Anaerobic digestion of grass and herbal ley                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  

Grass and herbal ley are increasingly being incorporated as feedstocks in biogas production via anaerobic digestion, with many turning to this source material due to its availability and low cost among other benefits.

However, turning to grass and herbal ley as the primary feedstock for your anaerobic digestion plant isn’t straight forward and comes with a number of considerations.

Here, PWCL’s AD Support Ross Henderson deep-dives into the anaerobic digestion of grass and herbal ley...

Grass and herbal ley are both feedstocks that are increasingly being used or considered for use in biogas production due to their availability and low cost; usually being harvested as a byproduct or residue of livestock farming. Harvested grass is relatively easily digested by livestock, and any excess harvested grass intended for livestock feed can be used instead for biogas production.

Herbal ley is becoming increasingly popular in anaerobic digestion and biogas production due to the positive environmental and biodiversity effects this has over other crops, alongside the green manure effect that herbal lay can provide, especially when red clover is introduced into the seed mix.

Good biomethane potential and cost effective

These materials have a good biomethane potential and are more cost effective than traditional energy crops such as whole crop maize or whole crop ryegrass. Energy crops require cultivation and fertilizer application on each crop which increases the cost of using these feedstocks for biogas production and they also compete for land area directly with other food crops or livestock feed, which in turn puts pressure on the human food supply chain.

However it is not as straight forward as feeding grass or herbal ley directly into an existing biogas plant. Most existing biogas plants have been designed to either co digest energy crops with food waste, manures, or other organic wastes and are therefore unsuitable for 100% grass or herbal ley feeding.

This is due to several factors such as HRT (hydraulic retention time), OLR (organic loading rate), floating layer formation, and lower biogas yields from energy crop only feed.

The nutrient in grass and herbal ley that contribute to biogas production are; protein, sugars, oils, hemicellulose, and cellulose. The sugars and oils are digested quickly and preferentially. The protein then follows alongside the digestion of hemi cellulose and cellulose. Only 5-10% of the hemicellulose and 2-5% of the cellulose is available for anaerobic digestion with the rest of the hemicellulose and cellulose passing through the digester undigested.

Treating the feedstock

To achieve good biogas yield from grass and herbal ley feedstock it is necessary to treat the feedstock to allow the enzymes in hydrolytic bacteria access to the glucose that is bound up in hemicellulose and cellulose, which is contained in the grass and herbal ley.

The amount of each of these nutrients in the grass or herbal ley is dependent on the plant variety and harvesting time. Harvesting in spring will yield higher quantities of protein and sugars, and less non digestible lignocellulosic material than harvesting in the autumn. Herbal ley is often only available for harvest later in the season due to environmental benefits of a later harvest on biodiversity that the herbal ley is planted to support. This make the use of herbal ley more challenging as it will have a lower biogas yield than if it was harvested earlier in the season.

Lignocellulosic material is made up of 3 main compounds. Cellulose, Hemi cellulose and lignin. Cellulose is an alpha linked polymer of glucose that can contain up to 15,000 glucose monomers. When the plant lays down this material in the plant cell wall, it is formed as a fibril that consists of multiple cellulose polymers closely packed in a structure resembling wire rope. Cellulose is said to be in a high crystalline state when it is bound up within a fibril. In this state is very difficult for cellulase enzymes to get to the alpha link bonds in the cellulose to break the monomer units apart. This along with the presence of lignin, which is a heterogeneous water repelling polyphenol with ethylpropane type links, that are not digestible in an anaerobic environment, makes the plant cell wall highly resistant to anaerobic digestion. This is why very little lignocellulosic material is digested by animals or in an AD plant. Only the hemicellulose is available for anaerobic digestion and even then only 5-10% can be reached because of the structure of the plant cell wall.

Currently most AD plants feeding grass or whole crop rye grass end up with a significant portion of the feedstock passing through the plant undigested and ending up as separated fiber at the back end of the plant.

This represents untapped biogas potential!

To increase the amount of hemicellulose and cellulose available for digestion, the highly structured lignocellulosic matrix needs to be broken up and the cellulose microfibrils need to be disrupted so they are no longer crystalline in nature.

This will allow the hydrolytic enzymes access to the hemicellulose and cellulose, which will break the alpha link bonds between the glucose monomers, which will be further digested into biogas. This significantly increases the volatile solids in the feed material available for biogas production, which can increase the biogas yield by up to 50%.

There are 3 main types of pretreatment;

• Chemical, such as sodium hydroxide treatment.
  
• Mechanical, such as a hammer mill or macerator
  
• Biological, such as molds and bacteria that produce ligase and cellulase enzymes
  

There is also the possibility of combining two or more of these pretreatments. To increase the feasibility of disrupting the highly ordered structure of the lignocellulose material to allow more enzyme activity on the feedstock material.

There are many benefits to the use of grass and herbal ley as a feedstock for biogas production, such as increased biodiversity, regeneration of soils, capture carbon and providing a good feedstock for biogas production.

The UK government is paying farmers to plant herbal ley due to these positive benefits so there is likely to be a significant increase in the availability of this feedstock in the future. If more AD plants are designed to use this material as a feedstock, then AD and herbal ley can play a key role in achieving net zero by increasing the production of biogas as well as future proofing feedstock supply for anaerobic digestion plants.