Intermediate Pyrolysis of Agricultural Waste: A Decentral Approach towards Circular Economy

Abstract

The sustainable utilization and recycling of secondary raw materials is the backbone of the Circular Economy (CE), which aims at the efficient conversion of resources and energy while minimising the impact on the environment. Intensive farming generates large amounts of waste streams such as litter and manure. At present, they are not recovered systematically in the sense of CE. Chicken manure is an interesting feedstock due to the content of nutrients, in particular of phosphorus, which makes chicken manure a suitable feedstock for fertilizer applications. However, such streams are often contaminated with antibiotics and other organic pollutants, which must be thermally destroyed before disposal or further utilisation. Certain technologies are arising with the aim of combining nutrients recycling and energy recovery. Among them, intermediate pyrolysis targets at decentral application for production of carbonized solids and organic vapours for fertilizer application and heat generation, respectively. The aim of this work is to evaluate the combined nutrients recycling and energy recovery for the example of pyrolysis of chicken manure in a proprietary screw pyrolysis reactor with integrated hot gas filtration. After a brief description of the bench-scale pyrolysis unit the feedstock is characterized. The effect of reactor temperature on yields and properties of the pyrolysis products is investigated experimentally. The chemistry and the mineralogy of the pyrolysis chars are evaluated applying a number of analytical techniques. The bio-availability of the main nutrients (NPK) is assessed adopting standard methods. Subsequently, the pyrolysis process is scaled-up and it is integrated in a CHP (Cogeneration of Heat and Power) unit for self-sustained operation of the pyrolysis reactor. The integrated system is evaluated in terms of energy production and of emissions. Finally, the “break-even price” of the pyrolysis char is provided as result of a techno-economic analysis.