Updates to the Hazardous Organic National Emission Standards for Hazardous Air Pollutants (HON) were finalized by the U.S. Environmental Protection Agency in May 2024 and took effect July 15, 2024. The rule updates federal air emissions requirements for chemical manufacturing facilities, tightening flare performance standards, expanding controls on ethylene oxide (EO) and other hazardous air pollutants, and establishing phased compliance timelines. Key requirements take effect in 2026 and 2027.
As a result, chemical manufacturers are rethinking how flare systems are designed, operated and permitted. The updated HON requirements focus on two core outcomes. Facilities must achieve reliable, high-efficiency destruction of hazardous pollutants during flaring and eliminate visible smoke during flaring events.
While the objective is clear, the path to compliance is not. These requirements introduce technical, operational and regulatory challenges that extend well beyond a simple equipment change. For many facilities, treating compliance as a flare-tip upgrade alone risks underestimated costs, operational disruption and extended project timelines. A site-specific, system-level strategy is essential.
The Flare Compliance Challenge
To achieve reliable, high destruction efficiency of hazardous pollutants without continuous monitoring, the EPA established a proxy requirement. Flare gas must maintain a minimum heating value of 270 British thermal units per standard cubic foot (Btu/scf), along with tighter requirements for pilot flame reliability and monitoring of assist steam or airflows.
For most refineries, this threshold is not a significant hurdle, as their flare gas typically has a high heating value. Flare gas streams typically contain sufficient hydrocarbons to maintain high heating value. Chemical plants, however, operate across a much wider range of flare gas stream compositions. Many facilities operate flare headers that contain high levels of nitrogen or other inert gases. This dilution lowers heating value and makes compliance more complex.
In these cases, supplemental fuel injection becomes necessary to raise heating value and sustain effective combustion. While straightforward in concept, this solution depends on several variables, including:
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Availability and cost of fuel gas, which can affect both capital planning and long-term operating expense.
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Capacity of existing flare hardware, including burners, pilots and control systems, to handle higher firing rates.
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Ability to accommodate higher combustion rates within the facility’s air permit, particularly in nonattainment areas or where emission margins are limited.
Each of these constraints can quickly complicate project execution.
In parallel, facilities often benefit from evaluating what enters the flare system in the first place. By identifying streams diluted with nitrogen or other inert gases, as well as other low-value streams, and rerouting them to alternative processing or recovery systems, operators can reduce the need for supplemental fuel while addressing the root cause of noncompliance. This approach can lower emissions, reduce operating costs and improve long-term system performance.
The second core requirement, eliminating visible smoke, introduces additional trade-offs. Compliance typically requires a steam-assist or air-assist system or a redesign using a sonic flare. Each option introduces downstream impacts. Full flare replacement is a major capital project, often requiring extended outages and competing for limited space within congested facilities. Steam-assisted systems can impose large instantaneous demands on steam systems, sometimes requiring capacity upgrades or diversion from other critical users.
Although flare performance is the focal point of the regulation, the most effective solutions often involve changes elsewhere in the plant. Fuel systems, steam networks and operating practices all influence whether hazardous pollutants are reliably and effectively destroyed under real-world operating conditions.
Additional Challenges for Ethylene Oxide Facilities
Facilities handling ethylene oxide face heightened requirements under the revised HON rule. The regulation tightens emission limits and requires capture and treatment of EO-containing vent streams, including emergency vents. For older facilities, compliance can be particularly capital-intensive.
Meeting these requirements may involve installing extensive piping networks, deploying EO-compatible flare systems or thermal oxidizers, and modifying wastewater treatment infrastructure. These changes must also account for stricter work-practice standards during startup, shutdown and malfunction events, which are no longer broadly exempt under the rule.
Air Permitting as an Overlooked Constraint
Air permitting frequently emerges as the most underestimated risk. Projects intended to improve environmental performance can inadvertently trigger major permit revisions. In nonattainment areas such as the Houston-Galveston-Brazoria region, even modest increases in nitrogen oxide emissions can require offset credits, public notice periods and extended regulatory review. These processes can add 18 months or more to project schedules and materially impact cost.
This reality reinforces the value of compliance strategies that minimize supplemental fuel use and limit incremental emissions. Early coordination among engineering, operations and permitting teams can mitigate downstream delays and regulatory surprises.
How Compliance Timing Shifted in 2025
Although the compliance dates remain unchanged for most facilities, EPA actions in 2025 introduced additional complexity. Under Clean Air Act provisions, EPA granted limited compliance extensions to a subset of affected sources while reconsidering specific implementation issues related to feasibility and sequencing of controls. Roughly 25% of facilities received up to a two-year extension.
But many facilities remain on the original compliance schedule for 2026 and 2027, creating a bifurcated landscape for operators. Some plants have additional time, but all facilities must still plan, engineer and permit solutions well in advance. Waiting for regulatory certainty before acting can compress schedules and increase execution risk.
Planning Beyond the Deadline
The revised HON requirements represent a structural shift for chemical industry. These are not discrete compliance actions but interconnected engineering programs with operational, regulatory and financial implications. Facilities that pursue narrowly scoped solutions risk discovering constraints too late to correct them efficiently.
Success depends on understanding how flare systems interact with fuel gas supply, steam generation, wastewater treatment and air permitting, and on tailoring solutions to the specific conditions of each site. The path forward favors a holistic approach and an engineering partner capable of navigating complexity to deliver compliant, reliable and economically sound outcomes.
