Pyrolysis of organic waste streams for the production of optimized biochar as an amendment in green roofs.

Abstract

Excessive amounts of stormwater, floods and the heat island effect in cities due the increasing urbanization, hence the loss of green areas is a worldwide problem. These issues could be altered by the use of green roofs. Nevertheless, green roofs still struggle with some serious problems nowadays, like an excessive amount of drainage water, an excess of nutrients in this water, plant mortality in dry periods and the fact that the production process of the components of these substrates, such as expanded clay, is not environmentally and energy-friendly. Therefore it is investigated in large scale field tests whether optimized biochar could be used as an additive in these substrates to counter these issues. As a first step, biochars are produced from 6 problematic, organic and nitrogen rich waste streams in a lab-scale pyrolysis reactor (40 g input/batch). After this, production is enlarged to a continuous pilot scale reactor (a few kg input/hour). The optimal pyrolysis temperature for the scale up was determined by producing and characterizing biochar at different temperatures (350, 450, 500 and 550 °C) in the lab-scale reactor. To evaluate the effect of upscaling and to compare the materials to each other, all biochars were characterized based on their yield, elemental composition and leaching, surface functional groups, morphology, water holding capacity and cation exchange capacity. Furthermore a techo-economic assessment for all biochars and their application in green roof substrates has been made. All biochars were tested in growth experiments in different ratios with commercially available substrates, both on small scale and large scale test fields. The results show up to double the amount of biomass on the green roofs for some of the fields with biochar. In this way two problems are being solved at once by using biochar that has been produced from a problematic waste streams as a soil amendment in green roof substrates.