When most people think of renewable energy, the mind goes straight to solar PV panels glistening on a sunny day, blistering winds moving wind turbines, and the traditional renewable infrastructure. While these technologies indeed are some of the most prominent renewable energy sources, industry innovators looking to usher in the energy transition are constantly thinking outside the box.
The definition used by the U.S. Department of Energy for renewable energy is “energy from sources that are naturally replenishing but flow-limited,” while clean energy is more broadly defined as “energy that is produced through methods that do not release greenhouse gases or any other pollutants.” So while the obvious power sources of solar, wind, hydropower, geothermal, and the like do of course qualify as clean and renewable sources of energy, the power sector is constantly looking at new energy sources that can accomplish these same goals through unconventional means as well.
With that in mind, energy stakeholders should prepare to start hearing more about this new addition to the clean energy tool box: renewable natural gas.
Natural gas is one of the most common fossil fuels out there, an energy type which is justly considered the antithesis to a clean, renewable energy source. So, what in the world does renewable natural gas mean? And if the fuel has a current buzz about it, will it live up to the hype?
Renewable Natural Gas vs. Fossil Gas
Traditional natural gas comes from underground drilling processes, such as fracking, that are meant to go deep into the earth to find, release, and capture the fossil fuel resources that have been stored beneath our feet for thousands of years or even longer. This gas drilling process often goes hand in hand with drilling for oil, so it’s no wonder the first reaction is to associate natural gas use with dirty, carbon-emitting fossil fuels. And whether that natural gas is burned at a power plant to create electricity or sent directly into buildings to heat homes or cook food, the greenhouse gas emissions associated with natural gas is a major issue: natural gas emits 117 pounds of CO2 for every million Btu of energy generated, so while that is only about half of the emissions of coal power, it still represents a major problem for the climate and emission reduction goals. Over the course of 2020, natural gas alone accounted for 1,651 million metric tons of CO2 emissions across the United States, twice as much as coal and 80% the total of oil.
If natural gas is such a problem, how can a new type of natural gas be the solution? To answer that question, renewable natural gas (RNG) can provide some clarity.
The difference between renewable natural gas and traditional fossil gas is in its origin. While natural gas is extracted from the ground, RNG is a product originating from the breakdown of organic waste material, such as food waste, lawn clippings, wood scraps, animal- and plant-based materials, and more. Taking it back to high school biology class, these organic materials naturally get broken down by bacteria via anaerobic digestion-- whether that’s taking place in a landfill, in a compost heap, or out in nature-- into a ‘wet’ waste product, and in the process they end up releasing methane, CO2, and other byproducts. The methane that gets released actually has a much greater impact to the climate than CO2, with it having over 80 times more greenhouse impact. However, the methane that is released by these natural processes are not typically accounted for in emissions tracking because they were so recently absorbed via natural growing processes, so releasing them back into the atmosphere is a net zero result. The methane (as well as the CO2 and other gases) will then simply be sequestered into the next batch of trees, crops, and plant or animal life in part of a perpetual cycle.
Creating Renewable Natural Gas
The rise of the RNG sector means there can now be a middle man in the natural carbon cycle that provides a net benefit to the state of emissions. While the decomposition releases methane, if that methane were to be captured and burned it would then only release CO2, which is significantly less harmful to the environment. The technology has been developed to capture the methane and other greenhouse gases during anaerobic digestion and send them to processing plants where the gases have impurities removed and ensure the resultant gas is as chemically identical as possible to fossil gas. When this conversion is successfully performed, the resultant gas (now fully formed RNG) is considered ‘pipeline ready’ can then be mixed in with existing natural gas supplies, sent through existing natural gas pipelines, used for transportation (DOE considers RNG to be an advanced biofuel under the Renewable Fuel Standard), and used directly for electricity generation or heating. When this RNG (which can sometimes also be referred to as biomethane or biogas) is ultimately burned by its end user, it releases CO2, but when comparing the emissions that would have been emitted via natural decomposition with these CO2 emissions upon use the result is actually net negative emissions and a positive result for the climate.
Another way to think about the difference between fossil gas and RNG is that natural gas has been stored for thousands of years or more, so burning that fuel means releasing carbon into the atmosphere that would have otherwise been sequestered indefinitely moving forward—emissions that were no long a part of the natural cycles. Burning renewable natural gas, though, is simply extracting useful energy out of a natural carbon cycle process, hence it being considered a clean and renewable energy source.
The raw biogas that gets sent to production facilities to become pipeline ready RNG can come from a number of sources. Landfills account for over 90% of all U.S. RNG production today. This sourcing of RNG comes as no surprise, given landfills are the third-largest source of human-related methane emissions across the country, so ‘landfill gas’ is a hotly tracked commodity. Over 500 landfill gas projects are in operation in the United States, with many of them even using the gas right on site to reduce their external power or gas dependency.
Livestock operations provide another significant source of biogas. Animal manure can be collected and delivered to anaerobic digesters to process and capture those naturally emitting gases, helping to create clean fuel while minimizing the climate impact of animal agriculture. In a very similar way, wastewater treatment plants are another growing source of biogas for RNG. These plants take solids from the wastewater streams and perform a similar process to extract usable biogas. Such resources are obviously ubiquitous, so much so that a study by the National Association of Clean Water Agencies and the Water Environment Federation found that 12% of U.S. power demand could be met by energy generated at U.S. wastewater treatment plants alone. Additional opportunity for harnessing biogas towards RNG production can also be found in smaller, niche operations like industrial sites, institutional campuses, commercial sites, and more.
Pros and Cons of Renewable Natural Gas
When it comes to the rise of RNG as an energy source, as with any emerging technology, pros and cons naturally arise.
To start with the positives, RNG is at its core an opportunity to reduce greenhouse gas emissions. Not only does the production of RNG eliminate naturally occurring emissions that contribute towards climate change, but the wider availability of RNG can and does directly displace the direct use of fossil gas and other fossil fuels. For every ounce of RNG used in electricity production, it is likely reducing by that same amount the coal or fossil gas that would have otherwise been burned. Similarly, the more RNG is used in heavy-duty trucks, the less demand will be available for diesel fuel and other petroleum products. Overall, RNG can actually be considered to be carbon negative, as the difference between allowing natural decomposition and burning the captured RNG isn’t just a one for one replacement. When decomposing naturally, it would emit methane, which again is a much more impactful GHG. But by combining it with CO2 and then burning it, it’s just the CO2 that gets emitted and the accounting for that can be carbon negative (which can be enhanced even further with the purchase of carbon offsets, which some RNG producers and consumers do to make their emission reduction impact even greater).
Taking these benefits a step further, RNG is unique in its ability to fill in gaps where other clean energy sources would be unavailable. According to SoCalGas, for example, replacing less than 20% of traditional gas supplies with RNG from sources like dairies, wastewater treatment plants, and landfills can reduce collective emissions the same amount as electrifying 100% of buildings by 2030. Not only can it accomplish this impressive feat, but that same study finds it can do so in a manner that is 3 times more cost effective. Further study finds that sufficient amounts of RNG can directly replace 4-7% of current fossil gas consumption. These examples show that while RNG is no silver bullet solution, it’s one more huge step in the right direction. And when compared with the more traditional renewable sources like solar and wind, RNG has the massive advantage that it can be burnt at any given moment, not being reliant on intermittent and unpredictable sources like the sun shining or the wind blowing. That unique feature demonstrates why RNG can be the perfect complement to other clean energy sources to truly capture gaps in the clean energy systems of today.
When comparing the possibilities of RNG compared with other emerging technologies in the clean energy transition, the renewable gas has another key advantage: it’s already available and in use. The industry doesn’t need to await any sort of breakthrough, and in fact a UC Davis study found that over 20% of California’s residential natural gas consumption today can be met from RNG from state’s existing organic waste streams. Not only that, but the infrastructure is also readily available. The transition to adopt RNG won’t be like the push to adopt electric vehicles where new infrastructure needs to be built everywhere, rather RNG can be put right into the pipelines and fueling stations that are already in use for fossil gas.
RNG also has a unique benefit in its opportunities for use that other clean energy sources don’t share. While RNG can be burned directly for electric generation, it can also be sent into homes and businesses as a heating fuel, minimizing the types of inefficient losses associated with power generation, transmission, and distribution. Further than that, transportation fuel is actually the most common use of RNG, when it substitutes for diesel gasoline in heavy-duty vehicles and trucks.
Outside of emissions, the proliferation of RNG provides additional environmental benefits. Production and use of RNG tends to be typically local in nature, meaning a region’s use of RNG will directly reduce the local waste streams in that same region and minimize the volume of landfill dumping. Additionally, the use of RNG will ease the stress on existing, carbon-intensive energy source. All of these free co-benefits are happening locally.
However, RNG certainly does not come without downsides and its own set of detractors. For one, the burning and use is still physically emitting carbon into the atmosphere. While those emissions may not show up in the accounting sheets, there is an argument to be made that the RNG sector is creating demand for more of these organic materials, making their growth and ultimate burning more prominent. For example, if the economics dictate RNG has sometimes caused farmers to grow corn or other crops specifically to be fuel, which defeats the purpose. When the food and plant scraps are natural byproducts, the emissions reductions are real, but when those crops are grown specifically to be fuel inputs then it eliminates those benefits.
Another potential concern with RNG is in its ability to act as a crutch for the traditional natural gas industry. By integrating RNG, traditional natural gas producers are being given license to continue their business (and emissions) as usual while adding a minority of renewably sourced gas. Some environmental advocates would argue that a more immediate shift to electrification would be a better long-term approach than allowing RNG to delay that switch.
In terms of clean energy solutions, RNG is also somewhat expensive and hard to scale. Where the resources for RNG are readily available, the fuel can be a valuable resource. But in the end, RNG will only be available as a single tool to reduce a portion of fossil fuel usage. Real energy transition needs other sources where RNG can simply be a complement.
The Future of the RNG Sector
According to the U.S. Energy Information Administration, 2019 saw 261 billion cubic feet were consumed in the United States, mostly via independent power producers but also from electric utilities, commercial sector actors, and industrial sector actors. However, that’s just the tip of the iceberg as studies suggest that 2,200 billion cubic feet (BCF) of RNG could be sourced from anaerobic digestion in the United States, which would be the equivalent to 11% of the daily U.S. natural gas consumption.
What’s notable about the RNG sector isn’t just where it sits now, but the pace at which it’s growing. Over the past five years, the number of RNG production facilities has tripled, with 115 now currently in production. That state of growth will only continue on strongly, as numerous states—among them California, Washington, and Oregon (all climate leaders in their own right)-- have integrated RNG into their official climate goals and targets.
Moving forward, RNG will still likely remain niche, not providing a snug one-size fits all solution. However, RNG projects must be evaluated for what resources are available locally and where they can displace otherwise fossil fuel use. Renewable natural gas will continue to be a valuable complementary clean energy source with a growing presence and environmentally-positive impact across the country and the world.
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