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Power to Gas in Anaerobic Digestion: Power to BioGas

How do we store excess energy during peak times? This is a big issue for energy providers, which needs to be addressed. Solving this issue will help the UK ensure it can supply enough energy to meet the UK's growing demand for energy. Power to Gas is a technology which could help solve this global issue. It has the potential to store energy in the order of TWhs of capacity, which is the greatest potential among all other storage technologies available to date.

The production of renewable hydrogen is considered to be the most important economical challenge with power to gas technology [1]. The utilisation of biological methanation technology will strongly depend on the development and optimisation of the electrolysis step in the process.

Power to Gas transforms renewable energy into a storable and safe carrier of chemical energy; hydrogen or methane. It uses our existing gas grid infrastructure which is an advantage to the technology. Below shows a simple step by step process for power to gas:

Step 2-4 can occur through two processes (1) Thermochemical methanation and (2) Biological methanation.  

(1) Thermochemical methanation occurs, as the name suggests through transforming chemical (and chemical compounds) with heat. Hydrogen and carbon dioxide are mixed with nickel based catalysts at 300 – 550⁰C. This process requires lower reactor volumes for certain feed gas flow although, full CO2 conversion cannot be achieved during this process due to limitations in thermodynamic equilibrium [2]. The system is exothermic which means energy is release by heat, therefore cooling is essential for the operation of Thermochemical methanation. Methanation can occur through three different processes;

  • Fixed bed methanation, which uses adiabatic fixed bed methanation where no heat is exchanged, however pressure is applied quickly to change the temperature.
  • Fluidised bed methanation, which uses fine catalysts particles are fluidised by gaseous reactants to remove heat from the reactor.
  • and Three Phase methanation, which occurs in a slurry bubble column reactor to enable good heat dissipation and control.

The chemical formula for this process is: CO2 + 4H2 ---> CH4 2H2O

(2) Biological methanation is an immature technology meaning there is still progress to be made through research and innovation to make it economically viable. In this process microorganisms serve as a biocatalyst [3]. These microorganisms are used in the methane-forming step to produce biogas. This process can occur at around 40-70⁰C and can deal with large fluctuations in power supply. This process is advantageous as the microorganisms used have a high tolerance for impurities, which are typically found in gases for feed gas methanation, such as, sulphur, ammonia and oxygen. The process has been improved by introducing continuous stirred-tank reactors as this can improve the efficiency of converting liquid into gas.

Diagram to show the biological methanation process [4]

There are two main processes to achieve power to gas in biological methanation and they are;

  • Methanation in situ digester, where, hydrogen is fed directly into the biogas digester;
  • Methanation in a separate/ external reactor, where, hydrogen is fed into the biological methanation tank separately to the biogas digester.

Biological methanation is being trialled in Germany and Denmark [4] as pilot schemes. As stated earlier, the biggest engineering challenge is supplying hydrogen to the microorganisms, as hydrogen is a poorly soluble liquid. There are several reactor concepts are being developed to reduce limitations when changing from liquid to gas.

Power to Gas is a fairly new technology and is currently being tried in a few places in Europe. There is a lot of potential for the power to gas technology to work with the AD process.

This blog was based on articles by; Manuel Gotz, Amy McDaniel Koch, Frank Graf called “State of the Art and Perspectives of CO2 Methanation Process Concepts for Power-to-Gas applications” and Green Gas Grids, Fraunhofer UMSICHT

At ADBA we're interested to learn more about Power to Gas technology and how it can be utilised in the AD process. If you have any comments, feedback or information you would like to share about the above article or Power to Gas in general. Please contact Emma Thomas on 0203 735 8118 or emma.thomas@adbioresources.org

 

[1] http://www.greengasgrids.eu/fileadmin/greengas/media/Downloads/GGG_SWOT_analysis_FINAL.pdf

[2] Thermodynamic Equilibrium: when two “bodies” one hot and one cold, achieve the same temperature are said to be in thermodynamic equilibrium. There are three conditions which need to be satisfied for equilibrium; mechanical, thermal and chemical.

[3] Biocatalyst: converts hydrogen and carbon dioxide to methane.

[4] http://www.greengasgrids.eu/fileadmin/greengas/media/Downloads/GGG_SWOT_analysis_FINAL.pdf

Posted in: Industry News, News for Members, R&D Updates

Tags: r&i, power to gas, biogs, methane, emma thomas, anaerobic digestion