Boilers and Process Heaters: virtual tour
Emissions
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 A
wide variety of pollutants may be emitted from boilers and process
heaters, including HAP’s, VOC’s, and criteria pollutants.
The HAP’s emitted from boilers and process heaters can be categorized
as either inorganic HAP (primarily acid gases such as hydrogen chloride
or hydrogen fluoride), organic HAP’s (such as benzene or polycyclic
aromatic hydrocarbon compounds [PAH’s]), and metallic
HAP (such as mercury or lead). Due to its health affects and different
emission characteristics, mercury is often analyzed separately from
non-mercury metallic HAPs. The types and amounts of pollutants emitted
from these sources depends greatly on the type of fuel being burned
in the combustion device.
Emissions from boilers and process heaters burning solids,
liquids, and gaseous fuels will also differ. Boilers and process
heaters emit a number of different types of HAP emissions.
In general, their formation is dependent upon the composition
of the fuel. The combustion quality and temperature may also
play an important role. The fuel dependent HAP emissions from
boilers and process heaters are metals, including mercury,
and acid gases. These fuel dependent HAP emissions generally
can be controlled by either changing the fuel property before
combustion or by removing the HAP from the flue gas after combustion.
Organic HAP, on the other hand, are formed from incomplete
combustion and are much less influenced by the characteristics
of the fuel being burned. The degree of combustion may be greatly
influenced by three general factors: time, turbulence, and
temperature. These factors are a function of the design of
the boiler or process heater which is dependent in part on
the type of fuel being burned. The different emission characteristics
will affect the type of air pollution controls that may be
used. Accordingly, the source category was divided into three
subcategories to consider these differences: solid fuel-fired
units, liquid fuel-fired units, and gaseous fuel-fired units.
The solid subcategory includes units that burn any amount of
solid fuel. The gaseous subcategory includes units that only
burn gaseous fuel, except during periods of natural gas curtailment.
The liquid subcategory includes the remaining units.
 Another
factor that affects emissions from boilers and process heaters
is the combustor design. The combustor design influences the
completeness of the combustion process and the formation of
organic compounds. Boilers with capacities less than 10 MMBtu/hr
use combustor designs (e.g., firetube or cast-iron) which are
not common in units above 10 MMBtu/hr. Large boilers generally
are field-erected using watertube combustor design with capacities
above 10 MMBtu/hr. The vast majority of these small units use
natural gas as fuel. Additionally, most existing State and
Federal regulations for boilers and process heaters do not
regulate units with a heat input capacity of less than 10 MMBtu/hr,
due to their low emissions. Accordingly, the three subcategories
were further divided into large units (watertube boilers and
process heaters > 10 MMBtu/hr capacity) and small units
(all firetube boilers and process heaters # 10 MMBtu/hr capacity)
to differentiate the combustor designs typically found in these
size ranges.
 A
third subcategory classification was also considered to distinguish
units that are operated infrequently, such as back-up or emergency
units. Back-up or emergency units only operate if another boiler
that is the regular source of energy or steam is not operating
(for example due to a shutdown for maintenance and repair).
Peaking units operate only during peak energy use periods,
typically in the summer months. The boiler database indicates
that these infrequently operated units typically operate 10
percent of the year or less. These limited use boilers, when
called upon to operate, must respond without failure and without
lengthy periods of startup. This subcategorization was made
because the limited use units, those with capacity utilizations
less than 10 percent, have a specialized use and operation
that are different from typical industrial, commercial, and
institutional units.
Thus, a total of nine subcategories were developed for this
source category:
- large solid fuel-fired boilers and process heaters
- large liquid fuel-fired boilers and process heaters
- large gaseous fuel-fired boilers and process heaters
- limited use solid fuel-fired boilers and process heaters
- limited use liquid fuel-fired boilers and process heaters
- limited use gaseous fuel-fired boilers and process heaters
- small solid fuel-fired boilers and process heaters
- small liquid fuel-fired boilers and process heaters
- small gaseous fuel-fired boilers and process heaters
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