a study on the calcination and sulfation behavior of limestone during ...

Juan chen et al.
Oxy-fuel combustion is a promising technology for coal-fired power plants to mitigate
its greenhouse gas emission in the short/medium term. In this technology, the flue gas is
recirculated to mix with the air-derived pure oxygen for combustion. The resulting CO2
concentration in flue gas can reach up to 95%. Since a large portion of flue gas is
recycled, the gaseous impurities such as SO2/SO3 within it can gradually accumulate in
a furnace, which in turn potentially causes extra fouling and tube corrosion problems
through the deposition of sulfates, as well as affects the purity of the CO2 which is
eventually sequestrated. To solve these problems, an efficient capture of SO2 for flue
gas clean-up is pivotal for an oxy-fuel system.
In-furnace capture of SO2 through the use of limestone is a promisingly cost-effective
manner for flue-gas clean-up. As has been confirmed for the conventional air
combustion, the sulfur removal of limestone involves two consecutive steps: calcination
of limestone and sulfation of calcium oxide, (Borgwardt 1970) according to
Calcinatio n : CaCO3
→ CaO + CO2
(1)
Sulfation : CaO + SO2
+ O2
→ CaSO4
(2)
Direct Sulfation : CaCO + SO + 0.
5O
→ CaSO + CO
(3)
3
2
The calcination of CaCO3 by reaction (1) is affected by two factors, CO2 partial
pressure and particle temperature. The high CO2 partial pressure in an oxy-fuel furnace
is expected to play a negative role. However, it is unclear if the increase in particle
temperature through radiative heat transfer from coal flame will offset this negative role.
Regarding the sulfation reaction, apart from reaction (2), the direct sulfation of
limestone according to reaction (3) is also considerable for sulfur capture, in particular
during oxy-fuel combustion. A generalised understanding has yet to be achieved in the
literature. Liu and his co-workers (2000) considered that, as the sintering extent of
limestone in CO2 is decreased substantially, the direct sulfation of limestone exhibits a
comparable rate to that of the sulfation reaction of calcium oxide in O2/CO2. Fuertes et
al (1994) and Snow et al (1988) suggested the kinetic control and fast diffusion of gases
for reaction (3). Our previous result also confirmed the large potential for reaction (3)
in O2/CO2 under certain conditions (Chen et al., 2011). In contrast, Iisa and Hupa (1990)
hypothesized the diffusion limit for the direction sulfation of limestone, and some other
researchers also proposed the comparable significance of reaction control and
diffusivity limit (Hajaligol et al. 1988; Iisa and Hupa, 1992). Moreover, the influence of
the coal-bound inorganic metals on the behavior of limestone in O2/CO2 mixture has not
yet been studied. At the typical pulverized coal-fired temperatures, the interaction
between calcium and alumina/silicon for the formation of molten eutectics is a
significant cause inhibiting the sulfation capture efficiency in air (Zhang et al. 2002).
The present <strong>study</strong> aims to address these knowledge gaps to clarify the transformation
behavior of CaCO3 in oxy-fuel combustion. In particular, the interaction of coal-bound
metals with limestone has been elucidated in detail, through the combustion of
limestone mixed with a low-rank lignite at different mass ratios. The lignite used is
Victorian brown coal rich in organically bound metals. Its combustion at high
temperature is expected to deliver plenty of fine and even ultra-fine ash particles which
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