Civil Engineering Reference
In-Depth Information
A cross-limited metering control by continuous monitoring of flue gas O
2
levels
can eliminate the effects of variations in fuel heating values and combustion air
conditions. The control system can also be preset with an O
2
level corresponding to
minimum fuel consumption. Oxygen analyzers using zirconium oxide sensors and
computers are currently used for reliable operation.
6.11
Boiler Emission and Environmental Pollution
Emission control is undoubtedly one of the main requirements in boiler plant
operation. Depending on fuels and boiler equipment, exhaust gases contain
by-products in different quantities, some of which are air pollutants. Small and
medium-sized boiler plants, which cannot economically accommodate abatement
systems, must cope with this problem by a correct choice of fuel and/or of burners
and auxiliary equipment.
Although every country has its own environmental protection regulations, a
general classification of major pollutants can be made as follows: sulfur dioxide,
nitrogen oxides, carbon monoxide, particulate matter, and hydrocarbons. In addi-
tion, other pollutants are always present depending on the particular fuel and
operating conditions.
Typical ranges of values of permitted pollutant concentrations in air emission
from boiler plants are reported in Table
6.8
for the above-listed pollutants. The
permitted values vary from country to country, but boiler plants must be conducted
so as to ensure values lower than those in the table.
6.11.1 Sulfur Dioxide
Sulfur dioxide SO
2
is formed during combustion (see Sect.
6.2
) by the combination
of sulfur contained in the fuel with oxygen from combustion air.
Table 6.8
Typical ranges
of values of permitted
pollutant concentrations
from combustion
Level of concentration
C
R
a
mg/Sm
3
ppm
b
Pollutants
Sulfur dioxide
1,500-2,000
525-700
Nitrogen oxides
c
50-200
102-410
Carbon monoxide
100
80
Particulate matter 100-150
a
Referred to dry gas and to a concentration of O
2
equal to 3%
in volume. To convert from the actual pollutant concentra-
tion
C
x
(with an oxygen concentration
W
x
) to the concentra-
tion referred to 3 % value of oxygen concentration, use the
following relationship:
C
R
¼
(18/(21
W
x
))
C
x
b
10,000 ppm
1 % in volume
c
Expressed in NO
2
¼
