Basics Of Biogas Upgrading
Several powerful incentives are driving biogas generators to consider upgrading their biogas to meet standards for pipeline injection and vehicle fuel markets. Producing renewable natural gas (RNG) can create opportunities to monetize lucrative RIN, or renewable identification number, credits under the federal Renewable Fuel Standard and generate carbon credits under state supported low carbon fuel standards.
As each biogas project is unique it can be a challenge to determine the best gas upgrading technology for the given situation. Digester biogas can have varying levels of carbon dioxide (CO2) and elevated hydrogen sulfide (H2S) to address. Landfill gas and gas from covered lagoon digesters can have elevated nitrogen (N2) and oxygen (O2) levels and landfills and municipal wastewater treatment plant (WWTP) digesters have siloxanes that need to be handled.
Biogas Stream Considerations
Biogas is usually fully saturated with water vapor and typically has from 40 to 60 percent methane (CH4) and 40 to 60 percent carbon dioxide (CO2). Finding the proper technology or combination of technologies to upgrade the gas from these modest methane levels up to 99 percent methane can be challenging. The main treatment goal of gas upgrading projects is to get the CO2 removed from the biogas to create RNG with 1 to 2 percent CO2. Actual specifications will vary based on end use and the specific requirements provided by the gas utility off-taker.
Selecting an upgrading system that can reliably meet the CH4, O2, CO2, nitrogen and H2S specifications — and perform reliably day in and day out — is critical for a successful relationship with one’s gas off-taker. At times, such as at a new project site, technologies may need to be selected without having extensive biogas characterization data. Selecting a biogas upgrading system that is robust and can handle varying gas quality and quantity and stay within specifications is paramount.
The preferred design approach is to design for existing biogas streams that allow for maximum data collection for the upfront characterization of the gas. Generally, gas is pretreated to remove water and H2S before the gas upgrading unit. Where appropriate, users should consider drying biogas through aggressive refrigeration techniques or desiccant drying. Hydrogen sulfide can be effectively removed by several techniques, including ferric chloride injection in the digester, chemical scrubbing, carbon filtration, iron-based media filtration or biological desulfurization. Municipal WWTP digester biogas and landfill gas containing traces of siloxanes requires pretreatment through media filtration. In all cases the pretreatment system needs to be evaluated to ensure that it does not interfere with the gas upgrading system that follows.
In terms of water vapor content, biogas from a digester has just emerged from a warm, liquid process so it is fully saturated with water. Water vapor, or steam, is dissolved in the gas to its highest degree possible and thus the gas is extremely damp and humid, containing about 6 to 12 percent water by weight. Warm gas holds more water vapor than cool gas. Dew point is the measure used to describe the condition where steam starts to condense into liquid. High dew point (i.e., >70°F) gas is indicative of very humid gas that is acceptable to send to “wet” upgrading systems. Low dew point (i.e.,