Slight carbonization as a new approach to obtain peat alternative

Abstract

Biochar has been widely studied to replace peat in growing media due to its suitable water holding capacity, non-phytotoxicity, stability, and some nutrients. Those features are most probably generated during biomass carbonization, because raw garden waste, common feedstock for biochar, generally has plant originated phytotoxic compounds and unbefitting pH, EC and poor water holding capacity as growing media. However, most studied biochar (pyro-char/hydro-char/torrefied-char) are deeply carbonized biomass resulting in other features, e.g., rich nanopores and/or severely degraded cellulose and lignin. Those features are less found in the conventional growing media, i.e., peat. Therefore, we assume that slightly carbonized products with properties between biochar and raw garden waste might perform as peat alternatives. Superheated steam (SHS) torrefaction is a recently proposed incomplete carbonization process, which could selectively boost biomass decomposition and regulate features favoring growing media. Temperature and residence time parameters were investigated to customize product properties to remove phytotoxicity compounds, increase water holding capacity, prolong half-life, and obtain proper pH and EC. After evaluating the properties of products, an optimized product generated in 5 min at 350 ℃ was selected. Hemicellulose and extractives have been extensively removed by 99.6% and 90.2%, respectively. The product had a pH of 6.81, EC of 344 µs/cm, water holding capacity of 0.507, germination index of 96.54% and half-life of longer than 100 years. Therefore, the product was investigated to replace peat as growing media at different rates with three horticultural plants and evaluated by dry weight, plant height, SPAD value, nitrogen content, and the number of flowers. The optimized products could replace up to 40 vol% peat and benefit plant growth.