Flare Gas Recovery

Flare gas recovery is a proven technology that recovers flare vent gas and compresses it into the plant fuel system. Instead of the energy of the vent gases being combusted in a flare, that energy is released into furnaces and boilers, reducing the amount of fuel required for a process, which in turn reduces the overall CO2 emissions from a plant. A well-designed flare gas recovery system (FGRS) can reduce the amount of gas flared by 90% or more. Another benefit of combusting the vent gases in a furnace/boiler is improved destruction efficiency and reduced NOx emissions.

Quick Discussion on Methane

Methane is a common vent gas component in the production, refining and petrochemical industries. The U.S. EPA has estimated the global warming potential (GWP) of methane to be 27 to 30 over a 100 year period. This means that 1 ton of methane in the  atmosphere will have a similar heat-capturing effect as 27 to 30 tons of CO2 over 100 years. While CO2 will remain in the atmosphere for thousands of years, methane on average is only in the atmosphere for 10 years. This means that the GWP of methane over a 10 year period is in the range of 270 to 300. When methane burns completely, it produces approximately 2.74 mass units of CO2 per mass unit of methane. Consequently, when methane escapes into the atmosphere, it captures around 10 times (29/2.74 = 10.6) more heat than if it had been combusted to CO2 over a 100 year period. Methane represents approximately 11% of all U.S. greenhouse gas emissions.

Quick Discussion on NO2

NO2 is another greenhouse gas and is estimated by EPA to have a GWP of 273 over 100 years. NO2 remains in the atmosphere for more than 100 years. NOx emissions from flares are typically unknown and unmeasurable. NO2 represents approximately 7% of all U.S. greenhouse gas emissions. (Note: Most NOx formed in combustion is NO but once in the atmosphere it converts to NO2.)

Many flares in the U.S. are permitted with an assumed destruction efficiency (DE) of 98%. If this assumption is correct, 2% of the vent gases are vented to the atmosphere. Assuming a vent gas composition of 100% methane, 2% venting means the heat absorption capacity of the flare emissions goes from 2.74 mass units of CO2 per mass of methane to approximately 3.27 mass units of CO2 equivalent (2.74 x 0.98 + 29 x 0.02) per mass of methane, a 19% increase. Combustion in a furnace or boiler is monitored and the amount of unburned hydrocarbon leaving a furnace is typically very low. By simply combusting the vent gas in a more controlled environment, the greenhouse gas emissions are reduced by around 16% (1 -2.74/3.27) based on the assumed DE and vent gas composition.

Besides increased destruction efficiency, furnaces/boilers have better control over NOx emissions. This is typically achieved by using low NOx burner technology, but some systems also use downstream NOx abatement such as selective catalytical reduction (SCR) and selective non-catalytic reduction (SNCR).

The need for flares is not eliminated with the installation of a FGRS for a couple of reasons. First, it is not practical to design a FGRS for very large emergency cases. The frequency of such large cases is low and the increased capital cost of the FGRS to handle those flowrates is not justified. Second, a power failure (often one of the largest relief scenarios) will also disable a FGRS. Hence a flare is always associated with a FGRS. A common method of attaching a flare to the flare header is with a liquid seal but valves can also be used for this purpose. A flare that is part of a system containing a FGRS is termed a “stand-by” flare. Stand-by flares offer a couple of opportunities to reduce greenhouse gas emissions.

Stand-by Flares and Nitrogen Purge

Stand-by flares do not typically receive vent gas. Since the flare is not receiving vent gas, the downsides of a nitrogen purge are mitigated. The main concern with the use of nitrogen as a purge gas is the potential to generate lean gas mixtures that do not burn well, venting some or all the hydrocarbons. If the flare is not receiving any vent gas, then there is no risk of creating a lean mixture and nitrogen can be used to keep oxygen out of the flare stack. The lean gas mixture concern would again occur if small flowrates of vent gas are discharged into the nitrogen purged flare stack. If the stand-by flare tip is steam assisted, another benefit of nitrogen purge is the ability to reduce the minimum steam flow to that necessary to keep the lines warm (warming rate) which is lower than the traditional cooling rate. If the stand-by flare tip is air assisted, another benefit of nitrogen purge is the ability to shut off all blowers which conserves energy.

Stand-by Flares and Primus

Stand-by flares also provide an opportunity to reduce pilot emissions. The Primus technology is a rapid pilot ignition system developed by JZ which will quickly ignite (5 seconds or less) a pilot even if the fuel in a fuel line has been displaced with air. The concern with such technology is reliability given the importance of flare ignition. An approach to reduce this concern involves leaving one pilot on and turning off the rest. Each day, a pilot will be ignited and once verified the previously ignited pilot will be turned off. The amount of pilot emission reduction will depend on the number of pilots. (A flare tip with three pilots would experience pilot emissions reduction of 66%.) This allows each Primus equipped pilot to be functionally verified once every few days. If the header pressure upstream of the stand-by flare reaches some elevated pressure, all the pilots would be ignited.

Staged Flares

Even if a FGRS is not installed, there are benefits to staging. A staged flare system consists of a small flare which receives the normal daily rates and a large emergency relief stand-by flare connected to the header via a liquid seal or valves. The smaller primary flare has lower purge requirements and is more efficient at smokeless combustion of vent gas than a single large flare tip designed for the full range of operation. Pilot count will increase with a staged flare. The emergency flare may have three to four pilots while the primary flare will typically have two or more. The use of Primus on the emergency flare could reduce the number of pilots in operation to be equal to or slightly fewer than a single large flare. The large emergency flare can take advantage of all the benefits of a stand-by flare (N2 purge, Primus).

Staged Multi-Point Flares

Multi-Point Ground Flares (MPGF) or in John Zink parlance (Linear Relief Gas Oxidizer – LRGO) have several advantages. A MPGF is a flare made up of many burners grouped into stages. These groups of burners are put into service and taken out of service depending on the relief rate. Typically, each row of burners will have two pilots with the first stage potentially having a pilot on every burner. The first stage of a MPGF will have a low purge requirement. Often the purge rate is set by the end users desired sweep rate within the header and not the rate needed to keep oxygen out of the first stage. When stages are taken out of service they are purged with nitrogen. There are many pilots associated with a MPGF. The Primus technology could be applied to a MPGF reducing the number of pilots in service. The largest benefit of a MPGF is the very high destruction efficiency associated with such flares. Many MPGF burners have been tested for emissions. It is not unusual for regulators to permit such flares for 99.5% destruction efficiency. (The majority of test data shows destruction efficiencies higher than 99.5%.) 99.5% DE is a 75% reduction in unburned hydrocarbon emissions compared to 98% DE. The downside of a MPGF is the need for the heating value of the vent gas to be relatively high (typically 800 Btu/scf). If the vent gases to be combusted have a low heating value, a supplement gas must be used to increase it. This requirement usually limits MPGFs to applications that have consistently high heating value vent gas.

Purge Reduction Devices

For decades, John Zink has supplied purge reduction devices. These devices come in two types: velocity seal (trade name Airrestor) and buoyancy seal (trade name Molecular Seal). These devices reduce the amount of purge needed to keep oxygen levels within the stack acceptable. Reduced purge means reduced emissions.

Hydrogen Fuel

The use of hydrogen is becoming more prevalent as companies work to reduce CO2 emissions. The only product of combustion from hydrogen is water. John Zink has provided 100% hydrogen flare pilots for decades, providing such pilots for space vehicle launch facilities. This is a proven technology.

Stella Technology

John Zink has developed a direct spark ignition system for flare tips (Stella). The purpose of the development was for pit flares located in remote locations that did not have access to utilities. By using solar battery systems, a modification to a pipe flare allowed a high-voltage spark to reliably ignite the flare tip. Testing was conducted over a range of vent gas exit velocities (from very low to Mach 1), a range of wind speeds, wind directions, with and without rain. The technology was tested in both a horizontal orientation (typical pit flare orientation) as well as a vertical orientation and ignition reliability was high for all configurations. (Note: Country-specific regulations may prevent the use of this technology.) The benefit of this technology is the absence of pilot gas and the associated emissions.

Flaring Prevention

The most beneficial way to reduce CO2 emissions from flares is to prevent process upsets which trigger flaring events.  Most major operators have analytical capabilities that can use historical data to predict potential flaring events allowing the events to be either reduced or avoided altogether.  

Partner with John Zink

With proven solutions like Flare Gas Recovery, Primus ignition systems, and staged flaring technologies, we provide the expertise and innovation you need to achieve compliance and sustainability goals. Ready to explore how our solutions can help your operation cut emissions and improve efficiency? Connect with our experts today.