The occurrence of calcium and magnesium in the ash from Zhundong

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The occurrence of calcium and magnesium in the ash from Zhundong coal combustion: Emphasis on their close juxtaposition Bin Fan, Dunxi Yu, Xianpeng Zeng, Fangqi Liu, Jianqun Wu, Lian Zhang, and Minghou Xu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04520 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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Energy & Fuels

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The occurrence of calcium and magnesium in the

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ash from Zhundong coal combustion: Emphasis on

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their close juxtaposition

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Bin Fana,b, Dunxi Yua,⁎, Xianpeng Zenga, Fangqi Liua, Jianqun Wua, Lian Zhangc, Minghou

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Xua,⁎

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a State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology,

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Wuhan 430074, China

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b School of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen

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333403, China

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c Department of Chemical Engineering, Monash University, Wellington Road, Clayton, Victoria

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3800, Australia

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Key words: coal combustion; Zhundong coal; ash formation; occurrence; juxtaposition of

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calcium and magnesium

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ACS Paragon Plus Environment

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Abstract: In addition to sodium (Na), calcium (Ca) and magnesium (Mg) are also abundant in

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Zhundong coals and play important roles in ash deposition. This work investigated the occurrence

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of Ca and Mg in the ash from combustion of a low-rank Zhundong coal. An unreported close

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juxtaposition of Ca and Mg in ash particles was disclosed and emphasized. The modes of

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occurrence of Ca and Mg in the coal were thoroughly characterized by chemical fractionation,

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computer-controlled scanning electron microscopy (CCSEM) and X-ray powder diffraction

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(XRD). Coal combustion was conducted in simulated air and at 1350 °C on a drop-tube furnace.

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The generated ash was carefully analyzed by XRD and CCSEM. The results showed that more

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than 55% of the Ca and Mg were present as exchangeable cations in the Zhundong coal. The

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remainder mostly occurred as calcite and silicates. The Ca and Mg in the combustion ash were

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dominantly contained in glass phases, suggesting their extensive interactions with aluminates and

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silicates. Major crystalline Ca- and Mg-containing phases, including Calcite, Lime, Periclase,

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Anhydrite, Portlandite, and Yeelimite, were detected by XRD. CCSEM results showed they were

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present as discrete ash particles and/or as combined with other inorganics. Besides, a close

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juxtaposition of Ca and Mg in ash particles was discovered. This was demonstrated by the

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significant production of a Ca-Mg-rich particle phase with the two metals being dominant

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constituents (>80%). It contained about 21% of the total Ca and 27% of the total Mg. The

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formation of this phase was accounted for by a new mechanism involving interactions between

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exchangeable Ca and Mg through particle coalescence, agglomeration and sintering. It was found

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that the Ca-Mg-rich particles were mostly less than 10 μm and the compositions of the fine

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particles were more heterogeneous. The significance of these findings was discussed.

36


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Energy & Fuels

1. Introduction The large-scale utilization of Zhundong coals in power plants is significantly restricted by severe

38

1-8.

39

slagging and fouling problems

These issues have been notoriously attributed to the high

40

contents of alkali and alkaline earth metals (AAEMs), especially Na, Ca and Mg 1, 7, 9-11. In the last

41

decades, most of the attention has been paid to the behavior of Na and its contribution to ash

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deposition during Zhundong coal combustion

43

troublesome species such as Ca and Mg. Different from the Na that is very volatile and whose

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partitioning is largely determined by the subsequent scavenging of the vaporized phases, the Ca

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and Mg are less volatile and tend to primarily remain in ash particles

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occur in the ash and how they are in juxtaposition (or association) 26, 28 with other mineral species

47

are the most critical information required in predicting their contribution to ash deposition.

10, 12-25.

However, little work is available on other

26, 27.

Therefore, how they

48

The occurrence of the Ca in coal ash has been extensively characterized 8, 20, 29-31. Nevertheless,

49

the information on the Mg is little available. The Ca in the ash can be present in various forms 30,

50

31.

51

include aluminosilicates, oxides, hydroxides, carbonates, sulphides and/or sulphates 31. However,

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due to mineral interactions prevailing during coal combustion, the specific occurrence of the Ca

53

can be much more complex. This is also the case for the Mg and other ash-forming species. It is

54

well recognized that, compared with high rank coals, low rank coals (e.g. Zhundong coals) are

55

generally richer in Ca and Mg 26, 30. What is of particular importance is that the Ca and Mg in the

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low rank coals are largely present as exchangeable cations. These species are highly dispersed

57

throughout coal matrix and their combustion intermediates are very reactive. Therefore, the

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exchangeable Ca and Mg are more prone to interact with other ash-forming species, compared

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with their mineral forms. As a result, the complexity of the occurrence of the Ca and Mg and their

Depending on coal properties and combustion conditions, the Ca-bearing phases in the ash may


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juxtaposition with other inorganics is expected to be greatly increased. This poses a big challenge

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for combustion scientists aiming to acquire such information.

62

Conventionally, the X-ray powder diffraction (XRD) technique has been used to determine the

63

mineralogy of the coal ash 31. The occurrence of the Ca and Mg in the ash can be inferred to some

64

extent. However, the important information on their juxtaposition with other mineral species in

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individual ash particles cannot be obtained. Different from the XRD, a bulk analysis technique,

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the advanced computer-controlled scanning electron microscopy (CCSEM) is capable of

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characterizing mineral species on a particle-by-particle basis

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size, composition, abundance of various mineral particles, but also the complex juxtaposition of a

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specific element with other inorganics within single particles 26, 28. Considering that ash deposition

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in boilers occurs primarily in the form of particles, the CCSEM data are of more relevance to

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practical conditions32,

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instrumentation in detailed characterization of ash-forming species 26, 28, 32-34.

34.

32, 33.

It can not only determine the

Therefore, the CCSEM has been recognized as a very useful

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The present work aims to investigate, primarily with both the XRD and CCSEM techniques, the

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occurrence of the Ca and Mg in the ash from combustion of a low rank Zhundong coal. Our recent

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work

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which were less associated with aluminosilicates. Special emphasis in this work was put on the

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finding of a close juxtaposition of the Ca and Mg themselves in the ash particles, which has not

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been reported before. Mechanisms for this phenomenon were also proposed and verified. The

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knowledge obtained is believed to be very helpful in understanding both the transformation and

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deposition of the Ca and Mg during Zhundong coal combustion.

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2. Experimental

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with the same coal identified considerable amounts of Ca-Mg-rich particles in the ash,


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Energy & Fuels

2.1 Fuel properties The Zhundong coal sample tested in this work has been used in previous studies

7, 35.

Its

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properties are presented in Table 1. The Zhundong coal is a sub-bituminous coal and characterized

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by high contents of moisture and volatile matter but a low content of ash (3.56%). The low-

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temperature ash (LTA) of the coal sample was prepared in a low temperature asher (EMS1050X,

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ProSciTech). Its oxide composition was determined by X-ray fluorescence spectroscopy (XRF,

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EAGLE III, EDAX Inc.). The normalized ash composition is shown in Table 1. It is striking that

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the ash is dominated by AAEMs and sulfur. Their contents as oxides total up to 79.8%. In addition

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to Na2O, CaO (37.69%) and MgO (10.58%) also account for significantly high fractions in the ash.

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The content of K2O is, however, very low. The contents of Al2O3 and SiO2 are only 8.7% and

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6.95%, respectively. Compared with the bituminous coal ash 7, the Zhundong coal ash is much

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more abundant in CaO and MgO but very deficient in Al2O3 and SiO2. The high alkaline nature of

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the Zhundong coal ash is believed to be responsible for its high deposition propensities 4, 9, 11, 36. Table 1 Properties of the Zhundong coal sample

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Proximate analysis (wt%, ad) Moisture

Ash

7.25

3.56

Ultimate analysis (wt%, ad)

Volatile

Fixed

Matter

Carbon

40.13

C

H

Oa

N

S

49.06

65.77

3.95

14.60

4.36

0.51

Normalized ash composition (wt%) Na2O

MgO

Al2O3

SiO2

P2O5

SO3

K2O

CaO

Fe2O3

4.27

10.58

8.7

6.95

0.71

26.73

0.51

37.69

3.86

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a by

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2.2 Experimental procedures

difference.


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Coal combustion was carried out on a lab-scale drop tube furnace (DTF). The detailed information on the DTF and experimental procedures was described elsewhere

7, 37.

During the

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test, the coal sample was injected into the furnace at a feeding rate of about 0.15g/min. The

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simulated air (prepared by mixing pure O2 and N2 at a volume ratio of 21:79) was provided at a

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flow rate of about 4L/min for combustion. The furnace temperature was maintained at 1350 °C.

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Under these conditions, the particle residence time in the DTF was estimated to be around 1.6 s

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and the loss on ignition measurements suggested complete combustion. The ash-laden flue gas

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was directed into a water-cooled probe at the outlet of the furnace. All of the ash particles were

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collected by glass fiber filters for further analysis. To verify the mechanisms of the juxtaposition

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of the Ca and Mg, a partially burned char sample was also collected and examined.

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2.3 Analysis techniques

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The occurrence of the Ca and Mg in the coal is of vital importance, as it largely determines how

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they would transform and occur in the ash. A variety of techniques are available for obtaining such

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information. Nevertheless, each technique has its merits and limitations 34. In this work, several

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complementary techniques were used to determine the occurrence and abundance of the Ca and

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Mg in the Zhundong coal sample. They included inductively coupled plasma mass spectrometry

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(ICP-MS, ELAN DRC-e, PerkinElmer Inc.), chemical fractionation, computer-controlled

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scanning electron microscopy (CCSEM) and X-ray powder diffraction (XRD, X’pert3 powder,

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PANalytical B.V.). The ICP-MS was used to determine the total concentrations of the Ca and Mg

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in the coal. The chemical fractionation technique was adopted to quantify their abundance in

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different modes. It is valuable in that this technique is capable of quantify inorganics (in non-

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mineral forms) that cannot be determined by the CCSEM 32. In the analysis, the coal sample was

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successively extracted by deionized water, ammonium acetate and hydrochloric acid. The


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Energy & Fuels

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leachates and the insoluble residues were subject to ICP-MS analyses. The detailed procedures can

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be found elsewhere

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work

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different forms: water soluble (water-soluble salts), ammonium acetate soluble (exchangeable

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cations), hydrochloric acid soluble (mainly carbonates) and insoluble residues (primarily silicates).

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The latter two forms actually consist of a variety of minerals and were further characterized by the

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CCSEM, which has been validated and widely used 7, 8, 38-42. The CCSEM system was based on a

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FEI Quanta 200 SEM equipped with an EDAX energy dispersive X-ray spectrometer (EDS). The

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analysis procedure was described in previous work

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identified according to the classification scheme used by Zygarlicke and Steadman

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scheme, mineral particles were classified based on their elemental composition rather than

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mineralogical properties. Therefore, the mineral types identified by the CCSEM did not

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necessarily represent the actual minerals as their names indicated. To emphasize this point, the

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mineral names are put into quotation marks when the CCSEM data are discussed in the following

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sections. To verify the minerals identified by the CCSEM, XRD analyses were also conducted.

35.

8

and the conditions were slightly different from those adopted in previous

The chemical fractionation technique generally classifies a specific element into four

40.

The mineral particles in the coal were 28.

In that

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The combustion ash was characterized by both the XRD and CCSEM techniques, so that the

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occurrence of the Ca and Mg can be more clearly determined. The mineralogy of the bulk ash was

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quantified by the XRD, as done in the previous work 8. The CCSEM analysis procedures were the

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same to those in coal characterization. Microanalyses of the ash particles were also carried out on

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a field emission scanning electron microscope (FE-SEM, Sigma 300, Carl Zeiss Microscopy Ltd.)

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equipped with an EDS (X-MaxN 80, Oxford Instruments). It was further used to characterize the

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ash particles evolved on the partially burned char surface.

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3. Results and discussion


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3.1 The modes of occurrence of Ca and Mg in the coal

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The ICP-MS data show that the total concentrations of Ca and Mg in the Zhundong coal sample

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are 9562 g/g and 1563 g/g, respectively. The chemical fractionation technique combined with

147

ICP-MS was used to obtain general information on the Ca and Mg occurring as non-mineral

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species and discrete minerals. In this characterization, the water soluble (WS) and ammonium

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acetate soluble (AS) modes indicate the Ca and Mg as non-mineral species, while the hydrochloric

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acid soluble (HS) and insoluble (IS) modes as discrete minerals.

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The weight fractions of the Ca and Mg in each mode are compared in Figure 1. It is seen that

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the fractions of the Ca and Mg in different modes decrease in the order of AS > HS > IS > WS.

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For both the Ca and Mg, the AS mode overwhelmingly predominates over other modes.

154

Specifically, the Ca in the AS mode accounts for ~55.13%, while the Mg in this mode accounts

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for an even higher fraction of ~65%. These results show that the Ca and Mg in the coal investigated

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are primarily present as exchangeable cations. Consequently, the fate of these exchangeable

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species would be very critical in the partitioning of the Ca and Mg in the ash. The amounts of

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exchangeable metals may vary widely with respect to coal rank. Finkelman et al. 43 investigated

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the leaching behavior of elements in ten coals of different ranks. It was found that low-rank coals

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were generally richer in exchangeable Ca and Mg than high-rank coals. Even for the low-rank

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coals, the amounts of exchangeable metals were seen to vary significantly as well. For example,

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the exchangeable cations in the Beulah-Zap lignite were about twice those in the Wilcox lignite,

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and about three times those in the Wyodak subbituminous coal. These results highlight that the

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modes of occurrence of inorganic elements (including the Ca and Mg) should be characterized on

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a coal-by-coal basis.


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Energy & Fuels

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It is seen in Figure 1 that the Ca and Mg in minerals (HS plus IS mode) account for lower

167

fractions (Ca: 38.2%; Mg: 31.29%) than those in the AS mode. This is apparently different from

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high rank coals, in which the Ca and Mg are mostly present as discrete minerals rather than

169

exchangeable cations 26, 30. The Ca and Mg in the HS mode are higher than those in the IS mode,

170

implying that a larger fraction of the Ca and Mg is present as carbonates. For both metals, the WS

171

mode only accounts for a marginal fraction, and is not expected to have significant influence on

172

their transformation.

173 174

Figure 1. Distributions of the Ca and Mg in different modes by chemical fractionation. (WS-

175

water soluble; AS- ammonium acetate soluble; HS- hydrochloric acid soluble; IS- hydrochloric

176

acid insoluble)

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The chemical fractionation technique roughly classified the Ca- and Mg-containing minerals

178

into two categories (i.e. carbonates and silicates) based on their solubility in the hydrochloric acid

179

(Figure 1). However, such information is insufficient for clarifying their specific occurrence in

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mineral particles. This was accomplished by using the CCSEM technique. The analysis results

181

show that there are only three particle phases containing detectable Ca and Mg (> 0.5 wt%), i.e.


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“Calcite”, “Ca-Rich”, and “Unclassified particles”. The normalized fractions of the Ca and Mg in

183

these mineral phases are compared in Figure 2. It is seen that the dominant Ca- and Mg-containing

184

mineral particles are the “Unclassified particles”, which consist primarily of complex silicates or

185

aluminosilicates. The “Calcite” phase is the only carbonate found in the Zhundong coal, which

186

contains a second large fraction of the Ca and a minor fraction of the Mg. Dolomite, another

187

important carbonate, is however not detected. The “Ca-Rich” particle phase only contains trace

188

amounts of the Ca and Mg ( 5%)

263

decrease in the order of "Unclassified particles" > "Iron Oxide" > "Dolomite" > "Calcite". Other

264

particle phases only account for minor fractions on an ash basis. The results clearly suggest the

265

extensive association of the Ca and Mg with other elements such as Si, Al and Fe, due to mineral

266

interactions. Such information is not implied by the XRD data (Figure 4(a)).


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Energy & Fuels

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Figure 4. Quantification of ash minerals by XRD (a) and CCSEM (b) 3.3 The juxtaposition of Ca and Mg in ash particles

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The most interesting finding in Table 2 and Figure 4 is that the particle phase “dolomite” (Ca-

271

Mg-rich particles) identified by the CCSEM is unexpected. Based on the classification scheme

272

adopted 28, the “dolomite” particles are those with a composition of Ca + Mg > 80%, Ca > 10%

273

and Mg > 5%. They contain Ca and Mg as dominant constituents and apparently suggest a close

274

juxtaposition of Ca and Mg in individual ash particles. The formed “dolomite” particles possess

275

similar composition to that of the real mineral dolomite, but are obviously not its derivatives. This

276

is because that both the CCSEM and XRD data (Figures 2 and 3) have clearly shown that the

277

Zhundong coal does not contain any dolomite. Therefore, there should be a new mechanism,

278

involving interactions between individual CaO and MgO particles, accounting for the formation

279

of the unexpected “dolomite” particle phase.

280

The quantitative CCSEM data are used to uncover the nature of the observed “dolomite” phase.

281

The distributions of the Ca and Mg in different particle categories (Table 2) are shown in Figure

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5. It is seen that the “dolomite” is the second major phase containing Ca and Mg, highlighting the


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prevalence and importance of their juxtaposition. Up to 21% of the Ca and 27% of the Mg occurs

284

as “dolomite” particles. Since the mineral dolomite is absent in the Zhundong coal (Figures 2 and

285

3), the origins of the “dolomite” identified in the ash need to be further examined. The fractions of

286

the Ca and Mg in the “Calcite” phase in the ash are comparable to those in the coal (Figure 2),

287

suggesting the independent evolution of the “Calcite” in the coal and it is not the origin of the

288

“dolomite” phase. Comparisons between Figure 2 and Figure 5 show that, there is an appreciable

289

increase of Ca (from 0.76% to 8.34%) and Mg (from 0.1% to 6.22%) in the “Ca-Rich” phase in

290

the ash, compared with those in the coal. There is also a significant increase of Ca (from 22.9% to

291

42.5%) and Mg (from 28.9% to 55.73%) in the “Unclassified particles” after combustion. Both

292

results show that it is more unlikely that the “dolomite” in the ash originates from these coal

293

minerals. Based on the above analyses, the “dolomite” phase is believed to be primarily formed

294

through interactions between the exchangeable Ca and Mg, as the water-soluble Ca and Mg only

295

account for a marginal fraction (Figure 1).

296 297

Figure 5. Distributions of the Ca and Mg in ash minerals by CCSEM


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Energy & Fuels

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The interactions between the Ca and Mg have little been investigated during coal combustion.

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Quann et al.27 postulated that a CaO-MgO melt with additional impurities was likely formed from

300

the exchangeable materials in coal combustion. However, no evidence was provided. In a later

301

work

302

organically bound metals during lignite combustion. The organically bound Ca and Mg were

303

observed to agglomerate to form large ash particles on the char surface. Nevertheless, how they

304

ended up in the ash and the underlying mechanisms were not discussed. The interaction between

305

Ca and Mg is a hot topic in the field of CO2 capture by CaO-based sorbents. To improve the

306

stability of CaO sorbents during the cyclic carbonation and calcination processes, MgO from

307

various sources has often been incorporated as an inert support material 48. Although the Tammann

308

temperatures (the minimum temperature at which the sintering occurs) of CaO (1285°C)

309

MgO (1290°C) 50 are far higher than the operating temperatures (650–950 °C) 51, agglomeration

310

and sintering could still take place between CaO and MgO particles even at a low temperature of

311

758°C 48. The study by Li et al.48 further suggested that a molecular level mixing of CaO and MgO

312

tended to accelerate particle sintering. These processes are somewhat analogous to those occurring

313

during coal combustion, and thus can shed light on the interactions between the exchangeable Ca

314

and Mg in the coal.

47,

the scanning electron microscopy was used to characterize the transformation of

49

and

315

In the Zhundong coal investigated, the Ca and Mg are primarily present as exchangeable cations

316

(Figure 1). These species are highly dispersed throughout the coal matrix and constitute a

317

molecular level mixing. At the early stage of combustion, the exchangeable Ca and Mg will be

318

released as atoms by breaking down the bonds between metals and the char matrix

319

exposed to the environment, the metal atoms will be rapidly oxidized into their corresponding

320

oxides (i.e. CaO and MgO) when oxygen is available

44-46.

52.

Once

Depending on the temperature and


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reaction time, the oxide nuclei on the char surface can grow into nano-particles by incorporating

322

new materials 53. Since the exchangeable Ca and Mg are well mixed in the coal, there are great

323

opportunities for their oxide particles to be in contact with each other

324

coalescence, agglomeration and sintering will take place. This can be clearly evidenced by a close

325

examination of the surface of the partially burned char (Figure 6). As shown in Figure 6(a), there

326

are a large number of nanometer protuberances and particles that are uniformly distributed on the

327

char surface. Their compositions are very similar, and dominated by the Ca and Mg with only

328

minor impurities (e.g. Na and S), as shown by a typical EDS spectrum in Figure 6(b). These

329

observations suggest apparent interactions between the exchangeable Ca and Mg and their close

330

juxtaposition within Ca-Mg-rich particles. Although both CaO and MgO are refractory oxides and

331

show a large miscibility gap 55, they could thermodynamically form CaO-MgO mixtures by cation

332

rearrangement

56.

It was verified by the observation of a CaO-MgO solid solution in dolomite

333

decomposition

57.

Therefore, the observed Ca-Mg-rich particles (Figure 6(a)) are most likely a

334

result of particle coalescence through solid solution, rather than a simple physical mixture of CaO

335

and MgO particles. As shown, the protuberances are believed to be the precursors of the particles,

336

as their morphologies and compositions are very similar. These materials are present as discrete

337

particles, but mostly as chain-like agglomerates with distinct component particles similar to

338

individual ones. The agglomerates consist of a various number of nano-particles and can have a

339

size up to 2 m. Such characteristics clearly imply the occurrence of particle agglomeration and

340

sintering.

54,

where particle


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341 342

343

(a) Particle morphology

(b) Typical particle composition

Figure 6. Microanalysis of the partially burned char

344

At the later stage of Zhundong coal combustion, the interactions between the minute CaO and

345

MgO particles are expected to be more significant than those in the carbonation and calcination

346

processes 48. This speculation is based on the facts that: (1) the exchangeable Ca and Mg in the

347

coal are well mixed on a molecular level; (2) the combustion temperature (1350°C) in this work is

348

higher than the Tammann temperatures of CaO and MgO; (3) the formed CaO and MgO are

349

expected to be very reactive 26; (4) the particles on the char surface will have greater opportunities

350

to come into contact with each other as the carbon is gradually consumed. For these reasons,

351

particle coalescence, agglomeration and sintering on the char surface will be highly favored. This

352

is verified by the inspection of the ash particles formed. Typical results are presented in Figure 7.

353

As shown in Figure 7(a), ash particles from Zhundong coal combustion consist primarily of two

354

morphologies, i.e. agglomerated clusters and spherical particles. The element mapping analysis

355

(Figures. 7(b)-(f)) shows that, compared with the clusters, the spherical particles are richer in Si

356

and Al. They are most likely formed through interactions between mineral aluminosilicates and

357

AAEMs during combustion. By contrast, the clusters are dominated by the Ca and Mg, with the


19

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Page 20 of 32

358

association of some Al (Figure 7(f)). They contain particles with sizes ranging from nanometer to

359

micrometer and with morphologies similar to those as observed on the char surface (Figure 6).

360

These results strongly suggest that the clusters are generated from agglomeration and sintering of

361

the CaO and MgO particles evolving during combustion. It is found that the observed discrete Ca-

362

Mg-rich particles or chain-like agglomerates in Figure 6 are absent in the Zhundong coal ash

363

(Figure 7(a)). The clusters show complex grape-like structures with the evidence of enhanced

364

particle sintering. This indicates extensive interactions between the CaO and MgO particles and

365

their close juxtaposition during Zhundong coal combustion.

366 367

Figure 7. Microanalysis of ash particles and elemental mapping

368

The mechanisms provided above are consistent with the close juxtaposition of Ca and Mg in the

369

“dolomite” particles, as observed in Figure 5. Since these particles are primarily formed from the

370

exchangeable Ca and Mg, there is an interest in the correlation between ash particle composition

371

and the relative ratio of the two metals in the coal. Figure 8 presents the size-dependent mass ratio

372

of Mg to Ca in individual “dolomite” particles (denoted as Rdolomite). The mass ratio of Mg to Ca

373

organically bound in the coal (denoted as Rcoal) is also depicted for comparison. It is seen that the


20

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Rdolomite of individual ash particles generally centers around the Rcoal. This result further suggests

375

that the exchangeable Ca and Mg are the sources of the “dolomite” particles. A closer examination

376

shows that the composition of the fine “dolomite” particles seems to be more scattered than the

377

coarser ones. It implies a more heterogeneous nature of these particles, which most likely results

378

from their different formation processes. Another finding from Figure 8 is that the “dolomite”

379

particles are mostly less than 10 m. Similar observations were also reported by Quann et al.47,

380

who found that the particles generated from the atomically dispersed alkaline earth metals could

381

coalesce and form ash droplets in the size range of ~1 to 10 m.

382 383

Figure 8. Comparison of the mass ratio of Mg to Ca. (Rdolomite: The mass ratio of Mg to Ca in

384

“dolomite” particles; Rcoal: The mass ratio of Mg to Ca organically bound in the coal)

385

3.4 Significance of the findings

386

The close juxtaposition of Ca and Mg in ash particles, as observed in this work, demonstrates a

387

new pathway of Ca and Mg transformation. It shows that, in addition to interactions with silicates,

388

they can also interact with each other to form Ca-Mg-rich particles through particle coalescence,


21

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389

agglomeration and sintering. Such information is very useful for the development of appropriate

390

models for the transformation of Ca and Mg during Zhundong coal combustion. Since interactions

391

between silicates and alkaline earth metals are prevailing during coal combustion, the survival of

392

a significant amount of the “dolomite” phase (Figure 5) is most likely due to the insufficiency of

393

silicates in the Zhundong coal (Table 1). This can be confirmed by re-examination of the data

394

presented in the previous work 8. In that work, kaolin was added when the same Zhundong coal

395

was combusted. The generated ash particles were also characterized by the CCSEM. The results

396

clearly showed that the Ca-Mg-rich particles, formed during sole combustion of the Zhundong

397

coal, nearly completely disappeared when even only 2% kaolin was added. This was apparently a

398

result of their scavenging by kaolin particles.

399

It is not an exception that the Ca and Mg are in juxtaposition in individual ash particles from

400

Zhundong coal combustion. Additional evidence can be deduced from the reported data on

401

international low-rank coals. Richards et al.

402

ash deposits for two Powder River Basin (PRB) coals, which also contained substantial amounts

403

of organically-bound Ca and Mg. The dominant phases in ashes from both coals were found to be

404

Ca-rich particles that were rich in Ca, Mg, Al and Fe. The mass fractions of Ca-rich particles in

405

Coal A fly ashes were 72.7% and 63.2% at 900 °C and 1300 °C, respectively. Those in Coal B fly

406

ashes were 51.2% and 45.5% at 900 °C and 1300 °C, respectively. These particles were believed

407

by the authors to be formed from the coalescence of the organically associated elements in the

408

coal, consistent with the observations in this work. Hurley and Schobert 59, 60 studied ash formation

409

during combustion of two subbituminous coals, i.e. Eagle Butte coal and Robinson coal. CCSEM

410

techniques similar to this work were adopted for sample characterization. The results

411

that the Eagle Butte coal initially contained about 1.3% “dolomite”, which totally disappeared at

58

investigated the mechanisms for the formation of

59

showed


22

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Energy & Fuels

412

the early stage of burnout. It suggests that the “dolomite” phases are mineral grains originally

413

present in the coal and tend to decompose and be scavenged by silicates during combustion.

414

However, new “dolomite” phases were formed again at late stages of burnout 60. They are most

415

likely generated from the organically-bound Ca and Mg in the coal, similar to the finding in this

416

work. For the Robinson coal that contained higher contents of aluminosilicates than the Eagle

417

Butte coal, the juxtaposition of Ca and Mg in the ash particles was not observed 59, 60. This result

418

provides further evidence for the important roles of aluminosilicates in scavenging the Ca and Mg.

419

This work and the studies mentioned above strongly suggest that the formation of Ca-Mg-rich ash

420

particles will be favored for coals that are rich in exchangeable Ca and Mg but deficient in silicates

421

and/or aluminosilicates. Its further generalization needs to be conducted in the future work.

422

This work also finds that the “dolomite” particles formed during Zhundong coal combustion are

423

mostly in the size range of < 10 m (Figure 8). It is consistent with the result that ash particles less

424

than 10 m were dominated by the Ca and Mg (totaling up to 66%) 7. The recent work 35 further

425

showed that the Ca and Mg were more abundant in the PM0.5-10 (particulates with an aerodynamic

426

diameter between 0.5 and 10 m) than in the PM0.5 (particulates with an aerodynamic diameter

427

less than 0.5 m). These fine ash particles contribute not only to particulate matter emissions, but

428

also to ash deposition on heat exchanger surfaces. Field studies 1, 3, 10 showed that the initial layer

429

of the ash deposits from Zhundong coal combustion was abundant in the Ca and Mg. This can be

430

accounted for by the preferential deposition of the Ca-Mg-rich particles formed. Once deposit,

431

these particles tend to interact extensive with the existing siliceous materials, forming molten

432

phases and aggregating ash slagging. The previous work 8 found that, in addition to Na, kaolin was

433

also capable of scavenging Ca and Mg during Zhundong coal combustion. It suggests that co-


23

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

434

firing with kaolin may also be an important strategy to alleviate ash deposition induced by the

435

exchangeable Ca and Mg in the coal.

436

4. Conclusions

437

Calcium and magnesium play significant roles in ash-related issues during Zhundong coal

438

combustion. The information on their occurrence in the coal ash is very critical to the

439

understanding of mineral transformation and ash deposition behavior. This was investigated in

440

combustion of a Zhundong coal with high contents of alkaline earth metals. The modes of

441

occurrence of Ca and Mg in the coal and its combustion ash were characterized by a combination

442

of several techniques involving chemical fractionation, computer-controlled scanning electron

443

microscopy (CCSEM) and X-ray powder diffraction (XRD). The following results were obtained.

444

(1) The Ca and Mg in the Zhundong coal were mostly present as exchangeable cations (>55%),

445

followed by silicates and calcite. Dolomite, an important Ca-Mg-rich mineral, was not detected

446

by any techniques.

447

(2) A number of crystalline Ca- and Mg-containing phases, including Calcite, Lime, Periclase,

448

Anhydrite, Portlandite, and Yeelimite, were identified by XRD. But most of the Ca and Mg

449

were contained in the glass phases, demonstrating their extensive interactions with siliceous

450

materials. These phases were shown by CCSEM to be present as discrete ash particles and/or

451

as combined with other inorganics. The Mg-containing phases in the coal were more inclined

452

to interact with other minerals than the Ca-containing phases, some of which were found to

453

evolve independently during combustion.

454

(3) About 27% of the Mg in the ash was found to be in close juxtaposition to about 21% of the Ca,

455

forming a distinct Ca-Mg-rich particle phase. The dolomite, absent in the coal, was excluded


24

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Energy & Fuels

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as its origin. A new mechanism responsible for the formation of these Ca-Mg-rich ash particles

457

was proposed, involving interactions between exchangeable Ca and Mg through particle

458

coalescence, agglomeration and sintering. It was verified by the characterization of partially-

459

burned chars and the final ash particles. It was further shown that the Ca-Mg-rich particles

460

formed were mostly less than 10 m and the fine particles were more heterogeneous than the

461

coarser ones. These findings were of significance in understanding the transformation of the

462

exchangeable Ca and Mg and their deposition on heat exchanger surfaces.

463

AUTHOR INFORMATION

464

Corresponding Author

465

* Dunxi Yu* Fax: +86-27-87545526. Email: [email protected]

466

* Minghou Xu* Fax: + 86-27-87545526. Email: [email protected]

467

Author Contributions

468

The manuscript was written through contributions of all authors. All authors have given approval

469

to the final version of the manuscript.

470

Funding Sources

471

National Natural Science Foundation of China (Grant Nos. 51520105008, 51676075 and

472

51661125011); Foundation of State Key Laboratory of Coal Combustion (Grant No.

473

FSKLCCA1807)

474

ACKNOWLEDGMENT

475

The financial supports from the National Natural Science Foundation of China (Grant Nos.

476

51520105008, 51676075 and 51661125011), and the Foundation of State Key Laboratory of


25

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 32

477

Coal Combustion (Grant No. FSKLCCA1807) are appreciated. The authors also acknowledge

478

the Analytical and Testing Center at Huazhong University of Science & Technology for the

479

assistance in sample characterization.

480

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The occurrence of calcium and magnesium in the ash from Zhundong

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