Power Plant Engineering

Fig. 7.10
furnace, where it is solidified and broken for removal. The slag layer that forms on the walls of the
cyclone provides insulation against too much heat loss through the walls and contributes to the effi-

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232
POWER PLANT ENGINEERING
ciency of cyclone firing. The high temperatures also explain the large production of NO, in the gaseous
combustion products. These gases leave the cyclone through the throat at right and enter the main boiler
furnace. Thus combustion takes place in the relatively small cyclone, and the main boiler furnace has the
sole function of heat transfer from the gases to the water-tube walls. Cyclone furnaces are also suitable
for fuel-oil and gaseous-fuel firing.
Initial ignition is done by small retractable oil or gas burners in the secondary air ports.
Like pulverized-coal systems, cyclone firing systems can be of the bin, or storage. or direct-firing
types, though the bin type is more widely used, especially for most bituminous coals, than in the case of
pulverized coal. The cyclone system uses either one-wall, or opposed-wall, firing, the latter being pre-
ferred for large steam generators The size and number of cyclones per boiler depend upon the boiler size
and the desired load response because the usual load range for good performance of any one cyclone is
from 50 to 100 percent of its rated capacity. Cyclones vary in size from 6 to 10 fv in diameter with heat
inputs between 160 to 425 million Btu/h (about 47,000 to 125,Oa_ kW), respectively .
The cyclone component requiring the most maintenance is the burner, which is subjected to
erosion by the high velocity of the coal. Erosion is minimized by the us; of tungsten carbide and other
erosion-resistant materials for the burner liners, which are usually replaced once a year or so.
EXERCISES
1. A sample of coal has the following molal analysis C 67.35%, H
2
26.26%, O
2
2.28%, N
2
0.57%, S 1.37%, H
2
O 2.17%. Write the complete combustion equation in stoichiometric air
and calculate the coal ultimate analysis, mass percent.
2. Write the complete combustion equation for the anthracite coal, assuming stoichiometric air
and find the dew point, degrees centigrade, of the combustion products if the total pressure is
I bar.
3. H
2
burns in pure oxygen in a chemically correct (stoichiometric) mixture. Write the combus-
tion equation and calculate (a) the mass of products per unit mass of H
2
, and (b) the lower
heating value of H
2
if its higher heating value is 61,100 Btu/lbm.
4. Calculate the higher and lower heating values, in Btus per pound mass, using the Dulong-
type formula, of the anthracite coal, if the total pressure is 1 atm.
5. A gaseous fuel that is derived from coal has the following ultimate volumetric analysis: H
2
47.9%, methane (CH
4
) 33.9%, ethylene (C
2
H
4
) 5.2%, CO 6.1%, CO
2
2.6%, N
2
3.7%, and O
2
0.6% It burns in 110 percent of theoretical air. Calculate (a) the volume flow rate of air
required per unit volume flow rate of the gas when both are measured at the same pressure
and temperature, and (b) the dew point of the combustion products, in degrees fahrenheit, if
the total pressure is 2 atm.
6. 10,000 U.S. gal of a fuel oil are burned per hour in 20 percent excess air. The fuel oil has the
following ultimate analysis by mass: C 87%, S 0.9%, H
2
12%, ash 0.1%. Write the combus-
tion equation and find the volume flow rate of air required, in cubic feet per minute, if the fuel
has a density of 7.73 Ibm u.s gal and the air is at t atm and 60°F.
7. A southern California natural gas has the following ultimate analysis by mass: H
2
23.3%.
CH
4
72% N
2
0.76%, and O
2
1.22%. The flue gases have the following volumetric analysis:
H
2
O 15.583% SSWc. CO
2
8.387%, O
2
3.225%, N
2
72.805%. Calculate (a) the percent

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FUELS AND COMBUSTION
233
theoretical air used in combustion and (b) the dew point, in degrees centigrade, if the flue
gases are at 2 bars.
8. A fuel oil composed only of carbon, hydrogen, and sulfur is used in a steam generator. The
volumetric flue gas analysis on a dry basis is: CO
2
11.7%, CO 0.440%, O
2
4.002%, SO
2
0.176, and N
2
83-682 Find (a) the fuel mass composition, (b) the air-fuel ratio by mass, (c)
the excess air used, in percent, and (d) the dew point, in degrees centigrade, of the flue gases
if their pressure is 2 bar.
9. A fuel oil burned in a steam generator has a composition which may be represented bv C
14
H
30
.
A dry basis flue-gas analysis shows the following volumetric composition: CO
2
11.226%. O
2
4.145% CO 0.863% N
2
83.766%. Write the complete combustion equation for l mol of fuel
and calculate (a) the air-to-fuel ratio by mass, (b) the excess air, in percent, and (c) the mass
of water vapor in the flue gases per unit mass of fuel.
10. A crushed bituminous coal to be used in a fluidized-bed combustion chamber caries in size
between 1/4 and 3/4 in and has a density of 80 Ibm/ft
3
. The coefficient of drag when fiuidized
is 0.60. Calculate (a) the minimum gas velocity that fluidizes all the coal if the gas is at
1600°F and 9-atm pressure, and (b) the pressure drop in the bed, psi. Assume that the coal in
the collapsed state has a height of 2 ft and a porosity of 0.25 and that the gas density can be
approximated by that of pure air.
11. 10,000 tons of coal are burned in a powerplant per day. The coal has an as-received ultimate
analysis of C 75%, H
2
5%, O
2
6.7%, H
2
O 2.5%, S 2.3 %, N
2
1.5%, ash 7.0%. It burns in
excess air in a fluidized bed combustor. Calculate (a) the mass of calcium carbonate to be
added, in tons per day, and (b) the mass of calcium sulfate to be disposed of, in tons per day.
(The molecular mass of calcium = 40.)
12. Write the chemical formula and sketch the molecule for the following hydrocarbons: (a)
ethane, (b) ethene or ethylene, (c) decane, (d) iso-decane (2,2,3,3 tetramethyl hexane), (e)
pentatriacontane (do not sketch), (f) isobutene (2-methyl propene), (g) I,5-heptadiene (the
numbers indicate the positions of the carbon atoms that precede double bonds), (h) cyclohexane,
(i) naphthalene, (j) 1-methyl napthalene (a methyl radical CH
3
attached to a carbon atom
instead of a hydrogen atom), (k) tetracontane (do not sketch), and (l) dotriacontahectane (do
not sketch).
13. A coal-oil mixture (COM) is composed of the bituminous medium volatility coal listed and a
distillate oil no. 2 that has an ultimate analysis on a mass basis of C 87.2%, H
2
12.5%, S 0.3%,
N
2
.

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