How Petrochemical and Organic Chemical Sectors Can Comply with Flare Gas Rules

Increasingly, national and local government bodies are requiring processing companies to monitor the various emissions from their plant stacks and flares to reduce the amount of pollution entering the atmosphere. Initially the focus was on oil refinery flares; more recently petrochemical and chemical plant flares are receiving attention as potential sources of hazardous air pollutants.

In March 2020, the EPA signed several Risk and Technology Review (RTR) rules, including National Emission Standards for Hazardous Air Pollutants (NESHAP): Generic Maximum Achievable Control Technology Standards for Ethylene Production (EMACT). The EPA decided that current requirements for EMACT flares are not adequate to ensure 98% destruction efficiency needed to meet standards.

As a result, EMACT flares are subject to the same flare definitions and requirements as refineries and must be continuously monitored. Existing ethylene crackers must comply three years after publication of the final rule in the Federal Register; new crackers, or facilities that started construction after October 9, 2019, must comply on initial start-up or date of publication of final rule, whichever is later.

In May 2020 EPA finalized amendments to the 2003 Miscellaneous Organic Chemical Manufacturing National Emission Standards for Hazardous Air Pollutants (NESHAP), known as MON. This adds monitoring and operational requirements for flares that control ethylene oxide emissions and flares used to control emissions from processes that produce olefins and polyolefins. It also allows facilities outside of this subset to opt into these flare requirements in lieu of complying with the current flare standards.

EPA estimates these finalized amendments will reduce hazardous air pollutant (HAP) emission by 107 tons per year, with reductions in ethylene oxide emissions of ~0.76 tons per year. EPA also estimates that emissions of HAP from flares will be reduced by ~ another 260 tons per year.

The above referenced rules follow those of November 2018 when the US Environmental Protection Agency (EPA) published amendments to Refinery Sector Rule (RSR) 40 CFR Part 63 affecting flares; refineries had to bring flares into compliance with new §63.670 ‘Requirements for Flare Control Devices’ by January 30, 2019.

The new requirements define five flare operating limits – combustion zone net heating value (NHV_CZ), dilution net heating value (NHV_dil), flare tip exit velocity, pilot flame presence, and visible emissions – and specifies an NHV_CZ minimum operating limit of 270 BTU/scf, based on a 15 minute block period¹. NHV_CZ can be calculated by measuring the net heating value of the vent gas (NHV_VG), making flare gas analysis a vital part of any compliance strategy. If the NHV_CZ approaches 270 BTU/scf, additional fuel gas such as propane or natural gas has to be added. This may then require the addition of steam to the flare, to avoid the production of visible emissions.

Analysis of flare gases presents a series of challenges, whatever process they originate from. Emissions are typically made up of complex mixtures of inorganic and organic species, and compositions and concentrations vary dramatically over time as process conditions change. Although many regulations simply require total heating value, total sulfur, or total hydrocarbon values to be recorded, measuring the concentrations of individual components helps identify the source of the emission, locating the problem to a specific part of the plant. Speed of analysis speed is also crucial, as the heating value of the flare can change quickly. Analysis times measured in minutes increase the risk of failing to meet emission standards.

Process Mass Spectrometry is particularly suited to the measurement of both flare and process gas streams in real time because it offers accurate, fast, multicomponent analysis on a single analyzer. Process Mass Spectrometers are gas analyzers based on magnetic sector mass spectrometer technology that provide lab-quality online gas analysis and process analytics.

Table 1 below shows an example of a flare gas stream containing hydrogen, nitrogen and hydrocarbons up to C₆. Analysis of these 19 components will typically be performed in under 30 seconds, allowing one mass spectrometer to monitor more than one flare, depending on the distances involved.