Hydrothermal co-carbonization of medium-density fiberboard with N-rich compounds to produce N-containing compounds and N-doped biochar: Product distribution, nitrogen transformation pathway and electrochemical performance

Abstract

High-value utilization of waste resources has received widespread attention. In this study, N- containing chemicals (NCCs) and nitrogen-enriched hydrochars were prepared by co-hydrothermal carbonization (Co-HTC) of waste medium-density fiberboard (MDF) and various nitrogen carriers. The influence of the nitrogen carrier type (chitosan, urea, or melamine) and the MDF: nitrogen carrier mass ratio on yield and production distribution were investigated. The formation mechanism and possible reaction pathways of NCCs in bio-oil and N-doped biochar were elucidated, and the electrochemical performance of N-rich hydrochar as a precursor for supercapacitor materials was explored. The results indicated that the introduction of an ammonia source during hydrothermal carbonization obviously increased the NCC content via the Maillard reaction and decreased the hydrochar yield, resulting in structures disordered and poor microporosity. The introduction of chitosan significantly promoted pyridine (35.50 %) and hydrochar (36.95 %) generation owing to the nitrogen fixation effect (N: 6.27 ∼ 15.56 %). Urea addition facilitated the formation of NCCs (89.92 %) and imidazoles/pyrazines (39.46 %). The addition of melamine, which has the most amino functional groups, significantly increased aniline compound formation. Moreover, the activated hydrochar derived from the ammonia source Co-HTC exhibited great electrochemical performance, and the prepared electrode material derived from MDF: chitosan = 1:5 achieved highest reversible specific capacitance of 174.5F/g at 0.5 A/g and 96.46 % initial capacitance retention after 10,000 cycles at 10 A/g due to developed mesoporosity, high specific surface area and enriched N/O functional groups. This study provides a novel sustainable strategy for the efficient and high-value utilization of waste MDF and improved the economic viability of MDF biorefineries.