Phosphorrecycling aus biogenen Reststoffen nach thermo-katalytischer Reformierung und Vergasung
Creators: name: Stenzel, Fabian
name: :Unav
Publisher: name: Fraunhofer-Gesellschaft
Year: 2022
Subjects: Klärschlamm
TCR
Phosphorrückgewinnung
TCR-Kohle

description: Phosphorus is a vital raw material. It is a component of human DNA and as such cannot be substituted. It is predominantly used as a fertilizer in agriculture. In this function, phosphorus secures the food supply for the world's population, and as this population increases, so does the demand for this raw material. However, phosphorus is currently obtained mainly from primary deposits, which are concentrated in only a few countries. Germany and Europe have no phosphorus deposits of their own and are completely dependent on imports. This results in a high degree of dependence. In addition, the quality of phosphate ores is declining. They have increasingly lower phosphorus contents and increasing uranium and cadmium contents. To reduce dependence from imports, phosphorus should be recycled. Phosphorus is found in biogenic residues. For example, it enters the plant components of residual material streams via application as a fertilizer and enters the excretions of humans and animals via the food chain. Material streams with a high phosphorus recovery potential are therefore digestate, manure and municipal sewage sludge. However, these materials have low transportability due to their high water content, which means that targeted demand-based fertilization is not always possible. In the case of sewage sludge, direct use in agriculture is additionally restricted due to the contained pollutants. The aim of the work was therefore to develop a concept that enables the most energy efficient possible utilization of sewage sludge with a simultaneously high phosphorus recovery rate. At the same time, it should also achieve the lowest possible transport costs and a high level of acceptance among the population. Finally, it must also be able to prevail over competing processes in terms of ecological and economic aspects. The decision was therefore made in favor of Thermo-Catalytic Reforming (TCR®), a two-stage thermochemical conversion process consisting of pyrolysis and post-reforming step. This process produces the three products oil, gas and char, which can be further utilized for energy purposes. In this process, the phosphorus is concentrated in the char and in the ash when it is used as an energy source in a gasifier for instance. Therefore, recovery options for phosphorus from the char and gasifier ashes were investigated as part of the work. Chars from sewage sludge produced at different process temperatures by TCR® process were used for the investigations. In addition, gasification residues were produced from some of the chars on a laboratory and pilot plant scale. Both were treated with different acids, such as hydrochloric, sulfuric and phosphoric acid, in order to analyze the redissolution behavior of phosphorus and, in this context, heavy metals, which are also present in the sewage sludge chars. The acids were added at different concentrations and solid-liquid ratios as well as varying residence times. Furthermore, the addition of potassium and sodium compounds as additives was tested at different proportions to optimize phosphorus redissolution and the gasification process. To achieve further reduction of heavy metals in the phosphorus product, treatment of the ashes and chars with ethylenediaminetetraacetic acid (EDTA) and post treatment of the phosphorus solution after acid leaching with a cation exchange resin (CER) were investigated. The results from the investigations have shown that the redissolution behavior of phosphorus from the chars is influenced by the process temperatures of the carbonization process. This decreases with increasing process temperature. If a potassium-based additive is added to the feedstock prior to carbonization, the redissolution is significantly improved. Likewise, the laboratory tests on the gasification of the chars showed the clear effect of the additives. Depending on the amount of additive added, conversion of up to one hundred percent could be achieved. In the pilot plant trials with the "ALPHA SuRo" gasifier, a higher reaction rate was shown with the chars with additives due to a higher char conversion and a higher synthesis gas flow. However, the conversion rate needs to be optimized here. With regard to phosphorus redissolution, the gasifier ashes without additive showed the best results. The lowest heavy metal redissolution rates were also observed here. When an additive was used, the basic content of the gasifier ashes was very high, as a result of which the pH values in the subsequent leaching tests were also quite high and thus only a low redissolution rate was achieved. By using EDTA and CER, it was shown that the heavy metals in the phosphorus product could be reduced by up to 67% and 47%, respectively. Based on the results, two technical concepts were developed for future implementation. Alternative A is based on a complete energetic utilization of the sewage sludge and thus also a gasification of the chars. In Alternative B, the focus is on material utilization. Phosphorus and heavy metals are separated from the char so that it can subsequently be used to further purposes. The heavy metals are separated from the chars or ashes and the phosphorus solution using EDTA and CER in order to obtain a clean phosphorus products at the end. Economically and ecologically in terms of greenhouse gas emissions (GHG), Alternative B performs better than Alternative A. This is partly due to the high investment costs for the gasifier in Alternative A, but also due to the GHG credits for the carbon storage in Alternative B. However, the GHG credits were only considered here via the stoichiometric factor for the stored carbon, as there is not yet an established methodology for evaluating carbon storage via chars. Once this is available, the ecological assessment should be re-examined. Compared to previously known phosphorus recovery processes, the approach considered here is in the middle range with respect to recovery costs at 10 €/kgP but still clearly above the market price. In contrast, there is a clear advantage in terms of GHG credits, taking into account the caveats mentioned above. For comparison, the Ash2Phos process was used here, which also has credits. However, these are lower, which is why Alternative B considered here performs better ecologically due to carbon storage. For a future economic optimization of the approach, future revenue potentials should also be taken into account. For Alternative B, only the compensation payments for carbon storage were currently taken into account. However, if the carbon were to be used as a raw material, additional revenues would be generated. Assuming a future revenue of about 500 €/t of char, the process would be competitive compared to phosphorus extraction from primary deposits.