Objective  The heating time in a hydrothermal carbonization process used to convert Miscanthus floridulus into biochar was varied to examine the chemical and microstructural differences in the resulting material for potential applications.

  Method  An experiment applying 200℃ hydrothermal carbonization on the perennial grass in a pressurized reaction cauldron for 0, 1.5, 3.0, 6.0, and 9.0 h was conducted. Properties including organic carbon, total nitrogen, total phosphorus, C/N ratio, ash, pH, yield, and loss of elements of the biochar were determined, and microstructure observed under a scanning electron microscope (SEM).

  Results  After the carbonization, the biochar had an organic carbon content ranging from 39.90% to 54.82%, a C/N ratio from 57.90 to 81.22, and a yield from 57.3% to 67.1%. The longer the treatment time, the higher the organic carbon, total nitrogen, C/N ratio, carbon loss, phosphorus loss and ash loss were in the biochar. The total phosphorus, total ash, pH and yield decreased as well. The rate of nitrogen loss peaked after 6 h of carbonization but was significantly lower at 9 h compared with other treatment time. Under SEM, the biochar appeared with numerous starch particles. Prolonged carbonization significantly charred the surface with disorganized, thickened, and stacked tissues. Most of the vascular bundles was damaged, the thin-walled cells blocked, and the edges thickened with clear outlines. The starch granules began to slacken followed by the appearance of many irregular-shaped or ellipsoidal microspheres that coalesced and fused together.

  Conclusion  The hydrothermal carbonization converted M. floridulus into biochar with changed chemistry and microstructure. Prolonged heating decreased the yield, pH, carbon, and phosphorus in the material. A carbonization time longer than 3 h could already significantly alter the properties of the biochar as observed under the experimental conditions.