An Experimental Study of the Effect of Coal Blending on Ash Deposition

An experimental study of the effect of coal blending on ash deposition

A. Rushdi*, A. Sharma, R. Gupta

Department of Chemical Engineering, Cooperative Research Centre for Coal in Sustainable Development (CCSD),

The University of Newcastle, Callaghan NSW 2308, Australia

Received 26 March 2003; revised 24 July 2003; accepted 20 August 2003; available online 25 September 2003

Abstract

In this study, deposition experiments with well characterized samples of Australian black coals and blends were conducted in a laminar

drop-tube furnace to assess the blends behavior and their potential to form ash deposits. Comparison of the results between the blends and

single coals shows that the blends behavior was not additive in nature. Some blends developed a thicker ash deposit layer with a maximum

thickness higher than 700 mm compared to a thickness of 300 mm for the source coals, whereas, other blends had a lower potential to form

ash deposits. This non-additive behavior of coal blends results from the interaction between ash particles within the deposit layer. Therefore,

the coal blends performance may not be the same as that of the source coals with same bulk composition.

Three techniques were used to explain the trends of the deposition experiments. The use of slag viscosity measurements and a slagging

index based on the ash bulk properties, were not successful. Thermomechanical analysis (TMA) test was conducted as another technique that

represents the thermal behavior of the ash deposits. The (TMA) penetration curves showed a reasonable matching trend with the deposit

thickness obtained in the deposition experiments.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Ash deposition; TMA; Slag viscosity and indices

1. Introduction

Many pulverized coal fired power plants were originally

usually designed to burn a fairly narrow range of fuels. In

many cases, these fuels are no longer fired. Therefore, more

care must be taken to maintain the quality of the coal going

to the power plant [1]. Consequently, utilities use fuel

specifications based on experience, usually based on the

performance of coals from a single source. This can be

achieved by blending of coals. Possible advantages for the

electric utilities are:

1. Reducing fuel costs

2. Controlling emission limits

3. Enhancing fuel flexibility and extending the range of

acceptable coals

4. Providing a uniform product from coal of varying quality

5. Solving existing problems such as poor carbon burnout,

slagging and fouling and improving boiler performance

However, although blended coals may conform to the

fuel specifications, their performance may not be interp-

olated linearly from that of the parent coals [1]. The use of

two or more coals blended to meet station requirements has

been successful at some sites, but has resulted in some

problems at others. Thus, questions are raised about coal

combustion, ash deposition and emission performances for

blended fuels and how the performance parameters of

individual coals can be combined for blended coals. During

coal blend combustion, there is no interaction between the

different ash particles during ash formation and transpor-

tation processes. The interaction amongst the different ash

particles takes place only on the deposit surface [2]. This

interaction can be highly non-linear for coal blends. This

explains why the behaviour of coal blends cannot be

predicted from the bulk properties of the parent coals.

So far, it is apparent that the effects of coal blending on

power plant performance are poorly understood, and also

the attempts to understand them are limited. There have

been a number of lab-scale and full-scale studies aimed at

investigating ash deposition problems for coal blends.

In Germany, one of the full-scale studies was carried out

to evaluate the influence of coal types on slagging, fouling,

0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2003.08.013

Fuel 83 (2004) 495–506

www.fuelfirst.com

* Corresponding author. Tel.: þ61-2-4921-7442; fax: þ61-2-4921-8692.

E-mail address: [email protected] (A. Rushdi).

http://www.fuelfirst.com

temperatures and gas concentrations in a 325 MW power

plant boiler using three coals and their blends [3]. This study

showed that the operation of the plant in the tests worked

well with all coals and blends, except for the case with the

regular coal. Also, the type of coals affected furnace

temperatures and the extent of furnace slagging depended on

the coals used.

Another lab-scale study examined the ignition, flame

stability, carbon burnout, slagging and fouling propensities

for three pairs of Australian black coals and blends [1]. This

study concluded that due to the chemical interactions

between ash particles in the blends, the propensity of

slagging and fouling of a blend cannot be linearly computed

based on weight percentage of component coals.

At Imperial College, three UK coals were blended with

two US coals and two South African coals using an

entrained flow reactor (EFR) [4]. The main conclusion from

this study was that the blends of UK coals with either US or

South African coals demonstrated non-linear behavior, with

enhanced slagging propensity frequently observed with

relatively small additions of an overseas coal to a UK coal.

Inorganic matter in coal is present in three distinct forms

as shown in Fig. 1. These forms are:

1. Organically bonded elements

2. Minerals included within the coal matrix

3. Excluded minerals

During coal combustion, the minerals in coal are

transformed into ash particles that are different in size and

chemical composition. Physical form of ash is classified in

three categories: fumes, fines and coarse particles [5]. The

fines are mostly generated from fragmentation of excluded

minerals or from expulsion of the mineral grains during

combustion. The large ash particles result mostly from the

excluded quartz or clay particles. The particle size distri-

bution of the ash influences the mode of transportation to the

heat transfer surfaces. Large particles are transported by

inertial impaction and turbulent diffusion, whereas, the fine

particles are transported by thermophoretic forces and

Brownian motion [5,6].

Ash deposition on the heat transfer surface is initiated

with the attachment of particles in a dry, semi-molten or

molten state depending upon the composition of the ash

particles, its particle size, the local heat flux, or conden-

sation of volatile constituents [7,8]. Small dry particles may

adhere to the surface due to surface changes. Semi-molten

and molten particles form as a result of fluxing of quartz by

alkalis, partial decomposition of pyrites, the chlorination or

sulfation of heavy metals and alkali earth, metals, or super

cooling due to a fast quench below the melting temperature

of the ash particles [8].

Particles in the free gas stream of the combustion gases

will be at a high temperature 1400–1500 8C. However, as

the particles approach the deposit surface, the decreasing

temperature gradient will cool them near the surface and

they may be molten when they impact the surface. In this

case, for the particles to adhere to the deposit surface they

must be sticky [2]. It has been suggested that a glue is

formed from alkali salts (Fe and Ca) that condense on the

cold tube surface [9]. It is known that the initial layer can

be enriched in sulphate materials or consist of simple

oxides [10].

These semi-molten particles may bond to the tube

surface by surface tension prior to freezing. Submicron

particles and products of condensation generally deposit

uniformly over the surface, whereas larger semi-molten

or molten deposits develop in irregular patterns depend-

ing upon the fluid dynamics and distribution of ash in the

gas stream.

As the initial layer builds up, the temperature of the

deposit surface increases. This temperature change affects

the physical structure of the deposit surface, in which, it

changes from a particulate surface to a partially molten

surface. This will provide a sticky surface on which other

particles can adhere. At this stage, sintering of the deposit

will occur to some extent depending on the state of melting

of the arriving particles. As the sintered deposit continues to

grow, the deposit surface temperature increases, and the

degree of sintering will increase [11]. When the deposit

temperature becomes very high the deposit surface will

become molten and flow. At this stage, the deposit may not

grow any further due to the liquid running off the surface.

Fig. 2 shows the development of the ash deposit layers.

Fig. 2. Schematic of the development of an ash deposit [11].

Fig. 1. The nature of mineral matter, included, excluded, and organically

associated elements.

A. Rushdi et al. / Fuel 83 (2004) 495–506496

The present paper presents an experimental approach to

study the effect of coal blending on ash deposition using a

vertical drop tube furnace (DTF) shown in Fig. 3. Also, it

aims to explain the behaviour of coal blends using the

techniques:

† Slag viscosity and slagging index techniques based on

the ash bulk properties

† Thermomechanical analysis (TMA) technique

2. Experimental procedures and analysis techniques

2.1. Experiments apparatus and conditions

In this study, the ash deposition experiments were

conducted using seven Australian black coals. Eight blends

were generated from those coals with different ratios. The

proximate, ultimate, and ash analyses for all coals and

blends are given in Table 1.

All the coals and blends were crushed and pulverized to

the pulverized fuel (PF) size (75% less than 212 mm). Due

to the difficulties in feeding fine coal particles less than

45 mm, all the coals and blends were wet sieved to remove

the fine particles less than 45 mm. Before running the

deposition experiments, all the coals and blends were dried

at 115 8C for 24 h after the wet sieving tests. Fifteen ash

deposition experiments were carried out. In each exper-

iment, the ash deposit samples were analyzed using

scanning electron microscopy (SEM) to get the average

deposit layer thickness around the alumina probe.

All the experiments were conducted in an electrically

heated laminar flow Drop Tube Furnace illustrated in Fig. 3.

Coal particles were introduced with the primary air into the

furnace using a mechanical feeder to avoid particle size

segregation at feed rates of 20–30 gm coal/h for 3 h.

Secondary air was introduced at a lower point to provide

mixing during the initial stages of coal combustion. An

excess air of 15–20% was introduced for all experiments to

enable a complete combustion environment. The combus-

tion temperature within the 1.2 m long drop tube section

was 1500 8C permitting complete burnout for the largest

coal particles.

At the furnace exit, ash particles were introduced into a

smaller furnace at the bottom of the large drop tube with an

operating temperature around 1000 8C. In this furnace the

rotating alumina probe 10.0 mm OD was oriented perpen-

dicular to the gas flow and the ash particles were deposited

onto the probe uniformly. A small motor was connected to

the probe to provide a low rotating speed 0.6 rpm to provide

a stable condition for the sticking of ash particles on the

probe surface. Other particles that did not intercept or stick

on the probe were collected in a separate tray located at the

exit of the small furnace, as shown in Fig. 3.

Fig. 3. Schematic diagram of the Drop Tube Furnace (DTF), the bottom furnace and the deposition probe.

A. Rushdi et al. / Fuel 83 (2004) 495–506 497

In all experiments, the collection probe was air-cooled. The

air ran through the alumina probe to keep the inner surface of

the probe temperature less than 700 8C to avoid any physical

changes to the probe structure due to the high temperature of

the bottom furnace and the depositing ash particles. Two

thermocouples were located at the inlet and outlet of the

collecting probe to control the temperature changes along the

probe as shown in the bottom section of Fig. 3.

At the end of each experiment, the collecting probe was

removed from the bottom furnace carefully to keep the ash

layer undisturbed. Then the 10 cm probe with the ash

deposit layer was embedded in resin for at least 24 h as

illustrated in Fig. 4.

2.2. Data analysis

The probes were sectioned and polished and analyzed

using scanning electron microscopy. Fig. 5 shows the cross

sectioned samples ready for analysis that on the left side

being coated with a carbon layer for backscattered electron

imaging.

Table 1

Analysis data of the four Australian black coal sets [13]

Coals and blends Set 1 Set 2 Set 3 Set 4

A AB

(67:33)

BA

(33:67)

B AE

(67:33)

EA

(33:67)

E C CD

(75:25)

DC

(50:50)

D F FG

(67:33)

GF

(33:67)

G

Proximate analysis (%, db)

Ash 9.6 9.0 8.4 7.8 8.9 8.1 7.4 3.2 5.6 7.9 12.7 19.2 17.01 14 12.7

VM 45.9 39.6 33.0 26.7 42.3 37.5 33.9 38.5 36.5 34.5 30.6 30.2 30.4 30.5 30.7

FC 44.5 51.4 58.6 65.5 48.8 54.4 58.7 58.3 57.9 57.5 56.7 50.6 52.6 54.6 56.6

Ultimate analysis (%, db)

C 70.0 72.5 75.0 77.5 72.3 75.5 77.8 76.3 75.3 74.3 72.4 62.4 65.9 69.5 73.0

H 5.3 5.1 4.8 4.6 5.2 5.0 4.9 5.0 4.9 4.8 4.7 3.5 3.9 4.3 4.7

N 11.0 1.2 1.5 1.8 1.2 1.5 1.8 2.1 2.0 2.0 1.9 0.9 1.3 1.6 1.9

S 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.4 0.4 0.5 0.6 0.2 0.3 0.4 0.5

O 13.7 11.8 9.8 7.9 11.9 9.6 7.8 13.1 11.8 10.4 7.7 13.7 11.6 9.3 7.2

Ash analysis (%)

SiO2 71.5 67.2 62.7 58.4 66.4 59.5 54.4 51.8 52.5 53.2 54.6 50.8 53.4 56.2 58.8

Al2O3 15.9 19.0 22.2 25.3 17.7 20.2 22.0 18.9 23.4 27.8 36.8 28.0 30.0 32.1 34.1

TiO2 2.2 2.1 1.9 1.8 2.2 2.2 2.2 1.3 1.4 1.4 1.6 2.2 2.1 2.1 2.0

Fe2O3 1.8 3.6 5.4 7.2 4.3 7.6 10.1 10.8 8.8 6.8 2.8 11.1 8.2 5.2 2.3

CaO 4.2 3.7 3.1 2.6 4.5 4.9 5.2 7.1 5.5 3.9 0.7 4.0 2.8 1.6 0.4

MgO 1.3 1.1 0.9 0.7 1.2 1.0 0.9 2.5 2.0 1.5 0.4 1.5 1.1 0.7 0.4

Na2O 0.58 0.47 0.36 0.25 0.49 0.38 0.29 0.77 0.65 0.52 0.27 0.48 0.37 0.27 0.16

K2O 0.27 0.61 0.96 1.30 0.79 1.48 2.00 0.42 0.57 0.71 1.00 0.14 0.52 0.92 1.30

Mn3O4 0.04 0.04 0.04 0.04 0.04 0.05 0.05 0.12 0.10 0.07 0.02 0.21 0.14 0.08 0.01

SO3 1.30 0.96 0.60 0.26 1.39 1.51 1.60 3.00 2.30 1.60 0.19 1.40 0.97 0.53 0.10

P2O5 0.06 0.60 1.16 1.70 0.24 0.47 0.65 2.20 1.69 1.19 0.17 0.02 0.08 0.15 0.21

SrO 0.12 0.13 0.13 0.14 0.14 0.18 0.20 0.16 0.12 0.09 0.01 0.03 0.04 0.06 0.07

BaO 0.09 0.09 0.09 0.09 0.09 0.10 0.10 0.09 0.08 0.07 0.05 0.03 0.03 0.04 0.04

ZnO 0.03 0.03 0.02 0.02 0.03 0.03 0.03 0.02 0.02 0.02 0.02 0.03 0.03 0.03 0.03

V2O5 0.09 0.08 0.08 0.07 0.10 0.10 0.11 0.03 0.04 0.04 0.05 0.05 0.06 0.08 0.09

ðBase=AcidÞSdry 0.028 0.046 0.052 0.082 0.041 0.054 0.078 0.101 0.075 0.047 0.029 0.062 0.072 0.072 0.062

Fig. 4. Schematic diagram of the probe with the deposits layer embedded in

an epoxy.

Fig. 5. Collecting probe polished samples with the deposit layer around the

probe covered by a resin layer, the sample on the left side covered with a

carbon layer. The average sample diameter is 29.5 mm.


2.3. TMA technique

This technique was used to study the thermal behavior of

ash from coals and blends and to compare the ashes in terms

of the potential to form deposits and assess the character of

ash deposits. It involves heating 50 mg ash sample of coals

or blends from 700–1600 8C at 5 8C/min under 100 gm load

and measuring the penetration of a ram into the sample [11,

12]. In this study, a lab-ash produced from all the coals and

blends under 815 8C for two and half hours were used to

generate the TMA percent of penetration curves.

2.4. Slag viscosity calculations ðhÞ

Slag viscosity was used in this study to try to explain the

results obtained from the deposition experiments. Eq. (1)

shows the expression for the standard viscosity curve. This

viscosity model is explained elsewhere [14].

log10

h

TD 2 TS

� �¼

14788

TD 2 TS

2 10:931 ð1Þ

Where

h : is the slag viscosity

TD : deposit temperature

TS : probe surface temperature.

Fig. 6. Two images of coal (A) show sites number 8 and 4, respectively with the deposit particles and the maximum thickness of the deposit layer.

Fig. 7. (SEM) images that show ash deposit layers for the two coals (A) and (B), respectively.

Fig. 8. Deposit layer thickness comparison for coal set (A–B).


2.5. Slagging index

Usually, the slagging indices are based on the ash bulk

analysis. One of these indices is based on the ratio of

the basic to acidic oxides as shown in Eq. (2):

Slagging Index ¼Basic

Acidic

� �Sdry ð2Þ

where

Sdry indicates the dry sulfur content in coal or blend

samples [8,15–17].

Basic: Basic Oxide contents in the form of CaO, MgO,

Na2O, K2O and Fe2O3 etc.

Acidic: Acidic Oxide mainly consists of SiO2, Al2O3 and

TiO2.

3. Experimental results and discussion

3.1. Deposit samples analysis technique

Using SEM, 11 images were taken for each sample to

cover the whole probe surface using the same magnification

for all the images. Fig. 6 shows two images of coal (A).

From each image the maximum deposit thickness was

obtained during the analysis as shown by the arrows. The

final deposit thickness was obtained by taking the average

thickness value from 11 locations of the tube surface all coal

and blend samples.

Fig. 9. (SEM) images that show ash deposit layers for the blends (AB) and (BA), respectively.

Fig. 10. (TMA) percent of penetration curves for coal set (A–B).

Fig. 11. (SEM) images that show a selected ash deposit layer for coal (E).

Fig. 12. Deposit layer thickness comparison for coal set (A–E).


3.2. (DTF) Deposition experimental results

3.2.1. (A–B) coal set

3.2.1.1. Experimental results. The first set of coals consists

of two Australian black coals (A) and (B). Both coals

have close values of ash content 9.6 and 7.8% (ad)

respectively, as shown in Table 1. Coal (A) is rich in

silica and to some extent in calcium oxides, while (B)

coal is rich in alumina and to some extent in iron oxides.

The following images show selected ash deposit layers

from the parent coals (Fig. 7).

From the ash deposition point of view, coal (B), which is

rich in iron oxides 7.20%, should create more ash deposition

problem than coal (A) due to the low melting point of iron

oxides, which may contribute in forming the initial deposit

layer as discussed earlier. The effect of pyrite on ash

deposition in this study is not significant due to the very low

sulfur content in the Australian black coals as shown in

Table 1. This behavior is predicted from the deposit layer

thickness comparison shown in Fig. 8. In this figure, the

maximum deposit thickness for coal (B) is 500 mm, which is

100 mm thicker than coal (A).

The blends (AB) and (BA) have compositions in between

the parent coals with respect to the ratio of blending. Fig. 9

shows ash deposit layers for the blends (AB) and (BA),

respectively. Blend (AB) has more iron and aluminum

oxides than (A) coal, which increased the potential to form

ash deposits for this blend. From Fig. 8, blend (AB) is

showing the worst case of ash deposition with a maximum

thickness around 700 mm. On the other hands, blend (BA)

has more silica than (B) coal comparing to other alkali

oxides. This blend showed the lowest potential to form ash

deposits with a maximum thickness of around 300 mm. This

behavior of coal blends provides a strong evidence of the

non-additive nature of coal blends; therefore, the coal

blends performance may not be the same as that of the

parent coals.

3.2.1.2. TMA comparison. To explain the above results of

(A–B) coal set, (TMA) tests were conducted for each coal

and blend. This test shows the potential of coals and blends

to form ash deposits by characterizing the melting properties

that can be related to the chemical composition of the coal.

All the (TMA) results will be presented with a maximum

temperature of 1000 8C. The reason for that is to match the

conditions of the deposition experiments in the bottom

furnace, where the operating temperature did not exceed this

temperature.

Fig. 10 shows the percent of penetration curves for the

parent coals and the blends of (A–B) coal set. These curves

show that the blend (AB) has the highest potential to form

ash deposits as observed from the (DTF) deposition

experiments [18].

For example, at 900 8C, the percent of penetration value

for (BA, A, B, and AB) was 7.37, 8.89, 12.00, and 16.00%,

respectively. These values show almost the same trend

predicted from Fig. 8. This means that the ash particles of

blend (AB) have the ability to melt at a lower temperature

due to the alkali oxides present which have low melting

points [18,19].

Fig. 13. (SEM) images that show ash deposit layers for the blends (AE) and (EA), respectively.

Fig. 14. (TMA) percent of penetration curves for coal set (A–E).


3.2.2. (A–E) coal set

3.2.2.1. Experimental results. The second set of coals

consists of two coals (A) and (E). Coal (E) has a lower ash

content than coal (A) 7.4% (ad) as shown in Table 1

(Fig. 11). Coal (E) rich in iron, calcium, and potassium

oxides. The following image in Fig. 12 shows a selected ash

deposit layer from coal (E).

If the ash content of coal (E) was the same of coal (B), coal

(E) would show more potential to form ash deposits due to the

higher alkali oxide content, especially 10.1% iron oxide

content comparing to 1.8% for coal (A). But, from Fig. 12, it

is obvious that this coal had developed a lower deposit

thickness 380 mm comparing to coal (A). This is due to the

lower ash content for this coal comparing with the coal (A).

The blends (AE) and (EA) have compositions in between

the parent coals with respect to the ratio of blending. Fig. 13

shows the ash deposit layers for the blends (AE) and (EA),

respectively. Blend (AE) has more iron and aluminum

oxides than (A) coal, which increased the potential to form

ash deposits for this blend. From Fig. 13, blend (AE) is

showing the worst case of ash deposition in this set of coal

with a maximum thickness around 620 mm. On the other

hand, blend (EA) has slightly more silica compared to coal

(E) 59.53 and 54.4%, respectively. This blend shows the

lowest potential to form ash deposits with a maximum

thickness around 340 mm.

3.2.2.2. TMA comparison. Fig. 14 shows the percent of

penetration curves for the parent coals (A) and (E) and the

two blends of (A–E) coal set. These curves show that the

blend (AE) has the highest potential to form ash deposits as

predicted from the (DTF) deposition experiments.

For example, at 900 8C, the percent of penetration value

for (EA, E, A, and AE) was 3.68, 5.71, 8.89, and 16.48%,

respectively. These values show almost the same trend

predicted from Fig. 12. This means that the ash particles of

blend (AE) have the ability to be molten at

lower temperatures. At 867 8C blend AE achieves 10%

penetration, while blend EA needs a temperature 927 8C to

reach that value [18].

3.2.3. (C–D) coal set

3.2.3.1. Experimental results. The third set of coals consists

of two coals (C) and (D) along with two blends (CD) and

(DC). The single coals have different ash contents, as shown

in Table 1; coal (D) has the highest ash content 12.7%, while

coal (C) has only 3.2%. In this coal set, the main factor to

determine the highest potential to form ash deposits was the

basic oxides content in each coal. Coal (C) has 10.8% iron,

7.1% calcium, 2.5% magnesium, and 0.77% sodium oxides

comparing to 2.8% iron, 0.7% calcium, 0.42% magnesium,

and 0.27% sodium oxides in coal (D) that is mainly rich in

alumina oxide. Fig. 15 shows two images for selected ash

deposit layers from coal (C) and (D).

Fig. 16 shows the huge difference in the prediction of the

ash deposit thickness for the two coals (C) and (D). During

the deposition experiments, coal (C) built a deposit layer

more than 700 mm in thickness, which is 60% thicker than

coal (D) deposits.

The first blend in this set is (CD) that consists of 75%

of coal (C) and 25% of coal (D). This blend causes less

Fig. 15. (SEM) images that show ash deposit layers for the two coals (C) and (D), respectively.

Fig. 16. Deposit layer thickness comparison for coal set (C–D).


ash deposits problem than coal (C), due to the increase in

the silica and alumina oxides. At the same time, it

produced deposit layer with 50 mm thicker than coal (D)

as shown in Fig. 16. From the practical point of view,

(DC) blend that has 50% from both single coals had the

lowest potential to form deposits in this set. The

maximum deposit thickness of this blend was around

220 mm, due to the higher content of silica and alumina

comparing with (CD) blend and (C) coal as shown in

Fig. 16. Fig. 17 shows selected ash deposit layers for the

blends (CD) and (DC), respectively.

3.2.3.2. TMA comparison. To validate the above results of

(C–D) coal set, (TMA) tests were conducted for each coal

and blend. Fig. 18 shows the percent of penetration curves

for the parent coals and the two blends. These curves show

that coal (C) has the highest potential to form ash deposits as

predicted from the (DTF) deposition experiments compar-

ing with coal (D) and the two blends.

For example, at 900 8C, the percent of penetration

value for (DC, D, CD, and C) was 1.23, 1.93, 3.49, and

14.25%, respectively. These values show almost the same

trend predicted from Fig. 16. This means that the ash

particles of coal (C) can melt at low temperatures due to

the higher content of basic oxides, which has low melting

points [18,19].

From the percent of penetration of coal (D), it can be

seen that when the temperature is less than 870 8C this

coal behaves similar to the (DTF) deposition experiment

trends. Beyond that temperature, coal (D) will have a

higher potential to form deposits with a maximum percent

of penetration of 9.72% at 1000 8C. This behaviour can be

explained on the basis of the chemical interaction between

the oxides beyond 870 8C producing different alkali

silicates with lower melting temperatures [7,8,15]. This

can increase the potential of coal (D) to form ash deposits

at higher temperatures.

3.2.4. (F–G) coal set

3.2.4.1. Experimental results. The last set of coals consists

of two coals (F) and (G) along with two blends (FG) and

(GF). The single coals have different ash contents, as

shown in Table 1; coal (F) has the highest ash content

19.2%, while coal (G) has 12.7%. Both coals have almost

the same silica content, but the main difference is in the

iron and alumina oxides. Coal (F) has the highest iron

oxide content, while coal (G) has higher silica and alumina

contents of 58.8% and 34.1%, respectively. Fig. 19

shows two images for selected ash deposit layers for the

parent coals.

Fig. 20 shows the deposit thickness difference between

coal (F) and (G). Coal (F) deposit layer is slightly higher that

(G) coal, due to the high iron oxide content.

The first blend in this set is (FG) that consists of 75% (F)

coal and 25% of (G) coal. This blend shows the highest

potential to form deposits as shown in Fig. 20 comparing to

the other blend and parent coals. (FG) has higher potassium

content comparing to coal (F), this may cause to form some

silicates or sulfates with low melting temperatures [7,15].

Fig. 21 shows selected ash deposit layers for the blends (FG)

and (GF), respectively.

Fig. 17. (SEM) images that show ash deposit layers for the blends (CD) and (DC), respectively.

Fig. 18. (TMA) percent of penetration curves for coal set (C–D).


Blend (FG) built a deposit layer with a thickness of

337 mm, which is 80 mm thicker than coal (F). The other

blend built a deposit layer that has almost the same

thickness as coal (F).

From the practical point of view, these two blends are not

recommended to be used in a real coal combustion plant,

due to the fact that they may create higher ash deposition

problems in the real furnace, which will affect the overall

performance of the plant.

3.2.4.2. TMA comparison. Fig. 22 shows the percent of

penetration curves for the parent coals and the two blends in

this coal set. These curves show that up to 875 8C all the

coals and blends have the same potential to form deposits as

shown with the arrow. Beyond that temperature, coal (G)

and (GF) blend show the highest potentials, while (FG)

blend has the lowest potential to form ash deposits.

Although the differences were small, the general trend of

the percent of penetration curves did not match well with the

deposition experimental results for this coal set.

4. Slag viscosities and slagging indices comparison

The slag viscosity and slagging index are based on bulk

ash analysis and can only be used to characterize ash

deposits. These bulk analyses have a non-additive behavior

in first place, and secondly the rate of deposition and

consequently the thermal performance of coals and blends

cannot be predicted by using these techniques [2]. Also, the

(TMA) technique is derived form the ash bulk analysis, but

as discussed earlier, this test was able to predict the blends

behavior due to the interaction between the blend ash

particles during the test. This particle–particle interaction is

a heterogonous interaction that reflects the real ash deposits

situation in PF boilers. While, in slag viscosity measure-

ments and slagging indices this interaction is totally

homogenous.

By using the viscosity model discussed earlier [14], coals

and blends viscosity values were generated. Fig. 23 is

showing the viscosity measurements for (C–D) coal set

only at 1000 8C. By analyzing these values, it can be seen

that the blend behaviour should be in between the parent

coals. The slag indices show the same trend as in Table 1.

So, it can be concluded that the slag viscosity measurements

cannot be used as an indicator to predict the slagging

behaviour of coal blends.

On the other hand, from the deposition experimental

results and (TMA) percent of penetration curves it was

shown that this was not the case and the blends could have a

higher potential to form ash deposits than the parent coals,

such as, (AB), (AE), and (FG) blends. In other cases, it

could have a lower potential, such as, (BA), (EA), (DC), and

(GF) blends.

As a summary, Fig. 24 shows a comparison between the

ash deposit thickness obtained form the deposition exper-

iments with the (TMA) percent of penetration, slag

viscosities, and slag index measurements for the coal sets.

It is obvious that the TMA percent of penetration curves are

Fig. 19. (SEM) images that show ash deposit layers for the two coals (F) and (G), respectively.

Fig. 20. Deposit layer thickness comparison for coal set (F–G).


showing the same trend of the ash deposit thickness

obtained form the deposition experiments.

There was an exceptional case for the blend (CD) when

the TMA percent of penetration lower than expected. By

analyzing the TMA percent of penetration curve in Fig. 18,

it shows that beyond the temperature 945 8C the blend (CD)

will have a lower potential to form ash deposits than the

source coal (D).

From the slag viscosity and index figures, it was obvious

that the general behavior of the blends should be in-between

the source coals in all the cases due to the ash bulk

properties and the trends were not following the exper-

imental deposit thickness trends.

5. Conclusions

From the deposition experimental results, it can be

concluded that the performance of the coal blends and

their potential to form ash deposits cannot be predicted

from the source coals. Some blends can have a lower

potential to form ash deposits during coal combustion,

while, other blends can build a thicker ash deposits

comparing to the source coals. This is due to the chemical

interaction between ash particles within the deposit layer

at higher temperatures that can lead to form ash deposits

with lower melting points that can enhance ash

deposition.

The attempts to assess the blends behavior using the slag

viscosity and the slagging indices techniques that is based

on the bulk ash analysis were not successful. These bulk

analyses can be used only to characterize the ash deposits

and not the ash deposition rate.

However, TMA techniques represent the thermal beha-

viour of the ash deposits. The (TMA) percent of penetration

curves of Australian black coals and blends used in this

study showed the same trend with the experimental ash

deposit thickness obtained from the (DTF) experiments. So,

the (TMA) test was found adequate in assessing the blends

behavior and their potential to form ash deposits due to the

heterogeneous ash particle–particle interaction during the

six hours (TMA) test period.

It is important to emphasis that all the deposition

experimental results were obtained under controlled opera-

tional conditions (deposit surface temperature was around

1000 8C). Further investigations are ongoing including

pilot-scale deposition experiments and computational fluid

dynamics (CFD) modelling.

Fig. 22. (TMA) percent of penetration curves for coal set (F–G). Fig. 23. Viscosity measurements for (C–D) coal sets.

Fig. 21. (SEM) images that show ash deposit layers for the blends (FG) and (GF), respectively.


Acknowledgements

The authors acknowledge the support of CCSD for Coal

in Sustainable Development, Australia for the present

research and Professor Terry Wall for his valuable advice.

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Fig. 24. Comparison between the ash deposit thickness obtained in the deposition experiments with the TMA percent of penetration, viscosity, and slag index

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An Experimental Study of the Effect of Coal Blending on Ash Deposition

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