Power Plant Engineering

Table 7.1 : Proximate and ultimate analysis of some U.S. coals
Analysis
Bituminous
mass percent
Anthractive
medium velocity
Subiluminous
Lagnite
Proximate
Fixed carbon
83.8
70.0
45.9
30.8
Volatile matter
5.7
20.5
30.5
28.2
Moisture
2.5
3.3
19.6
34.8
Ash
8.0
6.2
4.0
6.2
(Contd.)

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FUELS AND COMBUSTION
221
Analysis
Bituminous
mass percent
Anthractive
medium velocity
Subiluminous
Lagnite
Ultimate
C
83.9
80.7
58.8
42.4
H
2
2.9
4.5
3.8
2.8
S
0.7
1.8
0.3
0.7
O
2
0.7
2.4
12.2
12.4
N
2
1.3
1.1
1.3
0.7
H
2
O
2.5
3.3
19.6
34.8
HHV
Btu/lbm
13.320
14.310
10.130
7.210
7.4 COAL FIRING
Since the old days of feeding coal into a furnace by hand, several major advances have been
made that permit increasingly higher rates of combustion.
The earliest in the history of steam boilers were mechanical stokers, and several types are still
being used for small and medium-sized boilers. All such stokers are designed to continuously feed coal
into the furnace by moving it on a grate within the furnace and also to remove ash from the furnace.
99.99
99.90
99.50
99.00
98.00
95.00
90.00
80.00
70.00
60.00
50.00
40.00
30
20
10
5
0.05 0.1 0.2
0.5
1
2
5
10
20
50
100
270
140 60 50 4030 20 14 10 6 4
200 100
US standard sieve designation
Sieve aperture nm
F
inen
es
s
pr
es
ent
pas
s
ing
Fig. 7.1. Coal Sieve Analysis.
Fig. 7.1 Coal sieve analysis. (A) pulverized-coal sample; (B) coal range for cyclone firing; (C)
coal as fired.

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222
POWER PLANT ENGINEERING
Pulverized-coal firing was introduced in the 1920s and represented a major increase in combus-
tion rates over mechanical stokers. It is widely used today. To prepare the coal for use in pulverized
firing, it is crushed and then ground to such a fine powder that approximately 70 percent of it will pass
a 200 mesh sieve* (Fig. 7.1). It is suitable for a wide variety of coal, particularly the higher-grade ones.
Advantages of pulverized coal firing are the ability to use any size coal; good variable-load response; a
lower requirement for excess air for combustion, resulting in lower fan power consumption; !lower
carbon loss; higher combustion temperatures and improved thermal efficiency; lower operation and
maintenance costs; and the possibility of design for multiple-fuel combustion (oil, gas, and coal).
In the late 1930s cyclone furnace firing was introduced and became the third major advance in
coal firing. It is now also widely used though for a lesser variety of uses than is pulverized coal. In
addition to those advantages already mentioned for pulverized-coal firing, cyclone firing provides sev-
eral other advantages. These are the obvious savings in pulverizing equipment because coal need only
be crushed, reduction in furnace size, and reduction in fly ash content of the flue gases. Coal size for
cyclone furnace firing is accomplished in a simple crusher and covers a wide band, with approximately
95 percent of it passing a 4-mesh sieve (Fig. 7.1).
Most recently, fluidized-bed combustion has been introduced. In this type of firing, crushed par-
ticles of coal are injected into the fluidized bed so that they spread across an air distribution grid. The
combustion air, blown through the grid, has an upward velocity sufficient to cause the coal particles to
become fluidized, i.e. held in suspension as they burn. Unburned carbon leaving the bed is collected in
a cyclone separator and returned back to the bed for another go at combustion. The main advantage of
fluidized-bed combustion is the ability to desulfurize the fuel during combustion in order to meet air
quality standards for sulfur dioxide emissions. (Other methods are the use of low-sulfur coal,
desulfurization of coal before it is burned, and removal of SO
2
from the flue gases by the use of scrub-
bers). Desulfurization is accomplished by the addition of limestone directly to the bed. Fluidized-bed
combustion is still undergoing development and has other attractive features.