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TRANSCRIPT
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: [email protected] (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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