Efficient processes for biogas production from of organic waste and renewable raw materials

Everywhere, there are residual materials and waste that cannot be further utilized as animal feed or raw materials. However, all of these organic residues and bio-waste contain valuable energy. This energy is lost during processing in composting facilities; moreover, composting requires significant energy for aerating the compost material. Anaerobic digestion offers an energy-efficient and cost-saving alternative for the utilization and disposal of organic materials.

Use of organic residues from the food industry and biotechnology

In the production of food or in biotechnological processes, residual materials of the most varied composition are produced. Fraunhofer IGB develops individual and adapted solutions for the material and energetic use of organic residues that are generated as by-products. Organic residues that cannot be recycled are particularly suitable for anaerobic conversion into the end product biogas (methane and carbon dioxide).

Productivity increase of biogas plants

Biogas plants, in which above all renewable raw materials are digested into biogas, are an important decentralized component in the expansion of renewable energies. However, there is still a need for research to make biogas production as a whole as effective as possible and thus improve the biogas yield.

Biogas from waste 

In Germany, biogas is still mainly produced from renewable raw materials. In this context, fermentation is excellently suited for all wood-poor fresh and moist organic waste: from liquid manure and dung, to green waste and vegetable waste from municipal waste disposal, to food waste from trade and industry. Promoting the widespread use of these organic wastes should find its way into national strategies for the sustainable use of biological resources so that the cultivation of energy crops on arable land can be reduced. This would also create economic incentives for agricultural operators of smaller plants. Recently, already functioning plants have been decommissioned because the Renewable Energy Sources Act (EEG in German) remuneration ceases after 20 years of operation.

Biogas – an alternative to natural gas for baseload power supply

Biogas, or the biomethane purified from it, could also represent an alternative to ensure security of supply with methane ("natural gas") and reduce international dependencies. According to the German Biogas Association, there are currently around 9,600 biogas plants in Germany. With a share of eleven percent of renewable electricity generation, they supplied a good 50 terawatt hours of electricity in 2021 – as much as all photovoltaic plants in Germany (Federal Environment Agency, based on AGEE-Stat, as of 02/2022). This makes biogas an important storable supplement to volatile electricity from wind and sun.)

Efficient anaerobic digestion of organic residues

Anaerobic digestion processes can be used to operate plants directly at the point of origin of the residual biomass, ensuring the utilization of readily fermentable materials to produce biogas. The high-load process developed for increasing efficiency in sewage sludge digestion also promises significantly improved efficiency, short retention times and a high degree of degradation, as well as a high biogas yield, for use with organic residues.

Advantages of anaerobic digestion technology

• Production of biogas as an alternative to fossil fuels
• Contribution to climate change mitigation through reduction of greenhouse gas emissions
• Use of fermentation residues for soil improvement and sustainable phosphorus supply
• Inactivation of weed seeds, neutralization of ingredients (e.g. antibiotics) and hygienization of the residues (with thermophilic processes at approximately 55°C)

Fraunhofer IGB develops individual and adapted solutions for the material and energetic utilization of organic residues, which accumulate as by-products in industry and agriculture.

Investigation of fermentability and process design

To this end, we are investigating the fermentative conversion of the residues on a laboratory scale with regard to their biomethane potential. For the design of the new process according to the customer's specifications, we evaluate the various process parameters, identify weak points and optimize the process with regard to maximum product yield.

Optimization of biogas plants

We evaluate and optimize existing biogas plants regarding their performance and energy efficiency.

Analysis and consulting

With a technical and economic analysis of the production waste and by-products, we advise companies on how to make optimum use of any residual materials that arise, for example, by coupling anaerobic digestion with further conversion or recovery steps.

Optimized process control

For the design of the process, the type and composition of the substrates is crucial, since the properties of the feedstocks have the greatest influence on the degradation time. In addition to the process temperature, other important parameters we are investigating for optimal conversion of the feedstocks to biomethane are the residence time of the substrates and the organic loading rate in the reactor. The stability of the biological process is mainly influenced by the microorganisms available in the inoculum and the C/N ratio in the feeding regime, furthermore, by the availability of trace elements. Metabolic products such as ammonia and volatile fatty acids or toxins must also be taken into account in the process control.

Comprehensive analytics ensure the monitoring of the process, its optimization and the improvement of the process and energy efficiency of the plant. 

Anaerobic reactor technology

Characteristic of anaerobic conversion is the low growth of biomass, since most of the energy contained in the substrates is converted into the end product: methane. Therefore, to increase the conversion rate, the active biomass must be retained and concentrated in the reactor. This can be done by immobilizing the biomass on a support material in a fixed-bed reactor or by mechanical retention in a membrane bioreactor.

Liquid raw materials can be used without further pretreatment. Suspension reactors or fixed-bed reactors are used here.

For raw materials with a high solids content, e.g. the organic fraction of municipal solid waste, food waste and lignocellulosic biomass, anaerobic solid reactors are suitable.

For feedstocks containing fibers and lignocellulose, longer residence times and suitable pretreatment, for example with various mechanical, biological or thermochemical digestion processes, lead to better biogas yields.

Substrates suitable for biomethane production

• Food waste
• Cattle/pig manure and poultry manure
• Waste/residues from beverage and food industry
• Waste/residues from biofuel production
• Energy crops, e.g. corn, cup plant (silphium), miscanthus (after sufficient pretreatment), sugar sorghum
• Combination of different substrates