When boiler equipment does not meet state or federal emissions requirements, operators may require after-treatment systems. After-treatment systems contain emissions control catalysts, depending on the pollutant that needs to be controlled. Such systems can come as part of the boiler package from the manufacturer or be retrofitted to an installed unit.
There are two types of catalysts that can be incorporated into a system. For control of oxides of nitrogen (NOx), it is a selective catalytic reduction (SCR) catalyst, while, for carbon monoxide (CO), volatile organic compounds (VOCs), and formaldehyde (HCHO), it is oxidation catalyst.