Processing of Digestate from Biogas Plants

abstract

en

Following significant financial incentives in European countries, biogas plants have become a widespread technology for biomass conversion and electricity generation. Approx. 18 thousand plants are currently running in the EU with a total installed capacity of over 10 GW. Despite the undeniable advantages of this technology, there are still several issues that need to be addressed. One of them is the inefficient use of the heat produced during the cogeneration process. Usually, only 20-40% of the heat produced is used. The main reasons for this situation are the lack of appropriate technologies and the low-potential nature of the waste heat (around 90°C). Another problem is related to the treatment and use of wastewater (so-called digestate or fermentation residues). Digestate is a by-product of anaerobic fermentation of biomass and is particularly rich in nutrients such as nitrogen, potassium, or phosphorus. It is therefore mainly used as a natural fertilizer. However, the nutrient concentration is relatively low as the water content of the digestate ranges from 88 to 98 %. Since the production of digestate is significant, typically around 15 000 m3 per MW of installed capacity, plant operators have to face high costs for the storage and transport of this wastewater. For this reason, suitable technologies are being sought that can reduce the water content of the digestate and thus reduce its volume. Reducing the volume of the fermentation residues can have many economic and environmental benefits. These include a reduction in the need for storage space, fuel savings during digestate transportation, fewer passes of agricultural machinery over farmland, elimination of tanker passes through populated areas, noise reduction, reduced emissions, water savings for irrigation or other technical uses, minimization of nitrogen losses or meeting legislative requirements, in particular the nitrate directive. The present study focuses on addressing both – waste heat recovery and digestate thickening – using vacuum evaporation as the main treatment technology. Vacuum evaporators are robust and reliable devices using low-potential thermal energy as the main energy source. The output of the evaporator is thickened fertilizer and ammonia water that does not meet the requirements for discharge to surface water. For this reason, it is necessary to extend the evaporators with other technologies such as stripping, membranes, or acid scrubbers. The present work summarizes the latest findings in this field, both technical and economic. It is based on experience from biogas plant operation and experiments on a prototype thickening plant.

Following significant financial incentives in European countries, biogas plants have become a widespread technology for biomass conversion and electricity generation. Approx. 18 thousand plants are currently running in the EU with a total installed capacity of over 10 GW. Despite the undeniable advantages of this technology, there are still several issues that need to be addressed. One of them is the inefficient use of the heat produced during the cogeneration process. Usually, only 20-40% of the heat produced is used. The main reasons for this situation are the lack of appropriate technologies and the low-potential nature of the waste heat (around 90°C). Another problem is related to the treatment and use of wastewater (so-called digestate or fermentation residues). Digestate is a by-product of anaerobic fermentation of biomass and is particularly rich in nutrients such as nitrogen, potassium, or phosphorus. It is therefore mainly used as a natural fertilizer. However, the nutrient concentration is relatively low as the water content of the digestate ranges from 88 to 98 %. Since the production of digestate is significant, typically around 15 000 m3 per MW of installed capacity, plant operators have to face high costs for the storage and transport of this wastewater. For this reason, suitable technologies are being sought that can reduce the water content of the digestate and thus reduce its volume. Reducing the volume of the fermentation residues can have many economic and environmental benefits. These include a reduction in the need for storage space, fuel savings during digestate transportation, fewer passes of agricultural machinery over farmland, elimination of tanker passes through populated areas, noise reduction, reduced emissions, water savings for irrigation or other technical uses, minimization of nitrogen losses or meeting legislative requirements, in particular the nitrate directive. The present study focuses on addressing both – waste heat recovery and digestate thickening – using vacuum evaporation as the main treatment technology. Vacuum evaporators are robust and reliable devices using low-potential thermal energy as the main energy source. The output of the evaporator is thickened fertilizer and ammonia water that does not meet the requirements for discharge to surface water. For this reason, it is necessary to extend the evaporators with other technologies such as stripping, membranes, or acid scrubbers. The present work summarizes the latest findings in this field, both technical and economic. It is based on experience from biogas plant operation and experiments on a prototype thickening plant.

Biogas Plants; Liquid Digestate; Vacuum Evaporation; Ammonia Recovery; Anaerobic Digestion

30.08.2021

Volos

978-618-84403-5-7

2653-8911

8th International Conference on "Energy, Sustainability and Climate Crisis" ESCC 2021 - Book of Abstracts

8

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27–28

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