EPA MATS — the Mercury and Air Toxics Standards — requires every coal-fired electric generating unit above 25 megawatts to meet technology-based emission limits for mercury, acid gases (HCl, HF), non-mercury HAP metals, and organic HAPs, with compliance demonstrated through continuous emissions monitoring systems (CEMS), quarterly stack testing, or certified performance monitoring systems. The 2012 MATS Rule remains fully in force regardless of subsequent regulatory amendments, and compliance obligations include not only meeting emission limits but maintaining complete, retrievable records of monitoring data, performance evaluations, sorbent injection system maintenance, and semiannual compliance reports submitted to EPA through CEDRI. Plants that treat MATS as an emissions engineering problem rather than an ongoing maintenance documentation program consistently fail audits on record completeness, not emission levels. Sign up free on OxMaint to connect your MATS compliance monitoring and control equipment maintenance directly to structured, audit-ready work order documentation that satisfies inspectors on the first request.
EPA MATS · Clean Air Act · Coal Plant Compliance · HAP Emissions
EPA MATS Compliance for Coal Plants: Emissions Control Is Only Half the Battle
Meeting MATS emission limits for mercury, HCl, and HAP metals is necessary — but not sufficient. Inspectors also verify that your control equipment was maintained, your CEMS was calibrated, and your records are complete. This guide covers both.
86%
Mercury emission reduction achieved industry-wide since MATS took effect in 2015
25+ MW
Capacity threshold above which coal and oil-fired EGUs must comply with MATS
4 HAP streams
Mercury, acid gases, non-Hg HAP metals, and organic HAPs — each with separate monitoring requirements
Semiannual
Frequency of mandatory compliance reports submitted to EPA via CEDRI
Regulated Pollutant Streams
The Four MATS Pollutant Categories — Limits, Controls, and Monitoring
Each of the four MATS pollutant streams requires a distinct control technology, monitoring method, and maintenance record. Compliance gaps are most common where monitoring equipment maintenance records are incomplete — not where emission limits are exceeded.
Limit: 1.2 lb/TBtu (coal-fired) / 0.020 lb/GWh (oil-fired)
Control Technologies
Activated carbon injection (ACI) — primary sorbent technology for bituminous coal
Halogen-salt co-injection (sodium/calcium bromide) for enhanced Hg oxidation
Fabric filter or ESP for sorbent capture downstream
Monitoring: Hg CEMS or sorbent trap sampling system (30-day rolling average)
Limit: 0.002 lb/MMBtu HCl or 0.001 lb/MMBtu HF (coal-fired)
Control Technologies
Dry sorbent injection (DSI) — lime or sodium bicarbonate injection upstream of fabric filter
Wet flue gas desulfurization (FGD) — captures HCl as a co-benefit
Spray dry absorber (SDA) for mid-size units
Monitoring: HCl CEMS, HF CEMS, or quarterly stack testing with CPMS
Limit: 0.030 lb/MMBtu filterable PM surrogate (existing coal-fired EGUs)
Control Technologies
High-efficiency electrostatic precipitator (ESP) — must demonstrate consistent collection efficiency
Fabric filter (baghouse) — lower maintenance burden, higher first-pass capture rate
PM CEMS (continuous particulate monitoring) — compliance demonstration method
Monitoring: PM CEMS, quarterly stack tests, or CPMS for ESP/FF operation
Limit: 2.0E-05 lb/TBtu dioxins/furans (coal-fired EGUs)
Control Technologies
Low-temperature ESP or FF operation reduces D/F formation and capture
Carbon injection co-capture at lower flue gas temperatures
Combustion optimization — minimize incomplete combustion zones
Monitoring: Initial and periodic stack testing (not continuous monitoring required)
CEMS Calibration Failures Are MATS Violations. Document Every Service Event.
A CEMS data gap caused by a missed calibration creates a compliance deviation — regardless of actual emission levels during that period. OxMaint's CEMS maintenance tracking ensures every calibration, QA/QC event, and sensor replacement is documented, timestamped, and linked to the emissions monitoring record it supports.
Maintenance-Compliance Link
Why MATS Compliance Lives in Your Maintenance Program — Not Just Your Emissions Data
Inspectors reviewing MATS compliance examine more than your CEMS data printout. They trace the maintenance history of every piece of equipment whose failure could create a compliance deviation. These are the five equipment-maintenance links that determine your MATS audit outcome.
01
Activated Carbon Injection System — Feed Rate Records
The ACI system must deliver sorbent at the rate demonstrated during your performance test. Injector wear, hopper level failures, and ductwork plugging reduce feed rates below the demonstrated level — making your CEMS data invalid as a compliance demonstration. Inspectors ask for maintenance logs, calibration of the mass feed rate controller, and hopper service records to confirm the system was operated as tested.
OxMaint tracks: PM schedule for ACI feed system, mass flow calibration records, injector replacement history, and hopper inspection logs — all indexed under MATS equipment.
02
Fabric Filter / Baghouse — Bag Integrity and Differential Pressure
Bag failures in a pulse-jet baghouse create localized PM breakthrough that PM CEMS may not immediately detect at the stack level if the failure is in one compartment. Inspectors look at bag replacement records, differential pressure trending, and compartment-level inspection intervals to verify structural integrity of the primary control device for both non-Hg HAP metals and sorbent capture.
OxMaint tracks: Bag replacement work orders by compartment, differential pressure trending, leak test results, and scheduled compartment inspections with pass/fail documentation.
03
Mercury CEMS / Sorbent Trap — QA/QC and Calibration Frequency
MATS requires Hg CEMS to undergo relative accuracy test audits (RATA) at least annually and daily calibration drift checks. A CEMS that misses a required calibration period creates a data gap that must be filled with substitute data — which counts against your rolling 30-day compliance average. The calibration maintenance record is the first document inspectors request after the CEMS data printout.
OxMaint tracks: RATA scheduling, calibration drift check records, sensor replacement, and ECMPS submission timestamps — with alerts when QA deadlines are approaching.
04
Dry Sorbent Injection (DSI) — Lime Feed Rate and Nozzle Condition
DSI systems for HCl control are particularly sensitive to nozzle wear and moisture-induced bridging in the lime feed system. Performance drift in the HCl removal efficiency often traces back to a degraded nozzle pattern or reduced stoichiometric ratio — and the maintenance record linking observed HCl CEMS trends to corrective maintenance actions is exactly what auditors need to verify the system was operated in compliance.
OxMaint tracks: Nozzle inspection and replacement, lime silo level and quality logs, feed rate calibration records, and HCl trend correlation to maintenance events.
05
Semiannual Compliance Report — Traceability to Source Records
The compliance reports submitted to EPA's CEDRI system aggregate CEMS data, deviation periods, QA/QC test results, and startup/shutdown/malfunction events. Inspectors follow the numbers backward — from the CEDRI submission to the underlying source records. If the source records (maintenance logs, calibration sheets, stack test reports) don't match the reported values, the discrepancy becomes an independent violation.
OxMaint maintains: A complete source record library linked to each reporting period — accessible by date range and pollutant stream for rapid inspector response.
Compliance Monitoring Matrix
MATS Monitoring Requirements by Pollutant and Compliance Method
Plants can choose different compliance demonstration methods for each pollutant stream. This table shows what each method requires in terms of monitoring equipment maintenance and records.
Common Questions
EPA MATS Frequently Asked Questions
Does the 2025 repeal of the 2024 MATS amendments affect current compliance obligations?
No. EPA's February 2025 repeal of the 2024 amendments returns affected facilities to the 2012 MATS Rule standards, which remain fully in force. The 2012 rule's emission limits for mercury, acid gases, non-Hg HAP metals, and organic HAPs are unchanged. Plants that had been planning capital upgrades specifically for the 2024 amendments (PM CEMS installations, enhanced sorbent systems for lignite units) may now reassess those plans, but core 2012 MATS compliance obligations continue without modification.
What is a CPMS and when can it substitute for a full CEMS under MATS?
A Continuous Parameter Monitoring System (CPMS) monitors an operational parameter that is correlated with pollutant emissions — such as sorbent feed rate, pressure drop, or ESP power input — rather than measuring the pollutant directly. Under MATS, CPMS can be used as the compliance demonstration method for HCl and non-Hg HAP metals when combined with initial and periodic stack testing. The CPMS must demonstrate during the performance test that the monitored parameter corresponds to emission levels at or below the applicable limit.
OxMaint tracks CPMS calibration and correlation test schedules to keep this compliance pathway intact.
What qualifies as a MATS startup, shutdown, or malfunction event for reporting purposes?
Under the 2012 MATS Rule, startup and shutdown periods are defined in unit-specific terms and must be documented in the operating permit or compliance plan. Malfunction events — where emission limits are exceeded due to sudden, unavoidable equipment failure despite proper operation and maintenance — must be reported in the semiannual compliance report with root cause, corrective actions, and duration. A history of well-maintained control equipment significantly strengthens the "despite proper O&M" defense for any malfunction claim.
How does MATS interact with state air permits and Title V operating permit conditions?
MATS establishes federal floor standards under NESHAP 40 CFR Part 63 Subpart UUUUU. State air permits and Title V operating permits incorporate MATS requirements but may add state-specific conditions, different compliance schedule terms, or more stringent limits. In many states, MATS inspectors coordinate with state air quality division inspectors — meaning a single inspection may cover both federal MATS and state permit conditions.
Book a demo to see how OxMaint manages multi-agency compliance records under one asset record.
What are the most common reasons coal plants receive MATS notices of violation?
Based on enforcement data, the most frequent MATS NOV triggers are: CEMS data gaps from missed calibration periods (the most common), late or incomplete semiannual compliance report submissions, performance stack test failures after control equipment degradation, and failure to maintain operating records for sorbent injection systems in a manner traceable to emission limit compliance. Emission exceedances above the actual emission limits are far less common than documentation and monitoring failures.
MATS Compliance Starts With Maintenance Records
Your CEMS Data Is Only as Good as Your Maintenance Documentation Behind It
OxMaint connects your mercury, acid gas, and particulate control equipment maintenance directly to your MATS compliance program — creating a complete, retrievable record of every calibration, inspection, and sorbent system service event that inspectors need to verify your monitoring data is valid.
