Minna Sarvi, Johanna Laakso, Elina Tampio and Kimmo Rasa: Pyrolysis of side streams rich in nutrients

Current geopolitical situation and environmental concerns are strong drivers to enhance nutrient recycling. Phosphorus (P) and phosphate rock are on EU´s critical raw material list and nitrogen (N) fertilizer production relies largely on imported natural gas. Europe is divided into areas with excess P in intensive animal production areas, and those where P fertilizers is needed. This recalls continental scale rebalancing of nutrient streams. Thus, one driver for intensive biomass processing is the possibility to concentrate nutrients and/or make recycled fertilizer easy to store and transport longer distances. Industrial and municipal organic waste streams are available broadly, but concerns related to organic contaminants, such as pharmaceuticals, hormones and industrial chemicals, are increasing. Solution is sought from the Intensive processing technologies capable of reducing such risks.

Pyrolysis is a thermochemical process where biomass is heated above 350 °C in the absence of oxygen. It produces dry biochar that is easy to transport and store. Traditionally the technology is used to produce biochar from wood-based biomasses, but also sewage sludge, manure and nutrient-rich feedstocks from industrial sources can be used as raw materials.

In pyrolysis process organic matter transforms to more recalcitrant form, which contributes to soil long term carbon reserve in decadal or centennial time scale. Pyrolysis significantly reduces the content of many organic pollutants while P and potassium (K) are concentrated to the char fraction.

The other side of coin is that the processing requires energy intensive drying of the feedstock and investments in the technology. In addition, N is partly lost in the process, and plant availability of P and remaining N decreases. Also, non-volatile heavy metals present in the feed stock concentrate to the biochar, and relatively inert char does not offer food for soil microbes as original fresh organic matter does. Regardless of vast scientific literature, practical agronomic experience on the use of biochars produced from nutrient-rich biomasses is still rare.

It is no wonder that there is no consensus whether pyrolysis technology can facilitate nutrient recycling. And if it does, in what circumstances? Currently, there are existing industrial scale facilities for sewage sludge pyrolysis. One driver to this development has been a concern related to harmful substances, which hinders the agricultural use of sewage sludge. Sludge pyrolysis can be seen as an alternative method to sludge incineration with better possibilities for carbon and nutrient recycling.

In addition to pyrolysis, other thermochemical conversion technologies should also be considered in valorization of organic side streams. Hydrothermal carbonization (HTC) is a process suitable for liquid and slurry-like materials without drying. HTC is operated in moderate temperature (180–250°C) and pressure (15-25 bar). The degree of carbonization is lower than in pyrolysis, making the HTC char fraction less stable in soils than inert pyrolyzed biochar. More reactive HTC-chars may provide better food for soil microbes and have a positive impact on soil processes. HTC technologies could be potential for biomasses requiring “intensive hygienization” to remove for example health or plant pathogens. Suitability of HTC process for food industry side-streams, and the agronomic potential of the char-fraction, is currently being studied in Finland as a part of Horizon EU funded project DeliSoil (https://delisoil.eu/).

In the case of livestock manure, intensive processing is motivated largely by the need to reduce logistic costs. Solid-liquid separation facilitates the spreading of P rich solid fraction to the fields where P is needed. However, if longer distance transport is required, further processing with, for example, drying, pelletizing or even thermochemical conversion might be needed. Due to the current economic constraints, the increase in manure processing is not foreseen without stronger policy steering and appropriate incentives to take actions in practice. A Horizon EU funded project GREENHOOD is one timely project targeting to balance regional nutrient balance and co-develop governance solutions to support utilization of biobased fertilizers (https://greenhoodproject.eu/). In addition, an Interreg Baltic Sea Region funded CiNURGi -project will develop nutrient recycling and recovery strategies and policy coherency within the Baltic Sea region (https://interreg-baltic.eu/project/cinurgi/).

In conclusion, while the scientific literature on thermochemical processing of nutrient-rich biomass is vast, practical knowledge of generated recycled fertilisers remains limited, particularly in terms of long-term soil experiments. Variability in raw materials and processing conditions can significantly impact the properties of the resulting (bio)char product, highlighting the need for more precise categorization of both char products and raw materials. Further research is essential to establish a robust foundation for legislation and guidelines.