Mercury and Air Toxics Standards

MATS means the US Mercury and Air Toxics Standards, the Environmental Protection Agency rules that limit hazardous air pollutants from coal-fired and oil-fired electric utility steam generating units. The rule targets mercury, acid gases and non-mercury metal hazardous air pollutants, many of which are controlled through particulate capture and sorbent systems.

MATS was first issued in 2012 and has been revised through later EPA actions. For plant engineers, the practical effect has been sustained pressure to maintain high particulate-control performance, mercury-control performance and monitoring reliability.

Industrial implications

MATS compliance can involve:

• activated-carbon injection or other mercury-control sorbents;
• dry sorbent injection or flue-gas desulphurisation for acid gases;
• upgraded ESPs or fabric filters for particulate-bound metals;
• improved fly-ash handling and hopper evacuation;
• mercury, particulate and acid-gas monitoring or periodic testing;
• tighter operating procedures for startup, shutdown and malfunction conditions.

Because mercury and metals can be attached to particulate, a fouled or poorly cleaned collector can reduce compliance margin even if the upstream chemistry is unchanged.

Where fouling enters the compliance picture

Activated carbon and dry sorbents add solids to the gas stream. Those solids can increase dust loading, change ash resistivity, affect baghouse pressure drop, fill hoppers faster and alter ESP rapping behaviour. SCR operation can also influence mercury oxidation and downstream ammonium-bisulphate deposition, linking NOx control to particulate-control cleanliness.

Acoustic-cleaning relevance

Sonic horns do not control mercury chemistry directly. Their role is maintaining the hardware that provides the compliance margin: clean ESP internals, clear hoppers, stable baghouse compartments, open sorbent injection zones and reduced manual cleanout. During retrofit projects, acoustic cleaning may be justified where the added sorbent or new operating condition increases fouling risk.

Plant reliability context

MATS compliance depends on both chemistry and equipment availability. Activated carbon injection, dry sorbent injection, fabric filters, ESP upgrades and SCR operation can interact. More sorbent can improve mercury capture while increasing particulate loading and hopper duty. SCR catalyst can promote mercury oxidation, but the same reactor can suffer ash masking or ammonium-bisulphate fouling. A compliance plan therefore has to consider deposit handling, not only the pollutant reaction path.

Operators track mercury measurements, particulate control performance, sorbent feed rate, halogen addition, baghouse pressure drop, ESP power, hopper levels and ash disposal constraints. A failed hopper heater or bridged discharge can create an operating limit even when the chemical control system is working.

Acoustic cleaning is relevant as a reliability support. It can reduce ash build-up on ESP internals, help keep dry sorbent and fly ash moving through hoppers, and protect SCR or air-heater cleanliness where the MATS control strategy changes upstream chemistry. It should be justified through reduced excursions, fewer manual cleanouts, lower pressure-drop growth or improved abatement availability, not as a direct mercury-removal technology.

Related terms

• EPA NSPS
• Electrostatic precipitator
• Baghouse