Here we explain how to Eliminate CO2 from Biogas, (or CO2 Scrubbing). Biogas can be upgraded to pipeline natural gas quality for use as a renewable natural gas, but as part of the upgrading process it will be necessary to remove the carbon dioxide (CO2) from the biogas. This upgraded gas may also be used for residential heating and as vehicle fuel, so there are many good reasons for removing the CO2.
CO2 will always be found in biogas due to the fact that during anaerobic digestion, (i.e. degradation in the absence of oxygen), organic material is decomposed by bacteria forming a mixture of CO2 and CH4 with trace amounts of H2S and water vapour at saturation pressure.
Common methods used to eliminate CO2 from biogas are:
- water scrubbing (for example by using iron wool – see below.)
- membrane systems
- pressure swing adsorption (PSA) (see -below)
- chemical CO2 absorption
- amine gas treatment
- CO2 by cooling and recovering dry ice.
These systems also generally reduce the H2S and H2O content. Biogas treatment systems, also need to include feed compression on the un-pressurized raw biogas.
Trace components that are often present in biogas are water vapor, hydrogen sulfide, siloxanes, hydrocarbons, ammonia, oxygen, carbon monoxide, and nitrogen.
The Use of an Iron Wool Carbon Dioxide Scrubber
For home-size systems, a easy solution for getting CO2 from biogas is an Iron Wool Carbon Dioxide Scrubber Device, which is one which which absorbs carbon dioxide from circulated gas.
A carbon dioxide scrubber is a device which absorbs carbon dioxide (CO2). It is used to treat exhaust gases from industrial plant. via Wikipedia
An effective and efficient technology used in purifying raw biogas generated from the prototypic biogas production plant; this technology is otherwise known as Water Scrubbing technology.
The Scrubbing system consists of the Water scrubber with iron wool packed bed connected to a 500 litre water tank, and two tyre tubes which were used in storing the pre scrubbed (raw) biogas and the scrubbed (purified) biogas.
The water scrubber has an inlet for the entry of the raw biogas and a discharge for the exit of the scrubbed biogas. Raw biogas from the plant was stored in a tyre tube and directly fed into the Water scrubber housing the iron wool packed bed, the purified biogas from the exit was also collected into another tyre tube.
Samples of the gas mixture were taken before and after scrubbing and analyzed with the Pascal Manometric Glass Tube technique.
Results indicated that methane content of the scrubbed/ purified biogas was raised from 58% to 82% due to the reduction of Carbon dioxide and Hydrogen Sulphide. CO2 was reduced from 31% to 14% while H2 S was reduced from 1% to 0.4%.
The corresponding Energy content of the purified biogas was evaluated to be 41MJ/kg which is higher than that of the raw biogas which was evaluated to be 29MJ/kg. via Fundamentals of Renewable Energy
Biogas CO2 from Biogas and Hydrogen Sulphide Scrubbing
Is this How to remove Carbon Dioxide from biogas? Read on! is to put a section of 4″ PVC about a foot long in the line. Fill this with steel wool, the type used for scrubbing pots.
This will rust away similar to the metal above. It is very efficient because of the high amount of surface area presented by the steel wool.
Threaded connections make it easy to replace the wool. The easiest way to determine when to replace the wool is when you can smell the hydrogen sulfide at the burners.
For a farm-sized system, probably the best thing to use is a pair of old 200litre/44gallon drums.
Fill the drums with steel turnings or bashed up rusty tin cans or anything that is iron or steel. The easiest way to do that is to chop out the 3/4″ bung in the top and weld in a 4″ or larger pipe fitting.
The 2″ bung on the other side should have a length of the largest size of steel pipe that you can fit inside a 2″ pipe nipple, and that goes nearly to the bottom of the drum and that is the gas inlet from the gas bag.
The new and larger bung should be the gas outlet from the top of the drum with a 2″ reduction to plastic pipe, with which everything is connected. The wet biogas will flow slowly through the drum and the metallic iron and the rust will react with the Hydrogen sulfide.
You should have two of these drums made up and rotate them at suitable intervals to make sure that no H2S gas is getting through the system, or your intermediate gas compressor for Biogas CO2 Scrubbing will not last for very long. via Appropedia
Pressure Swing Adsorption (PSA) Systems for CO2 and Hydrogen Sulphide Scrubbing
Pressure swing adsorption (PSA) systems, can be thought of as being molecular-sieves for carbon. PSA has been described are the second most commonly used biogas upgrading technology in Europe, after water scrubbing which is most likely the most popular. A typical system is composed of four vessels in series that are filled with adsorbent media which is capable of removing not only the CO2 but also water vapour, N2, and O2 from the biogas flow.
Typically in order to eliminate CO2 from biogas, the PSA upgrading takes place over 4 phases: pressure build-up, adsorption, depressurization and regeneration. The pressure build-up occurs by equilibriating pressure with a vessel that is at depressurization stage. Final pressure build up occurs by injecting raw biogas. During adsorption, CO2, N2, and O2 are adsorbed by the media and the purified gas discharges as pure methane to a quality which will be far less corrosive and has a higher calorific value.
Recently developed gas-liquid membranes have been introduced, which operate at atmospheric pressures thereby reducing the energy consumption of compression. The use of specific solvent solutions allows the separation and recovery of the H2S and CO2.
Another approach to improving the economics of gas upgrading has been to recover the CO2 by cooling and recovering dry ice. This can then be sold as an industrial gas whilst the biogas is either used in its more concentrated form (80-90% CH4) or further refined to vehicle quality standard (>96% CH4).
An example of membrane use for the elimination of CO2 from Biogas follows:
“Silicone (PDMS) has a very high permeability to CO2 compared to methane. This difference in permeability facilitates the transfer of CO2 preferentially over methane, thus enabling the removal of CO2 from a natural gas stream. Similarly other contaminants such as hydrogen sulfide (H2S) and water (H2O) can be readily removed from methane. The separation factor for CO2/CH4 is approximately 3 for a 50/50 gas mix, indicating that the ratio of the mass fraction of CO2 to CH4 in the feed/retentate is three times smaller than the ratio in the permeate.
Natural Gas purification is simple and straightforward as shown in the figure below. The feed gas is supplied to one side of the membrane though the feed port, and depending on the feed pressure, a vacuum supply to the opposite side of the membrane may be necessary at the permeate ports. The contaminants with higher permeability than natural gas will permeate the membrane much faster than the methane, thereby stripping the feed from these contaminants. The high purity natural gas will exit at the permeate port. The permeate can be vented or flared as necessary.”
Mongabay News, Pre Combustion CO2 Capture
Why It Is “Green” to Eliminate CO2 from Biogas?
The purification of biogas for higher value applications, such as vehicle use or pipeline-quality, is established to be in general a more sustainable and environmentally friendly option than conversion of the energy to electricity as the energy losses from power generation and the transmission losses for electricity are in general higher than for direct uses of the gas as fuel. Also, the AD Process is uniquely able to produce transport fuels, whereas other renewable energy sources such as wind turbine, and hydro-power don’t, which suggests that it is best utilized to meet the demand for transport fuels. The energy efficiency for transport fuel use of upgraded biogas is particularly good where the upgraded gas is used to fuel vehicles working on the came facility, or operating from the same facility.
The technology for biogas upgrading is relatively new and improvements to the upgrading processes are ongoing to reduce methane losses and improve energy efficiency.