An Introduction to Biomethane

Biogas that has been upgraded by removing hydrogen sulphide, carbon dioxide and moisture is known as biomethane. Biomethane is less corrosive than biogas, apart from being more valuable as a vehicle fuel. The typical composition of raw biogas does not meet the minimum CNG fuel specifications. In particular, the COand sulfur content in raw biogas is too high for it to be used as vehicle fuel without additional processing.

Liquified Biomethane

Biomethane can be liquefied, creating a product known as liquefied biomethane (LBM). Biomethane is stored for future use, usually either as liquefied biomethane or compressed biomethane (CBM) or  since its production typically exceeds immediate on-site demand.

Two of the main advantages of LBM are that it can be transported relatively easily and it can be dispensed to either LNG vehicles or CNG vehicles. Liquid biomethane is transported in the same manner as LNG, that is, via insulated tanker trucks designed for transportation of cryogenic liquids.

Compressed Biomethane

Biomethane can be stored as CBM to save space. The gas is stored in steel cylinders such as those typically used for storage of other commercial gases. Storage facilities must be adequately fitted with safety devices such as rupture disks and pressure relief valves.

The cost of compressing gas to high pressures between 2,000 and 5,000 psi is much greater than the cost of compressing gas for medium-pressure storage. Because of these high costs, the biogas is typically upgraded to biomethane prior to compression.

Applications of Biomethane

The utilization of biomethane as a source of energy is a crucial step toward a sustainable energy supply. Biomethane is more flexible in its application than other renewable sources of energy. Its ability to be injected directly into the existing natural gas grid allows for energy-efficient and cost-effective transport. This allows gas grid operators to enable consumers to make an easy transition to a renewable source of gas. The diverse, flexible spectrum of applications in the areas of electricity generation, heat provision, and mobility creates a broad base of potential customers.

Biomethane can be used to generate electricity and heating from within smaller decentralized, or large centrally-located combined heat and power plants. It can be used by heating systems with a highly efficient fuel value, and employed as a regenerative power source in gas-powered vehicles.

Biomethane to Grid

Biogas can be upgraded to biomethane and injected into the natural gas grid to substitute natural gas or can be compressed and fuelled via a pumping station at the place of production. Biomethane can be injected and distributed through the natural gas grid, after it has been compressed to the pipeline pressure. In many EU countries, the access to the gas grid is guaranteed for all biogas suppliers.

One important advantage of using gas grid for biomethane distribution is that the grid connects the production site of biomethane, which is usually in rural areas, with more densely populated areas. This enables the gas to reach new customers. Injected biomethane can be used at any ratio with natural gas as vehicle fuel.

Biomethane is more flexible in its application than other renewable sources of energy.

The main barriers for biomethane injection are the high costs of upgrading and grid connection. Grid injection is also limited by location of suitable biomethane production and upgrading sites, which have to be close to the natural gas grid.

Several European nations have introduced standards (certification systems) for injecting biogas into the natural gas grid. The standards, prescribing the limits for components like sulphur, oxygen, particles and water dew point, have the aim of avoiding contamination of the gas grid or the end users. In Europe, biogas feed plants are in operation in Sweden, Germany, Austria, the Netherlands, Switzerland and France.

PSA System for Biogas Upgradation

Pressure swing adsoprtion, also known as PSA, is emerging as the most popular biogas upgradation technology in many parts of the world. A typical PSA system is composed of four vessels in series that are filled with adsorbent media which is capable of removing water vapor, CO2, N2 and O2 from the biogas stream.

During operation, each adsorber operates in an alternating cycle of adsorption, regeneration and pressure buildup. Dry biogas enters the system through the bottom of one of the adsorbers during the first phase of the process. When passing through the vessel, CO2, N2 and O2 are adsorbed onto the surface of the media. The gas leaving the top of the adsorber vessel contains more than 97% CH4

Biogas upgradation through PSA takes place over 4 phases: pressure build-up, adsorption, depressurization and regeneration. The pressure buildup is achieved by equilibrating pressure with a vessel that is at depressurization stage. Final pressure build up occurs by injecting raw biogas. During adsorption, CO2 and/or N2 and/or O2 are adsorbed by the media and the gas exits as CH4.

Depressurization is performed by equalizing with a second pressurizing vessel, and regeneration is achieved at atmospheric pressure, leaving a gas that contains high concentrations of CH4 to be re-circulated. During the regeneration phase, the bed must be regenerated by desorbing (or purging) the adsorbed gases. Purging is accomplished by reducing the pressure in the bed and back-flushing it with some of the concentrated gas product. The gas pressure released from one vessel is used by the other, thus reducing energy consumption and compressor capital costs.

Special adsorption materials are used as a molecular sieve, preferentially adsorbing the target gas species at high pressure. The adsorbent media is usually zeolites (crystalline polymers), carbon molecular sieves or activated carbon. Aside from their ability to discriminate between different gases, adsorbents for PSA-systems are usually very porous materials chosen because of their large surface areas.