Fractional evaporation is a process for treating agricultural substrates with simultaneous recovery of nitrogen in the form of a highly concentrated ammonia solution. In the process, over 95 % of the ammonia nitrogen load is recovered in various condensate fractions, which can be fed to a demand-oriented utilisation or treatment depending on the quality.
The spreading of digestate and liquid manure is associated with considerable environmental impacts due to the nitrate contamination of groundwater and presents farmers and operators of biogas plants with major challenges with regard to their material flow management. Fractional evaporation offers a treatment of the substrates, which on the one hand reduces the volume and thus saves storage capacities, but on the other hand also recovers the nitrogen in the form of a highly concentrated ammonia solution, which can potentially also be used outside agriculture.
The principle of fractional evaporation method, which was investigated in the research project “KompaGG-N”, is based on the generation of condensate streams with different qualities in terms of nitrogen content. As a result of the lower vapour pressure of ammonium compared to water, a condensate with a concentration between 2,000 - 5,000 mg NH4-N/L results depending on the initial content at the beginning of the evaporation. In this process, > 50 % of the original nitrogen load is transferred to about 10 % by volume of the condensate (or 95 % NH4-N). This first fraction can be used, for example, for fertiliser production or further concentrated by means of rectification.
The second fraction is already significantly depleted of nitrogen, but still requires treatment. Treatment can be carried out by means of reverse osmosis or biological processes, for example.
Depending on the volume reduction, the 3rd fraction can be used to produce a condensate that is almost free of nitrogen and can be discharged, fogged or used for operational purposes depending on local and regulatory requirements.
Compared to conventional evaporation processes, in which NH4 is usually recovered as an ammonium sulphate solution, fractional evaporation can save considerable amounts of operating materials in the form of sulphuric acid.
The main factors influencing the achievable volume reduction are the dry substance content in the input and the available thermal energy, which is provided by CHP waste heat. Due to evaporation under vacuum, process temperatures < 80 °C are also possible. In the case of a multi-stage design, the efficiency can also be further increased by multiple utilisation of the supplied heat. To achieve the highest possible volume reduction, a dry matter content of between 4 - 6 % is recommended in the input, so that upstream mechanical separation should be provided if necessary.
