Encyclopedia of Renewable Energy. James G. Speight

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      Biogas – Upgrading

      Biogas upgrading principally refers to removal of the carbon dioxide from the biogas to increase energy density and decrease needed storage volumes of finished product. Moisture, sulfur compounds, organo-silicon compounds (siloxanes), and other impurities in biogas or fuel gas are usually removed as well to meet gas quality specifications, and because they can cause problems in gas handling equipment and damage engines and emissions controls. Biomethane from upgraded biogas or landfill gas can be compressed or liquefied and used as a fuel in compressed natural gas (CNG) or liquefied natural gas (LNG) vehicles.

      Processes available for separating carbon dioxide from the methane in biogas include adsorption methods such as scrubbing with water, Selexol (polyethylene glycol ether), and amines, pressure swing absorption (PSA), membrane separation, and cryogenic separation. However, 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.

      Typically, biogas is usually fully saturated with water vapor and typically has from 40 to 60% methane (CH₄) and 40 to 60% v/v carbon dioxide (CO₂). Thus, the choice of an appropriate technology or combination of technologies to upgrade the gas from these modest methane levels up to 99% v/v methane can be challenging. The main treatment goal of gas upgrading projects is to get the carbon dioxide removed from the biogas stream to an acceptable level, typically on the order of with 1 to 2% v/v carbon dioxide. Actual specifications will vary based on end use and the specific requirements provided by the gas utility.

      Selecting an upgrading system that can reliably meet the methane-oxygen-carbon dioxide-hydrogen sulfide specifications 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. Therefore, selecting a biogas upgrading system that can handle varying gas quality and quantity and stay within specifications is essential.

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 hydrogen sulfide from the biogas stream before the gas the stream is fed to the upgrading process(es). 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. 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.

      In terms of water 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 approximately 6 to 12% w/w water. 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., >21°C, >70°F) gas is indicative of very humid gas that is acceptable to send to wet upgrading systems. Low dew point (i.e., <-29°C, <-20°F) gas is indicative of very dry gas that is acceptable to send to dry upgrading systems. However, each of the upgrading systems is capable of meeting gas pipeline or vehicle fuel specifications, either as standalone units or in combination with each other.

      There are four main technologies are used to create the so-called renewable natural gas stream from biogas, and these are (i) membrane separation, (ii) pressure swing adsorption (PSA), (iii) amine scrubbing, and (iv) water wash, also known as water scrubbing. Typically, these processes are usually deployed individually but can sometimes be installed in a series with one another, as needed for the given project requirements.

      The membrane system and the pressure swing adsorption system are somewhat similar as both are dry upgrading systems that involve a physical separation of the carbon dioxide and methane based on (i) molecular size, (ii) pressure, and (iii) ionic charge, if any. Water wash and amine systems are similar in that they are both wet upgrading systems and involve separating the carbon dioxide from the methane by solubilizing the carbon dioxide in a liquid solution while allowing the methane to remain in the gas phase.

       Membrane Separation

      The membrane separation technique uses polymeric membranes to separate the carbon dioxide from the methane in biogas while under high pressure. As a result of the process, there is (i) permeate gas, which is the gas that has traveled through the porous membrane and (ii) retentate gas, which is gas that is retained and collected at the end of the membrane fiber; this gas travelled down the path of the membrane lumen hole but did not travel through the porous membrane. In recent years, membrane manufacturers have improved manufacturing quality, improved membrane selectivity, and overall system methane recovery. These factors all lead to better overall system performance, improved economics, and increased opportunity for success.

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