Sources of Air Pollution / Emission Factors: Boilers
of AP-42 contains the emission factors for external combustion. The majority of
this section (and supporting documentation) is devoted to boilers.
The primary pollutant of concern in natural gas boilers is NOx. There
are two types of NOx formed by the combustion of natural gas in boilers. NOx formation
occurs by three fundamentally different mechanisms in boilers and other external
1. The principal
mechanism of NOx formation in natural gas combustion is thermal NOx. Thermal
NOx is formed during combustion when the nitrogen (N2) and oxygen (O2)
molecules react. Most thermal NOx formation occurs in the high temperature flame
zone near the burners. The formation of thermal NOx is affected by three factors:
(1) oxygen concentration, (2) peak temperature, and (3) time of exposure at peak
temperature. As these three factors increase, NOx emission levels increase. The
emission trends due to changes in these factors are fairly consistent for all
types of natural gas-fired boilers and furnaces. Emission levels can vary considerably
with the type and size of combustor and with operating conditions (e.g., combustion
air temperature, volumetric heat release rate, load, and excess oxygen level).
2. The formation
of prompt NOx occurs through early reactions of nitrogen molecules
in the combustion air and hydrocarbon radicals from the fuel. Prompt NOx reactions
occur within the flame and are usually negligible when compared to the amount
of NOx formed through the thermal NOx mechanism. However, prompt NOx levels may
become significant with ultra-low-NOx burners.
Nox formation stems from the evolution and reaction of fuel-bound nitrogen
compounds with oxygen. Due to the characteristically low fuel nitrogen content
of natural gas, NOx formation through the fuel NOx mechanism is insignificant.
The rate of CO emissions from boilers depends on the efficiency of natural
gas combustion. Improperly tuned boilers and boilers operating at "off-design
levels" decrease combustion efficiency resulting in increased CO emissions.
In some cases, the addition of NOx control systems such as low NOx burners and
flue gas recirculation (FGR) may also reduce combustion efficiency, resulting
in higher CO emissions relative to uncontrolled boilers.
The rate of VOC emissions from boilers and furnaces also depends on combustion
efficiency. VOC emissions are minimized by combustion practices that promote high
combustion temperatures, long residence times at those temperatures, and turbulent
mixing of fuel and combustion air. Trace amounts of VOC species in the natural
gas fuel (e.g., formaldehyde and benzene) may also contribute to VOC emissions
if they are not completely burned in the boiler.
Emissions of SO2 from natural gas-fired boilers are low because pipeline
quality natural gas typically has sulfur levels of 2,000 grains per million cubic
feet. However, sulfur-containing odorants, added to natural gas for detecting
leaks, lead to small amounts of SO2 emissions. Boilers combusting unprocessed
natural gas may have higher SO2 emissions due to higher levels of sulfur in the
Because natural gas is a gaseous fuel, filterable PM emissions are typically
low. Particulate matter from natural gas combustion has been estimated to be less
than 1 micrometer in size and has filterable and condensable fractions. Particulate
matter in natural gas combustion consists of larger molecular weight hydrocarbons
that are not fully combusted. Increased PM emissions may result from poor air/fuel
mixing or maintenance problems.
emission factors for the various pollutants from large (> 100 MMBtu/hr) and
small boilers (< 100 MMBtu/hr) are shown below. The factors are presented in
lbs./MCF natural gas input at 1000 Btu/cf. Carbon dioxide emissions are shown
in the second table as they are a greenhouse gas and are starting to receive more