As part of the purification process to transform biogas into RNG, some CH4 (methane) is stripped out as “tail gas,” along with the CO2 and other residual constituents such as VOCs, siloxanes, and H2S. This tail gas is typically routed to a flare or thermal oxidizer for destruction. The amount of methane removed during this process depends on several factors, including the technology used to upgrade the gas, the methane content required by the end user, and the cost benefit of additional methane capture considering the capital expense to achieve it.
Depending on the concentration of methane, as well as other VOC constituents there are several types of thermal oxidizers which are considered for controlling these "tail gas" emissions:
The most efficient thermal oxidizer for tail gas control, is a Regenerative Thermal Oxidizer (RTO). During operation of the standard two chamber RTO, the VOCs are directed into one of the system’s regenerators, an internally insulated vessel containing specialized ceramic media heat exchanger which allows thermal rate efficiencies up to 97%.
The contaminated gases are passed through the first regenerator where energy is transferred from the ceramic media to the gas in order to elevate the temperature. This elevated temperature approaches the ignition level for most solvents and is then directed from the ceramic bed into the combustion chamber. As the stream exits the ceramic bed and travels through the internally lined combustion chamber, minimal heat is added to ensure a proper oxidation temperature and a designed dwell time is maintained providing destruction of the VOCS at greater than 98% efficiency. The resultant clean, oxidized gases are directed to the second regenerator to absorb the heat energy before being released to the atmosphere.table of contents
A Direct Fired Thermal Oxidizer is used on applications that have very high solvent concentration levels, low air volumes, or particulate present. While offering the lowest capital expense investment, this is usually offset by high supplemental fuel usage required to operate.
A Recuperative Thermal Oxidizer integrates a heat exchanger into the design in order to minimize the fuel used for oxidation. By passing the hot exhaust gasses through a stainless-steel, non contact air-to-air primary heat exchanger, recovered heat is used to warm the incoming air to the oxidizer. These systems are more fuel efficient than a DFTO, however they are still less efficient than an RTO.
A Thermal Combustor System (TCS) combines the simplicity of an enclosed flare with the robustness and proven performance of a thermal oxidizer. The TCS are widely flexible and can confidently handle the vent gas that may or may not be within the flammable range. The TCS provides an extraordinary level of safety when compared to traditional flaring systems.
Renewable natural gas (RNG) is a term used to describe anaerobically-generated biogas that has been upgraded (or refined) for use in place of fossil natural gas. Raw biogas typically has a methane (CH4) content between 45 and 65 percent, depending on the source of the biogas, and must go through a series of steps to be converted into RNG. Biogas is treated to remove moisture, carbon dioxide (CO2) and trace-level contaminants (including siloxanes, volatile organic compounds (VOCs) and hydrogen sulfide (H2S), as well as reducing the nitrogen (N2) and oxygen (O2) content. Once purified, the RNG has a CH4 content of 90 percent or greater. RNG injected into a natural gas pipeline commonly has a CH4 content between 96 and 98 percent.
There are four main sources of biogas used to produce RNG in the United States. At each of these types of operations, biogas is produced as the organic materials are broken down by microorganisms in the absence of O2 (i.e., anaerobic conditions). “Organic” in this context means the wastes come from, or were made of, plants or animals.
Landfill gas (LFG) is generated in MSW landfills as the organic wastes decompose anaerobically. Instead of escaping into the air, LFG can be captured, converted and used as an energy resource. Applicable federal and state regulations require certain landfills to capture and destroy the LFG generated.
The US EPA Landfill Methane Outreach Program (LMOP) is a voluntary program that works cooperatively with industry stakeholders and waste officials to reduce or avoid methane emissions from landfills. LMOP encourages the recovery and beneficial use of biogas generated from organic municipal solid waste.
Stand-alone digesters are the newest source of RNG in the United States. These AD projects break down source separated organic material including food waste to generate biogas, which can be converted to RNG. Digesters that primarily process food waste can also co-digest other organic materials including yard waste.
As a substitute for fossil natural gas, RNG has many potential uses. RNG can be used as vehicle fuel, to generate electricity, in thermal applications, or as a bio-product feedstock. RNG can be injected into natural gas transmission or distribution pipelines, or it can be used locally (i.e., at or near the site where the gas is created).
Developing RNG resources is one way to diversify fuel supplies and increase fuel security, provide economic benefits to communities and end users, improve local air quality and reduce greenhouse gas (GHG) emissions.
The US EPA’s An Overview of Renewable Natural Gas from Biogas provides resource to promote and potentially assist in the development of RNG projects and provides technical information on how raw biogas is upgraded into RNG.