Measuring Flare Destruction Efficiency with Passive FTIR

Flares are an essential safety device for refineries, upstream oil and gas sites, and chemical plants. As such, they cannot be replaced. They are also used as pollution control devices for waste gas from various processes. For many years, regulatory control and enforcement of flares was based on emission estimates and engineering. However, due to recent advancements in remote sensing technology, direct measurement of flare emissions has become more widespread.

In the U.S., flares were moved to the forefront of the EPA’s enforcement efforts in 2009. Since then, several industrial scale tests of flare performance were conducted at refineries, chemical plants, upstream oil and gas sites, and flare manufacturer test facilities. Data from these tests have been used by the EPA to revise emission factors for flares and are incorporated into new flare operating requirements in the Refinery Sector Rule at 40 CFR 63.670. As a result of these initiatives, refinery flares in the U.S. have been equipped with instrumentation to measure the flow rates of vent gas, steam and air. Refineries have also installed analyzers to measure vent gas heating value.

In the past, measuring the combustion products from an industrial-sized flare was difficult and dangerous. However, recent technological advances have produced remote sensing instruments capable of measuring combustion products such as carbon dioxide, carbon monoxide, and select hydrocarbons without the safety hazards introduced by physically sampling a flare plume. One such instrument is the Passive Fourier Transform Infrared (PFTIR) analyzer.

CleanAir routinely uses a PFTIR analyzer to measure flare destruction efficiency. The PFTIR works by characterizing a flare plume’s chemical make-up (carbon dioxide, carbon monoxide, and total hydrocarbons) in units of concentration × pathlength. The PFTIR method used for the EPA flare tests was blind-validated against extractive sampling results during concurrent testing by the Texas Commission on Environmental Quality (TCEQ) and the University of Texas in 2010. The use of PFTIR technology with this PFTIR analytical method provides flare combustion performance data that have been accepted by the US EPA.

How it works

PFTIR analysis operates on the principle of spectral analysis of thermal radiation emitted by hot gases. “Passive” means that no “active” infrared light source is used. Instead, the hot gases of the flare are the infrared source. The spectrometer is a receiver only, and the measurement is based upon interpretation of the radiance spectra of the hot gases, as opposed to the traditional FTIR approach which is based upon an analysis of the absorption spectra created when IR passes through a gas.

Determining destruction efficiency can be broken into three steps:

Step 1: Calibrate

Before analyzing the flare plume, the PFTIR needs to be calibrated. There are four calibrations performed as part of a PFTIR test program.

•  Black Body Source: Calibrates the PFTIR in absolute units of radiance
•  IR Source: Accounts for the path between the flare plume and PFTIR
•  Cold Source: This is the “zero” calibration
•  Sky Background: Accounts for the path behind the flare plume

The black body, infrared (IR) source, and cold source calibrations are generally conducted once per day. Measurements for these 3 calibration sources use a collimator cart that is placed about the same distance from the PFTIR as the flare.

The sky background calibrations are conducted before and after each test run. To collect a sky background, the PFTIR collects raw data while aimed at the sky.

Step 2: Measure

A flare plume is a moving target, so a PFTIR operator must aim the PFTIR for an entire test run. Ideally, the PFTIR is aimed near the centerline of the flare plume about one flame length away from the flame tip. 

At this distance, all thermal destruction reactions are complete. Proper aiming of the PFTIR is critical to the acquisition of consistent, reliable data.

As an operator aims the PFTIR, it collects a spectrum about 40 times a minute. These individual spectra are averaged together into a composite. This is what is used to determine the concentrations of individual components in the flare plume.

The composite spectrum is the starting point for some fairly complex math and physics.The four calibration spectra are used to “filter out” background interferences and determine how hot the flare plume is. Speciated concentrations are reported for each composite spectrum after a complex data reduction process.

Step 3: Calculate

Destruction efficiency is a measure of how well the flare converts organic compounds to carbon dioxide and carbon monoxide. It is calculated from the concentrations of individual flare plume components.

Evolution of PFTIR Testing

PFTIR testing has evolved over the years. Early PFTIR projects were large, industry- and state-sponsored science experiments. They required multiple PFTIR analyzers, several continuous video feeds, and lengthy test runs. Previous validation testing has led to much simpler PFTIR tests. Now, a well-prepared site can conduct a flare test in only one day.

Who still tests flares?

PFTIR tests are shorter and less expensive than a decade ago. As a result, smaller facilities are using them to define flare- and site-specific operating limits and performance characteristics. Often, a facility or flare manufacturer will want to measure VOC destruction efficiency or carbon monoxide emissions. That data can ultimately be used to report lower emissions numbers. We know from a decade of flare testing experience that most flares can operate with destruction efficiencies greater than  99%. It may seem like a small change, but a flare operating at 99% DE emits only HALF of the VOC emissions as that of a flare operating at 98% DE.

Some flare types, most notably pressure-assisted tips, can exceed this threshold when operated within a certain envelope of process conditions. PFTIR testing is a useful tool for finding the boundaries of that envelope.