Encyclopedia of Renewable Energy. James G. Speight
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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.
For landfill gas or covered lagoon applications, separating nitrogen from methane is challenging with membranes; therefore, further gas polishing is required when there is a significant amount of nitrogen in the raw biogas. In addition, biogas oxygen levels can cause performance issues as membranes are not selective for separating oxygen. For example, sources of oxygen can be a less-than-airtight landfill or leaky covered lagoon, as well as leaking fittings on the suction side of compressing equipment.
A typical pretreatment option uses an activated carbon filter to remove hydrogen sulfide to a level <100 parts per million (ppm) and removal of volatile organic compounds (VOCs). Drying can be achieved by refrigerating the gas to 4 to 15°C (40 to 60°F) and capturing the resulting condensate. The volatile organic compounds should be free of any volatile organic compounds which can irreversibly foul or compromise some membrane types.
Pressure Swing Adsorption
The pressure swing adsorption (PSA) system is a batch process utilizing several vessels running in parallel under pressure. The heart of the process is an adsorptive medium, similar to activated carbon, which separates the constituents of the gas stream based on the molecular weight and size of the constituents. Predrying the gas ahead of the adsorbers to approximately 5°C (41°F) is required to keep the humidity out of the vessels to maximize their performance as dry adsorbers.
Carbon dioxide is preferentially adsorbed onto the media because it is a smaller molecule than methane and can permeate into the tiny pores in the carbon bed more easily and deeply. The methane goes through adsorber process columns relatively untouched, while the carbon dioxide does not. The adsorption process is reversible; thus, the carbon dioxide is eliminated during the regeneration cycle.
When a vessel has been saturated with contaminant gas, it is regenerated by reducing the pressure. At lower pressure, compounds are removed and desorbed from the media in a separate gas stream called “tail gas.” The common cycle time to saturation is 2 to 4 minutes. The tail gas is a low-quality gas stream that contains the previously adsorbed compound (such as carbon dioxide, hydrogen sulfide, and methane). Each vessel alternates its mode of operation from service, depressurization, regeneration, and finally repressurization before it returns to service mode.
Amine Scrubbing
The amine scrubbing system uses a two-step approach to upgrading biogas. The first step is adsorption followed by a second step of stripping or desorption. The amine portion of the scrubbing solvent molecule chemically reacts to the carbon dioxide in the biogas to retain it in solution. A common chemical deployed as the scrubbing solvent is monodiethanolamine (MDEA). The methane fraction of the biogas passes through the packed tower reactor untouched by the scrubbing chemical. High methane purities can be achieved in the recovered natural gas (>99.9% v/v).
In the second step, the scrubbing solution is heated to boiling to reverse the chemical reaction. The carbon dioxide in the fully packed stripper tower is disassociated from the scrubbing solution and discharged. High carbon dioxide purity in the off-gas can be achieved to accommodate potential for reuse of the carbon dioxide. The regenerated amine scrubbing solution is then cooled and reused back in the scrubbing tower in a closed loop system. Systems run at a relatively low operating pressure of approximately 0.5 to 3 psi. This low-pressure design affords equipment and operational savings as compared to systems that run at high pressure. Raw biogas with a high content (>300 ppm) of hydrogen sulfide is recommended to be pretreated by any one of several desulfurization techniques.
The recovered methane is dehumidified through a desiccant dryer and then pressurized to supply the