Industrial/Commercial/Institutional Boilers
and Process Heaters NESHAP Summary

3. What Hazardous Air Pollutants (HAP) Are Emitted and are to be Controlled under the Boiler MACT?

The US EPA has classified three of the HAP as human carcinogens and five as probable human carcinogens.

Health and Environmental Risk Assessment Resources
Search the following sites for more information on the HAP listed above.

US EPA HAPS Health Effects Notebook
US EPA Integrated Risk Information System
University of Vermont Safety Resources
NIOSH Pocket Guide to Chemical Hazards
WISER - the Wireless Information System for Emergency Responders

Because of the large number of HAP potentially present in emissions and the disparity in the quantity and quality of the emissions information available, the US EPA decided to use several surrogates to control multiple HAP in the final rule. This will reduce the burden of implementation and compliance on both regulators and the regulated community. The HAP, for the compliance purposes of the boiler MACT, are grouped into four common categories:

• Mercury
• Non-mercury metallic HAP
• Inorganic HAP
• Organic HAP

Next, compounds were identified that could be used as surrogates for all the compounds in each pollutant category. For the non-mercury metallic HAP, particulate matter (PM) was chosen to use as a surrogate. Most, if not all, non-mercury metallic HAP emitted from combustion sources will appear on the flue gas fly-ash. Therefore, the same control techniques that would be used to control the fly-ash PM will control non-mercury metallic HAP. Particulate matter was also chosen instead of specific metallic HAP because all fuels do not emit the same type and amount of metallic HAP but most generally emit PM. The use of PM as a surrogate will also eliminate the cost of performance testing to comply with numerous standards for individual metals.

However, the US EPA is sensitive to the fact that some sources burn fuels containing very little metals, but would have sufficient PM emissions to require control under the PM provisions of the proposed rule. In such cases, PM would not be an appropriate surrogate for metallic HAP. Therefore, in the final rule, an alternative metals emission limit is included. A source may choose to comply with the alternative metals emissions limit instead of the PM limit to meet the final rule.

For inorganic HAP, hydrogen chloride (HCl) was chosen to use as a surrogate. The emissions test information available indicate that the primary inorganic HAP emitted from boilers and process heaters are acid gases, with HCl present in the largest amounts. Other inorganic compounds emitted are found in much smaller quantities. Also, control technologies that would reduce HCl would also control other inorganic compounds that are acid gases. Thus, the best controls for HCl would also be the best controls for other inorganic HAP that are acid gases. Therefore, HCl is a good surrogate for inorganic HAP because controlling HCl will result in a corresponding control of other inorganic HAP emissions.

For organic HAP, carbon monoxide (CO) was chosen to use as a surrogate to represent the variety of organic compounds, including dioxins, emitted from the various fuels burned in boilers and process heaters. Because CO is a good indicator of incomplete combustion, there is a direct correlation between CO emissions and the formation of organic HAP emissions. Monitoring equipment for CO is readily available, which is not the case for organic HAP. Also, it is significantly easier and less expensive to measure and monitor CO emissions than to measure and monitor emissions of each individual organic HAP. Therefore, using CO as a surrogate for organic HAP is a reasonable approach because minimizing CO emissions will result in minimizing organic HAP emissions.


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