Power Plant Engineering

426 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 NOx. 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