Sulfur removal at high temperature during coal combustion

in furnaces: a review

Jun Cheng*, Junhu Zhou, Jianzhong Liu, Zhijun Zhou, Zhenyu Huang,Xinyu Cao, Xiang Zhao, Kefa Cen

Clean Energy and Environment Engineering Key Lab of Ministry of Education, Zhejiang University, Hangzhou 310027, China

Received 15 October 2002; revised 19 May 2003; accepted 19 May 2003

Abstract

This paper focuses on sulfur removal technologies in industrial grate furnaces (IGF) and pulverized coal fired boilers (PCFB)

with high flame temperature of 1200–1600 8C. The SO2 reduction without sorbents during coal combustion, thermal stabilities

of sulfation products, kinetics of sulfur retention reactions of sorbents, desulfurization processes, and sulfur removal under

unconventional atmospheres at high temperature are reviewed. It is proposed that some powdered minerals or industrial wastes

with effective metal components may be used as sorbents for sulfur removal to promote cost effectiveness. Because the main

reason that results in low desulfurization efficiencies in IGF and PCFB is the thermal decomposition of the conventional

sulfation product CaSO4 above 1200 8C, it is key to explore new sulfation products that are thermally stable at high

temperatures. It is also necessary to study the kinetic catalysis of alkali and transitional metal compounds on sulfation reactions

under the combustion conditions of IGF and PCFB. The two-stage desulfurization process, in which SO2 is captured by sorbents

both in the coal bed and the combustion gas, is promising for IGF, especially with the humidification of flue gas in a water-film

dust catcher. The staged desulfurization process combined with air-staged combustion, in which sorbents are injected into the

primary air field and upper furnace to capture SO2 under reducing and oxidizing atmospheres, is promising for PCFB. Flue gas

recirculation is also an effective desulfurization process under O2/CO2 conditions and can give a high desulfurization efficiency

of about 80% in furnaces.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: Sulfur removal; High temperatures; Coal combustion; Industrial grate furnaces; Pulverized coal fired boilers

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382

2. SO2 reduction without sorbents during coal combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382

2.1. Blending coals to control the sulfur content. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382

2.2. Self-desulfurization of coal ash during combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383

3. Thermal stabilities of sulfation products of sorbents at high temperature . . . . . . . . . . . . . . . . . . . . . . . 385

3.1. Alkaline earth sulfates as the stable desulfurization products . . . . . . . . . . . . . . . . . . . . . . . . . . . 386

3.2. Calcium aluminate sulfate as the stable desulfurization product . . . . . . . . . . . . . . . . . . . . . . . . . 386

3.3. Calcium silicate sulfate or Fe–Si–Ca melt enwraping CaSO4 as the stable products . . . . . . . . . . 388

4. Kinetics of sulfur retention reactions of limestones at high temperature. . . . . . . . . . . . . . . . . . . . . . . . 389

4.1. Calcinations and sintering of limestones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389

4.2. Sulfation kinetics of limestones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390

4.3. Kinetic catalysis of alkali compounds on sulfation reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . 390

0360-1285/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0360-1285(03)00030-3

Progress in Energy and Combustion Science 29 (2003) 381–405

www.elsevier.com/locate/pecs

* Corresponding author. Tel.: þ86-571-879-52889; fax: þ86-571-879-51616.

E-mail address: juncheng@cmee.zju.edu.cn (J. Cheng).

4.4. Kinetic catalysis of transitional metal compounds on sulfation reactions . . . . . . . . . . . . . . . . . . . 390

5. Sulfur removal technologies in industrial grate furnaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391

5.1. Desulfurization processes in industrial grate furnaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391

5.2. Influencing factors on sulfur removal in blending sorbents with coal on grates . . . . . . . . . . . . . . 392

5.3. Influencing factors on sulfur removal in coal briquettes combustion on grates . . . . . . . . . . . . . . . 393

6. Sulfur removal technologies in pulverized coal fired boilers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393

6.1. Desulfurization processes in pulverized coal fired boilers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393

6.2. Activation methods in preparation of reformed sorbents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394

6.3. Effects of particle size on sulfur removal in the limestone injection process . . . . . . . . . . . . . . . . 394

6.4. Effects of porosity structure on sulfur removal in the limestone injection process . . . . . . . . . . . . 394

6.5. Fouling and slagging problems on hot surfaces in the sorbent injection process. . . . . . . . . . . . . . 394

6.6. Influence of sorbent injection processes on dust catching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395

6.7. Reutilization of in-furnace desulfurization residues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395

7. Sulfur removal under unconventional atmospheres at high temperature . . . . . . . . . . . . . . . . . . . . . . . . 396

7.1. Desulfurization under reducing and oxidizing atmospheres. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396

7.1.1. Reducing and oxidizing atmospheres in industrial grate furnaces . . . . . . . . . . . . . . . . . . 396

7.1.2. Reducing and oxidizing atmospheres in pulverized coal fired boilers . . . . . . . . . . . . . . . 397

7.2. Desulfurization under O2/CO2 conditions by flue gas recirculation . . . . . . . . . . . . . . . . . . . . . . . 398

8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400

1. Introduction

Sulfur pollutants derived from coal combustion are

harmful to the environment. China is foremost in the

world with total SO2 emissions of 23.5 million tons in

1997 [1]. 50% of this comes from power station

pulverized coal fired boilers (PCFB) and 33% from

industrial grate furnaces (IGF). Among the various kinds

of desulfurization technologies, sulfur removal in furnaces

is competitive for controlling the SO2 pollutants derived

from coal combustion, due to the low capital and

operating costs. But it has met difficulties in becoming

popular commercially, because of the initially low

desulfurization efficiency. The main obstacle is that the

flame temperature in PCFB is about 1300–1600 8C and

that in IGF is about 1200–1400 8C, which are much

higher than the thermal stability temperature of the

sulfation product CaSO4. This paper focuses on the latest

development of desulfurization technologies in IGF and

PCFB that solve this problem. The SO2 reduction without

sorbents during coal combustion, thermal stabilities of

sulfation products, kinetics of sulfur retention reactions of

sorbents, desulfurization processes, and sulfur removal

under unconventional atmospheres at high temperatures

are reviewed. The pre-combustion desulfurization

processes such as coal washing, liquefaction and gasifica-

tion are beyond the present scope of discussion, as well as

the post-combustion desulfurization processes such as

scrubbing flue gas with various solutions. Readers

interested in the dry sorbent injection into the back-end

fields such as economizers or air-heaters with tempera-

tures of about 200–500 8C are referred elsewhere [2–6].

For sulfation phenomena in fluidized bed combustion

systems (FBC) with temperatures of about 800–900 8C, a

recent comprehensive review has been given by Anthony

[7] with further detailed information elsewhere [8–19].

2. SO2 reduction without sorbents during coal

combustion

2.1. Blending coals to control the sulfur content

A variety of low- and high-sulfur coals from various

sources can be blended in different proportions to meet

normal and optimal limits for SO2 emissions [20]. As sulfur

contents are directly proportional in blending, the blend

ratio of component coals can be determined based on sulfur

content to meet emission levels [21,22]. A simple goal

programming model is developed with an objective to

provide a decision support system. It determines appropriate

quantities of coal from different stockpiles for a consistent

feeding of blended coal while meeting environmental and

boiler performance requirements [23]. Although a sequence

of linear programs can give a blend ratio to achieve

the predicted sulfur content, it can not ensure a

good combustion or slagging performance of the blended

coal [24].

It is shown by experiment that some characteristic

parameters of a blended coal such as ignition temperature

and burnout efficiency cannot be predicted from component

coals by arithmetic averaging [25]. An overall grey

clustering model that takes into account the main

related parameters such as ash characteristics, mineral

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405382

transformation, and combustion parameters is proposed to

predict the slagging propensity of the blends. The results

suggest that when one coal is blended with another coal with

widely different reactivity or slagging potential, the slagging

grade of the coal blends changes significantly [26].

The mineral transformation during coal blends combustion

is affected by both the mineral species interaction and the

combustion behavior. Some combinations of component

coal’s mineral produce low-melting eutectic minerals at

high temperature and this is the main reason causing the

non-arithmetic averaging of ash fusion temperature of

blends [27,28]. A back-propagation (BP) neural network is

introduced into this coal blending optimization to accurately

predict some non-linear coal properties such as ash fusion

temperature. Compared to the traditional techniques such as

the ternary equilibrium phase diagrams and regression

relationships, the modeling process in the BP networks is

much more convenient and direct, for there is no necessity to

specify a mathematical relationship between the input and

output variables. Moreover, the trained BP model can

always achieve much better prediction results than

traditional methods [29,30]. As a complex regression

method in nature, the model can give rather precise

prediction for the trained coal samples, but less for the

new samples.

A Coal Quality Expert system has been developed by

ABB/CE company for controlling coal cleaning or blending

to provide clean coal products [31]. Based on a non-linear

programming model, a coal blending expert system is

developed to realize the multi-target optimization.

The characteristic parameters such as heating value, volatile

matter, ash content, sulfur content, ash fusion temperature,

ignition temperature and burnout efficiency are taken into

account in the system. In accordance with the actual blend

production process, the mixed discrete-variable

optimization design algorithm is employed to solve the

coal-blending project, which is based on BP neural network

models for some complex quality parameters of blends.

Application of this novel coal blending technology indicates

that it is much more useful and reasonable to guide blend

production than the traditional methods [32–34]. In order to

obtain an optimum blend ratio, a reasonable ratio index of

price to quality also can be introduced, simultaneously

considering the influences of heating value, volatile matter,

ash content and sulfur content [35].

As a viable cost-effective alternative to comply with

environmental considerations, power coal blending process

has been considered extensively. But it is subject to error

and insufficient to consider only the sulfur pollutant

emissions in coal blending, ignoring the changes in

combustion and slagging performances. Also, due to the

restrictions of component coals, it is difficult to realize

the optimization in each parameter of a blended coal. A

case-by-case evaluation must be made in order to determine

whether limitations imposed by blending coals are

acceptable to the user’s situations.

2.2. Self-desulfurization of coal ash during combustion

Most organic sulfur and pyrite in coal are oxidized and

converted to SO2 gas during combustion in furnaces. A small

part of the sulfur may be retained as solid compounds, due to

the contribution of alkaline components such as CaO, MgO,

Al2O3, Fe2O3, K2O, Na2O in coal ash. The alkaline sulfates

are dominant at lower temperatures under oxidizing

conditions, whereas most of them are sulfides under

reducing conditions [36]. It is necessary for boiler operators

to evaluate the conversion percentage of feed sulfur into

gaseous pollutants and select a proper desulfurization

process to meet the Clean Air Act. The desulfurization

property of coal ash during combustion is mainly affected by

the boiler shape, flame temperature, residence time in the

furnace, initial molar ratio of Ca/S and reaction activity of

alkaline components.

With a suitable furnace temperature of about

800–900 8C, a long residence time of particles and a good

gas–solid contact condition, a FBC gives a higher self-

desulfurization efficiency than a IGF or PCFB even for the

same coal. Due to the high flame temperature of about

1300–1600 8C and short residence time of about 1–2 s,

coal ash generally gives a desulfurization efficiency of lower

than 25% in a PCFB [37]. It is indicated that about 70% of

feed sulfur is turned into SO2 gas, less than 10% is retained

in the fly ash and less than 1% is held in the bottom ash.

On the other hand, about 60% of feed calcium is retained in

the fly ash and less than 10% is found in the bottom ash.

The XRD pattern of fly ash derived from Shenmu coal

combustion in a 1000 t/h PCFB is shown in Fig. 1. It is

obtained by calculation that the content of glassy non-

crystal phase is about 70%, the content of self-desulfuriza-

tion product CaSO4 phase is 3.4%, and the content of

remained active CaCO3 and CaO phases are, respectively,

4.7 and 2.2%. However, the content of melted non-crystal

phase is only about 26% and no CaSO4 phase is detected in

the bottom ash [38]. It is pointed out that calcium plays a

dominant role in sulfur retention of laboratory-prepared ash,

while the contributions of other elements are limited. But in

a PCFB, the contribution of calcium is reduced markedly,

while the roles of other alkaline elements are enhanced [37].

In my opinion, this statement is questionable. It is difficult

for the sulfates of other minor elements, such as MgSO4,

Al2(SO4)3, Fe2(SO4)3, K2SO4, Na2SO4 which are less

thermally stable than CaSO4, to act as the sulfation

products during coal combustion at high temperature.

Furthermore, it is found by X-ray powder diffraction

(XRD) analysis that the CaSO4 phase is the main sulfation

product retained in fly ash for a PCFB, and generally no

sulfates of other elements are detected. Therefore, calcium

should play a dominant role in sulfur retention of coal ash

not only in the laboratory but also in a PCFB.

The higher the steam load or flame temperature, the

lower desulfurization efficiency is. It is reported that the

self-desulfurization efficiency of Shenmu coal sharply

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405 383

decreases from 63 to 6% in a tubular furnace, with an

increase in furnace temperature from 800 to 1200 8C.

According to XRD analysis, the content of sulfation product

CaSO4 phase reaches 18% in Shenmu coal ash prepared at

800 8C, while the total content of remaining CaCO3 and

CaO phases is 22%. Neither active CaCO3 and CaO phases

nor sulfation product CaSO4 phase remain in Shenmu coal

ash prepared at 1200 8C, because of their complete

decomposition, as shown in Fig. 2. Most of the free calcium

ions Ca2þ in raw coal ash are converted into a part of the

melted glassy matter and entirely lose their activity [38].

A general trend can be found that the sulfur retention

efficiency of coal ash is promoted by an increase in molar

ratio of Ca/S, as shown in Fig. 3 [37,39,40]. However, there

Fig. 1. XRD pattern of fly ash derived from coal combustion in a 1000 t/h PCFB.

Fig. 2. XRD pattern of Shenmu coal ash prepared at 1200 8C.

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405384

is some scattering in regression of sulfur retention of

lab-prepared coal ash against Ca/S molar ratio, resulting

from that the calcium amount involved in sulfur capture

reaction is different as different coal is ashed and other

alkaline elements in coal may also have made contributions

to sulfur retention. The transformations of alkaline

components in a PCFB are different from those in lab

ashing process, mainly due to high combustion temperature

and short residence time of particles. For example, the ratio

of Ca/S in Shenmu coal (sulfur content ¼ 0.6%), Huangling

coal (sulfur content ¼ 1.1%) and Changguang coal

(sulfur content ¼ 4.3%) is, respectively, 1.5, 1.1 and 0.3.

In a lab-scale tubular furnace, Shenmu coal achieves a sulfur

self-retention efficiency of 23.8% at 1100 8C, which is much

higher than that of Huangling coal (13.3%) and Changguang

coal (13.7%) [38]. For Shikantai coal with a high molar ratio

of 4.0, a self-desulfurization efficiency of 28.4% is obtained

in a lab-scale PCFB [41]. The self-desulfurization efficiency

of Shenmu coal reaches 27–33% in a 1000 t/h PCFB [38].

It is valuable to further investigate the thermal behaviors

of minor elements in coal ash and their affinities for sulfur

during coal combustion. In a full-scale experiment, it is

difficult to obtain a mass balance between the feed sulfur in

coal and the discharge sulfur in flue gas, fly ash and bottom

ash, and so is the calcium that has the ability for sulfur

retention. To explain this non-balance needs further research.

3. Thermal stabilities of sulfation products

of sorbents at high temperature

The desulfurization capabilities of limestones are

strongly affected by thermal conditions, especially the

furnace temperature. The calcium-based sorbents can only

give low sulfur removal efficiencies during coal combustion

in IGF with the flame temperature of 1200–1400 8C or in

PCFB of 1300–1600 8C, that are much lower than in FBC of

850–900 8C. The key problem that controls the sulfur

removal efficiency is the thermal instability of the conven-

tional sulfation product CaSO4 above 1200 8C. In order to

develop highly effective sorbents suitable for sulfur removal

at high temperatures, which is of interest to boiler operators,

how to form thermally stable sulfation products is an

important issue. From the point of view of chemical

thermodynamics, strontium sulfate (SrSO4), barium sulfate

(BaSO4), calcium aluminosulfate (3CaO·3Al2O3·CaSO4)

and calcium silicate sulfate (Ca5(SiO4)2SO4) may act as

thermally stable sulfation products at high temperatures as

Fig. 3. Correlation of sulfur retention by coal ash with initial Ca/S molar ratio.

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405 385

well as CaSO4 and CaS, although these also have some

defects for sulfur removal during coal combustion which

needs further examination.

3.1. Alkaline earth sulfates as the stable desulfurization

products

Magnesium, calcium, strontium and barium all belong to

the alkaline earth metal group. Consequently, they have

similar chemical and physical properties, including the

sulfur retention capability. The thermal stabilities of

alkaline earth carbonates and sulfates gradually increase

from magnesium to barium. Because MgSO4 completely

decomposes at about 750 8C when carbon exists, it is not

suitable to act as the sulfation product under combustion

conditions in IGF or PCFB.

It is well known that CaCO3 that rapidly decomposes at

about 800 8C is often used as sorbent to capture SO2 to

produce CaSO4. It is shown by experiment that the

decomposition percentage of CaSO4 increases with the

increasing furnace temperature. It scarcely decomposes

below 1050 8C and only gives a decomposition percentage

of 13 wt% at 1150 8C. But the decomposition percentage

dramatically increases to 57 wt% at 1200 8C and 96 wt% at

1300 8C, which indicates that 1200 8C is the turning point of

the decomposition of CaSO4 [38]. It is known that the

decomposition of CaSO4 is accelerated in a reducing

atmosphere or at carbon surfaces. With some

impurities such as NaCl, SiO2 or steam, the initial

decomposition temperature of CaSO4 decreases, while the

decomposition rate increases. The traditional limestone,

Ca(OH)2, dolomite and dolomitic hydrate, respectively,

give desulfurization efficiencies of 30–45, 40–60, 40–60

and 50–65% in a furnace injection process [42]. The reason

why dolomite gives a higher desulfurization efficiency

than limestone is that inert MgO crystals disperse the

smaller CaO crystals, which leads to a higher porosity in

calcined dolomite during the sulfation procedure [43].

It should be noted that CaSO4 has three allotropes that

have different structures and chemical properties.

The metastable and soluble g-CaSO4 is derived from the

dehydration of CaSO4·2H2O at 130–200 8C. b-CaSO4 is

produced from g-CaSO4 at 300 8C. The a-CaSO4 obtained

at high temperature is thermally stable from 1210 8C up to

its melting point of 1495 8C [44]. However, which kind of

CaSO4 formed during coal combustion and how to promote

the yield of a-CaSO4 are interesting subjects and need

further research. In a normal Ca–S–O reaction system,

CaS that is thermally stable at high temperature up to

2400 8C in reducing atmospheres [45] can also act as the

sulfur retention product, besides CaSO4. CaS can be

produced from the reaction of CaO with H2S gas in a

reducing atmosphere, or from the reaction of CaSO4 with

carbon at coal particle surfaces. When CaO and some

additives are added into coal briquette at Ca/S ¼ 2, it is

found by XRD and X-ray fluorescence analysis (XRFA)

that the CaS phase is the main sulfation product and retains a

sulfur content of 4.7% in a desulfurization residue obtained

at 1300 8C. This is higher than the sulfur content of 1.8% in

raw coal ash [46]. Because there exist local reducing regions

under otherwise total oxidizing conditions in industrial

furnaces, it is possible to make use of the temperature

and atmosphere distribution to produce CaS or a-CaSO4 as

sulfation products, which can promote SO2 reduction from

the flue gas in IGF or PCFB.

If SrCO3 or Sr(OH)2 is used as sorbents to capture SO2

gas during coal combustion, SrSO4 that is thermally stable

to temperatures up to 1580 8C will act as the sulfation

product. Because the atomic weight of Sr is 2.2 times that of

Ca, strontium compounds are much heavier and more

expensive than the corresponding calcium compounds with

the same molar ratio to sulfur. It is still better to prepare new

sorbents composed of large amount of limestone and a little

of SrCO3. It is shown by experiments that SrCO3 can

promote the sulfur removal efficiency of limestone [47].

When 0.2 wt% SrCO3 and 8 wt% CaO are added into the

coal, a sulfation product SrSO4 phase is detected in

the combustion residue [48]. How to take effective measures

to enhance the yield of the thermally stable SrSO4 during

coal combustion needs further research.

Because BaCO3 rapidly decomposes at about 1300 8C

and BaSO4 is more thermally stable than SrSO4, it is

possible to use BaCO3 as a sorbent for sulfur removal to

obtain the sulfation product BaSO4 at high temperatures.

As shown in Fig. 4, adding BaCO3 with a molar ratio of Ba/

S at 2 gives a high sulfur removal efficiency of 44.5% during

coal combustion at 1250 8C in a tubular furnace. It is higher

than the low sulfur removal efficiency of 10.6% when

CaCO3 is added into the coal with a molar ratio of Ca/S at 2.

But due to the higher atomic weight of Ba, which is 3.4

times that of Ca, it is more practical to blend expensive

barium compounds with limestones for sulfur removal. It is

shown by experiments that barium ion Ba2þ has better

desulfurization capability than calcium ion Ca2þ at high

temperatures of 1200–1300 8C, while carbonate radicle

CO322 excels hydroxide radicle OH2. When BaCO3 content

decreases and CaCO3 content increases, the sulfur removal

efficiency of sorbents gradually declines, with a constant

sum of molar ratios of Ba/S and Ca/S at 2, as shown in Fig. 5

[38]. There is still much to do to reform the composition and

preparation of barium-based sorbents for more effective

sulfur removal.

3.2. Calcium aluminate sulfate as the stable

desulfurization product

In terms of powder diffraction files published by JCPDS

[49,50], calcium aluminate sulfate (3CaO·3Al2O3·CaSO4) is

a ternary compound. It is the main constituent in

sulfo-aluminous clinker, which is used as an expansive

agent in manufacturing expansive cement. It is synthesized

by heating a mixture of lime, alumina and CaSO4 at

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405386

1350 8C. As calcium aluminate sulfate has better thermal

stability than CaSO4, the possibility for the ternary

compound to act as the sulfation product during coal

combustion has been examined by various authors.

The crystal phase 3CaO·3Al2O3·CaSO4 is detected in

the desulfurization residue derived from coal briquette

combustion, as shown in Fig. 6. To a certain extent, the finer

reactant particles and longer reaction time, the more

3CaO·3Al2O3·CaSO4 is formed and less SO2 pollutant is

emitted [51]. Although a mixture composed of pure CaO,

Al2O3 and CaSO4 with the molar ratio of 3:3:1 gives a high

sulfur retention efficiency at about 1300 8C, an additive

Fig. 4. Desulfurization with barium compounds during coal combustion.

Fig. 5. Desulfurization with BaCO3 and CaCO3 during coal combustion.

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405 387

composed of CaO and Al2O3 gives only a low SO2 reduction

from the flue gas during coal combustion [52]. This is

mainly because of the other ensuing mechanisms that inhibit

the formation of calcium aluminate sulfate. Therefore, how

to enhance the yield of 3CaO·3Al2O3·CaSO4 during coal

combustion is a key problem, that still has many difficulties

to overcome in practice.

3.3. Calcium silicate sulfate or Fe–Si–Ca melt enwraping

CaSO4 as the stable products

The high-temperature phase equilibrium in CaO–SiO2–

SO3 system is important in the chemistry of

traditional cements. Calcium silicate sulfate Ca5(SiO4)2SO4

(from a very reliable JCPDS file: 26-1071) [44] and calcium

silicosulfate 2Ca2SiO4·CaSO4 (from an uncertain JCPDS

file: 18-307) [49] are interesting allotropes. It is pointed out

that Ca5(SiO4)2SO4 is synthesized from siliceous lime sand,

which contains 83 wt% CaCO3 and 13 wt% SiO2, with SO3

which is produced in the air-blast injection of fuel-oil with

3.8% sulfur content. It forms in the flame area between

the refractory lining (30% Al2O3 brick) and an outer layer

of larnite in a lime kiln, with a flame temperature of

1500–1600 8C and a kiln-wall coating temperature of about

1100 8C [53]. 2Ca2SiO4·CaSO4 is prepared from calcium

carbonate, crushed quartz and AnalaR CaSO4·2H2O which

are finely ground to pass a 300-mesh sieve, by ignition at

1150 8C for 150 h in a platinum–rhodium resistance furnace

[54]. Compared to 2Ca2SiO4·CaSO4, calcium silicate sulfate

Ca5(SiO4)2SO4 is more thermally stable and can act as a

sulfation product during coal combustion at high

temperatures. However, the formation mechanisms of

Ca5(SiO4)2SO4 during coal combustion, the reaction

schemes, conditions, extents and rates, are far from clear.

Unfortunately, the contribution of Ca5(SiO4)2SO4 to

retain sulfur during coal combustion at high temperatures

is uncertain. Most authors have paid attention to the Fe–

Si–Ca melt that physically enwraps the sulfation product

CaSO4 so preventing its thermal decomposition. It is found

that simultaneously adding Fe2O3 and SiO2 into CaO can

promote the sulfur retention efficiency from 46 to 65%

during coal combustion at 1200 8C. This results from

the formation of a new heat-resisting crystal phase

CaFe3(SiO4)2OH which enwraps the sulfation product

CaSO4 [55]. Because of its crystal structure, CaSO4 does

not form a solid solution with silicates at high temperatures,

but is enwrapped by silicates melt. Some individual phases

such as CaSO4, 2Al2O3·3SiO2 and a-Fe2O3 are found by

XRD analysis in the desulfurization residue derived from

coal combustion, but no new sulfation compound containing

silicon and ferrum is detected. It is shown by scanning

electron microscope (SEM) and energy dispersive X-ray

analysis (EDAX) that silicates with Si and Fe contents

closely enwrap CaSO4 and prevent it from decomposing at

high temperatures [56,57]. Which is the main sulfation

product of silicates, the Fe–Si–Ca melt enwraping CaSO4

or calcium silicate sulfate Ca5(SiO4)2SO4? It is still

unknown. The former is mainly formed through a physical

Fig. 6. XRD pattern of desulfurization residue derived from coal combustion.

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405388

flow and enwrapping process, while the latter is mainly

formed through a chemical combination process.

Because the high-temperature behavior of silicates during

coal combustion is complicated and the identification of

complex desulfurization phases needs advanced analysis

techniques, it is a challenge to clarify the reaction

mechanisms.

The above discussions are based on the viewpoint that

silicon compounds are beneficial to SO2 reduction from the

flue gas. On the other hand, it should be noted that SiO2

cannot only form silicates that enwrap the sulfation product

CaSO4, but also form a CaO–SiO2 phase that has a low

capacity for sulfur retention. The formation conditions of

calcium silicate sulfate and Fe–Si–Ca melt enwraping

CaSO4 are also not clear and affected by the variable

combustion conditions. Hence, adding silicon compounds

into the fired coal sometimes may be harmful to sulfur

removal under certain conditions. Whether the Fe–Si–Ca

sorbents are beneficial or harmful during coal combustion

depend upon factors such as heating rate, furnace

temperature and reaction activity of the silicates. It is

reported that simultaneously adding Fe2O3 and SiO2 into

CaCO3 with the same weight ratio of 0.1% promotes the

sulfur removal efficiency from 41.2 to 53.8% at a constant

heating rate from room temperature to 1080 8C, while

decreases the efficiency at a constant furnace temperature of

1080 8C [58]. When Na2SiO3·9H2O is added into CaCO3

with a weight ratio of 0.6%, the sulfur removal efficiency is

promoted from 16 to 33% at 1200 8C. But when the furnace

temperature is lower than 1100 8C, the sulfur removal

efficiency is decreased [59]. It is found that a kind of clay

mineral can promote the desulfurization efficiency of

CaCO3 from 16 to 31% during coal combustion at

1200 8C, but other clay minerals such as bentonite and

zeolite have little promotion effects for sulfur removal [60].

Because the silicon compounds as inert mineral ingredients

are harmful to coal combustion and promote the sulfur

removal in a limited range, it is not wise to add extra silicon

compounds into the fired coal. There are large amount of

silicon ingredients in coal ash, which can take part in the

desulfurization reactions during coal combustion. It is useful

to study how to activate these silicon ingredients to form

thermally stable sulfation products for effective sulfur

removal in furnaces.

4. Kinetics of sulfur retention reactions

of limestones at high temperature

4.1. Calcinations and sintering of limestones

The porosity structures of calcined limestones are

important for the high-temperature and short-time sulfa-

tion reactions in furnaces, thus it is important to study

the calcinations and sintering performance of limestones.

The calcination mechanisms of limestones vary with

particle diameters and furnace temperatures. In the

calcination procedure of limestones with particle diam-

eters of 9–16 mm, the rate-controlling step is chemical

reaction that follows the single-step nucleating mechan-

ism [61]. The decomposition of limestone with the

particle diameter of about 14 mm is mainly controlled by

chemical reaction at 1000 8C, while the decomposition of

limestone with the particle diameter of about 91 mm at

1100 8C and that with the particle diameter of about

152 mm at 1200 8C is mainly controlled by CO2 gas

diffusion [62]. It is assumed by a reformed partly

sintered globe model that calcination follows one class

of reaction dynamics in a furnace sorbent injection (FSI)

process [63]. A number of efforts have been made to try

to produce calcinated limestones with large surface areas.

The fragmentation behavior is a function of sorbent type,

particle size and particle temperatures in the range of

600–1600 8C, with the sorbent type being the dominant

parameter. It is found that dolostone is more susceptible

to breakage than limestone [64], and the decomposition

of calcium hydroxide produces larger CaO surface area

than calcination of limestone [65,66]. Calcining different

limestones under the same conditions yields different

surface areas and porosities [67,68]. Even for the same

limestone, the surface area varies with the calcining

conditions such as furnace temperature, duration and

particle diameter. It is reported that the surface area of

limestone firstly increases due to calcination and then

decreases due to sintering, with increasing duration at the

temperatures of 900–1100 8C. The calcined limestone

obtains a maximum surface area of 47 m2/g at 1100 8C,

which is higher than that of 27 m2/g at 900 8C [62].

The surface area of calcined limestone depends on the

calcination rate and sintering rate [69]. The sintering of

calcined limestone results in a decrease in surface area and

porosity, which leads to compact crystals with much lower

reaction activities [70]. The sintering characteristics of

limestones are strongly affected by furnace temperature,

residence time and reaction atmosphere, in which

temperature is the most important factor. In general,

sintering rate accelerates with an increase in temperature.

But the rate increases slowly below the critical Tamman

temperature which is 0.4–0.5 times the melting temperature

of solid, and increases rapidly above it. For CaO crystal

whose melting temperature is 2500 8C, the Tamman

temperature is about 1000–1250 8C. It is shown by

experiment that the surface area of calcined limestone

decreases sharply due to the increased sintering above

1100 8C [71]. Prolonged residence time also gives an

increase in sintering rate, especially when the temperature

is above 1000 8C. It is reported that the sintering extent of

CaO particles with diameters of 0.1–0.3 mm is enhanced

by an increase in temperature and duration [72].

The atmosphere and impurities also have important effects

on sintering properties of limestones. It is found that SO2

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405 389

and CO2 gases strengthen the sintering effects, as do

impurities such as Fe2O3, Al2O3 and SiO2 in the parent

limestones [73]. In a limestone injection process, the

presence of CO2, SO2, O2 or CO compositions in the flue

gas produces various degrees of sintering of CaO particles.

Extensive sintering occurs with CO2 compositions. When

O2 and SO2 gases exist in the flue gas, sintering of the dust is

reduced with a higher initial content of the sulfation

product CaSO4. With CO gas in the flue gas, the contrary

is found [74].

4.2. Sulfation kinetics of limestones

The mechanisms of high-temperature and short-time

sulfation reactions in a limestone injection process can be

well characterized by a two-stage process with a very fast

initial surface reaction and followed by a product layer

diffusion controlled step for CaO – SO2 reaction.

The reaction rate is very rapid and about 45% removal is

reached during the first 0.3 s. The reaction then

declines markedly, but a residence time of 2 s is still

necessary to obtain a reasonable SO2 reduction [43,75].

Although calcination increases surface area and utilization

of sorbents, an unreacted nucleus usually remains in the

partly sulfated CaO particle under furnace injection

conditions. This is attributed to plugging of intraparticle

pores and enveloping of particles by the product CaSO4.

Chemical reaction is a rate-limiting step only in the early

stage when a continuous product layer has not been formed.

For the bulk of the reaction when the product layer diffusion

is a rate-determining step, it is important to understand how

and where the solid and gaseous reactants interact.

One opinion is that SO2 and O2 penetrate inwards through

the product layer by gaseous or solid state diffusion, meeting

CaO at the interface between CaO and CaSO4 where

sulfation takes place. This reaction mode has been assumed

whenever the untreated core model or the grain model is

applied. Another opinion is that the solid reactant migrates

outwards through the product layer by ionic diffusion,

meeting SO2 and O2 at the outer surface of the CaSO4

product layer where reaction occurs. Evidence to support the

latter mechanism is from sulfation reactions at 1300 8C.

A combination of the two mechanisms is also possible,

that is chemical reactions may occur at both locations.

Accepting the assumption that chemical reactions take place

at the interface of the reactant and product layer, a

crystallization and fracture model is developed based on

free energy-work analysis. It is found that the product

‘layer’ formed in the early stage of the reaction is not a true

layer, but isolated nuclei and crystals. The ‘continuous’

product layer formed in the later stage is a monolayer of

individual crystals with pore size of 2–3 nm along the

boundaries. The product layer is more porous when

developed from larger stable nuclei formed during the

initial reaction at higher temperature and lower SO2

concentration [76–78].

The sulfation reactions are affected by many operational

factors, such as furnace temperature, residence time, SO2

partial pressure and molar ratio Ca/S. Also, the limestones

from different sources have different calcination and

sulfation performances. It is reported that Changshan

limestone has a better sulfur removal capability than

Jiawang limestone or Tongshan limestone under the same

conditions, due to the smaller particle diameter and larger

surface area resulted from calcination and impurities [79].

4.3. Kinetic catalysis of alkali compounds

on sulfation reactions

It is known that adding alkali compounds such as

NaCl into limestones can promote the sulfur removal

efficiency during coal combustion. It is reported that the

conversion percentage of CaO, which contains NaCl up

to 3 wt%, first increases to 26 wt% and then drops to

18 wt%, whilst the conversion percentage of raw CaO is

20 wt% at 850 8C. Maximum conversion percentage is

obtained when the NaCl content is about 2 wt% [80–82].

In general, the sodium compounds readily vaporize under

combustion temperatures. The reaction of CaO with

sodium induces particle fragmentation and large cracks,

which increases the number of CaO sites that are readily

available. In addition, a sufficient concentration of

sodium impregnated in CaO produces a thin surface

eutectic layer made up of NaCl/CaO that is highly active.

The presence of the liquid phase increases the diffusion

of SO2 in the product layer to inner unreacted CaO that

is inaccessible in the solid phase, and therefore promotes

the overall conversion [83,84].

In my opinion, alkali compounds mainly have a catalysis

effect on sulfation reactions below 1200 8C, where the

dynamic rate is the determining factor for sulfur removal.

When the furnace temperature is higher than 1200 8C,

the thermal stability of the sulfation products becomes the

controlling factor, while the dynamic rate is secondary.

Therefore, simply blending alkali compounds into the fired

coal probably gives little promotion for sulfur removal at

high temperatures in IGF and PCFB.

4.4. Kinetic catalysis of transitional metal compounds

on sulfation reactions

Transitional metal compounds such as FeCl3 and Cr2O3

can also accelerate CaO sulfation reactions during coal

combustion. FeCl3 acts as both a catalyst and an absorbent

in conversion of sulfur in coal to SO2. Due to the catalysis of

FeCl3, the activation energy required for SO2 formation is

reduced. This results in that the two peak temperatures of

sulfur emission are both reduced and their interval is

enlarged. Meanwhile, the first sulfur emission peak derived

from coal combustion with FeCl3 is higher than that without

sorbent. However, due to the subsequent absorption effect of

FeCl3, the second sulfur emission peak is dramatically

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405390

decreased, as shown in Fig. 7 [85,86]. Cr2O3 can promote

the sulfation rate of CaO because of the rapid diffusion of

SO2 into unreacted CaO sites through a liquid low-melting

eutectic phase resulted from the reaction of Cr2O3 with CaO

above 1000 8C [83]. Just like alkali compounds, transitional

metal compounds mainly exhibit their catalysis effects on

sulfation reactions below 1200 8C, where the dynamic rate is

the determining factor for sulfur removal.

5. Sulfur removal technologies in industrial grate

furnaces

5.1. Desulfurization processes in industrial grate furnaces

Simply blending limestones with feed coal at Ca/S ¼ 2

only gives a low sulfur removal efficiency of 15–20%

during coal combustion on grates, which cannot meet the

current requirements of SO2 emissions [45,87]. This is

mainly because of the thermal instability of sulfation

products above 1200 8C and the long duration about 1 h in

the coal bed. Also, the short residence time of SO2/H2S gas

in the coal bed, which is less than 1 s, cannot be ignored

as well as the poor contact between the gases and powder

sorbents.

Coal briquettes have been shown to be effective for

sulfur retention during combustion on grates. An industrial

briquette prepared from 87 wt% hardcoal, 7 wt% molasses

pulp and 6 wt% hydration limestone gives an effective SO2

reduction in a 46.5 MW traveling grate furnace [88]. It is

reported that a SO2 reduction of 40% from the flue gas is

obtained in a 4 t/h traveling grate furnace when reformed

CaCO3 is added into coal briquette [89]. In recent years,

biobriquettes prepared from coal and biomass under high

compression have been developed for clean coal

combustion in small boilers and stoves. The desulfurization

efficiency of biobriquettes is strongly affected by coal type

and it varies in the range 25–67% for eight experimental

coals [90].

For conventional grate furnaces, the combustion con-

ditions in the coal bed are not conducive to efficient sulfur

capture. More suitable conditions for sulfur capture exist

immediately above the coal bed, where combustion gas

temperature is below 1200 8C. Injecting sorbents into the

combustion gas gives higher sulfur removal efficiency rather

than blending sorbents with coal on the grate [91].

Sorbent injection processes for SO2 removal are available

and in continuing development. They are generally

characterized by low capital cost and modest SO2 removal

efficiency of 40–60%. The limestone injection process can

give SO2 removal efficiency about 37% in a 20 t/h traveling

grate furnace [92]. The efficiency of 65% can also be

obtained by a sorbent injection process in a 24 MW

traveling grate furnace [93].

As we know, the thermal conditions have great

influences on SO2 reduction during coal combustion. It is

reported that blending CaO with feed coal or injecting it

directly into the combustion gas only gives a sulfur

removal efficiency of 26.6 and 56.7%, respectively, [94].

A two-stage desulfurization approach that combines

blending and injecting processes is proposed, as shown in

Fig. 8. It promotes the sulfur removal efficiency to about

75% during coal combustion, because SO2 can be both fixed

by the blended sorbents in coal bed and captured by the

injected sorbents in combustion gas [94,95]. Applying

Fig. 7. Effect of FeCl3 on SO2 emission from coal combustion.

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405 391

the two-stage desulfurization process to a 10 t/h traveling

grate furnace gives an in-furnace SO2 reduction of 75–77%

and a total SO2 reduction of 85–90% after flue gas

humidification in a water-film dust catcher [96,97].

5.2. Influencing factors on sulfur removal in blending

sorbents with coal on grates

The added quantity, particle size and composition of

calcium-based sorbents only have certain effects on

the sulfur removal during coal combustion on grates. The

ideal molar ratio of Ca/S is two and its further increase has

little benefit. For example, when the molar ratio of Ca/S

increases from 2 to 6, calcium carbide residue gives little

promotion in sulfur removal efficiency from 23 to 24% [94].

When coarse limestones with particle diameters of 1–3 mm

are blended with feed coal at Ca/S ¼ 2, a SO2 reduction in

the flue gas about 24% is obtained. Even finer limestones

only give sulfur removal efficiency about 30% [98–101].

When sorbent particle size decreases from 75 to 0.1 mm, the

sulfur removal efficiency increases in a narrow range of 6%

[94]. It is favorable to prepare effective sorbents according

to the nature of the sulfur in coal [102]. Calcium carbide

residue as an industrial waste is mainly composed of

Ca(OH)2. It can effectively capture SO2 gas derived from

the oxidization of organic sulfur at 400–500 8C, but is apt to

undergo severe sintering at higher temperature. A limestone

mainly composed of CaCO3 can effectively capture the SO2

gas derived from the oxidization of pyrite at 700–800 8C,

but it hardly capture the SO2 gas released at lower

temperature. As shown in Fig. 9, a mixture of calcium

carbide residue and limestone with a weight ratio of 40:60

gives a good sulfur removal efficiency of 46% at the furnace

temperature of 1200 8C, which is much better than that of

any component [22,96]. When sorbents composed of

industrial wastes are added into the fired coal, a sulfur

removal efficiency of 40% is obtained in a 6 t/h traveling

grate furnace [103]. By XRD analysis, the thermally stable

Fig. 8. Schematic of a two-stage desulfurization process in a traveling grate furnace.

Fig. 9. Effects of calcium-based sorbents on sulfur release during

coal combustion.

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405392

phases of CaSO4, CaS, 3CaO·3Al2O3·CaSO4 and

Ca5(SiO4)2SO4 are found in desulfurization residue

derived from coal combustion on grates, as shown in

Fig. 10. How to promote the yield of the thermally stable

sulfation products is a key problem in resolving the low

desulfurization efficiency on grates [94].

5.3. Influencing factors on sulfur removal

in coal briquettes combustion on grates

Many factors such as coal size, molding pressure,

ignition pattern, particle size and composition of sorbents

have effects on the sulfur retention efficiency in coal

briquettes combustion. A large-sized coal briquette provides

a longer residence time of the SO2 gas in the particles and

more chance for SO2 capture by the limestone [104].

A briquette prepared under lower molding pressure with

finer sorbents has a better desulfurization capability.

Ignition at the bottom of briquette gives a higher sulfur

retention efficiency than ignition at the top, due to the larger

reducing region in the coal bed where CaS is formed as a

thermally stable sulfation product [105]. It is found that

calcium hydroxide and scallop shell both have better

desulfurization capabilities than limestone, because calcium

hydroxide has lower calcination temperature and

scallop shell has larger porosity after calcination [106].

Also, adding some alkali or transitional metal compounds

into limestone gives an increase in desulfurization efficiency

during coal briquette combustion [107].

In summary, if only chemical compositions or physical

properties of sorbents are improved, the ability to promote

sulfur removal efficiency is very limited during coal

combustion on grates. The two-stage desulfurization process

in coal bed and combustion gas is competitive with regard to

cost effectiveness. Although its market niche is likely to be

limited to smaller industrial boilers or existing boilers with

a short residual lifespan, the potential for application

depends on practical factors in plants.

6. Sulfur removal technologies in pulverized

coal fired boilers

6.1. Desulfurization processes in pulverized

coal fired boilers

Sorbent injection locations are critical for sulfur removal

from flue gas in PCFB. In order to determine the optimal

injection location that leads to the highest desulfurization

efficiency, it is essential to perform calculations based on

calcination–sulfation models for the actual industrial

conditions [108]. Several desulfurization processes have

been developed and demonstrated in experimental or

industrial PCFBs. A limestone injection multi-stage burner

process (LIMB) in which limestone is added to the periphery

of a ‘Low NOx’ burner typically reduces SO2 emissions by

30–50% at Ca/S ¼ 2 [45,109]. An upper-FSI process gives

a SO2 reduction of 30–40% at Ca/S ¼ 2 [110]. It is reported

that injecting limestone into a 10 t/h corner-tube furnace

gives an in-furnace SO2 reduction of 33% and a total

SO2 reduction of 60% after flue gas humidification in a

downstream wet precipitator [111]. A LIFAC process

(limestone injection into the furnace and activation of

unreacted calcium) has been developed by the Tempella

Company, which gives a total SO2 reduction of 70%

[112,113]. The largest full-scale demonstration unit of the

LIFAC process has been operated at SaskPower’s 300 MW

Poplar River Power Station in Canada since 1990. A

combined process comprising furnace limestone injection,

in-duct humidification and bag-filter capture gives a SO2

reduction of 80% in a pilot-scale PCFB [114].

It should be noticed that even for the same in-furnace

desulfurization process, differences in SO2 removal

efficiencies among pilot facilities probably occur due to

variations in the following various parameters: temperature

of the injection location, mixing of sorbent particles with

SO2 gas, residence time of sorbents in the furnace, cooling

quench rate of the combustion gases and the sorbent

properties [45]. In general, the turbulent velocity,

temperature field, particle and gas concentration

distribution, and sulfation reactions all have certain effects

on sulfur removal in furnaces [115]. It is found that the

potential core length for the particle-laden jet in a sorbent

injection process is nearly twice as large as that for the

single-phase flow. Recirculation of the particle-laden flow

occurs downstream from the axial location where all

concurrent flow is entrained into the diverging jet before

the jet hits the duct wall [116]. Owing to the great difficulties

in investigating the details of sulfation course by

experimental means under variable combustion conditions,

it is probably more feasible to perform numerical

Fig. 10. XRD analysis of desulfurization residue derived from coal

combustion on grates.

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405 393

simulations to clarify the complex gas–solid reaction

mechanisms in an in-furnace sorbent injection process.

6.2. Activation methods in preparation of reformed sorbents

In order to promote reaction activities of sorbents for

sulfur removal, it is effective to improve their physical

properties by controlling the operational parameters in the

preparation process. It is reported that introducing CO2 gas

into Ca(OH)2 suspension solution produces a reformed

CaCO3 with surface area of 10–70 m2/g, which has higher

sulfation capability than limestone [117]. When calcium

lignosulfonate or ethanol–water solution is used in the CaO

hydration process, the porous structure of calcium

hydroxide is improved and the particle size is decreased,

which leads to better SO2 capture [118]. Furnace sorbent

injection in a slurry form is found to increase sulfur removal

by lessening sorbent sintering due to evaporation of the

slurry droplets and by reducing sorbent particle size in the

slurrying process [119].

Because there are some alkali and alkaline earth

compounds in coal ash, it is feasible to make use of them

to prepare low-cost sorbents for sulfur removal. The key

problem is to activate the effective compositions in coal ash,

generally by a hydration process of Ca(OH)2 and coal ash

under elevated pressure. Hydration promotes the yield of

calcium aluminate silicate and results in an improved

microstructure. The sorbent surface area has a linear

dependence on the ratio of coal ash to Ca(OH)2, hydration

time and other operational parameters [120]. It is reported

that the ratio of coal ash to Ca(OH)2 have significant effects

on the surface area that varies in the range 2.5–64.3 m2/g

and on the calcium content that varies in the range

6–748 mg/l. It has little effect on the pore volume that

averages 1.1 cm3/g [121].

6.3. Effects of particle size on sulfur removal

in the limestone injection process

It is recognized that smaller sorbent particles give higher

SO2 reduction and CaO conversion. A sulfur removal

efficiency of 50% can be obtained by a furnace injection

process with limestone particle size of 5 – 100 mm.

The grinding cost and destruction of pore volume, if sorbents

are ground too fine, decide a minimum mean diameter of

approximately 5 mm [6]. A decrease in limestone mean

diameter from 10 to 1 mm promotes the SO2 capture from 40

to 50% at Ca/S ¼ 2 [42]. The sulfation reaction rate of

ultrafine CaO particles ðdp , 0:1 mmÞ is found to be

5 £ 102 2 5 £ 103 times higher than that of conventional

CaO particles ðdp . 1 mmÞ used in dry injection processes

[122]. It is speculated that diffusion resistance inside a

particle is eliminated with particle size of 1–2 mm, and

further reduction in particle size gives no additional benefit

[42,117].

6.4. Effects of porosity structure on sulfur removal

in the limestone injection process

The particle size and pore size distribution both have

important effects on high-temperature and short-time

sulfation reactions. It is reported that pore diameters of

5–30 nm are desirable for particles larger than 1–2 mm, as

shown in Fig. 11 [123]. For superfine limestones, the pore

volume located in larger pores that are greater than 5 nm

contributes to a high SO2 removal and CaO conversion,

in addition to other parameters such as particle size and

surface area [75]. The plate-like pore geometry developed

by some limestones and calcium hydroxides has been shown

to have several advantages over the cylindrical shape,

producing a higher sorbent reactivity and delaying pore

plugging. Very similar product layer diffusion coefficients

are obtained for calcium carbonate and for calcium

hydroxides, with activation energies ranging from 20 to

27 kcal mol21 [124]. A model based on fractal geometry

suggests that the average pore radius decreases with a

reduction of the fractal dimension, when the specific surface

area of calcined limestone increases. The optimum

utilization is achieved when limestone is calcined to a

structure with fractal dimension between 2.5 and 2.7 [125].

The sorbent utilization achievable for reaction times of

practical interest increases with surface area, because the

formation of a thin product layer over a large surface area

leads to a high conversion of the solid [126]. Some

distributed pore models and random pore models have

been developed to analyze the simultaneous calcinations,

sintering and sulfation processes suffered by small

limestone particles injected into the post-flame zone of

PCFB to reduce SO2 emissions [124,127].

6.5. Fouling and slagging problems on hot surfaces

in the sorbent injection process

The sorbents injected into combustion gas react with SO2

as the combustion gas cools, eventually escaping from the

furnace and being captured by the downstream dust catcher.

This process significantly increases dust burden and may

well enhance fouling and slagging on hot surfaces, as well as

having important implications for particulate collection and

ash handling equipments.

The pyrite, alkali and alkaline earth sulfates and

chlorides in coal ash promote the fouling and slagging on

hot surfaces, while in situ sulfation has some effects on

deposit formation during coal combustion [128]. A sintering

process under limestone injection conditions results in

superheater deposits, where CaO or partly sulfated CaO

particles grow together by interactions with the flue gas.

The gas composition, local temperature and initial degree of

sulfation have influences on the sintering tendency of CaO

particles, in which gas composition is an important

determinant. When air-cooled probes are inserted into the

superheater area of a 500 MW PCFB with time injection in

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405394

the upper furnace in order to collect samples of short-term

deposits, it is found that severe deposition is always

associated with high contents of either CaSO4 or CaCO3

in the deposit [74].

6.6. Influence of sorbent injection processes

on dust catching

A high-temperature sorbent injection has some effects on

particulate properties in the flue gas, such as electrical

resistivity, mass loading, size distribution, morphology and

cohesiveness. The particulate electrical resistivity increases

substantially and the electrostatic precipitator performance

degrades accordingly when a sorbent injection process is

applied to a boiler burning medium- to high-sulfur coal.

Using flue gas humidification or conditioning with SO3 may

bring the resistivity back to an acceptable level. The fly ash

and sorbent mixtures derived from a sorbent injection

process contain rougher particles and tend to be more

cohesive, compared to ordinary fly ash. Sorbent injection

does tend to shift the particle size distribution towards finer

particles [129]. Combining a downstream ceramic filtration

or a flue gas dry scrubbing with a furnace injection process

can improve the sulfur capture and dust collection

performance in a cost effective manner [130,131].

6.7. Reutilization of in-furnace desulfurization residues

In order to identify the potential disposal and utilization

options for the high-volume solid residues derived from coal

combustion with desulfurization processes, it is essential to

evaluate their granulometry, morphology, chemical

composition, mineralogy and behavior to water contact.

Their potential utilization is reported as road subbase,

landfill, embankment, brick material, cement components

and so on [132,133]. The dry desulfurization by-product

from a LIMB process contains substantial portions of

available lime and may prove amenable as a solidifying

agent with the fly ash [134]. The hydration reactions of FBC

ash are dominated by sulfate chemistry that is the formation

of gypsum and ettringite. The FBC ash treated via the

CERCHAR process and combined with pulverized fly ash

appears to make portlandite available for sulfo-pozzolanic

reactions, which result in superior performance in

application as a cement substitute [135]. The fly ash derived

from a conventional PCFB with a lime injection process and

from a circulating FBC with a limestone injection process

presents some similar features: fine granulometry, presence

of anhydrite phase and sulfate content. However, PCFB ash

has many spherical particles and a much higher lime content

due to the lower desulfurization efficiency, while most of

the trace elements in PCFB ash show an inverse concen-

tration-particle size dependence, compared to FBC ash.

The leachates obtained from both samples are rich in soluble

salts of CaSO4 and Ca(OH)2, but arsenic and selenium are

prevented from dissolving by a high lime content. In wetted

PCFB ash, the formation of ettringite crystals stabilizes

calcium and sulfate ions, while arsenate, selenate and

chromate anions are trapped in the crystals. The FBC ash

does not really harden because the lime content is too low,

Fig. 11. Model simulations of the impact of particle size and pore size distribution (10008C, 1000ppm SO2, 2s, Ca/S ¼ 1, 1CaO ¼ 0.5 in the

EFR) [123].

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405 395

and the leached selenium concentration is reduced in wetted

FBC ash [136].

In summary, the sorbent injection process is competitive

in controlling SO2 emissions derived from PCFB, with a low

cost, a simple process and average efficiency. It is necessary,

however, to further clarify the step-by-step courses in

high-temperature sulfation reactions based on the chemical

thermodynamics, kinetics and thermal conditions. In order

to commercially popularize the in-furnace desulfurization

technologies, it is important to solve the fouling, slagging,

ash loading and residue utilization problems.

7. Sulfur removal under unconventional atmospheres

at high temperature

7.1. Desulfurization under reducing and oxidizing

atmospheres

The reactions between limestones and SO2 under

periodically changing oxidizing and reducing conditions in

FBC boilers have been extensively studied by many authors

[137–167]. It is found that the dense bed is under reducing

conditions (PO2 , 10211 bar) for 80–90% of the time,

which may be caused by a bypass of the fluidization air in

bubbles and jets through the dense bed. The variable

parameters of alternating conditions include total reaction

time, cycle period, time fraction and composition of

reducing atmosphere, etc. The sulfation reactions under

alternating conditions follow a conversion circulation, as

shown in Fig. 12 [158]. The consumption of O2 gas and

reducing decomposition of CaSO4 have great effects on the

sulfation efficiency. The CaS produced in a reducing

atmosphere is further converted to CaSO4 or SO2 in an

oxidizing atmosphere. The relative importance of the two

competitive reactions depends on local temperature and

atmosphere, as shown in Fig. 13 [158].

7.1.1. Reducing and oxidizing atmospheres

in industrial grate furnaces

To meet the demand of SO2 reduction during coal

combustion, it is valuable to study the distribution of gas

compositions and bed temperature in grate furnaces. For a

fixed-bed grate furnace ignited at the bottom, the coal bed

can be divided into four layers from bottom to top, the ash

layer, reducing layer, oxidizing layer and fresh fuel layer.

That is to say, there exist changing oxidizing and reducing

atmospheres in the coal bed along the height direction.

As shown in Fig. 14, with an increase in height, the O2

concentration gradually decreases to nearly zero, while the

CO concentration gradually increases to some extent.

The CO2 concentration and bed temperature first increase

and then decrease, reaching the peaks at the interface

between the oxidizing and reducing layers [168].

In the coal bed of a traveling grate furnace, there exist

various oxidizing and reducing conditions, not only in the

height direction but also in the grate-traveling direction.

The coal bed is ignited at the top before an estimated

diagonal line where volatile matter begins to release and

ignite, and it is ignited at the bottom after the dividing line.

There exist both oxidizing and reducing layers in the coal

bed for the two ignition patterns, but the thickness of the

reducing layer for the bottom-ignition pattern is larger than

that for the top-ignition pattern, which is favorable to more

effective sulfur retention [98]. As shown in Fig. 15,

the turning points of the profiles of gas composition just

above the coal bed correspond to the transitions of

the various combustion regions in the coal bed. In the

grate-traveling direction, the O2 concentration firstly

decreases and then increases. On the other hand, the CO,

Fig. 12. Solid phase transformations taking place when limestones

are exposed to alternating oxidizing and reducing conditions in

FBC.

Fig. 13. Phase diagram for the system composed of CaO, CaSO4,

CaS, SO2, CO and CO2 in FBC.

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405396

H2 and SO2 concentrations firstly increase to their peaks and

then decrease [88,168].

The changing oxidizing and reducing conditions in the

coal bed make it possible to capture SO2 on grates more

effectively. The CaS phase produced in the reducing layer

and the CaSO4 phase produced in the oxidizing layer have

been identified in desulfurization residues derived from coal

briquette combustion [46,169]. How to make good use of

these oxidizing and reducing conditions to further promote

the sulfur retention efficiency is an interesting subject that

needs further research.

7.1.2. Reducing and oxidizing atmospheres

in pulverized coal fired boilers

SO2 emissions increase with the fuel equivalence ratio f in

fuel-lean combustion and decrease slightly when f . 1:2

[170]. The possibility of retaining sulfur in a solid form such as

CaS during pulverized coal combustion has been investigated

by studying the oxidation of 10 mm CaS crystals in a

laminar flow oxidation furnace under simulated coal

combustion conditions [151,171]. For the conditions

studied (1127 8C , T , 1477 8C; 0 , PO2 , 0.02 Mpa;

0 , t , 0.25 s), the oxidation results in the formation of

CaO and CaSO4. It is apparent that the intrinsic oxidation rate

of CaS to CaO (2 to 3 £ 1025 mol cm22 s21) is of the same

order of magnitude as the carbon oxidation rate for semi-

anthracites (3 to 5 £ 1026 mol cm22 s21). The CaS oxidation

can lead to: (1) a possible limiting retention level due to

protection afforded by the formation of a CaO–CaSO4 eutectic

at T . 1365 8C; or (2) possible full sulfur loss for the fine CaS

particles because of the high porosity of CaOproduct layer. The

experiments and thermodynamic predictions indicate that

sulfur retentions of 90% can be obtained at particle

temperatures above 1227 8C even with CaS fully exposed to

the flue gas, under fuel-rich combustion. Coal moisture has a

strong effect on the retention level due to the formation of

H2S. For fuel-lean combustion, retention can occur with

calcium dispersed within the burning coal due to a local

reducing atmosphere when carbon is present. Because the

oxidation rate of CaS is slower than that of most coal chars, high

reactive coals may be suitable for sulfur retention as CaS [172].

Based on equilibrium calculations for the coal/water/li-

mestone/air system (50 gas-phase and 6 solid-phase

chemical species), a staged sorbent injection process is

Fig. 14. Distribution of gas compositions and coal bed temperature

in the height direction.

Fig. 15. Distribution of gas compositions in the grate-traveling

direction. Fig. 16. Schematic of a staged sorbent injection process in a PCFB.

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405 397

proposed to control emissions derived from PCFB, as shown

in Fig. 16 [173]. Some sorbents are blended with feed coal

and injected by primary air into the furnace at high

temperature of 1500–1600 8C, while other sorbents are

directly injected into the upper furnace at lower temperature

of 1100–1200 8C. This process involves successive sulfur

retention as an intermediate product CaS in the high-

temperature reducing zone and as a final product CaSO4 in

the low-temperature oxidizing zone. This enhances the

in-furnace desulfurization efficiency to 80 – 85%. In

practice, the staged sorbent injection process is

accompanied by the air-staged combustion pattern that is

employed to reinforce the high-temperature reducing

atmosphere and reduce NOx emissions. The optimal values

for air excess coefficient and molar ratio Ca/S for the first

desulfurization stage in zone I are, respectively, 0.7–0.8 and

1–1.5, which gives a sulfur removal efficiency of 40–50%

in this case. With an increasing amount of O2 and cooling of

the flue gas, the conversion of CaS to CaSO4 increases and is

completed at an air excess coefficient of 1.2 in zone III.

This air requirement and Ca/S molar ratio of 2–2.5 are

optimal for CaSO4-based desulfurization.

It is promising to apply the staged sorbent injection

process combined with air-staged combustion pattern to

power station PCFBs. But it is necessary to further study the

sulfation mechanisms under varying reducing and oxidizing

conditions at high temperatures, especially the formation

and conversion of the intermediate product CaS and the final

product CaSO4.

7.2. Desulfurization under O2/CO2 conditions

by flue gas recirculation

A high CO2 concentration is beneficial to SO2 capture by

a furnace sorbent injection process during coal combustion,

compared to normal concentration at high temperatures and

usual sulfation times [174]. This can be explained by

minimizing sintering of CaO and plugging of reaction

product CaSO4 [175]. When the CO2 partial pressure is

higher than equilibrium, the calcination of limestone is

restrained, and limestone is subject to the direct sulfation

reaction: CaCO3 þ 1/2O2 þ SO2 ! CaSO4 þ CO2 [176].

The rate of direct sulfation does not decrease as much with

sulfation degree as it does CaO–SO2 sulfation, because

sintering is quite mitigated during direct sulfation of

limestone. The diffusivity in the product layer demonstrates

the high temperature dependence and hardly changes with

sulfation degree. This is associated with the fact that the

product CaSO4 layer is porous owing to CO2 formation

[177]. Because the particle size is found to have a very

strong influence on the rate of direct sulfation, which is

much stronger than that on the calcined sorbent reaction,

the conversion levels attained by larger particles are much

lower for the direct sulfation reaction. It is reported that the

conversion level of 53–62 mm Greer limestone can reach

about 79% during direct sulfation for long exposure time at

850 8C. This far exceeds the maximum allowable

conversion level (about 55%) when the pores of calcinated

limestone are completely plugged by the product CaSO4.

However, increasing the particle size from 53–62 to

88–105 mm decreases the conversion level by more than

50%. For the 88–105 and 297–350 mm particles, the effects

of particle size on the conversion curves are in quantitative

agreement on a relative basis [8]. It is reported that

increasing temperature under high CO2 concentration favors

reforming the microcosmic structure of calcinated sorbents

[178,179].

Coal combustion with O2/CO2 is promising because of

its high desulfurization efficiency, in addition to an

extremely low NOx emission and easy CO2 recovery.

In this novel process, 80% of the flue gas is extracted before

the dust catcher and is recycled into the furnace near the

primary air combustor. This results in much higher

concentration of CO2 and SO2 in the combustion gas, as

shown in Fig. 17 [180]. It is indicated that the

desulfurization efficiency in O2/CO2 pulverized coal

combustion, which reaches about 70–80%, is enhanced to

about four to six times over that of conventional pulverized

coal combustion, as shown in Figs. 18 and 19. It is mainly

attributed to the following factors: (1) the practical

residence time of SO2 is extended and SO2 is enriched

Fig. 17. Schematic of O2/CO2 coal combustion.

Fig. 18. Effect of temperature on system desulfurization efficiency

under 1.2 oxygen-fuel stoichiometric ratio, 8 s one-pass residence

time, (Ca/S) ¼ 5 and 1.0 £ 1025 m limestone diameter.

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405398

inside the furnace owing to the flue gas recirculation;

(2) CaSO4 decomposition is inhibited because of the high

SO2 concentration. These contributions are quantitatively

clarified as follows: below 1177 8C, the former contributes

above two-thirds, whereas above 1227 8C, the latter

contributes above two-thirds to the overall increase in

desulfurization efficiency [181]; (3) the direct sulfation of

limestone under high CO2 concentration; (4) the unreacted

calcium in the flue gas is reutilized by recirculation.

The system desulfurization efficiency in O2/CO2 pulverized

coal combustion maintains a high value over a wide range of

temperatures and particle residence times. The flue gas

recirculation decreases the flame temperature, which also

results in a lower yield of thermal NOx. After many cycles of

recirculation, the CO2 concentration in the flue gas reaches

about 80%, which can be extracted and purified by a special

installation before exhaust [175].

Although there still exist many unknowns about the

desulfurization mechanism in O2/CO2 pulverized coal

combustion systems, it is very promising and inspiring to

further develop this novel and artful process that gives a

high desulfurization efficiency of 70–80%. Because

current research on this new process is limited to small

lab-scale furnaces, it is necessary to carry out further

pilot-scale and full-scale experiments to evaluate its

feasibility for power station PCFBs from the technical

and economic view.

8. Conclusions

In-furnace desulfurization is a competitive technology for

controlling the SO2 pollutants derived from coal combustion,

due to the low capital and operating costs contrasted to flue

gas desulfurization techniques. This paper focuses on sulfur

removal processes in IGF and PCFB with high flame

temperatures of 1200–1600 8C, which have much lower

desulfurization efficiencies than those in fluidized bed

combustors. The SO2 reduction without sorbents during

coal combustion, thermal stabilities of sulfation products,

kinetics of sulfur retention reactions of sorbents, desulfuriza-

tion processes in IGF and PCFB, and sulfur removal under

unconventional atmospheres at high temperatures are

reviewed. It is proposed that some powdered minerals or

industrial wastes with effective metal components may be

used as sorbents for sulfur removal to promote cost

effectiveness. It is interesting to study the chemical

compositions, crystal structures, sulfation and kinetic

catalysis properties of the possible substitute sorbents.

Because the main reason for low desulfurization efficiencies

in IGF and PCFB is the thermal decomposition of CaSO4

above 1200 8C, it is key to explore new sulfation products that

are more thermally stable. The kinetic catalysis of alkali and

transitional metal compounds on sulfation reactions is also

important under combustion conditions, considering the

short residence time and decreasing surface area of

calcinated limestones at high temperature. Although SrSO4,

BaSO4, 3CaO·3Al2O3·CaSO4 and Ca5(SiO4)2SO4 are ther-

mally stable at temperatures over 1300 8C, it is difficult to

take effective measures to promote the yields of such

products during coal combustion in furnaces, due to the

low content of such reactants. In the normal Ca–S–O

reaction system,a-CaSO4 is thermally stable under oxidizing

conditions below 1495 8C and so is CaS under reducing

conditions below 2400 8C. It is promising to utilize the

temperature and burning gas distribution to produce CaS and

a-CaSO4 phases as sulfation products in furnaces.

Simply blending sorbents with feed coal to capture SO2

gas during combustion on grates gives desulfurization

efficiency lower than 40%, which also declines with an

increase in furnace size and flame temperature. Burning coal

briquettes with sorbents on grates or injecting sorbents into

the furnace can give moderate desulfurization efficiency that

are generally lower than 60%. It is proposed that the

two-stage desulfurization process, viz. SO2 is captured by

sorbents both in the coal bed and in the combustion gas,

is promising for IGF, which gives an in-furnace

desulfurization efficiency over 70% and a total SO2

reduction of higher than 85% after the flue gas humidifi-

cation in a water-film dust catcher. For PCFB, injecting

limestones into the furnace through a low NOx burner or by

a special nozzle only gives sulfur removal efficiency of

25–50%. Installing a large humidification tower before the

dust catcher, a LIFAC process can give an overall

desulfurization efficiency about 70% after the hydrated

activation of unreacted calcium. It is proposed that the

staged desulfurization process combined with an air-staged

combustion pattern, in which sorbents are injected into the

primary air field and the upper furnace to capture SO2 under

reducing and oxidizing atmospheres, is promising for PCFB,

and so is the desulfurization process by flue gas recirculation

under O2/CO2 conditions. It is valuable to study further

Fig. 19. Variation of system desulfurization efficiency with

residence time of particles at 1400 K under 1.2 oxygen-fuel

stoichiometric ratio, (Ca/S) ¼ 5 and 1.0 £ 1025 m limestone

diameter.

J. Cheng et al. / Progress in Energy and Combustion Science 29 (2003) 381–405 399

the reaction mechanisms and to make full-scale evaluations

of these two advanced dry processes, which can give high

desulfurization efficiencies about 80% in furnaces.

Acknowledgements

This project is subsidized by the Special Funds for Major

State Basic Research Projects (G1999022204) and

supported by the National High Technology Research and

Development Program of China (2002AA529122).

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