Everywhere cereal crops are grown, whether it be wheat for bread or barley for beer, whether it be in Denmark, Germany or Switzerland, there will always be a side-stream of straw. Primary straw (dry and stock stable) is a wonderful product, with such diverse derivatives as feed and bedding for animals, cellulose for packaging materials and even building materials, and as such should be used for industrial or agricultural value generating processes. Some of the primary quality straw is used in CHP plants converted from fossil coal to sustainable biomass. The shear amount of straw produced annually, and the logistics leaves large amounts un-utilized in the fields.
In 2020, 41.5% of the straw was left in the fields in Denmark, not being used for anything other than being plowed down. Plowing straw down comes with a price of increased unproductive greenhouse gas (GHG) emissions as the straw is decomposed, and the benefits of improving soil fertility and structure by returning a carbon source to the soil. But there are ways to realize the energetic value tied to the above mentioned unproductive GHG emissions, whilst not compromising the soil improvement properties of the straw. One of the answers are the production of biogas.
The 41.5% straw left in the fields of Denmark correspond to 2,276,300 tons, with standard biomethane potentials for straw equal on annual basis to 446,698,151 Nm3 or close to 5TWh. And this excluding the straw used as bedding material and without compromising any of the other utilization pathways of straw. By digesting straw in a biogas plant you realize and contain the energetic value stored in the crop, effective converting the unproductive GHG emission to a productive GHG emission, through a manmade recycling carbon loop. The indigestible parts of the straw will be returned to the fields with the digestate in the increased fertilizer product where it will build up long-term soil carbon
Straw is key to optimizing and increasing the biogas production from agricultural residues throughout Europe. As the market for good quality straw will ensure that the demand for these process pathways will be met, secondary quality and un-utilized straw should be freely used for biomethane production, realizing the energetic value tied within this golden resource.
