4 The record articles

Renewable Natural Gas – What Questions Should I Ask to Prepare for the Air Quality Permitting Process?

Posted: August 10th, 2022

Authors: John H. 


This is the first of a series of ALL4 articles addressing topics related to Renewable Natural Gas (RNG) projects. This article will focus on what questions project developers can ask to acquire vital information needed to prepare for the air quality permitting process. Asking the questions discussed below can help avoid significant project costs and delays.

RNG is defined by the U.S. Environmental Protection Agency (U.S. EPA) as “biogas that has been upgraded for use in place of fossil natural gas. The biogas used to produce RNG comes from a variety of sources, including municipal solid waste landfills, digesters at water resource recovery facilities (wastewater treatment plants), livestock farms, food production facilities and organic waste management operations.” Also, according to U.S. EPA, the number of new RNG facilities in the U.S. fleet has grown rapidly in the last five years, from 52 facilities operating in 2017 to 174 facilities operating in 2021. ALL4 expects this upward trend to continue.

RNG facilities are often subject to both state and federal air pollution control requirements which can affect the design, operation, and cost of such facilities. These requirements are identified in each state’s regulations and are reflected in permits issued by regulatory agencies, which are legally enforceable documents. It is important to note that permitting requirements can vary significantly from state to state.


Not all RNG plants are the same and need to be carefully evaluated on a case-by-case basis. While the underlying principles of creating RNG are generally similar, the way RNG is created (i.e., the recipe for creating RNG) can vary significantly from site to site. Therefore, RNG plant regulatory requirements can vary significantly. Some sources of that variability are:

  • Feedstock – Some examples of feedstock are manure, food waste, landfill waste, and sewage sludge. The type of feedstock can affect the chemistry of the raw biogas and therefore the equipment and processes needed to convert the raw biogas to RNG. For example, control of siloxanes may need to be addressed in the permitting process for certain feedstocks.
  • Fuels burned to provide heat and power – There are several fuels that a given facility may rely on such as raw biogas, pipeline natural gas, propane, and diesel fuel. The regulations for fossil fuels (e.g., diesel and propane) are often different than those from non-fossil fuels (raw biogas). For example, burning raw biogas in lieu of (fossil) pipeline gas can trigger different air toxics regulations in some states.
  • Emissions control equipment – hydrogen sulfide (H2S) control is commonly addressed in the air permitting process as it is often produced in the anaerobic digestion stage of producing RNG. The levels of H2S can vary significantly (i.e., up to several orders of magnitude) from site to site and therefore, the type and level of emissions control can vary accordingly. The level of pollution control is worthy of early consideration as it can affect the type of permit (i.e., minor source versus major source) that will be required for the facility.
  • Power availability – RNG facilities need power. In the event sufficient power from the electric utility grid is not available (i.e., three-phase power), a facility may need equipment to generate its own power, such as a reciprocating engine. Applicable regulations will depend on the type of engine (spark ignition versus compression ignition) and the type of fuel burned in the engine.


Here are a few questions we consider with our clients that address cost, schedule, and staffing requirements for planned RNG facilities.

  1. Do I need an air quality permit? Permits are typically issued by state regulatory agencies. Permit applicability typically depends on factors such as the desired RNG production level, type of organic matter that will be converted to RNG, type and size of equipment (e.g., boilers, generators, gas cleaning equipment), and the type of fuels to be burned (e.g., raw biogas, pipeline gas, propane, and diesel).
  2. How quickly can I obtain an air quality permit? Many states do not allow construction prior to issuance of a permit. Hence, it is very important to understand the timeframe between an air permit application submittal and permit issuance. Also important is understanding any factors that could delay the permitting process, such as public holidays, public opposition, and the backlog of previously submitted applications the state has in its queue for review.
  3. What fees will the regulatory agency charge? States typically charge a fee to process an air quality permit application (permit fee). Additionally, states require facilities to register their actual emissions annually and charge a fee for those emissions (registration fee). Awareness of these fees can help inform discussions around project design. For example, the installation of pollution control equipment (i.e., equipment that reduces the quantity of emissions released into the air) can reduce a facility’s emissions potentially qualifying it for a simpler air quality permit (with a lower permit fee) and reduce registration fees. These cost savings could potentially help pay for pollution control equipment!
  4. Will the permit allow me to build my project the way I want to build it? Many state permitting programs require air quality modeling to be performed to confirm project design parameters (e.g., effluent stack heights, building height/location, pollution control equipment) will enable the project to meet ambient (i.e., outdoor) air quality standards. Air quality modeling can be performed well in advance of submitting an air quality permit application to evaluate a project’s design and identify early-on if any significant design changes should be considered (e.g., raising/moving a stack, moving a building, changing the control efficiency of a pollution control device).
  5. Will I have to install pollution control equipment? Many state permitting programs require pollution control technology evaluations [e.g., Best Available Control Technology Evaluation (BACT)] which can impact the type of pollution control equipment selected for a project. Pollution control equipment capital and/or operating costs (and benefits) can be identified well in advance of submitting an air quality permit application.
  6. What do I need to do after my air quality permit is issued? Air quality permits typically include monitoring, testing, recordkeeping, and reporting requirements. The level of effort and cost to comply with each of these categories can vary based on the size and location (i.e., state) of a project. These requirements apply for the life of a facility (assuming no facility or regulatory changes occur), so it is important to ask questions about them early in the planning process.


No better time than the present to start asking questions! If I were in your shoes, I would start by enrolling an air quality permitting expert to get the conversation started. Alternatively, or additionally, I would reach out to the state regulatory agencies that issue air quality permits for the state(s) where you plan to construct RNG facilities.

The next article in this series will address questions that project developers can ask round Climate Change and Sustainability.

If you have any questions about air quality permitting for an RNG project, please reach out to John Hinckley at jhinckley@all4inc.com or by phone at (802) 359-7294.


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