Technologies for Removing Carbon Dioxide from RNG
In order for RNG to meet the standards required for use as vehicle fuel or injection into a natural gas pipeline it needs to have impurities such as carbon dioxide removed. There are currently four methods that are typically employed for CO2 removal from biogas during advanced treatment: 1) membrane systems; 2) solvent scrubbing; 3) water scrubbing; and 4) pressure swing adsorption (PSA).
According to the Argonne National Laboratory's Renewable Natural Gas database, as of 2018, CO2 removal technologies used by RNG projects located at US landfills are distributed as follows: 24% use solvent scrubbing, 24% use membrane systems, 10% use pressure swing adsorption (PSA), 7% use a combination of membrane systems and PSA, and 2% use water scrubbing technologies, while the remaining 33% use other or unknown technologies. The distribution for manure-fed anaerobic digestion based biogas to RNG projects for the same period varied somewhat, with 64% making use of membrane systems, while 12% use PSA, 6% use water scrubbing, and the remainder employing other or unknown technologies. The picture in Europe is somewhat different. Of the 80 RNG projects operating in 2014, 65% make use of water scrubbing or solvent scrubbing technologies to remove CO2, 23% use PSA, while only 11% use membrane systems.
CO2 Removal from Biogas using Membrane Systems
Membrane systems consist of a membrane filter that are rated according to their pore size. The pores allow the biogas to penetrate, but retain any particles that are larger than the pores. Membrane systems are capable of removing carbon dioxide and other impurities from the raw biogas to meet quality requirements stipulated for RNG.
Single-Pass Membrane System
CNG fuel used onsite to power vehicles generally doesn't need as high a heating value as CNG fuels injected into a natural gas pipeline. The quality requirements in terms of oxygen and other inert gases is also less stringent. For smaller applications such as these with lower biogas volumes, a single-pass membrane system, which captures between 65-80% of the methane, is often used. The remaining methane is emitted as tail gas and either flared onsite or blended any remaining biogas and used in combustible or piston engines to generate electrical power.
Multiple-Pass Membrane System
For large-scale onsite CNG vehicle fuel operations, a similar membrane technology as above is used, but rather than a single pass through the membrane, the process involves multiple passes through the membrane. As a result, the overall process, which typically captures between 96-99% of the methane, is much more efficient at capturing methane than single-pass membrane technology. The remaining tail gas needs to be flared using additional fuel due to the low heating value of the remaining biogas, which is unable to sustain combustion on its own.
CO2 Removal from Biogas using Pressure Swing Adsorption (PSA)
Pressure swing adsorption technologies use adsorbent media to capture impurities such as carbon dioxide and nitrogen as the biogas passes through under pressure, allowing methane (which has smaller molecules) to pass through. The media is depressurized once it becomes saturated, releasing the carbon dioxide and nitrogen out with the tail gas. PSA technologies can capture between 95-98% of the methane present in the biogas.
CO2 Removal from Biogas using Solvent Scrubbing
Solvent scrubbing is a process that uses either a chemical (e.g. Amine) or physical (e.g. Selexol) solvent to remove carbon dioxide and hydrogen sulfide from the biogas, allowing the methane component to pass through. The methane capture efficiency of solvent scrubbing is typically between 97-99%, but may be even higher for amine solvents, which are particularly effective at removing carbon dioxide from RNG.
CO2 Removal from Biogas using Water Scrubbing
Water scrubbing, or water washing, is a relatively simple CO2 removal process whereby the biogas is pumped into water under pressure. The water adsorbs the carbon dioxide, while the methane passes through. The water, along with the carbon dioxide adhering to it, is then depressurized, releasing the carbon dioxide which is emitted with the tail gas. The methane capturing efficiency of water scrubbing technologies is high, usually capturing more than 99% of the methane.
Which is the best Technology to use for CO2 Removal from Biogas?
As we can see, when it comes to CO2 removal from biogas, there are various options. There are pros and cons to using each of these technologies, which need to be assessed for each individual RNG project to ensure the most appropriate CO2 removal technology is employed according to the needs of the site. While all of these technologies are capable of producing RNG that meets the quality standards required for injection into a natural gas pipeline or for use as vehicle fuel onsite, the choice of selection is often a question of weighing up whether an upfront capital outlay or continuous operating expense would best suit that particular project and site.
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