An Overview of the RNG Purification Treatment Process

Depending on what feedstock is used in the digestion process, the raw biogas that is collected from the biodigester typically consists of between 45-65% flammable methane, with several impurities making up the balance. When the biogas is used to produce renewable natural gas (RNG), these impurities, which include carbon dioxide, oxygen, nitrogen, sulfur, hydrogen sulfide, siloxanes and volatile organic compounds (VOCs), need to be removed so that the methane content of the RNG is increased to at least 90% according to the specifications of the gas pipeline operator or end users, who may also stipulate other RNG quality requirements, such as heating value, pressure, temperature and moisture content.

Therefore, converting biogas to RNG is typically a multistep process, with several forms of purification undertaken at each stage of the treatment process, as outlined below:

1. Primary Treatment: where moisture and particulates are removed from the biogas;
2. Secondary Treatment: any remaining moisture is removed, along with contaminants, and the gas is compressed;
3. Advanced Treatment: carbon dioxide, oxygen, nitrogen and VOCs are removed, before the gas is compressed further.

After undergoing primary and secondary treatment, the biogas typically has a medium-Btu, with a heating value of around half that of fossil natural gas. However, this increases after undergoing advanced treatment, resulting in RNG that has a similar heating value to that of fossil natural gas.

During the advanced treatment stage of RNG purification, a small portion of methane is removed along with the carbon dioxide and other residual contaminants such as hydrogen sulfide, and destroyed by flaring or thermal oxidation. The amount of methane removed during this process depends on several factors, including the technology used during this stage of treatment, the methane content required by the end user, and weighing up the benefits of capturing the lost methane against the costs especially in terms of capital outlay of achieving this.

RNG Purification: Primary Treatment

During the primary treatment stage, basic moisture and particulates are removed from the biogas produced in the digester. The raw biogas is channeled through a knockout pot, a filtration system and a blower to reduce the moisture content. This is the only form of treatment required by landfill operators when destroying landfill gas by flaring; and is often the only treatment used by LFG energy operations who use medium-Btu LFG in applications that do not require a high heating value, such as boilers, kilns and leachate evaporators.

RNG Purification: Secondary Treatment

During the secondary treatment stage, any remaining moisture is removed using an after cooler which promotes condensation, together with impurities such as sulfur and siloxanes, before the gas is compressed. The type of impurities removed during this stage of treatment will depend largely on which contaminants are present, and their concentrations, as well as how/where the biogas will ultimately be used. Depending on its end use, the gas may be compressed during this stage of treatment if required. The secondary treatment process produces a medium-Btu gas with a lower heating value than fossil fuel gas which can be used for heating (e.g. boilers) or to generate power (e.g. turbines and engines).

RNG Purification: Advanced Treatment

In order to convert biogas into renewable natural gas it has to undergo more advanced treatment to remove contaminants such as carbon dioxide, oxygen, nitrogen, volatile organic compounds and siloxanes that may be present. The type of technologies used during advanced treatment vary and are largely dependent on the site and the project. We will discuss the different types of technologies used during advanced treatment in more detail, including their pros and cons, in future posts.

RNG Fuel Specifications

If the RNG produced is going to be injected into a pipeline for distribution to an end user further afield, the methane content specifications are typically higher than those required by onsite fuel stations that run their fleet vehicles on RNG. Consequently, RNG pipeline injection projects typically recover around 96-98% of the methane that would otherwise be lost during treatment. Gas pipeline operators also require low levels of oxygen and other inert gases in RNG injected into their pipeline. However, since these requirements mean that methane is removed and recovered from the tail gas, leaving only inert gases that do not readily combust, they need to be destroyed by flaring or thermal oxidation with the help of additional fuel.

According to the latest EPA report (July 2020): In the United States, CNG vehicle fuel project developers in the US generally design projects to meet the technical requirements set by the Society of Automotive Engineers Surface Vehicle Recommended Practice J1616â„¢ for Compressed Natural Gas Vehicle Fuel (SAE J1616). SAE J1616 sets minimum requirements for CNG fuel composition and properties to ensure vehicle, engine and component durability, safety and performance. It provides technical requirements for several fuel properties and potential constituents including CH4, sulfur compounds, O2 and particulate material. SAE J1616 references CARB's CNG commercial fuel composition for several specifications, including 88 percent CH4 (minimum) and 1 percent O2 (maximum).

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