Electricity and heat from farm manure

liquid manure is becoming increasingly important for biogas production

The new Fertiliser Ordinance for Germany has been in force since 1 May. It introduces longer periods in the autumn and winter months, during which fertilisation application on farmland is not permitted.

Although the designation of what are called 'red areas', in which nitrate concentration in the groundwater is particularly high, is only due to take effect from 1 January 2021, farmers in the affected regions are increasingly coming under pressure and are looking for solutions to deal with their excess liquid manure, which necessitates increased storage capacities in closed or open silo systems. As a result, liquid manure is becoming increasingly interesting as a substrate for biogas plants. The economic potential is very promising. In Germany alone, 140 million cubic metres are produced each year. So far, this has been applied on agricultural land in the form of liquid farm manure. Added to this is around 20 million tonnes of solid manure which can also be used as substrate in biogas plants.

High potential for use in energy production

Around 7,500 biogas plants in Germany are currently generating renewable electricity and heat using liquid manure. Most of this electricity produced by means of cogeneration, is fed into the grid. These plants  are joined by over 580 small liquid manure systems with outputs of up to 75 kilowatts for use on farms. These plants, which will be the focus of attention at EnergyDecentral, can ferment almost any organic substance. However, only around one quarter of the farm manure produced in Germany is fermented in biogas plants. Even this "Saves over two million tonnes of CO2 through the avoidance of methane emissions alone", states Sandra Rostek. The head of 'Hauptstadtbüro Bioenergie' sees an urgent need for action within the German Renewable Energy Act (EEG) in order to create planning security for new and existing plants and to increase the percentage of liquid manure fermentation to 60 percent – "A realistic target that would additionally avoid approximately three million tonnes of CO2", says Rostek. However, a decline in liquid manure fermentation is a possibility if the subsidy legislation and regulatory framework conditions remain unchanged. Above all, the special remuneration category needs to be further developed so that new, small liquid manure plants can receive compensation. "Otherwise, sustainable plant concepts will be hindered", warns the expert.

This is a risk that Dr Claudius da Costa Gomez also foresees. "If functional biogas plants are dismantled from 2021 onwards, a significant part of the climate-friendly energy supply will no longer be available", warns the German Biogas Association's managing director, and demands the further expansion of biogas potential in the area of liquid manure and waste fermentation. The volume of electricity generated from liquid manure is currently around four terawatt hours. This volume could be doubled if half the liquid manure still available was used. This is the result arrived at by the 'Current development and perspectives of biogas production from liquid manure and organic waste' research project commissioned by the German Environment Agency (UBA). Amongst other things, the authors developed a proposal for adapting the EEG subsidies to create more incentives for using liquid manure in biogas plants.

Technologies for efficient fermentation

New concepts for the economic operation of biogas plants are also required to increase the use of liquid manure fermentation. EnergyDecentral intends to demonstrate once again that the industry is not lacking in technical innovativeness. Biogas plant construction from conception and commissioning, gas usage and processing up to and including residue treatment: in Hanover, the exhibitors will be presenting a comprehensive portfolio for the entire digestion line and will be offering farm biogas plants with outputs between 30 and 75 kilowatts in modular designs. These systems are characterised by their compact dimensions and can be optimally integrated into any farm, even when space is limited. Fermenters of this size category can usually hold between 500 and 1,200 cubic metres of substrate. Around one third of the heat is recirculated for use in the biogas production. Thanks to modern local heating networks, the surplus can be used to heat agricultural buildings and livestock housing or to dry products such as cereals and straw.

In the future, high-load reactors should almost completely exploit the potential of the liquid proportion of separated slurry. To achieve this, scientists from Münster University of Applied Sciences are combining various substrates in the 'BioSmart' project: the key term is co-fermentation. This leads to synergy effects which further increase the potential of biogas. "It involves combining various residual substances from the agricultural and the industrial sectors and testing them in the laboratory", explains project engineer Lukas Wettwer. The project is focussing on five substrates, including liquid manure from cattle and pigs and waste water containing starch. The researchers aim to establish a detailed residual substance database. The team had already previously developed a high-load reactor for biogas plants in which the microorganism density is increased by means of innovative technology with degradation rate thus accelerated. So far, the reactors have been operated in a pilot plant. They are now due to be integrated into a two-line biogas plant at the Saerbeck bioenergy park in order to collect data for cost-effective implementation.

From fermentation residue to fertiliser pellet

For farmers who are focussing increasingly on the economical use of solid manure or liquid manure, the technologies for processing fermentation residues are equally as important as intelligent process organisation. The new methods to be presented at EnergyDecentral particularly make operating large biogas plants more efficient. The exhibitors' portfolio encompasses optimised reverse osmosis processes for increasing separation efficiency and innovative components for thermal refinement. In this process, modern systems use the waste heat from the combined heat and power plant to draw much of the moisture from the fermentation residue by vacuum evaporation. The drying process leads to high-quality fertilisers with high nutrient contents that can be further processed to form low-odour pellets.

The look ahead to EnergyDecentral shows that the exhibitors at the specialist international trade fair for innovative energy supply and sustainable energy production will offer biogas plant operators the opportunity to establish a long-term economic perspective for their operation. The Energy Collective Act (EnSaG) came into force at the end of 2018. With this statute, the size limit for small liquid manure plants was switched from installed capacity to rated power. This is sure to be a topic of discussion at the stands taking place on 09 and 12 February at the trade fair grounds in Hanover. This change enables farmers to install larger combined heat and power plants with an output of up to 150 kilowatts – thus offering access to flexible and needs-based operation.

08.05.20, MB/TW