Frequently Asked Questions
Renewable natural gas, or RNG, can be a made from a variety of organic feedstocks through a relatively straightforward but multi-unit operation processing plant. Many folks are interested in learning more about RNG and/or what Viridi Energy Solutions is offering the market. Accordingly, we’ve created this frequently asked questions (FAQ) page to answer all (and any) of your RNG questions. Keep reading to learn more.
Renewable Natural Gas: Frequently Asked Questions
What is RNG?
Renewable natural gas (RNG) is chemically like natural gas that we have all used in our homes and office buildings for heating, cooling, and cooking. The primary difference between the two is the source of the gas. Natural gas is derived from fossil sources, while RNG is derived from fresh organic materials like food waste, municipal biosolids, other biomass sources (wood waste, etc.) or livestock manure. A secondary difference is that, unlike natural gas, RNG contains only methane, and accordingly does not include other components such as ethane, propane, butane, etc., which are found in ‘natural gas’. Anaerobic digestion (“AD”) converts this organic residue into biogas that can then be converted into RNG. The biogas can be used to produce heat, power, and RNG. AD byproducts can also be produced such as fertilizer and biochar, and clean water that can be recycled.
Typically, the organic material used to create RNG would otherwise be a waste product. The conversion of this organic material into RNG starts with the either an anaerobic digestion process or a gasification/pyrolysis process, which converts organic material directly into biogas. Biogas is typically made up of methane, carbon dioxide, water vapor and trace gases such as oxygen and nitrogen. Once the biogas has been produced, it goes through an upgrading process to remove the non-methane components, which is then compressed and piped into a gas transmission line as RNG which is then used as a transportation fuel, displacing fossil fuels.
What is biogas?
Biogas can be derived either anaerobically with natural organisms or through pyrolysis/gasification. Anaerobic digestion is the largest biogas production unit operation in the U. S. Anaerobic digestion occurs in landfills and covered tanks. Anaerobic digestion (without the presence of oxygen) processes organic material over a period of time (typically >18 days hydraulic retention time). This anaerobic process allows acetogenic and methanogenic bacteria to form, breaking down organic material and creating biogas.
What is anaerobic digestion?
Anaerobic digestion is a process that occurs naturally in the environment or in a cows rumen. In the RNG industry, anaerobic digestion occurs by loading organic material (feedstock) into a vessel. It will be deprived of oxygen (anaerobic), heated to about 100F, and allowed a residence time of 18-30 days. AD can occur without heat, but the hydraulic retention time increases significantly. These factors (volatile organic solids + heat + time) enable the naturally occurring bacteria to form and convert almost half of the organic solids into biogas.
What is gasification?
Gasification is a technological process that can convert any carbonaceous (carbon-based) raw material such as woody biomass and biosolid residues (and yes, even coal) into fuel gas, also known as synthesis gas (syngas for short). When biomass is gasified it is often also called biogas. Gasification occurs in a gasifier, generally at high temperature/pressure vessel where oxygen (or air) and steam are directly contacted with the feed material causing a series of chemical reactions to occur that convert the feed to syngas and ash/slag (mineral residues). Syngas is so called because of its history as an intermediate in the production of synthetic natural gas. Composed primarily of the colorless, odorless, highly flammable gases carbon monoxide (CO) and hydrogen (H2), syngas has a variety of uses. The syngas can be further converted (or shifted) to nothing but hydrogen and carbon dioxide (CO2) by adding steam and reacting over a catalyst in a water-gas-shift reactor. When hydrogen is burned, it creates nothing but heat and water, resulting in the ability to create electricity with no carbon dioxide in the exhaust gases. Furthermore, hydrogen made from coal or other solid fuels can be used to refine oil, or to make products such as ammonia and fertilizer. More importantly, hydrogen enriched syngas can be used to make gasoline and diesel fuel. Polygeneration plants that produce multiple products are uniquely possible with gasification technologies. Carbon dioxide can be efficiently captured from syngas, preventing its greenhouse gas emission to the atmosphere and enabling its utilization (such as for Enhanced Oil Recovery) or safe storage.
Gasification offers an alternative to more established ways of converting feedstocks like biomass, and some waste streams into electricity and other useful products. The advantages of gasification in specific applications and conditions, particularly in clean generation of electricity, may make it an increasingly important part of the world’s energy and industrial markets.
What is pyrolysis?
Pyrolysis is one of the technologies available to convert biomass to an intermediate liquid product that can be refined to drop-in hydrocarbon biofuels, oxygenated fuel additives and petrochemical replacements. Pyrolysis is the heating of an organic material, such as biomass, in the absence of oxygen. Biomass pyrolysis is usually conducted at or above 500 °C, providing enough heat to deconstruct the strong bio-polymers mentioned above. Because no oxygen is present combustion does not occur, rather the biomass thermally decomposes into combustible gases and bio-char. Most of these combustible gases can be condensed into a combustible liquid, called pyrolysis oil (bio-oil), though there are some permanent gases (CO2, CO, H2, light hydrocarbons), some of which can be combusted to provide the heat for the process. Thus, pyrolysis of biomass produces three products: one liquid, bio-oil, one solid, bio-char and one gaseous, biogas or syngas. The proportion of these products depends on several factors including the composition of the feedstock and process parameters. However, all things being equal, the yield of bio-oil is optimized when the pyrolysis temperature is around 500 °C and the heating rate is high (1000 °C/s) fast pyrolysis conditions. Under these conditions, bio-oil yields of 60-70 wt% of can be achieved from a typical biomass feedstock, with 15-25 wt% yields of bio-char. The remaining 10-15 wt% is syngas. Processes that use slower heating rates are called slow pyrolysis and bio-char is usually the major product of such processes. The pyrolysis process can be self-sustained, as combustion of the syngas and a portion of bio-oil or bio-char can provide all the necessary energy to drive the reaction.
What is Biochar?
Biochar is black carbon produced from biomass sources [i.e., wood chips, plant residues, digestate solids, or other agricultural waste products] for the purpose of transforming the biomass carbon into a more stable form (carbon sequestration). Black carbon is the name of the range of solid residual products resulting from the chemical and/or thermal conversion of any carbon containing material (e.g., fossil fuels and biomass) (Jones et al., 1997). Biochar does not refer to a singular product with a given set of chemical and physical characteristics. Rather, biochar spans the spectrum of black carbon forms (Spokas, 2010) and it is chemically and physically unique as a function of the feedstock, creation process (pyrolysis unit), cooling, and storage conditions. This is an emerging market as both a carbon sequestering process and soil amendment or carbon feedstock replacement at cement kilns.
How are your projects financed?
To date, Viridi Energy Solutions has financed its RNG projects primarily with equity and, in some cases, debt and equity.
What types of digesters and gas upgraders do you use on your projects?
The RNG industry utilizes various technologies for digesters and gas upgrading systems. These are above-ground continuously stirred tank reactors (CSTR), in-ground horizontal plug flow, and covered lagoons. When selecting a digester, feedstock quality and composition, project size, project location, and digestate utilization are all decision-making factors.
The RNG industry typically uses various types of upgraders—including pressure swing adsorption, membrane, and amine scrubbing. Inlet biogas quality, outlet RNG quality, uptime, methane recovery, and operability are all factors in choosing a gas upgrader.
How do you earn revenue on your projects?
We generate revenue on our projects through three primary income streams:
1. LCFS (Low Carbon Fuel Standard Program) Credits – California’s LCFS program provides the project with credits for selling RNG into California and displacing fossil fuels in the transportation industry. These credits can be sold on the free market.
2. RIN (Renewable Identification Number) Credits – Our projects receive an EPA-issued RIN credit for each unit of RNG we produce. These credits can be sold on the free market.
3. Commodity natural gas sales
4. Fertilizer production
How do your projects impact local communities?
Our projects provide local communities with jobs, innovative infrastructure, energy projects, and increased natural gas capacity for the local electric grid or gas system.
What are the environmental benefits of our projects?
Our RNG projects deliver a true ‘win-win’ in driving sustainable agriculture with significantly less waste and improving economic development in rural communities, as well as solving a big problem for municipalities with respect to biosolids disposition from publicly owned wastewater treatment plants. Our projects are carbon negative.
What are the outputs from anaerobic digestion?
The outputs of anaerobic digestion fall into two categories: the gaseous and nongaseous outputs. The gaseous output is biogas, which is the main product that drives RNG projects. The nongaseous output is the leftover organic feedstock, known as digestate. The digestate contains water and undigested biosolids. Digestate maintains the nutrient values of the pre-digested feedstock, and the digestate biosolids still has a nominal calorific value of ~3,000 to 4,500 BTU/lb. This can be used directly on farm fields, or further processed into bio-fertilizer products, biochar and clean water.
Does anaerobic digestion reduce pathogens or odor from manure?
By keeping manure at an elevated temperature of around 100F for 20-30 days, pathogens are reduced by approximately 95%, particularly for Johne’s Disease (Mycobacterium avium).
The anaerobic digestion process can reduce odor by anaerobically treating manure in a controlled enclosed digester. Without a digester, odors can occur as bacteria convert organic solids in manure into volatile acids in an open environment.
How much will it cost for me to work with Viridi to install an RNG plant on my location?
No upfront capital is required when clients partner with Viridi. To provide feedstock to our RNG projects, we compensate our feedstock partners with upfront payments and ongoing payments throughout the operational term.
How long does it take to go from concept to project complete?
Viridi can generally reach terms within a few weeks of a presented opportunity. Because we can fund with all equity, the total timeframe from opportunity to a financial close is generally under 120 days. Once financially closed and a formal notice to proceed is issued to our EPC Contractor partner, the plant will generally be operational within 16 months.
What is Viridi’s business objective?
Unlike some of our competitors, Viridi’s objective is to buy and hold assets for a long time, generally 20 years or longer. This assures our partners relationship stability.
What is RNG used for?
RNG is chemically the same as natural gas and thus has the same uses. It is used for heating and as an available energy source for transportation fleets. According to the RNG Coalition, nearly 170 plants in the U.S. produce RNG, and while most Americans don’t yet realize it, over 50,000 buses and trucks run on RNG daily. For example, the New York MTA fuels its entire natural gas buses with RNG.
How is RNG beneficial?
Research shows that when all-climate benefits are considered together, RNG from dairy manure can reduce greenhouse gas emissions up to 350% when replacing traditional diesel fuel. Converting vehicles to run on net-carbon-negative RNG is a crucial lever in reducing U.S. greenhouse gas emissions.