Highly Efficient Processing of Fermentation Residue
Photos: Maabjerg Bioenergy, Jens Bach
World’s Current Largest Biogas Plant Works with Decanters from GEA
Hard cash: high phosphate content and high methane potential in the solids
Processing of the fermentation residue, i.e. dewatering, is an essential requirement if transportation of a large volume of water over long distances and at high cost is to be avoided. The processing of fermentation residue uses various processes: filtration by means of screw presses, belt filters, bowl filters, roller presses or sifters, sedimentation with tanks or hydrocyclones, or even centrifugation with decanters. All of these processes display specific pros and cons.
The comparatively greater investment costs of the decanter are more than absorbed by the lower total costs overall – the total cost of ownership. Other economic and procedural criteria also prove the advantages of the decanter. Specifically, the degree of separation of phosphate and nitrogen is significantly greater with the decanter than with other technologies. This has been permanently proven in benchmarking tests and by means of independent investigations.
This also means: high prices can be obtained on the fertilizer market for the concentrated solids with their valuable phosphate content. Another major factor is the high methane potential in the separated solids. This is of interest particularly if the slurry is to be used for incineration and energy generation.
Maabjerg Bioenergy uses five decanters
The advantages of separation with decanters are also recognized by the Maabjerg Bioenergy biogas plant in Holstebro, Denmark, which was rebuilt in 2012. Maabjerg Bioenergy is currently the largest biogas plant in the world. It was created through co-operation between farmers, local authorities and heating plants. On the one hand, the plant is intended to process and refine manure from agriculture; on the other hand, it is intended to provide heat and energy for the towns of Holstebro and Struer.
The biomass plant is designed to process around 500,000 tons of biomass per year from agriculture and the food industry. From this, it generates around 20 million cubic meters of biogas and accordingly an energy content of around 110,000 MWh of electricity and around 100,000 kWh of hot water. The carbon dioxide emissions here are reduced by 21,600 tons per year. Overall, the equivalent reduction of CO2 emissions in full operation should be around 50,000 tons per year in future, also due to the reduction of CO2 and methane emissions from the agricultural businesses. In order to transport the biomass to the plant in a way that is as environmentally friendly as possible and has low transport costs, Maabjerg is planning a pipeline system with radial design over an area of 16 kilometers, which will connect the largest biomass suppliers to the plant.
Without this supply system, up to 50 truck journeys would be required per day. For dewatering of the fermentation residue, Maabjerg has installed three UCD 535 decanters and two UCF 466 decanters from GEA. Because of the high level of phosphate and nitrogen separation by the decanters, the total annual reduction in nutrient surplus amounts to 400 tonnes of nitrogen and 450 tons of phosphate. This separating efficiency is achieved without the use of polymers. Because of the high methane potential in the separated slurry, burning the solids in an incinerator generates 22,000 MWh of energy per year. Phosphate and nitrogen remain in the ash, which can in turn be recycled.
Photo: Maabjerg Bioenergy, Jens Bach
Vital geometry
The correct bowl and scroll geometry are vital for the right design of the decanter. Here, GEA is able to bring all of its experience into play. As a result of the separation, the fermentation residue from a dry substance is dewatered from between around five and ten percent to around 30 percent. The phosphate content in the liquid fermentation residue, for example, is concentrated here from around 1.5 kilograms per ton up to between five and seven kilograms per ton in the dewatered solids and the nitrogen content from between around four and six kilograms per ton in the liquid fermentation residue to between eight and ten kilograms per ton of solids. The transport, which can be up to 500 kilometers in some regions, is thus made much easier, as less water is transported and trucks rather than tankers can be used. The processing therefore reduces the transport costs in addition to increasing the yield due to higher phosphate and nitrogen content as well as higher methane potential.
New: Two-stage separation
For the first time, GEA is offering two-stage separation. Here, in the second separation step, up to 95 percent – compared with around 75 percent without the use of chemicals – of the phosphate content in the solid fraction can be separated with the addition of a small chemical fraction – to bind the smallest particles of water soluble phosphate. This extremely economical variant is of particular interest in countries and regions that have an extremely high nutrient surplus and an accordingly high nutrient export rate, for example in Turkey or Mexico.
Many years of experience
GEA is able to draw on experience with the operation of biogas plants dating back to 1988. Since that time, over 6000 biogas plants of all sizes have operated in Germany. With a large number of construction projects, France, the Netherlands, Scandinavia and England as well as Eastern Europe and the USA are just discovering the many advantages of biogas generation. The decanters available from GEA for processing the fermentation residue handle outputs from three to 100 cubic meters per hour. With absolutely no use of chemicals, the decanters from GEA, which are excellently suited to this application, are able to guarantee a very high dry substance content of up to 30 percent in the solids.
