Promotion of methane production and degradation of pyrolysis oil during its co-anaerobic digestion process via addition of N-doping hydro-chars.

Pyrolysis of wastes usually produces toxic pyrolysis oil (PO), which has complex ingredients, including benzene series and long-chain macromolecule organic pollutants. Co-anaerobic digestion (co-AD) can be an economic and high-efficiency method for PO degradation and recovery of methane simultaneously, but complete degradation of PO has not been achieved yet. Addition of a hydro-char in the process is beneficial to PO degradation and methane production. In this study, to further enhance the effectiveness of the hydro-char, nitrogen (N) was doped into the hydro-char by plasma modification in a NH3 atmosphere; and the effectiveness of the N-doped hydro-chars for promoting PO degradation and methane production during the co-AD process were evaluated. The experimental results indicated that all the hydro-chars can reduce the biotoxicity of the PO, improve its degradation during the co-AD process, and increase the methane yield. Compared with the plain hydro-char (HC), the hydro-chars modified at ambient temperature (HC-NH3-P-25) and at 500 °C (HC-NH3-P-500) can help achieving complete PO degradation and increasing the methane yield more effectively. The anaerobic digestor containing the HC-NH3-P-500 had the highest apparent methane yield (169.03 mLCH4/mLPO) and highest COD removal rate (79.5%). The nitrogen content, specific surface area, and electron transfer capability are found to be the key factors affecting PO degradation and methane yield; and the HC-NH3-P-500 had the highest N-doping, most specific surface area and electron transfer capability, explaining its best performance. The microbial communities of the digestate with the addition of the hydro-chars were founded to be richer with Clostridia and Methanosarcina, which could enhance the electron transfer between different microorganisms and contribute to the PO degradation.