Chemical or Mechanical Purification
Gas separation is a technique deployed to split up gases to separate and purify multiple or single gas components. The environmental performance of biogas upgrading can be assessed via life cycle assessment (LCA) that studies midpoint (global warming potential, ozone layer depletion, etc.) and endpoint (damage to human health, resource availability, etc.) impact categories. LCA studies also consider the consumption of electricity, methane leakage, and heat demand to determine the overall environmental impacts of biogas on biomethane upgrading technologies. CleanCap’s approach helps differentiate technologies based on a specific projects’ needs.
Solvent or Amine Scrubbing
Gas sweetening is amine scrubbing deployed to convert ‘sour’ gas, meaning it contains H2S or CO2, to ‘sweet’ gas in conventional gas treating. Similar to CleanCap’s industrial CO2 capture process, renewable gas treating uses a specialty amine solution or physical solvent that has been sterically hindered such as Dimethyl Ether of Polyethylene Glycol (DEPG), to adequately perform in the presence of CO2 concentrations up to 50% by volume. Compared to other separation methods, scrubbing of raw biogas consumes less electricity, but requires a source of heat.
Pressure Swing Absorption (PSA)
Pressure Swing Absorption packages are used to separate mixed gas into individual gasses in a non-cryogenic process. The gasses are separated at near ambient temperatures and under pressure based on the species molecular characteristics and affinity for an adsorbent material. PSA can be used to recover methane, hydrogen, or to split oxygen and nitrogen from air. Specialized adsorbent materials such as zeolites, molecular sieves, and activated carbon absorb the recoverable gasses, but reject nitrogen. In biogas applications, they are also used as the last major piece of equipment in an upgrading plant in the form of a Nitrogen Rejection Unit (NRU). PSA packages can become complex to achieve high recovery rates. CleanCap is able to sort through the proper application of PSA units to ensure project goals are met.
Membrane Separation
Membrane separation selectively separates or fractionates gasses via pores in the molecular arrangement of a continuous structure. In biogas applications, many membranes are made of a long-term stable aromatic polyimide which is resistant against high temperatures and compaction caused by certain gasses and hydrocarbons under high pressure. Membrane packages consist of very few moving parts and are simple to operate and can be adjusted for changing gas qualities. Electrical costs can be high for membrane separation.
Cryogenic Separation
Cryogenic separation involves cooling the acid gasses to a very low temperature so that the targeted gas can be liquefied and separated. In most cryogenic technologies, pretreatment is required to remove moisture and other components that would result in hydrate formation in the cold section of the fractionation equipment. Typically, biogas applications use cryogenic means to remove the nitrogen gas from the methane utilizing boiling point differences. Due to the energy and capital costs, cryogenic separation is generally used in larger volume applications.