Power Plant Engineering

partly as NO
x
upon combustion and partly as N
2
. Staged combustion generally promotes the N
2
end
point over the NO
x
end point. Mineral matter in the oil can cause several problems, including deposits
and flyash emissions. The deposits can promote reactions, which affect the other pollutants, such as
SO
2
.
About 65% of the oil used in the US is used by the transportation sector that about 25% is used by
industry, and that most of the balance is used for residential and commercial heating. Very little is used
in the US for electrical generation. Most of the discussion in the paragraph immediately above pertains
to industrial, utility, and R/C burning of oil. However, the biggest sector involved in the control of
emissions from oil burning is the transportation sector, because the fuels burned by this sector (i.e.,
gasoline and diesel fuel) are mainly derived from oil. Thus, advancements in engine combustion tech-
nology and catalytic exhaust treatment are very important to the control of emissions from oil burning. A
major challenge is the development of automotive exhaust treatments which work well for fuel-lean
engine combustion. This is important because engines are thermodynamically more efficient when op-
erated with excess air. However, the present 3-way catalyst has a poor NO
x
reduction efficiency when
the engine is operated lean. Another major challenge is control of NO
x
and soot particulate from diesel
engines. The oil problem is being addressed with advanced combustion technology and exhaust particle
traps.
13.13 POLUTION DUE TO SOLID FUEL
The solid fuel of primary interest is coal. In power plant generally
coal is burned for electrical generation.
Coal is burned in several ways, depending on coal particle size. Lumps of coal, including coal
particles larger than about 0.25 inch in diameter, are spread on a grate and burned. Conceptually, this is
not unlike the burning of wood logs in a fireplace, though industrial and utility coal grate burners are
substantially more sophisticated than a fireplace. There is a limit to the size of a grate, (around the order
10 meters square is maximum). Grate coal burners are called Stokers. Stokers come in several forms,
and are used for small and medium size coal combustion systems.

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POWER PLANT ENGINEERING
Coal particles of about 1/8th inch diameter are burned in fluidized bed coal combustors (FBC).
In this system, the coal particles are injected into a bed of limestone or dolomite particles strongly
churned or agitated by blowing air through the bed from below. The bed has the appearance of "boiling."
The fluidizing medium is limestone (or dolomite), rather than ordinary sand (which could be used), so
that the sulfur from the coal is taken up by the limestone and converted into calcium sulfate particles
(CaSO
4
), which are periodically or continuously removed from the bed. Also, thermal NO formation is
very low because of the relatively low combustion temperature of the bed (about 800 to 900°C). In
addition, flyash emission is controlled to some extend by retaining the ash in the bed. However, the
nitrogen contained in the coal, i.e., the fuel-bound organic nitrogen, converts to nitrous oxide (N
2
O). If
the bed temperature were higher than 800-900°C, the fuel-bound nitrogen would convert to NO
x
. Fluid-
ized bed coal combustion is a relatively new commercial technology, which appears to be favored more
in Europe than the US.
Although the Stoker and Fluidized Bed technologies are important, the majority of the coal burned
in steam-electric power plants is burned in Pulverized Coal Combustion Furnaces. Pulverized coal is
coal, which has been ground (i.e., pulverized) into a fine dust, of about 70 micrometers (i.e., microns)
mean diameter. Pulverized coal combustors are suspension burners - that is, the coal dust is carried by
the furnace air and gases and burned in suspension.
In all solid fuel burners, the fuel undergoes heating and devolatilization as the first stage of the
burning process. Devolatilization is analogous to the vaporization process for the liquid fuel.
Devolatilization means that part of the solid fuel decomposes and forms gases and tars upon heating.
The fraction of the fuel, which forms volatiles, and the composition of the volatiles depend on the nature
of the fuel and the particle heating process. Typically, a combustion coal is about 50% volatile matter by
weight. The volatiles released from the coal are made up of the following components: CO, H
2
, light
hydrocarbon gases (such as methane, ethane, ethylene, and propane), oxygenated hydrocarbons, me-
dium molecular weight hydrocarbons, high molecular weight hydrocarbons called tars (which are vapor
at furnace temperatures), and inert gases (such as CO
2
and H
2
O). The volatiles burn via a mechanism
similar to that described above for the Alkanes. Typically, devolatilization occurs within 100 millisec-
onds, and the volatiles burning occur within a few milliseconds. The particles remaining after
devolatilization are composed of char (i.e., mainly carbon) and ash (i.e., mineral matter).
Following the release of volatiles from the solid fuel particle, it is possible for oxygen to diffuse
to the surface and oxidize the char particle. Char particle oxidation requires about 100 to several hun-
dred milliseconds of time. The furnace volume has to be big enough to accommodate this. The follow-
ing reactions happen at the char particle surface (including surfaces created by fissures in the particle):
C + (1/2) O
2
→
CO
C + O
2
→
CO
2
As the char particle burns away, the mineral matter imbedded in the coal as small inclusions gets
very hot, becomes molten, and fuses together to form liquid ash particles, which ultimately solidify.
Typically, 3 to 5 ‘big’ ash (i.e., flyash) particles form per original pulverized coal particle. Additionally,
the volatile mineral matter vaporizes during the devolatilization and char burning stages, and forms tiny,
sub-micron particles upon nucleation and condensation. The sub-micron particles can be more of a
problem than the ‘big’ (1 to 10 micron) particles, because the sub-micron particles tend to carry dispro-
portionate amounts of the toxic heavy metals found in coal. A toxic heavy metal emitted as a gas is
mercury.
It should be noted that coal contains about every element found in nature. Although C, H, and O
are the major elements found in coal, there can also be significant amounts of S and N. Some coals have

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POLLUTION AND ITS CONTROL
427
as much as 10% S by weight. The sulfur is contained in both the organic and inorganic fractions of the
coal. It is possible to remove some of the sulfur containing ‘rocks’ found with the coal using gravity
separation methods. Processes have been studied for removing additional amounts of S from coal; how-
ever, these processes are not widely used commercially.
Coal contains about 0.5 to 1.5% by weight N bound into the organic structure of the coal. When
the coal is heated, much of the N is released during the devolatilization stage as hydrogen cyanide
(HCN), other cyano species, and as ammonia (NH
3
). These species are rapidly converted into NO or N
2
,
and thus are not emitted directly from the combustor.
Some coals (e.g., British coals) contain chlorine, which upon combustion can be emitted as
hydrogen chloride (HCl).
Typically, coal has about 10% by weight mineral matter. This mineral matter is high in Si, Al, Fe,
and Ca and these are the primary elements found in the ‘big’ flyash particles. However, many other
inorganic elements are associated with the mineral matter of coal, including Mg, Na, and K. Toxic
metals are found in trace amounts in coal, including Ni, Pb, Cd, Cr, As, Se, and Hg. Generally, the toxic
metals are found associated with the sub-micron flyash, though Hg will be emitted as a gas. Radioactive
isotopes are also emitted from coal combustors. It has been estimated that more radioactivity is emitted
from a coal combustion power plant than from a nuclear power plant. Emission control is practiced as
follows for Pulverized Coal Furnaces:
A significant fraction of the installed cost of a coal-fired electrical generating station is devoted
to environmental control — about 30 to 40%. Also, a significant fraction of the operating and mainte-
nance cost is devoted to environmental control. Sulfur (i.e., SO
2
emission) is controlled by burning a
low sulfur coal, by pre-combustion coal cleaning, and by flue gas desulfurization.
NO