+----------------------------------------------------------------+
|+-----------------------+------+----+-----+----+------+--------+|
|| |CO_{2}| O | CO | H |CH_{4}| N ||
|+-----------------------+------+----+-----+----+------+--------+|
||Bessemer Furnace | 9.85|0.36|32.73|3.14| .. |53.92 ||
||Bessemer Furnace | 11.4 | .. |27.7 |1.9 | 0.3 |58.7 ||
||Bessemer Furnace | 10.0 | .. |26.2 |3.1 | 0.2 |60.5 ||
||Bessemer Furnace | 9.1 | .. |28.7 |2.7 | 0.2 |59.3 ||
||Bessemer Furnace | 13.5 | .. |25.2 |1.43| .. |59.87 ||
||Bessemer Furnace[47] | 10.9 | .. |27.8 |2.8 | 0.2 |58.3 ||
||Ferro Manganese Furnace| 7.1 | .. |30.1 | .. | .. |62.8[48]||
||Basic Ore Furnace | 16.0 |0.2 |23.6 | .. | .. |60.2[48]||
|+-----------------------+------+----+-----+----+------+--------+|
+----------------------------------------------------------------+
Until recently, the important consideration in the burning of blast
furnace gas has been the capacity that can be developed with practically
no attention given to the aspect of efficiency. This phase of the
question is now drawing attention and furnaces especially designed for
good efficiency with this class of fuel are demanded. The essential
feature is ample combustion space, in which the combustion of gases may
be practically completed before striking the heating surfaces. The gases
have the power of burning out completely after striking the heating
surfaces, provided the initial temperature is sufficiently high, but
where the combustion is completed before such time, the results secured
are more satisfactory. A furnace volume of approximately 1 to 1.5 cubic
feet per rated boiler horse power will give a combustion space that is
ample.
Where there is the possibility of a failure of the gas supply, or where
steam is required when the blast furnace is shut down, coal fired grates
of sufficient size to get the required capacity should be installed.
Where grates of full size are not required, ignition grates should be
installed, which need be only large enough to carry a fire for igniting
the gas or for generating a small quantity of steam when the blast
furnace is shut down. The area of such grates has no direct bearing on
the size of the boiler. The grates may be placed directly under the gas
burners in a standard position or may be placed between two bridge walls
back of the gas furnace and fired from the side of the boiler. An
advantage is claimed for the standard grate position that it minimizes
the danger of explosion on the re-ignition of gas after a temporary
stoppage of the supply and also that a considerable amount of dirt, of
which there is a good deal with this class of fuel and which is
difficult to remove, deposits on the fire and is taken out when the
fires are cleaned. In any event, regardless of the location of the
grates, ample provision should be made for removing this dust, not only
from the furnace but from the setting as a whole.
Blast furnace gas burners are of two general types: Those in which the
air for combustion is admitted around the burner proper, and those in
which this air is admitted through the burner. Whatever the design of
burner, provision should be made for the regulation of both the air and
the gas supply independently. A gas opening of .8 square inch per rated
horse power will enable a boiler to develop its nominal rating with a
gas pressure in the main of about 2 inches. This pressure is ordinarily
from 6 to 8 inches and in this way openings of the above size will be
good for ordinary overloads. The air openings should be from .75 to .85
square inch per rated horse power. Good results are secured by inclining
the gas burners slightly downward toward the rear of the furnace. Where
the burners are introduced over coal fired grates, they should be set
high enough to give headroom for hand firing.
Ordinarily, individual stacks of 130 feet high with diameters as given
in Kent's table for corresponding horse power are large enough for this
class of work. Such a stack will give a draft sufficient to allow a
boiler to be operated at 175 per cent of its rated capacity, and beyond
this point the capacity will not increase proportionately with the
draft. When more than one boiler is connected with a stack, the draft
available at the damper should be equivalent to that which an individual
stack of 130 feet high would give. The draft from such a stack is
necessary to maintain a suction under all conditions throughout all
parts of the setting. If the draft is increased above that which such a
stack will give, difficulties arise from excess air for combustion with
consequent loss in efficiency.
A poor mixing or laneing action in the furnace may result in a pulsating
effect of the gases in the setting. This action may at times be remedied
by admitting more air to the furnace. On account of the possibility of a
pulsating action of the gases under certain conditions and the puffs or
explosions, settings for this class of work should be carefully
constructed and thoroughly buckstayed and tied.
Natural Gas--Natural gas from different localities varies considerably
in composition and heating value. In Table 50 there is given a number of
analyses and heat values for natural gas from various localities.
This fuel is used for steam generating purposes to a considerable extent
in some localities, though such use is apparently decreasing. It is best
burned by employing a large number of small burners, each being capable
of handling 30 nominal rated horse power. The use of a large number of
burners obviates the danger of any laneing or blowpipe action, which
might be present where large burners are used. Ordinarily, such a gas,
as it enters the burners, is under a pressure of about 8 ounces. For the
purpose of comparison, all observations should be based on gas reduced
to the standard conditions of temperature and pressure, namely 32
degrees Fahrenheit and 14.7 pounds per square inch. When the temperature
and pressure corresponding to meter readings are known, the volume of
gas under standard conditions may be obtained by multiplying the meter
readings in cubic feet by 33.54 P/T, in which P equals the absolute
pressure in pounds per square inch and T equals the absolute temperature
of the gas at the meter. In boiler testing work, the evaporation should
always be reduced to that per cubic foot of gas under standard
conditions.