Bioenergy plants have the considerable advantage that they supply electricity and heat very flexibly in terms of time and space. This capability will be of considerable value in the electricity grid of the future, since the power yield from solar energy and wind turbines fluctuates considerably due to the weather conditions. Biogas plants are available throughout the country and the plants’ operating principle offers various starting points for feeding electricity into the grid as required.
In order to fulfil this task, precise gas management is required. In view of this, the "ManBio" research project was aimed at technically improving gas storage systems. Matthias Stur, project manager at the German Biomass Research Centre, Leipzig (DBFZ), received the German Agriculture Biogas Innovation Award in 2017 for his research work.

Mr Stur, what are the main advantages of using biogas to generate electricity and heat compared with other renewable energies?

Firstly, the biogas-based provision of gas, electricity and heat from biogas plants is weather-independent; secondly, the plants can be operated – depending on the selected mode of operation – with constant or fluctuating, demand-oriented energy provision to compensate for intermittent renewable energies. The supply of energy can thus be regulated within certain limits. If the biogas is upgraded to natural gas quality and fed into the natural gas grid as biomethane, the advantages of the large and comparatively inexpensive storage capacities in the natural gas grid can be exploited. In contrast to the electricity grid, the natural gas grid can already cope with the additional feed-in of renewably and regionally produced biomethane and can make a contribution to substituting fossil fuels. In addition to the temporal flexibility in utilising gas, the use of biomethane adds a further component: the consumption and production of the energy source are spatially decoupled. The place of utilisation is freely selectable – as long as there is access to the gas network.

Which storage systems are available for biogas?

The storage of biogas at the plant location provides short-term compensation for fluctuations and the resulting imbalance between gas production and gas utilisation.
The storage facilities can normally store the gas produced for a few hours should the gas utilisation temporarily fail.
Different systems are available on the market for storing biogas. Because of the need to fully utilise the net gas storage capacity in combination with several storage tanks, the pneumatically pre-stressed, integrated double-membrane system widely used in Germany has proven to be advantageous due to its functionality. Single-membrane, gas pressure-supported or mechanically pre-stressed systems or double-membrane, mechanically pre-stressed dry gas storage systems in the low-pressure range have comparatively poorer properties for combined applications. In general, integrated designs directly on the fermenter have proved better than separated versions, such as balloon or membrane cushion gas holders. This is due to the lower space requirement and the economic efficiency. For storing biomethane, on the other hand, only dry gas storage systems such as compressed gas cylinders in the fuel sector or the natural gas network in general are used. Wet gas storage systems such as bell or telescopic gas storage systems have failed to establish themselves in biogas practice.

It is important for the optimum utilisation of the biogas produced that the holder is not too full. How can this be ensured?

If the filling level is too high, sudden fluctuations in the gas utilisation, gas production and weather-related temperature fluctuations can no longer be compensated for and biogas is discharged via the safety devices.
The use of a filling level measuring system adapted to the type of gas holder enables the precise measurement of the filling level when operating biogas holders. However, the methods used in practice are not always suitable for accurately mapping the filling level. For example, it is not possible to measure the level of a pneumatically pre-stressed, double-membrane gas holder using a gas pressure measurement method, but it is certainly used in practice. In addition to the appropriate measuring technology, an important aspect, especially for actuating the switching points, is to integrate the correct filling level into the operation of the conversion unit (e.g. CHP and flare). A guaranteed gas extraction can thus avoid an excessively high level.

How can storage systems be optimally used?

In addition to precise level measurement, the exchange of gas between the holders and a specific pressure ratio in the storage system are important aspects in ensuring uniform use of the entire net gas storage capacity.
An optimum pressure ratio can be set in the system by operating several pneumatically pre-stressed, double-membrane gas holders in a network, whose internal pressures can be manipulated, for example, by adjustable supporting air blowers. This measure enables the entire net gas storage capacity of the combined storage system to be made available for subsequent biogas utilisation. In addition, forecasts that take into account the weather conditions and the biogas demand enable the operating regime to influence the biogas supply at an early stage. This means that the utilisation of the gas storage capacity can be adapted to the respective application. Under certain circumstances, this can lead to additional gas storage volumes being avoided.

What additional costs do operators incur for gas management?

In some cases, the operator can already improve the gas management without additional costs by making changes to the existing operational management. If this is not sufficient, the necessary measurement technology for suitable gas management must be purchased and installed. Costs can arise for procuring frequency converters and supporting air blowers, level measurement systems and their integration into the process control system, as well as various adaptations in the pipeline periphery. Costs can be saved by combining measures with planned maintenance work on the gas storage system. In addition, the costs incurred can be quickly compensated by the avoided gas losses.

According to the latest DBFZ figures, biogas plants in 2017 generated biogas quantities that will provide around 100 terawatt hours of (calorific value-related) energy for subsequent conversion paths such as electricity, heat or fuel generation. How much gas has so far escaped from these plants unused into the atmosphere and how high could the additional power generation potential be?

Only individual observations made at a few plants can be used for assessing gas losses; there has been no comprehensive study of the entire sector. There are plants which, besides the unavoidable emissions such as those from CHP plants, hardly have any additional emissions (less than 0.5 per cent of the volume of gas used), but there are also plants that emit significantly larger quantities due to overpressure events or leaks.

The ManBio project has already been completed. Can you describe the most important findings to me in a few words?

Considering the weather influence on the gas holder and the resulting avoidable biogas emissions are relevant for the plant efficiency, economic efficiency and greenhouse gas balance of the biogas plant.
The research teams investigated the display behaviour, suitability and limits of level measurement methods widely used in practice on a pneumatically pre-stressed, integrated double-membrane gas holder. Here metrological dead zones were detected with a staggered detection start in the lower display area.
The derived recommendations for operation, such as the recommended operation of the gas filling level below about 50 per cent, are primarily intended to reduce emissions.
With the support of a model incorporating the biogas production, temperature and pressure in the gas holder, as well as the weather conditions and gas utilisation, the filling level of the gas holder can be precisely predicted and thus controlled with foresight.
Contrary to the widespread and demonstrably incorrect approach taken by plant operators that a full gas holder is good, I hope that the results of the ManBio project will lead to the correct and, for all parties concerned, suitable setting of half-filled holders for operating biogas facilities.

Mathias Stur completed his studies at Leipzig University of Applied Science (HTWK Leipzig) in the Mechanical and Energy Engineering Faculty (General mechanical engineering – construction) in 2009. He then worked for about four years in the special mechanical engineering field as a design engineer and project manager. Since the beginning of 2013 he has been working at the DBFZ – Biochemical Conversion Department – Biogas Technology Working Group as a scientific researcher.

Picture Mathias Stur
© DBFZ 2019

„No full gas holder is a good gas holder!“

The conversion of biogas into electricity offers a way to compensate for fluctuations in the supply of wind and solar power through adapted production. Research biogas plant of the German Biomass Research Centre
© DBFZ Deutsches Biomasseforschungszentrum

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