Emission of suspended PM10 from laboratory-scale coal combustion

and its correlation with coal mineral properties

Lian Zhang, Yoshihiko Ninomiya *

Department of Applied Chemistry, College of Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai, 487-8501 Aichi, Japan

Received 10 October 2004; received in revised form 29 November 2004; accepted 28 March 2005

Available online 15 September 2005

Abstract

Four pulverized coals were subjected to combustion in a laboratory-scale drop tube furnace to investigate the emission of suspended particulate

matter smaller than 10 mm (PM10) and to study the correlation of PM10 emission with mineral properties of the coals. Combustion conditions of

1200 8C, 2.4 s and 20% atmospheric oxygen content were used and all the carbon was consumed under given conditions. The properties of PM10

were studied including its concentration, particle size distribution and elemental composition. Two typical sizes were also subjected to Computer

controlled scanning electron microscopy (CCSEM) analysis for determination of chemical species within them. To investigate the influence of

coal mineral properties, the metallic elements in the raw coals were divided into three parts: organically bound, included inorganic particles and

excluded ones. The results indicated that during coal combustion, about 0.5–2.5 wt% of inherent minerals changed into the suspended PM10. With

an increase in the coal ash content, the concentration of PM10 increased proportionally. The resulting PM10 had a bimodal size distribution with

two peaks around 2.5 and 0.06 mm, respectively. SiO2 and Al2O3 dominated the large mode around 2.5 mm, which is formed by the direct

transformation of inherent minerals. On the other hand, SO3 and P2O5 were prevalent in the small mode around 0.06 mm, which is formed by

vaporization of these two elements. For other metals found in PM10, the refractory metals were enriched in the large mode, with concentrations

proportional to their content in the excluded minerals in the raw coal. Volatile metals were however enriched in the small mode since, they react

with gaseous SO2 and P2O5 to form sulfates and phosphates in the solid phase. The study showed that experimental observations agree with

thermodynamic equilibrium considerations.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Coal combustion; PM10; Coal mineral properties; Direct transformation; Vaporization

1. Introduction

The emission of PM10, particulates less than 10.0 (m in

diameter, is one of the major sources of pollution from the

combustion of coal [1]. Over the past few decades, standards

for control of PM10 have become progressively stringent [2].

Even so, there are still many fine particulates that elude control

devices and are emitted into the atmosphere [1]. This is a major

motivation for studies on the formation of coal combustion-

derived PM10 as well as their control.

Many studies have been carried out to investigate the

formation of PM10 from both fundamental and practical

points of view. In principle, PM10 are formed from changes

in inherent metallic elements during coal combustion. In

coal, a portion of inherent metallic elements has diameters

0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2005.03.034

* Corresponding author. Tel.: C81 568 51 9178; fax: C81 568 51 1499.

E-mail address: ninomiya@isc.chubu.ac.jp (Y. Ninomiya).

smaller than 10.0 mm, which may directly transfer into PM10

without any chemical or physical changes. In addition, a

portion may undergo fragmentation to form fine particulates

as well, especially for refractory elements. This is termed as

the solid-to-particulate pathway governing the formation of

particulates larger than 1.0 mm [3–5]. On the other hand,

volatile metals initially undergo vaporization in the

combustion flame; the vapors produced subsequently

undergo nucleation, condensation, deposition and agglom-

eration to form particulates smaller than 1.0 mm; this is a

solid–vapor-particulate pathway [3–8]. This portion is toxic

because of the prevalence of heavy metals within the

particulates.

In addition to investigating the formation mechanisms,

parametric studies have been carried out to investigate the

influence of several factors affecting PM10 formation. Coal

combustion devices were thought to be paramount. The air

ratio, coal rank, and particle size of coal were of concern as

well. Coal combustion devices affect coal conversion

greatly. The higher the carbon conversion, the lower is the

Fuel 85 (2006) 194–203

www.fuelfirst.com

Table 1

Properties of the four coals used in this study

Ultimate analysis, daf, (wt%)

C H N SCOa

YZHS 77.85 1.75 1.60 18.80

YZLS 79.64 4.62 1.05 14.69

Baotou 77.05 4.82 1.25 16.88

Wangfg 88.52 5.47 2.22 3.79

Proximate analysis, as received, (wt%)

Ash Moisture Volatile Fixed carbona

YZHS 12.43 0.40 41.10 46.07

YZLS 7.29 4.01 34.00 54.70

Baotou 6.43 1.75 35.30 56.52

Wangfg 8.23 0.71 28.86 62.20

a By difference.

Table 2

Ash composition, wt% on basis of total ash

YZHS YZLS BT WFG

SiO2 11.98 27.91 32.88 34.07

Al2O3 5.77 11.14 6.71 17.24

Fe2O3 27.50 27.11 28.95 16.43

CaO 15.63 5.83 13.72 4.90

K2O 0.43 1.80 2.23 1.76

Na2O 0.17 0.29 0.68 0.71

TiO2 0.80 1.80 1.51 6.71

SO3 35.91 21.29 11.11 15.73

MgO 0.36 0.46 0.81 0.36

Cl 0.40 0.30 0.30 0.48

BaO 0.38 0.00 0.00 0.00

P2O5 0.14 1.21 0.29 0.47

MnO 0.22 0.29 0.44 0.21

ZrO2 0.13 0.26 0.00 0.29

CuO 0.08 0.08 0.13 0.24

ZnO 0.06 0.09 0.10 0.23

NiO 0.04 0.14 0.14 0.15

L. Zhang, Y. Ninomiya / Fuel 85 (2006) 194–203 195

PM10 concentration in the exhaust gas because less soot is

formed [9]. With increasing oxygen pressure, more of the

inherent metals directly transfer or vaporize to form PM10

[10,11]. Coal rank affects the distribution of inherent

minerals within as well as carbon conversion in combustion.

Accordingly, PM10 formation is affected as well [12].

Moreover, our previous study suggests that with decreasing

coal particle size, more of the inherent minerals change

into excluded particles, which preferentially transform into

PM10 [13].

There are still several little-understood mechanisms for

PM10 formation. Among them, the influence of coal properties,

especially its mineralogical composition, has not been

elucidated. Few studies have taken the heterogeneity of coal

minerals into account. Correspondingly, there is still a lack of

accurate guidelines for the selection of coal to minimize its

PM10 emission in industrial plants. Furthermore, the bulk

properties of PM10 have been studied until now whereas few

studies have given attention to its heterogeneity. A different

route, as discussed before, governs the formation of coarse

particles and fine/ultrafine particles. Accordingly, the sources

for the formation of different portions of PM10 must originate at

different parts of raw coal.

The present paper aims to link the formation of

suspended PM10 with the mineral properties of coal. PM10

was segregated into several groups according to elemental

composition. The coal mineral composition was analyzed by

several procedures to elucidate its heterogeneity: chemical

fractionation was conducted to quantify the contents of

organically bound elements in raw coals. Computer

controlled scanning electron microscopy (CCSEM) was

used to quantitatively determine the particle size distribution

and association of inorganic minerals in the coals. The

association of inorganic minerals is defined as included

particles existing in the carbonaceous matrix and excluded

particles not associated with the carbonaceous matrix [14].

The formation of each group of PM10 was linked with

different parts of inherent minerals quantified by the above-

mentioned analysis procedure. CCSEM speciation on two

sizes of PM10 and thermodynamic equilibrium calculations

were also conducted to further elucidate the relationship

between coal mineral properties and PM10 formation.

2. Experimental

2.1. Coal properties

Four Chinese coals, Yanzhou high-sulfur (YZHS), Yanzhou

low-sulfur (YZLS), Baotou (BT) and Wangfg (WFG), were

used in the investigation. The coals were ground to smaller

than 125 mm and dried overnight, prior to use. Their properties

are listed in Table 1. Clearly, the four coals have different

compositions with the ash content ranging from 6.43 wt% in

BT coal to 12.43 wt% in YZHS coal. The contents of the

volatile matter and fixed carbon vary greatly with the coal type.

The low-temperature ashing (LTA) ash compositions,

measured by X-ray fluorescence (XRF), are shown in

Table 2. The data further indicates the broad variation of the

coals selected for this study. SiO2 and Al2O3 are the most

prevalent oxides. The contents of the other major oxides,

including CaO, Fe2O3, SO3, P2O5, K2O and Na2O, however

vary greatly with coal type. Finally, the distributions of the

inherent minerals are listed in Table 3 for the four coals. The

percentage of organically bound metals was determined by

extracting raw coals with a solution of 1 M ammonia acetate.

Analysis of SEM images, obtained from CCSEM measure-

ments, was used for quantifying the included and excluded

inorganic minerals. It is noteworthy that the organically bound

metals are salts of organic acids in the raw coals. Thus, they are

too small to be detected by CCSEM due to the limitations of the

CCSEM. The included inorganic metals analyzed by CCSEM

are larger than 0.5 mm.

The mineral distribution varies greatly with the coal type.

YZHS coal has the greatest amount of organically bound

metals, the majority of which is sulfur. On the other hand,

WFG coal has the lowest amount of organically bound metals,

but a relatively large amount of included minerals, further

suggesting different particle size distributions of the minerals

in these coals.

Table 3

Distribution of inherent minerals in the four coals

Coal Total ash

(wt%)

Organically

bound (wt%)

Inorganic minerals (wt%)

Included Excluded

YZHS 12.43 4.97 0.96 6.50

YZLS 7.29 2.16 2.52 2.61

BT 6.43 3.91 0.40 2.11

WFG 8.23 1.25 2.72 4.26

0.5

0.6

0.7

g/g_

ash

YZHS YZLS BT WFG

L. Zhang, Y. Ninomiya / Fuel 85 (2006) 194–203196

2.2. Coal combustion procedure and collection of suspended

PM10

Coal combustion was carried out in a laboratory-scale drop

tube furnace, whose configuration and combustion procedure

have been described in detail elsewhere [15]. The reaction

temperature was maintained at 1200 8C, and the residence time

of gas in the furnace was 2.4 s for all runs. Coal was fed at

about 0.3 g/min into the furnace. An oxidizing atmosphere,

with 20% O2 and N2 being the balance, was used. All the coals

were burnt completely under these conditions.

The exiting gas, containing the solid products, was

initially quenched with N2 and collected by a water-cooling

probe. Subsequently, coarse ash particles were collected by a

cyclone. Meanwhile, the suspension of ultrafine particles

was further diluted with air, and immediately directed to a

Low-Pressure-Impactor (LPI) for size-segregated collection.

The LPI used here is composed of 13 stages having

aerodynamic cut-off diameters ranging from 0.03 to

12.1 mm. Each stage is composed of a filter above a

substrate and a substrate holder. The pressure after the final

stage is approximately 73.3 kPa.

The cut-off sizes of both the cyclone and the first stage of

LPI are noteworthy. The cyclone has a cut-off size of around

10.0 mm, which indicates that it collects the majority of coarse

particles having a sizeR10.0 mm. Meanwhile, a few of the finer

particles, smaller than 10.0 mm, were also collected by it. On

the other hand, the first stage of LPI collected particles around

its cut-off size of 12.1 mm. These collected particles were

added to those collected by the cyclone to form the total coarse

ash with a sizeR10.0 mm. In the present study, suspended

PM10 is defined as those particles collected by the other stages

of the LPI having a cut-off size smaller than 10.0 mm. The fine

particles collected by the cyclone were however ignored.

0.01 0.1 1 100.0

0.1

0.2

0.3

0.4

Con

cent

ratio

n, µ

Diameter, µm

Fig. 1. Particle size distribution of PM10 emitted by the combustion of the four

coals.

2.3. PM10 characterization

By the size-segregated collection using LPI, both the

concentration of PM10 and its particle size distribution were

obtained simultaneously. Each size of PM10 was also subjected

to XRF to quantify the elemental composition. Two typical

sizes, 2.5 and 0.13 mm, were also subjected to CCSEM to

determine their chemical forms. The procedures for CCSEM

analysis and the data interpretation have been previously

explained in detail [16].

2.4. Thermodynamic equilibrium calculation

To justify the formation routes of PM10 having different

sizes, thermodynamic calculations were carried out using the

Factsagee 5.1 software. The phase change of particulates

larger than 1.0 mm, the vaporization of volatile elements, and

chemical reactions among metallic vapors were investigated.

3. Results and discussion

3.1. Emission of PM10 in coal combustion and influence of coal

mineral properties

The emission of PM10 from the combustion of the four coals

is shown as a function of the particle size distribution in Fig. 1.

A bimodal distribution was obtained for each case regardless of

the coal type. The large mode is around 2.5 mm and the small

one around 0.06 mm. A 1.0 mm size acts as the boundary line

classifying PM10 into two groups: (1) a portion with size equal

to and greater than 1.0 mm, termed as PM1.0C hereafter, and (2)

particles smaller than 1.0 mm, termed as PM1. These findings

are similar to results reported elsewhere [17]. It can be

concluded that PM1C is derived from the direct transfer of

inherent minerals, whereas PM1.0 is mostly generated from the

vaporization of volatile metals within the coals. In addition, the

amounts of these two modes vary greatly with coal type,

implying their dependence on coal properties.

The influence of coal properties, specifically its ash content,

was initially plotted as a function of the percentage of ash

transformed into PM10 as shown in Fig. 2. A fairly linear

relationship was found, indicating that as the coal ash content

increased, the emitted PM10 concentration also proportionally

increased. In this study, it was found that about 0.5–2.5 wt% of

the inherent minerals were transformed into the suspended

PM10 during coal combustion, even after the collection of ashes

by the cyclone installed before LPI.

The distribution of individual elements is shown in Fig. 3.

The concentrations of SiO2 and Al2O3 were added together as

5 6 7 8 9 10 11 12 130.0

0.5

1.0

1.5

2.0

2.5

Wt%

of

coal

ash

tran

sfer

red

into

PM

10

Content of ash in raw coals, wt%

Fig. 2. Influence of inherent coal ash content on the percentage of inherent ash

transformed into PM10.

L. Zhang, Y. Ninomiya / Fuel 85 (2006) 194–203 197

the aluminosilicate group; Na2O, K2O and MgO were added

together as the alkali and alkaline earth compounds group; SO2

and P2O5 were added together due to the possibility that both

may act as negative ions to react with the other metallic vapors.

0

1000

2000

3000

4000

5000

6000

0.01 0.1 1 100

1000

2000

3000

4000

5000

6000

0.01 0.1 1 100

100

200

300

400

0.01 0.1 1 10Diameter, µm

Diameter, µm

Diameter, µm

Con

cent

ratio

n, µ

g/g_

ash

Con

cent

ratio

n, µ

g/g_

ash

Con

cent

ratio

n, µ

g/g_

ash

(SiO2+Al

2O

3)

(Fe2O

3)

(Na2O+K

2O)

Fig. 3. Distribution of individu

The results shown in Fig. 3 indicate that the majority of SiO2

and Al2O3 exist in PM1C and they are the dominant elements.

In the case of the other two refractory elements, CaO and

Fe2O3, the former mainly exists in PM1C and its low content in

PM1 implies negligible vaporization of calcium. Conversely,

Fe2O3 has a bimodal distribution and the two modes (around

0.06 and 2.5 mm, respectively), have comparable content,

indicating that vaporization of iron-based compounds is severe

compared to that of calcium. Besides the above-mentioned

elements, the existence of alkali and alkali earth elements was

also found in PM1, though they are present in relatively low

concentrations, implying the formation of alkali aluminosili-

cate salts in the larger particles.

Both SO3 and P2O5 dominate PM, having contents much

higher than those of alkali elements and Fe2O3. This implies

the formation of sulfates and phosphates during coal

combustion, which will be further discussed later.

Cl has a different distribution to the above-mentioned

elements. It has the largest mode around 0.2–0.5 mm, which

falls between the smaller mode, 0.06 mm, and the larger one

around 2.5 mm. Compared to SO2 and P2O5 in the 0.06 mm

0

1000

2000

3000

4000

5000

6000

0.01 0.1 1 100

1000

2000

3000

4000

5000

6000

0.01 0.1 1 100

100

200

300

400

YZHS

YZLS

BT

WFG

0.01 0.1 1 10Diameter, µm

Diameter, µm

Diameter, µm

Con

cent

ratio

n, µ

g/g_

ash

Con

cent

ratio

n, µ

g/g_

ash

Con

cent

ratio

n, µ

g/g_

ash

(CaO )

(SO3+P

2O

5)

(Cl)

al elements within PM10.

0 5 10 15 20 25 30 35 40 45 50 550

2000

4000

6000

8000

10000

PM1 vs. excluded mineralsPM1 vs. included minerals

Con

cent

ratio

n of

PM

10

Content of fine minerals (<10m) in raw coal, wt%

g/g_

ash

µ

Fig. 4. Relationship between the contents of PM1C with the content of minerals

in the raw coals.

0 5 10 15 20 250

2000

4000

6000

8000

Al2O

3

SiO2

Con

tent

in P

M1+

, g/

g_as

h

Content of excluded SiO2 and Al

2O

3in coal ash, wt%

Content of excluded CaO and Fe2O

3in coal ash, wt%

0 5 10 150

1000

2000

3000

4000

Fe2O

3

CaO

Con

tent

in P

M1+

, g/

g_as

hµ

µ

Fig. 5. Effect of excluded refractory metals smaller than 10.0 mm on their

concentrations in PM1C.

L. Zhang, Y. Ninomiya / Fuel 85 (2006) 194–203198

mode, the prevalence of Cl in the medium size range implies

the deposition of chloride on the medium-sized solid particles.

The results described above suggest that there are three

kinds of distribution of the elements in PM10. (1) SiO2, Al2O3,

CaO, SO3 and P2O5 have a single mode distribution; the former

three are prevalent in PM1C, whereas the latter two are

abundant in PM1. The transformation of the former three

metals was likely caused by the direct transformation of

inherent refractory metals within the coals. A solid-to-particles

pathway governs their transformation. Vaporization is however

the main formation route for SO3 and P2O5 in PM, the reactions

between them and the other vaporized metals allowed for the

formation of ultrafine solid particles following the solid–vapor-

particles pathway. (2) Fe2O3, Na2O, K2O and MgO have a

bimodal distribution. The transformation of Fe2O3 should be

governed by both the above-mentioned pathways, i.e. a portion

of it undergo direct transformation whereas the remaining

portion vaporizes and condenses into ultrafine particles. For the

other three metals, their presence in the large mode is likely

caused by adsorption of their vapors on the inherent minerals

such as kaolinite. In other words, the presence of these three

metals in PM10 likely resulted from their vaporization. (3) Cl

has a single mode around 0.5 mm, suggesting the agglomera-

tion of vaporized chloride or its deposition on the surfaces of

other solid particles.

As stated previously, the inherent minerals in raw coals are

generally assigned to two groups, organically bound and

inorganic. Inorganic minerals consist of included particles

embedded within the carbonaceous matrix, and the excluded

particles exist separate from the carbonaceous matrix. Coal

combustion is initiated by the release of volatile matter, which

is followed by char combustion. The formation of a flame

during char combustion causes a higher temperature around the

burning char than that of the furnace. The organically bound

and a portion of the included inorganic minerals within the char

then undergo vaporization leading to the formation of

submicron particles. Meanwhile, a portion of the excluded

minerals undergoes fragmentation in coal combustion, which

may lead to the formation of large particulates (in general

having size greater than 1.0 mm). The majority of included

inorganic minerals, however, undergo coalescence and

agglomeration to form coarse ash, having little effect on the

formation of large particulates. In this respect, it is plausible

that the formation of the large mode PM1C may be affected by

the content of the excluded minerals having sizes smaller than

10.0 mm. Meanwhile, PM1 may be affected by the contents of

both organically bound and included minerals smaller than

10.0 mm. Both of these will be discussed separately in the

following sections.

3.2. Formation of PM1C

The concentration of PM1C was plotted as a function of the

content of inherent minerals, smaller than 10.0 mm, as shown in

Fig. 4. No relationship was found for included minerals as a

function of the PM1C concentration. The included minerals

thus have little influence on the formation of the large mode

during combustion of these four coals. In contrast, the

relatively linear relationship for excluded minerals confirmed

their influence on the formation of PM1C.

The relationship between the content of individual elements

in raw coals (existing as excluded mineral particles smaller

than 10.0 mm) and their concentrations in PM1C is shown in

Fig. 5. These experimental results show that: (1) the transfer

rate of all elements except Ca is proportional to their content in

the raw coals; (2) the transfer rate of an individual element

depends on the elemental type.

The fitting lines for SiO2 and Al2O3 have the steepest slope

indicating that they have the greatest transformation rates

among all the elements. In addition, their similar gradient

100

L. Zhang, Y. Ninomiya / Fuel 85 (2006) 194–203 199

values suggest that these two elements have a nearly equal

transfer rate, which might be due to their co-existence as

aluminosilicate, e.g. kaolinite, in raw coals. The change in the

inherent aluminosilicate allowed the transfer of both into

PM1C.

The transformation of Ca is complex. The relationship

shown here, which is not very clear, implies that except for its

content, the reaction of Ca with other metals may be important

too. Though the extent of transformation of Fe is proportional

to its content in the excluded minerals, the nearly horizontal

fitting line suggests that less iron is transformed into PM1C

even for higher contents. The majority of iron might be

scavenged by aluminosilicate to form molten phases, which

undergo agglomeration to form coarse ash particles.

The chemical species in the large mode, 2.5 mm, were

quantified by CCSEM and the results are shown in Fig. 6. The

results shown are consistent with the elemental composition as

discussed above. That is, aluminosilicate dominates and its

salts, including calcium, iron and alkali elements, are relatively

prevalent too. For the YZHS coal, iron oxide and calcium

sulfate have minor contents. Both quartz and aluminosilicate

should be formed by direct transformation of the inherent

quartz and kaolinite in raw coals. Calcium sulfate should also

be the inherent mineral, as shown by CCSEM analysis on raw

coals reported elsewhere [15]. On the other hand, the existence

of aluminosilicate salts and iron oxide indicates that the

chemical reactions between different elements also play an

important role in the transformation of excluded minerals

during coal combustion.

The formation of iron oxide is likely caused by the oxidation

of pyrite. This reaction commences at 873 8C in an oxidizing

atmosphere, and magnetite and hematite are regarded as the

main products [4]. On the other hand, the formation of

aluminosilicate salts suggests that it is plausible that

aluminosilicate might react with the excluded calcium, iron,

and alkali vapors via collision in the gas atmosphere.

To better understand the formation mechanisms of

aluminosilicate salts, thermodynamic equilibrium calculations

were carried out to investigate the phases of these salts in the

combustion temperature windows. Calculations were con-

ducted by selecting the elemental composition of individual

Quartz

Al-silicate

Ca/Fe Al-silicate

Alkali Al-silicate

Iron Oxide

Ca sulfate

0 20 40 60 80 100Wt%

WFG BT YZLS YZHS

Fig. 6. Chemical species in the size of 2.5 mm.

particles as the input, which was obtained from CCSEM

analysis. An oxidizing atmosphere was used as the input as

well. The output was expressed in terms of percentage of liquid

phase in each particle. An average result was finally given for

each species. The calculated results are shown in Fig. 7. The Y-

axis unit is expressed as the weight percentage of liquid phase

in each compound. Less than 5 wt% of aluminosilicate was

maintained in the liquid phase, indicating a solid state for the

material. Therefore, the solid-to-particles pathway governs the

transformation of inherent quartz and kaolinite. About

10–40 wt% of Ca or Fe aluminosilicate was kept in a liquid

phase at 1200 8C, which indicates a semi-liquid phase, which

should be due to the reaction of calcium/iron and aluminosi-

licate on the surface of the latter compound. In the post-flame

zone, these partially molten particles condense again or

agglomerate with each other to form larger clusters. Hence, a

solid-semi-liquid-particles pathway should be the formation

route. Furthermore, more than 60 wt% of alkali aluminosili-

cates was kept as liquid phase. Clearly, the reaction of

vaporized alkali elements and aluminosilicate resulted in the

formation of a total melt phase, and the condensation of its

droplets led to formation of fine particulates in the exit gas.

Therefore, a pathway of solid–liquid-particles should be

adopted for the existence of alkali aluminosilicates in PM1C.

3.3. Formation of PM1

As suggested in Fig. 3, the major elements in PM are sulfur

and phosphorus. The influence of these two elements was

plotted as a function of the concentration of PM1, as shown in

Fig. 8. The linear relationship confirmed the significant

influence of these two elements.

For the other volatile elements shown in Fig. 3, in principle,

they may undergo physical and chemical changes after they

vaporize. In this respect, the formation of PM1 is discussed in

Sections 3.4 and 3.5.

700 800 900 1000 1100 1200 1300 1400 1500 16000

20

40

60

80Na/K Al-silicate

Ca/Fe Al-silicate

Al-silicate

Wt%

Reaction temperature, oC

Fig. 7. Predicted percentage of liquid amount in each phase in the size range of

2.5 mm.

0 1 2 543

6000

8000

10000

12000

14000

Con

cent

ratio

n of

PM

1,µ

g/g_

ash

Contents of S and P in raw coals, wt%

Fig. 8. Relationship between the formation of PM1 with the SO3 and P2O5

content in raw coals.

L. Zhang, Y. Ninomiya / Fuel 85 (2006) 194–203200

3.4. Vaporization of metals

Fig. 9 displays the degree of vaporization for the studied

elements, where the values for the Y-axis, i.e. the degree of

vaporization, was calculated using the following equation:

Degree of vapourization of Mð%Þ

Z Concentration of M in PM1=10½mg=m3N��

!exit gas amount½m3N=min��

!10K4=fcoal feed rate½g=min�

!concenration of M in coal½g=g�g

(1)

Here, M represents the individual elements listed on the X-axis

in Fig. 9. The vaporization of alkali elements was calculated on

the basis of their content in PM10; meanwhile the content of

other metals in PM1 was used for the vaporization calculation.

This is because a portion of the alkali metals is deposited on the

large mode as shown in Fig. 3. In addition, the results for each

metal are shown as a bar that represents the range of the degree

of vaporization for this metal. The upper end is the maximum

value and the lower end the minimum. The coal type

0

10

20

30

40

50

60

70

Al Si Ca Mg Fe K Na

Wt%

of

orga

nic

and

incl

uded

met

als

tran

sfer

red

into

PM

1

Fig. 9. Weight percentage of individual metals transferred into PM1.

significantly affected the vaporization of the elements as

discussed before.

The degree of vaporization of elements depends on the type

of element, which decreases in the order of NaOKOFeOMgOCaOSiOAl. The vaporization degree of Na ranges from

30 to 65%, the largest among the metals studied. It is followed

by K, for which the degree of vaporization ranges from 3 to

almost 20%. Fe has a similar range as that of K, suggesting it

has a relatively high degree of vaporization among the

refractory elements. 5% of Mg vaporized at most. Also, the

vaporization of Ca and Si is small, if not negligible. Al hardly

underwent any vaporization, accounting for its lowest value

shown here.

A thermodynamic equilibrium study was carried out to

justify the above conclusions from a qualitative viewpoint. For

simplification, a reducing gas atmosphere was adopted by

assuming that the oxygen diffusing to the char surface is

completely consumed. CO is the only product on the char

surface, and hence, a fixed partial pressure of CO, PCO, of

0.15 atm was used as an input. The content of each element

studied, M, and that of Si and Al in raw coal (both organically

bound and included discrete) were also used as the input. The

latter two elements are the most prevalent in char and they

could react with the vaporized metals in the char. A broad

temperature range from 1000 to 1800 8C was used for the

calculation considering the possibility that the char has a

temperature about 200–400 8C higher than that of gas.

The calculation results indicate that mullite (Al6Si2O13) is

the major compound formed in the char. SiO, Al and AlO are

the gaseous phases formed for elements Si and Al. For the other

two refractory elements, Ca and Fe, the majority of Ca is

scavenged by aluminosilicate to form various salts including

CaAl2Si2O8, Ca2Al2SiO7 and CaAl4O7. Meanwhile a small

amount of Ca is formed as vapor. Fe does not react with

aluminosilicate under the given conditions. It partitions into

solid Fe in the char as well as gaseous Fe. The formation of

Mg2Al4Si5O18 is also preferred. Its amount decreases gradually

as the temperature increases and consequently, more gaseous

Mg is formed. The formation of NaAlSi3O8(s) is only possible

at temperatures lower than 1300 8C. When the temperature

increases further, all the Na2O is transformed to gaseous Na.

The formation of KAlSi2O6(s) is however possible at

temperatures below 1600 8C and gaseous K is formed as its

equilibrium phase.

Fig. 10 shows the weight percent of the gas phases of

individual elements as a function of the reaction temperature.

Only the results for YZHS coal are shown here. Assuming that

the char temperature ranges from 1400 to 1600 8C, it is likely

that Na2O vaporizes completely, followed by K2O having 20%

of it vaporizes at 1400 8C, which goes up to 100% at 1600 8C.

SiO2 vaporizes as well to an extent ranging from 5 to 60%.

MgO and Fe2O3 vaporize to a relatively small extent, under

10%. In addition, CaO and Al2O3 do not vaporize since the

amounts of their gas phases are negligible.

Thus, from a qualitative viewpoint, the calculated results are

roughly the same as the results obtained experimentally. On the

other hand, there are differences between the calculated

0

20

40

60

80

100

1000 1100 1200 1300 1400 1500 1600 1700 1800Reaction temperature, oC

Wt %

tran

sfer

red

into

gas

0.0E+00

2.0E-02

4.0E-02

6.0E-02

8.0E-02

1.0E-01

Wt%

of

CaO

or

Al2

O3

tran

sfer

red

into

gas

Na2O K2O

SiO2

MgO

Fe 2

O3

CaO A

l 2O

3

Fig. 10. Predicted weight percentage of individual elements transformed into

gaseous phases (for YZHS coal).

Con

tent

in P

M1,

g/g_

ash

µC

onte

nt in

PM

1, g/

g_as

hµ

Con

tent

in P

M1,

g/g_

ash

µ

Fig. 11. Relationship between the content of inherent metals in the organic

matrix and their concentrations in PM.

L. Zhang, Y. Ninomiya / Fuel 85 (2006) 194–203 201

and experimental results from a quantitative viewpoint. One

plausible reason for this is that the calculations simplified the

elements as oxides in the char, rather than their original forms.

The original form of the metals is thought to be more important

for determining their vaporization [10]. Moreover, the

diffusion of the vaporized metals within the pores of the char

is another factor that might also lower the degree of

vaporization [3].

The relationship between the content of individual elements

in raw coal and their content in PM1 is shown in Fig. 11. The x-

axis in the figure refers to the content of individual metals in the

carbonaceous matrix of raw coals, and its unit is C.O.I. (content

of organically bound and included inorganic metal

(%10.0 mm)). A good linear relationship was not found for

several elements including SiO2, K2O, Na2O and MgO. This

suggests a complex process for the vaporization of these

metals. Even so, there is an upward trend wherein increasing

the C.O.I. of these elements resulted in the improvement of

their vaporization. For the other elements, including Al2O3,

Fe2O3 and CaO, their degree of vaporization is proportional to

their C.O.I. as expected. The elemental type affected its

vaporization as well. The gradient of the fitting line also varied

greatly with the elemental type, further indicating their

different vaporization rates during coal combustion.

3.5. Formation of chemical species in PM1

After leaving the char surface, the resultant metallic vapors

initially undergo re-oxidization to form oxides. These

subsequently react with the other constituents via their

heterogeneous agglomeration.

The chemical species for the particulate having the diameter

of 0.13 mm were quantified by CCSEM and the results are

shown in Fig. 12. The results show the existence of quartz,

silicates, sulfates and phosphates. Quartz should be formed by

the homogenous condensation of gaseous SiO. Silicates should

however be formed by chemical reactions between gaseous

SiO and other metallic vapors. The presence of iron oxide

indicates the re-oxidation of metallic iron. Sulfates are present

with the greatest amounts, suggestive of sulfation of the metals,

especially alkali elements. On the other hand, the amount of

phosphates is relatively low compared to solid P2O5. The effect

of reactions between P2O5 and other metals appears to be

minor. The existence of P2O5 likely resulted from the

condensation of its gaseous phase in the post-flame zone in

the furnace. The existence of SO2 can be attributed to the

formation of liquid sulfate droplets or ammonium sulfate,

considering the existence of moisture and nitrogen in coals.

The coal type affected the distribution of species as well. A

number of sulfates were formed for YZHS coal due to its high

sulfur content. The YZLS, WFG and BT coals follow it in

the decreasing order of sulfur content in the raw coals.

Quartz

Ca/Fe silicate

Na/K silicate

Iron Oxide

Ca Sulfate

Zn Sulfate

Na/K Sulfate

Sulfur oxide

Na/K Phoshpate

Phosphrous oxide

0 2010 30 40 50Wt%

WFG BT YZLS YZHS

Fig. 12. Chemical forms in the size of 0.06 mm.

L. Zhang, Y. Ninomiya / Fuel 85 (2006) 194–203202

This further suggests the significant influence of sulfur on the

formation of PM1.

For a better understanding of the formation of species for

particulate of 0.13 mm, the thermodynamic equilibrium

calculation was conducted with the elemental composition of

this size and an oxidizing atmosphere used as the input. The

results expressed as a percentage of species with this size are

shown in Fig. 13. Iron was not included in the results since all

the iron was re-oxidized into oxide whereas some of it reacted

with the other constituents as discussed above. Clearly, the

formation of quartz proved the condensation of SiO into SiO2.

The formation of K2SO4 (l) and the absence of Na2SO4 suggest

that alkali sulfate in this size was formed by the reaction

between K(g) and SO2, which allowed for the formation of

ultrafine droplets, and subsequently, fine round solid particles

after their condensation. In addition, the predicted variation in

K2SO4 content with coal type is similar to that observed using

CCSEM (as shown in Fig. 12). This suggests that this reaction

reached its equilibrium under the stated conditions. The

formation of alkali phosphate, Na3PO4, was also confirmed.

The absence of P2O5 indicates that all the gaseous phosphorous

should be used to form Na3PO4. The existence of P2O5 in

Fig. 12 however implies that the formation of Na3PO4 did not

reach its equilibrium. This was likely caused due to two

reasons: (1) a short residence time in the furnace; (2) a low

partial pressure of phosphorous vapor compared to sulfur.

Finally, the calculations also show the formation of a

--

Sulfur oxide

Phosphorous oxide

K sulfate(l)

quartz(s)

Na sulfate(s)

Na phosphate(s)

K silicate(l)

0 20 40 60 80 100Wt%

WFG BT YZLS YZHS

Fig. 13. Predicted quantitative distribution of species in the size range of

0.13 mm.

significant amount of K2Si4O9 (l). This is very different from

the experimental results of less silicate as show in Fig. 12.

Apparently, the formation of silicate is very slow and time-

consuming.

4. Conclusions

The present study leads to the following conclusions.

Suspended PM10 emitted from the combustion of four

different coals has a bimodal distribution with two peaks

around 2.5 and 0.06 mm, respectively. When the ash content

increased, the PM10 concentration also increased linearly.

About 0.5–2.5 wt% of inherent minerals changed into the

suspended PM10. Its amount is proportional to the ash content

in raw coal as well.

There are three kinds of elemental distributions in PM10.

SiO2, Al2O3, CaO, SO3 and P2O5 have a single modal

distribution. The former three elements dominate PM1C

having a size ranging from 1.0 to 10.0 mm and a peak around

2.5 mm. They are mainly formed by the direct transformation of

inherent minerals. SO3 and P2O5 are prevalent in PM1 with a

size range smaller than 1.0 mm and a peak around 0.06 mm.

These two elements were formed by their vaporization. Fe2O3,

Na2O, K2O and MgO have a bimodal size distribution. Both

direct transformation and vaporization govern the formation of

Fe2O3. A portion of the vaporized Na, K and Mg was captured

by the inherent aluminosilicate to form a large mode around

2.5 mm. Finally, Cl has a single distribution with a peak around

0.2–0.5 mm, which should be the result of the deposition of

chlorides on other solid particles.

PM1C was formed by the direct transformation of refractory

elements in raw coal. Quartz and aluminosilicates within this

portion were formed by transformation without phase change.

Meanwhile, calcium/iron aluminosilicate in PM1C was formed

by the reaction of calcium or iron with aluminosilicate, which

led to a sticky surface of the latter compound. Alkali

aluminosilicate was formed as melt droplets in coal combus-

tion and it condensed into large particulates in PM1C.

PM1 is rich in both sulfates and phosphates of vaporized

elements. The degree of vaporization of elements is determined

by their content (organically bound and included minerals

smaller than 10.0 mm) in the raw coals. The elemental type also

affects their vaporization greatly, which decreases in the order

of NaOKOFeOMgOCaOSiOAl. This is fairly consistent

with the predictions of thermodynamic equilibrium calcu-

lations. The vaporized metals reacted with gaseous SO2, P2O5

and SiO to form their compounds.

Acknowledgements

The author would like to thank Grant-in-aid for Scientific

Research on Priority Areas (B), 14380279, Ministry of

Education, Science, Sports and Technology, Japan, and the

Steel Industry Foundation for the Advancement of Environ-

mental Protection Technology for financial support. The first

author thanks the Japan Society for Promotion of Science,

JSPS, for the postdoctoral research fellowship.

L. Zhang, Y. Ninomiya / Fuel 85 (2006) 194–203 203

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