Drying of liquid digestate

Together with the company Arnold & Partner we have developed a process for energy efficient drying of digestate from biogas plants and/or slurry from animal manure.

The process is based on the superheated steam drying technology in combination with a vacuum evaporator using waste heat.

In a first step, the liquid digestate will be concentrated to a dry substance ratio of up to 20% by the use of a vacuum waste heat evaporator. This process step equals a 5 times volume reduction the concentrate compared to the digestate.

In a second step, the concentrate will be dried to an odourless granulate material by drying in a superheated steam atmosphere. During this process, a 13 times volume reduction compared to the liquid digestate will be achieved.

The energy required for the drying step can be recovered for the waste heat evaporator at a ratio of min. 60%. Further energy sources to feed the vacuum evaporator might be waste heat from a biogas plant etc. The drying process required a primary energy source such as natural gas or biomass. This is primarily due to generate the superheated steam used in the closed drying loop as well as to thermally oxidise the contaminants in the drying gas, which basically means to eliminate pollutants. 

 

Sketch of the process for digestate drying to fertiliser

Digestate drying flow diagram

The liquid digestate will be concentrated by the use of a vacuum evaporator. This is the most energy efficient method of water evaporation. Heat sources for this process step are biogas plants as well as the ecoDry system downstream. The concentrated fluid in a second step will be dried to a granulate material of 10% final moisture content in a superheated steam dryer. The dryer works at very low pollutants emissions. 

The granulate material can be used as a fertiliser.

Volume reduction by using evaporator and ecoDry system

Digestate drying volume reduction

The evaporation process will reduce the volume of the liquid digestate by 4-5 times. In combination with the ecoDry drying process, a 13 times total volume reduction will be achieved. Out of 1000 m3 of digestate only 73 m3 of granulated fertiliser will be obtained

The result: an odourless and dry fertiliser granulate

Granulat nach Trockner