Flare Monitoring Regulatory Compliance and Analyzers: An Analysis

Flare Monitoring Regulatory Compliance and Analyzers: An Analysis

Industry: Refinery    

Product: GC8000

In a boiler application, ammonia (NH3) gas is injected to remove nitrogen oxides (NOx) and reduce the NOx concentration in the stack flue gas. Conventional analyzers obtain the NH3 concentrations indirectly through a sampling system. The sampling system introduces time delays in the measurement and increases maintenance costs. Since the TDLS8000 Laser Analyzer does not use a sampling system, it solves such problems.
This technical note reviews the requirements and describes the three types of analyzers refineries can use to ensure compliance.

Flare Regulations

The new regulations set flare operating limits, require a flare management plan (FMP), and require a continuous parameter monitoring system (CPMS) plan. The new regulations also cover pilot flame monitoring, visible emissions, and flare tip velocity—requirements previously found in earlier versions of the rule.
Parameters that must be monitored in the FMP include:

• BTU
• Net heating value in the combustion zone (NHVcz)
• Net heating value dilution parameter (NHVdil)
• Vent gas composition

The FMP rule applies to a flare actively receiving perimeter assist air, or to a flare with the potential of operating above its smokeless capacity. A CPMS plan as defined in rule CFR 63.670 is required for each flare, and the owner or operator must have the CPMS plan available on site at all times.

Information required in the CPMS plan includes identification of the specific flare being monitored, flare type, parameters to be monitored, and the expected parameter range including worst case and normal.

The FMP rule specifically states, “For each flare, the owner or operator shall operate the flare to maintain the net heating value of flare combustion zone gas (NHVcz) at or above 270 British thermal units per standard cubic feet (BTU/scf) determined on a 15-minute block period basis when regulated material is routed to the flare for at least 15 minutes.”

The rule also states, “For each flare actively receiving perimeter assist air, the owner or operator shall operate the flare to maintain the net heating value dilution parameter (NHVdil) at or above 22 British thermal units per square foot (BTU/ft2) determined on a 15-minute block period basis when regulated material is being routed to the flare for at least 15 minutes.”

And most important, the CPMS must “…allow the (EPA) Administrator to confirm that the selected site-specific operating limit(s) adequately ensures that the flare destruction efficiency is 98 percent or greater or that the flare combustion efficiency is 96.5 percent or greater at all times.”

Combustion efficiency, net heating value in the combustion zone, and net heating value dilution parameter are calculated as defined in rule CFR 63.670.

Required data needs to be gathered, calculated, stored, and structured for reporting. This can be done with a new or an existing data management system. Data from analyzers used to monitor flares is typically sent to the data management system via some type of digital communications link such as Modbus, Ethernet, or fiber optics to handle the large amount of data.

Various parts of the regulations also require monitoring the pilot flame, visible emissions, flare tip velocity, opacity and emissions at the fence line. This note will concentrate on how to analyze for BTU, NHV, and composition.

Analyzers that can be used to meet these parts of the regulations include a Wobbe Index or BTU analyzer, a gas chromatograph, and a mass spectrometer.

Wobbe Index Analyzer

Wobbe Index or BTU analyzers (Figure 2) are widely used in refineries to measure BTU and hydrogen in various processes, and to analyze quality of fuel gases. The analyzer is a calorimeter that burns a sample to measure the oxygen content in flare gas and calculate the Wobbe index. The Wobbe index defines the heating value of gas expressed in BTUs per standard cubic foot. Since a Wobbe analyzer provides an instantaneous reading of BTU, it easily meets the requirements for a 15-minute response time. 

However, because it measures only BTU, it can only indicate that the flare is at, above, or below the limits. Since it does not measure any other components, it cannot provide any information regarding why a problem may exist.
Wobbe Index or BTU analyzer installation requires a sample conditioning system, an analyzer shelter, instrument air header, atmospheric vent header with a flame arrestor, power supply, and instrument cabling. All components must be designed for Class 1 Division 1, Group B/C/D, T3 inside, and Division 2 outside operation.

Typical installed cost of a Wobbe analyzer is about $215,000.
 

Gas Chromatograph 

Refineries use gas chromatographs (Figure 3) to analyze many different process streams, so operators are quite familiar with their operation, a major advantage of this technology. Some of the leading applications include component level concentration measurements, compositional analysis of finished products, and analysis mid-process.

All the TDL analyzers have been installed for over two years with no failures due to the TDLA’s to date. Some units were outfitted with the Yokogawa TDLS200 analyzer, while others were outfitted with the Yokogawa TDLS8000 models. There are other manufacturers of this technology but the user chose Yokogawa for this application.

A gas chromatograph (GC) measures components of the flare gas via a flow-through tube called a column. As the flare gas sample passes through the column, it reacts with a column filling—called a stationary phase—which separates the gas into various chemicals. Each chemical exits the column at a different time, where it is detected and identified. Measurement data can be calculated to provide BTU, which is critical for flare gas monitoring.

While a gas chromatograph provides a great deal of information about gas composition, it can be slow to provide this data. In fact, it might not meet the 15-minute response time rule, depending on the number of components and their composition.

Therefore, to meet the requirements for flare monitoring, a GC may need to be tuned to analyze only certain groups of components. For example, a GC might only analyze for methane, hydrogen, and carbon dioxide. After analysis, it produces a chart showing peaks and baseline for each component (Figure 4).
 

GC installation requires a sampling system with pneumatically actuated stream selection valves, an insulated and heated NEMA 4X fiberglass enclosure for the sampling system, a heated and insulated carbon steel enclosure for the GC, and a cylinder rack for carrier and calibration gases. All components must be designed for Class 1 Division 1, Group B/C/D, T3 inside and Division 2 outside operation.

Typical installed cost of a gas chromatograph is about $164,000
 

Mass Spectrometer

Since refineries typically are not familiar with mass spectrometers, a plant’s engineers and technicians would need to learn a new technology. A mass spectrometer has an ion source, a mass analyzer, and a detector. The ionizer converts the sample into ions, which are sent to the mass analyzer and the detector. The detector calculates the value of each ion to determine the quantity of each ion present, and provides BTU data.

A mass spectrometer is an expensive, complex analyzer, but it offers two advantages: First, a mass spectrometer can analyze 30 components in about 12 seconds, easily meeting the 15-minute response time rule; second, a mass spectrometer can be configured to handle multiple streams.

Since a refinery typically has multiple flares requiring monitoring, a single mass spectrometer could handle all of them—depending, of course, on the distance from the flares to the analyzer. A stream switching system could direct samples from each flare to the mass spectrometer on a rotating basis.

Installing a GC requires a sampling system, an insulated and heated NEMA 4X fiberglass enclosure for the sampling system, and an environmentally controlled steel enclosure for the analyzer. Gas cylinders are mounted remotely from the analyzer cabinet with pressure regulators, cylinder chains, and a rack for cylinders. Note that up to 12 gas cylinders may be required. All components must be designed for Class 1 Division 1, Group B/C/D, T3 inside and Division 2 outside operation.

Typical installed cost of a mass spectrometer is about $245,000 for a single flare. If multiple flares are to be monitored, a more complex sampling system is required, with correspondingly higher costs.

The comparison table summarizes the cost and design considerations for each of the three types of measurements.

Parameter	Wobbe Index	Gas Chromatograph	Mass Spectrometer
Response time	Instantaneous	Varies, can exceed 15 minutes	12 seconds
Components Monitored	Btu, Hydrogen	Up to 10 (limited by response tine)	30
Approximate Installed Cost	$$$	$$	$$$$
Refinery Familiarity	High	High	Low
Reliability	High	High	Mid
Operating Cost	Low	Mid	High
Calibration requirements	Low	Mid	High
Maintenance requirements	Low	Mid	Mid

Combining Technologies

In some applications, best results will require combining technologies. 
For example, in a two-flare refinery, Yokogawa used a BTU analyzer and a GC analyzer in combination.
The system had a single sampling system with two sample streams—one from each flare. The BTU analyzer provided rapid analysis of the flare gas streams for reporting compliance, while the common gas chromatograph provided compositional analysis for process trouble shooting.

In another refinery with three flares, Yokogawa installed a single mass spectrometer. It had a common sampling system with three sample streams—one from each flare. The mass spectrometer was capable of switching process streams fast enough to meet monitoring and reporting requirements.

Conclusion

The EPA rule, 40 CFR 63 Subparts CC and UUU, is forcing refineries to monitor flares. Fortunately, modern analyzer technology makes it possible to meet the requirements, generate the necessary reports, and stay in compliance.