Biogas – a way of storing renewable electricity?

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Many of the technological barriers to producing renewable electricity have now been overcome, with methods to harness wind, solar and tidal power now established, but storage methods are required in order to improve the utilisation of this energy. In this blog, Nikoletta Giantsiou, ATBEST researcher at University Duisburg-Essen, discusses a method of using biogas to do so. 

Anaerobic digestion (AD) is a well – established biological process which results in organic pollution reduction and bioenergy production. During the process organic wastes are degraded with simultaneous energy production in the form of biogas. Biogas is a mixture of mostly methane (CH4) and carbon dioxide (CO2). Usually, biogas consists of 50 -70% CH4 and 30-50% CO2. Carbon dioxide in the produced biogas is inert as a fuel and dilutes its’ energy content.  The low energy density of biogas renders biogas an inefficient energy carrier for long – distance transportation and energy storage. The low CH4 content limits the application of biogas. Currently, the most common utilization route is for electricity production, often combined with utilization of the excess heat (CHP units). In worst cases, biogas is burned through a flare.

Figure 1: Biogas reactor with emergency flare

Figure 1: Biogas reactor with emergency flare

Alternatively, after the removal of CO2, biogas can be upgraded to natural gas quality. CH4 content higher than 90% can increase the heating value giving more utilization possibilities as a renewable energy source. Upgraded biogas (biomethane) can be used as vehicle fuel, or it can be injected into the existing natural gas grid transporting biomethanefrom rural areas, where typically biogas plants are located, to urban areas where consumer density is higher.This widens up the opportunities in distant energy consumption locations.

The most common methods of biogas upgrading include water scrubbing, pressure swing adsorption, polyglycol adsorption, membrane separation, cryogenic separation and chemical treatment.

Figure 2: Membrane separation (http://www.heatandpowerltd.co.uk)

Figure 2: Membrane separation (http://www.heatandpowerltd.co.uk)

Generally, these methods are performed outside the anaerobic reactor and require investments in external equipment.Therefore, the cost is relatively high.The main disadvantage is that small amounts of CH4 are also removed, which can increase greenhouse gas emissions.

To circumvent these disadvantages, biological conversion of CO2 to CH4 for biogas upgrading can be achieved. In anaerobic digesters, hydrogen can be converted to CH4 by the action of hydrogenotrophic methanogens according to the following Equation:

CO2 + 4 H2 → CH4 + 2 H2O       ∆H = −165.0 kJ/mol

This is the Sabatier reaction in which microorganisms can bind CO2 with H2 and convert them to CH4. Carbon dioxide is already consisted in biogas.

Figure 3: Sabatier reaction

Figure 3: Sabatier reaction

Nevertheless, in order for the whole process to be considered as renewable, the H2 needed for the biogas upgrading should also be provided by renewable sources.  Wind and bioenergy are two of the most promising renewable energy sources.

The power production from wind mills vary due to weather conditions. Electricity demand vary as well.In times of overproduction a high percentage of the electricity from wind is judged to be a surplus. The potential is not fully utilized. Water electrolysis is an attractive way of exploiting excess electricity from wind mills, in order to produce hydrogen.

Electrolysis is an electrochemical process which takes place by conducting electricity via electrodes through water. The two electrodes, the anode and the cathode, have a positive and negative charge, respectively. The charge difference results in an ionization of the water molecules into hydrogen and oxygen ions. At the negatively charged electrode, the cathode, the positively charged hydrogen ions (H+) gather. At the positively charged electrode, the anode, the negatively charged oxygen ions (O2-) gather.

Figure 4: Electrolysis of wind production (http://www.hydrogennet.dk)

Figure 4: Electrolysis of wind production (http://www.hydrogennet.dk)

 

The produced hydrogen can then be stored in the existing gas network or used for fuel cell vehicles. However, hydrogen is a very light gas and contains much lower volumetric energy content compared to CH4. Storage costs of hydrogen are consequently high. As an alternative, hydrogen may be used for upgrading biogas to natural gas quality and thus making it storable in the gas network.

With our research project we intent to use the excess energy of renewable sources such as wind mills in order to produce high CH4 content biogas. That way, in times of energy oversupply the excessive energy could be converted into a storable gas.

More specifically, our objectives are:

- supply hydrogen to the organic waste feeding stream of the anaerobic digester with the goal to convert carbon dioxide from biogas into methane in situ.

- modify the anaerobic configuration so that H2 is used to upgrade the biogas efficiency, and to

- optimize H2 consumption by the hydrogenotrophic methanogens in anaerobic reactors.

Figure 5: Use of surplus electricity to biogas plants via hydrogen

Figure 5: Use of surplus electricity to biogas plants via hydrogen

With in situ biogas upgrading an amount of CO2 can be consumed, thereby, result in higher methane biogas content. This partial CO2removal can decrease the costs of upgrading biogas to natural gas quality. Finally, possible unconverted hydrogen mixed with methane, would improve the combustion properties of biogas as fuel. The storage cost of methane is lower compared to H2, due to its’ higher boiling point and higher volumetric energy density. Additionally, a number of countries already have natural gas infrastructure, which would make distribution of upgraded biogas feasible.

Overall, the expected outcome of this project is to implement an efficient process for converting the excessive renewable energy into CH4. This process will result in increased net CH4 production for biogas plants, decreased biogas upgrading costs and the possible use of biogas as an alternative to natural gas. The main benefit will be the use of the existing infrastructure system for storing electricity.

References:

  1. [Online]http://www.hydrogennet.dk/
  2. [Online]http://vav.griffel.net/filer/C_SGC2013-270.pdf
  3. A Single-Culture Bioprocess of Methanothermobacterthermautotrophicus to Upgrade Digester Biogas by CO2-to-CH4 Conversion with H2, Matthew R. MartinJeffrey J. Fornero, Rebecca StarkLaurens MetsLargus T. Angenent, Archaea, (2013), doi: 10.1155/2013/157529
  4. The Bio-Sabatier-Process and its potential for the electric industry, Dr. Ing. S. Schmuck, MSc.; Dipl.-Ing. B. Keser; Dr.-Ing. T. Mietzel, ICCE-2010

 

 

 

 

 

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