Page 153 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN006K-933 July 12, 2001 15:6

Fuel Chemistry 263

Flue gas desulfurization systems in principle use al-
CaCO 3 CaO + CO 2
(1) kaline reagents to neutralize the SO 2 and are classified
MgCa(CO 3 ) MgO + CaO + 2CO 2 .
2 as throwaway or regenerative types. This classification
is based on the product fate. While, in a throwaway pro-
Calcination, an endothermic reaction, occurs at tempera-
cess the product produced by absorbing medium is thrown
tures above 760 C. Some degree of calcination is thought
◦
away (discarded), in a regenerative process, the SO 2 is re-
tobenecessarybeforethelimestonecanreactwithgaseous
generated from the product.
sulfur dioxide. Calcined limestone is porous in nature due
Most of the flue gas desulfurization systems operating
to the voidage (pores) created by the expulsion of carbon
intheUnitedStatesuselimestoneorlimeslurryscrubbing.
dioxide.
In this system, limestone is finely ground (90% passing
Capture of the gaseous sulfur dioxide is accomplished
though a 325-mesh screen or 45 µm) and made into slurry.
via the following reaction, which produces a solid product,
This slurry is finely sprayed in the absorption (scrubber)
calcium sulfate:
column. This slurry absorbs SO 2 in water as shown here
CaO + SO 2 + 1/2O 2 CaSO 4 . (2)
SO 2 (g)+H 2 O <=> SO 2 ·H 2 O (dissolving gaseous SO )
2
The reaction of porous calcium oxide with sulfur diox- 3 −
+
ide produces a continuous variation in the physical struc- SO 2 ·H 2 O <=> H + HSO (hydrolysis of SO ).
2
ture of the reacting solid as the conversion proceeds. Be-
This slurry with absorbed SO 2 is sent to a retention tank
cause of the relatively high molar volume of CaSO 4 of
where the precipitation of CaSO 3 , CaSO 4 and unreacted
CaO, the pore network within the reactant can be pro-
CaCO 3 occurs. Calcium carbonate has low solubility in
gressively blocked as conversion increases. For pure CaO
water. Low pH promotes dissolution of CaCO 3 but low pH
prepared by the calcination of reagent grade CaCO 3 , the
also lowers solubility of SO 2 in the scrubber. Therefore,
theoretical maximum conversion of CaCO 3 to CaSO 4 has
a careful balance of pH is needed for this system. The
been calculated to be 57%. In practice, the actual conver-
following reactions take place in the retention tank, where
sion obtained using natural limestones is much lower due
to the nature of the porosity formed upon calcination. Cal- H + CaCO 3 <=> Ca 2+ + HCO 3−1
+
ciumutilizationsaslowas15–20mol%havebeenreported (dissolution of limestone)
in some cases, although utilizations of about 30–40 mol%
are typical. MgO will not react with sulfur dioxide at tem- Ca 2+ + HSO 3− + 2H 2 O <=> CaSO 3 ·2H 2 O + H +
peratures above 760 C; therefore, the sulfation reaction of (precipitation of calcium sulfate)
◦
dolomite is basically the reaction of sulfur dioxide with H + HCO 3−1 <=> CO 2 ·H 2 O
+
calcium oxide. (acid-base neutralization)
In pressurized fluidized bed combustion, however, the
partial pressure of CO 2 is so high that calcination does CO 2 ·H 2 O <=> CO 2 (g) + H 2 O (CO 2 stripping),
not proceed because of thermodynamic restrictions. For
the overall reaction being
example, at 850 C, calcium carbonate does not calcine if
◦
the CO 2 partial pressure exceeds 0.5 atmosphere. Under CaCO 3 + SO 2 + 2H 2 O CaSO 3 ·2H 2 O + CO 2 .
these conditions, the sulfation reaction is
CaCO 3 (s) + SO 2 + 1/2O 2 CaSO 4 + CO 2 .
B. Nitrogen Oxides
Increasing the pressure from one to five atmospheres Nitrogen oxides, NO and NO 2 , collectively known as
significantly increases the sulfation rate and calcium NO x , are formed during combustion in three ways. About
utilization. 85–90% of the NO x emitted from the combustion cham-
ber is NO and 5 or 10% as NO 2 . The three types of NO x
that form are by thermal, fuel, and prompt mechanisms.
3. Flue Gas Desulfurization
Nitrogen oxide emissions from coal combustion can
Dry sorbent use in the case of pulverized coal combustion occur from three sources. Thermal NO x primarily forms
units is not efficient because of the operating tempera- from the reaction of nitrogen and oxygen in the combus-
ture of the combustion chamber. At higher temperatures tion air. The Fuel NO x is a component that forms mainly
(>1100 C), CaO is known to sinter with a loss in the from the conversion of nitrogen in the fuel to nitrogen
◦
porosity and, therefore, the conversion. Also, at high tem- oxides. Prompt NO x is formed when hydrocarbon radical
peratures, CaSO 4 is not stable and decomposes to CaO fragments in the flame zone react with nitrogen to form
and SO 2 limiting this technology to FBC units. nitrogen atoms, which then form NO.