Toward the complete utilization of rice straw: Methane fermentation and lignin recovery by a combinational process involving mechanical milling, supporting material and nanofiltration

Abstract

Rice straw was mechanically milled using a process consuming 1.9 MJ/kg-biomass, and 10 g/L of unmilled or milled rice straw was used as the carbon source for methane fermentation in a digester containing carbon fiber textile as the supporting material. Milling increased methane production from 226 to 419 mL/L/day at an organic loading rate of 2180 mg-dichromate chemical oxygen demand/L/day, corresponding to 260 mLCH4/gVS. Storage of the fermentation effluent at room temperature decreased the weight of the milled rice straw residue from 3.81 to 1.00 g/L. The supernatant of the effluent was subjected to nanofiltration. The black concentrates deposited on the nanofiltration membranes contained 53.0–57.9% lignin. Solution nuclear magnetic resonance showed that lignin aromatic components such as p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) were retained primarily, and major lignin interunit structures such as the β–O–4-H/G unit were absent. This combinational process will aid the complete utilization of rice straw.

 

Keywords

  • Rice straw;
  • Mechanical milling;
  • Methane fermentation;
  • Nanofiltration;
  • Nuclear magnetic resonance

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