Production of biochar from the pyrolysis of spent mushroom substrate

Mushroom waste substrate is produced in large quantities, up to 5 kg per 1 kg of mushroom. It contains fungal spores, making landfill disposal not allowed; thus, mushroom waste substrate is currently accumulated. The objective of this study was to explore the conversion of mushroom waste substrate into biochar through pyrolysis, with the aim to apply the biochar in the cultivation of mushroom and for carbon sequestration. The study involved two types of mushroom substrate: fresh (e.g. the medium used to grow mushrooms) and spent (e.g. leftover material enclosing fungal mycelium). While the study is focused on the valorization of spent substrate, the fresh one was evaluated to explore the potential for future feedstock design. The experiments were conducted in a lab-scale horizontal mechanically fluidized bed reactor consisting of a 15L stainless-steel cylindrical chamber heated by an induction system. Pyrolysis batch tests were performed at 350, 450 and 550 C. The obtained biochars were quantified and characterized through proximate and elemental analyses and H/C ratio to evaluate their features and to determine the optimal conditions for further testing. As expected, biochar yields decreased by increasing temperature, and gas yields displayed an opposite trend. The achieved results were consistent for the spent and fresh substrates. In details, biochar yields were 48% at 350 C, 34% at 450 C and 28% at 550 C for the spent substrate; and 45%, 32% until 28% for the fresh one. Gas yields were 29% at 350 C, 35% at 450 C and 43% at 550 C for the spent substrate; and 27%, 38% until 43% for the fresh substrate. Bio-oil yields were 23% at 350 C, 31% at 450 C and 29% at 550 C for the spent substrate; and 28%, 30% and 29% for the fresh substrate. About the characteristics of biochar, the results are consistent with literature: the fixed Carbon content increased with temperature, from 61% at 350 C, to 74% at 450 C and 77% at 550 C, and correspondingly the volatile Carbon decreased from 31%, to 17% and to 12%; the ash content increased from 8% at 350 C, to 9% at 450 C and to 11% at 550 C. The biochars obtained at the three tested temperatures showed in all cases a H/C ratio below 0.7 (reaching values as low as 0.35) which makes them eligible for carbon offset credits. In conclusion, this study preliminarily demonstrates that mushroom substrate is a valuable feedstock for producing high-quality biochar via pyrolysis, with promising potential for carbon sequestration and as soil amendment. The results showed that higher temperatures improved biochar quality and stability by increasing the carbon and ash contents and reducing volatile matter. Further research is necessary to investigate how the biochar could be reintegrated into the mushroom cultivation process as a component of the growth substrate, following a circular economy approach that provides a sustainable solution to the significant challenge of mushroom substrate disposal.