Renewable Natural Gas (RNG)
RNG Facilities and Regenerative Thermal Oxidizers (RTOs)
With an increasing focus on Renewable Natural Gas (RNG), it is imperative to evaluate the variables governing abatement technology for tail gas. This evaluation is essential for designing the required equipment. The processes commonly employed in RNG upgrade facilities can result in significant variation in Volatile Organic Compound (VOC) content and airflow levels. When methane content is sufficiently high (around 8%), it can sustain self-combustion in a direct-fired oxidizer, necessitating different design considerations for streams with low methane content (<5%). In cases where methane content is in the lower range, utilizing a Regenerative Thermal Oxidizer (RTO) becomes a more efficient option to ensure proper operation.
The size of the RTO required depends on factors such as maximum VOC content and peak flow conditions. This often involves integrating a Hot Gas Bypass to release excess heat generated during high-capacity operations (typically implemented for streams with methane equivalent levels exceeding 1.25%). This step is necessary for maintaining thermal balance due to the heat generated during the combustion of organic elements within the tail gas stream.
VOC Breakdown with RTOs
Creating the right environment for Volatile Organic Compounds (VOCs) breakdown is crucial when designing a Thermal Oxidizer. Achieving this equilibrium is based on three factors: the duration of exposure to the destruction temperature, maintaining the required temperature in the retention chamber, and creating essential turbulence to enable thorough mixing of combustible elements with the necessary air for optimal combustion. When VOC concentrations are higher, it becomes possible to maximize airflow velocity with the heat exchange beds, and a Regenerative Thermal Oxidizer (RTO) can be used since efficient heat transfer becomes less critical due to surplus heat. It is essential to stay within the system’s pressure drop limitations. Precise calculation of retention time at peak flow rates becomes necessary to ensure that destruction efficiencies are met.
Design complexities arise at the lower end of the flow/VOC spectrum. Managing the fan’s turndown capabilities and maintaining a minimum airflow velocity through the beds for effective heat exchange and turbulence becomes challenging. In scenarios with lower heat content, the time spent in the retention chambers must increase to contribute to improved destruction conditions. Turbulence diminishes and can potentially lead to uneven airflow distribution throughout the heat exchange beds, subsequently reducing the efficiency of heat exchange mechanisms. In this situation, strategic stream management can address the turndown situation. Including accumulators to adjust the flow/VOC content can also be employed to avoid the need for increased fuel injection during combustion.
Designing and Sizing the Right RTO
Understanding the design considerations behind this downstream equipment supports RNG facility developers in creating financially beneficial facility operations. Knowing how much time is necessary at either end of the spectrum is crucial for understanding the system’s operational expenses. Establishing partnerships with companies specializing in custom designing and sizing Regenerative Thermal Oxidizers, such as Ship & Shore Environmental, proves beneficial in obtaining the most competitively priced solutions for acquiring and operating abatement equipment within this sector.
