.10

Na

0.6

0.05

P

0.15

0.27

Elemental analysis (g/kg)

a

Oxygen is calculated as a percentage difference.

Table 3. Properties of Fuels Used in this Study Fuel

Coal

Gross calorific value (MJ/kg d.b.) n. a.a

CNS 20.51

Proximate analysis (wt. %) Moisture

6.2

15.2

Ash

19.66

0.85


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Volatile matter a

29.34

75.5

n. a. – not analyzed

2.3. Equilibrium Tool, Databases, and Input Data Thermodynamic calculations have been performed with the FactSage™ software package version 6.3.42 Factsage™ is one of the largest fully integrated and commonly used software packages for predicting multiphase equilibria, and the proportions of the liquid and solid phases in a specified combustion atmosphere for a multicomponent system. For this study, the “Equilib” module was employed along with databases of pure substances (FactPS; FToxid; FTmisc; FTsalt; FTOxCN, FThall) and solution species listed in Table 4. Table 4. Databases and Solution Phases Used in Calculations Database

Phase (full name)

Liquid Solutions FToxid

DSlag-liq

FTsalt

FSalt-liquid

Solid Solutions FToxid

AMonoxide

FTsalt

FTsalt-CSOB

FTsalt

FTsalt-KCO

FTsalt

FTsalt-KSO

Databases of solutions for calculations were selected based on the results of our own experiments40,41 and on literature data5,13-15,19,22,24,25 confirming the presence of Ca, K and Mg carbonates in volatile and bottom ash. Among the available databases of liquid slag solutions, only DSlag-liq base contains carbonates (carbonates cannot constitute more than 40 wt% of a solution) in addition to oxides. Other databases available in the FactSage™ package allow for


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calculations for other groups of compounds such as oxides and sulfides – ASlag-liq, oxides and sulfates – BSlag-liq, etc. The author would like to point out that the selection of the DSlag-liq database was preceded by preliminary calculations, which show that the use of the DSlag-liq database slightly reduces potassium release compared to ASlag-liq. The use of the DSlag-liq database aims to reproduce the presence of unburnt carbon in a fluidized bed, that can react with volatile potassium from a newly added CNS lot. The possibility of the presence of sulfate and carbonate solutions in ashes was taken into account when selecting FTsalt-liquid, FSalt-CSOB, FTsalt-KCO, and FTsalt-KSO. The number of possible solid solutions was reduced to the minimum after the verification calculations were performed. It turned out that solutions from databases [FToxid-oPyr] and [FToxid-WOLL] were not formed, while solution [FToxid-OlivA] contained about 99 wt % Mg2SiO4. A reduction in solid solutions has been introduced, since the number of species that can be formed in the coal or biomass burning process (over the entire temperature range) is significantly higher than the maximum number of reagents in the Equilib module of FactSage ™ program of 1500. Therefore, for each of the fuels, preliminary calculations for temperatures of 650, 850 and 1050 °C were performed to eliminate species i.e. gases, pure solids, pure liquids, that are not formed in the temperature range under test. The species with the lowest activity values were rejected i.e. a 960 °C. The total ash masses (mtotal) decrease continuously as the combustion temperature increases for the examined λ, as shown in Figure 3(c). The highest decrease in mtotal, i.e. 2.23 g, was recorded for λ = 1.0 (650 → 1050 °C).


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Fig. 3. Effect of excess air ratio and temperature on mass of: (a) liquid slag; (b) solid ash; (c) total, from CNS


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The total weight of the ash formed (at each examined temperature) is higher for λ ≥ 1.0, compared to λ < 1.0. For λ ≥ 1.0, mtotal are close to each other at 650 °C, while there are distinct differences at 1050 °C. In summary, an increase in the combustion temperature decreases mtotal, which is confirmed in the literature reports.19,24,25 3.2. Transformation of Equilibrium Phases formed during Ash Fusion from Coal The shares of particular ash components formed at a given T and λ were expressed as weight percentages (wt%), and the results of calculations are presented for: liquid slags (Figure 4); solid ash (Figure 5); and in relation to the total weight of ash (Figure 6). As mentioned in section 3.1, in the case of coal combustion, the formation of liquid slag begins at T = 1050 °C. Fig. 4 shows the composition of the liquid slag for λ = 1.6, which is invariable for the analyzed λ. Liquid SiO2(l) (80 wt%) outweighs the other ash components, i.e. Al2O3(l), K2O(l), Na2O(l), TiO2(l), and CaO(l).

Fig. 4. Composition of liquid slag from coal combustion at 1050 °C


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Fig. 5(a)–(e) only includes the components in the solid phase with a content of min. 1.5 wt%, in order to ensure the transparency of the diagrams. Turning to the interpretation of the composition of ashes from coal shown in Fig. 5(a)–(e), it was observed that iron-containing compounds such as Fe-cordierite (Fe2Al4Si5O18(s)), iron(II)–sulfide (FeS(s)), and ilmenite (FeTiO3(s)) were detected only in ashes for λ ≤ 1.0 and additionally ulvite (Fe2TiO4(s)) for λ = 1.0. A change in λ (0.7 → 1.0) results in a reduction in wt% FeS(s) in ashes with increasing temperature (Fig. 5(a)–(c), Fig. 6(i)). This promotes the formation of Fe2Al4Si5O18(s) at lower temperatures (Fig. 5(a)–(c), Fig. 6(b)). The formation of FeS(s) and FeS2(s) in a fluidized bed during low-NOx combustion has been confirmed in other publications.5,11,22,26 The weight percentage of Fe2Al4Si5O18(s) in the ash is subject to an increase, and after reaching the maximum it remains constant up to 1040 °C. An increase in wt% of Fe2Al4Si5O18(s) at 1050 °C is a consequence of the decrease in solid ash mass and the beginning of slag occurrence in the liquid state. According to Fig. 5(a)–(e) and 6(a), the content of andalusite (Al2SiO5(s)) in the ash is practically invariable throughout its range for λ = 0.7, 1.2 and 1.6, or remains constant to 700 °C, and then decreases more intensely for λ = 1.0 than λ = 0.9. It can be seen when comparing the results of Fig. 5(a)–(e) that mullite (Al6Si2O13(s)) is formed instead of Al2SiO5(s). Mullite is characterized by a lower S/A index, as defined by equation (1), S⁄A = SiO ⁄Al O

(1)

and consequently, the SiO2 peak is observed in Fig. 6(c) at 780 °C (except for λ = 1.0). In the case of λ = 0.7, 1.2 and 1.6, these are the peaks of SiO2(s) content in the ash. From 800 – 1040 °C, the Al6Si2O13(s) weight share in ash is almost constant for λ ≥ 1.0.


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Fig. 5. Effect of temperature on composition changes in the solid ashes from coal for: (a) λ = 0.7, (b) λ = 0.9, (c) λ = 1.0, (d) λ = 1.2, and (e) λ = 1.6


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Stable ash constituents exhibiting slight changes in weight proportions for the examined λ are: leucite (KAlSi2O6(s)), anorthite (CaAl2Si2O8(s)), Mg-cordierite (Mg2Al4Si5O18(s)), Mn-cordierite (Mn2Al4Si5O18(s)), and albite (NaAlSi3O8(s)), as illustrated in Fig. 6(d)–(h). Ash components observed only for λ > 1.0 include: hematite (Fe2O3(s)); pseudobrookite (Fe2TiO5(s)), titanium dioxide (TiO2(s)), and calcium sulfate (CaSO4(s)). When λ ≤ 1.0, CaAl2Si2O8(s) is present in ash over the entire temperature range, but for λ > 1.0, it is only formed above 780 °C, as shown in Fig. 6(e). Under oxidizing conditions (λ > 1.0), the weight percentages of iron and titanium compounds are constant at temperature ranges of 650 ≤ T ≤ 810 °C: 7.5% Fe2O3(s); 1.7% TiO2(s) and 820 ≤ T ≤ 1040 °C: 4.26% Fe2O3(s) and 5.17% Fe2TiO5(s) (Fig. 5(d)–(e)). When λ > 1.0, calcium in ash is present in the form of two compounds – CaSO4(s) (T < 790 °C) and CaAl2Si2O8(s) (T > 790 °C), while for λ ≤ 1.0 it is only present as CaAl2Si2O8(s) over the whole temperature range, according to Fig. 5(a)–(e). Comparing Figs. 6(a) and 6(e), a decrease in Al6Si2O13(s) was noted above 900 °C with a concurrent CaAl2Si2O8(s) increase for λ = 0.7. Those compounds that allow sulfur retention in coal ash are: FeS(s) (λ ≤ 1.0) and CaSO4(s) (λ > 1.0) at the temperatures lower than 800 °C, since above 800 °C all sulfur is transferred to the exhaust gas.


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Fig. 6. Effect of λ on changes in weight shares of coal ash components (a-i)


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3.3. Transformation of Equilibrium Phases formed during Ash Fusion from Cedar Nut Shells The composition of liquid slag formed at λ = 0.7; 0.9; 1.0; 1.2; and 1.6 in the examined temperature range is shown in Fig. 7(a)–(e). Liquid slag in the form of a solution consisting mainly of K2O(l), K2CO3(l), SiO2(l) and Al2O3(l) is already formed at 650 °C, although the melting points of pure K2CO3(s), SiO2(s) (quartz) and Al2O3(s) (corundum) are: 901, 1423, and 2054 °C, respectively.18,44 The higher the temperature, the less K2CO3(l) there is in the liquid slag.

Fig. 7. Effect of temperature on composition changes in the liquid slag from cedar nut shells for: (a) λ = 0.7, (b) λ = 0.9, (c) λ = 1.0, (d) λ = 1.4, and (e) λ = 1.6


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Fig. 8(a)–(e) shows how the composition of the solid ash depends on λ. The basic difference is the absence of K2SO4(s) in the ashes for sub-stoichiometric conditions, which is consistent with literature experimental ash studies, e.g. on miscanthus.43 It can be concluded from equilibrium calculations that sulfur passes into the exhaust gas mainly in the form of H2S(g) (to a lesser extent in the form of COS(g)). The highest amount of H2S(g) in the exhaust gas (about 97 g) is formed at T = 650 °C (comparable values for λ = 0.7 and λ = 0.9). Fig. 8(a)–(e) shows that iron is present in ash in the form of iron(II) oxide (FeO(s)), magnesium ferrite (MgFe2O4(s)) and hematite (Fe2O3(s)), and the sum (FeO(s) wt% + MgFe2O4(s) wt% + Fe2O3(s) wt%) in solid ashes is similar for all examined λ. FeO(s) is the only significant component of solid ash (> 1.5 wt%) present only under sub-stoichiometric conditions. Over the entire temperature range, FeO(s) content is approximately 6 wt%. Although Fe2O3(s) is present in ash under sub-stoichiometric conditions, due to its low content ( 1.0). According to Fig. 8(a)–(e), phosphorus is present in the following compounds: Mg3(PO4)2 – over the entire temperature range, hydroxyapatite Ca5(PO4)3(OH) (s), when T < 900 °C, and tricalcium phosphate (Ca3(PO4)2(s)) crystalline phase – whitlockite, when T > 900 °C.


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Fig. 8. Effect of temperature on composition changes in solid ashes from cedar nut shells for: (a) λ = 0.7, (b) λ = 0.9, (c) λ = 1.0, (d) λ = 1.2, and (e) λ = 1.6


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An effect of λ and T on the changes in liquid slag constituents relative to the total ash mass is shown in Fig. 9(a)–(e). The apparent similarity of curves reflecting the changes in K2O(l) and SiO2(l) content can be seen in Fig. 9(a) and 9(c). For each λ, the curve representing the weight percentage change of K2O(l) and SiO2(l) has a local maximum. The observed increase in K2O(l) and SiO2(l) content above 1000 °C (λ > 1.0) results from the largest ash weight loss, as shown in Fig. 3(c). The temperature increase from 650 to 1050 °C is accompanied by a K2CO3(l) decrease in the ash (Fig. 9(b)) and an increase in Al2O3(l) and Na2O(l), according to Fig. 9(d) and 9(e). In solid ashes, sulfur is present only in saline solution consisting of: Na2SO4–K2SO4–Na2CO3– K2CO3. Over the entire temperature range the solution is dominated by K2SO4(s), the level of which is about 99 wt%, Na2SO4(s) level is about 0.7 wt% and carbonates are less than 0.3 wt%. K2SO4(s) in ash increases when 650 < T < 900 °C, and above 900 °C it drastically decreases according to Fig. 9(f). This can be explained by the increased process of volatilization of potassium contained in fuel19, rather than decomposition of K2SO4(s), which is possible above 1100 °C and increases with SiO2(s) content in the ash.46 Under sub-stoichiometric conditions, the following regularity can be observed: the increase in MgO(s) content in ash is accompanied by a decrease in the content of forsterite (Mg2SiO4(s)). An increase in combustion air excess (0.7 → 1.6) leads to an increase in the minimum Mg2SiO4(s) content in ashes which occurs at lower temperatures - as indicated by the marker in Fig. 9(g). Maximum contents of Ca5(PO4)3(OH)(s); Ca3(PO4)2(s), MgO(s), Mg3(PO4)2(s) and MnO(s) in ashes are noted for λ < 1.0 (Fig. 9(h)–(i), 9(k)– (l)). Phosphorus has a high affinity for calcium,29 forming stable compounds: Ca5(PO4)3(OH)(s); Ca3(PO4)2(s) in the ash.


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Fig. 9. Changes in the weight shares of components of cedar nut shell ash present in the liquid (a–e) and solid phase (f–l) depending on excess air


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The marked decrease in Mg3(PO4)2(s) content shown in Fig. 9(k) and observed at 890 °C (λ = 1.6), 900 °C (λ = 0.7) and 910 °C (λ = 0.9, 1.0, 1.2) is correlated with the temperatures of the beginning of Ca3(PO4)2(s) formation. Fe2O3(s) content in the ash (Fig. 9(j)) significantly increases with temperature up to 790 °C (λ > 1.0) and to 770 °C (λ = 1.0). Increasing λ (1.0 → 1.6) slows down an increase in MnO(s) content in the ash in the range of 650 ≤ T ≤ 940 °C, according to Fig. 9(l). 3.4. Mass Balance of Elements volatilised from Ashes The influence of the equilibrium combustion temperature and excess air ratios on the changes of weight percentages of sulfur in flue gas and ash has been shown in panels (a-e) – for coal, and (fj) for CNS of Figure 10. Since the sulfur distribution in combustion products is highly variable, sulfur balance drawings are added to the work. As shown in Figs. 10 (a)-(e), distribution of sulfur in coal combustion products is largely dependent on temperature and λ. Under substoichiometric conditions, the decrease in λ is accompanied by the expansion of the temperature range in which FeS(s) occurs. Most sulfur (61.5 wt %) is in ash at T = 650°C and λ = 0.7. In turn, as much as 50 % wt of S stays in ashes in form of FeS(s), if 690 ≤ T ≤ 770°C (λ = 0.7), T ≤ 690°C (λ = 0.9) and T = 650°C (λ = 1.0). Mass of FeS(s) in ash is controlled by the sulfur evaporation to flue gases. The dominant species in the flue gases is H2S(g) over the entire temperature range for λ < 1.0. Under stoichiometric conditions, there is more H2S(g) than SO2(g) at temperatures up to 940 °C and above 940 °C on opposite. COS(g), S2(g) and HS(g) are formed in the flue gases only for λ ≤ 1.0. Under over-stoichiometric conditions, sulfur is located in: CaSO4(s), SO2(g) and SO3(g) below 790 °C, while from 790 °C it is fully released to the gas phase mainly in the form of SO2(g) and to a lesser extent SO3(g).


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Fig. 10. Weight percentages of sulfur in flue gases and ashes from coal in panels (a-e), and – from CNS, in panels (f-j).


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To summarize the results shown in Figures 6 and 10, it can be concluded that sulfur in combustion products plays an important role in the formation of Fe and Ca aluminum silicates. Turning to the sulfur balance in the CNS combustion products shown in Figs. 10 (f)-(j) it was noted that sulfur under sub-stoichiometric conditions is a fully released gas phase consisting mainly of H2S(g) and relatively low shares of COS(g), SO2(g) and HS(g). In turn, the total sulfur retention in ash is observed for λ ≥ 1.0 below 850 °C according to Figs. 10 (h)-(j). Only when burning CNS with λ ≥ 1.0 at temperatures above 850 °C (that is above the fluidized bed) K2SO4(g) is formed in the flue gases. Interestingly, there are far more K2SO4(g) than SO2(g) in the flue gas under over-stoichiometric conditions. The equilibrium balance of coal combustion products and CNS has allowed the determination of the influence of temperature and λ on the retention of elements in ashes. Except sulfur, all elements are retained in coal ash when λ ≥ 1.0 and 650 ≤ T ≤ 1050 °C. Under reducing conditions above 850 °C, the phosphorus volatilised as shown in Fig. 11(a) and at 980 °C all P is in the gas phase. As shown in Fig. 11 (b) and (c) the phosphorus volatilise is promoted by λ increase. Even for sodium and potassium, there was a slight level of volatilisation (not more than 0.5 wt % at 1050 °C). Going to balance the elements while burning the CNS, only reduced potassium and sodium content in ash with increased temperatures were noted. Potassium retention in ashes is lower in λ > 1.0 compared to λ < 1.0 as seen in Figs. 11 (d)-(e), and (g)-(h). Most of the potassium is volatilized under stoichiometric conditions (as much as 62 %) according to Fig. 11(f). Raising the excess air ratio from 0.7 → 1.2 according to Figs. 11 (i)-(m) at T = 1050 °C translates to increase sodium in ash from 88 wt % to 92.7 wt %, which is in line with the results of the study.12


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Fig. 11. Influence of temperature and λ on the mass fraction (a-c) of phosphorus, (d-h) potassium, and (i-m) sodium, in gaseous, liquid and solid combustion products 4. CONCLUSION The analysis of the constituents of ashes from coal and CNS combustion for different combustion conditions (λ, T) prevailing in BFB was presented in this study. The following conclusions were made based on the results of equilibrium calculations:


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(1) An increase in combustion temperature from 650 to 1050 °C results in a reduction in the total ash mass (both from coal and CNS) and the mass of the liquid slag in CNS ash. (2) Compounds such as: Al2SiO5; Al6Si2O13; FeS; Fe2TiO4; Fe2TiO5; Fe2Al4Si5O18; CaSO4; TiO2, and Ca5(PO4)3(OH); Ca3(PO4)2; Mn2O3; Mn3O4; and MgFe2O4, constitute the constituents of coal and CNS ash, respectively, the formation of which is determined by the combustion temperature, i.e. none of these components are present throughout the whole examined temperature range. (3) The compounds in the solid phase formed only in the conditions of λ < 1.0; λ = 1.0 and λ > 1.0 can be isolated both in coal and CNS ash. In coal ash: λ ≤ 1.0 – FeS; FeTiO3; Fe2Al4Si5O18; λ > 1.0 – Fe2O3; Fe2TiO5; TiO2. In CNS ash: λ < 1.0 – FeO, and λ ≥ 1.0 – MgFe2O4; K2SO4; and Mn2O3. (4) The compounds that allow for the retention of sulfur in coal ash are: FeS (λ ≤ 1.0) and CaSO4 (λ > 1.0), and in CNS it is K2SO4 (λ ≥ 1.0). (5) The ratio of the excess combustion air of coal and CNS affects the temperature at which the formation of compounds such as Ca3(PO4)2, Al6Si2O13, Fe2Al4Si5O18, and CaAl2Si2O8 begins in the ashes. (6) Potassium retention in ashes from CNS is lower in λ > 1.0 compared to λ < 1.0.

ACKNOWLEDGEMENTS This work was financially supported by the AGH University of Science and Technology (D.S. no. 11.11.110.423). The author also thank Mr. Jacek Pasierb and Mr. Piotr Mondkiewicz, Department of Heat


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Engineering and Environment Protection – AGH University of Science and Technology, for the technical assistance in the experiments. AUTHOR INFORMATION Notes The author declares no competing financial interest. REFERENCES (1) Reinmöller, M.; Klinger, M.; Schreiner, M.; Gutte, H. Biomass co- firing. Relationship between ash fusion temperatures of ashes from hard coal, brown coal, and biomass and mineral phases under different atmospheres: A combined FactSage™ computational and network theoretical approach. Fuel 2015, 151, 118−123. (2) Hein, K. R. G.; Heinzel, T.; Kicherer, A.; Spliethoff, H. Deposit formation during the Cocombustion of coal-biomass blends. In Applications of Advanced Technology to Ash-Related Problems in Boilers; Baxter, L., DeSollar, R. Eds.; Plenum Press: New York, 1996; pp 353−366. (3) Nordin, A.; Öhman, M.; Skrifvars, B. J.; Hupa, M. Agglomeration and defluidization in FBC of biomass fuels - mechanisms and measures for prevention. In Applications of Advanced Technology to Ash-Related Problems in Boilers; Baxter, L., DeSollar, R. Eds.; Plenum Press: New York, 1996; pp 353−366. (4) Öhman, M., Nordin, A. The Role of Kaolin in Prevention of Bed Agglomeration during Fluidized Bed Combustion of Biomass Fuels. Energy Fuels 2000, 14, 618−624.


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Technology and Sustainable Development; Yue, G., Li, S., Eds.; Springer: Singapore, 2016; pp 603−607.


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Thermodynamic Prediction of the Effect of Excess Air Ratios and

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