Originating from small-scale farm-based anaerobic digestion plants, the landscape of anaerobic digestion (AD) in Germany and Europe has changed significantly over the past 20 years. Regarding Germany, this is due to the implementation of generous feed-in tariffs and unlimited priority feed-in for renewable energies.
This was regulated in the renewable energy laws of 2004 and 2009. Additionally, starting in the early 2000s, a specific subsidy for energy crop farming was introduced by the EU as an agro-political tool to avoid food overproduction and yet compensate the farmers to keep working their land. Energy crops include maize, corn, whole crop cereal, sugar beets and grass.
Massive boom in numbers of AD plants
This resulted in a massive boom in the total numbers of AD plants and the average installed capacity. Especially during the eight years from 2004 to 2012, more than 6,500 new AD plants digesting energy crops were commissioned in Germany alone.
Why were energy crops so attractive as a feedstock for AD installations? Well, they have a higher specific gas yield compared to slurry, the necessary agricultural technology is well established and similar to dairy farming and it offers farmers the opportunity to become independent from agricultural price cycles.
Compete with all other biomass plants
Major changes in subsidy policies in the German renewable energy law in 2014 and a complete redesign in 2017 threw all cards in the air again. Since 2017, new and exisiting AD plants have to compete with all other biomass plants via tender processes.
As a direct consequence, expensive feedstocks such as classic energy crops instantly became much less economically viable. Thus, operators are now looking into alternative feedstocks. Increased investment costs for AD plants also factors into economic calculations.
The rise took place in recent years due to an elevation in professionalism in the agricultural sector. Additionally, higher safety standards and technical requirements for AD plant equipment contributed to this development.
This is where manure and grass can come in. Good quality grass silage offers an impressive specific biogas yield of around 150 m³/t compared with the 200 m³/t of the corn silage, making it a viable and abundant alternative feedstock.
Opportunity
Grass and manure as feedstock for AD plants can have multiple financial and practical advantages over energy crops if selected and treated carefully. To take full advantage of its energy content, the grass silage for AD should follow similar quality standards as grass silage destined for dairy cows.
This can lead to direct agricultural competition between food and energy production, leaving behind only grass with low energy, high fibre and nitrogen content for anaerobic digestion.
Getting a hold onto high quality grass or grass residues has challenges of its own: it should be freshly cut, clean from impurities like wood branches, plastic or metal waste and available in quantities large enough to either ensilage it or directly feed it into the AD plant.
Collecting small amounts of grass and grass residues are both financially and logistically challenging, which is why specific contracts or agreements with larger, local facilities (for example, airports and golf courses) are recommended.
Restrictions to origin of substrate
Restrictions to the origin of the substrate also might cause issues: in some countries like Germany, residue grass from the roadside can be categorized as waste and must only be treated in specific, licensed waste digestion plants.
One big advantage of the market of grass residue over the one of maize and grain is that it takes place locally, bypassing the high commodity and transport prices. The farmer or operator could directly go for the cheap and available substrates like grass or manure originating from their own farm or the surrounding neighbours.
As a further positive impact, the nutrient cycle within the farm or the region is restrained and closed.
In Germany in 2015 only about 12 per cent grass silage and 1 per cent landscape grass of the total energy cop mass was digested compared to 73 per cent corn silage.
So although the digestion of grass is an innovative and efficient way of valorising this type of biomass, why is it not more utilised as a cheap, go-to feedstock? And what are the obstacles with digesting manure as part of the feedstock mix?
Issues and bottlenecks
One of the big hurdles with grass digestion is its contamination with pollutants such as sand and soil. Washing or sedimentation serve as effective but costly pre-treatment methods.
The presence of sand and soil causes a high amount of abrasion and general wear and tear in the solid feeding systems, the pumps and the stirrers. The digester has to be opened and cleaned more often compared to other AD plants to remove the sediment from the bottom of the tank.
As grass and also solid manure is an especially fibrous feedstock, it also has a tendency to form swimming crusts making more intensive and thorough stirring unavoidable. Combined with higher pre-treatment and maintenance efforts, this leads to both higher initial investment and higher running costs compared to other feedstocks.
Fresh grass should undergo mechanical treatment before ensilaging to make the following digestion as smooth as possible. To include a high amount of grass, up to 50 per cent of the total input mix mass, it should be shredded to particle sizes of 2 cm or less, in order to reduce problems with pumping, clogging and mixing.
High percentages of grass and manure in the feedstock can also lead to biological problems: high ammonia levels in the digester will inhibit bacteria activity which decreases or halts biogas production.
Perspective
So, what can policy makers do to make biogas production from manure and grass residues more attractive? The first step could be to emphasise the recognition of the triple emission reduction effect:
• Emission reduction in agriculture through manure treatment;
• Production of renewable energy;
• Production of fertiliser, partly replacing mineral fertiliser, saving energy for its production.
Additionally, a special compensation for biogas produced with a certain amount of grass residues can be considered. However, economic and climate policy related benefits will have to be defined.
Active awareness campaigns encouraging grass residue digestion, nutrient recycling and resource saving, as well as research activities in non-food crops for the purpose of soil improvements would also help the cause. Because every leftover grassland is a potential feedstock producer.
With all the presented issues, grass residues and manure still show a big potential as a viable alternative feedstock.
Authors: Michael Köttner, Tristan Gruszkos; IBBK Fachgruppe Biogas GmbH, Germany. IBBK Fachgruppe Biogas GmbH is a company from Southern Germany promoting and supporting the development of biogas and providing an efficient application for a sustainable future through national and international training courses, conferences as well as individual consultings and research projects.