For crop-based biogas plants, the cost for buying the crops is a predominant production cost and efficient systems for production, harvesting, transportation and storage are therefore of major importance. Furthermore, the competition on land between food and energy production has impacted the discussion. EU has decided to strongly limit the production of transportation fuel based on crops grown on arable land, a decision that affects crop-based biogas production. For this reason, it is interesting to examine ways to reduce substrate costs for crops as well as to find alternative crops that are not competing with food production.
Carina Gunnarsson, RISE (JTI/SP) has been the manager of Fresh and stored crops – a new way to organize all-year substrate supply for a biogas plant, a newly finished project within the collaborative research program Renewable transportation fuels and systems. The project has carried out a case study for two crop based biogas plants, Jordberga and Örebro, with the aim of reducing substrate costs by at least 10%. In order to achieve this, the crop supply was organized in a new way, combining fresh and ensiled crops, assuming that substrate costs could be reduced by feeding fresh crops into the biogas digester during the harvest period and thereby reduce costs for storage and avoid losses of dry matter during storage. For the two case studies around ten crops, available during several harvesting periods, were identified, and tested in scenarios with different land use and crop combination constraints These were also compared to a reference scenario without optimization, including the currently used crops.
Out of the different scenarios, two were proven to reduce substrate costs by 10% or more, whereas other scenarios showed that a certain combination of substrates increased the costs. The goal of the project was thereby reached but should also be seen in the light of current crop based biogas production that uses only a few crops. The project analysis included an increased variety of fresh and ensiled crops, which could persumably enhance the complexity of the the harvest, transport and storage systems. The possible advantages and drawbacks of this need to be studied further, as do site specific tests in order to study effects on the biogas process from feeding fresh substrates.