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Industrial Sources of Air Pollution / Emission Factors: Boilers

Section 1 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 combustion sources.

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.

3. Fuel 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.

Carbon Monoxide
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.

Volatile Organic Compounds
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.

Sulfur Oxides
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 natural gas.

Particulate Matter
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.

Boiler Emission Factors
The 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 attention.