Mechanical Decomposition of Substrates for Increase of the Biogas Yield

Besides encymes the mechanical decomposition of substrates is most widespread. We as biogas plant operators use mechanical decomposition of substrates as long as we operate plants. In case of liquid manure animals made the decomposition, the shredder at the harvest in case of silage. A decomposition beyond this requires considerable expense on energy and wear on the crushing machines or extruders. The question is, whether this expense pays off.

A pretreatment of substrates through mechanical decomposition is always then reasonable and appropriate, if substrates cause technical problems in the plants, for example blockages. Especially with the use of grass, straw, manure, particularly horse manure with a high straw part, upstream mechanical substrate decomposition techniques make sense.

A number of meanwhile established mechanical decomposition machines of different producers is on the market. Through grinding, defibration or percussive techniques the material should be decomposed and a considerable bigger contact surface be created. Of course this requires substantial forces. Hence the machines always show high electrical connection capacities.

The extruders and crushers are already in the investment rather expensive. Generally the operation costs are considerably underestimated by the operators. Besides the electricity demand for the drivetrain the wear of the crusher aggregates has a significant economic impact. A repeated replacement of the wear parts per year is not unusual. Within a few years more money for wear parts and reparatures can be spent, than the cost of the new machine. Lifetime, wear and electricity consumption of the machines depend, of course, on the material throughput and material quality.

If a substrate is only usable in a biogas plant through decomposition techniques at all, the costs must be added to the price of the substrate, in order to determine the profitability. Hereto on the one hand a realistic estimation of the operation costs of the machine is necessary and on the other hand an equally realistic estimation of the biogas yields is needed. Right here problems arise.

Particular precaution should be applied regarding percentages for higher biogas yields. As a rule this only serves the seller of the machine as a selling argument. The comparisons made in this connection are mostly rather unfair. If I refer to a biogas plant, not equipped and optimized for straw fermentation, and then make any optimization, I will always be better than without the optimization, no matter if there is a crusher for the straw or a specific hydrolysis. The cost-benefit analysis has to be made in the individual case between two competing technologies.

More gas by crushing results only when for example dry uncrushed straw is unfairly compared with dry, very fine crushed straw. If one would for example thoroughly soak the straw and optimize the nutrient composition, the advantage of the mechanical crushing would disappear quickly. The question is solely, what is more economic. The mere additional biogas yield only by crushing is as a rule so low, that the power demand for the crushing already consumes the advantage. Hence some operators already removed or decommissioned these machines.

In my experience, the crushing techniques are generally not profitable by the increase of the biogas yield. However, they can make sense if thereby cheap substrates as straw, grass from landscaping, horse manure or other substrates for a biogas plant can be made available at all.

From my point of view:

  • the benefit of crushing machines must be verified separately for each application case.
  • general data on additional biogas yields are not suitable for economic efficiency calculations. Very often they are unrealistic positive.
  • the additional biogas yield is mostly consumed by the operational costs.
  • The operational costs are often underestimated considerably (power consumption and wear).

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