Landfill gas flare systems often seem like a large “black box” just waiting for something to go wrong, with few people willing to understand the fundamentals of flare design and operation.

By Tim Locke

With some basic knowledge, you can learn simple, easy ways to minimize O&M time, stay in compliance, and minimize complaints about the flare. There are three common operational questions engineers and operators continue to ask. How can I get a greater turndown with my flare? How do I deal with low methane concentrations? And, how can I effectively control landfill gas to a third party user?

People from all aspects of the landfill industry have questions on how to minimize time and effort with landfill gas flare systems. Low flow rates, low methane concentrations, high oxygen concentrations, and third-party users can wreak havoc with the operation of flare systems. Understanding how these key elements affect the combustion process through landfill gas burner flow distribution and the flow of combustion air are critical to ease of operation and maintenance of the system.

Combustion 101
Combustion of landfill gas is primarily limited to the oxidation of methane (CH4), or in other words, the combination of methane, oxygen, and heat. This can be shown as the following equation:

CH₄ + 2O₄→ CO₂ + 2H₂O

The products of the above stoichiometric reaction are merely carbon dioxide and water. This reaction is very simple when looking at only the reactants on the left of the arrow and the products on the right. In reality, each molecule of methane and oxygen is broken down into individual atoms before they recombine into the product molecules. Very simplistically, the above reaction can be rewritten as follows:

CH₄+2O₂→C+H+H+H+H+O+O+O+O→CO₂ + 2H₂O

In order to make this reaction happen, heat is required to start the process. Once the methane molecule has been brought up to its autoignition temperature, enough heat is generated from the molecular bonds breaking to continue the process as long as methane and oxygen are present. After each molecule has been broken down into its individual atoms, those atoms then recombine into different molecules, in this case carbon dioxide and water.

The amount of oxygen required to oxidize methane has been determined and is defined as the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL), otherwise known as Flammability Limits. Since the oxygen required for this process is derived from atmospheric air, the UEL and LEL are stated as a function of the compound to be oxidized and air. For example, the flammability limits for methane are 5% to 15%. This means that a volume of air that contains between 5% and 15% methane is flammable.

Figure 1

Maintaining this mixture of methane and air is critical to the combustion process; if the mixture goes below the LEL or above the UEL, the combustion process ceases. This is precisely what happens when flares “rumble” or “vibrate.”

Once the proper amount of air and methane mix, the autoignition temperature has been reached, the combustion process is stable, and now the temperature must be controlled. This is done by modulating air dampers at the base of an enclosed flare (Figure 1).

Flare Turndown
The amount of turndown capable for a given enclosed flare is primarily based on the amount of combustion air entering the stack. As indicated in the operating temperature vs. combustion air graph (Graph 1), the larger amounts of combustion air correlate to decreasing operating temperatures. Therefore, the key to maintaining the desired operating temperature inside an enclosed flare at low landfill gas flow rates is to minimize the amount of combustion air entering the stack. This can be accomplished by the following:

• Fully closing both the manual and automatic air dampers;
• Removing one or more of the air dampers and replacing them with a steel blind plate;
• Replacing an air damper with a partial blind plate and a smaller air damper, generally 15% to 30% in size;
• Any combination of the above.

Graph 1

While decreasing the air flow into the stack will increase the operating temperature of the enclosed flare, the lower flow rates of the landfill gas will also have impacts on other aspects of normal flare operation. These issues are the initial burner ignition at a low flow rate, poor air and gas distribution, and a lower heat profile.

At extremely low flow rates, the landfill gas is not distributed evenly between the existing burners. Because of this, it becomes difficult to “cross light” from burner to burner to establish a stable flame during the initial start-up. Small amounts of landfill gas can flow unburned through certain burners, allowing gas to build up in the stack. When the light-off eventually takes place, it happens suddenly, with a contained explosion. In order to prevent this low-flow distribution issue, some of the burners can be replaced with blind flanges or in some cases the burner holes can be plugged in order to divert the gas flow to the remaining burners. If the goal is to have the original maximum design flow rate on the enclosed flare, it will not be possible to remove and blind burners.

Just as low landfill gas flow has poor distribution through the burners, the same is true with the required combustion air. The most critical factor in the combustion process is having the appropriate amount of air-to-landfill gas ratio for each burner. If burner tips are removed for proper gas flow distribution, the corresponding air flow around the removed burners should also be blocked. This can be accomplished by adding an insulated cover in the annular space of the removed or plugged burner tip. The cover should be installed so that combustion air can no longer enter the enclosed flare around the removed burner. With this cover in place, the air flow is now diverted to the burner(s) that require the combustion air.

Under extreme turndown conditions, it is impossible to maintain the normal operating temperature without modifications, even at the lowest standard thermocouple in the stack. Because of this, a new thermocouple must be added. There are two criteria for this: proper orientation and proper elevation. The most effective orientation for the new lower thermocouple is to place it directly over a burner, preferably a burner symmetrically placed in the middle of the remaining burners. The applicable requirements for retention time will govern over the proper elevation. Most air permits for enclosed flares require a 0.6 second retention time at the permitted operating temperature. For the lower stack thermocouple elevation, at least 0.6 seconds must be achieved at this elevation for the desired turndown. When the gas flow rate increases and the retention time drops below 0.6 seconds, the next higher thermocouple in the stack must be selected to control the operating temperature.

Low Methane Levels
Low methane concentrations in landfill gas are another common concern regarding the performance of a flare system. In general, most flare systems are capable of operating with methane concentrations as low as 25 to 30%. At this point, they will start to experience instability and eventually flame lift-off. Typical scenarios that can result in low methane concentrations are:

• Older, closed landfills
• Perimeter gas collection
• Collection system leaks

Older Landfills
Older, closed landfills progressively experience lower methane concentrations and gas flow rates over time. This creates two issues regarding flare operation. One is maintaining enough flow in order to achieve the operating temperature inside an enclosed stack, and the other is having enough methane to burn. The low flow rate turndown issue has already been discussed, but the flare can also be modified to operate with low methane concentrations. Since flame stability is a function of the flame propagation speed, the burner exit velocity can be reduced to maintain the operation of the flare. Flares are in operation today with methane concentrations as low as 15 to 18% using this technique. As the concentrations start to fall below 15%, flare systems can be set up to operate on timed cycles, allowing the methane to build up during down periods.

Perimeter Gas Collection
Perimeter gas collection can also lead to low methane concentrations, especially when it is primarily used to keep gas from migrating offsite. In addition to the lower methane concentrations, it is common to experience higher levels of oxygen in the gas due to the perimeter gas wells being on the boundary of the landfill, thus pulling in ambient air. From the standpoint of the flare, the higher oxygen concentrations will generally help the combustion process, although it has been known to cause rumbling and vibrations in some instances. Otherwise, flare turndown in this scenario should be handled in the same manner as indicated for closed landfills.

Collection System Leaks
System leaks from the gas wells, collection headers, and sumps can create an influx of air into the landfill gas. A small influx of ambient air due to a leaking point in the collection system will create an oxygen-laden gas very similar to that of a perimeter gas system. In this case, the flare can handle the lower methane concentrations in the same manner. However, if a substantial collection system breach occurs allowing the oxygen concentration to exceed 15%, then a combustible mixture is likely present. At low flow rates and during shutdowns, burnback can occur in the burner manifold or stack in the case of a candlestick flare, and cause material or structural damage.

Gas To A Third-Party User
Whether you have an existing landfill gas flare system or require a backup for a gas utilization project, interfacing with a primary user can be difficult. In addition to the critical communication required between the flare control panel and the gas utilization source, it is important to optimize the performance of the overall system.

A gas utilization project is normally designed to accept a preset, not-to-exceed amount of landfill gas where the flare system is designed to accept all the gas a landfill can generate. Because of this flow rate difference, one of two things generally occurs. Either the flare system acts as a backup and burns gas that the utilization project does not require, or the flare is not utilized properly and the gas is not fully extracted from the landfill. In order to design a proper interfacing system, the following three items need to be addressed:

• Supplying gas to the utilization project
• Flare turndown
• Communication between the flare and the utilization project

Gas From the Landfill
A gas project usually requires a positive displacement compressor to pressurize the landfill gas in order to meet the requirements of the internal combustion engine, turbine, pipeline, etc. While it is possible that this same compressor could pull directly from the landfill, it is more effective to send gas to the compressor from the landfill gas blowers, unless an elaborate control scheme is put into place.

If the compressor were to extract gas directly from the landfill, the vacuum generated on the collection system must always be less than the capability of the landfill gas flare blowers. This is possible under ideal conditions; however, since the compressor is likely to be a positive displacement device, it will get the flow it wants or keep increasing the vacuum until it does. If the vacuum increases to the limit of the landfill gas blowers, then it becomes impossible to extract all the available gas from the landfill. Even a slight increase in the vacuum from the compressor can change the dynamics of the collection system.

A more common approach is to let the flare system blowers extract gas from the landfill as shown in Fig. 2. In this case, a vacuum transmitter on the collection system sends a signal to a variable frequency drive (VFD) on the gas blower motor. A pressure controller then maintains a constant vacuum on the system to allow all of the gas to be extracted, regardless of the condition. Once all the gas is collected, it can then be sent either to the gas utilization project, the flare, or both. This is done by adding a pressure transmitter in the gas utilization header and a pressure control valve. The pressure controller in this scenario maintains a constant pressure to the compressor to meet its demand. Any excess gas will go automatically to the flare. This simple control system will also require less maintenance and is easier to operate than the alternative.

Flare Turndown
In instances where the gas utilization facility uses almost all the landfill gas available, the flare must be capable of turndowns in excess of the original design. The turndown modification in this case is more critical, because the flare must also be capable of operation at the maximum flow rate. With this criterion, the most common solution is to add a thermocouple at an elevation of approximately 8 feet to 10 feet for use only at extremely low flow rates. This additional thermocouple will allow the flare to maintain the operating temperature at a lower flow rate than the original design. If the air permit for the flare does not specify a minimum operating temperature, then the flow rate can easily be decreased further.

PHOTO: PERENNIAL ENERGY

The key to maintaining desired temperatures inside an enclosed flare is to minimize the amount of combustion air entering the stack.

In extreme turndown conditions, the minimum flow rate to the flare must be maintained or the operating temperature cannot be controlled. This can easily be achieved with either a mechanical or a software minimum stop placed on the pressure control valve. The minimum stop set point for the control valve must be configured in the field and should not allow the valve to close past what is required for the minimum flow condition. If the minimum stop is not incorporated into the pressure control valve, then the flare will automatically shut down on low operating temperature or flame failure, unless this shutdown is disabled.

Communication
Communication between the flare system and the gas utilization project can be critical to both operations. The main items that need to be incorporated into the flare control panel to maximize the operation of the overall system are:

Blower Operation. Landfill gas blowers normally shut down when the flare shuts down. Under the gas utilization scenario, the blowers must remain in operation even if the flare shuts down. Therefore, logic must be added to the flare control panel such that both the flare and gas compressor must shut down in order to shut down flare blowers.

Figure 2.

In addition to changing the blower shutdown logic, the logic for the pressure control valve must also be incorporated. For example, in the case of a flare shutdown, the pressure control valve will close, fully diverting all the gas to the compressor. Since the compressor cannot take all the gas available, the pressure control loop of the blower must now be changed from vacuum-control to pressure-control based on the discharge header pressure transmitter.

Pressure Control. Since the compressor for the gas utilization project is likely a positive displacement machine, it can easily overcome the incoming header pressure if its flow demand increases. If this does happen, it is possible to pull landfill gas out of the flare—and possibly even air into the system—before it has a chance to shut down. In addition, if the compressor demand is greater than the amount of gas available, it will pull through the landfill gas blowers, causing an over-extraction on the landfill.

In order to compensate for the scenario described above, a low-pressure alarm and low-pressure shutdown should be added to the flare control logic. Therefore, if the pressure in the blower discharge header decreases to a certain level below the operating set point, an alarm signal is sent to the gas utilization facility.

If the pressure continues to decrease below the pressure set point, the landfill gas blowers will shut down.

Status of Operation. The gas utilization project will require trained operating technicians for the facility. Since the flare system will not have this type of operating personnel, it is important that the flare-control panel report all the required status back to the gas facility. Also, just as important, the gas facility must provide certain information to the flare-control panel.

The flare-control panel, as a minimum, must receive a signal from the gas facility that it is ready to begin operation. This will allow the actuation of the pressure control valve. While other status signals can be received from the gas facility, they are not required for a fully functional system.

As for the gas facility, it can retrieve any status available from the control panel, including flare status, flow rate, pressure, temperature, or vacuum.

Conclusion
By understanding the basic design concepts of landfill flares and learning a few common fixes, it is possible to make logical, step-by-step changes to improve the performance of a landfill gas flare system.

Applying this knowledge can minimize time spent on operation and maintenance, help achieve compliance with environmental regulations, and reduce downtime, saving time and money.

MSW Management contributor Tim Locke is the manager of the Biogas Control Group for John Zink Co., LLC.

MSW - March/April 2006