Straw to biogas

It was previously believed that lignin-cellulosic raw materials such as straw or wooden chips are unsuitable for biogas production. However, pre-treatment with steam explosion allows the use of these materials for anaerobic digestion.

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Technology Description

The technology of straw steam explosion in biogas application is relatively new. However, the technology is well known in the paper industry for hundreds of years. Also the same process is applied in straw-to-ethanol production. 

Dry straw is chopped and mixed with water and filtrate from a biogas plant or sewage. Wet straw is periodically fed and heated to temperature of 150-180 ℃ at which lignin melts freeing cellulose and the hemicellulose skeleton. During the cooking process at high temperature and a pressure of 5-8 atmospheres, the raw material is completely saturated with water. All pores between the fibers are filled. After the cooking process is completed, the pressure is dropped to atmospheric in a fraction of a second. This leads to the water turning into steam creating the effect of an explosion shredding the fibers.

Breaking the fibers leads to an increase in the surface area of the raw material. In addition, a break occurs at the molecular level. Complex sugars are broken down into simple ones. After steam explosion, straw is processed in biogas reactors like maize silage.

Straw has advantages over maize silage as a substrate for biogas. Maize silage requires the construction of a silage storage. While straw can be stored in the open air. Moreover, storing straw in the open air improves its performance.

In growing maize for silage, the farmer loses the opportunity to produce grain. If farming grain, the farmer receives both grain and straw which is the substrate for a biogas plant. For biogas production, any kind of straw is suitable such as wheat, sunflower, and rapeseed. In addition to straw crop residues, corn stover, husks, sawdust, wood waste, and dung bedding can also be used. Even old, 2-3-year-old straw is suitable.

We at Zorg Biogas have experience of 6 steam explosion units with a capacity of 25 tons of straw per day each. In total, they produce biogas in a volume sufficient to feed generators with an electric power of 6 MW. We have also experience with other, not less effective, methods of straw pre-treatment like milling and prelletizing. 

Distinctive features:

  • used in conjunction with high-load reactor technology
  • the same steam explosion is applied in ethanol-out-of-straw production
  • no straw storage is required
  • high biogas yield
1 to 20 MW
power capacity

Suitable substrates:

  • wheat straw
  • rapeseed straw
  • sunflower straw
  • rye straw
  • corn stover
  • sunflower husk
  • sawdust

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