What Landfill Operators Should Know about RNG Technologies
By Jason Pennypacker, Project Development Director with Waga Energy
Every WAGABOX® unit utilizes a proprietary low-pressure cryogenic distillation process.
As a landfill operator, you are most likely aware of all the industry activity focused on upgrading landfill gas to renewable natural gas. Perhaps you have an existing legacy beneficial use project generating electricity or directly sending landfill gas to an end user in an industrial process. Back in the day, these were the best uses of landfill gas, generating respectable returns for the investment you made in your gas collection system. Or perhaps you missed those windows of opportunity because the volume of landfill gas generated at the time did not attract much attention from developers gathering the lowest hanging fruit.
Now, “The Times, They Are a -Changin” (Bob Dylan)
According to the US EPA, landfills are responsible for contributing 16% of all US methane emissions. As technologies such as satellite imaging measuring methane emissions evolve, the industry can expect even more pressure (pun intended) to increase the collection efficiencies of landfill gas collection systems (https://www.epa.gov/ghgemissions/methane-emissions). Landfill operators will be expected to increase financial investments and implement new solutions to meet these expectations.
Each new challenge also creates new opportunities. Economic, environmental, and social pressures are driving the industry towards next generation solutions. Government policies responding to an awareness of the impact of fossil fuels on climate change and energy independence are promoting investment in alternative energies. One of these newer energies is renewable natural gas.
Functionally, renewable natural gas is a low-carbon drop-in replacement for fossil fuel natural gas. Every unit of landfill gas energy upgraded to renewable natural gas displaces an equivalent unit of higher carbon fossil fuel natural gas. Renewable natural gas derived from landfills can be used to heat homes, for transportation, and industrial processes. It can utilize existing infrastructure that is already serving existing customers.
That sounds great, but isn't landfill gas a “dirty gas?" Isn't it very expensive to clean up the landfill gas so that it can be injected into a pipeline, or the local gas distribution system. Doesn’t it take a lot of landfill gas to support an RNG project? The short answer to these questions is “no.”
Why is it difficult to upgrade landfill gas to renewable natural gas?
When a contemporary gas collection system applies vacuum to the well field system, air gases, such as nitrogen and oxygen, are introduced into the waste mass. These air gases combine with the landfill gas, which is composed of methane and carbon dioxide. Residual contaminants such as VOC's, H2S, and other chemicals are also collected in the landfill gas. Natural gas system operators have strict guidelines to adhere to requiring high percentages of methane and limiting the other contaminants and components found in landfill gas.
Each landfill operator has to determine the appropriate level of vacuum their system can handle to optimize the amount of landfill gas collected to achieve compliance and odor control objectives. However, achieving these targets often reduces the landfill gas quality needed by legacy technologies, making upgrading landfill gas to renewable natural gas difficult.
The First Generation of Landfill Gas Upgrading
Although upgrading landfill gas to renewable natural gas might be seen as a newer solution, there are landfills in the US that have been doing this for over 40 years. The pioneers of this industry adapted technology from the oil and gas industry and applied this technology to landfill gas upgrading.
One of the most difficult components to remove from methane is nitrogen. The reason for this is that a nitrogen molecule is approximately the same molecular size as a methane molecule. Legacy technologies allow the inert nitrogen molecule to pass through the upgrading process with the energy containing methane molecule. Until recently, landfill gas upgrading technology only utilized membranes, solvents, and media filled pressurized vessels to separate the unwanted components of landfill gas from the valuable energy.
There are thermodynamic principles limiting these legacy technologies from accepting landfill gas with higher levels of nitrogen that are often present when a landfill is applying the appropriate level of vacuum to achieve the highest compliance and odor control metrics.
One commonly used technology is Pressure Swing Adsorption, or “PSA”. PSA systems can be designed using either a Kinetic Process or an Equilibrium Process. In either case, the separation of nitrogen from methane is performed based on differences in pressure and specialized media.
A challenge with using PSA is that these systems are sized to accept around only 10% nitrogen. If the landfill gas exceeds this limit, a PSA system must reduce cycle times and recycle the product gas back through the system to achieve the desired outlet quality. These responses to higher nitrogen increase electrical consumption, which is the highest input cost in upgrading landfill gas. Another approach is to increase the amount of methane in the waste gases that do not become product gas, which reduces the overall methane recovery, increases site emissions, and reduces the amount of revenue the facility can generate.
Another technology, although less common, is the use of polymeric membranes to separate nitrogen from methane. These membranes separate nitrogen from methane based on their molecular sizes in a pressurized environment.
Using these legacy technologies creates challenges for a landfill operator committed to incorporating the best landfill gas collection practices. Unless a landfill is fully closed and capped, as I stated before, air gases are introduced into the landfill gas as vacuum is applied. This is especially true at the working face, where much of the methane is released into the atmosphere.
These legacy technologies typically require landfill gas to contain over 50% methane and less than 10% nitrogen. Over the years, there have been several new technologies introduced that have incrementally improved the ability to process slightly lower quality landfill gas into pipeline quality renewable natural gas.
The Next Generation of Landfill Gas Upgrading
Cryogenic distillation uses these same principles of thermodynamics; however, separation of methane from nitrogen is achieved based on temperature instead of pressure. At very low temperatures, methane will liquify before nitrogen. This allows methane to be separated and accumulate in the bottom of a vessel while the nitrogen remains in a gaseous state. This separation occurs regardless of the amount of nitrogen in the landfill gas. This technology is designed to accept the typical gas composition fluctuations that occur in landfill gas.
Cryogenic distillation of landfill gas can occur at either low or medium pressure. Although separation occurs at approximately the same temperature in both systems, the process steps are very distinct. Medium pressure systems cannot accept oxygen, or an explosive gas mixture can result; therefore, it requires its removal through the addition of upstream oxygen removal equipment. This introduces the need for an additional water removal step, which is a byproduct of removing oxygen. There is also a requirement to add additional compression to reach medium pressure prior to entering the cryogenic process, further increasing the electrical consumption.
Low Pressure Cryogenic Distillation
Ten years ago, a new technology was introduced for landfill gas upgrading to solve this dilemma. In fact, this technology applies a gas separation process that has been utilized for over 100 years to produce medical and industrial gases.
This technology is known as Low Pressure Cryogenic Distillation. Low Pressure Cryogenic Distillation has many advantages over medium pressure cryogenic distillation or traditional PSA systems.
Low Pressure Cryogenic Distillation can simultaneously remove nitrogen and oxygen from landfill gas. These systems can accept landfill gas containing up to 30% nitrogen and still achieve pipeline quality requirements. No additional compression is needed after the initial compression to remove carbon dioxide, which results in the lowest electrical consumption compared to the other technologies.
Due to its ability to accept and separate the normal levels of oxygen in landfill gas, an oxygen removal unit is typically not required for most pipeline specifications. This reduces process steps, the associated equipment, and the overall footprint of the facility, while simultaneously increasing the overall uptime of the facility.
As landfill operators balance obligations to maintain regulatory compliance, meet increasing expectations to avoid unnecessary methane emissions, and reduce odor nuisances, it is important to consider solutions to utilize future landfill gas generation that can also support these objectives that are critical to the daily landfill operations. With the right solution, it is possible to sustainably and profitably achieve all of the obligations above.
For more information on discussing a landfill gas to RNG project, reach out to Jason Pennypacker with Waga Energy at Jason.pennypacker@waga-energy.com.