Turning residual biomass into a resource rather than waste has previously been difficult. A novel process produces methane with very high yield.
Lignocellulosic biomasses, organic household waste, waste from food industries, and pig manure are the main raw materials in the growing Danish biogas industry. However, some 40-50 per cent of the energy content is still present in the residual sludge after the biogas has been produced. A new process developed in an interdisciplinary project involving several groups at DTU Chemical Engineering is able to convert biomass into methane with very high carbon efficiency: Close to 100 per cent of the carbon present in the raw materials ends up in the methane. Methane is a valuable fuel and has the further advantage of being storable in the already established Danish infrastructure for natural gas.
As a first step, the biomass is converted into ‘syngas’—which is a mixture of H2, CO2, and CO—through gasification. This is a commercially available process. However, syngas in itself is not an ideal product.
“While syngas is used for instance as a resource for combined heat and power production, this involves a number of practical challenges, since the syngas cannot be stored and has to be used when it is produced,” Associate Professor Ioannis V. Skiadas explains.
“By pairing the combined heat and power production with the fermentation of syngas, the processing of residual biomasses can always follow the optimum path. That is, the production will be easily diverted to either combined heat or power, or to storable biofuels, thus satisfying the supply and demand of the biomass and energy markets at the given time.”
A microbial process is the key
When the demand for heating and electricity is high, the syngas may be exploited mainly through combined heat and power, but when the heating and electricity demand is low, the syngas will be fermented to storable biofuels like methane. The fermentation takes place at atmospheric pressure and very mild temperatures (30 - 60 ˚C), and therefore it is very simple and comes with very low operational costs. This makes the technology particularly suitable for relatively small gasification plants, which is the usual case for Denmark.
Therefore, the real novelty in the project is the proven ability to process the syngas further into methane using fermentation, which is a microbial process.
A number of industrial fermentation processes rely on pure microbial cultures where a single microbial species is used—often in a genetically engineered version—to perform a very specific task. For instance, this is a common strategy in the pharmaceutical industry. In contrast, the fermentation in this project at DTU Chemical Engineering is based on mixed microbial consortia.
“Pure cultures require costly procedures, for instance sterilizing equipment and feedstock, to avoid contamination from other species. This can be justified if you manufacture a high value product like an active pharmaceutical ingredient, but for a product like methane you need something cheaper,” Associate Professor Hariklia N. Gavala explains.
However, using mixed microbial consortia is not just hoping for the best. The team has developed enriched microbial cultures for this specific purpose.
Pilot reactor is in operation
Another factor which strongly contributes to the close to 100 per cent methane yield, is the development of thermodynamically driven enrichment processes and reactor operations, not least a trickle bed reactor. This has helped to further develop the process to include hydrogen (which can be produced with renewable electricity from wind turbines) and biologically convert the remaining CO2 content of the gases into methane as well.
Proof-of-concept for the reactor has been achieved in a lab-scale version, and a 30 times up-scaled pilot version is now in operation—both with close to 100 per cent methane yield.
In practice, any type of biomass will be relevant, Ioannis V. Skiadas explains:
“This could be straw, wood pellets, residual sludge from biogas production, etc. Whenever you have a
bio-resource that is rich in carbon and poor in water, gasification into syngas will be possible.”
Next step: Higher value chemicals
Further, production of methane could be just the first step in utilization of tricky biomass resources, Hariklia N. Gavala notes:
“We have also shown the possibility to produce ethanol and other chemicals from mixed microbial consortia. This could be an even more promising utilization of the difficult biomass fractions, as these chemicals have higher value than methane. ”
A new enabling technology has been the development of combined biomimetic membranes and diabatic distillation for the purification of the produced ethanol.
“The next step will be implementation in industry, possibly at a biogas, combustion, or gasification facility. We hope to attract interest from one or more companies as soon as we have the final report ready,” says Associate Professor Jens Abildskov.
The project has created a general overview of existing and new technologies.
“We have managed to create an innovative benchmarking study, for recovery of ethanol as an example of a higher value chemical. The project clearly shows how energy use can be reduced by applying the right knowhow. We have also improved the understanding of reactor design which will reduce the size of future full-scale plants,” concludes Associate Professor Philip Fosbøl.
The gasification/bio-conversion technology platform for production of methane and possibly other chemicals has been developed at DTU Chemical Engineering in the SYNFERON project (Optimised SYNgas FERmentatiON for biofuels production).
Partners in the SYNFERON project are DTU Chemical Engineering, Danish Gas Technology Centre, Aquaporin A/S, Biosystemer ApS, Highterm Research GmBH (Germany), and Bioeconomy Institute at Iowa State University (USA). Of the initial total budget of EUR 2.8 million, EUR 2.3 million was financed by Innovation Fund Denmark, while the partners have contributed with the remaining EUR 0.5 million. Professor Georgios M. Kontogeorgis, DTU Chemical Engineering, was overall coordinator.
The SYNFERON project was formally completed by summer 2019, but efforts and collaborations continue with the aim of implementing the developed technology platform.
At DTU Chemical Engineering, the SYNFERON project has been highly interdisciplinary involving staff from the AT CERE centre, the PROSYS centre, and the Pilot Plant.