HPAC Engineering, Jan. 2014, by Thomas Fitzpatrick.
http://hpac.com/heating/what-now-complying-major-and-area-source-boiler-rules (free registration required)

"Covering both new and existing boilers, which are divided into 19 subcategories, the rules [EPA] establish numerical limits for emissions of five pollutants: carbon monoxide (CO), hydrogen chloride (HCl), mercury (Hg), and filterable particulate matter or total selected metals. There are various requirements regarding start-up and shutdown to meet, and most boilers are required to have a tune-up; meanwhile, a one-time energy assessment is required for affected units."