What would it be like to get from A to B in a relaxed manner without harming the climate? The first thought that springs to mind is to buy an electric car. However, rare earths such as dysprosium or neodymium are needed to manufacture the batteries. Other elements such as lithium or cobalt must also be used to produce the batteries. These elements are generally in short supply and also have problematic suppliers, such as China for rare earths or the Congo for cobalt (Dicks, 2020).
So what now?
Is there an alternative that is both environmentally friendly and saves time when refueling?
The answer is a resounding YES! The fuel is called biomethane. What exactly biomethane is, how it is produced and which steps are necessary to get to the finished end product is explained in this article.
Dicks, H. (December 9, 2020). Biomethane - an attractive fuel for the mobility transition.
Here the question arises: How is the environmentally friendly fuel produced from manure (slurry/manure) and renewable raw materials?
First of all, residual materials, renewable raw materials, slurry/manure and waste materials from agricultural production operations and producers are collected by collectors, usually dealers or suppliers, and then transported to the biogas plant. This step is illustrated in Figure 1.
Figure 1 : 1st step in the value chain (agriportance GmbH, 2022)
Fermentation of these substances takes place in this plant. During methane formation, microorganisms break down organic matter in an oxygen-free (anaerobic) environment, releasing biogas. This is a water vapor-saturated gas mixture, which essentially consists of methane (CH4) and carbon dioxide (CO2). Other components in addition to water vapor are trace gases such as nitrogen, oxygen, hydrogen, hydrogen sulphide and ammonia. In biogas and sewage gas plants, these anaerobic fermentation processes are technically applied and biogas that can be used for energy is produced with the highest possible efficiency.
The process steps of the biogas plant can be roughly explained in four steps:
These processes are shown again in Figure 2.
Figure 2: Anaerobic degradation of organic material to biogas (G fermenters, AB acetogenic bacteria, HAB homoacetogenic bacteria, SAO synthropic acetate oxidizers, AM acetoclastic methanogens, HM hydrogenotrophic methanogens) (Martin Kaltschmitt, 2016).
Martin Kaltschmitt, H. H. (2016). energy from biomass. springer.
In order to produce biomethane from biogas, the gas mixture must be purified. This takes place in the biogas upgrading plants. This step in the value chain is shown in Figure 3
Figure 3: Step 2 in the biomethane value chain (agriportance GmbH. (June 2022). Workshop_THG_Bilanzierung_V.0.1.12.)
Various processes are used for processing, such as amine scrubbing, pressure swing adsorption or pressurized water scrubbing (DWW):
In amine scrubbing as a chemical absorption process, purification is similar to pressurized water scrubbing. Here, the biogas flows under slightly increased pressure through an amine-water solution in countercurrent, whereby the CO2 reacts with the scrubbing solution and passes into it. The amine solution achieves a higher loading than water, which reduces the amount of detergent that needs to be circulated. The exhaust air contains only small amounts of methane, which is why no lean gas cleaning is usually required. Fine desulphurization is recommended in order to maintain the capacity of the washing performance in the long term. Amine scrubbing is very energy-intensive, as large amounts of process heat are required to regenerate the amine solution (Braune, Naumann, Postel, & Postel, 2015).
The DWW process utilizes the different solubility of methane and carbon dioxide in water at varying pressure. The previously compressed biogas flows through the absorption column from bottom to top. This is usually designed as a trickle bed reactor in which water flows through the gas in countercurrent. This allows basic and acidic components of the biogas, especially carbon dioxide and hydrogen sulphide, to be dissolved and any dust and microorganisms to be separated. The purified gas leaves the column with a purity of 90 to 99% methane by volume. In addition to CO2, the exhaust air contains around 1 % methane by volume, which must be separated via lean gas treatment. The electricity requirement is high compared to other CO2 separation processes due to the circulation of the scrubbing water and the required biogas compression. Prior drying of the gas is not required (Braune, Naumann, Postel, & Postel, 2015).
In pressure swing adsorption (PSA), gas mixtures are separated by adsorption on activated carbons, molecular sieves or carbon molecular sieves. Drying, fine desulphurization and compression of the biogas are required before DWA. The cooled, dehydrated gas then flows through the adsorbent (molecular sieve or activated carbon), on which the CO2 adsorbs. The methane-rich product gas is then expanded and fed to a second column, in which the adsorption is repeated with the introduction of ambient air. Regeneration of the absorbent is achieved by reducing the pressure with the aid of a vacuum pump. The extracted CO2-rich gas still contains methane and must therefore be fed to a lean gas aftertreatment (Braune, Naumann, Postel, & Postel, 2015).
However, the biomethane that has now been purified by the processes must meet various requirements before it can be fed into the natural gas grid:
Theoretically, any biogas plant can feed the biomethane produced from the biogas into the natural gas grid. In practice, the construction of the feed-in plant and the pipelines to the gas grid must be checked by the respective grid operator. As the costs for the construction of feed-in plants and pipelines must be borne by the quantities of gas sold, feed-in generally only makes sense for plants that are located close to the natural gas grid. DVGW and DIN guidelines define corresponding key figures for feeding biomethane into the natural gas grid and using it as a fuel. Therefore, before biomethane can be fed into the natural gas grid, it must be brought to the natural gas quality required at the respective location. These qualities vary from region to region, especially with regard to the required calorific value and pressure (Braune, Naumann, Postel, & Postel, 2015); (agriportance GmbH. (June 2022). Workshop_THG_Balancing_V.0.1.12.)
The biomethane that has now been purified and meets the requirements can now be used by the consumer.
agriportance GmbH. (June 2022). Workshop_THG_Balancing_V.0.1.12.
As described at the beginning, biomethane can be used for transportation. Even if the automotive world becomes more electric: Natural gas or CNG is and will remain a more environmentally friendly alternative to gasoline and diesel. However, the choice of vehicle models is limited (new cars with CNG/natural gas equipment "ex works": Audi, Fiat, Seat, Skoda, VW) and the filling station network can be expanded (Figure 4). Like crude oil and coal, natural gas is a combustible organic raw material. It consists of around 85% methane. Compressed natural gas (CNG) and liquefied natural gas (LNG) are available. The latter is liquefied at minus 164 degrees Celsius and is only used in commercial vehicles. Gaseous CNG is most commonly used in passenger cars. Biogas is also increasingly being used at filling stations. It is stored in the vehicle in tanks with an operating pressure of 200 bar. There are around 820 filling stations in Germany where CNG is available. Biogas is a particularly environmentally friendly variant. By increasing the methane content, it can be brought up to the same quality level as natural gas. Such biomethane is suitable as a fuel for natural gas vehicles without restriction. Many natural gas filling stations in Germany already offer biomethane pure or mixed with natural gas as a fuel (ADAC, Erdgas/CNG - ein Antrieb mit Zukunft?, 2022)
Figure 4 Biogas filling stations in Münster and the surrounding area (ADAC, Finden Sie die günstigste Tankstelle)
ADAC. (January 11, 2022). Natural gas/CNG - a drive with a future? Germany.
ADAC. (no date). Find the cheapest filling station. Germany.
Bibliography
ADAC. (January 11, 2022). Natural gas/CNG - a drive with a future? Germany.
ADAC. (no date). Find the cheapest filling station. Germany.
agriportance GmbH. (June 2022). Workshop_THG_Balancing_V.0.1.12.
Dicks, H. (December 9, 2020). Biomethane - an attractive fuel for the mobility transition.
Martin Kaltschmitt, H. H. (2016). energy from biomass. springer.