Fuels for Furnaces.

FUELS

The selection of the best fuel should be based upon a study of the
comparative prepared costs, cleanliness of operation, adaptability to
temperature control, labor required, and the effects of each fuel upon the
material to be heated and upon the furnace lining. Attention must be
paid to the quantity to be burned in each burner, the atmosphere (fuel /
air ratio) desired in the furnace, and the uniformity of temperature distribution
required, which determines the number and the location of the
burners. Common methods of burning furnace fuels are as follows:

Solid Fuels (Almost Entirely Bituminous Coals)

Coal was once a common fuel for industrial furnaces, either hand-fired,
stoker-fired, or with powdered coal burners. With the increasing necessity
for accurate control of temperature and atmosphere in industrial
heating, coal has been almost entirely replaced by liquid and gaseous
fuels. It can be expected that methods will be developed for the production
of a synthetic gas (natural-gas equivalent) from coal.

Liquid Fuels (Fuel Oil and Tar)

To burn liquid fuels effectively, first it is necessary to atomize the oil
into tiny droplets which then vaporize and burn. Atomization can be
accomplished mechanically or with the aid of steam or air. With heavy
oils and tar, it is important to maintain the proper viscosity of the oil at
the atomizer by preheating the fuel.
For larger industrial burners, combustion air is supplied by fans of
appropriate capacity and pressure. Combustion air is induced with some
smaller burner designs.

Gaseous Fuels

Burners for refined gases (natural gas, synthetic gas, coke-oven gas, clean
producer gas, propane, butane):
Two-pipe systems: Include blast burners (open or closed setting), nozzle
mixing, luminous flame, excess air (tempered flame), baffle, and
radiant-tube burners, all for low-pressure gas and air.
Premix systems: Air and gas mixed in a blower and supplied through
one pipe.
Proportioning low-pressure mixers: Air and gas supplied under pressure
and proportioned automatically (air aspirating gas or gas inspirating
air). The resulting mixture is burned in tunnel burners, radiant-cup,
baffle, radiant-tube, ribbon, and line burners.
Pilot flames are generally used to ensure ignition for gas and oil
burners. Insurance frequently requires additional safety provision in two
main categories: an interconnected pressure system to prevent lighting
if any burner in a zone is open, and burner monitors using heat or light
to permit ignition.
Burners for crude gas (raw producer-gas, blast-furnace gas, or cokeoven
gas):
Simple mixing systems with large orifices and simple mechanisms
which cannot become clogged by tar and dirt contained in these gases.
Separate gas and air supplies to the furnace, with all mixture taking
place within the furnace.

Coal and its Types

Coal is a black or brownish-black combustible solid formed by the 
decomposition of vegetation in the absence of air. Microscopy can
identify plant tissues, resins, spores, etc. that existed in the original
structure. It is composed principally of carbon, hydrogen, oxygen, and
small amounts of sulfur and nitrogen. Associated with the organic matrix
are water and as many as 65 other chemical elements. Many trace
elements can be determined by spectrometric method D-3683. Coal is
used directly as a fuel, a chemical reactant, and a source of organic
chemicals. It can also be converted to liquid and gaseous fuels.



Meta-anthracite is a high-carbon coal that approaches graphite in
structure and composition. It usually is slow to ignite and difficult to
burn. It has little commercial importance.
Anthracite, sometimes called hard coal, is hard, compact, and shiny
black, with a generally conchoidal fracture. It ignites with some difficulty
and burns with a short, smokeless, blue flame. Anthracite is used
primarily for space heating and as a source of carbon. It is also used in

electric power generating plants in or close to the anthracite-producing
area. The iron and steel industry uses some anthracite in blends with
bituminous coal to make coke, for sintering iron-ore fines, for lining
pots and molds, for heating, and as a substitute for coke in foundries.
Semian thracite is dense, but softer than anthracite. It burns with a
short, clean, bluish flame and is somewhat more easily ignited than
anthracite. The uses are about the same as for anthracite.
Low-volatile bituminous coal is grayish black, granular in structure and
friable on handling. It cakes in a fire and burns with a short flame that is
usually considered smokeless under all burning conditions. It is used for
space heating and steam raising and as a constituent of blends for improving
the coke strength of higher-volatile bituminous coals. Low volatile
bituminous coals cannot be carbonized alone in slot-type ovens
because they expand on coking and damage the walls of the ovens.
Medium-volatile bituminous coal is an intermediate stage between
high-volatile and low-volatile bituminous coal and therefore has some
of the characteristics of both. Some are fairly soft and friable, but others
are hard and do not disintegrate on handling. They cake in a fuel bed and
smoke when improperly fired. These coals make cokes of excellent
strength and are either carbonized alone or blended with other bituminous
coals. When carbonized alone, only those coals that do not expand
appreciably can be used without damaging oven walls.
High-volatile A bituminous coal has distinct bands of varying luster. It
is hard and handles well with little breakage. It includes some of the best
steam and coking coal. On burning in a fuel bed, it cakes and gives off
smoke if improperly fired. The coking property is often improved by
blending with more strongly coking medium- and low-volatile bituminous
coal.
High-volatile B bituminous coal is similar to high-volatile A bituminous
coal but has slightly higher bed moisture and oxygen content and is less
strongly coking. It is good coal for steam raising and space heating.

Some of it is blended with more strongly coking coals for making
metallurgical coke.
High-volatile C bituminous coal is a stage lower in rank than the B
bituminous coal and therefore has a progressively higher bed moisture
and oxygen content. It is used primarily for steam raising and space
heating.
Subbituminous coals usually show less evidence of banding than bituminous
coals. They have a high moisture content, and on exposure to
air, they disintegrate or ‘‘slack’’ because of shrinkage from loss of
moisture. They are noncaking and noncoking, and their primary use is
for steam raising and space heating.
Lignites are brown to black in color and have a bed moisture content
of 30 to 45 percent with a resulting lower heating value than higher-rank
coals. Like subbituminous coals, they have a tendency to ‘‘slack’’ or
disintegrate during air drying. They are noncaking and noncoking. Lignite
can be burned on traveling or spreader stokers and in pulverized
form.
The principal ranks of coal mined in the major coal-producing states
are shown in Table 7.1.3. Their analyses depend on several factors, e.g.,
source, size of coal, and method of preparation. Periodic reports are
issued by the U.S. Department of Energy, Energy Information Agency.
They provide statistics on production, distribution, end use, and analytical
data