The problem with today's biogas plants

Why are common biogas plants running on energy crops, as they have been built at least 4,000 times in Germany so far due to the clearly incorrect orientation of the legislator, not suitable to utilise lignocellulosic residues from agricultural plant and/or animal production with sufficient material and energy efficiency by means of methane fermentation?

→ The solution: Repowering by Aleia


Distinctive features of common biogas plants running on energy crops

  • Ensiling of animal feed, such as maize, grass, beet or whole plants
  • Purchase of maize or grain from the trade or intermediate stacking in the plant area
  • Use of animal excrements from poultry, cattle and/or pig breeding as manure, dry manure or slurry to obtain the so-called "manure bonus"
  • Renunciation of the use of suitable biogenic waste materials according to the Ordinance on Biowaste in order to obtain the bonus for using energy crops
  • Feeding of the comminuted or uncrushed and dry substance-rich feedstock via plug screws into one or more parallel flow stirred fermenters
  • Ensuring the limited circulability of the fermentation substrate in the fermenters by reducing the average dry matter content in the fermenter to below 10 % by supplying drinking water, process water, surface water, leachates from the silage stores and/or biofiltrates to the fermentation stage
  • Transfer of the fermentation substrates discharged from the main fermenter to downstream post-fermenters, which are also mechanically mixed large tanks
  • Discharge of the fermentation residues into downstream stirred and usually gas-tight covered fermentation residue stores
  • Use of the predominantly liquid fermentation residues with dry matter contents of between 5 and 6 % as organic liquid fertilisers during the fertilisation campaigns on predominantly nearby agricultural land
  • Desulphurisation of biogases primarily by means of air injection into the fermenters and/or fermentation residue stores, combined with fine desulphurisation in absorption facilities with activated carbon or iron compounds to ensure the usability of exhaust gas catalysts in the interest of the so-called "formaldehyde bonus"
  • Use of the biogas at the plant location to ensure the thermal process energy supply, connected to drying facilities and/or satellite CHP plants, which are connected to the biogas plant close to heat consumers by so-called micro gas lines in the interest of a high energetic degree of utilization and to claim the so-called "CHP bonus"

Technical deficits of such designed and implemented systems

Unsuitable conditions for the reliable reproduction of a special bacteria culture

The fully mixed flow-through fermenters used would only be suitable for obtaining specially adapted cultures if the quantity of regrowing germs is very significantly higher than the fermentation substrates discharged from the respective fermenter in the same unit of time. Up to now, this cannot be guaranteed in any known case with the known extremely long generation times for bacteria that can use cellulose and lignocellulose organic input materials.

Insufficient fermentation time

As an approximation, the maximum residence time of portions of the medium used under fermentation conditions can be determined as twice the difference between the mean and minimum residence time. Since it is known that in fully mixed systems the minimum residence time of parts of the substrate used is between 1 hour and 1 day and the mean residence time can be easily determined as the quotient of the fermentation volume of the plant and the daily input quantity, it is suggested that both the mean and the maximum residence time must be sufficient for effective fermentation.

Example: The fermenter volume of 3,000 m³ connected in series leads to an average residence time of 3,000 m³ : 75 m³/d = 40 days at a daily input quantity of 75 m³.

The minimum residence time is indisputably less than 1 day, so that the maximum residence time can be determined as 2 x (40 -1) = 78 days.

While it is well known that fermentation times longer than 30 days hardly make any significant contribution to the fermentative degradation of the biomass, the contributions from the substrate fractions, which were discharged from the anaerobic system before a residence time of 30 days was reached, are irretrievably missing.An exception would be if the fermentation residues were subjected to a further fermentation treatment.

Temporally limited availability of the fermentation technology

The predominantly used fermenter construction forces a regular interruption of operation at least for

  • necessary repairs to the stirring mechanisms
  • removal of sediment reducing the active fermenter space. Such sediments are the result of mineral contamination of the feedstock
  • removal of floating layers hindering the gas release. Those floating layers are made up of light portions of the used input materials

Unnecessarily large use of land

The usable volume of the stirred fermenters, which are mostly built as as loosely reinforced concrete tanks, is limited for at least two technical reasons: both with increasing overall height and with increasing diameter, the ring tensile forces to be absorbed by the cylindrical outer wall increase, which limit the possible tank dimension due to the crack widths in the loosely reinforced concrete construction that must be avoided. Furthermore the fermenter dimension is limited by the limited circulation capacity of the stirring mechanisms.

Unnecessarily large treatment and stacking capacities

The thixotropy of the fermentation substrate in methane fermenters is primarily determined by its dry matter content (DM). Due to the limited efficiency of the stirring mechanisms used, the average DM content of the fermentation substrates of 10 % is therefore not normally exceeded. To ensure sufficient average residence times of the fermentation substrate in the anaerobic environment, this requires unnecessarily large apparatus sizes and leads to unnecessarily large quantities of fermentation residues, which in turn require correspondingly large stacking spaces.

Insufficient energetic use of the input materials

Effective fermentative degradation of common biogenic feedstocks for methane fermentation can be measured in a simplified way by the proportion of organic matter in the fermentation residue discharged from the secondary fermenters.

Under technically acceptable conditions, the fermentation residues from advanced methane fermentation plants have organic dry matter (oDM) contents in the dry substance of the fermentation residues of no more than 60 %. The fermentation residues of conventional biogas plants, on the other hand, are characterised by oDM contents of between 78 and 87 %. From this it can be concluded that the potential of the biogenic input materials that can be converted to biogas is only insufficiently used.

Insufficient usability of the nutrient potential

While the nutrient potential in the form of potassium and phosphorus compounds reaches the fermentation residues which can be used for fertilisation practically undiminished, this is not completely successful for the nitrogen and sulphur compounds contained in the input materials. Even if the storage losses, especially those of whole plant silages, are neglected, the biological integration of the volatile sulphur compounds into the fermentation substrate leads only incompletely to the desulphurisation of the biogases obtained, so that at least 20 to 30% of the volatile sulphur compounds are deposited on absorption media prior to the energetic utilisation of the biogases and are cost-effectively combined and disposed of with nutrient losses.

Although the nitrogen inventory of the input materials reaches the agricultural land to be supplied with plant nutrients when ammonia emissions during storage, handling and spreading of the fermentation residues are neglected, only a comparatively small proportion of the nitrogen compounds introduced into the fermentation process has been converted into the ammonium form due to the lack of adequate anaerobic treatment. The nitrogen inventory which has not been converted to the ammonium form is not effective for the direct supply of plants with fertiliser. Large proportions of these nutrients are regularly lost for plant nutrition and pollute the environment by being washed out into ground or surface water.

Questionable environmental benefit

The claimed environmentally friendly extraction of organic NPKS fertilizers that are particularly available to plants and the CO2-neutral generation of energy for use hardly stands up to strict scrutiny. The energy gain from whole plant silage in the amount of at best 30 MWh/ha is opposed by fossil energy expenditures for the following procedures

  • for seed production,
  • for seedbed preparation,
  • for the order,
  • for the use of plant protection products,
  • for the use of pesticides,
  • for the harvest,
  • for the production of shredded material,
  • for the storage of the shredded material,
  • for the construction of biogas plants,
  • for biogas plant operation,
  • for plant dismantling,
  • for the transport of harvested crops,
  • for the transport of fermentation residues and
  • for the replacement of nutrient losses by mineral fertilizer.

A positive eco-balance is therefore difficult or impossible to prove, if only because of the questionable energy balance.

Direct environmental pollution also results from the remaining odour emissions, from the plant-related transport and from the mechanical stress on the arable land required exclusively for the production of energy crops due to the transport of harvested material from and fermentation residues with comparatively low dry substance contents to the arable land.