The gasification of sewage sludge, in other words incineration under oxygen deficiency conditions , offers up exciting possibilities. Phosphorus is found in sewage sludge as a phosphate, i.e. a stable molecule containing oxygen. This oxygen can be separated using high temperatures, oxygen deficiency conditions and the presence of a reducing agent such as carbon, thus creating pure phosphorus or phosphorus suboxides. This can then be separated and used to produce phosphoric acid and then, for example, fertilisers. The gasification process encompasses high temperatures and oxygen deficiency conditions. Carbon acts as a reducing agent in the sewage sludge, which may need to be supplemented with additional carbon, e.g. from biomass, pre-treated residual materials or coal.
Gasification thus provides the technical foundation for extracting the phosphorus from the sewage sludge and is also an important part of developing carbon cycles. Gasification results in a gaseous mixture that mainly contains carbon monoxide (CO), hydrogen (H2), water (H2O), carbon dioxide (CO2) and phosphorus (P or PxOy). The phosphorus can be separated from this gas using a washing process. The residual gas is purified until a synthesis gas (CO + H2) is created that can be used to produce primary energy sources, fuels or important raw materials for the chemical industry.
In contrast to other disposal methods, the advantages of gasification are manifold. Sewage sludge can be thermally treated and phosphorus and carbon recycled as part of a single process step. Problematic trace elements, such as medicine residues, heavy metals or micro-plastics that cause problems in agriculture when sewage sludge is disposed of, can be safely treated in the gasification process.
Significant research and development work is needed to turn this method of recycling reusable materials from sewage sludge into a reality.
How the fuel (mixture of sewage sludge and a carbon carrier) required in the gasifier can be introduced into it must first be clarified. Processes for introducing fuel into a gasifier do already exist. However, these need to be qualified and tested to determine their suitability for the fuel combinations used in this project. Moreover, suitable processes and process conditions for the extraction of phosphorus from the producer gas of the gasifier must be developed. Last but not least, the necessary operating parameters in the core components of the process chain, i.e. gasification, need to be determined on the basis of experiments. Only then efficient gasification and extraction of the phosphorus can be facilitated.
The required R&D phases are being pressed ahead within a project by RWE Power. To this end, all permits required for the construction and operation of the plant will be gathered and a technical concept for the fuel input system drawn up, put into practice in a testing facility and tested. In addition, a detailed engineering is being prepared for the whole plant to ensure that procurement, assembly and construction of the whole plant can be started at at the beginning of the next project phase in 2020. A technical concept for the washer to extract phosphorus from the raw synthesis gas will be developed and tested by the end of 2019.