Page 152 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN006K-933 June 29, 2001 12:14

262 Fuel Chemistry

motion, molecular diffusion, and inertial impaction are, to sulfur dioxide emissions are from electric utilities burn-
some extent, controllable by the design of the unit. The ing fossil fuels. Sulfur, 97–99%, in the fuel forms SO 2
chemical factors associated with the ash, particularly the and a small fraction of it is oxidized to SO 3 . The equi-
alkalies in the flue gas which are condensed on the sur- librium level of SO 3 in fuel lean combustion products is
face of fly ash particles and the sintering characteristics determined by the overall reaction
of these particles depend upon the mineral species in the
coal. Two alkalies, sodium and potassium, are generally SO 2 + 1/2O 2 <=> SO 3 .
held responsible for superheater and reheater fouling, with
The equilibrium constant (Kp = 1.53 × 10 − 5 e 11,760/T
sodium given the most attention.
atm −1/2) indicates that the SO 3 formation is not favor-
A fouling index has been developed to correlate sinter-
able at combustion temperatures. SO 3 concentration in the
ing strength more closely with total ash composition:
flue gas is typically 1–5% of the SO 2 .
Fouling Index = Base/Acid × Na 2 O, The SO 2 control techniques can be classified into
precombustion, during combustion, and postcombustion
where the base is computed as Fe 2 O 3 + CaO + MgO +
methods.
Na 2 O + K 2 O, and the acid as SiO 2 + Al 2 O 3 + TiO 2 is ex-
pressed as weight percentage of the coal ash.
The fuel ash properties which boiler manufacturers gen- 1. Precombustion Control Methods
erally consider important for designing and establishing
Precombustion control technologies can be fuel switching
the size of coal-fired furnaces include:
or fuel desulfurization. Since sulfur dioxide emissions are
directly proportional to the amount of sulfur in the fuel,

The ash fusibility temperatures switching to a low sulfur fuel is a choice when permitted.

The ratio of basic to acidic ash constituents However, fuel switching may not be an alternative if the

The iron/calcium ratio regulations require a certain percentage of reduction in

The fuel–ash content (mg/MJ) SO 2 emissions regardless of the fuel sulfur content. Fuel

The ash firability desulfurization involves reduction of sulfur from the fu-
els prior to combustion. Sulfur in coals is present in three
In addition to the furnace design, the ash content and char- forms: pyritic, organic, and sulfatic. Pyritic sulfur is the
acteristics also affect the superheater, reheater, and con- sulfur that is present as a mineral pyrite (FeS 2 ). The den-
vective pass heat transfer surface design features. These sity of pyrite is higher than coal. Therefore, when coal is
characteristics along with a few others translate into the crushed and washed in water, the particles that are rich
relative furnace sizes. It is recognized that the ash deposi- in the pyrite sink and the particles that are rich in carbon
tion is much better understood if the inorganic constituents (organic) float. The sinks are rejected and the floats with
in the coal are known rather than the ash composition af- less sulfur are used as clean coal. The degree of desulfu-
ter oxidation of the minerals. With the advancement in the rization depends on the distribution of sulfur in the coal.
computing power and image analysis in the late 1980s The finer the pyrite particles are, the more difficult the
and early 1990s, Computer Controlled Scanning Elec- coal is to clean. These characteristics are of a coal that can
tron Microscopy (CCSEM) has been a very useful tool be evaluated by washability curves based on a standard
in predicting ash behavior in furnaces. The determination float and sink analysis. The organic sulfur is present in
of organically associated inorganic constituents is being coal bonded to the organic structure of the coal. This form
made by chemical fractionation. These advanced charac- of sulfur is finely distributed throughout the coal matrix,
terization methods are now being more effectively used to and therefore is not possible to remove by physical coal
help understand and solve the ash depositional problems in cleaning.
furnaces.

2. During the Combustion Control Method
VII. ENVIRONMENTAL ISSUES
In an FBC system, limestones or dolostones are introduced
IN COAL COMBUSTION
into the combustion chamber along with the fuel. In the
combustion chamber, limestones and dolostones undergo
A. Sulfur Dioxide Emissions
thermal decomposition, a process commonly known as
Coal contains sulfur and nitrogen which vary typically calcination. The decomposition of calcium carbonate, the
between 0.5–5% and 0.5–2%, respectively. Sulfur upon principal constituent of limestone, proceeds according to
combustion forms sulfur dioxide. About 65% of the total the following equation: