Waste to Fuel: On the making of renewable natural gas

Natural gas remains one of the most-used alternative fuels in Louisiana. According to Louisiana Clean Fuels’ (LCF) 2018 Annual Report, in 2018 alone, natural gas (CNG) was responsible for offsetting over 5 million gasoline gallon equivalents (GGEs) in the state [1].

Though the majority of natural gas used in Louisiana comes from fossil-sources, a small percentage is renewable natural gas (RNG), which is chemically identical to natural gas from fossil-sources but comes instead from renewable sources such as food waste, manure, and other decomposing organic materials.

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St. Landry Parish Solid Waste creates and uses RNG in central Louisiana; they collect methane (CH4) from their landfill which is then cleaned and used as RNG to fuel their trash trucks. St. Landry’s RNG project, which began operation in 2012, was the first of its kind in the United States. Over the last two years, LCF has worked closely with St. Landry to create an operations manual for their RNG project. Once the manual is complete, we will begin work on an RNG curriculum to teach the process to other landfills and increase the amount of RNG used in Louisiana.  

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RNG has grown in popularity across the United States in recent years, with the number of operational projects increasing from 60 projects at the end of 2017 to 89 projects at the end of March 2019 and the total production capacity of these projects increasing by approximately 25% over that same 15 month period. Additionally, a recent study by the Colorado Energy Office found that by utilizing RNG, Colorado can replace 24% of its annual diesel usage, eliminating 1.4 million metric tons of carbon dioxide (CO2) every year [3]. Interest in RNG has also dramatically increased, and there have been numerous articles written in the last year to explain the generalities of RNG production. Considering our working relationship with the St. Landry Parish project, LCF is uniquely situated to shed light on some of the deeper complexities of the subject, and our hope is to provide insight into how these types of projects actually operate. This article will hopefully serve as a midpoint between a 30,000 ft view and a deep technical dive.

The Process

RNG can be created from several different sources or feedstocks, but three of the most common are landfills, farms, and wastewater treatment plants. There are three basic steps to producing usable RNG: raw gas collection, conditioning/cleaning, and distribution. These steps are the same regardless of feedstock; in this article, we’ll be focusing on how this process works for landfills.

Collection

In a landfill, trash, which includes organic matter (food waste, paper waste, farming waste, etc.), is placed in a pit, called a cell, which is specially prepared to receive and contain trash with as little environmental impact as possible. The trash is dumped into the cell and covered with dirt, clay, and eventually an impermeable liner to prevent odors, liquids, and landfill gas (LFG) from escaping. This produces an anaerobic (oxygen-free) environment within the landfill. Most modern landfills are classified as Sanitary Landfills, which are designed to reduce their environmental impact as much as possible. This includes trapping the LFG and burning it.

Each landfill cell will hold a certain amount of trash before it’s completed and covered with an impermeable liner (basically a gigantic plastic sheet) that traps all the LFG. Wells are then drilled into the cell to collect that gas. These wells are large pipes with holes in them, allowing the LFG to pass through but not the solid trash. The pipes are all connected to each other in a huge network called a well-field and then connected to a vacuum pump that literally sucks the gas out of the landfill. Each well has its own wellhead, which is a valve that can control the amount of suction at that well without needing an individual vacuum pump for each well.

If the LFG isn’t removed from the landfill, pressure will build, eventually releasing methane and other harmful gases into the atmosphere. If the pump pulls too hard on the wells, air can get into the landfill, introducing nitrogen and oxygen to the landfill. The presence of oxygen inhibits anaerobic digestion and can contribute to the formation of landfill fires. Landfill fires release harmful emissions and are very difficult to extinguish, sometimes burning for weeks or even years. Nitrogen intrusion can also be very problematic due to the difficulty of separating nitrogen from the LFG during the conditioning process. Pulling too hard on the wells introduces nitrogen and oxygen, and not pulling hard enough releases raw LFG into the atmosphere. This balancing act is one of the most important pieces of any landfill RNG project.

Traditionally, the LFG is simply dried and then sent to a flare to burn all the methane and other unwanted compounds, reducing their environmental impact. Burning methane converts it to carbon dioxide, which is less harmful as a greenhouse gas, and also helps to remove harmful gases such as Volatile Organic Compounds (VOCs), a class of chemicals which includes carcinogens such as benzene. In a landfill RNG project, the LFG is diverted from the flare and sent to a separate conditioning/cleaning facility to be made into usable natural gas. Under EPA regulations, landfills above a certain size are required to collect and flare the LFG they produce [4]. This is one of the big benefits of a landfill RNG project: the first step in the RNG process – collecting the gas – is something the landfills are already required to do.

Conditioning/Cleaning

The level to which the gas must be cleaned is heavily dependent on its end-use. If the gas will be put into a pipeline, it must be cleaned according to the specifications of that pipeline. If the intention is to use it for electricity production, there are separate requirements for that as well. To use RNG as a vehicle fuel in natural gas vehicles, the gas needs to meet Society of Automotive Engineers (SAE) standards.

The makeup of raw LFG is generally about half methane and half carbon dioxide. The gas can also contain unwanted by-products that must be removed, including hydrogen sulfide, siloxanes, carbon monoxide, VOCs (which can include benzene, butane, ethane, or a massive variety of carbon-based compounds), along with many other unwanted chemicals. The specific mix of these chemicals is heavily dependent on the feedstock. For example, if someone throws old air-conditioning equipment into a landfill, the LFG may temporarily include freon (a type of VOC), which will also need to be removed. Weather also affects raw gas quality; both of these factors require the conditioning process to be robust enough to maintain the product gas specs year-round.

The first step in conditioning is to remove hydrogen sulfide (H2S). This is usually done by chemically filtering the gas through a special type of activated carbon or other filtration media that strips out the hydrogen sulfide. Hydrogen sulfide needs to be filtered out for two major reasons:

• Hydrogen sulfide can form sulfur oxides (SOx) when burned. SOx is a criteria pollutant under National Ambient Air Quality Standards (NAAQS) [6]. SOx emissions have effects on lung health and can contribute to the formation of both harmful particulate matter in the atmosphere and acid rain.
• To protect the CO2 membranes later in the process (explained below).

The second step in conditioning is to remove Volatile Organic Compounds (VOCs). This is also often done through chemical filtration with a different type of activated carbon. VOCs are filtered out for two major reasons:

• Many VOCs have negative health effects, and some are carcinogenic.
• To protect the CO2 membranes and siloxane filters later in the process.

VOCs can include butane, ethane, and propane, all of which are allowable and often desirable in pipeline natural gas, but they are removed along with other VOCs during conditioning to protect the siloxane media and carbon dioxide membranes.

Often, the third step in RNG conditioning is to remove siloxanes. These are silicon compounds that generally come from cosmetics, which means they’re present in landfills and wastewater treatment facilities, but not in farms Siloxanes can be filtered out by passing the LFG through a special type of silica pellet, by condensation, or by water/solvent absorption. In the case of silica pellets, the pellets will also absorb VOCs and hydrogen sulfide, so they can become saturated and lose effectiveness if these are not effectively filtered out first.

Siloxanes are not toxic; they’re removed to protect other equipment down the line. When siloxanes are burned, they produce non-toxic silicon powder which can clog sensors in a natural gas engine and act as an abrasive. This is so dangerous that a single tank of bad natural gas in a CNG vehicle can cause catastrophic engine failure. Siloxanes can also damage electricity generators if they use microturbines or post-combustion catalysts. Since siloxanes don’t damage other filtration and don’t harm all use-cases, this step is sometimes skipped depending on the intended end-use of the gas [7].

The fourth step is to remove carbon dioxide. Carbon dioxide makes up between 40-60% of the raw gas, so it may need to be removed to raise the BTU content to meet the specific needs of the use-case. Some use-cases can handle 50% methane, and carbon dioxide filtration is skipped, but this isn’t common. Carbon dioxide can be filtered out in two main ways:

CO2 Filtration Membranes

These membranes are essentially tiny plastic tubes with holes approximately the size of carbon dioxide molecules that act as a physical filter, separating methane from carbon dioxide. As the raw gas is pumped through the tubes at a fairly high pressure (>100 PSI), the carbon dioxide escapes through the holes in the tubes, and what’s left is a gas with a higher concentration of methane.

One of the benefits of using a membrane is that it’s a purely physical filtration method; a CO2 membrane will effectively last forever. A drawback is that the membranes are really sensitive to VOCs and hydrogen sulfide. Any breakthrough of VOCs or hydrogen sulfide will poison the CO2 membranes, rendering them useless. CO2 membranes are also very expensive, potentially costing tens of thousands of dollars, even for a small project. This is why so much care is given to separating VOCs earlier in the process.

There are many types of CO2 membranes, some of which can be used to filter hydrogen sulfide as well as carbon dioxide. St. Landry Parish Solid Waste uses the type described above.

Pressure-Swing Adsorption (PSA)

For this method, there is a surface that selectively adsorbs carbon dioxide, but only at certain pressures. The gas is brought to that pressure, the carbon dioxide molecules attach to the surface, the remaining gas (which is higher purity than the raw gas) is stripped away, and then the pressure is changed so that the filtered carbon dioxide releases from the surface. PSA filtration is an active filtration process, requiring a more complex system than passive filtration by a membrane. Both systems have various advantages and disadvantages that may make one more attractive than another for a specific project.

While it isn’t common, nitrogen may also need to be removed from the gas. Nitrogen is mostly inert, so it only needs to be removed in order to increase the product gas BTU content. Given that nitrogen is inert, it is fairly difficult to remove through chemical filtration, though it is possible. It is also difficult to remove nitrogen through physical filtration as is done for carbon dioxide because nitrogen molecules are close to the same physical size as methane molecules. An expensive, but effective filtration method is to liquefy the natural gas; since nitrogen’s boiling point is much lower than that of methane, nitrogen will remain as a gas that can be separated from the liquefied methane. This is more common in very high-volume projects, whereas smaller projects are more likely to use PSA filtration systems [8].

Additionally, it is worth noting that a conditioning facility will have various heat exchangers, compressors, and gas dryers throughout these steps to prepare the gas for the next step of the process. Depending on the use-case, the conditioner may also add an odorant, ethyl mercaptan, to the product gas.

Distribution

Since the RNG is now chemically identical to fossil natural gas, this section will be brief. Here are the three main use-cases:

For onsite vehicle-fueling, there will be a large, low-pressure (50-150 PSI, in the case of St. Landry’s RNG project) gas storage tank that the conditioners will fill as they clean the gas. This low-pressure storage will feed a series of high-pressure tanks (around 4500 PSI) that are compressed by dedicated, high-pressure compressors as-needed. These high-pressure tanks fuel the vehicles directly and are filled by the compressors as-needed.

For pipeline injection, the gas is compressed to match the pressure of the pipeline, which usually ranges between 400-1200 PSIG (gauge-pressure). In this case, there may be a low-pressure storage tank to act as a buffer, and the compressors will connect directly to the pipeline [9].

For on-site electricity generation, there is generally a storage tank to act as a buffer, a compressor to fill this storage tank, and a pipe connecting to the gas generators.

The Future of RNG

The use of RNG is increasing across the United States as more and more projects are developed. As of the writing of this article, there are 38 RNG projects currently under construction [10], with that number likely to grow. LCF looks forward to working with partners like St. Landry Parish Solid Waste and other Clean Cities coalitions as RNG use continues to grow across the country.

More Information

For more information and resources on RNG please visit:

• The Department of Energy’s Alternative Fuels Data Center at afdc.energy.gov/fuels/natural_gas_renewable
• The Coalition for Renewable Natural Gas at www.rngcoalition.com

For a thorough technical description of Landfill Gas RNG Project Development, check out the EPA Landfill Gas Energy Project Development Handbook at https://www.epa.gov/lmop/landfill-gas-energy-project-development-handbook-files. For more information on St. Landry Parish Solid Waste’s RNG project, check out the 2018 MotorWeek episode on the project at www.fuelsfix.com/2018/03/st-landry-parish-turns-garbage-into-renewable-natural-gas.

Sources

[1] LCF Publishes 2018 Annual Report Data: https://louisianacleanfuels.org/blog/id/381

[2] EPA Overview of Greenhouse Gases: https://www.epa.gov/ghgemissions/overview-greenhouse-gases#methane

[3] Colorado Energy Office Study Finds State Could Eliminate 1.4M Metric Tons of Emissions Annually by Utilizing Renewable Natural Gas: https://energyoffice.colorado.gov/press-release/colorado-energy-office-study-finds-state-could-eliminate-14m-metric-tons-emissions

[4] EPA Basic Information About Landfill Gas: https://www.epa.gov/lmop/basic-information-about-landfill-gas

[5], [9] Pipeline Gas Specifications: https://www.sciencedirect.com/topics/engineering/pipeline-gas-specifications

[6] NAAQS Table: https://www.epa.gov/criteria-air-pollutants/naaqs-table

[7] Performance and Economics of Currently Available Technologies for Removal of Siloxane from Biogas: https://www.scsengineers.com/scs-articles/performance-economics-currently-available-technologies-removal-siloxane-biogas/

[8] Pros and cons of different Nitrogen Removal Unit (NRU) technology: https://www.sciencedirect.com/science/article/abs/pii/S1875510012000170

[10] Argonne National Laboratory’s Renewable Natural Gas Database: https://www.anl.gov/es/reference/renewable-natural-gas-database

About St. Landry Parish Solid Waste

St. Landry Parish Solid Waste Disposal District provides residential and commercial solid waste collection and disposal, as well as operation of the St. Landry Parish Landfill and Recycling Centers. For more information, please visit slpsolidwaste.org.

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